Atolls are a special type of coral reef complex formed in tropical seas by a ring of reef coral enclosing a lagoon. The characteristic features of an atoll include a reef rim, from 100 to 500 m across, which is mainly awash at high tide, and flattish islands (motu), which remain a few meters above sea level and on which people may live. The word atoll itself comes from the language of the Maldive Islands, a chain of large atolls in the Indian Ocean (Fig. 1). Atolls range in size from a few to as many as 40,000 km2, and lagoon depths range from almost nothing to as much as 100 m.
The minimum water temperature for the approximately 600 atolls around the globe is about 20 °C, a temperature that prevails from about 24° N to 29° S. Reef-building corals are partly dependent on their embedded photosyn-thesizing algal symbionts for adequate oxygen, and thus grow healthily only in lighted waters less than a few tens of meters deep. Slow-growing, deep-water coral reefs, without algal symbionts, are known to exist at depths down to about 6000 m. Many atoll lagoons are connected directly to the sea by inlet channels, but few channels are wide or deep enough to be navigable by anything but canoes. Atolls with wide, deep channels are favored for tourism and for administrative headquarters. People from other atolls in the region tend to migrate to these centers, and overcrowding is a problem on some. The main source of income on most atolls is copra from coconuts, and tourism is important on a few.
Between the ocean and the lagoon, the reef comprises an outer, wave-resistant algal ridge a few tens to a hundred meters wide at sea level, against which the ocean swell breaks. Seaward, the ridge drops steeply off into deeper, less turbulent water, and corals thrive along these slopes. The outer part of the algal ridge is commonly notched to a depth of 10–20 m by a reticulate network of narrow (1–3 m) grooves and surge channels that funnel water across the reef rim toward the lagoon. Behind the algal ridge is a reef flat, close to low tide level, where a pavement of living and dead coral and coral debris from the size of sand grains to the size of boulders extends lagoonward. Reef debris thrown onto the flat during typhoons lies in patches and windrows 1–2 m high in some places.
Large and small islands, termed motu, surmount the reef flat in places and rise to as much as 4 m above sea level (Fig. 2). They are commonly edged by a steep scarp toward the sea, created by waves beating against the island. It is only on these islands that the people of the atolls can live, where they are safely above the surf. The motu are weathered remnants of reef flats dating from the time of a regionally (in low latitudes) higher stand of sea level, about 2000–4000 years ago. The highstand resulted from the isostatic response to a redistribution of ocean waters after deglaciation of the northern continental ice sheets, beginning about 18,000 years ago. Between motu are shallow channels (hoa) that direct washover from ocean waves toward the lagoon. Shallow lakelike depressions, termed faros, pock reef flats on some atolls. These are the now-drowned remnants of karstic depressions dissolved by meteoric waters during the last lowstand of sea level. In addition to carefully collecting and storing rainwater, atoll islanders depend on a precarious groundwater supply beneath the motu, which accumulates as a freshwater lens floating on the denser saltwater beneath. Seepages of this water are used to water vegetable gardens, and wells tap the lens for potable water.
Atoll lagoons are mainly of fairly normal ocean salinity, except for completely landlocked lagoons surrounded by relatively high islands. These lagoons may include shallow salt pans and briny lakes. Detailed charts of atoll lagoons commonly show patch reefs, some rising to sea level but others submerged so as to constitute hazards to navigation. Sediment builds up in the lagoon almost entirely during sea-level highstand times, when ocean waves can wash fine sediment across the reef. Shells of organisms living in the lagoon add to this thickness. At lowstand times, rain falling into the perched lagoonal depression may drain to the surrounding sea through erosional channels cut through the reef rim.
