Late Quaternary changes in biogenic opal fluxes in the Southern Indian Ocean

Late Quaternary sedimentary and paleoenvironmental conditions in the southern Indian Ocean have been reconstructed from radioisotope and proxy element profiles (biogenic opal and organic carbon) measured on five sediment cores taken along a transect across the Indian sector of the Antarctic Circumpolar Current. Dissolution-corrected opal rain rates were used to reconstruct past changes of opal productivity for this region. Records from these five cores indicate that opal productivity during glacial periods was lower than presently recorded south of the Antarctic Polar Front (APF), probably due to increased ice cover. North of APF, opal productivity was slightly greater during glacial periods than during the Holocene, probably in response to (1) the northward migration of the APF by approximately 5° latitude, (2) a northward transport of Si from the Antarctic Zone, and (3) an increase of Fe, necessary for opal-producing organisms, via upwelling and the erosion of the Kerguelen Plateau. We also invoke a decoupling between opal burial and organic carbon flux to the seabed to explain the variation in buried Si/C ratio between glacial and interglacial sediment. This decoupling is principally explained by better organic carbon preservation in the glacial sediments due to strong sediment focussing. An increase in glacial export paleoproductivity is not supported by the data, implying that bioproductivity variations in the Southern Indian Ocean are unlikely to have contributed to the glacial drawdown of atmospheric CO2 inferred from ice core data.