Quaternary Research 75 (2011) 366–370

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Quaternary Research

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Synthesis of studies of palsa formation underlining the importance of local environmental and physical characteristics

Matti Seppälä

Department of Geosciences and Geography, University of Helsinki, Finland article info abstract

Article history: This review presents a synthesis of four decades of palsa studies based on field experiments and observations Received 3 December 2008 mainly in Fennoscandia, as well as laboratory measurements. Palsas are peat-covered mounds with a Available online 12 November 2010 permanently frozen core; in Finnish Lapland, they range from 0.5 to 7 m in height and from 2 to 150 m in diameter. These small landforms are characteristic of the southern margin of the discontinuous permafrost Keywords: zone. Palsa formation requires certain environmental conditions: long-lasting air temperature below 0°C, thin Permafrost snow cover, and low summer precipitation. The development and persistence of their frozen core is sensitive Thermal conductivity Active layer to the physical properties of peat. The thermal conductivity of wet and frozen peat is high, and it decreases Cyclic development significantly as the peat dries and thaws. This affects the development of the active layer and makes its Snow depth response to climate change complex. The insulating properties of dry peat during hot and dry summers Palsa moderate the thawing of the active layer on palsas. In contrast, humid and wet weather during the summer Buoyancy causes deep thawing and may destroy the frozen core of palsas. Ice layers in palsas have previously been interpreted as ice segregation features but because peat is not frost-susceptible, the ice layers are now reinterpreted as resulting from ice growth at the base of a frozen core that is effectively floating in a mire. © 2010 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction with a cover of peat from 1 m or less to 7 m thick.” According to this definition, peat remains one of the primary constituents of a palsa. What is a palsa? This was the title of an article published by Link Washburn (1983b, p. 42) concluded his review: “Peat is commonly Washburn (1983b). Washburn based his review on a wide range of regarded as a critical constituent, either as a core or as a cover over literature that “reveals a confusing variety of usages of the term.” mineral core (no matter how thin the cover).” He proposed the The etymology of palsa was explained by Seppälä (1972): “The following definition: “Palsas are peaty permafrost mounds, ranging from term palsa was originally used by the Lapps and northern Finns, and in c. 0.5 to c. 10 m in height and exceeding c. 2 m in average diameter, their languages (Lappish and Finnish) means a hummock rising out of comprising (1) aggradation forms due to permafrost aggradation at an a bog with a core of ice.” It is a morphological term describing a certain active layer permafrost contact zone, and (2) similar-appearing mire type, which is also a special ecological habitat. It has no genetic degradation forms due to disintegration of an extensive peat deposit.” meaning. Lundqvist (1969, p. 208) defined palsas as “mounds of peat The main objectives of this paper are to test these early ideas of the and ice occurring on bogs in the subarctic region.” composition of palsas, and to synthesize four decades of field Åhman (1977, p. 145) in his thesis of northern Norway argued experiments and observations on palsas, with a focus on the that: “The presence of peat is merely an environmental consequence environmental characteristics and the physical properties of peat because of the physical processes being more favourable in bog areas. that affect palsa formation. The formative processes of these peat However, the physical processes are the same if the palsa formation hummocks and their inner structure are discussed in detail. takes place in a bog or elsewhere.” He (p. 144) extended the definition of palsas: “According to this view a palsa is a hillock or a more Palsa environment elongated rise in the ground, formed by the build-up of segregated ice in soil, minerogenic or peat or in combination.” Nevertheless he Palsas are found on mires in the zone of discontinuous permafrost indicated that: “Today, palsas are defined as hillocks with a frozen in the circumpolar area of the northern hemisphere. This type of core formed by a build-up of segregated ice in mineral soil, or in peat, environment does not seem to exist on the southern hemisphere, presumably because of lack of sufficiently cold dry land. Mires in Tierra del Fuego, for example, do not contain permafrost. Palsas are climatic morphological features. What are the char- acteristics climatic parameters in palsa regions? The answer is difficult E-mail address: matti.seppala@helsinki.fi. because only sparse meteorological records are available from these

