[MIS20-P07] Mineralogical and geochemical study of pelagic sedimentary rocks in the Permian / Triassic boundary of the Mino Terrane, central Japan
Keywords:Permian-Triassic boundary, bedded chert, Mino terrane, Panthalassa ocean, ocean anoxia
The study section, namely NF1212F, consists of Upper Permian chert (Changhsingian) and Lower Triassic (Induan) black claystone intermittently with thin chert beds. A total 25 claystone samples were obtained from the section following the sample number of Sano et al. (2012). Powder x-ray diffraction (XRD) analysis shows that the claystones below the PTB contained quartz, magnetite, illite and chlorite, while all the claystone above the boundary only contained quartz and illite. The result of XRF and ICP-MS analysis reveals that redox-sensitive trace-element (vanadium [V], chromium [Cr], molybdenum [Mo], and uranium [U]) increase in uppermost Permian chert beds. This trend suggests that strong reducing conditions developed before the end-Permian mass extinction. The increasing trend in Mo/U ratio from 150 cm to 25 cm below the PTB indicates shift from anoxic to euxinic conditions. This stratigraphic interval represents ca. 100 kyr based on the δ13Corg correlation with Meishan section in South China, or ca. 200 kyr based on the sedimentation rate of bedded cherts estimated by Algeo et al. (2011).