Permafrost Landform Degradation Over More Than Half a Century, Macmillan/Caribou Pass Region, NWT/Yukon, Canada

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 supplemented 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 suggests 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 dominates 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 different 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, including descriptions of surface cover (e.g. plant community, bare peat, mineral exposures) and evidence of landform 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 interruption were the most common reasons for loss of data. -1.5 D2

-2 D6

3 RESULTS Temperature ( º C) -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 (150 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 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. They occurred in clusters of two or three up to Tundra. The first vegetation type was only found in the 10. Many were in a degrading state, with a central Yukon and the area between the isolated trees was dom- remnant separated by a water-filled moat from a well- inantly Erect Deciduous Shrub Tundra. Most features defined rampart. Channels commonly drained these in the NWT were dominated by Erect Deciduous Shrub moats and since there was seldom any indication of Tundra or Decumbent Deciduous Shrub/Graminoid overland flow to the feature, groundwater springs must Tundra at higher elevations. Moister features were be the source. In some cases the moat water was turbid, dominated by Salix planifolia and Polytrichum spp. indicating sediment mixing associated with upwelling MAPT fluctuated during the record period and springs. Cores taken from two features were fine- to increased in some palsas (Figs. 2 & 3). Most had MAPT coarse-grained mineral material with massive ice at warmer than 2°C while some were above 1°C. depths below 3 m. These characteristics suggest that The lower elevation sites had the greatest temperature they were hydraulic pingos (Mackay, 1998) (Fig. 1). rises while the higher sites appear to have had the most Of the 91 palsas and peat plateaux investigated during stable and consistent temperatures (but recent data gaps site visits since 1974, some had mineral sediment expo- could have influenced this). sures. Most, however, had continuous peat cover with their thaw layers confined to the peat. Coring of 10 3.2 Feature status determined that peat thickness can vary considerably on a feature and among features in the same peatland. Features were placed in one of four status categories: Skaret (1995) reported on five features where the depth degraded, disequilibrium (degrading), dynamic equilib- to mineral varied: 175 cm to 320 cm (4 cores), 72.5 cm rium (stable) or disequilibrium (aggrading). A degraded to 300 cm (5 cores), 138 cm to 210 cm (4 cores) feature could be found on earlier aerial photos but had 180 cm to 195 cm (2 cores) 60 cm to 150 cm (3 cores). disappeared by the time of the site visit; a thermokarst Horvath (1998a) cored two palsas in Porsild’s field pond usually occupied its former position. The other and found peat depths to be 490 cm and 300 cm with categories were defined by evidence of submergence no mineral sediment. Her findings were similar to or emergence since, with few exceptions, the features those for the same field in 1979 (Kershaw and Gill, occurred in wetlands. Degrading landforms had sub- 1979). In the Dale Creek field, peat depths were 80 cm merged portions, often with upland species such as and 105 cm on one feature and 95 cm on another. In Betula glandulosa, Salix spp., Cladonia spp. or Cetraria

545 2 0 1944 7.301m2 1974 4.824m2(66%) 1994 2.916m (40%)

Read mile 10 post 230 1 1 Palsa 1 1 lb A 2 2 2 Vehicle -0.5 3 4 4 tracks Cross-section 9 5 A 5 5 6 6

BP Road -1 Gravel pit 7 7 Sources: 8 8 National Air Photo Library, Ottawa 0 40 80 120 160 metres Canol T21C-12. 13 (1944); A23924-4,5 (1974) HF Field notes (1994) -1.5 GF Figure 4. Palsas in Porsild’s field have decreased at a rate Temperature ( º C) of 1% a1 -2

