Palsa Dynamics in a Subarctic Mountainous Environment, Wolf Creek, Yukon Territory, Canada

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Palsa dynamics in a subarctic mountainous environment, Wolf Creek, Yukon Territory, Canada

T.L. Coultish & A.G. Lewkowicz Department of Geography, University of Ottawa, Ottawa, Ontario, Canada

ABSTRACT: More than 20 palsas, up to 4 m high and 50 m long, are present in a fen at an elevation of 1250 m a.s.l. in Wolf Creek, southern Yukon (60°30N, 135°13W). Palsa cryostratigraphy typically consists of a peat layer of 10 to 20 cm, underlain by organic sands and silts containing segregated ice. The annual air temperature is 4°C and permafrost is present on lower valley slopes adjoining the palsa field. Mean snow depths of 21 to 66 cm were measured on the palsa summits at the end of winter. A variety of analyses show that palsas have developed continually at the site from 150 years ago to present. The dynamics are at least partly linked to beaver activity. Water levels are strongly affected by the construction and destruction of beaver dams across the valley floor, and high water levels have resulted in the degradation on some of the oldest forms by block collapse. Mound development has occurred where ponds have drained.

1 INTRODUCTION Consequently, widespread degradation of palsas has been linked in several studies to regional climatic ame- Palsas are found throughout circumpolar regions, most lioration (e.g., Laberge & Payette 1995, Matthews et al. commonly in areas of discontinuous permafrost. There 1997, Sollid & Sørbel 1998, Zuidhoff & Kolstrup 2000). has been considerable discussion in the literature sur- The objectives of this paper are to describe the rounding the term palsa (e.g., Washburn 1983). For the structure and dynamics of a group of palsas in Wolf purposes of this study, a palsa is defined according to Creek, Yukon Territory, and to investigate the climatic the IPA Multi-Language Glossary: a peaty permafrost and non-climatic factors that affect them. The forms mound possessing a core of alternating layers of segre- are the most southerly reported palsas in the Yukon gated ice and peat or mineral soil material (van Territory, and as such, might be expected to be partic- Everdingen 1998). ularly sensitive to climatic variation. Palsas are cyclical in nature, and can be classified as aggrading, stable or degrading (Cummings & Pollard 1990). The main factors recognized as affecting their 2 STUDY SITE growth or decay are snow, peat cover, vegetation and regional climate. The Wolf Creek Research Basin, (centred at 60°30N, The depth of snow is generally viewed as a critical 135°13W), occupies an area of 195 km2. It is situated factor for palsa development, as the tops of the mounds 20 km south of Whitehorse, Yukon, within the sporadic are frequently wind-swept in winter, allowing ground permafrost zone (Figure 1). Basin elevations range heat to escape and permafrost to aggrade (Cummings & from 800 to 2250 m a.s.l. with the treeline occurring Pollard 1990, Seppälä 1990). The presence of peat is important due to its variable thermal conductivity when Yukon Territory, Canada N Permafrost zones frozen and unfrozen (Williams & Smith 1989). The Continuous Discontinuous loss of peat by wind abrasion or fire can result in palsa Sporadic degradation (Zoltai 1972). Vegetation plays a role in the 0 200 400 km growth stage of a palsa: changes in albedo during vege- Whitehorse tation succession may cause a negative heat budget Wolf Creek (Railton & Sparling 1973). Vegetation also alters the way in which snow is distributed on a palsa. Low vegetation traps snow and reduces ground heat loss in winter 012 km Elevation (m a.s.l.) (Kershaw and Gill 1979), but trees on a palsa can inter- 700-1200 1200-1500 cept snow allowing greater heat loss and permafrost 1500-2100 Rivers & lakes aggradation (Zoltai & Tarnocai 1971). Vegetation can Palsa site also help stabilize the surface mechanically, preventing Wolf Creek Research Basin palsas from collapsing (Cummings & Pollard 1990). Figure 1. The Wolf Creek study site and permafrost zones Palsas are frequently found in areas where annual in Yukon Territory, Canada (permafrost zones modified air temperatures are close to 0°C (Harris 1982). from Heginbottom et al. 1995).

