Coral microatolls and 20th century sea level in the eastern Indian Ocean

Coral microatolls are discoid intertidal corals that are limited in their upward growth by subaerial exposure during low tides. Microatoll upper surface morphology preserves a filtered record of changes in the height of living coral (HLC), the upper limit to which corals can grow, and by proxy a historical record of former constraining water levels. Chronologies for these variations in HLC were established in this study using annual skeletal density bands revealed when skeletal slices were X-radiographed, supplemented by annual fluorescent bands visible when samples were illuminated with ultra-violet light. The upper surface morphologies of two large microatolls from separate reef-flat sites on the Cocos (Keeling) Islands are well correlated and indicate that the upper limit to coral growth has fluctuated by more than 5 cm since the early 1900s. The upper surfaces of these microatolls also indicate that there has been little net rise in sea level in the eastern Indian Ocean during the 20th century. Microatoll surface morphology suggests that average rates of sea-level rise in the eastern Indian Ocean over this period were less than 0.35 mm yr−1, a rate considerably lower than the rate of average global sea-level change determined from aggregated tide-gauge data. The broad surface undulations do not appear to correlate directly with either El Niño-Southern Oscillation events or occurrence of the Indian Ocean dipole mode of ocean-atmosphere circulation. Microatolls provide a simple and effective method for extrapolating broad variations in sea level beyond the tide-gauge record in remote mid-oceanic settings.