Fe and S isotope ratios in sedimentary pyrite (δ56Fepyr and δ34Spyr) are extensively used in reconstructions of the coupled biogeochemical Fe, S, C and O2 cycles and investigations of sub-seafloor microbial activity. However, an underlying assumption that δ34Spyr variation is related to global S-cycle drivers is refuted by a growing number of observations. To elucidate the controls on δ56Fepyr and δ34Spyr values, we combine (i) bulk and microscale multi-isotope (Fe and multi-S) analyses of well-understood Pleistocene marine sediments, and (ii) a coupled microbial-diagenetic model, which accounts for the transport and (bio)reaction of C, S and Fe species.

Rather than global Fe-cycle and S-cycle drivers of isotopic variation, the studied sediment cores demonstrate clear controls by local variations in depositional parameters like the sedimentation rate, organic matter and reactive Fe content, and reworking by currents. Incorporating these insights into a microbial-diagenetic model, and using gridded datasets of the relevant depositional factors, we reproduce modern δ34Spyr spatial patterns and distributions compiled from the sedimentary rock record. In agreement with the bulk and microscale observations, the model microbial S isotope fractionation remains uniformly large under modern marine conditions, and the main determinants of isotopic variations in pyrite are the depositional parameters. These factors govern the degree of sulfate drawdown and isotopic distillation, and the degree of porewater sulfide accumulation, and through these, the distribution of δ34Spyr.

With the understanding that mostly local depositional parameters control the isotopic composition of pyrite, coupled δ56Fepyr, δ34Spyr and Δ33Spyr measurements by secondary-ion mass spectrometry (SIMS) offer a way forward. Such measurements provide a wealth of information about microbial activity, pyritization pathways, burial diagenesis and the dynamics of the depositional system.