North Indian Ocean Circulation Since the Last Deglaciation as Inferred From New Elemental Ratio Records for Benthic ForaminiferaHoeglundina elegans

The evolution of intermediate circulation in the northern Indian Ocean since the last deglaciation has been reconstructed from two marine cores located at intermediate depths off the southern tip of India (MD77-191) and in the northern Bay of Bengal (BoB) (MD77-176). Benthic foraminiferal delta C-13, seawater carbonate ion concentration ([CO32-]) estimated from the Sr/Ca, and paleotemperature reconstructed on the basis of the Mg/Li of aragonite benthic speciesHoeglundina eleganswere used to trace the evolution of past intermediate-deepwater masses and to constrain ocean-atmosphere exchanges during the two-stage increase in atmospheric CO(2)across the last deglaciation. The intermediate water [CO32-] was mainly affected by changes in the ocean alkalinity inventory, associated with the modulation of atmospheric CO(2)on glacial-interglacial timescales. Higher benthic foraminiferal delta C-13, depleted [CO32-], and decreased benthic-planktonic(14)C age offsets at intermediate water depths suggest a release of deep-sea CO(2)to the atmosphere through the Antarctic Intermediate Water (AAIW) in the Southern Ocean during the 17-15.2 and 12.6-10.5 cal kyr BP time intervals. In addition, the decreasedH. elegansMg/Li record seems to reflect an increased contribution of cold water mass during the 17-15.2 and 12.6-11.9 cal kyr BP intervals and throughout the Holocene. In contrast, two warm events occurred in the 15-13.3 and 11-10.3 cal kyr BP time intervals. During the late Holocene, a decrease in the intermediate water [CO32-] indicates a contribution to atmospheric CO(2)rise since 8 cal kyr BP, due to the depleted global ocean alkalinity and/or the variations in surface productivity (at least for MD77-191).