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PERMAFROST AND PERIGLACIAL FEATURES | Frost Mounds: Active and Relict Forms
Frost Mounds: Active and Relict Forms N Ross, Newcastle University, Newcastle upon Tyne, UK ã 2013 Elsevier B.V. All rights reserved. This article is a revision of the previous edition article by C. Harris and N. Ross, volume 3, pp. 2200–2207, ã 2007, Elsevier B.V.
Introduction Frost mounds are ice-cored hills or hummocks found in areas of permafrost and/or seasonally frozen ground. They comprise pingos, palsas, lithalsas, and seasonal frost mounds and blisters. There is significant diversity in their scale, host material, mechanism of formation, ground-ice type, and origin. One process, however, is common to all frost mounds: the subsurface formation of ground ice (injection or segregation ice) generates upward forces able to heave and deform overlying frozen materials, resulting in the upward doming of the ground surface. The collapse of frost mounds can result from mechanical processes (e.g., extension of the overburden material to expose the ice core) or climate forcing (e.g., thickening of the active layer into the ice core). Melting of an ice core over a prolonged period is liable to lead to the collapse of a frost mound. As the ice core melts out, significant sediment redistribution toward the margins of the landform can occur through mass movement and surface wash. These processes (thaw and redistribution of sediment) have the potential to generate annular ring-like ridges or ramparts surrounding central water-filled depressions. Not all frost mounds, however, generate a geomorphological signature of their demise; preservation potential is highly dependent on local factors, particularly the host material. Nevertheless, numerous examples of circular or nearcircular enclosed depressions have been recognized in presentday temperate areas. Many of these landforms were initially interpreted as the remains of pingos formed during Quaternary cold periods. Wisely, however, some researchers were more cautious (Sparks et al., 1972), and many landforms once interpreted as pingo ‘scars’ have since been reinterpreted as the remains of other types of frost mounds (see Pissart, 2000, 2003 and references therein), or even as landforms formed by glacial processes (Iannicelli, 2003). This article describes (i) the growth and collapse of active frost mounds in present-day periglacial environments, and (ii) Quaternary landforms (‘ramparted depressions’) interpreted as relict frost mounds. It focuses on those contemporary frost mounds that are likely to have a long-term geomorphological signature (i.e., pingos, mineral palsas, and lithalsas).
Modern-Day Frost Mounds Pingos Pingos are perennial frost mounds found in the continuous and discontinuous permafrost zones (Figures 1 and 2). They are typically produced by the injection and subsequent freezing of water within near-surface sediments or bedrock, resulting in the formation of a massive ground-ice body and the heave of overlying materials.
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The injection of water into near-surface permafrost to form a pingo ice core capable of deforming units of overlying frozen sediments (often several meters thick) and dome the land surface requires high subsurface water pressures. These pressures can develop in two ways: either (i) in a closed system, as a result of water expulsion from saturated coarse-grained sediments during refreezing of a talik (a zone of unfrozen sediment within continuous permafrost); or (ii) in an open system, because of artesian water pressures within a subpermafrost aquifer (Holmes et al., 1968; Mackay, 1998; Mu¨ller, 1959). In permafrost environments, both processes lead to the upward injection and freezing of water. Pingos are categorized on the basis of their process of formation: the first type is referred to as ‘closed-system pingos’ (or hydrostatic pingos) and the second type as ‘open-system pingos’ (or hydraulic pingos). The former occur in lowland settings within the continuous permafrost zone, generally in localities where surface lake drainage has occurred, while the latter are more common in valley-bottom and footslope localities in both discontinuous and continuous permafrost.
Closed-system pingo formation Much of our understanding of the nature and origin of closedsystem pingos derives from the seminal studies of Mackay in the area northeast of the Mackenzie Delta, Canada (Mackay, 1998). Here, permafrost is developed in a thick sequence of deltaic sands, often mantled by clays and silts. In these lowland areas, surface water accumulates in summer to form pools and lakes. If the lake depth is greater than the winter ice thickness, then the lower parts of the lake will remain liquid throughout the year and the mean annual lake bottom temperature will be above zero. This will have a strong thermal influence on the underlying permafrost, and a thaw bulb or talik will develop below the lake (Figure 3). The lakes of this region are typically impounded by perennially frozen but unconsolidated sediments. As a result, the lakes are highly susceptible to sudden drainage events generated by thermal erosion around the lake shore and by masswasting processes. When lakes drain, the underlying sediments are no longer insulated by the overlying water body, and so the talik begins to refreeze. Permafrost aggrades both from the surface downwards and from the lateral margins of the talik inwards (Figure 3). When the unfrozen saturated sands in the talik freeze, the 9% volume increase associated with the phase change from liquid water to ice results in expulsion of approximately 9% of the porewater ahead of the advancing freezing front. This expelled water migrates, under hydrostatic pressure, to the zone of least confining stress, usually below a residual pond within the drained lake basin (Figure 3). Freezing from the surface downwards then initiates the formation of the pingo ice core.
