Calcitic foraminiferal data confirmed by cadmium in aragonitic Hoeglundina: Application to the Last Glacial Maximum in the northern Indian Ocean

Abstract

A core‐top study of cadmium uptake into the shells of the aragonitic benthic foraminifera Hoeglundina elegans demonstrates that the Hoeglundina Cd/Ca partition coefficient is close to 1.0 throughout the ocean. Cd uptake by Hoeglundina is far less depth dependent than that of calcitic benthic foraminifera. Furthermore, manganese carbonate does not precipitate on these aragonitic shells, allowing for the recovery of Cd estimates in some samples where calcitic species are spoiled by contaminating overgrowths. Because Cd incorporation into Hoeglundina shows little depth dependence, a comparison of calcitic and Hoeglundina Cd data can be used to verify the assumption that the depth dependence observed for calcitic benthic foraminifera is time invariant. This comparison has been undertaken in downcore and last glacial maximum (LGM) samples from the northern Indian Ocean. Aragonitic and calcitic foraminiferal estimates for Cd in the LGM ocean are in excellent agreement. This result indicates upper ocean estimates of LGM Cd are reliable despite a factor‐of‐two variation in Cd uptake by calcitic species over this depth range. Cd and δ13C data indicate that the uppermost waters of the LGM Arabian Sea were strongly nutrient depleted relative to today but that the deepest waters of the northern Bay of Bengal had nutrient concentrations at least as high as or higher than modern levels. A strong vertical gradient of increasing nutrients with increasing depth 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 nutrients increasing from south to north. The LGM deep Arabian Sea and intermediate‐depth Bay of Bengal were nutrient depleted compared to the modern ocean; however, Cd and δ13C data indicate that nutrient depletion in the northern Indian Ocean did not exceed that of the deep eastern tropical Pacific except during the latest part of oxygen isotope stage 2 and (according to Cd data only) the early stages of deglaciation. During oxygen isotope stage 3 and early stage 2, Cd in the northern Indian Ocean and Cd in the eastern tropical Pacific were indistinguishable, although both regions have lower Cd at these times compared to the Holocene.