Isotopic Effects During Methane Oxidation

When 13C and D isotopes of ch4 shift in the same direction, for example when methane becomes more enriched in both 13C and D along a spatial or temporal gradient, the variation is referred to as sympathetic. Sympathetic

Tropical wetlands Fens

Rice

Bogs

Animals Landfills

Sewage and liquid waste

Combustion and Coal biomass burning

Amazon floating Gas venting meadows

Figure 6.2 A representation of the mass-weighted isotope mass balance approach for atmospheric methane inputs (redrawn from Chanton etal., 2000). The representation is similar to a 'see-saw' in that both the size of the bar and distance from the fulcrum point influence the importance of the source. The x-axis (the solid line resting on the fulcrum) represents the S'^C of the methane source, and the total size of the bars is roughly related to the source strength in this representation. Bar thickness represents the range of the source. The position of the fulcrum represents the isotopic composition of the global methane input, which is determined from the values of atmospheric methane corrected for isotopic fractionation associated with removal processes. The sizes of the bars in this representation are for illustrative purposes for the most part and only roughly depict the mass of the fluxes.

variations are typically considered to indicate the activity of methane-oxidizing bacteria, methanotrophs (Fig. 6.3). These bacteria preferentially consume ch4 containing the lighter isotopes, leaving residual ch4 that is enriched in the heavier isotopes, 13C and D. The fraction of ch4 that is oxidized can be calculated from changes in ch4 isotopic composition across a spatial gradient from anoxic to oxidizing conditions (Happell et al, 1994; Liptay et al., 1998; Chanton et al., 1999; Chanton and Liptay, 2000).

Sympathetic shifts in the 13C and D isotopic composition of methane associated with oxidation were first reported by Coleman et al. (1981) in incubation studies, and subsequently have been verified in a number of field studies. The amount of the D shift is always greater than for the C shift, because the mass change of 1 is proportionally greater for H than it is for C. This means that the fractionation factor, aH for hydrogen-deuterium is larger than the fractionation factor for carbon, aC. Isotopic fractionation in this case is a kinetic process so the term a is defined as

where k^ refers to the first-order rate constants for the reaction of 12CH4 or CH4, and kn refers to the rate constants of 13CH4 and ch3d. When a is expressed in this manner, it is greater than 1 since the molecules containing the lighter isotopes react faster than those containing the enriched or heavier elements; estimates of aC for aerobic methane oxidation vary from 1.008 to 1.031 (Reeburgh, 1996; Liptay et al, 1998; Chanton and Liptay, 2000).

Figure 6,3 Effect of methane oxidation on 51SC and <5D of CH4. Samples from several landfills in New England (Liptay et at, 1998). Filled circles represent anoxic zone methane which has not been subject to oxidation. Chamber samples (open squares) captured emitted methane which has experienced the activity of methane-oxidizing bacteria. The slope of the line fit to the data is 3.8.

Figure 6,3 Effect of methane oxidation on 51SC and <5D of CH4. Samples from several landfills in New England (Liptay et at, 1998). Filled circles represent anoxic zone methane which has not been subject to oxidation. Chamber samples (open squares) captured emitted methane which has experienced the activity of methane-oxidizing bacteria. The slope of the line fit to the data is 3.8.

Some measurements of the size of the A5D/A513C shift for ch4 oxidation are tabulated in Table 6.1. Interestingly, observations from the field are smaller than initial laboratory results. Further investigation comparing 813C and <5D fractionation driven by methanotrophy in lab and field studies is needed.

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