Mass-dependent fractionation of quadruple stable sulfur isotope system as a new tracer of sulfur biogeochemical cycles

Sulfur isotope studies of post-Archean terrestrial materials have focused on the ratio 34S/32S because additional isotopes, 33S and 36S, were thought to carry little information beyond the well-known mass-dependent relationship among multiple-isotope ratios. We report high-precision analyses of Δ33S and Δ36S values, defined as deviations of 33S and 36S from ideal mass-dependent relationships, for international reference materials and sedimentary sulfides of Phanerozoic age by using a fluorination technique with a dual-inlet isotope ratio mass spectrometer. Measured variations in Δ33S and Δ36S are explained as resulting from processes involve branching reactions (two or more reservoirs formed) or mixing. Irreversible processes in closed systems (Rayleigh distillation) amplify the isotope effect. We outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle, including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation. The isotope systematics discussed here cannot explain the much larger variation of Δ33S and Δ36S observed in Archean rock records. Furthermore, Phanerozoic samples we have studied show a characteristic Δ33S and Δ36S relationship that differs from those measured in Archean rocks and laboratory photolysis experiments. Thus, high precision analysis of Δ33S and Δ36S can be used to distinguish small non-zero Δ33S and Δ36S produced by mass-dependent processes from those produced by mass-independent processes in Archean rocks and extraterrestrial materials.