Radiocarbon in corals from the Cocos (Keeling) Islands and implications for Indian Ocean circulation

Annual bands of a Porites coral from the Cocos (Keeling) Islands, eastern Indian Ocean, were analysed by radiocarbon for 1955-1985 AD. A rapid oceanic response of the site to bomb C-14 is found, with a maximum D C-14 value of 132 parts per thousand in 1975. This value is considerably higher than those for the northwestern Indian Ocean, suggesting that surface waters reaching Cocos are not derived from the Arabian Sea. Instead, D C-14 values for Cocos and those for Watamu (Kenya) agree well over most of the study interval, suggesting that the South Equatorial Current carries C-14-elevated water rather than C-14-depleted water westward across the Indian Ocean. This implies that oceanic upwelling in the northwestern Indian Ocean is spatially confined with little contribution to the upper limb of the global thermohaline circulation.