Atmospherically-derived mass-independent sulfur isotope signatures, and incorporation into sediments

The discovery of sulfur mass-independent fractionation (S-MIF) in Archean sediments has motivated new work on atmospheric sulfur chemistry. Laboratory experiments showed that SO2 photolysis produces S-MIF at wavelengths both shortward of and longward of the SO2 photodissociation wavelength of 220 nm. It has been argued that the underlying S-MIF mechanism at wavelengths <220 nm is SO2 self-shielding. Additional S-MIF signatures associated with SO2 photolysis are possible due to isotopologue-dependent variations in absorption intensity and dissociation probability, which must be evaluated through new spectral measurements. Here, I claim that SO2 photoexcitation, near-UV CS2 photolysis, OCS photolysis, non-statistical sulfur allotrope reactions, and surface reactions during thermochemical sulfate reduction are all unlikely sources of the largest Archean S-MIF, signatures with arguments presented for each proposed source. A potential problem with the theory proposed here is that large mass-dependent fractionation (MDF) accompanies S-MIF during SO2 photolysis. The range of delta S-34 values is similar to 100%. in photochemically produced elemental sulfur, which exceeds the delta S-34 range observed in Archean rocks by a factor of similar to 3-5, and represents a weakness of the photochemical theory for the origin of Archean S-MIF. A combination of chemical and biogenic MDF processes may have acted to reduce the delta S-34 range of SO2 photolysis products. MDF during reactions that form elemental sulfur in the atmosphere and during aqueous phase reaction of HS- with Fe2+ and FeS to form FeS2 may have reduced delta S-34 values by similar to 40 parts per thousand relative to atmospheric SO. A simple mixing model suggests that a mixture of FeS2 in sediments from both elemental sulfur (yielding pyrite with delta S-34>0 and Delta S-33>0) and from bacterial sulfate reduction (BSR) of BaSO4 (yielding pyrite with delta S-34<0 and Delta S-33<0) may contribute to reducing delta S-34 from photochemical values to the observed range in Archean pyrites. (C) 2009 Elsevier B.V. All rights reserved.