Multi-model Insights into δ13C-CH4 from Arctic Permafrost Thermokarsts

Methane is experiencing an accelerating increase in the atmosphere globally. Of the tools researchers have to diagnosis and determine the cause of rapidly changing sources and sinks of methane, its carbon isotope composition, δ¹³C-CH₄, is a promising option to reduce uncertainty and provide constraints on methane atmospheric inversions. A building consensus in literature points towards wetland emissions as the driving force behind increase emissions, yet our ability to be prescriptive of the wetland δ¹³C-CH₄ flux remains uncertain. Of wetlands, northern permafrost and its thaw features add additional complexities just as they add a large potential carbon stock for future methane release. Early models attempting to determine the δ¹³C-CH₄ of permafrost thaw predict that these landscapes will be isotopic endmembers, more depleted than any source on the planet. How does this prediction hold up against observations when downscaled to the site level?

We present an intercomparison between observations and two isotope-enabled methane production models targeting the thaw feature Big Trail Lake outside Fairbanks, Alaska. We compare the isotope-enabled version of the terrestrial ecosystem model–methane dynamics module (isoTEM) to the Arctic Lake Biogeochemistry Model (ALBM) with an added isotope mass balance. We benchmark both model runs against methane eddy-flux data and flask-collected methane isotope measurements onsite. Through this multi-model-observation intercomparison, we evaluate model mismatch of δ¹³C-CH₄ flux at Big Trail Lake and evaluate how model physics can be improved to better capture permafrost thaw δ¹³C-CH₄ flux for use in constraining atmospheric inversions.