115 / 2025-04-01 08:44:37

Ocean redox records from deep-sea sedimentary rocks of the Early Triassic Smithian-Spathian pelagic Panthalassa

Smithian–Spathian boundary,Iron speciation,Pyrite framboids,Redox-sensitive elements

Theme 3: Earth-Life Co-evolution Through Time > Session 3f: Rise of Modern-type Ecosystems in Triassic

At the Permian-Triassic boundary, associated with the greatest mass extinction of the Phanerozoic, radiolarian cherts are absent in the low-latitude pelagic deep-sea Panthalassa (Paleo-Pacific). The geological record of this period can be seen in the accretionary complexes of Japan, where the sedimentary strata are mainly composed of siliceous claystone, which contains few siliceous fossils and is dominated by clay, and black claystone, which is enriched in organic matter. It was originally thought that the black claystone beds formed only once, in the latest Permian to earliest Triassic (e.g. Isozaki, 1997). However, recent research has shown that they formed three times (Muto, 2021), during the latest Permian to earliest Triassic (Induan), at the Smithian-Spathian boundary (the middle of the Olenekian), and at the Olenekian-Anisian boundary between the Early and Middle Triassic.

We provide new geochemical data for a Smithian–Spathian succession from an accretionary complex of central Japan (Inuyama, Aichi Prefecture). 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 bottom waters 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 (suggested by Sun et al., 2012) 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.

At the Permian-Triassic boundary, associated with the greatest mass extinction of the Phanerozoic, radiolarian cherts are absent in the low-latitude pelagic deep-sea Panthalassa (Paleo-Pacific). The geological record of this period can be seen in the accretionary complexes of Japan, where the sedimentary strata are mainly composed of siliceous claystone, which contains few siliceous fossils and is dominated by clay, and black claystone, which is enriched in organic matter. It was originally thought that the black claystone beds formed only once, in the latest Permian to earliest Triassic (e.g. Isozaki, 1997). However, recent research has shown that they formed three times (Muto, 2021), during the latest Permian to earliest Triassic (Induan), at the Smithian-Spathian boundary (the middle of the Olenekian), and at the Olenekian-Anisian boundary between the Early and Middle Triassic.

We provide new geochemical data for a Smithian–Spathian succession from an accretionary complex of central Japan (Inuyama, Aichi Prefecture). 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 bottom waters 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 (suggested by Sun et al., 2012) 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.