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

WashU affiliated authors: R.N. Bryant, D.A. Fike (Dept. of Earth and Planetary Sciences)

Abstract: Understanding variation in the sulfur isotopic composition of sedimentary pyrite (d34Spyr) 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 d34Spyr has remained underappreciated, and stratigraphic variations in d34Spyr were interpreted mostly to reflect global changes in biogeochemical cycling. We present two coeval d34Spyr 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 d34Spyr track short-term variations in local sedimentation and are completely absent from the abyssal record. In contrast, a long-term ~30‰ decrease in abyssal d34Spyr reflects regional changes in ocean circulation and/or sustained pyrite formation. These results highlight strong local controls on d34Spyr, calling for reevaluation of the current practice of using d34Spyr stratigraphic variations to infer global changes in Earth’s surface environment.