[M-IS11] Expedition Earth: discovering our planet's past and future through scientific drilling

[MIS11-P01] Monsoon-influenced variations in calcareous and siliceous plankton community structure and upper- water column stratificatio of the western Bay of Bengal during the past 80 kyr

★Invited Papers

*Yuki Ota¹, Hodaka Kawahata^2,3,1, Junichiro Kuroda^2,3, Atsushi Suzuki¹, Toyoho Ishimura⁴ (1.National Institute of Advanced Industrial Science and Technology, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Department of Earth and Planetary Science, The University of Tokyo, 4.National Institute of Technology, Ibaraki College)

Keywords:Sediment record, Biogeochemical data, Bay of Bengal, Planktonic δ18O

To enhance 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 in the western Bay of Bengal that extended to approximately 80 kyr before the present. The records of Globigerinoides ruber sensu stricto δ¹⁸O minus Neogloboquadrina dutertrei δ¹⁸O(Δδ¹⁸O_r–d) were used to investigate changes in upper-ocean stratification. In addition, mass accumulation rates (MARs) of CaCO₃ and biogenic silica were used as proxies of calcareous and siliceous productivities. 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 Δδ¹⁸O_r–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.