PERMAFROST AND PERIGLACIAL FEATURES | Frost Mounds: Active and Relict Forms
Figure 1 View of a closed-system pingo, Mackenzie Delta, Canada. Photograph by Charles Harris.
Figure 2 Open-system pingo, Adventdalen, Svalbard. The pingo has a height of 30 m and a diameter of around 200 m. Photograph by Neil Ross.
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If the rate at which expelled porewater accumulating beneath the bottom of the pingo ice core exceeds the downward rate of freezing of the water, then a subpingo water lens develops (Figure 3). In this instance, the hydrostatic pressure is able to support the overburden stress and cause further uplift and deformation of the overlying ice core and frozen overburden (Mackay, 1998). Monitoring of closed-system pingos indicates that high hydrostatic pressures in subpingo water lenses can mechanically fracture the overlying materials, leading to (i) rupturing of the pingo sides, (ii) high-angle faulting of overlying frozen sediments, (iii) formation of surface springs, and (iv) development of winter icings. Closed-system pingos in northwest Canada may exceed 40 m in height, though the vast majority are <20 m high (Mackay, 1998). Basal diameters up to about 250 m have also been recognized. The diameter and plan form of closedsystem pingos depend on the size and shape of the original talik, which is often circular or elliptical beneath thaw ponds, but can be markedly linear beneath elongated large water bodies, leading to the formation of linear, ridge-like pingos (Pissart and French, 1976). Importantly, closed-system pingos have the potential to grow extremely rapidly. Data collected from a pingo in northwest Canada, which began growing soon after 1950, showed rapid initial growth that then slowed (Mackay, 1998). However, Mackay was careful to warn that generalizations on pingo Lake
Permafrost
Unfrozen saturated sands
Permafrost
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Lake drains Residual pond Pore water under hydrostatic pressure due to porewater expulsion
Permafrost aggradation
Permafrost aggradation
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Dilation cracks Subpingo water lens at <0 ⬚
Pingo ice
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Collapsing pingo
Pond
Thawing pingo ice
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Figure 3 Formation of closed-system pingos. (a) Development of a talik beneath a thaw lake. (b) Lake drainage leading to refreezing of the talik and pore water expulsion as the permafrost table advances inwards. (c) Progressive freezing of a pressurized subpingo water lens leading to thickening of the pingo ice core and pingo growth. (d) Pingo collapse due to partial thaw of pingo ice beneath a central pond. Reproduced from Mackay JR (1998) Pingo growth and collapse, Tuktoyaktuk Peninsula area, Western Arctic Coast, Canada: A long term field study. Ge´ographie Physique et Quaternaire 52(3): 271–323, with permission.
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growth rates based on this sole example were probably inappropriate. Only some of the pingos monitored in his studies showed a progressive uplift: some pingos ruptured, causing water loss from subpingo water lenses and subsequent subsidence, while others showed up-and-down oscillations (Mackay, 1998).
Open-system pingo formation In contrast to the closed-system type, open-system pingos are most common in areas of marked relief, often in lower valley sides or in valley bottoms (Figure 2). The first detailed description of open-system pingos was by Mu¨ller in East Greenland (Mu¨ller, 1959), but long-term studies of open-system pingo growth and decay have not been undertaken. Much less is therefore known, from direct observations, about their origins and internal structure than for closed-system forms. Many open-system pingos occur in areas with thin and/or discontinuous permafrost. They have been described in central Alaska (Holmes et al., 1968), the Yukon Territory, Canada, the Tibetan Plateau, and central Yakutia, Russia. However, they are also found in areas of continuous permafrost, and have been recorded in Greenland (Mu¨ller, 1959; Worsley and Gurney, 1996) and Svalbard (Liestøl, 1977). Open-system pingos develop within a range of superficial sediments, including fluvial and slope deposits, and some even develop in weathered bedrock (Holmes et al., 1968; Liestøl, 1977; Mu¨ller, 1959). Open-system pingos are often somewhat smaller than their closed-system counterparts and are reported as single, isolated features and in clusters or groups of mutually interfering landforms of different ages. The most widely cited model of open-system pingo formation is that of Holmes et al. (1968), who observed that nearly all pingos of Central Alaska lie on gentle to moderate slopes near the slope base in minor valleys and in gently sloping valley-bottom locations. Their preferential development on south- and southeast-facing slopes was attributed to thinner permafrost on slopes with these aspects. Holmes et al. (1968) suggested that local groundwater flow was crucial in determining the location and likelihood of open-system pingo development (Figure 4). Pressurized groundwater at intermediate depths beneath the valley sides has a tendency to flow toward the ground surface within subpermafrost unfrozen sediments or through bedrock fractures. Groundwater flow is restricted, however, by the base of the overlying permafrost, which acts like an aquiclude, confining upward flow (Figure 4). But near the base of the slopes – where the artesian pressure of groundwater sourced from the more elevated parts of the topography is greatest, and where thicker permafrost below the valley floor blocks the lateral flow of subpermafrost groundwater – water is forced upwards toward the ground surface. This water will either freeze in the near-surface to form a pingo, or – if it is too warm or the permafrost is locally absent – it will issue from the ground surface as a spring. Whether one or the other develops appears to be a very delicate balance of local thermal and geological conditions: Holmes et al. (1968) reported active and apparently perennial springs in association with some open-system pingos, a feature also characteristic of opensystem pingos in Greenland and Svalbard. In Eastern Greenland, open-system pingos are formed in glaciogenic materials, alluvial sediments, and bedrock in the
continuous permafrost zone, where mean annual air temperature is around 10 C and permafrost is recorded to a depth of 220 m. Here, the hydraulic system leading to pingo growth is probably fed, to some degree, by deep intra- or subpermafrost groundwater (Mu¨ller, 1959; Worsley and Gurney, 1996), rather than by significant amounts of recharge from seasonal surface melt. The postulated deep origins for these waters is supported by measurements of warm (>0 C) and highly mineralized water discharging from perennial groundwater springs close to pingos in northeast Greenland; in winter, the springs form large surface icings. These characteristics suggest geothermal warming of ground waters, perhaps as they percolate along structurally controlled seepage zones and through taliks determined by zones of high geothermal heat flow (Worsley and Gurney, 1996). Interestingly, some Greenland pingos occur in localized clusters. This tendency has been attributed not only to the importance of regional hydrogeology in determining localities suitable for pingo development, but also to the evolution of very localized groundwater pathways through time. On Svalbard, open-system pingos are also common in valleys. A simple but elegant model that suggested a close link between the pingos and polythermal glaciers on Svalbard was proposed by Liestøl (1977). Although permafrost is thick (100–200 m) and continuous beneath the inland parts of Svalbard, Liestøl (1977) recognized that it thins rapidly beneath the margins of the polythermal glaciers, and is probably entirely absent beneath the thicker warm-based ice beneath glacier accumulation areas. Subglacial meltwaters are therefore able to recharge groundwater where permeable strata underlie the warm-based parts of these glaciers. This provides a major source of meltwater to maintain (and pressurize) the groundwater throughout a significant part of the year, despite the continuous nature of the surrounding permafrost. Beyond the glacier margins – in a system akin to that responsible for the development of Alaskan open-system pingos (Figure 4) – this glacier-sourced groundwater is able to migrate through confined subpermafrost aquifers, within which considerable artesian pressures can be generated. In large, broad valleys such as Reindalen (Figure 5), this artesian pressure is sufficient for groundwaters to reach the near-surface, where freezing can generate open-system pingo growth. As suggested for Greenland, groundwater flow paths in Svalbard may also be influenced by the geological structure and geothermal heat. Deep percolation and geothermal heating of groundwater are indicated by the warm surface springs in parts of western Svalbard (Liestøl, 1977). Many pingos in Svalbard also show surface discharge of water (Figure 2) whose elevated salt content significantly lowers the freezing point. Other similarities may exist between the open-system pingos of Greenland and Svalbard, inasmuch as deep groundwaters in Greenland are also likely to have their initial origins in glacial meltwaters, probably from the Greenland Ice Sheet. A subtype of open-system pingo has been identified in lowlying areas below the maximum Holocene sea level in Svalbard and Greenland. These landforms presumably date from the late to mid-Holocene and formed where, following relative sea-level fall, permafrost has aggraded in newly exposed and highly frost-susceptible fine-grained marine muds that favor
PERMAFROST AND PERIGLACIAL FEATURES | Frost Mounds: Active and Relict Forms
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Permafrost Injection ice
Active layer
Subpermafrost groundwater flow (a)
Tension cracks Mass movement on pingo sides
Pingo ice core
(b)
Degrading pingo
Pond
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Figure 4 Cycle of formation and decay of open-system pingos. (a) Percolation of subpermafrost groundwater within a confined aquifer and injection into thin permafrost, initiating formation of a pingo ice core. (b) Formation of cracks due to tensional stresses developed as the surface is stretched by updoming of pingo center. (c) Development of central crater with pond as the ice core decays. Modified from Holmes GW, Hopkins DM, and Foster HL (1968) Pingos in central Alaska. United States Geological Survey Bulletin 1241-H: H1–H40; Ballantyne CK and Harris C (1994) The Periglaciation of Great Britain. Cambridge, UK: Cambridge University Press.
ice segregation and frost heave (Ross et al., 2007). As a consequence, these pingos may be intermediate between closed- and open-system types. Such near-shore pingos, however, also share many characteristics with lithalsas.
Pingo collapse
Figure 5 Ross.
