Sedimentary parameters control the sulfur isotope composition of marine pyrite

Reconstructions of coupled carbon, oxygen, and sulfur cycles rely heavily on sedimentary pyrite sulfur isotope compositions (δ34Spyr). With a model of sediment diagenesis, paired with global datasets of sedimentary parameters, we show that the wide range of δ34Spyr (~100 per mil) in modern marine sediments arises from geographic patterns in the relative rates of diffusion, burial, and microbial reduction of sulfate. By contrast, the microbial sulfur isotope fractionation remains large and relatively uniform. Over Earth history, the effect of increasing seawater sulfate and oxygen concentrations on sulfate and sulfide transport and reaction may explain the corresponding increase observed in the δ34S offset between sulfate and pyrite. More subtle variations may be related to changes in depositional environments associated with sea level fluctuations and supercontinent cycles.

Deconvolving microbial and environmental controls on marine sedimentary pyrite sulfur isotope ratios

Reconstructions of past environmental conditions and biological activity are often based on bulk stable isotope proxies, which are inherently open to multiple interpretations. This is particularly true of the sulfur isotopic composition of sedimentary pyrite (δ34Spyr), which is used to reconstruct ocean-atmosphere oxidation state and track the evolution of several microbial metabolic pathways. We present a microanalytical approach to deconvolving the multiple signals that influence δ34Spyr, yielding both the unambiguous determination of microbial isotopic fractionation (εmic) and new information about depositional conditions. We applied this approach to recent glacial-interglacial sediments, which feature over 70‰ variations in bulk δ34Spyr across these environmental transitions. Despite profound environmental change, εmic remained essentially invariant throughout this interval and the observed range in δ34Spyr was instead driven by climate-induced variations in sedimentation.

Strong local, not global, controls on marine pyrite sulfur isotopes

Understanding variation in the sulfur isotopic composition of sedimentary pyrite (δ³⁴S_pyr) is motivated by the key role of sulfur biogeochemistry in regulating Earth's surface oxidation state. Until recently, the impact of local depositional conditions on δ³⁴S_pyr has remained underappreciated, and stratigraphic variations in δ³⁴S_pyr were interpreted mostly to reflect global changes in biogeochemical cycling. We present two coeval δ³⁴S_pyr records from shelf and basin settings in a single sedimentary system. Despite their proximity and contemporaneous deposition, these two records preserve radically different geochemical signals. Swings of ~65‰ in shelf δ³⁴S_pyr track short-term variations in local sedimentation and are completely absent from the abyssal record. In contrast, a long-term ~30‰ decrease in abyssal δ³⁴S_pyr reflects regional changes in ocean circulation and/or sustained pyrite formation. These results highlight strong local controls on δ³⁴S_pyr, calling for reevaluation of the current practice of using δ³⁴S_pyr stratigraphic variations to infer global changes in Earth's surface environment.