General trends in sulfur isotope fractionation produced by natural communities of sulfate reducing prokaryotes studied in flow-through reactors - Implications for the interpretation of the geological record
Stam, M. (2010). General trends in sulfur isotope fractionation produced by natural communities of sulfate reducing prokaryotes studied in flow-through reactors - Implications for the interpretation of the geological record, in: Stam, M. Sulfur isotopes as a tracer for biogenic sulfate reduction in natural environments - A link between modern and ancient ecosystems. Geologica Ultraiectina, 316: pp. 127-148
In: Stam, M. (2010). Sulfur isotopes as a tracer for biogenic sulfate reduction in natural environments - A link between modern and ancient ecosystems. Geologica Ultraiectina, 316. PhD Thesis. Utrecht University, Faculty of Geosciences: Utrecht. ISBN 978-90-5744-178-3. 184 pp., meer
In: Geologica Ultraiectina. Universiteit Utrecht: Utrecht, meer
Interpretation of sulfur isotope variations throughout the geological record relies heavily on biogenic isotope fractionation effect data obtained for modern environments. A consistent set of flow-through reactor experiments have been completed throughout this study to determine sulfate reduction rates (SRRs) and sulfur isotope fractionation effects (e) produced by natural communities of sulfate reducing prokaryotes (SRP) hosted in sediments from new and diverse geochemical settings. These include a brackish tidal estuary, a hypersaline soda lake and a shallow marine hydrothermal system. Data from each of these sites are reviewed and compared in this chapter. Additional new data from a fourth site in the freshwater area of the River Schelde in Belgium are also presented. When considering all sites together SRR ranged from 5 to 179 nmol cm-3 h-1, with corresponding isotope fractionation effects (e), measured using the difference in d34S between sulfate and the corresponding sulfide, of 5 to 43 ‰ that fall within the range predicted by the standard fractionation model of Rees (1973). Isotope fractionation is distinct at each sampling site and differences are most likely linked to electron donor availability and microbial community size and structure, although salinity and cellular energetics may also play a role. Although no clear relationship was found overall between SRR and e, greater isotopic variability was found at relatively low SRR below 20 nmol cm-3 h-1. At high SRR isotope fractionation reaches a minimum of between 5 and 15 ‰ and does not fall to the smaller values expected from the Rees model. The compiled data indicate that relatively small amounts of isotope fractionation (< 20 ‰) are common for microbial communities in sediments in the absence of competition from other metabolic processes, especially under conditions of high sulfate reducing activity. A relatively small isotope effect for microbial sulfate reduction under these close to optimum conditions conflicts with large variations in d34S measured in sedimentary rocks through time. The larger natural variability thus requires additional explanation such as cycles of oxidation and reduction or may imply that the optimum growth conditions of laboratory experiments are not representative of most sedimentary environments. Microbial sulfate reduction may have been more widespread than previously thought on the early Archean Earth where a lack of competition from other heterotrophic metabolisms enabled optimum growth of the SRP at high SRR, with correspondingly small amounts of isotope fractionation that are now difficult to detect.
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