Methane sinks

There are only two important sinks that serve to destroy methane. The first is the oxidation of methane by aerobic bacteria in soils whereas the second and the most important sink is reaction (oxidation) with hydroxyl radicals in the atmosphere. Biological oxidation of methane in soils is responsible for 6-10% of the global source strength. Oxidation dne to the reaction of methane with hydroxyl radicals in the atmosphere, however, accounts for the remaining 90% (Cicerone and Oremland, 1988). An estimated 500 Tg year is removed from the atmosphere each year over 95% of the annual emission is removed through these two primary sinks (Khalil et al., 1992). 

In general there is a homogenous consumption rate of methane in soils as opposed to a highly variable methane emission rate within specific geographical regions. Both methane prodnction and consumption are microbially mediated. Methane oxidation rates in soil are controlled in part by nonbiological factors, primarily redox status, and soil porosity. Generally, the rate of methane diffusion into the soil controls the rate of consumption (Keller et al., 1990). 

Methane oxidation cannot occur in the absence of oxygen therefore, in wetland soils the process is only important in the surface oxic-anoxic boundary or within the plant root rhizosphere. However, aerated soil regions or oxic layers have been demonstrated to consume as much as 

80-90% of methane produced (Bender and Conrad, 1992 Oremland and Culbertson, 1992a). Oxidation within a wetland soil is facilitated by rhizospheres where oxygen is transported from the plant to its roots. Florida Everglade studies have shown that the oxidation of methane by the rhizosphere was small compared to that oxidized at the sediment-water interface (King et ah, 1990). Studies performed in rice paddies have shown rhizospheric oxidation of up to 80% of the methane available (Holzapfel-Pschorn and Seiler, 1986 Sass et al., 1990 Schutz et al., 1989). 

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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. 

The first global CH4 budgets were compiled by Ehhalt (1974) and Ehhalt and Schmidt (1978), who used available published information to estimate emissions of CH4 to the atmosphere. They considered paddy fields, freshwater sources (lakes, swamps, and marshes), upland fields and forests, tundra, the ocean, and enteric fermentation by animals as biogenic sources. Anthropogenic sources included industrial natural gas losses and emission from coal mining, and were considered to be free. Observations of CH4 placed an upper limit on anthropogenic sources. Oxidation by the OH radical, as well as loss to the stratosphere by eddy diffusion and Hadley circulation, were presumed to be methane sinks. In spite of lack of data, this work correctly identified the major atmospheric sources and did... 

Staffelbach T, Neftel A, Stauffer B, et al. 1991. A record of the atmospheric methane sink from formaldehyde in polar ice cores. Nature 349 603-605. 

Our knowledge of the sinks of atmospheric methane can be summarized as follows. Model experiments made under laboratory conditions show that the CH4 uptake of soil is not important (Ehhalt, 1974). This means that soil microorganisms do not oxidize CH4. This also means that we have to look for methane sinks in the atmosphere. 

Table 4-8 summarizes methane emission rates as estimated by different authors. Most source estimates sum to a total of about 500 Tg/yr. An exception is the value derived by Sheppard et al. (1982) from a classification of natural ecosystems similar to that described in Section 11.2.4 for the turnover of biogenic carbon. Even this estimate, however, agrees with the others within a factor of 2-3. We think that is too high, mainly because it cannot be compensated by the known methane sinks, which are discussed next. 

The other sinks for methane gas are reaction with soil and loss to the stratosphere (see more details on stratospheric chemistry and methane sink in Chapter 6). 

The decrease in density between water and ice has a number of important implications for the world around us. Ice floats because it is less dense than liquid water. This is not true of any other liquid/solid equilibrium. Solid methane sinks in liquid methane and solid ammonia sinks in liquid ammonia. Floating ice means that ponds and lakes freeze from the top down, allowing fish and other biota to live protected from the cold weather of winter. If water froze from the bottom up, life as we know it would not have evolved on Earth. 

Khalil, M. A. K., M. J. Shearer, and R. A. Rasmussen. 1992. Methane sinks and distribution. In M. A. K. Khalil (ed.) Atmospheric Methane Sources, Sinks, and Role in Global Change. NATO ASI Series 1, Vol. 13. Springer-Verlag, Berlin, pp. 168-180. 

By beginning with methane, the diamonds formed have only in them. These tiny diamonds may then be used as the carbon source to form large (5 mm) single crystals by growth from molten catalyst metal in a temperature gradient. The resulting nearly pure crystals have outstanding thermal conductivities suitable for special appHcations as windows and heat sinks (24). 

Example 8 Ejfective Gas Emissivity Methane is hiimed to completion with 20 percent excess air (air half-saturated with water vapor at 298 K (60 F), 0.0088 mols H20/mol dry air) in a furnace chamber of floor dimensions. 3 X 10 m and heights m. The whole surface is a gray-energy sink of emissivity 0.8... 

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... 

This means that the observed change in M mainly reflects a change in the source flux Q or the sink function. As an example we may take the methane concentration in the atmosphere, which in recent years has been increasing by about 0.5% per year. The turnover time is estimated to be about 10 years, i.e., much less than Tobs (200 years). Consequently, the observed rate of increase in atmospheric methane is a direct consequence of a similar rate of increase of emissions into the atmosphere. (In fact, this is not quite true. A fraction of the observed increase is probably due to a decrease in sink strength caused by a decrease in the concentration of hydroxyl radicals responsible for the decomposition of methane in the atmosphere.)... 

Displacing the methane tied up in deep unmineable coal adds another small carbon sink to the portfolio of options and this process is called enhanced coal bed methane recovery. When injected into a coal bed, C02 can replace adsorbed methane. By doing so, coal beds can serve as a C02 reservoir and a source for methane production (Parson and Keith, 1998). This method is attractive in the sense that most of the injected C02 will be immobilized by either physical or chemical adsorption on the coal surface. 

To set up the simulation, we use the thermodynamic dataset from the calculation in Section 18.5, which was expanded to include mackinawite (FeS). As before, we suppress the iron sulfide minerals pyrite and troilite, and decouple acetate and methane from carbonate, and sulfide from sulfate. We set the aquifer to include a small amount of siderite, which serves as a sink for aqueous sulfide,... 

The hydrothermal chemistry of methane also provides another buffering control on the global biogeochemical carbon cycle by serving as the site of reactions that act as sources and sinks of methane. Examples of source reactions are... 

Table 8.1 Estimates of the global methane budget (Tg CH4 year ) from different sources and sinks...

Houweling S, Kaminski T, Dentener F, Lelieveld I, Heimann M. 1999. Inverse modeling of methane sources and sinks using the adjoint of a global transport model. Journal of Geophysical Research-Atmospheres 104 26137-26160. 

Taylor JA, Brasseur GP, Zimmermann PR, Cicerone RJ. 1991. A study of the sources and sinks of methane and methyl chloroform using a global three-dimensional Lagrangian tropospheric tracer transport model. Journal of Geophysical Research 96D 3013-3044. 

Jones HD, Kesler SE, Furman FC, Kyle JR (1996) Sulfur isotope geochemistry of southern Appalachian Mississippi Valley-type depopsits. Econ Geol 91 355-367 Jprgensen BB, BOttcher MA, Lflschen H, Neretin LN, Volkov 11 (2004) Anaerobic methane oxidation and a deep H2S sink generate isotopically heavy sulfides in Black Sea sediments. Geochim Cosmochim Acta 68 2095-2118... 

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