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 S-34/S-32 because additional isotopes,S-33 and S-36, were thought to carry little information beyond the well-known mass-dependent relationship among multiple-isotope ratios. We report high-precision analyses of Delta S-33 and Delta S-36 values, defined as deviations of S-33 and S-36 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 Delta S-33 and Delta S-36 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 Delta S-33 and Delta S-36 observed in Archean rock records. Furthermore, Phanerozoic samples we have studied show a characteristic Delta S-33 and Delta S-36 relationship that differs from those measured in Archean rocks and laboratory photolysis experiments. Thus, high precision analysis of Delta S-33 and Delta S-36 can be used to distinguish small non-zero Delta S-33 and Delta S-36 produced by massdependent processes from those produced by mass-independent processes in Archean rocks and extraterrestrial materials. (c) 2006 Elsevier Inc. All rights reserved.