Geographic variability in freshwater methane hydrogen isotope ratios and its implications for global isotopic source signatures.

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From: Biogeosciences(Vol. 18, Issue 11)
Publisher: Copernicus GmbH
Document Type: Article
Length: 564 words

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Abstract :

There is growing interest in developing spatially resolved methane (CH.sub.4) isotopic source signatures to aid in geographic source attribution of CH.sub.4 emissions. CH.sub.4 hydrogen isotope measurements ([delta].sup.2 H-CH.sub.4) have the potential to be a powerful tool for geographic differentiation of CH.sub.4 emissions from freshwater environments, as well as other microbial sources. This is because microbial [delta].sup.2 H-CH.sub.4 values are partially dependent on the [delta].sup.2 H of environmental water ([delta].sup.2 H-H.sub.2 O), which exhibits large and well-characterized spatial variability globally. We have refined the existing global relationship between [delta].sup.2 H-CH.sub.4 and [delta].sup.2 H-H.sub.2 O by compiling a more extensive global dataset of [delta].sup.2 H-CH.sub.4 from freshwater environments, including wetlands, inland waters, and rice paddies, comprising a total of 129 different sites, and compared these with measurements and estimates of [delta].sup.2 H-H.sub.2 O, as well as [delta].sup.13 C-CH.sub.4 and [delta].sup.13 C-CO.sub.2 measurements. We found that estimates of [delta].sup.2 H-H.sub.2 O explain approximately 42 % of the observed variation in [delta].sup.2 H-CH.sub.4, with a flatter slope than observed in previous studies. The inferred global [delta].sup.2 H-CH.sub.4 vs. [delta].sup.2 H-H.sub.2 O regression relationship is not sensitive to using either modelled precipitation [delta].sup.2 H or measured [delta].sup.2 H-H.sub.2 O as the predictor variable. The slope of the global freshwater relationship between [delta].sup.2 H-CH.sub.4 and [delta].sup.2 H-H.sub.2 O is similar to observations from incubation experiments but is different from pure culture experiments. This result is consistent with previous suggestions that variation in the [delta].sup.2 H of acetate, controlled by environmental [delta].sup.2 H-H.sub.2 O, is important in determining variation in [delta].sup.2 H-CH.sub.4 . The relationship between [delta].sup.2 H-CH.sub.4 and [delta].sup.2 H-H.sub.2 O leads to significant differences in the distribution of freshwater [delta].sup.2 H-CH.sub.4 between the northern high latitudes (60-90.sup." N), relative to other global regions. We estimate a flux-weighted global freshwater [delta].sup.2 H-CH.sub.4 of -310 ± 15 0/00, which is higher than most previous estimates. Comparison with [delta].sup.13 C measurements of both CH.sub.4 and CO.sub.2 implies that residual [delta].sup.2 H-CH.sub.4 variation is the result of complex interactions between CH.sub.4 oxidation, variation in the dominant pathway of methanogenesis, and potentially other biogeochemical variables. We observe a significantly greater distribution of [delta].sup.2 H-CH.sub.4 values, corrected for [delta].sup.2 H-H.sub.2 O, in inland waters relative to wetlands, and suggest this difference is caused by more prevalent CH.sub.4 oxidation in inland waters. We used the expanded freshwater CH.sub.4 isotopic dataset to calculate a bottom-up estimate of global source [delta].sup.2 H-CH.sub.4 and [delta].sup.13 C-CH.sub.4 that includes spatially resolved isotopic signatures for freshwater CH.sub.4 sources. Our bottom-up global source [delta].sup.2 H-CH.sub.4 estimate (-278 ± 15 0/00) is higher than a previous estimate using a similar approach, as a result of the more enriched global freshwater [delta].sup.2 H-CH.sub.4 signature derived from our dataset. However, it is in agreement with top-down estimates of global source [delta].sup.2 H-CH.sub.4 based on atmospheric measurements and estimated atmospheric sink fractionations. In contrast our bottom-up global source [delta].sup.13 C-CH.sub.4 estimate is lower than top-down estimates, partly as a result of a lack of [delta].sup.13 C-CH.sub.4 data from C.sub.4 -plant-dominated ecosystems. In general, we find there is a particular need for more data to constrain isotopic signatures for low-latitude microbial CH.sub.4 sources.

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Gale Document Number: GALE|A664891292