Smithers, Scott G., and Woodroffe, Colin D. (2000) Microatolls as sea-level indicators on a mid-ocean atoll. Marine Geology, 168 (1-4). pp. 61-78.

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Abstract

Microatolls are discoid corals with flat upper surfaces that develop when upward coral growth is constrained by exposure at low tide. They have been widely viewed as important and relatively precise indicators of modern and paleo sea-level positions because of their sensitivity to the water/air interface, though this has rarely been directly established by survey to a precise datum. This study involved an accurate survey of 282 microatolls growing in a range of intertidal environments (open reef flats, interisland passages, lagoons) across the Cocos (Keeling) Islands, a mid-ocean atoll in the Indian Ocean. The upper surface elevation of modern microatolls was found to vary through more than 40 cm across the environments in which these corals occur on Cocos, representing more than 30% of the spring tide range. Much of the variation reflects subtle ponding at low water levels across extensive low-gradient reef surfaces associated with interisland passages. Microatolls in open reef flat habitats are typically constrained within the narrowest and most consistent elevation range around the atoll. The environment in which individual fossil microatolls occurred when they were alive is significant in reconstructing former sea levels.

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