Distribution of diatoms in sediments of the northern Indian Ocean: Relationship to physical oceanography

Abstract

Diatoms in surface sediments and selected downcore intervals of the Indian Ocean confirm variations in the northern Indian Ocean monsoon and constrain its initiation to the late middle Miocene. Presence/absence of diatoms in surface sediments reflects surface water productivity and is influenced by the hydrochemical boundary at about 10–12°S. South of this boundary, diatom productivity is low due to the nutrient depleted subtropical gyre, but productivity in the northern Indian Ocean is high. Three geographically distinct assemblages occur in the northern Indian Ocean, two of which are related to the Southwest Monsoon, while the third is related to the Northeast Monsoon. Of the 19 samples examined at the 18,000 yr B.P. level, only one contained enough diatoms for analysis. This supports previous reports that the Southwest Monsoon was shut down or weakened during this time. Similarly, productivity in the eastern tropical Indian Ocean would also be affected by the absence of runoff into the Bay of Bengal restricting the plume of low salinity water flowing to the west of Sumatra. Studies of surface sediment and 18,000 yr B.P. diatoms point to a strong relationship between the reversing monsoon system and diatom presence. By studying diatom occurrence in sites which have been north of the hydrochemical boundary during the middle and late Miocene we can learn something of the history of the reversing monsoon system. DSDP Site 238 records the initiation of this system in the late middle Miocene and a further strengthening in the middle late Miocene.