[M-IS15] 古気候・古海洋変動

[MIS15-P10] 東部南太平洋チリ沖深海底における過去14万年間の炭酸塩保存・溶解変動

*粕谷 拓人¹、岡崎 裕典²、岩崎 晋弥⁴、長島 佳菜³ (1.九州大学 大学院理学府 地球惑星科学専攻、2.九州大学 大学院理学研究院 地球惑星科学部門、3.海洋研究開発機構　地球環境部門 、4.ブレーメン大学海洋科学研究センター)

キーワード：南大洋、炭酸塩補償、炭素循環、炭酸カルシウム、チリ沖

The Southern Ocean has been suggested to play a key role in the 80-100 ppm decreases in atmospheric carbon dioxide concentration (pCO_2,atm) in the glacial intervals compared to the interglacial through enhanced salinity stratification, dust-borne iron fertilization, and carbonate compensation. In particular, ocean alkalinity increase resulting from CaCO₃ dissolution in the glacial Southern Ocean seafloor might have significantly contributed to lowering pCO_2,atm. However, the lack of sedimentary CaCO₃ record from the Pacific sector of the Southern Ocean prevents comprehensive understanding of the CaCO₃ dissolution effect on the decrease of pCO_2,atm. Here, we present new CaCO₃ burial records from MR16-09 PC3 and International Ocean Discovery Program Site U1543, derived from off Chile, the eastern South Pacific, which cover the past 140 kyr. We measured the bulk CaCO₃ contents and the sieve-based weight (SBW) of a planktic foraminiferal species Globorotalia inflata and conducted scanning electron microscope observation of Globigerina bulloides. We further calculated the mass accumulation rate (MAR) of CaCO₃. The CaCO₃ contents and the MAR showed pronounced orbital scale changes, i.e., the burial increased during the interglacial periods in Marine Isotope Stage (MIS) 5 and 1. In contrast, there was no preservation during the glacial intervals in MIS 6 and 4-2. In MIS 5d, 5b, and 5/4 boundary, bulk CaCO₃ contents and the SBW of G. inflata showed significant decreases. Simultaneously, the G. bulloides shells frequently showed cracked surfaces and broadened pores, i.e., ultrastructure breakdown. These changes suggest CaCO₃ dissolution events occurred on the suborbital scale, during the three cold periods of MIS 5. These dissolution events were probably caused by the undersaturation with respect to calcite in the sediment porewater or the bottom water. Sedimentary bromine (Br) is a proxy of total organic carbon, which increases respiration CO₂ in the sediment porewater and leads to the CaCO₃ dissolution. The X-ray fluorescence (XRF) scanned Br records in the two core sites, MR16-09 PC3 and U1543, exhibited inconsistent patterns suggesting the undersaturated porewater was not the principal reason for the CaCO₃ dissolution events. Therefore, it is more likely that corrosive bottom-water intrusion into the eastern South Pacific was the primary factor controlling the events.