Microscale iron and sulphur isotopic compositions reveal pyritization pathways during early diagenesis

Sedimentary pyrite iron and sulphur isotope compositions (δ⁵⁶Fe_PYR, δ³⁴S_PYR, Δ³³S_PYR) are commonly used to reconstruct global ocean properties and the evolving oxidation state of Earth’s surface, motivating exploration of impacts of diagenesis on pyrite-based proxies. Along with auxiliary petrographic and porewater data, we present coupled microscale δ⁵⁶Fe_PYR-δ³⁴S_PYR-Δ³³S_PYR in accumulating sediments on the oxic margin of the Black Sea. The coevolution of microscale δ⁵⁶Fe_PYR-δ³⁴S_PYR-Δ³³S_PYR distributions provides insight into porewater S species production, consumption, and buildup on pyritization pathways. “Early” pyrite is characterized by low δ⁵⁶Fe_PYR and δ³⁴S_PYR values consistent with microbially-mediated iron and sulphate reduction and iron (oxyhydr)oxide sulphidization at low sulphide-to-iron ratios. In contrast, “sulphidic zone” pyrite displays distinct late-stage morphologies and higher δ⁵⁶Fe_PYR and δ³⁴S_PYR, which reflect sulphide accumulation at the sulphate-methane transition zone and direct sulphidization of residual iron phases. We propose that coupled δ⁵⁶Fe_PYR-δ³⁴S_PYR-Δ³³S_PYR distributions constrain pyritization pathways and microbial and physico-chemical aspects of the depositional environment.