Mass-Independent Fractionation of Sulfur Isotopes in Archean Sediments: Strong Evidence for an

Mass-independent fractionation (MIF) of sulfur isotopes has been reported in sediments of Archean and Early Proterozoic Age (.2.3 Ga) but not in younger rocks. The only fractiona-tion mechanism that is consistent with the data on all four sulfur isotopes involves atmo-spheric photochemical reactions such as SO2 photolysis. We have used a one-dimensional photochemical model to investigate how the isotopic fractionation produced during SO2 pho-tolysis would have been transferred to other gaseous and particulate sulfur-bearing species in both low-O2 and high-O2 atmospheres. We show that in atmospheres with O2 concentra-tions,1025 times the present atmospheric level (PAL), sulfur would have been removed from the atmosphere in a variety of different oxidation states, each of which would have had its own distinct isotopic signature. By contrast, in atmospheres with O2 concentrations$1025 PAL, all sulfur-bearing species would have passed through the oceanic sulfate reservoir be-fore being incorporated into sediments, so any signature of MIF would have been lost. We conclude that the atmospheric O2 concentration must have been,1025 PAL prior to 2.3 Ga. Key Words: Anoxic Archean atmosphere—Mass-independent fractionation—Atmospheric sulfur cycle. Astrobiology 2, 27–41