The origin of the ring-around-a-lagoon form has attracted the attention of geologists for two centuries, beginning with Charles Darwin (1842), who never actually visited an atoll during his Beagle voyage but made a painstaking study of charts, supplemented by his own observations on other coral-surrounded islands. He showed that tropical islands displayed a series of reef forms, from fringing reefs that cling to the shore, to barrier reefs separated from the island shore by a lagoon, to atolls, with a lagoon but no bedrock island. From his experiences in South America, where his hikes took him to shelly marine deposits high on the Andes slopes, connoting tectonic uplift, he reasoned that there must be a compensating subsidence in the Pacific basin and that the array of reef forms–fringing, barrier, atoll–must be due to regional subsidence of the islands. According to Darwin, the original fringing reef would grow upward, keeping pace with the relative rise of sea level during island subsidence, but as subsidence continued, the original fringing reef would continue to grow upward, and coral growth would be inhibited in the less turbulent and less fertile waters inland from the reef front. The perimeter would keep up with the rising sea level, whereas the more interior parts of the reef would fall further and further behind, resulting in a lagoon, first behind a barrier reef and then within an atoll. (Fig. 3)
Since Darwin's theory was published, in 1842, the reality of island subsidence has been confirmed by the drilling of atolls in several places, starting with a testing expedition by the Royal Society to the Pacific atoll of Funafuti, in the Ellice Islands, in 1896-1898, The drill reached a depth of 340 m, at a level near the Pleistocene-Pliocene boundary (~2 million years old according to Ohde et al. 2002) in dolomitized reefal deposits, a finding that demonstrated rapid subsidence of the island foundations. Later drilling at Bikini and Enewetak (Eniwetok) atolls in the Marshall Islands, as part of the nuclear bomb testing program, reaffirmed the subsidence theory. The history of reef construction at Enewetak goes back some 50 million years, to Eocene times.
Difficulties with Darwin's hypothesis for the origin of the lagoon provoked others (e.g., Daly 1915) to advocate other schemes, mainly in hypothesizing planation (including dissolution) of carbonate banks during lowered sea-level periods followed by construction of a perimeter reef during the following sea-level rise, thus leaving a central lagoon. These hypotheses were disproved by drilling at Bikini, Enewetak, and Mururoa, as part of nuclear bomb testing, which showed no such erosive platform.
The Darwinian hypothesis for the origin of the lagoon has been replaced gradually by the dissolution, or karst, theory, in which the lagoonal depression is caused by dissolution of the reef carbonate by rainwater during periodic lowstands of sea level in the Pleistocene, when the sea level dropped repeatedly by about 100 m, on a 100,000-year time scale. The freshwater dissolved a hollow in the emergent carbonate bank, a hollow that was later filled with seawater when the sea level rose again. The two most important controls on lagoon depth are rainfall catchment area and average rainfall. The bigger the atoll and the rainier the climate, the deeper the lagoon. Present-day global rainfall patterns (Fig. 4) probably reasonably reflect rain patterns (wet north and south of the equator, with an equatorial dry belt in the eastern Pacific) during drier glacial times. Erosion of the atoll rim, generally by about 8 m, occurs during lowstands, but the rim is rebuilt during sea-level rise, and then still more rim carbonate is added during slow tectonic subsidence at highstand times. At Enewetak atoll, drilling shows that since Eocene times, a flattish carbonate bank built up to a thickness of about 1400 m, keeping pace with tectonic subsidence, but interrupted from time to time by periods of emergence, when soils developed. The drilling and seismic work there also showed that the atoll form itself likely did not develop until Pleistocene times, when sea-level fluctuations were large and glacial intervals long, whereas earlier sea-level fluctuations were of lesser magnitude and duration. Atolls are thus mainly a phenomenon of the Pleistocene.
Atolls and their peoples are threatened today both by wave attacks on the fragile motu and by rising sea levels associated with global warming. These combine to reduce atoll relief to ephemeral patches of storm debris on reef flats awash at high tide. This is to say nothing of the threats to healthy coral growth posed by high water temperatures and possible increasing acidification of the ocean. Very high carbon dioxide levels probably occurred during the Cretaceous, a time of global warmth but also of very healthy populations of calcareous animals and plants. Whatever trends toward major acidification in the Cretaceous world existed were partly offset by dissolution of calcareous sediments on the deep seafloor. Ultimately, the same balancing may happen in the modern ocean, but this will take time. Over the shorter term, Waterworld awaits.
Darwin and Geologic History / Makatea Islands / Marshall Islands / Motu / Reef Ecology and Conservation
- The glacial control-theory of coral reefs. Proceedings ofthe American Academy of Arts and Sciences 51: 51-251. 1915.
- The structure and distribution of coral reefs. , ed. Berkeley: University of California Press. 1962.
- Impact of mid-Holocene hydro-isostatic highstand in regional sea level on habitability of islands in Pacific Oceania. Journal of Coastal Research 19: 19-502. 2003.
- Origin of atoll lagoons. Geological Society of America Bulletin 113: 837-854. , and ., 2001.
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