Title:
Depositional Controls on Sulfur Isotopic (δ34S) Records:
Rethinking Stratigraphic Trends & Geobiological Interpretations

Speaker:
David A. Fike
Department of Earth & Planetary Sciences, Washington University

Abstract:
Stable isotopic data (e.g., δ13C or δ34S) provide a framework for understanding microbial ecology today and for reconstructing both global redox budgets and microbial metabolic activity 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 the impact that depositional environment (facies) has on δ34S signatures in the rock record. To better understand the origin of this dependence, we then examine the spatial signature and magnitude of isotopic variability in modern marine sediments as a function of the ambient depositional conditions. 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 δ34S trends may be related to changing depositional environment rather than changes in the global biogeochemical sulfur cycle. Further, this environmental dependence can also help explain coeval but discordant deep-time δ34S data from within and between sedimentary basins. Together, these observations enable us to refine our interpretations of δ34S 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.