Reactive iron controls sulfur partitioning between pyrite and organic matter in sedimentary rocks

Sulfur isotopic values (δ³⁴S) of pyrite and organic matter (OM) in sediments are widely used for the reconstruction of the sulfur and oxygen cycles as well as pathways of OM preservation. Currently, significant uncertainties persist regarding the mechanism by which sulfur is partitioned between pyrite and OM and its effect on the δ³⁴S record of rocks. Here, we present experimental analysis of iron, carbon, sulfur and δ³⁴S values in marine and lacustrine rock samples. The experimental data was compared with published data of rock samples from the Mesoproterozoic era (1.6 Ga) to the Paleogene period (23 Ma). We also developed a kinetic model to simulate the evolution of δ³⁴S of pyrite and OM under different environmental conditions. Our analysis reveals linear relationships between δ³⁴S value of pyrite and its isotopic difference from δ³⁴S value of organic sulfur (R² ranging from 0.43 to 0.96) for large δ³⁴S ranges of pyrite (110 ‰) and organic sulfur (93 ‰). These ranges and linear trends cannot be explained by variations in the δ³⁴S value of seawater sulfate or by the isotopic fractionation associated with microbial sulfate reduction. Rather, local conditions that change the ratio between reactive iron and sulfate are shown to control the δ³⁴S values of organic sulfur and pyrite and their isotopic gap. This is because reactive iron pyritization rapidly captures isotopically light H₂S generated by microbial sulfate reducers in the early stages of diagenesis, leaving behind heavy H₂S that reacts with OM. In some environments, the isotopic gap between organic sulfur and pyrite correlates with the OM content in the rock, reflecting the critical role of reactive iron in OM preservation via sulfurization. Hence, the role of iron on the partitioning of sulfur between OM and pyrite in sedimentary environments is essential for reconstructing the sulfur cycle and its interaction with the carbon cycle in geological sequences.