Sediments methane formation

In the case where the anaerobic processes take place under conditions where consumption of Ss by the sulfate-reducing biomass and the fermenting biomass must be considered, Equation (7.10) expresses the total anaerobic hydrolysis rate. This equation is based on the assumption that methane formation in sewers without sediment normally can be neglected (Section 3.2.2). 

Surfactants such as LAS and NPEO have been found in compartments with low oxygen content, such as anaerobic sludge digesters or anaerobic continental and marine sediments [14,15,18-25]. One of the possible causes of this persistence is the inhibition of the anaerobic digestion [17,26,27]. Battersby and Wilson [27] observed inhibitory effects of NP at 50 mg CL-1 on methane formation in a survey of the anaerobic biodegradation potential of organic chemicals in digesting... 

Macgregor, A.N. and Keeney, D.R., 1973. Methane formation by lake sediments during in vitro incubation. Water Res. Bull., 9 1153—1158. 

In near-shore regions where organic matter flux to the sediments is high or bottom water oxygen concentrations are low and horizontal sediment transport does not dominate, sulfate reduction and subsequent methane formation are important processes. Early measurements of S04 and CH4 in marine porewaters indicated that methane appears only after most of the SO " has been reduced... 

The formation or release of methane from clathrate hydrates can have a profound impact on the isotopic value of surface ocean HCO3 . As outlined above, methane formation impacts the value of Aq. The rapid oxidation of methane released from sediment reservoirs influences atmospheric pC02 in a manner similar to volcanic emissions, except that the 5 C value of CO2 converted to methane is much lower than the typical 5 C value of volcanic or metamorphic CO2. Intuitively, the 5 C value of the surface ocean HCO3 will be lowered by the release of sediment methane. A more formal... 

The degradation of organic matter via fermentative pathways to small organic molecules such as lactate, butyrate, propionate, acetate, formate, Hj and CO is very important in marine sediments, since these compounds are the main substrates for sulfate reduction and partly for methane formation. 

Since methane formation is shifted at least as strongly as sulfate reduction towards sediments with high organic deposition, an equally small fraction of the global biogenic methane production presumably takes place in the deep sea below 1000 m... 

Methanogenic bacteria or eukaryotes are not known to exist. The primary sourees of methane formation in marine sediments are from the splitting of acetate (CH COO) and from the reduction of CO by hydrogen (Eq. 8.6 and 8.7) ... 

Methanogens require neutral pH conditions to function optimally. Methane prodnction occnrs only after a sequential reduction of other inorganic electron acceptors (e.g., Fe +, Mn +, SO ) it is produced at redox levels below which sulfate in soil is rednced. Methane prodnction is inversely related to sulfate concentration in wetland soils and sediments. High snlfate levels snch as those found in seawater can inhibit methane formation (DeLaune et al., 1983). 

It is believed that the bacteria that synthesize methane produce methylcobalamin as an intermediate in the synthesis. Thus, waters and sediments in which anoxic decay leading to methane formation is occurring provide the conditions under which methylmercury production occurs. In neutral or alkaline waters, volatile dimethylmercury, (CH3)2Hg, may be formed. 

Oremland RS, JP Zehr (1986) Formation of methane and carbon dioxide from dimethylselenide in anoxic sediments and by a methanogenic bacterium. Appl Environ Microbiol 52 1031-1036. 

Because free gas (or gas-saturated water) is less dense than either water or sediments, it will percolate upward into the region of hydrate stability. Kvenvolden suggested that a minimum residual methane concentration of 10 mL/L of wet sediment was necessary for hydrate formation. The upward gas motion may be sealed by a relatively impermeable layer of sediment, such as an upper dolomite layer (Finley and Krason, 1986a) or the upper siltstone sequence, as in the North Slope of Alaska (Collett et al., 1988). Alternatively, permafrost or hydrate itself may act as an upper gas seal. These seals can also provide traps for free gas that has exsolved from solution, and the seals can subsequently act to provide sites for hydrate formation from the free gas. 

Using innovative experiments, Tohidi and coworkers (2001) and Anderson et al. (2001) have shown that hydrates can be formed in artificial glass pores from saturated water, without a free gas phase. They found that with significant subcooling the amount of hydrate formation was proportional to the gas solubility carbon dioxide formed more hydrates from a saturated solution than did methane. Further, the maximum amount of methane hydrate formation was fairly low— about 3% of the pore volume—a value consistent with the amount of hydrates in sediment. 

Figure 7.9 (from Roberts, 2001) gives an overview mechanism of hydrate formation in the Gulf of Mexico as a function of the flux of methane through sediments ... 

However, other researchers suggest that considerably less methane can be generated by in situ hydrate production. Hyndman and Davis (1992) indicated that an unaccountably high concentration of gas was required for hydrate formation. Minshull et al. (1994), Pauli et al. (1994), and Klauda and Sandler (2005) suggest that for in-place formation, under the best conditions the maximum amount of hydrate that can fill the sediments is 3%. 

Sediment stratigraphy controls the hydrate distribution at Hydrate Ridge. The methane-rich migration pathway of Horizon A provides enriched hydrate formation relative to other sediments. In Figure 7.26, the remote sensing logs (gamma ray, density, RAB, and Archie water saturation) are most sensitive to hydrates. 

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