Methane biogenic

Of the two sources of methane (biogenic and thermogenic) in natural hydrates, most of the gas is biogenic, that is, from bacterially generated methane, with anecdotal exceptions. 

Atmospheric hydrocarbons produced by living sources are called biogenic hydrocarbons. Vegetation is the most important natural source of non-methane biogenic compounds. Several hundred different hydrocarbons are released to the atmosphere from vegetation sources. Other natural sources include microorganisms, forest fires, animal wastes, and volcanoes. 

An estimate of the annual methane flux into the atmosphere can be calculated by adding the sinks and the annual increase. These data (Table II) indicate that a flux of 375-475 trillion tons(Tg) per year would be required to account for an annual increase of50-60 trillion tons (7). Estimates of sources of atmospheric methane indicate that up to 83% is biogenic in origin (5). The other abiogenic... 

Large accumulations of shallow, biogenic methane are estimated to account for 85% of northern Ontario s forest rings (Hamilton et al. 2004). Forest rings have also been reported to form over accumulations of bitumen, coal and dissolved hydrogen sulfide (Hamilton ... 

About 20% of the World s natural gas is of biogenic origin. Biogenic methane commonly occurs in recent freshwater or marine sediments and results from various bacterial fermentation processes. Typical 813C for methanes from marine sediments... 

These redox reactions are abiogenic, whereas the methane sinks are thought to be biogenic, such as the anaerobic oxidation of methane by archaea as observed at the Lost City vent fields (Figure 19.20). Microbial production of methane has also been observed at this site. 

According to Rice and Claypool (1981), over 20% of the world s natural gas accumulations are of biogenic origin. Biogenic methane commonly occms in recent anoxic sediments and is well documented in both freshwater enviromnents, such as... 

Whiticar MJ, Faber E, SchoeU M (1986) Biogenic methane formation in marine and freshwater environments CO2 reduction vs. acetate fermentation-isotopic evidence. Geochim Cosmochim Acta 50 693-709... 

Biogenic particles which comprise primary (fungal spores, bacteria, viruses, plant debris) and secondary organic aerosol (SOA) from biogenic non-methane VOCs are part of the commonly measured organic carbon fraction. Model results [51] indicate a... 

Ocean sediments with hydrates typically contain low amounts of biogenic methane. 

Sediments with Hydrates Typically Have Low Contents of Biogenic Methane... 

The biogenic methane is generated from anaerobic degradation, accompanied by a sulfate-methane interface (SMI), which can be used to determine the upper boundary of hydrate formation depth (Pauli et al., 2005). 

Methane formed by biogenic processes ranges in < 13C from about — 60%c to —85%c, while methane from thermogenic processes ranges from —25%c to —55%c (Hunt 1979, p. 25). 

Figure 7.4 shows the reduction in sulfates and the corresponding growth of both the parent carbonates and the offspring methane with subbottom depth. The methane production is parallel but lower in isotope production than the carbonates. In Figure 7.4 the sulfur isotope (< 34S) content is defined in an identical manner to Equation 7.2 with the replacement of the fraction 13C/12C by 34S/32S in both the numerator and the denominator, using Canon Diablo meteoritic troilite as a standard. The < 34S value increases from 20-60%c before substantial biogenic methane is produced. 

There are three current models of hydrate formation in the literature (1) in situ formation from biogenic methane, (2) formation from free (perhaps recycled) gas traveling upward, and (3) formation by upward mobile water which exsolves the gas used for hydrate formation. Each model is discussed briefly in Sections 7.2.3.2.1-7.2.3.2.3. 

Hydrate formation by in-place biogenic methane Kvenvolden and Barnard (1983) and Brooks et al. (1985) followed the Claypool and Kaplan (1974) suggestion that free methane can be generated in place using the diagenetic mechanism indicated above. Brooks et al. (1987) indicate that twice the methane solubility amount can be achieved by in situ production. 

---