Abstract
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 geographic differentiation 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 have refined the existing global relationship between δ2H–CH4 and δ2H–H2O by compiling a more extensive global dataset of δ2H–CH4 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 δ2H–H2O, as well as δ13C-CH4 and δ13C–CO2 measurements. We found that estimates of δ2H–H2O explain approximately 42 % of the observed variation in δ2H–CH4, with a flatter slope than observed in previous studies. The inferred global δ2H–CH4 vs. δ2H–H2O regression relationship is not sensitive to using either modelled precipitation δ2H or measured δ2H–H2O as the predictor variable. The slope of the global freshwater relationship between δ2H–CH4 and δ2H–H2O 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 δ2H of acetate, controlled by environmental δ2H–H2O, is important in determining variation in δ2H–CH4. The relationship between δ2H–CH4 and δ2H–H2O leads to significant differences in the distribution of freshwater δ2H–CH4 between the northern high latitudes (60–90∘ N), relative to other global regions. We estimate a flux-weighted global freshwater δ2H–CH4 of −310 ± 15 ‰, which is higher than most previous estimates. Comparison with δ13C measurements of both CH4 and CO2 implies that residual δ2H–CH4 variation is the result of complex interactions between CH4 oxidation, variation in the dominant pathway of methanogenesis, and potentially other biogeochemical variables. We observe a significantly greater distribution of δ2H–CH4 values, corrected for δ2H–H2O, in inland waters relative to wetlands, and suggest this difference is caused by more prevalent CH4 oxidation in inland waters. We used the expanded freshwater CH4 isotopic dataset to calculate a bottom-up estimate of global source δ2H–CH4 and δ13C-CH4 that includes spatially resolved isotopic signatures for freshwater CH4 sources. Our bottom-up global source δ2H–CH4 estimate (−278 ± 15 ‰) is higher than a previous estimate using a similar approach, as a result of the more enriched global freshwater δ2H–CH4 signature derived from our dataset. However, it is in agreement with top-down estimates of global source δ2H–CH4 based on atmospheric measurements and estimated atmospheric sink fractionations. In contrast our bottom-up global source δ13C-CH4 estimate is lower than top-down estimates, partly as a result of a lack of δ13C-CH4 data from C4-plant-dominated ecosystems. In general, we find there is a particular need for more data to constrain isotopic signatures for low-latitude microbial CH4 sources.
Highlights
IntroductionMethane (CH4) is an important greenhouse gas that accounts for approximately 25 % of current anthropogenic global warming, but we do not have a complete understanding of the current relative or absolute fluxes of different CH4 sources to the atmosphere (Schwietzke et al, 2016; Saunois et al, 2019), nor is there consensus on the causes of recent decadalscale changes in the rate of increase in atmospheric CH4 (Kai et al, 2011; Pison et al, 2013; Rice et al, 2016; Schaefer et al, 2016; Worden et al, 2017; Thompson et al, 2018; Turner et al, 2019)
Rstandard where R is the ratio of the heavy isotope to the light isotope, and the standard is Vienna Standard Mean Ocean Water (VSMOW) for δ2H and Vienna Pee Dee Belemnite (VPDB) for δ13C. δ values are expressed in per mil (‰) notation
Our analysis suggests that annual δ2Hp may be a better predictor for wetland δ2H–CH4, while seasonal δ2Hp may be a better predictor of inland water δ2H– CH4
Summary
Methane (CH4) is an important greenhouse gas that accounts for approximately 25 % of current anthropogenic global warming, but we do not have a complete understanding of the current relative or absolute fluxes of different CH4 sources to the atmosphere (Schwietzke et al, 2016; Saunois et al, 2019), nor is there consensus on the causes of recent decadalscale changes in the rate of increase in atmospheric CH4 (Kai et al, 2011; Pison et al, 2013; Rice et al, 2016; Schaefer et al, 2016; Worden et al, 2017; Thompson et al, 2018; Turner et al, 2019). Freshwater ecosystems are an integral component of the global CH4 budget. They are one of the largest sources of atmospheric CH4 and are unequivocally the largest natural, or non-anthropogenic, source (Bastviken et al, 2011; Saunois et al, 2019). Gaining a better understanding of freshwater CH4 emissions on a global scale is of great importance for understanding potential future climate feedbacks related to CH4 emissions from these ecosystems (Bastviken et al, 2011; Koven et al, 2011; Yvon-Durocher et al, 2014; Zhang et al, 2017). It is necessary in order to better constrain the quantity and rate of change of other CH4 emissions sources, including anthropogenic sources from fossil fuels, agriculture, and waste (Kai et al, 2011; Pison et al, 2013; Schaefer et al, 2016)
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