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.

微生物硫酸盐还原 (MSR) 和微生物铁还原 (MIR) 是两种主要的厌氧代谢途径，占现代海洋沉积物中厌氧有机物降解的大部分。由于在热力学上更为有利，现代海相沉积物中孔隙水Fe ²⁺聚集区通常覆盖在孔隙水H ₂ S 聚集区之上。氧化还原反应的序列统称为“氧化还原后者”，它已被用作重建地球历史上海洋氧化还原景观的尺度。为了了解早期寒武纪海洋的氧化还原分布，我们在这里分析了同沉积和成岩黄铁矿（FeS ₂) 位于华南扬子地块早寒武世水井沱组黑色页岩中的碳酸盐结核。碳酸盐结核外围的同沉积黄铁矿层、浸染黄铁矿和成岩黄铁矿聚集体具有几乎相同的硫同位素组成，无法通过海水硫酸盐的定量还原来解决。相反，来自硫化（富含H ₂ S）海水的均质 H ₂ S 源可能是主要的硫源，而硫铁矿沉淀是通过硫化海水中的H ₂ S 扩散来维持的。此外，同质的黄铁矿硫同位素也意味着沉积物孔隙水中的 MSR 可以忽略不计，这反映了硫化海水中的低硫酸盐浓度。另一方面，Fe的富集碳酸盐结核中的²⁺意味着沉积在含铁（Fe ²⁺富集）孔隙水中，这是由于活性 MIR 将碎屑氧化铁还原为 Fe ²⁺所致。因此，早寒武世硫化洋可能以海水中广泛的 MSR 和沉积物孔隙水中的 MIR 为特征，这不同于现代硫化海洋，如黑海，主要表现为沉积物中的 MSR。这种反向氧化还原曲线的发展可能归因于通过热力学不稳定的硫化水柱的颗粒氧化铁的陆地供应和硫化海水中的低海水硫酸盐浓度，这阻止了沉积物中的 MSR。最后，我们的研究表明，早期寒武纪海洋中的氧化还原曲线可能是多种多样的。