Open-system pingo, Reindalen, Svalbard. Photograph by Neil
Pingo degradation is normally initiated by the development of radial dilation cracks and concentric cracks caused by tensional stresses arising from bending and extension of the frozen overburden during pingo growth (Figures 4 and 6; Mackay, 1988). Sediment covering the ice core is lost because of slumping and solifluction of the active layer on the steep pingo sides (Babinski, 1982; Mackay, 1988), leading to the exposure of the pingo ice core, which is then highly susceptible to melting in summer months.
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PERMAFROST AND PERIGLACIAL FEATURES | Frost Mounds: Active and Relict Forms
Figure 6 Closed-system pingo in the Mackenzie Delta, Canada, showing development of radial dilation cracks caused by tensional stress generated by upward growth. The exposed ice core is susceptible to thawing, initiating pingo collapse. Photograph by Charles Harris.
Figure 7 Frozen lake within the crater of an open-system pingo, Reindalen, Svalbard. Photograph by Neil Ross.
Once melting of the ice core begins, the landform can be susceptible to an important positive feedback. As the ice core melts, a summit crater is generally formed, surrounded by a complex arrangement of ridges and mounds. These ridges may impound meltwater from the melting ice core, to form a lake. Crater lakes, many with perennial groundwater springs and outflow streams, have been reported from active pingos in Canada (Mackay, 1988), Greenland (Mu¨ller, 1959), Svalbard (Liestøl, 1977), Mongolia (Babinski, 1982), and Alaska (Holmes et al., 1968) (Figure 7). Because of the high thermal conductivity of water, melting of the underlying ice core can occur rapidly beneath these water bodies. The lake will also generally increase the rate at which lateral (thermal) erosion of the surrounding frozen ridge occurs. As degradation progresses, the lake increases in diameter, further increasing (i) the surface area of the ice core subject to melt and (ii) the length of crater-lake shoreline subject to thermal erosion. Some processes, however, slow the rate of degradation. Intermittent, sometimes seasonal, breaching of crater lakes will result in outflow from, and possibly drainage of, the enclosed basin. If a crater-lake drains completely, then the rate of landform degradation may reduce significantly. Inward movement of thawing sediment from the crater rim is also
Figure 8 A collapsed closed-system pingo, Mackenzie Delta area, Canada. The pingo has an external diameter of approximately 250 m. Photograph by Ross Mackay.
possible once degradation is well advanced, and this may also slow the degradation of the ice core by insulating it from warm air temperatures. Complete landform degradation often leaves a pond surrounded by a circular, sometimes ice-cored, ridge of sediment. Although mature collapsed pingos with ramparts have been described from the contemporary permafrost zone (Figure 8; Babinski, 1982; Holmes et al., 1968; Mackay, 1988; Mu¨ller, 1959; Worsley and Gurney, 1996), there have been few investigations of their internal structures. Solifluction and slumping are clearly important processes in their development, as they result in a net transfer of sediment to the base of the mound where it accumulates to form a circular ridge. In addition to mass-movement processes, however, short-lived streamflow events may also play an important role in the redistribution of sediment necessary to form rampart ridges around collapsing pingos. Small-scale outlet channel forms, associated with small alluvial fans, have been observed around the periphery of several collapsing pingos (Mackay, 1988; Mu¨ller, 1959).
Other Frost Mounds In addition to closed- and open-system pingos, arrays of smaller seasonal and perennial ground-ice mounds have been recognized. This section describes seasonal frost mounds, palsas, mineral palsas, and lithalsas. Because seasonal frost mounds and palsas have limited long-term preservation potential, emphasis is given here to mineral palsas and lithalsas, which are more likely to be represented in the Quaternary record.
Seasonal frost mounds Ephemeral frost and icing mounds or blisters (Pollard, 1988), usually less than 5 m high, form in permafrost regions as a result of winter freeze-back of the active layer, trapping suprapermafrost groundwater fed by springs. Mounds are mantled with a thin layer of peat and soil, underlain by mineral soils, and contain bubble-rich or clear ice cores. These are sometimes layered and sometimes arched over a water-filled chamber within which sufficiently high hydrostatic pressures are developed in winter to uplift and deform the frozen overburden. Such landforms have a low preservation potential in the
PERMAFROST AND PERIGLACIAL FEATURES | Frost Mounds: Active and Relict Forms
Figure 9 Palsa mire, northern Finland, with low palsa mounds in the background and a decayed and collapsed mound in the foreground. Photograph by Matti Seppa¨la¨.
geological record and are unlikely to be found, or easily identified, in areas formerly affected by permafrost.
