A sulfate control on marine mid-depth euxinia on the early Cambrian (ca. 529-521Â Ma) Yangtze platform, South China

Excess highly reactive iron (FeHR) in deep water with a hydrothermal origin may have had a control on the mid-depth euxinia, given geochemical and geological evidence for basinal hydrothermal activities. On the other hand, a sulfate limitation on bacterial sulfate reduction (BSR) and sulfide generation was suggested by a weak correlation (R2 ≤ 0.22) between pyrite iron versus FeHR and total organic carbon (TOC) form anoxic deep-water sections and high TOC (averagely 6%) in the deepest-ferruginous-water Longbizui section. Furthermore, a spatial gradient was observed in sulfur isotopic composition of pyrite (averagely from 0.2‰ to 21.0‰), consistent with a lateral gradient of sulfate in concentration or/and in isotope from shore to distal waters on the transgressive Yangtze platform. We accordingly proposed that two distinct sources - riverine input and ocean reservoir provided sulfate for BSR dominantly in near shore and distal areas, respectively, and the euxinic retreat observed for the regression can be explained as a response to the progressive sulfate consumption of the ocean reservoir by intensive BSR and pyrite formation during preceding transgression. Therefore, our results support a sulfate control on the spatiotemporal distribution of mid-depth marine euxinia on the Yangtze platform in early Cambrian when complex metazoans rapidly evolved.