Paper No. 0
Presentation Time: 1:45 PM
MICROBIAL SULFUR ISOTOPE FRACTIONATION IN MODERN SEDIMENTS AND THROUGH EARTH HISTORY
Analysis of the stable sulfur isotopes in sulfate
and sedimentary sulfide can identify the activity of sulfate-reducing bacteria
in marine sediments through Earth history. Sulfate-reducing bacteria prefer
to reduce sulfate containing 32S compared to sulfate containing
34S,
and therefore sedimentary sulfides depleted in 32S were likely
formed by sulfate-reducing bacteria. In modern sediments and in sediments
deposited through the Phanerozoic, sulfides are however, generally depleted
in 32S by more than can be explained by sulfate reduction alone.
Additional fractionation to that of sulfate reduction must occur in these
sediments, probably during the reoxidation of sulfide to sulfate. The oxidative
part of the sulfur cycle is very complex and generally no isotope fractionations
are associated with the direct biological and chemical oxidation of sulfide
to sulfate. The only processes known to deplete sedimentary sulfide in
32S during the oxidative part of the sulfur cycle are the microbial
disproportionation of elemental sulfur, thiosulfate and sulfite. The disproportionation
processes might, therefore, be important in Phanerozoic sediments.
In the Precambrian sedimentary sulfides are not
depleted by more than about 45, which is also the maximum fractionation
yet observed during the microbial reduction of sulfate. Additional isotope
fractionation during the oxidative part of the sulfur cycle was probably
not important in the Precambrian. I will discuss which processes within
the sulfur cycle might dominate in Phanerozoic sediments and in the Precambrian.
Sedimentary sulfides depleted in 32S by more than 45 are first
encountered in sediments around 0.8 billion years old. This increase in
fractionation might correlate with an increase in the O2 concentration
in the atmosphere at this time.