[B-CG07] Phanerozoic biodiversity change: radiation and extinction

[BCG07-06] Marine osumium isotope records during the Carnian (Late Triassic) “pluvial episode” in the pelagic bedded cherts

*Yuki Tomimatsu¹, Tatsuo Nozaki^2,3,4,5, Honami Sato⁵, Yutaro Takaya⁶, Jun-Ichi Kimura⁷, Chang Qing⁷, Hiroshi Naraoka⁸, Manuel Rigo⁹, Tetsuji Onoue⁸ (1.Graduate school of Science and Technology, Kumamoto University, 2.Submarine Resources Research Center, Research Institute for Marine Resources Utilization, Japan Agency for Marine-Earth Science and Technology, 3.Frontier Research Center for Energy and Resources (FRC- ER), Graduate School of Engineering, Tokyo University, 4.Department of Planetology, Graduate School of Science, Kobe University, 5.Ocean Resources Research Center for Next Generation, Chiba Institute of Technology, 6.Department of Resources and Environmental Engineering School of Creative Science and Enginerring, Waseda University, 7.Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Scienc and Technology, 8.Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, 9.Department of Geosciences, University of Padova)

Keywords:Carnian Pluvial Episode, Osmium isotope analysis, Late Triassic, Jurassic accretionary complex, microfossils, Geochemistry

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.