0033-5894/$ – see front matter © 2010 University of Washington. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.yqres.2010.09.007 M. Seppälä / Quaternary Research 75 (2011) 366–370 367 regions and there are no long-term climatic observations of the palsa mires. Moreover, the mire conditions are not simple, linear functions of regional climatic conditions; they cannot be easily extrapolated from standard observations at meteorological stations in the region even if they are nearby. Local climate patterns form a mosaic of several climatic parameters: temperature, precipitation, humidity, wind, cloud cover, and in and out-going radiation. These all contribute to the overall local climatic regime. It is difficult to define how they affect palsa formation. For example topography, aspect, and soil properties influence local ground surface climatic conditions, which individually and collectively influence palsa behavior. We can define the range of conditions required for palsas to form provided long-term, detailed observations are available and are processed using adequate statistical techniques (e.g. multifactor analysis). This would provide a sound basis for understanding where and why palsas form only in certain regions. In general palsas are associated with the natural limit for permafrost formation, mean annual air temperature (MAAT) no higher than 0°C, and commonly lower than that (Washburn, 1980). In Figure 1. New palsas on Vaisjeäggi palsa mire, Utsjoki, Finland, in 2003. Photo O. Ruth. Sweden, J. Lundqvist (1962) characterized the general thermal requirement of palsas as more than 200–210 days a year with air temperatures below 0°C; the southern limit of this region corresponds filled thermokarst depressions (Luoto and Seppälä, 2002, 2003). to a MAAT of −2°C to −3°C. G. Lundqvist (1951) constrained the Strong winter storms may also cause peat abrasion at the palsa surface Swedish distribution of palsas to the zone having temperatures below (Seppälä, 2003); the frozen core of palsas loses their insulating peat −10°C for 120 days. Similar observations were made in northern layer and may collapse. Norway by Åhman (1977), but he differed by delimiting the region as Water must be available for palsas to form. Small creeks often cross having temperatures below −8°C for 120 days. A spatial analysis of the palsa mire and bring water and keep the mire peat moist. Too the distribution of palsa mires in northern Fennoscandia (Luoto et al., much water, however, is fatal for palsas. If the palsa mire is flooded for 2004) concluded that the optimum areas for palsas have low annual extended periods, the frozen core of palsa thaws and the palsa precipitation (b450 mm) and a MAAT between −3°C and −5°C. collapses (Fig. 2H). The results above give only broad guidelines defining the distribu- Though we know very little of the radiation conditions on palsa tion of palsas; they do not exist everywhere where climatic conditions mires, solar energy controls the ground surface temperature, peat are broadly suitable. Numerous local factors contribute the formation of formation and active-layer formation which, in turn affect the palsas. Ground surface temperature is the critical factor for permafrost permafrost. Local conditions are critical when modelling palsa formation (e.g. Haeberli 1973) and snow can control it. This was formation or considering the distribution of palsas. demonstrated with an experimental palsa study in the field (Seppälä, 1982, Seppälä, 1995). With repeated snow clearance on a mire, the frost Physical properties of peat on palsa formation penetrated the peat so deeply that it did not completely thaw during next seven years. The importance of thin snow cover for deep frost Palsa formation depends sensitively on the thermal properties of formation is well-known (Fries and Bergström, 1910; Seppälä, 1982, peat. Laboratory studies of frozen palsa peat samples extracted in 1990, 1994, 2004), and yet very little is known about the depth winter from northern Finland provided new insights into palsa distribution of snow on the mires as the only information that is maturation (Kujala et al., 2008). The thermal conductivity, k, generally available are in the immediate vicinity of meteorological measured with a thermal needle probe, varied between 0.23 and stations. A 1-m thick layer of snow is such a good insulator that the 0.28 W/mK in unfrozen peat dried at room temperature in the ground-surface temperature in the winter in Finland (and regions with laboratory, and between 0.43 and 0.67 W/mK in frozen peat collected similar climate) remains at a constant 0°C (Seppälä, 1994). in the field with a water content of 65%. For unfrozen water saturated Snow depth on palsa mires can be measured in the field, but peat samples, k ranged from 0.41 to 0.50 W/mK; after freezing k available data are sparse. Moreover, the wind conditions that distribute ranged from 1.48 to 1.49 W/mK. The temporal variation in temper- snow unevenly are still unknown; defining these conditions would ature in palsa depends especially on the amount of latent heat require long-term local monitoring, which is a difficult task. Topography involved in freezing/melting and on the thermal diffusivity k/ρC (e.g., Seppälä, 2002) and many local obstacles cause local snow scour where ρ is the density and C, the volumetric heat capacity, which and drift. Strong winds in the winter can remove much snow, creating increases with the water content (Kujala et al., 2008). conditions suitable for the development of new palsas (Fig. 1). Palsa formation starts when wind thins the snow layer on a mire and Railton and Sparling (1973) delimited Canadian palsa regions as frost penetrates deep into the water saturated peat with high thermal those with annual snowfall of less than 120 cm. This figure is only valid conductivity. The mire surface then rises up as a small blister. The surface for very cold regions, however, as permafrost can only form under much of the palsa embryo dries and the vegetation cover changes. Dry surface thinner snow in warmer regions. For example, in Finnish Lapland, frost peat with low thermal conductivity protects the frozen core during the can penetrate to depths of 70 cm in the winter with 20 cm of snow on a summer. With autumn rains, the water content of the surface peat mire in region about 70°N latitude (Seppälä, 1990). increases. This supports deeper freezing and the palsa begins to rise. Luoto and Seppälä (2002) reported that palsas in northern Finnish Several authors (e.g. Åhman, 1977; Seppälä, 1988; Allard and Lapland were restricted to an altitude band between 180 and 390 m Rousseau, 1999) observed ice layers in many drillings and have above sea level. The thickness of the peat layer in mires (minimum interpreted these thin ice layers in palsas as segregation ice. Laboratory 50 cm in Finnish Lapland) is one of the primary controlling factors of tests of frost heave, however, showed that the surface peat of palsa is palsa formation (Seppälä, 1988). At elevations above 400 m the peat is not frost-susceptible: ice-lenses were not observed, suggesting that too thin to protect the frozen core from thawing during the summer segregation ice growth within the peat is unlikely (Kujala et al., 2008). (Luoto and Seppälä, 2002). At lower elevations, palsa mires were AnandAllard(1995)found no segregation ice in most peat types, in characterized by thicker and wetter peat layers, often with water- contrast with silts that are commonly frost-susceptible. 368 M. Seppälä / Quaternary Research 75 (2011) 366–370

