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.