Palsas Palsas are low permafrost mounds, up to around 7 m high and commonly up to 30 m in diameter (Figure 9). Pissart (2002) described palsas as “perennial mounds covered in peat, situated in the discontinuous permafrost zone and due chiefly to segregation ice fed by cryosuction.” Palsas develop in peatland mires in the circumpolar discontinuous or sporadic permafrost zones, and are normally observed in groups or ‘fields’ (Gurney, 2001). They form as a result of differential winter frost penetration. Deeper frost penetration in areas with thinner snow cover leads to frost heave due to ice segregation. The more deeply frozen peat is frost-heaved above the surrounding landscape, and in summer its surface layers dry, forming an effective insulating cover that limits the depth of thaw and preserves a frozen core beneath the palsa. During succeeding winters, snow is blown from the upper surface of the growing palsa but accumulates in the low-lying wetter areas, causing further differential frost penetration, ice segregation, and palsa growth (Gurney, 2001). Palsas eventually decay when erosion of their sides and loss of peat cover from their top cause the frozen core to melt and collapse (Gurney, 2001). This often forms ponds (Figure 9) that are occasionally surrounded by a low ridge. Such ponds, however, rapidly fill with peat and their preservation potential is extremely low. All stages of cyclic palsa evolution (growth, maturity, and collapse) can commonly be observed in the same mire, though there is recent evidence for climatically driven degradation of palsa mires in Scandinavia and North America.
Mineral palsas and lithalsas The frozen core of a palsa may extend through the peat cover into underlying frost-susceptible sediments. These sediments are heaved upwards by the formation of segregation ice, fed by cryosuction, to form a domed mineral core (Gurney, 2001; Pissart, 2002). The peat cover of such ‘mineral palsas’ may be <10 cm thick and they appear to form part of a continuum of landforms caused by local redistribution of winter snow, differential frost penetration, and associated differential heave. At one end member are palsas formed entirely in peat, and at the
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Figure 10 Decaying mineral palsas or lithalsas with central ponds and raised rims, 20 km inland from the eastern coast of Hudson Bay, near Umiujaq, northern Quebec, Canada. The features are developed in marine silts. Photograph by Michel Allard.
other are low (<10 m high) perennial frost mounds developed in sediments with little or no peat cover (Gurney, 2001; Pissart, 2002). To distinguish them from other types of palsa, circular to oval mounds formed in sediments with no peat cover have been named ‘lithalsas’ (Pissart et al., 1998). Mineral palsas and lithalsas occur in extensive contiguous fields, with younger features commonly disturbing older ramparts and depressions (Figure 10). They are particularly common in Scandinavia and North America. Although normally developed in fine-grained lacustrine and marine sediments (Allard et al., 1987; Pissart et al., 1998), they can also form in areas with marine sands, in gravel, till, and stony glaciomarine diamicton (Allard et al., 1987). Besides being associated with low-lying zones of latitudinal permafrost, lithalsas have also been recognized in high-mountain environments. Wu¨nnemann et al. (2008) mapped lithalsas at elevations >4500 m in the Himalayas. Unlike palsas developed entirely in peat, mineral palsas and lithalsas are more likely to generate enclosed rimridges during their collapse (Gurney, 2001; Pissart, 2002). Because they are composed of mineral sediments, these landforms, like collapsed pingos, have much greater preservation potential than palsas.
Mineral palsa and lithalsa collapse The growth of radial and concentric cracks, small-scale landsliding, thaw consolidation, and solifluction have all been reported from mineral palsas and lithalsas (Allard et al., 1987; Calmels et al., 2008a,b). Like the collapse of pingos, the collapse of these smaller landforms results in the development of thermokarst depressions surrounded by ring-ridge ramparts (Figure 10). These are composed of mineral sediments accumulated by mass movement, as well as through lateral compression due to the growth of segregation ice (Calmels and Allard, 2008). There is some evidence for a general trend of mineral palsa and lithalsa degradation in response to twentieth-century warming, although some studies emphasize the influence of local hydrology on their growth or decay. For example, Lewkowicz and Coultish (2004) showed that mineral palsa formation was controlled by the influence of beaver on the
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PERMAFROST AND PERIGLACIAL FEATURES | Frost Mounds: Active and Relict Forms
local environment: frost-mound degradation followed flooding due to dam construction, and growth occurred after dams were breached and the ground surface was exposed. This unusual example clearly shows the influence that local hydrology has on frost-mound development, particularly in the sporadic or discontinuous permafrost zones.
Active frost mounds: a summary Frost-mound growth depends on a complex interplay of processes and environmental conditions. Growth is strongly controlled not just by the presence or absence of permafrost (and hence the climatic regime), but also by the regional and local hydrogeological conditions, glacier meltwaters, geological structure, and the geothermal heat flux. Such complexity must be considered when interpreting Quaternary landforms that have all the hallmarks of relict frost mounds.
