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 δ34S values is ∼ 100‰ in photochemically produced elemental sulfur, which exceeds the δ34S range observed in Archean rocks by a factor of ∼ 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 δ34S 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 δ34S values by ∼ 40‰ relative to atmospheric SO. A simple mixing model suggests that a mixture of FeS2 in sediments from both elemental sulfur (yielding pyrite with δ34S > 0 and Δ33S > 0) and from bacterial sulfate reduction (BSR) of BaSO4 (yielding pyrite with δ34S < 0 and Δ33S < 0) may contribute to reducing δ34S from photochemical values to the observed range in Archean pyrites.