163 at approximately 1300 m a.s.l. (Janowicz 1999). The perimeter locations every 3–5 m around the base. Peat basin consists of three main ecological zones: boreal plateaux and beaver dams were also surveyed. spruce and pine forests at lower elevations; sub-alpine Each palsa was classified qualitatively according to spruce forest and shrub willow and shrub birch domi- size, vegetation cover and evidence of change. Aggrad- nated scrublands at mid-elevations; and predominately ing forms are small (0.5–1.5 m in height), have bare low shrub tundra and bare surfaces at high elevations peat or grass-covered surfaces, and may exhibit dila- (Francis et al. 1999). tion cracks. Stable and degrading palsas are larger The study site is nearly all contained within NTS map (1.5–4.0 m in height), and are covered by grasses and 105 D/6 (scale 1:50,000). The palsa field is located on well-established shrub birch and willow colonies. the NW side of Coal Ridge (maximum elevation 1600 m Degrading palsas show signs of block collapse, usually a.s.l.), in an 800 m-wide valley trending NE–SW, at an occurring alongside water. elevation of approximately 1250 m a.s.l. This study site The dynamics of the palsa field were tracked was chosen in part because of its extensive aerial pho- through time using aerial photographs taken in 1946, tographic record, as well its proximity to Whitehorse, 1966, 1987, 1995, and 2001. This interpretation and which allows access to a long climate record. annual growth ring counts for shrubs on mound sum- Wolf Creek has a subarctic continental climate char- mits were used to determine the minimum ages for acterized by large seasonal temperature amplitudes, the palsas. However, smaller forms in 1946 may not low relative humidity and low precipitation. The mean have been recognized due to the scale of the photos annual precipitation is 300 to 400 mm with 40% falling (~1:20,000), especially if they subsequently egraded. as snow (Janowicz 1999). The MAAT measured at the The depth to the frost table was measured with a study site from April 2001 to April 2002 was 4°C. standard frost probe on the summit of each palsa and up Maximum and minimum hourly temperatures over this to four shrubs were cut for annual growth ring counts. period were 25°C and 36°C respectively. The site is Since shrubs grow on the mounds themselves, but not affected by shading from Coal Ridge to the east and an on the adjacent valley bottom, growth ring counts give unnamed mountain peak at 1900 m a.s.l. to the west. This the minimum ages of the palsas. The largest shrubs reduces potential incoming solar radiation in summer by were selected for sampling as it was expected that about 5% relative to a horizontal surface and may be a they would be the oldest. To ensure that the maximum factor in permafrost development at the site. Like other age of the shrubs was recorded, the “burl” or “root- parts of the basin, the valley probably also experiences crown” of the shrubs was excavated from just below the air temperature inversions in winter (Janowicz 1999). ground surface (Beschel and Webb 1962). The Wolf Creek Research Basin has been the focus Seven of the palsas were cored with a hand-driven of a number of hydrological, geophysical, ecological, 75 mm diameter CRREL auger. The cored sections and biological studies, but little work has been con- were extracted from the barrel and described in the ducted on permafrost. A preliminary study suggests field immediately. The description included grain- that permafrost occupies approximately 25 to 32% of size, Munsell colour, and cryostratigraphy (Murton the basin, predominately on north-facing slopes and at and French 1994). Core samples were shipped back high elevations (Seguin et al. 1999). unfrozen to the laboratory where grain-size, organic Palsas typically exist as isolated bodies of per- content and moisture content analyses were conducted. mafrost surrounded by unfrozen sediments in the spo- Gravimetric rather than volumetric moisture contents radic permafrost zone. In Wolf Creek, the lower valley are presented because the porosity of the sediments is sides adjacent to the palsa field also are underlain by not known and cannot be calculated because of the permafrost. Although generally rare at this elevation organic content. The specific conductance of the in the basin, high ice-content permafrost is also present supernatant water was measured using a TDS Tester on the east side of Coal Ridge at 1200 m a.s.l. in sedi- 3™ w/ATC to 25°C. Temperatures in the palsas were ments surrounding thermokarst lakes. measured using a steel probe equipped with a YSI 44033 thermistor which was allowed to stabilize before 3 METHODS readings were taken. Resistances were recorded using a Fluke multimeter which allowed a precision of The main fieldwork was conducted in July–August, 0.02°C at temperatures close to 0°C. 2001. Visits to the site also took place in April 2001 and April 2002 