A Global Atlas of Atolls 103226246X, 9781032262468

Table of contents :
Cover
Half Title
Title Page
Copyright Page
Table of Contents
Preface
Acknowledgments
Authors
1 A Global Atlas of Atolls
2 Methods
3 The Polynesian Pacific and the Atolls of the Tuamotu Archipelago
4 The Atolls of the Society Islands, French Polynesia
5 The Atolls of the Cook Islands
6 The Atolls of the Southern Central Pacific
7 The Atolls of Tokelau
8 The Atolls of Tuvalu
9 The Atolls of Kiribati
10 The Atolls of Hawai'i and the Pacific Remote Islands of the United States
11 The Micronesian Pacific and the Atolls of the Marshall Islands
12 The Atolls of the Caroline Islands
13 Melanesia and the Atolls of New Guinea
14 The Atolls of the Solomon Islands
15 The Atolls of Fiji
16 The Atolls of the Coral Sea
17 The Atolls of the North West Shelf of Australia
18 The Atolls of Indonesia
19 The Atolls of the South China and Sulu Seas
20 The Atolls of the Central Indian Ocean
21 The Atolls of the Western Indian Ocean and the Red Sea
22 The Atolls of the Western Atlantic
23 A Summary of Atolls and a View into the 21st Century
Index

Citation preview

A Global Atlas of Atolls

Scattered like dots rising from the deep across vast expanses of the world’s tropical and subtropical oceans, atolls with their turquoise lagoons and reefs teeming with colorful marine life have captured the public imagination. They also have been the homeland of millions of people for millennia as various groups of migrants spread across the far reaches of the Pacific, Indian and Western Atlantic regions. Developed from recently available satellite data, A Global Atlas of Atolls presents high-quality details of 476 atolls across the globe, characterizing aspects of the atoll rim, the lagoon, and their coral reef communities in unprecedented detail. In synthesizing and enhancing understanding of these unique seascapes, this volume provides a distinct compendium of descriptions and images as well as documentation of the environmental conditions of winds, waves, and tides and a summary of the background literature for each atoll area. There is no comparable work. After an introduction that includes a glossary of terms, each atoll is documented in the form of an atlas written for scientists, but accessible to any diver or reader interested in these spectacular reef-island habitats. This book also describes some current challenges and perspectives on their future. It will be useful as a reference work for marine scientists, while providing a minimum of technical jargon for those who are not scientists, but who enjoy reading about exotic places with unusual attributes.

‘A remarkably comprehensive and informative account of coral atolls—one of our world’s most enigmatic, beautiful but endangered marine habitats. A scholarly account, yet easy to read and understand. It is a highly informative text and a reference for anyone engaged in coral reef research.’ Professor Charles Sheppard, University of Warwick, UK

A Global Atlas of Atolls Walter M. Goldberg

Emeritus Professor of Biological Sciences, Florida International University, Miami, FL, USA

Eugene C. Rankey

Professor of Geology, University of Kansas, Lawrence KS, USA

Designed cover image: Aerial photo of Pukapuka Atoll in the Northern Cook Islands, central Pacific with its triad of islands that reach a maximum of 4 m elevation and its distinctive stingray-like rim with its westfacing tail. The lagoon, up to 70 m deep, is carved into pocket-like divisions called reticulations that are thought to represent erosion during lower stands of sea level. Photo courtesy of Ewan Smith First edition published 2024 by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742 and by CRC Press 4 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN CRC Press is an imprint of Taylor & Francis Group, LLC © 2024 Walter M. Goldberg, Eugene C. Rankey Reasonable efforts have been made to publish reliable data and information, but the authors and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged, please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www.copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. For works that are not available on CCC please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. ISBN: 978-1-032-26246-8 (hbk) ISBN: 978-1-032-26247-5 (pbk) ISBN: 978-1-003-28733-9 (ebk) DOI: 10.1201/9781003287339 Typeset in Garamond Pro by codeMantra

Table of Contents Preface .............................................................vi Acknowledgments ..........................................vii Authors ......................................................... viii 1 A Global Atlas of Atolls ............................. 1 2 Methods ...................................................25 3 The Polynesian Pacific and the Atolls of the Tuamotu Archipelago .........27 4 The Atolls of the Society Islands, French Polynesia ......................................79 5 The Atolls of the Cook Islands.................87 6 The Atolls of the Southern Central Pacific ......................................................97 7 The Atolls of Tokelau ............................. 101 8 The Atolls of Tuvalu ...............................105 9 The Atolls of Kiribati ............................. 115 10 The Atolls of Hawai’i and the Pacific Remote Islands of the United States ......143 11 The Micronesian Pacific and the Atolls of the Marshall Islands ................155



Preface Scattered like dots rising across vast expanses of the world’s tropical and-subtropical oceans, atolls with their turquoise lagoons and reefs teeming with colorful and diverse marine life have been objects of enchantment and fascination since the time of Darwin more than 180 years ago. Their appearance as variously shaped specks on the ocean surface belies their support, often by massive volcanic platforms rising thousands of meters from the ocean floor. Some barely reach the surface, but waves, tides, and long-range swells may influence the development of reefs followed by islands perched like a crown at or near the edge of the platform. On most atolls, the side facing the wind forms stronger islands and reefs than the leeward side. A lakelike lagoon of various sizes and shapes, ranging from irregular dinnerplates to shallow or deep bowls, is surrounded by islands forming a rim, and to a large degree, the nature of that rim determines the lagoon’s characteristics. When essentially sealed from regular oceanic exchange, the lagoon can exhibit fewer species or even biological impoverishment. Alternatively, they can be enriched and diverse as protected habitats, especially if islands develop channels, passes, and a more open structure. These dynamics are just beginning to be unravel, but the specifics of lagoon circulation are unknown for most atolls. Atolls have served as human steppingstones in exploration and migration across equatorial oceans and have been the homeland for millions of migrants for millennia, but with nutrient-poor soils, few edible

plants can be grown. And freshwater, another essential requirement for the support of human life, is by no means a guaranteed characteristic of the atoll environment. At the same time, because of their precarious position, commonly a few meters above sea level, inhabited atoll islands are often prone to flooding and contamination of water supplies by sea-level rise. Accordingly, some atoll residents are among those who have or may yet become ‘climate refugees’. Developed from recently available satellite data, A Global Atlas of Atolls presents high-quality details of 476 atolls across the globe, characterizing aspects of the atoll rim, the lagoon, and their coral reef communities in unprecedented detail. In synthesizing and enhancing our understanding of these unique seascapes, this volume provides a distinct compendium of descriptions and images as well as documentation of the environmental conditions of winds, waves, and tides and a summary of the background literature for each atoll area. There is no comparable work. After an introduction that includes a glossary of terms, each atoll is documented in the form of an atlas written for scientists but accessible to any diver or reader interested in these spectacular reef-island habitats. This book also describes some current challenges and perspectives on the future. It will be useful as a reference work for marine scientists while providing a minimum of technical jargon for those who are not scientists but who enjoy reading about exotic places with unusual attributes.

Acknowledgments We thank the knowledgeable individuals listed below for their expertise and their effort for reviewing the chapter areas of their expertise. This book is better for their insights. The responsibility for the contents, however, is ours and ours alone. Serge Andréfouët Institut de Recherche pour le Développement, France Pat Colin Coral Reef Research Foundation, Palau Charles (Chip) Fletcher University of Hawai’i, USA Murray Ford University of Auckland, New Zealand Rodrigo Garza-Perez National Autonomous University of Mexico, Mexico Eberhard Gischler Goethe-University, Frankfurt am Main, Germany Eko Haryono Research Center for Marine Science and Technology, Indonesia Kirby Morejohn Ministry of Marine Resources, Cook Islands Patrick Nunn University of the Sunshine Coast, Australia

Mick O’Leary University of Western Australia, Australia Mika Perez Department of Economic Development, Natural Resources and Environment, Tokelau Tomas Tomascik Independent research scientist Tion Uriam Ministry of Information, Communications & Transport, Kiribati Georg Warrlich Shell Global Solutions International Arthur Webb Tuvalu Coastal Adaptation Project (TCAP); Resilience & Sustainable Development Unit, United Nations Development Programme Jody Webster University of Sydney, Australia We are grateful to DHI for access to and use of atmospheric and wave data from the Metocean Data Portal (https://www.metocean-on-demand.com/#/ main) and the Global Tide Model (via MIKE 21). Remote-sensing data - PlanetScope, Sentinel, or Landsat data - were generously provided through Planet‘s Education and Research Program.

Authors Walter M. Goldberg graduated from the American University, Washington, D.C. with a Bachelor of Biological Science degree and was awarded a Ph.D. in Biological Oceanography by the University of Miami’s School of Marine and Atmospheric Sciences. He is currently a Professor Emeritus at Florida International University in Miami, where he was a faculty member for 40 years and the youngest in the Department of Biological Sciences to begin his career there at the age of 27. While on the faculty, he taught a variety of courses ranging from Electron Microscopy for graduate students to Marine Science for non-majors. In addition to teaching and research, Walter served as a department chair at a time when Biological Sciences at FIU was half the size it is now. He is the author of more than 50 professional papers, mostly on the formation, structure, and biochemistry of coral skeletons. He has also written two books in addition to this one: The Biology of Reefs and Reef Organisms (UChicago Press, 2013) and The Geography, Nature and History of the Tropical Pacific and its Islands (Springer World Regional Geography, 2018). After retirement, he taught scientific writing at FIU for an additional decade. He now lives in Stuart, FL with his wife Rosalie whom he met when she was a freshman at AU. They have two boys and two granddaughters. Walter has always been involved in sports. At AU, he was the captain of the wrestling team and was a conference champion at 123 lbs. He played racquetball and squash for 25 years and now is an avid pickleball player. His nom du sport is Walt, not Walter.

Eugene C. Rankey is a Professor of Geology at the University of Kansas, where he has taught since 2008. A geologist by formal training, Gene’s research program focuses on understanding geological, chemical, physical, and biological aspects of the oceans, how they shape the seascapes of tropical marine and coastal systems, and expression of comparable processes in the ancient rock record. In addition to remote-sensing analyses from around the globe, field efforts have ranged from the South Pacific (including the Cook Islands, French Polynesia, and Kiribati), Southeast Asia, and the Caribbean and included numerical modeling of dynamics of shoreline and atoll systems. Gene graduated with a B.S. degree from Augustana College (IL), a M.S. from University of Tennessee, and a Ph.D. from University of Kansas; he worked at Exxon Production Research Company, Iowa State University, and the Rosenstiel School of Marine and Atmospheric Sciences at the University of Miami before returning to Kansas. He has written more than 60 nerdy scientific papers, edited several volumes, and served as an editor and/or an associate editor for five journals. He lives in Paola, Kansas, near the geographic center of the U.S., and far from the ocean, where he claims to be a soccer (football) star in the city’s adult league, straightens headstones in the church cemetery, and lifts modest weights. His four kids are grown and scattered across the globe, but his dogs miss him when he travels to see his island friends.

A Global Atlas of Atolls Atolls are low to the water, confined, and often distant from other more hospitable places. The Lapita people (Chapter 3), the first atoll colonists, came to the atolls from the nearby Solomon Islands, perhaps as an experiment about 2,700 years ago, bringing with them familiar foods from larger islands that did not exist on atolls. At that time, the Tuamotu group and others in the central Pacific were still underwater and would require about another 1,700 years before being occupied (Chapter 3; Spriggs, 1991; Dickinson, 2023). Europeans ‘discovered’ atolls more than two millennia later, and the first to lay eyes on them were less than impressed with islands having a saltwater lake in the center. Long journeys required re-provisioning, and atolls were typically not the best places to take advantage of natural resources. After sailing for more than two months we finally espied land. The two islands were small, uninhab‑ ited, and afforded neither water nor sustenance of any kind-only birds, trees, and many sharks. We named them the Unfortunate Islands. –Antonio Pigafetta aboard Magellan’s ship Trinidad January 24, 1521. By contrast, Charles Darwin was stunned with the beauty of atolls: Everyone must be struck with astonishment, when he first beholds lagoon islands, vast rings of coral-rock, often many leagues in diameter, here and there surmounted by a low verdant island with dazzling white shores, bathed on the outside by the foaming breakers of the ocean, and on the inside surrounding a calm expanse of bright, pale green water. —Charles Darwin aboard HMS Beagle, 1842 Charles Darwin was fascinated by atolls even though he had only seen one of them. During his voyage aboard HMS Beagle, he began laying a foundation for describing and interpreting how atolls may have formed, generating a series of important scientific papers on the subject, along with controversies

that continue to this day. In the post-Darwinian era (assuming there is such a time), we have learned quite a bit about atolls, but have become no less enchanted by them. Scientific interest in atolls is not surprising even if they occupy a mere fraction of the ocean floor area constituted by other kinds of coral reefs. Atoll islands are flat, eliminating all the variables commonly associated with altitudinal differences. They are tropical or subtropical and oceanic, eliminating significant temperature differences. They are formed of calcium carbonate derived from plants and animals, eliminating significant geological differences. They are structurally simple, minimizing complexities of hydrology and rainfall. Their terrestrial flora and fauna are numerically small and simple, and even though that is not the case underwater, they are spatially limited. Other factors including island size and distance from neighbors, the impact of waves, storms, and tides all point to atolls as natural laboratories. If overlaid with human occupation, culture, and history, atolls become fascinating places. Much of the initial 20th century scientific interest in atolls was generated by the testing of nuclear weapons. That process, however, began classic scientific work that served as the foundation of modern reef science, especially on Enewetak and Bikini in the Marshall Islands. Atoll geology and mapping of lagoons as well as interest in the formation of spur and groove systems and the role of coralline algae in reef building began with Tracey et al. (1948) and Emery et al. (1954). The first successful drilling to an atoll volcanic basement on an atoll was documented in Ladd and Schlanger (1960). The structure of fish communities on Enewetak, including their habitats and an analysis of their diets (Hiatt and Strasburg, 1960), was among the first of many such studies carried out on atolls. The discovery that symbiotic relationships allow a highly complex but isolated reef ecosystems to exist while surrounded by nutrient-poor water also began there (Odum and Odum, 1954). A scientific journal called Atoll Research Bulletin, dedicated to their study and cited profusely herein, began in 1951 and continues to the present as a valuable reference, broadening our knowledge of atolls worldwide.

DOI: 10.1201/9781003287339 -1

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A quick search online will reveal lists of famous atolls, as well as those designated as the largest, most captivating, and the most beautiful. Many of these locales are advertised as exotic diving destinations that are isolated and untouched. Others are privately owned and have become commercial destinations as on Gaafaru in the Maldive Islands, or Île Desroches in the Seychelles where one can be married at a Four Seasons hotel. Others are touted for the privacy they afford, including the Brando on Tetiaroa in French Polynesia, where actor Marlon Brando wanted to ensure that his atoll would be a model of ecological consciousness (Sachet and Fosberg, 1983). And it is, but it is also a very expensive way to be alone. Atolls include coral reefs that host some of the most diverse and complex ecosystems on the Earth. But unlike the vast expanses of more famous barrier reefs, atolls are comparatively small and composed of islands strewn like dots in the ocean across the broad expanses of tropics, most of which are seldom visited by outsiders and are poorly known to the wider world. We identify 476 atolls in this volume in the pages that follow, along with descriptions of different types of atolls illustrated in the following chapters. We approach the subject of atolls visually, one-by-one, archipelago-by-archipelago, and from the broad perspectives of geography, biology, and geomorphology, to produce the first world atlas of atolls. Beyond describing each atoll, we document the physical factors, including wind, waves, and tides, and summarize the scientific literature for each archipelago. This atlas is based on science and is intended to be a reference work useful to the scientific community. However, we attempt to minimize jargon and write for a wide audience including divers, armchair travelers, those with an interest in natural history, and people who enjoy maps, as well as reading about and seeing beautiful places.

Satellite imagery We have made extensive use of the Allen Coral Atlas (‘the Atlas’), and attempted to be consistent with it wherever possible, but we also referred to the satellite imagery for guidance. The Atlas is a unique collection of satellite images and technologies that map and analyze important elements of the world’s coral reef environments (Allen Coral Atlas, 2022). The team is managed by the Arizona State University Center for Global Discovery and Conservation Science, along with partners from Planet Labs, the University of Queensland, the University of Hawai’i, and the National Geographic Society, all of which are funded by the charitable contributions of the late Paul G. Allen and his philanthropic organization, Vulcan Inc. More information on this effort is included in Lyons

Figure 1.1 A Dove nanosatellite with its telescope, camera, solar panels, and communication antenna in operational mode. Courtesy of Planet Labs. (https://www. satimagingcorp.com/satellite-sensors/other-satellitesensors/dove-3m/.)

et al. (2020), Roelfsema et al. (2021) and Kennedy (2021). The Atlas primarily uses data from Planet Labs, which has developed constellations of about 130 nanosatellites called Doves (Figure 1.1) that have been launched with continual improvement from the International Space Station (Planet Labs, 2017). Each Dove is solar powered, weighs 5 kg (11 lbs), and collectively, they can take multispectral images with a resolution of less than 4 m2/pixel for an area of ~200 million km2 every day. Each image in the Allen Atlas is a single image or a mosaic selected for sparse cloud cover and the highest levels of water clarity. Images also are subjected to a number of analytical adjustments including orthorectification, in which they are corrected for distortions from the sensors and from the angle between the satellite and the imaged position on Earth, to accurately position features. Algorithms are also applied to mask clouds and correct for atmospheric distortion, surface reflection, and sun glint. Corrections for water conditions include those for light attenuation due to absorption with increasing depth, light scattering, and reflection from the bottom. The optical images presented here use the visible spectrum (red, green, and blue wavelengths), which corresponds with human perception. More information on this technology can be found in Sayfin (2020) and Kopacz et al. (2020). The Atlas has mapped nearly all coral reef areas within 30° north and south of the equator. In addition to providing optical satellite images (Figure 1.2a), the Atlas provides classified geomorphic attributes (thematic maps) of each reef. Using all four spectral bands (blue, green, red, and near infrared) of the Dove satellite system, spectral reflectance properties of land and marine areas can document atoll benthic (bottom) properties objectively and over large areas. Thus, changes in concentrations of photosynthetic pigments in coral, algae, and seagrass, as well as light scattering by inorganic materials and sand or rubble derived from

A Global Atlas of Atolls

Figure 1.2 Views of Allen Atlas data. Vuladdore Atoll, Paracel Chain, South China Sea displayed in (a) satellite imagery, with the Allen Atlas overlay of the thematic maps of (b) geomorphological classes and (c) benthic habitats. (See also Vuladdore Atoll, Chapter 19.) Image credit for parts b and c: Allen Coral Atlas.

organisms, permit use of spectral reflectance data to determine the nature of atoll reef and reef-associated habitats. This process yields color-coded thematic maps (Figure 1.2b) that define the geomorphic regions, including deep outer reef slope, the reef crest and rim, the reef flat, and the attributes of the lagoon including its patch reefs, pinnacles, and reticulate reefs (all of which are defined below). Not every atoll includes every geomorphic class recognized by the Atlas. The geomorphic maps of the Atlas display these data for water depths

up to approximately 15 m. Areas that have a defined class are referred to as ‘coded’ in the following chapters: other undefined or unmapped areas are ‘uncoded’. Similarly in the Allen Atlas, zones are mapped as benthic habitats (Figure 1.2c), with colors describing the extent of coral and coralline algae development (collectively), and other bottom types. In the description of individual atolls, we distinguish reefs that are shallower than the geomorphic deep outer reef as the shallow outer reef. We also report the widths

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of these reef zones with the understanding that such measurements do not indicate biological diversity. We generally describe the biological communities where the literature is sufficient. The Atlas also distinguishes seagrass, rock, rubble, and sand (Roelfsema et al., 2002); as with the geomorphic classes, not every atoll includes every defined benthic habitat. The depth delineated by these biological zones is limited to about 10 m below sea level limited by the penetration of light. The details of the analytical methods can be found in Lyons et al. (2020) and Kennedy et al. (2021). The imagery is also capable of detecting reef stress, in the form of coral bleaching, in real time (Xu et al., 2020). With all that being said, we note that the Atlas is a work in progress. There are places in the text that at the time of this writing are miscoded including atolls where the lagoon is shown extending to deep water, and areas that are uncoded where they should be according to reported depths or from observation by us or others. We attempt to reconcile the Atlas data with the satellite imagery where possible.

Atoll structure and formation An atoll can be loosely described as an annular reef and its islands that may protrude a few meters above sea level and encircle a deeper lagoon. This skeletal definition deserves more flesh, and the one offered by Wiens (1962) provided some: an atoll is a more or less continuous coral reef that emerges from the open ocean or is slightly submerged and surrounds a distinctly deeper lagoon or several lagoons without volcanic islands, and whose upper seaward slopes rise steeply from a generally volcanic foundation too deep for the light-dependent growth of reef corals. Because the greatest number of atolls are formed atop deep, subsiding volcanic platforms in the Pacific Ocean, this definition is a good start (but see below). Darwin conceived of a theory of atoll formation aboard HMS Beagle in 1836. His idea was that atolls formed on volcanic islands that became dormant, which then began to cool and slowly sink, a process known as subsidence. The Pacific Ocean has aligned chains of submarine volcanoes called seamounts, some of which almost reach the surface or even become active volcanic islands. These features form from arguably stationary ‘hot spots’, places where small, hot plumes rooted deeply in the Earth’s mantle rise through oceanic crust. Much of the Pacific Ocean is on the Pacific tectonic plate, which is moving, and trails of volcanic seamounts and atolls record its absolute motions over several stationary hotspots. In the case of the Hawaiian-Emperor seamounts, the chain is nearly 6,000 km long and has lasted for at

least 82 million years (O’Connor et al., 2013). The island of Hawai’i, near the southeastern end of the chain, is the current active site for this hotspot, and islands, atolls, and seamount become progressively older to the northwest. Other chains and hotspots exist but are not as easily followed. Nothing was known about plate motions or hotspots in Darwin’s lifetime, but the idea of subsidence made sense, and if Darwin was right, it should be possible to show that old atolls have the calcareous remains of shallow-water reef organisms hundreds of meters below the sea surface, in water depths where they cannot have lived or grown. To test this prediction, boreholes that collected core samples were drilled by the Royal Society of London on Funafuti Atoll (Tuvalu Archipelago) between 1896 and 1898, 14 years after Darwin’s death. One of them reached a depth of 340 m, and although it encountered no volcanic material, it was clear that the core contained shallow-water calcareous remains at its base. Darwin’s theory was probably correct! The volcanic-basement hypothesis remained unconfirmed until the U.S. drilled two boreholes on Enewetak Atoll (Marshall Islands) in 1952 in preparation for nuclear testing. There, volcanic material was finally encountered below the reef debris at a depth of nearly 1,500 m below sea level (Figure 1.3). Darwin was right! There was a second part to Darwin’s observations and interpretations. While visiting Tahiti, a high volcanic island in French Polynesia, in November 1835, Darwin climbed a local peak for a better view and saw nearby Moorea with its surrounding barrier reef, lagoon, and central volcanic island. At that moment, he thought of subsidence as the starting point for what is now the classic view of atoll formation—that different types of reef morphologies form in a succession that ends with atolls (Figure 1.4). Coral reefs initiating growth at sea level on the flanks of volcanic islands are called fringing reefs. He anticipated that with subsidence of the conical volcano, the area of the volcanic island above sea level would shrink. Simultaneous vertical growth of the reef would form a lagoon between the island and the now-offshore barrier reef. He considered an atoll to represent a final stage, when the volcanic peak sank beneath the sea surface, leaving only a lagoon in the center, surrounded by a reef that grew on the outer surface (Figure 1.3). There are clear instances where such a succession can be inferred. For example, Aitutaki in the Cook Islands (Figure 1.5a) is an example where the volcanic structure, 119 m high, still protrudes from the lagoon. Presumably, when the volcano finally subsides completely, Aitutaki will be an atoll, but right now it is an almost-atoll, and that is an actual

A Global Atlas of Atolls

Figure 1.3 After an initial failure to reach volcanic substrata on Tuvalu in 1896, 56 years later, boreholes drilled on Enewetak Atoll succeeded penetrating through shallow-water coral growth of nearly 1,500 m thick to reach the volcanic base. (After Goldberg, 2013 as cited.)

Figure 1.4 The origin of atolls envisioned by Darwin. A fringing reef on a volcanic shore is followed by subsidence and development of an offshore barrier reef with the development of a lagoon. With continued subsidence, the volcanic island of an almost-atoll may ultimately be submerged, resulting in an atoll. (Modified from and courtesy of the U.S. Geological Survey.)

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term used by scientists. There are other islands on the southern portion of the reef that are composed of both volcanic basalts and coral material suggesting that coral growth began occurring here before volcanic activity had entirely ceased (Stoddart and Gibbs, 1975). On Maupiti (Figure 1.5b and c) in the Society Islands (French Polynesia), the central island is 372 m high and represents the eroded tip of a shield volcano that formed 4.5 million years ago. It is currently surrounded by a lagoon 1,500 m wide and is separated from the open ocean by a barrier reef as Darwin might have predicted. Indeed, Darwin was able to see all three reef types (fringing, barrier, and atoll) in several places near Tahiti so that subsidence and the sequence of reef formation that stems from it became known as Subsidence Theory. Exploring Isles (Vanuabalavu; Figure 1.5d) is another almost-atoll in the Lau Islands of Fiji, but it has had a complex history. The largest island is 4.8 by 22.5 km long and 283 m high at the western side of the lagoon (Agassiz, 1899), but the oldest exposed rocks are limestone followed by an alternation of volcanic and limestone strata that underwent subsidence beginning some 4 million years ago. However, the layers of this rocky pousse café also demonstrate considerable uplift and tilting. The result, at least for the largest island, is a confusing geological picture with volcanic rock exposed only at the center. Nonetheless, other islands on the rim of Exploring Isles (e.g., Munia on the south side) are clearly volcanic (Nunn, 1987; Nunn et al., 2002). Lastly, there is Clipperton Island, an almostatoll in the Eastern Pacific that would qualify as an atoll but for a 29 m high volcanic rock at the SE margin (Figure 1.5e). The rest of the island is carbonate rock and sand that is only a few meters above sea level but is closed to ordinary tidal exchange. Thus, the lagoon tends to be dominated by evaporation and is hypersaline (Sachet, 1962). Coral reefs require warm, clear tropical waters and abundant sunlight for their growth due to the near-universal presence of microscopic photosynthetic algae called zooxanthellae that occur within coral tissue. These symbiotic plants are required by their coral hosts and are key factors to the survival of both since they are sensitive to warmth and sunlight among other environmental factors. Therefore, extensive and diverse reef-building corals typically are limited to the upper 20 m of water, and lesser construction by reef communities is largely restricted to the upper 80 m, limited by light availability. Darwin was aware that reef corals are limited to shallow-water environments. However, the discovery of glacial episodes during the Pleistocene epoch (~2.6

million years to ~12,000 years ago) and the recognition that ice growth led to sea levels about 125 m lower than present during the most recent glacial period about 18,000 years ago were not known. This discovery led to what was referred to as the Glacial Control Theory (Daley, 1915) in which wave erosion and abrasion of volcanic platforms during past low sea levels (glacial episodes) caused sculpting and planation on atolls and volcanic islands. This model posited that erosion was independently responsible for different types of reef morphology, suggesting that as sea level rose again during glacial melting, reefs covered the erosional landscapes. Barrier reefs were thought to have formed on erosional terraces, whereas atolls were preceded by platforms that had become leveled. By contrast, the alternative Antecedent Platform Theory holds that any high spot that occurs in shallow water within the tropics is a potential coral reef foundation if ecological conditions permit. These reef foundations or platforms might be formed by erosion, volcanic eruption, or other geological processes or combinations of processes, and accordingly, they may occur independently of sea-level changes. The interesting aspect of these ideas is not only the distinction from glacial control but also that different atoll shapes may be determined or influenced by the configuration of the pre-existing platform (Hoffmeister and Ladd, 1944). A modification, the Antecedent Karst Theory (Purdy and Winterer, 2001, 2006), suggests that reef types are unrelated and that antecedent platforms are the result of karst, a landscape formed by limestone dissolution and erosion by the acidic rainwater (and groundwater) during low stands of sea level. In this model, reefs around the edges of islands or atolls are less prone to solution than the atoll interior, and this differentiation leads to barrier lagoon formation on exposed continental or island margins, or a bowl-like depression in the center parts of islands. An atoll is thought to form with subsequent marine flooding during global warm periods. These erosional foundations may control the morphology of modern reefs. Subsidence theory and its genetic sequence of reef development were being challenged. However, despite numerous examples of three types of reefs in the same area, a sequence showing the transformation of a fringing reef to a barrier reef and then into an atoll is rare. However, recent drilling in Tahiti has documented a geological record that confirms a transition from fringing to barrier reef associated with the post-glacial rise in sea level (Blanchon et al., 2014). Atoll scientists today recognize that no one model alone can explain the genesis and growth of all atolls through the millennia. A general conceptual model recognizes that reef and atoll initiation requires some

A Global Atlas of Atolls

Figure 1.5 An assembly of almost-atolls. (a) Aitutaki, southern Cook Islands. The large island at the north is volcanic and rises to an altitude of 119 m; the others include a mix of volcanic islands and islands made of reef-derived fragments. (b) Maupiti, Society Islands group, French Polynesia. The central island is 372 m high and is the eroded tip of a shield volcano that formed 4.5 million years ago. (c) Field photo from flank of volcano, Maupiti (almost-atoll), French Polynesia. View to the south and east includes the barrier reef and low motu of Pitiahe and Tiapaa. (Photo by Gene Rankey.) (d) Exploring Isles, Lau Islands of Fiji, aka Vanuabalavu: The elongated, 283-m-high island on the western side is a subsided volcano that has recently been uplifted and tilted along with a reef limestone coating, now exposed above sea level. Southernmost Munia Island is more clearly volcanic. (e) Clipperton Island in the Eastern Pacific technically qualifies as an almost-atoll due to volcanic rock that is 29 m high and located at the SE side (white arrow). Otherwise, the atoll surface is composed of carbonate. Images a, b, d, and e © 2021-2022, Planet Labs PBC.

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mechanism to produce a shallower-water platform in the surrounding deeper, open ocean. This relative high spot can be a subsiding volcanic foundation, as postulated by Darwin (as on Funafuti and Enewetak described above), a non-volcanic foundation (e.g., Alacrán in Mexico and Hogsty in the Bahamas), or even the isolated, vestigial highs of older, once-more-extensive reef systems (e.g., the Maldive Islands; Droxler and Jorry, 2021). Sea-level rise and fall during interglacial and glacial periods can also provide foundations from eroded or partly dissolved reef platforms.

Atoll structure We use a number of geological and biological descriptive terms in this atlas that define the composition and structure of atolls (Figure 1.6 and Table 1.1). We also define different types of atolls and attempt to distinguish atolls from other atoll-like structures. Classic atolls, occasionally called Darwinian atolls, already have been described as mid-ocean reefs with

a central lagoon above a subsiding volcanic platform surrounded by deep water. In the Allen Coral Atlas and in this volume, the term reef platform refers to the underlying substrate upon which coral growth occurs, and we operationally define the platform as the portion of the reef that supports coral growth and is visible in satellite imagery. Although the extent of this area that is visible depends on water clarity among other factors, it is generally limited to depths of up to 15 m in the geomorphic views of the Allen Atlas. We include that area in the reported measurements in cases where the visible platform is wider than what the Atlas has coded.

The reef rim A portion of the reef platform projects into shallow water less than a few meters deep and includes islands (if any are present). This region forms a calcareous barrier separating the lagoon from the open ocean and is referred to here as the rim of the reef, which we define for atolls as the shallow-water

Figure 1.6 Overview of atoll zonation and rim terminology as described in the following pages and the glossary. (Illustration by Elena Hartley, elabarts.com.)

A Global Atlas of Atolls

Table 1.1 Glossary of Reef Terminology Almost-atoll: An atoll-like structure with residual volcanic material, sometimes residing on the rim, but more often as a volcanic peak protruding from the lagoon. The presumption is that continued subsidence of the volcanic platform will ultimately result in a true atoll. Channel: A shallow cross-reef opening, often between islands or groups of islands, that may allow exchange of openocean water into and out of the lagoon. Large channels may be hundreds of meters wide, but they are shallow. Closed atoll: An atoll rim in which there are no passes, shallow channels, or functional hoa. Water exchange is dependent on percolation through the rim, storm activity, or atmospheric exchange. Crustose coralline algae: Reef-building red algae that can form dense sheets of calcium carbonate over and around other reef-building organisms, forming a durable and resistant reef binding agents. They can also form ‘rhodoliths,’ round, mobile balls of encrusting red algae. Drowned atoll: Reef with an atoll rim morphology, but at a depth below 25 m, suggesting that it is unable to keep up with sea-level changes and likely exhibits limited coral growth. Atolls at these depths are not commonly visible in satellite imagery. Flat (reef flat): An extensive area of shallow water behind the reef crest, often composed of rubble cemented by reef organisms, especially crustose coralline algae. Shallow depressions in the reef flat, such as tide pools, moats, or troughs, may allow for the development of small coral-associated communities despite the open and unprotected condition of these environments. Forereef or outer reef slope: The portion of the reef platform including spur and groove systems at the shallow end, and extending into deeper water, commonly with an increasing angle, to the dropoff. The outer reef at the deeper end may extend to depths of 60 m or more; eroded terraces may mark the position of past lower sea levels. Hoa: Polynesian term for storm-excavated channels across the reef rim that allows water to flow into and out of the lagoon, especially at higher tide levels. Hoa are typically 50–100 m wide and no more than 2–3 m deep (Stoddart and Fosberg, 1994). As used here, hoa occur between motus or between a motu and an island. Karst: A landscape formed by carbonate rocks that have partly dissolved or eroded due to exposure to rainwater. Many karst systems are highly porous. Lagoon slope: Area of increased gradient, from several degrees to near the angle of repose, passing from shallower water near the rim into the deeper water of the lagoon. The term ‘back reef slope’ is sometimes used synonymously but is generally inappropriate for an atoll where there is no back side. Shallow lagoons may lack a pronounced lagoon slope, and others may lack the development of coral communities at the lagoon slope. At the flank of the lagoon and deeper than the sand apron, coral communities may occur and can be referred to as a lagoon slope. Leeward: Atolls commonly develop an asymmetry due to the influences of varied intensities of wind and waves from different directions. The leeward side is that margin of an atoll that is more protected from these influences and generally faces a lower energy environment. As a result, the leeward margin is typically less well cemented, develops fewer or smaller islands, and is commonly lower in biological diversity. Motu: Polynesian term for islets that are separated by shallow, narrow hoa. Motus are small, commonly are less than a hectare in area, and may be rounded or elongated perpendicular to the margin. On non-Polynesian reefs, we refer to such structures as islets. They are typically vegetated and consist of coarse-grained sediment, coral gravel, and cobbles. Other more extensive subaerial vegetated features on the rim are common on atolls and are simply referred to as islands. Larger islands are oblong, follow the contours of the underlying reef rim, and are usually concave toward the lagoon. Wider breaks between motu or islands, e.g., areas that are flooded at high tide, are referred to as channels, or if sufficiently widely spaced, the reef flat. Unvegetated sand islands are referred to as cays (Stoddart and Steers, 1977). Open atoll: An atoll rim that is open to exchange with the surrounding seawater, typically through deep passes and hoa, or an incomplete rim. Outer reef: A synonym for the forereef. Passes: They occur naturally as deep entrances through the atoll rim into the lagoon through which there are major exchanges of water that cycle in and out of the lagoon with the tides. Passes, sometimes called ava in Polynesian, are arbitrarily defined by us as ≥5 m deep. Shallower gateways to the lagoon may referred to as hoa (in Polynesia) or channels elsewhere. Pavement: Reef and reef-derived material cemented by exposure to freshwater, commonly shaped by sea-level changes over thousands of years. The pavement can form a hard, rocky substrate that is exposed on the seafloor or be covered by sediment or islands. Pinnacle: Steep, cone-like projections thought to be the product of erosion during low stands of sea level but can be enhanced by coral growth in their shallow depths. Pinnacles may rise from even the deepest parts of the lagoon and may exhibit recent coral growth or may be covered with sediment. Platform: The underlying foundation upon which atolls, their coral growth, islands, and lagoon are built. All zones described here occur on the platform. The platforms that are measurable in Allen Atlas Imagery are limited to a depth of roughly 15 m. (Continued)

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Table 1.1 (Continued )  Glossary of Reef Terminology Reef crest: The surf zone and the shallowest portion of the reef that approaches atoll islands or the reef flat. Reef crests in the Indo-Pacific often are capped with crustose coralline algae and may pass oceanward into spur and groove systems. Caribbean atolls may also include reef crests, but red algae commonly are replaced by other wave-resistant organisms such as robust corals. Remnant lagoon: Small inactive, pond-like lagoons found on islands that have been uplifted or are otherwise limited in their capacity to exchange water with the open ocean. Remnant lagoons can be diluted with rainwater and become brackish or may become hypersaline or dry brines during droughts. Reticulate reefs: A series of interconnected, net-like ridges found in some lagoon systems. Many represent karst that formed during lower sea level but now are covered with corals or a sediment veneer. Some lagoons form only partial reticulations, whereas others form ridges that do not intersect. These structures may represent spatial self-organization, related to feedbacks during reef growth. Rim: As defined here, the shallowest portion of the reef platform that extends from the surf zone to the edge of the lagoon, including the reef crest, reef flat, and islands, if any. Some classifications include the sand apron or inner reef flat as part of the rim as well. Sand apron: The inner, lower energy part of the rim in which sediment typically accumulates and coral and red algal growth is less common. This term is used by some to represent the shallowest area of the lagoon system and by the Allen Coral Atlas as the innermost portion of the reef flat. The apron can reach above the range of local tides, and in the case of shallow lagoons, the apron may be hundreds of meters to kilometers wide. Shelf atoll: Non-Darwinian atolls that have formed on continental shelves. Shingle: Reef-derived sediment of size coarser than sand, commonly formed of rounded fragments of broken corals. Many high-energy beaches and islands consist of shingle or sand-shingle mixes. Slope atoll: Atolls that occur on continental slopes. Spur and groove system: Shallow portion of the forereef that develops a regular promontory (spur) and valley (groove) arrangement that is heavily solidified by crustose coralline algae and other encrusting organisms. These structures are thought to form in response to hydrodynamic forces and serve as breakwaters for the reef rim. Submerged atoll: Atolls that do not form significant islands or whose rim fails to project above the level of the tides. Submerged atolls described in the Allen Atlas are no deeper than 15 m, although on occasion some may in sufficiently clear waters to greater depths. Waveward: The side of an atoll system exposed to the most energetic wind and storm-generated waves, typically the opposite of the leeward side. Also referred to as the windward or upwind side, although the windward and waveward margins are not necessarily the same. Table reef: Reefs with islands that rise from the deep sea to intertidal levels, where they are typically covered with seawater at high tide but are nearly flat and do not possess a lagoon.

portion of the platform extending from the reef crest and reef flat to the lagoon. The rim is the shallowest portion of the atoll, and it can vary considerably in spatial, or map-view, morphology. The classic definition of an atoll rim stems from Darwin’s view that they are annular, and this description is commonly used to describe them even though we find only a few where such a geometry can be applied. Indeed, even if we include oval or egg-shaped atolls, the total comes to about 50. Atoll rim morphology is more often ellipse-like (Stoddart 1965) including elongated, bent, pointed, or otherwise modified ellipsoids as we describe them. However, the atoll rim often deviates significantly from an ideal geometric form because morphology is dependent on the nature of the antecedent platform or modifications to it such as instability due to erosion and fracture of the limestone cap, collapse due to submarine landslides (Terry and Goff 2013; Poupardin et al. 2017), or spatially variable rates of lateral reef expansion. The result is a range of rim

shapes that cannot be characterized by a single configuration or even by a narrow range of geometric forms. In that respect, we agree with Wiens (1962) who came to the same conclusion in his treatise on atolls. For example, Rose and Nikufetau atolls are square (American Samoa and Tuvalu, respectively). South Minerva (Tonga) forms a figure eight, Nupani (Santa Cruz Islands) is pentagonal (with a tail); Pukapuka (Northern Cook Islands) has a tail as well but is triangular. In addition, there are those that are shaped like a bonnet (Arno, Marshall Islands) or a spindle (Barque-Canada Atoll, South China Sea). With a little imagination, it is possible to see some that resemble a ballerina’s shoe (Hao, Tuamotu Islands), a shark (Karang Kaledupa, Indonesia), a fedora (Likiep, Marshall Islands), a hot water bottle (Ninigo, north of New Guinea) and even a uterus (Mioswundi, West Papua). The rim, including the islets and larger islands, lies upon a foundation of cemented or lithified coral and carbonate gravel referred to as a pavement. This rocky

A Global Atlas of Atolls

substrate can extend slightly above high tide and may be up to 2 m thick. Pavements are typical of the exposed (waveward) side of Pacific atolls, although they may occur on the leeward side as well. The components are thought to be the product of storm debris that has become cemented by exposure to freshwater during sea-level changes that have occurred over the last 1,000–3,000 years (Woodroffe et al., 1999; Montaggioni et al., 2021). Atoll rims are affected by waves generated by a variety of forces, from local wind waves to large swells generated over thousands of kilometers away by major wind belts, such as the trade winds, to local or distant low-pressure systems or cyclones (Hoeke et al., 2013; Wasserman and Rankey,  2013; Shope et al., 2016). Because dominant local wind direction and the direction from which the largest waves propagate may not correspond, here, the more active side of the atoll is referred to as the waveward side. Because reefs grow best where there is vigorous water movement and open-ocean water of low turbidity with consistent temperature and salinity, they usually are better developed on the waveward sides of atolls than on the more sheltered, leeward sides. These more protected sides of atolls tend to lack a high coral population density, and reefs are more poorly consolidated by crustose coralline algae and other binding agents. In addition, there tend to be

fewer or smaller islands, which contribute to rim asymmetry that is often readily visible in aerial and satellite imagery. By contrast, there are atolls, such as South Maalhosmadulu in the Maldives, that are influenced by monsoonal wind reversals that may form more symmetrical atolls (e.g., Kench et al., 2009). Over time, the waveward rim may accumulate sediment above sea level to form reef islands. These range from simple, unvegetated accumulations of sand called cays, although they can include coarser, less mobile carbonate material called shingle as well. If these combinations of sand and shingle are stable for some time, they can form the beginnings of vegetated islands (Stoddart and Steers, 1977). On Pacific atolls and elsewhere, these vegetated islands commonly include iconic coconut palms, Pandanus (screw pine), and broadly branched Pisonia trees which, if not removed for coconut plantations, serve as nesting habitat for many island bird species (e.g., Burger, 2005). Islands can be elongated and continuous, stand up to 2–4 m above sea level, and occupy large parts of the rim. Alternatively, they can be broken up into a series of smaller islets called motu in the Polynesian Pacific (Figure 1.7). Atoll islands are dynamic features whose shorelines respond to changing wave conditions on decadal or even seasonal timescales (Kench and Brander, 2006; Rankey, 2011).

Figure 1.7 Motu and hoa on Rangiroa Atoll, Tuamotu Archipelago, French Polynesia. Ocean water from surf (right) is carried by narrow channels called hoa in Polynesian through to the lagoon (left). (Image courtesy of © Philippe Bacchet French Polynesia. See also Rangiroa, Chapter 3.)

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Between the motu are shallow, narrow furrows called hoa in Polynesian, and they are thought to be the product of excavation by cyclonic storms (Stoddart and Fosberg, 1994). Hoa are typically 50–100 m wide and only 1–2 m deep, but they often are vital to lagoon circulation, especially if they allow seawater to be flushed from the lagoon during tidal cycles (Figure 1.7). They also contribute to the transport of sand into and out of the lagoon. There is an extensive hoa vocabulary. There are functional hoa that readily exchange water with the open ocean or allow water transport to the lagoon only during high tides. These contrast with semi-functional hoa that are active only during storms due to blockage by sediment at the lagoon end. Conversely, dry hoa, sometimes called paleohoa, are non-functional and are blocked by cemented sediment, uplifted islands, or the deposition of ridges by storms (Stoddart and Fosberg, 1994). In contrast to hoa, passes occur naturally as deep entrances through the atoll rim into the lagoon through which there are major exchanges of water that cycle in and out of the lagoon with the tides. Passes, sometimes called ava in Polynesian, are arbitrarily defined by us as ≥5m deep. The Allen Atlas provides a view of a natural pass 2 km wide and 54 m deep (Colin et al., 1986) on the western side of Helen Reef in Palau, western Caroline Islands. Other large, natural passes include the northern end of Majuro which is 46 m deep, and the multiple passes through South Malé in the Maldives that range from 30 to 70 m depth (Yamano et al., 2002; Suzuki and Kawahata, 2003). By contrast, relatively shallow gaps between islands or island groups are referred to here as channels. Some channels have been artificially created or enhanced and it can be difficult to distinguish origins in satellite imagery. Several atoll rims have been excavated or blasted across the reef to provide access for small to large boats (‘boat channels’) from the open ocean to the lagoon as in Funafuti Atoll in Tuvalu (Yamano et al., 2007). Rims with passes that are sufficiently open to oceanic water exchange and are flushed with some degree of frequency may develop significant community structure including reefs composed of coral or other calcifying organisms. Reef rims that allow such exchanges are referred to as open atolls. Those without passes and with rims that are well defined are referred to as closed atolls. However, local conditions including higher tides or wind and storm surge may carry ocean water across low-lying reef flats or through channels and low spots in the rim with a degree of frequency that may allow the development of significant reef communities even if there are no passes through the rim. This may be especially true

of atolls with relatively deep lagoons. On the other hand, the nature or health of the lagoon biota is often unknown. Therefore, we often presume that any atoll whose rim is low to the water and is apparently closed (no passes, low spots or channels) is likely to undergo some exchange with oceanic water. If the rim of such atolls projects above normal tides, it may be referred to here as semi‑closed, especially if it presents a relatively deep lagoon. There are several examples of these in the Tuamotu Archipelago and the South China Sea among other places.

Submerged rims On some atolls, including most in the Fijian Archipelago, on the North West Australian Shelf, among others, the rim surrounding the lagoon may not project above mean high tide, but their shoal waters still define a clear separation of shallow- and deep-water ocean areas, typically with passes, channels, and low areas that commonly surround a shallow lagoon. In this regard, we may differ from some authors who exclude atolls from those whose rims are within a few meters of the surface but fail to emerge above it. Such reefs have been described using a variety of names (Goldberg, 2016), but we refer to them here as submerged atolls. There are also atoll-like forms that fail to exhibit vigorous reef growth and are at depths that are typically ≥25 m or more (Abby and Webster, 2011). These relatively inactive reefs are distinguished as drowned atolls. However, with satellite resolution limited to about 15 m, we do not report on many such atolls here. We note, however, that intermediate cases exist. These include atolls that are submerged and yet have a few small terrestrial areas that project from a considerably larger rim, much of which is submerged or deeper than 25 m. Bellona and Chesterfield atolls in the eastern Coral Sea are excellent examples (Chapter 12), as well as Polowat, Pulusuk, and Minto in the Caroline Islands (Chapter 14). Others such as Cato and Mellish reefs (western Coral Sea) develop single islets on a larger rim that we classify as ‘essentially submerged’ whereas many others without islands are likely flooded at high tide. We also referred to these as submerged without creating a special category for them. In addition, there are partly drowned atolls such as the Chagos Bank in the Indian Ocean or Ayawi north of West Papua that develop one or more small islets on one end, whereas the rest of the platform is much larger and is submerged to depths of more than 25 m (see Chapters 13 and 20).

Reef rim zonation and terminology In general terms, the reef rim represents the shallowest areas of atolls and includes islands. These areas are

A Global Atlas of Atolls

characterized by hard substrates dominated by coral or red algal growth or lithified rocky surfaces such as the reef pavement described above. There are a number of terms used to describe different reef zones of the rim, areas that differ in their degree or energy and tidal exposure, depth, and the morphology of the underlying reef platform. We use the terminology of Kennedy et al. (2000), or modifications thereof as described below, to describe reef zones in order of occurrence from the lagoon to the deep ocean. The Reef Flat is typically a rubble-strewn, gently sloped or level intertidal platform that generally is composed of coral gravel cemented by coralline algae and other calcareous reef organisms. Flats occur adjacent to beach sand if islands are present (Figure 1.8a) or extend close to the lagoon if islands are absent. A narrow flat may be tens of meters wide, but in atoll groups such as the Marshall Islands and Tuvalu, they often extend to 500–1,000 m, and in the Tuamotu and the Maldive Islands, reef flats as wide as 2,000 m are common (Blanchon, 2011). Energetic, wave-exposed reef flats may develop narrow inlets called surge channels, as well as coarse sediment, rubble, and blocks of coral limestone, that have been deposited by storms (Figure 1.8b and c). All of these are commonly bound together by biotic or abiotic agents (Montaggioni et al., 1987 and as described below) and may be exposed at low tide depending upon the gradient, rubble size, and tidal amplitude. At low tide, the reef flat may also be only partially exposed or covered with a few centimeters of water. Hardy corals and fishes and other organisms may survive brief exposure or may be restricted to tide pools or larger troughs or moats if they can tolerate large variations in temperature, salinity, rainfall, and sediment deposition. This zone represents the upper physical limit for vertical coral reef growth and further development is limited to lateral growth, as allowed by the extent of the platform. The Allen Atlas distinguishes an inner reef flat that differs from this description by having a lower direct exposure to waves that leads to calmer conditions, the accumulation of sediment, and a lower suitability for corals and algae (see discussion of reef sand apron below). The Reef Crest is the seaward break point for incoming waves and may be considered as the outermost part of the reef flat (Figure 1.9a). It extends from shallowest parts of the reef system, where vertical coral growth has proceeded to the maximal extent allowed by water depth and tidal variation, out through the area where breaking waves occur. Reef crests dissipate an average of 86% of the open-ocean wave energy and 70% of that from incoming swells and are therefore important in protecting islands and lagoon communities from the direct impact of storms

(Ferrario et al., 2014; Kennedy et al., 2020). In the Indo-Pacific, the waveward reef crest is composed primarily of certain red algae that lay down thick and dense calcareous sheets and are uniquely resistant to abrasion, breakage, and the crushing forces typical of this zone (Figure 1.9b). These reef-forming agents are referred to as crustose coralline algae (CCA) and their importance in reef construction is well known (Littler and Littler, 2013). Such wave-resistant calcareous structures extend into other shallow-water portions of the rim, including surge channels in the reef flat (Figure 1.8c), and most significantly, below the reef crest as described below. In much of the Caribbean, in contrast, this red algal portion of the rim is absent, and the organisms that compose the crests in that region are more variable and dependent on the degree of wave exposure (Geister, 1977). The Reef Slope is the outer, seaward sloping reef, also referred to the forereef. This zone extends from below the reef crest and into deeper water gradually or more rapidly in the case of many atolls. The reef slope is often broken into subzones that are dependent on conditions of wave energy, depth, and light, but on the waveward sides of many Indo-Pacific atolls, the transition from the reef crest to the reef slope is accompanied by spur and groove systems that include regular comb-like CCA-covered (Figure  1.9c) reef promontories meters to tens of meters high (spurs), and rubble- or sand-filled notches called grooves. Corals become a more conspicuous component of spurs below the level of the tides and in deeper water (Figure 1.9d) where they act as natural breakwaters that absorb considerable wave energy (reviewed by Duce  et  al.,  2016). However, despite their prominence, the details of the formation and maintenance of these structures are still debated and are beyond the scope of this work. This highly energetic region is often obscured in satellite imagery by breaking waves and marks the outermost seaward point of the reef rim. Below the shallow portion of the reef slope, wave energy decreases but light levels remain high. The result is typically an increase in coral diversity, and at a depth of approximately 15–30 m, the highest diversity of corals and their branching, mound-like, and plate-like forms are found (e.g., Huston, 1985). This Goldilocks zone is depicted in Figure 1.10a. However, the Allen Atlas geomorphic view extends to only 15 m and thus characterizes only the upper portion of this high diversity region. Beyond 30 m, the slope of the atoll forereef increases, in places to near vertical, where corals assume a predominantly plate-like form to capture light more efficiently. This deep reef zone is called a wall or dropoff (Figure  1.10b). Corals and certain algae in this subzone

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Figure 1.8 Character of reef rims. (a) Beach and reef flat with reef debris exposed at low tide. Beru Atoll, Gilbert Islands, Kiribati. (b) Coarse rubble on the reef flat close to the reef crest. Aranuka Atoll, Gilbert Islands, Kiribati. (Photos (a) and (b) by Gene Rankey.) (c) Reef rim at Reao Atoll, including a satellite view of the reef flat and adjacent environments on the waveward side of Reao. Note surge channels extending into the reef flat (*) and to the submerged spur and groove system (S & G), described in more detail in Figure 9.1. (Aranuka and Beru atolls are further described in Chapter 9 and Reao in Chapter 3.)

A Global Atlas of Atolls

Figure 1.9 Spur and groove systems. (a) Satellite view of the spur and groove system of Vahanga Atoll (Tuamotu Islands, French Polynesia) as it projects above the tide, into the reef crest breaker zone, and below low tide. (b) Crustose coralline algae at the reef crest form thick, reddish-pink sheets of dense, wave-resistant calcium carbonate, shown here exposed at low tide on Aranuka Atoll, Gilbert Islands, Kiribati. (Photo by Gene Rankey). (c) Several species of coralline red algal growth on spur growth at Starbuck Island, Southern Line Islands, Kiribati. (Courtesy of Maggie Smith, Scripps, UC San Diego.) (d) Underwater view of spur and groove system, Palmyra Atoll, Northern Line Islands, Kiribati. (Image courtesy of JS Rogers, Stanford University.) (See Vahanga (Chapter 3), Aranuka and Starbuck (Chapter 9), and Palmyra atoll (Chapter 10), respectively, for details.)

persist on steep slopes to depths of more than 100 m as on Enewetak Atoll (Colin et al., 1986).

Lagoons: variation and zonation The presence of a lagoon inside the rim is characteristic of atolls and is the reason why they were once called lagoon islands. These features vary considerably in both area and depth, and this information is documented for each atoll in the following chapters of this volume where the data are available. In most cases, the lagoon accounts for the largest proportion of atoll area, and larger lagoons are commonly deeper than smaller ones, but this is not always the case. As an example, the lagoon of Rangiroa Atoll in the Tuamotu Archipelago (French Polynesia) has a lagoon area of 1,580 km2 and a maximal depth of 70 m. By contrast, the lagoon of Sapwuahfik Atoll in the eastern Caroline Islands is far smaller with an area of 86.5 km2, but the lagoon is up to 159 m deep (Purdy and Winterer, 2001) and may be the deepest of any atoll. The shallower parts of lagoons may vary in composition. On many atolls, the rim behind the reef crest, the reef flat, and islands is a shallow, lower energy

zone where sediment including rubble, gravel, and especially sand produced on the reef crest is driven toward the deeper lagoon or its periphery by waves, tides, and currents. This area is often referred to as a reef sand apron, which can be more than a km wide and covers 20% of the reef platform on average (Rankey and Garza-Pérez, 2012). Many geologists and geomorphologists include the sand apron as part of the reef flat (see the above discussion of that area), whereas others consider it as part of the lagoon; all agree that it is a transitional zone. It can be partially exposed depending on the amplitude of the tide, storm conditions, and angle into deeper water. It may develop sediment bars, dunes or ripples in response to wave and current forcing (Figure 1.11a). Atolls with shallow lagoons commonly include wide sand aprons as on several atolls including Kure at the end of the Hawaiian Island chain and Midway Atoll whose lagoons appear to be filling with sediment (Isaack and Gischler, 2015). Conversely, on many of the Caroline and Marshall Islands where deep lagoons are found, the sand apron is narrow and is an apparent testimony of the ability of such lagoons to accommodate considerable sand transport.

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Figure 1.10 Outer reef slope, Swallow Atoll (aka Pulau Layang-Layang or Danwan), Spratly Chain, South China Sea (Chapter 19). (a) Shallow outer reef slope (below reef crest) with diverse coral groups including platy and branching forms. (b) Deep outer reef slope showing the edge of the wall or dropoff. (Photos by Gene Rankey.)

However, there is additional evidence to suggest a reverse process of transport of sediment from the lagoon toward the reef platform, indicating the dynamic nature of the apron (e.g., Ortiz and Ashton, 2019; Rankey, 2021). Coralline features within the lagoon (and elsewhere) generally include patch reefs, which refer to isolated areas of reef that are detached from the main reef system perhaps by sand, erosion, mortality, or growth, and they are generally small but large

enough, perhaps at least 40–50 m2, to be distinguished individually by satellite imagery. However, patch reefs may also be defined by morphology (e.g., single, coalesced, linear, or reticulate). They can also be defined by location (e.g., lagoon patch, reef flat, outer reef patches) or by morphology (e.g., knolls, mounds, bommies) or even by their pinnacles if they arise from deep lagoon waters (Kennedy et al., 2020). We refer to discrete reef areas as patches and elongated linear patches as ribbons. We may also use the

A Global Atlas of Atolls

term ‘patchy’ to describe discontinuities in atoll features such as sand deposits, the reef crest, or deep outer reefs. Atolls with sufficiently protected areas including the lee side of shorelines, islands, or lagoons may develop seagrass meadows like in several of the northern Gilbert Islands (Figure 1.11b). Extensive seagrass areas have also been reported from inner lagoons or behind vegetated islets of Kwajalein and several other atolls in the Marshall Islands (McKenzie et al., 2021). Indonesian atolls may form extensive seagrass meadows in the lagoon or in protected areas of the reef flat (Tomascik et al., 1997; this volume). In the Western Atlantic, large areas of Alacrán, Glovers, and Lighthouse atoll lagoons are covered with seagrass, and on Turneffe Atoll, the largest in the Caribbean, the lagoon contains 36,643 ha of these flowering plants (Wabnitz et al., 2008; Fedler, 2018). Seagrasses host a greater abundance of burrowing organisms than surrounding bare soft bottoms because the extensive root systems serve as refuges from predation, provide greater habitat complexity, increased food supplies, more stable substrata, and create hydrodynamic conditions that are especially favorable for larval

settlement. Bivalve molluscs are especially abundant in these habitats, including those that are exploited for human consumption (Paulay, 2000). On some atolls with shallow and protected lagoon shorelines, mangrove communities may become well developed. On Turneffe in the Caribbean, almost all of the lagoon is red mangrove habitat, and due to restricted circulation, there are no lagoonal patch reefs (Gischler and Hudson, 1998). While atolls are more often bereft of mangroves (e.g., most of French Polynesia), some form more modest mangrove communities on reef flats, inland, and on the margin of lagoons (Spalding et al., 2010). Inland mangroves occur on the islands of Tuvalu and are particularly extensive in the protection of reeftop islands or in and around some of the almost land-locked lagoons (Woodroffe, 1987). On Beru Atoll in the southern Gilbert Islands, a well-developed mangrove community occurs in the shallows of the lagoon as it does elsewhere in Kiribati (Figure 1.11c). Mangroves provide significant benefits for atoll environments that include shoreline and sediment stabilization by their extensive root systems, which in turn provide resistance to the erosive effects

Figure 1.11 Shallow lagoon environments. (a) Ripples and small dunes indicate sand transport on the sand apron, here exposed at low tide. Aranuka Atoll, Gilbert Islands, Kiribati (Chapter 9). (b) Nearshore seagrass meadow in a shallow water lagoon, Aranuka Atoll, Gilbert Islands, Kiribati. (c) Nearshore mangroves, Beru Atoll, Gilbert Islands, Kiribati. Mangroves can stabilize shorelines and prevent erosion. (d) Ghost shrimp mounds with scattered seagrass in the deeper water, low-energy lagoon, Aranuka Atoll, Gilbert Islands, Kiribati. (Photos by Gene Rankey. See details in Chapter 9.)

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of wind, waves, and currents. The roots also provide a filtration effect that improves lagoon water quality. The high production of leaves and leaf litter additionally promotes food web complexity and ultimately serves as a source of resources for local communities. However, they are also vulnerable to human impacts, especially increasing population densities, and because mangroves are primarily intertidal, they are additionally vulnerable to sea level rise (Ellison et al., 2017). The floors of lagoons protected from strong wind, waves, and currents represent the lowest energy environment of many atolls, and in these quiet areas, fine sediment accumulates. Lagoon soft-bottom communities commonly are dominated by small gastropods, polychaete worms, and various crustacean groups that differ in their feeding habits, but all are burrowers that intensely rework the sediment. One group, ‘ghost’ shrimp of the family Callianassidae ejects sediment from burrows that often reach depths of two meters. This creates the irregular hallmark mounds on the lagoon floor, which can reach reach a height of up to 20 cm as shown in Figure 1.11d (e.g., Suchanek et al., 1986). Due to intense sediment turnover, these

organisms contribute significantly to both lateral and vertical sediment mixing. They also increase oxygen levels within the sediment and can influence ecosystem functions including lagoonal nutrient exchange and community structure (Koike and Mukai, 1983; Posey, 1986). Where lagoon waters are sufficiently circulated, a shallow patch reef community can develop in the shallower parts of the lagoon. Some authors refer to this region as a back reef, but reef communities that develop behind the reef crest are given the same name. We use lagoon slope (= back reef slope of the Allen Atlas which does not pertain to atolls) to describe the shallow margin of lagoon and the slope into the deeper parts of the lagoon system. Patch reefs are common in lagoon shallows, especially near openings through the rim or in the lee of a submerged rim where there is enhanced exchange of seawater. However, they vary extensively in area, vertical relief, and coral diversity, according to local conditions. Glovers Reef lagoon is 5–18 m deep and is studded with more than 860 randomly distributed patch reefs (Figure  1.12) that vary in composition and are open to ocean exchange due to three windward channels through the

Figure 1.12 Well-developed patch reefs in the of Glovers Reef, Belize, western Caribbean. (See Chapter 22.) Image ©2021, Planet Labs PBC.

A Global Atlas of Atolls

rim (Gischler and Lomando, 1999). Patch reefs on atolls with deeper lagoons may continue downslope to a typical depth of about 20 m where they may be widely dispersed or coalesce into larger ridge- or ribbon-like structures (Blanchon, 2011). Many atolls develop steep, cone-like projections called pinnacles that may rise from even the deepest parts of the lagoon. The Allen Atlas shows that many of the Tuamotu Atolls prominently display these peaks, and Andréfouët  et al. (2020) count 1,618 of them in Raroia Atoll (Figure 1.13a). The abundance of pinnacles in this region and the species richness of corals, molluscs, and other organisms are correlated with a lagoon that freely exchanges seawater through openings in the rim, especially passes and

submerged reef flats. Atolls with larger, open lagoons in the Tuamotu group are particularly well suited to high pinnacle numbers and lagoon species diversity (Adjeroud et al., 2000). Wherever they occur, these structures are either covered with sediment and are depicted in yellow by the Allen Atlas or they exhibit relatively recent coral growth and are encircled or covered by coral/algae. Pinnacles are thought to have formed during the last glaciation or earlier but are likely to be the result of successional growth during interglacial periods and erosion by waves during glacial-stage low sea levels (Montaggioni et al., 2019). Some atoll lagoons display a series of ridges that are sometimes interconnected in a honeycomb-like pattern creating separate basins within the lagoon referred

Figure 1.13 Lagoon pinnacles and reticulate reefs. (a) Raroia Atoll (Tuamotu Archipelago, French Polynesia), displaying >1,600 reef pinnacles in the deep lagoon. See also Raroia, Chapter 3. (b) Caroline Island (aka Millennium) with well-developed net-like pattern of reticulated reefs in its lagoon and other benthic features as described by the Allen Coral Atlas. (See also Kiribati, Chapter 9.)

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to as cells or pools. These are referred to as reticulate reef structures (Figure 1.13b). While found on fringing reefs and in barrier reef lagoons, this arrangement is so distinctive and common among Pacific atolls that Guilcher (1988) recognized them as a distinct group. Examples include Atafu (Tokelau), Manihiki (Cook Islands), Mataiva (Tuamotu Islands), Kanton (Kiribati), and Pearl and Hermes (Hawai’i) among many others. Reticulate reefs are also present on Alacrán reef in the Yucatan region of the Gulf of Mexico. There is considerable variation among such reticulate systems. Some display intersecting ridges that are incomplete and occupy part of the lagoon, while others form more or less parallel ridges that do not intersect and do not form pools. We describe such lagoons in the context of each atoll in the chapters that follow. While a karst origin has been proposed for these formations, some authors have suggested that reticulate reefs emerge as a result of feedbacks between organisms and their environments, as a process of self-organization (Schlager and Purkis, 2014). Many islands are former atolls whose small and shallow lagoons have filled with sediment ­(Figure  1.14a) as part of their natural life cycle. Tikei (Tuamotu Archipelago, French Polynesia) is one of several small atolls with shallow lagoons that have been filled with coral blocks and sand due to cyclonic and other storms during the last few 1,000 years (Salvat, 2009). The center remains as a shallow sink that collects freshwater, but the rim is only slightly higher than the interior (Agassiz, 1900). A discontinuous ring of coral occurs on the outer reef along the northern half of the island (Allen Coral Atlas). In other cases, the lagoon may become isolated as one or several ponds that have become isolated from the surrounding ocean. Depending on location and perhaps the season, these ponds may become brackish or hypersaline. Some authors refer to islands with these characteristics as table reefs, although this term originally referred to reefs with islands that rise from the deep sea to intertidal levels, where they are typically covered with seawater at high tide but are nearly flat and do not possess a lagoon (Tayama, 1935). We refer to islands with reduced or diminished lagoons as atoll islands with remnant lagoons, and we classify them separately from other such atolls. Likewise, some small islands may arise from reefs but neither the reefs nor the islands develop an interior lagoon. Kili, Jabat, and Jemo in the Marshall Islands are examples of such landforms that are not atolls. Uplifted atolls are on the other end of the atoll scale where lagoons typically do not exist (although there are exceptions presented in Section 1.2). The island of Makatea, for example, (also in

the Tuamotu group) is uplifted as much as 113 m above sea level (Figure 1.14b). There are prominent cliffs on all sides up to 75 m high that began about 2 million years ago, associated with the activity of the nearby Tahiti volcanic complex. Makatea is now a favorite destination for rock climbers. The foreground shows the rubble-strewn reef flat ~10 m wide solidified with coralline algae; pools remain at low tide, while seawater covers the reef flat from about 0.3 m (as shown) to about a meter deep. The plateau-like summit is replete with large solution features (karst) that range from potholes to large sinkholes 75 m deep, testifying to long periods of exposure to rainwater. Phosphate mining that continued until the 1960s added to the moonscape-like quality of the island’s upper surface. A fringing coral community surrounds the island on the outer reef extending 100 m from the cliff base (Montaggioni et al., 1987; Montaggioni and Camoin, 1997).

Non-Darwinian atolls Shelf and slope atolls As outlined above, classic Darwinian forms occur on deep-water, open ocean, subsiding volcanic platforms, and to some workers, these are the only true atolls. However, there are relatives of the atoll family that have become widely accepted as atolls, whereas others are confused with them and should be separated. In general, there are two additional classes of atolls that do not form on open-ocean, volcanic foundations, but have the same morphological characteristics. The most common of these are atolls that occur on continental shelves. Australia’s North West Shelf, for example, developed a large barrier reef about 10 million years ago, when it subsequently underwent rapid subsidence. Growth on much of the reef complex could not keep up with subsidence, and although most reefs drowned, the barrier did provide a foundation for the growth of several existing atolls (e.g., McCaffrey et al., 2020), as described in Chapter 17. Some shelf atolls offshore Belize, western Caribbean, include reef-associated deposits more than 500 m thick formed from an ancient submarine ridge that broke into fault blocks and began to subside (Gischler and Hudson, 1998). Others grow rapidly from a gently dipping carbonate substrate, including Alacrán reef in the Gulf of Mexico on the Yucatan continental shelf, which rises from water 50 to 60 m deep (Liddell and Tunnell, 2011). In addition, there are two Caribbean atolls (Courtown and Albuquerque cays) that arise from volcanic foundations at a depth of 1,000 m (Diaz et al., 1996).

A Global Atlas of Atolls

Figure 1.14 Filled lagoons and uplifted atolls. (a) The lagoon of Tikei (Tuamotu Archipelago, French Polynesia) has filled and is now a subtle low point in the center of the island. Image © 2022, Planet Labs PBC. (b) The former atoll Makatea in the Tuamotu Archipelago, French Polynesia, uplifted by nearby volcanic activity about two million years ago, now displays cliffs up to 75 m high and a total elevation of 113 m. Note the rubble-strewn reef flat with darker tide pools in the foreground. Photo courtesy of C. Serra and © La Direction de l’Environment de La Polynésie française (DIREN).)

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Other atoll-like features are quite varied. The platforms that form the bases of many atolls in the South China Sea initially developed during rifting and block faulting of the continental shelves, where they were once-more-extensive reefs and paleo-atolls (Steuer et al., 2014; Wu et al., 2016). Many present-day atolls there represent the hearty survivors of several drowning events since the Miocene (~23–5.3 million years ago). Most of these features in that region are shelf atolls, although Wang (1998) describes ten that formed on continental slopes and are called slope atolls. Likewise, Tomascik et al. (1997) describe a large number of atolls on the continental shelves of Indonesia, a geologically complex region where there have been relatively few studies. Muraras Atoll, 90 km off the coast of Borneo, rises from a depth of 300 m and is an example of an Indonesian slope atoll, and there are several others on the North West Shelf of Australia (Wilson, 2013). Additionally, the Maldive Islands in the Indian Ocean appear to have formed on non-volcanic, flat-topped carbonate terraces on the continental slope that developed coral communities in response to multiple changes in sea level related to ice sheet formation and melting over the last few million years (Droxler and Jorry, 2021).

Banks, shoals, and other problems of terminology In his monograph, The Coral Reef Problem, Davis (1928) categorized any coralline buildup lying back from the edge of continental shelves as a bank reef. The term ‘bank’ is also often applied to reefs and other shallow areas of the seabed that fail to reach the surface. Geomorphically, a bank can be defined as an isolated or cluster of elevations of the sea floor over which the water depth is relatively shallow but is safe for navigation (Ban and Sung, 2019). This general definition often does not work well for coral reefs. Some coral banks occur in cold or deep water and never were tropical or shallow (Freiwald, 2002). Conversely, atoll-like structures with submerged rims in warmer waters are sometimes also described as banks or bank atolls, whether they are hazards to navigation or not (Vecsei, 2000; Woodroffe and Biribo, 2011). In addition, there are traditional nautical descriptions that have been applied to submerged atolls that include naming them as shoals or reefs, neither of which adequately distinguishes them, but these names are indelibly applied to standard maps and charts. The effect of this elastic and vague terminology is considerable. More than 90% of the atolls in the South China Sea, nearly all of those in Fiji, and eight of ten in the Western Atlantic include few or no islands, but whose rims and

lagoons are clearly defined (Goldberg, 2016). Such cases are included in the imagery and descriptions presented here. Categorization of partially rimmed atolls on continental shelves is even more vexatious. For example, partly rimmed atolls including Chinchorro, Roncador, and Serrana in the Western Atlantic are still referred to as banks, as well as three in the South China Sea, among others. There are also atolls such as Ayawi in Indonesia that present a small island on one end, while most of the remaining rim lies at a depth of 28 m or more. This might be called a partly drowned atoll. Velasco Reef in Palau is partially rimmed as well, but while it reaches sea level to the south, the rim is generally 15 m deep and it is considered a drowned atoll (Guilcher, 1988, Colin, 2008). Portland Bank in the Gambier Islands (French Polynesia) is currently at a depth of −50 m and continues to sink (Pirazzoli, 1985). This is more clearly a drowned atoll, but these distinctions are sometimes difficult indeed.

A note on oceanographic terms and processes Oceanographic influences on atolls are quite varied across the globe. Thus, in addition to describing the distribution of atolls and providing remote sensing images of each of them, we also provide data that characterize wind, waves, and tides for examples from each area. Methods are described in detail in Chapter 2, but here we provide a brief overview. Wind data are discussed in the context of wind roses. These graphs illustrate the direction from which winds blow (azimuth) and the speed of those winds (by color). Wave data are documented as peak waves or the most energetic waves of the entire wave spectrum. Akin to wind data, waves are displayed as plots documenting the direction from which waves propagate and their significant wave height, a standard oceanographic metric representing the average height of the largest 1/3 of waves. Tides are illustrated for a representative month. Tides are complex, but can be classified as semidiurnal, diurnal, and mixed. Semidiurnal tides include two high tides and two low tides every 24 hours and 50 minutes (a lunar day). In contrast, diurnal tides have only one high and one low tide during a lunar day. Mixed tides describe those with a mixture of diurnal and semidiurnal, expressed as two highs and two lows each day, but exhibit a marked inequality (e.g., a low tide and a high low tide each lunar day). Most atolls experience semidiurnal tides, and a few have strictly or dominantly diurnal tides (e.g., the South China Sea, the central Caroline Islands, and

A Global Atlas of Atolls

the Gulf of Mexico). However, mixed tides also occur across wide areas with atolls (e.g., NW Hawai’i, parts of the Caroline and Indonesian islands, and areas around the Solomon Islands).

References Abby E, Webster JM 2011. Submerged reefs. In: Hopley D (ed.) Encyclopedia of Modern Coral Reefs: Structure, Form and Process. Springer Science+Business Media BV, Dordrecht, Netherlands, pp. 1058–1062. Adjeroud M, Andréfouët S, Payri C, Orempülle J 2000. Physical factors of differentiation in macrobenthic communities between atoll lagoons in the Central Tuamotu Archipelago (French Polynesia). Mar. Ecol. Progr. Ser. 195: 25–38. Agassiz A 1899. The islands and coral reefs of Fiji. Bul. Mus. Comp. Zool. 33: 1–150. Agassiz A 1900.The cruise of the Albatross. Science 11: 92–98. Allen Coral Atlas 2020. Imagery, maps and monitoring of the world’s tropical coral reefs. https://doi.org/10.5281/ zenodo.3833242 Allen Coral Atlas (2022). Imagery, maps and monitoring of the world’s tropical coral reefs. doi.org/10.5281/zenodo.3833242 Andréfouët S, Genthon P, Pelletier B et al. 2020. The lagoon geomorphology of pearl farming atolls in the Central Pacific Ocean revisited using detailed bathymetry data. Mar. Poll. Bull. 160: 111580. https://doi.org/10.1016/j.marpolbul.2020.111580 Ban H, Sung HH 2019. Visualization of uncertain boundary features. Intl. J. Geospat. Environ. Res. 6: 1–26. Blaise G, Guille G, Guillou et al. 2002. The island of Maupiti: The oldest emergent volcano in the Society hot spot chain (French Polynesia). Bull. Soc. Géol. 173: 45–55. Blanchon P 2011. Geomorphic zonation. In: Hopley D (ed.) Encyclopedia of Modern Coral Reefs: Structure, Form and Process. Springer-Verlag Earth Science Series, pp. 469–486. https:// doi.org/10.1007/978-90-481-2639-2 Blanchon P, Grenados-Correa M, Abbey E et al. 2014. Post-glacial fringing-reef to barrier-reef conversion on Tahiti links Darwin’s reef types. Sci. Rep. 4: 4997. https://doi.org/10.1038/ srep04997 Burger AE 2005. Dispersal and germination of seeds of Pisonia grandis, an Indo-Pacific tropical tree associated with insular seabird colonies. J. Trop. Ecol. 21: 263–271. Colin PL 2008. Velasco Reef: A sunken atoll with unusual “remnant” beds of the seagrass Thalassodendron ciliatum. Coral Reefs 27: 395. Colin PL, Bell LJ, Patris S 2008. Helen Reef 2008: An overview. Tech. Rept. Coral Reef Foundation. www.coralreefpalau.org Colin PL, Devaney DM, Hillis-Colinvaux L, et al. 1986. Geology and biological zonation of the reef slope, 50–360 m depth at Enewetak Atoll, Marshall Islands. Bull. Mar. Sci. 38: 111–128. Daley RA 1915. The glacial-control theory of coral reefs. Proc. Am. Acad. Arts Sci. 51: 157–251. Darwin C 1842. The Structure and Distribution of Coral Reefs. University of California Press, Berkeley. Davis, WM 1928. The Coral Reef Problem. Special Publication 9, American Geographical Society. AMS Press, New York. Diaz J, Sanchez JA, Zea S, Garzon-Ferreira J 1996. Morphology and marine habitats of two southwestern Caribbean atolls: Albuquerque and Courtown. Atoll Res. Bull. 435: 1–33. Dickinson, WR 2003. Impact of mid-Holocene Hydro-Isostatic highstand in regional sea level on habitability of Islands in Pacific Oceania. J. Coast. Res. 19: 489–502. Droxler AW, Jorry SJ 2021. Origin of modern atolls: Challenging Darwin’s deeply ingrained theory. Ann. Rev. Mar. Sci. 13: 537–573. Ducea S, Vila-Concejoa A, Hamylton SM et al. 2016. A morphometric assessment and classification of coral reef spur and groove morphology. Geomorphol. 265: 68–83. Ellison JC, Mosley A, Helman M 2017. Assessing atoll shoreline condition to guide community management. Ecol. Indic. 75: 321–330. Emery KO, Tracey JI Jr., Ladd HS 1954. Geology of Bikini and nearby atolls. U.S. Geol. Surv. Prof. Pap. 260: 1–265. Fedler T. 2018. The value of Turneffe Atoll blue carbon. Turneffe Atoll Trust. https://static1.squarespace.com/

st at ic /5989ba8 4 ebbd1a4 09ec 8c 6 6 4 /t /62fd 22ba 68f f3c411eb20400/1660756666867/Turneffe+Atoll+Blue+Carbon+FINAL+2018-+10-30-18.pdf Freiwald A 2002. Reef-forming cold-water corals. In: Wefer G et al. (eds.) Ocean Margin Systems. Springer, Berlin, Heidelberg, pp. 365–385. https://doi.org/10.1007/978-3-662-05127-6_23 Geister J 1977. The influence of wave exposure on the ecological zonation of Caribbean coral reefs. Proc. 3rd Int. Coral Reef Symp. 1: 23–29. Gischler E, Hudson JH 1998. Holocene development of three isolated carbonate platforms, Belize, Central America. Mar. Geol. 144: 333–337. Gischler E, Lomando AJ 1999. Recent sedimentary facies of isolated carbonate platforms, Belize-Yucatan system, Central America. J. Sed. Res. 69: 747–763. Goldberg WM 2013. The Biology of Reefs and Reef Organisms. University of Chicago Press, Chicago. https://doi.org/10.7208/ chicago/9780226925370.001.0001 Goldberg WM 2016. Atolls of the world: Revisiting the original checklist. Atoll Res. Bull. 610: 1–47. Graham NAJ, Spalding M, Sheppard CRC 2010. Reef shark declines in remote atolls highlight the need for multi-faceted conservation action. Aquatic Cons. Mar. Freshw. Ecosyst. 20: 543–548. https://doi.org/10.1002/aqc.1116 Guilcher A. 1988. Coral Reef Geomorphology. Wiley, Chichester, NY. Hiatt RW, Strasburg DW 1960. Ecological relationships of the fish fauna on coral reefs of the Marshall Islands. Ecol. Monogr. 30: 65–127. Hoeke RK, McInnes KL, Kruger JC et al. 2013. Widespread inundation of Pacific islands triggered by distant-source windwaves. Glob. Planet. Change 108, 128–138. Hoffmeister JE, Ladd HS 1944. The antecedent platform theory. J. Geol. 54: 388–402. Huston MA 1985. Patterns of species diversity on coral reefs. Ann. Rev. Ecol. Syst. 16: 149–177. Isaac A, Gischler E 2015. The significance of sand aprons in Holocene atolls and carbonate platforms. Carbonates Evaporites 32: 13–25. Kench PS, Parnell KE, Brander RW. 2009. Monsoonally influenced circulation around coral reef islands and seasonal dynamics of reef island shorelines. Mar. Geol. 266: 91–108. Kennedy EV, Roelfsema CM, Kovacs E et al. 2020. Reef cover classification. https://reefresilience.org/wp-content/uploads/ REEF-COVER-CLASS-DEFINITIONS.pdf Kennedy EV, Roelfsema CM, Lyons MB et al. 2021. Reef Cover, a coral reef classification for global habitat mapping from remote sensing. Sci Data 8: 196. https://doi.org/10.1038/ s41597-021-00958-z Koike I, Mukai H. 1983. Oxygen and inorganic nitrogen contents and f luxes in burrows of the shrimps Callianassa japonica and Upogebia major. Mar. Ecol. Prog. Ser. 12: 185–190. Kopacz JR, Herschitz R, Rony J 2020. Small satellites an overview and assessment. Acta Astronautica 170: 93–105. Kumar S, Kruger J, Begg Z, Handerson, E et al. 2013. Multibeam Bathymetry Survey Rangiroa, French Polynesia. SPC Applied Geoscience and Technology Division (SOPAC), Suva. Ladd HS, Schlanger SO I960. Drilling operations on Enewetak Atoll. U.S. Geol. Survey Prof. Paper 260–Y. Liddell WD, Tunnell JW 2011. Mexican coral reefs. In: Buster NA, Holmes CW (eds.) Gulf of Mexico: Origin, Waters and Biota Vol. 3 Geology. Texas A&M University Press, College Station, TX, pp. 341–354. Littler MM, Littler DS 2013. The nature of crustose coralline algae and their interactions on reefs. Smithson. Contr. Mar. Sci. 39: 199–212. Lyons MB, Roelfsema CM, Kennedy EV 2020. Mapping the world’s coral reefs using a global multiscale earth observation framework. Remote Sens. Ecol. Cons. 6: 557–568. McCaffrey JC, Wallace MW, Gallagher SJ 2020. A Cenozoic barrier reef on Australia’s North West shelf. Global Planetary Change 184: 103048. McKenzie LJ, Yoshida RL, Aini JW et al. 2021. Seagrass ecosystems of Pacific Island countries and territories: A global bright spot. Mar. Poll. Bull. 167: 112308. https://doi.org/10.1016/j. marpolbul.2021.112308 Montaggioni LF, Camoin GF 1997. Geology of Makatea Island, Tuamotu Archipelago, French Polynesia. In: Vacher HL,

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Quinn T (eds.) Geology and Hydrogeology of Carbonate Islands. Elsevier Science BV, Amsterdam, pp. 453–474. Montaggioni LF, Collin A, James D et al. 2019. Morphology of fore-reef slopes and terraces, Takapoto Atoll (Tuamotu Archipelago, French Polynesia, central Pacific): The tectonic, sea-level and coral-growth control. Mar. Geol. 417: 106027. https://doi.org/10.1016/j.margeo.2019.106027 Montaggioni LF, Gabrie C, Naim O et al. 1987. The seaward margin of Makatea, an uplifted carbonate island (Tuamotus, Central Pacific). Atoll Res. Bull. 299: 17. Montaggioni LF, Martin-Garin B, Salvat B et al. 2021. Coral conglomerate platforms as foundations for low- lying, reef islands in the French Polynesia (central south Pacific): New insights into the timing and mode of formation. Mar. Geol. 4437(7535): 106500. https://doi.org/10.1016/j.margeo.2021.106500 Nunn PD 1987. Late Cenozoic tectonic history of Lau ridge, southwest Pacific, and associated shoreline displacements: Review and analysis. NZ J. Geol. Geophys. 30: 241–260. https:// doi.org/10.1080/00288306.1987.10552620 Nunn PD, Ollier C, Hope G et al. 2002. Late Quaternary sea-level and tectonic changes in northeast Fiji. Mar. Geol. 187: 299–311. O’Connor JM, Steinberger B, Regelous M et al. 2013. Constraints on past plate and mantle motion from new ages for the Hawaiian-Emperor Seamount Chain. Geochem. Geophys. Geosys. 14: 4564–4584. Odum HT, Odum EP 1955. Trophic structure and productivity of a windward coral reef community on Eniwetok Atoll. Ecol. Monogr. 25: 291–320. Ortiz AC, Ashton AD 2019. Exploring carbonate reef flat hydrodynamics and potential formation and growth mechanisms for motu. Mar. Geol 412: 173–186. Paulay G. 2000. Benthic ecology and biota of Tarawa Atoll lagoon: Influence of equatorial upwelling, circulation, and human harvest. Atoll Res. Bull. 487: 1–41. Pigafetta A. 1969. The Voyage of Magellan: A Narrative Account of the First Circumnavigation (Translation). Yale University Press, New Haven. Pirazzoli PA 1985. Bathymetric mapping of coral reefs and atolls from satellite. Proc. Fifth Intl. Coral Reef Symp. 6: 539–544. Posey MH. 1986. Changes in a benthic community associated with dense beds of a burrowing deposit feeder Callianassa cali‑ forniensis. Mar. Ecol. Prog. Ser. 31: 15–22. Poupardin A, Heinrich P, Frère A et al. 2017. The 1979 submarine landslide-generated tsunami in Mururoa, French Polynesia. Pure Appl. Geophys. 174: 3293–3311. Purdy EG, Winterer EL 2001. The origin of atoll lagoons. Geol. Soc. Am. Bull. 113: 837–854. Purdy EG, Winterer EL 2006. Contradicting barrier reef relationships for Darwin’s evolution of reef types. Int. J. Earth Sci. 95: 143–167. Rankey EC 2011. Nature and stability of atoll island shorelines: Gilbert Island chain, Kiribati, equatorial Pacific: Atoll shoreline change, equatorial Pacific. Sedimentology 58: 1831–1859. https://doi.org/10.1111/j.1365-3091.2011.01241.x Rankey EC 2021. Platform-top reef sand apron morphodynamics and the half-empty bucket. Sediment. Geol. 412, 105825. https://doi.org/10.1016/j.sedgeo.2020.105825 Rankey EC, Garcia-Pérez JR 2012. Seascape metrics of shelfmargin reefs and reef sand aprons of Holocene carbonate platforms. J. Sed. Res. 82: 53–71. Roelfsema CM, Lyon M, Murray N et al. 2021. Workflow for the generation of expert-derived training and validation data: A view to global scale habitat mapping. Frontiers in Marine Sci‑ ence: Special Issue Remote Sensing for Applied Coral Reef Science and Management. https://doi.org/10.3389/fmars.2021.643381 Roelfsema CM, Phinn SR, Dennison WC 2002. Spatial distribution of benthic microalgae on coral reefs determined by remote sensing. Coral Reefs 21: 264–274. Sachet MH 1962. Geography and land ecology of Clipperton Island. Atoll Res. Bull 86: 29–30. Safyan M 2020. Planet’s Dove satellite constellation. In: Pelton J (ed.) Handbook of Small Satellites. Springer, Cham, pp. 1–17. https://doi.org/10.1007/978-3-030-20707-6_64-1 Salvat B 2009. Dominant benthic mollusks in closed atolls, French Polynesia. Galaxea 11: 1–10. Schlager W, Purkis S 2014. Reticulate reef patterns- antecedent karst versus self-organization. Sedimentol. 62: 501–515. https://doi.org/10.1111/sed.12172

Shope JB, Storlazzi CD, Erikson LH, Hegermiller CA 2016. Changes to extreme wave climates of islands within the Western Tropical Pacific throughout the 21st century under RCP 4.5 and RCP 8.5, with implications for island vulnerability and sustainability. Glob. Planet. Change 141: 25–38. https:// doi.org/10.1016/j.gloplacha.2016.03.009 Spalding M, Kainuma M, Collins L 2010. World Mangrove Atlas, 2nd ed. Earthscan, London. Spriggs, M 1991. Nissan, the island in the middle. Summary report on excavations at the north end of the Solomons and the south end of the Bismarcks. Report of the Lapita homeland project, pp. 222–243. Steuer S, Franke D, Meresse F et al. 2014. Oligocene–Miocene carbonates and their role for constraining the rifting and collision history of the Dangerous Grounds, South China Sea. Mar. Petr. Geol. 58: 644–657. Stoddart DR 1965. The shape of atolls. Mar. Geol. 3: 369–383. Stoddart DR, Fosberg FR 1994. The hoa of Hull Atoll and the problem of hoa. Atoll Res. Bull. 394: 1–26. Stoddart DR, Gibbs PE 1975. Almost-atoll of Aitutaki: Reef studies in the Cook Islands, South Pacific. Atoll Res. Bull. 190: 1–158. Stoddart DR, Steers JA 1977. The nature and origin of coral reef islands. In: Jones OA, Endean R (eds.) Biology and Geology of Coral Reefs. Academic Press, New York and London, vol. 4, pp. 60–106. Suchanek T, Colin PL, McMurtry G, Suchanek CS 1986. Bioturbation and redistribution of sediment radionuclides in Enewetak Atoll lagoon by callianassid shrimp: Biological aspects. Bull. Mar. Sci. 38: 144–154. Suzuki A, Kawahata H 2003. Carbon budget of coral reef systems: An overview of observations in fringing reefs, barrier reefs and atolls in the Indo-Pacific regions. Tellus 55B: 428–444. Tayama R 1935. Table reefs, a particular type of coral reefs. Proc. Imp. Acad. Japan 11: 268–270. Terry JP, Goff J 2013. One hundred thirty years since Darwin: ‘Reshaping’ the theory of atoll formation. The Holocene 23: 615–619. Tomascik T, Mah, AJ. Nontji A, Moosa MK 1997. The Ecology of the Indonesian Seas Part Two. Periplus, Hong Kong. Tracey JI Jr., Ladd HS, Hoffmeister JE 1948. Reef of bikini, Marshall Islands. Bull. Geol. Soc. Am. 59: 861–878. Vecsei A 2000. Database on isolated low-latitude carbonate banks. Facies 43:205–222. Wabnitz CC, Andréfouët S, Torres-Pulliza et al. 2008. Regional-scale seagrass habitat mapping in the wider Caribbean region using Landsat sensors: Applications to conservation and ecology. Rem. Sens. Environ. 112: 3455–3467. https://doi. org/10.1016/j.rse.2008.01.020 Wang G 1998. Tectonic and monsoonal controls on coral atolls of the South China Sea. Spec. Pubs. Intl. Assn. Sedimentol. 25: 237–248. Wasserman H, Rankey EC 2014. Physical oceanographic influences on sedimentology of reef sand aprons: Holocene of Aranuka Atoll (Kiribati), equatorial Pacific. J. Sed. Res. 84: 586–604. Wiens HJ 1962. Atoll Environment and Ecology. Yale University Press, New Haven. Wilson B 2013. The Biogeography of the Australian North West Shelf: Environmental Change and Life’s Response. Elsevier, Burlington MA. Woodroffe CD 1987. Pacific island mangroves: Distribution and environmental settings. Pac Sci. 41: 166–185. Woodroffe CD, Biribo N 2011. Atolls. In: Hopley D (ed.) Ency‑ clopedia of Modern Coral Reefs: Structure, Form, and Process. Springer, Dordrecht, Netherlands, pp. 51–71. Woodroffe CD, McLean RF, Smithers SG, Lawson E 1999. Atoll reef-island formation and response to sea-level change: West Island, Cocos (Keeling) Islands. Mar. Geol. 160: 85–104. Wu S, Zhang X, Yang Z et al. 2016. Spatial and temporal evolution of Cenozoic carbonate platforms on the continental margins of the South China Sea: Response to opening of the ocean basin. Interpretation 4: 1–19. Xu Y, Vaughn NR, Knapp DE 2020. Coral bleaching detection in the Hawaiian Islands using spatio-temporal standardized bottom reflectance and Planet Dove satellites. Remote Sens. 12: 3219. https://doi.org/10.3390/rs12193219 Yamano H, Kayanne H, Matsuda F, Tsuji Y 2002. Lagoonal facies, ages, and sedimentation in three atolls in the Pacific. Mar. Geol. 185: 233–247.

Methods In this section, we describe the general geomorphic and habitat features of atolls. The location maps that are used in the following section of this atlas are taken from http://maps.fiu.edu/gis/goldberg/ atolls, an interactive website using ArcGIS Online. Remote-sensing images are either Planet data (3-m resolution) for smaller atolls that can be covered in swaths from 1 day or Sentinel-2 data (10-m resolution) for larger atolls, mostly from 2021-2022. Atolls are identified and labeled according to the nature of their rims as defined in the previous chapter so that atolls with open rims (i.e., those with passes) are distinguished from those that are closed. The degree of closure may result in altered oxygen or salinity depending upon the rim and lagoon architecture. In many cases, the rim may be closed but is composed of reef flats that are low to the water and may undergo submergence and tidal flushing. These are referred to as semi-closed atolls. We also recognize those with completely submerged rims as well as those with remnant lagoons. We used the unannotated optical satellite images in this atlas because of their clarity, but we verbally describe the Allen Atlas geomorphic and benthic views in the text. However, we find it important to toggle in and out of those annotated views to ensure that all the area intended for measurement is included. Platform area is always measured with all mapping elements turned on. Conversely, the view of the reef crest required turning off the coral/algae layer in the benthic view. In addition, an Allen Atlas layer referred to as Reef Extent more inclusively depicts the extent of the shallow “coral reef environment,” broadly defined. This layer maps the extent of all benthic classes including but not limited to reefs (Roelfsema pers. comm., 2023), and, using algorithm improvements, extends classification into water deeper than those covered by previous benthic or geomorphic habitat maps. We measured the extent (width) of class elements using the measuring tool in the Atlas on transects, the number of which varied by the perimeter of the atoll. We used as few as six for smaller atolls and as many as 20 or more for those that were larger. On occasion,

2 coding by the Atlas overestimated the extent of the reef environment according to its visibility in Planet’s imagery and was ignored, whereas reef environment that was clearly visible but not coded was included. Notably in several instances the pinnacles coded as reef rimmed in the lagoon overlap with and are difficult to separate from one another and from the lagoon slope, especially in smaller lagoons. We did not attempt to quantify those reefs for that reason. In other instances, terrestrial features occasionally were coded as reef and submerged rims were coded as part of the lagoon. We recognize that the Atlas is undergoing an evolutionary process and these issues remain to be resolved, but it is the current state of the art. Because the Allen Atlas often did not code spur and groove systems clearly, Google Earth or uncoded Planet images were used to examine the extent of spur and groove systems where image quality was sufficient. The coding of the outer reefs gives their lateral extent which may be relatively wide or narrow. We assume that narrow reefs are steeply inclined and may refer to Sailing Directions or published literature to describe their composition. Widths do not otherwise refer to biological richness. We used the Allen Atlas classes to describe biological and geomorphic elements of the rim and lagoon as well, although we referred to the uncoded imagery for reference for the reasons described below. Lagoon areas were rounded to the nearest km2 or to the nearest 0.1 km2 for atolls less than 200 km2, although we alternatively used other sources cited in the text for such measurements. Lagoon area measurements typically included the submerged portion of the sand apron, and in some cases, the apron was measured separately from the deeper lagoon. In other cases, deep and shallow elements of the lagoon were measured for comparison. Lagoon measurements on atolls with submerged or partially submerged rims can be challenging and descriptions may be accompanied by adverbs of uncertainty including about, roughly, or approximately. When measuring the lagoon, we left the geomorphic view turned on, allowing us to omit the

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inner reef flat. In some lagoons, these flats are convoluted and are more of a challenge to avoid, but our measurements typically have smaller lagoon area compared with the occasional equivalent data by others. Lagoon depth maxima are taken from the comprehensive account by Purdy (2001), or from the literature as cited. We describe the parts of the component reef systems when permitted by the literature as cited. Latitude and longitude shown on each image are taken from the Allen Atlas GIS, detected by placing the cursor near the center of the lagoon. Distances from one atoll to another are reported as nearest reefto-reef, not center-to-center. The width of the rim of the reef is measured from the reef crest to the outer (oceanward) edge of the lagoon.

Wind data Meteorological parameters (wind vectors at 10 m) are derived from the Climate Forecast System Reanalysis (CFSR), National Centers for Environmental Prediction of the National Oceanic and Atmospheric Administration (NCEP/NOAA), which from 2011 and onwards has a spatial resolution of 0.2°. The wind data herein represent the 5-year interval between March 30, 2014, and March 30, 2019, and they therefore include both El Niño and La Niña states. Due to the 5-year interval, infrequent but major storm events may be difficult to resolve. Wind data are presented here as rose diagrams for illustrative atolls of each archipelago or region. Rose diagrams illustrate the direction (0°–360°) and the frequency (width of the petal) from which winds approach an area; colors reflect the relative proportion of winds from that direction. For many areas, dominant winds come from a relatively narrow azimuthal range, but the strongest winds may come from a different direction because of the impact of cold fronts or tropical depressions.

Wave data Wave data are provided by the DHI Global Wave Model dataset. This model uses the CFSR wind data from NCEP/NOAA, analyzed using the MIKE 21 Spectral Wave Model and calibrated and validated with buoy and satellite data. The flexible mesh of the model has varied cell sizes, but most are between 20 and 100 km. The wave data represent the

hourly data for the 5-year interval between March 30, 2014, and March 30, 2019, the most recent data available at the time of analysis, and the same time period as the wind data. They therefore include both El Niño and La Niña states. As for wind data, major but infrequent storm events may be difficult to resolve. Wave data are presented here as rose diagrams. Rose diagrams illustrate the direction (0°–360°) and the frequency (width of the petal) from which waves approach an area. Each petal is also proportionately color coded by the significant wave height, or the average height of the largest 1/3 of waves. Note that, although waves may be infrequent from a given direction, the waves that come from that direction may be large. These sorts of anomalies are caused by passage of cold fronts, local tropical depressions, or swell that travels great distances from more temperate regions.

Tide data Tides are predicted based on impacts of the gravitational forces exerted by the moon, the sun, and the rotation of the earth. The tide data illustrated herein are derived from the Global Tide Model via MIKE 21 by DHI. Developed by Technical University of Denmark (DTU) Space (DTU10), this model is calculated using constituents including the semidiurnal M2, S2, K2, and N2, the diurnal S1, K1, O1, P1, and Q1, and the shallow water M4. Data utilize TOPEX/Poseidon, Jason-1, and Jason-2 satellite altimetry data from the past 17 years for analysis of sea-level residuals, which are input for calculating harmonic constituents. Data are provided at a resolution of 0.125° × 0.125°. For this publication, we extracted data from near representative atolls of each archipelago for July 1–August 1, 2020, through a full lunar month. These data represent predictions for open-ocean conditions; tides observed at any specific location on an atoll will vary because the tides propagate differently onto each shallow-water atoll; additionally, water levels can change with sea-level change, ENSO variability, seasons, and  with  local atmospheric pressure or wind conditions.

Reference Purdy EG 2001. Supplement to Purdy EG, Winterer EL 2001. Origin of atoll lagoons. GSA Bull. 113: 837–854.

The Polynesian Pacific and the Atolls of the Tuamotu Archipelago Polynesia is composed of more than 1,000 islands spread over an area of more than 2 million  km2 forming a triangular area that extends from New Zealand to the south, to Easter Island to the east, and to Hawai’I to the north. Polynesian ancestors, the Lapita people, set out from Taiwan and settled Remote Oceania between 3,200 and 3,000  years ago, although there is evidence of Lapita settlements in the Bismarck Archipelago as early as 4,000 years ago. The Lapita ancestors became skilled seafarers who shared a common culture, language, art forms, houses, and a panoply of deities. Sites of early Polynesian migration to Tonga and Samoa left few footprints to indicate the pathways and timing of the early voyages from there to the rest of Polynesia. The conflicting dates from ethnography, linguistics, tool or building similarities, and even DNA analysis of the Pacific rat further confuse this history (see reviews by Kirch, 2000; 2010). However, based on Polynesian genomic analysis, a migration to the Society Islands from the Cook Islands and from there to the islands of the northwest Tuamotu group occurred about 1,100 years ago (Ioannidis et al., 2021). High islands were preferred for obvious reasons, but some atolls that may have been mere rest stops became permanently occupied if freshwater could be found. Occupation of many atolls, however, was temporally constrained by falling sea levels that occurred between 900 and 1,200 years before the present time. Before that, many low islands and atolls would have been under water (Pirazzoli and Montaggioni, 1986; Dickinson, 2009). Polynesian atolls (excluding the New Zealand area) occupy a roughly triangular area, about 1.4 million km2, of the south, central, and northern Pacific Ocean. The base includes of atoll-bearing Polynesia includes Tuvalu in the west, followed by

the Cook and Society Islands in the center, and the Tuamotu Archipelago to the east. Tokelau, the Phoenix islands, and the Line islands occupy the center, and the Northwest Hawaiian Islands form the northern apex (Figure  3.1). We begin with the Tuamotu group.

The Tuamotu and associated archipelagoes The Tuamotu Archipelago (‘distant islands’) represents the largest concentration of atolls in the world and extends for about 1,500 km (Figure 3.2). In addition to the Tuamotu group, the southeastern atolls are often included administratively as the Gambier Islands, and the two are often referred to collectively as the Tuamotu-Gambier Islands. Other atolls shown in Figure 3.2 that extend the area farther to the south and to the southeast are described below. Of the 73 atolls shown in this figure, only 26 atolls are formed with open lagoons, most of which are found in the northern third of the chain. Another 47 atolls develop closed or semi-closed lagoons, four of which include waters with markedly altered salinity or other chemical conditions as shown by color codes. The northwestern Tuamotu Islands are formed on volcanoes that rise from the summit of a long submarine plateau (the Tuamotu Plateau) at a depth of 1,500–3,000  m, rather than from the surrounding waters at a depth of 4,000–4,500  m. This volcanic basement of the islands in the northwest is covered with 500–800  m thick carbonate because they have been subsiding for at least 50 million years. In addition to the main axis of the plateau, there are four smaller ridge systems at the northwest that have developed at right angles and are oriented northeast–southwest.

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Figure 3.1 Atolls within Polynesia cover a vast region of the Pacific that range from the Tuamoto Archipelago to the southeast, the Gilbert Islands in the west, and the Northwest Hawaiian Islands. The atolls are color coded with respect to the condition of the rim and lagoon including rims that are open, closed, submerged, closed with alteration of the lagoon waters, or closed with remnant lagoons.

The seven atolls here were formed on erupting fissures generated from fracture zones. The northernmost ridge is occupied by Manihi and Ahe, the second contains Takaroa and Takapoto, the third supports Takume and Rarioa, and the southernmost is occupied by Amanu, all of which have the same orientation with respect to their rims and lagoons (Montaggioni et al., 2019). Second, a more northerly volcanic ridge system to the northeast includes Puka-Puka and Napuka atolls (Figure 3.2). Lastly, a third chain extends as a subparallel track south of the Tuamotu Plateau extending from the Duke of Gloucester Islands (four atolls from Herheretue to Nukutepipi) to the southeastern Tuamotu group, expanding the Tuamotu chain to the southeast including atolls from Tenararo to Temoe. These atolls originated as small volcanic islands that formed on top of a broad marine volcanic plateau. Two additional atolls (Ducie and Oeno) are part of the Pitcairn Islands (Figure 3.2 inset). These southeastern atolls are younger than those of the Tuamotu Islands, perhaps originating

on volcanics only a few million years old, as described later. These age differences suggest that the atolls of the archipelago have had dissimilar origins and a complex history (Montaggioni and Camoin, 1997; Clouard and Bonneville, 2005; Bonneville, 2009).

Regional climate and oceanography The Tuamotu Islands are bounded in the north at 14.66°S and at the south at 23.22°S, essentially at the Tropic of Capricorn (−23.50°). Despite the extensive areal coverage of 13,500  km2, less than 5% is dry land. As of 2017, 16,881 people lived in the Tuamotu Archipelago (Institut de la statistique de la Polynésie française, 2017), including 2,709 who live on Rangiroa, the most populated and the largest of the atolls. The dominant surface current is the trade winddriven South Equatorial Current, which forms part of a counterclockwise gyre that washes through the

Atolls of the Tuamotu Archipelago

Figure 3.2 Atolls of the Tuamotu and associated archipelagoes. Atolls to the south include the Tuamotu, Gambier, and Duke of Gloucester Islands and are either closed or semi-closed. Oeno and Ducie atolls among the easternmost Pitcairn Islands are shown in the inset. The predominant current system is the counterclockwise flow of the South Equatorial Current.

archipelago (Rougerie et al., 1997a). Rainfall is typically between 130 and 190 cm/year with a mean of 178 cm per  year for the islands. However, the rainfall distribution throughout the year is uneven, with about 65% during the wet season (November to March, the austral summer). For example, in January, an average rainfall of about 23 cm falls. However, during the austral winter (April to October), monthly rainfall levels drop by 66% so that in August, for example, only about 7.5 cm of rain falls on average. In addition, there can be considerable variability of rainfall from year to year (Intes and Caillart, 1994). Few trees grow well in these environments, but coconut palms are widespread, in part because they occur naturally and more often because they have been planted. In French Polynesia and elsewhere in the tropical Pacific, coconut palm plantations exploded from the late 19th century on, where the unfortunate technique was to cut down all the native vegetation, allow it to dry, and then set it afire. Newly planted coconut trees were then spaced 100–250 per hectare. Not only were these plantations subject to the boomand-bust cycle of the global copra trade but the trees are susceptible to cyclone damage and to the process of aging. Many that have been planted in French

Polynesia are now too old to bear fruit (Bourdeix et al., 2009). Nonetheless, a close Google Earth view of almost any of the islands—even the motu—in the Tuamotu Archipelago will reveal how widespread these plantations are. Shaped by the archipelago’s position in the trade wind belt, surface winds range from northeasterly to southeasterly 250 days of the year or more (Intes and Caillart, 1994) (Figure 3.3). The austral summer winds are generally moderate, but during the austral winter, trade winds can generate waves between 1 and 3  m from the east and southeast in the Tuamotu Archipelago. Swells from distant storms and the Southern Ocean can reach the Tuamotu Islands, and their exposed atoll margins and shores (which can be either windward or leeward) may be more heavily pummeled by these waves than with those produced by local winds. Storms from the northern hemisphere in the austral summer or from the southern hemisphere during the austral winter can generate swells in excess of 4 m high. However, atolls located on the flanks of the Tuamotu chain may dissipate storm waves from different directions by blocking or reflecting them, thereby providing atolls on the opposite side with some protection due

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Figure 3.3 Trade winds range from northeasterly to southeasterly 250 days of the year or more. Swell from distant storms and the Southern Ocean can reach the Tuamotus, where exposed atoll margins and shores may be pummeled more by these than by local wind-generated waves. The tides are semidiurnal (two highs and lows per day) and generally less than 50 cm range, such as those from Rangiroa shown here.

to an atoll ‘shadow effect’ (Andréfouët et al., 2012). Thus, even nearby groups of northern and southern atolls may have a different wave climate. In addition, cyclonic storms that pass near the area can produce waves more than 10 m high basically from any direction (Intes and Caillart, 1994), although direct hits by cyclones are infrequent except during El Niño periods (Larrue and Chiron, 2010). Wave data from near Rangiroa (Figure 3.3) illustrate these patterns and include the most frequent larger waves from the southwest and northwest. All these processes and events may have considerable influence on ocean water exchange with atoll lagoons. Lagoon water residence (turnover) time is complex and depends to a large extent on the degree of rim closure, depth, and wind and wave conditions. The structure of the rim—its passes, hoa, and reef flat—is key in determining this factor. More open lagoon rims allow swells or local storm waves and winds to exchange water with the lagoon most readily, but large atolls and those with closed and deep lagoons may require years to fully exchange lagoon water (Andréfouët et  al., 2001; Pagés and Andréfouët, 2001). Likewise, residence time in a closed atoll may depend on storm and swell conditions that wash over the rim, and in the absence of such wave activity, the equilibrium between rainfall and evaporation as well as the permeability of the submerged rim structure may be the most important factors (Andréfouët et al., 2001).

Tides in the Tuamotu Islands are semidiurnal. At Rangiroa, the spring tidal range is 40 cm, but in the vicinity of Tikehau 13 km to the west, the amplitude of spring tides is only 15  cm, and at neap tide, the amplitude is almost zero (Intes and Caillart, 1994). Likewise, tidal amplitude is only 30 cm in Ahe lagoon in the northwest of the Tuamotu group. These small (microtidal) variations in amplitude are due to proximity to an amphidromic node near Maupiti (Society Islands to the west) where tides rotate at a focal point and the total tidal variation is typically 20 cm or less (e.g., Rankey, 2021). However, even these small tidal ranges are sufficient to create a jet through the single pass in the rim of Ahe Atoll, a dynamic that significantly alters oceanic-water exchange time in the lagoon (Dumas et al., 2012), as described later. By comparison, the tidal range at Mangareva and Temoe at the opposite end of the archipelago—the islands in the Tuamotu group farthest away from the amphidromic node—increases to about 70 cm (Pirazzoli, 1987).

The Tuamotu atolls The Tuamotu Islands are divided into small administrative groups or larger groups related to their positions within the chain. Here, we describe the atolls in six groups from northwest to southeast. These include the Palliser Islands, the northern islands, two central island groups, the southeastern Tuamotu atolls, and the Pitcairn group.

Atolls of the Tuamotu Archipelago

The Palliser Islands From the northwest, the first group is the Palliser Islands, named after the Comptroller of the British Navy by Captain James Cook who sighted some of them (noted by an asterisk in the next sentence) in 1774. There are ten atolls in this group, described in the following order: Mataiva, Tikehau, Rangiroa, Arutua*, Apataki*, Aratika, Kaukura*, Toau*, Niau, and Fakarava. Interestingly, of the ten Palliser Islands, only Kaukura and Toau were sighted by Cook (Quanchi and Robson, 2005). The most northwesterly of the Tuamotu Islands is Mataiva (Figure 3.4), a small ovoid atoll about 10  km long and 5  km wide. The platform area is 49.9 km2 and it is surrounded by an outer reef that is

about 360 m wide to the southeast where it is best developed. The rim is asymmetrical and is narrower to the north (0.2–0.5 km) where the rim includes a continuous island; by contrast, it is wider (1.0–1.5 km) in the south, where the rim is punctuated by a cluster of several hoa and is exposed to southerly swell. A few patch reefs are associated with the hoa reef flats. Mataiva means ‘nine eyes’ in Tuamotuan, a reference to nine hoa on this part of the atoll. A small channel penetrates the northwestern, leeward rim, and although it is sometimes referred to as a pass, it is only about 0.5  m deep (Sailing Directions, 2017). According to Salvat (2009), this pass is the product of both natural and anthropogenic processes. The entire island was uplifted by 3.5 m in the past few thousand  years, like the others in the

Figure 3.4 Remote-sensing images of the atolls of Mataiva, Tikehau, and Rangiroa. Images © 2021, Planet Labs PBC.

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NW Tuamotu group, due to oceanic crust flexure and bulging associated with volcanic activity from nearby Tahiti. For comparison, Makatea, a raised atoll 135 km to the southeast of Mataiva (see Figure 1.14b), is closer to the highest part of the bulge and is uplifted 113 m (Montaggioni et al., 1987). This atoll is closed. The lagoon is highly reticulated by an inner reef flat and covers an area of approximately 18.6  km2 (eliminating as much of the inner reef flat as possible). It is partitioned into about 70 pools ranging from 100 m to more than 2 km in diameter and 4–8 m deep (Delesalle et  al., 1985; Pagés and Andréfouët, 2001). The Allen Atlas suggests that pools in the center, inboard of the southern hoa, have developed patch reefs. This interpretation is consistent with Delesalle et al. (1985), who report normal salinity in the vicinity of the hoa, whereas in more distant pools, the salinity values can vary seasonally, and perhaps this explains why lagoon salinity has been reported as brackish (Rougerie et al., 1997a). Nonetheless, the lagoon is generally sandy, while those in the east are similar but include some coral/algae development. The water renewal time, the period that it takes for waves and tides to force oceanic water through the rim and completely flush the lagoon, is estimated to be 21 days (Pagés and Andréfouët, 2001). Tikehau (Figure 3.4) is the native Polynesian word for ‘paradise,’ or ‘a place of peace’ (Young, 1899). The island of Tikehau also is known as Porutukai; this term is linguistically like other Polynesian phrases including Polutu (Samoan), Mbulotu (Fijian), and Bulotu (Tongan), which variably mean a place of departure to the underworld or the underworld itself. This atoll is 40 km east of Mataiva and is about 27 by 20 km. The platform area is 351 km2 (Purdy, 2001). The platform has been uplifted 4  m, bringing previously underwater reefs well above sea level (Montaggioni et al., 1987; Dufour et al., 2001). The outer reefs are generally narrow and are less than 300  m wide. The rim is narrow, 0.6–1.2 km wide, and includes a long, single island in the northeast and a wider but shorter island with an airstrip in the southwest. A reef flat occupies the southwestern rim. There is a single pass about 90 m wide and 3.7–11 m deep at the west-facing reef rim (Sailing Directions, 2017). The rest of the rim (southeast, northeast, and northwest) is composed of numerous scattered low motu and hoa. A very narrow sand apron runs around the lagoon, which has an area of 394  km2 and an average depth of 28 m; the maximum depth is 40  m and the estimated renewal time is 60 days (Pagés and Andréfouët, 2001; Purdy, 2001). There are numerous reef-rimmed pinnacles in the southwestern lagoon associated to the pass by proximity. Fewer pinnacles

occur in the protected lee of the island at the northeast. Both areas with pinnacles are associated with the lagoonal slope. Additional pinnacles appear in the center lagoon that are too deep to code. Rangiroa (Figure 3.4) is the largest atoll in the Tuamotu group and is one of the largest in the world. It is irregularly shaped, narrowing to the  southeast, and is 81 km long and 33 km wide at its longest axes. Its platform covers 1,762  km2 (Purdy, 2001), but its total land area is only about 170 km2. Like other Palliser Islands, Rangiroa has been uplifted 3.5 m from its submerged condition 2,500–1,000  years before present (Andréfouët et al., 2008; Montaggioni et al., 2021). The rim is surrounded by a narrow outer reef band that is wider to the southeast where it is commonly2 60–325 m wide. The rim is 1.6 km wide at the southeastern corner of the atoll but is only about 0.5 km wide elsewhere. The windward rim is northwest–southeast and is composed of multiple hoa with motu that exhibit an ocean-facing elevated beach ridge composed of coarse coral shingle and rubble or blocks (Duvat et al., 2021). There are two passes on the northeast-facing margin. The more northwesterly of the two is Passe Avatoru, which is about 300 m wide and has a charted depth of 14 m, although the Allen Atlas suggests some shoaling by sand and coral at the lagoon entry. The east pass, Passe Tiputa, is about 200  m wide and 8.7 m deep (Sailing directions, 2017). The leeward, southern, and western rims with their numerous, small motu are exposed to swell, especially during the austral winter (Vollbrecht et al., 2021). A narrow (100–300 m wide) sand apron surrounds the Rangiroa lagoon, which measures 1592  km2 and has an average depth of 45 m with a maximum depth of 70  m; its renewal time is estimated to be 155 days (Pagés and Andréfouët, 2001; Purdy, 2001). The Atlas shows some minor patch reef and sand pinnacle development associated with the southwest and southeast lagoon slope but some areas are too deep to code. Reef-rimmed pinnacles occur scattered through the lagoon center. Arutua Atoll (Figure 3.5) is about 34  km east of Rangiroa and is ovoid with a somewhat concave side to the northwest. The platform is about 27 by 30 km, covers 590 km 2 , and it is surrounded by a narrow band of deep outer reef that is less well developed on the windward eastern and northeastern margins. The shallow outer reef is best developed on the west. There, small islands form on the rim, which is consistently less than 500  m wide. The northwestern rim is composed of numerous elongated or rounded motu and hoa, whereas the southwestern and southern rims are primarily a reef flat. Here, mixes of coral development and sandy inner reef flats constitute broad rims, commonly greater than

Atolls of the Tuamotu Archipelago

Figure 3.5 Remote-sensing images of the atolls of Arutua, Apataki, Aratika, and Kaukura. Note the broad reef and reef sand apron on the southwest-facing margin that faces into Southern Ocean swell. Images © 2021, Planet Labs PBC.

750 m wide. A single, small channel in the southeast is passable only by small vessels. The lagoon covers an area of 516 km 2 with an average depth of 35 m; its renewal time is estimated to be 60 days (Pagés and Andréfouët, 2001). A band of patch reefs occurs on the lagoon slope extending from the northeast to (and into) the channel on the southeast, as well as on the western lagoon slope. Sand and coral pinnacles are clustered in the deeper western lagoon and extend across it, and although they are numerous, they are not as densely packed in the center and the east compared with the west. Some are elongate, more than 1,000 m long, and appear to be aligned, perhaps by currents in the lagoon. Apataki Atoll (Figure 3.5) is 17.5 km east of Arutua and has a distorted rectangular shape with rounded southeast and southwest sections. Its axes are about 25 km wide (east to west) and 35 km long (north to south). The platform covers 753 km2 (Purdy, 2001) and its outer reefs are narrow but surround the atoll

more or less uniformly. Rim islands are best developed to the north and the east. The southern and western rims are primarily a reef flat and sand apron; many areas are more than 500–750  m and include several motu. There are two passes on the western rim. Passe Tehere in the northwest is about 130  m wide and 5.8  m deep (Sailing Directions, 2017), whereas the one on the southeastern rim is narrower. Both passes are lined with coral development as extensions of the outer reef, but the southern pass appears to be blocked on the lagoon side. A very narrow and submerged shallow lagoon area surrounds the deep lagoon, which is 683 km2 with an average depth of 33 m and a maximum depth of 50 m. The renewal time is estimated to be 101 days (Pagés and Andréfouët, 2001; Purdy, 2001). The lagoon slope narrowly surrounds the basin; all the pinnacles (~30) are covered with dense, healthy corals (Andréfouët, pers. com.) and are coded as reef by the latest version of the Allen Atlas.

