Monsoon-influenced variations in plankton community structure and upper-water column stratification in the western Bay of Bengal during the past 80 ky

Abstract

To enhance our understanding of the forcing factors of marine biological community structure in the Bay of Bengal, we studied proxies for biogenic carbonate and silicate production in the western Bay of Bengal and compared them with proxies for upper-ocean stratification and strength of the Indian summer monsoon (ISM). Specifically, we investigated the record in a sediment core from the western Bay of Bengal that extended to approximately 80 ky before the present (BP). The records of Globigerinoides ruber sensu stricto δ18O minus Neogloboquadrina dutertrei δ18O (Δδ18Or–d) were used to investigate changes in upper-ocean stratification. We reconstructed variations in mass accumulation rates (MARs) of CaCO3 and biogenic silica (BSi) to discuss plankton community structure. Greater difference between Globigerinoides ruber sensu stricto δ18O and Neogloboquadrina dutertrei δ18O during the marine isotope stage (MIS) 1 and 5a indicate intervals with less saline and more stratified conditions. In contrast, the relatively small difference between both planktic foraminiferal δ18O during MIS 2 and 4 indicates diminished stratification during these periods. Elevated carbonate production during MIS 2 and 4, demonstrated by the increase in CaCO3 MAR, could have been driven by lower freshwater influx to the western Bay of Bengal resulting from lower ISM precipitation, and thus the increased mixing and upwelling of deep nitrate into the photic zone. The relative increase in BSi MAR during periods of high ISM precipitation and decreased salinity of surface waters suggests a limited recovery in biological siliceous production under interglacial conditions, possibly due to enhanced fluvial SiO4 delivery to the ocean. Fluctuations of Δδ18Or–d during MIS 3 indicate that freshwater stratification was moderate and insufficient to restrict upwelling. This might have induced moderate biogenic silica and carbonate productions during this period.