Reconstruction of Early Triassic ocean redox conditions based on framboidal pyrite from the Nanpanjiang Basin, South China

Widespread oceanic anoxia has been implicated as an important factor in the PTB (Permian Triassic boundary) mass extinction and the delayed recovery of Early Triassic marine ecosystems. An investigation of framboidal pyrite in the Bianyang section (Nanpanjiang Basin, South China) suggests that euxinia/dysoxia peaked during the Induan, late Smithian to earliest Spathian and late Spathian. These anoxic episodes show a relationship, albeit imperfect, to major episodes of climatic warming during the Early Triassic that were associated with intensified oceanic stratification, reduced marine productivity and organic carbon sinking fluxes, as well as diminished burial fluxes of organic carbon and reduced sulfur. In contrast, intervals of better-oxygenated marine conditions were associated with episodes of relative climatic cooling during the early to middle Smithian and mid-Spathian. The degree of ventilation of the thermocline region, in particular, had a profound effect on marine biotas, with intervals of improved ventilation resulting in increased global diversity among ammonoids and conodonts and increased local abundance of foraminifera in the Nanpanjiang Basin. These observations suggest that oceanic redox fluctuations played an important role in the delayed recovery of Early Triassic marine ecosystems, and, specifically, that episodic expansion of oceanic oxygen-minimum zones (OMZs) resulted in repeated setbacks to the recovery process, a pattern that persisted until the late Spathian.