Sulfur Biogeochemistry—Past and Present

12 V. BrüchertFigure 4. Depth profiles of concentration and sulfur isotope composition of dissolved sulfate and sulfide in organic-rich sediment from the anoxicNamibian shelf. Steep sulfate and sulfide concentration profiles are the consequence of high bacterial sulfate reduction rates and absence of sulfideoxidation. Isotope profiles of dissolved sulfate and sulfide are correspondingly steep. Note that this system is open to diffusion. The isotopedata were fit to a diffusion-reaction model to calculate the diffusive flux of dissolved 32 S and 34 S-sulfate, from which an isotope composition of+15.2 ‰ versus VCDT for the diffusive flux of sulfate across the sediment-water interface was calculated. The resulting bulk sediment isotopefractionation is only 5‰, suggestive of a predominantly incomplete-oxidizing sulfate-reducing community.by incomplete-oxidizing sulfate-reducing bacteria. Both findingsare consistent with the measured sulfate reduction rates in thishighly active near-shore coastal upwelling sediment.Whole-Sediment Incubations in Arctic EnvironmentsIn a third study, isotope fractionations were determined inanoxic bag incubation experiments of whole sediment from fjordstations around Spitsbergen at 79°N (Station J, Smeerenburgfjorden),using methods described in Hansen et al. (2000). Thesediment temperature was only 0.5 °C. Isotope fractionationswere determined following the decrease in sulfate concentrationwith time and the associated shift in sulfur isotope compositionof dissolved sulfate according to methods described in Habichtand Canfield (1997). The isotope fractionation factors (ε) werecalculated using the Rayleigh equation (Brüchert et al., 2001).Isotope fractionations between 9‰ and 18‰ were determinedfor surface sediment (0–3 cm depth) (Fig. 5). Sediment from the3–6 cm sediment depth interval, however, yielded a fractionationof 35‰. These results are consistent with the molecular ecologicalanalysis of the community of sulfate-reducing bacteriain the upper 10 cm of sediment at this station, based on 16SrRNA-targeted FISH with oligonucleotide probes (Ravenschlaget al., 2000). Their results indicated the overall dominance ofcomplete-oxidizing bacteria of the nutritionally versatile clusterDesulfosarcina/Desulfococcus spp. (>70%). However, incomplete-oxidizingbacteria, such as Desulfovibrio and Desulfotaleaspecies, were relatively more abundant in the uppermostcentimeter, consistent with the lower whole-sediment isotopefractionation. The whole-sediment sulfate reduction rate did notcorrespond to the change in isotope fractionation. These ratesvaried only between 5 and 15 nmol cm −3 day −1 , with the higherrates in sediment intervals with higher fractionations.Sulfur Isotope Fractionation Associated with CoupledSulfate Reduction and Anaerobic Oxidation of MethaneA unique marine environment is that associated with theanaerobic oxidation of methane (Valentine and Reeburgh, 2000).There is molecular genetic evidence that this process is catalyzedby a syntrophic association of Archaea and sulfate-reducing bacteria(Boetius et al., 2000). Hydrogen and formate have been proposedas potential interspecies transfer agents. In this environment,the substrate spectra for the sulfate-reducing bacteria are defined