Evolution of stable carbon isotope compositions for methane and carbon dioxide in freshwater wetlands and other anaerobic environments

Two types of distribution for alpha(C) Values are observed in anaerobic environments when delta(13)C-Sigma CO2 and delta(13)C-CH4 values are measured across gradients of depth or age of organic debris. The type-I distribution involves a systematic increase in or, Values with depth as a result of decreasing delta(13)C-CH4 and increasing delta(13)C-Sigma CO2 values. This behavior corresponds to a progressive increase in the prevalence of methanogenesis by the CO2 reduction pathway relative to acetate fermentation. Utilization of autotrophically formed acetate by methanogens would also cause an increase in or, values. The type-II distribution occurs when both delta(13)C-CH4 and delta(13)C-Sigma CO2 values decrease with depth, resulting in approximately constant alpha(C) values. This condition corresponds with a strong dependence of methanogens on porewater Sigma CO2 as a carbon source by way of either the CO2 reduction pathway or utilization of autotrophically formed acetate. Freshwater wetlands possess both types of a, value distribution. Wetlands with type-I distributions exhibit curves with slopes that vary probably as a function of deposition and preservation of labile organic carbon. An abundance of labile substrates in anaerobic soils yields steeper curves because aceticlastic methanogenesis predominates and delta(13)C-CH4 and delta(13)C-CO2 values are high. Diminished transfer of labile carbon to the methanogenic zone results in an increased prevalence of the CO2 reduction pathway, yielding low delta(13)C-CH4 values and shallowly sloping curves. Aerobic oxidation of organic matter or decay involving sulfate reduction produces CO2 with low delta(13)C values, which also will contribute to shallowly sloping curves. The size of the dissolved CO2 pool can influence the sensitivity of delta(13)C-CO2 values to change during methanogenesis. Regression curves of delta(13)C-CH4 and delta(13)C-CO2 values from four wetlands with type-I distributions intersect at delta(13)C-CH4 = -40.7 +/- 6.1 parts per thousand (1 sigma) and delta(13)C-Sigma CO2= -23.9 +/- 4.8 parts per thousand (1 sigma). These values are similar to delta(13)C values for methyl and carboxyl moieties within acetate produced by anaerobic degradation of fresh C-3 plant matter. A low abundance of acetate during aceticlastic methanogenesis will result in minimal expression of metabolic kinetic isotope effects (KIEs) and production of CH4 and CO2 with delta(13)C values similar to the intramolecular distribution of sedimentary acetate. The type-II distribution is prevalent in marine environments, probably because of substrate depletion in the sulfate reduction zone. The type-I distribution does occur in marine settings where deposition rates of organic matter are high. Landfills possess only the type-I distribution of a, values and exhibit unusually steep curves, possibly because methanogenesis occurs predominantly from acetate produced by fermentation at mesophilic temperatures. The high abundance of acetate in landfill leachate may permit varying degrees of expression of the KIE associated with aceticlastic methanogenesis. Outgassing of (CO2)-C-12 may contribute further to the steepening of or, curves in landfills and other anaerobic environments possessing a type-I distribution. Defining the type of alpha(C) distributions in different wetlands could reduce uncertainty in estimating the delta(13)C value of CH4 emissions. Hence, the prevalence of type-I vs. type-II or, distributions in wetlands may have practical importance for the refinement of global CH4 budgets that rely on C-13/C-12 ratios for mass balance. Copyright (C) 2000 Elsevier Science Ltd.