Degradation, stabilization, and initial aggradation of permafrost following an arctic tundra fire

The Anaktuvuk River fire severely burned more than 1,000 km2 of arctic tundra in northern Alaska in 2007. Since the fire burned tundra vegetation and organic-rich soils underlain by a range of permafrost ground ice conditions, long-term observations of the landscape can help understand how the Arctic will change as more area burns. Some outstanding questions in this regard are: What is the trajectory and longevity of fire-induced permafrost degradation, and can permafrost in burned tundra landscapes stabilize following disturbance? Post-fire thaw-related effects at the Anaktuvuk River tundra fire through 2014 included the development and stabilization of localized active layer detachment slides and retrogressive thaw slumps, but the widespread and ongoing degradation of ice wedges. In this study, we update observations on near-surface permafrost in the Anaktuvuk River tundra fire burn area through 2021 using ground temperature measurements, cryostratigraphic studies, optical satellite imagery, and repeat airborne LiDAR data. We focus on a 50 km2 mosaic of ice-rich permafrost terrain in the Yedoma silt belt region of the burn. Annual mean ground temperature data collected at a depth of 1 m at burned and unburned observation sites show that the permafrost was 2.2C warmer in the burned area 7-8 years following the fire, but only 1.1C warmer 12-13 years post-fire. Several permafrost boreholes drilled in ice wedge troughs and polygon centers 14 years post-fire revealed the presence of a thaw unconformity that was overlain by a 10-25 cm thick ice-rich intermediate layer, indicating aggradation of permafrost following thermokarst development. Remote sensing-based change detection of surface water in thermokarst pits and repeat analysis of airborne LiDAR data collected in 2009, 2014, and 2021 add further supporting evidence for the cessation of thaw subsidence of the ice-rich permafrost terrain. Taken together, our observations highlight that the initial degradation of ice-rich permafrost terrain in the first ten years following the Anaktuvuk River tundra fire was followed by a period of permafrost aggradation and terrain stabilization. The result is a post-fire landscape that shows evidence of being disturbed by fire-induced thaw, but one that is likely more resilient to future disturbance.