[M-IS30] 遠洋域の進化

[MIS30-02] Non-sulphidic anoxic conditions in the end-Early Triassic deep sea

*高橋 聡¹、山崎 慎一²、小川 和広³、海保 邦夫³、土屋 範芳² (1.東京大学理学系研究科地球惑星科学専攻、2.東北大学環境科学研究科、3.東北大学理学研究科)

キーワード：遠洋域深海相, 海洋無酸素, 微量元素, 三畳紀, 炭素同位体比

This study focuses on an upper Lower Triassic (Spathian) to lowermost Middle Triassic (Anisian) section representing the central Panthalassic deep sea. Analysed organic carbon isotope ratio (δ13Corg) records from the section demonstrate that lower values in the Spathian increase by up to 6‰ at the Spathian–Anisian transition. This trend accords with the carbonate carbon isotope (δ13Ccarb) record from shallow water carbonate sections. The end of the Early Triassic trough of δ13Corg consists of black chert deposits sandwiched by black claystone beds in the study section. In these black-coloured beds and underlying siliceous claystone beds, higher concentrations of redox-sensitive elements, such as Mo and V, and coinciding low sulphide sulphur isotope ratios (δ³⁴Ssulphide) and previously reported sulphur-bearing organic compounds are present, suggesting anoxic deep water. As enrichment factors of Mo are not higher than the typical sulphidic trend, these anoxic conditions did not become sulphidic. Oxygen-poor conditions coinciding with a carbon isotope trough have been also reported in late Early Triassic strata from shallow-water sections. These coincidences imply global environmental perturbations related to the delayed recovery of life after the end-Permian mass extinction.