An 80 million year oceanic redox history from Permian to Jurassic pelagic sediments of the Mino-Tamba terrane, SW Japan, and the origin of four mass extinctions

摘要

Fabric and pyrite framboid size analysis of Permian to Jurassic samples from the Mino-Tamba terrane of Japan provide an 80 myr redox history from the Panthalassa Ocean. Fully oxygenated conditions dominated but were punctuated by three phases of anoxia (euxinia) during the Permo-Triassic boundary, the late Early Triassic (Spathian Stage) and the late Early Jurassic (Toarcian Stage). A Permo-Triassic superanoxic event of 10 myr duration is thus resolved into a complex redox history of dysoxic-anoxic background conditions punctuated by short-lived euxinic pulses. Both the end-Permian and Toarcian anoxic episodes began abruptly with the development of siliceous daystones, characterized by a framboid-dominated pyrite fraction and no organic C enrichment, interpreted to record euxinic conditions. These facies are overlain by organic-rich shales with slightly larger framboids, more crystalline pyrite and bioturbation indicating accumulation beneath a better-ventilated, higher productivity water column. Thus, an initial phase of ocean-wide stagnation was followed by improved circulation attributed to the onset of warm saline bottom water circulation. Five radiolarian crises during our study interval show a complex relationship with redox conditions. The latest Permian mass extinction coincided with the onset of ocean euxinia but two other crises (Middle Permian and end Triassic), occurred during fully oxygenated intervals. Radiolarian radiation events are also not consistently linked with redox conditions: diversification in the early Late Permian coincides with the onset of weakly dysoxic conditions, late Early Triassic and late Early Jurassic radiations coincide with euxinic events, and the basal Jurassic radiation occurred during a fully oxic phase.