Relict Frost Mounds Enclosed depressions surrounded by annular ring-ridges or ramparts have been interpreted as remnant landforms that mark the former location of Quaternary frost mounds. They are frequently found in clusters, and, in Western Europe, are sited on valley bottoms, at slope–valley interfaces, and in some plateau locations. Various terms have been introduced to describe these relict features. Given the complexities of interpretation, however, terms such as ‘relict frost mound’ or ‘ramparted depression’ are preferred to those with genetic connotations such as ‘pingo scar.’ Useful reviews of ramparted ground-ice depression sites of Pleistocene age from northwest Europe and North America are given by Flemal (1976) and Pissart (2003). In Quaternary ramparted depressions, the ramparts may contain overturned, deformed, and tilted stratified sediments overlain by mass-movement deposits showing crude radially dipping stratification (Pissart, 2000; Ross et al., 2011). The presence of these structures often provides important evidence for an origin related to the decay of ground-ice mounds. The classic site where Pleistocene ramparted depressions were first detailed is on the Hautes Fagnes Plateau, Belgium. Here, enclosed, peat-filled depressions surrounded by distinct ramparts are widespread. The ramparts are commonly less than 1 m high, but some reach a height of 5 m (Pissart, 2000, 2003), and the thickness of basin infill varies between 1 and 7.5 m, with diameters of up to several hundred meters. These features were first interpreted as the remains of open-system pingos of the Younger Dryas age, dated by radiocarbon, tephra, and pollen stratigraphy methods (Pissart, 2000, 2003). The absence of an obvious aquifer to supply groundwater subsequently led Pissart to conclude that segregation ice rather than injection ice was responsible for the development of these landforms, and that they represent former lithalsas rather than open-system pingos. Their ramparts are thought to have formed by lateral compression and accumulation of slumped sediment (Pissart, 2000). In Scandinavia and the British Isles, relict ramparted depressions (Figure 11) formerly interpreted as open-system pingos have also been reinterpreted as lithalsa remnants (Pissart, 2000, 2003). Pissart (2002) argued that the known spatial characteristics of contemporary pingos (widely spaced,
Figure 11 Relict ramparted depression thought to mark the former location of a Quaternary frost mound, Llanpumsaint, Wales, UK. Photograph by Neil Ross.
low number per unit area) and lithalsas (groups of closely spaced landforms, often very numerous, with a high concentration per unit area) allow simple interpretation of relict Pleistocene landforms. Interpretation of ramparted depressions becomes more complicated, however, when it is accepted that in past and present glaciated regions burial and subsequent melting of marginal glacier ice can form landforms topographically similar to those produced by the collapse of frost mounds. Extensive areas with ramparted depressions in the northern North American Prairies have long been interpreted not as ground-ice-related phenomena but as the product of subglacial and supraglacial processes (Flemal, 1976; Iannicelli, 2003; Mollard, 2000). Similar interpretations have been proposed in Scandinavia. Icebergs, grounded by lake drainage, have also been used to explain ring-ridges (Mollard, 2000). Therefore, while many European researchers have favored a periglacial origin for ramparted depressions, the North American literature tends to favor a glacial origin (Flemal, 1976). Contemporary studies in permafrost regions aimed at assessing the processes of pingo decay and collapse are therefore critical for the accurate identification and classification of relict Pleistocene ramparted ground-ice depressions. Recent studies of ramparted depression in Europe have been more aware of the risks associated with assuming that ramparted depressions represent a relict permafrost landform and have also assessed potential glacial origins (Ross et al., 2011). Even when detailed sedimentological and geophysical data are available from particular sites, however, precise interpretation is frequently difficult, given (i) the range of frost-mound types identified in modern cold environments and (ii) the array of glacial processes that can lead to the burial and melt-out of glacier ice, and therefore the formation of topographic depressions with enclosing ridges or ramparts.
