CALCITIC FORAMINIFERAL DATA CONFIRMED BY CADMIUM IN ARAGONITIC HOEGLUNDINA - APPLICATION TO THE LAST GLACIAL MAXIMUM IN THE NORTHERN INDIAN-OCEAN

A core-top study of cadmium uptake into the shells of the aragonitic b enthic foraminifera Hoeglundina elegans demonstrates that the Hoeglund ina Cd/Ca partition coefficient is close to 1.0 throughout the ocean. Cd uptake by Hoeglundina is far less depth dependent than that of calc itic benthic foraminifera. Furthermore, manganese carbonate does not p recipitate on these aragonitic shells, allowing for the recovery of Cd estimates in some samples where calcitic species are spoiled by conta minating overgrowths. Because Cd incorporation into Hoeglundina shows little depth dependence, a comparison of calcitic and Hoeglundina Cd d ata can be used to verify the assumption that the depth dependence obs erved for calcitic benthic foraminifera is time invariant. This compar ison has been undertaken in downcore and last glacial maximum (LGM) sa mples from the northern Indian Ocean. Aragonitic and calcitic foramini feral estimates for Cd in the LGM ocean are in excellent agreement. Th is result indicates upper ocean estimates of LGM Cd are reliable despi te a factor-of-two variation in Cd uptake by calcitic species over thi s depth range. Cd and delta(13)C data indicate that the uppermost wate rs of the LGM Arabian Sea were strongly nutrient depleted relative to today but that the deepest waters of the northern Bay of Bengal had nu trient concentrations at least as high as or higher than modern levels . A strong vertical gradient of increasing nutrients with increasing d epth existed in both basins during the LGM. A west to east gradient of increasing nutrient concentrations also is evident at all depths. In the Bay of Bengal, there is evidence for an enhanced gradient of nutri ents increasing from south to north. The LGM deep Arabian Sea and inte rmediate-depth Bay of Bengal were nutrient depleted compared to the mo dern ocean; however, Cd and delta(13)C data indicate that nutrient dep letion in the northern Indian Ocean did not exceed that of the deep ea stern tropical Pacific except during the latest part of oxygen isotope stage 2 and (according to Cd data only) the early stages of deglaciat ion. During oxygen isotope stage 3 and early stage 2, Cd in the northe rn Indian Ocean and Cd in the eastern tropical Pacific were indistingu ishable, although both regions have lower Cd at these times compared t o the Holocene.