Oceanic anoxia in Panthalassa during the Early Triassic Smithian–Spathian transition

The Smithian–Spathian transition during the Early Triassic was associated with significant environmental change, including global warming and oceanic anoxia. However, understanding ocean redox conditions in the pelagic Panthalassa during this period has been challenging due to a lack of well-dated sedimentary sections. Here, we provide new geochemical data for a Smithian–Spathian succession from an accretionary complex of central Japan (Inuyama, Aichi Prefecture), which was deposited in a low-latitude abyssal location. Conodont index fossils and carbon isotope stratigraphy reveal that the studied sections span the middle Smithian to lowermost Spathian (Momotaro-Jinja lower section), and the middle part of the Spathian (Momotaro-Jinja upper section). The Smithian–Spathian boundary is placed within a thick (∼40 cm), organic-rich, black claystone layer in the lower part of the section. We reconstruct water column redox conditions using pyrite framboid size ranges, Fe speciation and redox sensitive trace elements. During the middle to late Smithian, we find that the water column changed from oxic to ferruginous anoxic, with a transition to euxinic during the latest Smithian to earliest Spathian. During the middle Spathian, the water column fluctuated between oxic and euxinic conditions. We suggest that late Smithian seawater warming contributed to deoxygenation of Panthalassa whilst surface productivity and organic matter fluxes were low, leading to non-sulfidic ferruginous anoxia. Productivity then increased during the latest Smithian to earliest Spathian, thereby stimulating microbial sulphate reduction and inducing widespread ocean euxinia.