Recent Developments in Frost-Mound Research Several key factors have limited our understanding of frostmound-related processes. These are (i) a lack of natural exposure (of both the internal structures of frost mounds and the surrounding host material), (ii) the cost and logistical challenges associated with creating artificial exposures (e.g., by drilling) in remote high-latitude permafrost environments, (iii) too great a focus on site-specific studies, and (iv) a lack of long-term monitoring. This section outlines a series of recent studies that have begun to address these research shortcomings. To overcome the lack of natural exposure of frost-mound internal structure, and the costs associated with drilling, several recent studies have applied geophysical tools (ground penetrating radar, electrical resistivity, seismic, and electromagnetic
PERMAFROST AND PERIGLACIAL FEATURES | Frost Mounds: Active and Relict Forms
techniques) to image the internal structures of frost mounds (Fortier et al., 2008; Ross et al., 2007; Yoshikawa et al., 2006). Because of the contrasting physical properties of unfrozen water, ice, and host geological materials, these techniques offer significant potential for characterizing the internal structures, ice content, and host materials of frost mounds, which can then be used to infer the likely mechanisms of formation. The most comprehensive of these studies is that of Yoshikawa et al. (2006), who integrated a suite of ground and airborne geophysical data with borehole data to constrain the internal structures of open-system pingos in Alaska. These authors showed that two-dimensional (2D) electrical resistivity tomography seems to be the most successful single technique for imaging pingo internal structures (although it is best combined with others). Because pure ice is highly resistive, electrical resistivity is normally extremely useful for delineating the massive ice core in pingos; values of >10 000 O m are common for pingo cores (Yoshikawa et al., 2006). Ross et al. (2007) have shown, however, that the response of the pingo core can be highly dependent on the host material and the type of ice (injection or segregation ice). Near-shore open-system pingos in Svalbard are characterized by very low values of resistivity (<2000 O m) for permanently frozen ground. These results were attributed to pingo growth by segregation-ice development within fine-grained, saline marine clays (Ross et al., 2007), a mechanism not dissimilar to that by which lithalsas form. Research into present-day frost mounds has tended to be rather focused on site-specific investigations and has, arguably, ignored the ‘bigger picture.’ Small-scale Geographic Information System (GIS) and remote-sensing studies of high-latitude environments show considerable potential for addressing this shortcoming and for contributing toward our understanding of frost-mound development. By far, the best example of this is the study of Grosse and Jones (2011), who compiled a geodatabase of over 6000 pingos from openly accessible Russian map and remote-sensing datasets, and used GIS tools to analyze the spatial distribution of these landforms across Northern Asia. By integrating open-access permafrost, geological, climatological, hydrological, and elevation datasets, they were able to (i) quantify the key factors for pingo development across northern Asia (near-surface geology and hydrology), (ii) delimit the range of mean annual ground and air temperatures that the pingos in the region of study were found in, (iii) propose that 34% of the pingos studied are located in areas where climate models predict permafrost thaw by 2100, and which may initiate pingo degradation, and (iv) provide an upto-date figure for the number of pingos on Earth (>11 000). Thanks to the work of Mackay (1998), closed-system pingos have been the subject of long-term monitoring studies, revealing key data concerning both internal structures and the way in which this type of frost mound forms and degrades. Until recently, however, comparable monitoring studies had not been undertaken on other types of frost mound. In 2000, a long-term monitoring and investigative study of a lithalsa was initiated in Quebec (Calmels et al., 2008a,b and references therein). During the period of monitoring (2000–2005), the lithalsa showed a pattern of long-term degradation, including the initial stages of rim-ridge formation (Calmels et al., 2008a). Tomodensitometric scanning of cores from the monitored lithalsa revealed unparalleled information regarding the ice
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and gas content within the landform, while ground temperature measurements were used to infer the processes of groundwater migration and ice-lens growth (Calmels et al., 2008b). Longterm monitoring studies such as this provide critical information, and if we are ever to fully understand the mechanisms that drive the evolution of other frost mounds (e.g., open-system pingos), it will only be achieved through systematic investigations similar to those undertaken by Calmels et al. (2008a,b) on lithalsas. The quantitative studies outlined above demonstrate that frost-mound research has clearly explored new and exciting avenues in recent years, not just by embracing and applying modern and novel techniques (geophysics and GIS) but also by doing the ‘basics’ correctly (through the setting up of longterm monitoring programs). These studies have provided important information regarding the processes responsible for the growth and collapse of modern-day frost mounds, information that is critical for making accurate interpretations of the likely mechanisms responsible for the formation of Quaternary ramparted depressions.
See also: Permafrost and Periglacial Features: Permafrost; Thermokarst Topography.
References Allard M, Seguin MK, and Le´vesque R (1987) Palsas and mineral permafrost mounds in northern Quebec. In: Gardiner V (ed.) International Geomorphology, pp. 285–309. London: John Wiley. Babinski Z (1982) Pingo degradation in the Bayan-Nuurin-Khotnor Basin, Khangai Mountains, Mongolia. Boreas 11: 291–298. Calmels F and Allard M (2008) Segregated ice structures in various heaved permafrost landforms through CT scan. Earth Surface Processes and Landforms 33: 209–225. Calmels F, Allard M, and Delisle G (2008a) Development and decay of a lithalsa in Northern Quebec: A geomorphological history. Geomorphology 97: 287–299. Calmels F, Delisle G, and Allard M (2008b) Internal structure and the thermal and hydrological regime of a typical lithalsa: Significance for permafrost growth and decay. Canadian Journal of Earth Sciences 45: 31–43. Flemal RC (1976) Pingos and pingo scars: Their characteristics, distribution and utility in reconstructing former permafrost environments. Quaternary Research 6: 37–53. Fortier R, LeBlanc A-M, Allard M, Buteau S, and Calmels F (2008) Internal structure and conditions of permafrost mounds at Umiujaq in Nunavik, Canada, inferred from field investigation and electrical resistivity tomography. Canadian Journal of Earth Sciences 45: 367–387. Grosse G and Jones BM (2011) Spatial distribution of pingos in northern Asia. The Cryosphere 5: 13–33. Gurney SD (2001) Aspects of the genesis, geomorphology and terminology of palsas: Perennial cryogenic mounds. Progress in Physical Geography 25: 249–260. Holmes GW, Hopkins DM, and Foster HL (1968) Pingos in central Alaska. United States Geological Survey Bulletin 1241-H: H1–H40. Iannicelli M (2003) Reinterpretation of the original DeKalb Mounds in Illinois. Physical Geography 24: 170–182. Lewkowicz AG and Coultish TL (2004) Beaver damming and palsa dynamics in a subarctic mountainous environment, Wolf Creek, Yukon Territory, Canada. Arctic, Antarctic, and Alpine Research 36: 208–218. Liestøl O (1977) Pingos, springs, and permafrost in Spitsbergen. Norsk Polarinstitutt A˚rbok 1975: 7–29. Mackay JR (1988) Pingo collapse and paleoclimatic reconstruction. Canadian Journal of Earth Sciences 25: 495–511. Mackay JR (1998) Pingo growth and collapse, Tuktoyaktuk Peninsula area, Western Arctic Coast, Canada: A long term field study. Ge´ographique Physique et Quaternaire 52: 271–323. Mollard JD (2000) Ice-shaped ring-forms in Western Canada: Their airphoto expressions and manifold polygenetic origins. Quaternary International 68–71: 187–198.
