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

[1] To investigate the controls on the delta D isotopic composition of biogenic CH4 in terrestrial wetlands, we collected a series of samples for delta D-H2O, delta D-CH4, C-13-CH4, and delta C-13-DIC (dissolved Sigma CO2) along a N-S transect across Alaska from 60 degrees N to 70 degrees N latitude from 7 to 15 August 2001. The delta D-H2O and delta D-CH4 values varied from -108% to -161 parts per thousand and from -308 parts per thousand to -394 parts per thousand, respectively, from south to north and were significantly correlated, indicating the significant influence of the delta D-H2O on the delta D of terrestrial CH4 collected along a latitudinal spatial gradient. Additionally, the apparent fractionation factors (alpha, alpha(A) -B = R-A/R-B, where R = C-13/C-12 or D/H) for H2O -> CH4 (alpha(D)) and DIC -> CH4 (alpha(C)) varied from 1.26 to 1.42 and from 1.035 to 1.084, respectively, and were inversely correlated. We conclude that while delta D-H2O is a critical factor controlling delta D-CH4 across latitudes, the CH4 production mechanism is also responsible for variation in the delta D-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 delta C-13 and depleted in delta D 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 delta D-CH4 on the delta D of environmental H2O indicates that specific fields on plots of delta D-CH4 versus delta C-13-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 delta D-CH4 values cannot be associated with production mechanism in an absolute sense. The production mechanism effect is in addition to the effect of the delta D-H2O.