Changing Permafrost Landscapes

Lithalsa degradation, thermokarst distribution, and landscape evolution, North Slave region, Northwest Territories

Tuesday, November 19, 2019 - 4:20pm to 4:40pm Theatre Three


P.D. Morse (Presenting)
Geological Survey of Canada
S.A. Wolfe
Geological Survey of Canada
A.C.A. Rudy
Northwest Territories Geological Survey

The Great Slave Lowland and Great Slave Upland ecoregions of the subarctic Canadian Shield are influenced by permafrost that has developed in a time transgressive manner throughout the Holocene with lake-level recession. Thermokarst in the region is commonly associated with the degradation of ice-cored mounds (lithalsas), which are numerous. Here we use site descriptions and air photos to document the distinct geomorphic signatures associated with degrading lithalsas and develop a conceptual model for lithalsa degradation, which builds upon an earlier model of lithalsa formation. Two main processes dominate lithalsa degradation: (i) subsidence indicated by ponded water with partially submerged standing dead trees, and (ii) colluviation of thawed sediments toward the lithalsa margin that results in a rampart. Applying these diagnostic criteria to satellite image analysis, we found that lithalsas were once more widespread at higher elevations (Great Slave Upland), but the majority have degraded. The results suggest that lithalsas have been vulnerable to thaw throughout the Holocene, and explain, in part, the reduction of lithalsa abundance with increasing elevation. The conceptual model suggests that soil hysteresis effects would likely prevent re-initiation of lithalsa formation if permafrost were to re-aggrade in the future. Finally, high counts of intact lithalsas, degraded lithalsas, and thermokarst ponds within 15 m elevation of the Great Slave Lake shoreline indicate that this terrain is the most sensitive to climate warming and should be avoided by new infrastructure development.