Contrasting genesis of lower Cambrian carbonate concretions in the Yangtze Block: Insights into seawater–porewater redox profiles

Early Cambrian oceans were widely anoxic and locally euxinic, with pronounced water-column redox stratification. However, redox coupling between bottom waters and sediment porewaters remains poorly constrained. Here, we focus on two distinct types of carbonate concretions and their host black shales from the lower Cambrian Qiongzhusi Formation in the Yangtze Block, South China. We performed in situ trace-element analyses of calcite and δ 34 S analyses of sulfide and sulfate minerals. Our results indicate: (1) host-shale geochemistry records predominantly anoxic bottom waters with intermittent euxinia. Z-2 formed during intensifying water-column sulfidity, whereas Z-1 formed during its subsequent weakening. (2) Pyrite largely preserves early diagenetic sulfur isotopic signals despite later fluid modification. The early formation of the two concretion types is interpreted to have been associated predominantly with bacterial sulfate reduction (BSR) and microbial iron reduction (MIR), respectively. Progressive 34 S enrichment from disseminated pyrite to marginal pyrite aggregates records sustained porewater BSR under limited sulfate supply. In contrast, low and relatively homogeneous pyrite δ 34 S values indicate that H 2 S was derived mainly from euxinic bottom waters and that porewater BSR was restricted. (3) Integration of host-shale redox evolution with concretion pyrite records reveals two seawater–porewater redox profiles. One is a normal profile dominated by porewater BSR, whereas the other is a reverse profile characterized by euxinic bottom waters and MIR-dominated ferruginous porewaters. These findings link bottom-water redox changes to porewater microbial processes. They also provide new constraints on benthic redox heterogeneity and marine biogeochemical cycling in low-sulfate early Cambrian oceans.