Abstract:
We propose that the hydrogen isotope composition of recently produced microbial methane, δD(CH4), in sulfate-poor, shallow freshwater environments, is directly related to the hydrogen isotopic composition of the system water δD(H2O). As δD(H2O) varies globally, systematic differences in δD(CH4) as a function of δD(H2O) should be observed.From available mean paired measurements from 46 sites, the relationship for δD(CH4) and δD(H2O) in the natural environment can be defined as δD(CH4) = 0.675δD(H2O) -284%% (p < 0.0001). This relationship is statistically distinct from that generated by considering three separate laboratory-based anaerobic inoculations that contain similar methanogenic communities to the natural freshwater samples and therefore, are likely to produce methane by similar metabolic pathways: δD(CH4) = 0.444δD(H2O) -321%% (p < 0.0001). We suggest that the relationship arising from the laboratory incubations defines the δD(CH4) of methane produced at source in shallow freshwater environments. We can approximate that 50% of the variation in natural δD(CH4) samples can be explained by δD(H2O), with isotopic fractionation postproduction, or mixing with gas already fractionated likely responsible for most of the noise in the natural system and difference of the natural sample relationship to the laboratory relationship. Methanogenic pathway may also influence δD(CH4), but the foundations for this hypothesis need to be reconsidered, and field and laboratory data exist that do not support it. The relationships presented here describe δD of methane from only shallow (subsurface) freshwater environments; paired δD(CH4)-δD(H2O) values from other environments (e.g., marine, glacial drift) suggest that a different relationship is needed to describe the influence of δD(H2O) on δD(CH4).