Evolution of stable carbon isotope compositions for methane and carbon dioxide in freshwater wetlands and other anaerobic environments
Two types of distribution for {alpha}{sub c} values are observed in anaerobic environments when {delta}{sup 13}C-{Sigma}CO{sub 2} and {delta}{sub 13}C-CH{sub 4} values are measured across gradients of depth or age of organic debris. The type-I distribution involves a systematic increase in {alpha}{sub c} values with depth as a result of decreasing {delta}{sup 13}C-CH{sub 4} and increasing {delta}{sup 13}C-{Sigma}CO{sub 2} values. This behavior corresponds to a progressive increase in the prevalence of methanogenesis by the CO{sub 2} reduction pathway relative to acetate fermentation. Utilization of autotrophically formed acetate by methanogens would also cause an increase in {alpha}{sub c} values. The type-II distribution occurs when both {delta}{sup 13}C-CH{sub 4} and {delta}{sup 13}C-{Sigma}CO{sub 2} values decrease with depth, resulting in approximately constant {alpha}{sub c} values. This condition corresponds with a strong dependence of methanogens on porewater {Sigma}CO{sub 2} as a carbon source by way of either the CO{sub 2} reduction pathway or utilization of autotrophically formed acetate. Freshwater wetlands possess both types of {alpha}{sub c} value distribution. Defining the type of {alpha}{sub c} distributions in different wetlands could reduce uncertainty in estimating the {delta}{sup 13}C value of CH{sub 4} emissions. Hence, the prevalence of type-I vs. type-II {alpha}{sub c} distributions in wetlands may have practical importance for the refinement of global CH{sub 4} budgets that rely on {sup 13}C/{sup 12}C ratios for mass balance.
- Research Organization:
- Univ. of Western Ontario, London, Ontario (CA)
- OSTI ID:
- 20020877
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 64, Issue 6; Other Information: PBD: Mar 2000; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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