The global influence of the hydrogen isotope composition of water on that of bacteriogenic methane from shallow freshwater environments

We propose that the hydrogen isotope composition of recently produced microbial methane, delta D(CH4), in sulfate-poor, shallow freshwater environments, is directly related to the hydrogen isotopic composition of the system water delta D(H2O). As delta D(H2O) varies globally, systematic differences in delta D(CH2) as a function of delta D(H2O) should be observed. From available mean paired measurements from 46 sites, the relationship for delta D(CH4) and delta D(H2O) in the natural environment can be defined as delta D(CH4) = 0.675 delta D(H2O) -284 parts per thousand (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: delta D(CH4) = 0.444 delta D(H2O) -321 parts per thousand (p < 0.0001). We suggest that the relationship arising from the laboratory incubations defines the GD(CH4) of methane produced at source in shallow freshwater environments. We can approximate that 50% of the variation in natural delta D(CH4) samples can be explained by delta D(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 delta D(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 delta D of methane from only shallow (subsurface) freshwater environments; paired delta D(CH4)-delta D(H2O) values from other environments (e.g., marine, glacial drift) suggest that a different relationship is needed to describe the influence of delta D(H2O) on GD(CH,). Copyright (C) 1999 Elsevier Science Ltd.