-2.5 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 Figure 3. Mean annual permafrost temperatures at 150 cm depth for the lowest (BP and HF) and highest elevation palsas. An exponential fit line is shown for each data set. spp. beneath water where they were in poor vigour or were dead. Bare peat, calving peat blocks and/or exposed frozen sediment were also taken as evidence of permafrost degradation. Emergence was indicated by wetland species such as Eriophorum spp. or Carex aquatilis on the elevated surfaces of the features (usu- Figure 5. A low-elevation feature (A – HF) has shrunk ally on the sides). Submergence confirmed degrada- 50% while one almost 400 m higher (B – GF) lost 90% tion of the permafrost while emergence was interpreted of its area. 1944 and 1974 margin positions based on aerial as permafrost aggradation in the palsa core. A few fea- photos. 2001 based on field surveys supplemented by tures exhibiting evidence of both aggradation and degra- oblique aerial photos taken by the author dation and so were classified as stable, in dynamic equilibrium. 4 DISCUSSION Because of the limited coverage of the earliest aerial photographs, not all features could be tracked over 4.1 Distribution and site characteristics the record period. However, interpretation of the 1940s photography and more recent images revealed no Palsas and peat plateaux occur throughout the study newly-formed or incipient features. During fieldwork area from the lowest elevations to high cirque bot- over almost 30 years, stringers and hummocks in most toms. Wherever peat has accumulated, palsas and peat palsa peatlands in the study area were checked for plateaux can be found. In some cases the peat is dis- permafrost but none was found. Seasonal frost was continuous on portions of the features while in other periodically discovered early in the thaw season but cases it is thicker than the height of the feature. No it later thawed (e.g. a 56-cm-high stringer adjacent relationship between peat thickness and feature status to a palsa had a temperature of 11°C at the end of was found. Degradation was just as likely for a feature August). with a thick peat surface layer as it was for one with Detailed measurements in Porsild’s field, just east only a thin surface peat deposit. of Macmillan Pass (Fig. 1) indicate that since 1944 Vegetation structure on features varied from multi- the field has lost 60% of the areal extent of palsas layered to a simple ground cover. Trees were common (Fig. 4). Several features completely degraded leaving in the Yukon at the lowest elevations but were absent thermokarst ponds behind (Fig. 4: 9 and 10 by 1974; in the Northwest Territories. The multi-layered shrub 3, 6, 7 and 8 by 1993) while others had broken into communities were often associated with continuous smaller units as their permafrost cores degraded fruticose lichen ground cover. Graminoid cover was (numbers 3 and 4 by 1974 and 1 by 1993). dominant on features at the highest elevations. Vegeta- A feature (HF) at 1260 m with an Erect Deciduous tion structural complexity declined with increasing ele- Shrub Tundra plant cover and one 15 km to the north- vation. Within a peatland, features with tall shrub cover east (GF) and almost 400 m higher had a Graminoid had deeper thaw depths than those lacking erect shrubs Tundra cover. Yet both features exhibited dramatic (Skaret, 1995). However, at the study area level, thaw areal reductions in a 57-yr-period (Fig. 5). depth did not seem to be related to height of plant cover.

546 Since 1990 permafrost core temperatures in several features have risen. This has been measured at the lowest and highest elevation features (Fig. 2 & 3). It appears that for at least the last decade permafrost warming has occurred regardless of elevation.

4.2 Feature status

The majority of features in the study area can be classified as being in a disequilibrium degrading state with the next most common group in a degraded state. Areal decreases of between 1% and 1.5% a1 in palsa/peat plateau fields were the norm. Horvath (1998a) determined that the rates varied between the periods 1949 to 1972 and 1972 to 1981 but that palsas were contin- uously degrading. Figure 6. Enlargement of a portion of an aerial photo Extensive searches in the peatlands of the study area (National Air Photo Library, Ottawa, T21 #23 V), with the over almost 30 years have failed to discover permafrost feature identified as an “embryonic palsa” (Harris and in small-scale features other than those resulting from Nyrose 1992). Palsa margins in 1944, 1990 (Harris and the degradation of formerly larger features. In contrast, Nyrose 1992), and 1998 (ground survey and oblique air Harris and Nyrose (1992) identified a feature as an photos by author). “embryonic palsa” which, if correct, would indicate an aggrading disequilibrium state. Analyses of aerial What triggered the disequilibrium between climate photography and site visits in July and September and permafrost, and when this occurred, are important 1998, however, provide evidence that contradicts this questions. Evidence from the peat in palsas such as classification: 5-m-long tree trunks and stumps in growth position 1 The feature was recognizable on the 1944, 1949, 14C dated at 6380 130 BP (AECV #1103C) and 1963, and 1974 aerial photography. In 1944 it was 3440 40 BP (pers. com. L.D. Arnold) suggests that approximately 10 times larger than in 1990 (Fig. 6) climate during these periods was amenable to tree when its dimensions were 12 m by 26 m (Harris growth when at present it is not (at least east of the bor- and Nyrose, 1992). Over the nine thaw seasons der). Presumably permafrost aggraded into the area at between 1990 and 1998 the feature further degraded the time when the climate was hostile to tree growth. by approximately 50%. The presence of the White River Volcanic Ash as a 2 The dominant plant cover on the feature was an layer in many features (Kershaw and Gill, 1979) sug- Erect Deciduous Shrub Tundra – Betula glandulosa gests that they emerged from their peatlands since 14C with a Cladina spp. and Cetraria spp. ground cover 1147 BP (Clague et al., 1995). Clearly, the climatic (Harris and Nyrose, 1992). Under the water sur- conditions prevailing at the time of permafrost aggra- rounding the feature in 1998 were dead and dying dation and since then have varied. But without records plants of these species. The Betula-Cladina vegeta- we can only extrapolate from proxy sources such as tree tion type, which dominates the palsas in the Erect rings and evidence of plant community changes pre- Deciduous Shrub Tundra, has still not reestablished served in peat deposits. Furthermore, one cannot dis- on Canol disturbances that are over half a century count the possibility that multiple periods of permafrost old (Harper and Kershaw, 1996). This suggests that aggradation produced the landscape we have today any features dominated by this type of plant cover (sensu Zoltai, 1993). Nevertheless, the majority of per- must be at least 50 years old and probably much mafrost landforms are degrading in the region. This older. Thus drowning of plants without evidence of process has dominated for at least the past half century. flooding by beaver or other agents suggests subsi- dence of features as their permafrost core degrades. 5 CONCLUSIONS Based on the above points, the feature identified as an “embryonic palsa” by Harris and Nyrose (1992) All the evidence accumulated over the past half cen- must be a remnant of a much larger feature. Classifying tury points to a widespread degradation of permafrost the state of this feature and others in the vicinity as features in the Macmillan/Caribou Pass region. A rise disequilibrium degrading extends the area of degrading of between 0.5°C and 1.4°C in MAPT since 1990 for permafrost into the Yukon. Thus this condition prevails features at different elevations, with different peat and on a regional scale. plant cover characteristics corroborates this conclusion.

