Jeffrey Chanton, Lia Chaser, Paul Classer, Don Siegel
Methane is the ultimate end-product of anaerobic respiration of organic matter. In terrestrial freshwater systems it is formed by two main pathways, fermentation of acetate ch3cooh CH4 + C02 (6.1)
and reduction of CO2 with hydrogen
which results in Eq. 6.4, overall.
Note that CO2 reduction results in overall net CO2 production when the production of hydrogen is accounted for. Methane production via CO2 reduction does not consume CO2. Also, acetate can be written as 2CH2O, so Eq. 6.4 describes both pathways.
Acetate, H2 and CO2 are generated in organic-rich environments by the respiration of anaerobic and fermentative bacteria higher in the anaerobic food web. Methane is the perfect waste product: it is non-toxic and relatively insoluble in water so it forms bubbles and is rapidly removed via ebullition (Martens and Klump, 1980) or transported via vegetation (Dacey, 1981; Chanton and Dacey, 1991). When ch4 migrates to an oxygen interface (Brune et al, 2000), it is rapidly consumed by methano-trophicbacteria (King, 1992) andean supportchemosynthetic communities (Paull et al, 1989; Martens et al, 1991).
An important functional relationship of low temperature geochemistry is that there are coincidental shifts in the ¿>13C and <5D of methane isotopic composition relative to the methane production pathway and/or to the
Figure 6.1 Scheme of variations in ¿>'3C and <5D associated with methane oxidation and production mechanisms. The lower right corner of the graph represents acetate fermentation while the upper left corner represents CO2 reduction. Oxidation effects leave residual methane enriched in and D isotopes.
effects of microbially mediated methane oxidation (Fig. 6.1). The purpose of this chapter is to illustrate these shifts as they occur in terrestrial environments and to consider the factors that influence this relationship, including anthropogenic impacts (e.g., landfills). As seen in Fig. 6.1, changes in 13CH4 alone are ambiguous, forced either by methane oxidation (Barker and Fritz, 1981) or variation in the methane production mechanism (Sugimoto and Wada, 1993). Information on variation in <5D of ch4 should strengthen any interpretation of 13C data (see for example Woltemate at al, 1984; Whiticar andFaber, 1986; Martens «¿a/., 1992; Kelley etal, 1995; Bellisario etal, 1999). While C and H isotope systematics can be useful for diagnosing a wide variety of processes leading to methane production (Whiticar et al., 1986; Whiticar, 1993, 1999), this chapter will focus only on microbial methane produced or consumed in low temperature settings.
Knowledge of the isotopic composition of source methane emitted from natural and anthropogenic systems is helpful for developing a global budget for methane sources and sinks (Fig. 6.2) (Stevens and Engelkemeir, 1988; Wahlen et al, 1989; Tyler, 1991). As stated by Miller (2004), a better understanding of the processes responsible for determining isotopic fractionation during both production and consumption of methane will allow better constraints on our estimates of the sources and sinks in the global methane budget.
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