Résumé / Abstract
Stable isotopic data (e.g., d13C or d34S) provide a framework for understanding biogeochemical cycling today and for reconstructing both global redox budgets and environmental change over Earth history. These reconstructions are often based on the assumption that the values measured at a given stratigraphic section are globally representative. However, recent reports of coeval but divergent isotopic data in the stratigraphic record call into question how these proxies are most commonly interpreted (i.e., as directly reflecting seawater chemistry). Here we examine d34S signatures in sedimentary pyrites in modern marine sediments as a function of ambient depositional conditions. Varying depositional conditions, particularly 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 d34S trends may be related to changes in the local depositional environment rather than changes in the global biogeochemical sulfur cycle. Further, this environmental dependence can also help explain coeval but discordant deep-time d34S data from within and between sedimentary basins. Together, these observations enable us to refine our interpretations of d34S 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.
Depositional Controls on Sedimentary Sulfur Isotopic (d34S) Records: Rethinking Stratigraphic Trends & Geobiological Interpretations