Global-ocean circulation changes during the Smithian-Spathian transition inferred from carbon-sulfur cycle records

Early Triassic marine ecosystems experienced multiple environmental perturbations and a delayed biotic recovery following the end-Permian mass extinction. The Smithian-Spathian boundary (SSB), in the late Early Triassic, marks a major shift toward a less intensely warm climate, ameliorated marine environmental conditions, and the onset of a more sustained recovery of marine faunas. Accumulating geochemical evidence indicates that these developments were accompanied by large changes in global-ocean circulation during the Smithian-Spathian (S-S) transition. In the present study, tandem carbon and sulfur isotopic records were used to investigate changes in the marine carbon and sulfur cycles across the SSB and their potential relationship to global-ocean circulation changes. First, we conducted a carbon-sulfur isotope study at two deepwater SSB sections (West Pingdingshan and Jiarong) in South China. High-resolution delta C-13(carb)-delta S-34(CAS) profiles show large positive excursions and strong correlations through the S-S transition, suggesting control by co-burial of organic matter and pyrite. Second, we reviewed global carbon-sulfur isotope studies through the Smithian and Spathian substages. Similar to carbonate carbon isotopes, sulfate sulfur isotopes also show vertical gradient changes during the S-S transition. We link these changes to an expansion of oceanic oxygen-minimum zones (OMZs) in response to a cooling-driven re-invigoration of global-ocean circulation that triggered enhanced productivity on platform margins. OMZ expansion may have contributed to a second-order extinction among conodonts and ammonoids at the SSB, but the long-term shift toward more vigorous ocean circulation and better-ventilated watermasses is likely to have facilitated the overall recovery of marine biotas during the Spathian Substage of the late Early Triassic.