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

To investigate the controls on the δ D isotopic composition of biogenic CH 4 in terrestrial wetlands, we collected a series of samples for δ D‐H 2 O, δ D‐CH 4 , δ 13 C‐CH 4 , and δ 13 C‐DIC (dissolved ΣCO 2 ) along a N‐S transect across Alaska from 60°N to 70°N latitude from 7 to 15 August 2001. The δ D‐H 2 O and δ D‐CH 4 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 δ D‐H 2 O on the δ D of terrestrial CH 4 collected along a latitudinal spatial gradient. Additionally, the apparent fractionation factors ( α , α A–B = R A /R B , where R = 13 C/ 12 C or D/H) for H 2 O → CH 4 ( α D ) and DIC → CH 4 ( α C ) varied from 1.26 to 1.42 and from 1.035 to 1.084, respectively, and were inversely correlated. We conclude that while δ D‐H 2 O is a critical factor controlling δ D‐CH 4 across latitudes, the CH 4 production mechanism is also responsible for variation in the δ D‐CH 4 . If the isotopic values of the precursors of methane, H 2 O, DIC, and organic matter are relatively constant, CH 4 produced by acetate fermentation will be enriched in δ 13 C and depleted in δ D relative to CH 4 produced via the CO 2 reduction pathway. Production mechanism variation will be particularly important in controlling variations in CH 4 isotopic composition with depth. The strong dependence of δ D‐CH 4 on the δ D of environmental H 2 O indicates that specific fields on plots of δ D‐CH 4 versus δ 13 C‐CH 4 may not always accurately represent the isotopic composition of CH 4 produced by CO 2 reduction in northern wetlands. Because of this variation at high latitude, we assert that δ D‐CH 4 values cannot be associated with production mechanism in an absolute sense. The production mechanism effect is in addition to the effect of the δ D‐H 2 O.