Microbial sulphate reduction

See Rosing, M.T., 1999, 13C-depleted carbon microparticles in >3700-Ma sea-floor sedimentary rocks from West Greenland, Science 283 674-676 Shen, Y., Buick, R., and Canfield, D.E., 2001, Isotopic evidence for microbial sulphate reduction in the early Archaean era, Nature 410 77-81 and Shidlowski, M.A., 1988, A 3800-million-year isotopic record of life from carbon in sedimentary rocks, Nature 333 313-318. 

Shen Y., Buick R., and Canfield D. E. (2001) Isotopic evidence for microbial sulphate reduction in the early Archaean era. Nature 416,11-U. 

Microbial sulphate reduction is of great antiquity (Shen et al. 2001), as may be sulphur reduction by hydrogen. Sulphate chemistry gives... 

Littke, R., Luckge, A. and Welte, D.H., 1997b. Quantification of organic matter degradation by microbial sulphate reduction for Quaternary sediments from the northern Arabian Sea. Naturwissenschaften, 84 312-315. 

Fig. 3 illustrates a conceptual model of operative redox environments in the Tono region since the present geological structure formed (i.e., at least over the past few tens of thousands of years) [19, 21]. Microbial sulphate reduction, oxidation of organic matter and pyrite precipitation appear to be dominant reactions controlling the redox chemistry of sedimentary porewaters. 

Ramsay, J. A., Li, H., Brown, R. S. Ramsay, B. A. (2003). Naphthalene and anthracene mineralization linked to oxygen, nitrate, Fe(III) and sulphate reduction in a mixed microbial population. Biodegradation, 14, 321-9. 

The isolated deep waters of the Baltic have probably always had low oxygen concentrations. However, the declining trend over recent years means that, in some areas, oxygen concentrations have fallen to zero (anoxic). Under anoxic conditions, respiration of organic matter by microbial sulphate (SO4-) reduction has produced hydrogen sulphides (HS ) (plotted as negative oxygen in Fig. 6.30). 

Reaction paths by which organic carbon could be sequestered in cements include bacterial sulphate reduction or direct microbial oxidation of organic matter, oxidation of methane, and/or the thermal degradation of organic matter (see Curtis, 1977 Irwin et al., 1977). In our samples the depth of cementation was at least several hundred metres, which is deeper than the depth to which marine sulphate survives (Hesse, 1990). Therefore, the organic carbon must have been derived from the oxidation of methane or, more likely, from the thermal degradation of organic matter. 

Degradation of OM is the main biogeochemical process taking place in recently deposited sediments (Berner, 1980). Depth distributions of microbially mediated electron transfer reactions within sediments are determined by their corresponding redox potentials (Stumm and Morgan, 1996). A distinction between an oxic layer (oxic respiration), a suboxic layer (denitrification, Mn(III,IV) and Fe(III) reduction) and an anoxic layer (sulphate reduction and methanogenesis) is usually made (Froelich et al, 1979). 

Respiratory sulphate reduction ideally takes place when all other electron acceptors are exhausted, but significant overlap may occur between the zones of microbial Fe(III) reduction and sulphate reduction due to kinetic constraints, as discussed before. Sulphate concentrations typically decrease to zero within the upper sediment layer (Fig. 1). In freshwater sediments, reduced S formed mainly by reduction of pore-water sulphate, is predominantly present as inorganic S in the form of AVS. Although pyrite is the most stable sulphide mineral, its formation in permanently submerged freshwater sediments is subject to controversy (Rickard et al., 1995). Because, contrary to marine sediments (S-dominated), there is an excess of Fe liberation over HS production in freshwater sediments (Fe-dominated), FeC03 as well as FeS may control pore-water Fe concentrations in the anoxic sediment layer. 

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