547 For degradation to occur on a regional scale, the fac- Kershaw, G.P. 1978. Rock glaciers in the Cirque Lake area tors responsible must be operating at the same level. of the Yukon-Northwest Territories. The Albertan The only environmental forcing factor that operates at Geographer, 14:61–88. this scale is regional climate. Despite potential buffer- Kershaw, G.P. 1984. Tundra plant communities of the ing by precipitation and the snowpack, this suggests Mackenzie Mountains, N.W.T. and the floristic characteristics of long-term surface disturbances. In that climatic warming has been affecting permafrost R. Olson, R. Hastings and F. Geddes (eds.), Northern in the study area over at least the past five decades. Ecology and Resource Management, pp. 239–309. University of Alberta Press, Edmonton. ACKNOWLEDGEMENTS Kershaw, G.P. 1990. Responses to 35-year-old surface disturbances in tundra ecosystems. In P.J. Smith and E.L. Jackson (eds.), A World of Real Places, pp. 95–114. Support for these long-term studies has come from Department of Geography, University of Alberta, many and diverse sources. Natural Sciences and Engi- Edmonton. neering Research Council, Imperial Oil Limited Kershaw, G.P., and D. Gill. 1979. Growth and decay of pal- (University Research Grant), Canadian Circumpolar sas and peat plateaus in the Macmillan Pass-Tsichu Institute and University of Alberta have all provided River area, Northwest Territories, Canada. Canadian financial assistance. Michael Fisher crafted Fig. 1. Journal of Earth Sciences, 16:1362–1374. Many graduate and undergraduate students have par- Kershaw, G.P., and K.D. Skaret. 1993. Microclimatic char- ticipated in phases of the field work, in particular acteristics of palsas along an altitudinal gradient, Brent Ambrose, Celesa Horvath, Linda, Eric and Geoff Mackenzie Mtns. NWT, Canada, Permafrost, Sixth Kershaw, Monique McIlhargey, Elaine Seir and Kevin International Conference Proceedings, Vol. 1, pp. 338–343. South China University of Technology Skaret Logistical support has been provided by Norman Press, Beijing. and Barb Barichello, Oldsquaw Lodge; Blair and Bea Laberge, M.-J., and S. Payette. 1995. Long-term monitor- Jensen, Ursus Aviation, and the RWED station at Camp ing of permafrost change in a palsa peatland in Northern 222 – Jim, Sammy and Keith Hickling, Alasdair Veitch, Quebec, Canada: 1983–1993. Arctic and Alpine Ken and Lois John. Comments from the anonymous Research, 27:167–171. reviewers and the Associate Review Editor on an earlier Mackay, J.R. 1998. Pingo growth and collapse, Tuktoyaktuk version of the paper are appreciated. Peninsula area, western Arctic coast, Canada: A long- term field study. Géographie physique et Quaternaire, 52:271–323. REFERENCES McIlhargey, T.M. 1994. Stratigraphy, sedimentology and microclimate of turf-banked solifluction lobes, Clague, J.J., S.G. Evans, V.N. Rampton, and G.J. Woodsworth. Macmillan Pass, NWT. MSc thesis, Alberta. 1995. 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