Controls on the hydrogen isotopic composition of biogenic methane from high-latitude terrestrial wetlands

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Abstract

[1] To investigate the controls on the δD isotopic composition of biogenic CH4 in terrestrial wetlands, we collected a series of samples for δD-H2O, δD-CH4, δ13C-CH4, and δ13C-DIC (dissolved ΣCO2) along a N-S transect across Alaska from 60°N to 70°N latitude from 7 to 15 August 2001. The δD-H2O and δD-CH4 values varied from −108‰ to −161‰ and from −308‰ to −394‰, respectively, from south to north and were significantly correlated, indicating the significant influence of the δD-H2O on the δD of terrestrial CH4 collected along a latitudinal spatial gradient. Additionally, the apparent fractionation factors (α, αA–B = RA/RB, where R = 13C/12C or D/H) for H2O → CH4 (αD) and DIC → CH4 (αC) varied from 1.26 to 1.42 and from 1.035 to 1.084, respectively, and were inversely correlated. We conclude that while δD-H2O is a critical factor controlling δD-CH4 across latitudes, the CH4 production mechanism is also responsible for variation in the δD-CH4. If the isotopic values of the precursors of methane, H2O, DIC, and organic matter are relatively constant, CH4 produced by acetate fermentation will be enriched in δ13C and depleted in δD relative to CH4 produced via the CO2 reduction pathway. Production mechanism variation will be particularly important in controlling variations in CH4 isotopic composition with depth. The strong dependence of δD-CH4 on the δD of environmental H2O indicates that specific fields on plots of δD-CH4 versus δ13C-CH4 may not always accurately represent the isotopic composition of CH4 produced by CO2 reduction in northern wetlands. Because of this variation at high latitude, we assert that δD-CH4 values cannot be associated with production mechanism in an absolute sense. The production mechanism effect is in addition to the effect of the δD-H2O.

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