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Sulfur isotope fraction in an estuarine sediment during enhancement or partial inhibition of microbial sulfate reduction - A preliminary study
Stam, M. (2010). Sulfur isotope fraction in an estuarine sediment during enhancement or partial inhibition of microbial sulfate reduction - A preliminary study, 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. 107-126
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., more
In: Geologica Ultraiectina. Universiteit Utrecht: Utrecht, more

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Document type: Dissertation

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Abstract
    Large microbial sulfur isotope effects of up to 70 ‰ between coexisting sulfate and sulfide reservoirs have been observed in the nature but cannot be reproduced in sediment incubation and pure culture experiments which fractionate up to only 47 ‰. The origin of the excess fractionation in nature is unclear but may be linked to very low rates of sulfate reduction, geochemical variability in for instance type of electron donor, or repeated cycles of oxidation and reduction. In this study the range of measured sulfate reduction rates at a brackish tidal estuary in the Netherlands is expanded by adding compounds that are known to enhance or inhibit microbial sulfate reduction, to investigate the potential for more isotopic variability than found under the site optimum conditions that were used in previous experiments (Chapter 2). Electron donors for sulfate reducing prokaryotes, lactate and acetate (10 mM), were used to increase potential sulfate reduction rates (SRRs), whilst rate reductions were achieved by adding variable concentrations of the group VI oxidized anions chromate, selenate, molybdate and tungstate (0 to 10 mM). Sediments were incubated in flow-through reactors at temperatures from 10 to 30°C. Lactate addition resulted in a 14 fold increase in SRR, whilst isotope fractionation remained comparable to values obtained for the natural substrate. Acetate addition had a negligible effect on SRR but gave more variability, up to 8 ‰, in isotope fractionation when compared against the natural substrate data. Inhibition of SRR with SeO42-, MoO42- and WO42- was complete at concentrations above 5 mM where no isotope effects could be measured. Isotope fractionation was suppressed with a maximum of 12 and 18 ‰ with increasing concentrations from 0 to 1 mM of MoO42- and SeO42- respectively, whilst WO42- and CrO42- showed smaller changes in SRR and isotope fractionation due to strong adsorption of these compounds into the sediment. The total variability in isotope data induced by enhancers and inhibitors of sulfate reduction, 5 to 32 ‰, does not extend the range that is possible across the SRR that would be normally experienced in this sedimentary environment.

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