[B-CG06] 地球史解読：冥王代から現代まで

[BCG06-13] 前期三畳紀スミシアン-スパシアン境界期の海洋無酸素化イベント

*高橋 聡¹、山北 聡²、武藤 俊³、小嶋 智⁴、海保 邦夫⁵、山崎 慎一⁶、土屋 範芳⁷、熊 怡俊⁸、ポールトン サイモン⁸、ウィグナル ポール⁸、松本 廣直⁹、板井 啓明¹¹、藤林 恵¹⁰ (1.名古屋大学大学院環境学研究科、2.宮崎大学教育学部、3.産業技術総合研究所、4.岐阜大学工学部、5.東北大学大学院理学研究科、6.東北大学大学院環境科学研究科、7.八戸工業高等専門学校、8.リーズ大学地球環境学部、9.筑波大学生命環境系、10.九州大学工学部、11.東京大学大学院地球惑星科学専攻)

キーワード：大量絶滅、コノドント、酸化還元環境、海洋富栄養化

The Smithian–Spathian transition during the Early Triassic was associated with significant environmental change, including global warming and oceanic anoxia. However, understanding ocean redox conditions in the pelagic Panthalassa during this period has been challenging due to a lack of well-dated sedimentary sections. Here, we provide new geochemical data for a Smithian–Spathian succession from an accretionary complex of central Japan (Inuyama, Aichi Prefecture), which was deposited in a low-latitude abyssal location. Conodont index fossils and carbon isotope stratigraphy reveal that the studied sections span the middle Smithian to lowermost Spathian (Momotaro-Jinja lower section), and the middle part of the Spathian (Momotaro-Jinja upper section). The Smithian–Spathian boundary is placed within a thick (~40 cm), organic-rich, black claystone layer in the lower part of the section. We reconstruct water column redox conditions using pyrite framboid size ranges, Fe speciation and redox sensitive trace elements. During the middle to late Smithian, we find that bottom waters changed from oxic, through ferruginous anoxic, to euxinic during the latest Smithian to earliest Spathian. During the middle Spathian, the water column fluctuated between oxic and euxinic conditions. We suggest that late Smithian seawater warming contributed to deoxygenation of Panthalassa whilst surface productivity and organic matter fluxes were low, leading to non-sulfidic ferruginous anoxia. Productivity then increased during the latest Smithian to earliest Spathian, thereby stimulating microbial sulphate reduction and inducing widespread ocean euxinia.