Predominant microbial iron reduction in sediment in early Cambrian sulfidic oceans

Abstract Microbial sulfate reduction (MSR) and microbial iron reduction (MIR) are two major anaerobic metabolic pathways, and account for the majority of anaerobic organic matter degradation in modern marine sediments. Because it is thermodynamically more favorable, the zone of porewater Fe2+ accumulation usually overlies the zone of porewater H2S accumulation in modern marine sediments. The sequence of redox reactions is collectively known as the ‘redox latter’, which has been used as a yardstick in reconstructing the marine redox landscape in Earth's history. To understand the redox profile in the early Cambrian oceans, here we analyzed syndepositional and diagenetic pyrite (FeS2) in carbonate concretions hosted in the black shale of the early Cambrian Shuijingtuo Formation in the Yangtze Block, South China. Syndepositional pyrite laminae, disseminated pyrite and diagenetic pyrite aggregates in the periphery of carbonate concretions have nearly identical sulfur isotope compositions, which cannot be resolved by quantitative reduction of seawater sulfate. Instead, a homogeneous H2S source from sulfidic (H2S-enriched) seawater might be the major sulfur source, and pyrite precipitation was sustained by H2S diffusion from sulfidic seawater. In addition, the homogeneous pyrite sulfur isotopes also imply negligible MSR in sediment porewater, reflecting low sulfate concentration in sulfidic seawater. On the other hand, the enrichment of Fe2+ in carbonate concretions implies deposition in ferruginous (Fe2+-enriched) porewater, resulting from active MIR that reduced detrital iron oxides to Fe2+. Thus, the early Cambrian sulfidic ocean might be characterized by extensive MSR in seawater and MIR in sediment porewater, which differs from the modern sulfidic ocean, such as the Black Sea, showing predominantly MSR in sediment. The development of such a reverse redox profile might be attributed to the terrestrial supply of particulate iron oxides that passed through the thermodynamically unstable sulfidic water column and low seawater sulfate concentration in sulfidic seawater that prohibited MSR in sediment. Finally, our study indicates that the redox profile might be diverse in the early Cambrian oceans.