Processes and emissions of CH4 and CO2 from subarctic and arctic thaw ponds: Insights from stable isotopes

The expansion of permafrost thaw ponds, a widespread feature of Canadian arctic sedimentary basins, is accelerating due to climate change. Permafrost soils store half of the global below ground organic carbon stock, which is now being mobilized as peat soils thaw and erode. Thaw ponds are large emitters of methane and carbon dioxide. Our studies of organic-rich thaw ponds in the Canadian Arctic (Bylot Island, north Baffin Island) and Subarctic (Kuujjuarapik, Quebec) reveal substantial fluxes to the atmosphere of CO₂ (up to 326 mmol CO₂ m^-2 d^-1) and CH₄ (up to 15 mmol CH₄ m^-2 d^-1) in summer, using wind-based model estimations (excluding ebullition). Carbon and hydrogen isotopes indicate that acetoclastic fermentation is the dominant methanogenic pathway in these ponds (e.g., δ¹³CH₄ ca. -55 to -75 %; δD-CH₄ ca. -350 %). In contrast with many peatland sites, methanogenesis in thaw ponds does not transition to the hydrogenotrophic pathway. In one type of arctic ponds colonized with thick cyanobacterial mats, active photosynthesis during summer drags down atmospheric CO₂ (negative flux down to -18 mmol CO₂ m^-2 d^-1) and potentially influences the δ¹³CH₄. Trends in the isotope data, in concert with O₂, confirm the importance and variability of aerobic methane oxidation on dissolved methane concentrations in the ponds. The combined mechanisms of methanogenesis and methylotrophy modulate the release of important greenhouse gases from these environments.