Microatolls and recent sea level change on coral atolls

Abstract

MICROATOLLS are colonies of corals, commonly Porties, which are dead on top but living around their perimeter, and are found in intertidal environments on coral atolls. They can grow to several metres in diameter. Their upward growth is constrained by sea level through prolonged exposure at the lowest spring tides^1,2, and their dead upper surfaces have been limited by past sea levels. They act as natural recorders of sea level, which is of particular significance for coral atolls thought to be susceptible to inundation and erosion if sea level rises in response to global warming. X-radiographs of vertical slices through microatolls from the Maldives and Cocos (Keeling) Islands (Indian Ocean) and Kiribati (Pacific Ocean) record sea-level fluctuations over the past few decades. There is a high degree of reproducibility between adjacent corals, although on Cocos we noted geographical variation in the pattern of change around the atoll. The majority of microatolls sampled on these atolls record a slight fall in sea level over the past ten years.