1st winter 1st summer

Snow

Frozen peat Water

Peat

Silt A B

2nd winter 2nd summer Rain water

Ice

C D

3rd winter 3rd summer

E F

nnd summer collapsed stage

Water

Frozen core

Frozen silt G H

Figure 2. General model of the formation of the frozen core palsa with ice layers.

Water migrating down from the thawing active layer downwards active layer cannot easily permeate through several ice layers to to the freezing front (An and Allard, 1995) may partly explain a single sustain the downward growth of the frozen core. thin ice layer in palsas, which is often found a few cm below the base It is well-known that in the zone of discontinuous permafrost, of the active layer, but it does not readily account for the series of ice- palsa-scale permafrost mounds appear to ‘float’ in surrounding rich layers characteristic of the frozen core of palsas. Water from the unfrozen mire (e.g., Brown, 1968; Zoltai, 1972; Špolanskaya and M. Seppälä / Quaternary Research 75 (2011) 366–370 369

Enseyev, 1973; Kershaw and Gill, 1979). Moreover, Outcalt and (Seppälä, 1976, 1983). In September and October, even when freezing Nelson's (1984) computer simulation of buoyancy and snow cover starts at the surface, thawing at depth continues for a few centimetres. effects on palsa dynamics suggests that palsa hummocks rise when Although, the highest air temperatures in Lapland are recorded in July, the surrounding mire surface has thawed of seasonal frost. This has the base of the active layer reaches its deepest level in October or even been experimentally demonstrated by Seppälä (1982). in early November (Fig. 3). Seppälä and Kujala ((2009); Fig. 2) also suggested that buoyancy The depth of the active layer on palsas tends to increase with their plays an important role in the formation of ice-rich layers in the early height, presumably because taller palsas are free of snow longer and, stage of palsa development. The formation of ice layers in the palsa hence, have more time to thaw and receive more solar radiation; this core is tied to Archimede's law. Water can exist at the base of the frozen effect is moderated by the lower conduction of heat through the core, and hence the core can effectively float, as long as the water thicker and drier peat (Fig. 3). In Finnish Lapland, for palsas rising pressure there equals the pressure at the same level in the mire. This more than 2 m above the mire surface, the active layer ranges from 60 pressure is ρm ghm where ρm is density, g the acceleration due to gravity to 75 cm; on low (~1 m in height) palsas, 40–50 cm; and on new and hm the depth in the mire. For representative densities (unfrozen palsas (ca 30 cm in height), only 25–30 cm at the end of thawing −3 peat: unsaturated −55% water by weight – ρd =467 kg m and season. This is consistent with isostasy. The thicker frozen core is, the −3 saturated −80% water by weight – ρm =1100 kg m ; and saturated larger is its lifting capacity and although the overlying active layer − 3 frozen peat: ρf =1019 kg m ), a frozen layer of saturated peat 1 m tends to be thicker, it also tends to be drier and easier to lift. thick capped by 20 cm of relatively dry peat would rise 19 to 22 cm Much as permafrost formation is influenced by vegetation cover − 3 above the mire surface for ρd =467 and 300 kg m , respectively. This (Brown, 1966), so are the active-layer depths on palsas (Rönkkö and result emerges from equating the pressure under the frozen layer Seppälä, 2003). High shrubs protect the frozen core. The active layer and the pressure at the same depth in the mire, as follows ρ m ghm = on palsas is thinner under vegetated surfaces than under a bare peat ρ f ghf +ρd ghd, where hf and hd are the thicknesses of the saturated surface. The thinnest active layer was found below the Betula nana frozen peat and the cap of relatively dry peat, respectively. The 19 to shrubs and thickest under the lichen-covered surface. The active layer 22 cm freeboard, the elevation of the top of the dry peat relative to the tends to be thicker on palsas that have been eroded considerably or mire surface, (hf +hd −hm), corresponds well with observations at that are collapsing than on uneroded palsas. Vaisjeäggi, Utsjoki in Finnish