Timing the Origin of Microbial Sulfate Reduction Using Four Isotope Systematics

Details Timing the Origin of Microbial Sulfate Reduction Using Four Isotope Systematics Timing the Origin of Microbial Sulfate Reduction Using Four Isotope Systematics

May 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.v42a..06r&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #V42A-06

Mathematics

Logic

0300 Atmospheric Composition And Structure, 0400 Biogeosciences, 1000 Geochemistry

Scientific paper

Dissimilatory sulfate reduction (DSR) is a pervasive microbial process that converts the weathering product of pyrite back into sulfide. In doing so, it consumes organic matter and thus links the global carbon and sulfur cycles. In modern natural environments, the sulfide produced by DRS is depleted in 34S by 10- 85‰, a range which overlaps other geological and microbial processes. Nevertheless, the consensus view, until recently, has been that DRS was a relatively late addition to the repertoire of microbial energy- producing mechanisms, at least at the planetary scale, and its introduction may have coincided with the first rise of atmospheric oxygen, ca. 2.3 Ga ago. The discovery of anomalously or "mass-independently" fractionated (MIF) sulfur isotopes in Archean sulfides and sulfates provides an additional tool to trace the history of DSR. According to photochemical models for the origin of MIF, sulfide is formed from the reduction of insoluble elemental sulfur that fell from the atmosphere into low-energy sedimentary environments such as those accumulating organic-rich shales; this sulfur was anomalously enriched in 33S and depleted in 36S (Δ33S > 0, Δ36S < 0). Conversely, the oxidized product of photochemical processing left the atmosphere as H2SO4 and accumulated in solution in oceans and lakes; it was anomalously depleted in 33S and enriched in 36S (Δ33S < 0, Δ36S > 0). Although these MIF signals may be destroyed by dilution or mixing, their retention in early Archean sulfates from three continents identifies the isotopic starting point for DSR, if it existed. Similarly, the presence of positive MIF signatures of Archean pyrites finely dispersed in organic-rich shales as young as 2.5 Ga, is evidence against DSR in environments where one would expect sulfate reducing microbes to abound. Using these and other criteria, there is limited evidence for DSR throughout much of the Archean.

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