when snow surveys were completed 4 RESULTS AND DISCUSSION on several of the palsas. During the summer field season, a topographic 4.1 Palsa descriptions survey of the palsas was undertaken with a Nikon NE-10LA Digital theodolite and 5 m stadia rod. The Two groups of palsas were present at the study site. The highest points on each palsa were surveyed, as were north group has fifteen palsas and the south group has 164 Gravimetric Moisture Cryostratigraphy Grain-size (%) Specific Conductance nine, as well as several small forms (0.5 m high) that Content (%)(Solid line) ( S/cm) 0 100 200 300 0 50100 0500 250 were not included in the survey. Here we present bore- 0 hole logs from typical examples of degrading and aggrading forms. Palsa 7A is degrading. It is 3.0 m high and its southwest side is experiencing block collapse. A 10 m long 50 exposure of mineral soil shows curved bedding running parallel to the ground surface. This indicates that the mound formed by upheaving of horizontal sediments during permafrost aggradation. The vegetation on Palsa 100 7A consists mainly of shrub willow with some shrub birch, and grasses and/or sedges. Mound 7A can be Frost Table seen on all the aerial photographs dating back to 1946, Aug.1, 2001 810 giving the form a minimum age of 56 years. The oldest 150 shrub sampled was 80 years old. Allowing 5–10 years for initial colonization (Walkerton et al. 1986), the Depth (cm) 670 results from the annual growth ring analysis extend the 670 age of the mound to a minimum of about 90 years. 200 The stratigraphy of Palsa 7A (Figure 2) was exam- ined by cleaning part of the exposure to a depth of 2 m and then coring until auger refusal occurred. Beneath 610 250 a 10 cm peat cover, sediments were organic-rich and 360 dominated by sands and silts with small amounts of clay and gravel. The frost table was at 135 cm depth. Temperatures below this were very close to 0°C, with 300 a value of 0.05°C at the base of the borehole, at a level lower than that of the adjacent stream. Ice The cryostructure consisted of short, discontinuous, wavy, lenticular ice lenses that were 1 mm thick, 350 interpersed with larger sub-horizontal lenses up to 16 Ice 0 5 10 Gravel Clay 20 mm thick. The visible ice content increased with Sediment Loss on Ignition (%) Sand Silt depth, as the ice lenses became larger. From 310 cm to Peat (Dashed line) 363 cm, the core was mainly ice with thin sediment Figure 2. Core of Palsa 7A depicting cryostratigraphy, layers (see Figure 2). moisture content, organic content, grain-size, and the spe- The specific conductance was variable down-core, cific conductance of supernatant water. with the lowest values coinciding with ice layers, presumably as a result of salt exclusion during the complex to its south and west (see Figure 4). Palsa 10 freezing process (Brouchkov 2002). The highest values, is mainly covered by grasses, but a few shrub willow ranging from 670–810 S/cm, were found in the upper seedlings have started to colonize its south side. The 66 cm of frozen soil. Below this, the specific conduc- oldest shrub sampled was 7 years old, giving this palsa tance values dropped, and generally increased with a possible minimum age of 12–17 years. depth. An increase in solute concentration with depth The stratigraphy of Palsa 10 (Figure 3) was relatively can occur due to the migration of water and salts with simple: 20 cm of peat overlay organic sands and silts the downward advancement of the freezing front containing very small amounts of clay (5% through- (Brouchkov 2002). The high values measured in the out) and fine gravel (1%). The frost table occurred upper 66 cm of frozen sediments suggest that this sec- at a depth of 36 cm on August 5th 2001 and soft tion may have thawed later in the summer, or in certain unfrozen sediments were encountered beneath the base warmer years. of core at 146 cm. It can be concluded that Palsa 7A was formed by Temperatures measured in the frozen part of the segregation processes. However, the potential influence borehole were very close to 0°C. A slight positive of groundwater pressures was revealed when the bore- gradient was present in the underlying unfrozen hole partially filled with water within one day of sediment, with a temperature of 0.6°C at 230 cm drilling to 1 m above the surrounding stream level. depth. Palsa 10 had a gradual increase in visible ice Palsa 10 is aggrading. It is less than 1 m high, content and gravimetric moisture content down-core, approximately 7.5 m long and 3 m wide. This small from 15% and 65% respectively, just below the frost palsa is located in the lee of a 4 m high, large palsa table, to 40% and 110% immediately above a large 165 Cryostratigraphy Gravimetric Moisture Grain-size (%) Specific Conductance Table 1. Mean snow depths (cm) on the north sides (N), tops (T) and Content (%)(Solid line) (␮ S/cm) 0 100 200 300 0 50 100 0 250 500 south sides (S) of selected palsas in Wolf Creek. 0

