Long-Term Permafrost Degradation and Thermokarst Subsidence in the Mackenzie Delta Area Indicated by Thaw Tube Measurements

摘要

Long-term measurements (1991-2016) from 17 thaw tubes in the Mackenzie Delta region were analyzed to investigate changes in thaw penetration (TP), ground surface (GS) elevation relative to a stable benchmark, and active-layer thickness (ALT). TP increased significantly during the study period at 10 of the sites, at a median rate of 0.5 cm a~(-1) (min: 0.2, max: 1.5 cm a~(-1)). Significant subsidence occurred at 10 sites, at a median rate of 0.4 cm a~(-1) (min: 0.2, max: 0.8 cm a~(-1)). These results indicate long-term regional permafrost degradation and surface subsidence in the western Canadian Arctic. About 5 to 38 cm of permafrost have thawed over 25 years at the sites, and ～5 to 20 cm of excess ground ice have melted. ALT increased significantly at only 5 of the sites and decreased at 4 sites. In ice-rich ground, increases in TP can more than double increases in ALT. For example, at a site near Inuvik, ALT increased by only 19 cm between 1994 and 2016, but TP increased by about 40 cm and was accompanied by 20 cm of surface subsidence. At sites in ice-poor ground, negligible settlement occurs, and increases in TP and ALT are similar. In ice-rich terrain, the degradation of permafrost cannot be detected by probing for active-layer thickness alone.