Preprints
https://doi.org/10.5194/bg-2020-410
https://doi.org/10.5194/bg-2020-410

  13 Nov 2020

13 Nov 2020

Review status: this preprint is currently under review for the journal BG.

Global geographic variability in freshwater methane hydrogen isotope ratios and its implications for emissions source apportionment and microbial biogeochemistry

Peter M. J. Douglas, Emerald Stratigopoulos, Jenny Park, and Dawson Phan Peter M. J. Douglas et al.
  • Earth and Planetary Sciences, McGill University, Montreal, H3A 0E8, Canada

Abstract. There is growing interest in developing spatially resolved methane (CH4) isotopic source signatures to aid in geographic source attribution of CH4 emissions. CH4 hydrogen isotope measurements (δ2H-CH4) have the potential to be a powerful tool for spatial resolution of CH4 emissions from freshwater environments, as well as other microbial sources. This is because microbial δ2H-CH4 values are partially dependent on the δ2H of environmental water (δ2H-H2O), which exhibits large and well-characterized spatial variability globally. We compiled a comprehensive global dataset of paired CH4 δ2H and δ13C measurements from freshwater environments, including wetlands, inland waters, and rice paddies, comprising a total of 131 different ecosystems, and compared these with measurements and estimates of δ2H-H2O. We found that the estimated δ2H of annual precipitation (δ2Hp) explained approximately 35 % of the observed variation in δ2H-CH4, and that the relationship between δ2H-CH4 and δ2Hp led to significant differences in the distribution of freshwater δ2H-CH4 between the northern high latitudes (60–90º N) relative to other global regions. Residual variability in δ2H-CH4 is partially explained by differences in the dominant methanogenic pathway and CH4 oxidation, as inferred from carbon isotope fractionation between CH4 and carbon dioxide (αC). Our results imply that hydrogenotrophic methanogenesis is characterized by a steeper slope of δ2H-CH4 vs. δ2Hp than acetoclastic methanogenesis, a pattern that is consistent with previous predictions. Biogeochemical sources of variability in δ2H-CH4 are reflected in apparent differences between different freshwater ecosystems, with relatively high values in rivers and bogs, and low values in fens and rice paddies, although more data is needed to verify whether these differences are significant. To estimate how changes in the spatial distribution of freshwater emissions would influence global atmospheric CH4 isotopic measurements, we developed a bottom-up mixing model of global CH4 δ2H and δ13C sources, including spatially resolved signatures for freshwater CH4 sources. This model implies that changes in high-latitude freshwater CH4 emissions would have an especially strong influence on global source δ2H-CH4. We estimate that global CH4 emissions sources have a combined δ2H value of −277±8 ‰, which is consistent with top-down estimates based on atmospheric measurements. In contrast our estimated δ13C value of −56.4±1.4 ‰ is not consistent with atmospheric measurements, suggesting possible errors in either emissions inventories or estimates of sink fluxes and isotopic fractionations. Overall our results emphasize the value of δ2H-CH4 measurements to help constrain atmospheric CH4 budgets.

Peter M. J. Douglas et al.

 
Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

Peter M. J. Douglas et al.

Data sets

Data for geographic variability in freshwater methane hydrogen isotope ratios and its implications for emissions source apportionment and microbial biogeochemistry Douglas, P. M. J. Stratigopoulos, E., Park, J., and Phan, D. https://doi.org/10.6084/m9.figshare.13194833.v1

Peter M. J. Douglas et al.

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Short summary
Hydrogen isotopes could be a useful tool to help resolve the geographic distribution of methane emissions from freshwater environments. We analyzed a global dataset of freshwater methane hydrogen isotope ratios, and found significant geographic variation linked to precipitation isotopic composition. This geographic variability could be used to resolve changing methane fluxes from freshwater environments, and provide more accurate estimates of the relative balance of global methane sources.
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