Sulfate reduction and methanogenesis

In sediments that lie in coastal waters, organic carbon levels are high enough to support denitrification, iron respiration, sulfate reduction and methanogenesis. As shown in the idealized profile presented in Figure 12.3b, the depth of O2 penetration in organic-rich sediments is typically so shallow as to make the zones of aerobic respiration. 

Relative rates of sulfate reduction and methanogenesis in lakes of varying trophic status are claimed to indicate that sulfate reduction rates are limited by the supply of sulfate (4, 5, 13). According to this hypothesis, at high rates of carbon sedimentation, rates of sulfate reduction are limited by rates of sulfate diffusion into sediments, and methanogenesis exceeds sulfate reduction. In less productive lakes, rates of sulfate diffusion should more nearly equal rates of formation of low-molecular-weight substrates, and sulfate reduction should account for a larger proportion of anaerobic carbon oxidation. Field data do not support this hypothesis (Table II). There is no relationship between trophic status, an index of carbon availability, and rates of anaerobic... 

The studies cited do not clarify what factors determine rates of sulfate reduction in lake sediments. The absence of seasonal trends in reduction rates suggests that temperature is not a limiting factor. Rates of sulfate reduction are not proportional to such crude estimates of carbon availability as sediment carbon content or carbon sedimentation rate, although net reduction and storage of reduced sulfur in sediments often does increase with increasing sediment carbon content. Measured rates of sulfate reduction are not proportional to lake sulfate concentrations, and the relative rates of sulfate reduction and methanogenesis in a variety of lakes do not indicate that sulfate diffusion becomes limiting in eutrophic lakes. Direct comparison of diffusion and reduction rates indicates that diffusion of sulfate into sediments cannot supply sulfate at the rates at which it is reduced. Neither hydrolysis of sulfate... 

Martens, C.S., and Klump, J.V. (1984) Biogeochemical cycling in an organic-rich coastal marine basin. 4. An organic carbon budget for sediments dominated by sulfate reduction and methanogenesis. Geochim. Cosmochim. Acta 48, 1987-2004. 

Sinke A. J. C., Comelese A. A., Cappenberg T. E., and Zehnder A. J. B. (1992) Seasonal variation in sulfate reduction and methanogenesis in peaty sediments of eutrophic Lake Loosdrecht, The Netherlands. Biogeochemistry 16, 43—61. 

Oremland, R.S. and Taylor, B.E., 1978. Sulfate reduction and methanogenesis in marine sediments. Geochim. Cosmochim. Acta, 42 209—214. 

Snppress other microbial processes that regnlate organic matter decomposition (e.g., sulfate reduction and methanogenesis)... 

Thus, as a sandstone passes through early burial environments characterized by sulfate reduction or methanogenesis, Fe/Mn reduction or silicate hydrolysis, and a source of calcium, there is ample opportunity for the precipitation of carbonate minerals. Sulfate reduction and methanogenesis during shallow burial are inevitable processes if the system contains bacteria and organic material. Likewise, Fe/Mn reduction and silicate hydrolysis are common processes in early burial (Berner 1980). It would be a rare sandstone/shale system that during shallow burial would not have the necessary components for sulfate reduction or methanogenesis and for Fe/ Mn reduction or silicate hydrolysis. Thus, the key to the formation of early carbonate cements typically may be the availability of Ca" and Mg". ... 

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