The potential for spatial resolution of freshwater methane emissions using hydrogen isotope measurements

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) could 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 dependent on the δ²H of environmental water (δ²H-H₂O), which exhibits large and well-characterized global spatial variability. 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 125 different ecosystems, and compared these measurements with estimates of δ²H-H₂O. We found that estimated δ²H-H₂O explained approximately 35% of the observed variation in δ²H-CH₄, and that the relationship between δ²H-CH₄ and δ²H-H₂O led to significant differences in mean freshwater δ²H-CH₄ between low- (0-30 °N), mid- (30-60 ºN), and high-latitude (60-90 ºN) ecosystems. In contrast, we did not observe significant latitudinal differences in freshwater CH₄ δ¹³C values in this dataset. Residual variability in δ²H-CH₄ is partially explained by methanogenic pathways and CH₄ oxidation, as indicated by carbon isotope fractionation between CH₄ and carbon dioxide. These biogeochemical sources of variability are reflected in apparent differences in δ²H-CH₄ 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 differences in δ²H-CH₄ between ecosystems. To estimate how changes in the spatial distribution of freshwater emissions would influence global atmospheric CH₄ isotopic measurements, we developed a model of global CH₄ δ²H and δ¹³C sources and sink signatures, including spatially resolved signatures for freshwater CH₄ sources. This model implies that increases in high-latitude freshwater CH₄ emissions would have a particularly strong influence on global δ²H-CH₄. We also examine how changes in the spatial distribution and flux of freshwater CH₄ emissions would influence co-variation between atmospheric CH₄ δ²H and δ¹³C, and compare these patterns with trends in atmospheric CH₄ isotopic measurements.