Wintertime production and storage of methane in thermokarst ponds of subarctic Norway

Abstract. The ongoing climate change in permafrost areas can trigger abrupt thaw processes, leading to the formation of thermokarst lakes and ponds. These water bodies, especially in organic-rich areas, are recognized as strong methane (CH4) emitters during the ice-free periods and have the potential to accumulate large CH4 amounts in and below the ice, which is released during the ice melt. We estimated winter CH4 bottom flux in nine shallow ponds within two permafrost peatlands in Northern Norway, Iškoras and Áidejávri, during the 2023–2024 ice cover season. The cumulative winter CH4 bottom flux ranged from 0.6 to 24 g CH4-C m−2 which at the Iškoras site contributed up to 40 % to the annual CH4 budget. The heterogeneity of the CH4 wintertime accumulation is related to pond depth, differences in vegetation, and the thermokarst pond formation age. The latter was investigated using a space-for-time substitution approach along chronosequences of thermokarst formation spanning more than 70 years. The winter CH4 bottom flux increased from 2 mg CH4-C m−2 d−1 in two-year-old pond to 106 mg CH4-C m−2 d−1 in a pond formed between 30 and 60 years ago. Pond that formed more than 70 years ago and is currently experiencing sedge regrowth exhibited large winter CH4 bottom flux of 59 mg CH4-C m−2 d−1, while older ponds dominated by Sphagnum mosses showed 4 to 10 times smaller CH4 bottom fluxes.