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Aratika Atoll (Figure 3.5) is approximately 68 km east of Apataki. It is rounded on the western side and indented on the east forming a triangular protuberance to the northeast. It is about 17 km across through the apex and 20 km at its widest. The platform area is 184 km2 and it is enveloped by an outer reef that reaches more than 200 m to the northeast but is somewhat narrower elsewhere. Islands have formed on the rim from the northwest to the south, although there are sections of reef flat and a pass on the indented eastern side. Neither this pass nor a second pass on the northwestern rim are suitably wide or deep enough to allow the passage of larger vessels. The remainder of the rim on the west and southwest is composed of outer and inner reef flats that are consistently more than 500 m wide and are more expansive than other margins. The lagoon is surrounded by a narrow and submerged shallow area; the deep lagoon is 151 km2 and up to 30 m deep (Purdy, 2001). Sand or sand and reef-rimmed pinnacles, some of which are distinctly elongated, are scattered throughout the lagoon. A second, parallel chain of Palliser atolls lies to the south of those just described, and these have a different geomorphology, perhaps due in part to the protective effect of their brethren to the north (Andréfouët et al., 2012) and the lack of protection from southern swells. This chain is composed of atolls that tend to be elongated and oriented from northwest to southeast. The westernmost of them is Kaukura Atoll (Figure 3.5), located about 25  km south of Arutua. It is sausage shaped with a pointed end to the southeast, about 15 km wide at the middle and 48 km through the long axis. The 531-km2 platform has been uplifted 3.5 m, like others in the NW Tuamotu Islands (Montaggioni et al., 1987). An outer reef is best developed, up to 400  m wide, on the southwest-facing margin, which corresponds to the waveward side of the atoll (Rankey and Garza-Pérez, 2012). Outer reefs are also prominent around the southeast corner of the atoll but continue along the waveward side. The northeast side of Kaukura represents the windward side and it develops a shallow and deep outer reef consisting of scattered, narrow ribbons. The northwest and southeast margins have the largest islands (Raitahiti at the northwest and Faro at the southeast) with several small and elongated ones between them. There are no passes, and the rim is closed. The sand apron is markedly asymmetrical (Rankey and Garza-Pérez, 2012). The northeast-facing, windward section is relatively narrow, generally ranging from 200 to 800  m wide, whereas the waveward, southwestern side is more typically 1–1.9 km wide, far broader than atolls farther north that have been described previously. These sand aprons surround the

lagoon, which covers an area of 421 km2. The lagoon slope to the northwest is a sandy extension of the apron that grades into sandy pinnacles approximately 2.5 km from Raitahiti Island. Large populations of giant clams (genus Tridacna) are found in the lagoon and occur in sand as well as on hard substrata (e.g., Andréfouët et  al., 2005). They will be described in more detail below. The lagoon basin is crowded with sand-topped, reef-rimmed pinnacles. Toau Atoll (Figure 3.6) has a rectangular shape, about 36 × 20 km at the longer axes and is about 28 km east of Kaukura. The platform is 652 km2 in extent (Charpy et al., 1997). As on Kaukura, the platform is surrounded by an outer reef that extends from the southeastern to the northwestern, waveward sides, where the rim is composed of a rock and rubble reef flat rather than the sandier Kaukura. The outer reefs occur all around the platform, but they are somewhat wider to the south and southeast where they may exceed 300 m. The shallow reefs become more strongly developed on the windward northeastern outer reef flats. The east-facing rim from the northwest to the southeast includes small islands that are more elongated toward the southeast. There are three passes, with two to the southeast, including Passe Otuni that is up to 8 m deep and is wide enough for large vessels. Nepo Pass on the northwest side is wide, but not deep enough for large vessels (Sailing Directions, 2017), as it has become partly filled with sand due to the development of an apron on the lagoon side. The lagoon covers 561 km2 (Charpy et al., 1997) and the deeper lagoon and slope contain numerous pinnacles, several of which reach the surface as shallow reef or sand islands. The pinnacles are more concentrated to the northeastern part of the basin. The lagoon is up to 24  m deep (Purdy, 2001) and has numerous sand and coral pinnacles through the deeper center. Niau (Figure 3.6) is a small, polygonal atoll about eight km in diameter, 30 km southwest of Toau. Its platform is 58.5 km2, and because it is closer to the edge of the Tuamotu Plateau, it has been raised 7.5 m more than the other uplifted atolls described previously in the Tuamotu group. The outer reefs are widest along the southern and southeastern edge of the platform where they reach more than 400  m from the rim, less so to the northwest sectors. The lagoon is 32.6  km2 in extent. Because of the elevated rim, the lagoon is closed, and as exchange with oceanic water is infrequent, it is brackish (25‰–32‰). The lagoon is also shallow with a mean depth of 2 m and a maximum depth of 6  m (Delrieu-Trottin et  al., 2018). The dark green color of the lagoon is due to the presence of cyanobacterial mats, called kopara in Polynesian, that are several meters thick. The mats are rich in phosphorous, and after decomposition,

Atolls of the Tuamotu Archipelago

Figure 3.6 Remote-sensing images of the atolls of Toau, Niau, and Fakarava. Images © 2021, Planet Labs PBC.

they lead to deposits of the phosphate-rich mineral apatite (Rougerie et al., 1997b). There is also a population of milkfish in the lagoon which grow to a kilogram. These fish are common in brackish water and remnant lagoons across the Pacific, where they feed on cyanobacteria. They in turn are consumed by the local population, and since there are few food sources in such lagoons, they are also commonly the subject of aquaculture (Delrieu-Trotten et al., 2018). Fakarava Atoll (Figure 3.6) is 51 km west of Niau where it forms a large and rectangular, 55 km long

by 18 km wide, and has a northwest–southeast orientation. The platform is 1,246 km2 in extent (Andréfouët et al., 2001) and has outer reef that is about 250–300 m wide to the south and southeast, and narrower to the north and northeast, similar to its neighbors. Narrow islands develop along the northern and eastern sides where the rim is about 0.5 km wide; the remainder of the west-facing rim is a reef flat, roughly 1.5  km wide and is composed of a combination of rubble, rock, and sand. A narrower reef flat occurs on the southeastern margin. There are two passes

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through the rim; the northwestern pass is suitable for vessels drawing less than 2.7 m. The southern pass is 1.4 km wide and has been cleared to a depth of 10 m (Sailing Directions, 2017) and it is flanked by low motu and hoa. Surrounding the lagoon, a sand apron is about a kilometer wide from the northwest to the south on the waveward margin where it is mixed with considerable rubble. On the opposite island-bearing side facing the wind, the apron can be up to 300 m wide, but is more typically a third of that. The lagoon covers an area of 1,112  km2 with an average depth of 45 m; its renewal time is estimated to be 75 days (Pagés and Andréfouët, 2001). It has few shallow areas and there are numerous sand and sand/coral pinnacles that are clustered in the center west, but some in the northwest and southwest are represented as well. Some of these near the center break the surface and one forms a vegetated motu of about 2 ha.

The central Tuamotu Islands The second group of the Tuamotu group lies in the center of the chain where atolls tend to be tightly clustered. Several were visited or sighted in 1820 by Russian Admiral Thaddeus Bellingshausen who assigned Russian names, usually those of admirals and generals, to each. It is true that the good captain was not the first to sight some of the Tuamotu Islands that he recorded in his logbook, but he did discover some that were new to Europeans including Mataiva, Fakarava, and Tikehau (Barratt, 1999), which are now credited to Captain Cook as members of the Palliser group. Perhaps the lack of acclaim for those discoveries motivated Admiral Bellingshausen’s belief that the entire chain should be named ‘the Russian Islands’ (Debenham, 2016). However, the only eponym that has stood the test of time is that of the Raevsky (Raeffsky) Islands—and there remain three of them—that were named after a Russian general who fought against the French as they invaded Russia during the sixth Napoleonic War. Leaving aside the irony of that honorific, we include 16 atolls in this group, many of which either were visited or sighted by Bellingshausen in the center of the chain (these are marked with an asterisk; Barratt, 1999) or are adjacent to them and are included for the sake of geographic propinquity. In order of presentation, these are as follows: Kauehi, Taiaro, Raraka, Katiu*, Makemo*, Taenga*, Nihiru, and Marutea Nord that fall along the northern rim of the Tuamotu Plateau. The more southerly line of atolls in this group includes Faaite*, Anaa at the extreme southern side of the Tuamotu Plateau, Tahanea*, Mototunga*, and Haraiki. The Raevski trio, Tepoto

Sud*, Tuanake*, and Hiti are small atolls sandwiched between Makemo in the north and Mototunga in the south (Figure 3.2). We arbitrarily drew the line for this second group at a natural break that occurs in the plateau southeast of Nihiru, Marutea Sud, and Haraiki. Kauehi Atoll (Figure 3.7) is ovoid, oriented northwest to southeast, and lies about 34  km from Aratika. The platform is about 24 by 15  km, with an area of 343 km2 (Charpy et al., 1997; Adjeroud et al., 2000). It is lined by a continuous outer reef that extends 175–190  m wide to the northeast and up to 285  m to the southeast. Islands are found on much of the rim; motu are common to the northwest and north, and these pass on the eastern, windward atoll flank into a single island about 24 km long and mostly about 300–500 m wide (except for a small protuberant area 1.2 km wide at the northeast where a village is located). A reef flat comprises part of the western rim, although this area also includes numerous motu. Narrow, elongated islands occupy much of the southwestern rim and they are interrupted in the southwest by single pass, Arikitamiro, that is about 300 m wide and 15 m deep (Adjeroud et al., 2000). A narrow sand apron, generally ~200  m wide, occurs along most of the lagoon except for along the western islands where there are ribbon-like reefs adjacent to the island shores on the lagoon side. Reefs also extend from the outer reef into the pass in the southwest. The lagoon area is 315 km2 with an average depth of 35 m and a renewal time of 77 days (Pagés and Andréfouët, 2001). Numerous coral-rimmed pinnacles occur, especially along the eastern and southern sides. The deeper central lagoon contains fewer such pinnacles. Nonetheless, Adjeroud et al. (2000) found a diverse community that included 22 coral genera in the Kauehi lagoon. Taiaro Atoll (Figure 3.7) is about 49 km northeast of Kauehi. The platform is ovoid, less than 6 km long and 4  km wide, and covers an area of 18.3  km2. It has an outer reef that is continuous around the rim where it is mostly 150–175 m wide to the north and southeast (coded somewhat wider by the Atlas), and narrower west–northwest clockwise. Most of the rim is composed of an essentially continuous island, ranging in width from 150 m to about 600 m. Although there are apparent hoa, all but one has been blocked by storm debris and they are not functional; the one that may be open is likely functional only during storms. In addition, Taiaro was uplifted by about 1 m during the ninth century (Galzin et al., 1998). This atoll is closed. With rainfall totals of about 110 cm/year, the 12 km2 lagoon has become hypersaline, with salinity of about 43 ‰, compared with 36.5 ‰ in the open ocean. Adjeroud et al. (2000) found only one coral genus represented here, but there is a surprisingly high diversity of

Atolls of the Tuamotu Archipelago

Figure 3.7 Remote-sensing images of the atolls of Kauehi, Taiaro, and Raraka. Images © 2021, Planet Labs PBC.

fishes in the lagoon. The mean lagoon depth is 15 m, the maximum depth is 27 m, and the renewal time is estimated to be 4.8 years (Galzin et al., 1998; Pagés and Andréfouët, 2001). This atoll is uninhabited and also privately owned (Salvat and Myer, 2009). Raraka Atoll (Figure 3.7) is oval, 27 km long and 19 km wide, and lies about 18 km southeast of Kauehi. The platform covers 402  km2 and it is marked by an outer reef that extends northwest–southeast where it is about 175–250  m wide and is narrower elsewhere. The outer reef is essentially absent on the windward, northern, and eastern sections of the platform. The rim is 300–400  m wide, and islands are

best developed on the northwest and northern side, although islands are also present on part of the eastern side. The northeastern rim is composed of windward motu, whereas the remainder of the rim (west to southeast) is a reef flat that includes tapered or fusiform motu. A single pass between islands in the northwest is about 45 m wide but only 5 m deep (Sailing Directions, 2017). There is a reef at the lagoon end of this channel, and there are small reef areas associated with motu on the western and southwestern reef flat. A sand apron 200–450  m wide occurs around the lagoon except on the western side, where it is wider. Shallow sand also extends along several ridges that

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Figure 3.8 Remote-sensing images of the atolls of Katiu and Faaite. Images © 2021, Planet Labs PBC.

project from the apron area into the deep lagoon. The lagoon area is 355  km2. There are pinnacles in the lagoon mostly northwest–southeast clockwise along with others that rise from the deep lagoon. Katiu (Figure 3.8) is another ovoid atoll with a northwest–southeast elongation. It has a long axis of about 24  km and is less than 13  km wide. The platform covers 263 km2 and bears a circumferential outer reef that is roughly 150–200 m wide with areas of lesser development to the southeast and west. Rim islands are developed most extensively between the northeastern side, where there is an airstrip, and the southeastern side. The rim here is from 350 to just over 600 m wide. The remainder of the rim is a south-facing reef flat that is 600–900  m wide with scattered motu, especially on the northwestern side where some reef growth is apparent on the southwest reef flats associated with motu.

There are two passes. The one facing the northeast between elongated islands (Pakata Pass) is about 30 m wide and at least 3.3 m deep; the other in the west is smaller. Both are usable only by small boats (Sailing Directions, 2017). In addition, both passes are associated with reef development. The lagoon area is 229 km 2 and is enveloped partly by a sand apron about 180–250 m wide on the east and southeast and 400–500  m wide at the northwest, west, and southwestern sides. The lagoon depth is up to 20 m deep (Purdy, 2001) and includes prominent narrow coral- and sand-covered ridges that protrude more than 1 km from the shallow lagoon slope, especially in the southwest. The lagoon slope includes reef-covered pinnacles that are especially concentrated toward the southwest; fewer pinnacles occur to the eastern and southern slopes but many occur in central lagoon.

Atolls of the Tuamotu Archipelago

Figure 3.9 Remote-sensing images of the atolls of the uplifted atoll of Anaa, as well as Tahanea, and Motutunga. Images © 2021, Planet Labs PBC.

Faaite Atoll (Figure 3.8) lies 56 km southwest of Raraka and 22  km southeast of Fakarava. Its platform is 271  km 2 in extent. An outer reef extends around the reef circumference and ranges from 150– 200 m wide, although it is more consistently developed to the north. Reefs to the south are generally about 100 m wide. The rim is composed of an inner and outer reef flat that is mostly 700–1,100 m wide. Slender islands with motu and a few hoa occupy the windward northern and northeastern rim, which is mostly 300–400  m wide, and there is a single y-shaped channel at the northwest that is reportedly 50  m wide with a depth of at least 3.5  m (Sailing Directions, 2017). The Allen Atlas clearly shows reef growth through the channel as an extension of the outer reef. The lagoon extent is 224 km 2. Numerous reef-rimmed pinnacles occur throughout the lagoon

and are more tightly clustered in its western half. Purdy (2001) records the depth maximum as 20 m. Anaa Atoll (Figure 3.9) is about 65 km southwest of Faaite. It is roughly 28 km long, 7 km at its widest, and is elongated northwest– southeast, like many of the southern Tuamotu Islands. However, Anaa includes a hook-like bend to the northwest and is raised 6 m more than other atolls at the edge of the Tuamotu Plateau (Montaggioni et  al., 1987; Pirazzoli et al., 1988a). This elevation is due to its passage over the tectonic flexural bulge associated with volcanic activity from nearby Tahiti, the same process that elevated Makatea to 113 m. Anaa lies along the same platform edge and is taking the same path that Makatea took (Figure 3.1). Indeed, Anaa in now in the same position that Makatea was three million years ago and will likely be uplifted to the same

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extent several million years from now (Pirazzoli and Montaggioni 2018). The 178 km2 platform includes outer reefs that are more or less uniform around the platform and range from about 150–250  m wide. The rim is closed and develops reef-rimmed islands on most of its extent. The remainder of the rim includes hoa that are not functional because of the uplift, although passageways through a reef flat at the northeast near Tukuhora Village allow for exchange of lagoon water and spawning aggregations of bonefish to pass in and out of the lagoon (Filous et  al., 2020). The lagoon covers an area of 106  km2, has an average depth of 4  m, and has an estimated renewal time of 9 days. About 15  km2 of the northwest lagoon is partly partitioned by rock and rubble forming what is known locally as the ‘Petit Lagoon’ as opposed to the remainder, called the ‘Grand Lagoon’ (Filous et al., 2020). The lagoon shallows are dominated by sand that expands to cover much of Petit Lagoon and divides Grand Lagoon in half along a shallow ridge. Small reefs are associated with the reef flat in the northeast and on the inner face of some islands in the western lagoon. Much of the western lagoon bottom is shallow and covered with sand, and the mid- and southern lagoon shallows are surrounded by sand. The deepest lagoon areas occur to the southeast, where pockets up to 10 m deep are found (Purdy, 2001). Although it lies on the flank of the archipelago, Anaa Atoll was once the home of the most powerful tribe in the Tuamotu Islands. At their peak, they reportedly had more canoes than all the other islands combined! The first European to discover Anaa was Pedro Fernándo de Quirós in 1606. It was the first inhabited island he encountered in the tropical Pacific, which he named La Conversion de San Pablo according to the date of arrival on the Catholic calendar. However, given the positioning equipment of the day and the absence of charts, chief pilot Gaspar Gonzalez de Leza believed this to be Hao Atoll farther south, whereas others initially thought this was Tahiti which lies farther west, but given the description of the island and the cruise track, it was very likely Anaa (Markham, 1904). James Cook was the next European to encounter Anaa in 1769. He described the atoll as ‘a double range of low woody islands [motu] joined together by reefs to form one island in the form of an ellipsis or oval, with a lake in the middle of it. The small islands and reefs that circumscribe the lake have the appearance of a chain, and we therefore gave it the name of Chain Island’ (Hawksworth, 1773). More recently, Tahiti’s royal family—the Pomare Dynasty— claims Anaa as its birthplace. Current Anaa residents are known for their javelin throwing skills and their sustainable bonefish fishing program.

Tahanea Atoll (Figure 3.9) is 15 km east–southeast of Faaite and is shoe shaped. The platform covers 643 km2 and displays an outer reef from the west to southeast (from heel to the toe) where the rim is a reef flat dotted with several hoa, some of which develop reef growth around them. The largest island occurs in the northwest and is about 2.6 km long by roughly 50 m wide. The remainder of the north-facing rim is a combination of slender islands, sand, and rocky bottom, which is 350–450 m wide. There are three passes separated by two islands in the northeast across distance of 4.5 km (the laces). The centermost pass is Teavatapu which is about 320 m wide and 11– 13 m deep. The two on either side of it are impinged by coral growth and may be hazardous (Sailing Directions, 2017). The lagoon is 545 km2 with an average depth of 45 m and a renewal time estimated to be 59 days (Andréfouët et al., 2001). The shallow lagoon reef sand apron extends to about a kilometer wide to the south and narrows to less than 200 m wide to the north. A few reef islets occur within the apron especially to the southeast along with patch reefs and ribbons along the shoe opening and at the heel counter (western lagoon). The deeper slope is associated with coral-rimmed pinnacles that are especially abundant in the west and southeast. Additional pinnacle development occurs in the deeper lagoon basin. As on Fakarava, some of these break the surface, and one forms a vegetated motu of about 2 ha. Mototunga Atoll (Figure 3.9) is located about 20 km southeast of shoe-shaped Tahanea. It is ovotriangular with the wider side to the southwest. The platform is 13 km from there to mid-base and about 15 km at its widest. The platform area is 163 km2, and it is surrounded by an outer reef that is generally about 250–275 m wide but narrows to about 150– 175 m wide to the north. The rim along most of the east, the south, and the west sides is composed of an inner and outer reef flat. The reef flat is narrowest in the east, where it is about 500 km across, and widest in the south, where it is ~1.3 km across. The west side that is about 800 m wide has the most numerous reef flat motu and hoa, many of which are associated with reef growth. The northern and northeastern rims are composed of small islands, motu and hoa, with coral growth on their reef flats. The largest islands cover 65 ha (in the northeast) and 61 ha (in the northwest). There are two passes through the rim, both on the north-facing, and it appears to have substantial coral growth that extends to the edge of the lagoon. The lagoon area is 126  km2 and is surrounded by a shallow, sandy edge less than 100  m wide to the east. Numerous reef-rimmed pinnacles are scattered across the lagoon. We are unaware of depth data for the lagoon.

Atolls of the Tuamotu Archipelago

There are three small, oval or ovoid atolls 25– 40 km northeast of Mototunga and 15–33 km southeast of Katiu. These are the Raevsky Islands referred to above, all of which lie in the center of this group of atolls. They all are surrounded by a shallow outer reef that is least well developed on the eastern (windward) side. The closest one to Mototunga is Tepoto Sud (Figure 3.10) whose platform area is 7.5 km2. An outer reef generally 150–200 m wide surrounds most of the platform, but it is somewhat more weakly developed to the north. Two islands with an area of 1.2 km2 occupy the north and northeast half of the island that are unequally divided by the pass. The southern rim is

a rock and rubble-strewn outer reef flat that is 1,000– 1,100 m wide, interspersed with elongated motu and hoa. A narrow band of sandy inner reef flat occurs at the eastern edge of the lagoon. The lagoon area is approximately 1.4 km2 and has a wide sand apron around it that represents about 59% of the lagoon area. Isaac and Gischler (2015) calculated the apron as 29% of the platform size, although they used a different platform area than that reported here. The lagoon has an average depth of 5 m and is renewed in a matter of hours or days depending on swell conditions (Pagés and Andréfouët, 2001). Reef growth occurs in the pass. Adjeroud et  al. (2000)

Figure 3.10 Remote-sensing images of the atolls of Tepotu Sud, Tuanake, and Hiti. Images © 2021, Planet Labs PBC.

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found ten genera of coral in lagoon reefs. Nonetheless, Salvat (2009) considered this lagoon closed. We tentatively mark it here as partly open. Tepotu Sud Atoll has been rumored to be the site of treasure, stolen from a church in Pisco, Peru, and buried there by four thieves in the 19th century. Although one recent explorer found some medallions from South America on the atoll, no mother lode of gold, silver, or jewels has been found. Tuanake Atoll (Figure 3.10) is 16 km northeast of Tepoto Sud and forms a rounded D shape with the flat side facing south. The platform extent is 41.6 km2 and it is 6 km wide at the flat side by about 7 km in diameter. The outer reef extends around most of the atoll and is about 250 m wide. A few small reef areas on the windward reef flats. Motu and one hoa occur to the north, and slender islands that extend from the northwest to more than halfway down the eastern rim. The remainder of the windward east (southeast rim) is composed of motu. The southern and western rims are composed of inner and outer reef flats, primarily with elongated motu and hoa. There are no passes, and the lagoon is closed, although there is a small, submerged area in the middle of the southern rim. The lagoon is 26  km2 in area, has an average depth of 25 m, and a renewal time of 17 days (Pagés and Andréfouët, 2001). A narrow, submerged apron in the shallow lagoon is up to 200 m wide in the northeast, but less than 100  m wide elsewhere. Isaac and Gischler (2015) found that the apron comprises about 5% of the platform area using a smaller platform area than that reported here. The deep lagoon slope and lagoon develop multiple reef-rimmed pinnacles. Hiti Atoll (Figure 3.10) is seven  km southeast of Tuanake and is an oval that is 6.8 km along the main axis and about 5 km wide. The platform area is 29.27 km2. The outer reef is generally about 200 m wide, narrowing to about 150  m to the north and northwest. The distribution of islands from northwest to southeast is also like Tuanake. Islands 2 km long in the northwest corner are followed to the east by windward motu and another island 1.6 km long at the north-most bend. Another island 3.3 km long is followed by a smaller 0.5 km long island on the east. The lagoon is 15  km2 with an average depth of 10 m and a renewal time of 3 days (Adjeroud et al., 2000; Pagés and Andréfouët, 2001). The southeast- to the west-facing rim includes inner and outer reef flats up to 1,200 m wide, making the rim quite asymmetrical. Large parts of it are covered with sand. There are no passes, and the lagoon is closed. The shallow lagoon from west to southeast is bordered by a narrow, submerged apron that widens at the northwest, where some reef develops. Isaac and Gischler

(2015) calculated an apron area of about 14% of the platform using a smaller total area than that used here. The lagoon develops plateaus in the northeast. Several reef-rimmed pinnacles occur on the slope as well as three near the lagoon center. Adjeroud et al. (2000) found 11 genera of corals in the lagoon, which is consistent with a rapid renewal rate comparable to the lagoon at Tepoto Sud. Makemo Atoll (Figure 3.11) is elongate, 62– 68 km long and of variable width but it is generally keyhole shaped. It lies parallel to the northern edge of the Tuamotu Plateau, in contrast to the smaller atolls just described (Figure  3.1). Because of this position and orientation, it is open to extreme storm waves, especially on the northern rim, where numerous boulders have been dislocated and deposited on the reef flat. Some of these measure 130 m3 and weigh more than 300 metric tons (Lau et al., 2014). In addition, distal swell from the southwest occasionally floods the lagoon, raising water levels up to 2.75 m, flooding villages, and contaminating groundwater (Canavesio, 2019). The platform area is 716 km2 (Purdy, 2001) which includes the surrounding outer reefs that are generally 175–250  m wide. The windward, northeastern rim is 400–500  m wide and supports elongated islands that are interspersed with motu. In addition, there are two passes. The one in the northwest, Passe Tupuhiria, is 90 m wide and 7 m deep, and it may be blocked by reef growth. The other pass, 50 km to the east on the north rim, is Passe Arikitamiro, which is 160 m wide and 14–27 m deep (Sailing Directions, 2017), but it is divided into three channels by coral growth. The Allen Atlas shows that it is flanked on both sides by coral that extends from the outer reef, wraps around the opening, and then extends 13 km along the shallow lagoon toward the west and another 5  km toward the east. Although some motu occur toward the northwest, the south-facing, waveward margin from the southwest to the southeast is a 1-km-wide reef flat with few motu, and most of the rim is reef or sand. In contrast, on much of the north-facing margin, the apron is narrow, mostly 200–300  m wide especially where islands occur. The lagoon area is 603 km2 with an average depth of 18 m, a depth maximum of 60 m, and an estimated renewal time of 15 days (Purdy, 2001; Pagés and Andréfouët, 2001). The passes are flanked by reefs. The deep slope and lagoon present numerous reef-rimmed pinnacles that are widely distributed throughout the lagoon system. Ovotriangular Taenga Atoll (Figure 3.11) is located about 35 km northeast of Makemo. The platform is close to 11  km from south-facing apex to mid-base, and about 27 km at its widest, parallel to

Atolls of the Tuamotu Archipelago

Figure 3.11 Remote-sensing images of the atolls of Makemo, Taenga, and Nihiru. Images © 2021, Planet Labs PBC.

the base and covers an area of 173 km2 (Purdy, 2001). Outer reefs surround the platform and are widest to the southwest where they extend up to about 250 m from the rim. A spur and groove system surrounds the entire atoll. The western rim is primarily a reef flat, 350–550 m wide, that is bisected by a 30-m-wide pass. Although the channel may be at least 1.5  m deep (Sailing Directions, 2017), it may be occluded at the lagoon end by coral growth and rock, as well as a sand bank. The remainder of the rim, which is about 400 m wide, is composed of windward motu between the northwest and the southeast, although there are two small islands in the northwest totaling 56 ha and small reefs on their lagoon side. The lagoon is 172  km2; the deepest portions are 14  m (Purdy, 2001), but this depth likely is an underestimate. A sand apron is up to 200  m wide, but commonly is less than 70 m wide, as it extends on the south-facing rim between the northwest and the southwest. More than ten narrow ridges extend into the lagoon

perpendicular to the southern rim, and another 4–6 penetrate from the northeast. Most of the ridges that are shallow enough likely are enveloped by reef growth. Reef-rimmed pinnacles and ridges also are found in the deep lagoon. Nihiru Atoll (Figure 3.11) is polygonal, extending about 14  km wide along both main axes. Located about 30 km southeast of Taenga, the platform covers 100  km 2 (Adjeroud et  al., 2000; Dufour et al., 2001), and it is surrounded by an outer reef system that is commonly 250–280 m wide but expands to the southeast to more than 325 m and narrows in part of the southern platform to about 125  m. A spur and groove system is found along the south and the east rim. The rim is composed of elongated or rounded reef-rimmed islands and motu, especially on the windward northeastern and northwestern sides. The eastern rim appears to be discontinuous with sediment partly covering a rocky substrate. The waveward, southeastern to

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the southwestern sides include inner and outer reef flats, most of which are dotted with motu and hoa, as well as a 59-ha island in the southwest corner. An artificial channel has been constructed through the rim just north of this island. Small, shallow reefs develop seaward of the eastern rim. There are several hoa at the northwest, but there are no passes, and the rim is closed. The 80 km2 lagoon has an average depth of 20 m and an estimated renewal time of 17 days (Pagés and Andréfouët, 2001). Numerous pinnacles are found throughout most of the lagoon basin, especially in

the southern half. Adjeroud et  al. (2000) found 18 coral genera associated with the inner reef flats and pinnacles of this atoll. Pearl oysters were at one time greatly diminished by overexploitation but have recovered in more recent years (Andréfouët pers. com.). Marutea Nord (Figure 3.12) forms a rounded triangle-like atoll, 43 km long and 19 km wide at the northeast-facing ‘apex.’ It is located on the northern line of these atolls about 25 km southeast of Makemo and 32  km southwest of Nihiru. The platform is 534 km2 in area and is surrounded by outer reefs that are about 250–300 m wide to the south and west, the

Figure 3.12 Remote-sensing images of the atolls of Marutea Nord and Haraiki. Images © 2021, Planet Labs PBC.

Atolls of the Tuamotu Archipelago

waveward side, and narrow to about 150–200 m wide elsewhere. The rim develops few islands, but there is a 5.5-km-long line of them at the north side of the eastern apex; there are also a few motu in the north, and there is a line of gray sand and rubble ridges with a few motu at the southeast corner of the atoll. Reefs develop on the flats immediately north and south of the eastern facing apex but are less pronounced on the lagoon side of the islands to the north. The rest of the rim along the waveward south and west is a reef sand apron 750–1,100 m wide. According to Sailing Directions (2017), these reef flats are completely submerged. The lagoon covers 450  km2, and although there is a small (likely artificial) channel at the apex, there are no passes; numerous reef-rimmed pinnacles occur in the lagoon, especially to the northwest and south where they are clustered. We do not have information on the depth of this lagoon. Because of those features and the high diversity of bivalves (Salvat, 2009), this lagoon is regarded as semi-closed. Haraiki (Figure 3.12) is a triangular atoll 44 km southwest of Marutea Nord and 95  km southeast of Mototunga. It is the last of the southern line of group 2 atolls. The rim is 4.9  km through the apex and 7.6 km at its widest. The platform area is 26 km 2 and is surrounded by an outer reef that is generally 200–350 m wide but extends to more than 500 to the south where a prominent pass is found (see below). The rim on the north consists of a single island 7.9  km long and 690  m at its widest. Reef growth occurs on the island’s perimeter. A large pass about 285 m wide occurs at the south-facing apex. We are not aware of its depth, but there is expansive reef growth at the entrance that may form a barrier to the entrance of the lagoon. Flanking the pass are two islands, each about a kilometer from the pass. The one to the northeast is 28 ha; the other to the northwest is 32 ha. Reef growth flanks the channel opening. The rim is open. The lagoon area is 10 km 2 , the average depth is 14 m, and the approximate renewal time is 3 days (Pagés and Andréfouët, 2001). A sand apron up to 100  m wide occurs to the northwest and narrows east–west clockwise with a 525  m gap to the southeast that coincides with a ridge. The lagoon slope develops pinnacles to the west and several reef pinnacles occur in the deep lagoon as well. Adjeroud et al. (2000) found a diverse suite of 15 coral genera in this lagoon.

The south-central Tuamotu Islands This group of 18 atolls takes a southeasterly course across about 700  km of the Tuamotu Archipelago. We begin with Rekareka (Figure 3.13), whose

nearest neighbor is Nuhiru, about 90  km to the northwest. This small ovoid atoll is elongated on a northwest–southeast axis. It is 5.6 by 2.3  km, and its platform covers an area of 7.3  km2. Outer reefs surround the platform, although a prominent shelf with deep reef and ridges in the southeast is the widest and extends more than 500 m from the rim. A large, vegetated island up to 140  m wide to the east and 650 m to the west dominates the rim. The windward, eastern portion of the island is a gray sandy ridge with a narrow strip of vegetation that is 80–100 m wide. Much of this appears to be a coconut plantation. A 76-ha island on the south-facing margin is flanked on either side by hoa that may only be functional during storms. The rim is closed. The lagoon is 0.7 km2, about a meter deep, and because only 1.7% of the rim allows water exchange (Torréton et al., 2002), the lagoon is poorly flushed and has an estimated renewal time of about 81 days (Adjeroud et al., 2000; Pagés and Andréfouët, 2001). Correspondingly, elevated water temperatures and depressed oxygen levels provide a challenging environment (Salvat, 2009). Tekokota (Figure 3.13) is a small, egg-shaped atoll about 4.5 km long by 2.5 km wide located about 85  km southwest of Rekareka. It is elongated on a northwest–southeast axis and is located about 48 km southeast of Marutea Nord. The platform area is 9.9 km2. Its outer reef is especially well developed to the south where it is about 350 m wide. Outer reefs elsewhere around the rim range from 170–295  m wide. The eastern rim includes a single vegetated island that is about 1  km long, 270  m wide at the center, tapering at both ends. The island is flanked by gray sand and rubble ridges that become especially prominent on the southern half of the eastern rim. The southwest-facing rim is about 250 m wide and is composed of a reef flat with several elongated motu. The rim at the southeast is submerged due to tilting of the platform but it has formed a spur and groove system that is open to the lagoon. The breadth of this opening is about 700 m, but it is shallow, only about a meter deep (Torréton et al., 2002). Although the lagoon appears to have an area of 5.1 km2 (Dufour et al., 2001; Torréton et al., 2002), there are relatively large rocky areas of inner and outer reef flats that encroach its boundaries. If these are considered, the lagoon area is closer to 4.0 km2. The average depth is about 3 m, and because nearly 60% of the rim is available for exchange with ocean water, it has an estimated renewal time of about 7 hours (Pagés and Andréfouët, 2001). This atoll is open. Some reef extends into the lagoon through the open rim at the southeast, but otherwise the lagoon bottom is sand and small patch reefs.

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Figure 3.13 Remote-sensing images of the atolls of Rekareka, Tekokota, and Hikueru. Images © 2021, Planet Labs PBC.

Hikueru Atoll (Figure 3.13) is located 24  km southwest of Tekokota. It is ovoid, about 15 km long by 9 km wide, and has the same northwest to southeast orientation as the others described above. The platform covers 107 km2 (Dufour et al., 2001; Torréton et al., 2002) and it is surrounded by an outer reef that ranges from 150–250 m wide. The northeast rim is about 400 m wide and includes an island about 6 km long that widens to roughly 700 m wide to the southeast. The trend of this island continues west and northwest along the entire eastern rim, where a gray ridge of sand and rubble forms the seaward edge of the vegetated island, smaller islands, and then motu and hoa at the northwest corner of the atoll. The rim of the northwest, the west, and the southeast parts of the atoll is open to southerly swells

and is a reef flat, with few islands. There are scattered coral/algal patches on the reef flats along the east and north segments of the rim, including within the confines of a narrow, artificial channel located immediately east of the northwestern island. The rim is semi-closed. The shallow lagoon is enveloped by a submerged sand apron that is 250–400 m wide along the northeast, 600 m wide to the southwest, and less than 50 m wide in the northwest. The lagoon covers 82  km2, the mean depth is 25 m, and the estimated renewal time is 37 days (Pagés and Andréfouët, 2001). The primary reef material in the lagoon is associated with the many pinnacles, most of which occur on the broad lagoon slope that is especially prominent in the center and the southwest parts of the atoll. Several

Atolls of the Tuamotu Archipelago

pinnacles reach the surface. Despite the semi-closed morphology, this lagoon has been the site of several mass mortalities that have had a serious effect on its biota, likely due to exceptionally calm weather and a large phytoplankton bloom (Adjeroud et al., 2001). Reitoru (Figure 3.14) is a reverse D-shaped atoll about 50 km southwest of Hikueru. The platform is 3  km wide bisecting the east-facing base, reaching 5 km at the widest, and occupies an area of 16.1 km2. The outer reef surrounds this atoll and most of it extends 125–160 m from the rim, although it is about 225–325  m wide to the south. The rim from the northwest and the east is capped by islands that lie on a gray-colored ridge of sand and rubble. There is a shallow gap about 20 m wide in the rim just north of the southwestern-most island on the east, and another one ~1.7 km south that is about 30 m wide. A reef flat that is 30–60 m wide occurs along the entire eastern rim, oceanward of the islands and ridges. The remainder of the rim extending west–southeast is a reef flat that exceeds 300 m width at the south tip of the atoll. The rim is semi-closed. The lagoon area is 9.1 km2 and includes a broad expanse of shallow sand apron that is 700 m wide in the southwest and more than a kilometer wide in the south. This submerged sand area is reduced to 70– 160 m wide along the east and is essentially absent to the north. Inside the shallow sand belt, a deeper lagoon covering 5.1 km2 corresponds to the area given by Salvat (2009), who also found that this lagoon is a favorable environment for giant clams of the genus Tridacna described in more detail below. The lagoon slope contains a scattering of reefs. We do not have a maximum depth for this lagoon.

Tauere Atoll (Figure 3.14) is a square with rounded corners that is just over 4.3  km across. Its nearest neighbor is Tekokota 109 km to the west. The platform area is 20.5 km2 and it has an uneven distribution of the outer reef around its borders. The outer reefs range from 170–240 m wide and is broadest at the southeast corner. A spur and groove system surrounds this atoll. The rim along the northeast side is a 3.9-km-long island. Counterclockwise, this is followed by a hoa and another smaller island in the north, a 540-m-wide gap that appears to be at sea level, and another small island at the northwest corner of the atoll. A reef flat occupies the western rim where there are motu and hoa, but at least a kilometer-long section appears to be near sea level. The southwestern and southern rims are composed of low islands, interrupted by a 23-ha island at the south corner. There are a few small and scattered reef patches seaward of these islands. The lagoon is 8.1 km2 and the rim is closed, but Andréfouët and Chauvin (2005) found that nearly 6% of the rim is open to exchange. Both the shallow lagoon on the southwest and the lagoon slope are convoluted with plateaus, and both areas develop a considerable reef system. We suspect that wave and tide activity may be sufficient to quickly renew the water in the lagoon, but we are unaware of the lagoon depth or data that would confirm this expectation. We mark this lagoon as semi-closed. Marokau Atoll (Figure 3.15) is located 72  km southeast of Reitoru and 46  km southeast of Hikueru. It forms a warped triangle with a depressed base facing northwest. It is about 16 km from midbase to the opposite, southeast-facing apex, and roughly 22 km at its widest parallel to the base. The

Figure 3.14 Remote-sensing images of the atolls of Reitoru and Tauere. Images © 2021, Planet Labs PBC.

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Figure 3.15 Remote-sensing images of the atolls of Marokau and Ravahere. Images © 2021, Planet Labs PBC.

platform area is 256 km2 (Adjeroud et al., 2000; Dufour et al., 2001) and the broadest outer reef area is the southwest where it ranges from 235 m to as much as 630  m. The remaining reef areas are commonly 135–175 m wide. The rim extending from the northwest-facing side to the northeast and southeast sides is occupied by small islands and hoa formed atop a ridge of graycolored rubble and sand. The island-bearing portion of the rim ranges from about 350 to 520  m at its widest point near the northeast apex of the triangle. There are two narrow, shallow channels through the eastern islands and numerous hoa among the north and northwest islands. A reef flat about 40–60  m wide occurs along the eastern rim oceanward of the islands. A reef flat up to a kilometer wide extends

along the southwest side of the atoll, where only a few motu occur, near the middle of that side. The rim is semi-closed. A pronounced reef sand apron up to 500 m wide flanks the southwestern reef flat but is reduced to 80–200 m wide on the east/northeast atoll side and narrows even more at the northwest-facing rim. The lagoon covers 213  km2 with an average depth of 30 m and has a renewal time of 55 days (Pagés and Andréfouët, 2001). Pinnacles with reef development occur along the northwestern, western, and southern lagoon slopes, but are most common towards the southwest and south margin. The deep lagoon at the atoll center and northeast displays fewer pinnacles. Salvat (2009) described a diverse bivalve community from here.

Atolls of the Tuamotu Archipelago

Ravahere (Figure 3.15) is a boomerang-shaped atoll just over 2 km southeast of Marokau. It is about 6.5 km at its widest, from the southeast-facing apex to the west-facing base, and 21 km long in a curved sweep from the southwest to the northwest. The platform area is 74.2 km2 and it is surrounded by a nearly continuous deep outer reef, except for a 1.8 km gap in the southwest and a 550 m gap in the northwest. The outer reef is continuous around the atoll and is up to 330 m wide to the south–southeast and ranges from 110–210 elsewhere. The rim between the northwest and the southeast apices is composed of a stretch of about 5  km of motu followed by a single island nearly 12  km long. Most of this part of the rim is about 300 m wide, but its southern terminus expands to about 500 m. A reef flat, mostly 40–60 m wide, flanks the islands along the eastern rim. The southeastern to the southwestern rim is composed of islands and motu. Most of the rim northwest and west is a reef flat less than 400 m wide with motu and hoa along much of its length. The rim is without passes, but because the reef flats are weakly developed along the southernmost 5 km of the west, we regard it as semi-closed. The lagoon area is 45.4 km2 including two enclosed pockets, one in the north with an area of 4.3  km2 and the other measuring less than 0.25  km2. Both are isolated by a narrow rock and sand ridge. A broad reef sand apron that is 250–450 m wide occurs in the western shallow lagoon; the sand apron widths are narrower elsewhere. Sporadic, small reef patches are found along the west lagoon, although more occur in the lee of the eastern islands. Some pinnacles occur in the deeper lagoon, but fewer than 15 of them are associated with reef development. We do not have a depth for the lagoon. From Ravahere, we move west where we encounter Hao and Amanu. Hao Atoll (Figure 3.16) is interesting for several reasons including the question of who among the European explorers discovered it. Pedro Fernándes de Quirós is often quoted as its discoverer in 1606, but as pointed out earlier in this chapter, Captain Quirós was not at all certain where he was at the time, and he logged the wrong latitude for Hao (e.g., Salmond, 2010), making credit for its discovery difficult to ascertain. There is no doubt that Louis-Antoine de Bougainville discovered Hao on March 24, 1768, and he named it Île de la Harpe in his belief that it resembled the musical instrument. However, Bougainville was given a decidedly unfriendly reception by the resident Polynesians (who likely arrived more than 800 years earlier) and being unable to find bottom by sounding to 100 fathoms (more than 180 m), even when close to shore, he was persuaded not to land (Dunmore 2002). Captain James Cook sailed

by Hao in the following year and named it Bow Island, thinking it resembled the arrow-launching device (Young, 1899; Buck, 1953). Hao is a large atoll and neither captain could have seen it all, especially en passant, hence the misnomers. When viewed from space, Hao looks more like a shoe, but the one we described previously (Tahanea) is more masculine. This one is longer, more elegant, and more feminine like a ballerina slipper. The long axis of Hao is oriented northwest–southeast, and it is a little more than 57 km from heel to toe and about 15 km at its widest. Hao is located 111 km from Ravahere. The platform area is 578 km2 including a narrow outer reef that is generally 150–200 m wide but expands to 250–300 m wide along parts of the west. The windward, northeast rim is composed of a single island nearly 26 km long (across the opening of the slipper) and includes an airstrip. Another island occurs from the toe 16  km along the top of the slipper. The widest part of the rim is 1.7 km at the toe. Between the two islands, the windward rim is composed of motu. The windward side is typically about 200 m wide, although it is about 340 m wide where the main settlement of Otepa is located. The northern rim, about 10 km from east to west, is composed of small islands, and between them is a pass, about 300 m wide, reaching a depth of at least 6.4  m (Sailing Directions, 2017). The remainder of the rim (from heel to toe) is a reef flat with motu, which is about 500 m across. The rim is open. The lagoon is 497 km2 with a mean depth of 40 m and a residence time estimated to be 90 days (Pagés and Andréfouët, 2001). The maximum known depth is 61 m (Purdy, 2001). Reefs occur through the pass in the northwest and continue around the shallow lagoon for 5 km to the west and more than 10 km to the east and south. More reef ribbons occur along the eastern just south of the slipper opening and flanking both sides of the lagoon northwest from the toe. There are numerous coralliferous pinnacles in the deep lagoon including those that are elongated along the sole and toward the toe. Amanu Atoll (Figure 3.16), Hao’s neighbor 19 km to the northeast, is sausage shaped and exhibits a northeast-southwest orientation, which is distinct from the other atolls described thus far in the Tuamotu Islands. There are seven atolls that have arisen from erupting fissures generated by fracture zones within the main Tuamotu line and Amanu is one of them. The remaining six will be described in the next section. This atoll is about 31.5 km long and 9.5 km wide. The platform area is 244  km2 (Andréfouët ­ et al., 2001). Outer reefs are widest to the east–southeast where they are 220–285 m wide but are otherwise narrower and become are more poorly developed

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Figure 3.16 Remote-sensing images of the atolls of Hao, Amanu, and Nengonengo. Images © 2020-2021, Planet Labs PBC.

to the northeast. The rim includes vegetated islands on all sides. The northeast rim develops the largest island, which is about 6  km long and 570  m at its widest. The northwest-facing rim of the primarily consists of small, closely spaced islands and motu. Toward the southwest on this margin, two passes are separated by a small island. The more southerly of the two is 45 m wide and is about 5 m deep; the more northerly one is narrower ‘and is not recommended’ for passage (Sailing Directions, 2017). Both are encroached by shallow outer reef growth. The southeastern rim is primarily composed of reef flat with

hoa and motu and is about 500  m wide. There are small ribbons of reef growth along the central portion of this rim. The rim is open. The lagoon area is 210 km 2 , with a mean depth of 30 m, a maximum depth of 62 m, and an estimated renewal time of 39 days (Pagés and Andréfouët, 2001; Purdy, 2001). The shallow lagoon is rimmed by sand northwest–southwest clockwise where it is mostly about 100  m wide. The lagoon slope in the southwest develops patch reef ribbons. Nengonengo Atoll (Figure 3.16) is located 54  km southeast of Ravahere and 102  km from

Atolls of the Tuamotu Archipelago

Hao. It is ovotriangular with one apex pointing toward the northwest and its short base facing the southeast. The distance from base to apex is about 14  km, and the atoll is about 9  km at its widest, near its base. The platform covers 91 km 2 and is completely encircled by an outer reef that is generally 225–275  m wide although it narrows to 200  m wide to the northwest and expands to 325 m to the east. The northwestern rim is capped by a 6-km-long island and is separated by a channel from a second island that is a little more than a kilometer long. The channel is roughly 160 m wide and ~2  m deep. However, coral growth accounts for that depth along nearly half of the extent of the pass (Sailing Directions, 2017) and the Allen Atlas clearly shows the outer reef penetrating and spreading as if to fill the channel. A low area of the rim that is about 80 m wide flanks the smaller northeastern island and appears to be sufficient to allow additional exchange. There is a limited area of coral/algal growth on the reef flat adjacent to this low area. The rim is open. A third island 2 km long at the southeast bears an airstrip. The distance between this island and the pass is 6.8 km, most of which is a low, sand and rubble ridge, 200–300 m wide composed of motu. The remaining rim of the west and south is a combination of reef flat and motu and is up to a kilometer wide. The lagoon is 67 km2 in extent and reef growth especially near the pass. Reef development is also associated with pinnacles on the lagoon slope, especially in the northwest. Additionally, there are scattered pinnacles rising from deeper water. We are unaware of any depth data for this lagoon. Manuhengi (Figure 3.17) is a small, egg-shaped atoll that lies about 68 km southeast of Nengonengo. It was discovered by the English navigator Samuel Wallis in 1767 and it is currently uninhabited and privately owned. Manuhengi is oriented northwest– southeast and is 5.6  km long and about 3.4  km at its widest. The platform is 16.6 km2 in extent and is surrounded by an outer reef system that is widest to the west where it is generally 200–285 m wide. The eastern reefs are about 135–170 m wide but narrow to the northeast and south. The shallow reefs extend onto the reef flat around most of the atoll. The rim is capped by a gray sand and rubble island that is vegetated between the north and the southeast; it ranges from 250 to 550 m wide. There is an apparent coconut plantation in the north. The rim of the west side has about the same range of widths, with less vegetation. The southern rim is composed of motu, but there are two shallow gaps in the rim divided by a single motu in the southeast. The largest is 30 m wide and the other is perhaps 20 m wide, and both

may be partly occluded by rock, rubble, or sand. The lagoon area is 8.7  km2 with reef growth along its shallows and slope, and small pinnacles are present in the center of the lagoon. We do not have any depth information for this lagoon system. Paraoa (Figure 3.17) is a small ovotriangular atoll with a south-facing apex and a north-facing base located about 54 km east of Manuhengi. It is about 4.4 km from apex to mid-base and 6.8 km at its widest. The platform covers 27.2 km2 and it is surrounded by an outer reef up to 330 m wide to the southeast that narrows to about 100 m wide to the east. The rim is dominated by a single island on the north-facing base that extends northwest–southeast more than 10 km. The remainder of the rim at the south to the west is composed of motu and reef flats. The outer reef flats in the east and along the southwest have motu and are associated with patchy coral/algal reef. There are no passes, and the rim is closed. The lagoon is 15.8 km2. A narrow band of submerged sand and reefs are found to the west in the lagoon shallows. A well-developed reef system is also present as extensions along the north and south lagoon slope. Pinnacles with coral are found in the deeper middle lagoon. Pinnacles are also present in the deep eastern lagoon, but fewer of them are associated with reefs. We are unaware of depth data for the lagoon, but Salvat (2009) indicates that giant clams (genus Tridacna) are abundant. We tentatively mark this lagoon as semi-closed. Ahunui (Figure 3.17) is a rounded atoll with a flattened side facing northeast, and that lies 94  km southeast of Paraoa. It is 5.4  km bisecting the base and 6.3 km at its widest. The 30.6- km2 platform develops an outer reef that is generally about 200 m wide but narrows to about 100 m to the east. However, a spur and groove system between 30 and 50 m wide is found around the atoll. The east-facing rim is occupied from north to south by a single island that is 8.9 km long and mostly about 200 m wide, but which expands to 400 m at the southern tip. The remainder of the rim is composed of motu, but there are no obvious functional hoa or passes. The rim is closed. There are also no settlements or air facilities. The lagoon is 19.2  km2 in extent and develops eight reef-rimmed pinnacles in the lagoon center. Salvat (2009) indicates that there are appreciable Tridacna maxima and pearl oysters, among other bivalves, but we are not aware of any quantitative data or depth information. The next three atolls form a triangle northeast of Ahanui. Vairaatea Atoll (Figure 3.18) is about 121 km northeast of Ahunui and is teardrop shaped with the single apex pointing west. From there, the longest axis is 6.8 km; the north-to-south axis across the drop is 4.8  km. The 25.3  km2 platform is surrounded by outer reefs that vary from 150–250  m

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Figure 3.17 Remote-sensing images of the atolls of Manuhengi, Paraoa, and Ahunui. Images © 2021, Planet Labs PBC.

wide north–west to 35–400 m wide east–southwest, becoming narrow (100–135 m) through part of the northwest. The rim is composed primarily of two islands. The largest is C-shaped, extends from north to south along the bottom of the drop, and is 7.2 km long. There is a small settlement on the north of this island, but there is no airstrip. The second island is 4.1 km long and extends from the north to the west where it curves, forming a reverse J-shape. A reef flat that is 1.1  km long occurs between the islands on

the northwest-facing rim. The southern rim is composed of a 3.5-km-long reef flat with scattered motu and hoa. Reefs are clearly developed on the outer reef flats here. The rim is closed but is open to exchange through both reef flat areas. The lagoon area is 12.8  km2. Reefs encircle the basin and are best developed to the north and northeast where they form pinnacles rising from the deep lagoon floor. We have no depth information for this lagoon.

Atolls of the Tuamotu Archipelago

Vahitahi (Figure 3.18) is located about 71  km northeast of Vairaatea. It forms an elongated oval shape that is oriented east–west. Its long axis is 8.4 km, and it is about 2.8 km at its widest toward the east. The platform is 22.5 km 2 and is surrounded by an outer reef about 125  m wide and the entire atoll is surrounded by a spur and groove system. An island 5.7  km long forms on the northeastern rim and curves around the eastern side. West of this, motu and hoa occur on the northern rim, along with two shallow channels 200–600 m wide. A small island occurs on the west tip, where there is a small settlement and an airstrip. The lagoon is 8.1 km2 in extent and is generally shallow and reticulated. Reefs occupy the shallows to the east and west. Most of the lagoon includes reef-rimmed pinnacles. The western

lagoon is sandy and develops a few reef areas. We do not have any depth records for this atoll. Pinaki Atoll (Figure 3.19) is round, about 2.2 km in diameter, and is located about 54 km east–southeast of Vairaatea. An outer reef up to 350 m wide to the south surrounds the 4.9 km 2 platform. The rim is composed of one island of 1.3  km 2 that circumscribes the lagoon, but for a west-facing 200-m-wide hoa that appears elevated above sea level and is blocked at least partially by rock and rubble. It likely does not permit oceanic exchange with normal tides and is closed, although there are populations of T. maxima, as described below. The lagoon covers 0.60 km 2 , and most of it is very shallow although no specific depths have been reported to our knowledge. Most of the lagoon is mapped

Figure 3.18 Remote-sensing images of the atolls of Vairaatea and Vahitahi. Images © 2021, Planet Labs PBC.

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Figure 3.19 Remote-sensing images of the atoll of Pinaki. Image © 2021, Planet Labs PBC.

as sandy, replete with ridges and associated with islands that break the surface, suggesting mapiko, a Polynesian term for high density shell mound aggregations formed by Tridacna that often can be seen in satellite imagery. Van Wynsberge et  al. (2016) report 4–12 T. maxima per m 2 , a lower density than for neighboring, T. maxima-rich atolls. Coding by the Atlas indicates reefs are present in the shallows and on the margins of the lagoon entrance.

The northern and northeastern Tuamotu Islands We introduce the 13 northernmost of the Tuamotu Islands here, starting with the northwesternmost pair, Ahe and Manihi, and then to another pair of atolls, Takaroa and Takapoto. These four are sometimes referred to as the King George Islands as explained below. Rarioa and Takume are the last of the paired atolls. All three pairs are elongated northeast– southwest rather than the more typical orientation of the Tuamotu Archipelago. As mentioned in the description of Amanu, Hao’s neighbor, this orientation is due to eruptions from fissures generated by fracture zones that formed at right angles to the main

Tuamotu trend (Montaggioni et al., 2019). This position changes their waveward sides to the west and northwest and the leeward side to the east and southeast. We then encounter Napuka and Puka-Puka, which are occasionally lumped together as the Disappointment Isles, as further explained below. These are followed by a chain of five including Fangatau, Fakahina, Takatoto, Pukarua, and Reao, the outermost of the Tuamotu group in the northeast and a convenient place to draw a boundary. Ahe Atoll is at the northwestern edge of the Tuamotu group. It is ovoid with an elongated southeast end; it is about 24  km to the northeast side and about 10  km at its widest expanse (Figure 3.20). The platform is 171  km 2 in extent (Purdy, 2001) and develops an outer reef that is commonl6y 200–250  m wide although some areas narrow to 90–150  m wide, especially to the southwest and southeast. A narrow band of spur and groove occurs along the eastern and the western parts of the platform. A 14-km-long island extends along the northern rim from the northwest to the northeast, reaching about 550  m wide where the airstrip is located. A few reefs occur on the reef flats to the northwest. A 200-m-wide pass about 11  km from the airstrip on the western rim is up to 11 m deep

Atolls of the Tuamotu Archipelago

Figure 3.20 Remote-sensing images of the atolls of Ahe and Manihi. Images © 2021, Planet Labs PBC.

(Dumas et  al., 2012). The remainder of the rim from the west consists of motu with hoa, some of which are blocked, as well as several that are 10– 300  m wide and less than a meter deep (Dumas et al., 2012); some are open to ocean exchange with the lagoon. There is a 1.1-km 2 arcuate island on the southwestern rim that is followed to the east along the southern margin by small islands (one of which includes a village) and motu separated by hoa. The eastern rim is closed. The lagoon area is 145 km2, with an average depth of 41 m, a maximum depth of 71 m, and a renewal time of 34 days (Pagés and Andréfouët, 2001; Andréfouët et al., 2020). Oceanic exchange with the lagoon has been studied intensively owing to the importance

of Ahe in pearl oyster aquaculture (Andréfouët et al., 2012). Most of the circulation is driven by tides that force water in and out of the lagoon through the northwest pass. Even though the tidal range reaches only 30  cm, strong currents at 2 m/sec (more than 7 km/hour) create a jet-like circulation that rapidly renews the lagoon waters. However, the lagoon volume can be viewed as a three-cell system that does not respond to wind and tide forcing uniformly or as forcefully as it does in the vicinity of the pass. The overall lagoon renewal time is estimated to be 252 days, but this figure does not consider all the dynamics of wind, wave, and tide or the response of basin morphology to these forces as described in detail by Dumas et al. (2012).

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The lagoon area includes a shallow, 47-ha sandy basin (excluding inner reef flat) at its southwestern point that is partly isolated from the main lagoon by a rock and rubble ridge. The shallow sand continues from this small basin along the lagoon margins for 2 km to the northeast and 4 km to the east. Patch reefs occur as ribbons in the shallow lagoon and on the reef flats along the north and northwest and continue to the northeast, where they are associated with plateaus. The lagoon slope is studded with numerous plateaus and reef pinnacles that are especially prominent to the southwest. However, most of them (141) are associated with the lagoon deeper than 20 m that extends throughout most of the basin (Andréfouët et al., 2020). Many of these break the surface where they are rimmed with coral/algae and topped with rock and rubble. It should be noted that Andréfouët et al. (2020) found 65 deep reticulated basins using bathymetry in Ahe lagoon, but these are too deep to be visible in optical satellite imagery. Manihi Atoll (Figure 3.20) is located 30 km east of Ahe. It is elongate with a point at its southwest (toward Ahe) and a short, flattened northeast-facing side. The atoll is 28 km long and 8 km wide covering an area of 201 km2 (Purdy, 2001). The outer reefs are quite variable in extent. Along the northwest platform for example reefs extend from 200–285 m wide but narrow to about 100 m wide to the southwest; long sections of reef to the southeast are narrower than 100 m. The distribution of islands is similar to Ahe. There is a narrow 9-km-long island lying on a gray sandy ridge forming the northeast part of the atoll. A low point in the rim that is 260 m wide occurs in the northwest and appears to be a point of oceanic exchange. This area is flanked by smaller islands and motu (some with possibly functional hoa) on the northwest-facing margin. A west-facing arcuate island that is 4.3 km long and 1.7 km wide at the southwest extent includes an airstrip, and a village is established on a smaller, adjacent island. Between the two is a narrow pass about 60  m wide that shoals at the lagoon entrance, leaving a navigable channel about 40 m wide, with a depth of 3 m (Sailing Directions, 2017). The shoaling and narrowing are due primarily to coral/algal growth. The southeast-facing rim is composed of small islands and motu with numerous hoa 40 to more than 400  m wide. It is not clear if these hoa permit ocean water exchange with the tides or only occasionally. The rim width here is about a kilometer. The lagoon extends across 165 km2, with a mean depth of 30 m, a maximum depth of 37 m, and an estimated renewal time of 130 days (Pagés and Andréfouët, 2001). The northwest lagoon is lined with reef ribbons which extend from the inner reef flat to

the shallow lagoon, as on Ahe. The lagoon shallows are flanked by a narrow edge of sand extending from the northeast lagoon more than half the length along the southeast flank and about 6.5 km along the northwest side of the lagoon. Most of the lagoon slope displays reef development with pinnacles rising from the deep lagoon. As on Ahe, many of these reefs reach the surface and they are rimmed with coral/­algae and are topped with sand, rock, and rubble. We are aware of no studies of lagoon dynamics comparable to Ahe, but the stock of pearl oysters is comparatively poor and limited to the upper 10 m of pinnacles (Andréfouët et al., 2016). Takapoto Atoll (Figure 3.21) is located about 70  km southeast of Manihi and it is about 20  km long by about 7 km wide at the center. The platform area is 104 km2 and an outer reef mostly about 200 m wide occurs along the northwestern flank. The platform from north–southwest appears to be less well developed and are most often 100  m wide or less. However, a continuous spur and groove system about 20–40 m wide occurs here, and consistent with the mapping, these are evident in the Allen Atlas along the northwestern rim. About 2/3 of the northwestern rim is occupied by a 21-km-long island that extends around the south and about 1/3 of the way along the southeast coast. The island is 300–480 km wide and includes a village and an airstrip at the southeast corner. A second island 1.6  km long is separated by a 40-m-wide gap that is spanned by a causeway and connects to the village. There are reef areas on the flats seaward of this settlement. A third island that is 10  km long curves around the northeastern rim. These islands are extensively planted with coconut palms. Motu are interspersed among these islands, a few of which are separated by gaps 20–40  m wide along the east that may allow oceanic exchange with the lagoon. There are no passes, and the rim is closed. The lagoon is 81  km2 in extent, with an average depth of 25  m, a maximum depth of 43  m, and the estimated renewal time of 268  days (Pagés and Andréfouët, 2001). This long interval of time corresponds to a higher salinity in the lagoon that ranges between 37‰ and 39.5‰, depending on precipitation. The average salinity here is 38.2‰ (Charpy, 1996) which is about 7% higher than the surrounding ocean but this does not seem to be an impediment for high lagoon densities of the giant clams and pearl oysters (Salvat, 2009). Ribbons of patch reefs occur flanking the northwestern islands facing the lagoon, although some are associated with the shallow lagoon accompanied by a narrow band of sand. Plateaus along part of the southeastern flank of the lagoon slope and in the south occur where the lagoon is partly partitioned into a reticulated basin. Most

Atolls of the Tuamotu Archipelago

Figure 3.21 Remote-sensing images of the atolls of Takapoto and Takaroa. Images © 2021, Planet Labs PBC.

reef development associated with the lagoon is in the 194 pinnacles that occur in waters deeper than 20 m (Andréfouët et al., 2020). The lagoon currently has developed a substantial population of oysters (Andréfouët pers. com.). Takaroa Atoll (Figure 3.21) is just over 9  km northeast of Takapoto. It is elongated northeast– southwest like the others in this group, but it is also polygonal, about 26  km long and about 6.5  km at its widest. The platform covers 114  km2 (Purdy, 2001). A deep and shallow outer reef occurs along its northwestern side and extends onto the reef flat

in several places. In the Allen Atlas thematic maps, the outer reefs to the north-northwest are generally 180–200 m wide, whereas those to the southeast are more often 100 m wide or less, similar to Takapoto. A spur and groove system 20–30  m wide occurs close to shore along the northwest, and continues around the southern platform and all the way along the southeast. As is typical for this group, the northeastern rim includes a single island, which in this case is over 11  km long and about 1.2  km wide at its widest. The northwest-facing rim is composed of smaller islands and motu extending about 12  km,

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some with accompanying hoa, after which there is an airstrip and a village. A pass 55  m wide with a depth of at least 12 m occurs about 2.3 km south of the airstrip, which shoals to about 3  m toward the lagoon (Sailing Directions, 2017). The Allen Atlas shows that the pass narrows and includes coral/algae on the south flank, and the lagoon entrance (which requires a sharp turn to port) is further obstructed by coral growth. A 3.3-km-long interval of motu separates the pass from a second island, 1.2  km long, that curves around the southwest rim back toward the southeast. The southeast-facing rim is composed of island and motu with about 20 hoa that are mostly 20–50 m wide. The lagoon covers 86  km2, with a mean depth of 26 m, a maximum depth of 48 m (rounded from ­A ndréfouët et al., 2020), and a renewal time of 76 days (Pagés and Andréfouët, 2001). This ­lagoon is about the same size and depth as its neighbor Takapoto, but exchanges ocean water 3.5 times faster, and it is a more welcoming habitat for the pearl oyster, although the stocks are relatively low and are found predominantly in shallow water between 1 and 10 m depths (­A ndréfouët et  al., 2016). The southwest lagoon is partitioned by a sand and coral-­bearing outer reef flat into an 82-ha shallow basin that terminates as a sandy shore. Reefs in the lagoon center include 246 reef-rimmed pinnacles rising from the deeper (>20 m) lagoon. Many of these break the surface where they appear as rock with sand and rubble. Pinnacles are also habitat for pearl oysters although they are still recovering from a mass mortality that occurred in 2014 (Monaco et al., 2021).

The four atolls just described are those that are the farthest north in the Tuamotu Islands and were named the King George Islands by John Byron in 1765. As Commodore Byron headed to the northwest with the copper-bottomed HMS Dolphin, he had been chased away from more than one island (see below) but was finally successful in landing on two of them, which he promptly named King George’s Islands in honor of King George III (Hawksworth, 1775). It should be noted for the purpose of clarity that the island of Tahiti was also briefly titled for King George in 1767 by another English captain, as was an island in the South Shetland group near the Antarctic Peninsula. The coordinates given in Byron’s logbook do not help determine which of the Tuamotu Islands were King George’s, but Gallagher (1964) suggests they were Takapoto and Takaroa, although Ahe and Manihi are also possible. Perhaps in the spirit of historical compromise, all four of these atolls have been accorded the royal eponym. Raroia is located about 315  km from Takaroa, although its nearest neighbor is Nihiru, which lies 56 km to the southwest. This atoll is ovoid, 40 km long, and about 13  km at its widest (­Figure 3.22). The platform area is 421 km2 and its outer reefs are about 150–200  m wide except to the south where they extend 100  m or less from the rim. Although the waveward, side is to the northwest, an account by Newell (1954) indicates that this atoll is surrounded by a conspicuous spur and groove zone that is 50– 100 m wide. The northeastern rim is capped by motu with hoa that extend 20 km around the rim and may be

Figure 3.22  Remote-sensing ­images of the atolls of Raroia and Takume. Images © 2021, Planet Labs PBC.

Atolls of the Tuamotu Archipelago

functional only during storms. The mid-northwestern rim is the location an airstrip with a pass located about 3.5 km to the north of it. The pass is 20 m wide and 6.2 m deep (Sailing Direction, 2017). South of the airstrip and around a west-facing elbow, the rim is punctuated by islands and motu, some of which may be functional. A 4.7-km-long island completes the western rim and is followed by a reef flat at the south that is about 5 km long and 650–850 m wide. There is some reef development on the flats at the middle of the eastern rim associated with an island. The lagoon is 368  km2 in extent (rounded from Andréfouët et  al., 2020). The northeast and southwest ends of the lagoon contain sand deposits that are about 450 m wide. The slope is narrow. The average depth of the lagoon is 32 m, whereas the maximum recorded depth is 68 m. Depths greater than 20 m are skewed toward the western flank although numerous coralliferous pinnacles, 1,434 of them as assessed by Andréfouët et al. (2020), populate much of the lagoon basin. However, there are an additional 184 pinnacles have bases in shallower water. The atoll of Raroia also is of broader, popular science note, as the Pacific landing place of Thor Heyerdahl in 1947. Heyerdahl was a Norwegian adventurer knowledgeable in ethnology, zoology, and geography. To demonstrate the plausibility of an east-towest pattern of ancient settlement of remote Pacific Islands, following the trade winds, he set out from South America on a small, balsa wood raft, Kon‑Tiki. Over 100 days and 8,000 km later, the raft crashed onto the reef at Raroia, simultaneously fulfilling and shocking Heyerdahl, who had nearly drowned twice as a child and was fearful on the water. A 1951 documentary captured the essence of the voyage, and although physical, cultural, and DNA evidence demonstrate a west-to-east migration and settlement, counter to his hypothesis, Heyerdahl is remembered as a colorful explorer. Takume Atoll (Figure 3.22) is about 9 km northeast of Raroia. That atoll and the nearby Takume were known as Napaite, which means ‘the twins,’ by the locals prior to European exploration. It is 21 km long and a bit more than 4 km at its widest on a platform of 72  km2. Similar to the other atolls with a northeast–southwest orientation, the outer reefs along its northwestern margin are 150–200 m wide and are better developed than the southeast where they are commonly less than 100 m wide. The rim supports two relatively large islands. The one in the southwest, Ohomo, covers 81 ha and includes an airstrip. The other, at the northeastern end, is Taheto, covers 1.3  km2 on a rim that is 1.6  km wide. The northwest-facing rim is composed of motu and hoa as well as reef flats from 400 to more than 900 m wide

that likely permit ocean exchange with the lagoon. The southern rim is a 3-km-long reef flat that extends from the airstrip in the southwest to the northeast. The eastern rim is composed of motu and hoa, some of which may be functional. Reefs on the flats along southeast flank of the rim occur in association with motu, as well as close to the reef crest. The lagoon is 41  km2 in extent (rounded from Andréfouët et al., 2020). The shallow areas are lined with reef ribbons that merge onto the inner reef flat. Such reefs are less common in the southwest lagoon and northeast lagoon, where sand is prominent in the lee of Taheto and adjacent islands. The lagoon depth averages 20 m, and this depth extends through most of the center of the basin to the maximum of 58 m. Most reef development is associated with 56 pinnacles in lagoon found at depths greater than 20 m (Andréfouët et al., 2020). Napuka is polygonal, oriented west–east, and is a distant atoll, 193 km northeast of Takume (Figure 3.23). It is about 11.5 km long and 5.8 km at its widest; the platform extent is 42.4 km2. It is bordered by outer reefs about 150 m wide along all sides accept the south, where they are 50–100 m wide although there is a spur and groove system along this edge. The rim is dominated by a 4.7-km2 island about 11 km long that extends from the northwest to the southeast. Another island in the west is 1.5 km2 in extent and is the site of a small village with an airstrip. A 1.5-km-wide gap separates the two northwestern islands, reflecting a reef flat with a rubble bottom and some nearshore reef ribbons. The south rim is composed of islands and motu with reef flats 60–600 m wide. The reef flat of the southeast is 370 m wide. The lagoon area is 20.3 km2. About 1 km2 of the western lagoon is partially closed by a ridge that is part of the shallow lagoon. The lagoon center supports a series of plateaus and reef-bearing pinnacles. Some of these features are elongated and others widen as they reach the surface, where they become flattened and sparsely vegetated; a few reach an area of up to 3 ha. Pinnacles diminish in extent but continue toward the eastern lagoon that ends in a sandy cul-de-sac with little reef development. We have no depth information on this lagoon. Although there are no passes, the lagoon has been characterized as semi-closed due to the high densities of giant clams found in this lagoon, as they are in several other closed atoll lagoon systems in this area (e.g., Van Wynsberge et  al., 2017). However, these same atolls are often susceptible to mass mortalities due to events such as prolonged periods of low wind or swell from the south that diminish or impede lagoon water renewal.

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Figure 3.23 Remote-sensing images of the atolls of Napuka, Puka-Puka, Fangatau, Fakahina, and Takakoto. Images © 2021, Planet Labs PBC.

Puka-Puka Atoll (Figure 3.23) is northeasternmost of the Tuamotu Archipelago and there is some evidence that Magellan may have encountered this island en route to Asia on January 24, 1521, but it could have been Fangahina or Fangatau just as easily (Fitzpatrick and Callaghan, 2008). This atoll occasionally is included as one of the Disappointment Islands, although it is doubtful that Byron ever sailed near it. The first Europeans to describe Puka-Puka without doubt were two Dutch merchants, LeMaire and Schouten, who followed the footsteps of Quirós exploring the Pacific. In 1616, they landed on a remote island and reported seeing three dogs there that

neither barked nor made any noise. These were Tahitian canines, now extinct, but Dog Island, as they named it (Honden Eiland in Dutch), is generally regarded as Puka-Puka (Young, 1899; Douglas, 2014). Puka-Puka is ovotriangular with a rounded apex facing the southeast. From there to the center base that faces northwest, it is about 6.6  km long and 4.3 km at its widest. The platform area is 23.1 km2. There are some outer reef areas to the northwest that are separated as ridge-like projections 100 m or more from the outer reefs. These were not included in calculating the platform area. The outer reefs are typically 200–275 m wide except to the southwest where

Atolls of the Tuamotu Archipelago

they extend to more than 300 m. Reef development also occurs on the outer reef flats, especially northeast–south. A spur and groove system appears to surround this atoll but is somewhat obscured to the east by wave activity. The rim is composed primarily of a 7-km-long island that extends on the northern margin from the northwest to the southeast, where it becomes wider and curves, as it does on Napuka. The island is about a kilometer wide to the north, where an airstrip is located. This island is separated by a 1.5-km-long reef flat from a second island with a village on the west. The remainder of the rim along the south is about 8 km long and is composed of reef flats interspersed with islands, motu, and apparently non-functional hoa. The reef flat on the southeast rim is about 400 m wide, similar to Napuka. There are no passes, and the rim is closed. The lagoon is about 1.9 km2. It is skewed toward the northeast rim, and it is very shallow, perhaps only a few meters deep (Rougerie, 1995). It is divided into three pools that have an extensive and dissected apron. The northwestern pool is 1.4 km2 and has four stream-like spillways from the southwest, where water from storms appears to have overtopped the rim. The middle lagoon is the smallest (0.15  km2) and is separated from the others by ridges. There are no spillway connections. The third, southeastern pool covers an area of about 0.3 km2 and has 2–3 spillways facing the south. There are numerous ridges and pinnacles, many of which break the surface. Fangatau is about 130  km east of Takume and 183  km southwest of Puka-Puka. This atoll is a D-shaped atoll with the 8-km-long flat side facing north and it is just over 3 km wide bisecting the base (Figure 3.23). The platform area is 25.2  km2 and it is surrounded by outer reefs that are generally 170– 180 m wide. Small areas to the southeast and southwest are 300 m wide, whereas adjacent areas develop reefs that are 100 m wide or less. Reef material occurs on the reef flats around the atoll with less widespread occurrences on the north than elsewhere. The rim is composed of two large islands that are separated by a 3.8-km-long stretch of motu and hoa along the southern rim and a 400-m-wide gap with two motu and three hoa on the north-facing rim. The 10–12-m-wide artificial channel through the reef flat on the southwest includes no passes. The lagoon could be regarded as closed (Salvat, 2009) or semi-closed, due to hoa along the south, which are sufficient to exchange seawater with the lagoon and thereby support a very large giant clam population (see below). The lagoon is 8.7  km2 in area and about 20  m deep. Andréfouët et  al. (2005) found that 40% of the lagoon (4.1  km2) is less than 6  m deep. These sandy shallows are home to an estimated population

of more than 20 million giant clams that attach their shells to the substrate, and by constructing mounds (Gilbert et  al., 2006a) or ridges, they have become the primary reef builder in this lagoon. Indeed, the shallow northeastern part of it (about 1.6 km2) is separated from the rest by a large deposit of shells, and most of the main lagoon is isolated from the shoreline by another such deposit. Reef-rimmed pinnacles and ridges occur in the deeper lagoon. Fakahina (Figure 3.23) is an egg-shaped atoll located about 73  km southeast of Fangatau. It is 8.6  km long and 5  km at its widest. The platform is 38.1 km2 and is surrounded by an outer reef that is about 160–200 m wide northwest–east clockwise. The remainder is quite variable and mostly narrower. A spur and groove system around rings most of the atoll. Coral/algae occur on the reef flat between the northeast and the southwest, with more occasional patches along the north rim. The rim is dominated by a single island nearly 14 km long, up to 860 m wide to the east, and 810 m wide to the west where there is an airstrip. The island is separated by a 4.4-km-long series of motu and hoa. The hoa do not appear to be functional daily, but there is a 300-m-wide low point in the rim (traversed by a causeway) where oceanic exchange clearly takes place. The lagoon area is 18.6  km2. An apron extends from the airstrip at the west to the east, becoming widest in the north where it extends up to 200  m. The apron is submerged along the southeast and south atoll, where it becomes part of the shallow lagoon. A relatively small number of pinnacles occur in the lagoon center. Salvat (2009) reported substantial populations of giant clams and pearl oysters in the lagoon here, but we have no depth reports. Tatakoto Atoll (Figure 3.23) is 274 km south of Puka-Puka and 228 km southeast of Fakahina. It is an elongate ovoid atoll and oriented west–east. The 45.7-km2 platform is surrounded by an outer reef that is generally 150–200 m wide, although parts of the southern reef are 275–325  m wide whereas the eastern reef is quite narrowly developed. A spur and groove system occurs around most of this atoll’s perimeter. The rim is composed of a C-shaped, 7-km2 island that is open to the south and widens as it wraps around the east and west ends of the atoll. A village and an airstrip are found to the west, where the island is a kilometer wide. The eastern side is 580 m wide, otherwise, the island is 300–400 m wide. The southern rim is 12 km long and is composed of motu and hoa, several of which at the center and westward, toward the airstrip, appear to be open. There are no passes, and the rim is closed, but see below. There are reefs that appear on the flats at the southwest and small ribbons near the island to the northeast.

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The lagoon covers 17.5  km2. It is shallow and is being infilled by sand from the south (Pirazzoli et al., 1988b). Several reticulated basins appear as plateaus and reef flat ridges in the western half of the lagoon. In addition, there is a 36-ha shallow area in the eastern lagoon that is mostly sand. Reefs can be found in areas on the lagoon slope along the shallow areas at the margins. As described for Fangatau in the previous section, this lagoon is a major habitat for large numbers of giant clams, T. maxima, whose shells contribute to the carbonate production of the lagoon and are especially prominent as shell mounds (mapiko) on shallow ridges and in the shallow eastern and western ends. Dense populations of these bivalves, as many as 544 individuals per m2, were recorded from this lagoon, although they were later devasted by a mass mortality that reduced the population considerably (Gilbert et  al., 2005; Andréfouët et  al., 2013). The lagoon depth has neither been reported to our knowledge nor have the lagoon dynamics, but the center is greater than 10  m deep (Van Wynsberge et  al., 2017), and the openings through the southern rim that support the coral and giant clam communities suggest that the lagoon is semi-closed. However, the closed rim and its effect on water exchange during calm weather likely was a factor in the mass mortality as well (Van Wynsberge et al., 2017). Giant clams, T. maxima, are common in the Tuamotu group, especially within the closed atolls of Fangatau and Tatakoto described above and many others in the southeastern Tuamotu group described below. The bivalves are shown wedged into cracks and crevices of the reef, often in high densities one next to the other (Figure 3.24a). The ridged shells eventually become i ncorporated into the reef framework and contribute, in some cases significantly, to the process of reef building. They are particularly abundant in closed lagoon systems where they build shell mounds and what appear as dotted ridges in satellite imagery as in the shallow lagoon of Tenararo Atoll (Figure 3.24b). There are ten species in the genus Tridacna, and despite the name ‘giant clam’ T. maxima typically grows to about 20 cm, its larger cousin Tridacna gigas grows to a meter or more. Thus, T. maxima often is called the ‘small giant clam.’ Giant clams attach to the substrate by means of a tangle of high-strength proteinaceous threads located at the shell hinge that faces the rocky substratum. Juveniles secrete compounds near the byssus that enable them to bore into rock, becoming completely embedded in the process, but older individuals outgrow their borings. Their multicolored and greatly enlarged mantle tissue is oriented toward the sun where their single-celled photosynthetic

symbionts (zooxanthellae) produce dissolved organic matter, especially glucose and amino acids, as well as oxygen for their hosts. Because of their dependence on sunlight, these species are restricted to waters less than 10 m deep. In addition, giant clams can absorb dissolved nitrogen and particulate matter from seawater (Fitt, 1992). How T. maxima achieves high abundance in many of the closed atolls in the Tuamotu Archipelago is unclear. There are many structural, hydrodynamic, and nutritional variables within closed systems, as well as exploitation by fishers for food. Thus, their conservation is often a concern. For more information, see the review by Neo et al. (2015, 2017). Pukarua is an elongate, ovoid atoll with a flattened side facing the northeast (Figure 3.25). Its nearest neighbor is Tatakoto, 169 km to the northwest is similar in many respects. It  is about 16  km long and 4.4 km wide bisecting the flattened northeast-facing side. The platform is 56.9 km2 and is surrounded by an outer reef that is mostly 150–250 m wide and a spur and groove system. The reef flats along the northeast rim have a few coral patches, but they become numerous on the southwest rim. The rim is dominated by a single island covering 6.3 km2. It is 16.7 km long and generally 250–350 wide but expands as it wraps around the northwest and southeast margins, like the large island of Tatakoto. There is an airstrip near the center-north part of the island and a small village in the northwest. The southwest side is composed of motu and hoa, most of which are functional at high tide (Salvat, 1971), and there are reefs closely associated with those islets on the outer reef flats. There is also an open reef flat that is more than 500  m wide to the southeast, which includes a rubble bottom. There are no passes, and the atoll appears to be closed; nonetheless, like Tatakoto, Fangatau, and other Tridacna-rich environments, the lagoon may also be considered semi-closed based on its biological characteristics. The lagoon is 29.1 km2 and up to 35 m deep (Salvat, 1971) with reticulations that subdivide it into numerous basins. The reticulated ridges and pinnacles among them are rimmed by reefs and break the surface forming a few (20 m and five basins are formed by reticulated ridges (Andréfouët et al., 2020). The shallow lagoon is rimmed by a sand apron that is up to nearly 400 m wide on the north but is up to 1,000 m wide on the south-facing margins. The lagoon quickly descends into the deep lagoon basin which serves as important habitat for pearl oysters and pearl farming (Andréfouët et al., 2020). Several pinnacles are also found here. Îles Maria (Figure 4.4) is an atoll that square but with rounded edges that widens to the southwestern side. The platform is 3.9 km from the rounded northeast as it bisects the flattened southwest and is 4.1 km parallel to the base where it is widest. The platform area is about 14.3 km2 and it is surrounded by outer reefs that extend more than 500 m from the rim to the south. The western side reefs are least developed. Patchy reefs occur on the outer reef flats in several areas. The rim is composed of four islands, the largest of which is 81 ha in the northeast. A smaller island is located 91 m from the first and occupies the southeast corner. The area between these islands is occupied by a reef flat with a rock and rubble ridge in the middle. A third island is near the southwest corner 1.9 km from the one on the southeast. The distance between islands here is

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Figure 4.4 Remote-sensing images of the atolls of Scilly, Mopelia, and Îles Maria. Images © 2021, Planet Labs PBC.

covered by a rocky reef flat with some reef patches on the southwest. The fourth island is 620 m lagoonward of the third island and is connected to it by several shallow sand ridges that lie on an extension of the outer reef flat. The western rim is a submerged reef flat about 2.5 km long without islands or motu. The rim is closed. The lagoon area is very shallow, mostly sand and rubble, and occupies an area of about 3.0 km2.

References Andréfouët S, Genthon P, Pelletier B et al. 2020. The lagoon geomorphology of pearl farming atolls in the Central Pacific Ocean revisited using detailed bathymetry data. Mar. Poll. Bull. 160: 111580. https://doi.org/10.1016/j.marpolbul.2020.111580 Collin A, Archambault P, Planes S 2014. Revealing the regime of shallow coral reefs at patch scale by continuous spatial modeling. Front. Mar. Sci. 1: 65. Che LM, Andréfouët S, Bothorel V et al. 2001. Physical, chemical, and microbiological characteristics of microbial mats

Atolls of the Society Islands

(KOPARA) in the South Pacific atolls of French Polynesia. Can. J. Microbiol. 47: 994–1012. Guillou D, Maury RC, Blais S et al. 2005. Age progression along the Society hotspot chain (French Polynesia) based on new unspiked K-Ar ages. Bull. la Société Géologique Fr. 176: 135–150. https://doi.org/10.2113/176.2.135 Lecchini D, Bertucci F, Fogg L et al. 2021. Marine biodiversity of a pristine coral reef in French Polynesia. Island Stud. J. 16: 1–16. https://doi.org/10.24043/isj.150

Neall VE, Trewick SA 2008. The age and origin of the Pacific Islands. Phil. Trans. Roy. Soc. B 362: 3293–3308. Sachet MH 1983. Natural history of Mopelia Atoll, Society Islands. Atoll Res. Bull. 274: 1–37. Sachet MH, Fosberg FR 1983. An ecological reconnaissance of Tetiaroa Atoll, Society Islands. Atoll Res. Bull. 275: 1–67. Sailing Directions 2017. Pacific Islands, pub. 126, 11th ed. National Geospatial-Intelligence Agency, Springfield VA. Salvat B 1983. La faune benthique du lagon de l’atoll de Scilly, archipel de la Société. J. Soc. Oceanistes 77: 5–15.

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The Atolls of the Cook Islands There are 15 Cook Islands to the west of French Polynesia which have a total land area of about 240 km2 scattered over 2.25 million km2 of the South Pacific Ocean. They form a widely spread northern group where there are five atolls and a more compact southern group composed of volcanic islands, uplifted reef islands, completely submerged reefs, and an almost atoll, but with only two atolls. The Southern Cook Islands region has a complex and unsettled geological history. Initially thought to be the product of a single hot spot originating in the Austral Islands, more recent evidence suggests that the plate traveled northwest over two or possibly three hot spots at different times. Unfortunately, the hot spot tracks are short, and some islands show two distinct volcanic episodes separated by several million years, whereas other areas hundreds of kilometers apart underwent simultaneous volcanic eruptions (Rose and Koppers, 2019; Jackson et al., 2020). The Southern Cook Islands are younger than their northern counterparts and include several uplifted islands ranging in age from 6 to 27 million years. The Northern Cook Islands lie on the Manihiki Plateau (Figure 5.1), an area that has been tectonically stable for many millions of years. The plateau itself, about the size of Texas, formed 110 million years ago (Hoffman, 2009) and the atolls residing on it have been building carbonate for 650,000–460,000 years (Hein et al., 1997). They currently constitute one of the principal breeding sites for seabirds in the Central Pacific. Today, the Cook Islands are notable not only for their high islands and atolls but also for the wealth of minerals in the deep seas between these features. These waters can include high abundances of deep-sea ferromanganese nodules (>25 kg/m2), perhaps as much as 12 billion tons. These nodules represent precipitations of manganese oxide and ferric oxyhydroxide from ambient bottom waters. Many appear to nucleate on shark’s teeth or shell fragments and incorporate them

as they grow in place. These nodules, growing at a rate of millimeters per million years, are enriched in cobalt, titanium, tellurium, niobium, rare-earth elements, as well as yttrium, platinum, and zirconium (Hein et al., 2015), elements that are utilized in green technology, high-tech applications, and energy generation. With their abundance in the Cook Islands EEZ and prices of some of these elements on the order of US$10,000,000 per ton, their presence has captured the interest of mining companies and raised the concern of environmental groups.

Regional climate and oceanography Lying between 8°S and 23°S latitude, the Cook Islands are dominated by currents of the South Pacific Gyre primarily from the northeast and north (Figure 5.1), and although the trade winds from the east dominate (Figure 5.2), weather patterns are quite complex. Considering long-term averages in the Northern Cook Islands, winds blowing to the southwest meet and converge with northwesterly winds that are dominant in the Southern Cook Islands. The region where these two wind fields meet, the South Pacific Convergence Zone (SPCZ), markedly influences the climate of the Cook Islands and determines rainfall patterns. On average, from November to April, the SPCZ brings unsettled conditions and rain to the Cooks and forms the wet season. In contrast, from May to October, the SPCZ weakens and lies north of the Southern Cook Islands, bringing the southeast trade winds over that group, ushering in the dry season. The El Niño-Southern Oscillation (ENSO) shifts the location of the SPCZ (Rongo and Dyer, 2014), however, moving it northward with El Niño phases and southward during La Niña events; extreme El Niños can shift it as much as 10° toward the equator,

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Figure 5.1 Seven atolls are found within the Cook Islands, five of which are either closed or semi-closed. The South Equatorial Current is predominant and flows primarily from the north and northeast.

causing drastic changes. El Niño phases bring wetter than normal conditions in the Northern Cook Islands, whereas in the Southern Cooks, they commonly bring drier than normal conditions. For example, the marked El Niño years of 1982–1983, 1997/1998, 2010, and 2015 brought drought to Rarotonga, the capital, in the Southern Cooks. We discuss ENSO in more detail in Chapter 9. At Palmerston Atoll in the Southern Cooks, winds are dominantly moderate to brisk from the east to southeast, reflecting the southeasterly trade winds (Figure 5.2). The Cook Islands are struck by cyclones with a frequency of 1.8 per year since 1970, with a major recurrence interval of every 3.8 years in the Southern Cooks and every 8.8 years, less common, in the Northern Cooks (De Scully, 2008). Although wind-generated waves are the most common, powerful waves commonly come from distant storms to the southwest and northwest as well

(Figure 5.2). Waves commonly are less than 3 m, but larger waves may come from the southeast, southwest, and northwest. Tides in the Southern Cooks are semidiurnal, with spring (neap) tidal range of 76 (40) cm at Palmerston Atoll (Figure 5.2). The tides of the Northern Cooks are slightly different, with spring tidal range of 85 cm and neap range of 35 cm at Pukapuka.

The atolls of the northern Cook Islands Penrhyn (Figure 5.3), also called Tongareva, is the northernmost atoll of the Northern Cook Islands and is the only one that is not situated on the Manihiki Plateau. Penrhyn is polygonal atoll. Its longest axis between the northwest and southeast apices is about 25 km, and it is 13.2 km at its widest, northeast– southwest. The platform area is 235 km2. A narrow

Atolls of the Cook Islands

Figure 5.2 Winds in the Cook Islands most commonly are from the east. Although wind-generated waves are the most common, powerful waves come from distant storms to the southwest or northwest. Tides in the Southern Cooks are semidiurnal, with spring (neap) tidal range of 76 (40) cm at Palmerston Atoll, shown here. The tides of the Northern Cooks are slightly different, as discussed in the text.

band of outer reefs occurs around the rim from northeast–southeast in a clockwise direction and are poorly developed but areas to the west and south develop reef ranging from 400–700 m wide. A spur and groove system appears to be present around the rim although on part of the east, available imagery does not permit an adequate assessment. The platform includes apices with deep reef shelves on the northwest, northeast, and southeast. The first two open into the lagoon, whereas the apex on the southeast does not; an additional opening occurs on the west-facing margin. The NW & NE channels are open with a depth of more than 3 m (British Admiralty 997). The rim includes numerous motu and islands, the longest of which are two, each about seven km long on the southwest-facing rims. Moananui, the western of the two long islands, includes an airstrip and the main settlement. Just north of the village, the west-facing rim is a reef flat with a channel (West or Taruia Pass) 120 m wide, which has a reported depth of 4.3 m (Sailing Directions, 2017). The northwest- and northeast-facing

apices are both shallow channels about 600 m wide and 3 m deep. Between these two, the rim is generally less than 500 m wide and includes a series of islands and motu, separated by hoa. The rim east of the northeast apex-channel to the eastern apex is broadly similar, although a few hoa appear to be deeper and are associated with flood ‘deltas.’ The southeastern and southern rims are reef flats, about 10 km long and locally exceed 700 m width; there are several other smaller reef flat areas on the margins. This atoll is open. The lagoon area is 196 km2 and the maximum depth is 67 m (Purdy, 2001). The shallow lagoon and intermingled inner reef flats are sandy. Numerous reefrimmed pinnacles, rise from the lagoon slope and the deep lagoon in these areas, and many of them break the surface at low tide. There are fewer such pinnacles in the lagoon center. There was a large population of black-lipped pearl oysters here, although the population has been depleted by overfishing and they do not occur below 36 m (Sims, 1992). Shallow habitat (90% of the island’s energy needs (Tollefson, 2014).

Climate and regional oceanography Situated around 9° below the equator and 500 km north of Samoa and about 600 km northwest of Pukapuka in the Cook Islands, Tokelau is isolated, and without an airstrip or deep-water entry to the shelter of their lagoons, they are seldom visited. In addition, because the islands drop sharply into waters 4,000 m deep, there is no safe anchorage for vessels on the oceanside. Residents rely on twice monthly visits from Samoan cargo ships that transfer people, food, and other supplies to shore by barge ­(Fergusson, 2015). The islands are within the path of the slowly moving South Equatorial Current (Figure 7.1). The NE trade winds predominate for most of the year, and they tend to drive the westerly flowing equatorial

Figure 7.1 Tokelau, a territory of New Zealand, is composed of three atolls with closed rims located about 9° below the equator. The currents are primarily from the northeast as part of the South Pacific Gyre.

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current more to the west-southwest up to one knot for most of the year (Singh et al. 2019). The weather from November to March often is unsettled and is susceptible to high winds and rough seas due to westerly or northwesterly monsoon winds and accompanying swells. In July, the SE Trade Winds may predominate, and although calmer weather may take place, large significant wave heights from the southeast occur about 50% of the time (Figure 7.2). Tides in Tokelau are mixed semidiurnal with highs and lows of different amplitudes. The spring tide range is about 1.3 m and a neap tide range is about 0.6 m (Figure 7.2). The islands are within the path of the Pacific cyclone belt, and although direct hits by cyclonic storms are infrequent, waves from storms hundreds of kilometers away can produce devastating effects on these low-lying islands (McLean and d’Aubert, 1993). Tokelau is situated within a steep rainfall gradient between the wet South Pacific zone to the west and the very dry equatorial zone to the northeast where rainfall is often less than 1,000 mm per year.

Rainfall averages 2,660–2,770 mm annually but falls primarily in the last and first quarters of the year. In addition, the annual rainfall is subject to air pressure movements that can extend dry or wet months that change the average rainfall amounts considerably (McLean and d’Aubert, 1993). Ground water is present as a freshwater lens that is minimal, especially on Nukunonu, and most freshwater is derived by collection from metal roofing and subsequent storage in wells (Taloa et al., 1992). In addition to seasonal storms and cyclones, ENSO-driven droughts (see details in Chapter 9) are the two most common natural hazards affecting Tokelau at sub-decadal timescales. The northernmost of the Tokelau atolls is Atafu, a truncated triangle-like atoll measuring 8.3 km from northwest to southeast and 5.5 km from the southwestern-facing apex to the middle of the base (Figure 7.3). The platform area is 30.3 km2. The entire platform is surrounded by an outer reef that is generally about 300 m wide. A reef-covered promontory extends 900 m from the rim to the east and is

Figure 7.2 The NE trade winds predominate for most of the year. The weather from November to March often is unsettled and is susceptible to high winds and rough seas due to westerly or northwesterly winds and accompanying swells. Tides in Tokelau are mixed semidiurnal, as shown by this example from Nukunonu.

Atolls of Tokelau

Figure 7.3 Remote-sensing images of the atolls of Atafu, Nukunonu, and Fakaofo. Images © 2021, Planet Labs PBC.

surrounded by a spur and groove system. Patch reefs occur on the reef flat on the east and southwest. The eastern rim is dominated by a single island 7.7 km long interrupted by two high sand ridges. This island curves on the northern and southern ends. A 750-m-long reef flat interrupted by two motu occurs on the north. The northwest corner is occupied by a 51-ha boomerang-shaped island that is the site for the atoll settlement. The western rim is primarily a reef flat, whereas the southern rim is composed of motu

and hoa. We have no information concerning ocean water exchange through the rim and we tentatively classify this atoll as closed. The lagoon area is 14.3 km2 and is surrounded by a sand apron that is best developed along the east in the lee of the largest island where reefs also appear to be developed. The inner reef flat along the western lagoon edge is also associated with patch reefs. The shallow lagoon is sandy and occurs as a shallow shelf on the northwest as well as the southwest where patch

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reefs occur. The lagoon includes a reticulate pattern of sediment-covered ridges and pinnacles that are rimmed with reef material. The lagoon depths can reach 30 m (Perez, pers. comm.). Nukunonu Atoll is about 90 km southeast of Atafu and forms a rounded rectangle with a flat side facing north (Figure 7.3). The longest axis north– south is 14.2 km, and the longest east to west distance is 10 km. The platform area of 116 km 2 is outlined by a band of deep and shallow outer reef about 300 m wide that extends prominently onto the reef flats everywhere except on the north. A spur and groove system is visible on the eastern platform and on part of the north. The rim on the east is composed of islands, including a claw-like one with its own lagoon on the northeast, followed to the south by a series of small islands, motu, and hoa. The most continuous island is 6.7 km long and occupies most of the eastern rim. The rim on the north is a sandy reef flat, whereas the remainder of the rim is composed of sandy flats dotted with small islands. The entire population of Nukunonu lives on a pair of islets located on the southwestern rim that are joined by a bridge. The village location is located by a prominent reef- and sand-covered arm that enters the lagoon. A channel about 10 m wide has been cut through the reef flat to the island, on the ocean side. This is the Port of Nukunonu. The rim is closed. The lagoon area is 94.9 km2. The shallow portions are surrounded by a narrow band of sand. A shallow shelf on the northeast dips gently into deep water as a lagoon slope but is steeper elsewhere. The deep lagoon includes numerous isolated, subcircular - coral-covered pinnacles, several of which are capped with sediment. The maximum depth for this lagoon is roughly 50 m (Perez, pers. comm.). Fakaofo is a polygonal atoll taking the form of a warped triangle (Figure 7.3). It is about 57 km southeast of Nukunonu and is 13.4 km from the north through the southern-facing apex. The widest part is about 9 km parallel to the northwest-facing base. The platform area is 68.9 km2 and is rimmed by band of outer reef that generally extends about 350 m from the rim. The shallow outer reef is continuous and more prominent. A spur and groove system completely surrounds this atoll. The rim is dominated by a nearly continuous 6-km-long island facing northeast that curves at the ends as on Atafu. The northwest-facing rim is a sandy reef flat nearly 9 km long

that ends with a 39-ha island (Fakaofo Island) on the west where there is a small settlement. The southwestern rim is a reef flat with scattered small islands, one of which is Fale, a 5.2-ha islet densely packed with homes and other buildings. This is the main settlement on the atoll. An artificial channel mostly 10 m wide has been cut into the reef flat and leads to a wharf where supplies from small boats can be offloaded. The southwestern rim ends on the south with a 32-ha island that encloses a small lagoon. The reef flats extending more than a kilometer northwest and northeast of this island are coded as sandy and terrestrial. The remainder of the rim on the east is composed of small islands and motu on a reef flat. The rim is closed. Reefs are well developed on the eastern and southwestern reef crest, less so on the northwest. The lagoon is 48.2 km2 and includes a narrow and sandy shoal area on the periphery. The deeper lagoon forms numerous coral rimmed ridges and pinnacles, some of which are elongated or egg shaped, and are capped with sediment. A few ridges reach across the lagoon as partial reticulations, forming a few enclosed or partially enclosed basins. The maximum lagoon depth is 79 m (Passfield 1998).

References Fergusson H 2015.Shallow water lagoon and reef fishes observed at Nukunonu Atoll, Tokelau. Atoll Res. Bull. 607, 1–34. Koppers AAP, Staudigel H 2005. Asynchronous bends in the Pacific seamount trail: A case for extensional volcanism? Science 307: 904–907. Koppers AAP, Staudigel H, Phipps Morgan J et al. 2007. Nonlinear40Ar/39Ar age systematics along the Gilbert Ridge and Tokelau Seamount Trail and the timing of the Hawaii-Emperor Bend. Geochem. Geophys. Geosyst. https://doi. org/10.1029/2006GC001489 Mclean R, d’Aubert AM 1993. Implications of climate change and sea level rise for Tokelau. United Nations South Pacific Regional Environment Programme. SPREP Reports and Studies 61 South Pacific Environmental and Regional Programme, Apia Western Samoa. Oni R, Addison EJ 2009. Ethnoecology and Tokelauan fishing lore from Atafu Atoll, Tokelau. SPC Traditional Marine Resource Management and Knowledge Information Bulletin 26. Passfield K 1998. A report of a survey of the marine resources of Fakaofo Atoll, Tokelau. SPC, Noumea New Caledonia http:// coastfish.spc.int/Countries/Tokelau/Fakaofofishery.pdf Singh KA, Kutty SS, Khan MGM 2019. Wind energy resource assessment for Tokelau with accurate Weibull parameters. 9th Intl. Conf. Power and Energy Syst., pp. 1–6 http://repository. usp.ac.fj/12769/1/Wind_Energy_Resource_Assessment__ Tokelau.pdf Taloa F, Collins D, Humphreys S 1992. National Reports to United Nations Conference on Environment and Development, Brazil. https://www.sprep.org/att/irc/ecopies/countries/ tokelau/6.pdf Tollefson J 2014. Energy: Islands of light. Nature News 157: 154–156.

The Atolls of Tuvalu Tuvalu, formerly known as the Ellis Islands, is an independent island nation within the British Commonwealth. The original inhabitants of these islands were Polynesian, unlike those of Fiji to the south. There are nine islands within the boundaries of Tuvalu, eight of which are atolls including three atoll islands with remnant lagoons that will be described here. The Tuvalu Island chain is roughly linear, about 685 km long and lies within 5°S–10°S of the equator (Figure 8.1). They are part of the Austral-Gilbert-Marshall Island chain of volcanic peaks that date from about 64–47 million years ago (Jackson et al., 2015) and are capped with thick carbonate deposits.

The southernmost of the group is Niulakita, a small reef island which lies about 595 km north of the large Fijian island of Vanua Levu and will not be included in our descriptions. However, it is worth noting that this island was rich in phosphatic limestone and was mined for its fertilizer value to exhaustion (Rogers, 1989). It is also interesting that Niulakita is the emergent remains of a much larger, poorly known atoll rim, and that a second such atoll, completely submerged, lies just 23 km to its northeast. In addition, Niulakita includes the highest point in Tuvalu, which is 4.6 m above sea level. Despite its topographic dominance, Niulakita had no permanent residents, so that in the minds

Figure 8.1 The island nation of Tuvalu is composed primarily of eight atolls located between 5°S and 10°S of the equator. Rims are open or closed although there are also three atolls with remnant lagoons. The currents are in the path of the South Equatorial Current, the northern part of the South Equatorial Gyre.

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of many, the chain includes only eight atolls. This dynamic is the source of the name ‘Tuvalu,’ which means ‘group of eight,’ even though there are nine. All the atolls in Tuvalu include high gravel ridges on the windward side, low sand ridges on the leeward side, and a central depression (the lagoon or its remnants) between the two. Although these atolls in their current form were built on volcanic foundations, the islands are geologically young, having been formed during the last 3,000 years, and they are very small low islands, even by low island standards (McLean and Hoskins, 1991). The elevation of these atolls has implications for their susceptibility to sea-level rise (further details in Chapter 23).

Regional climate and oceanography Lying within 5°S–10°S of the equator, Tuvalu is situated outside of the dry equatorial belt. Mean annual rainfall ranges up to 350 cm/year to south but decreases to 270 cm/year to north (Thompson, 1987). As such, the three northernmost atolls of the group, Nanumanga, Niutao, and Nanumea, are more drought-prone than their southern counterparts. Two of these three are atolls with remnant lagoons as described below. Extreme droughts are most common in La Niña years such as 1999 and 2011, when the government declared a state of emergency (Paeniu et al., 2017). Tuvalu lies within the trade wind belt of the southeast Pacific so that winds mostly commonly have an easterly component, from the northeast and southeast (Figure 8.2). During the winter months (May–September), the dominant easterly to southeasterly winds are stronger, whereas the more gentle summer (November–March) winds have a more northerly component. Nonetheless, during the summer, westerlies can also occur (Barstow and Haug, 1994). Most months include waves with average heights between 1.6 and 1.9 m, but Tuvalu is affected by swell from the north during the summer and the south during the winter (Barstow and Haug, 1994). Wind waves are common, but the most energetic waves come from the southwest (Figure 8.2). Lower wave heights occur in the northwest of the group, because these atolls are sheltered from northerly swell by the Gilbert Islands of Kiribati. ENSO also impacts the wave climate, with more common south to south-westerlies during El Niño years. See Chapter 9 for further details on ENSO and El Niño. Outside of the main cyclone belt, Tuvalu has been directly struck by roughly 30 cyclones in the

last 80 years. Cyclones tend to be in their formative stages in the oceans here, so strong winds and high waves are uncommon. Tides of Tuvalu are semidiurnal, with pronounced spring-neap inequality. Spring tides of Funafuti range by roughly 1.9 m, whereas neap tides are less than 0.4 m (Figure 8.2). Nukulaelae Atoll (Figure 8.3) is located 156 km northeast of Niulakuta. It takes the form of an elongated oval with compressed sides. The platform has a northwest–southeast orientation and is 11.5 km long northwest–southeast, 5.5 km northeast–southwest at its widest parallel to the north end, and it is surrounded by outer reefs that range from about 200 m-400 m wide. The platform area is 43.4 km2, the windward eastern rim develops two long, vegetated islands, both about 120 m wide. The north island is 4.3 km long and its northern terminus expands to about 220 m, whereas the southern island is 5.8 km long and broadens at its southern terminus to more than 380 m. The two islands are separated in the middle by a 1.5-km-long reef flat that supports several motu. The rim south–northwest is occupied by a reef flat. There are motu and one small (1.7 km2) island to the south and several more to the northwest, but there are no islets of any kind to the west. There are no passes or channels, and the rim is closed. The inner reef flats are sandy except for an area of rubble to the northwest. The lagoon area is 23.1 km 2 and displays a sandy margin that overlaps extensively with the inner reef flat, shallow lagoon, and lagoon slope. The northwest and southeast basin ends develop the widest sand margins where there is a more extensive shallow lagoon than elsewhere. The aprons here reach widths of 900–1,000 m. Small reef areas occur in the shallow lagoon to the south. Several pinnacles are found in the deep lagoon to the north and south. Some are sand-covered, most are associated with reef development. The maximum depth of the lagoon is 30 m (Smith et al., 1990). Funafuti Atoll (Figure 8.3) is ovopolygonal, irregularly round, but with a narrow neck projecting to the south. It is located 112 km to the northwest of Nukulaelae and measures 26 km northeast–southwest and about 20 km west–east. The platform area is 257 km2 including a submerged reef area of about 1.5 km2 that projects to the west. The platform is surrounded by an outer reef that is least well developed to the northeast where it is 100-175 m wide. The western outer reefs are wider, commonly 400400 m, but in areas of the southwest and west they extend to more than 700 m wide. The shallow outer reefs extend to the reef crest and around the islands

Atolls of Tuvalu

Figure 8.2 Tuvalu lies within the trade wind belt, and winds mostly commonly have an easterly component, from the northeast and southeast. The dominant easterly to southeasterly winds are stronger during winter, whereas the more gentle summer winds have a more northerly component. Wave heights average between 1.6 and 1.9 m, augmented by swell from the north during the summer and the south during the winter. Tides are semidiurnal with pronounced spring-neap inequality, as at Funafuti shown here.

on the rim. The rim to the east is composed of an island 12.4 km long, most of which is about 100 m wide, but expands in the center to a 700-m-wide apex where the densely settled capital city of Funafuti and its international airport are found. The remainder of the east rim is composed of several long and narrow islands that are separated by two submerged rim sections to the southeast. The northern one constitutes a channel for small boats. The southern end is semi-enclosed by several similar elongated islands. The west rim is primarily a reef flat that is punctuated by two passes (ava in Polynesian). The one near the middle is 18 m deep but only 160 m wide; the second located to the northwest is 480 m wide but only 5.8 m deep (Figure 8.3). North of this ava is an island and areas of submerged rim to the northwest, followed by a reef flat with motu on the north. The rim is open, and its perimeter is marked by a spur and groove system. While Funafuti is by far the largest atoll in Tuvalu, the land area totals 2.8 km2, which is about 1% of the platform area.

On the northwest-facing margin, two passes, Te Ava Kumkum and Te Ava Tepuka, are closed (see zoom in Figure 8.3). These, like Te Ava Fuagea, represent marked protrusions from the rim into the lagoon, almost like tidal deltas. They are associated with deeper channels, flanked by reefs, although the passes appear to be blocked by reef growth. The inner reef flats are primarily rock to the east and primarily rubble with patches of sand to the west. The most conspicuous area of sand is on the shallow inner flats to the south. Reefs line the outer edge of the inner reef flat, and in some areas, overlap with the shallow lagoon. The lagoon area is 216 km2 and only to the south in an area of about 4 km2 is shallow lagoon and its slope conspicuous. Reef-covered pinnacles are most common to the western lagoon, especially in the vicinity of the passes. Larger, more isolated ovoid reefrimmed pinnacles are found to the east of the lagoon center. Most of the lagoon is, covered with sand, and

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Figure 8.3 Remote-sensing images of the atolls of Nukulaelae and Funafuti. Images © 2020, Planet Labs PBC.

is 30–50 m deep, reaching 52 m at its maximum depth (Purdy, 2001; Sauni, 2003). Funafuti was the location that Darwin’s subsidence theory was given its first test by the Royal Society of London in 1896, 14 years after Darwin’s death. It was chosen because it represented all the features considered ‘typical’ of atolls, including clearly defined windward and leeward margins, abundance of coralline red algae, a rich population and a wide diversity of coral species, and the evenness of the entire platform, among other features (Armstrong et al., 1904). Three bore holes attempted to reach a volcanic basement beneath the overlying limestone.

The deepest drill during the first attempt was to a depth of 32 m. A second expedition was launched in the following year and this bore went as deep as about 213 m but still only limestone was retrieved. The last and final attempt on Funafuti was made in 1898 and the drill bored to 340 m, but no volcanic material was encountered. However, these efforts did not go for naught. During these expeditions, the atoll surface was completely mapped, flora and fauna were collected, and the lagoon sediments, reefs, submarine topography were all characterized. Indeed, the assembled information made Funafuti the best-known atoll on Earth at the time (Rogers, 1991).

Atolls of Tuvalu

Figure 8.4 Remote-sensing image of the atoll of Nukufetau. Images © 2021, Planet Labs PBC.

Nukufetau Atoll (Figure 8.4) exhibits a squarelike morphology, canted toward the northeast. The platform is about 17 km north–south, 8.3 km at its widest, and occupies an area of 122.9 km2. The platform is outlined by outer reefs that are 250-300 m wide to the east and more variable to the west where similar widths occur, but some areas extend to 500 or 600 m. The shallow reefs are also situated on the reef crest and reef flats. The rim is characterized by at least one island in each corner. The eastern rim is a gravel ridge occupied by two narrow islands totaling 9.5 km in length but only about 170 m at the widest except the southern terminus of the southern island that expands to an area of 1.2 km2. The southwestern rim is a reef flat. The southwest corner area contains three motu and four islands, one of which supports a settlement. The western rim is a reef flat with a small area of submerged rim and two passes. The southern is Teafua Pass (also called Te Ava Lasi), 4.9 m deep and nearly a kilometer wide. The second, called Entrance Inlet, is north of the first and is separated from it by a 3-km-long reef flat. The name is a misnomer as it is Y-shaped, only about 150 m wide with a dead end bearing right or a tortuous turn to the left (Sailing Directions, 2017). Nonetheless, it too has an elongated, tidal delta shape, as the three passes on

Funafuti. The northeast rim is composed of a reef flat with a small island on the corner and five motu toward the northeast corner. A spur and groove system appears to surround the atoll, despite poor imagery for the southeast. The rim is open. The inner reef flat is occupied in many areas by reefs, especially to the east and in the vicinity of the passes to the west. The southwest and southeast corners are sandy, intertidal reef flats. The lagoon area is 92.7 km2, and like Funafuti to the south, the area occupied by shallow lagoon and slope appears to be minimal, although small slope areas are present to the southeast, the southwest and west of the lagoon margins where a few patch reefs are located. The lagoon margin forms an apron that appears to be 70–90 m wide and expands in three of the four corners, but most of those areas are inner reef flat with minimal overlap into the shallow lagoon. The apron does not appear to be present to the northeast. However, a bathymetric and seismic investigation of the lagoon shows continuous belt of carbonate sand that extends to the deeper lagoon to 15–20 m deep on the western side and to 35 m in the southeast (Radke, 1986). A bathymetric survey (Smith, 1992) shows that much of the lagoon shallows and slopes (5–30 m deep) are 500–1,000 m

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wide, with narrow values to the east and wide values to the south and north. The slope to the west is more gradual lagoonward of Entrance Inset, where about 4,000 m must be traveled before 30 m depths are encountered. Most of the lagoon floor is 35–40 m deep where it is covered with a muddy carbonate sediment (Radke, 1986). Low relief mounds and pinnacles are more common than larger ones and are found rising from the southern lagoon deeper than 40 m and in the far northern central deep area. Scattered reef plateaus and pinnacles are found near the middle of the basin, some of which reach near the surface; however, many of these larger pinnacles were destroyed by the U.S. Navy to make the lagoons navigable for warships during World War II (Radke, 1986). The same was done in Funafuti’s lagoon. The maximum depth is 55 m in an area located in the eastern central basin (Smith et al., 1990; Smith, 1992). Vaitupu is one of three atolls in Tuvalu that can be characterized as atoll islands with remnant lagoons that are in the process of being filled by sediment

and are largely cut off from typical oceanic exchange. Others have referred differently to this type of geomorphology. For example, Scott and Rotondo (1983) referred to Vaitupu as a part-raised atoll with an enclosed lagoon. Kruger (2008) called it a hybrid between a low-reef island and an atoll, and McLean and Hosking (1991) referred to it as a table reef with atoll-like characteristics because of its high ratio of island to platform area on one hand, with restricted connections to the ocean on the other. We defined the term ‘table reef’ differently from the latter authors, using it to describe an intertidal flat-topped reef without a lagoon, as referenced in Chapter 1 of this work. Vaitupu is a pear-shaped atoll (Figure 8.5) whose platform is oriented northwest–southeast where it is 6 km long and about 3 km at its widest. The platform area is 12.5 km2 and it is surrounded by outer reefs that range from about 100 m wide to the southeast to about 450 m wide to the northwest. A spur and groove system is found around the rim. The lagoon

Figure 8.5 Remote-sensing images of the atolls of Vaitupu, Nui, and Niutau. Images © 2021, Planet Labs PBC.

Atolls of Tuvalu

is near the island center and covers an area of 76 ha. Several shallow sand channels in the eastern (windward) gravel ridge mark the occasional exchange paths with the surrounding ocean, likely from swell and storm waves. A shallow lagoon and a lagoon slope are visible, and a deeper area of about 40 ha can be distinguished where reef growth is found. The maximum depth of this lagoon is 12 m (Purdy, 2001). The main settlement is found to the west of this lagoon where a 30-m-wide boat channel has been cut into the 210-m-wide reef flat to allow delivery of supplies to the inhabitants. There is no airstrip. A second T-shaped lagoon of roughly 20 ha is found near the northern end of the island and forms a single channel to the ocean toward the east. Xue (2005) reports a depth of less than a half meter at low tide. Various mangrove species are found around the peripheral areas of the lagoons and these account for 1.2% of the total land area (Woodroffe, 1987). Nui is an ovoid atoll (Figure 8.5) with an overall north–south orientation 168 km northwest of Vaitupu. Its long axis is 8.6 km northwest–southeast through the lagoon center and 3.2 km at its widest to the south. The platform area is 23.6 km2 and it is surrounded by outer reefs that extend to about 400 m to the northwest and to about 200 m wide elsewhere. The shallow reefs extend onto the reef crest where it is well developed clockwise, northwest–south (windward). The rim is composed of seven polygonal islands with a few motu that have formed on the eastern rim and includes at least eight hoa. The southernmost island, Te Kolo Kolo, is curved so that part of it lies to the east, but most of it forms to the south–southwest where it curls toward the lagoon. The main settlement is here, along with a 300-m-long, ca. 10-m-wide artificial channel has been cut through the reef flat. There is no airstrip. The remainder of the rim to the west is a rubble-dominated reef flat. Whereas the inner reef flats are wide and extensive, almost no reefs are mapped on them by the Allen Atlas. The lagoon area is 3.6 km2 including mangrove strands that develop in the sheltered southeastern corners. The northern half of the basin is shallow and there are five plateaus, none of which appear to be associated with patch reefs. However, there are numerous knolls and pinnacles especially in the southern half of the basin in addition to others in the lagoon. We are aware of no descriptions of the lagoon biota or depth. We tentatively mark the lagoon as closed. Niutau is an ovoid atoll island (Figure 8.5) 121 km north–northeast of Nui. The platform is 5.1 km northwest–southeast and 2.0 km at its widest north– south through the middle. The deep and shallow outer reefs are surprisingly well developed on the platform periphery, extending 200–350 m from the

rocky outer reef flats. The platform area is about 4.4 km2. Shallow reefs are found on the reef crest at multiple points around the island, but especially northeast–south; a spur and groove system occurs around the entire island. A settlement has been established to the northwest but there is no airstrip. Two lagoons occur inland but only one with an area of about 9 ha is clearly defined by the shoreline and is readily distinguished by a causeway built across the southeastern end. Extensive mangroves occur here, apparently dominated by a single species (Woodroffe, 1991). Unlike Vaitupu, the lagoon has no obvious connection to the surrounding ocean, however, there are tidal inflows through fissures in the reef platform that raise and lower the water level of the lagoon (Wester et al., 1992). Nonetheless, Krüger (2008) reports that the lagoon is brackish although he gives no indication of the salinity values. We are not aware of any reports of the lagoon biota or depths. Nanumanga is an almond-shaped atoll island 113 km southwest of Niutao (Figure 8.6). The platform is 4.2 km north–south and about 1.8 km east– west through the middle. The platform area is 6.5 km 2 and it is surrounded by outer reef growth that extends more than 400 m to the northeast of the island. A reef crest is well developed north–southwest which is covered extensively by coral/algae, and a spur and groove system occurs around the island. A settlement is found to the west and is without an airstrip. Five lagoon pockets are found on Namumanga, all of which are shallow and mostly occupy less than one hectare. However, the largest occurs to the north and covers an area of about 26 ha. Its salinity is close to 30‰ placing it on the border between brackish and saltwater (Sinclair et al., 2012). Westerer et al. (1992) characterize the lagoon(s) on Nanumanga as porous to oceanic exchange as is the case for Niutao. This lagoon is surrounded by mangrove growth that extends to the south and incorporates one of the smaller lagoons. Mangroves cover 9.2% of the total land area (Woodroffe, 1987). Indeed, all the lagoon areas here are surrounded by mangroves and appear in satellite imagery as figured by Woodroffe. Nanumea is a boomerang-shaped atoll (Figure 8.6) 140 km northwest of Nanumanga. The platform is 13 km long through the center and it is about 2.6 km at its widest through the middle. The platform area is 27.0 km2 and it is surrounded by outer reefs that extend more than 500 m to the northwest and 350 m to the southeast. The rim is defined primarily by three islands. The northwesternmost is Lakena, a long oval with an area of 1.2 km2. The opposite side of the atoll is occupied by Y-shaped Nanumea Island with an area of 14.5 km2. The south fork is occupied by

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Figure 8.6 Remote-sensing images of the atolls of Nanumanga and Nanumea. Images © 2021, Planet Labs PBC.

Nanumea Village that is adjacent to a coral-flanked artificial boat channel 450 m long, 18 m wide, and 2 m deep (Xue, 2005) that was cut through the reef flats. The rim is otherwise closed. The north fork is adjacent to a third small island. There are two additional motu near the middle of the atoll supported by rocky extensions of the inner and outer reef flats that in effect bisect the lagoon except for a shallow 20-m-wide passage. The lagoon to the northwest is 1.4 km2 and it is rimmed by reef growth at the margin of the rocky lagoon slope and in several areas by reef patches surrounding and within the deep lagoon. The lagoon to the southeast is about 2.1 km2 and it is separated from the adjacent lagoon by about 200 m. Much of it

is filled with reef growth except for an area of about 28 ha in the middle constituted by areas of the deeper lagoon. The southernmost 36 ha of the lagoon is a sandy slope that is rimmed by coral growth. The lagoon is at least 15 m deep (Sauni, 2003).

References Armstrong HE, Blanford WT, Bonney TG et al. 1904. The Atoll of Funafuti. Borings into a Coral Reef and the Results. Report of the Coral Reef Committee of the Royal Society. The Royal Society of London, Harrison and Sons, London. Barstow SF, Haug O 1994. Wave climate of Tuvalu. SOPAC Technical Report 203, 26 p. Jackson MG, Koga KT, Price A et al. 2015. Deeply dredged HIMU submarine glasses from the Tuvalu Islands, Polynesia: Implications for volatile budgets of recycled oceanic crust. Ge‑ ochem. Geophys. Geosyst. 16: 3210–3234.

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Krüger J 2008. High-Resolution Bathymetric Survey of Tuvalu. EU EDF 8 SOPAC Project Report 50. Pacific Islands Applied Geoscience Commission: Suva, Fiji, vi + 58 p. + 9 charts. McLean RF, Hosking PL 1991. Geomorphology of reef islands and atoll motu in Tuvalu. S. Pac. J. Nat. Sci. 11: 167–189. Paeniu L, Holland E, Miller C et al. 2017. Rainfall trends, drought frequency, and La Nina in Tuvalu: A small island state in the Pacific Ocean: J Environ Anal Toxicol. 7: 5. https://doi. org/10.4172/2161-0525.1000501 Purdy EG 2001. Supplement to Purdy EG, Winterer EL 2001. Origin of atoll lagoons. GSA Bull. 113: 837–854. Radke BM 1986. Bathymetric and seismic features of Nukufetau lagoon, Tuvalu: An appraisal of submarine phosphate potential. CCEP/SOPAC Technical Report 57 PE/PU. 9, 1–28. Rogers KA 1989. Phosphatic limestones from Tuvalu (Ellis Islands). Econ. Geol. 84: 2252–2266. Rogers KA 1991. A brief history of Tuvalu’s natural history. S. Pac. J. Nat. Sci. 11: 1–14. Sauni S 2003. The status of coral reefs of Tuvalu. Coral reefs in the Pacific, Status and Monitoring, Resources Management. https://horizon.documentation.ird.fr/exl-doc/pleins_textes/ divers14-11/010032226.pdf

Scott GAJ, Rotondo GM 1983. A model to explain the difference between Pacific Island atoll types. Coral Reefs 1: 139–150. Sinclair P, Atumurirava F, Samuela J 2012. Rapid drought assessment Tuvalu. SOPAC Technical Report PR38, Secretariat of the Pacific Community: Suva, Fiji. Smith R 1992. Bathymetric map of Tuvalu - Nukufetau lagoon. 1:12500. SOPAC Bathymetric Series Map 4. Smith RB, Rearic DM, Saphore E et al. 1990. Survey of Nukulaelae and Nukufetau lagoons. SOPAC Tech. Report 105, Lands Dept., Tuvalu. Thompson CS 1987. Rainfall at Tuvalu, Tokelau and the Northern Cook Islands and its relationship to the Southern Oscillation, New Zealand. J. Geol. Geophys. 30: 195–198. https://doi. org/10.1080/00288306.1987.10422184 Wester L, Juvik JO, Holthus P 1992. Vegetation history of Washington Island (Teraina), Northern Line Islands. Atoll Res. Bull. 358: 1–50. Woodroffe CD 1987. Pacific island mangroves: Distribution and environmental settings. Pac. Sci 41: 166–185. Woodroffe CD 1991. Vegetation of Tuvalu. So. Pac. J. Nat. Sci. 11: 82–128. Xue C 2005. Causes of land loss in Tuvalu, a small island nation in the Pacific. J. Ocean Univ. China 4: 115–123.

