BIOGENIC SILICA ACCUMULATION RATE DURING THE HOLOCENE IN THE SOUTHEASTERN INDIAN-OCEAN

Three box-core transects were selected in the southeastern Indian Ocean to establish the variability of biogenic silica accumulation rate in the main subsystems of the Southern Ocean. Cycladophora davisiana and delta-O-18 chronologies, previously calibrated by C-14 dates, and biogenic silica contents determined by X-ray diffraction analysis were used to calculate accumulation rates. Concurrently, a transfer function was used to quantify the silica loss during the biogenic particulate accumulation in the deep-sea sediments. The average rate of biogenic silica rain on the sea-floor, calculated from the accumulation rate and the amount of dissolved biogenic silica, ranges from less than 0.1 to 16 g opal cm-2 ka-1. During the Holocene, biogenic silica has accumulated at the highest rates on the Southeast Ridge, south of the Polar Frontal Zone. To the north and south, the accumulation rate drops where summer sea surface temperatures are above 8-degrees-C or lower than 2-degrees-C. Biogenic silica dissolution is maximum in marginal sea ice zone. Accumulation rates of biogenic silica can be a useful index to estimate changes of palaeoproductivity in the southeastern Indian Ocean, although there is no strict proportionality between accumulation and silica rain rates.