Drivers of Sedimentary Pyrite δ34S Values - at Present and in the Past

The most important sulfur sink from a global redox-perspective is diagenetic pyrite produced in marine sediments. The amount and isotopic composition of this pyrite is thought to reflect environmental and physical properties of the ocean. This includes sulfate reduction rate, sulfate concentration, sedimentation rates, organic carbon and reactive iron concentrations and reactivities, porosity of the sediment etc. Our goal is to identify the main drivers that can explain the majority of observed sulfur isotopic composition in pyrite. To this end, we use a diagenetic model and calculate theoretical profiles for organic carbon, reactive iron, and a number of sulfur species and their isotopes in marine sediments. We calibrate our model using 216 sedimentary cores from a wide range of environmental conditions and locations from across the world. The model allows us to calculate burial rates and isotopic composition of pyrite in marine sediments on a global scale as well as infer drivers of the Phanerozoic pyrite d34S record. We show that isotopic composition of pyrite is determined by only three variables: the ratio of sulfate to organic carbon, the ratio of reactive iron to organic carbon, and the porosity. Based on this, we reinterpret Phanerozoic d34S trends as recording a shift in the locus and environmental conditions where pyrite is formed, rather than a change in microbial sulfate reducer fractionation.