LOI not determined April 6, 2001 April 1, 2002

Palsa Stage N T S Avg N T S Avg Frost Table Aug. 5, 2001 10 A –––– 20 28 52 33 50 20 A 18 21 31 23 35 42 47 41 9A S –––– 100 33 43 59 9C S –––– 50 49 69 56 12 S 33 56 19 36 72 64 57 64 Depth (cm) 7A D –––– 73 43 82 66 8D–––– 93 66 68 76 100 23 D –––– 64 60 73 66

Growth stage: A aggrading; S stable; D degrading.

Table 2. Snow depths reported in the literature. 146 >146 cm unfrozen sediment 0 5 10 Gravel Author Location Snow Depths Ice Silt Loss on Ignition (%) Sand Sediment Clay (Dashed line) Peat Cummings N Québec Palsas 0–45 cm, average 10 cm and Pollard Surrounding fen 48–175 cm, Figure 3. Core of Palsa 10 depicting cryostratigraphy, 1990 average 69 cm moisture content, organic content, grain-size, and the spe- Kershaw and Macmillan Mature palsas averaging 40 cm cific conductance of supernatant water. Gill 1979 Pass, NWT Young palsas averaging 7.5 cm Seppälä 1990 N Finland Palsa sides 20–50 cm (Nov/Dec) ice lens at the base of the core. The specific conduc- up to 140 cm (Apr) Palsa tops 3–20 cm (Nov/Dec) tance of Palsa 10 was highest beneath the frost table decreased to 0–5 cm (Jan) (140 S/cm) in soil that is likely part of the active Zoltai and N Manitoba No trees on palsa 53 cm layer, and lowest within the 22 cm thick ice lens Tarnocai 1971 Some trees on palsa 38 cm (20 S/cm). Dense trees on palsa 14.5 cm The cryostructure consisted of short, discontinuous, wavy, lenticular ice lenses 1 mm in thickness, Shrub willow and birch on the Wolf Creek palsas are 10–20 mm long and spaced 5–10 mm apart at the top effective at trapping snow, resulting in summit snow of the core. Down-core, the thickness of the lenses depths of 21–66 cm, values that are unusually high increased slightly to 1 mm and they were spaced only when compared to those reported in the literature 1.5 mm apart. A 20 mm thick ice lens occurred at a (Tables 1 and 2). Aggrading palsas with their cover of depth of 62 cm, and a 22 cm thick body of ice was grasses, have slightly lesser depths than stable or present from 124 cm to 146 cm. The latter was relatively degrading forms. In contrast to Seppälä’s (1994) obser- clear and contained 20 mm long tubular bubbles. Two vations, the sides of the palsas in Wolf Creek did not 5 mm thick sediment layers traversed the ice immedi- always have greater snow depths. Based on observa- ately above the base of the core. tions in the valley, winds that redistribute the snow The cryostratigraphy of the bulk of the core indicates are mainly from the southwest. Consequently, snow that segregation processes were operational and hence depths were generally greater on the north sides than that the form is a palsa. The massive ice at the base of on the south sides of the palsas. The aggrading forms, the core could be of segregation or intrusive origin, nos. 10 and 20, are in lee of much larger palsas and their based on form alone. However, ice lenses up to 15 cm south sides have greater snow depths because the thick have been reported in the literature (Forsgren drifts from the larger forms extend onto them. 1968, Seppälä 1980), and An and Allard (1995) predict These results suggest that although low snow that a thick segregated lens should be formed at bottom depths may contribute to the initial formation of pal- of a palsa. This origin is also suggested by the thin sas, they are not critical for the preservation of the sediment layers within the ice. forms. This appears to be possible because climatic conditions are sufficiently cold to produce permafrost 4.2 Snow depths in the surrounding area and on valley slopes, even at typical snow depths. Snow acts as an insulator, and can inhibit permafrost development. Snow depths were measured on two of 4.3 Dynamics of palsas over time the palsas in April 2001, and on an additional six in April 2002. Snow depths could not be recorded on the The aerial photographs show palsas present throughout adjacent valley bottom as it was covered by an icing in the past 55 years. There has been moderate change both winters. over this period (Figure 4): 10 forms present in 1946 are

