[M-IS11] 地球掘削科学

[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

*太田 雄貴¹、川幡 穂高^2,3,1、黒田 潤一郎^2,3、鈴木 淳¹、石村 豊穂⁴ (1.産業技術総合研究所、2.東京大学大気海洋研究所、3.東京大学理学系研究科地球惑星科学専攻、4.茨城工業高等専門学校)

キーワード：堆積物記録、生物地球化学データ、ベンガル湾、浮遊性有孔虫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.