Last interglacial reef and subsidence of the Cocos (Keeling) Islands, Indian Ocean

Poorly consolidated Holocene sands and coralline limestones beneath reef islands on the Cocos (Keeling) Islands are underlain by a well-lithified coralline limestone 8-11 m below sea level. Uranium-series ages of 118,000 ± 7000 yrs B.P. (bulk sample) and 123,000 ± 7000 yrs B.P. (sample from which secondary cement was removed) on a coral from within this lithified unit indicate that it was formed during the Last Interglaciation (oxygen isotope substage 5e). The depth at which the reef is encountered supports Darwin's hypothesis that this atoll is undergoing gradual subsidence. Elevation of a reef of this age varies on other islands on the Indo-Australian plate, but an average rate of subsidence of 0.1 mm yr-1 is implied for the Cocos (Keeling) Islands. This is significantly faster than the subsidence rate expected from thermal contraction of the lithosphere at this location and suggests that additional factors have to be considered to account for the subsidence. © 1991.