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Degradation of organic carbon stored in permafrost may represent an additional source of atmospheric greenhouse gases (GHGs) in a warming climate. However, there is no clear understanding of how seasonal freeze–thaw affects gas permeability and emission of methane in permafrost soils, in part due to the lack of chemical and physical characterisation of the soils. Here, we report vertical profiles of CO₂, CH₄ and other soil properties with resolutions of 0.05–0.1 m depth from five soil cores, drilled to 0.9 m depth, in Alaskan permafrost during the early spring of 2013, when the ground was frozen under snow cover. Two cores from tundra and black spruce bog showed sudden increases in methane concentration (up to 416 µmol litre⁻¹_soil), indicating excess CH₄ trapped in gas‐impermeable soil layers during the freezing period. Active‐layer cooling in late autumn, producing a frozen surface and relatively …