Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Permafrost landform degradation over more than half a century, Macmillan/Caribou Pass region, NWT/Yukon, Canada G.P. Kershaw Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada ABSTRACT: Aerial photography, ground photos and descriptions from the early 1940’s have been supple- mented by sequential aerial photography and field sampling to examine permafrost landform degradation over more than 25 years. Seven study sites, extending over a distance of 75 km and an elevation range of 550 m, have been intensively monitored since 1990 and regular reconnaissance-level visits have been made to an additional 91 features. Features indicative of degradation were evident on the earliest aerial photography and on ground photography from the same period. Without exception, peatlands with palsas and peat plateaux have continued to experience permafrost-landform degradation, including the complete decay of some features, areal reductions, and the development of thermokarst depressions. Degradation rates, established from sequential photography, average 1% per annum over the last half century. However, it is possible to find features that are degrading along one margin but are stable or possibly aggrading elsewhere. The regional nature of permafrost degradation sug- gests that climatic factors initiated processes of permafrost degradation sometime between 800 and 1944 AD. 1 INTRODUCTION 1.2 Study area and objective 1.1 Previous studies The area of study extended from approximately 20 km inside the Yukon Territory, Canada through Macmillan Studies of periglacial landforms in the Macmillan and Pass to 50 km inside the Northwest Territories, just Caribou Pass areas first started in 1944 (Porsild, 1945). beyond Caribou Pass (Fig. 1). The Yukon section was Over the years, these have focused on solifluction lobes below timberline and was dominated by open Picea (McIlhargey, 1994), rock glaciers (Kershaw, 1978), and glauca and Abies lasiocarpa stands. Above timberline palsas and peat plateaux (Harris and Nyrose, 1992; and below 1560 m asl, the mountains were cloaked Horvath, 1998a, b; Kershaw and Gill, 1979; Kershaw in Erect Shrub Tundra dominated by Betula glandulosa and Skaret, 1993; Skaret, 1995). Studies on the latter with a Cladina mitis, C. stellaris, C. rangiferina, (Kershaw and Gill, 1979) concluded that some features Cetraria cuculata, C. nivalis and C. islandica ground had degraded in areal extent by 34% to 100% between cover (Harper and Kershaw, 1996; Kershaw, 1990). 1944 and 1978. Horvath (1998a) estimated degradation Above 1560 m, the tall shrubs were replaced by of 36.4% and 19.7% for two palsa fields 4 km apart Decumbent Deciduous Shrub/Graminoid Tundra. These between 1949 and 1981. The mean annual rate of systems were dominated by Salix polaris (Davis, 1998; change was different for the 1949 to 1972 and the 1972 Kershaw, 1984). Peatlands varied from Carex aquatilis- to 1981 record periods – 1.4% and 0.5% for one field dominated fens to Sphagnum spp.-dominated bogs on and 0.7% and 0.4% respectively, for the other. These areas where drainage was impeded. results are at odds with Harris and Nyrose (1992) since The study objective was to locate and determine the they concluded that palsas in peatlands on the Yukon status of palsas and peat plateaux in the region and to side of Macmillan Pass were “stable or growing” and determine if there was a common set of characteristics that the features presented “a sequence of develop- exhibited by the features. If there was a common status ing peat islands culminating in mature palsas”. They then it could be concluded that factors affecting the classified one of their study features as an “embryonic features were similar throughout the region, whereas palsa”. differences would suggest more site-specific environ- There are numerous studies from non-mountainous mental influences. terrain supporting the assertion that degradation dom- inates Canadian palsa fields from the Mackenzie River (Zoltai and Tarnocai, 1975) across the interior (Thie, 2 METHODS 1974; Vitt et al., 1994) to northern Quebec (Laberge and Payette, 1995). Some note that it is also possible to 2.1 Photogrammetry discover evidence of aggradation but that permafrost degradation dominates (Laberge and Payette, 1995; Vitt Various aerial photographs were used but not all mis- et al., 1994; Zoltai, 1993). sions covered the entire study area. The earliest images 543 early photography was from 26–28 August 1944 (1:16,000, #T18, T19, T20 and T21) and was also trimetrogon imagery. Photography from 1948 to 1949 was not sharply focused but did cover much of the study area (1:31,680, #A-11445 and A-11486). Another mission flown in 1949 was used (1:31,200, #A12228, A12232, A12233, A12343, A12246, A12247, A12248, A12255, A12188) but