On the occurrence of the giant diatom Ethmodiscus rex in an 80-ka record from a deep-sea core, southeast of Sumatra, Indonesia : implications for tropical palaeoceanography

Abstract

Environmental changes recognised in a 80-ka record obtained from a deep-sea core taken at a water depth of 2542 m offshore southeast Sumatra conclusively indicate, for the first time, that 'blooms' of the giant diatom Ethmodiscus rex occurred in the Indian Ocean during the last glacial period, with the largest numbers coinciding with the Last Glacial Maximum. Previous occurrences of E. rex in both the Indian and the western Pacific Oceans could not be dated due to the paucity of foraminifers and contamination by diagenetic carbonate found in association with the diatoms. We use a reliable δ18O record to date the core. The interpretation of terrigenous clays and trace metals in the core sediments, together with carbon isotopes measured on planktic foraminifers, suggests that conditions that favoured the diatomaceous blooms were a combination of changes in hydrological regime in the region caused by a substantial increase in salinity near the surface, and coinciding with a rise in nitrate levels near the surface. No major upwelling was recorded during glacial times. During the glacial period, the Indonesian Archipelago, as other tropical regions, was much drier, thus preventing a low-salinity 'cap' to occur at the surface of the oceans in contrast with the Holocene. In addition, monsoonal winds were absent, thus permitting the ocean to be permanently stratified, with high levels of silica and nutrients near the surface. The glacial mode of productivity, represented by the 'blooms' of E. rex added considerable amounts of biogenic silica to the sediment, but is not reflected by 'classic' productivity proxies like biogenic barium. Therefore, we have to distinguish between a 'classic' productivity mode as observed in the photic zone today and a glacial 'deep' productivity, dominated by E. rex, which may have utilised nutrients from a wider depth range.