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

Abstract

Given the high oxygen demands of complex metazoans, a sound understanding of marine redox conditions is essential to reconstruct the factors that lead to the dramatic increases in ecosystem complexity and biological diversity in the early Cambrian. In this study, we report new Fe–Mo–C–S geochemical data from black shale-rich strata of the early Cambrian continental margin Yangjiaping section in the Yangtze platform, South China. Integrated Fe speciation and Mo abundance data from the Yangjiaping section, in combination with previously reported data from Shatan (inner shelf), Songtao (slope) and Longbizui (basinal) sections, suggest mid-depth euxinic waters coexisting with ferruginous deep waters and oxic surface waters. This redox structure is similar to conditions previously suggested for the Ediacaran Yangtze platform and other Proterozoic marine basins, indicating that Proterozoic layered ocean redox structure in some basins persisted into the early Cambrian. Furthermore, the Fe–Mo data demonstrate that the mid-depth euxinia was widespread during a marine transgression but shrank gradually along with widespread shelf oxygenation when the basin evolved into subsequent regression.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.