Coral Microatolls and a Low-resolution Record of Sea-level Changes

Corals are particularly useful indicators of sea level over a range of time scales. Most corals grow in shallow water and radiometric dates, even if they are on corals that are in their growth position, are indicative, rather than definitive, of sea-level position. Coral microatolls are disc-shaped corals that have grown into the intertidal zone and whose lateral growth has been limited by exposure at low tide. Microatolls are living on their outer margin but are predominantly dead on their upper surface. Microatolls in their growth position are fixed biological sea-level indicators and can be used to indicate previous limits to coral growth. The upper surface of these individual coral colonies track sea level and provide low-resolution records of interannual sea-level variations at decadal to millennial scale that have been used for detailed reconstructions of Holocene sea level in those parts of the Indo-Pacific region which experienced a mid-Holocene sea-level highstand. Microatolls were first described from the Cocos (Keeling) Islands by Guppy (1889), who initially termed them minature atolls. The geographical variation in microatolls around the Cocos (Keeling) Islands has been mapped in detail by Smithers and Woodroffe (2000), and it has been shown that fields of microatolls occur in three different habitats: on reef flats, in inter-island passages, and in the lagoon. Microatolls have played an important role in deciphering the pattern of mid- and late Holocene sea-level change, and a comparison of the elevation of radiocarbon-dated fossil microatolls with their living counterparts indicates that the sea was higher relative to Cocos around 3000 years ago (Figure 1). Microatoll morphology has been described from the northern Great Barrier Reef (Scoffin and Stoddart, 1978) where microatolls have been used to constrain former sea levels (Figure 2). Wherea