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The Atolls of Kiribati Kiribati is an oceanic nation in the central Pacific that includes the Line Islands and the Gilbert Islands, chains that extend north and south of the equator, and the Phoenix Islands, located just south of the equator longitudinally between the other two groups (Figure  9.1). Together, the three island groups cover an area of 3.5 million km2, an expanse greater than the continental U.S. However, the total land mass of Kiribati is 811 km2, a little less than the area of Tahiti, and only 22 of the 33 atolls and other islands are inhabited.

Regional climate and oceanography Because of their equatorial position, direct hits by cyclones are rare in most of Kiribati, although the southern and northernmost islands are subject to such storms. Kiribati typically experiences a wet season from November to April and a dry season from May to October. Rainfall is influenced by the movement of the Intertropical Convergence Zone (ITCZ), a region of clouds that seasonally migrates north and south of the

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equator. However, the rainy season on a given atoll is also dependent on the position of the island relative to the equator (Figure 9.2a). The Line Islands are a good example of this position effect. On Malden Island just south of the equator, rainfall exceeds 100 mm per month only in April and May. The annual rainfall is only 255 mm per year (10 inches) but a shallow lagoon persists. On nearby Starbuck Island, rainfall is about 77 cm (30 inches) per year and the lagoon is nearly dry (Toomey et al., 2016). The pattern is similar, although rainfall is slightly higher on Kiritimati just north of the equator, and in general, the farther from the equator in the Line Islands, the more rainfall. Teraina (Washington) Atoll, for example, has four times the rainfall that comes to Kiritimati even though it is only about 400 km away (Perry, 1980). A similar pattern occurs in the Gilbert Islands in western Kiribati where rainfall is lowest between 2°N and 3°S. Periodic droughts (see La Niña; Figure 9.2) often make the dry season even dryer than normal, and months can pass without precipitation. Kiribati’s climate is additionally affected by several factors that make the equatorial Pacific a very large,

Figure 9.1 The Republic of Kiribati straddles the equator in the Pacific and extends more than 4,200 km west–east. The country includes the Gilbert Islands to the northwest, the equatorial Phoenix Islands in the center, and the Line Islands to the east. The total number of atolls within the republic is 26. Atolls of Tuvalu described previously are also shown to the southwest. The predominant current system in the region is the west-flowing South Equatorial Current.

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Figure 9.2 Climate patterns of the equatorial Pacific. (a) The north–south movement of the Intertropical Convergence Zone (ITCZ) generally controls the formation of wet and dry seasons in Kiribati, although rainfall also varies considerably with island position from the equator. The figure shows the ITCZ position north of the equator during the northern hemisphere summer. (b) El Niño and (c) La Niña are opposite phases of the El Niño-Southern Oscillation (ENSO) cycle, with La Niña as the cold phase of ENSO and El Niño as the warm phase. These plots show representative monthly average sea-surface temperature relative to normal during (b) El Niño and (c) La Niña phases. Equatorial regions around Kiribati become warmer, rainier, and stormier than normal, with winds from the west during El Niño phases, relative to La Niña phases with normal trade winds. Figures courtesy of NASA Jet Propulsion Laboratory.

dynamic, and complex oceanic province. The islands lie within the path of the Pacific Equatorial Divergence, a parting of surface waters caused by trade winds and the Earth’s rotation, that drives upwelling of cool, nutrient-rich water from depths of 50–100 m to the surface near the equator. This dynamic in turn supports a rich phytoplankton community and a diverse tropical ecosystem. The predominant winds for most of the region are the southeast trade winds, which power the South Equatorial Current (Figure 9.1). The productivity derived from equatorial divergence and the rainfall patterns from the Intertropical

Convergence Zone are dependent on the westward flowing trade winds that force the warm equatorial waters toward the western Pacific. This warmer, less dense water literally piles up 40–50 cm higher in the western Pacific than in the east. The warmest and highest sea level area is north of the island of New Guinea, known as the Pacific Warm Pool. This area covers more than 30 million km2 and is permanently >28°C. It is the largest source of atmospheric water vapor, the region of the highest rainfall levels on Earth, and is often referred to as the heat engine of the globe ­(Niedermeyer et al., 2014; DeDeckker, 2016).

Atolls of Kiribati

El Niño-Southern Oscillation For reasons that are still unclear, the southeasterly trade winds that hold the warm pool in the west periodically diminish in intensity, allowing eastward, downhill migration of surface water along the equator. As this happens, the western Pacific becomes dry and fire prone, while the equatorial regions around Kiribati become warmer (Figure 9.2), rainier, and stormier than normal, sea levels increase by tens of centimeters, upwelling ceases, and there is a general decline in productivity. This oceanic-atmospheric state is known as El Niño. Not all El Niños are created equal. Major El Niño events are distinguished by the higher water temperatures that expand east and the length of time they remain until they retreat. Intensity is measured in Degree Heating Weeks (DHW), the number of weeks that temperatures remain above the maximum monthly mean, calculated over a 12-week period. Thus, if the temperature is 2°C above the normal summer maximum for 6 weeks, the corresponding measure would be 2°C × 6 weeks, DHW = 12. El Niños usually remain in place for 9–12 months, but some major events remain as ecological disruptions lasting 2 years or more. More recent El Niños, such as those in 1982–1983, 1997–1998, and 2015–2016, have each set records for DHW. Bleaching, the loss or diminution of coral photosynthetic symbionts, typically occurs during such exceptional heating events. El Niño conditions may simply abate with a return to normal, but often they are replaced by wind reversals, with stronger than normal trade winds that increase to the west. This process not only induces cold water upwelling but also decreases water temperatures below normal, and because of associated water vapor declines, it causes extreme droughts in Kiribati as well. El Niño and La Niña are opposite phases of the El Niño-Southern Oscillation (ENSO) cycle, with La Niña as the cold phase of ENSO and El Niño as the warm phase. Neutral conditions between the two are sometimes referred to as La Nada. Although ENSO shifts have impacts across the globe, as an equatorial Pacific nation, Kiribati lies at the center of the action. Its impacts therefore are especially pronounced on its atolls.

Equatorial siphoning and emergence Pleistocene time (2,580,000–11,700 years ago) is divided into glacial (cold) and interglacial (warm) periods, the last of which occurred about 125,000 years ago. The last glacial period began about 10,000 years later. During that cold interval, as large ice sheets slowly formed and expanded, seawater was gradually sequestered in the glaciers and global sea levels fell by

120–125 m. This fall caused carbonate platforms of modern atoll provinces to emerge from below the sea. Glacial melting and retreat began about 11,600 years ago, ushering in the modern Holocene Epoch, during which sea level began to rise. By about 9,000 years ago, reef growth restarted as Holocene sea levels rose above the remnants of interglacial paleoreefs. By about 5,000–1,500 years ago, sea levels had risen to their highest levels, 1–3 m above present, submerging the older reefs and allowing new reef growth to begin in shallow waters (­Grossman et al., 1998). However, the shrinking mass of ice near the poles and the resultant uplift there caused the deeper mantle to flow toward the poles. This flow away from the equator decreased the gravity in that region, and the bulge of seawater previously held there gravitationally begin draining away from tropical regions, causing an apparent fall in sea level. These processes are referred to as equatorial siphoning (Dickinson, 2004; ­Figure 9.3). Although this effect was regionally variable, mid-Pacific atolls have experienced a 1–2 m drop in sea level during the past 500–1,000 years, exposing to the atmosphere many reefs that had grown up to the higher sea levels (­ Mitrovica and Milne, 2002; Dickinson, 2009), thereby forming islands. In Kiribati, for example, the Line Islands display a number of these emergent islands presently represented by atoll islands with remnant lagoons (Keating, 1992).

Atolls of the Line Islands There are three groups of islands in Kiribati that we have incorporated into this chapter. The first is the Line Islands, the second is the Phoenix Islands, and the last is the Gilbert Islands. The Line Islands are a seamount chain rising from oceans more than 5,000 m deep that extends for 4,500 km across the Central Pacific Basin. The age of these seamounts ranges from 88 to 50 million years, but their origin remains controversial (e.g., Pockalny et al., 2019). Only 11 of these seamounts reach the surface to form atolls, islands with no lagoons, or atolls with remnant lagoons that stretch across 2,350 km (­Figure  9.4). Two of these, Kingman Reef and Palmyra Atoll to the north, are associated with the U.S. and will be considered in Chapter 10. The rest are within the Republic of Kiribati and include six that will be covered here. The International Date Line extends 2,500 km eastward to include the Line Islands in the same time zone as the rest of Kiribati, making them the world’s farthest forward time zone, the same time on the clock as Hawai’i, but a day ahead. As described above, the most common winds are associated with easterly trade winds. Although these generate wind waves propagating to the west, the

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Figure 9.3 After the last ice age, glacial melting began and caused sea levels to rise, reaching their highest levels of 1–3 m above present about 5,000–1,500 years ago. However, the shrinking mass of ice near the poles caused uplift there, followed by a flow of deep mantle toward the poles. This flow away from the equator caused an apparent fall in sea levels called equatorial siphoning. Although this effect was regionally variable, on mid-Pacific atolls, it caused an apparent 1–2 m fall in sea level during the past 500–1,000 years. Kiribati’s Line Islands display three of these emergent islands presently represented by atoll islands with remnant lagoons. (Modified with permission after Grossman et al., 1998 as cited.)

most powerful waves in the Line Islands come from either the southwest or northwest (Figure 9.5). Tides in the Line Islands are dominantly semidiurnal, with spring tidal range of roughly 60 cm during spring tides and 20 cm for neap tides (Figure 9.5). The southernmost of the Line Islands (and the farthest south anywhere in Kiribati) is Caroline, a boomerang-shaped atoll with a north– southwest orientation (Figure 9.6) located ten degrees south

of the equator. This atoll, which is only about 3 m above sea level and is isolated, is about 560 km to the north of Mataiva Atoll in the Tuamotu Archipelago and 785 km to the southeast of the nearest Line Islands. The southwestern side is blunt compared with the more rounded north-facing end. The platform is 13 km long taking a curved path, about 2.6 km at its widest, and has a total area of 25.4 km2. The outer reef is best represented north and along the east for

Atolls of Kiribati

Figure 9.4 Kiribati’s Line Islands are part of a seamount chain that extends 2,350 km across the central Pacific, extending from Teraina north of the equator to Caroline Atoll near 10°S. Only six reach the surface to form atolls, including three with remnant lagoons. The South Equatorial Current predominates in this region.

about 4.5 km where it is 180–190 m wide. The remainder is generally less than 100 m wide although a 300 m-wide reef is found to the southeast. A spur and groove system occurs around the entire rim as well as a continuous reef crest (Barrot et al., 2010). The rim is composed of about 39 forested islets and motu, the most significant of which are Nake Island to the north (1.2 km2), South Island (1.1 km2) to the opposite end, and Long Island in the middle of the eastern rim (93 ha, 4.1 km long, about 250 m wide). Long Island appears to have been dissected by storms compared with charts. The atoll rim is raised slightly above the low tide level, with water exchange into and out of the lagoon occurring over the reef flat and through shallow channels along the reef rim. Water

circulation within lagoons is primarily affected by surf, tidal fluctuations, and winds (references in Barrot et al., 2010). The lagoon is enclosed and is reticulated into at least 28 partly or completely enclosed basins. The lagoon area is 22.5 km2, and by necessity, this includes narrow (6–30 m wide) reef flat ridges that separate the basins from one another. The basins, which average 8.8–13.7 m deep, are each rimmed by high-diversity coral growth that extends to the lagoon slopes. Giant clams (Tridacna maxima) are also common in shallow reef areas. The basin interiors are dotted with patch reefs that are interconnected as ‘line reefs’ in the deeper center of the lagoon that include small knolls and pinnacles. The deeper lagoon basins to the

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Figure 9.5 The easterly trade winds are most common in the Line Islands, although the most powerful waves there come from either the southwest or northwest. Tides in the Line Islands are semidiurnal, with modest amplitudes as illustrated by data from Starbuck.

north, south, and south-central areas are up to 33 m deep and are covered with fine sand but still harbor large coral platforms (Barrot et al., 2010). We characterize this lagoon as semi-closed. Starbuck is an atoll island located 785 km northwest of Caroline Atoll, and it could not be more different. The island, named after a Nantucket whaling family, is essentially treeless and is dominated by herbs and grasses (Figure 9.6). Emerging as much as 5 m above sea level, the platform is shaped like a warped triangle with its longest axis 12 km long west–east including a 70-ha submerged area to the east. The platform is 4.8 km at its widest, north–south and the total area is 30.8 km2. The outer reefs occur around the island and are 400 m wide to the southwest. Elsewhere they are about 100 m wide or less except for an uncoded area of about 58 hectares that extends more than a km to the east. Shallow reefs also extend onto the outer reef flat to the southeast and southwest. The rim consists of coral blocks and ridges of fine sand. The center of the island is depressed and rather than a single lagoon, there are now several small hypersaline ponds, remnant lagoons covering an area of about

9.3 km2. The ones toward the eastern end communicate with the ocean by a tunnel, but many of them are dry or muddy and are ‘encrusted by thousands of tons of the purest kind of salt are found in various forms, coarse and fine.’ (Wiens, 1962; Keating, 1992). Guano deposits were once mined here in the mid- to late nineteenth century, but now Starbuck is a protected wildlife sanctuary, primarily for nesting seabirds, where access required a permit from the government of Kiribati (Lovell et al., 2002). Malden is another atoll island about 4°S of the equator and 203 km northeast of Starbuck, named after a British naval officer and surveyor who examined the island in 1825. The platform is roughly the shape of an equilateral triangle with two apices and a third northeast-facing apex (Figure 9.6). The long axis from apex to the mid-base is about 9 km. The platform is 9.9 km at its widest including a submerged reef to the southeast. The platform area is 41.8 km2. The outer reef is roughly 100–180 m wide except for reef platforms that extend to the northeast for a distance of about 650 m and another to the southeast that stretches to more than a km. Shallow

Atolls of Kiribati

Figure 9.6 Remote-sensing images of the atolls of Caroline, Starbuck, and Malden. Images © 2021, Planet Labs PBC.

reefs are more expansive but are essentially restricted to the outer reef flats and a spur and groove system that surrounds the island. The entire perimeter of the island is composed of a ridge composed of coral debris up to 10 m above sea level. A sandy beach occurs on the lee (west) side of the island. The interior of the island is vegetated primarily by grasses, shrubs, and large fields of sea purslane (Sesuvuim portulacas‑ trum) surrounding the lagoon (Pierce et al., 2015). Trees were likely more prominent in the past, but few remain due to human intervention. From 1860 to 1927, Malden’s high-phosphate fossil guano deposits were mined more extensively than other islands in the region for use in Britain as garden fertilizer. A railway was built around the island, as were houses for as many as 180 workers and supervisors. Pigs, goats, and poultry were imported to the island for food; feral cats and house mice also made their way to the island, and cats are still a problem for nesting seabirds (Watkins and Batoromaio, 2014; Pierce  et  al.,  2015). A graveyard for workers is also still visible on Malden. About 30 years after

the end of mining (1957–1962), the British returned to Malden to test atomic and hydrogen bombs. The northern and northwestern margins are still littered with rusting vehicles, fuel drums, and dump sites. There has been little systematic radiological monitoring of the test sites so that the extent and significance of ongoing contamination is unclear (Alexis-Martin et  al.,  2022). And with that as background, we note that Malden is a protected wildlife sanctuary, primarily for nesting seabirds, where access requires a permit from the government of Kiribati (Lovell et al., 2002). The lagoon is hypersaline with an area of about 12 km2, not including areas of periodically flooded mud flats, some of it colored brownish pink due to the growth or decay of filamentous algae. As on Starbuck, the lagoon maintains connections to the ocean through fissures in the reef platform (Wester et al., 1992). Although it is shallow, only about a meter deep (Purdy, 2001), its volume and area fluctuate covering between 29% and 42% of the land area depending on rain variability (Pierce et al., 2015).

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Kiritimati was visited on Christmas Day in 1777 by Captain James Cook, who named it Christmas Island. However, in Gilbertese, the language of Kiribati, the letter ‘s’ is not used in writing. The reason for this omission is that the first transliterator of the oral language, the missionary and linguist Hiram Bingham II, had a typewriter on which the ‘s’ key was broken. He replaced it with ‘ti’ (as in ‘nation’). In this way, Christmas Island became Kiritimati. Shaped polygonally but with a long claw open to the northwest, Kiritimati is the largest atoll in the world in terms of land area, 360 km2, and it is certainly the largest atoll in Kiribati (Trichet et al., 2001; Figure 9.7). The platform is about 46 km long from the southeast through the center of the ‘claw,’ and 31 km wide through North-west Point and South-east Point (Figure 9.7 inset). The total platform area is 697 km2 which is composed of a carbonate cap atop volcanic material that is 30 to more than 120 m thick. This mostly Pleistocene and Holocene growth was interrupted by sea level changes with little evidence of subsidence (Morrison and Woodroffe, 2009). The current platform occurs with two indented coastlines, Wreck Bay to the east and Vaskess Bay to the west. The platform is outlined by an outer reef that is about

200–300 m wide except to the northwest where it is about 800–1300 m wide for a distance of about nine km along the shore. The rim is generally elevated 2–3 m above sea level as a narrow expanse of coarse coralline debris up to 4 m high to the northeast and 5 m high near the Bay of Wrecks. The interior is low relief although sand dunes up to 11 m elevation are present (Keating, 1992; Woodroffe and McLean, 1998) Kiritimati is in the dry equatorial climate zone and receives an average annual rainfall of only 766 mm (30.2 inches) per year. The natural vegetation is a savanna with scattered shrubs, and an irregular scrub which in some locations forms a scrub forest. In addition, the soil is nutrient poor and lacks trace elements necessary for plant production (Morrison and Woodroffe, 2009). Nonetheless, there are enormous seabird populations, especially of sooty terns, perhaps the world’s largest, although La Niña droughts can have a lasting effect on both seabird populations and the vegetation (Muller-Dombois and Fosberg, 1998; Thaman and Tye, 2015). The total land area is about 359 km2, a figure that is dependent on a complex lagoon area, which in turn reflects cessation of reef growth due to periods of emergence followed

Figure 9.7 Remote-sensing image of Kiritimati Atoll. Image © 2021, Planet Labs PBC. Inset map courtesy of R. John Morrison, University of Wollongong.

Atolls of Kiribati

by development of storm ridges and the blockade of seawater entering the lagoon (Ryan et al., 2021). The land area also accounts for Kiritimati having the largest land mass of any atoll. A population of a few thousand people inhabit this island. The others covered to this point in this chapter are uninhabited. About 389 km2 of the island is composed of a highly complex lagoon system that can be divided into two parts. The largest, open to the northwest to tidal exchange, is 160 km2. It is sandy, shallow, about 4 m deep (Purdy, 2001) and develops complexes of broad tidal flats, peninsulas, linear patch reefs and motu (Valencia, 1977). The lagoon entrance is between London and Paris on either side of Cook Island (Figure  9.7 inset) where patch reefs with seagrasses ­ are found. In addition, both communities occur on the inland side of this lagoon. Much of the island is, however, made up of a reticulate network of exposed ancient reefs that more or less impound over 500 brackish to hypersaline lakes occupying an area of about 168 km2 (Thaman and Tye, 2015). These reefs represent the effect of lowered sea levels that affected different reefs at different times (depending on their elevation) during the last ca. 3,000 years. Thus, the present-day emergence of the fossil reefs in the interior of Kiritimati Island may have resulted from a lowering of water levels in the lagoon, as the reticulate lagoonal reefs were shut off from the open ocean (Ryan et al., 2021). This  atoll has thus become the world’s largest with respect to land area (Watson et al., 2016). Many of the smaller reef-locked lakes away from the large open one become hypersaline through evaporation, although during periods of heavy rainfall the salinity of some can become so reduced that they become freshwater or brackish. Some of these lakes that are at higher elevations on Kiritimati are independent of rainfall and remain fresh or brackish due to connections with subsurface freshwater. On the other hand, about a quarter of the 500 lakes are isolated and are only a few centimeters to a few meters deep so that when rainfall is low, they become evaporation basins where salinity levels can reach 300‰, about nine times normal seawater (Keating, 1992; Saenger et al., 2006). Larger isolated lagoons, Manulu, for example, is hypersaline (~55‰) even though it appears to be open to the main lagoon albeit by tortuous passageways. Isles Lagoon is more clearly isolated and develops a salinity of 125‰ (Figure 9.7 inset). Both are inhabited by milkfish, an edible oceanic fish that appears to have few osmotic difficulties migrating between pools with high salinities to feed on brine shrimp and brackish pools to feed on cyanobacterial mats (Creer, 1980).

Tabuaeran (Fanning) is an ovoid atoll with a depressed northeastern face (Figure 9.8), located 279 km northwest of Kiritimati. The platform is 17.6 km long northwest–southeast, about 12 km at its widest to the south with a total area of 165 km2, not including reefs peripheral to the platform edge. An outer reef surrounds the platform, and is best developed to the northwest where it is about 500 m wide including areas that are deeper than 15 m. The rim is composed of three main islands. Napari curves west–north and is 12.5 km long and 0.5–1.3 km wide. An airstrip and a small village have been established on this island. The island of Terine is about 12 km long and occupies the rim west–south where it ranges from 0.5 to 1.4 km wide. Its several villages account for most of the ca. 2,000 residents of this atoll. A pass between these two islands is found center west and is 5.8 m deep and 46 m wide (Sailing Directions, 2017). Reef growth occurs within this pass. Terine is followed by a sequence of small islands to the south and southeast terminate with a shallow channel that separates them from Tabuaeran Island on the eastern rim that is about 11.2 km long and 870 m at its widest. This island is partially vegetated but with large areas that are exposed as bare substrate. The northeast rim is a sandy reef flat about 750 m wide that is open to oceanic exchange and is followed to the south by a series of motu and small islands connected by a narrow ridge about 1.3 km long. The lagoon area is about 89 km2 and is reticulated, outlined by rubble-covered ridges that form multiple basins. The northern and southern lagoon waters are typically turbid and coral development is sparse but the waters of the central lagoon east and south of the western pass are clear and have both a high abundance and diversity of both massive and encrusting coral species (Maragos, 1974). These communities occur on the lagoon slope and extend into pockets of deep lagoon that are up to 16 m deep (Purdy, 2001). The shallower areas of the lagoon slope are also covered extensively with seagrass so that roughly a third of the lagoon area (~28 km2) is associated with coral, seagrass, or both. The current state of the coral communities around and within Tabuaeran and elsewhere in Kiribati has been affected by periods of warm-water intrusion due to El Niño as well as by fishing pressure due to relatively large human populations (e.g., Carilli et al., 2017). Teraina is an egg-shaped atoll island (Figure 9.8) that lies 136 km northeast of Tabuaeran. At 4.7°N of the equator, it is the northernmost island within Kiribati. The climate is influenced by the intertropical convergence, and although it is only 2° farther

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Figure 9.8 Remote-sensing images of the atolls of Tabueran and Teraina. Images © 2020-2021, Planet Labs PBC.

north than Kiritimati, it receives almost four times as much rain. The platform is about 7.5 km along the long axis, and it is about 3 km at its widest through the middle. The eastern end of the island is somewhat wider than the west. The platform area is 21.4 km2 including coral areas to the north, northeast, and southwest. The total land area is 14.2 km2. An outer reef surrounds the island and is generally about 300 m wide, but expands to more than 500 m wide to the northwest. A spur and groove system extends around most of the island and extends to more than 300 m wide to the windward east. The island is rimmed by beach sand that is generally about 60 m wide and reaches a height of nearly 5 m. The interior of the island is heavily forested, primarily but not exclusively composed of coconut trees planted in the early 20th century, that passes to a depression containing a potable freshwater lake surrounded by peat bogs. There is no seawater connection, and the lake level is maintained by rainfall

although there is a narrow outlet to the east that allows freshwater outflow. The lake is 8.3 km2 and about 10 m deep with two rush-dominated peat bogs to the west. East Bog is found on the western margin of the lake and is 3.2 km2 where the peat is up to 2.75 m thick. The other called West Bog is located 700 m to the west and is about 3.9 km2. The two are connected by a drainage channel. The peat is uniformly 70 cm thick and has developed a tree island in the middle, mostly composed of coconut. The uniform thicknesses suggest that this bog formed on the flat surface of an old reef flat. The peat dates from 860 to 1,150 years before present (data in this paragraph from Wester et al., 1992).

Atolls of the Phoenix Islands There are eight islands in the Phoenix Island chain (Figure 9.9), all of which originate from a cluster of seamounts rising from waters of 4,000 to more than 6,000 m depth northeast of the Tokelau Islands.

Atolls of Kiribati

They occur from 2.4°S to 4.7°S of the equator, and all of them are part of the Phoenix Islands Protected Area or PIPA, established in 2008 (World Heritage Datasheet, 2010; Rotjan et al., 2014). This region constitutes nearly 12% of Kiribati’s territory and is the largest marine protected area in the world, nearly the size of California. It also has been declared as a UNESCO World Heritage site. Nonetheless, as of late 2021, the Kiribati Cabinet decided to open the area to commercial fishing, in hopes of generating $200 million/year in tuna fishing licenses. Only three of the eight in the Phoenix chain are true atolls: Kanton, Orona, and Nikumaroro, and as described below, all of them develop restricted lagoon circulation with concomitant accumulation of carbonate sediment. The other five (Enderbury, Birnie, Rawaki, Manra, and McKean) are atoll islands with remnant lagoons that are now elevated 2–2.4 m above sea level with pond-like, shallow depressions. They often are reported as brackish, although perhaps due to droughts as described above, they also have been reported to be hypersaline or almost dry, and in the case of McKean, nearly filled with mud and guano. While hardly a tropical paradise, under British administration, the Gilbertese were considered too prolific for their islands’ carrying capacity, and thus, a scheme was hatched by the Commissioner of Native Lands to move a few thousand of them to the uninhabited Phoenix Islands (Maude, 1937). The result of this plan is evidenced by the current human population of these islands,

which is zero, except for a few dozen engaged in the management of PIPA on Kanton Atoll. Tides of the Phoenix Island group have a semidiurnal pattern with a spring tide range of approximately 1.3 m (Figure 9.10), larger than the Line Islands to the east. Winds are predominantly from the eastern quadrant and drive the South Equatorial Current westward (Figure 9.9). Waves of 1.5–3 m significant wave height from the southeast are most common, especially during the southern hemisphere winter, although swell from the north–northwest of significant wave height up to 3–6 m high occurs during the northern hemisphere winter (Figure 9.10). During El Niño phases, brisk westerly winds drive waves from the west and northwest as well. All the Phoenix Islands are dry, with great fluctuations in annual rainfall. However, the southern group (Manra, Orono, and Nikumararo) has higher levels of rain than the more northerly ones (Stoddart and Fosberg, 1994a). Kanton Atoll is located 2.8°S of the equator and it is the northernmost of the Phoenix Islands. The platform takes the form of a warped triangle (Figure 9.11). The long axis is about 17 km northwest–southeast, and it is about 7 km at its widest; the total area is 73 km2 (Purdy, 2001). The platform is surrounded by well-developed outer reefs that are about 340–440 m wide. The rim is almost completely circumscribed by a single, narrow island 34 km long and mostly 300 m wide or less. An airstrip is located on the northwest. A hoa separates this island from a second, smaller one in the northwest, and that is followed by a single

Figure 9.9 There are eight islands in the Phoenix Island chain of Kiribati, all of which originate from a cluster of seamounts rising from waters 4,000 m to more than 6,000 m deep about 3°S–5°S of the equator. Four atolls include remnant lagoons, and the remainder develop rims with varying degrees of closure. All are included within the Phoenix Islands Protected Area, a UNESCO World Heritage site.

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Figure 9.10 Winds are primarily from the east and drive the dominant South Equatorial Current to the west. Southeasterly waves of 1.5–3 m are most common, especially during the southern hemisphere winter, although swell from the north–northwest up to 3–6 m high occurs during the northern hemisphere winter. Tides of the Phoenix Islands develop a semidiurnal pattern as shown here for Kanton Atoll.

Figure 9.11 Remote-sensing images of Kanton Atoll. Image © 2021, Planet Labs PBC.

Atolls of Kiribati

deep channel on the western side (blasted to a depth of 10 m during World War II) that penetrates the rim (Obura, 2011); others on the west were filled during causeway construction that also occurred during World War II. A spur and groove system surrounds Kanton, except where the channel was constructed. The 48.3-km2 lagoon is shallow with a mean depth of 6 m and a maximum depth of 26 m (Obura, 2011). It was closed but now is semi-closed. Tidal flushing of the lagoon is most efficient near the channel and water traveling through it is near oceanic salinity. However, evaporation exceeds the renewal rate in most of the lagoon. As a result, the mean salinity is near 40‰ (compared with a normal seawater salinity of 35‰–36‰) and areas farthest from the channel have been reported as biologically diminished (Henderson et al., 1978). Nonetheless, the lagoon is

extensively reticulated with reef-covered ridges following the contours of the lagoon slope. Multiple polygonal pools that form within the ridge system have developed extensive reefs, apparently filling much of the lagoon. Seagrass is present in the shallow lagoon northwest to southwest. Orona is about 200 km southwest of Kanton (Figure 9.12). The platform is rectangular with an apex and small lagoon on the northeast side. The long axis is 10.3 km northeast–southwest, and it is 5.3 km at its widest. The platform area is 44 km 2 (Purdy, 2001) and it is surrounded by an outer reef about 360–380 m wide occurs southeast–west and narrows elsewhere except for a small area to the east that is about 450 m wide according to the Allen Atlas. A prominent easterly point in the platform extends the reefs to 540 m from the reef crest

Figure 9.12 Remote-sensing images of the atolls of Orona and Nikumaroro. Images © 2021, Planet Labs PBC.

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seaward. The shallow reefs extend into the reef crest everywhere except along the northwest, but the entire atoll is surrounded by a spur and groove system. The rim is composed of two relatively long islands. The eastern island is 8.4 km long, including the curve that extends this side of the platform, and up to 450 m wide. This island curves to the north and south rim. The island on the southwest is 8.5 km long and up to 620 m wide, and it curves to the north and south as does the eastern island. The remainder of the southern rim is composed of three smaller islands and four hoa. The northern rim is composed of multiple islands, with motu and hoa extending about 5.2 km between the major islands. Some of these hoa appear to be blocked by sediment, but others may be functional although they are quite narrow, mostly 8–10 m wide (Figure 9.12). A sand apron up to 850 m wide surrounds the 27.2 km2 lagoon, which includes a 35-ha embayment on the northeast that is dominated by fine sand. The maximum lagoon depth is 16 m and the bottom is composed of coral rubble and silty sand (Obura, 2011). The largest pinnacles that rise from the bottom are sand-covered remnants, although those in the shallow southwestern lagoon appear to be actively growing and are coded as reef by the Allen Atlas. Tridacna clams are common here compared with the other atolls in this group. However, ocean exchange with the lagoon is highly restricted (Stoddart and Fosberg, 1994a; Obura, 2011); the lagoon is closed. Nikumaroro is the third and last of the true atolls in the Phoenix group, located 252 km west of Orona. It forms a warped triangle with rounded edges that extends 6.7 km northwest–southeast and about 2.7 km at its widest (Figure  9.12). The platform is 14.3 km2 and the rim ranges from 0.5 to 1.3 km wide. An outer reef surrounds the platform although it is best developed northwest–southwest clockwise where it ranges from 170–420 m wide. The west and most of the southwest reefs are variable but generally narrower. The reef is quite narrow on the west where it drops from the intertidal to more than 40 m in some places within 50 m (Obura, 2011). A spur and groove system surrounds the atoll. There is a single island 15 km long that encloses the rim except for a sand channel about 100 m wide and less than 1 m deep. A single relict hoa is present on the southern margin, and there is a 12-m-wide artificial channel through the reef flat on the west. The lagoon is closed. The lagoon area is 4.9 km2 and its maximum depth is 4 m. The shallow 1.4 km2 section on the southeast is sandy, as is a 40-ha area on the northwest. Circulation is poor as evidenced by water that is often

opaque and milky (Obura, 2011). However, the Allen Atlas indicates patch reefs on the margin of these sandy areas. The deeper central lagoon constitutes about 42% of the lagoon area, almost all of which is mapped as seagrass. Nikumaroro is reputed to be the location where Amelia Earhart may have crashed during her 1937 attempted global circumnavigation. However, despite years of unsuccessful efforts to find her Lockheed Electra 10E twin engine aircraft, including a National Geographic expedition, no conclusive evidence has been found. Enderbury Island is about 70 km southeast of Kanton and was named after Samuel Enderby, the well-known owner of a London whaling company whose name became attached to the island despite the misspelling (Stanton, 1975). A ship called the Samuel Enderby also made its way into Herman Melville’s Moby Dick. Enderbury forms an elongated triangle with rounded apices. The long north–south axis is 5.3 km and it is 2 km at its widest across the south-facing base. The platform area is 8.7 km2 and it is surrounded by an outer reef that is generally 175– 225 m wide, although the southwest and southeast sections of the base are more extensive and the platform includes a 900-m-long submerged shelf to the northeast that is swept by currents and has a well-developed reef (Figure 9.13). A spur and groove system occurs on the west and north rim but is not clearly visible elsewhere due to surf conditions. The lagoon is 2.1 km2 and is reported as shallow, muddy, and brackish (Obura, 2011), but it is also subject to hypersalinity as evidenced by its shoreline that in places are encrusted by salt (Dana, 1890) and reduction in size due to drought conditions, likely associated with La Niña. Most of the observations on this lagoon suggest that the lagoon is a shallow evaporation basin and likely never exceeds a meter deep (Brown and Gulbrandsen, 1973; Stoddart and Fosberg, 1994b). The orange color may be due to pigments from halobacteria that are common components of such environments (e.g., DasSarma et al., 2020). The island is quite barren save patches of grass and a few mangrove trees, but it was one of several Phoenix Islands claimed under the U.S. Guano Act of 1856. Under this law, any American citizen could claim any uninhabited island to extract rocky phosphate-rich avian guano that was used for fertilizer at the time. There were interesting claims and counterclaims between U.S. Guano Company and the Phoenix Guano Company that resulted in one group escorting the other off the island. However, Phoenix Guano did indeed attempt to extract an estimated 100,000 tons of phosphate-rich rock between 1860 and 1871, even though it was of inferior quality

Atolls of Kiribati

compared with the Peruvian variety (Skaggs, 1994; Goldberg, 2018). Rawaki Island (Figure 9.13) is 75 km southeast of Starbuck and rises from the same seamount chain as Enderbury and Kanton. The emergent portion is ovotriangular and is 1.3 km from the northwest-facing apex and about a kilometer at its widest across the southeast-facing base. The platform area is approximately 5 km2 including a reef area that extends about 420 m seaward from the northwest rim and another

submerged promontory extending 325 m from the southeast rim. Column-like patch reefs surrounding the island are covered with a high density of corals, and at a depth of 15–20 m, a sand-covered bottom flows into large canyons with coral-covered spurs and sandy grooves that slope steeply into deep water (Obura et al., 2011). There is no Allen Atlas coverage of this 3.2 km2 island, which is rocky and treeless, but it is covered by herbs and grasses, and it makes excellent landing and nesting sites for seabirds as well

Figure 9.13 Remote-sensing images of the atolls of Enderbury, Rawaki, Birnie, Manra, and McKean. Images © 2020-2021, Planet Labs PBC.

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as nesting sites for turtles. Guano was mined there for several years. The island is open to heavy surf from the eastern waveward side (e.g., Figure 9.13). A shallow, remnant lagoon of about 74 ha is present and has been reported as brackish (Obura, 2011). Named after London whaling ship owner Alexander McBirnie, Birnie Island is the smallest of the Phoenix group. Located 90 km northeast of Rawaki, the platform of Birnie forms an elongated, rounded triangle that is about 2.6 km from north–south and 950 m at its widest (Figure 9.13). The platform area is about 2.1 km2 and includes well-developed shallow and deep outer reefs, especially at the southern and northern ends, that more than doubles the island size. The entire island is surrounded by a spur and groove system except perhaps on some parts of the east that are obscured by surf. The island is bean shaped and 3.0 km2 including a broad sandy beach on the northern and western sides. In relatively rainy years, the island vegetation (shrubs and grasses) turns a dark green (Google Earth imagery from 2005). There are no trees on the island. The remnant lagoon covers about 4 ha and has been reported to be brackish (Obura, 2011) although it has also been reported as hypersaline (reference in Wester et al., 1992). The lagoon is a deep green, suggesting cyanobacterial mats, and its measurement includes small areas that have become separated from the main lagoon. Manra Island (Figure 9.13) is about 100 km south of Bernie and Rawaki. The platform is ovotriangular and is 3.6 km from north–south, and about 4.6 km at its widest, east–west. The platform is 11.6 km2 and it is surrounded by an outer reef system that is best developed on the west, north, and east sides, where it ranges from about 200 m to 475 m wide, but narrows to about 100 m wide to the south–southwest (Allen Atlas 2023). As the reef becomes shallow it extends onto the reef flat on the west and south. In addition the island appears to be completely surrounded by a spur and groove system. The northern side is waveward and the southern side is exposed to southerly swells (Obura et al., 2011). The island is 9.1 km2 and is surrounded by a dense vegetation composed of shrubs and trees (Figure 9.13) that makes access to the 3.8-km2 lagoon difficult. The lagoon is brackish (Obura, 2011), but it also has been reported to be hypersaline (Wester et al., 1992). McKean Island is 126 km northeast of Nikumaroro and 234 km northwest of Orona. Its platform is about 4.8 km2 with the emergent portion accounting for 3.3 km2 (Figure 9.13). McKean was mined by the Phoenix Guano Company from 1859 to 1870 (Skaggs, 1994). The lagoon boundaries are poorly resolved in satellite imagery, but it is roughly 15 cm deep and partially filled with mud and guano. It is

reportedly recharged to some degree by tidal connections through the permeable rim (Dana, 1979). There is no Allen Atlas coverage of McKean. In addition to the land areas, there are two near-surface coral-rich platforms in the Phoenix group that have no islands. Winslow Reef is located approximately 230 km NW of McKean Island, and Carondelet Reef lies about 220 km SSE of McKean. These may represent submerged atolls, but the only description we are aware of is that the surface of Carondelet Reef is only 3–4 m below low tide. Although it should be resolvable using remote-sensing data, there is currently no supporting aerial or satellite imagery for either of these areas in the Planet archive, and therefore, they are not treated in this work.

The Atolls of the Gilbert Islands The Gilbert Islands form a chain approximately 780 km long (420 nautical miles) that is divided into north and south halves by the equator. The northern Gilberts are roughly bisected by 173° west longitude and extend to 3°N, whereas the southern islands extend to 2°S and curve to the southeast. Among these are 13 atolls, two with remnant lagoons and one with restricted lagoonal circulation that will be described here (Figure 9.14). In addition, there are three islands without lagoons that are not considered here. All the Gilberts are low-lying coral islands no more than 3.5 m above sea level (Richmond, 1993; Rankey, 2011) that rest upon volcanic foundations hundreds of meters below the surface. These seamounts rose 4,000 m from the ocean floor as active volcanoes 10–50 million years ago, and since that time, they have continued to slowly subside (Gillie, 1993; Sharma and Krüger, 2008). Despite being located far from any plate margins, faults, or other source of tectonic activity, the southern region of the Gilbert Islands was struck by a swarm of earthquakes between 1981 and 1984, the largest of which were magnitude 5.6–5.9 (Lay and Okal, 1983). This rare intraplate event is not only unusual and inexplicable but also is quite rare. There is additional evidence that a tsunami struck the northern end of the Gilbert chain in 1576 (Terry et al., 2021). The winds are dominated by the northeasterly trade winds, predominantly from the east (Figure  9.15). As an equatorial chain, the Gilbert Islands do not include marked seasonal changes in temperature, although squalls and rains are more common in the rainy (‘Aumeang,’ north/northeast winds) season from November to March than the dry (‘Aumaiaki,’ south/southeast winds) season. The available data and modeling (e.g., Barstow and Haug, 1994) suggest

Atolls of Kiribati

Figure 9.14 The Gilbert Islands of Kiribati form a chain approximately 780 km long that is divided into north and south halves by the equator and encompasses 13 atolls, ten of which develop open rims. Two are found with remnant lagoons and one is closed and develops a lagoon with altered salinity. The west-flowing South Equatorial Current is the dominant surface current.

that offshore significant wave height does not change markedly with the seasons, however. Nonetheless, traditional knowledge suggests that the spring tides during the ‘Aumeang’ season are higher, and this factor could impact wave energy that reaches the shoreline. Due to their equatorial positions, the Gilbert Islands rarely experience cyclonic storms, although a cyclone affected the northern islands in 1927–1928 (Sachet, 1957). More typically, 60%–80% of strong winds (greater than 60 km/hour) are between northwest and southwest, and these winds are also usually associated with squalls and showers. Likewise, wind waves are predominantly from the northeast or southeast depending on the season (Figure 9.15) and by extension, the eastern shores are the windward side of the Gilbert chain, where the islands and reefs are generally the best developed. While high seas from local winds are rare in these islands, swells from distant sources to the north or south may reach

the area and may cause beach erosion on the outer islands and lagoon beaches (Gillie, 1993). Tides near Aranuka (the geographic center of the Gilbert chain) are semidiurnal with marked variability; spring tidal range is ~2 m, whereas neaps are typically less than 60 cm (Figure 9.15). Rainfall varies among islands, depending on their location. More rain falls in the northern islands and decreases toward the south. Rainfall also varies markedly with ENSO shifts. For example, Onotoa, the chain’s southernmost atoll, receives an average of 1,250 mm (50”) of rain per year, but during El Niño conditions, that amount may increase by more than two-fold. During a La Niña year, Onotoa may receive only 150 mm (about 6”) of rain, with none during the first 6 months. Winds from the east may also become more forceful. The surface waters are dominated by the warm South Equatorial Current, although islands closer to the equator experience

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Figure 9.15 Winds are primarily from the east and wind waves are predominantly from the northeast or southeast, depending on the season. The eastern shores are the windward side of the Gilbert chain, where the islands and reefs are generally the best developed. Tides near Aranuka (the geographic center of the Gilbert chain) are semidiurnal with marked variability.

equatorial upwelling, the upward movement of deeper ocean water, described above. Figure 9.14 indicates the divergent movement of the current away from the equator. Once claimed by England, the Gilberts were taken over by the Japanese during the early part of World War II. Tarawa is perhaps most famous for the bloody battle fought on that atoll between U.S. Marines and 4,500 well-prepared and entrenched Japanese defenders in November 1943. The U.S. plan was to follow an intense naval bombardment of the Japanese base and airstrip on the island of Betio, the westernmost island on the southern apex of the triangular atoll, with an amphibious landing (the first of WWII). Marine Corps oceanographers expected the tides to rise in the early morning hours of November 20 to a level of 4 ft, sufficient for the 4-ft-draft Higgins boats to pass over the shallow intertidal flat. Oceanographers had failed to realize that the tides were still in a neap phase (cf. Figure 9.15), so that boats hit bottom and could not advance. Only amphibious, but non-armored, LVT ‘Alligators’ could advance closer than 500 m from shore, and those were quickly destroyed by Japanese shelling. Ultimately, sufficient Marine forces made it to the shore and fought the valiant Japanese defenders to the

death. The misprediction of tides and the resultant landing challenges thus played a pronounced role in this 76-hour battle. The number of Marine casualties was comparable to the campaign at Guadalcanal—but the latter lasted 6 months as opposed to 76 bloody hours on Tarawa—and included the highest casualty rate for Marines in WWII. Following a post-war period as a British protectorate, the Gilberts became part of the self-governing Republic of Kiribati in 1979. Today, Tarawa, now the capital, is the main population center of the islands with more than 63,000 residents (2020 census), the vast majority of whom live on the urbanized islets of South Tarawa (including Betio) on a total of 16 km2 of land. Although about 3,900 people per km2 is not among the highest atoll densities, this population is associated with construction of causeways and other construction projects that have exacerbated the effects of sea level rise, a topic that will be explored in Chapter 23. As a Micronesian microstate, Kiribati in general and the Gilbert Islands in particular suffer from limited size, isolation, and extremely limited native food sources. Among foods brought by aboriginal settlers were starchy roots (taro, giant taro swamp taro, yam, and arrowroot), two types of breadfruit, screw pine

Atolls of Kiribati

and coconut, although the latter two may have also been indigenous. Few plants are adapted to atoll soils, which are formed almost entirely from coral and coral-associated skeletal sand (predominantly calcium carbonate with some magnesium). Such ‘loamy sand soils’ are alkaline, coarse-textured, and are generally infertile (Woodroffe and Morrison, 2001). In addition, they do not hold water well above the water table. However, swamp taro is widely cultivated on atolls, including the Gilbert Islands, where pits are dug by hand down to the water table, which is only 1–2 m below the surface. The use of such plants is being promoted to divest from the consumption of high-energy, low-nutrient foods, including white flour, sugar, and polished rice (Thomas, 2003; Lyons et al., 2020). Most atolls of the Gilbert Island chain are elongated northwest–southeast, and include a well-developed reef rim, a rocky intertidal reef flat, and sandy islands on their eastern, windward margins. Coral reefs are well developed around most of these islands, especially on the outer reef slope. The western, leeward margins include less well-developed reefs and are commonly without islands. This side often remains submerged along with occasional small, islands, even at low tide. The lagoons are relatively shallow, ranging from 25 m deep, and several of them develop small mangrove forests which provide coastal protection and represent important fish nursery areas, as well as provide sources of wood and medicine (Rankey, 2011; Mangubhai et al., 2019). Makin is the northernmost atoll of the Gilbert group. It is an atoll island about 13 km long including an expanded area on the northeast (Makin Island) 2.7 km wide and 5.8 km long (Figure 9.16). The remainder of the platform south of Makin Island and joined to it includes three smaller islands on an arm 7 km long and about a kilometer wide. The entire platform area is 18.5 km2 and it is surrounded by a narrow reef system that is best developed on the north–northeast where it is about 300 m wide. Elsewhere it is generally 150–200 m wide. The shallow outer reefs additionally extend onto the outer reef flats along the eastern rim. In addition, a spur and groove system occurs all around this atoll. The reef flat on the northwest side of Makin Island is rocky and up to 550 m wide but is without reef development except for the spur and groove system. A 47-ha remnant lagoon occurs in the center of Makin Island, which is connected to the ocean by tides through a mudflat, despite the construction of a causeway that has reduced the volume of seawater exchange (Beretiteni, 2008; Figure 9.16). The 69-ha area northeast of the lagoon is semi-dry and is dominated by

grasses and sedges, with mangroves on the margins (Woodroffe and Morrison, 2001). Butaritari is about 6 km southwest of Makin and was called Big Makin at one time. The polygonal platform area is 446 km2 and is surrounded by an outer reef on the north and west that becomes patchier on the south and southeast (Figure 9.16). The northern and eastern rims are reef flats that together total 37 km long and include occasional motu and hoa. An 11-km-long island, mostly 250–350 m wide, occupies the southeastern rim and is connected by a causeway to the main island on the southern rim, which is roughly 25 km long and is variable in width but is about 350–450 m wide on either side of the airstrip. This island is distinguishable by two points that mark its southeast and southern rim margins. A spur and groove system is visible only on these two points. The leeward, western side lacks a continuous reef rim, and it has a broad opening with two prominent levee-flanked channels, all of which allow free flow of open-ocean water into the lagoon. A third elongated triangular island occurs to the northwest, where the rim is invaginated and extends as an extensive submerged shelf of shallow and deep outer reefs. Because Butaritari is the wettest and most westerly true atoll in the Gilbert group, the islands are associated with some of the most extensive areas of mangroves and freshwater swamps in Kiribati, estimated to cover about 166 ha (Thaman, 2016; Mangubhai et al., 2019). The lagoon area is 305 km 2 and it is 38 m at its deepest (Purdy, 2001). A sand apron is developed on all sides except the western rim. The shallow NW portion is populated with patch reefs. Numerous sand- and reef-covered pinnacles occur in the deep lagoon as well as at the end of and near the channel openings on the western margin. The southernmost channel is replete with reef growth and is mapped as part of the lagoon slope, as is the entrance to the more northerly one. Marakei is a triangular atoll with a north-facing apex that is 10.8 km from its south-facing base; it is 6.8 km long at its widest, parallel to the base (Figure 9.16). The reef platform covers 48.3 km2 and it is surrounded by outer reefs that are best developed on the northwest and least so on the south. The shallow outer reef extends into the reef crest along the eastern and southern rims. The rim consists of a single island that is 2.3 km wide at the north where there is an airstrip and narrows to 220 m on the south. Two hoa occur on the east and west rims, but they are not functional. The rim is closed, and Marakai is the only such atoll in the Gilbert group (Richmond, 1993). The lagoon covers 18.9 km2. The shallow areas are rubble and sand, and patch reefs occur along the edges,

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especially on the north. Most of the lagoon is shallow and sandy (according to the Allen Atlas) or muddy (Woodroffe, 2008), and at least some of the lagoon shallows range from freshwater to brackish. Scattered patch reefs occur in the shallow lagoon. A ridge system (partial reticulation) encloses two deeper sections of the lagoon where more normal-marine water occurs (Uriam, 2011). Numerous patch reefs occur in one of these deeper sections in the southeast lagoon closest to the shallow channel on the eastern side. A few pinnacles and a small ridge also occur here, but they are covered with sand. The lagoon at Marakei is therefore ‘mixohaline,’ that is, partly or occasionally freshwater or brackish, and marine.

Abaiang is a rectangle-like atoll about 31 km southwest of Marakei that is oriented northwest– southeast. Its long axis is 33 km, and the atoll is 13 km at its widest (Figure 9.16). The reef platform covers 385 km 2 and is surrounded by a deep and shallow outer reef, except on the northeast where outer reef development is poor for about 6 km. Reefs are also found on the outer reef flats on the northeast and on parts of the western rim. The rim is dominated by a 37-km-long J-shaped island on the east and south. The northern end of the rim is composed of small islands, with motu and hoa over a distance of 28 km, 10 km of which is a reef flat on the northwest. The western rim is primarily composed of reef flats with numerous

Figure 9.16 Remote-sensing images of the atolls of Makin, Butaritari, Marakei, and Abaiang. Images © 2021, Planet Labs PBC.

Atolls of Kiribati

channels. Bingham Channel in the middle of the western rim is about 480 m wide and 2.7 m deep (Sailing Directions, 2017). The others are narrower, but all these openings in the rim are penetrated by reefs. The lagoon area is approximately 278 km2 and has maximum depth of 27 m (Purdy, 2001). A broad sand apron occurs around the periphery and reaches up to three km wide on the western side. Patch reefs occur in the northern and in the shallow southern ends of the lagoon, whereas sand- and coral-covered pinnacles develop from deeper water. The pinnacle reefs correspond to the openings in the western rim and a ridge system surrounding a deeper section of the southern lagoon. Tarawa, home of the capital of Kiribati, is a triangular atoll resting on a platform of 421 km 2 (Purdy, 2001) (Figure 9.17). Much of the eastern rim is composed of narrow islands. The first large island is 17.5 km long and it is connected by a bridge to a second island that is 5.25 km long. The remainder of the eastern rim is composed of islands and motu mostly 350–500 m wide extending about 13 km to

a 6-km2 island on the southeast where the international airport is located. The southern rim extends 28 km west from this island including a connection by a causeway to the last island on the southwest. This is Betio (Figure 9.17), a 1.5 km2 strip of land that is home to more than 17,000 people. It is the most densely populated island in the Gilbert group, and it has a population density higher than Hong Kong. Most of the western rim is submerged by as much as 9 m (Paulay and Kerr, 2001) although shoals and multiple shallow channels occur along its length, and it emerges as an intertidal reef flat on the northwest. A ship channel about 300 m wide and 7 m deep (a pass) lies about 5 km north of Betio Island (Sailing Directions, 2017). An outer reef is found around most of the rim, but it is most extensive on the open western side. An outer reef is also well developed on the reef flat to the northwest and on the southwestern rim west of Betio Island. The lagoon covers about 349 km2 and is mostly at depths between 5 and 20 m, with a maximum depth of 25 m (Paulay and Kerr, 2001; Purdy, 2001). A sand apron is best developed on the northern and

Figure 9.17 Remote-sensing images of the Tarawa Atoll and the populous islet Betio. Images © 2021, Planet Labs PBC.

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especially on the eastern sides, where it reaches up to 3 km wide. Large patch reefs occur along the western lagoon and extend on a sandy lagoon slope toward the southeast where both patch reefs and pinnacles are found. The construction of causeways referred to above followed the growth of Tarawa’s human population. These have closed all the hoa on South Tarawa. The result is a lagoon with locally high turbidity, poor water quality, and a general decrease in coral reef development (Paulay and Kerr, 2001). Despite these impacts, Paulay (2000) reported seagrass beds occurring along the southern lagoon that support a commercial fishery of cockles and conchs, among other invertebrates. Mangrove habitat on Tarawa constitutes 57 ha (Mangubhai et al. 2019) or about 1.8% of 3200 ha total land area by Atlas measurement. High productivity appears to have been due to equatorial upwelling, perhaps abetted by nutrients

from human waste. Those seagrass communities are not mapped in the Coral Atlas, although they are present in the northern and part of the southwestern lagoon, suggesting a possible change since 2000. Maiana Atoll is 32 km south of Tarawa that is rectangle-like, similar to Abaiang, but with a 19-km-long axis facing northeast–southwest instead of northwest–southeast (Figure 9.18). The platform is about 11 km wide through the center and covers an area of 220 km 2. A shallow and deep outer reef slope surrounds the entire platform, and on the southwest, a deeper rocky shelf dips to the west/southwest. A single 31-km-long island, up to 1.2 km wide, forms on the rim andextends northwest–southeast. The last ~5 km on the northwest and southeast of the island are composed of narrow, recurved islets that are actively extending and protect a sandy, shallow area of about 21 ha where

Figure 9.18 Remote-sensing images of the atolls of Maiana and Abemama. Images © 2021, Planet Labs PBC.

Atolls of Kiribati

mangrove growth occurs (Rankey, 2011; Mangubhai et  al., 2019). The southwestern and western portions of the rim are without islands and form an intertidal to submerged, westward-dipping reef flat comprising 25% of the rim. The lagoon covers roughly 94 km2 and is primarily shallow and sandy with a maximum depth of 16 m (Purdy, 2001). The lagoon slope reefs are most strongly developed along the open western rim. Two deeper areas of the lagoon, comprising about 19 km2, flank the west and are partly divided by a ridge system. Interestingly, there is no notable coral development mapped by Allen inside the lagoon despite its open structure, although the visible images show scattered pinnacles and ridges across lagoon areas. Abemama is a trianguloid atoll, located 99 km southeast of Maiana. The platform’s long axis is about 25 km northwest–southeast and it is 13.5 km at its widest (Figure 9.18). The platform covers 292 km2 (Purdy, 2001) and it is flanked by a shallow and deep outer reef that extends around most of the atoll and even into the Western Passage (see below). This outer reef system appears to be somewhat less well developed along the eastern rim. Most of this rim is composed of two islands that are joined by a 100-m-long bridge and extend 28 km northwest–east. An airstrip is located on the northernmost island. The south– southeast rim is composed of three additional islands that extend more than 7 km. The southwest rim is 19 km long and consists of a shallow subtidal to intertidal reef flat, with a channel called South Passage that is 480 m wide at the entrance but becomes narrow and tortuous inside (Sailing Directions, 2017). A small, isolated island occurs on the westernmost protuberance of the atoll, and another occurs just east of the southern pass. The remaining rim on the northwest is a reef flat with a second channel, the 800-m-wide Western Passage that is at least 3.4 m deep in the center (Sailing Directions, 2017). The lagoon area is 164 km2 and reaches a depth of at least 24 m (Rankey, unpub. data). A sand apron circumscribes the shallow lagoon except for the South Passage and extends into the deep lagoon on the northern margin. The deeper center of the lagoon and the lagoon slope develop numerous patch reefs and pinnacles, particularly in the vicinity of the western channel. Aranuka Atoll forms a rounded triangle with a platform area of 92.7 km2 and represents the middle of the Gilbert chain (Figure 9.19), just 13 km north of the equator. Appropriately, Aranuka means ‘our middle’ in Gilbertese. The long axis measures 16 km from the west to the middle of the main island, and the platform is 11 km wide parallel to the eastern island. The outer reef is most strongly developed

along the southwest of the platform where it is about 600 m wide, and is least developed to the north. The northwest extends about 7 km2 as a submerged shelf that is partially outlined by deep and shallow outer reefs. Most of the center is likely too deep to code, but patch reefs are present (Rankey, pers. obs.). Spur and groove systems occur on the ocean side around most of this atoll. An island 11 km long and mostly about 700 m wide forms on the eastern, windward rim. The island expands to 1.3 km wide at both ends; an airstrip has been constructed near the northern end. Just to the north lies an east-west oriented, 2 km-long islet that tapers to a shallow hook. A 6-km2 island, Takaeang, occupies the western rim and is located about 4 km from the western platform edge and forms the leeward apex of the triangular rim. A sand spit along its northeast edge encloses a small, 14 ha mangrove area (Mangubhai et al., 2019). The entirety of the islands and shallow-water area covers 42 km2 and includes a large intertidal sand apron over 2,000 m wide toward the north and about 1,500 m to the southwest (Wasserman and Rankey, 2014). The main part of the lagoon is 17.2 km2. Large patches of seagrass occur in the shallows on the east, south, and southwest. Reefs expand along the southern lagoon flank from shallow to deep water, and they also occur as patch reefs in the shallow periphery and the lagoonal slope. Coral- and sandcovered pinnacles and short ridges fill large portions of the deeper lagoon and its slope, and patch reefs or knolls fill much of the space between them. The lagoon reaches up to 18 m deep and is connected to the open ocean by a single, artificially widened reef channel on the southwest (Purdy, 2001; Wasserman and Rankey, 2014). Nonouti is a large atoll 98 km southeast of Aranuka with a rectangle-like shape that expands into an apex on its southeastern border (Figure 9.19). The platform is 43 km from the apical end to the northwest, and it is about 18 km through the middle. The total platform area is 668 km2 (Purdy, 2001) which is surrounded by both shallow and deep outer reefs that are particularly well developed clockwise south– north, but that include an embayed area on the west. Submerged reef growth near the western platform margin outlines several small lagoon-like pockets between the deeper section of the western edge of the platform and the reefs nearshore. However, the northwest and northern ends of the platform each include ~9 km2 of reefs that may be too deep to be coded. The main island is about 34 km long and is aligned on the eastern, windward side with other smaller islets that recurve on their northern and southern ends, similar to those on Maiana Atoll. Hoa are more numerous

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Figure 9.19 Remote-sensing images of the atolls of Aranuka and Nonouti. Images © 2020-2021, Planet Labs PBC.

and broader to the north and west. The west, southwest, and south-facing rims include no islands; spurs and grooves form on the south- and west-facing rim margins, but do not appear well developed elsewhere. The southwestern rim is open and rocky area near the middle where it is about 5–6 km wide and more than 30 m deep, forming a marked embayment that opens the lagoon to fully marine conditions. The lagoon area is about 397 km 2 and is predominantly sand with a maximum depth of more than 35 m (Rankey, unpub. data). Numerous ridges, some up to 5 km long, occupy the shallow regions and the lagoon slope areas of the center as well as those to the south, east, and northwest. Despite the lagoon’s open condition, reefs are not mapped except for a confined area of ~20 km 2 on the lagoon slope to the southeast. However, many of the ridges are capped by reefs (Rankey unpub. data).

Tabiteuea is on the same seamount, aligned with Nonouti, but compared with the other trianguloid atolls in the Gilbert chain, it is more elongate (Figure 9.20). The platform is 39 km from Nonouti and is about 68 km long by 10 km wide, with a total area of 648 km2 (Purdy, 2001). The entire platform is surrounded by a deep and shallow outer reef, especially on the northwest where the outer reef appears to enter the lagoon. Spurs and grooves have developed along most of the eastern rim, but imagery is insufficient to assess the rest of the rim. The eastern rim is composed of an island 23 km long that varies from 2 km to less than 100 m wide. An airstrip has been constructed about 3.5 km from the southern end of this island, followed by a second island 4.9 km long that is connected by a causeway. The remainder of the eastern rim is a composed of a chain of motu and hoa about 44 km long. The southern flank of the rim is composed of intertidal reef

Atolls of Kiribati

Figure 9.20 Remote-sensing images of the atolls of Tabiteuea, Beru, Nikunau, and Onotoa. Images © 2021, Planet Labs PBC.

flats (Sailing Directions, 2017) with a shallow channel about 100 m wide in the middle. The western rim is composed of several scattered, isolated reef flats and is submerged, especially toward the southwest. The lagoon is primarily sand, but there are areas in the lee of the north island with large patches of seagrass. The channel on the south rim leads directly to a lagoon slope area of about 18 km2 that is dotted with pinnacles and is partially enclosed by a ridge system where reef development is considerable. Much of the lagoon is less than 10 m, but fishermen report that it exceeds 20 m in the protected region near the southeastern extent of the atoll (Uriam, pers. comm.). Beru Atoll (Figure 9.20) is about 100 km east of Tabiteuea. The platform area is 73.3 km 2 , 17 km long through the islands and reefs northwest–southeast,

and about 5 km at its widest. The area includes an outer reef that extends more than 600 m to the southwest and about 200 m wide to the southeast. In addition, a submerged 3 km 2 shelf projects from the southwestern rim and a second submerged area of about 19 km 2 extends the platform to the northwest. The rim is largely composed of a single island extending 14 km northwest–south which then recurves about 6 km forming a fishhook to the southwest. A narrow extension of the lagoon is found within the recurved area where mangroves of varying stature as well as seagrass communities ate found. Both areas form hypersaline ponds where cyanobacterial mats thrive. A similar area occurs as a depression to the southeastern end of the island. Small areas at both ends of the island are fenced for milkfish production

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(Tebano 2008). In times of drought these mats serve as an additional source of human food. Indeed another name for Beru Atoll is ‘Kang bokaboka,’ literally translated as ‘eats mud.’ The west–southwest rim is about 7.7 km long, which is formed by a reef flat that develops a broad sand apron toward the lagoon. The lagoon covers 9.8 km2 and it is deepest in the center, where it reaches up to 24 m (Rankey, unpub. data), much of which is covered with sand and rubble. A small number of patch reefs occur within the lagoon but almost all of them are associated with the inner reef flat. The spindle-shaped platform of Nikunau Atoll (Figure 9.20) is 17.6 km2 along the northwest–southeast axis. It is 3.4 km at its widest and about 900 m through the narrow neck in the center. The platform is about 31.5 km2 and includes adjacent satellite reef areas adjacent to the northwest. The north side of the island contains four shallow, brackish ponds (sometimes reported as hypersaline) that remain as lagoon remnants and cover an area of roughly 46 ha. Onotoa (Figure 9.20) is the southernmost atoll of the Gilbert chain. It is located 57 km southeast of Tabiteuea and about 70 km southwest of Beru and takes a wave-like form from the undulations of its eastern rim. The platform is 20 km long northwest– southeast, 7.4 km wide through the channel on the west, and covers an area of 134 km2. A shallow and deep outer reef surrounds the platform, but they are best developed on the west where they penetrate the westernmost channel. Spurs and grooves are visible along most of the east. The eastern rim is 25.5 km long and is composed primarily of two islands separated by a 1.4-kmlong causeway that traverses and closes a reef flat due to its supporting islets. The northernmost island is recurved and forms two connected ponds that surround a peninsula that supports most of a kilometer long airstrip. The southernmost part of the rim is formed by one island 2.6 km 2 , which also contains several pond-like enclosures and is separated from the east rim by a 300-m-wide reef flat. The west-facing margin is 16 km long and is separated from the South Island by multiple shoal areas and discontinuous reef rims separated by shallow channels several hundred meters wide. The most conspicuous of these is a channel in the westernmost elbow that is more than 1,200 m wide. The lagoon covers 68.4 km 2 and includes a deeper sub-basin that is up to 19 m deep (Uriam, pers. com.). It is reported to be studded with numerous coral heads (Sailing Directions 2017), many of which are visible on the Planet images.

References Alexis-Martin B, Bolton MB, Hawkins D et al. 2022. Addressing the humanitarian and environmental consequences of atmospheric nuclear weapons tests: A case study of UK and US test programs at Kiritimati (Christmas) and Malden Islands, Republic of Kiribati. Global Policy 12: 106–121. Barrot K, Caselle JE, Dinsdale EA et al. 2010. The lagoon at Caroline/Millenium Atoll, Republic of Kiribati: Natural history of a nearly pristine ecosystem. PLoS One 5(6): e10950. https:// doi.org/10.1371/journal.pone.0010950 Barstow SF, Haug O 1994. The wave climate of the Southwest Pacific: SOPAC (Pacific Islands Applied Geoscience Commission) Technical Rept. 206. Beretiteni T 2008. Makin. Republic of Kiribati Island Report Series. http://www.climate.gov.ki/wp-content/uploads/2013/01/ ​ ​ 1_MAKIN-revised-2012.pdf ​ Brown DW, Gulbrandsen RA 1973. Chemical composition of a saline lake on Enderbury Island, Phoenix Island group, Pacific Ocean. J. Res. US Geol. Surv. 1: 105–112. Carilli JE, Hartmann AC, Heron SF et al. 2017. Porites coral responses to an oceanographic and human impact. Limnol. Oceanogr. 62: 2850–2863. Creer D 1980. Observations on the reproductive state of milkfish populations (Chanos chanos) from hypersaline ponds on Christmas Island (Pacific Ocean). Proc. World Maricul‑ ture Soc. 11: 548–556. https://fdocuments.in/reader/full/ observations-on-the-reproductive-state-of-milkfish-populations-chanos Dana JD 1890. Corals and Coral Islands, 3rd ed. Dodd, Mead and Co., New York. Dana TF 1979. Species numbers relationships in an assemblage of reef-building corals: McKean Island, Phoenix Islands. Atoll Res. Bull. 228: 1–27. DasSarma S, DasSarma P, Laye V et al. 2020. Extremophilic models for astrobiology: Haloarchaeal survival strategies and pigments for remote sensing. Extremophiles 24: 31–41. DeDeckker P 2016. The Indo-Pacific warm pool: Critical to world oceanography and world climate. Geosci. Lett. 3: 20. 10.1186/s40562-016-0054-3 Dickinson WR 2004. Impacts of eustasy and hydro-eustasy on the evolution and landforms of Pacific atolls. Paleogeogr. Paeo‑ climatol. Paleoecol. 213: 251–269. Dickinson WR 2009. Pacific atoll living: How long already and until when. GSA Today 19: 4–10. https://doi.org/10.1130/ GSATG35A.1 Gillie RD 1993. Coastal erosion problems in the Gilbert Islands group Republic of Kiribati. SOPAC Technical Report 167, SOPAC Secretariat, 116 p. Goldberg WM 2018. The Geography, Nature and History of the Tropical Pacific and its Islands. 6: Pacific Islands and the Politics of Fertilizer. Springer Nature World Regional Geography Series, Cham. Grossman EE, Fletcher CH III, Richmond BM 1998. The Holocene sea-level highstand in the equatorial Pacific: Analysis of the insular paleosea-level database. Coral Reefs 17: 309–327. Henderson RS, Jokiel PL., Smith SV, Grovhoug JG 1978. Canton atoll lagoon physiography and general oceanographic observations. Atoll Res. Bull. 221: 5–14. Keating B 1992. Insular geology of the Line Islands. In: Keating BH, Bolton BR (eds.) Geology and Offshore Mineral Resources of the Central Pacific Basin. Earth Sciences Series. Springer-Verlag, New York, vol. 14, pp. 77–99. Lay T, Okal E. 1983. The Gilbert Islands (Kiribati) earthquakes swarm of 1981–1983. Phys. Earth Planet. Inter. 33: 284–403. Lovell E, Kitata T, Tekinaiti T 2002. Status report for Kiribati’s coral Reefs. https://horizon.documentation.ird.fr/exl-doc/ pleins_textes/divers14-11/010032214.pdf Lyons G, Dean G, Tongaiaba R et al. 2020. Macro- and micronutrients from traditional food plants could improve nutrition and reduce non-communicable diseases of islanders on atolls in the South Pacific. Plants 9: 942. https://doi.org/10.3390/ plants9080942 Mangubhai S, Lovell E, Abeta R et al. 2019. Kiribati: Atolls and marine ecosystems In: Sheppard C (ed.) The Indian Ocean to the Pacific Vol 2. World Seas, An Environmental

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Evaluation. Elsevier, Cambridge MA, pp. 807–826. https:// doi.org/10.1016/B978-0-08-100853-9.00054-3 Maragos JE 1974. Reef corals of Fanning Island. Pac. Sci. 28: 247–255. Maude ME 1937. Colonization of the Phoenix Islands by the surplus population of the Gilbert Islands, a Report to the Islands commissioner. Mitrovica JX, Milne GA 2002. On the origin of Late Holocene sea-level highstands within equatorial ocean basins. Quat. Sci. Revs. 21: 2179–2190. Morrison RJ, Woodroffe CD 2009. The soils of Kiritimati (Christmas) Island and comparison with previous studies. Pac. Sci. 63: 397–411. Muller-Dombois D, Fosberg FR 1998. Vegetation of the Tropical Pacific Islands. Springer, New York. Niedermeyer EM, Sessions AL Feakins SJ, Mohtadi M 2014. Hydroclimate of the western Indo-Pacific Warm Pool during the past 24,000 years. Proc Natl. Acad. Sci USA 111: 9402–9406. Obura D 2011. Coral reef structure and zonation of the Phoenix Islands. Atoll Res. Bull. 589: 63–82. Obura DO, Stone G, Mangubhai S et al. 2011. Baseline marine biological surveys of the Phoenix Islands, July 2000. Atoll Res Bull. 589: 1–61. https://doi.org/10.5479/si.00775630.589.1 Paulay G 2000. Benthic ecology and biota of Tarawa Atoll lagoon: Influence of equatorial upwelling, circulation, and human harvest. Atoll Res. Bull. 487: 1–41. Paulay G, Kerr A 2001. Patterns of coral reef development on Tarawa Atoll (Kiribati). Bull. Mar. Sci. 69: 1191–1207. Perry R 1980. Wildlife conservation in the Line Islands, Republic of Kiribati (formerly Gilbert Islands). Env. Cons. 7: 311–318. Pockalny R, Barth G, Eakins B et al. 2019. Multiple melt source origin of the Line Islands (Pacific Ocean). Geology 49: 1358–1362. Purdy EG 2001. Supplement to Purdy EG & Winterer EL 2001. Origin of atoll lagoons. GSA Bull. 113: 837–854. Rankey EC 2011. Nature and stability of atoll island shorelines: Gilbert Island chain, Kiribati, equatorial Pacific. Sedimentol‑ ogy 58: 1831–1859. Richmond BM 1993. Reconnaissance geology of the Gilbert Group, western Kiribati. SOPAC (Pacific Islands Applied Geoscience Commission) Tech. Report 77, 65 p. Rotjan R, Jamieson R, Carr B et al. 2014. Establishment, management, and maintenance of the Phoenix Islands Protected Area. Adv. Mar. Biol. 69: 289–324. Ryan EJ, Morgan KM, Kench PS et al. 2021. Fossil reefs reveal temporally distinct late Holocene lagoonal reef shutdown episodes at Kiritimati Island, Central Pacific. Geophys. Res. Lett. 48(8): e2020GL092113. https://agupubs.onlinelibrary.wiley. com/doi/10.1029/2020GL092113 Sachet MH 1957. Climate and meteorology of the Gilbert Islands. Atoll Res. Bull 60: 1–4. Saenger C, Miller M, Smittenberg RH, Sachs JP 2006. A physico-chemical survey of inland lakes and saline ponds: Christmas Island (Kiritimati) and Washington (Teraina) islands, Republic of Kiribati. Aquat. Biosyst. 2: 8. https://doi. org/10.1186/1746-1448-2-8 Sailing Directions 2017. Pacific Islands, pub. 126, 11th ed. National Geospatial-Intelligence Agency, Springfield VA. Sharma A, Krüger J 2008. High–Resolution Bathymetric Survey in Kiribati. EU EDF SOPAC Project Report 114. Pacific Islands Applied Geoscience Commission: Suva, Fiji, 56 p.

Skaggs JM 1994. The Great Guano Rush. St. Martin’s Griffin, New York. Stanton, W 1975. The Great United States Exploring Expedition. University of California Press, Berkeley. Stoddart DR, Fosberg FR 1994a. The hoa of Hull Atoll and the problem of hoa. Atoll Res. Bull. 394: 1–26. Stoddart DR, Fosberg FR 1994b. Flora of the Phoenix Islands, Central Pacific. Atoll Res. Bull. 393: 1–60. Tebano T 2008. A preliminary identification of cyanophyta/ cyanobacteria in the brackish milkfish ponds of Marakei, Nikunau and Kiritimati atolls, Republic of Kiribati. So. Pac. j. Nat. Sci. 26: 62–67. https://web.archive.org/web/20111106060656/ http://www.climate.gov.ki/library.html. Terry JP, Karoro R, Gienko GA et al. 2021. Giant paleotsunami in Kiribati: Converging evidence from geology and oral history. Island Arc. 30(1): e12417. https://doi.org/10.1111/iar.12417 Thaman RR 2016. Atolls of the tropical Pacific Ocean: Wetlands under threat. In: Finlayson CM (ed.) The Wetland Book. Springer Science + Business Media, Dordrecht. Thaman RR, Tye A 2015. Flora of Kiritimati (Christmas) Atoll, northern Line Islands, Republic of Kiribati. Atoll Res Bull 608: 1–73. Thomas FR 2003. Kiribati:” Some aspects of human ecology” forty years later. Atoll Res. Bull. 501: D1–D40. Toomey MR, Ashton AD, Raymo ME, Perron JT 2016. Late Cenozoic sea level rise of modern rimmed atolls. Paleogeogr. Paleoclimatol. Paleoecol. 451: 73–83. Trichet J, Défarge C, Tribble J. 2001. Christmas Island lagoonal lakes, models for the deposition of carbonate–evaporite–organic laminated sediments. Sediment. Geol. 140: 177–189. Uriam K 2011. Island Reports. Issues in relation to climate change. 254 p. https://repository.usp.ac.fj/7182/1/Island_reports.pdf Valencia MJ 1977. Christmas Island (Pacific Ocean): Reconnaissance geologic observations. Atoll Res. Bull. 197: 1–14. Wasserman HN, Rankey EC 2014. Physical oceanographic influences on sedimentology of reef sand aprons: Holocene of Aranuka Atoll (Kiribati), equatorial Pacific. J. Sed. Res. 84: 586–604. Watkins D, Batoromaio K 2014. Directory of wetlands of Kiribati. https://kiribati-data.sprep.org/system/files/Directory_ Wetlands_Kiribati_2014.pdf Watson MS, Claar DC, Baum JK 2016. Subsistence in isolation: Fishing dependence and perceptions of change on Kiritimati, the world’s largest atoll. Ocean Coast. Mgmt. 123: 1–8. Wester L, Juvik JO, Holthus P 1992. Vegetation history of Washington Island (Teraina), northern Line Islands. Atoll Res. Bull 358: 1–50. Wiens HJ 1962. Atoll Environment and Ecology. Yale University Press, New Haven, CT. Woodroffe CD 2008. Reef-island topography and the vulnerability of atolls to sea level rise. Global Planet. Change 62: 77–96. Woodroffe CD, McLean RF 1998. Pleistocene morphology and Holocene emergence of Christmas (Kiritimati) Island, Pacific Ocean. Coral Reefs 17: 235–248. Woodroffe CD, Morrison RJ 2001. Reef-island accretion and soil development on Makin, Kiribati, central Pacific. Catena 44: 245–261. World Heritage Datasheet 2010. http://world-heritage-data she ets. unep -wcmc.org/ datasheet/output/site/phoenix-islands-protected-area/​

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The Atolls of Hawai’i and the Pacific Remote Islands of the United States The United States casts a wide shadow in the Pacific (Figure 10.1) partly because of World War II and partly due to other conflicts, but several islands were claimed by the U.S. as part of the Guano Islands Act of 1856 or simply by the authority of the captain. The southernmost of the atolls in this group is Palmyra. It did not belong to the U.S., but in 1939, a suit was filed to take it over and it became the site of a naval base in preparation for possible hostilities with Japan; after the war, it was abandoned. In 2000, Palmyra was purchased by The Nature Conservancy and the U.S. Department of the Interior bought the adjoining reefs. Palmyra now houses a small research facility and has become a National Wildlife Refuge administered by the U.S. Fish and Wildlife Service (Maragos et al., 2008).

The atolls of the U.S. Line Islands Palmyra Atoll and Kingman Reef, its neighbor, are part of the U.S. Line Islands, and both are in the path of the westward flowing Equatorial Countercurrent (Hamann et al. 2004; Figure 10.1). The more southerly members of the group are part of Kiribati and were covered in Chapter 9. Palmyra (Figure 10.2) is about 5.9 km north of the equator, about 1,600 km southwest of Honolulu and 223 km from Teraina. It forms a warped rectangle about 19 km west–east including submerged and uncoded reef areas. The platform is 4.2 km north–south at its widest and covers an area of 53.9 km 2 . The coded reefs are about 6 km wide to the west of the rim, and up to 5 km wide to the east. The southern and northern reef margins are up to 350-400 m wide, but are narrower in much of the south. A well-developed reef crest occurs to the north and is accompanied by a spur and groove system to the north and east. The rim consists of

multiple islets and 1.1-km 2 Cooper Island which bears an airstrip. All terrestrial features are inboard of a reef flat about 900 m wide to the northeast. The pre-war atoll featured three large lagoons (western, eastern, and central), all of which were open to incoming tides, especially to the west. In 1940, however, a 60-m-wide ship channel was dredged and blasted. The spoils were used to create new islands while enlarging others. The central and western lagoons were then joined to create a turning basin and a causeway isolated the eastern lagoon. Previously, relatively cool ocean waters northeast– west could enter each lagoon successively and sink to the bottom, supplying the basins with oxygen. However, the closure of the natural channels to the lagoon impeded lagoon circulation and the dredged channel allowed strong tidal currents to stir up fine silt within them. These changes have extirpated corals and other reef organisms, and while there has been some recovery in the shallows, the lagoons below 25 m are oxygen deficient. See Goldberg (2018) for further details and references. The depth of the western lagoon ranges from about 20 to 40 m; the maximum depth is 53 m to the northeast. The center lagoon’s depth maximum is 51 m, and that of the western lagoon is 46 m (NOAA chart 83157). Kingman Reef (Figure 10.2) was claimed under the Guano Islands Act in 1860 by The American Guano Company. The Act was a response to the need for mineralized bird excrement as an agricultural fertilizer and it enabled U.S. citizens to temporarily take possession of islands with guano deposits as long as the islands were unoccupied and were not under the jurisdiction of another government. Interestingly, Kingman had no guano, but the U.S. retained its claim as a function of World War II in the Pacific. Ultimately, like Palmyra, Kingman came under the administration of the U.S. Fish and Wildlife

DOI: 10.1201/9781003287339-10

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Figure 10.1  The atolls of Hawai’i and the U.S. extend from about 28°N at Kure and Midway, the northernmost of atolls, to ­Kingman and Palmyra at about 5°N, and Wake about 1,800 km to the southwest of Kure. The west-flowing North Equatorial Current (NEC) dominates most of the area, but the Big Island of Hawai'i bifurcates the flow westward. This produces the North Hawai'i Ridge Current to the north of the archipelago and the variable eastward movement of the Subtropical Countercurrent and the Hawaiian Lee Countercurrent and their eddies along the southern side of the islands, depending on wind stress.

Service as a National Wildlife Refuge (see Maragos et al., 2008 for details). Kingman is a triangular atoll (Figure 10.2) located about 59 km northwest of Palmyra. It exhibits three reef flat sections up to 500 m wide that extend east, southeast, and to part of the southern rim. There are no islands, and the remainder of the atoll’s rim is submerged north–southwest counterclockwise. The lack of terrestrial surfaces may explain why no guano was found here. The rim is submerged. The outer reefs extend 20 km northeast–south and are up to 1400 m wide to the southeastern apex. The shallow outer reefs continue to the reef flat; spur and groove systems are visible northeast–south. The lagoon area is about 56 km2. The lagoon slope in the lee of the reef flats develops extensive ribbon reefs up to 180 m wide, but more often, they are about 60 m wide. The lagoon is mostly uncoded, although

reef-rimmed pinnacles can be seen rising from the basin to the southwest. The lagoon center is about 25–55 m deep, and the maximum depth is 82 m to the north (NOAA chart 83153).

Johnston Reef Johnston Reef (Figure 10.3) is an isolated atoll that rises from a depth of 4,950 m. It is located about 1,355 km northwest of Kingman Reef and the nearest land is French Frigate Shoals in the northwest Hawaiian Islands 804 km to the north. Johnston is one of the more isolated atolls in the world. At 16.4°N, it lies in the westward flow of the North Equatorial Current (NEC), but due to obstructed flow, a vortex wake is created around the atoll. The result of the North Equatorial Current flow interacting with the atoll geomorphology results in

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Figure 10.2 Remote-sensing images of Palmyra and Kingman atolls. Images © 2021, Planet Labs PBC.

a complex pattern of eddying and reversing currents around the shallow platform waters (Lobel and Lobel, 2008). In addition, the Hawai’i Lee Countercurrent, while embedded within the North Equatorial Current, responds to wind curls to the west of the Hawaiian Islands and generally flows eastward at latitudes near 20°N (Figure 10.1). The dynamics of the HLCC are not well understood, although it is strongest from August to January and is thought to be important in the recruitment of fish and invertebrate larvae from Johnston to the Hawaiian Islands (Kobayashim, 2006; Sasaki and Nonaka, 2006). Winds at Johnston are consistently moderate and from the east (Figure 10.3). Peak waves are more varied and include modes from the southwest (generally less than 3 m high), east, and northwest. Waves from the northwest–east include larger wave heights, with a marked proportion reflecting swell with heights more than 3 m. Tides are semidiurnal to mixed, with spring tidal range of 90 cm and neap tidal range of 21 cm (Figure 10.3).

The platform creates a rounded triangle that is 23.6 km southwest–southeast, 12 km north–south bisecting the base at its widest and covers an area of approximately 203 km2 (Figure 10.3). However, only about a third of this area is coded. The rest of the platform is tilted to the southwest due to landslides and attendant structural failures resulting in differential subsidence and submergence (Keating, 1987). The outer reefs are about 350–450 m wide to the northwest, including uncoded deep reefs. A total of 33 stony coral species has been recorded from Johnston. Corals are abundant only in depths less than 70 m below which coverage is limited to isolated colonies of a few species that persist to 165 m. Much of the reef to the northwest is awash at low tide where a well-developed reef crest, an emergent algal ridge, and a spur and groove system are found. The reef platform is slightly deeper behind the algal ridge, forming tide pools that remain covered at low water (Maragos and Jokiel, 1986). The rim supports four islets, some of which are dredge spoils, and 2.5-km2 Johnston Island on which is a 1,200 m long airstrip. The island

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Figure 10.3 Remote-sensing image of isolated Johnston Atoll. Images © 2022, Planet Labs PBC. Winds are consistently moderate and from the east. Peak waves are more varied and include modes from the southwest (generally less than 3 m high), east, and northwest. Waves from the northwest–east include larger wave heights, with a marked proportion reflecting swell with heights more than 3 m. The predominant flow is from the North Equatorial Current (Figure 10.1). Tides here are semidiurnal to mixed.

originally was 20 ha but was filled and enlarged to its present area by the U.S. Navy. From 1958 to 1963, the island became the site of high-altitude nuclear testing leading to contamination that persists today. In addition, the island was a chemical waste storage area for Agent Orange, Sarin, VX, and mustard gas

until 2002. The island remains heavily contaminated and is an EPA superfund site. It is managed by the U.S. Fish and Wildlife Service that has declared it a bird sanctuary, but entry for humans requires a special permit issued by the U.S. Air Force (Lobel et al., 2012; Goldberg, 2018). Of the entire 56.6-km-long

Atolls of the United States

rim perimeter of the atoll, only about 23% breaks the surface. The rim is submerged. The lagoon area is roughly 162 km2. Lagoon reefs are best developed to the south–southwest in the lagoon. The southwestern lagoon is semiprotected, deeper, and has a lower coral abundance except for a few areas near Johnston Island. The northeast lagoon is less well protected from wave activity but maintains a high species diversity and habitat complexity due to pinnacles with vertical relief, pits, platforms, and overhangs. The southeast lagoon lacks protection from wave activity and is deeper (10–20 m) than the rest. A few pinnacles and patch reefs are found there, but while benthic algae are common, corals are not (Maragos and Jokiel, 1986). The maximum depth is 27 m (Purdy, 2001).

The atolls of the Hawaiian Islands The Hawaiian Islands were initially settled between about 1,000 and 1,200 years ago, and by about 900 years ago, several lineages of voyaging Polynesian chiefs from the Marquesas, Tahiti, and Samoa were traveling back and forth between their home islands and those of Hawai’i. This is the largest archipelago in the Pacific with 16,692 km2 of land and a rich history of kingly marriages, alliances, wars, and conquests, as well as a rich archeological record to match (Kirch, 2002; 2011). In the early 19th century, however, Hawai’i’s Polynesian culture began to change with the influence of exploitation by merchant seamen and whalers on one hand, and New England missionaries on the other. One of the legacies that the missionaries left in their wake was the Missionary Party, a political organization that plotted with American business interests to overthrow the monarchy. Hawai’i was annexed by the U.S. as a territory in 1900 and was admitted to statehood in 1959 (details in Haley, 2014; Goldberg, 2016). The Hawaiian Islands are the exposed peaks of a Hawaiian-Emperor Seamount chain formed by the movement of the Pacific plate over a single persistent hot spot that is currently active on the Big Island of Hawai’i. The islands to the northwest are sequentially older, and beyond the major islands, the rest are low islands and atolls, commonly referred to as the Northwest Hawaiian Islands (NWHI). The southeast portion of the chain was inhabited prior to European contact by Polynesian societies, and Native Hawaiians recognize the islands as a sacred ancestral homeland from which life arises and to which spirits return after death (Kittinger et al., 2011). These islands are included within the Papahānaumokuākea Marine National Monument, a protected area of 1.5 million km2 that includes deep water seamounts,

banks, an almost-atoll, and four atolls. The latter structures are found within the northwest part of the NWHI from French Frigate Shoals (Kānemiloha‘i) at about 23.7°N latitude to Kure (Mokupāpapa) at the end of the chain at 28.4°N, a distance of about 1,350 km. The atolls herein are described using their common names from nautical charts, with the Hawaiian Lexicon Committee names noted parenthetically. The current systems around the NWHI are a complex mix of eastward and westward flows that are reasonably well known in the vicinity of the high islands but are poorly documented as they affect the NWHI. Flows to the west of Hawai’i include the highly variable eastward movement of the Subtropical Countercurrent (STCC), an extension of part of a larger gyral system to the west of the islands that exhibits seasonally stronger flows during the late spring and early summer between 18 and 21° north. The Hawaiian Lee Countercurrent is a branch of the STCC that continues to flow east near the southwestern Hawaiian Islands and is a response to local wind curl between latitude 19 and 20° north (Figure 10.1; Liu et al., 2003). By contrast, the major westward flows of the North Equatorial Current are pushed by the northeasterly trade winds that may exceed 70 km per hour. As the current approaches the Big Island of Hawai’i, it bifurcates. The flow to the north becomes the North Equatorial Ridge Current running west– northwest along the windward side of the islands. The flow to the south is affected by wind shear forming eddies on the lee side of the Big Island. Clockwise eddies tend to propagate to the northwest, whereas counterclockwise eddies tend to travel to the southwest. Despite their relatively long life, about 79– 110 days, and slow speed, they are difficult to track in the field and their net effects are not well documented (Qui et al., 1997; Calil et al., 2008). Eddies also form between the high islands and are pushed to the leeward sides, sometimes creating shorter lived clockwise and counterclockwise vortices that drift westward. Some of these eddies with the same rotational sign appear to be capable of merging and doubling their speed of rotation, perhaps feeding into the long-lived eddies generated from the Big Island and the North Equatorial Current (Barton, 2009). Given these complexities, it may not be surprising that the mean currents in the northern region of the Northwest Hawaiian Islands appear to be highly variable (Firing and Brainard, 2006). Winds of this archipelago are dominated by the persistent easterly–northeasterly trade winds (Figure 10.4), but include occasional winds from the north and west, and these can be strong. As at Johnston, peak waves are quite varied and

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Figure 10.4 Winds of the Hawaiian Archipelago are dominated by the persistent easterly–northeasterly trade winds and include a pronounced mode of large waves over 3 m high from the northwest that reflects North Pacific swell. At Pearl and Hermes Atoll, tides are mixed, dominantly diurnal with a weaker semidiurnal signal.

include a pronounced mode of large waves over 3 m high from the northwest that reflects North Pacific swell (Figure 10.4). Other modes include trade wind-generated waves from the east and waves from the south that are either southern swell or winter extratropical cyclone (‘Kona’) storm waves; both are less pronounced, generally less than 3 m height. Although tropical cyclones can traverse and impact the Hawaiian Chain as they travel from east to west, do not commonly have a marked destructive effect. Tides can be complex as they intersect and interact with the linear chain of islands and seamounts. At Pearl and Hermes Atoll (Holoikauaua) (Figure 10.4), however, they are mixed, dominantly diurnal with a weaker semidiurnal signal. Spring tidal range is 48 cm and neap tides are 23 cm. The first of the NWHI group and closest to Johnston Island is French Frigate Shoals (Kānemiloha‘i) (Figure 10.5), which like the other Northwest Hawaiian Islands forms atop a ridge from water more than 4,500 m deep. It appears as a crescent-shaped reef northwest–south on an oval platform where the southwestern–western margins are submerged. The platform is 35.4 km northwest–southeast, 24.9 km southwest–northeast, and covers an area of about 718 km 2 including a large area of uncoded reefs to the west and a smaller area to the southeast. The deep and shallow outer reefs are commonly 500–600 m wide to the

north–northeast, expanding to about 950 m to the southeast and about 6 km wide to the northwest where the reefs are deep and mostly uncoded in the Allen Atlas. The shallow outer reefs continue onto the reef flats north–northeast. The rim is composed of five vegetated sand islands totaling about 13 ha. Tern Island to the north was enlarged by the U.S. Navy to accommodate an airstrip (Amerson, 1971). The inner reef flats are sandy, up to 4.5 km wide to the east, and they extend northeast–south with a scattering of patch reefs toward the outer edges, mostly where rubble is common. About 22 km of the 90-km-long rim perimeter breaks the surface to the north–northeast. The rim is submerged. The lagoon area is approximately 462 km 2 including about 115 km 2 that is uncoded. Most of the coded section of the lagoon is described as deep. The lagoon area is 462 km2 and is approximately 37 m at its deepest (Amerson, 1971). The lagoon contains spider web-like extensions of the sandy inner reef flats that are especially prominent as a north–south ridge toward the eastern side of the basin. Ribbon reefs occur in association with these shallow areas flanking the western side of the ridge. Six sandy islets occur on this lagoon ridge, three of which are awash at high tide; the other three are sufficiently washed with salt water to preclude vegetation. Comte de La Perouse was the first to chart the atoll in 1799, and while in the lagoon, he discovered

Atolls of the United States

a pinnacle by almost running his frigate into it. La Perouse Pinnacle, as it is now known, occurs near the lagoon center and rises about 37 m above sea level. The pinnacle is volcanic (Amerson, 1971), and technically, that makes this structure an almost-atoll, but we include it here for a synoptic overview of Hawaiian atolls. Few reefs occur in the lagoon outside of the shallow areas coded as inner reef flat. Although cyclones are not common in this chain, they do have an impact, as evidenced by Hurricane Walaka in 2018, which passed over French Frigate as a Category 3 storm. During passage of that storm, East Island, an island that covered 11 acres and had served as a U.S. Coast Guard LORAN radio navigation station, was washed away. As the French Frigate Shoals serve as nesting ground for more than 95% of the Hawaiian green turtle population, and of those, more than half nested on East Island, this loss was devastating for the endangered green sea turtle population.

Recent (2022) images appear to suggest that the island is re-growing, although it is still not as large or vegetated as before the hurricane. Laysan (Kauō) is an ovo-rectangular atoll island with a remnant lagoon located about 590 km northwest of French Frigate Shoals (Figure 10.5). The 4.1-km2 island occupies the central part of a much larger submerged bank and is composed of reef-derived sand. The platform has been uplifted as shown by its elevation to 12 m above sea level and the tilted beds of coral along the shoreline (Athens et al., 2007). The deep and shallow outer reefs form irregular edges around the island with a platform area of 8.3 km2 mapped by the Allen Atlas, but the platform clearly extends beyond that in visible imagery. The reefs outline rocks and broad outer reef flats west–northeast where they reach 670 m from shore. Coral cover is modest, up to about 15% of the available substrate, and massive or encrusting species of Porites are most common (Kenyon et al., 2007a).

Figure 10.5 Remote-sensing images of the atolls of French Frigate and Laysan. Images © 2022, Planet Labs PBC.

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The island remained undisturbed until the late 19th century when it was mined for guano, nearly destroyed by the introduction of rabbits, and by feather poachers who killed hundreds of thousands of birds, an activity abetted by the fashion demands of Europe and America at the time. Only two of the five bird species found only on Laysan survive (Ely and Clapp, 1973; Rauzon, 2001). The lagoon or pond area is about 69 ha. There are no connections to the ocean and as the volume is subject to evaporation. Salinity increases are moderated only by rainfall which varies between 378 and 1,578 mm per year with dry periods during the summer. The pond salinity varies between 120‰ and 150‰ and the lagoon is always hypersaline although radiocarbon dating of cores suggests that its waters were fresh or brackish about 5,000 years ago. The lagoon is composed of two connected basins. The northerly one

is up to 6.5 m deep and the basin to the south at 5.6 m is a bit shallower, however, the salinity throughout the lagoon is uniform (Athens et al., 2007). The next closest reef area to Laysan is Neva Shoal, a shallow area of about 286 km2. The platform center forms a high sandy area about 15.7 km2 that connects with a single 1.3-km2 island (Lisianski Island, Papa‘¯apoho) to the northeast. An area of reef flat extends north and west of the island. Reef flats also spread southeast of the central sandy area as narrow web-like ridges to the southeast. The reef flats and ridges are the sites of most of the reef development on this shoal area. There is no rim enclosing the lagoon and we regard this system as a bank reef. Pearl and Hermes Reef (Holoikauaua) is an ovoid atoll (Figure 10.6) named for the British whaling vessel Pearl that ran aground there in 1822 and her sister ship Hermes that was wrecked while trying

Figure 10.6 Remote-sensing images of the atolls of Pearl & Hermes, Midway, and Kure. Images © 2021, Planet Labs PBC.

Atolls of the United States

to save the Pearl (Rauzon, 2001). The platform is 32.3 km northeast–southwest, 19.2 km west–east, and covers an area of approximately 443 km 2 including uncoded reef northeast–east and southwest. The atoll is surrounded by deep and shallow outer reefs that reach their greatest extent to the east where they are 1.9 km wide including uncoded reef. The reefs appear to wane toward the southeast. The deep reefs are primarily uncoded to the west, although shallow reefs extend to the reef flats. Both shallow and deep outer reefs are submerged to the southwest. Surveys by Kenyon et al. (2007b) found the highest coral cover, about 25% of the available reef surface, occurred on the outer reefs to the north and northeast, whereas reefs to the east covered only about 6% of the substrate. The outer reefs are dominated by massive and encrusting species of Porites as they are on Laysan, and the perimeter of the rim develops a spur and groove system around most of the atoll (Rooney et al., 2008). The rim is composed of rubbly reef flats that are up to 2 km wide to the northeast, extending more narrowly to the southwest. An island area of about 21 ha occurs to the south along with a smaller islet to the southwest. The rim is narrower and shallower in the north–northwest where there is far less rubble, but extensive shallow reef development. A low area in the rim about 12 km long occurs to the southwest (‘The Channel’, Kenyon et al., 2007b) and continues as a deep area into the lagoon. The rim perimeters are almost entirely encircled by a spur and groove system (Rooney et al., 2008). The rim is open. The lagoon area is approximately 292 km2. Most of it is coded as deep reef except for sand that accumulates in the northeast–east in a band up to 3.2 km wide. Smaller and intermittent sand pockets occur to the southeast and northwest. A 4.9-km2 sandy area west of the lagoon center is also prominent and is shallow that is about 3–6 m deep (NOAA chart 19461). Web-like rubbly reef flats appear to radiate into the deep lagoon as narrow ridges from the central sandy area and these ridges are colonized extensively with reefs. Rooney et al. (2008) and Page-Albins (2012) refer to these lattice-like shallow ridges as a reticulated reef, although they are distinct from the more typical honeycomb forms found more commonly in the Pacific. Kenyon et al. (2007b) found that two coral genera, Porites and Pocillopora, dominated the lagoon coral fauna, and that about 19% was the average coral cover. However, their survey diagrams make it clear that they were sampling the ridges. Sampling the deep lagoon in the surrounding areas may produce a different percent coverage. The lagoon was at one time rich with pearl oysters, but they were so heavily exploited for button manufacture during

the 1920s that the population crashed and has not recovered to pre-exploitation levels (Keenan et al., 2006). The maximum lagoon depth is 26 m (Purdy, 2001). The U.S. acquired Midway (Pihemanu) in 1859 by the declaration of a naval officer and it was first permanently occupied in 1903 as a relay station for the Commercial Pacific Cable Company. Its first transmission was the first message received around the world. Tons of topsoil were imported from California along with non-native plants to make the place feel more like home for the residents. By 1935, Pan American Airways had established a seaplane base as a stopover for their Manila to San Francisco flights. Buildings were added, including a hotel, but preparation for war soon took precedence (Bryan, 1973; Rooney et al., 2008). Midway Atoll is named for its position midway between San Francisco and Tokyo, a factor that became significant during World War II. It was attacked by Japan in June of 1942 but became famous shortly afterwards as the Battle of Midway, an air and naval attack by the U.S. that took place north of the atoll in which Japan lost a substantial part of the Imperial Navy, especially its aircraft carriers. American cryptographers had broken Japan’s code and knew in advance where they had concentrated their fleet. It was a major turning point in the Pacific war. Midway takes the form of a D with the flat side to the south (Figure 10.6). The platform is 9.7 km north–south, 11.5 km parallel to the base, and covers an area of about 99.8 km 2 including uncoded reef around the perimeter. The reefs are best developed to the northwest where they are up to 750 m wide. A spur and groove system is found around most of the atoll. The rim supports two islands, Sand and Eastern to the south that together account for an area of 627 ha that were enlarged, paved, and otherwise heavily modified during the war. Sand Island is now home to a U.S. Coast Guard base with an airstrip, a wastewater treatment facility, and an assortment of industrial facilities. Eastern Island with its abandoned airfield is now part of the Papahānaumokuākea Marine National Monument as a protected area and breeding grounds for millions of seabirds including the world’s largest population of Laysan albatrosses. The rim is surrounded by deep and shallow outer reefs that extend more than a kilometer wide south of Sand Island and about 2 km wide to the northwest, both including uncoded deep reefs. The reef flats have developed numerous patch reefs on all sides but the northeast. A low area in the rim to the west marked as Seward Roads shallows to less than 5 m and is open for small craft (NOAA chart 19482). An

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artificial channel was carved into the southern rim during the 19th century. The rim is open. The lagoon covers an area of about 43.1 km 2. The shallow lagoon occupies both sides of the deeply dredged midsection. The eastern shallows are about 3 km wide, and the western shallow lagoon area is 4.6 km wide. Both sides are covered with sand and form an apron as an incomplete halo separated from each other by the dredged lagoon (Purdy and Gischler, 2005). Small areas of reef flat that are exposed in these sandy areas are colonized by reef material. The maximum lagoon depth is 6 m (Purdy, 2001) not including the substantial areas that were dredged. Kure (Mokup¯apapa) is the northernmost atoll in the NWHI chain 86 km northwest of Midway. With its position at 28.4°N, it is also the northernmost atoll in the Pacific and the world. The platform is annular, 9.2 km north–south, 9.8 km west–east, and covers an area of 72.4 km2 including uncoded reef around the atoll perimeter (Figure 10.6). The reefs appear to be about 700–800 m wide to the north, east, and south, but narrow to about 450 m to the west when visible but uncoded reef is included. Reef growth is slower at this latitude during the winter months and there is some controversy concerning overall coral growth rates here, but the average outer reef coral coverage between 10 and 16 m is 9.7%, higher in the northwest–east and lower in the southeast–south. The dominant corals are small colonies of the genera Porites and Pocillopora. (Kenyon et al., 2008). The spur and groove system is well developed, up to 600 m wide to the north, and is present around most of the atoll perimeter. The rim is best developed in the southeast where it is up to 1 km wide and supports an 81-ha islet with an airstrip. The lagoon side of the islet is flanked by a series of sand dunes, some as high as 7.6 m (Woodward, 1972). The islet is encircled by a reef flat up to 1.1 km wide, but the flats are narrower, 400–600 m wide west– northeast, where patch reefs are far more common. The rim is submerged 2.7 km to the southwest where it is coded as lagoon slope and is densely populated with reef material. The lagoon area is 38.4 km2 and more than 90% is coded as a shallow lagoon, commonly with depths of 2–3 m (NOAA chart 19483). No reef material is indicated here. An area of about 3.7 km2 to the southwest is deeper, commonly 6–10 m, but is associated with coralliferous shallow ridges of the inner reef flat that run through it. Kenyon et al. (2008) found that coral coverage on central lagoon patch reefs was about 12% and was dominated by a single species of Porites. Because no reef areas are shown in the lagoon center, we suspect that the patch reefs they sampled were on these shallow ridges

west of center. Indeed, Kure’s lagoon has been filled almost entirely with wave-driven sediment except for the deeper area mentioned above and constitutes a large sand apron (Purdy and Gischler, 2005). The maximum depth of the lagoon is 15 m (Purdy, 2001).

Isolated Wake Island Wake Island is located 555 km northwest of Bokaak and is claimed by the Marshall Islands. On the other hand, the closest territory of the U.S. is Kure, 1,783 km to the northeast. Propinquity notwithstanding, Wake was taken by the U.S. from Spain in 1898 as part of the settlement of the Spanish-American War, and it is now administered by the U.S. Air Force. Located at 19.3°N latitude, the atoll is in the paths of the North Equatorial Current and the easterly trade winds that dominate the area for most of the year (Figure 10.7). Like the NWHI atolls, it is subjected to waves from the east and northwest; it does not, however, include common waves from the south. Tides are semidiurnal, with spring tidal range just over 1 m and neap tide range of 0.25 m. Wake is part of the Wake-Marcus seamount cluster on the Pacific plate and is the only one of them that rises 5,500 m to the sea surface (Yang et al., 2019). The platform takes the shape of a warped and rounded triangle that is 8.4 km northwest–southeast, 4.8 km at its widest northeast–southwest, and covers an area of 25.6 km2 (Figure 10.7). All measurements include uncoded deep outer reefs that encircle the atoll including deep outer reefs that are about 200 m wide on all but the western side where they are typically 90 m wide or less. The shallow outer reefs are narrow and difficult to separate from their offshore and onshore positions as they blend with a well-developed reef crest that extends north–south and around the southeast tip of the atoll. Areas of the reef flat are also encroached by shallow reef to the north and southwest. The shallow reefs to the south are about 75 m wide and appear to be poorly developed to the northwest. Live coral is most extensive to the north, where it is about 77%–81% of the available substrate. It is lowest to the south–southwest, where it is about 22%–27%, and variable to the east and west, where live coral coverage is about 35%–64% (Lobel and Lobel, 2008). The rim supports three islands forming a V shape that opens to the northwest covering about 6.5 km 2. The islands have been extensively modified for military use, including a 3,000 m long runway, a partially completed submarine channel, multiple causeways, and a small boat harbor that seals a channel to the lagoon with a seawall. About one-third of the atoll

Atolls of the United States

Figure 10.7 Remote-sensing image of Wake Atoll, one of the most isolated atolls in the world. Images © 2022, Planet Labs PBC. Easterly trade winds that dominate the area for most of the year and the main current is the North Equatorial from the east (Figure 10.1). Like the NWHI atolls, it is subjected to waves from the east and northwest, but here waves from the south are uncommon. Tides are semidiurnal. The atoll is administered by the U.S. Air Force by its airfield, which is primarily used as a mid-Pacific refueling stop for military aircraft and as an emergency landing area.

perimeter is occupied by a reef flat up to about 800 m wide that dominates the western and part of the northern rim where there are no islands. A spur and groove system is visible northeast–northwest. The lagoon area is 8.7 km2 as mapped by the Allen Atlas, and about 87% of this area is covered with sand, much of which becomes exposed during the lowest tides. Most of the lagoon is 3 m deep, where the bottom is mixed with occasional corals and abundant Tridacna clams. The waters are murky with poor visibility owing to low levels of exchange

with oceanic waters and closure of channels by construction between islands (Lobel and Lobel, 2008). The deepest area is about 40 ha to the western lagoon margin where it reaches 26 m (Purdy, 2001).

References Amerson AB Jr 1971. The natural history of French Frigate Shoals, Northwestern Hawaiian Islands. Atoll Res. Bull. 150: 1–383. Athens JS, Ward JV, Blinn DW 2007. Vegetation history of Laysan Island, Northwestern Hawaiian Islands. Pac. Sci. 61: 17–37.

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Barton ED 2009. Island wakes. In: Steele JH, Thorpe SA, Turekian KK (eds.) Ocean Currents. Elsevier, London, pp. 470–475. Bryan EH Jr. 1973. The Northwestern Hawaiian Islands, An Anno‑ tated Bibliography. U.S. Fish Wildlife Service. https://ecos.fws. gov/ServCat/DownloadFile/142975 Calil PHR. Richards KJ, Jia Y, Bigidare RR 2008. Eddy activity in the lee of the Hawaiian Islands. Deep‑Sea Res. II: Top. Stud. Oceanogr. 55: 1179–1194. https://doi.org/10.1016/j. dsr2.2008.01.008 Ely CA, Clapp RB 1973. The natural history of Laysan Island, Northwestern Hawaiian Islands. Atoll Res. Bull. 171: 1–361. Firing J, Brainard RE 2006. Ten years of shipboard ADCP measurements along the Northwest Hawaiian Islands. Atoll Res. Bull. 543: 347–364. Goldberg WM 2016. Atolls of the world: revisiting the original checklist. Atoll Res. Bull. 610: 1-47. Goldberg WM 2018. The Geography, Nature, and History of the Tropical Pacific and its Islands. Springer World Regional Geography Book Series. Springer Intl. Pub. AG, Cham, Switzerland. Haley JL 2014. Captive Paradise: A History of Hawai’ i. St. Martin’s Press, New York. Hamann IM, Boehlert GW, Wilson CD 2004. Effects of steep topography on the flow and stratification near Palmyra Atoll. Ocean Dynamics 54: 460–473. Keating BH 1987. Structural failure and drowning of Johnston Atoll, central Pacific basin. In: Keating BH, Fryer P, Batiza R et al. (eds.) Seamounts, Islands, and Atolls. Geographical Monograph Series. American Geophysical Union, Washington, DC, vol. 43, pp. 49–59. Keenan EE, Brainard RE, Basch LV 2006. Historical and present status of the pearl oyster, Pinctada margaritifera at Pearl and Hermes Atoll, Northwestern Hawaiian Islands. Atoll Res. Bull. 543: 333–344. Kenyon JC, Dunlap MJ, Aeby GS 2008. Community structure of hermatypic corals at Kure Atoll in the Northwest Hawaiian Islands: Stemming the shifting baseline. Atoll Res. Bull. 559: 1–25. Kenyon JC, Dunlap MJ, Wilkinson CB et al. 2007b. Community structure of hermatypic corals at Pearl and Hermes Atoll, Northwestern Hawaiian Islands: Unique conservation challenges within the Hawaiian Archipelago. Atoll Res. Bull. 549: 1–23. Kenyon JC, Wilkinson CB, Dunlap MJ et al. 2007a. Community Structure of hermatypic corals at Laysan Island and Lisianski Island/Neva Shoal in the Northwestern Hawaiian Islands: A new layer of scientific exploration. Atoll Res. Bull. 550: 1–30. Kirch PV 2002. On the Road of the Winds. University of California Press, Berkeley. Kirch PV 2011. When did the Polynesians settle in Hawai’i? A review of 150 years of scholarly inquiry and a tentative answer. Hawaiian Archeol. 12: 3–26. Kittinger JN, Dowling A, Purves AR et al. 2011. Marine protected areas, multiple-agency management, and monumental

surprise in the northwestern Hawaiian Islands. J. Mar. Biol. 2011: 1–17. Kobayashi D 2006. Colonization of the Hawaiian Archipelago via Johnston Atoll: A characterization of oceanographic transport corridors for pelagic larvae using computer simulation. Coral Reefs 25: 407–417. Liu Q, Wang S, Wang Q, Wang W 2003. On the formation of subtropical countercurrent west of the Hawaiian Islands. J. Geophys. Res.: Oceans 108: 3167. https://doi. org/10.1029/2002JC001366 Lobel PS, Lobel LK 2008. Aspects of the biology and geomorphology of Johnston and Wake atolls, Pacific Ocean. In: Riegl BM, Dodge RE (eds.) Coral Reefs of the U.S.A. Springer+Business Media BV, Dordrecht, pp. 655–690. Lobel PS, Schreiber EA, McClosky G, O’Shea L 2012. An Ecolog‑ ical Assessment of Johnston Atoll. Boston University Marine Lab. https://apps.dtic.mil/sti/pdfs/ADA571836.pdf Maragos J, Miller J, Gove J et al. 2008. U.S. coral reefs in the Line and Phoenix Islands, central Pacific Ocean: History, geology, oceanography, and biology. In: Riegl BM, Dodge RE (eds.) Coral Reefs of the U.S.A. Springer kmBusiness Media BV, Dordrecht, pp. 595–642. Maragos JE, Jokiel PL 1986. Reef corals of Johnston Atoll: One of the world’s most isolated reefs. Coral Reefs 4: 141–150. Page-Albins KN, Vroom PS, Hoeke R et al. 2012. Patterns in benthic coral reef communities at Pearl and Hermes Atoll along a wave exposure gradient. Pac. Sci. 62: 481–496. Purdy EG 2001. Supplement to Purdy EG, Winterer EL 2001. Origin of atoll lagoons. GSA Bull. 113: 837–854. Purdy EG, Gischler E 2005. The transient nature of the empty bucket model of reef sedimentation. Sediment. Geol. 175: 35–47. Qui B, Koh C, Lumpkin C, Flamant P 1997. Existence and formation mechanism of the North Hawaiian Ridge Current. J. Oceanogr. 27: 4431–444. Rauzon MJ 2001. Isles of Refuge: Wildlife and History of the North‑ west Hawaiian Islands. University of Hawai’i Press, Honolulu. Rooney JJ, Wessel P, Hoeke R et al. 2008. Geology and geomorphology of coral reefs in the Northwest Hawaiian Islands. In: Riegl BM, Dodge RE (eds.) Coral Reefs of the U.S.A. Springer kmBusiness Media BV, Dordrecht, pp. 519–571. Sasaki H, Nonaka M 2006. Far-reaching Hawaiian Lee Countercurrent driven by wind-stress curl induced by warm SST band along the current. Geophys. Res. Lett. 33: L13602. https://doi. org/10.1029/2006GL026540 Woodward PW 1972. The natural History of Kure Atoll, northwestern Hawaiian Islands. Atoll Res. Bull. 164: 1–317. Yang Z, Qian Q, Chen M et al. 2019. Enhanced but highly variable bioturbation around seamounts in the northwest Pacific. Deep Sea Res. Part I Oceanogr. Res. Pap. 156: 103190. https:// doi.org/10.1016/j.dsr.2019.103190

The Micronesian Pacific and the Atolls of the Marshall Islands Micronesia is one of the distinct regions of Oceania in the western Pacific that, while separate, overlaps in many ways with Melanesia to the south and Polynesia to the east. It extends from Palau and the Mariana Islands in the west to the Marshall Islands and the Gilbert Islands of Kiribati in the east. However, we do not consider the Mariana Islands here because they are without atolls, and we have already covered Kiribati as a separate geographic entity in Chapter 9. We focus here on the largest part of Micronesia, the Caroline and the Marshall islands in the western and central Pacific (Figure 11.1). This area stretches more than 3,500 km west–east and covers latitudes from just north of the equator to about 15°N. Although the languages spoken here are all Austronesian (derived mainly from those of maritime Southeast Asia), speakers from Palau fall within a different subgroup. The language of Palau is indicative of an Indonesian-

Philippine origin from settlements as early as 4,300 years ago, or at least by 3,250 years ago judging from radiocarbon pottery dates. By contrast, speakers from the Caroline and Marshall Islands fall into a cluster of several Austronesian subgroups referred to as Nuclear Micronesian indicating migrations not only from Southeast Asia but likely from the Solomon Islands and Vanuatu as well (Kirsch, 2010). Other evidence for Micronesian distinction includes certain types of songs, dancing, and clothing that are held in common among Micronesians, as are the matriarchal societies forming the core of ‘clans’ that can trace their lineages to the first settlers. On the other hand, genetic evidence from maternally transmitted mitochondrial DNA and paternally transmitted Y chromosomes has been controversial and has not determined a clear origin for the Micronesian people (Su et al., 2000; Friedlander et al., 2008). The high islands of the

Figure 11.1 Micronesian atolls include those of the Caroline and Marshall Islands that cover a wide swath in the western and central Pacific stretching more than 3,500 km west–east, covering latitudes from just north of the equator to about 15°N. A total of 66 atolls are described separately in the pages that follow. Parece Vela, an outlying atoll claimed by Japan, is also described. It is shown here for geographical convenience, but is not part of the Caroline or Marshall Islands.

DOI: 10.1201/9781003287339-11

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Caroline group east of Palau appear to have been settled 2,100–2,200 years ago, but the atolls would not have been colonized until 1,200–650 years ago, a time when central Pacific sea levels and temperatures fell associated with the Little Ice Age. Kapingamarangi in the Caroline group, for example, likely was not settled until about 1,000 years ago (Nunn, 2000), much later than the high islands.

The atolls of the Marshall Islands Named for John Marshall, a British sea captain, who explored the area in 1788, the Marshall Islands are a group of five islands (Jabwot, Jemo, Kili, Lae, and Mejit, not considered here) and 29 atolls lying east and mostly north of the Caroline group. Both are Micronesian, but rather than being spread out west– east as the Carolines are, the Marshall Archipelago is more compressed and oriented into two north–south chains. The westernmost is the Ralik Chain which

means ‘sunset’ in the Marshallese language and extends from Ebon in the south through Kwajalein, Rongerik, and Bikini in the middle, and Enewetak and Ujelang in the northwest—16 atolls in all. The eastern group of 13 atolls is called the Ratak Chain which means ‘sunrise’ in Marshallese and extends from Mili in the south to Wotje in the middle and Bokaak in the far north (Figure 11.2). Each chain maintains its own dialect and has its own minister, reflecting the days when each chain had its own paramount chief, but now both ministers report to the president who resides in Majuro. This is the capital where about 28,000 people live out of a total population of 87,000 in the Marshall Islands as of 2011 (CIA, 2022). As in the Federated States of Micronesia and Palau, the Republic of the Marshall Islands is a sovereign island nation aligned with the U.S. in a Compact of Free Association. As a member of the Association, the U.S. guarantees financial assistance

Figure 11.2 The Marshall Islands are divided into two parallel chains of seamounts, the western Ratak Chain and the eastern Ralik Chain. Of the 29 atolls here, all but two develop an open rim with well-circulated lagoons. The more northerly atolls are in the path of the North Equatorial Current, whereas the southerly ones are washed by the North Equatorial Countercurrent.

Atolls of the Marshall Islands

as well as defense authority and responsibilities. The evolution of this relationship followed a tortuous path. The Spanish were the first Europeans to explore the islands of Micronesia and claimed them in 1592 but made little attempt to gain administrative control over the islands. By the 19th century and after doing little with them, Germans began establishing businesses in the region and effectively shared power with Spain. However, the Spanish Empire collapsed after the Spanish-American War in 1898 causing them to sell their remaining possessions in Micronesia to Germany. Thus, the Marshall Islands, among others to the west, became part of New Guinea or Kaiser Wilhelmsland. However, German possession was short-lived. Japan seized the Marshall Islands in 1914, and after World War I, the group came under Japanese control in accordance with the League of Nations mandate. During its years of colonial rule, Japan began moving some of its population to the Marshall Islands, Palau, and the Mariana group, where in some cases they outnumbered the indigenous peoples. Japanese language and culture were taught in public schools, and in the 1930s, Japan claimed all the islands and began building air bases and other military installations on several atolls. This became their outer defensive rim at the start of World War II. After the war, the Marshall group as well as the Caroline Islands became part of the U.S. Pacific Trust Territory. A trust can be many things including an economic manipulation to prevent competition (think Teddy Roosevelt and trust busting in 1904 or the breakup of the Bell Telephone system in 1982). Or a trust can be a legal means of storing assets (think wills, trusts, and probate). However, the concept of a trust as created by the United Nations is different, allowing war-torn areas to be administered by a trustee while obliging them to guarantee human rights and social development without militarization. But the trustee for Micronesia, the U.S., refused and stated its intention to simply annex the islands ‘by right of conquest’ if the trust were not written to their specifications, which included the establishment of military bases (Fischer, 2002). The consequence of this would soon have a dramatic effect on the Marshall Islands as will be described later. The geological formation of the Marshall Islands is based on multiple episodes of volcanism involving multiple hot spots, uplift, reef building, and subsidence that created the seamount chains recognizable today. Deep drilling in 1951–1952 on Enewetak Atoll revealed sequences of reef growth and erosion 1,300–1,400 m below the surface. This showed that (a) reef growth took place during fluctuating sea levels, (b) shallow-water reef growth reef growth could

not have occurred at depths of 1,000 m or more, and (c) growth took place on a subsiding volcanic platform that was at least 50 million years old (Quinn and Saller, 2004). Thus, Darwin’s subsidence theory was finally confirmed after the partial success of the Royal Society’s attempts on Funafuti (Chapter 8). The proximity of islands facilitated voyages by early Micronesians, especially within the two chains. Flotillas of boats each carrying 50–100 people and supplies in the open sea were common according to reports of missionaries in the 1860s. These expeditions depended on navigators, usually relatives of the paramount chiefs, whose knowledge and skill were highly prized and were passed within extended families generationally. The training of navigators was facilitated by different types of stick charts, made from palm frond ribs tied together with coconut fibers. These were often small, but detailed representations of wind, wave, and current behavior around a small group of atolls or a single atoll. Larger charts called rebbelith were used to depict entire inland chains or the entire archipelago. Figure 11.3 shows a rebbelith with a combination of 30 islands and atolls represented by cowrie shells covering an area of more than 750,000 km2. In this orientation, Bikar (B) is shown in the upper left. Mili (M) is shown in the upper right, Ebon (E) is on the lower right, and Alinginae (A) is on the lower left. Note that distances are not depicted and are not intended to be. Only relative directions and generalized sea conditions around the atolls are shown.

