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 emitters during the ice-free periods and have the potential to accumulate high amounts of methane in and under the ice, which can be released during the ice melt. We estimated wintertime CH₄ storage and daily bottom flux in nine shallow ponds within two permafrost peatlands in Northern Norway, Iškoras and Áidejávri, during the 2023–2024 ice cover season. The wintertime CH₄ storage ranged from 0.6 to 24 g CH₄-C m⁻² and contributed up to 40 % of the annual CH₄ budget at the Iškoras site. The heterogeneity of the CH₄ wintertime accumulation is related to pond depth, differences in vegetation, and the thermokarst pond formation age. The latter has been 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 3 mg CH₄-C m⁻² d⁻¹ in two-year-old pond to 107 mg CH₄-C m⁻² d⁻¹ in a pond formed between 30 and 60 years ago. Ponds that formed more than 70 years ago and are currently experiencing sedge regrowth exhibited a high CH₄ bottom flux of 60 mg CH₄-C m⁻² d⁻¹, while older ponds dominated by Sphagnum mosses showed 4 to 10 times lower CH₄ bottom fluxes.