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

Abstract

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.