What Controls Sedimentary Pyrite δ34S Values? Insight From Theoretical Models and Modern Sediment Profiles.

Abstract:

Microbial sulfate reduction in anoxic porewaters of marine sediments generates hydrogen sulfide whose sulfur isotope composition (δ³⁴S) reflects the sum of biological, chemical, and physical processes. Hydrogen sulfide in porewaters subsequently forms iron sulfide minerals such as pyrite (FeS₂), which is preserved in the geologic record. Thus, the evolution of geologically preserved pyrite δ³⁴S values has long been utilized as a proxy for sulfur-cycle processes such as sulfate reduction rate and/or marine sulfate concentrations throughout Earth’s history. However, recent work suggests that preserved pyrite δ³⁴S values are strongly influenced by local physical parameters such as sedimentation rate and organic carbon reactivity, calling into question their utility as a proxy for global sulfur-cycle evolution. To test the relative importance of these controls, here we develop a theoretical model that predicts pyrite δ³⁴S values as a function of a limited number of non-dimensional governing variables. By comparing to a global suite of modern marine sediment profiles, we show that our model can accurately reconstruct observed values despite containing only three free variables. We conclude that decreasing organic carbon reactivity---potentially due to increasing depth of the sulfate reduction zone due to bioturbation---may have played a key role in the observed decrease in pyrite δ³⁴S values across the Phanerozoic Eon.