Abstract

Stable isotopic data (e.g., 13C or 34S) provide a framework for understanding microbial ecology today and for reconstructing both global redox budgets and microbial metabolic activity over Earth history. However, reports of coeval but divergent isotopic data in the stratigraphic record are becoming increasingly common, which calls into question how these proxies are most commonly interpreted (i.e., as reflecting seawater chemistry). Sulfur isotope records in particular are characterized not just by disparate 34S values, but also by differences in the spatial signature and magnitude of isotopic variability. Such discordant data suggest that we do not fully understand how isotopic signatures are generated or subsequently incorporated into and eventually preserved in the rock record. Here we examine the spatial signature and magnitude of isotopic variability in modern marine systems as a function of differential microbial metabolic activity and the surrounding depositional environment. Varying depositional conditions, particularly sedimentary reworking, sedimentation rate, and organic loading, are seen to play a major role in generating and modifying the isotopic signatures of sulfur phases in modern environments. These observations can be extrapolated to investigate records of sulfur cycling preserved in marine sediment cores and in ancient strata. The results suggest that many apparent secular 34S trends may be related to changing depositional environment rather than changes in the global sulfur cycle. Further, this environmental dependence can also help explain coeval but discordant 34S data from within and between sedimentary basins. Together, these observations enable us to refine our interpretations of 34S data that have the potential both to improve our understanding of modern depositional processes and to better constrain the behavior of the sulfur biogeochemical cycle over geological timescales.