166 Beaver damming has two main effects on palsa N 2001 7A dynamics at Wolf Creek. First, high water levels can cause palsa degradation. Several large palsas which

Weather no longer exist can be recognized on the 1946 aerial station 10 photographs (see Figure 4). The 1966 photographs show them surrounded by water ponded by beaver 1966 dams (Figure 4). These palsas had completely degraded by 1987. Beaver dams can also help in the development of new palsas. Beaver dam breach quickly drains water 1946 from an area, allowing permafrost to aggrade and palsas to form. On the 1987 photos, the southern group of palsas appear partially flooded. Today, four grass- 0 100 200 m covered palsas up to 1 m high are growing in this area. Figure 4. Maps of mounds (black) and surface drainage On the 1995 photographs, the northern group of palsas (grey) in the northern part of the main study site produced is again experiencing flooding. By 2001, most of these from aerial photographs. areas had drained, exposing fresh peat. Within these peaty areas, small aggrading palsas are present. 7

5 4.5 Effects of groundwater

3 Several observations demonstrate that groundwater

2 under pressure is present at the palsa site: (1) springs, Number of palsas 1 with water temperatures just above 0°C in late-

0 summer, occur at a break of slope 50 m from the 0 0-25 25-50 50-75 75-100 100-125 125+ stream; (2) the valley floor was covered by an icing in Shrub ages (years) both winters; (3) low icing mounds with dilation cracks Figure 5. Histogram of maximum annual growth ring ages were present in both winters. of shrubs on palsas (n 23). Note: Palsa 24 not sampled. The influence of groundwater on palsa dynamics is much more difficult to assess. At the very least, the no longer visible; 7 forms have developed since 1946. presence of groundwater means that the palsas have The shrub growth ring analyses show that the oldest a ready water supply for segregation processes. Ground- palsa is at least 160 years old, allowing 5–10 years for water may also be affecting three small, aggrading the initial colonization of Salix species. palsas that were encased in an icing to a depth of Evidence linking the formation of the oldest palsas 120 cm with only 5 cm of their summits exposed in to colder climatic conditions in the past is inconclusive. April 2002. Thermal conditions within the icing could A histogram of maximum shrub age (Figure 5) shows favour mound growth, as the ice cover is a much better a slightly higher frequency of palsas with shrub ring conductor of heat than snow. It is also possible that counts of 100–125 years. It is possible that this is the groundwater pressures are responsible for the initial result of increased palsa formation at the end of the upheaving of forms, with subsequent growth by ice Little Ice Age. However, shrub ring counts indicate segregation. However, thick bodies of ice that might only minimum ages because the shrubs sampled were be produced by injection were not present in any core the largest, but not necessarily the oldest, and some had immediately below the frost table, so there is no evi- rotten piths. It is also evident that palsas are forming dence to confirm this possibility. In contrast, several under current conditions, so that a colder climate is not other mounds located on the valley slopes near the necessary for their formation. springs appear to be partly the result of groundwater pressures. More study is needed to further understand 4.4 Effects of beaver damming the influence of groundwater on these mounds. The aerial photographs show dramatic differences in water levels in the valley floor, even though they were 5 CONCLUSIONS all acquired at about the same time of year. In the field, ten breached beaver dams were found traversing the We conclude the following: valley bottom at the site. These dams were too small to (1) The frost mounds described at the study site in see on the aerial photographs, but they clearly explain Wolf Creek are aggradational, perennial, and con- the large differences in water levels. tain segregated ice. They are identified as palsas.

