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TitleLithalsa degradation and thermokarst distribution, subarctic Canadian Shield
AuthorMorse, P DORCID logo; Wolfe, S AORCID logo; Rudy, A C A
SourceCold Regions Engineering 2019: proceedings of the 18th International Conference on Cold Regions Engineering and the 8th Canadian Permafrost Conference; by Bilodeau, J -P (ed.); Nadeau, D F (ed.); Fortier, D (ed.); Conciatori, D (ed.); 2019 p. 308-316,
Alt SeriesNatural Resources Canada, Contribution Series 20190038
PublisherAmerican Society of Civil Engineers
Meeting18th International Conference on Cold Regions Engineering and the 8th Canadian Permafrost Conference; Québec, QC; CA; August 18-22, 2019
Mediaon-line; digital
File formatpdf (Adobe® Reader®)
ProvinceNorthwest Territories
NTS85J/06; 85J/07; 85J/09; 85J/10; 85J/11; 85J/14; 85J/15; 85J/16
AreaNorth Slave Region; Great Slave Lake; North Arm; Behchoko; Yellowknife
Lat/Long WENS-115.3333 -114.4167 62.9667 62.4167
Subjectssurficial geology/geomorphology; environmental geology; geophysics; Nature and Environment; Science and Technology; Holocene; permafrost; ground ice; periglacial features; thermokarst; surface waters; lakes; water levels; remote sensing; photogrammetric techniques; airphoto interpretation; satellite imagery; modelling; subsidence; climate; Canadian Shield; Great Slave Lowland Ecoregion; Great Slave Upland Ecoregion; Climate change; lithalsas; colluvial and mass-wasting deposits; elevations; Phanerozoic; Cenozoic; Quaternary
Illustrationslocation maps; geoscientific sketch maps; photographs; histograms; bar graphs; models
ProgramClimate Change Geoscience Permafrost
Released2019 08 08
AbstractIn 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.
Summary(Plain Language Summary, not published)
Thaw of ice-rich permafrost causes the ground surface to settle and water often ponds on top. This is called thermokarst. Thermokarst is widespread in the North Slave region, Northwest Territories, and is usually associated with the degradation of ice-core permafrost mounds called lithalsas. Permafrost in the region developed progressively throughout the Holocene as lake levels declined. As a result, lithalsa age may relate to elevation, with some older features occurring at higher elevations whereas young ones have formed in historical time near the modern shoreline. As older lithalsas are more likely to have been subjected to degradation-inducing environmental conditions, we test our hypothesis that lithalsas used to be prevalent at higher altitudes, but have since been degraded. We develop diagnostic criteria for degrading lithalsas identified from geomorphic patterns determined from air photographs and site descriptions, use them to map lithalsas, accept our hypothesis, and develop a conceptual model for lithalsa degradation in this region.

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