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

Douglas, Peter M. J.; Stratigopoulos, Emerald; Park, Sanga; Phan, Dawson

There is growing interest in developing spatially resolved methane (CHinline-formula4) isotopic source signatures to aid in geographic source attribution of CHinline-formula4 emissions. CHinline-formula4 hydrogen isotope measurements (inline-formulaδ2H–CHinline-formula4) have the potential to be a powerful tool for geographic differentiation of CHinline-formula4 emissions from freshwater environments, as well as other microbial sources. This is because microbial inline-formulaδ2H–CHinline-formula4 values are partially dependent on the inline-formulaδ2H of environmental water (inline-formulaδ2H–Hinline-formula2O), which exhibits large and well-characterized spatial variability globally. We have refined the existing global relationship between inline-formulaδ2H–CHinline-formula4 and inline-formulaδ2H–Hinline-formula2O by compiling a more extensive global dataset of inline-formulaδ2H–CHinline-formula4 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 inline-formulaδ2H–Hinline-formula2O, as well as inline-formulaδ13C-CHinline-formula4 and inline-formulaδ13C–COinline-formula2 measurements. We found that estimates of inline-formulaδ2H–Hinline-formula2O explain approximately 42 % of the observed variation in inline-formulaδ2H–CHinline-formula4, with a flatter slope than observed in previous studies. The inferred global inline-formulaδ2H–CHinline-formula4 vs. inline-formulaδ2H–Hinline-formula2O regression relationship is not sensitive to using either modelled precipitation inline-formulaδ2H or measured inline-formulaδ2H–Hinline-formula2O as the predictor variable. The slope of the global freshwater relationship between inline-formulaδ2H–CHinline-formula4 and inline-formulaδ2H–Hinline-formula2O 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 inline-formulaδ2H of acetate, controlled by environmental inline-formulaδ2H–Hinline-formula2O, is important in determining variation in inline-formulaδ2H–CHinline-formula4. The relationship between inline-formulaδ2H–CHinline-formula4 and inline-formulaδ2H–Hinline-formula2O leads to significant differences in the distribution of freshwater inline-formulaδ2H–CHinline-formula4 between the northern high latitudes (60–90inline-formula N), relative to other global regions. We estimate a flux-weighted global freshwater inline-formulaδ2H–CHinline-formula4 of inline-formula−310 inline-formula± 15 ‰, which is higher than most previous estimates. Comparison with inline-formulaδ13C measurements of both CHinline-formula4 and COinline-formula2 implies that residual inline-formulaδ2H–CHinline-formula4 variation is the result of complex interactions between CHinline-formula4 oxidation, variation in the dominant pathway of methanogenesis, and potentially other biogeochemical variables. We observe a significantly greater distribution of inline-formulaδ2H–CHinline-formula4 values, corrected for inline-formulaδ2H–Hinline-formula2O, in inland waters relative to wetlands, and suggest this difference is caused by more prevalent CHinline-formula4 oxidation in inland waters. We used the expanded freshwater CHinline-formula4 isotopic dataset to calculate a bottom-up estimate of global source inline-formulaδ2H–CHinline-formula4 and inline-formulaδ13C-CHinline-formula4 that includes spatially resolved isotopic signatures for freshwater CHinline-formula4 sources. Our bottom-up global source inline-formulaδ2H–CHinline-formula4 estimate (inline-formula−278 inline-formula± 15 ‰) is higher than a previous estimate using a similar approach, as a result of the more enriched global freshwater inline-formulaδ2H–CHinline-formula4 signature derived from our dataset. However, it is in agreement with top-down estimates of global source inline-formulaδ2H–CHinline-formula4 based on atmospheric measurements and estimated atmospheric sink fractionations. In contrast our bottom-up global source inline-formulaδ13C-CHinline-formula4 estimate is lower than top-down estimates, partly as a result of a lack of inline-formulaδ13C-CHinline-formula4 data from Cinline-formula4-plant-dominated ecosystems. In general, we find there is a particular need for more data to constrain isotopic signatures for low-latitude microbial CHinline-formula4 sources.

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Douglas, Peter M. J. / Stratigopoulos, Emerald / Park, Sanga / et al: Geographic variability in freshwater methane hydrogen isotope ratios and its implications for global isotopic source signatures. 2021. Copernicus Publications.

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