Fig. 1.

Methane bubbling in an interior Alaska abrupt thaw lake. Methane bubbles released by ebullition from thaw bulbs beneath thermokarst lakes are seasonally trapped in winter lake ice forming white bubble patches. High bubbling rates of particularly strong ebullition seeps known as hotspots maintain ice-free holes in winter lake ice. Ebullition hotspots are indicative of abrupt thaw environments, where the rapid (decadal-scale) transformation of terrestrial permafrost to deep thaw bulbs beneath lakes fuels anaerobic decomposition of ¹⁴C-depleted soil organic carbon and the release of ¹⁴C-depleted CH₄ in bubbles^20,21. Diameters of the ice-free hotspot holes shown in this October 17, 2016 photograph are between 0.4 and 0.9 m