~(57)Fe Moessbauer analysis of the Upper Triassic-Lower Jurassic deep-sea chert: Paleo-redox history across the Triassic-Jurassic boundary and the Toarcian oceanic anoxic event

摘要

We investigated the paleo-redox change across the Triassic-Jurassic (T-J) boundary (～200 Ma) and the Early Toarcian oceanic anoxic event (T-OAE; ～183 Ma) recorded in the Upper Triassic to Lower Jurassic pelagic deep-sea cherts in the Inuyama area, Central Japan. The present ~(57)Fe Moessbauer spectroscopic analysis for these cherts identified five iron species, i.e., hematite (α-Fe_2O_3), pyrite (FeS_2), paramagnetic Fe~(3+), and two paramagnetic Fe~(2+) with different quadrupole splittings. The occurrence of hematite and pyrite in deep-sea cherts essentially indicates primary oxidizing and reducing depositional conditions, respectively. The results confirmed that oxidizing conditions persisted in deep-sea across the T-J boundary. In contrast, across the T-OAE, deep-sea environment shifted to reducing conditions. The first appearance of the gray pyrite-bearing chert marked the onset of the deep-sea oxygen-depletion in the middle Pliensbachian, i.e., clearly before the shallow-sea T-OAE.