Climate and regional oceanography Swell waves propagated over long distances from their source are shown as curved lines that represent their reflection as they encounter Kili Island (K) in the center of the arcs trailing to Jaluit (J) to the north and Namdrik (N) to the south. Short sticks such as those found near Kili represent the prevailing wind direction moving toward nearby atolls. In other places, swell is depicted as triangles that indicate how to navigate from one island to another nearby. The long diagonal lines represent the swell produced by the prevailing northeast winds. The cross lines are also significant and represent swell angles that are the same or are changing, often determined by lying down in the bottom of the boat to feel the movement. It should be noted that these charts are models—not maps—and those produced from different island groups may differ in their representation of currents, swell, and interaction with land (Ascher, 1995). Two ocean current systems dominate the Marshall Islands. A westward-flowing North Equatorial Current (NEC) is found between 10°N and 15°N

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Figure 11.3 Marshall Islands stick charts are indigenous models depicting a combination of 30 islands and atolls represented by cowrie shells covering an area of more than 750,000 km2. In this orientation, Bikar (B) is shown in the upper left, Mili (M) is shown in the upper right, Ebon (E) is on the lower right, and Alinginae (A) is on the lower left. Short sticks such as those found near Kili (K) represent the prevailing wind direction moving toward nearby atolls. The long diagonal lines represent the swell produced by the prevailing northeast winds. Wide-area charts like these were used to aid inter-island navigation, especially between chains. (Photo by Walter Goldberg.)

and bathes the islands in both chains from Ujelang, Wotho, and Likiep in the center to Bokaak in the north. The Marshall Islands between 5°N–10°N are in the path of the persistent, but weaker eastward flow of the North Equatorial Countercurrent (NECC) shown in Figure 11.2. Variations in the boundary of these two currents can be expected, especially when the winds are strongest from the northwest and the NEC’s range is expanded. In addition, large-scale eddies several 100 km wide are thought to occur to the center between these two current systems in an area approximately 4°N–10°N and may induce differing flows than those expected from the paths of the NEC and NECC, likely an effect of island proximity. Some eddies are subject to large-scale meandering or turbulence and may also be coupled with upwelling (Mao and Yoshida, 1955; Lukas, 2009). Winds in the Marshalls are dominantly from the east to northeast, reflecting the dominant trade winds (Figure 11.4). Most winds are fairly brisk, between 20 and 40 km/hour, although stronger winds can

be associated with passage of tropical depressions. Regional analysis (Holland et al., 1988) suggested that on average cyclones impact somewhere around the Marshall Islands on an average of once per year. More recent studies (e.g., Spennemann and Marschner, 1995) have suggested slightly lower recurrences, and that cyclone frequency is modulated by ENSO changes, with cyclones 2.6 times more likely during El Niño phases. Rainfall is varied across the area; on average, rainfall increases from northwest to southeast. For example, Enewetak in the northwest averages 150 cm of rainfall per year, Kwajalein near the center of the chain averages 260 cm/year, and Majuro to the southeast averages 356 cm/year. Precipitation is most abundant in the wet season (June to December) when some islands receive 75% of their rain. As in the other island groups, waves in the Marshalls are complex. Although the most common waves are from the east (Figure 11.4), they vary seasonally.

Atolls of the Marshall Islands

Figure 11.4 Winds in the Marshalls are mostly from the east to northeast, reflecting the dominant trade winds. Waves in the Marshalls are complex. Although the most common waves are from the east, they vary seasonally. Waves in the boreal winter (November–March) are more commonly larger than those in summer (July–September). Strong swell derived from tropical cyclones or extra-tropical storms in the northwest Pacific and south Pacific also can affect the Marshalls. At Kwajalein, tides are dominantly semidiurnal.

Waves in the boreal winter (November–March) are more commonly larger (mean 1.4 m) and from the northeast, whereas summer (July–September) waves are generally smaller and more from the southeast. Spring and fall waves (April–June, October) are transitional. Strong swell derived from tropical cyclones or extra-tropical storms in the northwest Pacific and south Pacific also can impact the Marshalls. Tides in the Marshall Islands are dominantly semidiurnal. At Kwajalein, tides have a spring tide range of 1.68 m and a neap tide range of 0.42 m (Figure 11.4).

The Ratik Chain of atolls The southernmost atoll in the Marshall Archipelago is Ebon, at the bottom of the Ratik Chain, located at 4.6°N, about 898 km southwest of Pingelap in the Caroline Islands, and 460 km northwest of Butaritari Atoll in the Gilbert Islands. Ebon is an annular atoll with a flattened side to the southwest (Figure 11.5). The platform is 13.5 km southwest– northeast, 13.0 km northwest–southeast, and covers an area of 136.6 km 2 . Deep and shallow reefs surround the platform and are up to 160 m wide to the northeast, where they are best developed. Satellite imagery shows a spur and groove system to

the north but is not adequate to confirm its presence around most of the atoll. The rim supports 16 islets and two islands, one of which is a little more than 1 km 2 . The other island is Ebon that stretches southwest–east for 12 km and covers 24.8 km 2 . The land areas are connected by a reef flat up to a kilometer wide to the west, where some patch reefs also occur, and the flats are interrupted by a delta-like channel that is narrow, tortuous, and subdivided. Three channels are occluded by reef growth but the main entrance to the north is at least 18 m deep (National Geospatial Intelligence Agency chart 81030). While the rim is technically open, Ebon is one of four atolls in the Marshall group that has a raised rim on the leeward side (see also Namdrik, Bikar, and Bokaak). Ocean water propelled by wind crosses the northeast reef flat and becomes ponded in the lagoon due to the limited outflow capacity of the narrow outlet. This condition is alleviated by the highest high tides that flood seawater through the leeward rim (Maragos, 1994). The lagoon area is 105.9 km2. The shallow lagoon is poorly developed but its slope forms reefs that outline the basin including the lee of islets to the west and the inner reaches of the lagoon entrance. Pinnacles and platforms up to 500 m wide rise from the deep regions of the slope and the basin bottom, the maximum depth of which is 33 m (Purdy, 2001).

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Namdrik (Namorik) is a trapezoidal atoll located 118 km southwest of Ebon (Figure 11.5). The platform is 5.6 km north–south, 6.5 km at its widest across the long base, and covers an area of 25.1 km2. Deep and shallow outer reefs surround the platform, and they are widest northwest–southwest clockwise and are up to 600 m wide to the northeast. The reefs are least developed to the west. The reef crest

is similarly disposed. The rim supports two forested islands, a small one to the northwest and the other covering much of the atoll perimeter extending midnorth–mid-west clockwise. Both are important nesting areas for hawksbill and green turtles and support relatively extensive mangrove communities (Thaman, 2016). About 18% of the rim is without islands and the reef flat is up to a kilometer wide as it opens

Figure 11.5 Remote-sensing images of the atolls of Ebon, Namdrik, and Jaluit. Images Ⓒ 2020-2021, Planet Labs PBC.

Atolls of the Marshall Islands

with gaps to the north and west. There are no channels or passes in the rim, and because the lagoon has no outlet, water driven by winds causes ponding as described on Ebon (Maragos, 1994). However, daily tides with springs to 2 m flush the lagoon (Clarke et al., 1996). We mark the rim as semi-closed. The lagoon area is 10.5 km2. The lagoon walls (shallows, slope, and part of the deep reef) form a sand apron up to 450 m wide to the south that is present around the lagoon except for a 2.4 km gap to the east. The shallows and lagoon slope to the north develop reefs in the lee of the islands. The southern lagoon additionally displays large plateau areas not associated with reefs. Other isolated reef areas are found to the west on the slope and the deep lagoon. The deepest areas of the slope and center lagoon develop eight large patch reefs, three of which reach the surface. Black-lipped pearl oysters are common on these peaks from 3 to 20 m depth but are not sufficient to support a commercial fishery (Clarke et al., 1996). We are not aware of any information regarding the depth of this lagoon. Jaluit Atoll (Figure 11.5) resembles a warped and extended diamond and is located 186 km east–northeast of Namdrik. The platform is about 60 km northwest–south, 35 km at its widest west–east, and covers an area of 909 km2. The platform is surrounded by deep and shallow outer reefs up to 380 m wide to the east and 200 m wide or less in the west. A well-developed spur and groove system surrounds the atoll judging from the available imagery. The rim supports more than 60 scattered islets and the island of Jaluit that stretches more than 20 km mid-east–southwest. The island supports mangrove growth in its lee that accounts for 4.4% of the land area (Crameri and Ellison, 2022). The western rim is the least elevated and supports only five islets but it includes multiple irregular extensions into the lagoon that are largely covered with sand and patches of seagrass. These areas also are outlined by reef growth. Three pass areas occur on Jaluit, first to the northeast with three openings, second to the southeast with three openings, and third with a single opening to the southwest. Details are shown in Defense Mapping Agency chart 81809 and all are adequate for vessels of all sizes. The rim is open. The lagoon area is about 752 km2. Sand deposits up to 650 m wide occur in the shallows and the slope to the northwest, narrowing rapidly to about 75 m wide and extending about 5 km to the south and east. Other prominent sand areas occur along the western lagoon, but many are separated by small breaks. Patch reefs occur in the shallows, but most of them occur on the slope and deep reef, where they

form a nearly continuous ribbon around much of the lagoon including areas around each of the three passes. The deep lagoon reef areas appear to rise from the uncoded deep lagoon slope. The lagoon basin also forms reef-rimmed pinnacles and platforms, many of which break the surface as isolated reef flat areas. The average lagoon depth is 37 m and the maximum depth is 53 m (Shepard, 1980). Ailinglaplap is a warped ovotriangular atoll located 127 km northwest of Jaluit (Figure 11.6). The platform is 26.5 km north–south, 51.2 km parallel to the base west–northeast, and covers an area of 861 km2. The platform is surrounded by deep and shallow outer reefs that are more than 300 m wide to the north, less so to the east and west, and narrow or patchy to the southwest. Spur and groove systems are visible to the southeast and southwest. The rim is up to 1.3 km wide and supports about 34 islets and four islands, including one to the west and northeast and two to the south. The rim to the southeast is submerged for 6.5 km and interrupted by a single islet. There are eight entrances to the lagoon, four of which are passes with hazards for vessels of various specified sizes and four others that are suitable for small vessels only (Sailing Directions, 2017). The rim is open. The lagoon area is 762 km2. A diagonal can be drawn from the northwest pass to the south pass to define the western lagoon. Reefs in the shallow lagoon are up to 150 m wide or less, overlap with areas of the deep lagoon, and tend to be patchy compared with the west. Sand deposits about 100 m wide occur sporadically on much of the lagoon shallows and are a bit more continuous around the northeastern lagoon border but fail to form an apron. Most of the lagoon center and west is about 45 m deep. Multiple reef-rimmed pinnacles as well as many small reef areas emerge from those depths to the west. Fewer of these reefs emerge from the center and eastern sections of the basin. The deepest part of the lagoon is 62 m (Purdy, 2001; Defense Mapping Agency chart 81729). Namu Atoll is funnel-like with its wide end toward the north and an expanded tube facing southeast (Figure 11.6). The platform is 56.4 km northwest– southeast, about 25 km at its widest, and covers an area of 491 km2. The platform is surrounded by deep and shallow outer reefs that are up to 270 m wide to the east and about 200 m to the north. The south and west are more typically 130 m wide. The northwest–northeast and southwest–southeast portions of the atoll are fringed by a spur and groove system. The rim supports about 53 islets and a small island to the northeast, all of which are connected by a reef flat about 600 m wide to the north and about

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Figure 11.6 Remote-sensing images of the atolls of Ailinglaplap, Namu, and Lib. Images Ⓒ 2020-2021, Planet Labs PBC.

250 m wide to the east. The western rim is generally narrower, shallower, and more porous. Three narrow passages to the lagoon are found to the west ranging from 90 to 135 m wide and all of them are blocked to some extent by shallow reef growth that requires navigation by sight (Sailing Directions, 2017; Defense Mapping Agency chart 81723). The rim is open. The lagoon area is 410 km2. Much of the western shallow lagoon is marked by patch reefs and plateaus.

Reefs are especially common in the passages. Deep reef areas near the passages to the west and the eastern lagoon are also notable. Sand deposits up to about 200 m wide occur to the west and continue to the east, where they are variable but mostly about 90 m wide. There are several small gaps in the distribution of sand around the lagoon perimeter as well as one to the northeast over a kilometer long, another to the northwest of about the same length, and a third to

Atolls of the Marshall Islands

the southwest about 300 m long. The lagoon deeper than 15 m presents coral-rimmed pinnacles near the three western entrances, especially to the south, as well as more than 20 m near the center of the lagoon. The maximum depth of the basin is 49 m (Purdy, 2001). Lib is a diamond-shaped atoll island with a remnant lagoon 65 km northwest of Namu (Figure 11.6). The platform is 2.6 km northwest– southeast, 1.7 km southeast–northeast, and covers an area of 2.5 km 2. The deep outer reefs are up to 200 m wide to the southeast and taper to about 90 m wide. Reefs appear to be absent to the west. The shallow outer reefs occur around the island, but extend to a well-developed reef crest that occurs everywhere but to the west. The island is about 98 ha and surrounds a pond-like lagoon that covers an area of roughly 12 ha. It has a maximum depth of 3.3 m and a salinity of about 20‰–50‰ depending on rainfall, with a mean value of about 27‰. The residence time of the lagoon water has been estimated as 31–34 days. Lib was an atoll about 3,000 years ago but then became closed due to sediment deposition. With time, the lagoon likely will become freshwater as it becomes increasingly isolated from ocean exchange (lagoon data from Myhrvold et al., 2014).

Kwajalein, the largest atoll in the Marshall Islands, serves as a naval base for the Ronald Reagan Ballistic Missile Defense Test Site and is home to a ground antenna and monitoring system for GPS navigation. While the military presence is fairly obvious, the most populated island on Kwajalein is Ebeye to the southeast. Here, about 15,000 civilian Marshallese are found on less than a third of a km2, a density of about 46,900 people per km2. The atoll takes the form of a pointed boomerang, with an elongated depressed base to the southwest (Figure 11.7). The platform length is about 139 km northwest–southeast with a waypoint to the atoll center and is about 34 km at its widest north–southwest. The atoll covers an area of 2,477 km2 and is surrounded by deep and shallow outer reefs that are up to 1.2 km wide to the east, 775 m wide to the north, 400 m to the northwest, and 250 m wide to the southwest. The peninsula-like northwestern platform displays a spur and groove system. The rim is generally about 250–500 m wide and supports at least 90 islets and 3 islands including 1.8-km2 Roi Namur to the north and 3.1-km2 Kwajalein Island to the north, both of which have airstrips, and 1.1-km2 Ebadon to the west. Multiple passes and channels occur around the atoll, most of which are found to the

Figure 11.7 Remote-sensing image of Kwajalein Atoll. Images Ⓒ 2019, Planet Labs PBC.

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west. The northwest rim is mostly submerged and extends 28 km. Short sections of the rim are submerged to the east and the rim is open. The lagoon area is about 2,242 km2, making it one of the largest in the world. The shallows do not display significant reef development and sand is deposited in patches to the east, north, and west toward Ebadon Island, where they can be more than 700 m wide. Sand accumulates least to the south side of the lagoon. Coral-rimmed pinnacles are common

along the south and northwest sides of the basin, particularly in the vicinity of channels and passes. The deepest part of the lagoon also displays multiple coral-rimmed pinnacles, primarily to the center and west. Most of the lagoon is 40–50 m deep (Defense Mapping Agency chart 81715) and the maximum lagoon depth is 61 m (Purdy, 2001). Lae is a small ovotriangular atoll located 75 km southwest of Kwajalein (Figure 11.8). The platform is 8 km from the east-facing apex to the west-facing

Figure 11.8 Remote-sensing images of the atolls of Lae, Ujae, and Wotho. Images Ⓒ 2020-2021, Planet Labs PBC.

Atolls of the Marshall Islands

base and is about 6.7 km parallel to the base at its widest. The platform covers an area of 28.7 km2 and is surrounded by deep and shallow outer reefs that are up to 380 m wide to the east, about 150 m wide to the north, 100 m wide to the south, and about 285 m wide to the west. The outer reefs are apparently least developed to the northwest. Satellite imagery from the east and part of the northeast reveals spur and groove formations. The rim is up to about 550 m wide and supports 14 islets, the largest of which is Lae to the southeast. Lae also includes the highest boulder ridge in the Marshall Islands, peaking at a towering 5.5 m above sea level. The rim has a single west-facing channel that is shallow and about 90 m wide and is suitable only for small vessels (Sailing Directions, 2017). The rim is open. The lagoon area is 17.1 km 2 . The shallows form a sand apron up to about 390 m wide to the southeast in the lee of Lae Island. Plateaus mark the boundary between the outer edge of the shallows and the lagoon slope northwest–southeast. Reefs are very well represented around most of the shallow lagoon slope, reaching 280 to more than 300 m wide toward the west and in the lee of Lae in the southeast. The lagoon is only partially charted and the maximum depth from Defense Mapping Agency 81557 is 75 m. Ujae is a spindle-shaped atoll located about 48 km west of Lae (Figure 11.8). The platform is 43 km northwest–southeast, about 13 km at its widest west– northeast, and covers an area of 265 km2. The deep and shallow outer reefs are about 280 m wide to the east, more variable to the west, in part due to reef growth that spreads across areas of the reef flat and into the shallow lagoon. A spur and groove system is found northeast–northwest, similar to Namu. The rim supports 12 islets including 92-ha Ujae to the southeast, where an airstrip is located. The reef flat is 300–600 m wide to the east. The rim to the west is shallower and is the site of the main channel to the lagoon, Bock Passage. This channel is bisected by a triangular area of reef flat at the lagoon entrance. It is useable by small craft with local knowledge, but only under favorable conditions (Sailing Directions, 2017). In addition, several sections of the western rim are submerged and may be considered as boat passages according to old charts (e.g., Witt, 1881). The rim is open. The lagoon area is 209 km2 and its shallows to the west develop patch reefs sporadically, some of which cannot be separated from growth on the reef crest. Fewer patch reefs occur on the shallows to the east. A sand apron up to a kilometer wide occurs in the shallows to the northwest as well as on the lagoon slope. It extends clockwise along the east to

the channel on the west side of the basin, as well as northwest–west for about 6 km. A 6.4-km gap occurs farther south along the western basin, where sand deposits are intermittent. More than 40 pinnacles emerge from the deep basin, most of them near the lagoon center where they are rimmed by reef. We do not have information concerning the depth of the lagoon, but perhaps unrelatedly, the Marshallese people have long considered that Ujae is the home of demons (Kelin, 2003). Wotho is 127 km northeast of Ujae, where it takes the forms of a warped triangle (or a cat’s back), concave to the east with ear-like islands projecting to the north (Figure 11.8). The platform is 20 km northwest–southeast, 11.4 km west–east across the base, and covers an area of 134.1 km2. The platform is surrounded by deep and shallow outer reefs up to 350–400 m wide north and east and variable but somewhat narrower to the west, where outer reef growth continues into shallow waters. A few patch reef areas are also found on the eastern reef flats. The rim supports at least 18 wooded islets, several sand cays, and two islands. These include Wotho to the northeast, where a dirt airstrip is located, and 1.1km2 Kabben Island to the south. The largest islands are littered with boulders, likely tossed up during storms (Maragos, 1994). The rim is up to 1.2 km wide northeast and about 300 m wide to the east, becoming narrower and shallower toward the southeast. The western rim is about 15 km long, and except for a shallow section of about 6.5 km, most of which is submerged. There are two passes to the west with depths of 9–18 m (Sailing Directions, 2017). The rim is open. The lagoon area is 97.8 km2. A few patch reefs occur in the shallow lagoon, but the dominant features are the shallow reach of the plateaus that become numerous on the lagoon margins and the sand apron that covers most of it. The apron reaches more than 800 m wide to the northwest and circumscribes all but a 4-km section of the northwest lagoon. The deepest part of the lagoon presents numerous reefrimmed pinnacles and platforms that rise toward the surface. Limited information is available concerning the depths in the lagoon, but it appears to be at least 37 m deep (Maragos, 1994). Rongerik is an ovotriangular atoll located about 194 km northeast of Wotho (Figure 11.9). The platform is 18.1 km from the southwest-facing apex to the middle of the north/northeast-facing base, 18.7 km west–east at an angle to the base, where it is widest and covers an area of 202 km2. Reef development continues around all sides of the atoll where it is generally 375–500 m wide, but expands to the southeast to more than a kilometer.

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Figure 11.9 Remote-sensing images of the atolls of Rongerik, Rongelap, and Ailinginae. Images Ⓒ 2020, Planet Labs PBC.

The rim is up to 1.5 km wide to the northeast and supports at least 13 islets, mostly northeast–southeast. About 8 km of the reef flat to the west is submerged. A single entrance occurs to the north that shows about 9 m depth but is marked for small craft only. The entrance to the southeast is Eniwetok Pass just south of 38-ha islet by the same name and shows depths of at least 18 m. Another unmarked opening (Bock Pass) that is 8.6 km from the southernmost point of the atoll is located on the submerged western rim and is at least 22 m deep. Two channels to the north are for small craft only (Sailing Directions, 2017; Defense Mapping Agency chart 81557). The rim is open. The lagoon area is 159 km2 including a 4 km2 triangular area of deep and shallow lagoon to the southeast that is semi-isolated from the main lagoon by reef growth. This small area develops patch reefs surrounded by 2.7 km2 of sand, while the remaining area to the west and south is occupied by seagrass. However, most of the shallows are occupied by sand deposits up to a kilometer wide and small

patch reefs. Most of the lagoon is 40–44 m deep, where a few platforms and pinnacles are found to the northwest and southeast associated with the deeper reaches of the slope. The deepest part of the lagoon is 49 m (Purdy, 2001; Defense Mapping Agency chart 81557). Rongelap is a polygonal atoll with a northeast-facing neck that reaches within 33 km of Rongerik (Figure 11.9). The platform is 51 km northeast–southwest, 43.1 km northwest–southeast at its widest, and covers an area of 1,122 km2. The deep and shallow outer reefs are 270–380 m wide north–east and are narrower and more variable to the west. The shallow reef extends to the reef crest where it is best developed clockwise west–south, and available imagery suggests that most, if not all of this atoll, has developed a spur and groove system. The rim supports at least 55 sparsely wooded islets and one island, 2.4km2 Rongelap to the southeast, where there is an airstrip. Nine entrances to the lagoon penetrate the rim, including three to the northeast, a submerged

Atolls of the Marshall Islands

rim area to the south, three to the southwest, one to the west, and a boat channel to the north. South Pass with depths of 33–51 m is the most readily navigable. Enybarbar Pass to the west carries 11–15 m depths and Northeast Pass is at least 18 m deep (Sailing Directions, 2017; Defense Mapping Agency chart 81564 charted only to the eastern lagoon). However, the western pass is unusual because lagoon water is constantly flowing oceanward from it, and it is likely a principal exit for wind-driven water to the western side of the lagoon (Von Arx, 1954). The rim is open. The lagoon area is 1,013 km 2 and its volume is ~35 km 3 (Von Arx, 1954). The shallows are interrupted or rimmed by small ridges and a few small patch reefs, but a sand apron, typically 300 m wide but up to 650 m wide to the southwest, is the most prominent feature. The apron extends to include the lagoon slope to the northeast–southwest as well. The lagoon slope to the northeast develops eleven reef-rimmed pinnacles (depths not charted). Other pinnacles emanating from the deep lagoon basin are scattered throughout, where the maximum depth is 62 m (Purdy, 2001). Ailinginae is about 14 km southwest of Rongerik and forms a tube-like shape with a rounded end facing east and a flat end facing west (Figure 11.9). The platform is 27 km west–east, 9.3 km at its widest, and covers an area of 160.0 km2. The platform is surrounded by deep and shallow outer reefs that are typically about 160 m wide to the east but extend to nearly 500 m to the northwest and 400 m to the southeast corners. Points along the northern platform show spur and groove development that is elsewhere obscured by surf on available imagery. The southern and western platform areas are narrower and more variable. The rim is a narrow reef flat about 300 m wide that is the base for at least 27 tree and bush-covered islets, most of which are to the south and southeast. The northern rim is without islands and is mostly 600–800 m wide. Two passes are found to the western side of the southern rim separated by a single islet. The westernmost, Mogiri Pass, is about 1,200 m wide and at least 7 m deep; the one to the east is encumbered by rocks and is for use only by small craft with local knowledge (Sailing Directions, 2017). Both entrances show broad areas of reef growth near the openings. The rim is open. The lagoon area is 115.3 km2. The shallows and slope develop a sand apron up to 1.5 km wide to the west and about 700 m wide to the southeast. The apron is continuous except in the passes. Small patch reefs and seagrass occur sporadically in the shallows to the north. Several reef-rimmed pinnacles rise from the deep basin or the deep slope, especially to the

east, where at least four of them reach the surface. Several more are found to the western lagoon. The maximum lagoon depth is 31 m (Purdy, 2001). Bikini Atoll is located about 88 km northwest of Ailinginae and it forms a warped box-like shape (Figure 11.10). Bikini became the name for the small two-piece beach apparel due to the efforts of a French clothing designer who named it after the atoll a few days after nuclear testing began, hoping that his design would be as explosive. However, the actual name of the atoll in Marshallese was ‘Pikinni’ meaning ‘coconut place.’ Bikini is about 40 km west–east, 26 km through the center at its widest, and covers an area of 729 km2. The platform is surrounded by deep and shallow outer reefs that are typically 300–400 m wide north–east, variable to the south and narrow, about 150 m wide to the northwest. A well-developed spur and groove system can be seen on the perimeter of most of Bikini. Their presence all around the atoll was noted by Munk and Sargent (1954) who found them typically at depths of 10–15 m, rising on the reef slope from there to the surf zone, where they either ended abruptly or continued as surge channels, or tunnels ending with ‘blowholes.’ The rim supports at least 16 islets and two islands including 2.3-km2 Bikini to the northeast and Enyu to the southeast, where an airstrip is located. The reef flats are mostly 300 m wide to the east, up to 2 km at to the north, narrowing to about 1.2 km to the northwest, where they display some unnaturally scalloped edges (see below). The southern margin is about 30 km long and, except for two islands that account for about 3.4 km, that part of the rim is submerged. Reef growth on the flats occurs seaward of and adjacent to the eastern islands. The lagoon entrance to the west of these two islands (Enerik Pass) is at least 53 m deep. Another just west of Enyu Island is about 1,600 m wide and at least 15 m deep. The reef flats become segmentally arranged to the southwest, where there are seven additional openings to the lagoon that can be used with caution (Sailing Directions, 2017). The rim is open. The lagoon area is 633 km2 and its volume is ~21 km3. The volume renewal time is 39 days, but that estimate is only during the winter trade wind season; the summer renewal time is estimated to be about twice the wintertime rate (Von Arx, 1954). The shallow lagoon is flanked by reef growth to the north and around the islands to the south. Less reef growth occurs to the east and west. A sand apron up to 800 m wide occurs to the west and is reduced to about 600 m wide to the east and north. The sand apron is weakly developed to the south and there are breaks, where channels and submerged reef occur. About 27 reef-rimmed pinnacles and platforms, some up to 750

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Figure 11.10 Remote-sensing images of Bikini Atoll. Images Ⓒ 2021, Planet Labs PBC.

m wide that are reef-rimmed, some capped with sand, occur in the middle and western lagoon. Other pinnacles too deep to be coded also occur here. The maximum depth is 60 m (Purdy, 2001). A survey of Bikini’s coral fauna 50 years after nuclear testing found that 28 species appear to have disappeared. Of these losses, 16 species were obligate lagoonal specialists and 12 were found more widely. However, 12 species were recorded from Bikini for the first time (Richards et al., 2008). Enewetak is an annular atoll with a flattened edge to the northwest located 306 km west of Bikini (Figure 11.11). The platform is about 40 km north– south, 34 km at its widest west–east, and covers an area of 1,051 km2. The deep outer reefs are up to 650 m wide to the east, more typically about 400 m wide. Outer reefs to the west are variable and are very narrow to the northwest where they are about 120–190 m wide compared with the east where they are more commonly 400–570 m wide including deep uncoded reefs. In general, branched stony corals are found to 60 m, where the slope is about 45°. Plate-like corals become more common to 90 m, where the slope is 60° or more, and may persist to depths of 100 m or more along with Halimeda and other calcareous algae. On the leeward side to the southwest, the shelf break is more rapid, increasing to 75°–90° below depths of just a few meters (Colin et al., 1986). Shallow outer reefs occur all around the platform where they grow without a visible boundary into the lagoon

or are found as patches associated with islets. Satellite imagery shows well-developed spur and groove systems around much of the atoll. The reef flat consists of a rocky pavement with a poorly developed coralline algae extending seaward and rubble extending toward the lagoon (Colin, 1987). The rim supports at least 39 vegetated islets and 1.4-km2 Enewetak Island to the southeast, where there is an airstrip. All the terrestrial features are found northwest–southeast, where the reef flats are up to 1.7 km to the northwest, up to 1 km to the northeast, and variable to the west but no more than 550 m wide. No islets are found to the northwestern reef flats and the 36-km-long western rim is partially submerged and the south rim is completely submerged for nearly 10 km. Two of the main entrances to the lagoon ranging from about 8 to 14 m deep are found to the west. A third opening found to the southeast is relatively narrow but 8.5 m deep (Sailing Directions, 2017). The lagoon area is 947 km2. Circulation is dominated by a wind-driven surface flow over the eastern (windward) reefs and renewal time is estimated to be 28 days (Atkinson et al., 1981). The shallow lagoon is widest to the west, where a sand apron 2.8 km wide is found and extends more narrowly all around, except for its absence to the south and southeast in the vicinity of the passes and submerged reef. The apron continues onto the lagoon slope as do reefs to the

Atolls of the Marshall Islands

Figure 11.11 Remote-sensing images of Enewetak and Ujelang atolls. Images Ⓒ 2021, Planet Labs PBC. On the Enewetak image, a yellow arrow points to a large crater near Enewetak’s north cape. This was Elugelab Islet, but it is now the Ivy Mike crater, 1,900 m across and 50 m deep. Immediately adjacent to Ivy Mike is the Castle Nectar test which formed a smaller crater. The remains of the two tests together form an unequal figure eight (yellow arrow). Radiologically contaminated soil was bulldozed from Enewetak and buried in a now-leaking concrete tomb. A massive concrete lid called the Runit Dome sits about 8 m above sea level and is easily seen in satellite imagery (white arrow).

southwest, east–southeast, and in smaller areas to the north. About 158 patch reefs reach the lagoon surface, some of them forming platforms up to 850 m wide; an estimated 3,000 pinnacle reefs rise to the subsurface from the bottom of the basin, where depths of 20–55 m are found and the maximum depth is 62 m (Purdy, 2001; Maragos, 2011; Sailing Directions, 2017). The deeper regions of the lagoon bottom vary.

Some areas include Halimeda-dominated sand with benthic algae including live Halimeda among other non-calcareous macroalgae. Burrowing callianassid shrimp were common in more finely grained sand. Hard bottom areas included some with nearly 100% coral coverage, including numerous mushroom corals, to other areas, where the rocky substrate appeared to be unoccupied (Colin, 1986).

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The last atoll in the Ratak Chain is Ujelang, which resembles a bow saw and lies 216 km southwest of Enewetak (Figure 11.11). The platform is 23.5 km across the blade to the southwest, 5.8 km at its widest bisecting the blade southwest–northeast, and covers an area of 104.9 km2. The platform is surrounded by outer reefs that are best developed east–south, where they are up to about 220–460 m wide, then narrow to about 125 m wide or less northwest–west. A small area of uncoded deep reef occurs to the southwest corner that extends that reef area by 170 m. A well-developed spur and groove system surrounds this atoll (see Blanchon, 2011). The rim supports at least 27 vegetated islets including 98-ha Ujelang to the south–southeast. The reef flats are up to about a kilometer wide around the western side of the atoll, narrowing to about half that to the east. Reef growth on the flats is largely confined to the narrow northeastern and southeastern tip. Two passages to the lagoon occur to the southwest. Wide Pass as it is called is farthest from the western end of Ujelang Islet and is about 480 m wide but less than 5 m deep. Narrow Pass is closer to Ujelang and is narrower, unmarked, and difficult to navigate (Sailing Directions, 2017). Both are better described as channels and display reef growth within them or across the entrances. The rim is open. The lagoon area covers 71.2 km2. The slope to the west develops a continuous ribbon reef about 70 m wide and continues along the blade-like south rim, where the ribbons become narrow and discontinuous until they are intercepted by outer reef growth that spreads into the lagoon. This occurs through low spots on the southern rim as well as the two channels. Discontinuous sand apron deposits are found around much of the lagoon and occur continuously around the easternmost 11 km, where they are more than 600 m wide and cover the shallow lagoon as well as the slope in some areas. Reef-rimmed pinnacles, platforms, and an arcuate ridge project from the uncoded deep lagoon, especially to the southeast, on and near the bottom of the lagoon slope. Much of the deep lagoon is 38–44 m deep, and the maximum depth is 49 m (Purdy, 2001; Defense Mapping Agency chart 81511).

A radiological vignette In February 1946, after World War II had concluded, the U.S. government asked the 167 men, women, and children of Bikini Atoll to relocate voluntarily and temporarily so that atomic testing could begin ‘for the good of mankind and to end all world wars.’ The families were given Rongerik as their new home even though it was 200 km away, one-sixth the size

of Bikini and had little food or freshwater—the most likely reason why no one lived there. Residents of Enewetak were also relocated for the same purpose but were placed on the much smaller and resource poor Ujelang Atoll. Between 1946 and 1958, 43 nuclear tests were carried out on Enewetak and 24 on Bikini. Tests were conducted in the air, the lagoon, in water tanks, on steel platforms, and on islands some of which were vaporized and no longer exist. Because so many tests were conducted on Enewetak, it became the focus for cleanup in the early 1970s. After 3 years, radioactive concrete debris, military equipment, coconut trees, and soil estimated to be about 84,000 m3 were collected, encased in concrete, and placed in a bomb crater on Runit Island. After decades of near starvation, the Enewetak people were allowed to return, but the cleanup was ineffective and left the northern half of the islands too contaminated for full use (Pevec, 2006). The massive concrete lid called the Runit Dome covers the now leaking encasement (Gerrard, 2015) which sits about 8 m above sea level and is easily seen in satellite imagery of Enewetak (Figure 11.11, white arrow). In addition, the large crater near the north cape was at one time Elugelab Islet. It is now the Ivy Mike crater, 1,900 m across and 50 m deep. Immediately adjacent to Ivy Mike is the Castle Nectar test which formed a smaller crater. The remains of the two tests together form an unequal figure eight (Figure 11.11, yellow arrow). Even though fewer nuclear tests were conducted on Bikini than on Enewetak, the Bikini tests were notable for other reasons. The Operation Castle tests were a series of six high-yield thermonuclear detonations, the first of which was Bravo conducted in early 1954. This thermonuclear (hydrogen) bomb was the largest device ever detonated in the atmosphere, equivalent to 15,000 kilotons of TNT, more than 800 times the power of the device dropped on Nagasaki. The crater produced by Bravo is 2,000 m in diameter and 76 m deep, and it is easily seen on satellite images of the atoll. Prior to the detonation, the blast was calculated to be 2.5 times smaller, but when the device was detonated, a cloud of coral debris rose to an altitude of 4,000 m and within a few minutes spread to about 100,000 m wide. But the wind had changed directions at that altitude, and instead of the fallout being carried west as expected, it drifted to the east, falling like a snow to people who had never seen it. The people of Rongelap (where Bikinians had been relocated) received radiation doses comparable to Japanese atomic bomb survivors (Blades and Siracusa, 2014). The contaminants, including high levels of radioactive iodine, cesium, strontium, americium, and plutonium, rained

Atolls of the Marshall Islands

down on Rongerik, Ailinginae, and even on Bikar more than 500 km away from ground zero (Corcho-Alvarado et al., 2021). To this day, bomb-produced americium and plutonium are present in the soils of these atolls, and while Bikar has always been uninhabited, thousands of Bikini islanders live elsewhere in the Marshall Islands due to persistent levels of contamination on their home atoll. Like Bikini, fallout also has contaminated Ailinginae, Rongerik, and Ujelang, which are currently uninhabited members of the northern Ralik Chain.

The Ratak Chain of atolls The southernmost of the Ratak Chain is Mili, an atoll shaped like the Big Dipper constellation (Figure 11.12). The platform is about 63 km northwest–southeast through the handle, 45.3 km southwest–northeast, and covers an area of 915 km2. We include the handle in this measurement even though the end of it is an area of about 15 km2 that includes a rubble strewn reef flat with about 20 vegetated islets surrounding a 2.3-km2 shallow lagoon, large parts of which dry at low tide

(Spennemann, 1996). This area is also called Nadikdik Atoll and is sometimes referred to as Knox Atoll, a secondary lagoon of Mili (Wiens, 1962), as is Toas Atoll and the larger Elato in the Caroline group (Chapter 12). Nadikdik is separated from Mili by a submerged reef, mostly less than 15 m deep and 3.6 km long that constitutes the proximal part of the handle. The Mili platform is surrounded by outer reefs that are generally wider but variable to the north, ranging from 400–500 m wide north–east compared with 60–120 m wide in the south. Spur and groove systems surround the perimeter of this atoll forming finger-like projections to the east while becoming wide or plate-like to the west. Shallow outer reefs continue their growth onto the reef flats in numerous locations. The rim is composed of well over 100 islets and 18-km-long Enejet Island to the south, as well as Mili to the southwest, a triangular, 3.2-km2 island with an airstrip. There are ten entrances to the lagoon including nine to the north and one to the southwest. The pass farthest to the west on the north side (Tokowa Pass) is reported to be the best, displaying widths of more than 300 m and depths of 10–35 m.

Figure 11.12 Remote-sensing images of the atolls of Mili, Arno, and Majuro. Images Ⓒ 2020-2021, Planet Labs PBC.

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Others are also navigable, but several are restricted to small craft (Sailing Directions, 2017; Defense Mapping Agency chart 81796). The rim is open. The lagoon area is about 780 km2 (782 km2 with Nadikdik lagoon). Sand patches, some 1.2 km wide, appear on the slope and deep lagoon to the west and a few small areas to the northwest and southwest. Sand also covers most of the lagoon of Nadikdik, where no reefs are coded. Significant numbers of ribbon-like reefs and large patch reefs occur on the lagoon slope. Additional sand- and coral-rimmed pinnacles and platforms rise from the lagoon slope especially in the vicinity of the passes and to the south and southwest of the lagoon basin. About 15 uncoded pinnacles rise from the western lagoon slope. A few more rise from the deep lagoon, where the maximum depth is 71 m (Purdy, 2001). Arno Atoll is located about 79 km north of Mili, where it takes the form of a double-drawstring purse (Figure 11.12) referred to as ‘horns’ by Wells (1951). The platform is 39.9 km southwest through the northeastern horn and 35.7 km south through the northern horn. The platform area is 460 km2. Outer reefs surround the platform and are best developed north– northeast, where they are subject to heavy swell and are up to 3.2 km wide to the tip of the northern horn, although they are more typically about 500 m wide (Sailing Directions, 2017). Reefs in the lee to the south and west are about 125–250 m wide. The rim includes more than 80 islands and islets. Three islands make up a large part of the rim northeast–west. Arno Island to the west is up to a kilometer wide, but follows the irregular rim to the southwest and south for 32.5 km and becomes very narrow. Older maps show Arno separate from Ine and another named island to the southwest–south (Hatheway, 1953), but they now appear to be joined with Arno. Similarly, Woen Island to the east is 10 km long, longer than shown on charts, as are Tinak and Langau Islands on the rim of the northeastern horn which now appear to be joined as one island 16 km long. Narrow sand dunes have been reported in the lee of these islands, and storm-driven boulder ramparts and ridges have been noted between them (Wells, 1951). Fosberg (1990) confirmed these islands to be joined by sand and gravel ridges. Indeed, drifting of these dunes appears to have cut off the horns from the main lagoon. A ring of mangroves flanks the protected lagoon side of both horns as well as the lagoon side of the island interiors, but mangroves are poorly developed here as in most of the Marshall Island group (Hatheway, 1953; Fosberg, 1990). The horn to the northeast is rimmed by a rocky reef flat with a single island (Tiniak–Langau described above) extending northwest–southwest

clockwise. The western rim is a reef flat with a lagoon entrance about 60 m wide in the middle. The rim partly encloses a shallow 10.0-km2 secondary lagoon. The lagoon slope is flanked on all sides except to the northeast by reefs up to 130 m wide. The uncoded deep lagoon contains four pinnacles that rise from a maximum depth of 34 m. The rim structure of the northern horn is a bit different and includes a scattering of 11 islets and the crescentic part of 1.2-km2 Bikarej Island to the south and southeast. The northern end of the horn is invaginated with reef growth and the remaining rim is shallow and about 300 m wide. The rim to the east is low and north of Bikaerej Island is connected to the sea (Fosberg, 1990) making it semi-enclosed. The secondary lagoon to the north is 4.4 km2 and develops sand deposits up to 350 m wide to the west and south in the shallows and on the slope. Sand to the east is narrow and discontinuous. Most of the coded lagoon is described as a lagoon slope and ribbon-like reefs flank much of the eastern side. The slope toward the center also displays reef areas up to 18 ha in extent. About 17 pinnacles, some of them reef-rimmed, project from the lagoon basin as do two platform areas up to 175 m wide. About 27% of the secondary lagoon is coded as deeper than 15 m, but we do not have depth data for this lagoon. The rim surrounding the main lagoon is punctuated by two passes to the northeast that are at least 15 m deep (Sailing Directions, 2017). The main lagoon area is 343 km2 and its rim is open. The shallow regions of the lagoon are found primarily to the west, where a sand apron more than 800 m wide occurs and narrows as it extends about 8.7 km to the northeast and about 7 km to the south. The lagoon east of these points is coded as a slope, where sand accumulates in patches. Reef-rimmed pinnacles and plateaus rise from the lagoon’s deeper reaches, many of which reach the surface and become especially prominent near the northeastern rim at depths of about 30 m, as well as low points in the rim and near the passes. Several reef-rimmed pinnacles are also found in the deep uncoded lagoon to the southeast at depths of about 40 m. Oval platforms that reach the surface and are up to 575 m wide are found to the northwest, where depths are about 40–55 m as they are generally in the western lagoon. The maximum lagoon depth is 60 m and is found toward the western center. The eastern lagoon by contrast is generally 25–40 m deep (National Geospatial Intelligence Agency chart 81791) and reef-rimmed pinnacles are found toward the south basin. Majuro is a slipper-shaped atoll located 18 km west of Arno (Figure 11.12). The platform is 39.7 km west–east (mid-counter to toe), 12.5 km north–south

Atolls of the Marshall Islands

at its widest, and covers an area of 369 km2 including a small area of uncoded reef to the northeast. The outer reefs are about 300–600 m wide to the north, where they are best developed and exposed to the northeasterly trade winds, and narrow to about 100–195 m wide to the west and east. The reefs to the south are narrower still and intermittent in their extent. Majuro is the capital of the Marshall Islands. Its population is about 28,000 (2021) and is concentrated to the southeastern part of the atoll, but the urbanized areas extend about 4.5 km to the north and about 8 km to the south, where an airstrip is located. The rim here is about 400–600 m wide and the population density is roughly 8,500 people per km2 (Ford, 2012). The urbanized area of Majuro covers about 24 km2 of ocean facing shoreline and about 26 km2 of lagoon shoreline both of which have been significantly altered by urbanization, shoreline armoring, causeway construction, landfill, and waste disposal (Pinca et al., 2005; Osawa et al., 2010; Ford, 2012). Similar challenges beset South Tarawa (Chapter 9) and other highly populated atolls. Majuro’s rim is closed to the south by Majuro Island. The east, the north–northeast, and the northwest are also closed by reef flat and about 50 islets. The rim to the west is a reef flat about a kilometer wide, where there are no terrestrial features. Most of the northern rim is submerged and two passes are found over a distance of about 8 km with Calalin Islet in the middle. The main entrance is Calalin Pass to the west of the islet and is 38–45 m deep, but it has ridge and shoal hazards within it. The channel to the east is narrower and can be used by small craft. (Sailing Directions, 2017; National Geospatial Intelligence Agency chart 81782). The rim is open. The lagoon area is 308 km2. Most of the coded lagoon is a peripheral slope, where there is a considerable development of reef to the northwest and southwest and to the north near the lagoon entrances. Uncoded, elongate pinnacles occur to the eastern lagoon along with a few reef-rimmed pinnacles in the center; however, most of the reef pinnacles occur to the southwest and rise from deep slope, and to the north projecting from the deep lagoon. Depths in both areas and the lagoon in general are 30–45 m and the maximum depth is 82 m (National Geospatial Intelligence Agency chart 81782; Purdy, 2001). Wind effectively mixes the lagoon water in about two weeks (Kraines et al. 1999). Aur atoll is 101 km north–northeast of Majuro, where it takes the shape of a distorted parallelogram (Figure 11.13). The platform is 31.2 km northwest– southeast, 20.2 km west–northeast, and covers an area of 278 km2. The deep and shallow outer reefs

are about 150–250 m wide around much of the perimeter. The shallow outer reefs spread to the reef crest to the north and east as well as to the reef flats, especially to the southwest. The rim includes 34 wooded islets including 98-ha Aur to the southeast and the 1.4-km2 island of Tabal to the east, both of which include unpaved airstrips. The connecting reef flats are about 450 m wide to the north and east, narrowing and becoming shallower to the west and southwest, where only two islets are supported. A spur and groove system is visible to the south of Tabal and around Aur to the southwest, but is otherwise unclear from the available imagery. Four entrances through the reef occur to the west, three of which are quite narrow but are passes about 12–15 m deep. The fourth is the northernmost of these called West Opening and is about 480 m wide and at least 16 m deep. A fifth pass occurs to the southwest, and it is about 90 m wide and about 9 m deep (Sailing Directions, 2017). The rim is open. The lagoon area is 237 km2. The shallows and adjoining plateaus develop a sand apron up to 325 m wide to the west that becomes narrower to the north and south and are discontinuous to the east and around the five passes. Patch reefs occur in the lagoon shallows to the east of the southwest passage but most occur as ribbons around the lagoon perimeter. About 12 reef-rimmed pinnacles rise from the deep lagoon slope and basin along with smaller and scattered reef areas to the center east. The northern half of the lagoon is about 42–45 m deep, whereas the southern lagoon is more variable but generally deeper, 45–72 m, including the maximum depth of 82 m (National Geospatial Intelligence Agency chart 81771; Purdy, 2001). Maloelap is about 18 km northeast of Aur Atoll, where it assumes the form of a warped triangle with a depressed base facing southeast (Figure 11.13). The platform is about 61 km northwest–southeast, 40 km east–west (islet to islet), and covers an area of 1,028 km2. The deep and shallow outer reefs are 250– 325 m wide clockwise north–south and generally 200 m wide to the west. The shallow outer reefs continue to the reef crest north–south and onto the reef flats, particularly to the west. The east– southeast and western rims and the part of the northeast develop spur and groove systems making it likely that this atoll is surrounded by them even though the totality of the imagery is insufficient to support this indication. The reef flats extend 400 to more than 600 m wide northwest–east and 300–500 m wide to the west, where they are discontinuous. The rim supports at least 62 wooded islets, including 82-ha Arik to the southeast, and two islands, including 2.2 km2 Kaben to the northwest and 1.5 km2 Taroa to the east, both

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Figure 11.13 Remote-sensing images of the atolls of Aur, Maloelap, and Erikub. Images Ⓒ 2018, 2019, 2021, Planet Labs PBC.

of which have unpaved airstrips. There are ten entrances to the lagoon, all to the southwest. The main channel is the southernmost of these and is 265 m wide and more than 37 m deep albeit with cautions noted by Sailing Directions (2017). The rim is open. The lagoon area is 932 km2. Occasional patch reefs occur in the shallows along with sand deposits up

to 300 m wide near Kaben Island to the northwest. The sand continues as a narrower band around the periphery of the lagoon but with several small breaks in addition to those near the passes. The lagoon slope is coded with ribbon-like reefs about 200 m wide. At least 30 reef-rimmed pinnacles rise from the uncoded deep reef slope and the lagoon basin, where the

Atolls of the Marshall Islands

depths are mostly 40–55 m and the maximum depth is 73 m (Sailing Directions, 2017; National Geospatial Intelligence Agency chart 81771). Erikub Atoll takes an elongated oval shape and is located 84.3 km west–northwest of Maloelap (Figure 11.13). The platform is 32.6 km northwest– southeast, 12.2 km at its widest, and covers an area of 314 km2. The outer reefs are 370–625 m wide northwest–southeast where they are best developed, and about 300–500 m wide to the east. Part of the eastern deep reefs is U-shaped with the trough deeper than 15 m; some of the outer edges are likewise deeper than 15 m. The shallow outer reefs extend shoreward to the reef crest northeast–south and occasionally to the reef flats as well. A spur and groove system is visible on most of the east and part of the south, but imagery elsewhere is not suitable for assessment. The rim supports 12 islets most of them east–southwest. The rim north–southwest is partly submerged, supports two islets, and is perforated by three entrances to the lagoon, the northernmost of which is 210 m wide and at least 16 m deep (Sailing Directions, 2017). The rim is open. The lagoon area is 275 km2. The shallow and coded deep lagoon are replete with reefs up to 240 m wide to the northeast, including a 40–60 m wide border of rock and sand between the shallow and deep zones. Fewer reef areas occur to the west except near the lagoon entrances. The entrances to the northwest are shown with narrow seagrass meadows in the deep lagoon area. Sand deposits occur sinuously around the southern half of the lagoon, but deposits to the northern half, although wider in places, are discontinuous. Most of the reef-rimmed pinnacles occur near the lagoon entrances. The maximum lagoon depth is 49 m according to a limited Marshall Islands survey. Wotje Atoll is shaped like a warped rectangle 9 km north of Erikub (Figure 11.14). The platform is 49.6 km west–east through Wotje Island, about 21 km north–south at its widest to the west, and covers an area of 798 km2. The outer reefs are generally 250–300 m wide to the north and east although smaller areas can reach more than 700 m wide. Reefs to the south are often less than 100 m wide although in some areas they can extend to 200 m. The reef dimensions to the west are variable. A spur and groove system appears to be present on all sides of the atoll. The rim supports at least 65 vegetated islets including 94-ha Ormed, as well as 2.5 km2 Wotje Island to the east, where an airstrip is located. The rim to the north is about 500 m wide and somewhat wider to the east, where more than half of the islets are found. Several bands of seagrass also occur to the north, and when summed, they are at least 8 km long and up to 450

m wide. The southern rim narrows to about 300 m wide and opens to two channels near the center. The western rim is narrow and shallow with a single pass to the northwest. All three entrances to the lagoon are deep and clear (Defense Mapping Agency chart 81604; Sailing Directions, 2017). The rim is open. The lagoon area is 716 km2. The shallows and lagoon slope areas accumulate sand 550–600 m wide behind Ormed Islet and Wotje Island spreading narrowly several kilometers northwest–south but with discontinuities. Likewise, sand accumulates southwest–northwest discontinuously. A deep reef extends more than 17 km to the north of Wotje Island and continues along the northern rim after a small gap of reef patches then expands to an area of lagoon slope 800 m wide in the lee of Goat Islet (fourth islet from the northwest) and forms an arch surrounding two large pinnacles. Numerous reef-rimmed pinnacles are found rising from the uncoded deep lagoon. At least nine of these reach the surface in the form of platforms, the largest of which is 18 ha. Most of these large pinnacles rise from a depth of about 45 m. Much of the lagoon is 40–55 m deep and reaches a maximum depth of 64 m (Defense Mapping Agency chart 81604). Likiep is a fedora-shaped atoll 64 km northwest of Wotje (Figure 11.14). The platform is 45.9 km across the brim northwest–southeast, 18 km from the brim to the highest point of the crown to the southwest, and covers an area of 493 km2. The platform is surrounded by outer reefs up to 975m wide including uncoded reefs to the northeast, where they are widest. Reefs to the south and west are generally 150–425 m. A spur and groove system appears around most of this atoll. The rim supports at least 61 wooded islets and three islands. Two to the northwest are Mato and Rikuraru with a combined area of 2.5 km2. Likiep Island, 1.3 km2, is located to the southeast and houses a small settlement and an unpaved airstrip. Most of the terrestrial areas are located northwest–southeast, where the rim is closed and connected by a reef flat that is 1 km wide to the east and 1.6 km wide to the northwest. The western rim supports seven islets, the largest of which is 80-ha Kapenor that extends farthest west and forms the edge of the crown. The rim north and south of Kapnor is partially submerged and forms several channel-like entrances to the lagoon. Northwest Pass is 160 m wide and at least 16 m deep. Another one is located about 5 km south of Kepenor and is about 350 m wide and 20 m deep. The Pass to the south is well defined, 320 m wide and 16 m deep (Sailing Directions, 2017; Defense Mapping Agency chart 81587). The rim is open. The lagoon area is 413 km2. Small patch reefs occur to the west and southwest in the shallow lagoon. A sand apron is best developed to the northwest in

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Figure 11.14 Remote-sensing images of the atolls of Wotje and Likiep. Images Ⓒ 2019-2021, Planet Labs PBC.

the shallows and on the slope, where it is up to 700 m wide, but tapers dramatically to less than 100 m wide to most of the west, 50 m wide to the east, and even less for 1.6 km to the north of Likiep Island. Reefs occur on the lagoon slope to the northwest, the pass to the south, and to the southeast, where they are up to 270 m wide. Pinnacles on the lagoon slope and coded deep lagoon rise to the surface in the form of sandy and reef-rimmed pinnacles that become especially numerous to the southeastern lagoon, where

depths of 15–45 m are common. The western lagoon is somewhat deeper, where depths are commonly 24– 50 m. The maximum lagoon depth is 56 m (Purdy, 2001; Defense Mapping agency chart 81587). Ailuk Atoll is located about 79 km northeast of Likiep taking a path through Jemo, an 8-km-long, flat-topped, steep-sided table reef with a single island to the west (Maragos, 1994). Ailuk takes the form of a rounded scalene triangle with a concave western rim (Figure 11.15). The platform is 28 km

Atolls of the Marshall Islands

Figure 11.15 Remote-sensing images of the atolls of Ailuk, Taka, Utrik, Bikar, and Bokaak. Images Ⓒ 2020-2021, Planet Labs PBC.

north–south, 17.4 km at its widest across the base, and covers an area of 251 km2. The outer reefs are typically 250–300 m wide to the east, where they are best developed, and about 150 m wide to the west, although they are variable. Although the imagery to the northwest is unclear, the remainder of the atoll is surrounded by a well-developed spur and groove system. The rim is up to 1.5 km wide to the north and supports at least 51 wooded islets including 70-ha Ailuk to the south, where there is a settlement and an airstrip. All but two of the islets occur on the eastern rim north–south. The western rim north of an elbow created by Aglue Islet is punctuated by three passes

to the lagoon. The best of these is Erappu, about 7 km north of the elbow. This entrance is about 160 m wide and at least 26 m deep, although the fairway is tortuous and divided into several branches by shallow waters (Sailing Directions, 2017; Defense Mapping Agency chart 81612). The rim is open. The lagoon area is 191.6 km 2 . Patch reefs develop on the shallows primarily in and around the southwesternmost pass and continue to the northeast. Considerably, more reef development occurs on the lagoon slope in the form of both ribbons about 250 m wide and extensive patches to the east and around the entrances to the lagoon north of Aglue

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Islet. A reef sand apron occurs in the shallows and the lagoon slope up to 480 m wide to the north and narrows considerably with breaks to the east and to the northwest. Numerous coral-rimmed pinnacles and a few oval platforms up to 270 m wide rise from the deep lagoon, where depths are commonly 40–46 m. The maximum lagoon depth is 53 m (Defense Mapping Agency chart 81612; Purdy, 2001). Taka is an elongated D-shaped atoll 75 km northwest of Ailuk (Figure 11.15). The platform is 13.8 km from its north-facing base to its south-facing apex, 16.1 km across the base at its widest, and covers an area of 142.7 km2. The rim is a reef flat that supports 7 wooded islets, the largest of which is 35-ha Taka. A reef crest extends northeast–south. The platform is surrounded by an outer reef about 350–400 m wide to the north and east, where the rim is best developed. Spur and groove assessment cannot be made with existing imagery. The rim is up to 1.1 km wide to the north and expands to the northeast and the northwest, where the rim is partially submerged. The western rim is submerged. A single pass about 90 m wide and at least 8 m deep occurs to the west (Sailing Directions, 2017). The rim is open. The lagoon area is 101.8 km2. The shallows are widest to the east, where a sand deposit up to 330 m wide is found extending to the southwest. The deep lagoon projects near the surface as additional reef areas in the form of reef-rimmed pinnacles and platforms near the western pass and from the lagoon, most of which is 40–50 m deep and is 51 m at its deepest (Defense Mapping Agency chart 81616; Purdy, 2001). Utirik is Taka’s sister atoll 7 km to its east, where it forms a rounded tringle with a partially concave eastern side (Figure 11.15). The platform is 14.8 km north–south to the mid-base, 12.5 km across the base, and covers an area of 106.2 km2. The platform is surrounded by outer reefs that are 150–400 m wide northeast–south. Reefs to the northwest and west narrow to about 90–150 m then expand near the mid-west to about 760 m where the rim is submerged. wide. The reef flat is up to 1.7 km wide, develops a reef crest northeast to mid-east, and connects five vegetated islets. The 1.8-km2 island of Utirik, supports a settlement and an airstrip. All these terrestrial areas are to the southeast. An additional (sixth) 60-ha islet, Aon, occurs to the southwest. A single entrance to the lagoon occurs 3.4 km north of Aon, where it is about 150 m wide and about 5 m deep, although the fairway is studded with coral heads (Sailing Directions, 2017). The rim is open. The lagoon area is 75.0 km2. A few patch reefs occur on the shallows to the north, northwest, and

south. However, most of the reefs mapped by the Atlas are found in the vicinity of Utirik Island to the southeast, extending 5 km to its north on the slope and deep lagoon and continuing to its south following the island curve. Several reef-rimmed pinnacles also form on the adjacent deep reef slope to the southeast, as well as a few more to the lagoon center. A sand apron is present around much of the atoll north–southwest. It is up to 1.5 km wide near the northern atoll apex, but generally 200–350 m wide elsewhere. The basin depths are typically 40–48 m including the southeast sector; the maximum depth is 49 m (Defense Mapping Agency chart 81616; Purdy 2001). Bikar (Pikaar) is an atoll lying about 99 km northeast of Utirik, where it forms a polygon with a fluted northern edge, similar to a turkey wing ark shell (Figure 11.15). The platform is 12.3 km north– south, about 8 km west–east at its widest, and covers an area of 62.8 km2. The platform is surrounded by outer reefs that are about 100 m wide to the north and up to 175 m to the east, where they are best developed. The outer reefs to the west are narrow and discontinuous, but Maragos (1994) reports ten common and abundant coral genera that are found on the lee side. The shallow outer reefs blend onto the reef flats, and charts note that the shallow reefs are awash at low water. The ocean margin to the east is formed by a well-developed spur and groove system, whereas the opposite side is extant but less well developed. A reef crest occurs to the north and part of the northeast. The rim is a reef flat up to 800 m wide to the east that supports five wooded islets northeast–south. The western rim is about 225 m wide and expands to more than 500 m wide, where a narrow pass through the rim is found that is a little more than 20 m wide. Although the entrance is about 8 m deep, the inner end shoals to half that depth with many reefs in the fairway and is suitable for small craft only (Sailing Direction, 2017). More significantly, the rim is raised as much as a meter above sea level on the leeward side so that when water is pumped into the lagoon by wave activity, the lagoon level becomes higher due to limited drainage though the narrow pass to the west. This ponding effect causes the lagoon water levels to remain elevated until the highest tide on the windward side completely submerges all perimeter reefs and lagoon flats, allowing free exchange of ocean water for a limited period. This ponding also allows corals and coralline algae to flourish in overhangs even though they are above the tide level (Maragos, 1994). The rim is open. The lagoon shallows are especially broad northwest–northeast, where they appear to bear few

Atolls of the Marshall Islands

reefs. Maragos (1994) noted that the tops of the shallowest reefs are smooth pavements of living crustose coralline algae, most common in wide northwestern shallow lagoon. A welldeveloped sand apron reaches about 1.9 km wide in northwest–neast. The width of the apron west, east, and south varies from about 500 to 600 m. The apron covers the plateaus and extends to sections of a rocky reef slope. Large populations of giant clams including two Tridacna species and the small giant clam Hippopus occur in the shallow lagoon. The rocky reef slope is extensive and supports large reef areas to the west and southwest, as well as a smaller area to the east. Maragos (1994) describes the lagoon walls as being filled with ribbon reefs dominated by live coral. Most of the lagoon is 18–20 m deep. The maximum depth is 24 m (Maragos, 1994). Bokaak (Taongi) is a crescent-shaped atoll located 280 km northwest of Bikar (Figure 11.15). The platform is 26 km tip to tip following a curved path northwest–southwest. It is 10.2 km at its widest west–east and covers an area of 118.2 km2. The outer reefs are 175–275 m wide northeast–southeast, where they are best developed, and about 90–130 m wide to the west. A spur and groove system flanks the eastern rim. It is less well developed to the west (although at present, only poor imagery is available for the northwest) and it is poorly developed to the SW. The rim supports eight islets and 2.5-km2 Sibybella Island, all to the central east–southeast. The rim is continuous but for a shallow boat channel to the west that is 6–10 m wide. The sea breaks over the rim to the east and flows into the lagoon with an easterly wind. The rim is semi-closed. The lagoon area Is 92.8 km2. Patch and ribbon reefs are noted in the shallows form elongate ribbon reefs around some of the lagoon periphery to the west-northwest. Despite the reduction in the number of coral species found on Bokaak likely due to its isolation, coral communities achieve very high level of abundance due to high light levels, transparent waters, shallow depths (see below), lack of suspended sediment, and only minor wave activity. Lagoon habitats include complex three-dimensional coral-dominated environments, with many overhangs, mounds, walls, and elevated ledges. In addition, very large populations of giant clams including two Tri‑ dacna species and the small giant clam Hippopus are found throughout the lagoon similar to those found in Bikar (Maragos, 1994). In addition, the leeward lagoon reefs are elevated and cause the level of the lagoon to be raised as described on Bikar. The lagoon is shallow and depths between 7 and 13 m are common (Defense Mapping Agency chart 81626) where

numerous pinnacles project from the slope, some rimmed with reef as coded by the Atlas, but most are not. A reef ridge is also found in the slope 800–900 m west of the islets near Sibybella. The maximum depth is 15 m (Maragos, 1994).

References Ascher M 1995. Models and maps from the Marshall Islands: A case for ethnomathematics. Historia Mathematica 22: 347–370. Atkinson MJ, Smith SV, Stroup ED 1981. Circulation in Enewetak Atoll lagoon. Limnol. Oceanogr. 26: 1074–1083. Blades DM, Siracusa JM 2014. A History of U.S. Nuclear Testing and Its Influence on Nuclear Thought, 1945–1963. Rowman and Littlefield, Plymouth UK. Blanchon P 2011. Geomorphic zonation. In: Hopley D (ed.) En‑ cyclopedia of Modern Coral Reefs, Structure, Form and Process. Springer, Dordrecht, the Netherlands, pp. 469–486. CIA 2022. https://www.cia.gov/the-world-factbook/countries/ marshall-islands/ Clarke RP, Sarver D, Sims NA 1996. Some history, recent developments and prospects for black-lip pearl oyster, Pinctada margaritifera in Hawaii and Micronesia. SPC/Fisheries 26 Information Paper 39, pp. 588–596. Colin PL 1986. Benthic community distribution in the Enewetak Atoll lagoon, Marshall Islands. Bull. Mar. Sci. 38: 129–143. Colin PL 1987. The physiography of Enewetak Atoll. In: Devaney et al. (eds.) The Natural History of Enewetak Atoll 1. The Ecosys‑ tem: Environments, Biotas, and Processes. U.S. Department of Energy, Oak Ridge, TN, vol. 228, pp. 27–36. Colin PL, Devaney DM, Hillis-Colinvaux L et al. 1986. Geology and biological zonation of the reef slope, 50–360 m depth at Enewetak Atoll, Marshall Islands. Bull. Mar. Sci. 38: 111–128. Corcho-Alvarado J, Guavis C, McGinnity P et al. 2021. Assessment of residual radionuclide levels at the Bokak and Bikar Atolls in the northern Marshall Islands. Sci. Total Environ. 801: 149541. https://doi.org/10.1016/j.scitotenv.2021.149541 Crameri NJ, Ellison JC 2022. Atoll island and coastal mangrove climate change vulnerability assessment. Wetlands Ecol. Man‑ age. 11: 1–20. https://doi.org/10.1007/s11273-022-09878-0 Fischer SR 2002. A History of the Pacific Islands. Palgrave Macmillan, London. Ford MR 2012. Shoreline changes on an urban atoll in the central Pacific Ocean: Majuro Atoll, Marshall Islands. J. Coast. Ed. Res. 28: 11–22. Fosberg FR 1990. A review of the natural history of the Marshall Islands. Atoll Res. Bull. 330: 1–100. Friedlander JS, Friedlander FR, Reed FA et al. 2008. The genetic structure of Pacific islanders. PLoS Genetics 4(3): e19. https://doi.org/10.1371/annotation/cbdd11a0–4a29-4e7c9e4e-c00a184c7777 Gerrard MB 2015. America’s forgotten nuclear waste dump in the Pacific. SAIS Rev. Intl. Affairs 35: 87–97. https://scholarship. law.columbia.edu/faculty_scholarship/3056 Hatheway WH 1953. The land vegetation of Arno Atoll, Marshall Islands. Atoll Res. Bull. 16: 1–68. Holland GJ, McBride JL, Nicholls N 1988. Australian region cyclones and the greenhouse effect. In Pearman GI (ed.), Greenhouse. Planning for a Climatic Change. EJ Brill, London, pp. 438–456. Kelin DA 2003. Marshall Islands Legends and Stories. Bess Press, Honolulu. Kirsch PV 2010. Peopling of the Pacific: A holistic anthropological perspective. Ann. Revs. 39: 131–148. Kraines SB, Suzuki A, Yanagi T et al. 1999. Rapid water exchange between the lagoon and the open ocean at Majuro Atoll due to wind, waves, and tide. J. Geophys. Res. 104: 15,635–15,653. Lukas R. 2009. Pacific Ocean equatorial currents. In: Steele JH, Thorpe SA, Turekian KA (eds.) Encyclopedia of Ocean Sciences, 2nd ed. Elsevier, Boston. Mao H-L, Yoshida K 1955. Physical oceanography in the Marshall Islands area. Geol. Surv. Prof. Pap. 260-R: 645–684. Maragos JE 1994. Description of reefs and corals for the 1988 protected area survey of the northern Marshall Islands. Atoll Res. Bull. 419: 1–88.

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Maragos JE 2011. Enewetak Atoll, Marshall Islands. In: Hopley D (ed.) Encyclopedia of Modern Coral Reefs, Structure, Form and Process. Springer, Dordrecht, the Netherlands, pp. 380–391. Munk WH, Sargent MC 1954. Adjustment of Bikini Atoll to ocean waves. Geol. Surv. Prof. Pap. 260-C: 275–280. Myhrvold CL, Janny F, Nelson D et al. 2014. Holocene closure of Lib Pond, Marshall Islands. PLoS One 9(3): e90939. https:// doi.org/10.1371/journal.pone.009093 Nunn PD 2000. Environmental catastrophe in the Pacific Islands around A.D. 1300. Geoarchaeology 15: 715–740. Osawa Y, Fujita K, Umezawa Y et al. 2010. Human impacts on large benthic foraminifers near a densely populated area of Majuro Atoll, Marshall Islands. Mar. Poll. Bull. 60: 1279–1287. Pevec D 2006. The Marshall Islands nuclear claims tribunal: The claims of the Enewetak people. Denver J. Intl. Law Policy 35: 221–239. Pinca S, Beger M, Jacobson D, Keju T 2005. The state of coral reef ecosystems of the Marshall Islands. The state of coral reef ecosystems of the United States and Pacific Freely Associated States. http://archive.iwlearn.net/sprep.org/att/IRC/eCOPIES/ Countries/Marshall_Islands/1.pdf Purdy EG 2001. Supplement to Purdy EG, Winterer EL 2001. Origin of atoll lagoons. GSA Bull. 113: 837–854. Quinn TM, Saller AH 2004. Geology of Anewetak Atoll, republic of the Marshall Islands. In: Vacher HL, Quinn TM (eds.) Geology and Hydrogeology of Carbonate Islands, Developments in Sedimentology. Elsevier, Amsterdam, vol. 54, pp. 637–666.

Richards ZT, Beger M, Pinca S, Wallace CC 2008. Bikini Atoll biodiversity resilience five decades after nuclear testing. Mar. Poll. Bull. 56: 503–515. Sailing Directions 2017. Pacific Islands, pub. 126, 11th ed. National Geospatial-Intelligence Agency, Springfield VA. Shepard FP 1980. Lagoonal topography of Caroline and Marshall Islands. GSA Bull. 81: 1905–1914. Spennemann, DHR 1996. Gifts by the waves: A case of marine transport of obsidian to Nadikdik Atoll and the occurrence of other drift material in the Marshall Islands. https:// marshall.csu.edu.au/Marshalls/html/PDF_downloads/ JCReport023.pdf Spennemann, DHR, Marschner IC 1995. The association between El Niño/Southern oscillation events and typhoons in the Marshall Islands. Disasters 19: 194–197. Su B, Jin L, Underhill P et al. 2000. Polynesian origins: Insights from the Y chromosome. Proc. Nat. Acad. Sci USA 97: 8225–8228. Thaman RR 2016. Atoll of the tropical Pacific Ocean: Wetlands under threat. In: Finlayson CL, Milton G, Prentice R et al. (eds.) The Wetland Book. Springer Science+Business Media, Dordrecht, pp. 1–25. Von Arx W 1954. Circulation systems of Bikini and Rongelap lagoons. Geol. Surv. Prof. Pap. 260-B: 265–273. Wells JW 1951. Coral Reefs of Arno Atoll, Marshall Islands. Atoll Res. Bull. 9: 1–29. Wiens HJ 1962. Atoll Environment and Ecology. Yale University Press, New Haven. Witt J. 1881. Die Marschall-Gruppe, Ann. Hydrographie IX. http://marshall.csu.edu.au/Marshalls/html/atolls/histmaps/ Ujae_1881.html

The Atolls of the Caroline Islands The Caroline Islands are clustered from about 5°N to about 10°N of the equator and extend approximately from Palau in the western Pacific to about 3,000 km to the east toward the Marshall Islands in the central Pacific. In all, there are more than 600 high and low volcanic islands including those that are coral capped. Among the latter group, we include 38 atolls (Figure 12.1). The Caroline group is divided into two political entities, Palau (Belau), an archipelago to the west that comprises about 340 islands including six that are oceanic, and three atolls, Helen, Kayangel, and Velasco (Ngeruangel). The rest of the Caroline group is composed politically of four Federated States of Micronesia (FSM). While self-governing, both Palau and the FSM are aligned with the U.S. in a Compact of Free Association, as is the Marshall Islands (Chapter 11). The westernmost of the four states within the FSM is Yap, named after its four high islands, perched on the edge of the 8,500-m-deep Yap trench. There are 11 atolls within Yap State. To the east, Chuuk State is named for Chuuk (formerly Truk) Lagoon, an almost-atoll with 19 high volcanic islands surrounded

by a barrier reef located in the center of the district. In addition, there are 12 atolls in Chuuk State. Pohnpei State is named for the rugged high island of the same name that reaches an altitude of 791 m. Mangroves surround its fringes, and because of its proximity to the Intertropical Convergence Zone, a dense rainforest covers its lower elevations, and a cloud forest begins as low as 450 m (Raynor, 1995). There are nine atolls in Pohnpei State. Kosrae is the fourth and easternmost state in the FSM and is composed of a single, isolated high island that is similar to (but smaller than) the island of Pohnpei. There are no atolls in Kosrae State. The Caroline Islands represent the emergent portions of the Caroline ridge that in turn are part of a separate Caroline plate (Weissel and Anderson, 1978). Geochronological studies of the Caroline Islands have suggested a hotspot origin for the central and eastern islands. Chuuk, with ages ranging from 14.8 to 4.3 million years, Pohnpei, ages 8.7–0.92 million years, and Kosrae, ages 2–1 million years, indicate volcanism with a progressive increase in age toward the west. However, the Caroline Islands do

Figure 12.1 The Caroline Islands are clustered from about 5°N to about 10°N of the equator and extend from the north of New Guinea in the western Pacific to about 3,000 km to the east toward the Marshall Islands in the central Pacific. There are more than 600 high and low volcanic islands including some that are reef capped. We include 38 atolls within Palau and four Federated States of Micronesia states (shown in white font). The dominant flow is the North Equatorial Countercurrent that moves north and south of Palau. The North Equatorial Current acts more like a system of westward-flowing eddies than a single current, and the South Equatorial Current near the equator is generally weak.

DOI: 10.1201/9781003287339-12

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not lie in a line within range of a hotspot and instead represent a broad zone on a ridge within the Caroline plate. Additionally, the overlap in ages suggests several sites with simultaneous volcanic activity, which is unlikely the result of a single hotspot. Furthermore, it is unclear what happened to the putative hotspot after the most recent volcanic events in Kosrae (Rehman, 2013). The western Caroline Islands (Palau and Yap) appear to have had a different and more complex history. Over the past 1–2 million years, the Caroline plate has moved westward, where it has been partly consumed by the Mariana, Palau, and Yap trench systems. In addition, volcanic activity, rifting, tilting, and seafloor spreading have created rises and troughs in the Caroline ridge to the east of the Yap trench (Zhang et al., 2020). The area to the west of the trench where the island of Yap and Ngulu Atoll are situated represents the uplifted eastern part of the Philippine plate. Farther to the south, the Yap trench converges with the Palau trench forming the boundary between the Caroline plate and the Philippine plate (Fujiwara et al., 2000). As a result, uplift formed the Palau-Kyushu Ridge to the southwest which features the Palau Islands and its associated atolls. However, the ridge extends to the southwest where Helen Reef is found, 515 km from the main islands (Figure 12.1). In addition, the ridge extends more than 1,000 km to the north toward Japan, where an isolated atoll not in the Caroline group is found and will be described below.

Climate and regional oceanography Two current systems dominate this area. The westward-flowing North Equatorial Current (NEC) is important for only the northernmost of the Caroline Islands (Ulithi) at about 10°N, but those waters are deflected south near the southern Philippines where they encounter the New Guinea Countercurrent (Figure 12.1). These combined flows deflect waters to the east forming the North Equatorial Countercurrent (NECC), the primary current system for the central Pacific from about 3°N to 8°N. The NECC conducts water from the Pacific warm pool north of New Guinea eastward over most of the Caroline Islands (Figure 12.1; Wang et al., 2016; Zhou et al., 2021). Kapingamarangi Atoll at 1°N (Pohnpei State) is an outlier with respect to ocean currents but may be in the path of the northern extent of the South Equatorial Current (SEC), the mean flow of which is as far north as 3°N (e.g., Lukas, 2009). However, the SEC is thin and weak near the equator, especially to the west of 155°E. While the oceanographic

conditions near Kapingamarangi, which is just east of this longitude, are not definitive, the surface flows may favor the influence of the SEC (Figure 12.1; Kessler et al., 2003). Located just north of the equator, the Caroline Islands’ wind and rainfall patterns are shaped by the ITCZ. The dominant northeasterly winds (Figure 12.2) are associated with considerable rainfall, but which varies from an annual average of 3.0 m in Yap to 4.8 m in Pohnpei (Bailey, 2015). Rainfall and winds show considerable temporal variability, as influenced by the El Niño Southern Oscillation (Fletcher and Sussman, 2014). During El Niño phases, winds are more commonly from the west and weaker than average. Rainfall is scarce and can decrease by as much as 80% from normal. During La Niña phases, winds are easterly and stronger, and rainfall tends to increase; mean sea levels are generally higher. Tropical cyclones are also influenced by ENSO. Within El Niño phases, they can initiate within the general area of the Kosrae and the Marshall Islands to the east. In contrast, they tend to form farther west (e.g., Yap State) during La Niña stages. The net result is that western atolls encounter more tropical cyclones than those to the east. For example, Sengupta et al. (2021) suggested that between 1900 and 2017, fewer than 50 cyclones were in close proximity of Pingelap, whereas Ulithi to the west saw more than 300. Waves for much of FSM are from the northeast and less than 2 m high (Figure 12.2 shows data from Ifalik); mean wave height generally increases from 1.0 m to the east to 1.8 m to the west (Sengupta et al., 2021). Peak waves can also be driven by swell from different directions, however. Tides at Ifalik (Yap State) are dominantly diurnal (Figure 12.2). There, spring tidal range is 1.05 m and neap tidal range is 0.44 m. To the east, Pingelap has mixed diurnal–semidiurnal tides, with spring tides of 1.55 m range and neap tides of 0.41 m (Figure 12.2). To the west near Palau, tides are also mixed diurnal and semidiurnal (Wolanski et al., 2004).

The atolls of the Caroline Islands Helen Reef (Figure 12.3) is the southernmost atoll in Palau at about 3º N and it is located closer to the Bird’s Head Peninsula, West Papua, than it is to the high islands of Palau. Helen was the name of a brigantine that struck the reef in 1824 with the loss of all aboard. The platform is 25.3 km north–southwest, about 11 km at its widest west–east, and covers an area of 168.5 km2. The platform is surrounded by outer reefs that are up to 300 m wide to the south but are more commonly 125–175 m wide. These

Atolls of the Caroline Islands

Figure 12.2 Located just north of the equator, wind, and rainfall patterns here are shaped by the ITCZ. The dominant northeasterly winds exhibit considerable temporal variability, especially when influenced by the El Niño Southern Oscillation. Waves for most of the islands are from the northeast and are less than 2 m high as shown for Ifalik (Yap State). Mean wave height generally increases from the east to the west. Tides at Ifalik are dominantly diurnal with a spring tidal range of 1.05 m. To the east, Pingelap (Pohnpei State) has mixed diurnal– semidiurnal tides, with spring tides of 1.55 m range.

reefs are dominated by table-like forms of Acropora, Heliopora (blue coral), and calcareous green algae (Colin et al., 2008). The reef crest is developed on all sides but is more continuous to the east. The rim supports a single 1.5-ha islet (Helen’s Island); the rest is a continuous reef flat up to a kilometer wide with expansive reef areas interrupted to the west by a single pass about 300 m wide and 2 km long with delta-like openings to the lagoon. The pass varies from 5 to as much as 54 m deep and is flanked by reef along its length and its openings. A nearly continuous but narrow reef crest is found from the northwest to the south clockwise. The rim is open. The lagoon area is about 118 km2 and is flushed by ocean water on a rising tide over the reef flat to the east and exits the pass to the west. That opening

is also the only exit from the lagoon, and on a falling tide, it produces a net outflow for the basin (Colin et al., 2008). The shallows bear few reefs, but a sand apron more than 3 km wide occurs to the north around Helen’s Island. The apron narrows variably as it extends about 450 m to the east. The western apron near the pass reaches 600–800 m wide but is more typically about 75 m. The lagoon slope develops reef communities up to 400 m wide to the north, 225 m wide to the east, 700 m wide to the south, and around the edges of the pass to the west. Reef communities are patchy to the northwest but are up to 100 m long. However, Colin et al. (2008) report limited coral diversity here on Helen Reef’s lagoon slope. About 30 sand and reef-rimmed pinnacles rise from the lagoon deeper than 15 m, especially south

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Figure 12.3 Remote-sensing images of the atolls of Helen, Kayangel, and Velasco. Images © 2018, 2021, Planet Labs PBC.

of the western pass. The maximum lagoon depth is 60 m (Purdy, 2001). Kayangel is a small, egg-shaped atoll located about 3.5 km northeast of Palau’s rim (Figure 12.3). The platform is 7.8 km north–southeast and 4.3 km northwest–east at its widest. The platform area is 26.5 km2 including uncoded reef. The outer reefs are typically up to 550 m northeast–southwest but most of them to the west appear to be narrower, 150–170 m wide. A narrow, nearly continuous reef crest occurs to the northern half of the atoll. The rim supports four islets and one island east–southeast totaling an area of about 1.6 km2. A channel to the west was widened and deepened in the early part of this century and lies in the lee of a reef flat extension. Another narrow

channel to the south can only be navigated by small craft and only at high tide. The reef crest is best developed seaward of these land areas and extends to the northwest as well. The island to the northeast (Kayangel Island) develops a lush seagrass community with patch reefs in its lee (Colin, pers. com.). The lagoon area is 11.3 km2. The shallow regions appear to develop a small reef area to the west and northwest, and little beyond that. The shallow lagoon and part of the deep lagoon to the west develops a sand apron up to 1,000 m wide in the southwest, less west–northeast clockwise, and continues elsewhere in the shallow lagoon and inner reef flat as patches with gaps. The maximum depth of the lagoon is about 12 m (Yamano, 2002) and 4.8 km2, but while

Atolls of the Caroline Islands

it is uncoded by the Atlas, it is primarily covered by sand with a few scattered small patch reefs (Colin pers. com.). Corals in the lagoon have been described as low diversity with low coral cover, except in the vicinity of the western channel (Birkeland et al. ND). We tentatively mark this atoll as semi-closed. Velasco Reef (Figure 12.3) is located about 9 km northwest of Kayangel. The platform is 36.6 km north–south, 13.7 km at its widest through the middle, and covers an area of 363 km2 most of which is submerged and is coded as a reef outline in the Atlas. Numerous sand channels penetrate the rim, representing what was likely a uniform reef flat (Colin, 2009). The rim presently is covered with sparse but uniform fields of macroalgae and small coral heads. Dense beds of seagrass occur on the northern and western rims at depths of 15–25 m (Colin, 2018); these are too deep to be coded by the Atlas. Multiple reef-rimmed pinnacles are associated with the deepest parts of the lagoon slope, especially to the east. The western section of the lagoon slope presents fewer of them, and more than 20 occur in the basin center. The lagoon bottom in places is up to 60 m deep with calcareous algal flats throughout. Much of the lagoon is composed of calcareous sand, particularly Halimeda flakes (Colin et al., 2008; Colin, 2009) and covers an area of 197 km. The southwestern portion of the rim forms a reef flat with an area of about 26 km2, about the same size as on Kayangel. Although the flats are essentially islandless, a small rubble islet that is of considerable importance as a turtle nesting site projects only about a meter above high tide. The outer reefs are about 375–475 m wide, expanding to more than 600 m wide to the southeast. A channel about 200 m wide, 35 m deep, and about 4 km long bifurcates and enters the lagoon from the west

Before leaving the atolls of Palau, we note an isolated atoll, Parece Vela, which translates to ‘looks like a sail,’ named in 1565 by a Spanish explorer. This atoll (Figure 12.4) is perhaps more precisely described as a scalene triangle with a rounded base and is located 1,360 km north–northeast of Kayangel at about 20°N atop the Kyushu-Palau Ridge (Micronesia ­ map, Figure  11.1). Parece Vela is also called Okinotorishima and is claimed by Japan, although that claim is disputed by China, South Korea, and Taiwan. This reef lies near the center of the northwestern Pacific subtropical gyre as the NEC turns to the north and forms the Kuroshio Current. The platform is about 4.8 km west–east, 1.7 km at its widest north–south, and covers an area of 5.7 km2 including uncoded reef west–southeast clockwise. Deep outer reefs extend about 285 m to the east, where they are best developed, and narrow to about 170 m wide to the south. The outer reefs descend at a 45° angle to an undetermined depth. Shallow outer reefs are almost entirely restricted to the wide eastern part of the platform where they are dominated by members of the genus Acropora. However, a relatively low number of total coral species occur compared with other areas in the Marianas and in Palau, possibly due to its isolation and high wave energy. The rim is composed of a submerged reef flat that supports two islets. The reef crest reaches low tide level and becomes dry during spring low tides (Kayanne et al., 2012). The lagoon area is about 3.9 km2. The slope contains patch reefs up 10 m wide and 1–5 m high that are covered with corals. Those to the center and west lagoon shallows can cover 20% of the available substrate. The lagoon shallows to the north and east are covered in part by with rubble and a band of sand and gravel. However, the central lagoon covering about 80 ha is

Figure 12.4 Remote-sensing image of Parece Vela Atoll. Images © 2021, Planet Labs PBC.

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and the maximum lagoon depth is 5.5 m (Kayanne et al., 2012). The next atoll heading east from Palau is Ngulu in Yap State, located 294 km east of Velasco Reef. This atoll takes the shape of a warped triangle with an east-facing base and a truncated and depressed apex to the west (Figure  12.5). The platform is 21.6 km west–east and about 36 km parallel to the base at its widest. The platform area is 461 km2 and is surrounded by Outer reefs up to 350 m wide to the southeast. There are also well-developed reef crests and outer reef flats in the same locations. A narrow reef crest occurs to the northern half of the atoll. The rim supports 19 islets on reef flats scattered northwest–southeast clockwise with a total area of 43 ha

(Intoh, 1981), although only five of them are vegetated including three to the south. The southernmost islet is Ngulu Island flanked by 27-m-deep West Passage to its north and 19-m-deep East Passage to its southeast (GPS nautical chart AU-408147). In addition, there are two islets that compose part of the rim entering the lagoon to the northeast. The eastern rim of this atoll is immediately next to the Yap trench and is submerged. There are multiple passes and channels through the rim on this atoll and it is open. The lagoon area is 412 km2 with narrow shallows or coded lagoon slopes. More than 40 reef-rimmed pinnacles are found on the lagoon slope near the 1.8-km-wide pass to the west, but there are few of them elsewhere. The maximum lagoon depth is 60 m (Purdy, 2001).

Figure 12.5 Remote-sensing images of the atolls of Ngulu, Zohhoiiyoru, and Alithi. Images © 2019-2021, Planet Labs PBC.

Atolls of the Caroline Islands

Ulithi (Figure 12.5) is the northernmost of the Caroline atolls and is about 10°N of the equator. It lies between the paths of the NEC and the North Equatorial Countercurrent, and thus, it may be affected by currents from multiple directions. It also lies very close to the southern edge of the typhoon belt, where the strongest cyclones are most common. The atoll is located 262 km northeast of Ngulu and takes the form of a bent funnel 38 km north–south following the lagoon center and 23.8 km at its widest to the north. The platform area is 376 km2, including Falalop Island with its airfield to the northeast, separated by a 64-m-deep channel from the main body of the atoll (National Geospatial Intelligence Agency chart 81209). The platform is surrounded by Outer reefs that are best developed to the northeast, where they extend to more than 800 m. The rim supports more than 30 islets on about 20 sections of reef flat. The largest islet is 85-ha Falalop. Fourteen channels lead to the lagoon from multiple directions and range from 1 to 15 m deep. The deeper channels are to the northeast. The lagoon is open. The lagoon area is 313 km2. The shallow areas are flanked by reef development and a sand deposit more than 400 m wide occurs north–northeast, becomes disjunct, and then narrower along the southeast– southwest lagoon margin. Scattered reef-rimmed pinnacles occur to the north and south lagoon, most of which are deeper than 15 m. From the chart, the mean depth is about 32 m and the maximum depth of 47 m occurs to the north. Zohhoiiyoru Atoll is bone shaped, 21.5 km long northwest–south following a curved path, is about 6 km at its widest, and covers an area of about 72 km2 (Figure 12.5). The rim is almost entirely submerged and is coded as reef with the exception of two islets each encircled by their own outer reef flats to the northwest. Outer reefs are developed between and around both islets. A series of small plateaus continue to the east. A distinct rim surrounds a lagoon of about 44 km2 that contains pinnacle-like structures near the center. However, except for the island areas to the northwest, the rim is submerged to depths ranging from 5 to 19 m, although most of it is 10–15 m below the surface. Zohhoiiyoru is often referred to as a bank on charts, but its clearly defined rim and lagoon suggest that it belongs to the submerged atoll category. The tilted condition of the platform may reflect the blocky nature of the rift zone associated with the collision of the Caroline and Philippine plates in that area (Lee, 2004). Sorol (Figure 12.6) is a bird-shaped atoll with a stout and sharp beak, not unlike a kingfisher, lying 193 km southeast of Ulithi and about 300 km east of Ngulu. The platform is 12.5 km from beak to tail with

an inflection point at the eyelike islet and is 3.1 km at its widest through the middle. The platform area is 20.5 km2 and is surrounded by outer reefs that are widest to the east where they are up to 575 m wide. The shallow outer reefs spread to a substantial extent onto the reef crest around the platform. A reef crest about 150 m wide occurs from head to tail (northwest–southeast) and is patchy but equally developed to the south and southwest. The rim supports 17 islets, mostly sand cays all lying on the northeast side. Five islets appear to be vegetated; the largest of these is 60 ha. The reef flat occupies all the head, as well as most of the back and tail (northwest–northeast), and dries at about 1.2 m. The south side is steep-to and is submerged (Sailing Directions, 2017). A narrow passage that crosses the reef flat to the south may allow small craft` at high tide, but the rim is closed. The submergence of the southern rim may be associated with its position adjacent to the Sorol Trough to the south of the atoll. This 4,000-m-deep basin is an important rifting center (Zhang et al., 2021) and is widest near Sorol Atoll (Figure 12.1), but also extends eastward through much of Yap State. The lagoon area is 8.4 km2. The lagoon slope develops reef areas, particularly to the south, where they may overlap from the reef crest and extend up to 275 m wide. Sand patches about 200 m wide to the northwest and twice that to the southeast are connected by narrower trails along the north and northwest to form a sand apron. These trails are intermittent to the south and southwest. The lagoon beyond 15 m depth develops reef-rimmed pinnacles on the lagoon slope to the southeast and a few others on the lagoon slope to the south. The maximum depth is 45 m (Purdy, 2001). Eauripik Atoll is located 325 km southeast of Sorol, where it is shaped like a rounded triangle with a south-facing base and a northeast-facing depressed apex (Figure 12.6). The platform is 11.0 km west– east, 3.0 km north–south, and covers an area of 21.0 km2. It is surrounded by outer reefs that are best developed to the south where they are up to 275 m wide. The shallow outer reefs extend to the reef crest to the west, north, and east where it can be up to about 100 m wide Three islets occur on the rim, one north and two east, that are surrounded by rocky reef flats. The western rim is 2.8 km long and 1.3 km wide with a prominent sand patch through most of the middle. There are no islets or islands here and this section of the rim is awash at high water (Sailing Directions, 2017). There are a few patch reefs on the west and east reef flats. The rim is closed. The lagoon area is 7.3 km2. The shallows and part of the deep lagoon areas are covered by a sand apron up to 90 m wide that extends to the inner reef flats to the east

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Figure 12.6 Remote-sensing images of the atolls of Sorol and Eauripik. Images © 2021, Planet Labs PBC.

and west. Plateaus mark the edges of the deep lagoon to the south, west and northwest. Coral patches and ribbons are found to the south on the lagoon slope, but there are few elsewhere. We are unaware of depth data for the lagoon. Woleai Atoll is located about 110 km northeast of Eauripik and is shaped like a warped figure eight (Figure 12.7). The platform is 11.2 km west–east, 11.1 km northeast–southwest, and presents a total area of 48.3 km2. Outer reefs up to 500 m wide flank all but the southern and southwestern side of the platform where they are submerged over a distance of 10.5 km. Two very small islets project from that submerged side. The shallow outer reefs are typically 150 m wide and are narrower than the deep reefs but overlap with the reef crest which is about 125 m wide to most of the north and east. The rim west, north, and east supports 17 islets and one triangular island (Woleai, 1.4 km2 in the northeast), where there is an airstrip. In addition to openings in the southern rim, there are two channels to the north. The rim is open. The total land area is 4.5 km2, representing a little more than 9% of the platform.

The lagoon area is 31.3 km2, but the submerged southern rim forms a ridge that divides it into two basins in which the western part is about twice the size of the eastern one. Reefs are found in the shallow lagoon in the lee of Woleai Island, two of the islets on the southern rim and around the deeper edge of the western lagoon. Sand deposits up to 1.2 km wide are found to the west and occur discontinuously around much of the western lagoon. A sand apron up to 600 m wide varies considerably as it extends clockwise around the eastern lagoon west–east. The lagoon deeper than 15 m include eleven pinnacles rise from the bottom of the larger lagoon, the depth of which ranges from 35 to 50 m. The smaller eastern lagoon deeper than 15 m occupies an area of only about 3 km2 where it is 20–35 m deep, but there is a pass south of the eastern reef flats where large vessels can enter (Dalzell and Smith, 1995; Sailing Directions, 2017). Ifalik Atoll (Figure 12.7) is located about 58 km south–southeast of Woleai and is shaped like a strap wrench. The platform is 4.6 km northeast– southwest, 3.5 km northeast–southwest, and covers

Atolls of the Caroline Islands

Figure 12.7 Remote-sensing images of the atolls of Woleai, Ifalik, and Faraulep. Images © 2021, Planet Labs PBC.

an area of about 9 km2 including uncoded reefs to the east, southeast, and south. The outer reefs surround the platform and as coded extend to about 400 m to the south and nearly twice that to the southeast. Elsewhere on the platform, they begin to narrow and become narrowest to the northwest where they are about 150 m wide. A well-developed reef crest up to 150 m wide occurs clockwise northwest–south. The rim supports 1.3-km2 Ifalik, the main island, which extends north–southeast, as well as an 11-ha islet to the south and a 3-ha islet to the west. Reefs develop in the lee of Ifalik. A pass flanked by reefs about 45 m wide and 5.5 m deep occurs between the two (Sailing Directions, 2017). The rim is open.

The lagoon area is 3.3 km2. The shallows and deep reef areas present a sand apron 500 m wide to the west, 725 m to the south, and about 100 m wide in the lee of Ifalik Island. The larger extent of sand from the south is the result of their continuation on a tangle of connected ridges that are found in the lagoon center. The deep lagoon surrounds these ridges, occupies an area of about 115 ha, and extends to a depth of 20 m (Purdy, 2001). There are scattered patch reefs in this part of the lagoon. Faraulep Atoll is located 145 km north of Ifalik and resembles a warped trapezoid with the long base facing the northeast (Figure 12.7). The platform is 4.9 km northwest–southeast, 4.7 km west–east at

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its widest, and covers an area of 10.7 km2 including uncoded reef northwest and east. The platform is surrounded by an outer reef that is best developed, northwest–southwest clockwise where it is typically 200 m wide, but extends 850 m at its easternmost point and 580 m to the northwest. The outer reefs are roughly 100 m wide to the leeward west. The reef crest is best developed northeast–southeast, where it is about 100 m wide, and the reef flats are more than 500 m wide in the same area. The rim supports three islets to the north, east, and south that are joined by a continuous reef flat interrupted by a 600-m-wide gap that includes two sections of reef flat and three narrow and shallow channels to the southwest. The center channel has been navigated by a 65-m vessel according to Sailing Directions (2017). We tentatively mark this atoll as open. The lagoon area is 3.2 km2. The shallows contain patch reef areas in the vicinity of the channels to the

southwest. A sand apron about 400 m wide to the north and west, narrowing to 180 m or less to the east, overlaps the shallow and the edges of the deep lagoon to 15 m. A few pinnacles can be distinguished near the lagoon center. The maximum lagoon depth is 20 m (Purdy, 2001). Olimarao is a D-shaped atoll (Figure 12.8) located about 177 km southeast of Faraulep. Its platform is 5.2 km northeast–southwest at its widest, 3.1 km northwest–southeast, and covers an area 11.8 km2. The outer reefs range from 275–400 m north–southwest with an extended reef area to the northeast that is 800m from the rim. The reefs are narrower (225–325 m) to the northwest. The reef crest is up to 125 m wide to the east, and the reef flats are up to 750 m wide to the southwest. Patch reefs occur to the west–southwest. The rim supports two islets, one to the northeast and the other to the west. The area between them is a reef flat that is continuous except

Figure 12.8 Remote-sensing images of the atolls of Olimarao, Elato, Toas, Lamotrek, and West Fayu. Images © 2019-2021, Planet Labs PBC.

Atolls of the Caroline Islands

for the submerged rim to the south, where there are two small segments of reef flat separated by three entrances to the lagoon. The westernmost of these is 2.5-m-deep channel, whereas the center is 6-m-deep pass and the easternmost is 4.5–5.5-m-deep pass (GPS nautical chart AU-408147). The lagoon area is 6.1 km2. A sand deposit up to 190 m wide to the southwest is discontinuous, but a sand apron up to 250 m wide occurs northwest– southeast. Patch reefs occur to the south in the lagoon shallows near the channel/passes. More than 67% of the lagoon area is too deep to code. However, while the GPS nautical chart referred to above shows a depth of about 30 m near the easternmost pass, there are only four other depths shown for the northwestern lagoon (18 m or less) and no depths are given for the center. Elato is a compound atoll (Figure 12.8) located 36 km southeast of Olimarao. The northern part is bowtie shaped and is quite shallow in the center, producing two separate lagoons, whereas the southern atoll, apparently separate, is sometimes referred to as Toas Atoll (described below). Weins (1962) refers to it as a ‘secondary lagoon’ of Elato because, although the two are separated by about 2.6 km, they are connected by a ridge that is at least 20 m deep (Sailing Directions, 2017) and about 650 m wide according to the Allen Atlas. The Elato platform is 9.6 km following a curved path north–southeast, 2.8 km at its widest to the north, and covers an area of 14.7 km2. Elato is surrounded by outer reefs up to 425 m wide to the north and east (including plateaus) and less so to the west where they are about 225 m wide. Consistent with this distribution, the reef crest is better developed to the east than to the west. The rim supports a 48-ha islet to the northeast. The reef flats are up to 700 m wide to the west, where there are several scattered patch reefs and are continuous except for a 90-m-wide pass to the east that is 5–11 m deep (Sailing Directions, 2017). The rim to the northeast is open. The southern rim is a reef flat extending counterclockwise northwest–southeast with a 17-ha islet to the south. The rim to the south is low to the water on the southeastern side, is covered by shallow reefs and is semi-closed. We mark Elato Atoll as open. The lagoon area to the northeast is 2.5 km2. The shallows form a sand apron northwest–northeast, where it is up to about 200 m wide but trails variably in both directions and develops gaps around the lagoon pass and areas to the west of it. The apron continues southwest of these gaps in the shallows of the southwestern portion, where it is up to about 200 m wide. Reefs encircle the 90 m pass opening, turn along the shallow lagoon and to the northeast for 1.3 km, and to the southwest for about 450 m.

The northern lagoon reaches a depth of 27 m (Sailing Directions, 2017). The southern lagoon area is 2.4 km2 and is connected to the northern one by a 93-ha channel composed almost entirely of shallow lagoon that is suitable only for small craft. There is no other entrance to it. The shallows to the northeastern and southeastern ends contain sand deposits that are discontinuous, and although there are additional sand deposits to the west, these do not constitute an apron. The maximum depth of the southern lagoon basin is 29 m deep (GPS nautical chart AU-408147). Toas Atoll, connected to Elato to the north, forms a warped ovotriangle 3.9 km north–south, 2.3 km at its widest northwest–east, and covers an area of 4.3 km2 (Figure 12.8). A well-developed reef crest occurs north–southwest. The platform is surrounded by outer reefs that extend up to 575 m wide to the east and up to 325 m to the west. The rim supports a single vegetated island to the southeast of about 18 ha, that is also describable as two islets 1,670 m long joined by a 60-m-wide sandy isthmus. The island is surrounded by a reef flat that is continuous and up to 750 m wide to the south. A low area in the rim occurs to the east that is about 90 m wide and 2 m deep (GPS nautical chart AU-408147). We regard this rim as semi-closed. The lagoon area is 1.7 km2. Patch reefs are found in the shallows to the southwest and northwest; a few small patches occur to the east. Sand deposits up to 150–200 m wide occur to the north and south, respectively, and spread from those points farther into the lagoon, but they are disconnected in several areas. Lamotrek is a rounded triangular atoll located 9.5 km east of Elato. The platform is about 7 km north–south, 15.2 km at its widest parallel to the base, and covers an area of 49.5 km2 (Figure 12.8). The perimeter of the reef is 33.5 km and about 10 km of that length (28%) is submerged, much of it to the southwest and northeast. The outer reefs extend 700 m from the rim to the east, 400 m to the north, and 200 m to the northwest. The rim supports three islets, each in different sections of the atoll to the north, the southeast and the west. The islets north and west are joined by a rock and rubble reef flat up to 650 m wide and somewhat less to the southeast. The lagoon area is about 32 km2 with pinnacles clustered to the northwest and southeast. The maximum lagoon depth is 50 m (Purdy, 2001). West Fayu (Figure 12.8) is an atoll shaped like a strap wrench, similar to Ifalik. Its nearest neighbor is Lamotrek, about 72 km to the southwest. West Fayu is distinguished from East Fayu which is in Chuuk State rather than in Yap and is a reef topped by two islands that are surrounded by very shallow water

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that would not seem to constitute a lagoon. The platform of West Fayu is 7.4 km southwest–northeast. A comet-like submerged and mostly uncoded reef extends tail-like 6.7 km to the east. The atoll is 3.1 km north–south and covers an area of 15.2 km2 including the tail. The platform is surrounded by outer reefs that are best developed to the south, where they are mostly 350–500 m wide compared with about 280 m wide to the north. The reef crest is best developed to the north and west; the reef flats to the east of the lagoon are about 1.6 km wide, where a single triangular islet is also found. The reef flats are continuous except to the southeast, where they become submerged for a distance of 1.1 km including a shallow area of reef development in the center. Passes on either side of this reef area are about 6 m deep to its southwest and about 8 m deep to its northeast (GPS nautical chart AU-40817). The rim is open. The lagoon area is 6.4 km2. The shallows contain a sand apron up to 450 m wide to the west that extends about 900 m to the northeast and to the south– southeast for about 2.7 km. Sediment deposits occur elsewhere as well, but they are smaller and discontinuous. Reefs spill into the shallow lagoon to the south with the sand apron and occur to the north as well as within the submerged area of the rim. The lagoon deeper than 15 m occupies an area of 4.8 km2 and reaches a maximum depth of 38 m (Purdy, 2001). Pikelot is a largely submerged, Y-shaped atoll located 98 km east of West Fayu. The platform is 4.3 km northwest–southeast, 2.2 km at its widest north–south, and covers an area of about 5.1 km2. The rim is up to 1,300 m wide to the east and is submerged save a 14-ha island to the west in the cleft of the Y. Reefs surround the island to a distance of 800 m and shoal patches with depths of 9.1–18.3 m lie within a radius of 1,600 m (Sailing Directions, 2017). The lagoon covers an area of about 33 ha, but its depth is unknown. The westernmost atoll in Chuuk State is Poluwat (Figure 12.9) and is located 278 km southeast of West Fayu. The emergent part of the platform is ovotriangular 4.5 km northwest–southeast and 2.8 km at its widest across the base, and covers an area of about 9.5 km2. The reef crest is best developed north–southwest. The rim is composed of three islets and two islands that cover 3.2 km2 of the platform. The lagoon is small, about 1.3 km2, with a pass depth of about 7 m, and a maximum depth of about 11 m (GPS Nautical chart AU-40817). However, Poluwat Atoll is the emergent part of much larger Uranie Bank (Figure 12.9) whose area is 270 m2 and maximum depth is 44 m. Thus, like Zohhoiiyoru, Polowat is the remains of a partly submerged, partly drowned atoll that we mark overall as submerged.

Pulap (Figure 12.9) is an ovotriangular atoll with a concave eastern vertex and a convex western vertex that lies about 32 km northeast of Poluwat. The platform is 13.7 km north–south, 5.5 km west–east through the middle, and covers an area of 49.3 km2. A 51-ha islet occurs to the north and is surrounded by a reef flat 7.7 km long. A second islet of about 5 ha occurs to the west and is surrounded by a reef flat that is separated from the northern islet by a 1.7 km length of submerged reef. A third islet shaped like clubs, handle-to-handle of about 34 ha is found to the south and is separated by submerged reef 4.9 km long from the western islet. The clubs and their reef flats are separated from the northern reef flat by a 10.7-km-long submerged reef that constitutes the entire eastern rim. The outer reefs similarly surround the western and southern islets, but the remainder of the rim, about 60% of the perimeter, is submerged. The rim is open. The lagoon area is about 32 km2 and the slope is about 450 m wide to the west, less to the east. The depth maximum is 35 m (Purdy, 2001). Lying close to the center of the Carolines, the residents of Pulap claim to be a cultural center, a source of many regional customs and ceremonies. They also purport to be key in the development of navigation skills. Pwollap, their name for the island, originates from ppwo (‘to be initiated as a navigator’) and lap (‘big’ or ‘important’) (Flinn, 1990). Other islanders recognize this heritage, and the Pulapese are quite proud of this history. Pulusuk (Figure 12.9) is about 72 km south of Poluwat and has much in common with it as a largely submerged atoll and is referred to on charts as Manila Bank. The emergent part occurs to the south, where the island of Pulusuk is found as a compressed oval. A 16-ha enclosed lagoon—a freshwater lake with salinities ranging from 0% to 3% according to Nelson and Cushing (1982)—is found to the north and frequently exchanges ground water through a fissure in the limestone bed. Deep and shallow outer reefs surround it and the area of the emergent part of the platform is about 5.3 km2. However, the submerged platform (including Pulusuk) is 272 km2 and takes the shape of a bottle opener with a curved end and a sand cay to the northwest, with the handle to the southeast ending with Pulusuk. The submerged rim is about 20 m deep, although the curved area dips to about 33 m (GPS nautical chart AU-40817). The lagoon is divided into a large area of 183 km2 that is separated from a small area of about 6 km2 by a narrow neck-like rim about 7.8 km long to the north. The lagoon depth varies from the limited information available through GPS nautical charts, but the northern hook region is about 13–18 m deep, whereas the maximum lagoon depth of 43 m (Purdy,

Atolls of the Caroline Islands

Figure 12.9 Remote-sensing images of the atolls of Poluwat, Uranie, Pulap, and Pulusuk. Images © 2018, 2021, Planet Labs PBC.

2001) is in the larger lagoon area. We mark this atoll as submerged. Namonuito (Figure 12.10) is a large triangular atoll with a truncated northern apex located 106 km from Pulap. The platform is 50 km north–south, 83 km across the south-facing base, and covers an area of 2,111 km2. The rim supports five islets, all north–southeast, and a single island to the southwest. The total land is 3.7 km2, less than 0.2% of the platform area. The outer reefs surround all six land areas and there are several points on the submerged eastern rim, where isolated reefs occur near the surface. The remainder of the rim to the south and west is submerged and is mostly coded as reef. The rim is

shallow and submerged, although there is a pass to the southeast with a depth of about 10 m according to U.S. Hydrographic chart 5417. The lagoon area is about 1,940 km2 with pinnacle-like structures scattered throughout, some of which are reef-rimmed. The average lagoon depth is about 46 m and the maximum depth is 66 m to the center east. Nomwin (Figure 12.10) is a D-shaped atoll with a pointed eastern end located about 138 km east of Namonuito. The platform is 17.8 km northwest–southeast, 27.3 km parallel to the base including the island and reef extension to the northeast and covers an area of 336 km2. Outer reefs up to 425 m wide occur to the north that narrow somewhat to the southeast and

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Figure 12.10 Remote-sensing images of the atolls of Namonuito, Nomwin, and Murilo. Images © 2021-2022, Planet Labs PBC.

become poorly developed to the west. The rim is composed of 11 reef flat sections that support 11 islets to the northeast–south and two more islets to the west. The largest reef flat section is to the east and covers 7.6 km2 including the two islets there. Most of the islets are closely associated with patch reefs. There are 11 openings to the lagoon including 18–38-m-deep passes to the west and shallower passes to the northeast and southeast (GPS nautical chart AU-40817). The rim is open. The lagoon area is 292 km2 and is accompanied by accumulations of sand that continue around the lagoon except to the southeast and among the breaks in the rim. These sandy areas are 900 m wide to the

east at its widest and narrows to about 200 m wide to the northwest. Most of the lagoon area is about 35 m deep. Several pinnacles or pinnacle-like structures, mostly reef-rimmed, emerge from the basin center, which is 52 m deep at its maximum (Purdy, 2001). Murilo Atoll (Figure 12.10) is shaped like a keyhole and is located about 11 km east–northeast of Nomwin. The platform is northeast–southwest, 20.3 km at its widest, and covers an area of 407 km 2. The outer reef is widest to the east–south clockwise where it is 300-450 m wide and is narrows to the west where it is about 150 m wide. The rim is composed of two long reef flats. One to northwest–east is 23 km long and supports two islets. The other

Atolls of the Caroline Islands

is 21.6 km long west–southwest and supports two islets. The southeast rim is about 32 km long and supports two islets and a few sand cays; however, the rim is otherwise submerged and is as shallow as 1 m and as deep as 9 m with an average depth of 5 m (GPS nautical chart AU-40817). Reefs develop in shallow areas along this part of the rim. The south end of the rim is a 4.8-km-long reef flat followed by a pass that shallows to about 15 m deep to its southwest. There are four additional gaps in the rim to the northwest that flank three islets. These constitute narrow channels with reef development, although the northernmost of these islets is flanked by passes on both sides that are more than 14 m deep. The rim is open. The lagoon area is 361 km2. The area northwest–northeast is about 25 km long and presents a sand apron up to 375 m wide. Sand accumulation

elsewhere is patchy, but reef material develops in the shallow lagoon whether they are sandy or not. The lagoon in the widest region (the northeast) is about 34 m deep and tends to become deeper to the southwest where it is about 41 m deep in several places (GPS nautical chart AU-40817). The deepest part of the lagoon, 51 m (Purdy, 2001), is here as well. Several reef-rimmed pinnacle-like structures rise to or near the surface, mostly from the center of the lagoon. Kuop Atoll (Figure 12.11) is located less than 3 km to the southeast of the reef surrounding Chuuk Lagoon and about 167 km south of Murilo. Shaped like a cat’s back, the platform is 21.3 km northwest–southeast, 7.3 km at its widest to the southeast, and covers an area of 111.9 km2. The platform is surrounded by outer reefs that are mostly 175 m wide. The shallow outer reefs blend into the reef crest

Figure 12.11 Remote-sensing images of the atolls of Kuop, Losap, and Namoluk. Images © 2019-2021, Planet Labs PBC.

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around most of the atoll. The reef flats are continuous northwest–southeast, where they are 22.7 km long and 1.2 km at their widest. The northeast-facing rim is continuous and supports at least four islets. The remainder of the rim, counterclockwise northwest–southeast, is a reef flat with six shallow channels and one islet to the southwest. One channel to the southeast may be navigable. The rim is open. The northwest corner of the outer reef includes an area, where the reef face has collapsed giving rise to large cracks as well as a complex of reef blocks that have fallen into deep water (Colin pers. com.). The lagoon area is 84.5 km2. A sand apron overlaps the inner reef flats as well as the shallow and deep lagoon and is up to 500 m wide. Patch reefs also overlap the inner reef flats and shallow lagoon to the west and the deep lagoon to southwest. The lagoon slope is about a kilometer wide to the northwest where it bears numerous small pinnacles. The slope to the southeast and east is 500–1,000 m wide with few pinnacles. Three pinnacles rise from near the center of the lagoon, which is up to 46 m deep (Purdy, 2001). Losap Atoll (Figure 12.11) takes the form of a warped ovotriangle and is located 77 km east–southeast of Kuop. The platform is 9.2 km north–south, 8.4 km at its widest west–east, and covers an area of 48.1 km2 including uncoded reef to the north, northeast, and southeast. The platform develops outer reefs 150–250 m wide to the north, up to 350 m to the east and narrowest to the werst where they are generally less than 100 m wide. Shallow reef growth continues to the reef crest, which is well developed around the northeast but also around most of the atoll. The western platform develops a spur and groove system. Two major reef flats surround most of the atoll. The larger is 16.4 km long, up to 2 km wide, and partly encloses the northern half; the other is 9.1 km long, 680 m wide, and partly encloses the southern half. Between these reef flats is an 80-m-wide channel to the east, which is about 9 m deep at the entrance and deepens quickly inside the rim (GPS nautical chart AU-40817). The rim is additionally punctuated to the west by five short sections of reef flat with corresponding channels between them. Four of these are narrow and shallow, but the third one from the south is 230 m wide and 4–7 m deep (Sailing directions 2017). All passages to the lagoon are flanked by reef growth. The rim is open. There are about ten islets that are supported by the rim. At 75 ha, Laol Island to the northeast is the largest; Losap Island is smaller, adjacent to Laol, and closer to the lagoon. It is densely populated (Manner and Sana, 1995) and the southern half of the islet is occupied by houses. The total land area of the atoll is a little more than 1 km2.

The lagoon area is 30.0 km2 and develops about 25 ha of seagrass to the north, and additional reef near Losap Island in shallow water and the deeper lagoon. Smaller reefs occur in the shallow lagoon to the south and in association with the channels and passes to the west. Sand bodies up to 200 m wide accumulate in the northern half of the shallow and deep lagoon with gaps in the passes, some of the channels, and in the lee of the islands. The lagoon slope is up to about 800 m wide to the south where several reef-rimmed pinnacles occur. The maximum depth of the lagoon is 67 m (Purdy, 2001). Namoluk Atoll (Figure 12.11) approximates the form of a rounded isosceles triangle and is located about 110 km southeast of Losap. The platform is 4.0 km northeast–southwest, 6.4 km across the base at its widest, and covers an area of 15.9 km2 including uncoded deep reef areas to the northwest and southeast. The platform is surrounded by an outer reef 180–240 m wide north–southeast clockwise wide, then narrow to the west. The shallow reefs grow around the atoll onto the reef crest, especially northwest–east. The southwestern side develops a spur and groove system that cannot be distinguished elsewhere on this atoll. The rim supports five vegetated islets having a total area of 1.4 km2: the three largest form the apices of the triangular atoll rim. The rim between islets is a continuous reef flat up to about 500 m wide. A few patch reefs occur to the north and in association with the large islet to the northwest. There are no channels or passes and the rim is closed. The lagoon area covers 7.7 km2. Ribbon reefs are found in the shallow lagoon in the lee of the large islet to the south and a line of patch reefs arises from uncoded areas to the southeast. A sand apron up to 200 m wide occurs in the shallow lagoon and is widest to the northwest. Sand also covers large areas of the reef flats to the east and southwest. The lagoon slope widens to 500–600 m in the lee of each of the three main islets. No pinnacles are noted on the slope or from  the  deepest reaches of the lagoon, where a maximum depth of 77 m has been recorded (Purdy, 2001). Etal Atoll forms a rounded triangle and is located 53 km southeast of Namoluk (Figure 12.12) The platform is about 7 km northwest–southeast, 5 km at its widest across the base to the northwest, and covers an area of 27 km2. The outer reefs are 200–290 m wide north–southwest clockwise and narrow to about 160–180 m wide to the west–northwest. Shallow reefs persist around the atoll as they continue onto the reef crest which is well developed on all sides. The rim is composed of 16 vegetated islets, 14 of which are concentrated on the northeast-facing rim. The

Atolls of the Caroline Islands

Figure 12.12 Remote-sensing images of the atolls of Etal, Lukunor, and Satawan. Images © 2018, 2020, Planet Labs PBC.

largest islet is 630-ha Etal to the south, where a small settlement is located. All are connected by a continuous reef flat up to a kilometer wide. An artificial channel has been dug across the reef flat about 350 m northwest of the south island’s western tip. The channel is about 10 m wide and about 320 m long, and it does not change the closed nature of the rim. The lagoon area is 16.9 km2. Ribbon reefs develop in the shallow lagoon and extend around the islands. A sand apron up to about 120 m wide occurs in the shallow lagoon southwest–northeast clockwise, becoming patchy and discontinuous to the east and to the south, especially where reefs are located. The lagoon slope is about 900 m wide to south and less than 200 m wide elsewhere. The deepest regions of

the lagoon are essentially featureless in the imagery, and we do not have any information concerning its depth. Lukunor Atoll (Figure 12.12) resembles a rounded scalene triangle and is located 14.4 km southeast of Etal. The platform is 12.8 km northwest– southeast, 7.6 km across the west facing base, and covers an area of 72.1 km2. The platform is surrounded by outer reefs that are roughly 230–280 m wide and narrow to about 200 m to the west. There are 16 islets and two small islands with settlements, including 1.4-km2 Lukunor Island to the southeast corner and Oneop Island to the southwest measuring less than 1.1 km2. All but two of the islets are found on the southern rim and reef flats, even though they are narrower

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(less than 300 m versus more than 500 m) elsewhere. A 6-m wide, 290 m long, artificial channel is visible about 300 m to the southeast end of Oneop in the southwest corner. The channel is very shallow but a natural entry to the lagoon, a pass 91 m wide and about 15 m deep, is found to the west of Lukunor Island. A shallow reef flat dries at 1.2 m and divides the entry east and west (Sailing Directions, 2017). The rim is open. The lagoon area is 53.3 km2. The shallow lagoon develops reef areas in the lee of Lukunor and Oneop islands, where they overlap the reef flats as well. Small patch reefs also are found in the lagoon to the northeast. A narrow sand apron 50–60 m wide extends around the lagoon on its shallows and the deep lagoon. The only apron gaps are in the pass and in the lee of Lukunor Island. The lagoon slope is about 3.5 km wide to the southeast and is roughly 30 m deep (GPS nautical chart AU-40817). Several reefrimmed pinnacles rise from the lagoon slope. An additional pair also rises from the deep lagoon basin to the surface. The maximum lagoon depth is 57 m (Purdy, 2001). Satawan (Figure 12.12) is the last atoll in Chuuk State and is formed as a warped rectangle located about 8.2 km from Lukunor. The platform is 32.1 km northwest–southeast, 17.3 km at its widest, and presents an area of 425 km2 including uncoded reef to the east. The platform develops outer reefs that are generally ­250–350 m wide and narrow somewhat to the east. There are 56 islets, all but two of which occur northwest–southeast. In addition, two islands occur to the southeast. Ta, 9.2 km long and no wider than 340 m, is the site of a 410-m-long airstrip. The only other island that exceeds 1 km2 is Satawan to the southeast where there is a settlement. Rock and rubble reef flats up to 950 m wide connect all the islands/islets except for two passes. The one to the north winds for about 3 km before an entrance inside the lagoon can be made. This pass is 228 m wide and 18 m deep. The main passage to the lagoon is found to the southwest, where it is at least 270 m wide and about 15 m deep (Sailing Directions, 2017). The rim is open. Reefs occur sporadically among the islets but become a bit more notable in the lee of the islands to the south. The lagoon area is 367 km2. Sand extending from the reef flats forms an apron in the shallow lagoon up to 230 m wide, although it is more often about 125 m wide with gaps found only in the channels. The lagoon slope includes reef material in the vicinity of the passes. About 25 sand and coral-rimmed pinnacles rise from the slope and from the bottom of the lagoon which is up to 77 m deep (Purdy, 2001). Satawan, Lukunor, and Etal are collectively known

as the Mortlock Islands. The residents here consider themselves distinct from the Chuukese and this distinction is a point of pride for them. There are eight atolls in Pohnpei State, the westernmost of which is Minto Reef (­ Figure 12.13), a distorted trapezoid atoll with a concave southwest-facing long base and a pointed northeast-facing short base located about 296 km northeast of Satawan. Minto is 7.2 km northeast–southwest, about 10 km northwest–southeast, and covers an area of 46.9 km2 including uncoded reef to the east. The platform is surrounded by outer reefs up to about 240 m wide to the northeast, most of which are not coded. Elsewhere to the east, uncoded deep reefs are about 110 m wide, but becomes narrower to the west. The rim is a reef flat up to about a kilometer wide with scattered coral patches to the west and fewer elsewhere. There are no islands, and the reef flat develops a 25-kmlong perimeter that is continuous except for 3.5 km to the southwest, where rim is submerged leaving about 11 small areas that break the surface. The rim is mostly awash except for a 1.8-m-high sand bank to the north (Sailing Directions, 2017). We mark this atoll as submerged. The lagoon area is 33.3 km2. A sand apron in the shallow lagoon is up to 120 m wide but is more often half that width. Sandy accumulations are discontinuous to the northwest, west, and southeast as well as to the southwest where the rim is submerged; most of these gaps are occupied by reefs. The lagoon slope is steep and most of the lagoon area is occupied by deep water, however, we do not have depth data for this lagoon. Oroluk Atoll, shaped a bit like the island of ­Hawai’i, is located 105 km southeast of Minto Reef (Figure 12.13). The platform is 33.6 km northwest– southeast, 26.9 km northeast–southwest, and it covers an area of 464 km2. The platform is surrounded by a narrow band of outer reefs that are about 135 m wide likely due to the sharp angle of descent. The rim is composed of a reef flat up to 1.7 km wide to the northwest, where a single 19-ha islet is found. There are no other islets or islands, but the rim is separated by 16 channels, 14 of which are to the west. Most passes vary in depth from 9 to 57 m although some are narrow and others have not been adequately swept (GPS nautical chart AU-40817). Most of Oroluk’s rim dries at low tide (Sailing Directions, 2017). The rim may be open, but because it is all but under water through most of the tidal cycle, we mark it as submerged. The lagoon area is 416 km2. The shallows are occupied by intermittent sand deposits and the bulge to the north develops an apron up to 180 m wide and about 14 km long. The northern and northwestern

Atolls of the Caroline Islands

Figure 12.13 Remote-sensing images of the atolls of Minto, Oroluk, Pakin, and Ant. Images © 2015, 2020, 2021, Planet Labs PBC.

shallow lagoon near the passes develops ribbon-like reefs that are intermittent elsewhere. The lagoon slope or deeper parts of the basin form about 20 pinnacle-like structures most of which are rimmed with coral. The maximum lagoon depth is 75 m (Purdy, 2001). Pakin Atoll (Figure 12.13) is located 263 km southeast of Oroluk, where it resembles a pointed ellipse. The platform is 9.9 km northwest–southeast point-to-point, 3.9 km at its widest, and covers an area of 26.0 km2. The platform develops outer reefs that are narrow around the rim and are up to about 200 m wide to the southwest. However, the platform to the northwest extends into deeper water where it is mostly uncoded although several isolated reef areas rise toward the surface. When this area is included, the platform area increases by about six km2. Reefs to the east–northeast are about 70 m wide where they and the reef crest are best developed. The reefs on this side of the atoll extend to a depth of 80 m, where they form a wave-cut notch at 100–110 m, then drop away, and become vertical (Rowley et al.,

2019). The west and southeast sides of Pakin are also nearly vertical. However, deep outer reefs in these locations appear to be more sparsely distributed. The steep bathymetry on these sides appear to favor the development of rich black coral and octocoral communities that flourish at depths 130 m or more on the west and southwest sides. Calcareous blue octocorals (Heliopora) are also quite common here, where they extend to a depth of 45 m (Rowley et al., 2019). The rim is composed of about 28 islets concentrated clockwise northwest–southeast, the largest of which is 70-ha Nikalap. All the islets are enclosed by reef flats up to 800 m wide, although the southwestern rim is a reef flat with no islands. A conspicuous area of reef development of about 57 ha occurs on the reef flats to the northeast, and there are a few patch reefs near the islets to the northwest. There are small channels in the reef flat to the west, one of which is 5–6 m wide near the center of the western rim that can be used at high tide by small boats (Rowley et al., 2019). However, there are no passes or channels that appear to be sufficient to affect lagoon circulation.

