科研之友
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摘要
The discovery of sulfur mass-independent fractionation (S-MIF) in Archean sediments has motivated new work on atmospheric sulfur chemistry. Laboratory experiments showed that SO(2) photolysis produces S-MIF at wavelengths both shortward of and longward of the SO(2) photodissociation wavelength of 220 nm. It has been argued that the underlying S-MIF mechanism at wavelengths <220 nm is SO(2) self-shielding. Additional S-MIF signatures associated with SO(2) 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 SO(2) photoexcitation, near-UV CS(2) 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 SO(2) photolysis. The range of delta(34)S values is similar to 100%. in photochemically produced elemental sulfur, which exceeds the delta(34)S 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(34)S range of SO(2) photolysis products. MDF during reactions that form elemental sulfur in the atmosphere and during aqueous phase reaction of HS(-) with Fe(2+) and FeS to form FeS(2) may have reduced delta(34)S values by similar to 40 parts per thousand relative to atmospheric SO. A simple mixing model suggests that a mixture of FeS(2) in sediments from both elemental sulfur (yielding pyrite with delta(34)S>0 and Delta(33)S>0) and from bacterial sulfate reduction (BSR) of BaSO(4) (yielding pyrite with delta(34)S<0 and Delta(33)S<0) may contribute to reducing delta(34)S from photochemical values to the observed range in Archean pyrites.
- 出版日期2009-9-30
