Tracing organoclastic sulfate reduction within the sulfate-methane transition zone: Petrographic and in situ sulfur isotope evidence from Early Silurian nodules

As the terminal zone for marine sulfate reduction, the sulfate-methane transition zone (SMTZ) drives anaerobic oxidation of methane coupled with sulfate reduction (AOM-SR), linking the carbon and sulfur biogeochemical cycles. This process indirectly influences the redox balance of surface geological environments. To investigate the biogeochemical characteristics within paleo-SMTZs, we examined two representative Early Silurian period nodules from South China. The diagenetic barite and 34S-enriched euhedral pyrite within these nodules indicate a close association with the SMTZ. Sedimentary microtextures reveal the authigenic growth sequence of framboidal pyrite, and significant δ34Spyr heterogeneity suggests a multistage formation for these nodules. In Type-1 nodules, δ34Spyr content at the edges is as low as 8.6‰—significantly less than the 18.8‰ observed at the centers. At the grain scale, the δ34S within individual pyrite grains ranges from −1.9‰ to 29.1‰. We propose that the formation of Type-1 nodules occurred in three stages: (1) Nodule embryos with 34S-depleted pyrite edges formed in the sulfate reduction zone via a diffusion-precipitation model. (2) Within the SMTZ, barite underwent dissolution and reprecipitation, fostering nodule growth by forming large diagenetic barite. Meanwhile, AOM−SR reduced both the residual 34S-enriched sulfate pool near the top of the SMTZ and sulfate released from dissolving barite, producing 34S-enriched, euhedral pyrite. (3) Below the SMTZ, sulfate depletion led to extensive replacement of barite by other minerals. The pronounced concentric structure in Type-2 nodules indicates multiple episodes of formation. The initial stage resembles that of Type-1 nodules, while needle-shaped minerals at the edges formed in response to vertical shifts within the SMTZ. Additionally, calcite, which is commonly associated with the SMTZ, is notably rare within these nodules. Instead, quartz replaces calcite as the nodule matrix and commonly undergoes pseudomorphic replacement of barite. We suggest that the substantial enrichment of quartz relative to calcite within nodules arises from microbial activity that alters pore-water pH and alkalinity, and serves as a petrographic imprint of organoclastic sulfate reduction within paleo-SMTZs.