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The lagoon area is 13.4 km2. Few reefs are found in the shallow lagoon, but a well-developed apron up to 600 m wide is found in the shallows as well as in much of the deep lagoon. The lagoon slope is most conspicuous to the northwest where it presents large areas of sand as well as large reef patches. Reefs are also found on the lagoon slope and include several reef-rimmed pinnacles near the deep slope margin and a few near the center of the basin. The maximum depth for this lagoon is 55 m (Purdy, 2001). The rim is closed, but considering the amount of reef development on the lagoon, we tentatively mark this atoll as semi-closed. Ant Atoll (Figure 12.13) lies about 25 km to the southeast of Pakin and is shaped like a compressed isosceles trapezoid. The platform is 12.3 km from the southwest-facing wide base to the northeastern facing short base. It measures 10.9 km parallel to the base and covers an area of 98.1 km2. The outer reefs are best developed along the northwest where they are about 250 m wide but only for about 3 km. The remaining reefs are narrow, typically about 100 m wide. The steeply sloping bathymetry of Ant is similar to that of its sister atoll Pakin, and both present deep-water populations of black corals and octocorals including the calcareous blue coral (Heliopora) on the south and northwestern sides that extend to a depth of 45 m (Rowley et al., 2019). The rim is composed of 12 islets, all but one of which occurs east–south and all are surrounded by a rock and rubble reef flat that is up to 500 m wide, but which reaches a kilometer wide to the north. The flats are continuous except to the south–southeast, where a ‘delta-like’ pass 90 m wide and about 15 m deep lies between two large, curved islets (Sailing Directions, 2017). The lagoon area is 75.1 km2. The shallow and part of the deep lagoon develop a sand apron up to 600 m wide to the northwest, but that varies and becomes reduced to as little as 60 m wide elsewhere. While there is little reef development in the shallow lagoon, reefs are commonly encountered on the lagoon slope, especially west–north where the rim is open and in the lee of the pass to the south. Reef-rimmed pinnacles rise from the deeper reef slope or the deep basin southwest–east clockwise. The maximum depth is 67 m (Purdy, 2001). We mark the rim as open. Sapwuahfik (previously Ngatik) Atoll (Figure 12.14) lies 118 km southwest of Ant, where it takes the form of a warped and rounded scalene triangle similar to Parece Vela described above. The platform is 21.4 km west–east–northeast, 9.7 km at its widest north–south, and covers an area of 124.6 km2. The platform develops outer reefs that are 190–200 m wide northwest–east, but are mostly narrower elsewhere except near the pass to the south (see below)

where the outer reefs are up to 350 m wide. The rim supports eight islets to the east (north–south) and one to the west, where a reef flat up to 2.4 km wide is found. The flats are continuous except to the south, where a single ‘delta-like’ pass about 115 m wide and between 55 and 110 m deep occurs. However, the 3-km-long pass is narrowed by reef growth about 500 m from the entrance and ends to the north with a cul-de-sac formed by reef flat. Reefs also flank both sides of the pass to the lagoon as they rise from an extension of the reef flat to its west and a submerged part of the rim coded as lagoon slope to the east. A turn to the right may allow passage over a part of the lagoon slope, where depths are marked to 34 m, but charts of this atoll are inaccurate (Sailing Directions, 2017). The rim is open. The lagoon area is 86.5 km2. The shallow areas develop small and scattered patch reefs. The predominant feature of the shallow and deep lagoon areas is a sand apron up to 500 m wide to the north, although it is about 70 m wide as it extends around the lagoon to either side of the pass. Reefs are best developed on the lagoon slope, especially to the west of the pass where reef-rimmed pinnacles rise from the deeper areas. The lagoon is about 55 m deep to the west and about 90–110 m deep around the channel entrance but is anomalously deep (149–159 m) to the center north and east. However, it should be noted that soundings in the wide northern half of the eastern lagoon have not been published (NGA chart 81427; Purdy, 2001). Mwoakilloa Atoll (aka Mokil) (Figure 12.14) is located 283 km northeast of Sapwuahfik and is a small atoll in the shape of a laterally compressed rectangle. The platform is about 5.2 km northeast– southwest, 2.6 km at its widest, and covers an area of 9.1 km2. Outer reefs extend up to 350 m wide to the northeast, where they are partly uncoded as are several smaller areas to the north and east. Generally, the outer reefs are 100 m wide or less. The rim is composed of three reef-rimmed islets totaling an area of about 1.4 km2. Two flank the lagoon to the north and the other lies to its south. The three are connected by a reef flat that is up to 860 km wide to the north. A few scattered areas of reef development occur to the eastern and western reef flats. The lagoon area is 1.8 km2 and it is wider to the north, where it is up to 60 m deep (Purdy, 2017). The remainder of the lagoon to the south is shallower, cone shaped, and is separated from the deeper area to the north by a ridge up to 90 m wide that is flanked by reefs. Apart from those areas there are but a few patch reefs around the lagoon. There are no passes to the lagoon other than two boat channels that are usable only at high tide (Sailing Directions, 2017). The rim is closed.

Atolls of the Caroline Islands

Figure 12.14 Remote-sensing images of the atolls of Sapwuahfik, Mwoakilloa, and Pingelap. Images © 2019, 2021, Planet Labs PBC.

Pingelap Atoll (Figure 12.14) is located 113 km southeast of Mwoakilloa and is a small atoll that resembles the continent of Africa. The platform is 4.7 km northwest–southeast, about 3.8 km west– east, and covers an area of 8.2 km2 including small areas of uncoded reef north–east. The outer reefs are best developed clockwise northwest–southwest, where they are 300 m wide. The west–southwest outer reef is not coded. The largest land area is 1.4km2 wrench-shaped Kahlap Island which curves and partly encloses the east end of the lagoon. A 360-m-long airstrip is found to its southwest that

terminates perilously close to the edge of the lagoon. In addition to Kahlap, there are two islets. The larger one borders the northern lagoon, whereas the second is club shaped and resides on the reef flat about 170 m west of the lagoon. The terrestrial areas total just over 2 km2 and the reef flat is up to 800 m wide to the west. There are no reefs coded on the reef flat. The lagoon area is 1.4 km2, and like Mwoakilloa, it is divided into a relatively shallow and a deep areas by a reef ridge. A single pinnacle rises from the deep lagoon, the maximum depth of which is 40 m (Purdy, 2001).

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The current population of Pingelap is about 250 individuals but at one time it was larger. Curiously, many of the population are color blind—not just the ordinary green-red color blindness—but instead a form called achromatopsia—a complete loss of color vision that is accompanied by a loss of visual acuity and an uncontrollable movement of the eyes themselves. To make matters worse, most achromatopsia victims suffer from photophobia due to a reversal of the normal pupillary response of constricted in the light and open in the dark to the opposite condition. As a result, the Pingelapese with this genetic defect have become nocturnal. Normally, this disease occurs in one of 20,000–50,000 individuals because a pair of mutated genes are required from each parent. However, on Pingelap, about 6%–10% of the population have achromatopsia and about 30% are carriers. The reason for the high percentage is both historical and genetic. In about 1775, a ferocious Typhoon Lengkeike struck the atoll and killed a large proportion of its residents either directly or subsequently due to starvation. One of the 20 or so survivors was a carrier of achromatopsia who, as a consequence of consanguinity and high rates of population replacement, passed the defective genes on. The result firmly inaugurated the disease in the remaining small population, making a rare condition common. Neurologist Oliver Sacks (1998) famously documented the nature and spread of this ­condition on Pingelap. Two other atolls are outliers within Pohnpei State due to their distance from the others. The first of these is Nukuoro, an annular atoll (Figure 12.15) about 210 km southeast of Satawan and 322 km southwest of Sapwuahfik. The platform is about 6.2 km north–south, 7.8 km west–east, and covers an area of about 39 km2. Outer reefs surround the rim and are generally 200–325 m wide, narrowing to about 170 m wide to the north. The rim supports 42 islets northwest–south, the largest of which is about 54 ha. The reef flats surrounding these islets are up to 600 m wide to the east and less than 480 m wide to the west where there are no islets. The reef flats are continuous except for a pass to the lagoon 27–37 m wide and at least 8 m deep (Sailing Directions, 2017); however, reef growth partly obstructs the entrance. There are few reefs on the flats or in association with the islets. The lagoon area is 25.2 km2. The shallows are extremely narrow with reefs. The upper lagoon slope develops a few patch reefs to the southwest and east– southeast near the pass. Pinnacles are found toward the edges of the slope and two 115–160 m wide rise from near the bottom of the lagoon forming shallow

areas toward the center and the north. The maximum lagoon depth is 99 m (Purdy, 2001). Kapingamarangi lies about 300 km to the south of Nukuoro where it takes a compressed annular form (Figure 12.15). At just over 1°N of the equator, this is the southernmost of the Caroline Islands and the last of the group. Despite its equatorial position, this atoll is subject to variable sea and swell from the northeast December through April, from the southeast May through November, and from the west October through January. Westerly waves of 1.5–1.8 m high occur about 10% of the time (Sailing Directions, 2017). The platform is about 9.2 km north–south, 11.9 km west–east, and covers an area of 76.2 km2. Outer reefs generally range from 150– 250 m wide with no obvious change from one side to the other. Shallow reefs are framed principally by coralline red algae and corals of the genus Acropora (McKee, 1956). However, the seaward edge of the atoll is steep (Sailing, Directions, 2017) and little explored. The rim supports about 30 islets northeast– south, none of which is larger than 35 ha. The reef flat is up to 850 m wide to the north, 800 m wide to the west, and is narrowest at about 300 m wide to the southwest, where there is considerable reef development. The rim is continuous except for a pass facing the southwest that is divided into three entrances to the lagoon by shallow areas. The southeast pass is 68 m wide and about 9 m deep, whereas the western one is wider and about the same depth but requires a 90° turn. The northwest passage is shallow and is for small craft only (Sailing Directions, 2017; GPS nautical chart AU-40817). Strong currents enter the channel and carry a tongue of sediment about 1,600 m into the lagoon (McKee et al., 1959). The rim is open. The lagoon area is 56.5 km2. Sand deposits occur west–northeast, but are often interrupted and are absent northeast–southwest. The shallow lagoon slope and the deeper reaches of the lagoon produce numerous patch reefs ranging in configuration from mounds, pinnacles or platforms 30 m or more above the bottom and 150 m or more wide. At least 18 of these larger platforms occur near but below the surface where dense thickets of branched corals surround and cover the tops. Those rising from the shallower areas of the lagoon often reach the surface, where they are commonly covered with sand or dead corals (McKee, 1959). Branched corals are common to about 30 m and sediments to that depth are composed of broken coral fragments. Flake-like fragments of the calcareous alga Halimeda occur to about 50 m and their distribution overlaps with foraminiferal sand to about 60 m. However, below

Atolls of the Caroline Islands

Figure 12.15  Remote-sensing images of the atolls of Nukuoro and Kapingamarangi. Images © 2021-2022, Planet Labs PBC.

about 60 m, sediments are unmixed and tend to form calcium carbonate mud suggesting a poor circulation in the deeper lagoon away from the passes (McKee, 1959; McKee et  al., 1959). The deepest part of the basin becomes flat to the center with a relatively large area that is roughly 55–75 m deep. The maximal lagoon depth is 79 m (Purdy, 2001).

References Bailey RT 2015. Quantifying transient post-overwash aquifer recovery for atoll islands in the Western Pacific. Hydrol. Process. 29, 4470–4482. Birkeland C, Ahser E, Goldbuu Y et al. No Date. Status of the coral reefs in the Pacific Freely Associated States. https://www. coris.noaa.gov/portals/pdfs/status_coralree_FAS.pdf Colin PL 2009. Marine Environments of Palau. Indo-Pacific Press, San Diego. Colin PL 2018. Thalassodendron ciliatum (Cymodoceaceae) in Palau: Occurrence, typhoon impacts and changes over time. Bot. Mar. 61: 537–546. Colin PL, Bell LJ, Partis S 2008. Helen Reef 2008: An overview. Tech. Report, Coral Reef Res. Found. www.coralreefpalau.org

Dalzell P, Smith A 1995. Something old something new: An approach to obtaining fisheries management information from a remote Pacific atoll. International Workshop on Traditional Marine Tenure and Sustainable Management of Marine Resources in Asia and the Pacific. International Ocean Institute – South Pacific, Suva, vol. 2, pp. 311–332. Flinn J 1990. We still have our customs: Being Pulapese in Truk. In: Linnekin J, Pover L (eds.) Cultural Identity and Ethnicity in the Pacific, pp. 103–126. University of Hawai’i Press, Honolulu. Fujiwara T, Tamura C, Nishizawa A et al. 2000. Morphology and tectonics of the Yap Trench. Mar. Geophys. Res. 21: 69–86. Intoh M 1981. Reconnaissance archeological research on Ngulu Atoll in the western Caroline Islands. Asian Perspectives 24: 69–80. Kayanne H, Hongo C, Okaji et al. Low species diversity of hermatypic corals on an isolated reef Okinotorishima, in the northwestern Pacific. Galaxea 14: 73–95. Kessler WS, Johnson GC, Moore DW 2003 Sverdrup and nonlinear dynamics of the Pacific equatorial currents. J. Phys. Oceangr. 33: 994–1008. Lee S-M 2004. Deformation from the convergence of oceanic lithosphere into Yap Trench and its implications for early-stage subduction. J. Geodynamics 37: 83–102. Lukas R. 2009. Pacific Ocean equatorial currents. In: Steele JH, Thorpe SA, Turekian KK (eds.) Encyclopedia of Ocean Sciences, 2nd ed. Elsevier, Boston. Manner HI, Sana D. The vascular plants of Losap Atoll. So. ­Pacific Study 15: 49–73.

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McKee ED 1956. Geology of Kapingamarangi Atoll, Caroline Islands. Atoll Res. Bull. 50: 1–38. Nelson SG, Cushing FA Jr. 1982. Survey of a brackish lake on Pulusuk with regard to its potential for fish culture. Univ. Guam Tech Rept. 77. Purdy EG 2001. Supplement to Purdy EG, Winterer EL 2001. Origin of atoll lagoons. GSA Bull. 113: 837–854. Raynor B 1995. Montane cloud forests in Micronesia: Status and future management. In: Hamilton LS, Juvik JO, Scatena FN (eds.) Tropical Montane Cloud Forests. Ecological Studies. Springer, New York, vol. 110, pp. 274–283. https://doi. org/10.1007/978-1-4612-2500-3_19 Rehman H 2013. Geological Origins of the volcanic islands of the Caroline group in the Federated States of Micronesia, Western Pacific. South Pacific Studies 33: 101–118. Rowley SJ, Roberts TE, Coleman RR, Spalding HL 2019. Pohnpei, Federated States of Micronesia. In: Loya Y, Puglise K, Bridge T (eds.) Mesophotic Coral Ecosystems. Coral Reefs of the World. Springer Nature, Switzerland, AG, vol. 12, pp. 301–320. Sacks O. 1998. Island of the Colorblind. Penguin Random House, New York. Sailing Directions 2017. Pacific Islands Publ. 126. National Geospatial Intelligence Agency, Springfield VA. Wang F, Li Y, Wang J 2016. Intraseasonal variability of the surface zonal currents in the western tropical Pacific Ocean:

Characteristics and mechanisms. J Phys. Oceanogr. 46: 3639–3660. Weissel JK, Anderson RN 1978. Is there a Caroline plate? Earth and Planet Sci. Lett. 41: 143–158. Wiens HJ 1962. Atoll Environment and Ecology. Yale University Press, New Haven. Wolanski E, Colin P, Naithani J et al. 2004. Large amplitude, leaky, island-generated, internal waves around Palau, Micronesia. Est. Coast. Shelf Sci. 60: 705–716. Yamano H, Kayanne H, Matsuda F, Tsuji Y 2002. Lagoonal facies, ages, and sedimentation in three atolls in the Pacific. Mar. Geol. 185: 233–247. Zhang Z, Dong D, Sun W et al. 2020. Investigation of an oceanic plateau formation and rifting initiation model implied by the Caroline Ridge and the Caroline Plate, western Pacific. Intl. Geol. Rev. 63: 193–207. Zhang Z, Dong D, Sun W, Zhang G 2021. The Caroline Ridge fault system and implications for the bending-related faulting of incoming oceanic plateaus. Gondwana Res. 92: 133–148. Zhou H, Liu H, Tan S et al. 2021. The observed North Equatorial Countercurrent in the far western Pacific Ocean during the 2014–16 El Niño. J. Phys. Oceanogr. 51: 203–220.

Melanesia and the Atolls of New Guinea The Melanesian islands extend from the island of New Guinea, the Solomon and Bismarck islands, Vanuatu, New Caledonia, and as far east as Fiji and Tonga—although the latter archipelago does not possess atolls (Figure 13.1). In the 19th century view of the Pacific Islands, native peoples were placed into cultural and ethnic categories, one of which was Melanesian, based primarily on dark skin color. However, language varies among and within these islands, ranging from Austronesian to 23 or more forms of Papuan, as well as different political, social, and cultural divisions that suggest a complex pattern of migration. Archeological and geochemical evidence indicates an initial settlement and expansion in New Guinea by hunter-gatherers at least 50,000 years ago and occupation of the highlands by 40,000–49,000 years ago (Shaw et al., 2011; Summerhayes et al., 2010).

Other analyses indicate a second important expansion to the Solomon Islands about 9,200 years ago and then to Vanuatu both with different mixtures of Austronesia and Papuan languages (Delfin et al., 2012). There is clear evidence of trade, cultural exchanges, and intermarriage between the western Polynesian and eastern Melanesian populations that result in a higher degree of genetic diversity compared with more isolated groups (Friedlaender et al., 2005; Friedlaender et al., 2007). In addition, atolls north of the Solomon Islands are primarily populated by Polynesians (Chapter 14), and that represents another interesting geographic twist. It follows that most anthropologists do not accept the unitary characteristics of the term ‘Melanesia’ or the boundaries it imposes. We use the term Melanesia simply to designate the geographic position in the western tropical Pacific.

Figure 13.1 The atolls of Melanesia extend from the island of New Guinea, including the Louisiade Archipelago to its southeast, the Solomon and Bismarck Islands, Vanuatu, New Caledonia, and as far east as Fiji and Tonga—although the latter archipelago does not possess atolls. We use the term Melanesia simply to designate the geographic position of its archipelagoes in the western tropical Pacific, where a total of 78 atolls are found.

DOI: 10.1201/9781003287339-13

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The geography of New Guinea and adjacent seas The Island of New Guinea is the mountainous northern margin of the Australian continent and is connected to it beneath the shallow waters of the Arafura Sea that now separate the two. New Guinea is the largest island in the western tropical Pacific and the second largest island in the world. Its landmass extends from about 1°S–11°S and 130°E–150°E and resembles a giant bird flying west. The eastern half is the independent nation of Papua New Guinea (PNG) whose southeast coast is concave and forms the Gulf of Papua. A forested, mountainous arm with peaks to 3,376 m, the Owen Stanley Mountain Range, represents the bird’s tail and extends farther southeast as it partially encloses the Solomon Sea to the south. This range was named after Stanley, a British naval officer and surveyor who explored and mapped parts of New Guinea during the mid-19th century. To the north, the Bismarck Archipelago islands of New Britain and New Ireland, along with the northeast coast of PNG, partly enclose the Bismarck Sea (Figure 13.2). Unlike the island of New Guinea, these seas and islands are the product of convergence between the Pacific and Australian plates and are thus formed by oceanic volcanic arc activity. The plate boundary itself is a complex system of microplates, each with their own separate motions (Davies, 2012). There are 13 atolls north of New Guinea, eight atolls in the Solomon Sea, and two within the Bismarck Sea for a total of 23 (Figure 13.2). The western half of the island is West Papua, a province of Indonesia whose westernmost extent includes the aptly named Bird’s Head peninsula. The adjoining Cenderawasih Bay forms north of the bird’s neck. There are four atolls that trend northwest of the bay opening into the Pacific Ocean (Figure 13.2). The Indonesian side of New Guinea is composed of an oceanic province to the north toward the bay and a continental province to the south where uplift includes snow-capped Puncak Jaya, the highest mountain peak on any island, reaching an elevation of 4,884 m. A glacier is also found here, but like the snow cap, the ice fields, which were once 32 m thick, have shrunk dramatically over the last decade and are expected to disappear entirely within the next 10 years (Permana et al., 2019).

Climate and regional oceanography The 23 atolls and the seas that we consider in this chapter are in a complex climatic and oceanographic environment located in the western part of the

Pacific, an area with the warmest waters in the global ocean. The predominant surface flow in the region is the western flow of the South Equatorial Current (SEC) as it is throughout most of the South Pacific Ocean. However, those flows become complex in the vicinity of the islands to the east of New Guinea. Part of the SEC diverges into the Bismarck Sea from the north, as well as through the Solomon Straits, and exits into the Coral Sea. Another arm of the SEC enters both the Solomon Sea from the south and washes through the atolls of the southeast Solomon Sea, an area called the Louisiade Archipelago, and then partly exits into the Bismarck Sea through the Vitjaz Straits (Figure 13.2). This overview does not consider the more complex oceanography of this region, where the SEC (or parts of it that are split by islands bordering the Coral Sea) is joined by other more local currents, including the New Guinea Coastal Current, the North Queensland Current, and the Gulf of Papua Current (Ganachaud et al., 2008; Zhai et al., 2014; Radenac et al., 2016). We also note that the Coral Triangle, the world’s center of biodiversity, begins here and includes reefs within the Solomon and Bismarck seas and those on the northern, eastern, and western sides of New Guinea. The triangle region contains 76% of the world’s reef-building coral species, 52% of Indo-Pacific reef fishes, the highest diversity of seagrass species, and the world’s largest expanse of mangrove forests despite occupying only 3% of the tropical Indo-western Pacific (Hoeksema, 2007; Allen, 2008; Williams et al., 2017; Asaad et al., 2018). A key element of the regional climate variability is the seasonal latitudinal migration of the Inter-Tropical Convergence Zone and South Pacific Convergence Zone (ITCZ and SPCZ, respectively). When the related wind convergence is north of PNG, roughly from May to October, southeasterly trade winds blow over the Coral, Solomon, and Bismarck seas, bringing maximum seasonal rainfall southeast of New Britain in the Solomon Sea. When the wind convergence is south of PNG, from about November to April, northwesterly monsoon winds predominate, bringing maximum seasonal rainfall to the west of New Britain in the Bismarck Sea and to PNG (Delacroix et al., 2014). In addition, the regional climate varies from year to year and is influenced markedly by El Niño-La Niña conditions (e.g., Cobon et al., 2016). These varied atmospheric conditions are reflected in wind roses from this region (Figure 13.3). To the east and south, around the Solomon Sea, winds are dominantly from the southeast and brisk, mostly greater than 20 km/hour. To the west and toward

Atolls of Melanesia and New Guinea

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Figure 13.2 Atolls are found to the north of the Island of New Guinea and to the east in and around the Louisiade Archipelago, Eastern Papua New Guinea. Additional atolls occur within the Bismarck and Solomon seas. The 23 atolls described here include a range of rim structures. The South Equatorial Current dominates the area to the north and south, but its flow is diverted into the adjacent seas as shown. The New Guinea Coastal Current reverses course seasonally with the monsoon. (Black and white Louisiade Archipelago map courtesy of Tim Skewes, http://www.timskewes.com.)

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Figure 13.3 Wind roses and the wave climate for Laualau and Rani lying north of PNG indicate waves from the open Pacific. Redlick, protected from the north, includes waves only to the southeast.

the equator, northwesterly winds are dominant and are most commonly less than 20 km/hour. A transition zone, with both southeasterly and northwesterly winds, occurs between the two endmembers in the western part of the Bismarck Archipelago. Waves change across the area as well (Figure 13.3). For example, at Redlick Atoll in the Solomon Sea, an area protected from the north by the Solomon Island chain, the most pronounced waves are from the southeast. In contrast, those atolls lying north of PNG typically include waves from the open Pacific to the northeast. The character of the tides also changes across this region (Figure 13.4). Tides of the atolls of the Solomon Sea and the western part of the Bismarck Archipelago are diurnal, with spring tidal range of around 1 m and neap tides of around 40 cm. In contrast, the atolls to the north of West Papua are mixed diurnal–semidiurnal. For example, at Mapia, tides include spring range of 1.84 m, whereas neap tides vary by 0.72 m.

The atolls of New Guinea and adjacent seas The Louisiade Archipelago is an eastward extension of the Owen Stanley Mountain Range composed of about 100 islands that are separated north and south by a channel that is about 1,400 m deep (GPS nautical chart AU-410151). The islands toward the mainland tend to be volcanic, while those to the east are

low lying and extensively rimmed and connected by barrier reefs such as the Calvados Island–Barrier Reef system that extends for 80 km (Figure 13.2). There are six atolls in this area, three of which are on the north side of the archipelago while three others occupy the south side. Three Deboyne Islands are located on the north side. The largest of these is Deboyne Lagoon which features an atoll-like configuration that includes Panapompom Island near the center, which is forested, 157 m high, and means Deboyne is likely an almost-atoll. Immediately to the southeast lies a pair of atolls, the larger of which is Redlick Reef, a rounded and truncated trianglelike structure whose platform is 9 km northwest– southeast and about 7.3 km at its widest (Figure 13.5). The platform area is 44.7 km2 and is surrounded by Outer reefs are variable north–southwest clockwise but are generally 170–300 m wide, whereas the west is less than 100 m wide. Shallow reefs continue to the reef crest in patches, primarily to the north. The rim supports a few small, scattered islets (The Redlick Islands) that together represent about 20 ha. A shallow area of the rim occurs to the west, but remainder is a reef flat with a small area of seagrass to the northwest. The rim is closed. The lagoon area is 33.2 km2. The shallow lagoon contains a sand apron that is a kilometer wide at its maximum to the west and 300–400 m wide to the west and north. The apron to the east is generally

Atolls of Melanesia and New Guinea

Figure 13.4 The character of the tides changes across this region. Tides of the Solomon Sea atolls and the western part of the Bismarck Archipelago are diurnal as shown for Budibudi Atoll. However, the atolls to the north of West Papua are open to the north and exhibit mixed diurnal–semidiurnal tides as shown for Mapia.

narrower, 200 m wide or less. Reef-rimmed pinnacles and patch reefs are present within the lagoon where many are capped with sand. The maximum lagoon depth is 18 m (Purdy, 2001). A small companion atoll is found about 1.5 km southeast of Redlick and appears not to be named on charts (e.g., US Navy Hydrographic Office 2955). However, a single islet, Mabui, is present which can be used for the atoll name, as islands often are. The platform forms a distorted D and is 4.3 km northeast–southwest, about 3.1 km at its widest and has an area of 7.5 km2 (Figure 13.5). The platform is surrounded by an outer reef that is generally about 150–250 m wide. However, along the southwestern rim, the reefs extend up to 450 m wide. Mabui Islet is 21 ha; there are no other islets. The rim is closed and is primarily a reef flat where reefs occur as patches, especially to the south-northwest clockwise. The Atlas maps seagrass beds to the west and northwest in the lee of the outer reefs, where they range from about 75 to 130 m wide. The lagoon area is about 3.1 km2. A sand apron up to 500 m wide is found to the east and west, where it overlaps with the lagoon slope and is about 70–100 m wide elsewhere. The lagoon slope occupies most of the lagoon basin and there are several patch reefs and pinnacles projecting from it. We have no depth data for this lagoon.

Conflict Atoll occurs with islands and reefs (Figure 13.2) on the north side of the Louisiade Archipelago, lying 64 km to the west of Redlick. It is ovotriangular, 29.4 km west–east, and about 11 km at its widest parallel to the base (Figure 13.5). The platform area is about 206 km 2 and it is surrounded by an outer reef that varies from 120–330 m to the north and from 250–350 m wide to the south. Shallow reefs are found along the southern rim, accompanied by a 28-ha seagrass area to the southeast and the west. The rim to the northeast bears five islets on a reef flat about 3.3 km long with channels 3.3 km wide and about 4 m deep to its northwest and 2.1 km wide, 7 m deep to its southeast. Another opening about 3 km wide to the southwest constitutes a 27-m-deep pass (Purdy, 2001; GPS Nautical Chart AU-5509P1). The rim is open and consists of about 23 islets, two of which form horn-like projections to the west. The largest is Irai (barely more than 1 km 2), followed by Panasesa (0.56 km 2), where there is an airstrip. The total island area of the rim is about 3 km 2. The lagoon area is about 170 km 2 . The shallow lagoon develops a few patch reefs near Panasesa Island and a few more south of Irai. A sand apron occurs in the lee of the reef flats and is about 700 m wide near Irai Island, about 500 m wide to the south, and narrower elsewhere. Most of the lagoon

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Figure 13.5 Remote-sensing images of the atolls of Deboyne, Redlick, Mabui, and Conflict. Images © 2021, Planet Labs PBC.

is deeper than 15 m and about 30 pinnacles rise from the deep slope or basin, two of which develop islets at their surface. The maximum lagoon depth is 41 m, which occurs near the lagoon center (GPS chart AU-410151). Bramble Haven (Figure 13.6) is an atoll that resembles a dissected square and is located on the south side of the Louisiade Archipelago about 41 km south of Conflict Atoll. The platform measures 16.7 km west–east, 10.7 km at its widest, and covers an area of 54.7 km2. The outer reefs are generally 1215–250 m wide but are wider (300–450 m) to the south. Shallow reefs are common on the reef crest and flats. The rim supports five wooded islands, all to the north, the total area of which is less than 80 ha. Three shallow

entrances to the lagoon occur to the north and an additional two to the southwest, however, only the south westernmost entrance is a pass and is 11 to about 18 m deep (Sailing Directions, 2020). The remainder is a reef flat. The rim is open. This atoll was named after HMS Bramble, a tender that accompanied HMS Rattlesnake as it surveyed the Archipelago under the leadership of Owen Stanley. The Bramble was able to enter and freely navigate the lagoon, and the atoll was thereafter named Bramble Haven (MacGillivray, 1852). The lagoon area is about 135.5 km2. The shallow regions have few patch reefs, but a sand apron extends up to 2 km wide to the northwest, about 1 km wide to the southeast and south, and is variable but

Atolls of Melanesia and New Guinea

less elsewhere. The lagoon slope is associated with several pinnacles, a few of which are rimmed with reefs. The deepest part of the lagoon descends to 33 m (Purdy, 2001; GPS chart AU-410151). Long Reef is located about 6 km to the west of Bramble Haven (Figure 13.6). The platform is conelike, wide to the west, narrow and truncated to the east. It is 39.5 km long northwest–southeast, about 10 km at its widest to the northwest, and covers an area of about 326 km2. The western boundary is indistinct and is composed of a series of patch reefs that separate Long Reef from nearby Anchorage Reefs and Sable Islet, three reef flat segments to the west (occasionally all referred to as Anchorage Reefs) that are not included in the platform area (see GPS chart AU-410155 and Sailing Directions, 2020). Kosmann Island and its reefs found to the northwest of Long Reef also are excluded from the atoll platform area,

as are the deep lagoon and plateau areas coded to the north of Long Reef. The platform develops outer reefs that are commonly 350–450 m wide to the south and 100–250 m wide to the north and east. The northwestern outer reefs are deep and patchy whereas the western reefs are apparently discontinuous. The rim adjacent to Anchorage Reefs and extending to the southeast is a rocky reef flat with no breaks, although a poorly surveyed area adjacent to Anchorage Reefs may allow transit to the Long Reef lagoon. The rim northwest–southeast is composed of reef flats that bear several shallow channels as well as one unnamed cay with an area of 2.6 ha to the northwest that is located less than 5 km south of Kosmann Island. In addition, there is another 1.4-ha cay, Lejeune Island, to the southeast near two channels. One channel is about 500 m west of the island; the other is 5.5 km to its east and is flanked on both sides by reefs. These

Figure 13.6 Remote-sensing images of the atolls of Bramble Haven and Long Reef, also showing Anchorage Reefs and Kosman Island. Images © 2021, Planet Labs PBC.

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may be open to the intrepid sailor, but neither of them has been adequately surveyed (Sailing Directions, 2020). We tentatively mark this atoll as open. The lagoon area is about 265 km2. The shallow lagoon includes patch reefs to the southwest, but otherwise is unremarkable except for a sand apron that is up to 2.3 km wide to the south, 900 m wide to the north, and up to 3.2 km to the northwest. The lagoon slope is associated with numerous pinnacles near the eastern and central basin, many of which are rimmed by reefs. We have a single depth of 27 m to the eastern end of the basin (Davis, 1928), but otherwise we do not have depth data for this lagoon.

Gallows Reef (Galogalohi) is located about 76 km northwest of Conflict Reef and about 99 km northwest of Long Reef (Figure 13.1). The platform is a pointed D-shape that is open to the west except for a single shoal area (Figure 13.7). Although sometimes referred to as horseshoe shaped, there are two disjunct rims that overlap to the east–southeast creating a partial double atoll. A pass between the rims is more than 500 m wide and has a depth of at least 25 m (GPS nautical chart AU-410151). With some imagination, the northern and eastern rim could form an upright and a crosspiece for the gallows, while the inner rim to the south could be seen as a rope. The platform

Figure 13.7 Remote-sensing images of the atolls of Gallows, Egum, and Budibudi. Images © 2021, Planet Labs PBC.

Atolls of Melanesia and New Guinea

is 9.6 km east–west, about 5.5 km north–south at its widest, with an estimated area of 41.8 km2. The rim is emergent only to the northeast and the eastern point, where a reef flat about 4 km long is found. Elsewhere, the platform consists of outer reefs reaching about 350–400 m to the south and 150–170 m wide to the east and north. The west-facing flank of the atoll is unrimmed. Submerged reefs connect most of those that are coded, except to the west. Indeed, almost the entire rim is submerged. The lagoon area can roughly be estimated by summing the inner reef slope areas (~36 ha) and subtracting it from the platform area. Areas coded as shallow lagoon are covered with sand and dotted with patch reefs. The eastern lagoon deeper than 15 m contains four coral areas totaling 31 hectares, and several small pinnacles that are associated with the lagoon slope, three of which are rimmed by reefs. We do not have depth data for this lagoon. Egum (Figure 13.7) is a large, isolated atoll about 115 km northeast of Gallows Reef. Egum is situated north of the Louisiade Archipelago amidst the Trobriand Islands to the north, Woodlark Island to the east, and the d’Entrecasteaux Islands to the west (Figure 13.1). The latter group was somewhat confusingly named after the same Rear-admiral Bruni d’Entrecasteaux for whom islands near New Caledonia were also dedicated, but this is no worse than the islands named for King George III in the Tuamotu Archipelago along with one of the South Shetland Islands near Antarctica. The Trobriand, Woodlark, and d’Entrecasteaux island groups represent trough, fault, and rift boundaries for the Solomon and Woodlark microplates that are trapped between the obliquely colliding Pacific and Australian plates (Davies et al., 1984). The area is actively tectonic and unstable. Egum Atoll is roughly annular, 32 km northeast– southeast, and 26.6 km at its widest approximately through the center. The platform area of 529 km2 is surrounded by outer reefs that are generally 150–200 m wide expanding to 250–300 m wide to the south–southeast. The rim to the north supports Yanaba Island which is 6.2 km2. Islets are found to the west of Yanaba and to the northeastern and eastern rim. These areas are reef flats that develop along with shallow reefs near the islands. The remainder of the rim west, east, and southeast is largely submerged. There are shallow entrances to the lagoon to the east and another two of unknown depth to the southwest. Sailing Directions (2020) indicates that the area northwest of Yanaba Island is 5.5 m deep and is the best lagoon entrance. The rim is open. The lagoon area is about 501 km2. The shallow lagoon areas clockwise west–east develop sand deposits that are about 500–800 m wide in the lee of

islands and islets but vary considerably between them. Sand occurs as isolated patches in the southern half of the lagoon, where shallow areas are separated by submerged reefs similar to Gallows. The lagoon slope is about 5 km wide to the north near Yanaba, where reef and pinnacle areas are visible. The deeper lagoon contains numerous isolated reef areas and pinnacles as well as two islets, Egum and Nasakori, in the middle (Sailing Directions, 2020). We do not have information on the depth of this lagoon. Budibudi is a small D-shaped atoll (Figure 13.7) that lies east of Woodlark Island (Figure 13.1). It is located 173 km east–northeast of Egum and is 5.3 km northwest–southeast and is 7.1 km at its widest. The platform area is 25.2 km2 including uncoded reef to the northeast and west, and it is surrounded by outer reefs that are best developed to the southwest where they are 400–425 m wide. The reefs are half that width to the east. The rim supports several islets to the south and one to the northeast. A single island to the east that is 1.9 km2 is the largest on the rim and develops a narrow band of seagrass and coral in its lee. The western rim is submerged allowing a free exchange of ocean water between the lagoon and the Solomon Sea. The rim is open. The lagoon area is about 14.5 km2 and its shallows north, east, and south are covered by a sand apron about 700 m wide near the east-facing island and about 550 m or less elsewhere. The lagoon basin contains three isolated shoal areas that are rimmed by reefs. We are unaware of depths reported for the lagoon. The Bismarck Sea is adjacent to and north of the Solomon Sea, and there are two poorly known atolls near the center of Bismarck, both south of Manus Island (Figure 13.2). This area is influenced by the South Equatorial Current and seasonally by the New Guinea Coastal current as described below. The first atoll in the Bismarck Sea is Kreis Reef, which a bowtieshaped atoll (Figure 13.8), sometimes is referred to as Circular Reef (Sailing Directions, 2017) for reasons that are unclear. The platform is 12.8 km northwest– southeast, 4.9 km at it’s widest to the south, and covers an area of 34.5 km2. The outer reefs range from 170–250 m wide. Numerous patch reefs with seagrass meadows associated with the outer reef flats on that side. Coral patches are also found on other reef flat areas, but they are comparatively less densely packed. The rim is without islands and is a continuous reef flat that nearly dries at low tide except for an open area that is ‘deep’ (Sailing Directions, 2017) and 480 m wide to the south that serves as a pass. We mark the rim as open. The lagoon area is 25.4 km2. The shallow areas develop a band of sand less than 85 m wide along the east that extends for about 4 km from the lagoon

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opening. Sand deposits become patchy elsewhere. The lagoon slope is narrow and drops quickly into the lagoon basin, where there are only a few reef pinnacles, most of which are rimmed with reefs. We do not have depth information for the lagoon. Sherburne Reef is the second atoll in the Bismarck Sea, and it consists of two atolls on the same plateau (Figure 13.8). The smaller of the two is shaped like a fishhook and is 2.9 km northwest–south and 1.2 km east–west at its widest. The platform is 1.8 km2 and develops outer reefs that range from 110–195 m wide, the southwest as the widest section of reef. The eastern reefs continue onshore and form a well-developed reef crest. The rim is a reef flat without islands that develops a deep pass 183 m wide into the lagoon to the west (depth not given, Sailing Directions, 2017). The lagoon

area is about 76 ha. The shallows are dotted with patch reefs bordering a sand apron up to 160 m wide to the south that extends counterclockwise to the northwest, where it is mostly 80 m wide. The larger Sherburne Reef lies about a kilometer to the south of the smaller one. This atoll is polygonal, 6.6 km northwest–southeast, and about 3.9 km at its widest. The platform area is 16.8 km2 and is surrounded by outer reefs that range from 140–220 m wide. The shallow reefs extend to the outer reef flats along most of the rim, but are best developed to the west. There are no islands, and the rim is a continuous reef flat except for an opening to the lagoon of about 430 m to the northwest that is navigable by small vessels. This reef nearly dries at low tide (Sailing Directions, 2017). The rim is open. The lagoon area is 13.1 km2. The

Figure 13.8 Remote-sensing images of the atolls of Kreis and Sherburne. Images © 2021, Planet Labs PBC.

Atolls of Melanesia and New Guinea

shallow lagoon forms patch reefs northwest–southeast clockwise. Similar to the smaller Sherburne, a sand apron about 200 m wide to the southeast extends along the east, where it becomes 100 m wide or less. Little sand accumulates to the west. The lagoon slope is narrow and most of the lagoon appears more than 15 m deep, but we have no information on the depth of the basin. The north coast of New Guinea is influenced by the South Equatorial Current and by the New Guinea Coastal Current (NGCC) that reverses course seasonally with the monsoon winds. During the austral summer, the NGCC is influenced by the southeastern monsoon that moves coastal waters to the northwest, whereas during the austral winter, the current flows in the opposite direction due to northwesterly monsoon winds (Kuroda, 2000, see also winds at Laualau in Figure 13.3). This seasonal reversal is shown in Figure 13.2 as a double-headed arrow. Several atolls are in a cluster north of New Guinea, the Ninigo Islands, that represent the western part of the Bismarck Archipelago. There are 21 islands here, six of which are atolls on the same plateau with deep water between them. A seventh atoll, Liot, emerges from a deeper plateau nearby (Figure 13.2). The southernmost of the Ninigo Island atolls is Awin, a small ovoid structure about 4 km northwest–southeast and 2.9 km at its widest (Figure 13.9). The platform area is 9.5 km2 and it develops an outer reef that is mostly less than 100 m wide but expands up to 225 m to the southwest. The reef flats in this area are occupied by about 24 ha of seagrass. A 450 m long part of the southern margin appears to lack deep reefs and shallow reefs are diminished here as well. Patch reefs occur on the inner reef flats to the southwest, as well as small areas to the west and northwest, where a sandy area up to 800 m wide occurs. The rim supports two islets, one to the east that is 80 ha and another to the south that is 10 ha with a small seagrass meadow in its lee. The remainder of the rim is a reef flat with no openings to the 3.4-km 2 lagoon. The lagoon slope contains relatively large areas of reefs associated with plateaus north–southwest clockwise. We mark the rim as semi-closed. The lagoon slope occupies most of the basin, where there are uncoded pinnacles. We do not have bathymetric data for this lagoon. Sumasama Atoll is about 15 km north of Awin and is triangular with a bulge to the northwest (Figure 13.9). The platform is 5.5 km north–southwest, 4.6 km northwest–southeast, and covers an area of 12.2 km2. The outer reefs are generally less than 100 m wide but extend from 150–250 m wide to the northeast and southwest. The rim is dominated by a

single 2.3-km2 island along the east that becomes up to about 800 m wide as it turns to the south. A narrow band of seagrass occurs in its lee. An islet with an area of about 2 ha occurs to the northwest near the lagoon. The remainder of the rim is a continuous reef flat that widens to the west and is largely covered by a sand deposit that is up to 1.2 km wide to the northwest and narrows to about 450 m wide to the southwest. Patch reefs occur near the lagoon edge near the islet to the northwest. Other reef patches occur with seagrass in the lee of the island. The lagoon area is 2.8 km2 and contains reef patches to the north and more extensively on the lagoon slope to the east and south. The lagoon deeper than 15 m displays many pinnacles. We tentatively mark this lagoon as semi-closed. Sama Atoll is about 5.5 km northeast of Sumasama, where it takes the form of a compressed oval 4.3 km long north–south and about 2.2 km at its widest (Figure 13.9). The platform area is 7.7 km2. Outer reefs occur around the platform and are mostly less than 100 m wide except to the northwest and a small area to the east where they extend 170– 195 m. The rim is mostly a semi-closed reef flat with two islets to the north that together occupy 734 ha and a third islet to the southeast that adds 2.9 ha. A few patch reefs and small areas of seagrass are associated with these islets. There are a few patch reefs on the inner reef flat with large sand patches north and south. There are no entrances to the lagoon, the area of which is 1.3 km2. Surrounding the lagoon is a reef sand apron with the same general patterns as on Sumasama, widest to the northwest and narrowest to the south. The lagoon slope and the deeper reaches of the lagoon include several pinnacles, some of which are sandy, and reef-rimmed, whereas others are covered by reefs. We do not have depths for the lagoon. Ninigo Atoll, the largest of this group, is located 8 km northeast of Sama. The platform resembles a hot water bottle 34.3 km northeast–southwest and about 18 km at its widest (Figure 13.10). The platform area of 443 km2 is surrounded by outer reefs that extend 190–400 m or more northwest–northeast, but both narrow to mostly 150 m or less elsewhere. The rim consists of three reef flat segments bearing 13 islets and a single island. The largest reef flat area is 42 km long, extends northwest–east, and supports eight islets. This area is followed clockwise by three shallow channels to the lagoon. A second reef flat segment to the south is about 14 km long and bears the island of Mal with an area of about 3 km2 along with Lau, a single islet near the northern end of this segment. A pass occurs to the north of Lau that is at least 11 m deep. The third reef flat

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Figure 13.9 Remote-sensing images of the atolls of Awin, Sumasama, and Sama. Images © 2021, Planet Labs PBC.

segment is 3.8 km long and supports a single islet to the southwest. The western rim is submerged except for three islets to the northwest. A pass with a depth of 8.2 m occurs between the northern two islets that are closest together (Sailing Directions, 2017). Patch reefs are near the islands to the northeast, especially on the inner reef. Seagrass areas are associated with islets, especially to the south and southwest.

The lagoon area is 359 km2 and numerous patch reefs occur in the shallows southeast–southwest and to the east. A sand apron is found north–southeast following the contours of the long reef flat segment. The apron is variable in width and up to 2.5 km wide to the northeast, tapering to the north and south from there. The lagoon deeper than 15 m bears numerous sand and reef pinnacles and plateaus, especially to

Atolls of Melanesia and New Guinea

the northeast. Six islands and shallow plateaus occur near the central lagoon, where seagrass patches also are mapped by Allen Atlas. We have depths only for the northern half of the lagoon, where the maximum is 14 m (GPS Nautical Chart AU-410151). Analtin (also referred to as Pelleluhu Atoll) is ovotriangular and is only about 600 m northeast of Ninigo (Figure 13.10). The platform is 11 km north–south, 8.1 km at its widest, and covers an area of 63.8 km2. The outer reefs surround the platform, and are generally 160–250 m wide. A reef crest is best developed to the northwest. The rim north–south supports ten islets and two islands. Those east–south are associated with patch reefs and/or seagrasses on one or both sides. The rim to the southwest and west is a continuous reef flat without passes or channels. The rim is closed. The lagoon area is 34.2 km2. The shallows and lagoon slope are covered by a sand apron up to 1,100 m wide to the west although the width is variable, narrowing to about 70 m wide to the south. The lagoon slope areas north–northeast are associated with reef ribbons. The lagoon deeper than 15 m develops pinnacles on the slope to the south and includes reefs

around plateaus that emerge from the lagoon basin. We do not have depth information for this lagoon. Laualau Atoll (also referred to as Heina Atoll) is annular, 5.5 km northwest–southeast, 4 km southwest–northeast, and lies 5.2 km from Analtin (Figure 13.11). The platform area is 14.5 km2 including uncoded reefs to the north and east. The outer reefs are about 150–270 m wide. Patch reefs develop on the west-southwest reef flats. The rim supports five islets including 96-ha Motum to the north. In addition, a single Island, 1.4-km2 Laualau, is about 300 m south of Motum and extends east–south. The remainder of the rim is a reef flat with no passes or channels. The rim is closed. The lagoon area is 6.1 km2. Ribbon reefs mark the lagoon slope to the northeast and the shallow lagoon to the southwest. A sand apron up to 300 m wide extends northwest–east counterclockwise and occupies the lagoon shallows to the northwest but thereafter spreads toward the lagoon slope and deep lagoon. We do not have depths for this lagoon. Liot Atoll takes the form of a rounded square about 20 km southeast of Ninigo (Figure 13.11). The platform is 3 km north–south, 2.9 km west–northeast at

Figure 13.10 Remote-sensing images of the atolls of Analtin and Ninigo. Images © 2021, Planet Labs PBC.

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Figure 13.11 Remote-sensing images of the atolls of Laualau and Liot. Images © 2021, Planet Labs PBC.

its widest, and covers an area of 5.9 km2. The outer reefs surround this atoll and are less than 100 m wide. Two large reef areas are located to the west and southwest inner reef flat. The rim is dominated by a single 1.3-km2 arcuate island that extends north–southwest and is up to 640 m wide. The island is forested, borders the lagoon, and is located about 540 m from the northern platform edge and about 420 m from southern platform edge. The remainder of the rim to the west is a reef flat up to 950 m wide that closes the lagoon. The lagoon area is 2.7 km2 and develops a shallow sandy ridge to the southwest, where lagoon slope reefs are found in addition to patch reefs along the shallow lagoon waters to the west. Most of the lagoon is not coded and we do not have depth data for it. Hermit Reef (not shown) is 50 km to the east-southeast of Liot and is an excellent example of an almost-atoll with four volcanic islands in the lagoon, the largest of which is about 4.8 km long and more than 900 m high. Four atolls are associated with islands to the north of Cenderawasih Bay, West Papua, where some that are closer to New Guinea are likely influenced by the NGCC as well as the South Equatorial Current (Figure 13.2). Mioswundi (Mios Woendi) is a uterine-shaped atoll located 7.8 km to the east of Biak Island and about 90 km northwest of West Papua (Figure 13.12). The platform is about 19 km west–east following a curved path and 10.6 km north–south through the middle. The platform area is 116.5 km2 and develops outer reefs that range from 150–300 m around most of this atoll although a length of 6.6 km along the western rim is 100 m wide or less. The rim supports eight terrestrial areas including six islets and two islands, Sandedori

to the northwest and Pai to the northeast. Almost all the islets and both islands are closely associated with patch reefs and seagrass meadows on their protected sides. The remainder of the rim is a reef flat, except to the south where there is a 3-km-wide lagoon entrance that is 9 m deep (Sailing Directions, 2020). The rim is open. The lagoon area is 52.6 km2 including the 1.8 km2 area south of Sandedori Island. The shallow lagoon and its slope are dotted with patch reefs, especially in the lee of islands and islets. Reef sand aprons occur around much of the lagoon reaching up to 1,250 m wide just west of Pai Island The lagoon deeper than 15 m presents reef-rimmed plateaus as well as several shallow reef areas that rise from the basin. We do not have depth data for this lagoon. Rani Atoll (Figure 13.12) is elongate and somewhat curved. It lies 92 km northwest of Mioswundi and less than 4 km offshore of the island of Biak, where the platform is about 21.1 km long northwest–southeast, 5 km wide, and covers an area of 66.4 km2, not including a 23 km2 submerged reef area to the northwest. The atoll is surrounded by an outer reef that is up to 480 m wide to the southwest and 50–170 m wide to the northeast. Scattered patch reefs form on the reef flats, especially to the northwest and on the outer reef flats around much of 3.4-km2 Yamnaisu Island to the southeast where there is a coconut plantation. Two small islands with a combined area of about 4 km2 include settlements and are located on the eastern rim. The remainder of the rim is a reef flat with no entry to the lagoon. The rim is closed. The lagoon is about 27.3 km2 and consists of an irregular area more than 15 m deep near the atoll

Atolls of Melanesia and New Guinea

Figure 13.12 Remote-sensing images of the atolls of Mioswundi, Rani, Avawi, and Mapia. Images © 2021, Planet Labs PBC.

center and a channel-like extension off to the northwest. There is no navigable channel entry to the lagoon center or to its northwest extension, and the two areas are separated by lagoon shallows and slope with scattered patch reefs. More numerous patch reefs are associated with plateaus to the eastern lagoon slope in the vicinity of the two small islands. The lagoon center deeper than 15 m contains plateaus and a few shallow areas, some of which are rimmed with reef. We do not have information on the depth of the lagoon. Ayawi is an atoll about 80 km northwest of Rani in the Pacific Ocean (Figure 13.12). The platform is

about 26 km northwest–southeast and 8.4 km at its widest. The platform area is 156.7 km2. One islet with an area of less than 6 ha remains on the surface to the southeast, and it is surrounded by coral patches. The rest of it is completely submerged. Purdy (2001) gives the lagoon depth as 28 m. Little else is known about this atoll. Mapia Atoll is about 100 km north of equatorial New Guinea. The platform takes the form of a warped isosceles triangle 16.0 km long and 6.8 km at its widest (Figure 13.12). The platform area is 63.3 km2 and it forms outer reefs that are 225–300 m wide mid-west to mid-east. Reefs to the southern half of the atoll vary

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from about 50–175 m wide. The rim supports two islets and two islands northwest–northeast, and a single island 4.4 km long and 400 m wide on the southern apex. All the islands and islets are densely packed with coconut palms. A winding channel 40–70 m wide and more than 3,200 m long is found to the west. Both sides are flanked by deep and shallow reefs and the center may be blocked by a reef flat, although Sailing Directions (2020) state that it may be possible for small craft to navigate. The remainder of the rim is a continuous reef flat with few if any reef formations. We regard the rim as semi-closed. The lagoon area is about 32.1 km 2 . The shallows and the lagoon slope develop a sand apron up to 1,100 m wide to the north, east, and south and 340 m wide or less to the west. The lagoon slope is clearly defined up to 750 m wide to the west and south and up to 620 m wide to the east. A ribbon of sand delimits the border between the slope and the

deep lagoon to the north and in part to the west. Pinnacles are prominent in the lagoon beyond the slope including several to the south and southeast that are rimmed by reefs. Only a few reef-rimmed or sand-covered pinnacles are found elsewhere in the lagoon, and we are unaware of any information regarding its depth. East and West Ayau (Figure 13.13) are atolls found 41 and 48 km north of Waigeo Island (see Figure 13.2), Raja Ampat National Park in West Papua and they are the northwesternmost of New Guinea’s atolls. Both lie on a small plateau although we are not aware of the depth between them. Little is known about them and there is no listing for these atolls in Sailing Directions. We describe them separately. West Ayau is shaped like a rounded triangle 19.4 km from the eastern apex to the western base, and 7.6 km at its widest. The platform area is 124 km2. The outer reefs vary from 75–360 m wide

Figure 13.13 Remote-sensing images of the atolls of East and West Ayau. Images © 2021, Planet Labs PBC.

Atolls of Melanesia and New Guinea

and are often more than 300 m wide to the northwest. The rim is without passes and supports a single 5.3 km2 island to the east that is rimmed by a narrow reef and about 1.5 km2 containing five species of seagrass (Irawan et al. 2020) that extend southeast to west clockwise. Three small reef areas occur on the western rim. The lagoon is about 47 km2 and forms an irregular border with a rubble inner reef flat where three very small reef-rimmed islets occur. The shallow and deep lagoon are sandy with numerous patch reefs. The reef growth suggests that some seawater exchange occurs, and we tentatively regard the rim as semi-closed. We do not have data for the lagoon depth. East Ayau is found less than 2 km to the northeast of West Ayau where it forms a warped hemisphere-like polygon 34.1 km north–south and 19.1 km at its widest through the center. The platform area is 413 km2. The outer reefs range from 150–400 m wide but the most commonly are 250–280 m. The rim supports nine scattered islets that total 241 hectares, the largest of which is 86 hectares. The inner reef flats present irregular borders and are composed of rubble and sand deposits which are wider to the east, where they can exceed 2 km in width, than the west. The lagoon area is about 272 km2 and is connected to the open ocean by two channels, each about 200 m wide that pass through the rim around an isolated section of reef on the northwestern platform. The rim is open. The channels are coded as reef and lead to a lagoon slope and deep lagoon area where several areas of reef are developed including a few reef ridges and multiple reef-rimmed pinnacles. We are not aware of any bathymetry for this atoll lagoon.

References Allen GR 2008. Conservation hotspots of biodiversity and endemism for Indo-Pacific coral reef fishes. Aquatic Cons. 18: 541–556. Asaad I, Lundquist CJ, Erdmann MV, Costello M 2018. Delineating priority areas for marine biodiversity conservation in the Coral Triangle. Biol. Cons. 222: 198–211. Cobon DH, Ewai M, Inape K, Bourke RM 2016. Food shortages are associated with draughts, floods, frosts and ENSO in Papua New Guinea, Agric. Syst. 145: 150–164. Davies HL 2012. The geology of New Guinea—the cordilleran margin of the Australian continent. Episodes 35: 87–102.

Davies HL, Symonds PA, Ripper ID 1984. Structure and evolution of the southern Solomon Sea area. BMR J. Austr. Geol. Geophys. 9: 170–199. Davis WM 1928. The Coral Reef Problem. Reprinted by AMS Press (1968), New York, p. 520. Delacroix T, Radenac M-H, Cravatte S et al. 2014. Sea surface temperature and salinity seasonal changes in the western Solomon and Bismarck Seas. J. Geophys. Res. 119: 2642–2657. Delfin F, Myles S, Choi Y et al. 2012. Bridging near and remote Oceania: mtDNA and NRT variation in the Solomon Islands. Mol. Bio. Evol. 29: 545–564. Friedlaender JS, Gentz F, Friedlaender FR et al. 2005. Mitochondrial genetic diversity and its determinants in island Melanesia. In: Pawley A, Attenborough R, Golson J, et al. (eds.) Papuan Pasts: Cultural, Linguistic, and Biological Histories of Papuan‑Speaking Peoples. Pacific Linguistics, Canberra, pp. 693–716. Friedlaender JS, Friedlaender FR, Hodgson JA et al. 2007. Melanesian mtDNA complexity. PLoS One 2(2): e248. https:// doi.org/10.1371/journal.pone.0000248 Ganachaud A, Gordeau, Kessler W 2008. Bifurcation of the subtropical South Equatorial Current against New Caledonia in December 2004 from a hydrographic inverse box. J. Phys. Oceanogr. 38: 2072–2084. Hoeksema B 2007. Delineation of the Indo-Maylayan Centre of maximum diversity: The coral triangle. In: Renema R (ed.) Biogeography, Time, and Place: Distributions, Barriers, and Islands, Springer, New York, pp. 117–178. Irawan A, Hafizt M, Hernawan UE 2020. Community structure and condition of seagrass meadows in Ayau Islands, Raja Ampat Regency. Mar. Res. Indonesia 45: 13–24 Kuroda Y 2000. Variability of currents off the northern coast of New Guinea. J. Oceanogr. 56: 103–116. MacGillivray J 1852. Narrative of the voyage of H.M.S. Rattlesnake, commanded by the late Captain Owen Stanley, R.N., F.R.G.S. & co. during the years 1846–1850 including discoveries and surveys in New Guinea, the Louisiade Archipelago, etc., Vol 1, T & W Boone, London. Permana DS, Thompson LG, Mosely-Thompson E, Mark BG 2019. Disappearance of the last tropical glaciers in the Western Pacific Warm Pool (Papua, Indonesia) appears imminent. Proc. Nat. Acad. Sci. USA 116: 26382–26388. Purdy EG 2001. Supplement to Purdy EG, Winterer EL 2001. Origin of atoll lagoons. GSA Bull. 113: 837–854. Radenac M-H, Léger F, Messié M et al. 2016. Wind-driven changes of surface current, temperature, and chlorophyll observed by satellites north of New Guinea. J. Geophys. Res. 121: 2231–2252. Sailing Directions 2017. The Pacific Islands Pub. 126. National Geospatial-Intelligence Agency, Springfield VA. Sailing Directions 2020. New Guinea Pub. 164, 15th ed. National Geospatial-Intelligence Agency, Springfield VA. Shaw B, Buckley H, Summerhayes G et al. 2011. Prehistoric migration at Nebira, south coast of Papua New Guinea: New insights into interaction using isotope and trace element concentration analysis. J. Anthropol. Archeol. 30: 344–358. Summerhayes GR, Leavesley M, Fairburn A et al. 2010. Human adaptation and plant use in highland New Guinea 49,000 to 44,000 years ago. Science 330, 78–81. Williams SL, Ambo-Rappe R, Sur S et al. 2017. Species richness accelerates marine ecosystem restoration in the Coral Triangle. Proc. Natl. Acad. Sci USA 114: 11986–11991. Zhai F, Hu D, Wang Q, Wang F 2014. Long-term trend of Pacific South Equatorial current bifurcation. Geophys. Res. Lett. 41: 3172–3180.

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The Atolls of the Solomon Islands With this chapter, we begin to explore the 16 atolls in the Solomon Island group and we add a seventeenth as we include the only atoll in Vanuatu (Figure 14.1). The Solomons are a complex of about 1,000 islands forming a double chain, oriented northwest–southeast

and extending between ~5°S and 11°S of the equator. The islands closer to New Guinea in the Bismarck Archipelago were occupied at least 39,500 years ago during the last glacial period. Although the gap between those islands and the Solomons is only about

Figure 14.1 A total of 17 atolls are found in the vicinity of the Solomon Islands. Two, Pinipir and Nissan, are associated with the Bougainville Rise, a block geologically associated with the island of Bougainville. Another eight atolls occur on the Ontong Java Plateau to the north (dark blue), the world’s largest submarine plateau. Three others are associated with the Santa Cruz Islands, and one other atoll is found in northern Vanuatu. All of these are on the Pacific plate that moves northwest and is separated from the Australian plate moving northeast, where three Indispensable Reefs with submerged rims are found on a submarine ridge. A branch of the South Equatorial Current dominates the area and is bifurcated by the Solomon Islands. The northern branch is strongest during the dry, strong trade wind season, but weakens during the wet season. Waters entering to the south side of the islands are weaker than those to the north.

DOI: 10.1201/9781003287339-14

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170 km, evidence suggests their occupation by about 12,000  years later (Wickler and Spriggs, 1988; Leavesley, 2006). The first European to discover the Solomon group was Álvaro de mendaña y Neira, the leader of a Spanish expedition who in the spirit of colonialism, commercialism, and proselytism explored them in 1568. Finding what were thought to be traces of alluvial gold on Guadalcanal, the captain named them Solomon’s Islands (Thompson, 1901), believing that perhaps he had found the source of King Solomon’s wealth. However, no gold or metal of any kind was found until the 1990s when volcanic-hosted gold was discovered and was mined briefly on Guadalcanal (Tolia and Petterson, 2005). The main group includes seven large islands that are bound to the north by the Pacific plate, which is moving to the northwest at about 10 cm/year, and to the south by the Australian plate, which is moving northeast at about 7 cm/year. In addition, the Solomon Islands are delineated by two trench systems formed by the collision of these two plates, areas where oceanic crust is consumed (Figure 14.1). The New Britain trench to the northwest and San Cristobal trench to the southeast mark the boundaries of the Australian plate and the Vitiaz trench marks the boundary of the Pacific plate, both at depths of  more than 5,000  m (Petterson, 2004). The resulting geology of the Solomon Islands is complex and involves sections that form their own  microplates causing fault patterns where vertical and horizontal  movement occur. The net result of these intraplate interactions is that the Solomon Islands are tectonically dynamic with volcanic activity above and below water, as well as frequent earthquake events up to magnitude 8, some of which generate tsunamis (Heidarzadeh et al., 2016). There are no atolls in the main Solomon Islands, but two, Pinipir and Nissan, are associated with the Bougainville Rise, a block associated with the same geological origin as the island of Bougainville itself (Davies, 2015). By contrast, eight other atolls including  Malum, Nuguria, Kilinailau, Takuu, Nukumanu, Ontong Java, Candelaria, and Sikaiana atolls are former volcanoes found on the Ontong Java Plateau (OJP) to the north, the world’s largest ocean plateau with an areal extent equivalent to that of Alaska and a crustal thickness of up to ~42  km (Petterson, 2004; Watson et al., 2017). The OJP and its atolls are shown in Figure 14.1. Despite being populated primarily by Polynesians, these atolls are part of the  Melanesian Solomon Islands. However, they are administered as the Autonomous Region of Bougainville by the government of Papua New Guinea as part of a peace agreement that ended a decade of rebellion and conflict between the two governments in 2000.

The Santa Cruz Islands are a group of volcanic islands located to the southeast of the  main group where three additional atolls are found: Nupani, Nakapu, and The Great Reef. Finally, a group of three atolls (The Indispensable group) occur on the Australian plate side of the Solomon Islands at the edge of the Coral Sea, and these are accompanied by two raised atolls, Bellona and Rennell, parts of a World Heritage Site.

Regional climate and oceanography The two  main seasons in the Solomon Islands are the dry, strong trade season (June–November) and the wet, weak-trade season (December–May) when winds may be from the west or northwest. During the wet season, the intertropical convergence zone passes through the area causing winds to weaken before reversing direction, thus creating monsoon conditions. At Nissan atoll to the north, rainfall averages roughly 3,000  mm/year (Spriggs, 1991). The Solomons are influenced by an average of between one and two cyclones annually, but those that do strike the atolls are often newly formed and so generally are weak. Currents are dominated by a branch of the South Equatorial Current (SEC) that varies in strength between seasons. To the north of the islands, the SEC is strongest during the dry, strong trade wind season, but during the wet season, the northwesterly SEC weakens and may even reverse its flow. Water flows entering the south side of the islands are weaker than those from the north and may also reverse flow during the monsoon season (Cravatte et al., 2011; Hristova and Kessler, 2011). Synoptic wind data (Figure 14.2) show that atolls to the west (Takuu) and north have stronger (but less frequent) winds from the west than the east, whereas areas to the south and east (Great Reef) include stronger easterly winds but less marked seasonal reversals. Waves can come from most directions (Figure 14.2); the wind waves approach from the east and southeast, but the largest waves (2014–2019) were from the northwest. It is possible that these wind reversals may contribute to the east–west symmetry in the atoll rims. Tides are mixed semidiurnal–diurnal, with two highs and lows of unequal heights daily. Spring tidal range at the Great Reef is 1.78 m, whereas neap tides vary by only ~0.6 m (Figure 14.2).

The atolls of the Solomon Islands The northwesternmost atoll of the Solomon Islands is Malum (Figure 14.3). The platform takes the form of a warped D and it is located 202 km northwest of

Atolls of the Solomon Islands

Figure 14.2 Atolls to the west and north (e.g., Takuu on the Ontong Java Plateau) may develop strong winds from the west, whereas areas to the south and east (e.g., Great Reef, Santa Cruz Islands) may include mostly easterly winds. Waves can come from most directions; the wind waves approach from the east and southeast, but the largest waves (2014–2019) were from the northwest. Tides are mixed semidiurnal–diurnal, with two highs and lows of unequal heights daily. Spring tidal range at The Great Reef is 1.78 m, as shown.

Bougainville, one of the main islands of the archipelago. The long axis is 13.6 km from northwest–southeast and it is 4.5 km at its widest point. The platform area is 43.9  km2 including outer reefs 125–230  m wide southwest–northeast counterclockwise, narrowing to about 100 m wide mid-west to northwest. The rim is dotted with about a dozen motu, none of which are more than 0.5 km2. A pair of them on the northwestern rim appears as Planaria-like crossed eyes and a small but distinctive crescentic motu occurs to the southeast. The rest of the rim is a reef flat with rubble on the outer flat followed by sand on the inner flat. Ribbon and patch reefs occur in association with some of the motu or on the flats in a few cases. The rim perimeter is composed of a spur and groove system. A narrow channel about 60 m wide opens to the lagoon slope to the southwest. There are no passes, and the rim is closed. The lagoon area is 26.6  km2

and it is surrounded in the shallow areas by a sand apron, typically 200–500 m wide. The lagoon slope, a prominent feature of the southwest basin, is included in the apron where it extends to  more than 800  m wide. A few pinnacles occur to the east-center basin along with 2–3 more to the northwestern and southeastern ends. We do not have a mean or maximum lagoon depth for this atoll. Nuguria Atoll (Figure 14.3) occurs on the same volcanic platform as Malum and is separated from it by a cleft 4 km wide. This atoll is much larger than its neighbor to the northwest and forms an irregularly shaped arc with a prominent bight to the northwest. The platform is 37 km long following a curved path northwest–southeast and about 9 km wide through the center. The platform area is 179.9 km2 and is surrounded by outer reefs that vary from 80–275 m wide although much of the east tends to be about 190 m

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Figure 14.3 Remote-sensing images of the atolls of Malum and Nuguria. Images Ⓒ 2021-2022, Planet Labs PBC.

wide. A reef crest also is well developed to the east as is a spur and groove system although the latter encircles the atoll. The northern 30-km perimeter of the rim is composed of a loop-like reef flat with two motu to the west. The eastern rim continues to the south with two shallow channels (depths unknown) separated by a 1.5-km-long section of reef flat. Several  motu and islands follow to the south on the reef flat for the next 13  km reaching Paopao Island, the largest on the rim. Paopao is 13 km long and extends mid-east to south, undulating in width from 40 to 600 m. The widest sections occur to the north, middle (where an airstrip is located) and to the south. The southern rim forms a beveled loop of reef flat that continues to the west ending with the 4-km2 island of Nuguria. The remainder of the western rim to the north of Nuguria consists of three islands and two small sections of

reef flats each separated by a total of seven channels, depths unknown. We mark the rim as open. The lagoon area is approximately 124 km2, that is about 7 km wide in the middle, narrowing to 2–3 km wide toward both ends. A sand apron occurs along the shallow lagoon northwest to the southwest near Nuguria Island. From there, the western rim continues to the northwest as a narrow strip about 250 m wide or less on the southwestern side but reaches about 1.4 km at its widest to the northeast. Narrow reef ribbons occur in the shallow lagoon flanking the seven channels on the western margin and the two on the eastern lagoon margins. The deepest region of the lagoon is about 30 m (Christianson, 1964) and is  marked by numerous pinnacles, some of which are fused and include small islands with areas of up to 10 ha as they reach the surface. The central deep

Atolls of the Solomon Islands

lagoon may be pitted with solution holes from previous subaerial exposure. Pinipir (Pinipel) Atoll (Figure 14.4) is located 121 km southwest of Nuguria and it is paired with Nissan, another atoll only about 2.5 km to its southeast. Collectively, these are referred as the Green Islands due to their densely wooded forests, and both are uplifted with Pinipir elevated to 101 m above sea level (Sailing Directions, 2017). The platform takes the shape of a lobster claw that is about 10 km northeast–southeast and about 4.3 km at its widest across the northern end. The platform area is 25.5  km2 with outer reefs surrounding the atoll generally 130– 170 m wide, although the Atlas codes it wider. The reefs to the southeast are best developed, reaching about 240 m from the shoreline, and least developed to the west. The rim is dominated by 5.5-km2 Pinipir Island that extends northwest–southwest on the east atoll flank and is 1.9 km wide to the north but narrows to about 200 m wide as it curves around the south. The remainder of the rim to the west is a reef flat with a few small islets on the northwestern end. The lagoon area is about 3.3 km2 and is found in the northern half of the platform (in the space between the pincers), where a ~200-m-wide pass that is about 6 m deep connects it to the open ocean (US

Navy Hydrographic Office chart 103). The rim is open. The shallow areas are surrounded by seagrass that extend from the intertidal to the inner reef flat, reaching up to 500 m wide to the northeast in the lee of Pinipir Island. The seagrass areas are followed by ribbon and patch reefs in the shallow lagoon, extending to the lagoon slope in some areas. A single 1.4-ha reef-covered pinnacle is found to the south of this lagoon which reaches a maximum depth of 33 m (US Navy Hydrographic Office 103). The southern half of the atoll is primarily a sandy inner reef flat that extends to the lagoon. Pinipir’s neighbor 2.5 km to the southeast is Nissan, a diamond-like, polygonal atoll (Figure 14.4) 15.5 km long northwest–southeast and 9.2 km at its widest with a total platform area of 88.1 km2. The outer reefs range from 200–390  m wide although coding is incomplete along the central part of the eastern platform. This windward eastern side presents a spur and groove system. The rim is dominated by Nissan Island that extends clockwise northwest— the  middle west. The island is about 1.0–1.8  km wide, 30 km long, and is heavily wooded. An airstrip has been constructed on the mid-eastern rim. Two islands occupy the remainder of the western rim to the north of Nissan, and each is separated by three

Figure 14.4 Remote-sensing images of the atolls of Pinipir and Nissan. Images Ⓒ 2021, Planet Labs PBC.

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narrow passes (South, middle, and North Channels, respectively) ranging from 5.5 to about 8  m deep (US Navy Hydrographic Office chart 1030) with reef material in the middle. The rim is open. The intertidal zone and the inner reef flats are associated with seagrasses, especially to the south. mangroves almost completely cover this area (Parkinson, 2010). The lagoon area is 46.6 km2. The inner reef flats and the shallow lagoon are associated with a sand apron that is up to 800 m wide to the northwestern margin and about 200 m to the southwest, but with narrow connections and short gaps between the two areas. Reef ribbons and patches line the  margins of the shallow lagoon. and the lagoon slope, especially to the south and west, less so to the east. most of the lagoon (82%) is deeper than 15 m with a maximum depth of 34 m (USN HO 1030).

Small pinnacles, some with reef patches, occur in the deep southern lagoon along with 2.5-ha Hon Island, coded as an outer reef flat with reef material on part of the rim. Kilinailau (Carteret Islands) is an ovoid atoll (Figure 14.5) about 25.5 km on its longest axis northeast–southeast and about 16  km at its widest, lying 120 km to the east–southeast of Nissan. The platform area is 302 km2 and it is surrounded by an outer reef slope that is 170–120 m wide along the eastern side and less than 100 m wide to the west. The rim is primarily a rubble and sand reef flat with five scattered motu no more than 52 ha each along with several smaller islets. Small patch reefs occur with seagrasses in the lee of the larger  motu. Reef ribbons also occur on the leeward western rim even where motu are absent. The rim is more than 99% submerged, but there are

Figure 14.5 Remote-sensing images of the atolls of Kininailau and Takuu. Images Ⓒ 2022, Planet Labs PBC.

Atolls of the Solomon Islands

three major channels: South, Southwest, and Northwest channels, respectively, none of which is deeper than 2.4  m (Sailing Directions, 2017). The center and northwestern channels are flanked by reefs on delta-like forms that open to the lagoon. The rim is open. The lagoon area is 246  km2 and a sand apron, mostly about 150  m wide or less to the southwest but up to 600 m elsewhere, occupies its shallows and ­extensively overlaps the inner reef flat. Ribbon-like reefs occur commonly on the shallow lagoon slope. The lagoon is associated with small pinnacles, some of which are ringed by reef. The area deeper than 15 m occupies 88% of the lagoon area and ­contains 12 sand- and reef-rimmed pinnacles, some of which cover up to 18 ha and break the surface. The ­lagoon displays a step-like terrace at 30–40  m depths, but most of it is 50–60 m deep (Christianson, 1964). Kilinailau Atoll has an interesting history of names including the Carteret Islands, the Sir Charles Hardy Islands, and the Tulun Islands. In the language of the Hanahan people of eastern Buka of the nearby Solomon Islands, tulun is the word for horizon, reflecting either the marked distance over the sea from Buka, the low-lying islands, or both. The atoll also has been referred to as the Nine Islands; the nine have been reduced since those days to six by an earthquake and erosion. Unfortunately, this trend appears to be continuing. As the residents noted their low-lying islands eroding and groundwater becoming more saline, in October 2007, the PNG government announced that islanders were to be moved forcibly to mainland Bougainville Island as ‘climate refugees’ to much media publicity (Connell, 2016). Families willing to relocate were even offered a hectare of land to grow cocoa seedlings. Nonetheless, by 2020, only ten families had been relocated and the population of the atoll has doubled. Takuu Atoll (Tauu) has ovo-annular configuration (Figure 14.5) and lies 160  km east of Kilinailau. The platform is 14.6  km northwest–southeast and is 14.3  km at its widest. The platform covers 130.6 km2, and similar to Kilinailau, its sister atoll to the west, the outer reef slope is  continuous and is  mostly 160–260  m wide but is narrower to the south. A spur and groove system surrounds the shallow and intertidal portions of the platform. The rim is primarily an intertidal reef flat with 13 or so motu to the east and one to the northwest. The largest of these motu is a triangular-shaped islet on the eastern rim which covers 74 ha. A nearby smaller motu to the north of the latter supports a small village. The total land area there is less than 100 ha. As on Kilinailau, reefs and seagrasses are found near and in the lee of the several of the eastern islands, as well as the one

to the northwest. There are three shallow channels to the southwest, west, and northeast as on Kilinailau, depths unknown, that lead to the lagoon. The widest to the southwest is 180 m, whereas the other two are 120 m wide. The rim is submerged. The lagoon area is 104  km 2 . The shallow areas are about 125 m wide or less and form a sand apron that overlaps extensively with the inner reef flat. Small patch reefs are found on the lagoon slope in the vicinity of the eastern islands. The slope deeper than 15 m is visible north–southeast in the lagoon basin, with a small patch to the south. A few pinnacles arise from the deeper regions of the lagoon, most of which are rimmed by reefs. Christianson (1964) describes the lagoon as terraced with steps at 15–20 m, 40–50 m, and a maximum basin depth of about 90 m. Nukumanu Atoll takes the form of an expanded D with its flat north–south axis to the west of 16.6 km and 20.5 km at its widest west–east (Figure 14.6). The platform area is 250  km2; its western, northern, and eastern margins are surrounded outer reefs that are about 220–275  m wide, but narrow to the northeast where they are  mostly 125–150  m wide. Although it is not apparent from the surface, the rim to the south has partly collapsed forming debris fields extending about 8 km from at the base of the atoll (Watson et al., 2017). The rim to the east is dominated by a single 2.4-km2 island, 8.9 km long that is mostly only about 100 m wide (except for a triangular expansion where it becomes a kilometer wide at the easternmost point). In addition, there are 25 motu including one to the south where there is a village. most of the rim with these motu are part of a unified reef flat with occasional patch reefs to the north and south. By contrast, five motu on the western rim cap four discontinuous reef flat sections that provide channels to the lagoon. The northernmost opening that is up to 1.2 km wide apparently constitutes a pass (Sailing Directions, 2017), but depths are not given. The rim is open. The lagoon area is 217  km2. The shallow lagoon to the northeast is rimmed by ribbon reefs and another occurs in the nearshore lee of the main island to the southeast. A sand apron is found in the shallow ­lagoon which spills over to the lagoon slope in several areas. The apron is widest in the lee of the southeastern island (about 750 m) and narrows in several areas of the shallow lagoon to as little as 150 m wide. The northwestern rim contains an additional 1.2-km2 area of a shallow lagoon embayment where the sand apron continues. This area opens through narrow gaps in the inner reef flat to the deeper lagoon. The lagoon slope includes reef growth and the western portion and is crowded with reef-rimmed pinnacles

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15–20 m deep... most of the lagoon, however, is about 30 m deep, roughly 40 m at maximum (Christianson, 1964) and supports six reef-rimmed pinnacles. Nukumanu Atoll is perhaps best known as the last confirmed position reported by Emelia Earhart for the Lockheed  model 10E Electra. She and her navigator, Capt. Fred J. Noonan, had departed from Lae, Papua New Guinea, on July 2, 1937, en route to Howland Island, 2.556 miles distant. Neither the aviator nor her plane have been recovered. The atoll of Ontong Java is one of the largest in the world and the plateau on which it resides is named after it. This boot-shaped atoll lies about 98 km to the south of Nukumanu (Figure 14.6) and forms an angle of about a 100° from the center of the north-facing shaft to the tip of the boot pointing to

the east. Following this path, the platform is 84 km long north–east and is 26 km at its widest through the heel. The platform area is 1,418 km2 and includes outer reefs that are about 190–465 m wide along the western and southern  margins, narrowing to about 150–175 m wide to the east. The rim is composed of about 30 scattered motu along with 2-km2 Luaniua Island to the east (the toe of the boot) and ~ 1-km2 Pelau Island to the north, both of which are occupied by villages. Islands account for less than 1% of the rim, most of which is a reef flat composed of rubble and sand. The rim is essentially submerged. Numerous bights mark the rim periphery of this atoll, and they are associated with slope failures and debris fields at depths of about 1,400  m near the volcanic base. These debris are far  more extensive

Figure 14.6 Remote-sensing images of the atolls of Nukumanu and Ontong Java. Images Ⓒ 2020-2021, Planet Labs PBC.

Atolls of the Solomon Islands

than those found around Nikumanu (Watson et al., 2017). There are seven passes to the lagoon, three on the western rim and four around the periphery of the toe (British Admiralty chart 3199; no lagoon depths given). Coral ribbons flank the channels and the island-like rubbly reef flats that lie lagoonward of them. Several channels to the north and south of the toe are flanked by delta-shaped extensions of the rim that project into the lagoon. The lagoon area is 1,265 km2 but is complex with numerous reef flat islands that project from the lagoon slope, especially near passes to the south and west that are included in this measurement. The lagoon has been described generally by Crean (1977) and the Allen Atlas as follows. The shallow lagoon (2–6  m deep) is composed of scattered corals on a sandy bottom that extends around the lagoon as a sand apron. The apron is up to 800  m wide at the southernmost point inside the rim but is  more

typically about 100  m wide. The lagoon slope (6– 25  m) is widest to the west and south. The lagoon floor is composed of coral-derived sand and gravel at a depth of 30–46 m. Numerous pinnacles, most of them reef-rimmed, rise from the deep basin. Candelaria (Roncador) is an annular atoll that develops a northeast-facing projection ­(Figure 14.7). Located about 78 km south of Ontong Java, the atoll is about 13 km northeast–south west and about 10 km at its widest. The platform is 97.0 km 2 and is surrounded by outer reefs that are 180–330 m wide, with somewhat greater widths to the southeast. The rim is primarily a rubbly reef flat about 700 m wide to the north and 500 m wide or less elsewhere. The rim is a reef flat and there are no islands; with no islands and no residents, chiefs from Ontong Java consider Candelaria theirs. Two breaks in the rim that occur to the southwest are 2.8  km apart and are referred to as passages

Figure 14.7 Remote-sensing images of the atolls of Candelaria and Sikaiana. Images Ⓒ 2021, Planet Labs PBC.

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(Sailing Directions, 2017). The northernmost passage is at least 250  m wide and is bisected by an arrowhead-shaped island (and two other shoal areas) but  may be navigable. The channel to the south is about the same width but shows uncoded reef growth between sand shoal areas. The lagoon area is 77.8  km 2 . The shallows are outlined by a sand apron 60–160  m wide. A prominent lagoon slope 1.2  km wide occurs to the northeast and is accompanied by a broader but deeper slope to the southwest. Several pinnacles are associated with the deeper lagoon slope, but only a few are apparent in the deepest basin which extends to a depth of 36 m (Sailing Directions, 2017). The atoll was mapped as Peregrino Roncador, ‘the Snoring Pilgrim,’ by the European explorer, Francisco Antonio mourelle de la Rúa. Mourelle describes how on the night of January 22, 1781, the crew of the frigate La Princesa heard ‘a frightful roaring’ as they came upon the reef ‘quite white with foam.’ Sikaiana Atoll (Figure 14.7) is located on the southeastern extent of the OJP about 450 km southeast of Candelaria. The platform of this ovotriangular atoll is 9.5 km west–east, bisecting the base and extending through the east-facing apex. It is 6.7 km at its widest across the west-facing base. The platform area is 30.4 km2 and is surrounded by a narrow band of outer reefs that are 70–125 wide. The deep outer reef slope is incomplete to the north and develops gaps to the south, whereas the shallow outer reef circumscribes the platform and ends near shore with a well-developed spur and groove system. The rim is a reef flat with three motu on the western triangular base that together comprise a land area of 41 ha. The east-facing Sikaiana Island is the largest at 1.3 km2 and is the only one that is inhabited. The atoll rim is closed. All four islands are vegetated and develop patch reefs on their protected side but cover a greater area near the larger island although little coral  material is coded on the reef flat. The lagoon area is 16.4 km2 and a sand apron up to 300 m wide, but more often 90–100 m wide, occupies its shallows. Note that the Allen Atlas lagoon is not coded and lacks a satellite image for this atoll. The lagoon deeper than 15 m accounts for most of its area where a few pinnacles are found. We have no information concerning the mean or maximum lagoon depth. Spalding et al. (2001) mention that Sikaiana is associated with a number of small islands around a 45-m-high remnant volcano and refer to it as a near-atoll. This description does not appear to fit any of the imagery that we have examined, and we can find no corroborating literature.

The Santa Cruz Islands (Figure 14.1) lie near a 90° bend in the boundary between the Pacific and Australian plates and are volcanic islands in an area of high seismic activity emanating deep from the nearby San Cristobal trench. There are four active above-water volcanoes in the group as well as others that are submerged. Three atolls or atoll-like structures are found about 425 km southeast of Sikaiana and 31–39  km northeast of the northernmost (and volcanically active) Santa Cruz Island of Tinakula. These are referred to as the Reef Islands. The first of these is Nupani, an ovotriangular atoll (Figure 14.8) 7.7  km long north–southeast, and 6.4  km at its widest along the south-facing base. The 25.0-km2 platform area is surrounded by outer reefs that are best developed to the north, east, and south where they are 330–420  m wide and  more than 500 m wide to the northeast. The western reefs are narrower, typically 130–185  m wide. A spur and groove system is found in the shallow waters of this atoll. most of the rim is an inner reef flat with no passes to the lagoon. The rim is closed. A single 5-ha motu occurs on the eastern rim and an island with an area of 1.4 km2 is found to the north. The lagoon area is 11.9 km2. The western rim contains wide areas of shallow lagoon with numerous patch reefs and sand patches. Ribbon reefs are found around  most of the northern, eastern, and western edge of the deeper lagoon along with a discontinuous series of sand patches. The basin deeper than 15  m constitutes about 71% of the lagoon area. A few coral-rimmed pinnacles rise from the deeper south lagoon basin. We do not have a reference for the mean or maximum depths. Nakapu is an ovoid atoll located about 30 km east of Nupani. The platform is 3.3 km northeast–southwest and about 2.5  km at its widest (Figure 14.8). The platform area is 7.0  km2 and is surrounded by outer reefs that are  mostly 250–350  m wide. The rim is composed of a single 1.5-km2 island to the northeast (more than 20% of the platform area); the remainder is an inner reef flat that is up to 550  m wide to the southeast. There are no channels or passes and the rim is closed. The island and the reef flats around it are closely associated with reefs. Patch reefs are found along the inner reef flat to the northwest. The lagoon area is 2.2  km2. The shallow lagoon is more than 400 m wide to the northwest where a sand apron is best developed, and it continues to the southeast and southwest where it overlaps with the lagoon slope. Here, it narrows to about 100 m wide. The lagoon center covers an area of about 80 ha and occurs as a slope less than 15 m deep judging from

Atolls of the Solomon Islands

Figure 14.8 Remote-sensing images of the atolls of Nupani, Nakapu, and The Great Reef. Images Ⓒ 2019-2021, Planet Labs PBC.

the appearance on optical imagery and coding by the Allen Atlas. Ribbon reefs are scattered in the middle of the deeper lagoon as well as peripheral to it toward the northwest. The Great Reef is an atoll complex (Figure 14.8) about 13  km southeast of Nakapu. It is not to be confused with Australia’s Barrier Reef, which is also great, but  much larger. The  main part of the atoll somewhat resembles a hammerhead shark that is 27.8 km long from head to tail (east–west) following

a curved path and 11  km at its widest. The eastern half of the southern platform edge is dissected by  multiple irregular openings to the lagoon. The platform area is 139  km2, not including adjacent reef flat islands or peripheral reef areas. The platform is rimmed by outer reefs that typically range from 250–350 m wide although areas to the west are up to 485 m wide and parts of the north are about 200 m. The rim is highlighted by a single island and an adjacent island fragment located to the east that together

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amount to an area of 5.7  km2 or 4.1% of the platform area. The rim is open. The lagoon is bordered by an inner reef flat that is most extensive to the north and the last 4 km of the tail. The lagoon area is about 99.3 km2. The shallow lagoon to the north is bordered by a sand apron up to 2 km wide to the north of the head, but it is more often less than 500 m wide. Patch reefs occur to the north in the shallow lagoon, but more of them are associated with the southern lagoon margin. The greatest concentration of reefs occurs in an area of about 6.8 km2 in the lee of the island to the east associated with the lagoon slope, which is coded as reef by the Atlas. The lagoon area deeper than 15 m is extensive but is associated with reef-studded plateaus. We are not aware of any depth data for this lagoon system. We note that in addition to the  main atoll, The Great Reef is associated with four small satellite atolls without islands ranging from 1.5 to more than 5 km from its southern rim, all of which are composed primarily of a comparatively wide south-facing reef flat. The westernmost atoll is elongate and curved with a platform of 3.25 km2. The lagoon area is 74 ha and is open to the north. most of it is shallow, but an area of about 10 ha forms a lagoon slope that is deeper, likely less than 15 m deep. Reefs surround the platform and enter the deeper lagoon to the north. Another such atoll lies just over 2 km to the east of the latter and is similar in shape but larger with a platform area of 6.8 km2. This atoll is surrounded by outer reefs that are well developed on all sides. The platform is composed of reef flats surrounding two lagoon areas. The westernmost is about 1 km2 and is surrounded by shallow and deep ribbon reefs where the depth is greater than 15 m, whereas the eastern lagoon is about 25 ha, an area that is covered by deep reefs but is likely less than 15 m deep. A small ovoid atoll to the east of the latter covers a platform area of about 1.5  km2 composed of reef flats. A lagoon area of 27 ha, likely more than 15 m deep, is surrounded by plateaus as well as outer reefs. The easternmost satellite atoll takes an ovotriangular form, is immediately adjacent to the previous one, and covers an area of 8.3  km2. The platform is surrounded by outer reefs followed by a reef flat that is most extensive to the south where it is about 1.2 km wide. The lagoon area is 3.1 km2 and displays prominent plateaus to the west that are dotted with reefs. most of the lagoon appears to be deeper than 15 m. The last atoll of this group is located on the northern extension of the New Hebrides (aka the Vanuatu) trench, an area that also includes the Santa Cruz Islands. However, while Rowa Atoll (Figure 14.9) is

part of Vanuatu geologically, it is the only atoll in that island arc archipelago. Rowa is about 390  km southeast of The Great Reef and forms a polygon to the north with arcuate handle extending to the south. The platform is 9  km northeast–southwest, 5.6 km at its widest west–east, and covers an area of 28.4 km2 including outer reefs. Reefs average about 370  m wide around the atoll but extend to 840  m to the west and as little as 190 m wide to the northwest. The rim consists of three reef-rimmed islets to the northeast and one to the northwest, the largest of which is 37 ha. The total land area is about 1.1 km2. The platform is primarily a sandy reef flat with a very shallow lagoon area of about 2.0 km2. The western rim is submerged and allows ocean water to enter three small areas in the western lagoon totaling about 42 ha that are coded as reef by the Atlas. All the atolls described thus far reside on the Pacific plate, but there are three Indispensable Reefs, which are part of the Solomon Islands that lie on the Australian plate (Figure 11.2) about 746 km west of Rowa or 522 km south of Sikaiana Atoll. The Indispensable was a French sailing vessel that was captured by the British during the French Revolutionary Wars in 1793. As she sailed in the service of the British East India Company, a discovery was made of dangerous low-lying reefs and a passage between the main islands of the southeastern Solomon Islands, both of which were named after the vessel. The Indispensable Reefs form a line trending northwest–southeast. None of the three has islands; they are all shallow and may be uncovered at low tide. This may be due to tilting and uplift at Bellona and Rennell Islands about 100 km to the north which are raised 79–110 m, respectively, likely associated with their proximity to the San Cristobal trench (Grover, 1962). Indispensable Reef North forms an elongated cylinder with horn-like projections, two to the north and one to the southwest (Figure 14.10). The platform is 20.1 km northwest–southeast and 7.9  km at its widest, southwest–northeast. The platform area is 108 km 2 where the outer reefs are generally up to 370  m wide to the southwest and 215–270 m wide to the northeast. The rim is submerged with an area of about 21  km2. A narrow west-facing channel is  may be partly blocked by shoal areas. A north-facing channel about 200 m wide appears to be navigable but. We are not aware of depths that may have been published for either of these channels. The lagoon area is about 80.6  km2. The shallow areas and part of the lagoon slope are marked by a sand apron up to 550 m wide to the southeast and about 300 m wide elsewhere. The lagoon slope is clearly visible to the

Atolls of the Solomon Islands

Figure 14.9 Remote-sensing image of the atoll of Rowa. Images Ⓒ 2021, Planet Labs PBC.

southeast and narrows to about 400–500 m wide on the southwest margin. Several sandy to reef-rimmed pinnacles and plateaus rise from the deep lagoon basin and from the outer edge of the lagoon slope to the southeast. We do not have a median or maximal depth for the lagoon. Indispensable Reef Middle is the largest of the three submerged atolls. It lies less than 5 km to the south of Indispensable North and is shaped like a section through a spade with a northwest-facing shaft and a southeast-facing blade (Figure 14.10). The platform is 62.2 km long following the center of the lagoon, 13.6 km at its widest, and covers an area of 416 km 2 . The platform is surrounded by an outer reef that is mostly 250–350 m wide to the southwest and 150–250  m wide to the east. The rim to the northwest of the blade is a series of shoal areas and shallow channels 12.4 km long, and there are two

channels to the east. The northernmost entrance is about 330 m wide with a shoal-water obstruction in the  middle. The second is separated from the first by a short section of reef flat and  measures about 190  m wide. This channel appears to be clear for passage, but we are unaware of data for its depth. The rest of the rim is a reef flat covering an area of 72 km 2 . The lagoon area is 344 km 2 . The shallow lagoon and upper slope support a sand apron typically about 300–400  m wide but expanding to about 880 m wide to the easternmost point and 1.2 km wide to the northwest. The lagoon deeper than 15 m constitutes the most extensive portion of the basin, and numerous reef-rimmed pinnacles rise from it. Indispensable Reef South is cylindrical and somewhat torpedo shaped, 21.3  km through the

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Figure 14.10 Remote-sensing images of the atolls of the Indispensable Reefs, North, Middle, and South. Images Ⓒ 20202021, Planet Labs PBC.

lagoon center northwest–southeast and 9.3 km at its widest (Figure 14.10). The platform area is 153.5 km2, and like its neighbor to the north, the platform is surrounded by an outer reef that is generally 175–325 m wide but extends to more than 800 m to the southeast. The shallow reefs continue onto the reef crest and on the reef flats in multiple areas. The rim is a rubbly reef flat with an area of about 27  km2 that is open through four channels along a 2.6-km-long area to the west. The lagoon is 128.4  km2. The shallow lagoon features are covered with a sand apron that is up to 2.4 km wide to the southeast. The rest of the apron is variable but mostly less than 400 m wide. The deep lagoon supports  multiple pinnacles and ridges that are coded as reef-rimmed and rise from a maximum depth of 35 m (Sailing Directions, 2017).

References Christianson S 1964. Lagoons of Nukuria and its neighboring atolls: A field reconnaissance. Geografisk Tidsskrift 63: 237–248. Connell J 2016. Last days in the Carteret Islands? Climate change, livelihoods and migration on coral atolls. Asia Pacific Viewpoint 57: 3–15. Cravatte S, Ganachaud AA, Duong Q-P et al. 2011. Observed circulation in the Solomon Sea from SADCP data. Progr. Oceaogr. 88: 116–13. Crean K 1977. Some aspects of the beche-de-mer industry in Ontong Java, Solomon Islands. SPC Fisheries Newsletter 15: 37–49. Davies H 2015. The geology of Bougainville. In: Regan AJ, Griffin HM (eds.) Bougainville before the Conflict. Australian National University Press, Acton AU, pp. 20–30. Grover JC 1962. Geology of Rennell and Bellona. The great uplifted atolls on the edge of the Coral Sea. In: Wilson EO (ed) The Natural History of Rennell Island, British Solomon Islands, vol. 4, pp. 103–119. Heidarzadeh  m, Harada T, Satake K et al. 2016. Comparative study of two tsunamigenic earthquakes in the Solomon Islands: 2015  mw 7.0 normal fault and 2013 Santa

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Cruz mw 8.0 megathrust earthquakes. Geophys. Res Lett. 43: 4340–4349. Hristova HG, Kessler WS 2011. Surface circulation in the Solomon Sea derived from Lagrangian drifter observations. J. Phys. Oceanogr. 42: 448–458. Leavesley m 2006. Late Pleistocene complexities in the Bismarck archipelago. Archaeology of Oceania: Australia and the Pacific islands 8: 189–204. Parkinson, R (2010). The German Solomon Islands, Together with Nissan and the Carteret Islands. Sydney University Press, Sydney. Petterson  mG 2004. The geology of north and central  malaita, Solomon Islands: The thickest and most accessible part of the world’s largest (Ontong Java) ocean plateau. In: Fitton JG, mahoney JJ, Wallace PJ et al. (eds) Origin and Evolution of the Ontong Java Plateau. Geological Society, London, vol. 229, pp. 63–81. Sailing Directions 2017. Pacific Islands Pub. 126. National Geospatial Intel. Agency, Springfield VA.

Spalding mD, Ravilious C, Green EP 2001. World Atlas of Coral Reefs. University of California Press, Berkeley CA, p. 320. Spriggs m 1991. Nissan, the island in the middle. Summary report on excavations at the north end of the Solomons and the south end of the Bismarks. Report of the Lapita Homeland Pro‑ ject, pp. 222–243. Thompson B 1901. The Solomon Islands by Alvaro de  mendaña in 1568 Translated from the Original Spanish. Hakluyt Society, London. Tolia DH, Petterson  mG 2005. The Gold Ridge  mine, Guadalcanal, Solomon Islands’ first gold  mine: A study in stakeholder consultation. Geol. Soc. London Sec. Publ. 250: 149–159. Watson SJ, Whittaker JM, Lucieer V et al. 2017. Erosional and depositional processes on the submarine flanks of Ontong Jave and Nukumanu atolls, western equatorial Pacific Ocean. mar. Geol. 392: 122–139. Wickler S, Spriggs m 1988. Pleistocene human occupation of the Solomon Islands, melanesia. Antiquity 62: 703–706.

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The Atolls of Fiji Fiji is an archipelago of more than 300 islands, originally populated by Austronesian people from New Guinea and the Solomon Islands as described in Chapter 13. These were among the groups referred to as Melanesians, dark-skinned people that occupied Fiji (and Tonga) as their southeasternmost outposts in the Western Pacific. The islands were first encountered by Europeans through the agency of Abel Tasman, a Dutch explorer who came upon these islands quite by chance in February 1643 aboard his vessel the Heemskercq. The two largest islands are Viti Levu and Vanua Levu to its northeast (Figure 15.1). There are 34 atolls of this archipelago in our account, but there are likely more, especially smaller ones that have been overlooked. We tentatively include a few with islets of unknown composition in the lagoon. Almost all the atolls in this archipelago are submerged and are either exposed at low tide or are only slightly deeper. Although they are remarkable in that respect, they are not unique as described in later chapters. We accept the possibility that these atolls may have resulted from solution by exposure during low stands of sea level (e.g., Purdy and Winterer, 2006), but we maintain a broad view of atoll morphology irrespective of origin as described in Chapter 1. We also note that the atolls of Fiji are among the most poorly known of the large Pacific Island groups. Most of Fiji’s atolls are part of the Lau Archipelago and are associated with the elongate Lau Ridge, a remnant volcanic island arc that was active until about 5–7 million years ago when it was joined to the Tonga Ridge (Nunn, 1994; Neall and Trewick, 2008). The Lau Ridge currently rises more than 3,000 m above the surrounding seabed (Packham, 1978). The neighboring islands of Tonga are part of an active volcanic island arc that is currently being uplifted as the Tonga Trench (Figure 15.1), the edge of the Tonga microplate, continues to dive under a complex of plate junctions that include Fiji. The two ridge systems gradually parted due to seafloor spreading between Tonga and the Lau Islands. Although most of the Lau volcanic activity ceased millions of years ago, some islands continued erupting from 2.5 million to 280,000 years ago (Nunn, 1999) producing the complex history that characterizes this region.

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The northern Lau islands generally appear to have been uplifted more than those to the south, including some that are currently over 300 m above sea level. One of these, Vanuabalavu (Figure 15.1), is a large island shaped like a question mark. It has a volcanic spine 23 km long and 4 km wide that is elevated to 283 m in the center. However, to the north, the volcanic slopes end abruptly at the base of limestone cliffs that rise vertically, averaging 122 m above sea level. This part of Vanuabalavu is heavily forested and is composed of a jagged limestone replete with sinkholes. Ladd and Hoffmeister (1945) describe the initiation of the island’s formation by volcanism dating from the early Miocene about 20 million years ago. A succession of limestone deposits followed, initially in water at least 200 m deep, and later, by a more coralliferous limestone in shallow water. By the end of the uplift period, the island was at least as high as it is now, exposing the limestone to subaerial solution and wave activity. Uplift was then followed by a second sequence of volcanic activity that lasted for millions of years (Nunn, 1999). A second period of volcanic activity was followed by subsidence so that the island was about 40 m lower than it is today. A younger limestone was then deposited on the submerged parts, leaving the higher regions of the island with only the older, heavily eroded limestone. Sea-level changes during the Pleistocene also left telltale signs, ultimately flooding deep sinkholes that are now bays and initiating the reefs surrounding the island. Vanuabalavu is now subsiding again relative to the rest of the Lau Ridge, at rates fast enough to cause erosion due to recent sea level rise, while others in the vicinity are rising (Nunn et al., 2002), thus producing barrier reefs, atolls, and almost-atolls, occasionally next to one another. Interestingly, Vanuabalavu is one of several high volcanic islands enclosed within a 53-km 2 lagoon by a barrier reef and is thus an almost-atoll once named Exploring Isles. The latter name is the consequence of the U.S. South Seas Exploring Expedition, also known as the Ex. Ex., commanded by Lieutenant Charles Wilkes. The expedition included a squadron of six vessels that was tasked with providing accurate charts of several Pacific

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Figure 15.1 The 34 atolls in Fiji archipelago are all located between 16°S and 24°S. Most are located on the Lau Ridge, where most develop rims that are near sea level but remain submerged. Nine atolls are found north of the Lau group and are at times referred to as the Ringgold Isles, several of which are associated with the young oceanic crust of the Fiji Plateau. Two other atolls are isolated from both groups.

islands including some of the Tuamotus, as described previously (Figure 3.25), and entered Fijian waters in 1840. Others also attempted to map Fiji. Lieutenant William Bligh made notes and took the positions of Viti Levu and Vanua Levu, the main islands, after having been cast adrift in a lifeboat by the mutineers aboard HMS Bounty. The stretch of water between these two islands is still referred to, perhaps a bit insincerely, as Bligh Water. Later, the eponymous Russian Admiral Fabian Gottlieb von Bellingshausen also made notes of the islands in 1820, but the Exploring Expedition was the first scientific entry into what were called the Cannibal Islands (see Smith, 2013 for details).

The reef islands and atolls of Fiji as presented here are typically given names in Fijian preceded by the word Cakau, which means ‘reef ’ in Fijian and was once spelled phonetically as Thakau, because in the Fijian language, C sounds like TH. Today, however, it is spelled exclusively as Cakau (Nunn pers. comm.). In our descriptions below, we have taken out such references so that Cakau Momo, for example, is referred to as ‘Momo Reef or Momo Atoll.’ It may also be noted that there are numerous variants in the spelling of Fijian place names. For example, Qelelevu is referred to on some older maps as Nggelelevu, because in the Fijian language, Q sounds like NGG (Sprague, 1987). Lastly, there are atolls

Atolls of Fiji

that have retained English names on maps and charts despite the existence of perfectly appropriate Fijian ones. We briefly derive the history of behind these English names whenever possible, and when referred to as a reef on charts such as Defense Mapping Agency 83500, we refer to them as a proper name (e.g., Heemskercq Reefs described below). The atolls of Fiji occur between 16°S and 24°S, and thus, they lie in the heart of the trade wind belt. As such, whether in the north or south part of Fiji, atolls are dominated by easterly winds. Rainfall varies markedly from the high islands, which can receive 200–300 cm per year near the coast and up to 600 cm in the mountains, to the smaller islands and atolls, which average 150–300 cm per year. The winter months from May to October include winds from the southeast and are generally drier. November to

April, the summer months, includes more precipitation and winds that are more easterly; these months are also cyclone season. Less than two tropical cyclones annually hit Fiji on average, nonetheless, in 2020 alone, three tropical cyclones, Tino, Harold, and Yasa, devastated parts of Fiji. Although winds commonly are from the east and southeast, waves come from a range of directions (Figure 15.2). For the atolls to the south (e.g., Minerva South), the most energetic waves largely are from the southwest, but those to the north (e.g., Matacucu Reef) are affected by strong waves from the southeast, south to southwest, and northwest (Figure 15.2). Tides are semidiurnal (Figure 15.2). At Matacucu, spring tidal range is 1.71 m and neap tidal range is 0.6 m. Tidal range decreases slightly to the south, with spring range of 1.53 m at Minerva South.

Figure 15.2 The atolls of Fiji lie in the heart of the trade wind belt. Although winds commonly are from the east and southeast, waves come from a range of directions. For the atolls to the south (e.g., Minerva South), the most energetic waves largely are from the southwest, but those to the north (e.g., Matacucu Reef) are affected by strong waves from the southeast, south to southwest, and northwest. Tides are semidiurnal. At Matacucu, spring tidal range is 1.71 m, but tidal range decreases slightly to the south, with spring range of 1.53 m at Minerva South.

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The Ringgold atolls The northernmost nine of the Fijian atolls are referred to as the Ringgold Isles, named after Lieutenant Cadwalader Ringgold, who captained the brig Porpoise as it plied these waters as part of the Exploring Expedition (Smith, 2013). Some of these most northerly Fijian atolls are in line with the eastern tip of the large island of Vanua Levu and may be associated with the young oceanic crust that composes the Fijian Plateau (Figure 15.1). The first is Matacucu Reef (Matathuthu) (Figure 15.3), a bean-shaped atoll that is slightly canted to the northwest. The platform is about 11 km along a southeast–northwest axis and about 6 km at its widest normal to that. The platform area is 41.2 km2. Most of the rim is submerged; the shallowest parts appear to lie on the north- and

east-facing margins. The platform is composed of outer reefs that are widest to the east where they extend more than 700 m wide, narrowing to about 300–400 m elsewhere. The rim is rocky and forms a well-developed reef crest north–northeast, whereas the rim to the northwest is open for about 1.6 km. The lagoon is about 30 km2 and develops small reef areas to the west and southeast. A sandy lagoon slope 1500 m wide occurs on the north slope along with other small sand patches. Most of the lagoon slope is coded as reef. A few deep reef pinnacles occur to the southeast and the maximum depth is 35 m (Agassiz, 1899). Vuthovutho Atoll (Vuthocutho, Figure 15.3) is about 7 km northeast of Matacucu and takes the shape of a warped square about 10 km northeast– southeast. It is 7.8 km northeast–southwest. The

Figure 15.3 Remote-sensing images of the atolls of Matacucu, Vuthovutho, and Qelelevu. Images © 2019, 2021, Planet Labs PBC.

Atolls of Fiji

platform area is 52.4 km2. The rim is submerged, although numerous images with breaking waves on the eastern and northern margins suggest shallower reefs. As on Matacucu, the rim is deepest to the west where the lagoon is open to the open ocean. The lagoon area is 37.8 km2 and the shallow areas form reefs near the lagoon slope to the north and east. The eastern and part of the northwestern margins are marked by a submerged sand apron 350–400 m wide. The lagoon deeper reef plateaus to the north. The lagoon is 27 m deep at its maximum (Agassiz, 1899). Qelelevu Atoll (Nggelelevu, Figure 15.3) is ovopolygonal with apices to the north and east. The platform is 24.7 km west–east and 12 km north–south at its widest. The platform area is 189 km2 and its margins are marked by an outer reef that is best developed to the north where it is up to 375 m wide. The northern and eastern sides also forms a well-developed reef crest. The rim is composed primarily of a reef flat where the shallow outer reef is better developed and extends from offshore. The eastern rim develops an island with an area of 1.5 km2 that includes uplifted limestone deposits with elevations of 5–18 m, from west to east (Sand et al., 2007), and a smaller island 1 km to its northwest. The marginal inner and outer reef flats are wider and more continuous on the northern margin. The southwestern platform is open and is punctuated by four passes among areas of shallow outer reef. The lagoon is 157 km2 with numerous patch reefs on the shallows to the north and the east. The shallow lagoon is composed of a sand apron up to 2.5 km wide to the east, becoming narrower elsewhere from 1.9 km to 250 m, generally broader on the northern margin. The deeper parts of the basin include more than 20 reef-capped pinnacles and the maximum depth is 37 m (Purdy, 2001). The remaining Ringgolds are a group of four, the first of which is 19 km southwest of Qelelevu and is called the Heemskercq Reefs (Figure 15.4) after Tasman’s ship, where he negotiated a pass not much wider than the vessel, only because he was otherwise trapped by wave breaks. Heemskercq (currently referred to as Nukusemanu) is more or less sausage shaped with narrow and extended ends north–south, and two 5–6 km projections on the east-facing rim. The platform is 57 km north–south and 12.4 km at its widest to the north. The total area is about 331 km2 including Heemskercq’s southern extension of 10 km2 that is about 10 km long and called Nanuku Reef (Figure 15.4). The outer reef is varies in extent and on average is 440 m wide to the east and 415 m wide to the east, although a small area is more than a kilometer wide. The platform center is deeper than the rest and is coded as reef that appears to merge with the lagoon slope. Two islets, one on the north and the

other in the center of Nanuku at the south, and a few small sand cays mark the emergent part of the rim which is almost entirely submerged with shallow reefs that extend to the lagoon. Sand deposits are relatively common to the northeast, parts of the east, and to the south. Most of the rim is a complex interlacing of reef flat, shallow lagoon, sheltered reef slope, lagoon slope, and plateaus 1.3–1.6 km at their widest on the northern and southern platform. A pass through the rim occurs just north of the easternmost projection of the east face. The lagoon area is roughly 261 km2 and can only be estimated by tracing the lagoon slope and marginal plateaus. Scattered pinnacles are found throughout the deep lagoon, which is 95 m deep at its maximum (Agassiz, 1899). About 2 km west of the central part of Heemskercq’s western rim lies Pitman Reef, a small ovoid atoll 2 km northwest–southeast and 1.1 km west–east (Figures 15.4 and 15.5). The platform area is 2.1 km2, qualifying Pitman as the smallest atoll in the Fijian Archipelago. The platform is surrounded by outer reef that are mostly 155–250 m wide but narrows to 130 m to the northwest. Shallow reefs continue onto the rim and the southeastern section develops a wide reef crest. The rim consists of separate rocky reef flats to the north, south, and west platform areas. There are no islands, and the reef is awash at low tide (Sailing Directions, 2017). The lagoon area is 1.3 kmincluding the shallow lagoon and its adjacent slopes. The deeper lagoon occupies an area of 45 ha and is about 33 m deep (Agassiz, 1899). We also note Budd Reef, a large almost-atoll located 12 km southwest of Pitman where, as described by Agassiz, a crater about 800 m in diameter and 44 m deep rises more than 180 m above sea level in the northeastern lagoon. Another small atoll, Adolphus Reef (Figures 15.4 and 15.5), is found 9.5 km east of Heemskercq. Adolphus is an annular atoll that includes a northwest-facing projection. The platform is 8 km northwest–southeast and it is also 8 km southwest– northeast. The platform area is 43.7 km2 and it is surrounded by a complex of shallow and deep outer reefs. The rim is submerged to a depth ranging from about 2 to 7 m (Agassiz, 1899). The rim is discontinuous to the southwest. Nonetheless, the reef breaks heavily to the northwest and southeast (Sailing Directions, 2017). The composition of the rim is a combination of reef-covered plateaus and shallow reefs. The lagoon is roughly 25.8 km2 and it is about 37 m deep at its maximum (Agassiz 1899). A few pinnacles occur to the north and a scattering of deep reefs and reef-covered plateaus occur elsewhere in the lagoon. A deeper sand apron up to 900 m wide occurs to the northwestern basin and tapers as it approaches the southwest break in the rim.

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Nukubasanga Reef (Figures 15.4 and 15.5) is a small, oval atoll 2.6 km from Adolphus. The platform is 4.3 km northeast–southwest, 2.6 km at its widest, and its total area is 10.3 km2. Outer reefs are about 300–550 m wide north–south and narrow to about 200 m wide to the west. The northeastern reef extends to 850 m wide. The rim is composed of a rocky reef flat extending north–southwest and is open to the west. A 19-ha islet forms to the northeast. The lagoon area is 2.8 km2 and has patch reefs in the shallows although most of it is sandy. A small islet occurs to the west and small patch reefs occur toward the lagoon center. Two small atolls are found west of Nanuku Reef (the southern tip of Heemskercq). First is Motua Lailai (Figures 15.4 and 15.6) lying 9 km west of Nanuku. The platform of this egg-shaped atoll is 2.2 km east–west, about 1.5 km north–south, and covers an

area of 2.6 km2. The outer reefs are 175–200 m wide west–east clockwise and reefs southeast–southwest are wider, generally 380–560 m. The rim is a broad rocky reef flat up to 400 m wide on the waveward southeast–southwest, much of which is coded as shallow reef that surrounds the atoll; however, the rim is miscoded by the atlas as open to the north. We mark it as semi-closed, unlike most of these reefs that are essentially submerged. The lagoon covers an area of about 64 ha. The lagoon slope displays a sand apron that is up to 200 m wide to the west and tapers to 60 m wide or less to the east. The deeper central area is about 45 ha (about 70% of the lagoon area) and includes patch small reefs but we do not have mean or maximal depths. Motua Levu (Figures 15.4 and 15.6) is 4.6 km west of Lailai Reef. The atoll is square with a

Figure 15.4 Overview of remote-sensing image of the Heemskercq Reefs, including Pitman, Adolphus, Nukubasanga, Nanuku, and Motua Lailai. Images © 2020, Planet Labs PBC.

Atolls of Fiji

Figure 15.5 Remote-sensing images of the atolls of Pitman, Adolphus, and Nukubasanga. Images © 2021, Planet Labs PBC.

concave side and two projections to the northwest. The platform is 3.6 km northwest– southeast, 3.2 km northeast–southwest, and occupies an area of 7.7 km 2 including outer reef areas. The outer reefs are about 175–250 m wide except to the north where they narrow to about 150 m wide. The rim is submerged to the west and northwest but is dotted with reef patches, except for 2.9 km along the south–southeast where the rim is consolidated and better developed than elsewhere. An abbreviated sand apron occurs to the southeastern corner and extends about 1.3 km where it is up to 150 m wide. The deep lagoon occupies about 51% of the lagoon area where it is reported to be 46 m deep. While

there is no entry to the lagoon, the northwest rim is submerged enough to allow passage of small boats (Agassiz, 1899). An isolated (1.2 ha) pinnacle rising from the deep lagoon is capped by reefs. Several large plateaus to the north and a few pinnacles with numerous reef patches are also prominent features of the lagoon.

The Lau atolls The Ringgold Islands are separated from the Lau Islands by the 13-km-wide Nanuku Passage on the southern end of Heemskercq Reefs. Wailangilala ­Atoll (Figure 15.7) is the northernmost of those on

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Figure 15.6 Remote-sensing images of the atolls of Motua Lailai and Motua Levu. Images © 2020, Planet Labs PBC.

the south side of the passage. The platform is ovoid with an apex to the northeast, is 7.4 km northeast– southwest, 4.8 km at its widest west–east, with a total platform area of 25.0 km2. Wailangilala is surrounded by outer reefs that are 390–550 m wide to the east and southeast and narrow to 135–160 m to the west. The northeastern reef projects to at least 800 m wide. The shallow reefs continue onto the rim where a reef crest and patch reefs form around most of the periphery. A 240-m-wide, 12-m-deep, reef pass occurs to the west (Sailing Directions, 2017) where it is flanked by reefs. The lagoon area is 14.7 km2. A submerged sand apron is best developed on the northeast, lagoonward of the islands where it is more than 400 m wide. The apron continues as a narrower strip mostly about 100 m wide, to the east, south, and west for more than 6 km. Sand deposits on the western margin

north of the pass and to the north are narrow and patchy. The shallow lagoon displays numerous patch reefs on the east and southwest margins. The deep lagoon area is 10.9 km2 and extends to 42 m deep (Sailing Directions, 2017), where the Allen Atlas codes a few reef-rimmed pinnacles. We note that Wailangilala island, about 46 ha in extent, is for sale at press time. Duff Reef (Figure 15.7) is located about 14 km east of Wailangilala. This atoll was named after a London Missionary Society ship that nearly ran aground on it in 1797. The platform is shaped like a bottle opener 11.7 km north–south and about 3 km west–east, with 2.8 km of the southwestern rim submerged and open to the lagoon. The platform area is 37.5 km2 including outer reefs that are 600–1100 m wide to the northwest and east, and 60–135 m

Atolls of Fiji

Figure 15.7 Remote-sensing images of the atolls of Wailangilala and Duff. Images © 2021, Planet Labs PBC.

wide to the west. The widest reefs are clearly visible on optical Planet images but to a large extent they appear too deep to code. Part of this uncoded area is a spur and groove system surrounds the rim and is extensively developed clockwise northwest–south where it is 400–700 m. The rim is at or below sea level and is without islands except for a 4-ha sand cay on the southeast. The lagoon is 20.4 km2. A sand apron typically 120–250 m wide occurs in the shallow lagoon west–south clockwise, although it is discontinuous in several small areas, and it is absent to the south– southwest. About 12 reef-covered pinnacles occur as a circle in the deep lagoon, just north of the rim opening in the southwest. The maximum depth is 20 m (Purdy, 2001). Williamson, Dibbles, and Bell reefs lie to the south of Wailangilala and Duff. We do not know if they are named after people or ships. Williamson Reef (Figure 15.8) is a small atoll that lies about 20 km southwest of Duff. The reef takes the shape of a rectangle with rounded edges and a concave-out southern rim. The platform is 2.4 km east–west, 1.5 km north–south, and has a total area of 3.7 km2. The outer reefs are 160–285 northeast–south clockwise and narrow to 140–175 m to the west. The rim is

composed primarily of a rubbly reef flat on all but the western side where it submerged. There are no islands and most of this reef is dry at low water (Agassiz, 1899). The lagoon area is 1.7 km2 and develops as reef patches in the shallow lagoon. An abbreviated sand apron about 200 m wide occurs on the southeast corner. The deep lagoon occupies 34 ha where it is 24 m deep at its maximum (Purdy, 2001) and is apparently too deep to code. Dibbles Reef (Figure 15.8) is located about 8 km northeast of Williamson and appears as a compressed ovoid atoll that is about 4 km east–west and about 2.5 km north–south. The platform area is 7.5 km2 and is surrounded by an outer reef that is 420–625 m to the east, 100–170 m wide to the west. The shallow reefs also extend onto the outer reef flats west– northeast. The reefs on the southwestern platform are somewhat patchy, and this follows the structure of the rim, which develops a reef crest and flats that reach up to 700 m wide north and east but are narrower and submerged or at sea level on the southwest. There are no islands. The lagoon is 4.7 km2 and develops a deep and shallow reef about 300 m wide on the lagoon margin west–northeast, in parallel to the distribution of reefs on the outer reef flat. The

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Figure 15.8 Remote-sensing images of the atolls of Williamson, Dibbles, and Bell. Images © 2021-2022, Planet Labs PBC.

deep lagoon occupies 3.1 km2 and is up to 27 m deep (Purdy, 2001) where no features are coded, but appears to host several patch reefs. Bell Reef (Figure 15.8) resembles a warped triangle with its base to the northeast, and it is located 6.3 km southeast of Dibbles. The distance between the apex on the northeast that bisects the base is about 4 km and the platform is 3 km at its widest parallel to the base. The platform area is 7.8 km2 including outer reef growth extending 345–460 m northwest–south clockwise, and 180–250 wide southwest–west. The shallow reef extends to the outer reef flats and to the shallow lagoon as well. The rim is mostly submerged, lacks island development, and is open to

the northwest. An uninterrupted reef crest occurs north–east clockwise where the rim forms a rock and rubble breakwater. The lagoon area is approximately 3.9 km2. Patch reefs and ribbons are found in the shallow lagoon. The deeper lagoon extends to 22 m (Purdy, 2001) and includes knolls and pinnacles that are too deep to code to the southeast. Traveling south from Bell Reef, we pass to the south side of the Vanuabalavu group and encounter five more atolls. The northernmost of them is Malevuvu Reef (Figure 15.9), located 46 km southeast of Bell. This atoll takes a sickle shape oriented with its long axis north–south. The platform is 5.1 km north–south, 3 km through the middle

Atolls of Fiji

Figure 15.9 Remote-sensing images of the atolls of Malevuvu and Vekai. Images © 2021, Planet Labs PBC.

east–west, and has a total area of 11.0 km 2. The outer reefs are best developed to the northeast–east where they are about 450 m wide, narrowing southeast– southwest, then becoming irregular to the west ranging from 280–450 m wide. As on other atolls of this group, the rim is broadest to the east (the windward side), and in this case to the north and south as well, where there is a reef crest and a well-developed rocky outer reef flat. The lagoon area is 5.5 km 2, the shallow part of which develops a sand apron north–south on the eastern flank that is about 180–350 m wide. Patch reefs surround the shallow lagoon, although they are also closely associated with the inner reef flat in some areas. Deeper waters occupy about 56% of the lagoon area, which is 26 m at its maximum (Purdy, 2001).

