HR: 0800h
AN: B21D-0912    [Abstracts]
TI: A Revised Isotope Fractionation Model for Dissimilatory Sulfate Reduction in Sulfate Reducing Bacteria
AU: Benjamin, B
EM: ben.brunner@alumni.ethz.ch
AF: Astrobiology Research Group, JPL, Caltech, Pasadena, Ca 91109 United States
AU: * Bernasconi, S M
EM: stefano@erdw.ethz.ch
AF: Geological Institute, ETH-Zurich, Zurich, 8092 Switzerland
AB: Sulfur isotope fractionation during dissimilatory sulfate reduction is related to the stepwise reduction of sulfate to sulfide within the cells of the bacteria. The magnitude of fractionation is dependent on the interplay between different reduction steps in a chain of reactions. One of the most intriguing questions in sulfur isotope geochemistry stems from the observation that in natural environments, sulfides are commonly depleted in $^{34}$S by -45\permil to -70\permil relative to sulfate whereas maximum sulfur isotope difference between produced sulfides and sulfate of around -46\permil have been obtained in laboratory cultures. A maximum fractionation of 47\permil was also predicted by the model of sulfate reduction introduced by Rees (1973). The Rees model is commonly accepted but since its introduction, new information about sulfate reduction and isotope fractionation processes has become available in the literature that demands an update of some of its assumptions. We present a improved model for bacterial sulfate reduction which includes revised fractionation factors for the sulfite-sulfide step, a multi-step reduction of sulfite to sulfide including reverse flows and an exchange flux of sulfide between the cell and ambient water. With this model we show that, contrary to the model of Rees (1973), isotope fractionations well in excess of -47\permil are possible. Therefore, some of the large sulfur isotope fractionations observed in nature may be explained without the need of alternate pathways involving the oxidative sulfur cycle as proposed by Canfield and Thamdrup (1994). In particular, we speculate that large fractionations should occur under hypersulfidic conditions and substrate limitation. We obviously do not disregard the involvement of processes related to the oxidative cycle of sulfur in near-surface environments, but our model suggests that this is not a prerequisite condition to obtain large isotope fractionations. References: Canfield D. E. and Thamdrup B. (1994) Science, 266, 1973-1975. Rees (1973) Geochimica et Cosmochimica Acta, 37, 1141-1162
DE: 4840 Microbiology
DE: 4870 Stable isotopes
DE: 4803 Bacteria
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting