A theoretical model of the stable sulfur isotope distribution in marine sediments

A diffusion-diagenesis model of the sulfur cycle is developed to calculate theoretical distributions of stable sulfur isotopes in marine sediments. The model describes the depth variation in δ³⁴S of dissolved sulfate and H₂S. and of pyrite. The effects of sulfate reduction, sulfate and H₂S diffusion. and of sedimentation are considered as well as the bacterial isotope fractionation and the degree of pyrite formation. Under open system conditions of sulfur diagenesis the isotopic difference, ΔSO^2?₄ — H₂S, tends to increase with depth being smaller than the bacterial fractionation factor near the sediment surface and larger in deeper layers. The two isotopes in SO^2?₄ or in H₂S do not diffuse in the same proportion as they occur in the porewater. This explains why sulfur, which is incorporated from seawater sulfate by diffusion and precipitation as pyrite, can be enriched in ³²S relative to the seawater sulfate. The model calculations demonstrate the importance of taking the whole dynamic sulfur cycle into account before drawing conclusions about sulfur diagenesis from the stable isotope distribution.