Vekai Reef is a small ovopolygonal atoll with a bight on the southeastern rim (Figure 15.9) located 19 km southwest of the southern end of Malevuvu. The longer axis of the platform measures 3.7 km northwest–southeast, 3.3 km northeast–southwest, and constitutes a total area of 8.6 km2. The outer reefs are widest to the south–southeast where they are 270–300 m wide; the remainder is about 175–260 m wide. The atoll develops a spur and groove system north, east, and south; the imagery is unclear to the west. The rim is developed as a reef crest and rocky flat clockwise northeast–west. The rim is submerged for 1.7 km to the northwest. There are no islands. The lagoon area is 5.3 km2. The shallow lagoon is surrounded by numerous patch reefs and sandy areas (except to the northwest). There is a 0.8-ha rock of unknown composition to the lagoon edge of the inner reef flat near the bight. Waters deeper

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than 15 m occupy more than 80% of the lagoon which extends to a maximum depth of 31 m (Purdy, 2001). No details are shown there by the Atlas. Three small atolls lie in a chain southwest–northeast of Vekai, the first of which is ­ Lasemawara Reef (Figure 15.10), about 28 km to the southeast of Vekai. This reef is ovotriangular with a somewhat concave-in base facing northwest. The distance from mid-base to the apex to the southeast is 2.6 km, and it is 3 km northeast–southwest parallel to the base. The platform area is 6.3 km2 including outer reefs that average about 180 m wide, narrowing to about 125– 165 m to the southwest. The rim is outlined by

a reef crest that is best developed north–mid-west following the surf pattern evident on many images. The rim is an islandless rocky reef flat up to 830 m wide. The inner reef flat bears ribbon-like patch reefs on its inner edges to the northwest and south. A sand apron up to about 400 m wide occurs in the southeast, narrowing to the north and west. The deep lagoon occupies 45 ha and is up to 11 m deep according to Purdy (2001) and displays a few pinnacle-like structures. Nokeva Reef (Figure 15.10) is about 4 km northeast of Lasemawara. The irregularly shaped platform is 6.7 km2 and is about 4.4 km on its longest axis northeast–southwest and 2.2 km at its widest

Figure 15.10  Remote-­sensing images of the atolls of Lasemawara, Nokeva, and Tabu. Images © 2021-2022, Planet Labs PBC.

Atolls of Fiji

The British colonization of Fiji The British interest in Fiji began with the discovery of sandalwood trees and the precious oils they contained. Ironically, the secret was let out by one of the survivors of the Argo wreck thinking that he would make a fortune by keeping the trees a secret. But it was not very long before American and British entrepreneurs and missionaries became interested in Fiji. In the wake of the Civil War, the price of cotton had drawn thousands of white settlers to Fiji as the ‘perfect place’ to grow cotton. They acquired land, almost universally using guns, alcohol, and intimidation, and began enslavement of Fijian hill tribes. The new residents of Fiji did not ingratiate themselves with the natives. John Brown Williams, an American agent, was appointed by the U.S. State Department to oversee U.S. interests in Fiji. During the July 4 celebration in 1849, the wadding from a saluting cannon struck and set fire to the agent’s home. Fijians quickly set about collecting belongings in the home for themselves. Williams’s trading store was looted, and a European settlement was burned to the ground. This and other incidents involving American interests prompted Williams to sue the local Fijian King Cakobau for tens of thousands of dollars, which the King was unable to pay. Mr. Brown then threatened to depose the monarch using the U.S. Navy for the purpose. As the debt began to mount with additional fines and compound interest, Cakobau pleaded his case with the British Wesleyan Missionary Society, Methodists who had become very powerful in Fiji. Indeed, Cakobau converted to Christianity and established a constitutional monarchy dominated by European settlers as a means of retaining a semblance of control. When the new constitution collapsed, Cakobau offered Fiji to be ceded to Britain in exchange for remaining an influence in Fijian affairs. Eventually, Britain took the king up on his offer in 1874 and Fiji became a British colony, which lasted until 1970 when Fiji became an independent nation. west–east. The outer reefs are generally 195–325 m wide but expands to the northwest where it is 385– 535 m wide. A spur and groove system is found on the eastern rim. The eastern rim north–south clockwise is bounded by a reef crest and a rock and rubble reef flat 850–950 m wide. A 12-ha area of the reef occurs on the reef flat to the northeast. The western rim is pointed at the west and south and is discontinuous to the northwest. Much of this reef dries at low water (Sailing Directions, 2017). The lagoon area is 1.8 km2. Two small areas of the lagoon to the northwest are enclosed from the shallow lagoon by a plateau system and are open to the surrounding sea by narrow channels. These are coded as deep lagoon and are replete with reef growth. Another is outlined by ribbon reefs. Additional ribbons occur on the periphery of these three areas, much of it overlapping with the inner reef flat. A sand apron up to 600 m wide occurs in the shallow lagoon and extends north–south. Purdy (2001) gives the maximum depth of the deeper areas as 11 m. Tabu Reef (Figure 15.10) is 9 km northeast of Nokeva and is shaped as a rounded square with a concave northeast-facing side. The platform is 5.6 km north–south and 5.5 km west–east and covers an area of 18.8 km2. The platform edge is marked by outer reefs that extend an average of 310 m from the rim and are widest to the north and west where they are more than 400 m wide. The rim is marked by a reef crest and a rock and rubble reef flat extending north–southwest where a spur and groove system also is found. The western rim is essentially submerged for 2.4 km. The lagoon area is about 2.4 km2

and includes a few patch reefs in the shallows, mostly to the south and north. Two small, somewhat deeper areas to the west are rimmed by reef. The shallow lagoon forms a sand apron up to 900 m wide that extends clockwise northwest–southwest. The lagoon slope displays three knoll-like structures. The lagoon basin is 20 m deep at its maximum (Purdy, 2001) and is uncoded. Between these small atolls and the next group lies the Lakeba Passage which is roughly 700–1,000 m deep (Defense Mapping Agency 83500). The distance from Tabu on the north of the passage to Reid Reef on the other side is 30 km to the southeast. Reid Reef (Figure 15.11) is arrowhead shaped with the tip pointing east. The outer reef varies from 195–580 m wide, averaging 360 m with the widest reefs located in part of the west and part of the north. The platform area is 92.0 km2. The rim is marked by a reef crest that is best developed to the east and south followed by a rock and rubble reef flat about 600 m wide on the north, south, and east apices. The rim to the west is open for 3.2 km and is constituted mostly by shallow reef and channels to the lagoon. The lagoon is 74.7 km2 and its shallow waters are dotted with patch reefs, many of which overlap with the inner reef flat. A sand apron forms to the north, east, and south mostly in the shallow lagoon with some overlap on the inner reef flat. It is more than 800 m wide to the east, but more often the apron is about 250 m wide. The lagoon deeper than 15 m contains numerous plateaus with patch reefs on their surfaces. In addition, three reef-rimmed islets of unknown composition project from the lagoon. Two of these appear to build

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Figure 15.11 Remote-sensing image of Reid Atoll. Images © 2021, Planet Labs PBC.

from the lagoon slope to the north and the third is located to the east. Agassiz (1899) describes them as rising 10, 20, and 60 feet (3.1, 6.1, 18.3 m) from the lagoon, probably as limestone structures. The lagoon is 38 m deep at its maximum (Purdy, 2001). Bukatatanoa, also known as the Argo Reefs, are sometimes divided into north and south because of a deep, but narrow (~1.3-km-wide) channel between them. The English name is derived from the schooner Argo that was on its way from Sydney to China with goods for sale in 1800, but the vessel was wrecked on the reefs named after her. The crew was able to make their way to an island near Vanua Levu where they were discovered by Fijians. All but two survived and eventually escaped. North Argo Reef (Figure 15.12) includes a platform of 110.7 km2 and is bell-shaped with the mouth facing southwest and is 15.8 km along the north–southwest axis and 11.2 km at its widest parallel to the base. The outer reefs are generally 555– 920 m wide to the east and 170–380 m wide to the west although one section of the west extends to 1400 m and another to the southeast is 2150 m wide. The rim to the northeast develops a reef crest; the east is dotted with areas of reef crest and reef flats but is otherwise dissected into submerged channels. The western and southwestern rim is largely open, and is

marked by a few largely submerged reef flat areas and a pass that occurs to the northwest. The lagoon area covers about 85 km2 where numerous patch reefs and ribbons are found in its shallows, especially to the northwest and parts of the east. A sand deposit occurs to the shallows to the northeast. An islet is found to the northeast on the inner edge of the shallow lagoon. The deep lagoon occupies 74 km2 of the lagoon area. A reef-rimmed 95-ha island of unknown composition rises from a deep portion of the lagoon slope. The lagoon basin is dotted with small plateaus that are reef rimmed. The maximum depth recorded for the lagoon is 38 m (Purdy, 2001). South Argo Reef (Figure 15.12) is a polygonal atoll with an elongate northeast–southwest orientation, where the platform is about 38 km long and about 21 km at its widest northeast–southwest. The platform area is about 528 km2, almost five times the size of its neighbor to the north. The outer reefs ranged from 150–350 m wide with the wider measurements occurring south–southeast. The rim is 99 km long and forms a nearly continuous belt of rock, rubble, and sandy reef flats northeast–southwest along 2/3 of its perimeter. The remaining 33 km of the perimeter on the western and northeastern platform edge is open and is composed of submerged reef knolls and plateaus. A 5.8-km-long segment of the northern rim is curiously

Atolls of Fiji

Figure 15.12 Remote-sensing images of North and South Argo atolls. Images © 2021, Planet Labs PBC.

arranged as an emergent arc facing northwest–southwest, where it marks the closest approach of this atoll to the southern rim of North Argo Reef. This segment also is surrounded by a series of reef-studded plateaus. The lagoon area is 444 km2, almost all of which is deeper than 15 m. A few plateaus occur in the lagoon but only along the south basin by contrast with North Argo Reef. A few deep reef-rimmed pinnacle-like structures emerge in the central and west, and two rock islets of unknown composition are found to the north-central lagoon. The maximum depth is 60 m (Purdy, 2001). Lekaleka Reef (Figure 15.13) is a small atoll 24 km south of South Argo Reef on a path that passes through the volcanic lagoon island of Oneata which may represent an almost-atoll. Lekaleka is egg shaped with a slightly concave southern face. It is 2.7 km at its widest, about 2.2 km north–south, and a platform area of 4.2 km2. The platform is surrounded by outer reefs that that range from 85–195 m wide northeast– southeast and 250–275 m wide southwest–northeast clockwise. The rim is developed as a reef flat north–southwest with equal bands of rock and rubble toward the lagoon up to 400 m wide to the east. A reef crest forms northeast–south. The western rim is a submerged reef

with reef-flanked channels that lead to the lagoon. Coral patches are scattered on the reef flat to the east and southeast. There are no islands. The lagoon is 2.7 km2. Many patch reefs occur in shallow areas, especially to the northwest–south. A submerged apron up to 300 m wide occupies the shallow lagoon and part of the slope to the east; it narrows to about 100 m wide to the west. The lagoon area is 1 km2 and is up to 18 m deep (Defense Mapping Agency chart 83500). Motu Reef is a V-shaped atoll with the apex pointing northeast; the western platform is open to the lagoon (Figure 15.13). Motu is about 8 km northeast–west and 7 km at its widest, north–south across the west. The platform area is about 36 km2. The rim is a rock and rubble reef flat that is well formed clockwise northeast–south, less so to the north and northwest. There are no islands, an irony for an atoll of this name. The development of outer reefs follows the same pattern as the rim and are 470–600 m wide with somewhat greater widths to the north. The lagoon area is about 28 km2. Patch reefs are found on the inner reef flats to the east and north followed by a shallow sand apron typically about 400 m wide on the margins, increasing to about 800 m wide flanking the northeast apex. The lagoon slope extends

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along the southeast and north, narrowing to a ribbon to the northeast where a single reef-rimmed pinnacle is found. Five or six other reef-rimmed pinnacles are found on the deep slope. The maximum lagoon depth is 42 m (Purdy, 2001). Vau Reef is an ovotriangular atoll (Figure 15.13) about 4 km east of Motu that is 3 km from a northeast-facing apex to the base and 2.4 km parallel to the base where it is widest. The platform area is 5.0 km2. The outer reef covers the entire platform periphery where it is 130–285 m wide, the wider extent occurring the southwest and parts of the southeast. The rim is composed of a rocky reef flat that extends clockwise northeast–west. A reef crest occurs primarily northeast–west clockwise and is patchy elsewhere. The rim width is about 500 m to the northeast and

southeast and narrows to about 100 m elsewhere. The lagoon is 1.7 km2 and is mostly a shallow and sandy with a few patch reefs to the east and northwest. A 1,100-m-long rocky, reef-covered ridge flanks a deeper lagoon area 20 km2 that also occurs to the northwest. The depth of the lagoon is not known, but Agassiz (1899) characterized the lagoon water as ‘impounded,’ suggesting that oceanic exchange is limited despite the absence of any obvious barriers. The depth of the main lagoon also is not known. Vuite Reef is a small ovoid atoll 30 km west– northwest of Motu with a concave side and two projections to the north (Figure 15.14). The platform is 4.5 km north–south and about 3.2 km at its widest. The platform area is 15.1 km2. The outer reefs range from 215–300 m wide south–north clockwise and

Figure 15.13 Remote-sensing images of the atolls of Lekaleka, Motu, and Vau. Images © 2021-2022, Planet Labs PBC.

Atolls of Fiji

Figure 15.14 Remote-sensing images of the atolls of Vuite, Levu, and Vuata Vatoa. Images © 2021-2022, Planet Labs PBC.

narrowed to 60 m or less northeast–southeast. The rim is best developed as a rock and rubble reef flat on the south–southwest but extends to the northeast and west as well. The reef crest extends northeast–southeast clockwise. Patch reefs are common on the reef flat, especially east–southwest. The rim to the west is submerged and consists of a series of reefs separated by narrow channels to the lagoon. The rim here is miscoded as part of the lagoon. The lagoon area measured from the satellite image is 9.6 km2. The shallow lagoon is developed as a submerged sand apron especially northeast-southwest where it is about 335– 400 m wide and is poorly developed to the northwest. The maximum lagoon depth is 29 m (Purdy, 2001). Levu Reef is an atoll with a warped V-shaped rim (Figure 15.14) about 22 km south of Motu Reef. The platform is about 7.9 km from an east-facing apex to the open western boundary, and it is about 4 km at its widest north–south. The platform area

is 22.7 km 2 including adjacent ridges and uncoded reef to the northwest, north, and south. The outer reefs are generally 300–725 m wide to the north and 425–790 m to the south. The eastern reefs are about 150–325 m wide. The rim is composed of a rock and rubble reef flat that is 650 m wide to the east, narrowing to about 150 m wide to the north and south. A reef crest is well developed on the eastern half if the rim. The western rim is composed of a submerged outer reef 1.6 km long. Shallow reefs to the northwest followed by a 700 m wide portion of the rim that is also submerged. The lagoon area is about 13.9 km2. Patch reefs are found on the inner reef flat to the north and east. A sand apron about 550–950 m wide occurs to the east in the shallow lagoon and narrows to about 100–400 m wide toward the west. Patch reefs occur in the sandy shallow lagoon to the east and north. The lagoon slope extends from near the middle of the

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lagoon and extends with variable widths north and south to the western rim. About 42% of the lagoon is beyond 15 m deep and the maximum depth is 24 m (Purdy, 2001) where a few reef-rimmed pinnacles are found to the west. There are four additional atolls on the Lau Ridge that are isolated from the others. Vuata Vatoa is a small oval atoll (Figure 15.14) 110 km south of Levu Reef. The platform is about 7 km north–south and 4 km west–east. The platform area is 28.4 km2 including adjacent plateaus and deep reef areas. A narrow band of outer reefs 65–155 m wide surrounds the platform. The rim is almost continuous north– west as a rubbly reef flat except for about 1,100 m to the northwest where the rim is notched with narrow channels. Patch reefs are found to the east and north. We mark the rim as submerged. The lagoon is 16.1 km2 and presents a sand apron primarily in the shallow lagoon that overlaps in some areas with the inner reef flat. The apron is a kilometer wide to the southern basin, narrowing to about half that to the west and east. The lagoon slope forms with pockets of shallower water are rimmed by reefs. We have no data on the lagoon depth. Vuata Ono is an ovotriangular atoll (Figure 15.15) that is 104 km to the southwest of Vuata Vatoa. It is 6 km on its longest axis northeast–southwest and 3.8 km at its widest, northwest–southeast. The 14.2km 2 platform is surrounded by outer reefs that are best developed southeast–south where they are 300370 m wide. The rim is composed of a rocky reef flat that is widest east–west clockwise where it is 350–480 m wide. A well-developed reef crest also

occurs here. The rim narrows to the north and is accompanied by a diminished reef crest. There are no islands. The lagoon is approximately 4.9 km 2. Some patch reefs develop to the north on the inner reef flat and to the north in the shallows. A sand apron covers most of the inner reef flat and shallow lagoon and it is up to a kilometer wide to the southwest. The lagoon slope is a narrow band that is coded as reef. A lagoon area of 1.3 km 2 that is deeper than 15 m is present to the western basin. We do not have a lagoon depth average or maximum for this atoll. We note in passing that the possible almost-atoll of Oni-i-Lau with three volcanic islands in the lagoon lies 10 km to the northeast of Vuata Ono. These islands represent a collapsed volcanic cone that reaches an altitude of 79 m (Clark 2009). The last two atolls on the Lau Ridge are the two Minerva Reefs, named after a whaling vessel that was wrecked on one of them in the early 1800s. A territorial dispute between Fiji and Tonga has yet to be settled and we cover them here in the context of the Lau Ridge. Minerva Reef North is an annular atoll (Figure 15.16) 6.8 km north–south and 6.5 km west–east. The 34.8-km2 platform is essentially surrounded by a narrow band of outer reefs 115–185 m wide northwest–southeast clockwise. The reefs are 45– 115 m wide southeast–west. The ref crest is best developed to the northwest and north–northeast. The rim is a low-lying rock and rubble reef flat with large areas occupied by reefs on the northern outer reef flat and especially on the southwest–southeast inner reef flat. The rim dries at low tide. A pass to the lagoon of unknown depth is on the northwestern rim and is

Figure 15.15 Remote-sensing images of Vuata Ono Atoll. Images © 2021, Planet Labs PBC.

Atolls of Fiji

about 320 m wide. The lagoon area is 22.9 km2. Patch reefs are found in the shallow lagoon to the north, many of which are connected to those on the outer reef flat. The southern half of the shallow lagoon is occupied by a sand apron about 300 m wide that overlaps with areas of the inner reef flat. About 73% of the lagoon is deeper than 15 m and the maximum depth is 27 m (Sailing Directions, 2017). No details are coded in this deeper area. Minerva Reef South is located about 32 km southwest of Minerva North. The platform is an incomplete figure eight representing the conjunction of two volcanic peaks (Figure 15.16). The total platform area is 26.1 km2 and it is surrounded by a narrow band of outer reef that varies from 45 m wide to the west to 185 m to the northwest. The rim is low to the water and is composed of rock and rubble that extends to about 550 m wide to the southeast. The western part of the platform is shaped like a laboratory (Erlenmeyer) flask with the neck facing east and is composed of a wide rock and rubble reef. This area

dries at low tide prior to the eastern rim due to small elevational differences (Sailing Directions, 2017). The lagoon is 9.1 km northeast–southwest, and is 4.3 at its widest and forms an area of 4.5 km2. A thin but continuous reef ribbon marks the northern and narrow eastern edge of the outer reef flat. The western lagoon develops a sand apron up to 600 m wide to the south and rubble appears to partially block the entrance of the western lagoon to the east. The lagoon area is about 5.8 km2 and is about 13 m deep (British Admiralty chart 948). A sand apron occurs northwest to east and lies between the shallow lagoon and the lagoon slope. The easternmost part of the figure-eight platform is annular and measures 4.1 km north–south, 4.5 km west–east, develops an area of 5.8 km2 and a depth of about 13 m (British Admiralty chart 948). The perimeter of the eastern rim is 13.6 km and about 550 m of that is submerged to the northwest and serves as an entrance to the lagoon. However, reefs are shown on British Admiralty chart 948 in the

Figure 15.16 Remote-sensing images of the atolls of Minerva Reef North and Minerva Reef South. Images © 2021-2022, Planet Labs PBC.

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Figure 15.17 Remote-sensing images of the atolls of Momo and Navatu. Images © 2022, Planet Labs PBC.

entrance, whereas the depth is not. The rim forms a reef crest to northwest–east, forming a rock and rubble flat that widens to about 400 m to the east. The eastern lagoon is wider and deeper than its westerly counterpart (10.8 km2 and 23–27 m deep, respectively near the entrance). The shallows are marked by patch reefs northwest–northeast as well as to the southwest followed by sand deposits northeast–south that become patchy elsewhere.

Other Fijian atolls There are two atolls that are not associated with the Lau Ridge. The first of these is Momo Reef (Figure 15.17), an egg-shaped atoll located 50 km east–northeast of Ovalu Island, part of Viti Levu. Momo Reef is 3.4 km north–south, 2.5 km west– east, and a total platform area is 7.1 km2. The rim is submerged except to the south where a rock and rubble reef flat has formed and is marked by outer reef 110–320 m wide with the lower values to the south. A pass to the north is about 250 m wide. This opening has resulted in the name of Horseshoe Reef on some charts, although better examples of this configuration can be found elsewhere. The rim is rock and rubble to the south and is coded as reef to the north. The remainder of the rim is submerged close to the surface. The lagoon is about 5 km2 and is up to 22 m deep (Agassiz, 1899). A narrow sand apron perhaps 100 m wide occurs on the southern–southwestern lagoon margin.

The second atoll not on the Lau Ridge is Navatu Reef (Figure 15.17), a D-shaped atoll located 85 km west of Vuite. The platform is 5 km north– south, 3.8 km west–east, and the area is 17.9 km2. The platform edge is composed of outer reefs east– southwest clockwise where they are 195–355 m wide and narrow to the west where they are least developed 150–160 m wide. The rocky rim develops a reef crest northeast–southwest clockwise and an outer reef flat that is best developed in the same directions and dries at 0.6 m during low tide (Sailing Directions, 2017). Patch reefs occur to the west and northeast. The lagoon is 11.4 km2 in area. Fingers of reef extend from the reef flat to the shallow lagoon around much of the lagoon periphery. The inner reef flat reefs occur as broad ribbons west–northeast with patch reefs, clockwise east–south. A sand apron in the shallow lagoon is best developed on the southeast–southwest where it is 165 m wide, narrowing to the north. The apron is absent in the north where the rim is weakly developed. The lagoon slope displays a few reef-rimmed pinnacles. Most of the lagoon (78%) is deeper than 15 m; the maximum depth is 29 m (Purdy, 2001). Little is known about Ceva-I-Ra (Figure 15.18), which is located about 675 km southwest of Vatua Ono astride the New Hebrides Trench, considerably west of the other Fijian Islands. While it is administered by Fiji, it is not part of it either geologically or geographically. The ovotriangular platform covers an area of 4.9 km2 and is surrounded by outer reefs that

Atolls of Fiji

Figure 15.18  Remote-sensing image of Ceva-I-Ra Atoll. Images © 2019, Planet Labs PBC.

are 250–320 m wide to the north and the southwest. Elsewhere they are 100 m wide or less. The rim is not properly coded but appears to be a rubbly reef flat that develops a reef crest up to 120 m wide to the south and is submerged except for a sandy islet about 7 ha in extent and about 1.8 m high (Sailing Directions, 2017) near the center of the platform. This area is surrounded by a narrow band of reef extending to the northwest. The reef flat develops bands of reef to the north and multiple bands to the south of the island. A shallow lagoon covering about 13 ha is found about 220 m east of the island and includes patch reefs to its north and east. We mention in passing North Astrolabe Reef, a fine example of an almost-atoll located 53 km south of the city of Suva, Viti Levu where the lagoon bears a single volcanic rock.

References Agassiz A 1899. The islands and coral reefs of Fiji. Bull. Comp. Zool. 33: 1–151. Clark G 2009. Early ceramic sites in Southern Lau, Fiji. Archaeol. New Zealand 52: 81-89.

Ladd HS, Hoffmeister JE 1945. Geology of Lau, Fiji. BP Bishop Museum Bull. 181: 399. Neall VE, Trewick SA 2008. The age and origin of the Pacific Islands: a geological overview. Phil. Trans. Roy. Soc. B 363: 3293–3308. Nunn PD 1994. Oceanic Islands. Blackwell, Oxford. Nunn PD 1999. Late Cenozoic emergence of the islands of the Lau-Colville ridge, southwest Pacific. Geol. Soc. London Special Pub. 146: 269–278. Nunn PD, Ollier C, Hope G et al. 2002. Late Quaternary sealevel and tectonic changes in northeast Fiji. Mar. Geol. 187: 299–311. Packham GH 1978. Evolution of a simple Island arc: The LauTonga Ridge. Exploration Geophysics 9: 133–140. Purdy EG 2001. Supplement to Purdy EG and Winterer EL. Origin of atoll lagoons. Geol. Soc. Am. Bull. 113: 837–854. Purdy EG, Winterer EL 2006. Contradicting barrier reef relationships for Darwin’s evolution of reef types. Intl. J. Earth Sci. 95: 143–167. Sailing Directions 2017. Pacific Islands, Publ. 126, 12th ed. National Geospatial Intelligence Agency, Springfield VA. Sand C, Valentin F, Bole J et al. 2007. Report and preliminary analysis of the first Archaeological survey of Naqelelevu Atoll, Northeast Fiji. J. Polynesian Soc. 116: 407–432. Smith JS 2013. The bound[less] sea: wilderness and the United States Exploring Expedition in the Fiji Islands. Env. Hist. 18: 710–737. Sprague R 1987. The United States Exploring Expedition in Fiji. Occ. Pap. 31. Center for Pacific Islands Studies, School of Pacific and Asian Studies, University of Hawai’i, pp. 12–49. http://hdl.handle.net/10125/15538

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The Atolls of the Coral Sea The Coral Sea is a basin in the southwestern Pacific that is bounded by Australia’s Great Barrier Reef to the west, the island of New Guinea to the north, the Solomon Islands in the northeast, the islands of Vanuatu in the east, and the Tasman Sea to the south. In the previous chapter, we covered the northeastern Coral Sea atolls that belong to the Solomon Islands. Now we begin to explore those that are in the territorial waters of Australia and France. A considerable diversity of atoll morphology occurs here. The Melanesian French atolls of Bellona, Chesterfield, and Huon, for example, are large, irregular structures with discontinuous and partly submerged rims, many passes, and deep lagoons. On the western side of the Coral Sea, and conversely, the Australian atolls of Merite, Elizabeth, and Cato, for example, are smaller and nearly annular with submerged or essentially submerged rims and shallow lagoons. We list 25 atolls in the Coral Sea (Figure 16.1).

Climate and regional oceanography The northwest Coral Sea, on the Queensland Plateau (data from Willis Island 16.3°S, 150°E), includes persistent southeast trade winds during winter (May– November). During summer (December–April), the ITCZ shifts and displaces the trade winds to the south, resulting in weaker northeasterly winds dominating from December to February (Orme, 1977). Both winds and peak waves are from the east to southeast (Figure 16.2). Given the rare landmasses and islands in the area, long-term rainfall records are relatively uncommon. In general, however, rainfall changes through the year, from the wet season of January– April to the driest month in August (Orme, 1977). Relatively high rainfall occurs throughout the year in the northeastern Coral Sea, such that Ouvéa Atoll has a mean annual rainfall of almost 1,700 mm (Vilas et al., 2019).

Figure 16.1 Atolls of the Coral Sea include those off the eastern edge of the Australian plate, west of the Loyalty Ridge north of New Caledonia, and on the Chesterfield Plateau, a high-standing feature of the Coral Sea. The South Equatorial Current represents the western part of the South Pacific Gyre and is dominant in this region. The SEC bifurcates near the Australian continent turning northward as the Hiri Current and southward forming the East Australian Current.

DOI: 10.1201/9781003287339-16

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Figure 16.2 The Queensland Plateau in the northwest Coral Sea presents persistent southeast trade winds during winter. During the austral summer, the ITCZ shifts and displaces the trade winds to the south, resulting in weaker northeasterly winds. Both winds and most energetic waves are from the east to southeast. Tides are diurnal to mixed, with two highs and lows of very unequal heights daily, as shown by data from Marion Atoll.

Coral Sea represents the western part of the South Pacific gyre (Orme, 1977). One important oceanic driver is generated by the westward-moving South Equatorial Current. As this current crosses the Coral Sea and encounters the Australian landmass, it splits into the northward-moving Hiri Current (North Queensland Current) and the southward-flowing East Australia Current. The latter flows to the south until it encounters the West Wind Drift, where it is deflected back to the east (Figure 16.1). These circulation and rainfall patterns generate spatial changes in sea-surface temperature and salinity (Maes et  al., 2013; Jaffries, 2013). To the north, near the Solomon Sea, waters include relatively warm and freshwater (mean temperatures of greater than 28°C, salinities less than 35‰), but become relatively cool and salty (mean temperatures of less than 28°C, salinities greater than 35‰) in the subtropical south. In contrast to the dominant interannual variability related to ENSO along the equator, the Coral Sea region is influenced more by seasonal variability about the means (Maes et al., 2013). The Coral Sea region is struck by tropical cyclones, with an average of fewer than 1/year. Although there is considerable variability, many systems travel westward and poleward through this area, although they

may track back eastward once they go south of 15°S. The most intense storms tend to form between 8°S and 12°S and during La Niña ENSO phases (Flay, 2006). Tides are diurnal to mixed, with two highs and lows of very unequal heights daily. Spring tidal range at Marion Atoll is 1.95 m, and neap tides vary by only ~0.66 m (Figure 16.2). The high tidal range over plateaus such as the Coral Sea Plateau can generate powerful tidal currents of 1 knot (~0.5 m/s) or more (Orme, 1977).

The Melanesian Coral Sea atolls The French group of atolls and islands is parallel to the large island of New Caledonia where they lie atop the Loyalty Ridge, a 1,300-km-long zone of uplift associated with the consumption of the Australian plate as it is drawn under the edge of the Pacific plate. The convergence zone is marked by the Vanuatu trench (Figure 16.1). Two islands that are closest to the trench, Maré and Lifou, were Pleistocene atolls that are now uplifted islands with elevations of over 100 m. Adjacent to them is Ouvéa, an atoll whose eastern rim is also uplifted to 43 m, but tilting has preserved the lagoon and the rim to the southwest is submerged. A smaller companion atoll, described

Atolls of the Coral Sea

below, continues the Loyalty Ridge to the northwest of Ouvéa and is similarly tilted. Ouvéa Atoll (Figure 16.3) is located about 100 km northeast of New Caledonia and is shaped as a rounded triangle with a flared eastern edge where the 131-km2 Ouvéa Island is located. The platform is 44 km west–east, about 35 km at its widest across the base, and covers 1,003 km2. The deep and shallow outer slope reefs surround the closed eastern rim and become discontinuous following the remaining platform outline to the west, with reefs separated by passes. The outer reefs are about 200 to as much as 660 m to the north and roughly 100–390 m wide to the south–southwest. Much of the atoll is rimmed with a spur and groove system. The rim is dominated by Ouvéa Island as described above and is accompanied by smaller adjacent islands to its north and south as well as by about 30 other small islands or islets scattered along the discontinuous remainder of the rim that sometimes are referred to as the

northern and southern Pleiades. The total rim area is about 200 km2. The lagoon area is 787 km 2 . The shallow lagoon is divisible into a nearshore zone that is primarily sandy in the lee of Ouvéa Island. This area grades into a shallow sandy slope 10–20  m deep (cf. Bozec et al., 2004) and is widest to the northeast and narrows along the southeast of Ouvéa Island The lagoon slope to the southeast is broad and supports numerous patch reef clusters. A deep lagoon 20 m or more deep forms the central-western lagoon and is approximately 180  km 2 or about 23% of the lagoon area. The maximum lagoon depth is 44 m (Purdy, 2001). Beautemps-Beaupré Atoll is located about 20 km northwest of Ouvéa where it takes the form of a warped D that is 13  km through the center west– northeast and 14  km at its widest north–south (Figure 16.3). The platform area is about 117  km 2 including the western platform deeper than 15  m

Figure 16.3 Remote-sensing images of the atolls of Beautemps-Beaupré, Ouvéa, and Pétrie. Images © 2021, Planet Labs PBC.

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that is visible in satellite imagery. The outer reefs extend about 235–635  m to the northwest, and 200–470 m southwest–southeast. Narrower reefs in general are to the north and east where they range from 135–280 m wide. The rim is rubbly, less than 200  m wide, and extends northwest–southwest clockwise. The western rim is submerged except for a 47-ha islet to the south-southwest that has been uplifted to 14 m (Maurizot et al., 2020). The only other islet is found on the eastern rim and has an area of less than 2 ha. The lagoon area is about 105 km2. Patch reefs primarily occur in the shallow areas. Sand in shallow water occurs discontinuously rather than in the form of a shallow apron. The lagoon deeper than 15 m develops an extensive sandy slope similar to Ouvéa that extends more than 3.5 km northwest–southwest and occupies most of the basin. The maximum lagoon depth extends to 29 m (Purdy, 2001). We note that Andréfouët et  al. (2009) classify this structure as a bank reef. Pétrie Reef (Figure 16.3) takes the form of a warped pair of tongs located about 260 km to the northwest of Beautemps-Beaupré. Despite the distance, both are part of the Loyalty group and are regarded by Andréfouët et al. (2009) as banks rather than atolls. The platform is 91.7 km2 and is surrounded by outer reefs ranging up to 295–530 m wide to the east. A low, islandless rim is best developed northwest–south and the reef crest is best developed to the northeast and the southeast. The rim to the southwest is discontinuous and submerged, whereas it is open for about 6 km to the northwest, however, the northernmost part of it is miscoded. The rim is generally submerged. The lagoon is about 77 km2 and is primarily more than 15 m deep. A sandy slope extends into the shallows along the eastern rim. We are unaware of bathymetric data for this lagoon. Several ridges are found in the vicinity of the Loyalty Ridge and the New Caledonian Ridge underlying the island of the same name is an important one. The northern termination of the New Caledonia ridge forms an oceanic arc, the d’Entrecasteaux Ridge, that trends west–east and is distinct from the basement of the New Caledonia ridge (Collot et al., 1988; Andréfouët et al., 2009). There are a few names that are noteworthy for his region. Rear-admiral Bruni d’Entrecasteaux was the leader of an expedition to the Coral Sea in 1791 along with distinguished hydrographer C.F. Beautemps-Beaupré (atoll named in his honor described above) and navigator Jean-Michelle Huon de Kermadec whose surnames are eponymously attached to rivers, New Zealand islands, a Tasmanian pine tree, a deep ocean trench, and an atoll as described below.

The d’Entrecasteaux Ridge supports a complex of five atolls, the largest of which is called Surprise, lying about 45  km northwest of the New Caledonian Barrier Reef system. We note in passing that the name ‘Surprise’ has nothing to do with the d’Entrecasteaux expedition. Instead, it is derived from a ship of the same name that was the means of transport for Anglican missionaries who were given a one-way ticket out in 1869 when the French government demanded them to leave its territory. Considering the mutual antagonism between the two countries at the time, that demand should not have been surprising. Surprise Atoll is located 118  km west of Pétrie Reef. The platform is an oval structure that is 33.7  km northwest–southeast and 19.4  km northeast–southwest at its widest (Figure 16.4). The platform area is 500  km 2 and much is surrounded by outer reefs that vary considerably with an average of 520  m wide but they extend from 175–995  m on the northeast alone. The rim is low to the water but is continuous northeast–southeast and becomes discontinuous southwest–northwest. It develops a reef crest to the center of the eastern rim but is essentially submerged except for two islets and two sand cays to the northwest and Ile de Surprise to the southwest. Multiple passes occur south–northwest counterclockwise, four of which allow access to the lagoon by vessels (Sailing Directions, 2017). The rim to the west is completely open and submerged for 7 km. The lagoon area is 437 km 2. Its shallows and part of the lagoon slope develop a sand apron up to a kilometer wide along with scattered patch and ribbon reefs to the east where the rim is continuous. The lagoon slope and a few pinnacles extend up to 2 km wide along the east, but the largest part of the basin is deep, typically 35–52 m, with a sandy bottom that contains corals and calcareous algae (Garrigue et al., 2000). Pelotas is a small bean-shaped atoll located about 4 km southeast of Surprise (Figure 16.4). It is 9.8 km north–south, 3.7 east–west at its widest, with a total area of 34.0 km2. The platform is surrounded by outer reefs that are best developed on the east side where they are about 290–575  m wide. The northwestern outer reefs are mostly 150–255  m wide. The rim is up to 500  m wide and extends north– southwest; the western rim is discontinuous with a 150-m-wide channel or passage in the center. There are no islands. The lagoon area is 25.5  km2. Patch and ribbon reefs encircle the shallow lagoon, more of them to the western margin than the east. Sand is deposited as an apron more than 500 m wide northeast–southwest, especially to the east on the deep lagoon slope with some areas about 350  m wide to

Atolls of the Coral Sea

Figure 16.4 Remote-sensing images of the atolls of Surprise and Pelotas. Portail, a bank reef, is also shown. Images © 2021, Planet Labs PBC.

the east that trail discontinuously to the south. A lagoon slope about 1.7 km wide is found to the north. The bottom topography is divided by ridges. We do not have a reference for the biotic composition of the benthos or a maximum depth, although the bottom features are visible in satellite imagery. Portail is a satellite bank reef of Surprise Atoll, located about 16  km to its west. The reef is narrow and arcuate, about 12 km measured north– southeast, and about 5.9  km at its widest (Figure 16.4). The platform area is roughly 52  km2. There

are no islands, and the eastern rim is drowned except for a 21-ha area of deep outer reef and plateau. The reefal part of the platform is surrounded by a deep outer reef slope and a congruent shallow reef. The lagoon area is undefined, as there is no annular reef, but the area east of the reef rim has a large number of reefs occurring between the shallow lagoon and the slope to the west. A sand pocket about 450 m wide occurs in the elbow of the lagoon to the south. We do not have a depth for this lagoon. Note that Andréfouët et al. (2009) categorize this structure as a bank

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reef. We tend to agree, and it is not counted here as an atoll. Huon Atoll is the second largest of the d’Entrecasteaux group (Figure 16.5). It is located about 11  km to the northwest of Surprise and is J-shaped, similar to a fishhook, measuring 38.5 km north–south, 13 km at its widest, with an 11.7-kmlong gap in the rim to the west. The platform is about 343  km 2 and is surrounded by outer reefs that are generally wider to the east where they are 310–470 m than to the west and about 270 m wide to the west although there are areas up to more than 500 m wide to the northwest and northeast.

The rim forms a reef crest along the east and develops a reef flat up to 375  m wide with a single sandy island to the east that is about a kilometer long and about 100  m wide. The western rim is composed of three sections of reef flat that separate three passes, the southernmost of which is 500  m wide and ‘very deep’ (Sailing Directions, 2017) in addition to the fishhook-like gap that is open to the surrounding ocean to the northwest. The lagoon is about 300 km 2 and develops patch and ribbon reefs that are scattered mostly within the shallow parts of the lagoon. Sand is found in the shallow and deep lagoon areas of the emergent western rim. A few

Figure 16.5 Remote-sensing images of the atolls of Huon, Merite, Gros Guilbert, and Petit Guilbert. Images © 2020-2022, Planet Labs PBC.

Atolls of the Coral Sea

pinnacles occur to the narrow northern section of the lagoon. More than 90% of the lagoon is deeper than 15 m and it reaches a maximum depth of 46 m (Purdy, 2001). Three satellite atolls are found to the east of Huon. Merite, located about 6 km from the southern end of Huon, is egg shaped, 4.6  km northeast–southwest and 3.3  km northwest–southeast at its widest (Figure 16.5). The outer reefs The platform area is 14.7 km2. The outer reefs are widest to the north–northwest where they are about 440 m wide; elsewhere they range from 215–380 m wide. The rim develops a reef crest primarily east– southeast and is a continuous reef flat, that is up to 550 m wide to the east, with no islands or patch reef development. The lagoon area is 7.1 km2 and develops a broad area of shallow and deep lagoon to the northeast basin that is primarily covered by sand although shallow patch reefs are found northwest–north. The sand forms an apron about 580 m wide to the northeast, narrowing to about 100 m to the southwest. The northwestern lagoon margin accumulates little sand where the rim is narrowest. The shallow lagoon is more than 800 m wide to the east although deeper areas that are rimmed by reef occur within it. A deeper area of about 80 ha is near the lagoon center, but we do not have a maximum depth and although a number of patch reefs are evident in the imagery, no pinnacles are coded. Two other small atolls occur 15–22 km northeast of Merite and 10–17 km east of Huon. These are the Guilbert Atolls, Gros, and Petit Guilbert, respectively. Gros Guilbert (Figure 16.5) is annular, 5.4 km northeast–southwest, 4.5  km northwest–southeast, and covers an area of 18.8 km2. The platform is surrounded by, outer reefs extending 330–485 m wide north–northeast 190–350 elsewhere. The rim is a reef flat that is continuous north– southwest and is up to 600 m wide. A reef crest occurs east–southeast. Most of the western rim is either partly open to the northwest or it is open as it is to the southwest. The lagoon area is 12.7 km2. Reef ribbons and patches are found in the shallows to the southeastern and northwestern lagoon margins. A sand apron up to 600 m wide occurs to the northeast and expands to about 1.5 km wide to the southwest. Most of the deeper lagoon is represented by a lagoon slope covering an area of about 6.9 km2. We do not have a maximum depth for this lagoon. Two pinnacles occur in this deeper area. Petit Guilbert Atoll (Figure 16.5) is egg shaped and is located about 2.6 km from Gros Guilbert. The platform is about 2.5  km northwest–southeast, 2.3  km northeast–southwest, and covers an area of 6.0 km2 including outer reefs that are widest north– east where

they are 525–760 m wide. Elsewhere they are 300– 550 m wide. The rim is organized as a continuous reef flat clockwise from north to south and it is especially well developed in the middle (east) where it is up to 800 m wide due to the extent of the inner reef flat. The reef crest is best developed east–south clockwise. The western rim is submerged with shallow openings to the 1.2-km2 lagoon area. Small patch reefs are found in the shallow lagoon to the northeast, but most of the lagoon is sandy and shallow, likely 10 m or less, and without significant reef development. We do not have specific depth information for this lagoon. The next area to consider is the Chesterfield Plateau, a high-standing feature of the Coral Sea, the foundation of which is supported by five extinct volcanoes. The plateau consists of two large atoll platforms with a carbonate layer 200–300 m thick that is connected by a saddle (Missègue and Collot, 1987; Andréfouët et al., 2009). The first of these is Chesterfield (Figure 16.6), an atoll that is curved and polygonal, located 414 km southwest of Huon. The platform is 136  km curving northeast–southeast, and it is about 63  km at its widest northwest–east. The platform area is 4090 ± 120 km2, the uncertainty owing to the drowning of more than half of the eastern rim. The platform is surrounded by outer reefs that are quite variable but are generally 300–575 m wide to the east and 130–35 m wide to the west. The emergent rim began post-glacial growth about 6,000 years ago and stopped about 2,500 years later when sea level stabilized (Andréfouët et  al., 2009). Thus, the rim is primarily a reef flat with 14 islets scattered across the surface, none of which are larger than about 22 ha (e.g., Long Island to the southwest) and the total land area is about one km2 (Cohic, 1957). Most of the reef flat sections are connected by visible but submerged reefs except for the drowned reef to the east as noted above. The rim is submerged. The lagoon area is roughly 3,859 km2. The shallow lagoon is sandy, especially to the east, and ribbon reefs develop along the inner edge that rapidly descend to depths greater than 15 m. Large sections of the lagoon are between 37 and 53 m deep, but several areas to the east are deeper (Sailing Directions, 2022). Numerous reef-rimmed pinnacles and ridges occur in the deep lagoon. Bellona is the second atoll on the Chesterfield Plateau (Figure 16.6) and is about 75  km to the southeast of Chesterfield Atoll. The platform of Bellona is 192 km northwest–southeast and narrows to 114 km northeast–southwest. The platform area is about 8,561  km2, more than double the size of Chesterfield, but most of the 476  km perimeter of the rim is submerged, and the northern and central eastern satellite imagery of the rim is indistinct. Most of the

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Figure 16.6 Remote-sensing images of the atolls of Chesterfield, Bellona, and Bellona South. Area of Bellona South is shown in yellow inset. Images © 2021, Planet Labs PBC.

57  km length of the emerged rim dries at low tide and are reef flats. A single islet occurs on the western rim. The deep and shallow outer reefs are about 125–250  m wide seaward of the emergent rim. Behind the inner reef flats are lagoon shallows that are sandy and flanked by reefs (e.g., Bellona Reef South, Figure 16.6). The lagoon area  is approximately 8208 km2 with depths that are similar to Chesterfield Atoll (Andréfouët et al., 2009), but we have no specific bathymetric data for Bellona. Few pinnacles are coded in this lagoon.

The Australian Coral Sea atolls The remaining Coral Sea atolls occur on an extended region off the eastern Australian continental shelf and is outside the realms of Melanesia and Polynesia. This area was subjected to sea floor spreading along an ancient and extinct rift system or systems, coupled to differential subsidence and sea level changes that produced the Queensland Plateau and other continental shelf extensions to the south (Davies et  al., 1989; McKenzie et al., 1993). These tectonic changes

Atolls of the Coral Sea

provide the foundation for 15 atolls, all with submerged rims. Atolls to the north are influenced by the South Equatorial Current that continues its westward flow, but then bifurcates offshore as one arm turns north toward the Solomon Sea (atolls there to be considered later). The nearshore boundary currents are influenced by the Australian landmass so that a second bifurcation occurs near the continental slope at about 15°S. The southern arm turns south and becomes the East Australian Current between 10°S and 15°S, while the Hiri Current (named for the traditional trade route of the Motu People of Papua New Guinea) turns north at about 15°S. (Burrage 1993; Wijeratne et  al. 2018; Figure 16.1). The Hiri Current thus likely influences circulation near the atolls found on the western edge of the Queensland Plateau, whereas the more southerly and easterly atolls are likely influenced by the South Equatorial Current and the East Australian Current.

The southernmost of the group includes two atolls, both of which are situated on a seamount chain, the Lord Howe Rise, at about 30°S latitude, a marginal environment for the development of coral reefs. Woodroffe et  al. (2004) examined both atolls, and despite being coral-capped volcanic structures rising from 2,000  m depths, noted some possible differences between them and those of other more tropical atolls. Elizabeth Atoll is located about 640  km southeast of Brisbane Australia and is ovotriangular, 9.4  km northwest–southeast and 7.5  km across the base northeast–southwest (Figure 16.7). The platform area is 55.0 km2 including uncoded deep reef northwest–east where outer reefs extend 900 m to at least 1,200  m from reef crest to a depth of 40 m. Outer reefs on the remaining platform average about 150 m wide. A well-developed spur and groove system is found in depths of less than 8 m (Woodroffe

Figure 16.7 Remote-sensing images of the atolls of Elizabeth and Middleton. Images © 2020-2021, Planet Labs PBC.

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et al., 2004) and is accompanied by a reef crest that occurs mainly to the southeast. The rim is without islands and is composed of a reef flat that is continuous clockwise east–northwest and is interrupted by a 5-km-long section of the rim to the north which dries at 2.6 m low tide (Sailing Directions, 2022) and ends with a 200-m-wide opening to the lagoon. The lagoon covers 37.6 km2 but about 75% of that area is a sandy inner reef flat. The lagoon area proper (without the inner reef flat) is about 9 km2 and most of it is confined to the southwest, southeast, and inside the opening in the rim. Deeper areas occur as irregular patches toward the center where they are partially closed by reticulations of the rocky inner reef flat. These pockets are sandy and less than 15 m deep (Woodroffe et al., 2004) Middleton Atoll (Figure 16.6) is located about 47  km north of Elizabeth. Its distinctive kidney shape is 9.3 km west–east and 6.4 km north–south where it is widest. The platform covers 47.7  km2 and is slightly smaller than Elizabeth. The platform includes the uncoded area between the renal lobes (‘The Sound’ on charts) to the north. The outer reefs (coded and uncoded) are about 500  m wide to the windward northeast where they are best developed about 300 m wide to the east, and less than 100 m wide to the southwest. The shallow outer reefs are better developed to the southeast where they are 300 m wide, as they are on Elizabeth Atoll. Woodroffe et al. (2004) report that the reefs to the south form spur and groove systems to a depth of 9 m and drop steeply to 30 m. The rim is a continuous reef flat except to the north where a 3.2-km-wide area is open to the Coral Sea. The flats develop patch reefs to the west and south. The lagoon area is about 30 km2 but most of it is occupied by a shallow extensive, sandy inner reef flat that form patch reefs to the west and southeast. The lagoon itself is 12.4 km2 and is covered with rubble and sand. Woodroffe et al. (2004) report lagoonal patch reefs that form reticulations, but these are not visible in satellite imagery. The lagoon depths appear to be shallow and similar to those of Elizabeth Atoll. The remaining atolls of the Australian east coast are a considerable distance from Elizabeth and Middleton and rise about 2,000 m from various plateaus. The first of these is Kenn Plateau, considered as part of the northern Lord Howe Rise, with an area of about 56,000 km2 and an average depth of 1,890 m. Coral-capped seamounts that rise from this plateau and reach the surface as atolls include Cato, Kenn, and Frederick atolls (Keene et al., 2008). Cato Atoll is located about 760 km northwest of Middleton Atoll and is ovotriangular, about 4  km west–east, and about 2 km at its widest (Figure 16.8).

The platform area is approximately 8.3 km2 and includes uncoded deep outer reef areas to the north and the east. The outer reefs to the northwest extend 925 m and is the widest reef area whereas the northeast is 160 m wide and is the narrowest. The rim develops a reef crest on all sides but the north, and a single 13-ha sandy island inside a reef flat that extends to 500 m wide to the northwest and represents a few percent of the rim area; the rest of this system arguably can be described as essentially submerged. The lagoon area is 1.4 km2, and almost all of it is shallow and sandy with patch reefs to the north, west, and south. The maximum depth is 4 m (Purdy, 2001). Wreck Reef (Figure 16.8) lies between Cato and Kenn atolls. It has been included as an atoll in some compilations, but we consider it a bank reef system, so it is not described in detail. Kenn Atoll (Figure 16.8) is located 218 km north of Cato and it is about 21  km north–south and roughly 13  km wide west–east. This reef forms a warped triangle, but its western rim is deeply submerged. The estimated platform area is 108 km2. The rim is composed of four reef flat areas including one to the north, two to the east—one of which forms an eastward elbow-like projection—and a smaller shoal area to the south/southwest. None of these develop islands but each is outlined by an outer reef slope that is up to 420  m wide, although they are more commonly about 325  m wide. Shallow reefs spread onto the reef flats of the elbow-like shoal and the southern shoal. The lagoon area is approximately 79 km2, including its shallow areas bordered to the east by reef flat that commonly include patch and ribbon reefs. The lagoon slope is coded as reef with a few isolated pinnacle-like structures. The basin drops to a 68 m-deep (Purdy 2001) where two additional pinnacles are found. Frederick Atoll (Figure 16.9) is the third member of the Kenn Plateau atoll group. It lies about 138 km west–northwest of Kenn and takes the shape of the Big Dipper asterism. Three shallow-water areas without islands outline this atoll. The northernmost composes the handle, the larger curved area is the bottom of the dipper, and a small shoal area (‘Ridge Rock’ on charts) marks the top. The outer reefs 250–500 m from the rim to the east. The lagoon area is 45.2 km2 and is widest in the lee of the large, curved shallow water area to the southeast. The shallow lagoon is flanked by patch and ribbon reefs on the two larger shoal areas but is otherwise too deep to be coded. The maximum depth for this lagoon is 34 m (Frederick Reef chart, n.d.). The Marion Plateau covers 77,000  km2 and lies at a depth of 300–800 m. It is a deep extension of the Australian continental shelf off the central

Atolls of the Coral Sea

Figure 16.8 Remote-sensing images of the atolls of Cato, Kenn, and Wreck. Images © 2019, 2021, Planet Labs PBC.

Queensland coast. Its eastern margin includes two large atolls, Saumarez to the south and Marion to the north, both of which are east of the southern Great Barrier Reef (Keene et  al., 2008). Saumarez is located about 92  km southwest of Frederick Atoll. The platform takes an ovotriangular form that is roughly 38 km northeast–southwest, 25.5 km at its widest, and covers approximately 678  km2 (Figure 16.9). These measurements are estimates because the northern and western margins of the platform include deep reefs that are uncoded and are indistinct on optical satellite data. The southeast-facing rim is bound to the east by outer reefs ranging from 435– 980 m wide. It is composed of 10 reef flat areas that extend northeast–southwest, the largest of which is 18.7 km long and about 500 m at its widest. Two of the smaller reef flat areas, one to the north and one to the southwest, develop small sand cays, but otherwise there are no islands. The lagoon area is about 512 km2. The outer reef flats are dotted with patch reefs as well as ribbon reefs, but the lagoon quickly deepens to a maximum depth of 44 m (Purdy, 2001). The other atoll on the Marion Plateau is the large and aptly named Marion Reef (Figure 16.9) located about 300 km northwest of Saumarez. The platform is bean shaped, 42 km north–south and about 22 km wide through the concave western center to the eastern edge. The platform of about 879 km2 is outlined

by nine shoal areas, most of which are reef flats including one to the east–south that is about 17.5 km long. The remaining shallow areas are mostly submerged and roughly outline the western platform. All shoal areas are outlined by outer reefs that are about 250 m wide. The shallow reef continues onto the outer reef flats, especially to the north of the 17.5-km-long reef flat where there are submerged gaps. The lagoon area is about 759 km2. The shallow lagoon in the lee of the emergent reefs is dotted with patch reefs, especially the shallows to the east. The narrow lagoon slope gives way to the largest part of the lagoon basin, which is dotted with multiple reef-rimmed pinnacles and reaches a depth of 55 m (Purdy, 2001). Mellish Reef (Figure 16.10) is an atoll that projects from the Mellish Rise, a large (200,000 km2), dissected, northeasterly facing plateau that separated from the Australian continental margin and other plateaus more than 50  million years ago. Mellish is located 411 km northeast of Marion and is boomerang shaped with a platform 12 km long following a curved path. It is 2.3 km at its widest and covers an area of 29.4 km2. Outer reefs surround the platform and are mostly 300–450  m wide, but a large part of the west ranges from 650–1000 m wide. The entire rim is a reef flat that is submerged or awash at high tide. The lagoon area is approximately 18.5 km2. The shallow lagoon is sand and rubble and is up to

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Figure 16.9 Remote-sensing images of the atolls of Fredrick, Saumarez, Marion, and Mellish. Images © 2019-2021, Planet Labs PBC.

15 m deep. However, a long and narrow, reef-rimmed lagoon area of 84 ha that is 43  m deep at its maximum is found to the northwest (GPS Nautical Chart AU-4611P2). Six atolls are found on the Queensland Plateau, an ovotriangular area of about 165,000  km2 lying between 14°S and 19°S latitude. This plateau is separated from the Great Barrier Reef at the continental shelf edge by a 200 km wide and 2,800 m deep channel, the Queensland Trough (Dunbar et al., 2000). Lihou Reef, a large atoll found at about 19°S near the southeast edge of the Queensland Plateau from which it

rises steeply from depths of more than 1,000 m (Keene et al. 2008). Located 172 km north of Mellish Reef, Lihou takes an elongated ovoid shape (Figure 16.10) whose platform is 103 km long northeast–southwest, 36 km at its widest, and covers an area of 2,358 km2. Outer reefs 300–450 m wide surround the emergent portions of the rim, which is composed of about 40 reef flat segments, a few of which develop small islets or sand cays, some reaching 1.8–2.4 m high (Sailing Directions, 2022). However, about 40% of the western rim is submerged, including about 35 km to the southwest. The lagoon area is 2,182 km2, and while

Atolls of the Coral Sea

Figure 16.10 Remote-sensing images of the atolls of Lihou, Flinders, and Flinders South. Images © 2019, 2021, Planet Labs PBC.

its shallower regions display many patch reefs, most of the lagoon is deep and presents multiple isolated pinnacles. The lagoon basin is about 64 m deep around the center where the maximum depth is 70 m (GPS Nautical Chart AU-3318151). Flinders Reefs are a poorly known pair of atolls located 294 km west of Lihou near the southwest of the Queensland Plateau (Figure 16.10). The larger of the two is North Flinders, a U-shaped atoll with isolated reef areas to the northwest and north. The platform is 38.6 km northwest–southeast, about 27 km at its widest near the south-facing base and covers an area of 820 km2. As with the other atolls in this region, the rim consists of reef flat segments, in this case seven of them, some with sandy cays. The outer reefs to the south and southwest are up to 800  m

wide and narrow to the east where they are generally 265–350 m wide. About 27 km of the northern rim is deeply submerged, as is about 11  km of the rim to the southwest. The lagoon area is approximately 886 km2. The reef flats are sand and rubble. The lagoon slope is narrow, partly reef-rimmed, and gives way to a deep lagoon basin where the maximum depth is 50  m (GPS Nautical Chart AU-4615P1). The center of the U-shaped basin contains numerous reef pinnacles. A companion atoll, Flinders South, lies about 7 km away from the larger one and is separated from it by a channel 800–900  m deep that descends to the plateau surface making these atolls two separate bathymetric features (Orme, 1977). The platform is ovoid, 16 km long northwest–southeast, 7.5 km at its widest, and covers an area of 81.8 km2.

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Four reef flat segments define the northwestern rim, whereas the southeastern rim is defined by a single 14-km-long reef flat. A 7.6-km-long gap occurs in the rim to the northeast and another 6.6 km long occurs to the southwest, but both are clearly defined by outer reefs that encircle the atoll and are about 500–800 m wide to the southeast. A reef crest is best developed to the southeast as well but is somewhat farther south of the widest reefs. The lagoon area is about 68.8 km2 and shoal areas are covered extensively by patch reefs. The lagoon basin is uncoded. The maximum depth is 31 m (GPS Nautical Chart AU-4615P1). Holmes Reefs include three adjacent atolls. The first, Holmes Reef West (Figure 16.11), is about 114  km northwest of North Flinders and is shaped like a strap wrench. The platform is about 16  km curving from the southwest to the northeast and is 7.5 km at its widest to the north. The platform area covers 63.2  km2 and is defined by a series of rock and rubble reef flats with unvegetated sand cays. The

western rim (the strap) has a single shoal area within its 6.4-km curved length. Although much of the outer reef there are uncoded and submerged, the reefs are clearly defined and at 1250–1500  m wide, they are the most extensive. The lagoon area is 43.6 km2, including the lagoon area of the handle. There are numerous reef areas in the shallow lagoon, as well as sand pockets to the north and south of the strap area. The remainder of the lagoon has a 1.6-km-wide slope to the south. The maximum depth of the lagoon basin is 27  m (GPS Nautical Chart AU-4615P2). In addition, a bowl-shaped reef flat area is found to the south of the wrench handle that occupies an area of 9.7 km2 and has a sand-filled shallow lagoon covering an area of 5.3 km2. Reefs flank the perimeter of the reef flat as well as the opening of the lagoon to the north. Holmes Reef East (Figure 16.11) is located 8.5 km east of Holmes West and is D-shaped, measuring 14.5  km north–southwest and 13  km west–east.

Figure 16.11 Remote-sensing images of the atolls of Holmes Reef West, Holmes Reef East, and Flora. Images © 20202022, Planet Labs PBC.

Atolls of the Coral Sea

Figure 16.12  Remote-sensing images of the atolls of Bougainville and Osprey. Images © 2019-2020, Planet Labs PBC.

The platform area is 117.5 km2 and is surrounded by outer reefs that extend up to 275–330 m to the south and southwest; the western rim is a submerged reef that is clearly visible and mostly coded as reef. The lagoon area is 98.2 km2, the shallow parts of which present numerous reef fingers that extend from the outer reefs as well as patch reefs and sand patches that occur east–south. Most of the lagoon is deeper than 15 m and reaches a maximum depth of 42 m (GPS Nautical Chart AU-4615P2). Two small atolls occur to the north and south of Holmes Reefs. The first of these is Flora Reef (Figure 16.11), a small ovotriangular feature that is truncated to the northeast and located about 26  km to the southwest of Holmes Reef West. Flora measures 5.6 km northwest–southeast, 4.6 km parallel to the base, and its platform covers an area of 21.0 km2. The platform is a rock and rubble reef flat surrounded by outer reefs that extend 370 to more than 400 m southeast–south. The northwest-facing platform

margin is about 3.3 km long and is submerged. The lagoon area is about 15 km2 and contains fingers of shallow outer reef that extend to the lagoon shallows to the south, along with a few patch reefs elsewhere. A sand apron up to 900 m wide forms in the shallow lagoon northwest–southwest. The maximum lagoon depth is in the center and is only 12.4 m according to GPS Nautical Chart AU-4615P2 but is uncoded by the Atlas. A second small atoll is found rising from the Queensland Plateau about 123  km northwest of Holmes. This is Bougainville Reef, a bowl-like structure with a surface 3 km wide northeast–southwest and 5.3  km at its widest (Figure 16.12). The platform area is 12.0 km2 and is rimmed by a narrow band of outer reefs up to 160 m wide on the windward north, with lesser development to the leeward southwest. Reefs are also well represented on the rocky reef flats, which are up to 690 m wide to the southeast. There are no islands; the rim is continuous

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around the lagoon and dries at half tide (Sailing Directions, 2022). The lagoon area is 6.1 km2 and is reef rimmed, shallow, likely less than 15 m, and mostly sandy with patch reefs to the northwest. Osprey Reef (Figure 16.12) is an atoll found on the northwestern end of the Queensland Plateau as it rises steeply from over 2,000  m about 300  km northwest of Holmes and 136  km east of the Great Barrier Reef. Shaped similarly to the outline of a unicornfish, this atoll is about 27 km from the tip of the head (North Horn) to the tail, northwest–southeast, and it is about 10.3 km at its widest dorso-ventrally. The platform area is about 185 km2 and is composed of a reef flat without islands that is awash in most places and is surrounded by outer reefs that vary from 250 m wide on the east to more than 680 m wide in some areas of the west. The shallow reef continues onto the outer reef flat to the east. The rocky flats are continuous except for a pass to the west about 800 m wide and 9.1–18.3 m deep (Sailing Directions, 2022). Numerous patch reefs occur on the reef flats around most of the rim. There are also channels in the rim to the west south of the pass and to the atoll rim to the southeast. Passes and channels notwithstanding, the entire rim is submerged. The lagoon area is 152.5 km2 and its shallows contain many patch and ribbon reef areas, but these are restricted to the northern half of the lagoon. A sand apron forms to the northwest and extends for about 11 km. It is about 1.9 km wide inside North Horn, but tapers to about 250 m on the western side. The taper is more dramatic to the northeast where the apron is only about 75 m wide, and then it becomes patchy. Similarly, an apron forms in the south lagoon, covering a distance of about 24 km. It is about 800 m wide to the southeast and southwest corners, then tapers to about 250 m wide on both sides to the north. Most of the lagoon area is deep however, eight pinnacles are found to the southeast, and a few others along the northeastern slope. The maximum lagoon depth is 43 m (Leis, 1994).

References Andréfouët S, Cabioch G, Flamand B, Pelletier B 2009. A reappraisal of the diversity of geomorphological and genetic processes of New Caledonian coral reefs: A synthesis from optical remote sensing, coring and acoustic multibeam observations. Coral Reefs 28: 691–707.

Bozec Y-M, Gascuel D, Kulbicki  m 2004. Trophic model of lagoonal communities in a large open atoll. Aquat. Living Resour. 17: 151–162. Burrage DM 1993. Coral Sea currents. Corella 17: 135–145. Cohic E 1959. Report on a visit to the Chesterfield Islands September 1957. Atoll Res. Bull. 63: 1–11. Collot JY, Rigolot P, Missègue P 1988. Geologic structure of the northern New Caledonia Ridge as inferred from magnetic and gravity anomalies. Tectonics 7: 991–1013. Davies PJ, Symonds PA, Feary DA, Pigram CJ 1989. The evolution of the carbonate platforms of northeast Australia. SEPM Special Publication No. 44, pp. 233–258. Dunbar GB, Dickens GR, Carter RM 2000. Sediment flux across the Great Barrier Reef shelf to the Queensland Trough over the last 300 ky. Sediment. Geol. 133: 49–92. Flay SA 2006. Climatology of Queensland landfalling tropical cyclones: Evaluating instrumental, historical, and prehistorical records. PhD thesis, James Cook University, 226 p. Frederick Reef chart n.d. Australian Hydrographic Office. https://en.wikipedia.org/wiki/Frederick_Reefs#/ media/File:Frederick_Reef_Chart1.jpg Garrigue C, Richer de Forges B, Laboute P et al. 2000. Paléo-Surprise: Paléoenvironments et bioécologie de l’atoll de Surprise, Nouvelle-Calédonie. Inst. Recherche Developp. Nouméa Rapports de Missions Sciences de la Mer 25. Keene J, Baker C, Tran M, Potter A 2008. Geomorphology and sedimentology of the east marine region of Australia. Geoscience Australia Record 2008/10, Canberra, 262 pp. Leis J 1994. Coral Sea atoll lagoons: Closed nurseries for the larvae of a few coral reef fishes. Bull. Mar. Sci. 54: 206–227. Maes C, Dewitte B, Sudre J et  al. 2013. Small-scale features of temperature and salinity surface fields in the Coral Sea. J. Geophys. Res. 118: 5426–5438. Maurizot P, Collot J, Cluzel D, Patriat  m 2020. The Loyalty Islands and Ridge, New Caledonia. In: Maurizot P, Mortimer N (eds) New Caledonia: Geology, Geodynamic Evolution and Mineral Resources. Geological Society of London Memoirs, London, vol. 51, pp. 131–145. McKenzie JA, Davies PJ, Palmer-Julson A et  al. 1993. Cenozoic evolution of carbonate platforms on the northeastern Australian margin: Synthesis of leg 133 drilling results. Proc. Ocean Drill‑ ing Progr. 133: 763–770. Missègue F, Collot J-Y 1987. Étude géophysique du Plateau des Chesterfields (Pacifique sud-ouest). Résultats préliminaires de le campagne ZO E200 du N/O Coriolis. C.R. Acad. Sci. 304: 279–283. Oceandots 2010. https://web.archive.org/web/20101223063133/ http://oceandots.com/pacific/coralsea/mellish.php Orme GR 1977. The Coral Sea Plateau—A major reef province. In: Jones OA, Endean R (eds.) Biology and Geology of Coral Reefs Vol. 4: Geology, Academic Press, New York, pp. 267–306. Purdy EG 2001. Supplement to Purdy EG & Winterer EL 2001. Origin of atoll lagoons. GSA Bull. 113: 837–854. Sailing Directions 2017. Pacific Islands Pub. 126 National Geospatial Intel. Agency, Springfield VA. Sailing Directions 2022. East Coast of Australia and New Zealand. Pub. 127 National Geospatial Intel. Agency, Springfield, VA. Vilas MP, Adams MP, Ball MC et  al. 2019. Night and day: Shrinking and swelling of stems of diverse mangrove species growing along environmental gradients. PLoS One 14(9): e0221950. https://doi.org/10.1371/journal.pone.0221950 Wijeratne S, Pattiaratchi C, Proctor R 2018. Estimates of the surface and subsurface boundary current transport around Australia. J. Geophys. Res. Oceans 123: 34444–3466. Woodroffe CD, Kennedy DM, Jones BG, Phipps CVG 2004. ­Geomorphology and late Quaternary development of Middleton and Elizabeth reefs. Coral Reefs 23: 249–262.

The Atolls of the North West Shelf of Australia The North West Shelf of Australia, including that of the Kimberley region, is an extensive tropical to subtropical continental shelf bordering the northeastern Indian Ocean. This area can be divided into three sections often referred to as the inner, mid, and outer ramps. The inner ramp is shallow and commonly includes fringing reef systems around islands despite spring tides that may reach heights of more than 10 m, turbid waters that are generated from river outflow, and cyclones that average three per year (Collins and Testa, 2010; Kordi and O’Leary, 2016). The inner ramp deepens to the middle and outer ramps, areas more than 120 m deep, and ultimately slopes relatively steeply into deeper water off the continental shelf. In the Miocene about 16 million years ago, this area developed into a barrier reef system similar in extent to the present-day Great Barrier Reef off of Australia’s east coast. However, about 10 million years ago, the shelf underwent rapid subsidence and left a series of isolated reefs and atolls of limited extent in its place (McCaffrey et al., 2020). Today, these offshore reefs emerge tower-like rising 200–400  m to the surface about 300–400 km from shore. Some of them form banks or shoals, but in addition, there are eight shelfedge atolls between 18°S and 12°S (Figure 17.1). The offshore reefs, in contrast to their nearshore relatives, are immersed in clear, low-nutrient waters with little land-derived sediment input.

Climate and regional oceanography Winds are quite variable and monsoonal. During the austral summer (December–February) wet season, they are from the west to southwest, but weaken in the fall. They ultimately reverse to become dry east-southeasterly trade winds in the austral winter (April–September). The result is a distinctly bimodal distribution (Figure 17.2), but the strongest winds, those more than 40 km per hour, are from the west to northwest.

Peak waves most commonly are less than 3 m high and from the southwest; waves larger than that are generally from the open Indian Ocean to the west to northwest (Figure 17.2). On average, three cyclones impact this area annually. Most originate as low-pressure systems to the north, near Indonesia, in the Arafura Sea or Timor Sea, and travel west to southwest. These cyclones can generate waves that approach from any direction, but the largest waves come from offshore due to the greater fetch, and the open water over which wind may blow continually in the same direction. Tides are semidiurnal but vary markedly along the coast, reaching more than 9  m in some places. Even in the open ocean near the Scott Reef, spring tides approach 4  m, with neap tides around 1  m (Figure 17.2). The primary surface currents in the area originate in the western Pacific and emerge through the several passages into the Indian Ocean shelf as part of a complex exchange called the Indonesian Throughflow. The main parts of this throughflow affecting the Australian outer shelf reefs occur through the Timor Sea and join the wind-driven movement of the Holloway Current flowing near the outer shelf. The movement of this current is augmented by southwesterly trade winds that are strongest in March–September although there are also occasional reversals or other changes in the current flow within season due to anomalous wind or heat events (Adamo et  al., 2009; Wilson, 2014). Although many authors have regarded the Holloway as a seasonal current, Bahmanpour et  al. (2016) found the flow is year-round but seasonally variable. Although the Holloway Current and the Indonesian Throughflow are the main systems that are known to affect the reefs of the offshore North West Shelf reefs, the Eastern Gyral Current with origins in the Indian Ocean also plays a role as it bifurcates into northern and southern arms near the Dampier Peninsula. The southern arm contributes to boundary currents south of the area of interest, while

DOI: 10.1201/9781003287339-17

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Figure 17.1 Eight atolls occur on the NW Australian shelf edge between 18°S and 12°S where offshore reefs are immersed in clear, low-nutrient waters with little land-derived sediment input. All develop rims that are close to the surface and are submerged. The primary surface currents in the area emerge through the several passages from Indonesia into the Indian Ocean shelf as part of a complex exchange called the Indonesian Throughflow.

the northern arm functions to recirculate Indonesian Flowthrough waters over the North West Australian Shelf (Phillips et al., 2021; Figure 17.1).

Atolls of the North West Australian Shelf The atolls of the North West Australian Shelf are divided into three groups based on their deep shelf location. The more southerly group of three, Imperieuse, Clerke, and Mermaid reefs, is located west of the Dampier Peninsula on an area referred to as the Rowley Shoals, a marine protected area where fishing is restricted or prohibited. These reefs are similar in size and shape but rise from increasing depths to the northeast with the base of Imperieuse at 230 m, Clerke at 390  m, and that of Mermaid at 440  m (Collins, 2011). The calcareous green alga Halimeda makes a substantial contribution to the sediment of these reefs. In addition, despite similarities in terms of wind, wave, and tidal regimes, the three reefs exhibit increased lagoon infilling from Mermaid in the

north to Imperieuse to the south (Collins and Testa, 2010). The three lagoon systems are challenging environments for reef growth due to the development of prominent reef flats that are exposed at low tide and restrict lagoon flushing despite 1–3 narrow channels through their rims. This restriction of flow exposes coral populations to more variable thermal regimes compared with the outer reefs. Nonetheless, reefs are well developed in these systems. The branched staghorn coral Acropora tenuis in particular forms thickets and is a hardy and common component of both the lagoon and outer reef slope environments (Thomas et al., 2022). Imperieuse Reef (Figure 17.3) is located about 350  km west of the Dampier Peninsula city of Broome where it takes the shape of a Bartlett pear with the narrow end to the north. The platform is 18.3 km north–northeast–south–southwest, 9.7 km at its widest west–east, and presents an area of 117.3 km2. The platform is surrounded by outer reefs that are about 240–480 m wide mid-west–southeast

Atolls of North West Australia

Figure 17.2 Winds of the North West Australian Shelf are quite variable and monsoonal. Although they include a distinctly bimodal distribution, the strongest winds, more than 40 km/hour, are from the west to northwest. The most energetic waves most commonly are less than 3 m high and from the southwest; waves larger than that are generally from the open Indian Ocean to the west and northwest. Tides are semidiurnal but vary markedly along the coast, reaching more than 9 m in some places; near the Scott Reef, spring tides approach 4 m, as shown.

counterclockwise and about 140–395 m wide to the east. The coral coverage at a sample depth of 12  m varies from 23% to 40%. The rim is composed of a rocky reef flat 1–1.6 km wide to the west and northeast where a well-developed reef crest is found along the west. The intertidal coral coverage is variable but reaches up to 45% of the available space (Richards et al., 2018a). The eastern rim is about 400–600 m wide and opens by a very narrow channel to the northeast. A single sand cay, Cunningham Islet occurs to the north. Large areas of the reef dry at low water (Sailing Directions, 2022) and the rim is submerged. The lagoon is partitioned by rubbly reef flats into three basins along with deeper areas near the center. These include coded reefs throughout much of their area. The larger basin to the west is about 30  km2 not including an inner reef flat area of about 5 km2 that is flanked by a series of flat-topped, coalescent coral reefs (Collins and Testa, 2010). Sand deposits occur around the deeper lagoon and cover an area of about 3 km2 within the northern section of this basin. The two lagoon basins along the eastern edge are 4–5 km2 and are both deeper than the larger, western basin. Although we do not have specific depth data,

the lagoons as coded by the Allen Atlas are presumed to be less than 15 m deep. The shallow eastern margins are flanked by a sand apron. The lagoon system is thought to have become infilled with sediment due to import by wave and tide activity and retention by the closed nature of the rim (Collins, 2011). Clerke Reef (Figure 17.3) is an ovoid atoll located 41  km northeast of Imperieuse. Rising from a depth of 390  m, the platform is 16.7  km north– south, 7.6 km west–east at its widest, and covers an area of 97.8 km2. The platform is outlined by outer reefs that typically range from 315–460  m wide to the south and west. The eastern reefs are narrower, ranging from 160–250  m wide. The mean level of coral coverage around Clerke has been estimated at 37% ± 10% at a sample depth of 12 m. The reef crest is best developed to the west–north where a well-cemented reef pavement with a high coral cover occurs (Richards et al., 2018a) as shown by the Atlas. The rim is composed primarily of a rocky reef flat although a sandy cay, Bedwell Islet, is found to the northeast along with three narrow channels that penetrate the rim that range from about 5 to 40 m wide. Reef growth occurs on the outer reef flats mostly in the west–northeast.

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Figure 17.3 Remote-sensing images of the atolls of Imperieuse, Clerke, and Mermaid. Images Ⓒ 2021, Planet Labs PBC.

The lagoon area is about 63 km2, 26 km2 of which is designated as shallow with expansive deposits of sand along the west and to the south. The remaining lagoon is partitioned into three oval basins totaling 37 km2 that are somewhat deeper—but still less than 10 m deep (Collins and Testa, 2010)—and are partially infilled by sediment. Reef growth occupies much of the southern end of the two smaller basins to the northeast. Reefs within the largest of the three deeper basins in the western lagoon form within its northern and southern reaches with little reef growth in the middle. Mermaid Reef (Figure 17.3) is an ovoid atoll 28 km northeast of Clerke. The platform rises from waters 440 m deep, is 15.2 km north–south, 7.6 km

west–east, and covers an area of 87.7 km2. A narrow band of outer reefs 140–200 m wide surrounds the atoll although a 2-km long section to the west ranges from 270–300 m wide. A survey of the southeastern offshore reefs at 12 m revealed a high level of coral coverage, with a mean of 26.9% ranging as high as 56% of the substrate (Richards et al., 2018a). In general, Mermaid reefs develop a coral cover of 29% and appear to be more diverse than the coral fauna on the other Rowley Shoals reefs (Veron, 1986; McKinney, 2009). The rim bears a sandy cay to the northeast and a nearby channel that is 250 m wide and 8 m deep (Grimaldi et al. 2022) and opens to the lagoon. The remainder of the rim is a rocky reef flat that dries at low tide. The western side is 1.5–1.8 km wide, whereas

Atolls of North West Australia

the rim to the east is about 400–600  m wide. The shallow outer reefs continue their growth onto the reef flats, especially on the eastern (protected) side. The open ocean-facing west supports an extensive spur and groove system (Collins and Testa, 2010) but has fewer scattered reef patches on the flats. The rim is submerged. The lagoon area is about 48 km2 including a shallow extension partitioned by inner reef flat to the north–northwest. The lagoon is otherwise free of partitions and is open to oceanic swell predominantly from the southwest, as well as spring tides that range to heights of about 4 m. The resulting flows generally enter the lagoon from the west and exit to the east, and although the lagoon can be characterized as a tidally dominated system over the long term, models suggest that episodic swell/wave-induced circulation is one to two orders of magnitude greater than the tidal influences (Grimaldi et al., 2022). A reef sand apron up to 350 m wide occurs in the lagoon margins to the southwest but is more commonly 100–200  m wide with minor breaks to the

southeast. This lagoon displays the least sediment infill of any of the Rowley Shoals reefs and achieves a maximum depth of 20 m (Collins, 2011). Patch and ribbon reefs are scattered throughout the lagoon but are more frequent to the west and in the southern half. Thickets of Acropora dominate the lagoon reefs (Richards et al., 2018a). Scott and Seringapatam are two isolated atolls near 14°S that rise from 400 to 700 m from a terrace on Australia’s northwest continental slope. They are part of a reef chain near or on the terrace edge that extends north to another cluster of atolls in the Timor Sea (Figure 17.1) described below. Scott Reef (Figure 17.4) is the better described of the two and is composed of two reefs, Scott North and Scott South. These reefs are at the edge of the continental shelf, where just off their western rims the bottom abruptly descends to a depth of 2,000  m. Scott South is arcuate and U-shaped, open to the north, and is located 392 km northeast of Mermaid Reef. The platform is 27 km wide west–east, 20 km north–south, and covers an area of approximately

Figure 17.4 Remote-sensing image of the Scott atolls, composed of two reefs, Scott North and Scott South. Images Ⓒ 2022, Planet Labs PBC.

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423  km 2 including two detached, unvegetated islets, one to the northwest and the other to northeast. The platform’s outer reefs are rimmed to the north by an outer reef 160–450 m wide, and to the south by outer reefs that range from 280–450  m wide. A small section of Scott South to the northeast joins the larger structure and is connected to it by a submerged reef 850 m wide. The outer reef assemblages are composed of encrusting coralline algae with minor contributions by corals. The reef crest is rocky and mostly submerged at low tide allowing the development of patch reefs, but it is strewn with boulders and rubble reflecting the high energy environment of strong tides, swells, and cyclonic storms (Collins et al., 2011). The rim is composed of a reef flat more than 2.4 km wide to the west and about 1.4 km wide to the east. Sections of the rim form double crests with a low-lying area between which may be the result of pre-existing topography or erosion during low sea level stands (Collins et  al., 2011). There are a few scattered patch reefs on the flats. The northern rim is open, but large parts of it dry at low tide (Sailing Directions, 2022) and this reef is generally submerged. The U-shaped lagoon is 35–55 m deep with Halim‑ eda as a common component of large sandy areas, amidst isolated coral knolls and stunted coral communities. However, sheltered lagoon slopes and some flat areas at depths of about 40 m develop Acropora thickets along with other corals, sponges, and sparse seagrass to form a community of moderate diversity (McKinney 2009; Collins et al., 2011). Scott North is pear shaped and is separated from Scott South by a channel that is 5  km wide and more than 360 m deep. The platform is 17 km north–south, 15 km west–east, and covers an area of 162.3 km2. The outer reefs are 320–450 m wide to the west and southwest and about 150–290 m wide to the east. The slope is initially gentle and then becomes irregular to about 30 m but is heavily dissected with surge channels and is dominated (65%–95% of the substrate) by rock and rubble. Stony corals are occasional and are often massive or encrusting (Morrison, 2009). The shallow reefs are continuous and grow onto the reef flat and forms numerous patch reefs, especially to the east. The rim is composed of rock and rubble reef flats that range from 1 to 1.5 km wide to the west and 1.1–0.6 m to the east, both of which become almost dry at low tide (Sailing Directions, 2022). The reef flat is a mixture of scattered large boulders and occasional corals. There are no islands, and the rim is punctuated by two channels. The one to the northeast is about 115  m wide but shoals to about 2 m. In addition, a triangular section of reef flat partially blocks the opening to the lagoon.

The channel to the southwest is narrower, poorly charted (GPS Nautical Chart AU314122), and opens to the lagoon with a delta-like sediment lobe, perhaps representing the prominence of southwesterly swells. The rim is submerged. The lagoon area is 114.8 km2. The shallows present a reef sand margin which are up to about 2 km wide north–southwest but are more commonly 300–400  m wide. The apron narrows considerably to the east where it is about 150 m wide or less. Coral patches cluster within the apron to the north, west, and southeast, but also form narrow tracts parallel and perpendicular to the southeastern lagoon slope as well as in the deep central lagoon. The morphology of the bottom here is complex and more biologically diverse than elsewhere with numerous channels at 15–20  m associated with coral knolls and plateaulike reef areas. Coral patches are common between 1 and 10  m depths and include Acropora thickets as well as scattered encrusting and hemispherical corals. However, the Acropora community incurred losses after suffering mass bleaching events in 1996 and again in 2016 here and elsewhere on the NW Australian shelf atolls, including Seringapatam and Ashmore reefs described below (Gimour et al., 2019). The maximum lagoon depth is 34 m (Purdy, 2001). Seringapatam Reef (Figure 17.5) is located 24 km northeast of Scott North and was named after a whaling ship built in India whose captain made note of this small semicircular atoll en passant in 1839. The platform is 8.7 km north–south, 7.2 km west–east, and covers an area of 52.8 km2. Outer reefs surround the platform and are 225–400 m wide west–southeast counterclockwise, and 140–220  m wide elsewhere. The subtidal outer reef develops a spur and groove system that extends to a depth of 12–15  m and is deeply undercut and cavernous. There is also a moderate cover of stony corals at these depths. The outer reef to the northeast drops to nearly vertical from 30–40 m to about 200 m with few stony corals, but large numbers of soft and branched octocorals (Wilson, 1985). The rim is a reef flat about 1.2 km wide to the west and about 700 m wide to the east and is rocky on the outer reef flat with a clearly developed zone of boulders composed of reef limestone around the perimeter. These may represent the reef crest that surrounds this atoll... Most boulders were eroded by storms, but others have been created by agents of biological erosion—burrowing barnacles in this case ( Wilson, 1985). The reef flat is composed of coral rubble. The shallow rim develops a coralline algal veneer but coverage by living coral is low, only 3%–7% (McKinney 2009; Collins, 2011). Wilson (1985) by contrast found the intertidal to be covered with

Atolls of North West Australia

Figure 17.5 Remote-sensing image of Seringapatam Atoll. Images Ⓒ 2021, Planet Labs PBC.

leafy brown algae. There are no islands at or above sea level and there is a single narrow channel to the northeast, about 80 m wide at the seaward entrance but only 2.5 m deep making it usable only by small vessels at high tide due to torrents of water exiting the lagoon when the tide is low (Wilson, 1985). The reef dries at low tide (Sailing Directions, 2022) and is submerged. The lagoon area is 29.8  km2. The inner reef flat is well represented by reefs along the eastern rim. The lagoon shallows develop a reef sand apron up to 500  m wide in the east, narrowing to about 75  m wide to the south, that extends to the lagoon slope in several areas. The apron is generally at a depth of about 5 m. Reef growth is found encircling the deeper lagoon adjacent to the apron and includes dense and extensive patches of staghorn (Acropora) corals. The bottom of the lagoon is very irregular with numerous coral patch reefs, mounds, and pinnacles, but these only rise about 2 m where the maximum depth is 27  m. The patches are quite diverse and contain

branching, foliose, and massive corals despite a thick, silty bottom that is easily disturbed (Wilson, 1985). Three additional atolls lie on the northwestern Australian shelf close to the Timor Sea in an area referred to as the Sahul Shelf. The Sahul region is complex and connects the Australian continental shelf with that of New Guinea. It is generally shallow and tidally dominated, meaning that bottom sediments at depths of 100 m are mobilized by 4-m-high spring tides that generate rotating tidal currents as they sweep across its surface. The shelf has been subjected to numerous forces including uplift, subsidence, and faulting due to its proximity to the 3,000-m-deep Timor Trough where the north-moving the Indo-Australian plate collides with the Timor plate. The western edge of the shelf includes a subsection known as the Ashmore Platform, a carbonate shoal area with a number of carbonate knolls, banks, and atolls rising from it (Weinberg et  al., 2010). The atoll called Ashmore Reef (Figure 17.6) is the largest of these structures

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Figure 17.6 Remote-sensing images of the atolls of Ashmore, Cartier, and Hibernia. Images Ⓒ 2021, Planet Labs PBC.

to reach the surface. It lies 183 km northeast of Seringapatam but only about 140 km from the Indonesian Island of Timor (Figure 17.1) where it may be more affected by the year-round flow of the Indonesian Flowthrough than by other current systems (Molcard et  al., 1994). The climate is monsoonal with prevailing westerly and northwesterly winds during the rainy season from November to March, but because of its position at about 12°S, cyclonic storms are much less common than at Scott and other reefs described previously. The dry season is dominated by the southeasterly trade winds (May– September) which drive the flow of the Holloway Current (Wilson, 2014). Ashmore Reef and its companion, Cartier Reef, are designated as National Nature Reserves. Ashmore in particular is a biologically and ecologically diverse marine system that supports many species of reef corals and fishes, some of which are found only in this region. It is the most diverse region for sea snakes in

the world with over 13 species (not including recent enigmatic declines) and supports large productive seagrass pastures which feed high densities of endangered green sea turtles and a small population of dugongs. The endangered loggerhead and hawksbill turtles are also found on the reef. Over 90 species of birds have been recorded on the reef and islands which form the largest breeding sites in the region for numerous species of sea birds (Whiting, 2000). The platform, which resembles a rounded triangle, is 25.6  km west–east, 13.3  km north–south through the middle, and covers an area of about 227  km 2 . Ashmore differs from the other shelfedge reefs described in this chapter by its west–east orientation rather than north–south and by rising from a relatively shallow depth of about 100  m (Berry, 1993). The outer reefs are generally 300– 500 m wide to the east and 230–440 m wide southwest–west clockwise. However, there are areas to the southeast and south that project 600  m from

Atolls of North West Australia

the rim and 1300  m to the north where much of it is uncoded but visible. The northwestern (lower energy) margin displays diminished outer reefs in four areas that form openings to the lagoon. The outer reefs to the north are steeply inclined at about 35°, with walls that are profusely covered (28.3%– 31%) with hard coral growth. The reef flat to the south, by contrast, is gently sloped and is composed of loose rubble followed shoreward by an extensive sand flat with seagrasses that are not coded by the Allen Atlas. The northern reef flat is similar to that of the south but lacks the extensive sand flats found to the south (Berry, 1993). A large reef area up to 600  m wide occurs to the east where hard coral coverage at a depth of 12  m is 32.7% (Richards, et  al., 2018a). Soft corals are abundant, but stony corals are dispersed or are found in small, isolated patches. The rim is composed of reef flats, more than 3 km wide to the west, and about 1.5 km wide to the south and east. The windward side of the platform is to the south. The reef crest is well developed by coralline algae and is unbroken along the south and southeast where it is up to 900 m wide. The intertidal slopes gently forming a shelf about 150 m wide where a spur and groove system is found (Berry, 1993; Hooper, 1994). The lagoon area is about 145 km2 and forms eastern and western basins separated by an extensive inner reef flat system with numerous patch reefs, especially to the south, some of which reach the surface during low spring tides. Patch reefs also are found on the northern and northwestern inner reef flats. The eastern lagoon is heavily silted, does not form a lagoon slope, and is about 10 m deep (Hooper, 1994; Rees et al., 2003). Because of the large breaks in the reef, there is no impounding of water in the western lagoon on outgoing tides as on the Rowley Shoals reefs, and to a lesser extent, Scott and Seringapatam Reefs (Berry, 1993). This lagoon develops reefs and patches throughout the basin where waters are clear, open to oceanic exchange, and is typically 10–25 m deep with a maximum depth of 46 m (Berry, 1993; Sailing Directions, 2022). Cartier Reef (Figure 17.6) is a small, egg-shaped atoll located 50 km southeast of Ashmore. The platform rises from a depth of about 200  m, is 5.9  km west– east, 2.6 km north–south at its widest to the west and covers an area of 14  km2. The outer reefs are 375 to more than 500 m wide west–southeast clockwise, and 170–250  m wide elsewhere. The northern outer reefs drop off steeply to 50 m and support a profusion of soft as well as hard corals before reaching a sandy shelf with reef patches (Berry, 1993). Hard coral cover in this area was about 20% at 8–10 m (Richards et al., 2009).

The rim to the south–southwest develops the most consistent reef crest. The reef flat here grades into a wide expanse of biologically impoverished sand which reaches a 1.5-km2 unvegetated, reef-rimmed sand cay to the center of the platform. The sand to the south of the island dries at low tide. Likewise, the outer reef slope is wide, gently sloping, and is scoured by surge resulting in little coral growth. The shallow reef to the north forms a spur and groove system and a consolidated pavement that develops into a reef flat about 500 m wide with a few living corals along with a sparse cover of seagrass. The lagoon is a remnant system covering an area of about 1.3 km2. Its depth is about 8 m (Smith et al., 2002). We have no information concerning its formation or biota. Hibernia Reef (Figure 17.6) is the northernmost of the atolls on the Sahul Shelf where it is perched on the edge and rises 100  m on all sides but drops to 300  m to the northwest. Located 31  km northeast of Ashmore, Hibernia forms a rounded triangle 7.5 km west–east, 2.6 km at its widest to the east, and covers an area of 15.3 km2. The outer reefs surround the platform except for a gap to the north–northeast where the rim is submerged. The outer reefs to the southwest–southeast are 410–560  m wide. Those extending west–northeast clockwise are irregular, but generally range from 95–315 m wide. Coral growth is most extensively developed on the southern and western sides (Hooper 1994) and partially overlaps with maximum reef width. Similarly, Richards et al. (2018a) found that coral cover was highest to the southeast (35.7%) and lowest (18.7%) to the northeast. Interestingly, a new species of the calcareous blue octocoral Heliopora (Heliopora hiberniana) has been described from Hibernia Reef and is one of several corals only known to occur on offshore northwestern Australian reefs (Richards et al., 2018b). The rim is composed of a reef flat that is 1.3 km wide to the west and is accompanied by a reef crest that is more than 200 m wide (not shown by the Atlas), and although the crest appears to continue to the north, it is only developed as a coralline algae rim to the west and south, the directions of the prevailing swell (Hooper, 1994). The rim to the northeast is sufficiently submerged for 3.5 km for small vessels to cross, and that area is replete with shallow reefs that spread into the lagoon. There are no cays or islands; the rim dries at low tide (Sailing Directions, 2022) and is submerged. The lagoon area is 6.8  km2 and presents a slope that is extensively colonized by branching coral thickets on the western and eastern margins (Hooper, 1994). The Atlas shows reef development

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around the margins of the deep lagoon. The basin develops large patch reef pinnacles rising from the lagoon floor at a depth of 30–50  m. Pinnacle-like reefs are coded to the western and eastern basin areas. The lagoon depth maximum is more than 60 m in various locations (Hooper, 1994).

References Adamo ND, Fandry C, Buchan S, Domingues C 2009. Northern sources of the Leeuwin Current and the “Holloway Current” on the North West Shelf. J. Roy. Soc. West. Austr. 92: 53–66. Bahmanpour MH, Pattiaratchi C, Wijeratne WMS et al. 2016. Multi-year observation of Holloway Current along the shelf edge of North Western Australia. J. Coast. Res. 75: 517–521. Berry PE 1993. Historical background, description of the physical environments of Ashmore Reef and Cartier Island and notes on exploited species. Rec. West. Austr. Mus. 44: 1–11. Collins LB 2011. Geological setting, marine geomorphology, sediments and oceanic shoals growth history of the Kimberley region. J. Roy. Soc. West. Austr. 94: 89–105. Collins LB, Testa V 2010. Quaternary development of resilient reefs on the subsiding Kimberly continental margin, Northwest Australia. Brazilian J. Oceanogr. 58: 67–77. Collins LB, Testa V, Zhao J, Qu D 2011. Holocene growth history and evolution of Scott Reef carbonate platform and coral reef. J. Roy. Soc. West. Austr. 94: 239–250. Gimour JP, Cook KL, Ryan NM et al. 2019. The state of Western Australia’s coral reefs. Coral Reefs 38: 651–667. Grimaldi CM, Lowe RJ, Benthuysen JA et  al. 2022. Wave and tidally driven flow dynamics within a coral reef atoll off Northwestern Australia. J. Geophys. Res.: Oceans 127, e2021JC017583. Hooper JNA 1994. Coral reef sponges of the Sahul Shelf–a case for habitat preservation. Mem. Qld. Mus. 36: 93–106. Kordi MN, O’Leary M 2016. Geomorphic classification of coral reefs in the northwestern Australian shelf. Regional Stud Mar. Sci. 7: 100–110. McCaffrey JC, Wallace MW, Gallagher SJ 2020. A Cenozoic barrier reef on Australia’s North West shelf. Global Planet. Change 184: 103048. https://doi.org/10.1016/j.gloplacha.2019.103048 McKinney D 2009. A survey of the scleractinian corals at Mermaid, Scott, and Seringapatam reefs, Western Australia. Rec. West. Austr. Mus. 77: 105–143. Molcard R, Fieux M, Swallow JC et al. 1994. Low frequency variability of the currents in the Indonesian channels (Savu-Roti and Roti-Ashemore Reef). Deep Sea Res. 41: 643–1661.

Morrison PF 2009. Subtidal habitats of Mermaid Reef (Rowley Shoals), Scott and Seringapatam reefs, Western Australia. Records West. Austr. Mus. 77: 29–49. Phillips HE, Tandon A, Furue R et al. 2021. Progress in understanding of Indian Ocean circulation, variability, air-sea exchange, and impacts on biogeochemistry. Ocean Sci. 17: 1677–1751. Purdy EG 2001. Supplement to Purdy EG, Winterer EL 2001. Origin of atoll lagoons. GSA Bull. 113: 837–854. Rees M, Colquhoun J, Smith L, Heyward A 2003. Surveys of Trochus, Holothuria, giant clams and the coral communities at Ashmore Reef, Cartier Reef and Mermaid Reef, Northwestern Australia. Australian Institute of Marine Science. https://citeseerx.ist.psu.edu/viewdoc/ download?doi=10.1.1.392.9752&rep=rep1&type=pdf Richards Z, Bryce M, Bryce C. 2018a. The composition and structure of shallow benthic reef communities in the Kimberley, north–west Australia. Records West. Austr. Mus. 85: 75–103. Richards Z, Yasuda N, Kikuchi T et  al. 2018b. Integrated evidence reveals a new species in the ancient blue coral genus Heliopora (Octocorallia). Sci. Rep. 8: 15875. https://doi. org/10.1038/s41598-018-32969-z Richards Z, Chong-seng K, Pratchett M, Hobbs J-P 2009. Ashmore Reef National Nature Reserve and Cartier Island Marine Reserve Marine Survey 2009. James Cook University. Sailing Directions 2022. North, West, and South Coasts of Australia. National Geospatial Intelligence Agency, Springfield VA. Smith L, Rees M, Heyward A, Colquhoun J 2002. Stocks of trochus and bêche de mer at Cartier Reef: 2002 surveys. Australian Institute of Marine Science. Thomas L, Underwood JN, Rose NH et al. 2022. Mechanisms of ecological divergence with gene flow in a reef-building coral on an isolated atoll in Western Australia. Sci. Adv. 8: eabl9185. Veron JEN 1986. Reef-building corals. Rec. West. Austr. Mus. 25: 27–35. Weinberg C, Westphal H, Kwol E, Hebbeln D. 2010. An isolated carbonate knoll in the Timor Sea (Sahul Shelf, NW Australia): Facies zonation and sediment composition. Facies 56: 179–1993. Whiting S 2000. Management and Research Issues at Ashmore Reef National Nature Reserve. Biomarine International, Darwin 29. https://www.agriculture.gov.au/sites/default/files/ documents/ashmore-management.pdf Wilson B 2014. Kimberly marine biota. History and environment. Records of the West. Austr. Museum 84: 1–18. https://doi. org/10.18195/issn.0313-122x.84.2014.001-018 Wilson BR 1985. Notes on a brief visit to Seringapatam Atoll North West shelf, Australia. Atoll Res. Bull. 292: 83–100.

The Atolls of Indonesia Indonesia is an archipelago, sometimes referred to as the Maritime Continent, composed of about 13,500 tropical islands, five of which are large and include Java, Sumatra, Kalimantan (Borneo), Sulawesi, and the western half of New Guinea (Chapter 13), along with about 30 groups of smaller islands. The spatial extent of the country is deceptively large and stretches about 5,100 km east to west. By comparison, the distance from San Francisco to New York is about 4,100 km. Indonesia is a transitional country, lying between the Pacific and Indian oceans. It also lies amidst the convergence of the Indo-Australian, Eurasian, Pacific, and Philippine plates that are moving in different directions and converge in some areas (summarized by Verstappen, 2010). In addition, smaller plates, some of which lie under a single island or small archipelago, and numerous trenches dissect the margins of the archipelago resulting in very complex geology including the formation of microplates and microcontinents further described below. Plate collisions form mountain ranges on islands and are responsible for about 400 volcanoes that are especially prominent along its southern border at the boundary of the Nusa Tenggara (Lesser Sunda) Islands where the Australian plate dives beneath them. The most explosive of the numerous volcanic eruptions that have occurred there in the last 10,000 years include Mt. Tambora (Figure 18.1), a volcano that expelled about 140 billion tons of magma in 1815. The plumes were propelled to an altitude of 43 km and an estimated 71,000 people died during or in the aftermath of the eruption. In addition, about 60 million tons of sulfur were released to the atmosphere forming a global veil of sulfate as an aerosol that caused anomalously cold weather and summer frosts in Europe, Russia, Canada, and the U.S. in 1816, the year without summer. Crop failures, livestock deaths, malnutrition, and even typhus epidemics became common as a result (Oppenheimer, 2003; Luterbacher and Pfister, 2015). Indonesia lies within the Coral Triangle, the center of biological diversity for the world’s marine ecosystems. With about 75,000 km2 of coral reefs,

Indonesia also has one-eighth of the world’s coral reefs, features that are especially prominent east of Kalimantan (Cesar, 1998). Conversely, the waters of Sumatra, Java, and most of Kalimantan are subject to river outflow, suspended sediments, and estuary-like conditions where nearshore patch and fringing reefs may occur, but atolls are absent. Reefs in these waters are also subject to earthquake and tsunami damage as well as pollution effects from population centers (Hoeksema and Putra, 2000; Edinger et al., 2008; Browne et al., 2019). Although a few atolls occur to the northeast of Kalimantan, almost all of them occur around Sulawesi, including its gulfs and the adjacent Flores and Banda seas. Some additional atolls are found in the Makassar Strait, whereas others flank the Island of Halmahera to the east of Sulawesi’s northern arm (Figure 18.1). We account for a total of 43 atolls in the region. Many of Indonesia’s atolls are unlike those of their Pacific counterparts, and therefore, we generally follow the lead of Tomascik et al. (1997) and their comprehensive listing.

Climate and regional oceanography The Indonesian Archipelago serves as a gateway for waters from the tropical North Pacific to the Indian Ocean. The primary movement of seawater is regulated by the Indonesian–Australian monsoonal winds that maintain a pressure difference between the Pacific and Indian oceans. Most of the southbound flow, about 80%, passes from the semi-enclosed Sulawesi and Sulu seas to the north through the Makassar Strait west of Sulawesi then eastward into the Flores and Banda seas. Most of the remaining Makassar Strait waters and those from other entry points (not shown) flow around the eastern Banda Sea, where they remain inside the Banda Island arc, and then exit to the Indian Ocean through several passages in the Nusa Tenggara Islands and the Timor passage north of Australia to become the Indonesian Throughflow (Figure 18.1; Gordon, 2005; Sprintall et al., 2019).

DOI: 10.1201/9781003287339-18

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Figure 18.1  Although a few atolls occur to the northeast of Kalimantan away from river outflow, almost all of them occur around Sulawesi, including its gulfs and the adjacent Flores and Banda seas. Some additional atolls are found in the Makassar Strait to the west of Sulawesi, whereas others flank the Island of Halmahera to the east of Sulawesi’s northern arm. Waters from the tropical North Pacific move into the archipelago by the Indonesian–Australian monsoonal winds. About 80% of the southbound flow passes through the semi-enclosed Sulu and Sulawesi seas to the north, and then through the Makassar Strait (black arrows). From there, Pacific inflow is guided into and around the eastern Banda Sea, where it is circulated and mostly contained until it exits to the Indian Ocean through several passages in the Nusa Tenggara Islands, and the Timor passage north of Australia to become the Indonesian Throughflow (white arrows).

Tropical cyclones in the seas of Indonesia near the equator are rare, but they can occur. For example, in 1973, an unnamed cyclone formed in the Banda Sea and became the Southern Hemisphere’s deadliest cyclone. Although it was only a Category 3 storm, an estimated 1,500 fishermen were killed by the cyclone on Palu’e Island alone, news that did not reach the capital Jakarta for a month. Rainfall varies with location relative to the Indian and Pacific oceans and Asian and Australian continents and related ocean circulation, the monsoons, ENSO state, sea-surface temperature, and proximity to land and mountains (Aldrian and Susanto, 2003). Nonetheless, in general terms, atolls of the Flores and Banda seas south of Sulawesi have rainfall of 160 cm/year or less, but are strongly influenced by the monsoons, with a wet northwest monsoon (November–March) and a dry southeast monsoon (May– September) (Figure 18.2). The other end member

includes areas further east in the Halmahera Sea and south, which includes generally greater rainfall (160 cm/year or more) with a peak in June–July and lowest rainfall in November–February. The El Niño state tends to be accompanied by drier conditions and droughts. Waves are sheltered from open Indian and Pacific oceans’ waves, thus the inner seas where most atolls occur have generally small waves. In most of the area, significant wave heights average less than 1 m (e.g., Figure 18.2), with direction that varies with the monsoon (Rizal and ­Ningsih, 2022). Wind speeds are greatest during the southeast monsoon, and wave heights follow a similar pattern. For example, in the Banda Sea, winter (January–February) waves average 0.5–0.75 m in height, whereas summer (May–August) waves are 0.75–1.25 m. The tides of Indonesia are complex and vary considerably in amplitude and phase across the vast

Atolls of Indonesia

Figure 18.2  Waves are sheltered from the open Indian and Pacific oceans, thus the inner seas where most atolls occur are exposed to generally small waves, as shown here for Karang Kaledupa. Wind speeds are greatest during the southeast monsoon, and wave heights follow a similar pattern. Indonesian tides are complex and vary considerably in amplitude and phase across the vast oceanic area with its passes and complex bathymetry. In general, the tides of Indonesian seas with atolls covered herein are mixed with prevailing semidiurnal tides except for the southwestern atolls near Kulukalukuang. There, tides also are mixed, but include prevailing diurnal tides.

oceanic area with its passes and complex bathymetry. In general terms, however, the tides of Indonesian seas with atolls covered herein are mixed with prevailing semidiurnal tides (see Figure 18.2, data from Karang Kaledupa), except for the southwesternmost atolls near Kulukalukuang. There, tides also are mixed, but include prevailing diurnal tides (Figure 18.2). Maximum (spring) tidal ranges vary from just over 1 m in the south (Halmahera (1.12 m), Pulau Maringki (1.20 m), Kulukalukuang (1.31 m) to over 2.2 m elsewhere (Karang Kaledupa (2.17 m), Lucipara (2.23 m), Muaras (2.45 m)). Several terms are used to describe Indonesian reefs, and included in terms here. Some of the more common phrases include besar: large, great; gosong:

shoal, sand bank; karang: coral; kepulauan: archipelago, large islands; nusa: island; pasir: sand, sandy beach; pulau: island.

The atolls of Halmahera Located to the east of Sulawesi and west of the Bird’s Head of New Guinea, Halmahera is K-shaped, sparsely populated, and the largest of the Maluku Islands in the northeast of the Indonesian Archipelago. Halmahera is mountainous, volcanically active, and biologically rich with numerous environments including coral reefs, mangrove forests, seagrass beds, sandy beaches, rocky beaches, estuaries, lagoons, and deltas. Rainforests occupy two-thirds of

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the islands and are home to an array of plants and animals, many of which are found nowhere else such as Wallace’s bee, the largest bee in the world. Alfred Russel Wallace is a British biologist lived for years in the Halmahera region. After decades of work, he developed his own hypotheses on evolution and what was later called Wallace’s Line, an important biogeographic division of Indonesian islands in which the larger ones were connected to Asia during lowstands of sea level, but others including Sulawesi and Halmahera were isolated and differ considerably in the affinities of their animals and plants. These isolated islands between Indonesia and Australia are collectively referred to as Wallacea in his honor. Halmahera is part of its own highly active microplate that is colliding with other plates from two sides. The eastern side dips from collision with the Bird’s Head region of New Guinea, while the Molucca Sea, forming its own plate, is colliding with Halmahera from the west (Hall and Spakman, 2003). While Halmahera is not known for the number of its atolls, there are four that are found in the area. Pasir Raja lies on a platform that is about 6.5 km southeast of Tidore Island and about the same distance west of Halmahera where it rises from depths

of about 220 m. Two small atoll reefs occur on the platform (Figure 18.3) and are separated by depths of 40–80  m (National Geospatial-Intelligence Agency chart 73016). The northernmost of the two forms a rounded rectangle that is about 1200 m northeast–southwest, 780 m west–east, and covers an area of about 70 ha. The platform is composed of a relatively broad outer reef that is up to 400 m wide to the southwest and enters the 8.5 ha lagoon from the open northwest. The rim is coded primarily as a reef-covered reef flat and develops no islands. The more southerly of the pair is ovoid and lies less than a kilometer to the southeast. Its platform is about a kilometer long northwest–southeast, about 760 m at its widest, and covers an area of 75 ha not including reefs adjacent to the platform. The rim is a reef flat with rock and sand to the north and a small area of seagrass adjacent to the lagoon. The circular lagoon covers about 8 ha. The maximum depth of Pasar Raja’s lagoons is 37 m (Tomascik et al., 1997) and each of the Pasir Raja rims is submerged. Another atoll pair is located about 300 km southeast of Pasir Raja in the Halmahera Sea near the mouth of the Gulf of Weda (Figure 18.1). These are Sukar and Kokota. Sukar Atoll, the westernmost of the pair, is boomerang shaped (Figure 18.3) and

Figure 18.3  Remote-sensing images of the atolls of Pasir Raja, Sukar, and Kokota. Images Ⓒ 2021-2022, Planet Labs PBC.

Atolls of Indonesia

is 15.4 km west–southeast following a curved path, 6.1 km at its widest to the southeast, and covers an area of 49.3 km2. The outer reefs are weakly developed in their extent as coded and are up to 60 m wide to the northeast. A spur and groove system develops to the east and a few discontinuous outer reefs are found to the southwest. The rim is dominated by five wooded islands west–south clockwise and several islets to the southwest. The largest of the group, Pulau Sukar (or Pulau Weda, depending on the chart), is predominantly V-shaped and covers about 7 km2. The area enclosed by the rim and islands is a shallow lagoon that covers an area of 9 km2. A shallow channel about 90 m wide leads to deeper part of the lagoon in the southeast covering 3.7 km2 that Tomascik et al. (1997) mark as less than 5 m deep, but some charts indicate depths up to 9 m. The lagoon contains a few patch reefs but it is also largely covered with sand as is the rest of the lagoon. We tentatively mark the rim as semi-closed. Kokota Atoll (Figure 18.3) is located 3.5 km to the east of Sukar. The cleaver-shaped platform is 16.6  km northwest–southeast following a curved path, 4.6 km at its widest, and covers an area of 51.6 km2. The outer reef surrounds the platform but is quite variable in its development with the widest reefs facing east where they are 170–220 m wide. Like Sukar, a spur and groove system is visible but is more extensive extending northeast–southeast. The rim supports two islands and several islets clockwise northeast–northwest and several additional islets occur to the north. The inner reef flat is composed of rubble and sand. The only area of reef and seagrass development is in the lee of 3.8-km2 curved Pulau Gembird to the northeast. The inner reef flat contains small but numerous deeper areas that are rimmed by reef growth. The lagoon is subdivided by an extension of the inner reef flat. The main section is 12.3 km2 and is surrounded by extensive reefs that develop on the slope and surround about 18 sandy platform areas that rise from the deeper lagoon. Tomascik et al. (1997) give the depth for the lagoon as less than 5 m although it appears to be deeper. In addition, a shallow area to the south of the main lagoon is 1.5 km2 and is covered primarily by sand. The rim opens to the main lagoon by a shallow channel that is about 600 m wide. The rim is open.

The atolls of Sulawesi Few islands in Indonesia speak to the complex geology of the region more than Sulawesi. This unusually shaped, four-armed island in the center of the archipelago was formed by collision between the northward movement

of the Australian plate, the westward movement of the Pacific plate, and the south–southeast movement of the Eurasian plate. Each arm, south, southeast, north, and east (Figure 18.1) is the product of different complex processes, histories, and origins. More generally, western Sulawesi is influenced by the formation of the Makassar Strait, an area where Kalimantan at the edge of the Eurasian continental shelf collides with Sulawesi forming a seaway 100–200 km wide and up to 2,000 m deep (Brackenridge et al., 2020). Eastern Sulawesi, by contrast, is the product of collisions with fragments of the Australian plate. The colliding fragments, often referred to as microcontinents, are now in the form of archipelagoes, that either lifted reef terraces up to 300 m at points of impact or subsided and produced atolls (e.g., Wilson, 2008) to be described below. As a result of these plate (and microplate) motions, the island is actively deforming by means of multiple faults and is a center of volcanic activity.

The Gulf of Tomini Three atolls are found in the Gulf of Tomini, south of Sulawesi’s north arm. Because of the semi-enclosed, well-circulated nature of the area and its position near the center of the Coral Triangle, the Gulf is highly diverse. Small coral reef banks, atolls, fringing, and barrier reefs are common throughout the islands and provide multiple island habitats that are especially prominent within the Togian Island Archipelago. This area has a high coral diversity (about 264 species), particularly among the Acrop‑ ora group, as well as about 800 species of reef fishes (Miller et al., 2016 and references therein). East Atoll and Pasir Tengah are part of the high Togian Islands in the Gulf of Tomini. Indeed, it is that proximity that drove Bryan (1953) to leave them out of his Checklist of Atolls. East Atoll assumes an annular shape (Figure 18.4). The platform is small, about 1.5 km northwest–south, 1.2 km west–east, and covers an area of 1.6 km2. The rim is mostly submerged with two small areas totaling 93 ha that are emergent as a rocky reef flat and supports reef patches. The lagoon area is 79 ha and includes small reef ribbons. We do not have depth data for this lagoon. Pasir Tengah is located 3 km west of East Atoll where it assumes the shape of a warped D. The platform is 4.8 km to the west, 2.3 km through the middle at its widest, and covers an area of 8.7 km2. The rocky rim is emergent only southwest–southeast and three small segments to the north. The rim is submerged and is surrounded by reefs that are variable but up to 270 m wide to the south. The outer reefs

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Figure 18.4  Remote-sensing images of the atolls of Pasir Tengah, East, and Karang Lalanga. Images Ⓒ 2021-2022, Planet Labs PBC.

have been described briefly as steeply sloping (observed to 36 m) with 58% coral cover at 4–5 m and 56% at depths of 18–21 m (Yusef and Allen, 2002). The lagoon area is 6.2 km2 and for the most part is not coded. The maximum depth is 51 m (Tomascik et al., 1997). Karang Lalanga is located 125 km southwest of a Pasir Tengah where it assumes the shape of a box jelly with its concave side facing east (­Figure 18.4). The platform perimeter is 6.8 km long to the south, 2.3 km wide to the west, and about 5 km to the north. If the open sections to the east are joined, the area covered by this unusual atoll becomes about 12 km2. The outer reefs surround the platform on all but the east. The rim is a rocky flat that develops reefs along its southern and western sides. The lagoon contains a few reef-rimmed pinnacles and includes reefs that flank much of the shallow areas of the rim. Tomascik et al. (1997) give the lagoon depth maximum as 10 m,

although it deepens to more than 70 m depth to the poorly defined and open east.

The Banda Sea The Banda Sea supports several archipelagoes that are the home to most of Indonesia’s atolls. The northernmost group is defined by the Banggai Islands that extend eastward forming the northern boundary of the Banda Sea (Figure 18.1). This archipelago originally was part of a westward migrating microcontinent, a fragment of New Guinea (part of the Australian plate) that broke away and began colliding with Sulawesi about 3–5 million years ago (Hinschberger et al., 2005; Maulana et al., 2013). The Banggai Islands ­platform supports five atolls, three of which have no names on charts and are referred to here by number. The first of this group is No Name 1, a V-shaped atoll with a rounded base (Figure 18.5) located

Atolls of Indonesia

Figure 18.5  Remote-sensing image of No Name 1 Atoll. Image Ⓒ 2022, Planet Labs PBC.

