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 (δ²H_p) explained approximately 35 % of the observed variation in δ²H-CH₄, and that the relationship between δ²H-CH₄ and δ²H_p 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 (α_C). Our results imply that hydrogenotrophic methanogenesis is characterized by a steeper slope of δ²H-CH₄ vs. δ²H_p 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.

中文翻译：

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淡水甲烷氢同位素比率的全球地理变异性及其对排放源分配和微生物生物地球化学的影响

摘要。对开发空间分辨甲烷（CH ₄）同位素源签名以帮助CH ₄排放的地理源归因的兴趣日益浓厚。CH ₄的氢同位素测量（δ ² H-CH ₄）必须为CH的空间分辨率的有力工具_4吨从淡水环境的排放量，以及其他微生物来源。这是因为微生物δ ² H-CH ₄的值是部分地依赖于δ ²环境水的H（δ ² H-H ₂O），在全球范围内表现出较大且特征明确的空间变异性。我们编译成对CH的全面的全球数据集₄ δ ² H和δ ^13次从淡水环境，包括湿地，内陆水和稻田Ç测量，包含总共131个不同的生态系统的，并用δ测量和估计相比这些² HH ₂ O.我们发现，估计δ ²年降水量（δh的² ħ _p）在δ解释所观察到的变化的约35％的² H-CH ₄，和δ之间的关系² H-CH ₄和δ² ħ _p导致淡水的分布显著差异δ ² H-CH ₄相对于其它全球区域的北部高纬度地区（60-90ºN）之间。在δ残余变性² H-CH ₄部分地由在所述主导产甲烷途径和CH差异来解释₄氧化，如从碳同位素分馏CH之间推断₄和二氧化碳（α _Ç）。我们的结果意味着，氢营养产甲烷的特征在于一个δ更陡的斜率² H-CH ₄与δ ² ħ _p比碎裂产甲烷作用，这种模式与以前的预测相一致。在δ变异的生物地球化学源² H-CH ₄反映在不同淡水生态系统之间明显的差异，在河流和沼泽，和在沼泽地和稻田低值相对高的值，尽管需要更多的数据来验证这些差异是否是重大。若要估计在淡水排放量的空间分布的变化将影响全球大气CH _4次同位素测量，我们开发了一种自下而上的全球CH混合模型₄ δ ² H和δ ^13个C源码，包括淡水CH空间分辨签名_4个来源。该模型意味着在高纬度淡水CH变化_4个排放将对全球源的特别强的影响δ ² H-CH ₄。我们估计，全球CH ₄排放源具有组合的δ ²的-277±8 H值‰，这与一致的自顶向下的基于大气测量估计。相反我们的估计δ ¹³的-56.4±1.4 C值‰不一致与大气测量，表明在任一排放清单或水槽通量和同位素分馏的估计可能的错误。总的来说我们的研究结果强调δ值² H-CH ₄测量以限制大气中的CH ₄预算。