[B-CG07] 顕生代の生物多様化：放散と絶滅

[BCG07-06] パンサラサ海遠洋性層状チャートに記録されたカーニアン“湿潤期”における海水オスミウム同位体変動

*冨松 由希¹、野崎 達生^2,3,4,5、佐藤 峰南⁵、高谷 雄太郎⁶、木村 純一⁷、常 青⁷、奈良岡 浩⁸、リゴ マニュエル⁹、尾上 哲治⁸ (1.熊本大学大学院自然科学研究科、2.国立研究開発法人 海洋研究開発機構 海洋機能利用部門 海底資源センター、3.東京大学大学院工学系研究科 資源・エネルギーフロンティアセンター、4.神戸大学大学院 理学研究科 惑星学専攻、5.千葉工業大学次世代海洋資源研究センター、6.早稲田大学創造理工学部環境資源工学科、7.海洋研究開発機構 地球内部物質循環研究分野、8.九州大学 大学院理学研究院 地球惑星科学部門、9.パドヴァ大学)

キーワード：カーニアン湿潤化イベント、オスミウム同位体分析、後期三畳紀、ジュラ紀付加体、微化石、地球化学

The Carnian Pluvial Episode (CPE) is considered as a global environmental change and biotic crisis that occurred during the Carnian (Late Triassic). The climate during the CPE is characterized by a short-lived period of extreme rainfall caused by the maximal development of the Megamonsoon, and an extinction of marine taxa is known to have occurred during uppermost Julian (lower Carnian). Although these events have been considered caused by the Wrangellia Flood Basalt (FB) volcanism, existing studies have found little directly evidence to support this. Here, to clarify the linkage between the eruption of Wrangellia FB and the timing of environmental crises during the CPE, we investigate high-resolution microfossils biostratigraphy and paleo-seawater Os isotope profiles in an Upper Triassic bedded chert succession from the Jurassic accretionary complex of Japan. Our biostratigraphic based on conodonts and radiolarians and osmium isotope data show (i) a continuous decline in the early Julian, (ii) a persistence of lower steady-state in the late Julian, and (iii) an abruptly increase in the end of Julian. The decrease phase of initial ¹⁸⁷Os/¹⁸⁸Os ratios throughout the Julian indicates an increased delivery of unradiogenic Os from the eruption of the Wrangellia FB. Moreover, our geochemical data suggest that enrichment factors of the redox sensitive elements, such as V_EF and U_EF, abruptly increased and recovered in the end of the Julian, which presents first evidence of ocean anoxia in the latest Julian in the pelagic deep-sea Panthalassa. The ocean anoxia in the late Julian has been recognized from widespread deposition of black shales and organic rich marls in from intermediate to shallow water setting of Tethyan sections. Thus, this observation suggest that the ocean anoxia occurred between shallow continental margin and deep-Panthalassa basin at the end of the Wrangellia FB volcanism, and that was likely to have triggered extinction.