Cooling-driven oceanic anoxia across the Smithian/Spathian boundary (mid-Early Triassic)

The Smithian/Spathian boundary (SSB) represents a major climatic-oceanic-biotic event within the ~5-Myr-long recovery interval of the Early Triassic following the end-Permian mass extinction. The SSB was associated with pronounced cooling following the middle Smithian hyper-greenhouse, a ~ +2 to +8‰ positive carbon isotope excursion, and a second-order mass extinction, yet its underlying cause(s) remain poorly understood. Here, we review oceanic environmental changes during the middle Smithian to early Spathian interval and their potential mechanisms. In addition, we undertook an analysis of the carbon‑sulfur‑iron (C-S-Fe) systematics of the South Majiashan section, which was located on the paleo-western margin of the South China Craton during the Early Triassic, to better understand the causation of these environmental changes. This analysis revealed low δ13Ccarb (i.e., the N3 minimum) in the middle Smithian but rapidly rising δ13Ccarb (i.e., to the P3 maximum) and δ34SCAS within the SSB interval (i.e., late Smithian-earliest Spathian). Both the middle Smithian and SSB intervals are marked by development of marine euxinia, which we hypothesize had fundamentally different causes. The middle Smithian was associated with hyperwarming (the “Smithian Thermal Maximum”), which was probably triggered by a peak in magmatic activity of the Siberian Traps LIP. In contrast, the SSB interval was associated with global climatic cooling, which may have stimulated oceanic overturning circulation and upwelling on continental margins, leading to transient local increases in marine productivity and expansion of oceanic oxygen-minimum zones (OMZs). The concurrent positive excursions in δ13Ccarb and δ34SCAS at South Majiashan and other sections worldwide are evidence of enhanced burial of organic carbon and pyrite on a regional or global scale, as well as of a concurrent decline in atmospheric pCO2 that may have positively re-enforced the SSB cooling event.