We investigated the paleo-redox change across the Triassic-Jurassic (T-J) boundary (similar to 200 Ma) and the Early Toarcian oceanic anoxic event (T-OAE; similar to 183 Ma) recorded in the Upper Triassic to Lower Jurassic pelagic deep-sea cherts in the Inuyama area, Central Japan. The present Fe-57 Mossbauer spectroscopic analysis for these cherts identified five iron species, i.e., hematite (alpha-Fe2O3), pyrite (FeS2), paramagnetic Fe3+, and two paramagnetic Fe2+ 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.
