No abstract

The distribution of segregated ice and soluble ions in near-surface permafrost were investigated in hummocky terrain near Inuvik, Northwest Territories. Soil water content profiles from analyses of drill cores indicate that ice-poor permafrost developed beneath a permafrost table aggrading at approximately 4 cm/a, but an ice-rich zone, 10 to 20 cm thick, was observed beneath a permafrost table that had remained stable for about a decade. Ice-rich intervals 10 to 30 cm thick were observed immediately beneath both a thaw unconformity formed in 1981 and an older, deeper unconformity. In profile, the correspondence between zones of cation and ice enrichment suggests soluble materials were incorporated into permafrost during development of near-surface aggradational ice. Moisture enrichment below an experimentally degrading permafrost table was negligible. Similar ice contents beneath the present permafrost table and the deep thaw unconformity, and the preservation of ice-poor intervals immediately above the 1981 and deep thaw unconformities indicate limited vertical ice enrichment. The estimated rates of ice accumulation in two-decade-old permafrost are on the order of mm/a, but ice accumulation above older unconformities indicates that, in aggregate, these initial rates decrease with time. The ground ice and soluble cations sequestered in near-surface permafrost comprise an important pool of water and nutrients that may be released into the active layer during periods of deeper thaw.

In tundra uplands east of the Mackenzie Delta, retrogressive thaw slumps up to several hectares in area typically develop around lakes. Ground temperatures increase in terrain affected by slumping due to the high thermal conductivity of exposed mineral soils and deep snow accumulation in winter. Mean annual temperatures at the top of permafrost were several degrees warmer in thaw slumps (À0.18C to À2.28C) than beneath adjacent undisturbed tundra (À6.18C to À6.78C). Simulations using a twodimensional thermal model showed that the thermal disturbance caused by thaw slumping adjacent to tundra lakes can lead to rapid near-surface lateral talik expansion. Talik growth into ice-rich materials is likely to cause lake-bottom subsidence and rejuvenation of shoreline slumping. The observed association of thaw slumps with tundra lakes, the absence of active slumps on the shores of drained lakes where permafrost is aggradational and depressions in the lake bottom adjacent to thaw slumps provide empirical evidence that thermal disturbance, talik enlargement and thawing of subadjacent icerich permafrost can drive the polycyclic behaviour (initiation and growth of slump within an area previously affected by slumping) of lakeside thaw slumps. Figure 1 A) Large retrogressive thaw slump adjacent to a small tundra lake, Mackenzie Delta region (69807 0 04 00 N; 134810 0 59 00 W). An active slump in the foreground is developing within an older, stable slump scar, illustrating polycyclic activity. The headwall of the active slump is 3 to 4 m in height. Total disturbed area is greater than 5 ha. B) Multi-aged retrogressive thaw slumps north of Noell Lake, Mackenzie Delta region (68836 0 26 00 N; 133834 0 52 00 W). The total slump affected area is approximately 1 ha. 178 S. V. Kokelj et al.Figure 8 A) Maximum and minimum ground temperature profiles for undisturbed tundra and a stable vegetated slump surface, T7, Richards Island. B) Mean annual ground temperature profiles for undisturbed tundra and stable slump surfaces at T5 near Parsons Lake, and at T7 on Richards Island.

No abstract

Recent climate warming has activated the melt-out of relict massive ice in permafrost-preserved moraines throughout the western Canadian Arctic. This ice that has persisted since the last glaciation, buried beneath as little as 1 m of overburden, is now undergoing accelerated permafrost degradation and thermokarst. Here we document recent and intensifying thermokarst activity on eastern Banks Island that has increased the fluvial transport of sediments and solutes to the ocean. Isotopic evidence demonstrates that a major contribution to discharge is melt of relict ground ice, resulting in a significant hydrological input from thermokarst augmenting summer runoff. Accelerated thermokarst is transforming the landscape and the summer hydrological regime and altering the timing of terrestrial to marine and lacustrine transfers over significant areas of the western Canadian Arctic. The intensity of the landscape changes demonstrates that regions of cold, continuous permafrost are undergoing irreversible alteration, unprecedented since deglaciation (~13 cal kyr B.P.).