Refining the early Cambrian marine redox profile by using pyrite sulfur and iron isotopes

It is widely accepted that the emergence of metazoans in the Ediacaran-Cambrian transition might be triggered by oxygenation of the atmosphere-ocean system. However, some Ediacaran and early Cambrian macroscopic fossils (such as multicellular algae) were preserved under sulfidic conditions, challenging the conventional view of the prohibition of eukaryotes in the presence of H₂S. It was proposed that organisms might have lived in some episodic non-sulfidic intervals although the ocean was predominantly sulfidic. However, the hypothesized short-lived non-sulfidic intervals were hardly identified by traditional bulk sample analyses, which cannot provide sufficiently high temporal resolution to resolve the short-term oxygenation events. In this study, we analyzed pyritized sponge spicules and disseminated pyrites from black shales of the early Cambrian Shuijingtuo Formation in South China. We conducted in situ pyrite sulfur (δ³⁴S_py) and iron (δ⁵⁶Fe_py) isotopes as well as bulk sample pyrite sulfur isotope analyses. The intra-crystal or intra-spicule δ³⁴S_py variations reflect the evolution of seawater and porewater geochemistry during pyrite formation, providing a higher temporal resolution of redox fluctuation in the early Cambrian ocean. Our study indicates: (1) The bulk sample pyrite sulfur isotope and pyrite content data suggest diagenetic pyrite precipitation in sediment porewater with H₂S diffusion from the overlying sulfidic seawater; (2) Limited intra-crystal and intra-spicule δ³⁴S_py variations imply that both spicule and disseminate pyrites had a homogenous S source from sulfidic seawater, and dissimilatory sulfate reduction (DSR) in sediment porewater was negligible; (3) The in situ iron isotope analyses indicate active dissimilatory iron reduction (DIR) in sediment that generated excessive ferrous iron (Fe²⁺) to compensate for Fe²⁺ consumption by pyrite precipitation, allowing Fe²⁺ to accumulate in ferruginous porewater. Therefore, the early Cambrian ocean was characterized by active DSR in sulfidic seawater and DIR in ferruginous sediment porewater. In addition, our study demonstrates that the seawater might be persistently sulfidic, and does not support the presence of intermittent non-sulfidic intervals for the survival of organisms. If correct, it is hypothesized that some primitive sponges might have survived in sulfidic seawater, although the seawater H₂S concentration remains unknown.