167 (2) Snow depths on the palsas are greater than in pre- Creek Research Basin: Hydrology, Ecology, Environ- vious studies and the effects of wind in removing ment. Environment Canada: 125–134. snow in winter appears to be less critical at this Kershaw, G.P. & Gill, D. 1979. Growth and decay of palsas site than elsewhere. and peat meadows in Macmillan Pass-Tsichu River area, Northwest Territories, Canada. Canadian Journal (3) New palsas have continually developed over the of Earth Sciences 16(7): 1362–1374. past 150 years while older forms have degraded. Laberge, M.-J. & Payette, S. 1995. Long term monitoring of (4) Palsa dynamics at this site are strongly influences permafrost change in a palsa peatland in northern by beaver damming which affects water levels. Québec, Canada: 1983–1993. Arctic and Alpine Consequently, no relationship is likely between Research 27: 167–171. regional climatic change and palsa development. Matthews, J.A., Dahl, S.-O., Berrisford, M.S. & Nesje, A. 1997. Cyclic development and thermokarstic degradation of palsas in the mid-alpine zone at Leirpullan, ACKNOWLEDGEMENTS Dovrefjell, southern Norway. Permafrost and Peri- glacial Processes 8: 107–122. This project was funded by grants from the Northern Murton, J.B. & French, H.M. 1994. Cryostructures in per- Scientific Training Program, DIAND, and NSERC. The mafrost, Tuktoyaktuk coastlands, western Arctic Canada. use of the Geological Survey of Canada Sedimentology Canadian Journal of Earth Sciences 31: 737–747. Railton, J.B. & Sparling, J.H. 1973. Preliminary studies on Laboratory is appreciated, as is the field assistance the ecology of palsa mounds in northern Ontario. of Dr. Marcia Phillips, Andje Lewkowicz-Lalonde, Canadian Journal of Botany 51: 1037–1044. Christophe Kinnard and Mark Ednie. Rebecca Zalatan Seguin, M.K., Stein, J., Nilo, O., Jalbert, C. & Ding, Y. 1999. helped with shrub ring counts, and Rick Janowicz, Hydrogeophysical investigation of the Wolf Creek DIAND, provided logistical support. Mark Ednie watershed, Yukon Territory, Canada. In: J.M. Pomeroy undertook the solar radiation calculations. The com- and R.J. Granger (eds), Wolf Creek Research Basin: ments of the anonymous reviewers are appreciated. Hydrology, Ecology, Environment. Environment Canada: 55–78. Seppälä, M. 1980. Stratigraphy of a silt-cored palsa, Atlin REFERENCES Region, British Columbia, Canada. Arctic 33(2): 357–365. An, W. & Allard, M. 1995. A mathematical approach to Seppälä, M. 1990. Depth of snow and frost on a palsa modelling palsa formation: insights on processes and mire, Finnish Lapland. Geografiska Annaler 72A: growth conditions. Cold Region Science and Technology 191–201. 23(3): 231–244 Seppälä, M. 1994. Snow depths controls on palsa growth. Beschel, R.E. & Webb, D. 1962. 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