Cessation of ice-wedge development during the 20th century in spruce forests of eastern Mackenzie Delta, Northwest Territories, Canada

Abstract

Ice wedges are presently inactive in white spruce (Picea glauca) forests of eastern Mackenzie Delta as shown by the absence of vein ice above ice wedges, the maintenance of intact breaking cables, and the abundance of rootlets propagating across ridge-trough sequences. At spruce forest sites, near-surface ground cooling rates and minimum near-surface temperatures from the years 2003-2005 were above ice-wedge cracking thresholds. Ground thermal conditions associated with cracking were recorded at a tundra peatland with active ice wedges. Annual mean permafrost temperatures at the spruce forest sites ranged from -1.8 to -2.9°C, whereas at the tundra peatland, the permafrost was colder than -6°C. Although winter air temperatures are similar throughout the study region, deeper snow cover, thicker active layers, and warmer permafrost account for the more gradual seasonal cooling and warmer nearsurface temperatures recorded at the subarctic forest sites. The subtle ridge to trough relief, 12-35 cm of permafrost above wedge ice, roots up to 80 years old grown across ice wedges, and negligible tritium levels in wedge ice indicate that thermal contraction cracking in the spruce forests has been infrequent throughout much of the last century. The proximity of wedge ice to the base of the aggrading permafrost table and the absence of old spruce roots spanning icewedge troughs suggest that ice-wedge cracking did occur in the forest environments during the cold and dry conditions associated with the Little Ice Age and early part of the 20th century. When these ice wedges cracked, minimum temperatures at the top of permafrost were probably at least 3-8°C colder than presently observed and similar to present conditions at the tundra peatland.Résumé : Des coins de glace sont actuellement inactifs dans les forêts d'épinette blanche (Picea glauca) de l'Est du delta du Mackenzie tel que montré par l'absence de veines de glace au-dessus des coins de glace, le maintien intact des câbles de rupture et l'abondance de racines se propageant à travers les séquences crêtes-creux. Aux sites de forêts d'épinette, les taux de refroidissement et les températures minimales à proximité de la surface pour la période 2003-2005 étaient au-dessus des seuils de fissuration des coins de glace. Des conditions thermiques au sol, associées à la fissuration, ont été enregistrées dans une tourbière de la toundra contenant des coins de glace actifs. Les températures annuelles moyennes du pergélisol aux sites de forêts d'épinettes variaient de -1,8 à -2,9°C, alors qu'aux tourbières de la toundra la température du pergélisol est inférieure à -6°C. Bien que les températures de l'air en hiver soient semblables à travers toute la région à l'étude, le manteau nival plus épais, des couches actives plus épaisses et un pergélisol plus chaud expliquent le refroidissement saisonnier plus graduel et les températures plus chaudes enregistrées près de la surface aux sites forestiers subarctiques. Le relief subtil entre les crêtes et les creux, les 12 à 35 cm de pergéliso...

Permafrost and climate change at Herschel Island (Qikiqtaruq), Yukon Territory, Canada

“…The difference between T a and T g ranges from 0 to 3°C at the sites with low snow cover, but is 4°C or greater at site 5 (Table 3b). These differences are generally comparable to other data collected throughout the permafrost regions of Yukon Territory [Smith et al, 1998], but it is unusual to record differences of less than 1°C between these indices in the boreal forest [Karunaratne and Burn, 2004], and in the northern boreal forest near Inuvik, the difference is more than 5°C [Kokelj et al, 2007]. Indeed, if the air temperature at Collinson Head is, on average, warmer than at sea level, as discussed above, then, from time to time at windswept sites, T a may be higher than T g .…”

Field observations of syngenetic ice wedge polygons, outer Mackenzie Delta, western Arctic coast, Canada

“…Near the surface, the shoulders were close to right angles (Figure b), but as depth increased, they were increasingly deformed, probably from shear and diapiric uplift (Figure c) [ Mackay , ]. Stratigraphic deformation adjacent to the ice wedges also increased systematically with depth (Figure ), as observed alongside syngenetic wedges in the upper Mackenzie Delta [ Kokelj et al ., , Figure ].…”

Permafrost thermal state in the polar Northern Hemisphere during the international polar year 2007–2009: a synthesis

“…Peat and clay-silt overlie sand and gravel. The shrubby tundra vegetation favours the accumulation of thicker and lower density snow (Kokelj et al, 2007;Burn and Kokelj, 2009). As a result, the annual amplitude of temperature variations right below the ground surface at borehole RCT-2 is much smaller than at RCT-1 ( Figure 4B).…”