Lithalsa Degradation and Thermokarst Distribution, Subarctic Canadian Shield

In the North Slave region, permafrost developed in a time transgressive manner throughout the Holocene with lake-level recession, giving rise to the Great Slave Lowland and Great Slave Upland ecoregions of the subarctic Canadian Shield. Thermokarst in the region is commonly associated with degradation of numerous ice-cored mounds called lithalsas. Here we use site descriptions and air photos to document the distinctive geomorphic signatures associated with degrading lithalsas and develop a conceptual model for lithalsa degradation in this region, which builds upon an earlier model of lithalsa development. Physical degradation of lithalsas is dominated by two main processes: (i) subsidence indicated by the common occurrence of ponded water with partially submerged standing dead trees, and (ii) colluviation of thawed sediments toward the lithalsa margin that results in a rampart. According to these diagnostic criteria, satellite image analysis suggests that lithalsas were more widespread at higher elevations in the past, but the majority have degraded. This explains, in part, the reduction of lithalsa abundance with increasing elevation. The results suggest that lithalsas are vulnerable to thaw. Following from our observations and findings, we develop a conceptual model of lithalsa degradation. It suggests that soil hysteresis effects would likely prevent re-initiation of lithalsa formation if permafrost were to re-aggrade in the future.