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.

中文翻译：

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淡水甲烷氢同位素比率的地理变异性及其对全球同位素源特征的影响

人们对开发空间分辨的甲烷 (CH ₄ ) 同位素源特征以帮助 CH ₄排放的地理源归属越来越感兴趣。CH ₄氢同位素测量（δ ² H–CH ₄）有可能成为区分淡水环境和其他微生物来源的 CH ₄排放的强大工具。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），它在全球范围内表现出巨大且特征鲜明的空间变异性。我们通过编译来自淡水环境（包括湿地、内陆水域和稻田）的更广泛的δ ² H–CH ₄全球数据集，完善了δ ² H–CH ₄和 δ ² H–H ₂ O 之间现有的全球关系，包括总共 129 个不同的站点，并将这些与δ ² H–H ₂ O 以及δ ¹³ C-CH ₄和δ ¹³ C–CO _{2 的}测量值和估计值进行比较测量。我们发现δ ² H–H ₂ O 的估计值解释了观察到的δ ² H–CH ₄变化的大约 42%， 斜率比以前的研究中观察到的更平坦。推断的全球δ ² H–CH ₄与δ ² H–H ₂ O 回归关系对使用模拟降水δ ² H 或测量的δ ² H–H ₂ O 作为预测变量不敏感。δ ² H–CH ₄与全球淡水关系的斜率δ ² H–H ₂ O 与孵化实验的观察结果相似，但与纯培养实验不同。该结果与以前的建议相一致的是，在变型δ ²乙酸盐的H，受环境控制的δ ² H-H ₂ O，是在确定的变化重要δ ² H-CH ₄。δ ² H–CH ₄和δ ² H–H ₂ O之间的关系 导致淡水δ ² H–CH ₄分布的显着差异 相对于全球其他地区， 北部高纬度地区（60-90 ^∘ N）之间。我们估计通量加权的全球淡水δ ² H–CH ₄为− 310  ±  15 ‰，高于大多数先前的估计值。与CH ₄和 CO _{2 的}δ ¹³ C 测量值的比较表明，残留的δ ² H–CH ₄变化是 CH ₄氧化、产甲烷主要途径的变化和其他潜在生物地球化学变量之间复杂相互作用的结果。我们观察到明显更大的δ分布² H–CH ₄值，经δ ² H–H ₂ O校正 ，内陆水域相对于湿地，表明这种差异是由内陆水域更普遍的 CH ₄氧化引起的。我们使用扩展的淡水 CH ₄同位素数据集来计算全球源δ ² H–CH ₄ 和δ ¹³ C-CH ₄的自下而上估计，其中包括淡水 CH ₄源的空间分辨同位素特征。我们自下而上的全球源 δ ² H–CH ₄估计值 ( − 278  ± 15 ‰) 高于使用类似方法的先前估计值，这是由于来自我们的数据集的更丰富的全球淡水δ ² H–CH ₄特征。然而，它与基于大气测量和估计的大气汇分馏对全球源δ ² H–CH ₄的自上而下的估计一致。相比之下，我们自下而上的全球来源δ ¹³ C-CH ₄估计低于自上而下的估计，部分原因是缺乏来自 C ₄的δ ¹³ C-CH ₄数据-植物主导的生态系统。总的来说，我们发现特别需要更多数据来约束低纬度微生物 CH ₄源的同位素特征。