Process-based mapping of global wetland carbon isotopic signatures of methane

To effectively use measurements of δ¹³C of atmospheric CH₄ to constrain emissions of CH₄ by source sector in atmospheric transport model studies, the spatial and temporal distribution of source isotopic signatures must be known. But currently, such information on δ¹³C of CH₄ (δ¹³C-CH₄) emitted by wetlands is limited. Observations show a latitudinal gradient in wetland δ¹³C-CH₄ source signatures with higher values in tropics and lower values in the arctic. Here we incorporated a carbon isotope-enabled module into an extant biogeochemistry model to mechanistically simulate the latitudinal and inter-annual variability of global wetland δ¹³C-CH₄. The new model explicitly considers isotopic fractionation during methane production, oxidation, and transport processes. The model is then parameterized for low and high pH conditions of arctic, temperate, and tropical wetland ecosystems using observed data from field studies and extrapolated to global wetland ecosystems for 2000 to 2016. We estimate a flux-weighted global wetland δ¹³C-CH₄ with its seasonal and inter-annual variability. We find that the new model better matches field chamber observations than an empirical wetland δ¹³C-CH₄ map. The model also reasonably reproduces the regional heterogeneity of wetland δ¹³C-CH₄ in Alaska, consistent with vertical profiles of δ¹³C from NOAA aircraft measurements. Furthermore, we show that the latitudinal gradient of δ¹³C of atmospheric CH₄ simulated by a chemical transport model using the new wetland δ¹³C-CH₄ map reproduces the observed latitudinal gradient based on NOAA/INSTAAR global flask-air measurements. Lastly, we attribute the modeled latitudinal gradient of wetland δ¹³C-CH₄ to the soil organic carbon δ¹³C distribution, methane production, oxidation, and transport processes. We believe this study is among the first to use a process-based biogeochemistry model to map the global distribution of wetland δ¹³C-CH₄, which will significantly help atmospheric chemistry transport models partition global methane emissions.