Lapland (Seppälä and Kujala, 2009). Surprisingly, due to the physical properties of peat, high air Buoyancy forces support the frozen core, permitting a loose, water- temperature in the summer does not necessarily increase the active- rich layer to form in the saturated peat at the base of the frozen core. layer depth on palsas because dry peat is a good insulator. Rather, the Indeed, some drill holes through young palsas, with frozen cores that frozen core thaws most deeply in wet summers even if they are not had not yet reached the mineral soil, revealed water under pressure at particularly warm. This was dramatically demonstrated in a field the bottom of the frozen peat layer; water discharged spontaneously up experiment in Finnish Lapland (Seppälä, 1982). The man-made palsa to a half a meter above the palsa surface at Vaisjeäggi. As the frost survived seven years, until the summer of 1984, which was very wet penetrates during a subsequent cold period, this basal water would and the mire with the experimental plot was flooded until the end of freeze and form an additional ice layer in the palsa core. This process is September. No permafrost could be detected in the artificial palsa site repeated during the following years, forming new ice layers (Fig. 2), (Seppälä, 1988) in the following 24 yr. Much to their surprise, causing the palsas to grow as the buoyant ice layers in the peat thicken. however, investigators in the middle of August 2009 noticed a new Once the peaty core with ice layers reaches at the bottom of the mire natural palsa under formation at this particular location despite the and frost formation continues in silt or silty till layers, ice segregation general warming in the region. can start to play an important role in the development of the palsa. Summer temperatures in Lapland have been unusually high during the recent years. For example, in July 2004 meteorological station at Active-layer development Kevo (69°45′N lat) recorded the highest MMAT, 17.2°C (long term average for Kevo is 13.0°C); the record high temperature for Finland Environmental changes impact palsas primarily by their influence that year was 28.7°C at Kevo. However, this did not cause deeper on the active layer. The depth of seasonal thaw in Finnish Lapland has thaw. Some new palsas formed in this region on Vaisjeäggi mire, been measured since 1974 Seppälä (1976, 1983). Thawing on palsas Utsjoki in 2001 (Fig. 3). They survived until the wet and cool summer in Finnish Lapland generally starts at the end of May/beginning of June of 2008 thawed these new palsas, as observed in August. approximately when the snow cover disappears. Between 17th of May Several authors have reported recent thawing of palsas in the and 27th of July the sun does not set. southern margin of the palsa region in Fennoscandia (Matthews et al., The depth of active layer increases relatively quickly until the end 1997; Sollid and Sørbel, 1998; Zuidhoff and Kolstrup, 2000; Luoto and of June (Fig. 3) and continues at a lower rate to the end of August Seppälä, 2003) and in North America (Yoshikawa and Hinzman, 2003;

Figure 3. General active-layer development of three palsas with different heights in Finnish Lapland. 370 M. Seppälä / Quaternary Research 75 (2011) 366–370

Payette et al., 2004; Vallée and Payette, 2007; Fortier and Aubé-Maurice, Haeberli, W., 1973. Die Basis Temperatur der winterlichen Schneedecke als möglicher Indicator für die Verbreitung von Permafrost in den Alpen. Zeitschrift für 2008). This recent thawing is not necessarily related to climate Gletscherkunde und Glazialgeologie 9, 221–227. warming, however, because numerous environmental changes can Kershaw, G.P., Gill, D., 1979. Growth and decay of palsas and peat plateaus in the affect palsas: including heavy rains in summer, thick snow cover in Macmillan Pass-Tsichu River area, Northwest Territories, Canada. Canadian Journal of Earth Sciences 16, 1362–1374. winter, and changing ground water table. Detailed measurements are Kujala, K., Seppälä, M., Holappa, T., 2008. Physical properties of peat and palsa needed to determine and separate out the critical factors. Progressive formation. 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