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

• Published in: Geochimica et cosmochimica acta 2000-03, Vol.64 (6), p.1013-1027
• Main Authors: Hornibrook, Edward R.C., Longstaffe, Frederick J., Fyfe, William S.
• Format: Article
• Language: English
• Subjects: AIR POLLUTION; ANAEROBIC CONDITIONS; CARBON 13; CARBON CYCLE; CARBON DIOXIDE; DEPTH; ENVIRONMENTAL SCIENCES; GLOBAL ASPECTS; ISOTOPE RATIO; METHANE; METHANOGENIC BACTERIA; WETLANDS
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(99)00321-X

Two types of distribution for α C values are observed in anaerobic environments when δ 13C–ΣCO 2 and δ 13C–CH 4 values are measured across gradients of depth or age of organic debris. The type-I distribution involves a systematic increase in α C values with depth as a result of decreasing δ 13C–CH 4 and increasing δ 13C–ΣCO 2 values. This behavior corresponds to a progressive increase in the prevalence of methanogenesis by the CO 2 reduction pathway relative to acetate fermentation. Utilization of autotrophically formed acetate by methanogens would also cause an increase in α C values. The type-II distribution occurs when both δ 13C–CH 4 and δ 13C–ΣCO 2 values decrease with depth, resulting in approximately constant α C values. This condition corresponds with a strong dependence of methanogens on porewater ΣCO 2 as a carbon source by way of either the CO 2 reduction pathway or utilization of autotrophically formed acetate. Freshwater wetlands possess both types of α C 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 δ 13C–CH 4 and δ 13C–CO 2 values are high. Diminished transfer of labile carbon to the methanogenic zone results in an increased prevalence of the CO 2 reduction pathway, yielding low δ 13C–CH 4 values and shallowly sloping curves. Aerobic oxidation of organic matter or decay involving sulfate reduction produces CO 2 with low δ 13C values, which also will contribute to shallowly sloping curves. The size of the dissolved CO 2 pool can influence the sensitivity of δ 13C–CO 2 values to change during methanogenesis. Regression curves of δ 13C–CH 4 and δ 13C–ΣCO 2 values from four wetlands with type-I distributions intersect at δ 13C–CH 4 = −40.7 ± 6.1‰ (1σ) and δ 13C–ΣCO 2 = −23.9 ± 4.8‰ (1σ). These values are similar to δ 13C 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 CH 4 and CO 2 with δ 13C 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 su