again the images were not sharp. Photography from 1963 included the Yukon side of the study area (Spartan Air Surveys, 1:15,600, #A18131). On 10 and 11 August 1972, mining com- panies had multiple missions flown, each with a dif- ferent scale (Lockwood Survey Corporation Ltd., 1:6,000, 1:9,600, 1:12,000 and 1:24,000, #72–88), but much of the eastern portion of the study area was not covered. In August-September 1974, missions covered the entire study area at two scales (1:40,000 #A23932, A23933 and 1:12,000 #A23919 and 23924). In 1980, high elevation aerial photos were taken but the 1:66,000 scale (#A25586) was too small for study purposes. On 14 June 1981, another mission was flown over all but the western-most and eastern-most sections of the study area (1:12,000 #A25766). On 7 and 24 August 1981, (Pacific Survey Corp. 1:10,000 #81–135) the western- most portion of the study area was covered. In 1996 another mission was flow (#A28283) but 1:30,000 scale proved too small for precise measurements. 2.2 Field work Field investigations were initiated in 1974, but until 1990, there were years when no data were collected (i.e. 1975 and 1976; 1983 to 1989). Since 1990 when the first four automated microclimate stations were installed (Kershaw and Skaret, 1993), reconnaissance- level visits have been made to most features within the study area. Many had been provisionally identified on aerial photos and were visited to verify their landform classification and to describe their status. Photos, sketches and notes were taken at each feature, includ- ing descriptions of surface cover (e.g. plant community, bare peat, mineral exposures) and evidence of land- form status. Thaw layer depths were measured by prob- ing with a graduated stainless-steel rod (1 cm diameter) and by periodic checking with an electronic thermome- ter in a 92 cm stainless-steel probe. Coring (Zoltai, 1978) of features was done on a limited basis, often Figure 1. 1943/1944 locations of palsas/peat plateaux and when installing temperature sensors attached to the hydraulic pingos in Macmillan/Caribou Pass region. H: automated dataloggers. 1160, PF: 1390, HF: 1260 m, BP: 1272 m, DC (2 & 6): In 1990 four automated microclimate stations 1475 m, GF: 1621 m, SF: 1645 m. (Campbell Scientific Inc., CR10 and CR10X) were installed with thermocouples placed at a standard were from 10 and 25 May 1943 (1:27,000, #2–3025, depth of 150 cm in the palsa core (Kershaw and Skaret, R 306 and R 310). This trimetrogon mission covered 1993). The reference junction was attached to the wiring only the eastern-most section of the study area and there panel and the logger and a gel-cell battery were housed was snow on the ground. The most comprehensive, in a water-tight case. In subsequent years two loggers 544 were added at new sites. The mean annual permafrost 0 temperature (MAPT) was calculated from the daily means when a minimum of 340 days of data were avail- -0.5 HF able. Animal damage, sensor failure and power supply -1 BP C) interruption were the most common reasons for loss º of data. -1.5 D2 -2 D6 3 RESULTS Temperature ( -2.5 GF -3 3.1 Distribution and site characteristics SF -3.5 Palsas and peat plateaux are found as low as 1150 m asl 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 along the South Macmillan River, Yukon and up to Figure 2. Mean annual permafrost temperatures (Ϫ150 cm 1650 m on the Mackenzie Mountain Barrens, NWT depth) for the entire record period for all palsas with micro- (Fig. 1). They occur 55 km east of the Yukon border in climate stations. HF: 1260 m, BP: 1272 m, D2: 1473 m, D6: Caribou Pass. They are confined to peatlands where 1477 m, GF: 1621 m, SF: 1645 m. Record gaps were due to impeded drainage favours the accumulation of organics. animal damage, sensor failure or power interruptions. In the study area peatlands are found in valley bottoms and on the plateau surface of the Mackenzie Mountain Barrens. The tallest features rise approximately 15 m both cases, frozen mineral sediment was encountered above the adjacent peatland while the smallest was beneath the peat. On a peat plateau on the Mackenzie 35 cm high. The latter was a degrading feature in 1978 Mountain Barrens a core over 5-m-long was entirely (Kershaw and Gill, 1979) that had disintegrated by 1990. in peat. Multiple 150 cm cores (n ϭ 10) in the same Several areas near Caribou Pass and another, south feature never encountered mineral sediment. of Camp 222, had features lacking a peat cover. They Three broad vegetation types occur on the study fea- were generally less than 3 m high and were associated tures: Open Subalpine Forest, Erect Deciduous Shrub with cold-water springs at the base of slopes or alluvial Tundra and Decumbent Deciduous Shrub/Graminoid fans.