Rethinking Secular δ34S Records: Stratigraphic Trends and Biogeochemical Interpretations

Abstract:

Sulfur isotope ratio data (δ³⁴S) have been used to provide significant insights into global biogeochemical cycling over Earth history, providing a framework for reconstructing both global redox budgets and microbial metabolic activity. However, as the record of ancient oceanic conditions becomes better resolved, reports of coeval but divergent isotopic proxies are becoming increasingly common. These sulfur isotope records are characterized not just by divergent δ³⁴S 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 incorporated 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 depositional environment and differential microbial metabolic activity. Varying depositional conditions, particularly sedimentary reworking, 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 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 sulfur cycle. Further, this environmental dependence can also help explain coeval but discordant δ³⁴S data from within and between sedimentary basins. Together, these observations provide new insights that enable us to reflect on and refine our interpretations of chemostratigraphic δ³⁴S data that have the potential to constrain the behavior of the sulfur cycle over geological timescales.