Lithofacies and in-situ sulfur isotope characteristics of nodules across the Ordovician-Silurian boundary marine shale in South China: Indicative significance for sedimentary environment

As the terminal zone for marine sulfate reduction, the sulfate-methane transition zone (SMTZ) facilitates anaerobic oxidation of methane coupled with sulfate reduction (AOM-SR), integrating the biogeochemical cycles of carbon and sulfur. This process indirectly influences the redox balance of surface geological environments. To investigate the biogeochemical characteristics within paleo-SMTZs, we examined two representative nodules from the Early Silurian, South China. The diagenetic barite and ³⁴S-enriched euhedral pyrite within these nodules indicate a close association with SMTZ. The sedimentary microtextural evidence of the authigenic growth sequence of framboidal pyrite and pronounced heterogeneity δ³⁴S_pyr suggests a multi-stage genesis of nodules. In Type-1 nodules, δ³⁴S_pyr at the edges are as low as 8.6‰. significantly less than the 18.8‰ observed at the centers. At the grain scale, the δ³⁴S within individual pyrite grain 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 based on a diffusion-precipitation model; (2) within the SMTZ, barite dissolution and reprecipitation promote nodule growth, forming ³⁴S-enriched euhedral pyrite and causing strong heterogeneity in the sulfur isotope distribution within some pyrite grains.; and (3) below the SMTZ, sulfate depletion leads to extensive replacement of barite by other minerals. The pronounced concentric structure in Type-2 nodules indicates multiple formation episodes; the initial stage aligns with that of Type-1 nodules, while needle-shaped minerals at the edges formed in response to vertical spatial shifts within SMTZ. Additionally, calcite, typically associated with SMTZs, 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 over calcite within nodules results from microbial activity altering pore water pH and alkalinity, serving as a petrographic fingerprint of organoclastic sulfate reduction within paleo-SMTZs.