6 km northwest of Bangkulu Island. The platform is 11.1 km northeast–southwest, 5.9 km north–south at its widest, and covers an area of 44.8 km2. The emergent part of the rim is a reef flat that is outlined by reefs up to 300 m wide to the south. The inner reef flat is rock and sand with reef development that extends around its perimeter. There are no islands, the rim is submerged, and is open to the east. The lagoon covers an area of 28.6 km2 and is mostly rock and sand in the shallows flanked by reefs along the convergent northwestern and southeastern rims. Nautical charts suggest that the lagoon reaches depths of up to 29 m. No Name 2 takes the form of a bent oval (Figure 18.6) and is located on the south side of Bangkulu Island, 28.6 km southeast of No Name 1. The platform is 16.3 km northeast–southwest, 4.1 km west–east at its widest, and covers an area of 51.8 km 2. The northeastern section of the platform is primarily submerged. Reefs surround the rim and extend to the northeast where they are too deep to code and join the reefs extending from the deeper waters of the lagoon. The extent of the outer reefs is quite variable ranging from 150–490 m. The emergent part of the rim is a reef flat with a 21-ha islet to its northernmost extent. Seagrass patches occur

east and south of the islet. Nautical charts indicate depths of up to 21 m. Maringki is the largest atoll in this group and is located 4 km south of No Name 2. Maringki can be imagined as having a fish-like shape, with a rounded anterior to the northeast with a forked tail to the southwest (Figure 18.6). Narrow and discontinuous outer reefs that are up to 300 m wide outline the emergent parts of the platform, and these are overlapped with shallow outer reefs that continue onto the rim. The platform is 31.2 km northwest–south, 15.3 km west–east through the middle, and covers an area of 336 km2. Most of the platform is submerged. The largest islet is 21-ha and occurs to the west. Another unnamed reef flat area to the southeast supports two islets totaling 22 ha. Both reef flat areas also support seagrass communities. The lagoon covers approximately 258 km2 and is dotted with reefs along the platform margins and discontinuously along the lagoon slope, especially within the forked tail region to the south. The maximum depth is 59 m (Tomascik et al., 1997). Panteh is an ovotriangular atoll (Figure 18.7) located about 3.5 km from the eastern edge of Maringki. The platform area is about 35 km2. The outer reefs surround the platform and are up to 250 m

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Figure 18.6  Remote-sensing images of No Name 2 Atoll and Maringki Atoll. Images Ⓒ 2021, Planet Labs PBC.

wide. Google Earth imagery suggests weakly developed spurs and grooves on the southern margin. The triangular northernmost 11 km2 of the platform is submerged and is essentially uncoded. The remaining 24 km2 includes an uncoded cup-shaped reef flat area to the south flanked by more patchy reefs that develop toward the lagoon and to the northwest. The rim is submerged. The lagoon is approximately 21 km2 and is shallow to the south, followed by the lagoon slope; both these areas develop extensive reefs and the maximum lagoon depth is 24 m according to charts. No Name 3 is a rounded triangular atoll with an incomplete and depressed northwest-facing base (Figure 18.7) 9.5 km south of Panteh. The platform is 15.1 km northeast–southwest parallel to the base, 7.1 km east apex to mid-base at its widest and covers an area of 73.1 km2. The outer reefs that surround this atoll are up to 350 m wide to the east. The rim is composed of a reef flat with a narrow islet to the southwest and a cup-like island 1.2 km2 to the north. Both islet areas are surrounded by seagrasses. The rim is a 2.3-km-wide reef flat to the north

where it is widest and narrows to 1.5 km to the southwest. A 2.7-km-wide gap in the rim occurs in an arcuate re-entrant to the west where it is below sea level. The flats are dominated by reef development outside of the seagrass areas. The lagoon area is about 45.8 km2; the northern end is shallower than the rest and is covered with plateaus and deep reef pinnacles that are replete with reefs. The reef slope dominates the eastern lagoon and continues along the southwest where reef development is prominent along with deposits of sand. Nearly half of the lagoon is deeper than 15 m according to mapping by the Atlas, but Tomascik et al. (1997) give the depth for a No Name lagoon very close to our coordinates as less than 5 m (see also No Name 2 lagoon described above). Another cluster of islands to the south of the Gulf of Tolo extends about 60 km southeast into the Banda Sea from a coastal indentation referred to as Matarapé Bay (U.S. Navy Hydrographic Office chart 3070). A group of five atolls occur here, the westernmost of which is egg-shaped Pangadjarang (Figure 18.8). The platform is 5.4 km northwest–southeast, 4.6 km at its

Atolls of Indonesia

Figure 18.7  Remote-sensing images of the atolls of Panteh and No Name 3. Images Ⓒ 2021-2022, Planet Labs PBC.

widest, and covers an area of 20.7 km2. The rim is a horseshoe-shaped rock and rubble reef flat that develops reef areas most strongly southwest–southeast where it is about 1500 m wide. Small seagrass meadows are associated with the reef areas. The rim is without islands and is submerged and develops a channel of unknown depth about 300 m wide to the northeast that is flanked by reef growth. The lagoon covers an area of 9.8 km2. The shallow lagoon extends to the northwest where the rim is submerged and is covered with sand. The shallow lagoon around the rest of the rim develops intermittent and narrow sand deposits. The lagoon area extends to 30 m (­Tomascik et al., 1997). A small rectangle-like atoll is found 5.6 km east of Pangadjarang with no name listed on charts. This area is referred to here and by Tomascik et al. (1997) as No Name 4 (Figure 18.8). Its platform is 4.5 km northwest–southeast where it is 3 km wide. The northwestern platform narrows to 1.7 km wide. The platform area covers 10.2 km2 and develops outer reefs around the periphery that are mostly 130–140 m wide but reach 250 m to the southwest. A small area to the southwest extends to 600 m wide. Shallow reefs continue their growth onto the rocky reef flats, especially to the northeast, where they are up to 300 m wide and 2.5 km long. Reefs elsewhere on the flats are scattered but are still prominent to the southwest where the flats extend more than 1500 m. There are no islands or passes to the

lagoon, and the rim is submerged. The lagoon is oblong and covers an area of 1.7 km2. The shallowest lagoon margin slopes into the deeper lagoon where it develops a band of reefs about 70 m wide in most areas. A few pinnacles develop to the east. We do not have depth data for this lagoon. Belantang is an ovotriangular atoll located 2.4 km southeast of No Name 4 (Figure 18.8). The platform is 5.7 km northeast–southwest parallel to the base, 3.6 km from the west-facing apex to mid-base and covers an area of 13.8 km2. The platform is circumscribed by outer reefs as described above among this group of atolls. The rim is composed of a rocky reef flat with no islands or passes to the lagoon and is submerged. Reefs are scattered across the rim but become best developed southwest–southeast where they form a band about 300 m wide and 2.7 km long. The lagoon covers 3.4 km2 and forms a distinct shallow area that develops a reef sand apron up to 800 m wide to the northeast. The lagoon slope is widely surrounded by reefs; a plateau system co-occurs with reefs around the uncoded 90 ha deep lagoon. We do not have depth data for this lagoon. The shallow part of Bobubu is shaped like a question mark to the east where the rim is emergent but continues to the west–northwest where the platform is completely submerged but for a single 2-ha islet, the only one found on this atoll (Figure 18.8). The island supports a seagrass meadow and is surrounded

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Figure 18.8  Remote-sensing images of the atolls of Pangadjarang, No Name 4, Belantang, and Bobubu. Images Ⓒ 2021-2022, Planet Labs PBC.

by a small reef. The platform is 10.6 km northeast– west through the islet, 8.5 km northwest–southeast, and covers an area of 60.1 km2. The outer reefs around the platform are up to 260 m wide in parts of the southeast. Patch reefs are scattered across the reef flats; the remainder of the rim north and southwest is marked by sand and plateaus that support small reef areas. A larger reef area surrounds the islet to the west. Most of the rim is submerged. The lagoon covers approximately 34.6 km2 and is mostly uncoded except that parts of the slope are coded as reef as are

a few small reef platforms that rise from them. The maximum depth is 38 m (Tomascik et al., 1997). Padea Besar is an ovoid atoll located 5.5 km southeast of Bobubu (Figure 18.9). The platform is 9.2 km northeast–southwest, 5.1 km at its widest, and covers an area of 35.8 km2. Outer reefs are found around the atoll although mostly as narrow ribbons except for about 2.5 km along the northwest where they extend to about 150 m wide. The rim is ­primarily a rocky reef flat that supports a 4.8-km2 island to the south that is surrounded by seagrass meadows and patch reefs. The

Atolls of Indonesia

Figure 18.9 Remote-sensing images of the atolls of Padea Besar and Sapondra Utara. Images Ⓒ 2021-2022, Planet Labs PBC.

rim is primarily submerged as a rock and rubble flat that develops reefs especially to the northeast. A channel 60–80 m wide penetrates the platform to the west and connects with a 7-km2 lagoon in the platform center. A reef sand apron covers the shallow lagoon the slope areas. The slope is partly rimmed by plateaus and by sand and reefs up to a kilometer wide surrounding an uncoded area of 45 ha that is deeper than 15 m. Sapondra Utara is an isolated, sinusoidally shaped atoll (Figure 18.9) located about 46 km southwest of Padea Besar and about 20 km northwest of Wowoni Island near Sulawesi’s southeastern arm. The platform is 7.8 km west–east, 1.9 km north–south, and covers an area of about 11.6 km2 although only an area of 1.8 km2 to the northeast is emergent as a reef flat that supports a 7-ha islet to the east. The remainder of the rim is submerged as a reef that extends more than 5.5 km to the west from the island and about 3.4 km from its southwest, all of which is coded as reef that partly encloses a deep, uncoded lagoon with an area of roughly 4.9 km2. The maximum depth of the lagoon is 39 m according to charts.

The Tukang Besi Archipelago is an extension of Sulawesi’s southeastern arm that divides the Banda Sea into southern and northern basins (Figure 18.1). The islands here are included within the Wakatobi National Park, a 13,900 km2 protected marine area in the center of the Wallacea Region known as a hot spot of biodiversity within the Coral Triangle. Note that Wakatobi is a portmanteau derived from the four larger islands in the Tukang Besi Archipelago, Wangi-wangi, Kaledupa, Tomia, and Binongko. The reef morphologies exhibited by these and other islands in this area include a wide variety of reef morphologies ranging from table reefs to fringing and barrier reefs as well as seven atolls. In addition to considerable spatial heterogeneity, the area exhibits high connectivity with complex currents that facilitate genetic interchange while maintaining multiple semi-isolated reef and reef-associated communities. Coral cover in the park is about 22% and 396 species of reef-building corals are represented. There are also 590 fish species that have been recorded. Seagrass meadows are a major feature of the intertidal

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and coastal waters of the park, especially on its major islands and atolls where high-diversity communities of fishes and invertebrates have been recorded. In addition, larger islands develop small mangrove forests, and because of the water clarity in the park, seagrasses and fringing reefs grow contiguously and are highly connected as nursery grounds that represent one of many habitats and life cycle dependencies that maintain the diversity of reef biota found here (Unsworth et al., 2008; Clifton et al., 2010). Similar to the Banggai Archipelago described above, the Tukang Besi group rests on a westward-migrating microcontinent derived from the Birds Head Peninsula of New Guinea. This 1,000–4,000-m-deep platform broke away and began to collide with the large islands adjacent to Sulawesi’s southeastern arm and Sulawesi itself about 13 million years ago (Hinschberger et  al., 2005; Maulana et al., 2013). Several islands have been uplifted to 300 m or tilted due to the collision, in some cases exposing at least 14 step-like terraces, areas where reef growth was interrupted and subjected to erosion by changing sea levels (Borel-Best et al., 1985; Pedoja et al., 2018). Although understanding uplift has been the principal objective of detailed geological studies in this area, debate concerning the nature and origin of the atoll morphologies and whether subsidence has occurred (Madden et al., 2013 and refs. therein) has not been resolved. We describe the Tukang Besi atoll morphologies in the following section. The atoll closest to the southeastern arm of Sulawesi is Karang Kapotta (Figure 18.1), located to the east of Buton Island. The platform of this cleaver-shaped reef is 20.2 km northwest–southeast, 8.5 km west–east, and covers an area of 102.2 km2. The platform develops narrow outer reefs that are about 50–100 m wide with small areas extending to about 200 m wide. The reef flat is composed of rock and rubble with large areas of seagrass growth up to 850 m wide that fringe the shoreline. Shallow reef growth occurs on the reef flat but principally in pockets to the northeast. The northwestern rim, which constitutes about 31% of the atoll perimeter, is a submerged sand bank and includes two openings to the lagoon that exhibit reef development. The northwesternmost one is at least 6.7 m deep and is preferred (Sailing Directions, 2018); another opening about a kilometer wide is about 2.3 km to its south. Both develop reefs near the openings. There are no islands, and the rim is submerged. The lagoon covers an area of about 59.3 km2. Reefs occur in the shallow lagoon adjacent to the seagrass areas and to the southeastern half where isolated and shallow reef areas are surrounded by deeper water.

The lagoon slope is marked by a narrow reef ribbon leading to the lagoon basin where the maximum depth is 45 m (Tomascik et al., 1997). Karang Kaledupa is a shark-shaped atoll (Figure 18.10) located just 3.5 km southeast of Karang Kapotta where the depth between the two is 475 m (Defense Mapping Agency chart 73000). The platform is 49.1 km curving through the middle, 15.3 km at its widest near the ‘head,’ and covers an area of 413 km2. Outer reefs up to 400 m wide occur to the west, whereas those to the east may be up to 270 m wide, however, most the rim is surrounded by reefs that are far narrower. The rim is a rocky reef flat that is developed as seagrass meadows up to 2.5 km wide. There are no islands, and the rim is perforated by eight openings to the southeast, two of which are passes 16 m deep (Sailing Directions, 2018). The rim is intertidal and is essentially submerged. The lagoon covers an area of about 244 km2. The northwestern portion (the tail) is a 1–2-m-deep shallow lagoon that is isolated by ridges of inner reef flat and contains uncharacterized mound or knoll-like structures. This shallow area leads to an adjacent lagoon with a central, deep basin that opens by narrow channels to the Banda Sea and covers about 11 km2. The lagoon slope of the latter area is flanked by reefs and connects to the central lagoon by a narrow, reefrich canal. The central lagoon widens to 4.5–5.5 km mainly due to wide reef slope to its north and continues southward while narrowing to the east and west. The deep uncoded lagoon is infiltrated by reef flat fragments and a few pinnacles both of which are reef rimmed. However, the 14–32-m-deep central lagoon supports considerable reef development on its adjacent slopes. The head region is the widest part (about 8.8 km) of the lagoon and develops reefs flanking the margins as well as in pockets partly enclosed by reef flats. The maximum depth is 37 m (all depth data from Tomascik et al., 1997). About 5 km to the west of Karang Kaledupa lies another small ovotriangular atoll, No Name 5 (Figure 18.10), whose platform is 4.3 km northwest– southeast, 2.0 km parallel to the base, and covers an area of 8.4 km2. The outer reefs are up to 200 m wide to the northwest. The rim is rocky and the reef flat is an intertidal/subtidal environment almost entirely populated by a seagrass meadow. A rubbly area rims the lagoon to the southwest. The lagoon is 1.1 km2 with a 9-ha area to the southeast that is partly separated by a rocky ridge. The lagoon is coded as reef except for the center where a 14-ha area is found that is presumably deeper than 15 m, but we have no specific information for the lagoon depth of this atoll.

Atolls of Indonesia

Figure 18.10  Remote-sensing images of the atolls of Karang Kapotta, Karang Kaledupa, No Name 5, and Lintea. Images Ⓒ 2021-2022, Planet Labs PBC.

Lintea Atoll is shaped as a warped triangle–or perhaps a Neanderthal hand adze (Figure 18.10) and is located 2 km east of Karang Kaledupa. The platform is 15.7 km north–south, 14.1 km west–east, and covers an area of 94.9 km2. The periphery is outlined by a band of outer reefs up to 280 m wide in some areas but is least developed to the northwest and near Pulau Lintea (Lintea Island) to the east. The rim is dominated by seagrass meadows on the reef flats south of Pulau Lintea that are up to 1,100 m wide east–south and extend to the lagoon edge. Seagrass areas narrow to the west but continue along the lagoon edge

and become sparse to the north. Pulau Lintea on the eastern rim is 8.5 km2 and is uplifted as are all of the other Wakatobi Islands (Nugraha and Hall, 2018). Several terraces are visible although we are unaware of their descriptive details. An additional vegetated islet of 12 ha occurs on the northern rim, and a 150-m-wide channel of unknown depth occurs to the west. Seagrasses flank the channel and passage to the lagoon may be hampered by reef growth. The rim is tilted but primarily submerged. The lagoon covers an area of 35.3 km2. About 30% of the lagoon is shallower than 15 m and patch reefs are developed on

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Figure 18.11  Remote-sensing images of the atolls of Ndaa, Karang Koro Maha, and Karang Koka. Images Ⓒ 2021-2022, Planet Labs PBC.

the edges of the deep lagoon. The deepest part of the lagoon is 25 m (Tomascik et al., 1997). A small egg-shaped atoll, Ndaa (Figure 18.11), is found about 20 km to the northeast of Lintea. The platform is 1.9  km northwest–southeast, 1.4  km through the center, and covers an area of 2.3 km2. Outer reefs extend about 100 m to the east, 200 m to the south and about 50 m to the west. The rim is mostly a rocky reef flat, and sandy to the west where an 8-ha reef-rimmed island is found as well as a small seagrass area. The rim dries and is submerged by the tides (Sailing Directions, 2018). The lagoon covers an area of 32 ha and develops reefs around the shallow margins to the north and west. The center is uncoded. We do not have bathymetry data for this lagoon.

Karang Koro Maha is an annular atoll with a somewhat flattened southwestern aspect (Figure 18.11) located 25 km northeast of Pulau Lintea. The platform is 5.7 km north–south, 6.9 km at its widest west–east, and covers an area of 34.9 km2. A band of outer reef is up to 285 m wide southwest–southeast but becomes mostly narrow and patchy elsewhere. The rim is composed of rock, rubble, and sand reef flat that is 1150 m wide to the southeast and extends more narrowly to the east and north. Unlike other atolls in this area, the Allen Atlas codes almost no seagrass here. The inner reef flat southwest–northwest clockwise is flanked by a sand apron that locally reaches more than 700 m wide. The rim is open to the Banda Sea by two shallow channels that are suitable only for small craft (Sailing Directions,

Atolls of Indonesia

2018). There are no islands, and the rim is regarded as tidally submerged. The lagoon covers an area of approximately 22.2 km2. The shallows to the east develop a reef sand apron about 150 m wide that extends from the reef flat with minor breaks. Reefs develop on the slope to the southwest on shallow platforms that rise to the surface from it. Numerous pinnacles flank the western and southern lagoon sides. The lagoon is up to 34 m deep, as indicated on nautical charts. The easternmost atoll of the Tukang Besi group is ovotriangular Karang Koka (Figure 18.11), located 32 km southeast of Karang Koro Maha. The platform is 10.2 km northeast–southwest, 9.5 km parallel to the base at the widest, and covers an area of 62.4 km2. The outer reefs are up to about 400 m wide to the southwest and about 375 m wide at points along the east, but more often the reefs are 200 m wide or less. The platform is primarily a rock and rubble reef flat with little reef development except for three submerged areas located to the northeast, the southeast,

and the south. The northeastern area is at least 5.5 m deep and constitutes a passage to the lagoon (Sailing Directions, 2018). In addition, the western–southern rim develops three lagoon-like enclosures, the largest of which covers up to 22 ha. These areas are mostly coded as reef. The rim is without islands and is submerged. The lagoon covers an area of about 46.9 km2. The northern margin forms a slope with slender platform areas that rise near the surface where they are rimmed with reefs. The northern and western lagoon margins develop a continuous shallow sand deposit up to 750 m wide that does not continue to most of the east. A few pinnacles rise from the lagoon slope or from the deep lagoon that nautical charts suggest reaches depths of up to 50 m. The eastern Banda Sea contains a pair of atolls and a third that is found on its southeastern border (Figure 18.1). Karang Lucipara is the first of the pair and is located about 350 km east–northeast of Karang Koka where it takes an elongated and

Figure 18.12  Remote-sensing images of the atolls of Lucipara and Skaro. Images Ⓒ 2021, Planet Labs PBC.

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curved form (Figure 18.12). The platform is 8.7 km ­northwest–southeast, 3.2 km at its widest, and covers an area of 21.8 km2. The outer reefs surround this atoll but are less than 100 m wide everywhere. The shallow outer reefs continue their growth on the reef flats, especially to the southwest. The rim is a sandy reef flat that supports three vegetated islets, one to the northwest and two to the east. Two lagoons separated by a narrow ridge are found to the center. The northwesternmost lagoon is very shallow with an area of 1.2 km2 and is flanked and subdivided by rocky ridges that are covered by sand. The second lagoon to the southeast covers an area of 1.7 km2 and appears to be only a few meters deep. It is also dominated by sand. Reefs are not coded in either lagoon, but appear to form reticulate patterns in both. The rim is submerged. Karang Skaro is an ovoid atoll (Figure 18.12) located about 9 km southwest of Karang Lucipara. The platform is 4.6 km northwest–southeast, 2.5 km wide through the center, and covers an area of 8.6 km2. The outer reefs occur around the periphery, and like Karang Lucipara, they are narrow, mostly less than 50 m wide. The rim is submerged, without islands, and is dominated by rock, rubble, and sand. Shallow outer reefs continue their growth on the reef flat to the southeast to northwest clockwise, but elsewhere there is little reef growth on the submerged rim. The lagoon covers an area of about 2.0 km2 and,

is subdivided by ridges capped and flanked by reef growth. The lagoon near the center is described as less than 5 m deep (Tomascik et al., 1997). Meatimiarang is an oddly shaped atoll that resembles a short-tailed seahorse (Figure 18.13) located 304 km southeast of Karang Skaro and 123 km east–northeast of Timor (Figure 18.1). The platform is 26.4 km northwest–southeast following a curved path, 8.5 km through the center, and covers an area of 126 km2. The outer reefs surround the platform and are up to 330 m wide to the west and less than 200 m wide to the east. Sailing Directions (2015) indicate that these reefs rise rapidly from deep waters. The shallow outer reefs continue their growth onto the rocky margin of the rim along the east and the concave section of the southwest. The rim interior is composed primarily of rubble and supports two islets to the northwestern tip and 1.2-km2 densely wooded Pulau Meatimiarang to the southeast. Two apparently shallow passes to the lagoon 1.9 km apart are found to the southeast. The rim dries at low tide and is otherwise submerged. The lagoon is convoluted and covers 53.4 km2. Its northern extent is shallow and rocky with sand and rubble platforms that are surrounded by reef patches. This area is followed by shallow, sandy extensions of the reef that subdivide the remaining lagoon into two sections that are surrounded by a sand apron of variable width. Both areas develop a reef slope as well as

Figure 18.13  Remote-sensing images of the atolls of Meatimiarang and Sogari. Images Ⓒ 2021-2022, Planet Labs PBC.

Atolls of Indonesia

reefs that fringe small and shallow islands and several pinnacles. These lagoon areas are further separated from one another by a reef-rich area that is connected to the two passes to the southeast. Several additional atolls lie on ridges on the eastern border of the Flores Sea. However, Sogori Atoll (Figure 18.13) is found near the southeastern mouth of the Bone Gulf west of the island of Rahadopi and is isolated from the other atolls. Sogori rises more than 125 m from a submarine ridge built on a base of fused corals and biologically derived sediment (Solihuddin, 2017). The ovoid platform is 9.7 km north–south, 6.8 km west–east, covering an area of 47.3 km2. The platform develops reefs that are generally 150–170 m wide with sections that project up to 400 m wide; the outer reefs are submerged to the northeast. Corals of the genera Acropora and Porites are common. Most of the rim is crescentic (northeast–southeast anticlockwise) but the entire rim is tidally submerged (Solihuddin, 2017) and is composed largely of sediment with little reef development. The sedimentary stripes to the west are arranged as sand and gravel dune-like undulations where the rim is widest. Here, the rim is about 2.5 km wide and is open to the Bone Gulf and the prevailing swell (Solihuddin, 2017). The emergent parts of the rim are rocky, mostly to the north, where a single, narrow, inhabited island (Pulau Sogori) occurs as a sand cay surrounded by small reefs and seagrass areas. The island area is about 85 ha, and the low areas are inundated by seawater during spring tides. Small reef and seagrass areas surround the island. Additional but smaller emergent rocky areas are found to the west and south. The eastern rim is submerged, mostly as a shallow plateau that deepens to a 2-km-wide passage about 6 m deep to the northeast where the lagoon can be entered. A large area of the reef dries during low tides (Sailing Directions, 2018) and the rim is submerged. The lagoon area is 20.4 km 2 by Solihuddin’s estimate and is 6–20 m deep where the bottom is composed of shell and skeletal fragments. The lagoon slope develops a few pinnacle-like areas either associated with the slope or in the deeper basin.

Atolls of the Flores Sea Six atolls are found along the eastern border of the Flores Sea and are thought to represent the remnants of an old volcanic arc (Hamilton, 1979). Several of these are named ‘taka,’ a term that is similar to ‘karang’ and refers to a reef or a group of coral i­slands in the language of south Sulawesi ­(Tomascik, pers. com.). The northernmost of

the six is Taka Garlarang (Figure 18.14) which rises from a depth of more than 2,000 m on the side of a ridge that runs south from southern Sulawesi ­(Umbgrove, 1950). The platform is 123 km ­southwest of Sogori, where it forms a reverse C (open to the west), is 12.5 km west–east, 12.9 km north–south, and covers an area of 115.5 km 2 . The outer reefs are about 100–180 m wide to the north and widen to 250–300 m wide to the south. The outer reef flat is rocky and provides enough protection for seagrass growth behind it, especially to the east, but around the entire emergent perimeter as well. The inner reef flats are covered with rubble and sand. There are no islands, and the rim is submerged. The open section of the C-shape is about 4.3 km wide and is submerged and coded as reef. A narrower opening of about 500 m occurs in the lagoon to the east and is also shown with reef growth. The lagoon covers an area of 69.6 km2. The shallows are sandy and constitute about 72% of the entire lagoon. The lagoon’s maximum depth is 23 m according to charts. Taka Bone Rate (Figure 18.14) is an atoll that lies only 5 km to the west of Taka Garlarang. The platform is trapezoidal with its narrow end facing northwest and its wider basal end to the southeast where the platform is about 63 km long, 34 km at its widest toward the base, and covers an area of approximately 1,957 km2. This atoll is the largest in Indonesia and third largest in the world. The perimeter is marked by individual reef flat segments with islands and islets rather than a continuous reef flat. Most of the segments are 6 km or less apart, although a gap in the rim of about 17 km occurs to the west. Parts of the individual flats support islets, but many of them are dominated by sand and develop their own shallow lagoons, structures similar to those called faros that more commonly are encountered in the Maldive Islands (Chapter 20). A few of these faro-like perimeter rim segments cover more than 32 km2 although most are smaller. Several with ocean-facing reef perimeters develop seagrass areas in their lee. The lagoon area is roughly 1,584 km2 and contains at least 24 sandy reef flat structures, some of which are up to 59 km2 and contain their own small reef-flanked lagoons. The rims of lagoonal reef flats are often circled with reef material, and some develop patch reefs as well. The southeastern rims are stitched together by submerged reefs that extend 35 km. The lagoon contains deep passes between rim segments that range from 40 to about 70 m where the bottom is filled with extensive patch reef areas. The maximum lagoon depth is 77 m (Tomascik et al., 1997).

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Figure 18.14  Remote-sensing images of the atolls of Taka Garlarang and Taka Bone Rate. Images Ⓒ 2021-2022, Planet Labs PBC.

Taka Bone Rate, which means ‘coral islands over sand,’ is a very large structure. Because of its ­d iscontinuous rim and lagoon islands, it is sometimes referred to as an archipelago (Kepulauan Bone Rate), a large pseudoatoll, or both (e.g., BorelBest et al., 1985; Nienhuis et al., 1989). We are not aware of any criteria that would qualify Taka Bone Rate as a pseudoatoll, one with an atoll-like configuration not caused by subsidence (Agassiz, 1903). Indeed, the presumptive volcanic base (Hamilton, 1979) rising from depths of well over 1,000 m would argue at least tentatively for classification of this structure as an atoll in agreement with Tomascik et al. (1997). This atoll was designated as a national park in 1992 and as a UNESCO Biosphere Reserve in 2015. It is characterized by rich biodiversity, with coral reefs that include up to 261 coral species and 295 species of fish,

mangrove forests with up to 26 species, and 34 species of birds. Recent studies have suggested that its coral ecosystems were heavily degraded, however, at least in part due to blast fishing practices and the use of corals for building materials (Wulandari et al., 2022). Taka Lambaena is an atoll located about 34 km southeast of Taka Bone Rate and resembles a piscine-like polygon (Figure 18.15). It is 35.6 km west–east, 13.6 km at its widest through the center, and encloses a surface area of 272 km 2 . Outer reefs rim the platform and are up to 150 m wide to the north but are up to 300 m wide to the southwest. The rim is continuous southwest–southeast clockwise and is roughly 1,100 m wide but expands to the northwestern fin and east to the head where two islands are supported. The more northerly island covers 1.6 km 2 and the larger one 3 km to its south is 12.4 km 2 . Tomascik et al. (1997) refer to

Atolls of Indonesia

Figure 18.15 Remote-sensing images of the atoll of Taka Lambaena. Images Ⓒ 2021, Planet Labs PBC.

Taka Lambaena as an almost-atoll because the two coral islands to the east are up to 86 m high and the platform is tilted to the west. However, we tentatively retain it as a partially uplifted atoll pending further investigation. The rim is mostly open to the south where submerged areas allow access to the lagoon. In addition, a pass of unknown depth about 350 m wide is found to the southeast. Several areas of the rim, north, west, and south, support seagrass in the protection of a rocky outer reef flat. The remainder of the rim is covered with rubble with little reef development. The atoll is characterized as a drying reef (Sailing Directions, 2018), and because islands account for only about 16% of the rim perimeter, we regard the rim as submerged. The lagoon area is 188.6 km2. The shallows are dotted with patch reefs to the north as well as to the west of the islands on the lagoon slope. A sand apron 2–6 km wide occurs in the lagoon shallows southwest–northeast clockwise and extends to deeper waters west and east. A channel or pass to the southeast exposes the lagoon to flushing by the southeast monsoon and is flanked by reefs. A lagoon area of about 13 km2 occurs to the southeast and is partially enclosed by a rocky rim. The center of this area develops pinnacle-like structures that do not reach the surface and the surrounding rim is flanked by patch reefs. Tomascik et al. (1997) regard this lagoon area (depth unknown) as a separate system (i.e., a double lagoon) although we included it in the total area given above. A low rim to its southwest allows oceanic exchange with this area. The main lagoon slope has been described as extending to a maximum depth of less than 5 m (Tomascik et al. 1997), but the Atlas and optical imagery suggests that it is deeper.

Taka Bassi, also known as Marianne Atoll, rises 400–600 m from the Flores Sea where it is located about 50 km south of Taka Bone Rate. The platform takes the form of a rounded triangle with a south-facing apex measuring 4.2 km north–south, 5.4 km northeast–southwest, and covering an area of approximately 17 km2 including submerged reefs to the south (Figure 18.16). The outer reefs surround the northern periphery of the atoll clockwise southwest– southeast where they are up to 200 m wide. Reefs to the south are submerged and enclose a lagoon. Tomascik et al. (1997) regard this system as a double atoll. The northern section develops a largely shallow sandy lagoon constituting an apron with patch reefs scattered across the surrounding rocky rim. The lagoon covers an area of 3.7 km2 of which 81 ha to the north is coded as a deep lagoon. The lagoon of the submerged reef to the south is about 3.9 km2. Tomascik et al. (1997) give the depth as less than 50 m. This area if flanked by reefs. Kakabia is a small foot-shaped atoll with a small area to its northeast that is attached by a submerged reef. (Figure 18.16) located 62 km northeast of Taka Bassi on a ridge bordering the Flores Sea. The platform is 3.6 km northwest–southeast (toe–heel), 2.0 km at its widest, and covers an area of 4.9 km2. Outer reefs surround the platform and are up to 190 m wide to the southwest. The rim is a rock and rubble reef flat that supports a 42-ha reef-rimmed island to the west with additional reef areas on the flats to its west. Bryan (1953) did not consider Kakabia as a candidate atoll due to the island’s uplift to 38 m, but the rim is primarily submerged. The lagoon is shaped like a witch’s hat, covers an area of 1.2 km2, and is flanked by a reef about 125 m wide covering the lagoon shallows and

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Figure 18.16  Remote-sensing images of the atolls of Taka Bassi, Kakabia, Pasir Layaran, and Gosong Boni. Images Ⓒ 2021-2022, Planet Labs PBC.

slope. Nautical charts show depths in the lagoon of up to 78 m. Pasir Layaran is an unusual trianguloid atoll with a divided lagoon (Figure 18.16) that is located about 93 km south of Kakabia in the eastern Flores Sea. The platform rises from a depth of about 2500 m and it is 6.2 km northwest–southeast, 4.9 km west–east, and covers an area of 18.1 km2. Outer reefs narrowly surround this atoll and are up to 70 m wide. The rim is primarily a reef flat 2–3 m deep (Tomascik et al., 1997) covered with rock, rubble, and pockets of sand. There are no islands. Patch reefs are found to the northwest. A section of the rim up to 680 m wide divides the lagoon into two main sections. The western lagoon covers an area of 5 km2 and is subdivided by

a narrow, spine-like section of reef flat that is flanked by narrow reefs. A submerged reef area (depth unknown) about 225 m wide occurs to the northwest of this lagoon. The maximum depth here is 59 m (Tomascik et al., 1997). The D-shaped lagoon to the east covers an area of about 4 km2 including two partially enclosed pondlike areas to its southeast. In addition, a third lagoon area of 57 ha occurs to the south between the two main lagoons. All these lagoon areas are mapped by the Atlas as reef rimmed. Gosong Boni (Figure 18.16) is an atoll 64 km south–southwest of Pasir Layaran and about 20 km north of the island of Flores that is part of the eastern Nusa Tenggara chain. The reef is C-shaped, less than 2 km west–east as well as north–south and is open

Atolls of Indonesia

Figure 18.17  Remote-sensing images of the atolls of Sadapur, Sapoeka, Paternoster, and Sabalana, including Pulau Sabalana. Images Ⓒ 2021-2022, Planet Labs PBC.

(submerged) to the east. The platform area is 3.5 km2 and supports outer reefs around the platform perimeter that are up to 165 m wide to the northeast. The lagoon covers an area of about 2.6 km2 and develops reefs in the shallows everywhere but the eastern opening to the lagoon. The maximum depth is 84 m (Tomascik et al., 1997). Four atoll-like platforms occur near the western edge of the Flores Sea where they are oriented northeast–southwest and occur as part of a subsiding volcanic arc of Miocene age (Hamilton, 1979). The southernmost of these is Sadapur Atoll, with a polygonal shape that is difficult to categorize due to extensive submersion (Figure 18.17). The platform is about 18 km northwest–southeast, 10.8 km

north–south, and covers an area of roughly 117 km2. The rim is narrow, at most 900 m wide, and supports several small cays to the north and south. The eastern rim is submerged but visible on imagery. Most of the southern rim is submerged and poorly defined except for a section of the rim about 4 km long. The western rim is completely submerged. Roughly 35% of the rim perimeter is on or near the surface. The emergent portion of the rim is composed of a reef, rock, and rubble reef flat, all of which is narrowly rimmed by reef. There is almost no coding for the lagoon which is 100 m deep (Tomascik et al., 1997). Paternoster is a sinusoidally shaped atoll ­located about 21 km southeast of Sadapur (Figure 18.17). The platform is about 103 km long following a curved path,

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17 km at its widest through the middle, and covers an area of 1,103 km2. The rim is composed of 21 reefrimmed islands, islets, and shoal areas separated by submerged reefs that define the general outline of the lagoon. The reef flats on the islands, the largest of which is 6.8 km2, are immediately surrounded by seagrass and reef areas. More than half of the rim, especially to the south, is submerged. Several islands and shoal areas lie lagoonward of the rim although it is unclear if they are part of the rim or within the boundaries of the lagoon. Assuming all islands are part of the rim, the lagoon area is roughly 545 km2. It is uncoded and its maximum depth is 68 m (Tomascik et al., 1997). Sapoeka is an atoll that is located about 25 km northeast of Paternoster and where it is shaped like a fish net (Figure 18.17). The platform is approximately 48 km west–east, 31 km north–south, and covers an area of roughly 1,036 km 2. The platform area is well defined to the north and east but is ill-defined and uncoded to the west, where 15 km of it is submerged, and to the south, where it is submerged about 30 km, a total of about 35% of the rim. Reefs appear around the largest island (3.0 km 2) within the ‘handle’ to the east, a reef flat area extending around a 1.3-km 2 island (Pulau Sapoeka Besar) to the central north, and around the remaining approximately 15 islets and cays that comprise the emergent rim. The larger islands and islets that are rimmed by reefs enclose well-defined seagrass meadows. The lagoon is also uncoded except for a shallow area with a few islets and cays occupying an area of about 76 km 2 south of the central northern rim. These lagoonal shallows are surrounded by extensive reefs and develop seagrass meadows within their protected interiors. While the lagoon area cannot be estimated reliably, the maximum depth is 79 m (Tomascik et al., 1997). The northernmost of this atoll group is Sabalana which is similar to Sapoeka and presents itself as a larger, incomplete fish net with the same orientation about 25 km to its northeast (Figure 18.17). The platform is about 104 km west–east, 60 km northeast– south, and covers an area of roughly 3,275 km2. The boundaries of the eastern half of the rim are suggested by reefs, but the western platform is indistinct as most of the rim (about 56%) is submerged. The ‘handle’ area is the most extensive emergent part of the rim that covers an area of 163.3 km2 and surrounds eight islands and islets (four to the west and four to the northeast) with a total land area of about 12 km2. The largest of the islands, referred to as Pulau Sabalana on some charts, has an area of about 1.3 km2 and is found in the handle center (Figure 18.17). The rim surrounding the island extends to the northeast and is primarily a rock and

rubble flat. Seagrass is prominent close to shore. Pulau Sabalana is flanked by two shallow lagoon areas. The one to the northeast is about 10 km2 where most of it is covered by reef growth and seagrasses close to the island. The second lagoon area southwest of the island similarly encloses a smaller area that also is occupied by reefs and seagrasses. The handle is surrounded by a band of outer reefs that is more than 500 m wide along several areas of the perimeter. The western rim supports three islets with reef growth around them as well as one area that is coded as submerged reef. Most of the rim between these areas is submerged. The lagoon area is mostly uncoded and cannot be reliably measured, but its maximum depth is 70 m (Tomascik et al., 1997).

Atolls of the Makassar Strait Three atolls lie on banks north of Sabalana near the mouth of the Makassar Strait (Figure 18.1). Taka Rewataja (De Bril Bank) is an ovoid atoll (Figure 18.18) that lies about 53 km northeast of Sabalana near Sulawesi’s southern arm. The platform is 6.5 km north–south, 4.5 km west–east, and covers an area of 25.4 km 2 . Outer reefs surround the platform and are more than 300 m wide to the south and narrow approaching the north. The rim is a rock and rubble reef flat with extensive reef areas to the north, south, and much of the west. An extension of the reef flat extends across the lagoon as a rock and reef arm that essentially divides the 7.9-km 2 lagoon into two sandy basins. The westernmost basin develops sand deposits that extend to the west and part of the east. The smaller lagoon area to the east is flanked by a reef sand apron about 50 m wide. The submerged rim dries except for part of the southwest where a channel with depths of 2–4 m provides access to the deeper western basin (Sailing Directions, 2018). We are not aware of data for the depths of this lagoon system. Dewakang Besar lies about 73 km northwest of T. Rewataja and takes a V-shaped form to the north where the rim is emergent and an ovoid shape to the south where much of it is submerged (Figure 18.18). The platform is 23.4 km northeast–southwest, about 10 km west–east, and covers an area of about 179 km2. The northern V is surrounded by a narrow outer reef that is variable in extent but is best developed to the west of the rim where it is up to about 500 m wide. Most of the rim is a reef flat that is composed of rock, rubble, and sand. The nexus of the V supports an island covering 2.2 km2 and a seagrass area of about 3 km2 to its west that is encircled in part by reefs. An additional 84-ha islet occurs ten km southeast of the larger island where it is isolated

Atolls of Indonesia

Figure 18.18 Remote-sensing images of the atolls of Taka Rewataja and Dewakang Besar. Images Ⓒ 2021, Planet Labs PBC.

from the main rim and is flanked to the west and south and by reef and to the east by a sand deposit. The rim to the south of the islet is composed of a submerged sandy area that forms the eastern boundary of the lagoon. The western side is coded as a submerged reef, and the reef to the southwest is too deep to code and is essentially open. The rim is best characterized as submerged. The lagoon area is approximately 102 km2. Reefs occur within the apex of the V-shaped rim, as well as outside its opening on the lagoon slope and deep lagoon surfaces. The maximum lagoon depth is 28 m on nautical charts. Kulukalukuang (Figure 18.19) lies 56 km northwest of Dewakang Besar in an area built up mainly by sediment from the calcareous green alga Halim‑ eda on pre-existing topographic highs. Rising from a depth of about 600–1,000 m, this reef forms an irregular polygon pointed to the north. It displays raised margins that include about 15 shallow reef banks of various sizes and five reef-rimmed islands to the north although most of the rim to the south is depressed and the boundaries of the platform are difficult to measure precisely. The platform is approximately 92 km north–south, 65 km west–east, and covers an area of about 3,100–3,200 km2. The largest island is 35.5 km2 and the total area the five comprise is about 83 km2. The eastern islands are joined by a submerged but visible rim coded as reef. The islands surround a central depression 61 m deep; the structure has been described as a reef bank (Roberts et al., 1987) and as an atoll by Tomascik et al. (1997).

The atolls of Kalimantan Two atolls and a raised atoll are found to the west of the Sulawesi Sea and the northeast coast of Kalimantan (Figure 18.1). The southernmost of these is Muaras located 90 km east of the coast near the edge of the Makassar Strait facing the southwestern Sulu Sea (Figures 18.1 and 18.20). The platform takes the shape of a bent oval in which the southeastern end is submerged. The resulting form of the rim is that of a box jelly. The platform is about 41 km northwest–southeast taking a curved path, 8.7 km to the northwest at its widest, and covers an area of 266 km2. The outer reefs generally are about 250 m wide but extend as reefs and plateau up to 800 m wide to the southeast. The platform is steep sided to and drops to more than 183 m deep near shore (Sailing Directions, 2018). The shallow reefs appear to be formed as narrow bands that are limited to the northwest and southeast, although they continue elsewhere in scattered patches on the reef flats. The rim is dominated by sand, is up to 2.5 km wide to the northwest, and bears a wooded islet and a sand cay to the southeast. A low area between the islands appears as a channel but is not passable except for small craft. Another channel about 100 m wide occurs to the northwest, depth unknown. The rim is essentially submerged and dries at low tide (Sailing Directions, 2018). The lagoon covers an estimated area of 170 km2. A reef sand apron up to 1.8 km wide dominates the margins of the lagoon including shallow and deep areas, although it disappears

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Figure 18.19  Remote-sensing images of Kulukalukuang Atoll. Images Ⓒ 2021, Planet Labs PBC.

to the southeast where the rim is submerged. The deep sections of the lagoon are marked by reefs to the north beginning near the lagoon center and extending along the submerged armlike rim to the southeast. The maximum lagoon depth is 52 m (Tomascik et al., 1997). Maratua Atoll is located 14  km to the northwest of Muaras where it takes the shape of a slipper, pointed to the northwest and square to the southeast ­(Figure 18.20). The platform rises from a depth of 200 to 300 m where it is 30 km northwest–southeast, 9 km at its widest, and covers an area of 178.5 km2. The outer reef is narrow although it forms a band 7.8 km long and up to 250 m wide to the northwest. The leeward western also develops a narrow lagoonal system up to 400 m wide and more than 3 km long inside a raised rim where a rich assemblage of seagrass occurs but is not coded by the Atlas. The reef crest develops a mixture of massive and branching coral species and the reef drops to 30 m as an almost vertical wall ­(Tomascik et al., 1997). Reefs to the east are about 150 m wide and develop a high-energy side with a spur and groove system

10–20 m wide. The reef crest occurs at a depth of 5 m and is dominated in areas by Acropora either alone as monospecific stands or along with other stony coral species. Immediately to the east, the outer reef drops to a depth of 35 m (Tomascik et al., 1997). The rim is dominated by V-shaped 25-km2 Pulau Maratua with a perimeter length of 31.8 km. A village and an airstrip are found to the west. The vegetated island is uplifted to 120 m near the middle of the west side and is one of several karst hills pushed up by an underlying fault system leaving the north and northwest rims 50–80 m above sea level. There are in addition at least nine stratified lakes (marine subsurface, brackish surface) confined by the island, the largest of which averages 10 m deep (Becking et al., 2013). The eastern margin of the atoll is established by a pronounced fault; the downward movement of the southern end of the platform is caused by a subsidiary fault that crosses that atoll, and thus, Maratua is an open atoll (Haryano et al., 2022). Two passes about 150 m wide with delta-like fans are found to the east, one near the end of Pulau

Atolls of Indonesia

Figure 18.20  Remote-sensing images of the atolls of Muaras, Maratua, and Kakaban. Images Ⓒ 2021-2022, Planet Labs PBC.

Maratua, and another 7 km to the south where depths of up to about 22 m can be found (Sailing Directions, 2018). Both the passes are flanked by reef growth. The rim between the two passes displays a pattern of sand and rubble extending toward the lagoon as well as at least six islets; three additional islets are found to the southeast of the rim. The lagoon covers an area of about 75 km2. The bay-like enclosure formed by the island is shallow, about 0.5–5.0 m, and is partly fringed by mangrove patches with prop roots that are exposed at low tide (Becking et al., 2013). Two small islets and rocky areas surrounded by plateaus are found in this part of the lagoon. Nearly the entire lagoon is occupied by sand 4.6 km wide to the north, narrowing to half that to the south. However, reef development occurs to the north, center and south in this lagoon system. The maximum depth is 10 m (Tomascik et al., 1997).

The last of the Kalimantan group is Kakaban, a former atoll that has been raised to 60 m above sea level from 200 to 300 m below sea level. We include it here to complete the record. The platform takes the shape of a distorted trapezoid 7.7 km southwest of Maratua (Figure 18.20) which lies on the same submarine base. The platform is 7.4 km northwest– southeast, 2.7 km north–south, and covers an area of 13.4 km2. The outer reefs at the cliff base appear to be narrow and in part to deep to be coded. The emergent part of the rim is covered with a dense tropical forest. Tomascik et al. (1997) describe spur and grooves on the east side that develop on a gently sloping abraded reef flat 1–4 m deep and 40–100 m wide. The inner reef flat (intertidal to 1 m depth) is covered with a veneer of coralline algae and stubby branching corals. The lagoon covers an area of 4.6 km2 and is surrounded by a 4–8-m-wide fringe of mangroves. During

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the uplift phase, the lagoon lost all connection with the surrounding sea except for fissures, caves, and solution channels, but tides infuse the lagoon with about 2.5% of its volume forming a brackish system whose salinity is about 23‰–24‰, about two-thirds of normal seawater. In addition, the lagoon pH is relatively low (7.0–7.8) for a marine system (Becking et al., 2013). Nonetheless it is dominated by Halimeda, a marine calcareous green alga that in this environment forms meadows up to 1.5 m thick that are visible on imagery as white patches throughout the lagoon, especially to the north. Remarkably, there are several strictly marine invertebrates in the lagoon including shrimp, tunicates, sea cucumbers, sea anemones, sponges, and a small number of fishes. Two species of non-stinging jellyfish also are present including one that is the same as in the well-known Jellyfish Lake, Palau. The lagoon depth maximum is 11 m (Tomascik et al., 1997).

References Agassiz A 1903. The coral reefs of the tropical Pacific. Mem. Mus. Comp. Zool. Harvard Coll. 35: 1–460. Aldrian E, Susanto RD 2003. Identification of three dominant rainfall regions within Indonesia and their relationship to sea surface temperature. Int. J. Climatol. 23: 1435–1452. Becking LE, Cleary DFR, de Voogt NJ 2013. Sponge species composition, abundance, and cover in marine lakes and coastal mangroves in Berau, Indonesia. Mar. Ecol. Progr. Ser. 481: 105–120. Borel-Best M, Moll H, Boekschotten GJ. 1985. Investigations of recent and fossil coral reefs in eastern Indonesia. In: Proceed‑ ings of the Fifth International Coral Reef Congress. Tahiti, 27 May–1 June 1985, pp. 311–316. Brackenridge RE, Nicholson U, Sapiie B et al. 2020. Indonesian throughflow as a preconditioning mechanism for submarine landslides in the Makassar Strait. Geol. Soc. London Spec. Pubs. 500: 195–217. Browne N, Braoun C, Mcllwain J et al. 2019. Borneo coral reefs subject to high sediment loads show evidence of resilience to various environmental stressors. PeerJ 7: e7382. https://doi. org/10.7717/peerj.7382 Bryan EH Jr. 1953. Check list of atolls. Atoll Res. Bull. 19: 1–38. Cesar H 1998. Indonesian reefs: A precious but threatened resource. In: Hatziolos ME, Hooten AJ, Fodor M (eds.) Coral Reefs: Challenges and Opportunities for Sustainable Manage‑ ment. The World Bank, Washington DC, pp. 163–171. Clifton J, Unsworth RKF, Smith DJ 2010. Marine Conservation in the Coral Triangle: The Wakatobi National Park. Nova Science Publishers Inc. Edinger EN, Kolasa J, Risk MJ 2008. Biogeographic variation in coral species diversity on coral reefs in three regions of Indonesia. Diversity Distrib. 6: 113–127. Gordon AL 2005. Oceanography of Indonesian seas and their flowthrough. Oceanography 18: 14–27. Hall R, Spakman W 2003. Australian plate tomography and tectonics. In: Hillis RR, Müller RD (eds.) Evolution and Dy‑ namics of the Australian Plate. Geological Society of Australia Special Publication. Geological Society of Australia, Sydney, vol. 372, pp. 361–381. Hamilton W. 1979. Tectonics of the Indonesian Region. Geolog‑ ical Survey Professional Paper 1078. U.S. Dept. of the Interior, Washington DC.

Haryano E, Putro STJ, Barianto DH et al. 2022. The formation of an open atoll in strike-slip fault setting: New insight from Marutua Island, Kalimantan-Indonesia. Environ. Earth Sci. 81, 350. https://doi.org/10.1007/s12665-022-10469-8 Hinschberger F, Malod J-A, Réhault J-P et al. 2005. Late Cenozoic geodynamic evolution of eastern Indonesia. Tectonophysics 404: 91–118. Hoeksema BW, Putra KS 2000. The coral reef fauna of Bali in the centre of diversity. In: Proceedings 9th International Coral Reef Symposium, vol. 1, pp. 173–178. Luterbacher J, Pfister C 2015. The year without summer. Nat. Geosci. 8: 246–248. Maulana A, Watanabe K, Imai A, Yonezu K 2013. Origin of magnetite- and ilmenite-series granitic rocks in Sulawesi, Indonesia: Magma genesis and regional metallogenic constraint. Procedia Earth Planet. Sci. 6: 50–57. Madden RHC, Wilson MEJ, O’Shea W, O’Shea M 2013. Modern fringing reef carbonates from equatorial SE Asia: An integrated environmental, sediment and satellite characterization study. Mar. Geol. 344: 163–185. Miller MJ, Wouthuyzen S, Sugeha HY et al. 2016. High biodiversity of leptocephali in Tomini Bay Indonesia in the center of the Coral Triangle. Regional Stud. Mar. Sci. 8: 99–113. Nienhuis PH, Coosen J, Kiswara W 1989.Community structure and biomass distribution of seagrasses and macrofauna in the Flores Sea, Indonesia. Netherlands J. Sea Res. 23: 197–214. Nugraha AMS, Hall R 2018. Late Cenozoic palaeogeography of Sulawesi, Indonesia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 490: 191–209. Oppenheimer C 2003. Climatic, environmental and human consequences of the largest known historic eruption: Tambora volcano (Indonesia 1815). Progr. Phys. Geogr. 27: 230–259. Pedoja K, Husson L, Bézos A et al. 2018. On the long-lasting sequences of coral reef terraces from SE Sulawesi (Indonesia): Distribution, formation, and global significance. Q. Sci. Rev. 188: 37–57. https://doi.org/10.1016/j.quascirev.2018.03.033 Rizal AM, Ningsih NS 2022. Description and variation of ocean wave energy in Indonesian seas and adjacent waters. Ocean Eng. 251: 111006. https://doi.org/10.1016/j.oceaneng.2022.111086 Roberts HH, Phipps CV, Effendi L 1987. Halimeda bioherms of the eastern Java Sea. Geology 15: 371–374. Sailing Directions 2015. New Guinea. Pub. 164 Natl. GeospatialIntelligence Agency, Springfield VA. Sailing Directions 2018. Borneo, Jawa, Sulawesi, and Nusa Tenggara. Pub.163 Natl. Geospatial-Intelligence Agency, Springfield VA. Solihuddin T 2017. Atoll reef geomorphology of Sagori Island, SE Sulawesi: A reconnaissance study. Indonesian J. Geosci. 4: 181–191. https://doi.org/10.17014/ijog.4.3.181-19 Sprintall J, Gordon AL, Wijffels SE et al. 2019. Detecting change in the Indonesian seas. Front. Mar. Sci. 6: 257. https://doi. org/10.3389/fmars.2019.00257 Tomascik T, Mah AJ, Nontji A 1997. The Ecology of the Indonesian Seas Vol. 2. Periplus Group, Hong Kong. Umbgrove JHF 1950. Deep furrows on the continents and in the deep sea. In: Symphony of the Earth. Springer, Dordrecht, pp. 91–96. Unsworth RKF, Salinas De León P, Garrard SL et al. 2008. High connectivity of Indo-Pacific seagrass fish assemblages with mangrove and coral reef habitats. Mar. Ecol. Progr. Ser. 353: 213–224. Verstappen H. Th. 2010. Indonesian landforms and plate tectonics. J. Geol. Indonesia 5: 197–207. Wilson MEJ 2008. Reservoir quality of Cenozoic carbonate buildups and coral reef terraces. In: Proceedings, Indonesian Petroleum Association. https://archives.datapages.com/data/ ipa_pdf/079/079001/pdfs/IPA08-G-155.html Wulandari P, Sainal, Cholifatullah F et al. 2022. The health status of the coral reef ecosystem in Taka Bonerate, Kepulauan Selayar Biosphere Reserve, Indonesia. Biodiversitas 23: 721–732. Yusef S, Allen GR 2002. Condition of coral reefs in the Togean and Banggai Islands. In: Allen GR, McKenna SA (eds.) A Rapid As‑ sessment of the Togean and Banggai Islands, Sulawesi, Indonesia. Conservation International, Washington DC, pp. 27–37.

The Atolls of the South China and Sulu Seas The South China Sea (SCS) is located just north of the Coral Triangle but is still a biodiverse region with at least 3,365 species of fishes, a third of them reef fishes (Arai, 2015), and more than 500 species of reef corals (Huang et al., 2015). While coral reefs in so many places have been undergoing serious degradation due to ocean warming, crown-of-thorns starfish invasions, and overfishing, the SCS has suffered from other insults, especially those stemming from multinational territorialism. The SCS lies within the exclusive economic zones of China, Taiwan, Malaysia, Brunei, Indonesia (Kalimantan), Vietnam, and the Philippines. China claims all of it, and not surprisingly, there are multiple territorial and adversarial claims by neighboring countries involving fishing rights and oil and gas exploration. In addition, nations whose territories lie outside of the SCS consider freedom of navigation and strategic control of shipping as issues of critical international concern. In furtherance of China’s claims, several atolls have been fortified by construction of artificial islands converting them to military outposts and have so altered their natural state in some cases that it is questionable whether they are describable as atolls at all. In other cases, other nearby countries have made lesser modifications including runways, buildings, and artificial channels. Island construction involves intense dredging activity in which sand, gravel, and the reefs themselves are crushed and placed on the reef flats, burying everything on them. In addition, silt plumes that drift with waves and currents kill or injure reef architects and residents even at a distance from the dredge site and block sunlight necessary for their survival. As a result, two-thirds of the currently occupied group of atolls (regardless of the country of occupation) support proportionately smaller amounts of shallow reef compared with their unoccupied counterparts. In some cases, the damage is widespread (Asner et al., 2017). Further, the atolls and other areas that have been dredged are accompanied by fishing fleets, often under

military protection, to harvest giant clams and other shallow water species including marine turtles, sharks, large groupers, and wrasses destined for home markets (McManus, 2017). We discuss the nature and extent of modifications in the description of specific atolls below.

Regional climate and oceanography The SCS is the largest marginal sea in Southeast Asia. The basin is semi-enclosed, reaches more than 5,000 m deep, and includes broad, shallow continental shelves to the northwest (the Tonkin Gulf) and the large Sunda Shelf to the southwest that includes the area between mainland Malaysia and Kalimantan (Figure 19.1). Large volumes of freshwater enter the sea, most notably from the Pearl River (Zhujiang) in the north and from the Mekong River in the southwest. The entire area is influenced by the East Asia Monsoon, a seasonal monsoonal wind system. In the northern hemisphere winter, an equatorial pressure trough moves just north of Australia, and consequently, mainland Asia experiences a high-pressure system, establishing winds from the northeast in the SCS. In marked contrast, during the northern hemisphere summer, winds in the SCS are broadly from the southwest, driven by low pressure over mainland Asia and high pressure over Australia. These shifting winds result in seasonally distinct oceanographic patterns of waves and currents. Driven by the winds, waves reverse their direction seasonally, moving from the northeast in the northern hemisphere winter and from the southwest in the northern hemisphere summer. The stronger southwest monsoon winds drive larger waves during winter, resulting in large-scale anti-clockwise current circulation (Figure 19.2a). At this time winds can exceed 50 km/hr and monthly

DOI: 10.1201/9781003287339-19

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Figure 19.1 Map of South China Sea atolls. These include into a northerly group (the Paracel Islands) all of which form on bathymetric highs of the continental slope. Scarborough Atoll is an isolated exception that forms on the crest of a seamount. A southern group (the Spratly Islands) is also shown and is separated from the Paracel Islands by about 600 km. The Spratly Islands form a southerly group, part of which is on the continental slope. The remainder (divided by a line on the map) is composed of atolls that are from atop seamounts. We count a total of 51 atolls in the South China Sea. Four atolls found in the Sulu Sea are also shown, surrounded by the Philippine Islands of Palawan, Panay, and Luzon.

average significant wave height can be greater than 3 m (Figure 19.2c). The South China Sea Warm Current (SCSWC) is generated from this wind pattern. It originates in the SCS from the powerful and warm northward flow of the Kuroshio Current in the western Pacific and displays complex flow patterns as it moves to the northwest. Part of this flow moves into the SCS in the form of a loop current or by eddy formation (Liu et al., 2008) that contributes substantively to the formation of the SCSWC (Figure 19.2a). Winter winds from the northeast drive the Warm Current westward, and as they pile up toward the coasts, the flow is intensified near the shelf breaks of the Tonkin Gulf, Vietnam, and the Sunda Shelf (Nan et al., 2015; Yu et al., 2021). The wind-driven flows cause upwelling in the vicinity of islands within the SCS, dramatically increasing the feeding and catch of yellowfin and skipjack tuna, squid, and mackerel scad. Indeed, the yield

of the SCS fisheries may rival the annual catches from the nearshore regions of Peru and Chile, known as the world’s most productive upwelling ecosystem (Kempf and Chapman, 2016; Kempf et al., 2022). Water from the SCS flows out through the Karimata Strait between Kalimantan and peninsular Malaysia into the Java Sea (not shown) or continuing a circuit through the eastern SCS. Some of the eastward flow exits into the Sulu Sea primarily through the Mindoro Strait where it undergoes a strong counterclockwise flow during the winter (Cai et al., 2009; Qu et al., 2009). During the summer monsoon, the winds reverse course but are weaker compared with those of the winter as they drive waters into the SCS from the southwest (Figure 19.2c). The southern region from the Karimata Strait retains its northward flow up to about 11°N where it separates from the coast forming an eastward-moving Vietnam Offshore Current (Figure 19.2b). As these waters move to the east, cooler and nutrient-rich subsurface

Figure 19.2  The physical oceanography of the South China Sea is complex. (a) The entire area is influenced by the East Asia Monsoon. In the northern hemisphere winter, winds here are from the northeast. During the northern hemisphere summer, winds are broadly from the southwest. These stronger winds are evident in the wind roses from the area and drive large waves that can exceed 3 m. (b) Winter winds cause anti-clockwise current circulation that drives the entry of the powerful Kuroshio Current forming the South China Sea Warm Current westward, piling them up toward the coasts. Currents exit through the Karimata Strait or continue an eastward circuit, some of which exits into the Sulu Sea primarily through the Mindoro Strait where it undergoes a strong counterclockwise flow during the winter. (c) In the summer monsoon, the winds reverse course but are weaker, driving waters to the southwest. The southern region from the Karimata Strait retains its northward flow up to about 11°N where it separates from the coast forming an eastward-moving Vietnam Offshore Current. (d) Tides of the South China Sea are dominantly diurnal to mixed prevailing diurnal tides, as at Vuladdore Atoll, shown here. Tides and tidal currents can be complex, as they are influenced by tides from the adjacent Pacific, Indian, and other basins. In addition, they are funneled through passes and around atolls. (Continued)

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Figure 19.2 (Continued) The physical oceanography of the South China Sea is complex. (a) The entire area is influenced by the East Asia Monsoon. In the northern hemisphere winter, winds here are from the northeast. During the northern hemisphere summer, winds are broadly from the southwest. These stronger winds are evident in the wind roses from the area and drive large waves that can exceed 3 m. (b) Winter winds cause anti-clockwise current circulation that drives the entry of the powerful Kuroshio Current forming the South China Sea Warm Current westward, piling them up toward the coasts. Currents exit through the Karimata Strait or continue an eastward circuit, some of which exits into the Sulu Sea primarily through the Mindoro Strait where it undergoes a strong counterclockwise flow during the winter. (c) In the summer monsoon, the winds reverse course but are weaker, driving waters to the southwest. The southern region from the Karimata Strait retains its northward flow up to about 11°N where it separates from the coast forming an eastward-moving Vietnam Offshore Current. (d) Tides of the South China Sea are dominantly diurnal to mixed prevailing diurnal tides, as at Vuladdore Atoll, shown here. Tides and tidal currents can be complex, as they are influenced by tides from the adjacent Pacific, Indian, and other basins. In addition, they are funneled through passes and around atolls.

waters rise to the surface along the continental slope, creating a biologically rich fishery environment the particulars of which are poorly known. The South China Warm Current also reverses its course and spreads over a wider area, taking a more westward position toward the continental shelves while concomitantly spreading farther to the east (Hu et al., 2000; Fang et al., 2012). Summer circulation is weakly clockwise in the Sulu Sea during the summer monsoon (Cai et al., 2009). The stronger winds from the northeast winter monsoon are evident in the wind roses from the area (Figure 19.2c). Tides of the SCS are dominantly diurnal to mixed prevailing diurnal tides. At Vuladdore Atoll, spring tidal range is just over 1.6 m and neaps are around 50 cm (Figure 19.2d). Tides and tidal currents can be complex, as they are influenced by tides from the

adjacent Pacific, Indian, and other basins. In addition, they are funneled through passes and around atolls. Open-ocean waters of the SCS are generally more than 26°C mean annual sea-surface temperature (SST) with less annual variation than in the nearby Indian and Pacific oceans. Tropical cyclones are common in the area, with an average of ~10/year passing through the SCS, but generally are short-lived, lasting 3–4 days. Internal waves or waves that travel within the ocean (rather than at its surface) occur in this area. They are generated due to interactions among the tides of the SCS, tides of the open Pacific Ocean, and bathymetry. Most initiate in the Luzon Strait between the Philippines and Taiwan and propagate west into the SCS where they first encounter Pratas Reef.

The Atolls of the South China and Sulu Seas

Figure 19.3 Remote-sensing image of Pratas Atolls. This area is influenced by internal waves, or waves that move within the water column at density contrasts. The lower images show such low-frequency waves striking Pratas. Images © 2021, Planet Labs PBC.

We count a total of 51 atolls in the SCS including two drowned atolls. We also include in this chapter description of four atolls in the Sulu Sea (Figure 19.1).

The Paracel Islands The northernmost group of atolls includes the Paracel group (also called the Xisha or the Hoàng Sa Islands), all of which form on bathymetric highs on the continental slope of the northwestern SCS (Wang, 1998). We count nine atolls in this group and describe one drowned atoll. In our descriptions, we use the traditional Western names, and while recognizing that there are other names used in Asia, we use the Chinese names parenthetically as a result of the voluminous

scientific literature in the area that has been published by scientists from the People’s Republic of China. Pratas Reef (Dongsha), the northernmost of the group (Figure 19.3), is surrounded by submerged banks and emerges from a plateau 300–600 m deep on the continental slope about 310 km southeast of Hong Kong (Luan et al., 2019). The atoll is annular with a shallow submerged western rim that gives the reef flat a crescentic appearance (Figure 19.3). The platform is roughly circular, 24.0 km west–east, 21.9  km north–south, and covers an area of about 422  km2. The outer reefs surround the platform and are best developed to the east where they are up to 400 m wide. About 20 coral species are known from Pratas Reef including large Acropora thickets

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and massive Porites colonies. The outer reefs descend gradually to about 25 m and then form precipitous cliffs. The outer reef coral coverage is 34%–74% and appears to have fared well during the 1998 mass coral bleaching event, unlike the shallower reef areas (Dai, 2004; Chou, 2016). The rim is a crescentic reef flat that develops patch reefs on much of its surface. Spur and groove formations can be seen along the northern rim in Google Earth imagery. An isolated reef flat area to the west bears an island about 1.7 km2 in area with its own semi-enclosed reef-filled lagoon. The island also supports an airstrip and a small research station. The southwest rim is open to the lagoon for seawater exchange by tidal cycling with depths of about 6–15 m but rapidly shallows to 2–3 m (US Navy Hydrographic chart 2784). The reef crest is covered by 1–2 m of water, although some areas dry during spring tides. The crescentic inner reef flat deepens to a few meters and is dominated by dense seagrass beds and coral reefs composed of several species that occupy both intertidal and subtidal environments (Chou, 2016). The rim is submerged. The lagoon covers an area of about 258 km2 and contains wide sand deposits in many areas shallower than 8 m, with several areas that dry. The USNH chart shows a few areas that are 16 m deep. The lagoon displays numerous pinnacle reefs through most of its extent. Chou (2016) reports 116 patch reefs of various shapes and sizes. A high density of Porites species characteristically occupies the top surfaces of these reefs. The lagoon basin displays more scattered patch reefs compared with the slope areas, but seagrass beds continue their growth from the inner reef flat mainly to the west, north, and south. Even though internal waves occur within the water column, they are expressed at the surface. In some remote-sensing images (Figure 19.3), the refraction or bending of the wave train is clearly visible. Although the asymmetric trend of more advanced wave fronts to the south could reflect the direction of wave approach, even waves arriving directly from the east show this pattern. This greater speed has been interpreted to reflect the deeper water south of the atoll. The impacts of these waves have been captured in the field. Moored instruments suggest that they have period of about 4 hours and speed of 3 m/second in deep water. They also can be accompanied by decreases in temperature by up to 8°C. The passage of waves and their cold, nutrient-rich waters are commonly followed by microbial decomposition of particulate organic matter and decreased dissolved oxygen, and several days later, by phytoplankton blooms. North Reef (Beijiao) is an atoll 665 km southwest of Pratas Atoll and 207 km from Hainan Island

(China). The platform forms a warped oval 11.8 km west–east, 4.5 km north–south, and covers an area 48.5 km2 (Figure 19.4). Outer reefs surround the platform and are best developed west-northeast clockwise where they are 200–400 m wide and more than 600 m wide to the northwest. Outer reefs to the south and west are 375 m wide or less. The rim is without islands and is an asymmetrical rubbly reef flat about 4–5  km to the east narrowing to about 600 m as it surrounds the lagoon. The western rim is less than 200 m wide. A large band of reef about 1.8 km2 is found on the outer reef flat to the north and a 73-ha area of seagrass occurs with reefs on the wide eastern flats. A spur and groove system is visible to the north and northeast. The rim is submerged. The lagoon covers an area of about 17 km2 including a 15.9-km2 area isolated by reef flats to the east of the main lagoon. The shallow lagoon is unremarkable in terms of coding, but the 11-km2 slope and deep lagoon are rimmed by sand followed by a ribbonlike reef rim that is wider to the southwest. The basin forms ridges and pinnacles plateaus many of which are covered with reefs. Amphitrite Atoll (Xuande) is located about 70 km east–southeast of North Reef where it forms a warped oval (Figure 19.4) about 32 km northwest– southeast, 25 km west–east, and covers an area of roughly 392 km2 that rises from depths of 800 to 1,400 m. Most of the platform southeast–northwest is submerged, and the rim is discontinuous due to a variety of historical and active influences including antecedent topography and sea levels, wind-driven currents, water depth, and tectonic subsidence (Wu et al., 2020). The emergent portions of the rim include a 41-km2 tadpole-shaped reef flat (Qi Lianyu Shoal) that supports eight islets and a 5.7-km2 reef area occurs on the reef flats. Some of the smaller islets have been dredged and filled to construct a single islet. Another island, 3.5 km2 Yongxing, is supported by a 16 km2 comma-shaped reef flat located 9 km to the south that occupies the eastern rim. This area has been heavily modified to accommodate a dense settlement, two semi-enclosed harbors, and an airstrip. Collectively, these small terrestrial areas constitute the largest residential area in the SCS, and human activities such as garbage disposal, farming, and oil contamination from ships have resulted in a high abundance of potentially pathogenic bacteria (Zhang et al., 2020). The coded platform sections north and east are surrounded by outer reefs up to 600 m wide to the northeast where winter monsoonal winds are expected to dominate. Conversely, the reefs to the southeast are about 200 m wide. The southwestern rim supports a submerged bank, Yinli Shoal, the outer margins of

The Atolls of the South China and Sulu Seas

Figure 19.4 Remote-sensing images of the atolls of North, Amphitrite, and Crescent. Note that the westernmost Money Island is not included as part of the rim structure of Crescent. Inset: The Sansha blue hole on Crescent Atoll. © 2019, 20212022, Planet Labs PBC.

which are outlined by reefs a shown in the Atlas at depths close to 20 m. The rim is submerged. Depths of 40–60 m are found in the lagoon with large (uncoded) patch reefs to the southeast and center that rise about 20 m toward the surface (Wu et al., 2020). Tidal currents enter the lagoon to the west and northeast of Qi Linyu Shoal (Qijiang et al., 2021). Crescent Atoll (Yongle) is located 67 km southwest of Amphitrite and presents a large surface area of reef flats in the form of a rounded triangle with an apex to the southwest (Figure 19.4). The platform is 22.3 km southwest–northeast, 14.1 km parallel to

the base, and covers an area of about 278 km2. The platform is bounded to the northeast by outer reefs up to about 700 m wide. The rim is extensive and includes seven individual island-like reef flats connected by shallow ridges. A spur and groove system is visible to the northeast. Four of the reef flat areas support islets. Most islets are inhabited and some of them are 3–6 m above sea level. Duncan Islet (Chenhang) to the south has been modified to form a harbor and a military base (Sailing Directions, 2020). The largest reef flat area is a boomerang-shaped region to the east that includes 19-ha Drummond Islet

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Figure 19.5 Remote-sensing images of the atolls of Vuladdore and Discovery. © 2021-2022, Planet Labs PBC.

(Jinqing) and Sansha, a 301-m-deep blue hole (Figure 19.4), the deepest in the world, near the center (Li et al., 2018a). The southwesternmost reef flat area, Antelope Reef (Linyang), is additionally noteworthy. The reverse D-shaped 173-km2 reef flat here is bordered to the west by outer reefs about 250 m wide, an asymmetric sand apron wider to the west, and contains its own 7.5-km2 central lagoon. About 70% of the rim perimeter is at or above sea level, and we regard this atoll as open. Note that the westernmost island (Money Island) is isolated from and generally is not considered part of the rim structure. The Crescent lagoon proper covers an area of about 200 km2 and most of it (about 65%) is deep and uncoded except for the reefs associated with the emergent parts of the platform. The shallows and slope of the northeastern lagoon are relatively shallow and contain numerous patch and ribbon reefs. The maximum lagoon depth is 49 m (Wang, 1998). Coral diversity is highest on the outer reef slopes where most of the 144 species are branching forms, including 33 species of Acropora, although branching species of Pocillopora covered most of the reef slope area. The coral diversity of the lagoon is relatively low. Interestingly, the diversity of the small lagoon within Antelope (Linyang) Reef followed the same pattern where the slope diversity was relatively high.

Indeed, 83% of the coral species found around the entire atoll were found in this small lagoon system (Zhao et al., 2016; 2017). Vuladdore Atoll (Yuzhou) is located 27 km southeast of Crescent where it takes the shape of a bent oval with a pointed western end (Figure 19.5). The platform is 15.6 km west–east taking a curved path, 4.2 km at its widest north–south, and covers an area of 523 km2. The outer reefs surround the platform and are best developed to the north and northeast where they are up to 450 m wide including deep uncoded areas in parts of the north and east, and in a smaller area to the south. Outer reefs taper toward the west where they are about 200 m wide. Spur and groove systems form to the north and east, and reefs continue their development on the reef flat to the east but generally appear to be absent elsewhere. The rim to the north is a reef flat about 550 m wide, narrowing as it approaches the west where it is open. The rim to the east is about 3 km where it is widest. The southern rim is low to the water and is about 200 m wide. There are no islands, and the rim is submerged (Figure 19.5). The lagoon area is 27.3 km2 and much of it is dominated by a sand apron, especially to the east and to the west where it extends up to 2.9 km wide. The apron is diminished to the north where it is roughly 500 m wide before passing into the deep lagoon. The

The Atolls of the South China and Sulu Seas

Figure 19.6 Remote-sensing images of the atolls of Passu Keah, Bombay, and Scarborough. © 2021-2022, Planet Labs PBC.

deep lagoon and slope occupy an area of 8.3 km2 where reef-rimmed sandy ridges and pinnacles rise near the surface. The maximum lagoon depth is 16.8 m (Wang 1998). Discovery Atoll (Huaguang) is 17 km southwest of Vuladdore where it takes the form of a bent oval (Figure 19.5). The platform is 28.7 km east–southwest taking a curved path, 8.3 km at its widest to the east, and covers an area of 195.7 km2. An additional uncoded area of roughly 11–16 km2 occurs to the northwest. The outer reefs are about 270 m wide to the east–northeast but are mostly uncoded and are presumed to be deeper than about 15 m. The reefs extend around the atoll periphery with little overall change in extent. The shallow reefs continue their growth on the reef flats to the north where they are associated with well-developed spurs and grooves north–east. The rubbly rim is up to 1.5 km wide to the east and narrows to about 400 m wide elsewhere. The rim is perforated by two shallow channels to the south that are partially blocked by reef growth. In addition, the northwestern rim presents low areas to

the northwest and north that support seagrass and reef communities that enter the lagoon. The rim is without islands and is submerged. The lagoon covers an area of 146.9 km2. In the shallows, a reef sand apron that is narrower to the north and south reaches up to 2.5 km wide to the west and 1 km wide to the east. The lagoon slope is marked by reef growth, much of it in the form of ribbons although wider reef areas occur to the north and east. The lagoon develops a few marginal pinnacle-like structures, especially to the south, that are too deep to code and reaches a maximum depth of 30 m (Wang 1998). Passu Keah (Panshiyu) is an ovotriangular atoll located 18.5 km southeast of Discovery Reef (Figure 19.6). The platform is 8.5 km west–northeast, 4.4 km at its widest south–northwest, and covers an area of 30.3 km2. The outer reefs are up to 275 m wide to the northeast Spur and groove formations are visible north–southeast. The rocky outer reef flats and rubbly inner reef flats are up to 1.7 km wide to the east and support an 11-ha sand cay to the northwest.

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The reef crest is strongly developed to the southeast, whereas the reef flat is narrow, about 400 m wide. The reef flat and sand apron to the east is about 2 km wide, up to 1.3 km wide to the west and becomes narrower and patchy to the north. The reef flat is shallow, semi-closed, with little coded reef material. The lagoon area covers 13.5 km2. The shallows to the east develop a sand deposit that extends around the deep lagoon as an apron. The deep lagoon occupies an area of 6.9 km2. The basin contains sandy pinnacle-like structures some of which rise to or near the surface as shallow rock or rubble lagoon features. Many of these pinnacles are reef-rimmed, but the dominant feature is sand that completely blankets the lagoon, the deepest part of which is 11 m (Wang, 1998). However, the deep basin is not coded by the Atlas. Bombay Reef (Langhua) is located 65.8 km east of Passu Keah where it takes the form of an elongated oval that narrows to a point facing the northeast (Figure 19.6). The platform is 18.0 km northeast–southwest, 5.2 km north–south at is widest, and covers an area of 78.9 km2. The deep outer reefs are best developed to the northeastern tip where they extend more than 500 m and surround the platform. A well-developed spur and groove system is visible north–southeast. The rim is about 1.9 km to the northeast where it is best developed, about 300 m wide to the south and west, and is variable but generally narrower to the north. The western rim is partly penetrated by a channel about 15 m wide, but the rim is low, without islands, surrounds the lagoon, and is semi-closed. The lagoon area covers 49.8 km2. The shallow lagoon contains a sand apron to the north and supports several patch reefs to the east as well as two small seagrass areas. The deeper lagoon occupies 28.8 km2 and contains numerous reef-rimmed ridges, plateaus, and pinnacles, especially in the western half. Sand is mapped as prominent in the deep lagoon. The deep lagoon basin contains many sandy and reef-rimmed pinnacle-like structures within its boundaries not all of which are coded. The maximum lagoon depth is 14.8 m (Wang, 1998). Scarborough Reef (Huangyan) is an isolated atoll that, unlike the others described thus far in the SCS, is an oceanic feature that rises from the sea floor at depths of 3,000–4,000 m along with a series of seamounts, rather than originating on the continental shelf or slope (Zhao et al., 2016). Located 558 km southeast of Bombay Reef, it takes the shape of a rounded trapezoid or a flattened delta. The platform is 17.5 km parallel to the base, 9.8 km northeast–southwest, and covers an area of 130 km2. The atoll perimeter is surrounded by outer reefs up to about 350 m wide to the southwest, narrowing to about 170 m wide along the seaward northeastern face where corals are said to be abundant (Li et al., 2018b). Shallow reefs continue their growth on the reef

flats on all sides, less so to the north where the flats are up to 2 km wide compared with half that width or less elsewhere. No islands occur, but a channel about 320 m wide penetrates the rim to the southeast. Although the entrance is more than 9 m deep, reef patches cause the passage to become hazardously shallow as indicated by the Atlas (Sailing Directions, 2020). Even though the rim is in an environment where the tidal range is only about 1.2 m, the rim is submerged at high tide (Li et al., 2018b). The lagoon area is 99.6 km2 and surrounding waters are a rich fisheries ground that has been exploited by China and the Philippines for decades. Its shallows are surrounded by a sand apron, wider to the northeast, and contain numerous, small patch reefs, whereas the slope forms a series of plateaus that develop extensive reefs. The lagoon develops patch reefs and ridges that extend across the lagoon. The maximum lagoon depth is 19.5 m (Li et al., 2018b). Macclesfield Bank (Zhongsha) lies 136 km east of Bombay Reef and 310 km northwest of Scarborough. Its longest axis is more than 150 km and it is more than 75 km northwest–southeast. The platform area is about 23,500 km2 making it one of the largest documented drowned atolls in the world. The block upon which it has developed slopes steeply to about 2,500 m deep to the west and to 4,000 m to the east. The north, east, and south display numerous canyons, gullies, and slumps. The surface consists of a ring of shoals and reefs arranged along the outer rim that become shallowest to the northeast where depths of 10–20 m are found (Huang et al., 2020). Most of these areas are not coded by Atlas. Reed Bank (Lilu) is located about 600 km to the south of Macclesfield Bank in the Dangerous Ground region of the SCS (see below). Reed is a carbonate platform covering an area of about 8,900 m2 that formed on an ancient continental block and was likely attached to Macclesfield Bank before separating from it by seafloor spreading about 30 million years ago. Reed Bank is now submerged 9–45 m below sea level (NGA chart 91005) and displays a peripherally rugged topography, especially to the south and east where several submerged atoll-like reefs are outlined. However, Reed Bank is often referred to as a tablemount due to its relatively flat uppermost surface (Ding et al., 2013). Neither Reed nor Macclesfield banks are included as atolls in this chapter.

The Spratly Islands The Spratly group is found in the southern SCS where they lie in the path of major shipping lanes enroute to Southeast Asia between Malaysia and Palawan in the southern Philippines (Figure 19.2). Low islands and reefs just below or barely above sea level characterize

The Atolls of the South China and Sulu Seas

the area, which is appropriately referred to on nautical charts as Dangerous Grounds. Named after British whaling captain William Spratly in 1843, the Spratlys are located about 600 km to the south of the Paracel group where they are bordered by several nations which give them different names. For example, China refers to them as the Nansha Islands, Vietnam calls them the Trươ ǹ g Sa, and the Philippines refers to them as the Kalayaan. The Spratlys consist of islands, banks, table reefs, and atolls with a complex geology, some of which formed on submarine ridges rising 2,000–3,000 m to the surface. Scattered volcanoes occur within this area that are relatively young, and in addition, there are numerous older seamounts south and west of Reed Bank

that rise from waters of 3,000 m or more (Yan et al., 2006). Descriptions of individual atolls by the National University of Singapore (refs. below) suggest that the Spratly group of atolls can be divided into two groups as shown by the green dividing line in Figure 19.2. Of the 42 described below, nine are on the continental slope, whereas the remainder are oceanic.

Continental slope atolls of the Spratly Islands The northernmost of the continental slope atolls is North Danger Reef (Shuangzi), shaped like an irregular pointed oval (Figure 19.7). The platform

Figure 19.7 Remote-sensing images of the atolls of North Danger, West Thitu, and East Thitu. © 2021, Planet Labs PBC.

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measures 15.5 km northeast–southwest, 7.6 km at its widest through the middle, and occupies an area of 74.6 km2. The rim is composed of four emergent reef areas that characterize the rock and rubble rim northeast–southwest and combine to form a total surface area of about 8 km2. All of the emergent areas are rimmed by a reef crest and are joined by a submerged reef almost 500 m wide that continues around the atoll perimeter and into the lagoon. Two of these areas, Northwest Cay and Southwest Cay support islets separated by 3 km. The latter cay is densely populated and is the site of an artificial west-facing harbor. The reef flats to the northwest dry at low tide (Sailing Directions, 2020). The entire southeastern platform (15.8 km long) is submerged and when added to submerged areas to the northeast constitutes about 70% of the atoll perimeter. The lagoon is approximately 57 km2 and charts show that it deepens to 43 m to the southeast (NGA 93044). The rim is coded as reef, but it is not separable from the submerged outer reefs. East and West Thitu (Zhongye and Zhongya, respectively) are paired atolls 1.7 km apart that are located 34 km south of North Danger (Figure 19.7). The platform of West Thitu is polygonally shaped with an area of 45.6 km2 and is composed of six drying reefs and shallow patches to the north side. A 30-ha sand cay to the east is occupied by an airstrip that protrudes from it to the east and west. A second reef flat area occurs about 2 km to the northwest and supports a 1-ha sand cay. Both reef flats are joined by a spur and groove system that extends along the length of the northern platform joining the remaining reef flat areas, all of which are bordered by outer reefs that continue their growth into the lagoon. The westernmost reef flat area is referred to as South Reef, a U-shaped area forming a harbor that opens to the lagoon. The entire northern rim develops a spur and groove system. The southern rim is occupied by an arm of South Reef and a shallow area of reef and plateaus, but most of it is submerged with a deeper area near the center that comprises a pass to the lagoon up to 9.3 m deep. The entire platform perimeter is 34.4 km, of which 38% is essentially submerged. The lagoon slope is coded as reef but is inseparable from the coding of the submerged outer reefs. The deep lagoon is largely uncoded and is up to 34 m deep (Wang, 1998). East Thitu forms a rounded scalene triangle with an emergent area to the southwest about 2 km wide that is composed of three closely spaced reef flats that dry at low water (Sailing Directions, 2020). Another reef flat area of about 2.5 km 2 is found to the northeast. The remainder of the platform is submerged, 9.2 km 2 southwest–northeast, 3.9 km

south–north at its widest through the middle and covers an area of 20.7 km 2 where uncoded deep reefs occur. About 41% of the rim is submerged. The platform around the two emergent ends of the atoll is surrounded by outer reefs up to 285 m wide to the northeast, narrowing to about 100 m wide to the southwest. The shallow northeastern reef is marked by a well-developed reef crest followed by a broad reef flat. In addition, the entire submerged area to the north and almost all of it to the south are marked by spur and groove systems. Most of the lagoon is uncoded with a maximum depth of 27 m (Wang, 1998). Subi Atoll (Zhubi) is located 12.6 km southwest of West Thitu where it forms an oval with an extended western side (Figure 19.8). The platform is about 6.1 km northeast–southwest, 3.8 km at its widest, and covers an area of about 16.5 km2 including contiguous uncoded reef areas northwest–northeast. The outer reefs on the platform periphery extend more than 300 m wide to the northeast where a reef crest is found as well. The current condition of this atoll has been dramatically changed as China constructed a densely populated military base with a 3,100-m-long airstrip on its western rim. The rim has been enlarged by 74% with intense dredge-and-fill operations that at its peak moved a mean of 32,000 m3/day (Lee, 2015). The atoll, once essentially awash at low tide, has been transformed into a C-shaped terrestrial complex. The eastern rim is the only part of this atoll that has not been sealed in concrete, but the rim to the east, formerly enclosed by reef flat, has been cut and opened to more than 330 m allowing large vessels, including fleets of fishing vessels from China, to enter the lagoon. The lagoon itself covered an area of about 10.4 km2 in 2012 and displayed an apron that no longer exists. The lagoon currently occupies about 7.7 km2, a reduction of roughly 26%. Wang (1998) listed the maximum lagoon depth as 22 m, but it is now undoubtedly deeper. Loaita Atoll (Daoming) is located 34 km southeast of Subi where it takes the form of a ladle 74 km from the southwestern tip of the bowl to the end of the northeast-facing handle (Figure 19.8). The platform is about 13 km at its widest southeast–northwest and covers an area of about 546 km2. The platform outline is reasonably clear on imagery, even though only seven small reef flat areas mark the surface. However, the rim is clearly outlined as submerged reef in the Atlas. The largest of the emergent areas are 2.8 km2 to the southwest, around which spurs and grooves develop and a 2.0-km2 reef flat area, Menzies Reef, that forms to the northeast. Loaita Islet, the only vegetated and emergent part of this reef, occupies an

The Atolls of the South China and Sulu Seas

Figure 19.8 Remote-sensing images of the atolls of Subi, both in current state and prior to construction (inset), Loaita, and Tizard. © 2020-2021, Planet Labs PBC.

area of about 7 ha and is found on a reef flat area to the southeast. Small sand islets are present elsewhere on the rim, but approximately 92% of it is submerged and the shallow areas are awash at low tide (Sailing Directions 2020). The uncoded lagoon is divided into two by a submerged reef neck. The southeastern lagoon is commonly about 55 m deep, whereas the southwestern lagoon is commonly 62 m. The maximum depth is 79 m according to British Admiralty chart 1201. Tizard Bank (Zhenghe) is a large, mostly submerged atoll lying about 30 km south of Loaita. Its outline resembles the lateral view of a snail with a north-facing shell, an eyestalk to the northeast, and its head to the east (Figure 19.8). Rising from depths of

500–700 m, the platform is 57.6 km northeast–southwest, 18.2 km at its widest near the center, and covers an area of about 717 km2. The perimeter is about 166 km of which about 61% is essentially submerged . The emergent rim is composed of eight reef flat areas some of which have been given names on charts including Eldad Reef (the snail’s head), Petley Reef (the eyestalk), Sand Cay (east end of the shell), Itu Aba (west end of the shell), Gaven Reef to the west, and Namyit Island to the southwest. The outer reefs surrounding Eldad and Petley are about 625 m wide. A few reef flats support islets and small seagrass areas. Eldad Reef is surrounded by a well-developed spur and groove system as are two small islets to the north. The islands of Tizard Bank essentially constitute

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Figure 19.9  Remote-­sensing images of the atolls of Discovery Great, Fiery Cross, and Union. © 2020-2021, Planet Labs PBC.

a piecemeal multinational condominium. Itu Aba bears an airstrip built by the Taiwanese, whereas Sand Cay and Namyit have been heavily modified with buildings constructed by Vietnam. Most notably, Gaven Reef to the northeast has been expanded by dredge-and-fill operations that supports a Chinese military complex. This island has suffered significant reef damage (Asner et al., 2017). A solar panel array and administrative buildings, gun emplacements, radar facilities, and six wind turbines are among the improvements now found on this reef flat (AMTI, 2018). The lagoon area is approximately 575 km2 and is essentially uncoded although charts show at least six circular shallow areas that are 8–10 m deep in a raised area to the lagoon center. The maximum depth is 81 m (NGA chart 93044). Discovery Great Reef (Daxian) is a cylindrically shaped atoll located about 39 km southwest

of Tizard Bank (Figure 19.9). The platform is 15.3 km north–south, 2.1 km west–east at its widest, and covers an area of 25.1 km2. The platform develops outer reefs primarily to the southern half of the west, where they are commonly 250 m wide and in some areas are up to more than 400 m wide. Reefs continue their growth to the west on the reef flats along with seagrass patches. By contrast, reefs to the east are patchy and are mostly submerged, although a spur and groove system is visible on most of the shallow eastern platform. The reef flat is most extensive to the north where it occupies about 5.5 km2 then bifurcates around the lagoon to the south. The western flats are rocky, whereas the eastern flats are composed of rubble. The lagoon covers an area of 5.4 km2 and is primarily shallow with three small deeper areas. Little reef development is coded, and the dominant constituent is sand that is widely

The Atolls of the South China and Sulu Seas

distributed within the lagoon and on the shallows to the west. The middle section of the lagoon rim has been pierced by an artificial 60-m-wide channel that leads to three buildings and a small harbor-like structure connected by a wharf 280 m inside the rim. A shallow channel is also found across the northern reef flat that communicates with two islets near the center that are joined by a wharf-like structure. The construction was initiated by the Vietnamese during and after 2014 (AMTI, 2017). Artificial channels notwithstanding, we tentatively regard the rim as semi-closed. Fiery Cross (Yongshu) is located 93 km southwest of Discovery Great where it forms an elongated oval 26 km long oriented northeast–southwest. The platform is 7 km northwest–southeast and covers an area of 112.8 km2. The southwestern end, representing about 7% of its area, has been totally dredged and filled to form a military platform including a 3,000-m-long runway, a 100-m-wide channel through the rim to the southeast, and a 1.5-km-long entrance to a 500-km2 artificial harbor. Imagery also indicates that the base is equipped with jet fighters and bombers (AMTI, 2022a). The remainder of the rim is composed of four shallow, sandy areas and submerged reefs to the northwest. The southeastern rim is a shallowly submerged reef. Outer reefs still appear around the periphery of the platform where they are more than a kilometer wide. A spur and groove system is found along the entire northwestern platform even though construction buried the northwestern part of them in sediment in 2014. There are no reports on the post-construction condition of these reefs to our knowledge. Most of the southeastern rim is submerged. The lagoon covers roughly 78 km2 and while the lagoon slope includes a few pinnacle-like structures, most of it is uncoded with a maximum depth of 39 m (Wang, 1998). Union Bank (Quyuantan) is located about 133 km east of Fiery Cross and takes a form similar to Tizard Bank 30 km to the north except for the distinctive V-shaped shoal to the northeast (Figure 19.9). The platform is 55.7 km northeast–southwest, 14.1 km at its widest, and covers an area of 490 km 2. About 20 reef flat areas no more than 10 km apart constitute about 58% of the rim. All of them are surrounded by reefs as are about another 12 small areas that are shallowly submerged. Several of the reef flats enclose their own small lagoons that include small seagrass meadows. Spur and groove systems are visible adjacent to islands to the northwest. Notable rim areas include Whitsun Reef, the V-shaped northeastern end, and the southwesternmost reef flat, Johnson Reef. Several islets, small islands, and Johnson Reef bear dredge scars, and the

rim has been modified to accommodate construction. Hughes Reef is the fourth reef flat area south of Whitsun Reef. An area of about 7.6 ha has been dredged and supports a lighthouse and a two-story defense outpost built by China. Further construction is visible at Sin Cowe Island to the west. Nonetheless, we mark this atoll as submerged. The lagoon covers an area of approximately 372 km 2 and it is mostly uncoded, although it contains about eight submerged reefs and sand areas to the northwest. The maximum lagoon depth is 51 m (Wang, 1998).

Oceanic atolls of the Spratly Islands This group of atolls has been suggested as a distinct biogeographic and ecological subset of reefs in the SCS. Rising above the abyssal plains, they not only form an environment for unique coral species but they also provide a network of vertical and horizontal connectivity between shallow oceanic environments and those of the continental shelf (Huang et al., 2015). We count 41 such reefs in this group including one drowned atoll, all of which drop precipitously into deep water. Irving Reef (Cailun) is the northernmost of the oceanic group, located about 10 km northeast of Whitsun Reef (Union Bank) where it takes the form of an elongated, bent triangle terminating with a 3.3-km-long tail extending to the southwest (Figure 19.10). A few hectares of the tail are uncoded but most of it is only about 13 m deep (Sailing Directions (2020), and terminates with a 52-ha oval area composed of a reef flat 3–4 m deep surrounded by outer reefs. The entire platform is 9.9 km southwest–northeast taking a curved path, 1.8 km at its widest, and covers an area of 7.2 km2 Outer reefs surround the atoll and are wide and are more than 750 m wide to the northeast including uncoded areas deeper than 15 m. The platform is surrounded by a spur and groove system including the submerged tail. A few patch reefs continue their growth onto the rocky reef flats, along the eastern rim. The reef flats are most extensive to the north where they occupy an area of about 90 ha along the northwestern rim and generally dry in patches at low tide (Sailing Directions, 2020). The lagoon forms a warped triangle that covers an area of 1.7 km2. The shallows contain a reef sand apron more than 300 m wide that surrounds a lagoon slope area of 28 ha. The lagoon is 5–6 m deep and is characterized by a dense reticulate reef system that connects numerous shallow coral heads and patch reefs that are about 20 m wide (National University of Singapore, 2017). However, the Atlas codes no reef development in the lagoon.

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Figure 19.10 Remote-sensing images of the atolls of Irving, Jackson, and Seahorse. © 2021, Planet Labs PBC.

Jackson Reef (Jiesheng) is a box-shaped atoll (Figure 19.10) located 95 km southeast of Irving and about 8 km southwest of Reed Tablemount reefs. The platform is 11.8 km northeast–southwest, 11.2 km northwest–southeast, and covers an area of 82.6 km2. The platform is composed of reef flat areas that comprise most of the western rim, as well as the corners to the northeast and southeast. Each of these flat areas is surrounded by outer reefs that are commonly about 200–270 m wide. About 54% of the rim perimeter is submerged. Patch reefs occur on the reef flats, most of which also support small seagrass communities. The lagoon covers an area of approximately 66 km2. The slope has a few pinnacles associated

with it but most of the lagoon is uncoded and the maximum depth is 43 m (Wang, 1998). Moving east–northeast 205 km from Jackson and passing by two V-shaped reefs that are part of Reed Tablemount, isolated Seahorse Shoal (Haima) is encountered. This is a pear-shaped, drowned oceanic atoll (Figure 19.10) measuring 15 km northeast– southwest, 8.2 km at its widest, covering an area of about 92 km2. The rim is clearly visible in the imagery at a depth of about 8 m and is coded by the Atlas, but with few other details. The lagoon depth is 31 m (Sailing Directions, 2020). Livock Reef (Sanjiao) and Hopps Reef (Hejiao) are two small triangle-like atolls located 70–80

The Atolls of the South China and Sulu Seas

Figure 19.11  Remote-­sensing ­images of the atolls of Livock, Hopps, and Mischief. © 2021, Planet Labs PBC.

km from Union Bank where they occupy the same ridge system and are collectively referred to as the Southampton Reefs (Figure 19.11). Livock forms a warped triangle 4.8 km northwest–southeast, 5.8 km parallel to the base, and covers an area of 12.7 km2. Outer reefs about 300–320 m wide are best developed to the northeast and the southwest apices. Elsewhere on the platform edge, these reefs are roughly 150–200 m wide. The rim is a shallow reef flat about 300 m wide on most sides, except for the northwest where it extends toward the lagoon and is about 1.5 km wide. A small area of spurs and grooves is visible to the northeast. There are no islands or openings to the lagoon, and we regard the rim as

semi-closed. Shallow reefs continue their growth onto the reef flat more frequently to the west, especially to the northwest where they cover an area of about 17 ha. The lagoon covers an area of 6.1 km 2. The shallow margins and part of the lagoon slope form a sand apron more than 500 m wide to the southwest, narrowing to about 125 m to the east. By contrast, the eastern lagoon develops a slope about 275 m wide. An uncoded deep lagoon occupies about 2.4 km2 and reaches an unknown depth. Hopps Reef is located 5.5 km northeast of ­Livock where it forms a rounded triangle 2.5 km northeast– southwest, 1.4 km northwest–southeast, and covers an area of about 2.9 km2. The platform is surrounded

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by outer reefs that are best developed to the southwest where they are up to about 300 m wide and taper somewhat to the northeast and southeast from there. A few scattered patch reefs occur on the rock and rubble reef flat that, like Livock, covers most of the shallow interior of this atoll and is without islands or passages to the lagoon. We regard the rim as semiclosed. The lagoon is very shallow, covers an area of about 17 ha, and is covered by sand. Mischief Reef (Meiji) is an annular atoll located 35 km southeast of Livock and 250 km west of Palawan, the Philippines. The platform as currently configured is 9.3 km east, 6.5 km north–south, and covers an area of about 50 km2. Prior to construction in 2015, Mischief’s rim was 400–600 m wide to the north and 100–200 m wide to the south and was above water at low tide by up to 0.5 m in the north and 0.2 m to the south. The rim area was 8.5 km2. Three passages penetrated the rim. The lagoon was 36.1 km2 and depths were generally 20–25 m with a maximum depth of 28 m. Most of the lagoon (83%) presented depths of more than 10 m where there were 48 patch reefs (preconstruction data from Liu et al., 2022). There were no islands on the rim. Now, the northern half of the atoll has been dredged and filled with concrete military structures including a 3,000-m-long airstrip, five hangars including one for 16 combat aircraft, underground storage, and multiple radar domes to the west (AMTI, 2018). The 8.5-km2 reef flat has been reduced to about 3 km2 (Figure 19.10). The lagoon area has been reduced by about 5%. The channel closest to the airstrip has been deepened to more than 18 m and widened from about 125 m in 2013 to 300 m. The southern channel, about 115 m wide in 2013, is now about 260 m wide. The average lagoon depth is now about 26 m (Sailing Directions, 2020). Although the Atlas shows reefs at the periphery of the construction, we are not aware of reports on their current condition. The damage resulting from this atoll modification has been significant (Asner et al., 2017). Second Thomas Reef (Renai) is located 32 km southeast of Mischief where it takes the shape of a bow saw (Figure 19.12). The platform is 17.2 km northeast–south, 5.4 km west–east at its widest, and covers an area of 52.5 km2 not including the reefs unconnected to the platform. Reefs occur to the west of the platform where they are about 225 m wide and to the east where they are about half that width. We note that at press time this reef lacked a satellite image in the Atlas as do a group of six others noted below, although all of them are coded. Spur and groove systems occur to the northwest for about 400 m, then become widely spaced west of a wrecked

freighter on the northwestern rim. The rim is up to 1.3 km wide to the central west and presents five shallow passages to the lagoon, only one of which is unencumbered by reef growth. Several areas of the rim dry at low tide (Sailing Directions, 2020). Much of the eastern rim is narrow, typically 200 m wide or less, and is shallowly submerged except for several small reef flat areas. Patch reefs are scattered along the western rim and continue in the northwest to the lagoonal shallows. The rim is submerged. The lagoon area covers about 37 km 2. The shallows to the west and north are draped with a sediment apron typically 225 m wide but expanding to 1.5 km wide to the closed southern end. The eastern lagoon by contrast accumulates shallow sediment only in patches due to its discontinuous reef. Many pinnacle-like structures and plateaus occur in the lagoon, along with many rimmed with patch reefs, especially in the wider northern half. The maximum depth is 27 m (Wang, 1998). Boxall Reef (Niuchelun) is a small, pear-shaped atoll located 34 km southeast of Second Thomas (Figure 19.12). The platform is 2.0 km northeast– southwest, about 1.1 km west–east to the south, and covers an area of about 1.7 km2. The outer reefs are best developed to the northeast where they are about 130 m wide. Outer reefs to the west and south are narrower, about 50–100 m wide, and become discontinuous to the east. The rim is a relatively broad rock and rubble flat with little reef development that dries at low tide (Sailing Directions, 2020). The rim is without islands or passages to the lagoon and is semi-closed. The lagoon is shallow, less than 5 m deep (Wang, 1998), and occupies an area of about 10 ha with numerous shoal areas scattered throughout. Several additional small pools are isolated by inner reef flats. Part of the lagoon and some of the isolated pools to the north develop sand deposits. This reef is coded but lacks satellite imagery in the Atlas at the time of publication. Sabina Shoal (Xianbin) is a cleaver-shaped atoll (Figure 19.13) lying 143 km west of Palawan (the Philippines) and 57 km east of Second Thomas. The platform is 23 km northwest–southeast, 7 km northeast–southwest, and covers an area of 95.6 km2. The outer reefs are best developed to the north where they are about 170 m wide. The rim is a complex arrangement of more than 30 reef flat segments, at least 20 of which compose a ‘broken’ cleaver handle. The remainder outlines the blade. The largest reef flat segments are northwest–northeast where four of them cover an area of 4.1 km2. Many of these separate flats develop patch reefs on their surfaces. Several reefs are

The Atolls of the South China and Sulu Seas

Figure 19.12 Remote-sensing images of the atolls of Second Thomas, Boxall, and Sabina. © 2020-2021, Planet Labs PBC.

awash on the northeastern half of the rim and the southwestern rim is largely submerged by about 4–18 m (Sailing Directions, 2020). The rim is submerged. Although the lagoon area (inboard of the blade part of the rim) is more than 60 km2, except for the sand, coral lagoonal margins, and part of the slope, most of the basin is uncoded. The maximum depth is listed by Wang (1998) as 40 m.

Bombay Shoal (Pengbuo) is a small ovoid atoll located 45 km southeast of Sabina (Figure 19.13) about 2.5 km northeast–southwest, 1.8 km at its widest, and covers an area of 3.0 km2. The platform is surrounded by outer reefs that are about 100 m wide, narrowing to 50 m or less to the southeast. The reef flat is rocky, supports a few patch reefs, and is up to 380 km wide to the northeast, narrowing to less than

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Figure 19.13 Remote-sensing images of the atolls of Bombay, Royal Captain, and Northeast Investigator. © 2020-2021, Planet Labs PBC.

150 m wide to the east. Numerous surge channels are found in the reef pavement to the northwest (Google Earth imagery). No islands are present, and because the rim surrounds the lagoon and lacks passages, it is designated as semi-closed. The lagoon covers an area of about 1.5 km2 and is circumscribed by an area of about 50–70 m wide where reefs, sand, and seagrass communities are found. Most of the lagoon deeper than 15 m is uncoded and reaches a maximum depth of 33 m (Wang, 1998). This reef lacks a satellite image in the Atlas at press time. Royal Captain Reef (Jianzhang) is an oval atoll located about 51 km southwest of Bombay Shoal.

It lacks a satellite image in the Atlas at publication time but is coded. The platform is 4.3 km northwest– southeast, 2.7 km at its widest, and covers an area of 8.9 km2. The outer deep and shallow reefs are narrow, about 90 m wide or less on all sides. The rim to the north–southeast develops some widely spaced spurs and grooves. The reef flats are up to 400 m wide to the northeast and southwest where a distinctive reefrimmed spike-like extension is found to the southeastern rim protruding to the northwest into the lagoon. Large patch reefs are found on the flats to the northeast and south. No islands or passages through the low-lying rim are present and it is designated as

The Atolls of the South China and Sulu Seas

Figure 19.14  Remote-sensing images of the atolls of First Thomas, Alicia Annie, and Half Moon. © 2021, Planet Labs PBC.

semi-closed. The lagoon covers an area of 5.6 km2. The shallows are narrow and are occupied by sand deposits up to 180 m wide to the west with seagrass patches extending northwest–southeast. Most of the deeper ­lagoon is uncoded except for the edge of the slope and nine small reef-rimmed areas that project to or near the surface from a maximum depth of 30 m (Wang, 1998). Northeast Investigator Shoal (Haiku) is a small ovoid atoll with a pointed eastern end located 23 km northwest of Royal Captain. The platform is 2.2 km northwest–southeast, 2.6 km west–east, and covers an area of about 3.4 km2. The outer reefs are up to about 80 m wide to the northwest and east and are narrower elsewhere, becoming poorly developed to the northeast. The rim east–northwest clockwise develops large patch reefs, especially to the southeast where one reef area covers about 15 ha. The rim

dries in places (Sailing Directions, 2020), is without islands or passes, and the atoll is semi-closed. The lagoon covers an area of 1.6 km2. The shallower lagoon bears a reef sand apron up to 190 m wide to the west, narrowing elsewhere to 100 m wide or less. More than 20 mound-like structures rise toward the surface from the lagoon, some of which are rimmed by reefs. The deepest area of the lagoon covers about 20 ha where the maximum depth is 20 m (Wang, 1998). No satellite image of this atoll is in the Atlas at press time. First Thomas Shoal (Xinyi) is an atoll shaped like a warped cricket bat with an east–west orientation located about 56 km northwest of NE Investigator. The platform is 7.2 km west–east, 1.6 km at its widest, and covers an area of 8.5 km2 (Figure 19.14). The outer reefs are up to 100 m wide to the north but only about 10 m wide to the south. Widely spaced spurs

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and grooves are found to the north. The reef flats are about 750 m wide to the west but more commonly are about 250 m wide and surround the lagoon with no islands or passages. Patch reefs appear on the rim mostly to the south, although the rim dries (Sailing Directions, 2020) and is semi-closed. The lagoon covers an area of about 3.0 km2 and the shallows are mostly covered with sand. The atlas does not code for any reef material in the deeper lagoon which Wang (1998) marks as 12  m deep. The Atlas also lacks a satellite image of this atoll at press time. Alicia Annie Atoll (Xiane) is located about 48 km west of First Thomas where it forms a distorted teardrop (Figure 19.14) 7.9 km north–south, 3.8 km west–east at its widest, and covers an area of about 20.6 km2. The outer reefs are best developed southeast–southwest where they are about 250–295 m wide, and can extend to 575 m wide over short distances. The rim is a reef flat about 900 m wide to the north, tapering to about 300 m wide to the south as it surrounds the lagoon. Patch reefs occur southeast– west. There are no islands or passages, and the entire edge of the rim dries at 0.3 m (Sailing Directions, 2020). We mark this reef as semi-closed. The lagoon covers an area of 14.1 km2. Its shallows and much of the slope are covered with a reef sand apron up to 650 m wide to the south, narrowing to about 100 m wide to the east. The deeper lagoon areas display a few reef-rimmed pinnacles. The deepest part of the lagoon is uncoded and occupies an area of about 4.2 km2 where the maximum depth is 20 m (Wang, 1998). Half Moon Shoal (Banyue) is a parallelogram-shaped atoll located about 100 km southeast of Alicia Annie (Figure 19.14). The platform is 6.8 km northeast–southwest, 4.9 km north–south, and covers an area of 17.9 km2. The outer reefs are about 250–300 m wide southeast–southwest, with some areas extending to more than 400 m. Widely spaced spurs and grooves are found west–north clockwise. The rim is partly awash, about 450–550 m wide to the west, about 250 m wide to the east, and is without islands. Patch reefs occur northeast, southeast, and southwest. A 200-m-wide channel occurs to the southeast that is about 13 m deep (Sailing Directions, 2020). The rim is submerged. The lagoon covers an area of 9.3 km2. The shallows form sand deposits counterclockwise northeast–southeast up to about 600 m wide but are largely absent to the east, north of the lagoon entrance. The deep, uncoded lagoon forms about 20 small pinnacle-like structures that project near the surface where they are rimmed by reefs. Commodore Reef (Siling) is located 123 km southwest of Half Moon where it takes the form of an asymmetrical bow tie (Figure 19.15) due to the

arrangement of two lagoons separated by the reef flat. The platform is 12.7 km northwest–center and center–east, 4.3 km north–south at its widest, and covers an area of 27.7 km2. The outer reefs are better developed to the north and northwest where they are up to 300 m wide and taper to about 200 m to the northeast. The deep outer reefs to the south are up to 165 m wide, but are more often less than 100 m wide. Widely spaced spurs and grooves occur to the north. The shallow reefs continue their growth to the reef flats, especially to the northwest, southwest, and the narrow center. The reef flats are continuous, widest to the center of the rim where they are 2.5 km, followed by the northwest where they are 1.5 km wide. There are no islands, and the rim is semi-closed. The west lagoon covers an area of about 6.8 km2 and surrounded by a reef sand apron about over 500 m to the north and 150 m wide to the south that overlaps the shallows and the slope. A number of reef-rimmed pinnacle-like structures project near the surface from the basin of the deep west lagoon where the maximum depth is 15 m (Wang, 1998). The east lagoon covers an area of 2.3 m2 and likewise forms a reef sand apron up to 500 m wide that overlaps the shallows and the lagoon slope. The deep lagoon to the east may be shallower than the western lagoon but no separate depth is given by Wang (1998). A few small buildings have been constructed by the Philippines on the central reef flats (AMTI 2022b). Pigeon Reef (Tianlan) is about 76 km northwest of Commodore where it forms a rounded scalene triangle (Figure 19.15). The platform is 7.1 km northeast– southwest, 3.3 km at its widest northwest–southeast, and covers an area of 15.5 km2. The outer reefs are up to roughly 130  m wide to the north–northeast and somewhat narrower to the south. Widely spaced spurs and grooves occur to the west and north. The rocky reef flat surrounds the lagoon that is about a kilometer wide to the northeast and southwest and is about 350 m wide elsewhere. Patch reefs occur around the lagoon periphery, especially to the southwest. This atoll dries at low water (Sailing Directions, 2020), is without islands, and is semi-closed. The lagoon covers an area of 7.9 km2 including the shallows to the north and northwest that supports a broad reef sand apron up to 500 m wide to the north. The lagoon depths are not available from the literature, it is likely less than 15 m deep. This is the seventh and last atoll in this region where there is no satellite image that corresponds with the coded reef. Cornwallis South Reef (Nanhua) is an atoll located 49 km southwest of Pigeon Reef where it takes the form of a warped rectangle (Figure 19.15) measuring 9.1 km northwest–southeast, 4.4 km across the base to the southeast, and covers an area of 30.3 km2.

The Atolls of the South China and Sulu Seas

Figure 19.15  Remote-­sensing images of the atolls of Commodore, Pigeon, and Cornwallis South. © 2021, Planet Labs PBC.

The outer reefs are developed around the platform and are mostly 225–295 m wide, extending to 380 m wide to the south. The rim is a reef flat 1.3 km wide to the northeast and 400–700 m wide elsewhere. A spur and groove system is visible around much of the platform although it is widely spaced to the west and east and finger-like north–northeast. The rim includes a mix of patch reefs in several areas and rocky substrate which dries episodically. The rim is without islands and was closed until 2015 when two channels 70–90 m wide through the southwest and southeast rim were dredged through and buildings were constructed near them by Vietnam (AMTI, 2017). An additional shallow channel was carved into the rim to the northeast. Despite the channelization of the rim, we regard the reef as semi-closed.

The lagoon covers an area of 16.3 km2. A reef sand apron forms on the shallows extending to 1.2 km wide to the northwest and narrows to about 90 m wide to the west. The basin is shallow to the north and deepens slightly to the south where the maximum depth is listed by Wang (1998) as 9 m. The slope is narrow to the north and widens elsewhere. Several pinnacle-­like reefs rise from the lagoon floor, but the deeper lagoon codes few other reef areas. Alison Reef (Lisheng) lies 11 km northwest of Cornwallis South where it takes a piscine-like form pointing southeast (Figure 19.16). The platform is 11.5 km southeast–northwest, 6.8 km northeast–south at its widest, and covers an area of 77.3 km2. The outer reefs are best developed to the west–northwest where they generally range from 250–300 m wide,

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Figure 19.16 Remote-sensing images of the atolls of Alison and Pearson. © 2021, Planet Labs PBC.

extending to 370 m wide to the northwest. The platform is rimmed by spurs and grooves that are widely spaced to the northwest and more variable to the north. The rim is an asymmetrical rocky reef flat without islands 450–850 m wide to the north and opens by a single 560-m-wide pass that is about 9 m deep (Sailing Directions, 2020). The south rim is composed of eight reef flat segments, several of which are joined by shallow reefs or are separated by narrow passages. The southeastern rim is submerged, but the emergent portions do not dry. The flats develop patch reefs on all sides but to the west; the inner reef flats form seagrass meadows adjacent to the lagoon. The rim is submerged. The lagoon covers an area of 55.3 km2. The northern portion contains sand deposits in the shallows that reach up to 700 m wide to the northeast before passing to a sandy slope and deep lagoon. Reefs are developed in the shallow lagoon around most of the basin as well as on the slope. Multiple reef-rimmed pinnacles, mostly submerged, rise from the deep lagoon basin that appears to be more than 15 m deep, but Wang (1998) reports the maximum depth as 9 m. Pearson Reef (Bisheng) is a warped cricket bat-shaped atoll (cf. First Thomas) lying 27 km

northwest of Alison (Figure 19.16). The platform is 9.7 km northeast–southwest, 2.2 km north–south at its widest, and covers an area of 17.1 km2. The outer reefs are generally 250–300 m wide, commonly to the lower range along the north, while extending to more than 400 m wide to the southwest. A spur and groove system is visible to the north and shallow reef growth continues as patch reefs around the northern and southern extent of the reef flats. The flats are widest to the northeast (the handle) where they extend to about 2.5 km and about half that width to the southwest. A 3-ha islet to the northeast includes multiple buildings, wind turbines, and a helipad built by Vietnam. Recent dredge-and-fill operations have created an additional sand area joining the islet that appears to intend enlarging it eightfold or more (AMTI 2022c). A channel about 30 m wide and 350 m long has been cut into the rim to the northwest and reaches the lagoon. Three additional structures joined by catwalks with a helipad in the center have been built nearby. Despite the artificial channels, we regard this atoll as semi-closed. The lagoon covers an area of 3.7  km2. Its shallows include a sand apron up to 400 m wide that extends to the deeper lagoon.

The Atolls of the South China and Sulu Seas

The basin and its knoll and ridge structure is clearly visible in satellite imagery and the depth maximum is recorded as 27 m by Wang (1998). Three atolls lie to the west of Pearson and are collectively referred to as the London Reefs. Cuarteron Reef (Huayang) is located 85 km west–southwest of Pearson where it is sausage shaped (Figure 19.17) 5.8 km northwest–southeast, 1.6 km north–south, and covers an area of 8.5 km2. The platform is surrounded by outer reefs that are best developed northwest– southeast where they range from 250–300 m wide. The entire atoll is surrounded by mostly widely spaced spurs and grooves except where dredging has occurred. The rim is continuous, up to a kilometer wide and is composed primarily of a rock and rubble reef flat where three small patches of reef are found to the northeast. About 23 ha of the rim has been dredged by

China to support a large radar array, communications facilities, a lighthouse, and defensive emplacements and a 10.5-ha turning basin (AMTI, 2018). The rim surrounds a shallow 52-ha lagoon that is reduced by the incursion of the inner reef flats from the north. No depth data is available, but the lagoon is entirely shallow. There are no natural islands or entrances, and we regard the rim as semi-closed. East London (East Yinqing) forms a rounded isosceles triangle (Figure 19.17) 17 km southwest of Cuarteron, measures 13.4 km west–east, 4.4 km north–south, and covers an area of 42.7 km2. The outer reefs are widest to the east where they reach 450 m. The rim is widest to the north where it is up to about 1.6 km. The northern rim supports a spur and groove system and a well-developed line of patchy reefs up to 525 m wide. The southern rim, by contrast, is up to

Figure 19.17 Remote-sensing images of the atolls of Cuarteron, East London, and West London. © 2021, Planet Labs PBC.

