[M-IS14] 古気候・古海洋変動

[MIS14-P14] ヤンガー・ドリアス期の北西太平洋下北沖における底生有孔虫群集解析に基づく溶存酸素濃度復元

*丸田 典弥¹、岡崎 裕典¹、長島 佳菜²、佐川 拓也³、粕谷 拓人¹、熊 威誠¹、横山 祐典⁴、宮入 陽介⁴、木元 克典² (1.九州大学、2.海洋研究開発機構、3.金沢大学、4.東京大学)

Benthic foraminifera are suitable for monitoring the oxygen level of the bottom water because of their rapid responses to changes in dissolved oxygen. Ice sheets covered the northern part of North America and Eurasia during the last glacial period from 100 ka to 20 ka. After that, the ice sheets retreated during the last deglaciation from 20 ka to 7 ka with abrupt millennial-scale climate changes. Among them, the Younger Dryas (12.9 ka – 11.5 ka) was a remarkable cold event in the Northern Hemisphere. The trigger of the Younger Dryas event was the massive meltwater from North America to the North Atlantic, which hampered thermohaline circulation. The Younger Dryas event was observed in the Northern Hemisphere. In the western North Pacific off Shimokita, millennial-scale changes in the sedimentary faces were found during the last deglaciation, laminations during the Bølling-Allerød, 14.5 ka- 12.9 ka, and the Preboreal, 11.5 ka- 10.5 ka around the time of the Younger Dryas. These suggest drastic changes in dissolved oxygen conditions on the seafloor off Shimokita, i.e., dysoxic during the Bølling-Allerød and the Preboreal, whereas oxic during the Younger Dryas. In this study, we investigated the benthic foraminiferal assemblages from the Younger Dryas to Preboreal intervals in the SMK3-PC piston core (41°08.53'N, 142°24.58'E, water depth 1356 m) from off Shimokita obtained during the MR23-05 cruise of the R/V Mirai. Oxic ([O2] > 1.5 mL/L) taxa dominated during the Younger Dryas, and dysoxic ([O2] = 0.3-1.5 mL/L) taxa dominated during the Preboreal. Suboxic taxa ([O2] = 0.1-0.3 mL/L) had a relative abundance peak during the transition from the Younger Dryas to the Preboreal. These changes in benthic foraminiferal assemblages suggest an oxic seafloor condition off Shimokita during the Younger Dryas due to low surface water productivity.