Permafrost is an important physical component of the Northwest Territories, and has a profound effect on the hydrology, landscape and ecology of northern environments. The Northwest Territories falls within the continuous, discontinuous and sporadic discontinuous zones of permafrost and permafrost underlies the majority of NWT communities. An extensive area of subsea permafrost on the western Arctic continental shelf also developed during the last glacial period, when sea levels were lower and sediments were exposed to frigid air temperatures. The current and future conditions of permafrost are key considerations in the planning and management of virtually all infrastructure in the NWT.
The temperature and thickness of permafrost varies across the NWT with climate, vegetation cover and geology. The transition from continuous to discontinuous permafrost roughly coincides with the position of subarctic boreal-tundra transition, south of which permafrost generally occupies between 65 and 90 % of the landscape. In the southern NWT permafrost is classified as sporadic discontinuous, where thin lenses of frozen ground persist in peatlands at temperatures just below 0oC21.
The type and amounts of ice within permafrost13 determine the environmental and geotechnical implications of thawing. Ground ice in permafrost gives rise to unique landforms such as pingos, polygonal ground, palsas and lithalsas21. A zone of near-surface ice enrichment immediately beneath the base of the active layer is ubiquitous in fine-grained sediments throughout the western Arctic and Mackenzie Valley, and thus, the terrain is sensitive to disturbances that would cause the active layer to deepen14. Ice-wedges which underlie polygonal terrain are another common form of ground ice in the tundra. Extensive bodies of massive ground ice several metres thickness can be found in glacial deposits such as moraines and eskers.
Determining areas with ice-rich permafrost is important for planning sustainable infrastructure and predicting which landscapes are most sensitive to change if the permafrost thaws13. Some of the most abrupt changes to terrestrial and aquatic environments in the NWT have resulted from the degradation of ice-rich permafrost8. Thawing of ice-rich permafrost results in a thermokarst landscape which can modify drainage2, cause lakes or wetlands to expand, or conversely to drain22, and vegetation communities to change irreparably. Landsliding or slumping can contribute large volumes of sediment into lakes and streams, raising suspended sediment concentrations and adversely impacting aquatic habitat8,16,19. Thaw slumping and permafrost degradation can also influence lake or stream water chemistry by exposing previously frozen materials to leaching or by increasing ground water flow7,15.
Terrestrial ecosystems are also modified by the formation or degradation of ground ice. For example, heaving of the ground can cause hummocky terrain, and subsidence may produce “drunken forests”5. Both thaw slumping and collapse of peatlands can change moisture and chemical conditions in soils6. Permafrost disturbances can also expose mineral soils for colonization by disturbance-adapted species, which may contribute to changes in the composition of surrounding vegetation communities9. Thawing permafrost can also damage buildings, roads and other infrastructure facilities leading to increased maintenance and mitigation costs. There is also the possibility that thawing of permafrost could release significant amounts of carbon sequestered in permafrost11.
Multidisciplinary approaches are required to understand northern environmental systems and distinguish normal variation from the effects of climate change, and natural or anthropogenic disturbance1,18. The thermal state of permafrost is strongly influenced by climate, vegetation and snow cover1,17. The formation and degradation of ground ice has both geotechnical and ecological implications10,12. The strong links between permafrost and other components of the environment places the discipline of permafrost science in a unique position to serve a unifying role in the investigation of northern physical and biological processes and their responses to global change4. The high priority of permafrost issues in development and management of northern infrastructure highlights the importance of the discipline in engineering and northern geoscience2.
Tracking ground temperatures and active-layer monitoring in the NWT can give early-warning information on the degradation of permafrost. Delineating areas with ice-rich permafrost provides an indication of landscapes sensitive to change if the permafrost thaws. Trends in the areal extent of thermokarst, provide information on landscape change. Together, monitoring of these indicators provides critical information to resource managers, planners, proponents of resource development projects and those charged with maintaining NWT infrastructure.
Permafrost: Rock or soil that remains below 0°C for at least two consecutive years. Surface conditions including vegetation, organic cover and snow thickness can influence permafrost temperatures. Permafrost thickness is related to the air temperature, soil characteristics and the geothermal gradient as well as the geological history of the area.
Active layer: Surface layer of earth materials within permafrost that thaws and refreezes each year.
Ice-rich permafrost: Ground-ice content greater than the saturated moisture content of thawed soil is called “excess ice”. Ground ice features include polygonal terrain underlain by a network of ice wedges and the distinct ice-cored conical pingos of the Mackenzie Delta region.
Thermokarst: Land surface that forms as ground subsides due to thawing of ice-rich permafrost on slopes. Thermokarst processes may cause lakes to enlarge, peatlands to collapse and landslides or thaw slumps to develop.
Retrogressive thaw slump: Develop due to thawing of ice-rich permafrost on slopes. Thawing turns exposed ice-rich permafrost into a mud slurry which collapses to the base of the exposure and flows downslope.
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