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380 m wide and supports small, scattered reef patches. Two structures joined by a catwalk have been built by Vietnam on the reef flats to the west along with a shallow channel that crosses the rim. Other small platforms support towers and are serviced by shallow channels to the northern and the eastern rims (AMTI, 2021). Multiple C-shaped dredge scars are clearly visible on the southwestern rim where they cover a width of about 600 m. The rim is punctuated to the west by two natural channels about 40–50 wide separated by a triangular section of reef flat. The rim is submerged. The lagoon covers an area of 24.1 km 2 . The shallows to the east include a reef sand apron that is up to 600 m wide. The apron is up to 1.7 km wide to the west, but it is highly variable, and it is typically about 150 m wide to the south. The easternmost lagoon presents numerous pinnacle-like structures many of which are coded as reefs. The deep lagoon also contains scattered reef-rimmed pinnacle-like forms. The maximum depth is 14.6 m according to Wang (1998), little of which is coded by the Atlas. West London Reef (West Yinqing) forms a rounded triangle (Figure 19.17) 32 km to the west of East London with a northwest-facing base whose platform is 10.7 km west–northeast, 5.9 km north– south, and covers an area of 42.6 km2. The outer reefs surround the platform and are most extensive to the northeast where they extend more than 325 m. A spur and groove system lies northwest–northeast of the platform. The rim is composed of separate reef flat areas to the west and the east, along with two smaller flats to the north and one to the south. Most of the southeast and northern rim is submerged with reef growth. A small sand cay to the southeast has been developed by Vietnam into a small settlement with a dredged harbor protected by breakwaters facing the lagoon. In addition, there are several inhabited platforms to the north and west (AMTI, 2017). The lagoon covers an area of 24.9 km 2 with a slope from the northeast and northwest that supports scattered seagrass communities and is rimmed with elongated patch reefs. The lagoon occupies 13.2 km 2 and is marked a reef-rimmed slope and by about 25 pinnacle-like structures, several of which are rimmed by reef development. The maximum depth of the lagoon given by Wang (1998) is 18.3 m. Several uncoded pinnacles occur in the lagoon center. Ladd Reef (Riji) is a sausage-shaped atoll (Figure 19.18) located 56 km southwest of West London. It is the westernmost of the Spratly Island atolls. The platform is 6.6 km west–northeast, 1.9 km north–south, and covers an area of 10.1 km2 including uncoded reef to the northeast. The platform is surrounded by outer reefs that are best developed to

the northeast where they are up to 275 m wide. Outer reefs to the south are about less than 100 m wide and are composed of only deep reefs near a ca. 120-m-wide channel that was cut into the rim after 2016 by Vietnam. A spur and groove system forms northwest– northeast. The rim is coded mostly as a reef flat that extends into the western half of the lagoon. The northwestern reef flats and those to the northeast and east exhibit several patch reefs. A few small buildings can be observed to the northern flats along with a shallow channel through the rim. We tentatively mark this atoll as semi-closed (although see below). The entire lagoon area, including the very shallow western half coded as a reef flat, is shaped like a 5.4-km-long club, of which 3.5 km (the curved handle) is occluded by rock but is covered to a large extent by patchy reefs. The wider eastern section of the lagoon covers an area of 1.2 km2 including the easternmost end where patch reef growth and seagrass communities are found. This wide part of the lagoon is adjacent to the channel through the south rim and is coded as sandy, including part of the shallows to the north. An area of 37 ha deepens to 4–5 m where a reticulate reef system forms with numerous coral heads in the shallows (National University of Singapore, 2017). However, the Atlas does not show this, and as of September 2020, an armada of dredges were at work within the lagoon and may change the character of this atoll. Barque-Canada Reef (Baijiao) is located 184 km southeast of Ladd where it resembles a bird in flight heading northeast (Figure 19.18). The platform is 29.4 km southwest–northeast following the midline, 3.8 km at its widest, and covers an area of 60.3 km2. The platform is surrounded by outer reefs that are up to 500 m wide to the northeast and about 300–400 m wide to the southeast where a well-developed reef crest occurs. A spur and groove system is best developed mid-northwest–north but is visible around most of this atoll. The rim is a reef flat 1.7 km wide to the northeast and about a kilometer wide to the southwest and develops patch reefs on the reef flats northeast–southwest anti-clockwise. Nearly all the reef dries (Sailing Directions, 2020) and there are no islands, although there are three paired buildings and towers on the northwestern rim (north, central, and south) to the northwest, and the rim is semi-closed. The central lagoon covers an area of about 22 km2, and when the secondary lagoons are added (separated from the center by inner reef flats), the total lagoon area is 23.8 km2. The shallows and the adjacent deep lagoon are covered with a reef sand apron up to 1.4 km wide, much narrower on the southeast-facing flank, that surrounds the remaining ca. 6.7 km2 of the central lagoon where several plateaus rise toward the surface, many with reef growth. Wang (1998)

The Atolls of the South China and Sulu Seas

Figure 19.18 Remote-sensing images of the atolls of Ladd, Marivelas, Barque-Canada, and Erica (Enloa). © 2020-2021, Planet Labs PBC.

describes the maximum lagoon depth as about 4 m, whereas National University of Singapore (2017) finds it to be at least 8 m deep. The Atlas suggests that the lagoon may be more than 15 m deep, but we are not aware of any bathymetric charts for this atoll. Mariveles Reef (Nanhai) lies 64 km southeast of Barque-Canada and takes the form of a bow tie (Figure 19.18; compare with Commodore South). The platform is 10.5 km northwest–southeast, 3.2 km at its widest, and covers an area of 18.9 km2. The

outer reefs are up to 280 m wide to the southeast and the northeast. A discontinuous spur and groove system extends southeast–southwest. The rim is mostly about 300–500 m wide and includes rocky bottom and patchy reef extending clockwise around the rim southeast to northeast. A well-developed reef crest coincides with the wider outer reefs northeast–southeast. Several buildings have been constructed on the southeastern rim by Vietnam. Additionally, a 2-ha area that includes a breakwater-protected harbor, a

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helipad, and a 16-m-wide channel that penetrates the rim has been built on the central southwestern rim. Other than the artificial construction, the rim is without islands and is continuous. The rim is semi-closed. A shallow inner reef flat to the center gives rise to the appearance of two unequal lagoons. The larger is to the northwest and covers an area of about 7.2 km2, whereas the one to the southeast covers an area of about 1.4 km2. The northwestern lagoon develops a reef sand apron up to 700 m wide surrounding a lagoon that appears to be deeper than 15 m. The southeastern lagoon develops a striated sand deposit about 800 m wide that appears to emanate from the shallow area separating the two lagoon areas. A small area near the lagoon center is coded as reef. We do not have verifiable information concerning the depths of either lagoon. Erica, aka Enloa Reef (Boji), is a small ovoid atoll (Figure 19.18) located about 25 km northeast of Mariveles that is 3.6 km northeast–southwest, 2.9 km northwest–southeast at its widest, and covers an area slightly over 6 km2. The outer reefs are about 150–250 m wide around the perimeter of the platform. A spur and groove system borders the northern rim with rim patch reefs occurring predominantly east–southwest clockwise. The rim is a reef flat ranging from 300 to 650 m wide and bears no islands but develops a reef crest northwest–northeast. A naval station with a small harbor protected by breakwaters is found to the southeast on an artificial island (Sailing Directions, 2020). The rim is semi-closed. The lagoon covers an area of 1.9 km2, much of which is a shallow sandy apron that extends to nearly 500 m wide on the northwest, north, and northeast-facing margins. It passes to a somewhat deeper part of the lagoon 94 ha in extent. A small band of reef material is coded within the lagoon center, and we have no information on its depth maximum. Investigator Shoal (Yuya) is located 40 km east of Erica where it takes the form of a cleaver (Figure 19.19; compare with Sabina Shoals) that is 32.5 km west–east, 12 km north–south at its widest, and covers an area of about 198 km2. The outer reefs here are best developed to the southwest where they range from about 300 to more than 500 m wide. The rim north–northeast includes four small reef flat areas that are composed of shallow reef and seagrass, but is otherwise submerged, and in one section 1.5 km wide, the submergence is sufficient to serve as a passage to the lagoon. The southern rim is narrow, about 500–800 m wide, and except for a section of rim about 7 km long extending from the west, this section is mostly 5–18 m deep (Sailing Directions, 2020). The rim is submerged. The middle south rim extends as an 8.6-km submerged area composed of reefs and sand that represent a spur and groove system that blends into the lagoon with diffuse boundaries. The lagoon are indistinct and the area it covers is estimated

to be 167 km2. The narrow and shallow handle area is covered with sediment and supports a few small patch reefs. The larger lagoon is up to 45 m deep (Wang, 1998), and except for reefs that occur on the slope, few are coded in the deepest lagoon areas. Ardasier Bank (Andu) is an oddly shaped drowned atoll located about 19 km southwest of Investigator Shoal. The platform covers about 860 km2 and is approximately 72 km northeast–southwest following a curved path, and about 20 km northwest–southeast at its widest. The rim is shallowest to the southwest end of the platform where it forms Ardasier Reef (Figure 19.19 shows only the shallow part; see below). The northern end of the platform is about 6 m deep, and the rest extends to a depth of about 10–20 m. Most of the rim is coded by the Atlas. The northern rim is about 1.9 km wide. A series of plateaus extend about 24 km to the northeast of Ardasier Reef, many of which are associated with reef development. The plateaus re-emerge about 27 km farther to the northeast, about 17 km from the northeastern end of the platform where a shallow area of reefs and sand is found. The southeastern rim is submerged and is roughly 1.1–1.5 km wide. The lagoon covers an area of about 658 km2 and reaches a depth of 65 m (Wang, 1998; National University of Singapore, 2017). The lagoon is uncoded by the Atlas. Ardasier Reef (Guangzingzi) to the southwest of Ardasier Bank forms a beveled triangle 4.8 km northwest–southeast, 4.5 km northeast–southwest, and covers an area of 11.5 km2. The outer reefs surround this end of the platform and are best developed to the north where they extend 150–225 m.. A widely spaced spur and groove system extends the platform to the northeast. The rim is a reef flat, widest to the northwest where it is o about 600 m wide. Most of the southern rim is poorly developed for about 2 km to the southeast where it is open to oceanic exchange. Reefs are abundant on the reef flat as are seagrass communities that develop on much of the periphery of the shallow lagoon. Buildings, a protected harbor opening to the lagoon, and a helipad (similar to that found on Mariveles) have been constructed on the southern reef flat. Ardasier Reef is submerged. Sand deposits occur in the shallow lagoon and form a reef sand apron mostly surrounding a central section of the lagoon that is about 10 m deep. Dallas Reef (Guangzing) is located 9.6 km west of Ardasier Reef where it forms an ovoid atoll rounded and turned to the northeast and more narrowly rounded to the west (Figure  19.19). The platform is 9.4 km west–northeast, 2.1 km at its widest, and covers an area of 14.2 km2. This area is surrounded by outer reefs that are 150–175 m wide to the northwest, 145 m to the west, and about 100 m or less wide ­elsewhere. The rim is composed of a reef flat about a

The Atolls of the South China and Sulu Seas

Figure 19.19  Remote-­sensing images of the atolls of Investigator, Ardasier, Dallas, and Swallow. © 2019-2021, Planet Labs PBC.

kilometer wide to the east, 700 m wide to the west, and 300–400 m wide north and south that dry (Sailing Directions, 2020). The southern and eastern rims are nearly blanketed by reef growth which accounts for more than 90% of the rim area (Asner et al., 2017). An additional, isolated reef of about 18-ha occurs to the west. The rim is semi-closed. The lagoon covers an area of about 7.5 km2. The. The reef flats display reef growth to the northeast, west and especially along the southern rim. The northern rim develops fewer reefs. The inner shallow lagoon forms a sand apron about

200 m wide to the north that extends to the lagoon slope at a depth of 2–4 m; the southern apron is much narrower. The deep lagoon basin supports numerous ridges with multiple coral heads and coral ribbons 500 m long and reaches a depth of 6–10 m (National University of Singapore, 2017). Swallow Reef (Danwan) is administered by Malaysia where it is called Layang-Layang and is protected as a marine sanctuary by the Royal Malaysian Navy (but see below). This atoll is located about 27 km south of Dallas where it forms an elongated ovoid

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with a west-facing point (Figure 19.19). The platform is 7.5 km west–east, about 2.3 km at its widest, and covers an area of 11.2 km2. The outer reefs narrowly surround the atoll where they are mostly 150 m wide or less except to the west where they extend to about 350 m. The rim is a rubble reef flat 1.2 km wide to the west, 1.0 km wide to the east, and 250–350 m wide to the north and south. The shallow outer reefs extend to the reef flats in most of the rim, but especially to the east and the west. The southeastern rim is occupied by a 40-ha military complex including a 1,300-m-long runway, aircraft hangars, a naval station with an air defense system, a marine research facility, and a threestar dive resort (NBR, 2021). A 100-m-wide channel has been carved through the rim connecting to the lagoon about 200 m to the west of the complex, and another 40-m-wide channel is found to the southwest. In its natural state, the rim was semi-closed. The lagoon covers an area of about 5.3 km2 and is characterized by a reef sand apron, mostly about 100 m wide but up

to 280 m to the east and wider to the west. Patch reefs and seagrass areas are common around the shallow lagoon. The lagoon is up to 18 m deep (Tom Neal, pers. obs.) and occupies an area of about 3.5 km2. Royal Charlotte Reef (Huanglu) is a small D-shaped atoll (Figure 19.20) located 53 km southwest of Swallow. The platform is 2.2 km across the base, about 1.7 km at its widest through the middle, and covers an area of 3.1 km2. The outer reefs are 150–160 m wide extending west–east clockwise. The reefs narrow to the south and become deeper to the southeast where they are about 70 m wide. The shallow outer reefs continue their growth onto the rim primarily to the west. The rim is a reef flat up to about 600 m wide to the north and east and 300 m wide to the northeast and the southeast. The flats surround the lagoon without islands or passes, and the rim is semiclosed. The lagoon basin covers an area of about 78 ha and presents ridges and reefs, especially on the center. Several pinnacle-like structures are associated with

Figure 19.20 Remote-sensing images of the atolls of Royal Charlotte and Louisa. © 2022, Planet Labs PBC.

The Atolls of the South China and Sulu Seas

the deep slope. The lagoon reaches a depth of 8–10 m about 20 m from the inner edge of the reef flat and reaches a depth maximum estimated to be 15–20 m (National University of Singapore, 2017). The lagoon and much of the rim is uncoded by the Atlas. Louisa Reef (Nantong) is located 76 km southwest of Royal Charlotte where it assumes the shape of a distorted D (Figure 19.20) about 2 m northwest–southeast, 1.2 km at its widest north–south, and covers an area of 2.1 km2. The outer reefs are up to 125 m wide to the northeast, narrowing to about 50 m wide to the south and southeast. Some spur and groove development is visible north–east. The rim is rocky, about 300 m wide to the west, narrowing to the north and east. The flats are weakly developed to the south. Strong shallow reef development continues onto the reef flats northwest–southeast clockwise and surrounds a shallow, sandy basin that is coded as a reef flat. No lagoon is coded by the Atlas, but the flats likely flood the sandy basin at high tide and may be regarded as an atoll that is submerged with a very shallow lagoon (Wang, 1998). Rifleman Bank (Nanwei) is a drowned oceanic atoll 75 km south of Ladd Reef and 225 km northwest of Louisa (Figure 19.2). The platform is polygonal, 49 km northeast–southwest, 23 km west–east at its widest, and covers an area of about 750 km2. The largest shallow area is to the north, 10-km2 Bombay Castle Shoals,

which is accompanied by other smaller areas likely about 10–15 m deep. The lagoon is up to 82 m deep (Wang, 1998). Most of this structure is beyond the depth range of satellite imagery and it is not counted as an atoll here.

Atolls of the Sulu Sea Five atolls are found in the Sulu Sea which communicates with monsoonal current flows from the SCS (Figures 19.1 and 19.2). The first of these is Mapun (Figure 19.21), part of the Sulu Archipelago that constitutes the southern boundary of the Sulu Sea. The atoll is 64 km east of Kalimantan and is part of the Subutu Islands but is separated from them by channels that are 250 m deep to the west and 161 m to the east (NOAA chart 92010). The platform is an elongated oval 50 km north–south, 14.6 km at its widest, and covers an area of 540 km2. The platform is rimmed by outer reefs that are 1500 m wide to the northwest where much of it is uncoded but more often are about 100 m wide or less. The rim is composed of five islands to the east, the largest of which is Sitangkai that rises to about 12 m and supports a population of more than 37,000 as of 2020, most of whom are engaged in fishing. The rim is otherwise a reef flat that is open to the lagoon by a reefrimmed pass that is 450 m wide and about 13 m deep

Figure 19.21 Remote-sensing images of Mapun Atoll and surrounding islands. © 2021, Planet Labs PBC.

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to the northwest. The southern platform is separated from the main reef flat area by a 13–18-m-deep reef flanked pass that isolates a U-shaped reef flat of about 22 km2 from the rest. Reefs and seagrass communities characterize the central reef flats and those to the northwest. The rim is open. The lagoon covers an area of 259 km 2 and is divided into a northern section up to about 12.2 km wide and a narrow, channel-like area to the south 0.65–2.3 km wide. The northern lagoon is surrounded on three sides by a sandy rim in the

shallows that is more than 8 km wide but opens to the south as it connects with the southern lagoon, which has its own separate sand deposits. The region between sandy areas is replete with patch reefs that cover about 26 km 2, mostly on the lagoon slope. About 48 km 2 constitutes the lagoon area that is up to 16.5 m deep. Tubbataha Reefs are dual atolls composed of a north atoll and a separate and smaller south reef (Figure 19.22), both of which lie in the middle of the Sulu Sea on the Cagayan Ridge. The ridge represents

Figure 19.22 Remote-sensing images of the atolls of Tubbataha North, Tubbataha South, and Arena. © 2021, Planet Labs PBC.

The Atolls of the South China and Sulu Seas

a remnant volcanic arc that according to charts divides the Sulu into two distinct basins about 3,000 m deep to the west and 1,300 m deep to the east. Despite their isolated position, these reefs have been affected by El Niño warming, coral diseases, crownof-thorns starfish invasions, and worse, cyanide and blast fishing. The atolls were designated as a UNESCO World Heritage Site in 1993 and have achieved some recovery. Tubbataha Reefs are biologically rich with 415 species of fishes and 243 coral species, most of which are confined to the outer reefs. The southeastern leeward slopes carry on average 56% coral cover, whereas 30% of the northeastern windward slopes are coral covered, and the western slopes support 18% coral cover (Licuanan et al., 2017). Lagoon sites are less diverse with an average of 43 species. Monospecific stands of small-branched Acropora are common along with mixed coral assemblages in the shallows as well as spur and groove systems. Small giant clams (Tridacna crocea) populations are large with more than 30,000 individuals per km2 on the reef flats and the lagoon (Ledesma, 2001; Delarosa & Schoppe, 2005).

North Tubbahata takes the form of a warped oval forming a beak-like projection to the northeast. The platform is 17 km northeast– southwest, 6.6 km at its widest, and covers an area of 78.5 km2. The outer reefs undulate but are best developed along the southeast where they range from 200–420 m wide. The rim is occupied by about ten sand cays, very small islets that emerge during extreme low tide. The rim is enclosed by reef flats that are about 600 m wide except to the beak-like northeast where they are about 2.5 m wide and sandy. However, patch reefs are common around the northeastern half of the rim. The lagoon covers an area of 49 km2. Its shallows support a reef sand apron that is up to 500 m wide to the northeast, narrowing elsewhere. The shallow lagoon develops reef areas around the northeastern half. Several pinnacles emerge from the deep slope and are coded as reef rimmed. The deep, uncoded lagoon extends to 24 m (Purdy, 2001). The south atoll is a small version of the north, measuring 7.7 km northeast to southwest, about 4 km at its widest west to the beak-like east, and covers an area of 18.4 km2 including a small area of

Figure 19.23 Remote-sensing image of Cagayan Atoll. © 2021, Planet Labs PBC.

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uncoded reefs to the east. The outer reefs are narrow, generally less than 100 m wide except to the beaklike easternmost point where they reach 160 m. The flats are sandy and become about 1.7 km wide to the south, where a very small sand cay occurs, and 3.8 km wide west–east through the beak. The lagoon is shallow, depth unknown, covers an area of 4.5 km2 A reef sand apron up to 570 m wide occurs in the shallows, narrowing to 290 m wide to the south and becomes narrower to the east and west of the basin margin. The basin is studded with knolls and ridge-like structures that are uncoded. A few areas rise from the bottom and are coded as reef rimmed. . Both atolls are semi-closed. Arena is a small ovoid atoll (Figure 19.22) located 83 km northeast of North Tubbahata. The platform is 7.2 km northeast–southwest, 2.6 km northwest– southwest where it is widest, and covers an area of 15.2 km2. The outer reefs are widest to the southeast where they undulate but are up to about 230 m wide. Estacion et al. (1993) report that the outer reefs are covered up to 81% with various coral species to a depth of 20 m. The rim is an islandless, rubbly reef flat about 2.2–2.5 km wide on either side of the central lagoon where a few patch reefs are found. The rim is semi-closed. The lagoon is 1.2 km2, and despite the presence of several plateau-like structures, it is very shallow, covered with sand, and not coded for reefs. Cagayan Atoll is located 56 km northeast of Arena where it takes the form of a seahorse (Figure 19.23) rising from the northern part of the Cagayan Ridge. The platform extends 45.2 km northeast–southwest following a curved path, 8.6 km at its widest, and covers an area of 163.9 km2. The outer reefs are up to 300–450 m wide to the southeast and vary from 290 to less than 1000 m wide elsewhere. Reefs extend to the reef flats primarily to the eastern abdominal area and the caudal region to the southwest. Shallow reefs also occur in the snout to the northeast which are found on a submerged part of the rim. The rim is composed of 10.4-km-long Cagayancillo island and adjacent 1.9-km Nusa Island, both to the southwest. Cagayancillo is mostly flat and unforested with a population of about 6,900 people as of 2020. The remainder of the rim west–north is a rubbly reef flat. The snout area is complex and is composed of submerged reefs, two small reef flat areas, and terminates with a 78-ha reef flat supporting a 4-ha islet. The eastern rim is composed of small reef flat areas but is mostly submerged except for 5.7-km-long Santa Cruz Island to the southeast. The lagoon covers an area of about 102 km2 including deep lagoon pools surrounded by shallow lagoon ridges to the northwest, and part of the snout

outlined by small, uncoded, and deep lagoon areas. A reef sand apron up to 2.5 km wide to the northwest surrounds the lagoon and fills the caudal lagoon area to the southwest where it is 1.8 km wide. An additional sand bar occurs in the central lagoon that extends to 3.8 km long and is up to 1.7 km wide. Patch reefs occur as clusters on the lagoon slope in the neck and abdominal region as well as to the south of the centrally located sand bar. A few reef-rimmed pinnacle-like structures are also located in the abdominal region. Several areas of the lagoon are too deep to code. We are unaware of the maximum depth records for this atoll.

References AMTI 2017. Asia Maritime Initiative and Center for Strategic and International Studies, Washington DC. https://amti.csis. org/vietnam-builds-remote-outposts/ AMTI 2018. Asia Maritime Initiative and Center for Strategic and International Studies, Washington DC. https://amti.csis.org/ comparing-aerial-satellite-images-chinas-spratly-outposts/ AMTI 2021. Asia Maritime Initiative and Center for Strategic and International Studies, Washington DC. https://amti.csis. org/east-reef/ AMTI 2022a. Asia Maritime Initiative and Center for Strategic and International Studies, Washington DC. https://amti.csis. org/fiery-cross-reef/ AMTI 2022b. Asia Maritime Initiative and Center for Strategic and International Studies, Washington DC. https://amti.csis. org/commodore-reef/#OverviewImages-heading AMTI 2022c. Asia Maritime Initiative and Center for Strategic and International Studies, Washington DC. https://amti.csis. org/pearson-reef/ Arai T 2015. Diversity and conservation of coral reef fishes in the Malaysian South China Sea. Rev. Fish. Biol. Fish. 25: 85–101. Asner GP, Martin ME, Mascaro J 2017. Coral reef atoll assessment in the South China Sea using Planet Dove satellites. Re‑ mote Sens. Ecol. Conserv. 3: 57–65. Cai S, He Y, Wang S, Long X 2009. Seasonal upper circulation in the Sulu Sea from satellite altimetry data and a numerical model. J. Geophys. Res. 114, C03026. Chou Y 2016. Dongsha Atoll research station–a steady research platform in the South China Sea. Kuroshio Sci. 10: 23–27. Dai C-F 2004. Dong-sha Atoll in the South China Sea: Past, present and future. In: Islands of the World VII International Conference Kinman Island, Taiwan. Delarosa RG, Schoppe S 2005. Focal benthic mollusks (Mollusca: Bivalvia and Gastropoda) of selected sites in Tubbataha Reef National Marine Park, Palawan, Philippines. Sci. Dili‑ man 17: 1–10. https://sciencediliman.upd.edu.ph/index.php/ sciencediliman/article/viewFile/1/11 Ding W, Li J, Dong C et al. 2013. Carbonate platforms in the Reed Bank area, South China Sea: Seismic characteristics, development and controlling factors. Energy Explor. Exploit. 332: 243–261. Estacion JS, Palaganas VP, Perez RE, Alava MNR 1993. Benthic characteristics of islands and reefs in the Sulu Sea, Philippines. Silliman J. 36: 15–41. Fang G, Wang G, Fang Y, Fang W 2012. A review on the South China Sea western boundary current. Acta Oceanol. Sin. 31: 1–10. Hu J, Kawamura H, Hong H, Qi Y 2000. A review on the currents of the South China Sea: Seasonal circulation, South China Sea Warm Current, and Kuroshio intrusion. J. Ocean‑ ogr. 56: 607–624. Huang D, Licuanan WY, Hoeksema B et al. 2015. Extraordinary diversity of reef corals in the South China Sea. Mar. Biodiv. 45: 157–168. Huang X, Betzler C, Wu S et al. 2020. First documentation of seismic stratigraphy and depositional signatures of Zhongsha atoll (Macclesfield Bank), South China Sea. Mar. Petrol. Geol. 117, 104349. https://doi.org/10.1016/j.marpetgeo.2020.104349

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Kempf J, Chapman P 2016. Upwelling System of the World. Springer Nature, Switzerland. https://doi.org/10.1007/978-3-319-42524-5 Kempf J, Möller L, Baring R et al. 2022. The island mass effect: A study of wind-driven nutrient upwelling around reef islands. J. Oceanogr. 79: 161–174. https://doi.org/10.1007/ s10872-022-00673-2 Ledesma MC 2001. Monitoring Tubbataha Reef National Marine Park (TRNMP) and areas along the Cagayan Ridge. http:// tubbatahareefs.org/wp-content/uploads/2019/02/2001-Research-Report.pdf Lee VR 2015. South China Sea: Satellite images show pace of Subi Reef reclamation. https://thediplomat.com/2015/06/ south-china-sea-satellite-images-show-pace-of-chinas-subireef-reclamation/ Li T, Feng A, Liu Y et al. 2018a. Three-dimensional (3D) morphology of Sansha Yongle blue hole in the South China Sea revealed by underwater remotely operated vehicle. Sci. Rep. 8, 17122. Li K, Ke Z, Tan Y 2018b. Zooplankton in the Huangyan Atoll, South China Sea: A comparison of community structure between the lagoon and seaward slope. J. Oceanol. Limnol. 36: 1671–1680. Licuanan WY, Robles R Dygico M et al. 2017. Coral Benchmarks in the center of biodiversity. Mar. Poll. Bull. 114: 1135–1140. Liu J, Webster JM, Salles T et al. 2022. The formation of atolls: New insights from numerical simulations. J. Geo‑ phys. Res. Earth Surface 127, e2022JF006812. https://doi. org/10.1029/2022JF006812 Liu Q, Kaneko A, Su J 2008. Recent progress in studies of the South China Sea circulation. J. Oceanogr. 64: 753–762. Luan X, Ran W, Wang K et al. 2019. New interpretation for the main sediment source of the rapidly deposited sediment drifts on the northern slope of the South China Sea. J. Asian Earth Sci. 171: 118–133. McManus JW 2017. Offshore coral reef damage, overfishing, and paths to peace in the South China Sea. Intl. J. Mar. Coast. Law 32: 199–237. Nan F, Xue H, Yu F 2015. Kuroshio intrusion into the South China Sea: A review. Progr. Oceanogr. 137A: 314–333.

National University of Singapore 2017. https://cil.nus.edu.sg/ south-china-sea-satellite-mapping-project/ NBR 2021. https://map.nbr.org/2021/03/swallow-reef/ Purdy EG 2001. Supplement to Purdy EG, Winterer EL 2001. Origin of atoll lagoons. GSA Bull. 113: 837–854. Qijiang H, Gang L Xuemu W et al. 2021. Submarine geomorphologic features and genetic mechanism in the Xuande atoll, Xisha Islands. Haiyang Xuebao 43: 81–92. Qu T, Song T, Yamagata T 2009. An introduction to the South China Sea throughflow: Its dynamics, variability, and application for climate. Dyn. Atmos. Oceans 47: 3–14. Sailing Directions 2020. South China Sea and the Gulf of Thailand, Publ. 161. National Geospatial-Intelligence Agency, Springfield VA. Wang G 1998. Tectonic and monsoonal controls on coral atolls in the South China Sea. Spec. Publs. Int. Assn. Sedimentol. 25: 237–248. Wu S, Chen W, Huang X et al. 2020. Facies model on the modern isolated carbonate platform in the Xisha Archipelago, South China Sea. Mar. Geol. 425: 106203. https://doi.org/10.1016/j. margeo.2020.106203 Yan P, Deng H, Liu H et al. 2006. The temporal and spatial distribution of volcanism in the South China Sea. J. Asian Earth Sci. 27: 647–659. Yu Z, Metzger EJ, Fan Y 2021. Generation mechanism of the counter-wind South China Sea Warm Current. Ocean Modelling 167: 101875. https://www7320.nrlssc.navy.mil/ pubs/2021/zyu1-2021.pdf Zhang B, Li Y, Xiang S-Z et al. 2020. Sediment microbial communities and their potential roles as environmental pollution indicators in Xuande Atoll, South China Sea. Front. Microbiol. 11: 1011. https://doi.org/10.3389/fmicb.2020.01011 Zhao M, Yu K, Shi Q et al. 2016. The coral communities of Yongle Atoll: Status, threats and conservation significance for coral reefs in South China Sea. Mar. Freshw. Res. 67: 1888–1896. Zhao M, Yu K, Shi Q et al. 2017. Comparison of coral diversity between big and small atolls: A case study of Yongle Atoll and Linyang Reef, Xisha Islands, central South China Sea. Biodiv‑ ers. Conserv 26: 1143–1159.

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The Atolls of the Central Indian Ocean The atolls of the Lakshadweep Islands The northernmost atolls of the Indian Ocean are the Lakshadweep (formerly the Laccadive, Amindivi, and Minicoy) Islands (Figure 20.1). This archipelago is located between 8°N and 12°N and includes 36 islands, three reefs, five submerged banks, and 12 atolls located in the Arabian Sea 200–440 km west of India’s Malabar Coast. Most atolls of the group are small, elliptically shaped, and less than 10 km on their longest axis. The atolls rise from the Chagos–Laccadive Ridge, a system extending from the Arabian Sea at about 15°N to about 10°S, that supports not only the Lakshadweep atoll group but also the Maldive Islands and the Chagos Islands to the south. The ridge rises 2,000–2,700 m to the west and about 4,000 m to the more steeply inclined east. The ridge also forms east– west gaps in the Lakshadweep, the most prominent being the 200 km wide and about 2600 m deep 9° Channel that separates the southernmost atoll of Minicoy from the rest of the group (Prakash et al., 2015).

Climate and regional oceanography The area is substantially influenced by the Indian Monsoon system. The South-West (Summer) monsoon season runs from June to October. It includes the strongest winds and represents the rainy season accompanied by more than 60% of the annual precipitation. The average rainfall is 1,640 mm/year, but in general decreases from south to north. Cyclones moving from east to west affect the area, with an average of roughly 1 per year, most frequently in the post-monsoon season. The wave climate is dominated by Summer (SouthWest) monsoon (Figure 20.2). During this time, waves from the southwest and west can reach up to 5 m in height; otherwise, heights average ~1.4 m (Prakash et

al., 2015). Shielded by the Indian subcontinent, waves from the east are less common and smaller than in the more open ocean setting of the Maldives. Tides are in the area are mixed diurnal–semidiurnal, with two markedly unequal high tides and low tides daily (Figure 20.2). The spring tidal range is roughly 1.5 m and the neap tidal range is 75 cm. The major currents affecting the Lakshadweep group (Figure 20.1) include the Summer Monsoon Current (June–October) and the generally weaker Winter Monsoon Current (December–March). November and May are inter-monsoonal months. Winds and currents move nearshore sediments on the western sides of the islands to the north during the winter monsoon but with greater effect during the more energetic summer monsoon. The western rims are often submerged, exposing the lagoon to monsoonal currents. As a result, the western lagoons are shallow and often are covered or nearly filled with sediment. By contrast, the eastern rims, especially among the Lakshadweep Islands, are typically developed as islands 2–7 m high rising from a 1 to 2 km layer of coral debris followed by an overlying limestone conglomerate and fine sand. In general, the sedimentary succession is about 300 m thick and is underlain by volcanic rock (Shankar et al., 2002; Prakash et al., 2015). Corals typically are restricted to the outer reefs with a scattering in the sandy and often shallow lagoons which are typically 1–5 m deep (Wagle and Kunte, 1999). Most of the Lakshadweep atolls face the open Arabian Gulf and are submerged, whereas the easternmost members of the group face the Malabar Coast, receive some protection from the west, and are open. We describe the western group first. Cherbaniani is the northernmost of the western Lakshadweep atolls and takes the shape of a rounded scalene triangle (Figure 20.3). The platform is 12.7 km north–south, 4.9 km at its widest, and covers an area of 48.8 km2. The outer reefs found to the west are about 400–600 m wide, whereas those to the east are

DOI: 10.1201/9781003287339-20

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Figure 20.1 The northernmost atolls of the Indian Ocean are the Lakshadweep Islands. This archipelago is located between 8°N and 12°N and includes 13 atolls that rise from the Chagos–Laccadive Ridge. The area is substantially influenced by the Indian Monsoon system. The major oceanic flows include the Summer Monsoon Current (red arrows) and the generally weaker Winter Monsoon Current (blue arrows). The Maldive island chain is composed of 21 atolls situated atop the Chagos– Laccadive submarine ridge. North–south breaks in the ridge include the Kardiva Channel and the Suvadiva and Equatorial Channels. Most of the Maldive Islands are affected by monsoonal winds. The southerly Maldive atolls (Suvadiva and Addu) are in the path of the westward flow of the Equatorial Current and are subject to more variable monsoonal winds than atolls to the north. The Chagos Archipelago is a group of eight atolls found at the southern end of the Laccadive–Maldive–Chagos Ridge in the geographical center of the Indian Ocean about 5°S–7.5°S of the equator. During this summer monsoon, an Equatorial Countercurrent is stronger to the south and can be felt as far north as the northern Chagos Islands. During the winter monsoon, winds shift to southeasterly for the remainder of the year and the countercurrent is replaced by a weak and variable Equatorial Current.

Atolls of the Central Indian Ocean

Figure 20.2 Waves of the Lakshadweep group are dominated by the summer monsoon, when waves from the southwest and west can reach up to 5 m high; otherwise, heights average ~1.4 m. Because the atolls are shielded by the Indian subcontinent, waves from the east are less common. Tides in the area are mixed diurnal-semidiurnal, with two unequal high tides and low tides daily as shown by data from Kavaratti Atoll.

commonly about 300 m wide. A shallow spur and groove system occurs on the rim periphery northeast– southeast counterclockwise. The eastern side develops deeper and less well-developed spurs and grooves. The rim is a reef flat that extends up to 1.3 km wide to the west where numerous patch reefs are found. The rim to the east is narrower, develops fewer patch reefs, and forms elongated sand cays in the intertidal zone, but no vegetated islands are present. The rim is submerged and is largely sand. The deeper lagoon and lagoon slope cover an area of about 10 km2 most of which is flanked by a shallow sand apron on its west side over 1 km wide. The maximum lagoon depth is 5.5 m (Purdy, 2001). Byramgore Atoll is located 38 km southwest of Cherbaniani where it forms an oval with a flattened end to the south (Figure 20.3). The  platform is 21.5 km north–southeast, 8.0 km at its widest, and covers an area of 126.5 km2 including 17.7 km2 to the north that is submerged. The outer reefs are 450–525 m wide to the southwest. The rim is composed of two reef flat areas, one to the southwest, the other to the northwest with a broad area between that is essentially submerged. Reefs that border the rim appear to merge with those of the lagoon creating broad areas of reef development, especially to the southeast where the lagoon enlarges. A sandy apron to the south includes submerged dune-like structures can be seen in a small area to the west. Sand appears to

be transported from the west in areas coded as shallow lagoon to the center of the platform. The lagoon area is difficult to estimate and Purdy (2001) gives the maximum depth as 3.6 m, although satellite imagery suggests it is deeper than that. Bitra Par is 43 km southeast of Byramgore where it takes the shape of a rounded trapezoid 12.9 km north–south, 6.7 km west–east, and covers an area of 66.3 km2 (Figure 20.4). The outer reefs are found around the atoll and are best developed east–southwest clockwise where they range from 350–450 m wide. Outer reefs are less than 200 m wide elsewhere. The rocky reef flats are about 500 m wide southwest– northeast clockwise that dry at 0.3–0.9 m (Sailing Directions, 2022a). Reefs cover most of the flats and an 11-ha island (Bitra) is found to the northeast. A 500-m-wide channel forms to the northeast where the rim is submerged. This area and the shallows around it develop an extensive shallow reef system that likely make this passage suitable only for small craft. The eastern flats are narrower although patch reefs and seagrass meadows also are found here. The lagoon covers an area of 35.4 km2. The western and southern extents are shallow and include a well-developed reef sand apron that reaches up to 1.7 km wide, narrowing to the north. Scattered patch reef areas occur in the lagoon. We do not have a maximum lagoon depth for this atoll.

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Figure 20.3  Remote-sensing images of the atolls of Cherbaniani and Byramgore. Images © 2022, Planet Labs PBC.

Peremul Par is shaped like a brimless cap (Figure 20.4) located 32 km south of Bitra Par. The outer reefs are widest northwest–southeast clockwise where they extend 300–450 m from the rim. Reefs to the west are variable but are generally 200 m wide or less. Spur and groove systems occur around the island periphery but are best developed northwest. Shallow reef growth continues onto the reef flats mostly to the northwest. The rim to the northwest is about 500 m wide and is up to about a kilometer wide to the southeast. The flats dry at low tide. The eastern rim supports several sand cays, but island formation is negligible as on the other Lakshadweep atolls on the west of the Chagos–Laccadive Ridge described thus far (i.e., Cherbaniani, Byramgore, and Bitra Par). The lagoon area is 55.3 km2. The shallows and part of the slope are broad, up to 3.5 km to the north narrowing to about 2.9 km to northeast and support a reef sand apron up to 2 km wide. The lagoon includes more reef ridges and patch reefs in its western third. Roughly 10 km2 of the lagoon is uncoded despite the maximum recorded depth of 11 m (Sailing Directions, 2022a).

Bangaram Atoll is located about 26 km southeast of Peremul Par where it takes the shape of a warped parallelogram (Figure 20.4). The platform is about 10 km west–east, 7 km north–south at its widest, and covers an area of 52.8 km2. The platform is surrounded on three sides (north, east, and south) by outer reefs that are about 325–425 m wide, not including a deep outer reef to the south that forms a coralline shelf between Bangaram and Agatti (see below). The outer reefs to the south and southeast are best developed biologically where coral genera Acro‑ pora and Favia are most common. A reef crest is also best developed in those areas. The western platform is shallowly submerged and develops a large and elongated spur and groove system although smaller spurs and grooves are found elsewhere. Shallow reef growth occurs on the reef flats nearest the west. Otherwise, the reef flats are composed of rubble and exhibit few coded reefs, although Deshmukh et al. (2005) found coral coverage of about 5% confined to tide pools. The inner flats support two islets, Bangaram (59 ha) and Tinnkara (48 ha), as well as two smaller islets on the northeastern rim.

Atolls of the Central Indian Ocean

Figure 20.4  Remote-sensing images of the atolls of Bitra Par, Peremul Par, Bangaram, Agatti, and Suheli Par. Images © 2021-2022, Planet Labs PBC.

The lagoon covers an area of about 21.1 km2. Essentially, all of it and the surrounding inner reef flats are covered with sand. Some reef areas to the west and between Bangaram and Tinnkara are found along with scattered patch reefs that surround a deeper area of the lagoon covering about 3.7 km2.

The latter area is 10–20 m deep and contains submerged coral knolls 10–15 m in diameter that rise to within 0.5–1.5 m of the surface. These are composed primarily of Goniopora along with Acropora thickets, some of which attain a diameter of 3–4 m (Deshmukh et al., 2005). This atoll has one of the

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deepest lagoons (20–50 m) of the Lakshadweep atoll group (see also Minicoy Atoll below). Agatti Atoll is located about 10 km to the southwest of Bangaram and takes a hemispherical form (Figure 20.4). The platform is 9.7 km northeast–southwest, 4.2 km through the center at its widest, and covers an area of about 34.9 km2 not including submerged areas to the northeast. Outer reefs occur around the atoll perimeter and are generally about 500 m wide northeast–south clockwise although the northeast extends to about 1100 m wide and the south to 775 m wide. . The mean coral coverage of these reefs is 48.6%, although dead corals comprised an additional 21.6% due to events described below. Coral genera Acropora and Porites were among the most abundant genera (further details in Sreenath et al., 2015). Reef flats compose the rim all around the platform and are most extensive north–southwest counterclockwise where they are up to 1.8 km wide. The flats to the west exhibit two low areas, 1 km wide to the center, the other about 1.1 km to the northwest, considered the main lagoon entrance where well-developed spurs and grooves occur. The eastern rim supports oar-shaped Agatti Island with its population of 7,560 people, most of whom live on the 900-m-wide section (the blade). The long narrow handle area (the shaft) is about 200 m wide and includes an airstrip about 1,400 m long. A 300-m-wide gap occurs between the southwestern end of the island and Kalpitti, a 7-ha island to the south. This is the only one of the western Lakshadweep atolls that forms a large island on its eastern rim and the only one whose rim is open. A reef flat about 150 m wide appears to emanate from a shallowly submerged part of the western reef flat and connects with flats with those that surround the island. This arrangement partially divides the lagoon and is an area that is replete with reefs extending toward Agatti Island to the east as well as around most of the peripheral reef flats northwest–south clockwise. The lagoon covers an area of about 7.9 km2. Higher lagoon areas are given by others (cited below) who do not exclude the inner reef flats. Prior to 1998, the shallow lagoon slope (less than 2 m) supported coral coverage of 40%–50% of the available surface but was then subjected to a mass bleaching event that promoted a high cover of algal turfs (Cernohorsky et al., 2015). The northern section contains sand deposits and a few patch reef areas. The southern lagoon near Kalpitti, also sandy, contains massive corals of the genus Porites. Seagrass is reported to occur in this lagoon system at depths of 2–5 m, its deepest point, although this is not shown in the Atlas, possibly due to heavy grazing by marine turtles (Kaladharan et al., 2013).

Suheli Par takes an elongated ovoid form (Figure 20.4), pointed to the northeast and rounded to the southwest, located about 80 km southt of Agatti. The platform is 18.4 km northeast–southwest, 7.6 km at its widest, and covers an area of 98.5 km2. The outer reefs are up to 1700 m wide to the north. Outer reefs to the east are about 350 m wide although some areas extend to 500 m. The rim is least developed to the northwest where at least five short sections are submerged. The rim mid-west–north counterclockwise is composed of reef flats that dry at low tide (Sailing Directions, 2022). Valiyakara, a 36ha island, is found to the north and is followed to its south by a 4.9-km-long narrow sand cay that forms a ridge associated with the reef flats to the northeast. The rim is submerged. The lagoon covers an area of 60.3 km2, including 2-ha Cheriyakaya islet to the south. The central and northern lagoon contains an area of slope and deep lagoon that covers about 15.5 km2 and is densely packed with reefs, where the maximum depth is 10 m (Purdy, 2001). The surrounding shallow lagoon area is covered with a sand apron up to 2.8 km wide.

The atolls of eastern Lakshadweep Islands The eastern Lakshadweep atolls follow a zigzag line extending north–south and present one or more permanent, vegetated islands to the eastern side of the rim that are peopled by residents whose livelihood is supported primarily by fishing. Unfortunately, the consequence of high populations has led to intensive construction activity. The natural vegetation in some cases has been cleared for settlement. Goats and cattle have been introduced. Large fishing and cargo vessels now can enter lagoons because of cutting channels through the reef flats and dredging/deepening lagoons on some atolls to widen and deepen them, thus leading to high siltation rates (Menon and Pillai, 1996). The first of these eastern atolls is Chetlat located about 55 km northeast of Byramgore which takes the form of a rounded triangle with a curved northwestern side (Figure 20.5). The platform is 4.1 km north–south, 2.6 km west–east, and covers an area of 8.3 km2. The platform is surrounded by a relatively extensive area of outer reefs measuring up to about 750 m to the north–northeast and 950 m wide to the south. The rim is submerged to the west and north. A lagoon is present (Wagle and Kunte, 1999) but it appears to cover an area of about 1100 m2 that is to the west of the island (see below). We do not have a maximum depth for this area. The rim is open. Reefs and seagrass areas occur on the reef flats primarily to

Atolls of the Central Indian Ocean

the west. In addition, the northwest rim and much of the lagoon support an extensive area composed of the calcareous blue octocoral Heliopora (Sreenath et al., 2021). Mushroom corals of the genus Fungia are very abundant in the shallows of the lagoon (Pillai and Jasmine, 1989). A relatively large, wooded island 1.1 km2 oriented northeast–southwest and reaching about 2 m above sea level lies on the eastern rim. In

2016, Chetlat Island’s population was about 2,400 people. Kiltan Atoll is located about 38 km southeast of Chetlat where it takes the shape of an elongated oval, slightly curved in opposite directions toward the ends (Figure 20.5). The platform is 5.2 km northwest–southeast, 1.8 km at its widest, and covers an area of 8.4 km2. Outer reefs surround the platform

Figure 20.5  Remote-sensing images of the atolls of Chelat, Kiltan, Kadmat, and Kavaratti. Images © 2022, Planet Labs PBC.

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and are about 500–550 m wide to the west and 200– 225 m wide to the east. The shallow nearshore areas develop a spur and groove system. The intertidal areas are mostly devoid of living corals except for tide pools where isolated living colonies occur. Two shallow channels are found on the western reef flats to the northwest and center west. The reef flat limestone on Kiltan is elevated and exhibits erosion (Pillai and Jasmine, 1989). The eastern rim is dominated by the presence of an island that is 1.7 km2 and has been uplifted to 27 m (Sailing Directions, 2022). The rim is open. The lagoon is quite limited and is formed in small areas around the island and to the northwestern platform. Despite the shallowness of the lagoon (0.7 m, Purdy, 2001), corals of the genera Acropora (11 species) and Heliopora form large  clusters  in its shallows (Pillai and Jasmine, 1989). Kadmat Atoll is located about 33 km southwest of Kiltan where it takes a sausage-like form (Figure 20.5). The platform is 12.4 km northeast–southwest, 3.9 km at its widest, and covers an area of 34.8 km2. The outer reefs surround the platform and are best developed west and south where they undulate and are commonly 400–500 m wide but extend to more than 800 m wide to the southwest. A total of 45 coral species have been recorded from these reefs (Pillai and Jasmine, 1989). Outer reefs to the east are about 250 m wide. The western rim is a reef flat 350–750 m wide where reefs are found to the center and seagrass communities extend northwest–southeast. The eastern rim is dominated by coconut palm-covered Kadmat Island that is 9.2 km long northeast–southwest, 530 m wide, and covers an area of 3.4 km2. The rim is open. Approximately 5,400 people live in Kadmat (2011 census). The western reef flat develops numerous patch reefs. The shallow lagoon covers an area of about 3.8 km2 and an average depth of 2.3 m (Dhargalkar and Shaikh, 2000). The Atlas codes most of the southern lagoon area as a sandy inner reef flat whose area is not included. The northern lagoon is sandy although with the central region is rubbly. The shallow lagoon supports several species of Acropora with mushroom corals (Fungia) living among them (Pillai and Jasmine, 1989), although shallow reefs here suffered from considerable mortality during a severe bleaching event in 1998 (Arthur et al., 2006). About 52% of the lagoon is covered with seagrass at depths of 2–5 m and the resident green turtle population they support has been variable (Tripathy et al., 2002; Kelkar et al., 2013). These relatively large areas are not coded as such by the Atlas. Kavaratti is an atoll located 67 km south–southwest of Kadmat where it takes the form of a compressed D (Figure 20.5). The platform is 5.6 km

northwest–southeast, about 3 km at its widest to the northeast, and covers an area of about 15 km 2 including uncoded reefs to the southwest. The outer reefs are generally widest to the northwest where they are up to 975 m wide. Reefs to the southwest occupy a smaller extent of the platform but are more than 1500 m wide. Coral cover was about 19% in 2001 (Arthur  et  al., 2006). The rim is dominated to the east by lambchop-shaped Kavaratti Island with a 2011 population of 11,200. A reef flat extends around the island and continues clockwise to the mid-west. The rim is open. The lagoon is a 2.9 km2 trough between the outer reefs and the island and contains massive Porites and branched Acropora corals, especially in the northern half at depths of 1–3 m (Harithsa et al., 2005). The lagoon is just over 3 m at its deepest (Purdy, 2001). A channel has been dredged and blasted to the north of Kavaratti to introduce an inter-island ferry system and resulted in extensive coral mortality (Nowshad et al., 1998). Septic tank leakage and other population-related environmental issues continue to be problematic. However, seagrass and the resident turtle populations they support appear to remain stable with as many as 14 individuals reported in 2008–2009 (Kelkar et al., 2013). Large seagrass areas are not shown by the Atlas. Kalpeni Atoll is located 123 km southeast of Kavaratti where it resembles a warped crescent-like form (Figure 20.6). The platform is 11.6 km north–south, 3.7 km west–east at its widest, and covers an area of about 37 km2. Outer reefs surround the platform and are commonly 450 m wide to the west although one area is more than a kilometer wide. A shallow (1–2 m deep) trough to southwest is covered with seagrass meadows. The eastern outer reefs are about 250–270 m wide. The rim to the west is composed of reef flats that are continuous southwest–mid-west. The rim then becomes discontinuous for a distance of 3.8 km to the northwest. The rim to the east supports two main islands. Cheriyam is the northernmost of the two and stretches 2.9 km along the northeast rim and is about 375 m at its widest. The other, Kalpeni Island, occupies the southeast rim and is axe-shaped, 1.4 km wide at the blade to the south with a handle extending 3.3 km to the north. The total area of the two islands is 2.8 km2 and both are densely planted with coconut palms. Four islets that occur to the west of the blade account for an additional 4.3 ha. The rim is open. The population of these islands in 2011 was 4419. The lagoon covers an area of about 14.2 km2. Its shallows are covered with sand deposits that extend to the slope. Reefs are best developed on the slope near the lagoon center with additional patch reefs to the north slope. The maximum lagoon depth is 5 m (Nowshad et al., 1998).

Atolls of the Central Indian Ocean

Figure 20.6  Remote-sensing images of the atolls of Kalpeni and Minicoy. Images © 2022, Planet Labs PBC.

Minicoy is an isolated pear-shaped atoll located about 200 km southwest of Kalpeni and is the southernmost of the Lakshadweep Islands, separated from Kalpeni and Suheli Par to the north by the 9° Channel described above. The platform is 10.3  km northeast–southwest, 6.1 km west–east, and covers an area of 41.3 km2. The outer reefs surround the platform and are best developed to the northwest where they are up to 950 m wide. Reefs are about 250 m wide to the east. The rim is a continuous reef flat that extends midwest–northeast counterclockwise, sections of which become dry. The rim mid-west–northwest consists of short reef flat areas with shallow communications to the lagoon. The rim to the northeast is s­ubmerged covering 1.7 km and is the main entrance to the lagoon with a depth of up to about 4 m (­Sailing Directions, 2022). The reef flats support several ­ species of Porites and species branching species of Acropora (Menon and Pillai, 1996). The eastern rim supports 11.5-km-long Minicoy Island, shaped like a hockey stick. The blade is up to 900 m wide and narrows to about 800 m wide where most of the population is found (10,500 in 2011). The western end of the blade is recurved and serves as a small habitat for mangroves and their associates (Vargas and Pillai, 2005). The handle extends 3.2 km to the northeast of the settlement and is no wider than about 115 m. Considerable beach alteration has occurred due to the removal of natural vegetation by the Agriculture

Department (who thought it harbored rodents), for sand mining to produce lime and cement blocks for houses, and ironically, for more cement blocks used in armoring the beach to prevent the beach erosion caused by the other activities. The lagoon covers an area of 22.5 km2 and supports several species of Acropora and mushroom corals (genus Fungia) in the shallows (Menon and Pillai, 1996). Ridges and mounds are formed in the shallow western lagoon, some of which are coded as reef. The shallows are also dominated by sand deposits (broader to the west) that extend to the lagoon slope. A large portion of the latter area is covered by reef development and the Atlas codes a small area of seagrass in its center. However, Tripathy et al. (2006) report that more than 55% of the lagoon supports a seagrass community, the largest recorded among the Lakshadweep atoll group. The average lagoon depth is 4 m with a maximum depth of 15 m (Nasser et al., 1998).

The atolls of the Maldive Islands The Maldives are of note as they represent the linguistic origin of the word atoll. This English phrase is an Anglicized version of the Dhivehi ‘atholhu.’ In addition, the Maldives island chain is one of the largest and one of the most studied atoll groups in the world. The Maldives form a chain composed of 21 atolls, small islands, and four with lagoons that have

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become filled with sediment (Thoddoo, Kaashidoo, Foahmulah, and Alifushi). The group extends approximately 1,000 km from 8° Channel to Addu about 1°S with islands of a mean elevation of 1–1.5 m above sea level (Figure 20.1). These structures are situated atop the Chagos–Laccadive submarine ridge (hereinafter referred to as the ridge), the same system that supports the Lakshadweep atolls to the north. Many are more clearly associated with either the windward west (see below) or the eastern side of the ridge, where a central bathymetric low 300–500 m deep called the Inner Sea more clearly separates the two sides and extends from South Malosmadulu to Kolumadalu, a distance of about 300 km. North–south breaks in the ridge include the Kardiva Channel, more than 500 m deep, and two at the southern end of the Maldives including the Suvadiva and the Equatorial Channels, both about 2,000–2,300 m deep (Figure 20.1).

Climate and Regional Oceanography The Maldives lie within the doldrums, an area of rising warm air, and no major storms affect most of the archipelago although historically they have been susceptible to tsunamis generated from Indonesian earthquakes (Klosterman et al., 2014). However, models indicate

that the very steep bathymetry of these atolls may be extremely efficient in absorbing and redirecting many incoming tsunami waves (Rasheed et al., 2022). The paucity of major coastal impacts during the major 2004 Banda Aceh tsunami that devastated coastal areas of Sri Lanka and India is consistent with this notion. The major winds (Figure 20.7) influencing the climate of the Maldive Islands are monsoonal. Winds from the west predominate for 8 months of the year with energy levels that are 4.5–7 times that from other directions. In general, the western rims of Maldivian atolls are referred to as the windward side (Kench et al., 2006; Gischler et al., 2014). Tropical cyclones are rare, with an average of just over one per decade impacting the northern part of the Maldives. High-energy waves at South Malosmadulu (Figure 20.7) are most commonly from the west to southwest and can reach significant heights of up to 4 m, reflecting that these are remotely generated swell. In the northern part of the Maldives, large waves (heights up to 5 m) come from the west, but in the northeastern part, waves also can come from the northeast. Conversely, winter monsoons generate marked swell that affect the rims of the east-facing sides of the atolls. As a result of these complexities, the character of waves affecting any individual atoll or part of an atoll is influenced by their latitude,

Figure 20.7 The major winds influencing the Maldives are monsoonal. Winds from the west predominate for 8 months of the year with energy levels that are considerably higher than those from other directions. High-energy waves at South Malosmadulu are most commonly from the west to southwest and can reach heights of up to 4 m, reflecting that these are remotely generated swell. In the northern part of the Maldives, the character of waves affecting any individual atoll or part of an atoll is influenced by their latitude, location (east or west flank), the season, and shadow effects by other atolls. Tides at South Malosmadulu are mixed semidiurnal, with two high tides and two low tides per day, but with different strengths. Pronounced currents driven by the tides also propagate through and around the atolls.

Atolls of the Central Indian Ocean

location (east or west flank), the season, and shadow effects. Tides at South Malosmadulu (Figure 20.7) range by almost 1.0 m at spring tide and 40 cm at neap tide. They are mixed semidiurnal, with two high tides and two low tides per day, but with markedly different strengths. In addition, pronounced currents driven by the tides propagate through and around the atolls. These tidal currents flow through channels and induce sediment transport in high-velocity areas that can have a scouring effect. Silt and mud accumulate in other channels—even those within the same atoll—that are more protected and shadowed by the presence of other atolls (Rasheed et al., 2021). Thus, the sediment of lagoon floors can vary from coarse and coral-rich to mud depending upon the channel environment (Gischler, 2006). The southerly Maldive atolls (Suvadiva and Addu) are in the path of the westward flow of the Equatorial Current (Figure 20.1). While Addu is affected by monsoonal wind systems, they are more variable than those to the north and reach greatest average speed in May and October (Sailing Directions, 2022). In addition, while Addu lies in the fringes of the eastward-flowing South Equatorial Countercurrent, it becomes even better developed farther to the south. Similar to the Lakshadweep Archipelago, most atolls are located on either the west or the eastern side of the ridge. Depths of lagoons increase from north to south, commonly interpreted to be due to greater precipitation toward the equator which enhanced dissolution of limestone during sea-level lowstands. Wind and wind-induced waves and sediment transport are present-day factors in shaping the morphology of these atolls, particularly away from the equator where storms become stronger and increase the number of lagoonal reefs and channels, especially on larger atoll rims. Another feature of Maldivian atolls is the formation of rims composed of separate sections of reef flat, many of which form faroes, small circular reefs with their own lagoons that can be as deep as 30 m. Many faroes include extensive sand deposits. In addition, most of these atolls form faroes within the lagoon as well as on the rim. Thus, the atolls broadly define an elongate ring-like organization; this pattern may be reflected in the name, as the name ‘Maldives’ may be a derivative of the Sanskrit maladvipa, which means ‘garland of islands.’ Although the mechanics of their formation is controversial, faroes are most common in the central-north Maldives from about 3°N to 7°N, an area that overlaps with the pathways of the summer and winter monsoon wind reversals. Likewise, the number of 10–40 m deep cleft-like channels within the

reef margin increases southward from the equator and appears to be related to the influence of monsoons. Such channels allow water to flow around and through atolls as well as across the ridge, contributing to complex local circulation patterns (Gischler et al., 2014; Su et al., 2021). West-facing atolls tend to develop wider reef flats and sand deposits or aprons to the west, whereas the opposite is true for east-facing atolls, reflecting the predominant wave and current patterns impacting the respective side. Whereas we remeasured and recounted parameters in some cases, most of the numerical descriptions and the spellings of the considerably variable place names below follow Gischler et al. (2014). Although a through inventory of Maldivian corals was made by Pillai and Scheer (1976) of 241 species, mass mortalities and tsunamis have taken their toll. In 1997–1998, before the mass bleaching event, Maldivian reefs were dominated by hard and soft corals, followed by algae and sponges. Hard coral cover values remained around 50% until the massive mortality of 1998, when it dropped to  20m deep

Lighthouse

120 m

Crescent

301 m, world’s deepest

(Continued)

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Table 23.2 (Continued)  Summary and Characteristics of Atolls Militarized

French Polynesia

United States

China

Malaysia

Mururoa

Johnston

Crescent

Swallow

Fangataufa

Wake

Fiery Cross

Bikini

Gaven

Enewetak

Mischief Subi

High Human

Maldives

Marshalls

Kiribati

Population

North Malé

Majuro

Tarawa

Density

*

Privately Owned

Ebeye

French Polynesia

Maldives

Seychelles

Fiji

Anuanurunga

Gaafaru

Ile Desroches

Wailangilala

Manuhengi Marutea Sud Taiaro Tetiaroa

Selected Almost-Atolls

Aitutaki, Cook Islands Budd Reef, Fiji Clipperton Reef, Eastern Pacific Chuuk Lagoon, Caroline Islands Deboyne Islands, Louisiade Arch., New Guinea French Frigate Shoals, NW Hawai’i Hermit Islands, Admiralty Islands, New Guinea Mangareva Islands, SE Tuamotu Archipelago Maupiti, Society Islands North Astrolabe Reef, Fiji Oneata, Fiji Oni-i-Lau, Fiji Vanuabalavu, Fiji

* Does not include islands for sale that are part of an atoll, e.g., South Malosomadulu.

of interest. The ‘world’s largest atoll’ is often referred to, but that depends on the context. The Macclesfield Bank (South China Sea) would certainly qualify as the largest with a platform area of about 23,500 km2 and a lagoon 50–70 m deep, but it is completely drowned (Huang et al., 2020) and most of it is beyond the current imaging capacity of satellites. We therefore do not include it and instead offer the largest of the

atolls described in this volume in terms of platform size, land area, lagoon dimensions, and lagoon depth. Seagrasses often are not well represented on atolls except in the lagoon or the lee of islands or the leeward side of the rim. However, there are several atolls where seagrasses cover relatively large proportions of the lagoon, and these are listed in Table 23.2 along with outstanding atoll dimensions, an attribute

A Summary of Atolls and Their Future

commonly mentioned in the lay literature, often incorrectly. Geomorphologists are interested in other atoll features including reticulate lagoons as a means of assessing the effects of lowered Pleistocene sea levels and subaerial exposure, although other origins such as self-organization or colonization of dunes and other high spots by reef builders also are possible (Schlager and Purkis, 2015). Lagoonal reticulate reefs described in the text are listed in Table 23.2. Likewise, blue holes are water-filled vertical solution openings in the carbonate rock that exhibit complex morphologies, ecologies, and water chemistries. Three have been described from atolls and those from the text are included in the table. Mangroves are also limited in their distribution on atolls but are notable in a few places as listed in Table 23.2. Like seagrasses, they provide important ecological services including habitat for numerous species that use them for refuges, nurseries, and migration sites. They also prevent erosion using their extensive root systems, take up nutrients such as nitrogen and phosphorous, and through bacterial decomposition of their leaf litter provide the basis of a detrital food chain with a high level of productivity. Several atolls have been or are currently being used for military purposes and those described in the text are listed in Table 23.2 as are heavily overpopulated atolls from several atoll groups as described in the text and expanded below in terms of limited land, water, and food supplies. Lastly, we list as many atolls as we can find that are privately owned as a point of interest.

Atolls of the 21st century At least 30 countries claim atolls and islands on them, and in some areas, these claims conflict. In several nations, atolls represent the only habitable land masses. Although in detail these islands have varied geological origins, they—and the people and nations that inhabit them—share many common challenges. The purpose of this part of the chapter is to briefly introduce a few of the pervasive issues that face residents of atoll islands; by necessity, we cannot be exhaustive or comprehensive. Many of these issues are interconnected and represent formidable challenges to the people who live on atolls.

ENSO and coral bleaching We already have discussed El Niño (see Kiribati, Chapter 9) as it generally affects the sea-surface temperature and is associated with extensive coral bleaching. The 2015–2016 El Niño-associated warming event caused mass coral bleaching in all tropical

oceans and is one three such events that have occurred on a global scale (1982/83, 1997/98, and 2015/16) and during which some reefs experienced a massive 98% loss of their coral fauna (Vargas-Ángel et al., 2019). Even La Niña events that normally bring cooler waters to a number of the tropical areas have brought thermal anomalies and bleaching to atolls on the North West Shelf of Australia (Le Nohaïc et al., 2017). In addition, there are severe heatwaves and bleaching events that occur in non-El Niño years in disparate places such as Fiji, Papua New Guinea, and Rocas Atoll in Brazil (Oliver et al., 2018; Gaspar et al., 2021). Coral species that are naturally resistant to bleaching, their transplants from nurseries, and the rapid speed of recovery in areas that are not subject to repeated marine heatwaves are all positive signs but are works in progress. Projected declines in coral reef habitats in the face of climate change and population increases are beyond the scope of this work but are summarized by numerous publications (e.g., Douglas, 2003; Lesser, 2011; Morikawa and Palumbi, 2019; Eddy et al., 2021, among many others).

Freshwater In addition to bleaching and heatwaves that affect reefs everywhere, there are related challenges that are more immediately relevant to those who live on atolls: limited land and a paucity of freshwater due to high population levels, or droughts, or both. A sustainable supply of freshwater is an essential element for life on remote and often highly dispersed atolls; as Albert Szent-Gyorgi once said, ‘Water is life’s matter and matrix, mother and medium. There is no life without water.’ Indeed, the question of whether historical settlement occurred or not was often traceable to the quantity and quality of the freshwater supply. Even in cases where the supply may be sufficient, it may become curtailed due to a variety of causes, some of which are described below. Rainfall is a primary source of water for areas where it is adequate, and it is typical to collect rainwater from roof and gutter systems that direct rainfall to cisterns. The number of these systems, however, is often unknown as it is on Tarawa where it is estimated that only 28% of households (those equipped with a steel roof) are equipped with a rainwater tank and only about two-thirds of these are functional (Moglia et al., 2008). In addition, there are health considerations including fecal contamination by animals and from soil bacteria around the cistern including Legionella, a bacterium commonly found in soil and plant litters

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in tropical regions. Legionnaires disease can be contracted by simply showering with contaminated water. Storage of cisterns above ground with covers to protect from heat reduces ground contamination and water sterilization protocols have been implemented in many areas (Wallace and Bailey, 2014). In some areas, atolls have installed desalination systems to augment roof collection, but these are expensive to build, maintain to run reliably, and are commonly powered by diesel plants or by photovoltaic systems with storage batteries (e.g., Quon et al., 2021). Where rainfall is sufficient, the permeable carbonate sand and gravel substrate of atolls may allow the percolation of rain to an underground unconfined aquifer referred to as a freshwater lens. On atolls, lenses form a convex-shaped layer of fresh groundwater that floats above denser saltwater, with a gradual (brackish) mixing zone between the two layers that typically ranges from 2.5‰ to 95‰ of seawater salinity. The lens is usually between less than 2 and 29 m thick on atolls, but its thickness, size, and shape are strongly influenced by the island size, rainfall and evaporation rates, the nature of the subsurface sediments, and oceanic tidal forces (amplitude and current strength), among other factors (Figure 23.1; Werner et al., 2017). Nations such as the Republic of Maldives in the Indian Ocean, the Federated States of Micronesia, the Republic of Marshall Islands (RMI), and the Republic of Kiribati in the Western Pacific Ocean are especially vulnerable due to the prevalence of droughts (see below). Indeed, both the Marshall Islands and Kiribati are among the countries with atolls that may become uninhabitable in the event of worst-case drought scenarios. Average lens thickness in the Marshall Islands during typical seasonal rainfall is approximately 4 m, and 55% of the islands

form a lens less than 2.5 m thick (Barkey and Bailey, 2017). The vegetation on small carbonate islands generally consists of a limited range of bushes, grasses, and trees. On many atoll islands, the native vegetation has been partially cleared and replaced with food crops especially coconut palms in the form of plantations. The coconut tree is remarkably salt tolerant and can grow in water with relatively high salinity levels. In addition, the mature root system is deep, typically 2–3 m below the surface, and is highly efficient at extracting freshwater, especially compared with broadly branched, bird friendly trees that cannot compete with the removal of up to 150 L of water per day by mature coconut trees (Krauss et al., 2015). Compounding these characteristics, coconut plantations on atolls including many in Kiribati and the Marshall Islands consume about 50% of the annual rainfall. (Wallace and Bailey, 2017).

Stresses on the water supply One of the most critical natural stresses on water supplies is drought, especially due to ENSO events, and these are location dependent. The Marshall Islands and the western Caroline Islands commonly experience extended droughts during El Niño periods, whereas central Pacific atolls, Kiribati in particular, exhibit droughts during La Niña episodes. On Tarawa (Gilbert Islands of Kiribati), for example, such episodes can last up to 43 months and currently occur with a frequency of 6–7 years (White et al., 2007a). During these drought periods, freshwater lenses decrease by up to 50%, markedly diminishing the water supply and increasing the salinity of groundwater. Drought-related challenges are expected to be

Figure 23.1 Freshwater lens developed below an atoll island with transitional brackish layer and ocean water beneath it. The roofed feature represents a schematic well. Arrows represent general water movement—downward expansion and mixing of freshwater with salt water, upward flow from more permeable limestone bedrock, and lateral intrusion by tides. (Based on US EPA https://climatechange.chicago.gov/climate-impacts/climate-impacts-us-islands.)

A Summary of Atolls and Their Future

exacerbated by global climate change, as enhanced ENSO variability and intensity will result in more extreme and persistent droughts in some areas. Although the timing, intensity, and duration of droughts vary across the globe, they persistently have pronounced impacts. For example, a long La Nina drought impacted Tuvalu in 2011–2012. During this period, supplies from centralized water sources were rationed to 2.1 L/person/day, and over 60% of household bathed and washed clothes using solely well water that was brackish. Similarly, the Samoan government had to charter boats to supply Tokelau’s atolls with 97,000 L of water during the 2010–2011 drought (Lese et al., 2021). Droughts impact atoll agriculture as well. The crops that grow on atolls are naturally resilient but are also quite limited. During droughts, staples such as bananas, breadfruit, and swamp taro can wilt, die, or rot and be unfit for human consumption. Likewise, they can lead to increases in pests, such as parasites or weevils, and change nutrient values of some crops. These droughts can even affect the marine realm, as illustrated by expanded growth of the invasive seaweed (Sargassum polycystum) in Tuvalu during the La Niña drought of 2011 (N’Yeurt and Lese, 2015). The decomposition of the algae, in

turn, resulted in contaminated groundwater when it was driven onshore.

Increasing atoll populations Despite their small area, many atoll islands maintain high population densities that place great stress on water resources by extraction, land use, and contamination. An extreme example is the island of Malé in the Republic of Maldives (Chapter 20) where a population of 250,000 (30,000 individuals per km2) live, but the situation there is not unique. Betio in Kiribati (Chapter 9) and Majuro and Ebeye in the Marshall Islands (Chapter 11) also have population densities comparable to Tokyo or Hong Kong. These large, densely settled islands not only create high demands for water but also increase the risk of pollution. Fecal contamination caused by sewage system overflows, leakage from unsealed septic tanks, the use of pit latrines are common problems on heavily populated atolls. In locations without running water, residents use the beach as a toilet, one that flushes twice a day. Likewise, pig farming is common, and in Kiribati, for example, there is an average population of one pig per three people (Figure 23.2). One of the most important factors influencing groundwater

Figure 23.2 Photo of household with a pig in a pen, adjacent to an open water well. (Photo from Betio, Kiribati, by Gene Rankey.)

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contamination is the depth of the water table that may be less than 2 m below the surface. In porous and permeable reef-derived sediment, surface contaminants can reach groundwater in less than 2 hours. Indeed, fecal contamination of groundwater is a major source of gastroenteritis in atoll communities, causing high infant mortalities and outbreaks of diseases including hepatitis, typhoid, and cholera (White et al., 2007b). The location of landfill dumps and cemeteries on atolls is also a concern for water contamination (Spennemann, 1999). On some atolls such as Atafu in Tokelau, small, isolated islets are designated as burial grounds. On others such as Kiribati or the Cook Islands, however, it is common to bury the deceased on family land. In situations where crowding is extreme, burial plots are located immediately next to houses. A tradition remains that those graves are honored and not disturbed until no living person can personally recall the deceased. At that time, the land can be re-purposed.

Sea-level change Similarly, a central concern for most low-lying atoll nations is sea-level rise, which has been affecting many atoll systems for millennia; recall that many Pacific atolls were not settled before a sea-level fall a few thousand years ago exposed reefs and initiated islands. Predictions are that sea level may continue to rise by up to 2 m by end of the 21st century (Bamber et al., 2019). Given that the entire land mass of some nations such as the Marshall Islands, Tokelau, Maldives, and Kiribati lies less than a few meters above present-day sea level, a pronounced sea-level rise represents a threat that could simply overtake and drown entire islands and island nations. Yet, even before such an extreme outcome, sea-level rise could have pronounced effects. For example, high tides that at one time were not a threat are now flooding low-lying areas, including those that have high populations (Figure 23.3). An additional impact of higher sea level is an increase in frequency and intensity of storm-wave-induced overwash and island inundation. Higher sea level favors passage of larger waves across the reef flat to islands. In the case of storm waves, these larger waves can more frequently overtop the low-lying islands. Storm surge that accompanies tropical cyclones pushes seawater allowing large wind-driven waves to sweep either partially or completely across an atoll (Figure 23.3). Although on many atolls today the recurrence interval, or the time between successive events, is commonly sufficient for infrastructure and resources to recover, larger waves directly hitting

atolls with increased frequency will have two major effects. First, it can degrade or ruin water supplies in the freshwater lens. In one example of storm impact to an atoll’s freshwater lens, Cyclone Percy, a category 5 storm, swept directly across Pukapuka Atoll in the Northern Cook Islands in 2005 (Terry and Falkland, 2010). The total land area of the three islands on Pukapuka is 3.8 km2. With passage of the cyclone, almost all houses were destroyed or damaged. Flood conditions wiped out food resources as well as roof water catchment systems. Coconuts could not be used for emergency supplies as most had been torn from trees and rotted on the ground. The storm surge swept ocean water over the islets causing saltwater to penetrate to the aquifer as well as destroying the taro plantations. The signature of aquifer destruction involved the thinning of the freshwater lens and thickening of the brackish layer by 1–2 m. As a result, an emergency shipment of drinking water from the Red Cross had to be transported from American Samoa. The aquifer contamination lasted for at least 11 months and may have lasted longer had it not been for heavy rainfall in the months that followed the storm. A second impact of more frequent or more pronounced inundation is destruction of agriculture, as many of the crops grown on these islands are not tolerant of brackish water. For example, a common crop is swamp taro, a large herbaceous perennial plant that can reach up to 5 m in height with leaves a meter or more wide (Figure 23.4). The starchy corm can weigh up to 10 kg or more within a year or two of planting. Because of its hardiness, it has become a traditional food in atoll environments, and it is sometimes called the potato of the Pacific. It is cultivated in large numbers in natural depressions or in pits hand-dug to the water table. Low-lying central areas are a feature of many islands and are often locally excavated for production of taro. Although they are sometimes described as salt tolerant, most are not, even though there are some cultivars that can withstand up to 5‰ salinity. More commonly, optimal growth is achieved at lower salinities (Rao et al., 2014). These lower-lying areas are already subject to inundation, which seems certain to increase as the sea rises (Woodroffe, 2008). Another effect of sea-level rise is related to the very existence of islands as landforms. Although some islands include rocky outcrops, most atoll islands are made of sand and gravel derived ultimately from the nearby reef and reef flat. These ‘soft’ landforms are naturally at risk from erosion. Existing models indicate that sea level may exceed 2 m above present by the end of this century. For atoll islands with average

A Summary of Atolls and Their Future

Figure 23.3  (a) A king tide, which followed a storm surge, inundates residences in Majuro in the Marshall Islands in 2014. Credit: Karl Fellenius, University of Hawai’i Sea Grant, U.S. Geological Survey, https://www.usgs.gov/media/images/king-tidefollows-flooding-storm-surge. (b) Schematic of pre- and (c) post-inundation distribution of fresh- and brackish water on a small atoll island. Sea-level rise, storm surge, king tides, or a combination of the three can lead to overtopping islands and degrading the freshwater lens. (Modified from Storlazzi et al. (2018) as cited.)

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Figure 23.4 Photo of taro crops. These large plants thrive in low-lying areas, so are commonly planted in pits dug in the interior of islands. (Photo by Gene Rankey from Nonouti Atoll, Kiribati.)

elevations less than 2 m above present sea level, this poses an existential threat. Reefs, which form natural wave breaks, are already challenged by bleaching and disease, and thus may be unable to keep up with sea-level rise, leading to deeper water across the reef and reef flat. Likewise, as discussed above, with this deeper water, more wave energy will pass across the reef flat and threaten to erode islands (Figure 23.5a). In the worst-case scenario, with no new sediment supply (Perry and Morgan, 2017), this increased energy will erode the islands and wash them back to the sea (Storlazzi et al., 2011; 2015). To mitigate this possibility, many atoll governments attempt to modify their shorelines by offshore dredging to artificially replenish sediment to the shore. In some cases, artificial islands are created as shown for Subi Atoll in the South China Sea (Chapter 19). One extreme example is described below, but even smaller-scale cases can be deleterious as they include sand and coral rubble mined from the very islands or beaches they are designed to protect (e.g., Duvat, 2013; Minicoy Atoll Chapter 20). Another less pronounced—but widely used— method utilizes coastal engineering approaches to slow erosion. These sorts of approaches often take the form of seawalls made of coral rubble (Figure 23.5b) or concrete. These features, which rigidly armor the beach, are designed to hold the shore in

place. Nonetheless, they can have the effect of reflecting wave energy, which can increase erosion at the toe of the wall. Likewise, the changes in energy and decreased rate of longshore sediment supply past the end of the seawall can drive erosion there. In time, the seawall can enhance erosion of the remaining beaches on the same shoreline. Another approach utilizes groins, solid structures built perpendicular to the shoreline (Figure 23.5c) that either armor or slow longshore transport with the objective of mitigating shoreline erosion. Each groin acts effectively as a local ‘dam’ that traps sediment updrift and causes a deficit of sediment downdrift, leading to erosion. A net effect is destabilization of the entire shoreline, often resulting in chronic erosion. These sorts of negative impacts led the U.S. Army Corps of Engineers’ Coastal Engineering Manual to characterize groins as ‘probably the most misused and improperly designed of all coastal structures’ (US Army Corps of Engineers, 2002), and many governmental entities have outlawed their construction. The net result of these sorts of activities is that human attempts to modify shorelines can lead to more issues than they solve. In many instances, especially in populated areas, erosion of both lagoon and ocean shorelines has been attributed largely to human impacts (Webb and Kench 2010), as there

A Summary of Atolls and Their Future

Figure 23.5 (a) Erosion of a shoreline, Maiana Atoll, Kiribati. (b) Seawall made of coral rubble, Bauriki, Tarawa Atoll, Kiribati. (c)  Remote-sensing image of jetties, Maafushi, South Malé Atoll, Maldives. (Photos in (a) and (b) by Gene Rankey.) Remote sensing image © 2021, Planet Labs PBC.

is less change in uninhabited areas. Many authors suggest that additional modification will further disrupt natural processes and increase susceptibility of the islands to erosion from anticipated future sealevel rise (Ford, 2012; Biribo and Woodroffe, 2013; Duvat, 2018). Nonetheless, although sea level is rising in many areas, not all atoll islands are simply disappearing. To the contrary, at a global scale, there has not been a reduction of land areas on atolls, but rather an increase, although much of the expansion is related to large-scale reclamation projects (Holdaway et al., 2021). This general trend has been documented at the scale of individual atolls as well; several studies have suggested that—far from eroding away—many islands are increasing in size, and many atolls have seen increases in total land area during the past few decades of sea-level rise (Webb and Kench, 2010; Rankey, 2011; Kench et al. 2018; Duvat and Pillet, 2017; Duvat, 2018). The dynamics and persistence of atoll islands remains an active research topic.

Adaptation attempts The United Nations Framework Convention on Climate Change (UNFCCC) now has a policy instrument to assess and consider responses to losses arising from climate change, especially for small island states. This treaty, known as the Warsaw International Mechanism for Loss and Damage, recognizes that ‘the adverse effects of climate change includes that which can be reduced by adaptation’ (Barnett, 2017). Indeed, many atoll residents are adapting, and some efforts are described above. Nation-scale efforts have increased following development of the Kiribati Adaptation Program (KAP) around the turn of the century. Many atolls and atoll nations now have large, commonly international aid-driven adaptation programs. These might be considered to follow five avenues: (a) protect, (b) raise, (c) reclaim, (d) relocate, and (e) migrate. The scope of protective projects ranges from attempting to establish sustainable-yield freshwater resources to planting mangroves for

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Figure 23.6 Mangrove planting effort, South Tarawa, in May 2012. These mangroves are the stand that Ban Ki-moon, Secretary-General of the United Nations (2007–2016), helped in planting. None survive today. (Photos courtesy of Arthur Webb, Tuvalu Coastal Adaptation Project.)

shoreline protection (Figure 23.6). Although mangroves are known to resist erosion and flooding, planting has met with mixed success on atolls due to lack of maintenance or poor positioning. Tarawa is a good example of both, where success was achieved in the sheltered water to the east but failed on the exposed north side of the atoll. Low elevation is an additional issue of where to plant due to increase in the length of time that the mangrove is inundated, which causes oxygen stress to most mangrove species (Ellison, 2020). More recent ‘raise and reclaim’ tactics have similarly focused on in situ adaptation and included more

centralized approaches including defining plans for long-term expansion and preservation of land by elevating government islands. In one of the first largescale adaption developments for small island nations, the government of Kiribati presented at the 2017 UN Climate Change Conference specific plans for relocation of up to 35,000 people to a 300 ha reclaimed marshy area on Tarawa Atoll. The project, which is expected to take 30 years, would cost US$273 million for land reclamation alone. Nonetheless, it remains on hold as of 2023. These costs are staggering for small island states. But residents see adaptation as akin to survival, thus

A Summary of Atolls and Their Future

imperative, and worthy of the expense. By some accounts, Maldives government spends more than 50% of the nation’s budget on adaptation. One innovative ‘relocate’ solution was creation of an artificial island—known as Hulhumalé and nicknamed the ‘City of Hope.’ Located near the capital city of Malé, Hulhumalé was created by sand dredged from the ocean floor. The island towers a full 2 m above sea level—more than twice the elevation of Malé—and may ultimately be home to nearly as many people as in Malé itself once completed by 2035. In the extreme cases, however, some residents may need to leave their homes as migrants. For example, to start transferring residents and culture elsewhere, many Pacific Island countries have bilateral agreements with larger neighbors such as Australia, New Zealand, and France to educate, train, and even host island residents as guest workers abroad. The former president of Kiribati, Anote Tong, formalized this approach as the Migrate with Dignity (MWD) Strategy. He envisioned a means to relocate entire communities, mitigating the environmental threats while preserving the culture of the I-Kiribati into the future. These sorts of initiatives have been successful; for example, as many as 9,000 Tokelauans live in New Zealand, a much greater number than the less than 2,000 who remain on those atolls. In another extreme, in 2009, Maldives discussed the possibility of a land purchase in another country. In 2014, the government of Kiribati purchased the Natoavatu Estate in Fiji for economic development and food security, although the government unofficially suggested that it perhaps could host up to 70,000 I-Kiribati. They paid AU$9.3 million at the time, a price that was four times the average per-acre price of comparable sales, and to date, no I-Kiribati have moved. They do, however, raise cattle there. The fact that these sorts of discussions of mass relocation and island/nation un-inhabitability are even on-going and becoming more common at international gatherings raises questions as to what would happen if an atoll or a group of them disappear completely. The initial salvos of this struggle have been manifested in courts, where residents claim migrant status as climate refugees. Representatives from Kiribati in 2015 even requested a status of ‘climate citizenship’ for future climate migrants before the United Nations Human Rights Council, but to date no court has granted such a status. The prospects outlined above provide a dim outlook for atoll futures that are mirrored as part of the ‘disappearing islands’ narrative in the popular press. On some atolls (and elsewhere on larger tracts of land), there are those who may believe the scientific prediction of climate change but are confused

by its complexities. A few other believers in science see atolls as sacrificial laboratories. ‘Wishful sinking,’ as some have put it, occurs as a perverse desire to see a few atolls disappear for the planet to be saved ­(Farbotko, 2010). There are also those who disbelieve either because they distrust scientists or because they believe God’s promise in Genesis that the earth will never again be flooded (Rudiak-Gould, 2011). Yet even for those who do follow the science, there are many gaps in observations and predictive knowledge that require filling. Changes in rainfall, the nature or impact of increasing cyclone frequency or intensity, the duration, and the frequency or intensity of meteorological and agricultural drought are also difficult to predict. In addition, the science is complex and involves the accumulation of risk from multiple drivers, only some of which have been mentioned here (e.g., Lese et al., 2021; Duvat et al., 2021). Atolls are still natural laboratories of sorts, now mostly as climate-change canaries. At one time, the physical isolation of atolls was their saving grace. Not anymore. Atolls tell us that as human populations become larger and more concentrated, the local atoll environment becomes degraded. Thus, a distinction can be drawn between those atolls and islands that are populated and those without people (Sandin et al., 2008; McCaulay et al., 2013; Biribo and Woodroffe, 2013). Yet, in addition to these local factors, challenges such as anthropogenic climate changedriven variability in ocean acidification, sea level, and sea-surface temperature, the pervasiveness of trash and microplastics, and widespread overfishing make clear that solutions must be global in scale. The resolution of issues—if even possible—will require not only action from citizens of atolls but also effort from the global community. We hope that this volume will provide a small step toward recording the delicate form and nature of these reefs and atolls as they now appear and motivate citizens of the world to focus effort to ensure that they continue to be the iconic structures and homelands that can be inhabited and appreciated by many future generations.

References Bamber JL, Oppenheimer M, Kopp RE et al. 2019. Ice sheet contributions to future sea-level rise from structured expert judgment. Proc. Natl. Acad. Sci. USA 166: 11195–11200. Barkey BL, Bailey RT 2017. Estimating the impact of drought on groundwater Resources of the Marshall Islands. Water 9(1): 41. https://doi.org/10.3390/w9010041 Barnett J 2017. The dilemmas of normalising losses from climate change: Towards hope for Pacific atoll countries. Asia Pac. Viewp. 58: 3–13. Biribo N, Woodroffe CD 2013. Historical area and shoreline change of reef islands around Tarawa Atoll, Kiribati. Sustain. Sci. 8: 345–362. https://doi.org/10.1007/s11625-013-0210-z

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Douglas AE 2003. Coral Bleaching—how and why. Mar. Poll. Bull. 46: 385–392. Duvat V 2013. Coastal Protection structures in Tarawa Atoll, Republic of Kiribati. Sustain. Sci. 8: 363–379. Duvat V, Magnan AK, Perry CT et al. 2021. Risks to future atoll habitability from climate-driven environmental changes. Wiley Interdiscip. Rev.: Clim. Change 12 (3), e700. https://doi. org/10.1002/wcc.700 Duvat VKE 2018. A global assessment of atoll island planform changes over the past decades. WIREs Clim. Change 10: e557. https://doi.org/10.1002/wcc.557 Duvat VKE, Pillet V 2017. Shoreline changes in reef islands of the Central Pacific: Takapoto Atoll, Northern Tuamotu, French Polynesia. Geomorphology. 282: 96–118. Eddy TD, Lam VWY, Reygondeau G et al. 2021. Global decline in capacity of coral reefs to provide ecosystem services. One Earth 4: 1278–1285. Ellison J 2020. Mangrove ecosystem-based adaptation: Advice on improved success. Human Ecol. 30: 37–40. Farbotko C 2010. Wishful sinking: Disappearing islands, climate refugees and cosmopolitan experimentation. Asia Pacific View‑ point 51: 47–60. Ford M 2012. Shoreline changes on an urban atoll in the central Pacific Ocean, Majuro, Marshall Islands. J. Coast. Res. 28: 11–22. Gaspar TL, Quimbayo JP, Ozekoski R et al. 2021. Severe coral bleaching of Siderastrea stellata at the only atoll in the South Atlantic driven by sequential marine heatwaves. Biota Neotropica 21: e20201131. https://doi.org/10.1590/1676-0611-BN-2020-1131 Holdaway A, Ford M Owen S 2021. Global-scale changes in the area of atoll islands during the 21st century. Anthropocene 33: 100282. Huang X, Betzler C, Wu S et al. 2020. First documentation of seismic stratigraphy and depositional signatures of Zhongsha atoll (Macclesfield Bank), South China Sea. Mar. Petrol. Geol. 117: 104349. Kench PS, Ford MR, Owen SD 2018. Patterns of island change and persistence offer alternate adaptation pathways for atoll nations. Nature Commun. 9: 605. https://doi.org/10.1038/ s41467-018-02954-1 Krauss KW, Duberstein JA, Cormier N et al. 2015. Proximity to encroaching coconut palm limits native forest use and persistence on a Pacific atoll. Ecohydrology 8: 1514–1524. https://doi. org/10.1002/eco.1601 Le Nohaïc M, Ross CL, Cornwall CE et al. 2017. Marine heatwave causes unprecedented regional mass bleaching of thermally resistant corals in northwestern Australia. Sci. Rep. 7: 14999. https://doi.org/10.1038/s41598-017-14794-y Lese V, Kiem AS, Mariner A et al. 2021. Historical and future drought impacts in the Pacific islands and atolls. Climate Change 166: 19. https://doi.org/10.1007/s10584-021-03112-1 Lesser MP 2011. Coral bleaching: Causes and mechanisms. In: Dubinsky Z, Stembler N (eds.) Coral Reefs: An Ecosystem in Transition. Springer, Dordrecht, pp. 405–420 McCaulay DJ, Power EA, Bird DW et al. 2013. Conservation at the edges of the world. Bioll. Conserv 165: 139–145. Moglia M, Perez P, Burn S 2008. Water troubles in a Pacific atoll town. Water Policy 10: 613–637. Morikawa MK, Palumbi SR 2019. Using naturally occurring climate resilient corals to construct bleaching-resistant nurseries. Proc. Nat. Acad. Sci USA 116: 10586–10591 N’Yeurt ADR, Lese V 2015. The proliferating brown alga Sar‑ gassum polycystum in Tuvalu, South Pacific: Assessment of the bloom and applications to local agriculture and sustainable energy. J. Appl. Phycol. 27: 2037–2045. https://doi.org/10.1007/ s10811-014-0435-y Oliver ECJ, Donat MG, Burrows MT et al. 2018. Longer and more frequent marine heatwaves over the past century.  Nat. Commun. 9: 1324. doi: 10.1038/s41467-018-03732-9

Perry CT, Morgan KM 2017, Bleaching drives collapse in reef carbonate budgets and reef growth potential on southern Maldives reefs. Sci. Rep. 7: 40581 Quon H, Allaire M, Jiang SC 2021. Assessing the risk of Le‑ gionella Infection through showering with untreated rain cistern water in a tropical environment. Water 13: 889. https:// doi.org/10.3390/ w13070889 Rankey EC 2011. Nature and stability of atoll island shorelines: Gilbert Island Chain, Kiribati, Equatorial Pacific. Sedimentol. 58: 1831–1859. Rao S, Taylor M, Jokhan A 2014. In vivo screening of salinity tolerance in giant swamp taro (Cytosperma merkusii). South Pac. J. Nat. Appl. Sci. 32: 33–36. Rudiak-Gould P 2011. Promiscuous corroboration and climate change Translation: A case study from the Marshall Islands. Global Environ. Change 22: 46–54. Sandin SA, Smith JE, DeMartini EE et al. 2008. Baselines and degradation of coral Reefs. PLoS One 3(2): e1548. https://doi. org/10.1371/journal.pone.0001548 Schlager W, Purkis S 2015. Reticulate reef patterns- antecedent karst versus self-organization. Sedimentol. 62: 501–515. Spennemann DHR 1999. No room for the dead. Burial practices in a constrained environment. Anthropos 94: 35–56. Storlazzi CD, Elias E, Field ME, Presto MK 2011. Numerical modeling of the im-pact of sea-level rise on fringing coral reef hydrodynamics and sediment transport. Coral Reefs 30: 83–96. http://dx.doi.org/10.1007/s00338-0110723-9. Storlazzi CD, Elias EPL, Berkowitz P 2015. Many atolls may be uninhabitable within decades due to climate change. Sci. Rep. 5, 14546. Storlazzi CD, Gingerich SB, van Dongeren A et al. 2018. Most atolls will be uninhabitable by the mid-21st century because of sea-level rise exacerbating wave-driven flooding. Sci. Adv. 4(4): eaap9741. https://doi.org/10.1126/sciadv.aap9741 Terry JP, Falkland AC 2010. Responses of atoll freshwater lenses to storm-surge overwash in the Northern Cook Islands. Hy‑ drol. J. 18: 749–759. US Army Corps of Engineers 2002. Coastal Engineering Man‑ ual. EM 1110-2-1100. US Army Corps of Engineers, Washington, DC. Vargas-Ángel B, Huntington B, Brainard RE et al. 2019. El Niño-associated catastrophic coral mortality at Jarvis Island, Central Equatorial Pacific. Coral Reefs 38: 731–741. Wallace CD, Bailey RT 2014. Sustainable water catchment systems for Micronesian atoll communities. J. Am. Water Res. Assn. 51: 185–199. Wallace CD, Bailey RT 2017. Geohydraulic factors governing atoll islands groundwater resources. J. Hydrol. Eng. 22. https:// doi.org/10.1061/(ASCE)HE.1943-5584.0001499 Webb AP, Kench PS 2010. The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis of island change in the Central Pacific. Global Planet. Change 72: 234–246. Werner AD, Sharp HK, Galvis SC et al. 2017. Hydrogeology and management of freshwater lenses on atoll islands: Review of current knowledge and research needs. J. Hydrol. 551: 819–844. White I, Falkland T, Metutera T et al. 2007a. Climatic and human influences on groundwater on low atolls. Vadose Zone J. 6: 581–590. White I, Falkland T, Perez P 2007b. Challenges in freshwater management in low coral Atolls. J. Clean. Prod. 15: 1522–1528. Woodroffe C. 2008. Reef island topography and the vulnerability of atolls to sea-level rise. Global Plant. Change 62: 77–96.

Index

I

Note: Bold page numbers refer to tables and italic page numbers refer to figures. A Abaiang Atoll 134, 134–135 Abemama Atoll 136, 137 Actéton Islands 66 Addu Atoll 368, 369 Adolphus Reef 243, 244 Agatti Atoll 353, 354 Ahe Atoll 54–56, 55 Ahunui Atoll 51, 52 Ailinginae Atoll 166, 167 Ailinglaplap Atoll 161, 162 Ailuk Atoll 176–178, 177 Aitutaki in Cook Islands 4, 7, 406 Alacran Reef 8, 17, 20, 391, 392, 393, 399, 400 Albuquerque Atoll 393–394, 394 Aldabra Atoll 377, 382–383, 382, 404 Aldabra Islands 378, 382–383, 382 Alicia Annie Atoll (Xiane) 333, 334 Alison Reef (Lisheng) 335–336, 336 Allen Atlas 2, 3, 8, 12, 18, 25, 32 Allen Coral Atlas 2, 8, 20 almost-atoll 5 formation of 7 list of 406 Alphonse Atoll 379–380, 381 Amanu Atoll 49–50, 50 Amirante Islands Atolls 377–381 Amphitrite Atoll (Xuande) 318–319, 319 Anaa Atoll 39, 39–40 Analtin Atoll 217, 217 Ant Atoll 199, 200 antecedent karst theory 6 antecedent platform theory 6 Antilles Current 391 Anuanuraro Atoll 75, 75–76 Anuanurunga Atoll 74–75, 75 Apataki Atoll 33, 33 Arabian Sea 349 Arafura Sea 206 Aranuka Atoll 137, 138 Aratika Atoll 33, 34 Ardasier Bank (Andu) 340, 341 Ardasier Reef (Guangzingzi) 340 Arena Atoll 344, 346 Argo Reefs 252 Ari Atoll 363, 364 Arno Atoll 171, 172 Arrecife Alacranes (Alacrán Reef ) 399, 400 Arutua Atoll 32–33, 33 Ashmore Reef 284, 284 Astove Atoll 382, 384 Atafu Atoll 102–103, 103 Atlantic atolls 392 atoll islands 1, 11, 20, 105, 110, 111, 117, 120, 125, 133, 163, 407, 408, 409, 413 atoll lagoons 17, 19, 30, 59, 97 Atoll Research Bulletin 1 atoll rims 8, 10–12 atolls characteristics of 4–23, 404–406 atolls, of 21st century 407–415

freshwater 407–408 increasing atoll populations 409, 409–410 sea-level change 410–413, 411–412 stresses on the water supply 408–409 atoll structure 8–20, 9–10 drowned atolls 12 and formation 4–8, 5, 7 lagoons: variation and zonation 15–20, 16–19 reef rim 8, 9–12, 14 reef rim zonation and terminology 12–15 submerged rims 12 Aur Atoll 173, 174 Australian Coral Sea atolls 268–276 Awin Atoll 215 Ayawi Atoll 219, 219 B Bahamas 391, 400, 401; see also Hogsty Reef Banda Sea 292–303 Bangaram Atoll 352–354, 353 Banggai Archipelago 298 Barque-Canada Reef (Baijiao) 338–339, 339 barrier reef 4, 5 Bassas da India Atoll 385–386, 386 Bay of Bengal 350 Beautemps-Beaupré Atoll 263, 264 Belantang Atoll 295, 296 Bellingshausen Atoll 82, 83 Bellingshausen, Thaddeus Admiral 36 Bellona Island, uplifted atoll in the Solomon Islands 224, 234 Bellona Atoll 12, 267–268, 268 Bell Reef 248, 248 Beru Atoll 14, 17, 17, 139, 139–140 Beveridge reef 97, 99, 99 Bikar (Pikaar) Atoll 177, 178–179 Bikini Atoll 167–168, 168 Birnie Island 129, 130 Bismarck Sea 213 Bitra Par Atoll 351, 353 bivalve 17, 62, 63 Blenheim Atoll 370–371, 371 Bligh, William, Captain 79 Bobubu Atoll 295, 296 Bokaak (Taongi) Atoll 177, 179 Bombay Reef (Langhua) 321, 322 Bombay Shoal (Pengbuo) 331–332, 332 Bougainville Reef 275–276, 275 Boxall Reef (Niuchelun) 330, 331 Bramble Haven Atoll 210–211, 211 Brando, Marlon 2 Brazil 401–402; see also Rocas Atoll British colonization of Fiji 251 Budibudi Atoll 213–214 Bukatatanoa Atoll 252, 253 Butaritari Atoll 133, 134

Byramgore Atoll 351, 352 Byron John, Commodore 58 C Cagayan Atoll 345, 346 Candelaria (Roncador) Atoll 231, 231–232 Caribbean Current 391 Caribbean Sea 392 Caroline Islands map 181 Carondelet Reef 130 Cartier Reef 284, 284, 285 Cato Atoll 270, 271 Cayman Current 391 Central Indian Ocean map (Lakshadweep, Maldives, Chagos) 350 Ceva-i-Ra Atoll 258–259, 259 Chagos Archipelago 368–374 Chagos–Laccadive Ridge 349, 350 Cherbaniani Atoll 349, 350, 352 Cheriyam Atoll 356 Chesterfield Atoll 12, 267, 268 Chetlat Atoll 354–355, 355 Chinchorro Bank 399–400, 399 Clerke Reef 279–280, 280 Clipperton Island 7 closed atolls 12 Cocos-Keeling Atolls 374–375 Commodore Reef (Siling) 334, 335 Comoros-Amirante Islands 377 Conflict Atoll 209–210 continental slope atolls, of Spratly Islands 322–327 Cook Islands 7, 27, 87, 88, 97 Aitutaki almost-atoll in 4–5 Northern 87 Southern 94 Cook Islands map 88 Cook, James, Captain 49, 79, 122 coralline algae, crustose (CCA) 1, 3, 9–10, 11–13, 20, 70, 98–99, 168, 178–179, 282, 285, 311, 363–364, 386, 395, 402 Coral Sea Atolls 261, 261, 262–268 Coral Sea map 261 Coral Triangle 206 Cornwallis South Reef (Nanhua) 334–335, 335 Cosmoledo Atoll 382, 383–384 Courtown Cays 394, 395 Crescent Atoll (Yongle) 319–320, 319 Cuarteron Reef (Huayang) 337, 337 Cyclone Percy 410 cyanobacteria 35, 81, 97, 123, 130, 139, 375 D Dallas Reef (Guangzing) 340, 341, 341 Darwinian atolls 8 Darwin’s subsidence theory 4 Degree Heating Weeks (DHW) 117, 118 d’Entrecasteaux Ridge 264

Desroches Atoll 379–380, 380 Dewakang Besar Atoll 308, 309, 309 Dhaalu (South Nilandu) Atoll 364, 365 DHW see Degree Heating Weeks (DHW) Dibbles Reef 247–248, 248 Diego Garcia Atoll 373–374, 373 Disappointment Islands 54, 60 Discovery Atoll (Huaguang) 320, 321 Discovery Great Reef (Daxian) 326–327, 326 diurnal tides 22 Dove satellite system 2 drowned atolls 9, 12, 22 Duff Reef 246, 247 Duke of Gloucester Islands 28 Ducie Atoll 70–72, 71 world’s southernmost atoll 72 E East and West Ayau Atolls 220–221, 220 East and West Thitu Atolls 323, 324 East Atoll 291, 292 East Australian Current 261 Eastern Lakshadweep Islands Atolls 354–357 East Gyral Current 278 East London (East Yinqing) Atoll 337, 337–338 East Thitu Atoll 323, 324 Eauripik Atoll 187–188 Ebon Atoll 159–160, 160 Egmont Atoll 372, 373 Egum Atoll 212, 213 Elato Atoll 190, 191 Elizabeth Atoll 269, 269–270 Ellis Islands see Tuvalu El Niño 26, 30, 87–88, 106, 116, 117, 123, 125, 131, 158, 182, 206, 288, 345, 383, 407, 408 El Niño-Southern Oscillation (ENSO) 106, 116–117, 158, 407 and coral bleaching 407 shifts 117 Enderbury Island 128–129, 129 Enewetak Atoll 168–170, 169 ENSO see El Niño-Southern Oscillation (ENSO) equatorial siphoning 117 Erica, aka Enloa Reef (Boji) 339, 340 Erikub Atoll 174, 175 Etal Atoll 196–197, 197 Europa Atoll 387, 387 F Faafu Atoll (North Nilandu) 363, 364 Faaite Atoll 38, 39 Fadiffolu Atoll 361, 362 Fakahina Atoll 60, 61 Fakaofo Atoll 104 Fakarava Atoll 35–36, 35 Fangatau Atoll 61 Fangataufa Atoll 71, 72 Faraulep Atoll 189–190, 189

418 A Global Atlas of Atolls

Farquhar Atoll 384, 385 fault block atolls 20, 391–392, 396 Federated States of Micronesia (FSM) 181 Felidhoo Atoll 365, 366 Fiery Cross Atoll (Yongshu) 326, 327 Fijian Archipelago 12 Fiji map 240 First Thomas Shoal (Xinyi) 333–334, 333 Flinders Reefs 273–274, 273 Flora Reef 275 Flores Sea Atolls 303–308 Frederick Atoll 270–271, 272 French Frigate Shoals (Kanemiloha‘i) 148, 149 fringing coral community 20 fringing reefs 4, 5 Funafuti Atoll 106–108, 108 G Gaafaru Atoll 364, 366 Gallows Reef (Galogalohi) 212–213, 212 Geyser Reef 385 Giant clams (Tridacna maxima) 62–63, 63, 119 Gilbert Islands 17, 17, 115, 155 Gilbert Islands Atolls see Kiribati Gilbert Islands map glacial control theory 6 glacial melting 117, 118 Global Tide Model 26 Glorioso Archipelago 385 Glovers Reef 396 Goidhoo Atoll 361–364, 362 Google Earth 25, 29, 130, 294, 318, 332 Gorgonian corals 391, 393, 395–399, 400 Gosong Boni 306, 306–307 Great Chagos Bank 369 Great Reef 233 Gros Guilbert Atoll 266, 267 Guano Islands Act 143 Gulf of Carpenteria 205 Gulf of Honduras 392 Gulf of Tomini 291–292 H Haddumati Atoll 367, 368 Half Moon Shoal (Banyue) 333, 334 Halimeda 168–169, 185, 202, 278, 282, 309, 312, 384, 387, 391, 398, 400 Halmahera Atolls 289–291 Halmahera Sea 290 Hao Atoll 49, 50 Haraiki Atoll 44, 45 Hawaiian-Emperor seamounts 4 Hawaiian Islands map 144 Hawaiian Lee Countercurrent 144 Hawai’i, Hawaiian Islands 4, 15, 20, 23, 28, 117, 145–149 Heemskercq Reefs 243, 244 Heina Atoll see Laualau Atoll Helen Reef 12, 182 Hereheretue Atoll 75, 76 Hibernia Reef 284, 285–286 Hikueru Atoll 46–47, 46 Hiri Current 261 Hiti Atoll 41, 42 HMS Actaeon 66 HMS Beagle 1, 4 HMS Bounty 240 HMS Bramble 210 HMS Dolphin 79 HMS Rattlesnake 210 hoa 11, 12, 11 dry, or paleo- 12

Hogsty Reef 399, 400–401 Holloway Current 278 Holmes Reef West 274–276, 274 Holoikauaua Atoll 150–151, 150 Hopps Reef (Hejiao) 328, 329, 329 Huon Atoll 266–267, 266 Heyerdahl, Thor 59 I Ifalik Atoll 188–189, 189 Ihavandiffulu Atoll 359, 360 Île Desroches 2 Îles Eparses 385–387 Îles Maria 83–84, 84 Imperieuse Reef 278–279, 280 Indian Monsoon system 349 Indian Ocean, northernmost atolls of 349, 350 Indispensable Reef Middle 235, 236 Indispensable Reef North 234–235, 236 Indispensable Reef South 235–236, 236 Indonesia throughflow 277, 288, 374 tides of 288–289 tropical cyclones in 288 Indonesia map 288 Indonesian Throughflow currents 2 77, 288 Intertropical Convergence Zone (ITCZ) 115–116, 182–183, 206, 261, 262 Investigator Shoal (Yuya) 340, 341 Irving Reef (Cailun) 327, 328 ITCZ see Intertropical Convergence Zone (ITCZ) J Jackson Reef (Jiesheng) 328, 328 Jaluit Atoll 160, 161 Johnston Reef 144–147, 146 K Kadmat Atoll 355, 356 Kakaban Atoll 311, 311–312 Kakabia Atoll 305, 306, 306 Kalimantan Atoll 309–312 Kalpeni Atoll 356, 357 Kanton Atoll 125, 126, 127 KAP see Kiribati Adaptation Program (KAP) Kapingamarangi Atoll 202–203, 203 Karang Kaledupa 298, 299 Karang Kapotta 298, 299 Karang Koka 300, 301 Karang Koro Maha 300, 300 Karang Lalanga 292, 292 Karang Lucipara 301, 301–302 Karang Skaro 301, 302 Kardiva Channel 363 Katiu Atoll 38, 38 Kauehi Atoll 36, 37 Kaukura Atoll 33, 34 Kavaratti Atoll 355, 356 Kayangel Atoll 184, 184–185 Kenn Atoll 270, 271 Kili Island 157 Kilinailau (Carteret Islands) 228–229 Kilinailau Atoll 228, 228–229 Kiltan Atoll 355, 355–356 Kingman Reef 143–144, 145 Kiribati El Niño Southern Oscillation 116, 117 equatorial siphoning and emergence 117 Gilbert Islands 130–140 Line Islands 117–124 Phoenix Islands 124–130

Kiribati Adaptation Program (KAP) 413 Kiribati Gilbert Islands map 131 Kiribati Line Islands map 119 Kiribati map 115 Kiribati Phoenix Islands map 125 Kiritimati Atoll 122–123, 122 Kokota Atoll 290, 291 Kolomadulu Atoll 367, 368 Kreis Reef 213–214, 214 Kulukalukuang Atoll 309, 310 Kuop Atoll 195, 195–196 Kure Atoll (Mokupapapa) 150, 152 world’s northernmost atoll 152 Kuroshio Current 315 Kuroshio Loop Current 315 Kwajalein Atoll 163, 163–164 L Ladd Reef (Riji) 338, 339 Lae Atoll 164–165, 164 lagoon floors of 18 measurements 25 pinnacles and reticulate reefs 19 slope 9, 13, 18 soft-bottom communities 17 variation and zonation 15–20, 15–18 waters 18 Lagoonal reticulate reefs 407 Lagoon slope 13, 9 Lagoon water residence time 30 Lamotrek Atoll 190, 191 La Niña 26, 106, 115, 122, 128, 131, 182, 262, 407–409 Lapita people 1, 27 Laualau Atoll 217, 218 Lau atolls 245–258 Laysan Atoll (Kauo) 149–150, 149 Lekaleka Reef 253, 254 Levu Reef 255–256, 255 Lib Atoll 163 Lighthouse Reef 397–398, 398 Lihou Reef 272, 273 Likiep Atoll 175–176, 176 Line Islands (Kiribati) 117, 118, 119 Line Islands (U.S.) 143 Lintea Atoll 299, 299, 300 Liot Atoll 217–218, 218 Livock Reef (Sanjiao) 328, 329, 329 Loaita Atoll (Daoming) 324–325, 325 Long Reef 211–212, 211 Losap Atoll 195, 196 Louisa Reef (Nantong) 342, 343 Louisiade Archipelago 208 Lukunor Atoll 197, 197–198 M Mabui Atoll 209, 210 Macclesfield Bank (Zhongsha) 322, 406 Maiana Atoll 136–137, 136 Majuro Atoll 12, 156, 158, 171, 172–173 Makanudu Atoll 360, 360–361 Makassar Strait atolls 308–309 Makemo Atoll 42, 43 Makin Atoll 133, 134 Malaku Atoll 367–368, 367 Malden Atoll 120–121, 121 Maldive Islands 2 Malevuvu Reef 248–249, 249 Maloelap Atoll 173–175, 174 Malum Atoll 224–225, 226 mangroves 17, 17, 404 Manihi Atoll 55, 56 Manihiki Atoll 90–91, 91 Manihiki Plateau 88

Manra Island 129, 130 Manuae Atoll 93, 94 Manuhengi Atoll 51, 52 Mapia Atoll 219–220, 219 Mapun Atoll 343, 343 Marakei Atoll 133–134, 134 Maratua Atoll 310, 311 Maria Est Atoll 68–70, 69 Marianne Atoll see Taka Bassi Maringki 293, 294 Marion Reef 271, 272 Mariveles Reef (Nanhai) 339–340, 339 Marokau Atoll 47–48, 48 Marshall Archipelago 156 Marshall Islands 1, 13, 15 Marshall Islands atolls 155–158, 155–179 radiological vignette 170–179 Ratak Chain of atolls 170–179 Ratik Chain of atolls 159–170 Marshall Islands map 156 Marutea Nord Atoll 44, 44 Marutea Sud Atoll 68, 69 Matacucu Reef (Matathuthu) 241, 242, 242 Mataiva Atoll 31, 31 Matureivavao Atoll 66, 67–69 McKean Island 130 Meatimiarang Atoll 302, 302 Mekong River 313 Melanesia map 207 Melanesian Coral Sea atolls 262–268 Melanesian islands 205, 205 Mellish Reef 271, 272 Merite Atoll 267 Mermaid Reef 280–281, 280 Micronesia map 155, 185 Middleton Atoll 269, 270 Midway (Pihemanu) Atoll 151 Migrate with Dignity (MWD) Strategy 415 Mili Atoll 171, 171 Minerva Reef North 256–257, 257 Mindoro Strait 314–315 Minerva Reef South 257–258, 257 Minicoy Atoll 357, 357 Minto Reef 198, 199 Mioswundi Atoll (Mios Woendi) 218, 219 Mischief Reef (Meiji) 329, 330 mixed tides 22 Momo Reef 258, 258 monsoonal wind reversals 11 Mopelia Atoll 83, 84 Morane Atoll 71, 72 Mototunga Atoll 36–37, 39 motu 11, 12, 9, 225–232 Motua Lailai 244, 246 Motua Levu 244–245, 246 Motu One see Bellingshausen Atoll Motu Reef 253–254, 254 Mozambique Channel 377, 385–387 Mozambique Current 378 Muaras Atoll 309, 311 Murilo Atoll 194–195, 194 Muroroa volcanic alignment 72 Mururoa Atoll 72–73, 73 Mwoakilloa Atoll (aka Mokil) 200, 201 N Nakapu Atoll 232–233, 233 Namdrik Atoll (Namorik) 160, 160–161 Namoluk Atoll 195, 196 Namonuito Atoll 193–194, 194 Namu Atoll 161–163, 162 Nanumanga Atoll 111, 112 Nanumea Atoll 111–112, 112

Index

Napuka Atoll 59, 60 Navatu Reef 258, 258 Ndaa Atoll 300, 300 NECC see North Equatorial Countercurrent (NECC) Nengonengo Atoll 50, 50–51 Nepo Pass 34 New Guinea, Bismarck and Solomon seas map 207 New Guinea Coastal Current (NGCC) 207, 215 New Guinea Island 207 Ngatik Atoll see Sapwuahfik Atoll (Ngatik) Ngulu Atoll 182, 186, 186 Niau Atoll 34–35, 35 Nihiru Atoll 43–44, 43 Nikufetau Atoll 10 Nikumaroro Atoll 127, 128 Nikunau Atoll 139, 140 Ninigo Atoll 215–217, 217 Ninigo Islands 215 Nissan Atoll 227, 227 Niulakita 105 Niutau Atoll 110, 111 Nokeva Reef 250–251, 250 Nomwin Atoll 193–194, 194 No Name Atoll 1 292–293, 293 No Name Atoll 2 293, 294 No Name Atoll 3 294, 295 No Name Atoll 4 295, 296 No Name Atoll 5 298, 299 non-Darwinian atolls 20–23 banks, shoals, and other problems of terminology 22 oceanographic terms and processes 22–23 shelf and slope atolls 20, 22, 322, 391 Nonouti Atoll 137, 138, 412 North Argo Reef 252, 253 North Brazil Current 391 North Cocos-Keeling Atoll 375, 375 North Danger Reef 323–324, 323 Northeast Investigator Shoal (Haiku) 332, 333 North Equatorial Countercurrent (NECC) 158, 181, 182 North Equatorial Current (NEC) 144, 144, 145, 146, 147, 156, 182 North Hawaii Ridge Current 144 North Malé Atoll 364–365, 366 North Malosmadulu Atoll (Raa) 361, 361 North Reef (Beijiao) 318, 319 Northwest Hawaiian Islands (NWHI) 147 atolls 152, 153 nuclear testing 4, 73–74, 146, 167, 170 Nuguria Atoll 225–227, 226 Nui Atoll 110, 111 Nukubasanga Reef 244, 244 Nukufetau Atoll 109–110, 109 Nukulaelae Atoll 106, 108 Nukumanu Atoll 229–230, 230 Nukunonu Atoll 104 Nukuoro Atoll 202, 203 Nukutepipi Atoll 74, 75 Nupani Atoll 232, 233 Nusa Tenggara (Lesser Sunda) Islands 287 North West Australian Shelf map 278 NWHI see Northwest Hawaiian Islands (NWHI)

O oceanographic influences on atolls 22 Olimarao Atoll 190–191, 190 Olohega in Tokelau 97 Olosega in Samoa 97 Onotoa Atoll 139, 140 Ontong Java 230–231 Ontong Java Plateau 224 Oroluk Atoll 198–199, 199 Orona Atoll 127–128 Osprey Reef 275, 276 Ouvéa Atoll 263, 263 P Padea Besar 296, 297, 297 Pakin Atoll 199, 199–200 Palliser Islands see Tuamotu Palmerston Atoll 88, 93, 94 Palmyra Atoll 143, 145 Pandanus (screw pine) 11 Pangadjarang Atoll 294, 296 Panteh Atoll 293–294, 295 Papua New Guinea (PNG) 206, 208, 208 map 207 Paracel Islands 317–322 Paraoa Atoll 51–52, 52 Parece Vela Atoll 185–186, 185 Pasir Layaran 306, 306 Pasir Raja 290, 290 Pasir Tengah 291–292, 292 Passe Otuni 34 Passe Tehere 33 Passu Keah (Panshiyu) 321–322, 321 patch reefs 16, 17, 18 Paternoster Atoll 307, 307–308 pavement 9, 10, 11, 13, 168, 279, 285 Pearl and Hermes Reef (Holoikauaua) 150, 150–151 Pearson Reef (Bisheng) 336–337, 336 Pelleluhu Atoll see Analtin Pearl River 313 Pelotas Atoll 264–265, 265 Penrhyn Atoll 88–90, 91 Peremul Par Atoll 352, 353 Peros Banhos Atoll 372, 372–373 Petit Guilbert Atoll 266, 266 Pétrie Reef 263, 264 Phoenix Islands Atolls see Kiribati phosphate mining 20 photosynthetic pigments 2 Pigeon Reef (Tianlan) 334, 335 Pikelot Atoll 192 Pinaki Atoll 53–5, 54 Pinctada margaritifera 72 Pingelap Atoll 201, 201–202 Pinipir (Pinipel) Atoll 227, 227–228 pinnacles 9, 19 Pisonia trees 11 Pitman Reef 243, 244 Pitt Bank 373, 373 Planet remote-sensing data 2 Platte Atoll 381, 381 Pleistocene epoch 6 PNG see Papua New Guinea (PNG) Poluwat Atoll 192, 193 Polynesia map 27, 28 ancestors 27 Portail Atoll 265, 265–266 Pratas Reef (Dongsha) 317, 317 Providence Atoll 384–385, 384 Pukapuka Atoll 10, 91–92, 92 Puka-Puka Atoll 28, 60, 60–61

Pukarua Atoll 62, 64 Pulap Atoll 192, 193 Pulusuk Atoll 192–193, 193 Q Qelelevu Atoll 240, 242, 243 Queensland Plateau 261, 272 Quirós, Pedro Fernández, Captain 40, 49, 74 QuitaSueño (Quita Sueno, Quito Sueno) 396, 396–397 R Raitahiti Island 34 Rakahanga Atoll 90, 91 Rangiroa Atoll 31, 32 Rani Atoll 219, 219 Raraka Atoll 37, 37–38 Raroia Atoll 58, 58–59 Rasdhoo Atoll 363, 364 Ravahere Atoll 48, 49 Reao Atoll 62–63, 64 Redlick Reef 208 Reed Bank (Lilu) 321 reef crest 3, 10, 13, 15 reef extent, definition in Allen Atlas 25 reef flat 9, 13, 352–353, 361, 374, 379 reef-locked lakes 123 reef platform 8 reef rim 8, 10–12, 11 zonation and terminology 12–15 reef sand apron 15, 17, 10, 33 reef slope 13, 15, 15 reef terminology 9–10 Reitoru Atoll 47, 47 Rekareka Atoll 45, 46 remnant lagoons 10, 20, 25, 35, 105, 106, 110, 117, 120, 125, 130, 133, 149, 163, 375 remote-sensing images 25 Republic of the Marshall Islands 156 reticulate reefs 19, 19–20 ridge atoll 387 Rifleman Bank (Nanwei) 343 rim islands 33 rimmed atolls 22 Ringgold, Cadwalader Captain 242 Ringgold Isles 243, 245 atolls 242–245 Rocas Atoll 401–402, 401 Roncador Atoll 394, 395 Rongelap Atoll 166, 166–167 Rongerik Atoll 165–166, 166 Rose Atoll 10, 97, 99, 99 Rowa Atoll 234, 235 Royal Captain Reef (Jianzhang) 332–333, 332 Royal Charlotte Reef (Huanglu) 342–343, 342 Runit Dome 170 S Sabalana Atoll 307, 308 Sabina Shoal (Xianbin) 330–331, 331 Sacks, Oliver 202 Sadapur Atoll 307, 307 Saloman Atoll 371, 371 Sama Atoll 215 Santa Cruz Islands 232 Sapoeka Atoll 307, 308 Sapondra Utara 297, 297 Sapwuahfik Atoll (Ngatik) 200, 201

satellite imagery 2, 2–4, 3 Saumarez Atoll 271, 272 Scarborough Reef (Huangyan) 321, 322 Scilly Atoll 83, 84 Scott North 282 Scott Reef 279, 281 Scott South 281–282 SCSWC see South China Sea Warm Current (SCSWC) Seagrass areas 404 Seahorse Shoal (Haima) 328, 328 sea-level rise adaptation attempts 413–415, 416 seamounts 4 Second Thomas Reef (Renai) 330, 331 semidiurnal tides 22 Seringapatam Reef 282, 283 Serrana Atoll 395, 396 shallow lagoon 45 shelf atolls 20, 282, 391 Sherburne Reef 214 shingle 10 Sikaiana Atoll 231, 232 Society Islands map 80 Sogori Atoll 302, 303 Solomon Islands atolls 224–236 Solomon Islands 1, 155, 223, 223 Solomon Islands map 223 Solomon Sea 205 Solomon Straits 206 Sorol Atoll 187, 188 South Argo Reef 252, 253 South Central Pacific map 98, 99 South China Sea Warm Current (SCSWC) 314, 315 South China and Sulu seas map 314 South Cocos-Keeling Atoll 374, 375 South Equatorial Countercurrent (SECC) 350, 377, 378 South Equatorial Current (SEC) 101, 105, 115, 119, 126, 131, 206, 207, 223, 240, 261, 350, 378 Southern Central Pacific, atolls of 97–99, 98, 100 South Malé 365, 366 South Malosmadulu (Baa Atoll) 361, 361 South Pacific Gyre 28, 29, 79, 80, 87, 98 Speakers Bank 370, 371 Spratly Islands 322–343 continental slope atolls of 323–327 oceanic atolls of 327–343 Spratly, William 323 spur and groove systems 13, 14 Starbuck Atoll 120, 121 STCC see Subtropical Countercurrent (STCC) St. Francoise Atoll 380–381, 381 St. Joseph Atoll 379, 380 Stoddart, David 382 Subi (Zhubi) Atoll 324, 325 submerged atolls/rims 12 subsidence 4, 6 subsidence theory 6 Subtropical Countercurrent (STCC) 144, 147 Suheli Par Atoll 353, 354 Sukar Atoll 290, 291 Sulawesi Atolls 291–308 Sulu Sea Atolls 343–346 Sumasama Atoll 215, 216

419

420 A Global Atlas of Atolls

Summer Monsoon Current 350 Surprise Atoll 264, 265 Suvadiva Atoll 368, 369 Suwarrow Atoll 92–94, 93 Swain, Jonathan, Captain 97 Swains Island 97 Olohega in Tokolau 97 Olosega in Samoa 97 Swallow Reef (Danwan) 341–342, 341 T Tabiteuea Atoll 138 table reefs 20 Tabuaeran Atoll (Fanning) 123, 124 Tabu Reef 250, 251 Taenga Atoll 42–43, 43 Tahanea Atoll 39, 40 Tahiti 79 Taiaro Atoll 36–37, 37 Taka Atoll 177, 178 Taka Bassi 305, 306 Taka Bone Rate 303–304, 304 Taka Garlarang 303, 304 Taka Lambaena 304–305, 305 Taka Rewataja 308, 309 Takaroa Atoll 57, 57–58 Takume Atoll 58, 59 Takuu Atoll (Tauu) 228, 229 Tarawa Atoll 132, 135–136, 135, 173, 407, 408, 413 Tatakoto Atoll 60, 61–62 Tauere Atoll 47, 47 Tekokota Atoll 45, 46 Tematangi Atoll 73, 74 Temoe Atoll 69, 70 Tenararo Atoll 66, 66–67 Tenarunga Atoll 66, 67 Tepoto Sud Atoll 41–42, 41 Teraina Atoll 123–124, 124 Tetiaroa Atoll 80, 82, 82 tide data 26

tides, types of 22 Tikehau Atoll 31, 32 Tiladummati-Miladummadulu Atoll 359–360, 360 Timor Sea 277 Tizard Bank (Zhenghe) 325, 325 Toas Atoll 190, 191 Toau Atoll 34, 35 Tokelau map 101 Tongareva (Penrhyn) Atoll 88 Tonkin Gulf 313 Trade winds 11 reversal 117 upwelling 117 Tridacna maxima 51, 53, 54, 62, 63, 63, 67, 68, 70, 74, 90 Tuamotu Archipelago (‘distant islands’) 27–30, 29, 30 Tuamotu atolls 30–76 Tuamotu chain 29 Tuamotu-Gambier Islands 27 Tuamotu Islands 1, 27–28, 30 Duke of Gloucester Islands 28, 72, 74, 75 King George Islands 58 northern and northeastern islands 54–65 Palliser Islands 30–36 Pitcairn Islands 28, 30, 70, 72 South Central Islands 45–54 southeastern 65–76 Tuamotu Islands map 29 Tuanake Atoll 41, 42 Tubbataha Reefs 344–345, 344 Tukang Besi Archipelago 297 Tupai Atoll 81–83, 82 Turneffe Atoll 398–399, 398 Tuvalu map 105 U Ujae Atoll 164, 165 Ujelang Atoll 169, 170

Ulithi Atoll 186, 187 Union Bank (Quyuantan) 326, 327 United Nations Climate Change Conference (2017) 413 United Nations Framework Convention on Climate Change (UNFCCC) 413 United States South Seas Exploring Expedition 64, 64, 239–240, 242 uplifted atolls 20, 21, 31–32, 34, 36, 149, 182, 227, 239, 243, 262, 264, 298, 305, 310, 356, 372, 392 upwelling 116–117, 132, 136, 158, 314, 377, 385, 392 U.S. Coast Guard LORAN radio navigation station 149 U.S. Line Islands atolls 143–144 Utirik Atoll 177, 178 V Vahanga Atoll 66, 67 Vahitahi Atoll 53, 53 Vairaatea Atoll 53, 53 Vaitupu Atoll 110–111, 110 Vanavana Atoll 65, 66 Vanuabalavu almost-atoll 239, 240 Vanuatu Atoll 155 Vattaru Atoll 365, 366, 367 Vau Reef 254, 254 Vekai Reef 249–250, 249 Velasco Reef 184, 185 Victory Bank 372, 373 Vietnam Offshore Current 314, 315 Vitiaz Straits 206, 223 volcanic basement of islands 1, 27, 157 Vuata Ono 256–257, 256 Vuata Vatoa Atoll 255, 256

Vuladdore Atoll (Yuzhou) 320, 320, 321 Vuthovutho Atoll 242–243, 242 W Wailangilala Atoll 246, 247 Wake Island 152–153 Wake-Marcus seamount cluster 152 Wallis, Samuel, Captain 79 wave data 26 wave-resistant calcareous structures 13 waveward 11 westerly winds 80, 125 western Caroline Islands 12, 182, 408 Western Lakshadweep Islands atolls 349 West Fayu Atoll 190, 191–192 West London Reef (West Yinqing) 337, 338 Wilkes, Charles, Captain 64 Williamson Reef 247, 248 wind data 22, 26 wind-generated waves 88 Winslow Reef 130 Winter Monsoon Current 350 Woleai Atoll 188, 189 Wotho Atoll 164, 165 Wotje Atoll 175, 176 Wreck Reef 270, 271 Y Yap Atoll 182 Yucatan Current 391 Z Zohhoiiyoru Atoll 186, 187 zooxanthellae 6