A diatom and benthic foraminiferal record from the South Tasman Rise (southeastern Indian Ocean): implications for palaeoceanographic changes for the last 200,000 years

The last two glacial/interglacial cycles represented in core MD88-779 in the southeastern Indian Ocean over the South Tasman Rise (47°50.69'S, 146°32.75'E; 2260 m water depth) have been analysed. An oceanographic reconstruction for this region is presented, based principally on the recovery of benthic foraminiferal and diatom assemblages. The most striking feature of the microfossil record from core MD88-779 is the 'lack' of diatoms during glacials, despite evidence of high oceanic productivity. Benthic foraminiferal data suggest significant increases in ocean surface productivity during glacial periods and, in particular, during isotopic stages 2, late 3 and 6. For these periods of elevated surface productivity, substantial, and at times total, dissolution of diatom frustules occurred. We propose that increased influx of aeolian dust and especially associated iron during glacial periods may have reduced the intake of silica in diatoms, thus resulting in less silicified diatoms. Additionally, winnowing is suggested to have removed remaining frustules from the sedimentary record. Increased productivity at the sea-surface is indicative that both the Subtropical Front and the Subantarctic Front may have shifted northward during glacial periods and that the Subantarctic Front was near the coring site on the South Tasman Rise for these periods. We also postulate, based on diatom floral evidence, that southward-moving eddies generated by the subtropical East Australian Current may have intermittently reached the coring site even during glacial periods.