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Mu¨ller F (1959) Beobachtungen u¨ber Pingos. Meddelelser om Grønland 153: 1–127. (translated by Sinclair DA, National Research Council of Canada, Ottawa, TT-1073, 1963). Pissart A (2000) Remnants of lithalsas of the Hautes Fagnes, Belgium: A summary of present day knowledge. Permafrost and Periglacial Processes 11: 327–355. Pissart A (2002) Palsas, lithalsas and remnants of these periglacial mounds: A progress report. Progress in Physical Geography 26: 605–621. Pissart A (2003) The remnants of Younger Dryas lithalsas on the Hautes Fagnes Plateau in Belgium and elsewhere in the world. Geomorphology 52: 5–38. Pissart A and French HM (1976) Pingo investigations, north-central Banks Island, Canadian Arctic. Canadian Journal of Earth Sciences 13: 937–946. Pissart A, Harris S, Prick A, and Van Vliet-Lanoe¨ B (1998) La signification paleoclimatique des lithalsas (palses minerales). Biuletyn Peryglacjalny 37: 141–154. Pollard WH (1988) Seasonal frost mounds. In: Clark MJ (ed.) Advances in Periglacial Geomorphology, pp. 201–229. Chichester: Wiley. Ross N, Brabham PJ, Harris C, and Christiansen HH (2007) Internal structure of open system pingos, Adventdalen, Svalbard: The use of resistivity tomography to assess
ground-ice conditions. Journal of Environmental and Engineering Geophysics 12: 113–126. Ross N, Harris C, Brabham PJ, and Sheppard TH (2011) Internal structure and geological context of ramparted depressions, Llanpumsaint, Wales. Permafrost and Periglacial Processes 22: 291–305. Sparks BW, Williams RBG, and Bell FG (1972) Presumed ice depressions in East Anglia. Proceedings of the Royal Society A327: 329–343. Worsley P and Gurney SD (1996) Geomorphology and hydrogeological significance of the Holocene pingos in the Karup Valley area, Traill Island, northern east Greenland. Journal of Quaternary Science 11: 249–262. Wu¨nnemann B, Reinhardt C, Kotlia BS, and Riedel F (2008) Observations on the relationship between lake formation, permafrost activity and lithalsa development during the last 20 000 years in the Tso Kar Basin, Ladakh, India. Permafrost and Periglacial Processes 19: 341–358. Yoshikawa K, Leuschen C, Ikeda A, et al. (2006) Comparison of geophysical investigations for detection of massive ground ice (pingo ice). Journal of Geophysical Research 111: E06S19.
Introduction Frost mounds are ice-cored hills or hummocks found in areas of permafrost and/or seasonally frozen ground. They comprise pingos, palsas, lithalsas, and seasonal frost mounds and blisters. There is significant diversity in their scale, host material, mechanism of formation, ground-ice type, and origin. One process, however, is common to all frost mounds: the subsurface formation of ground ice (injection or segregation ice) generates upward forces able to heave and deform overlying frozen materials, resulting in the upward doming of the ground surface. The collapse of frost mounds can result from mechanical processes (e.g., extension of the overburden material to expose the ice core) or climate forcing (e.g., thickening of the active layer into the ice core). Melting of an ice core over a prolonged period is liable to lead to the collapse of a frost mound. As the ice core melts out, significant sediment redistribution toward the margins of the landform can occur through mass movement and surface wash. These processes (thaw and redistribution of sediment) have the potential to generate annular ring-like ridges or ramparts surrounding central water-filled depressions. Not all frost mounds, however, generate a geomorphological signature of their demise; preservation potential is highly dependent on local factors, particularly the host material. Nevertheless, numerous examples of circular or nearcircular enclosed depressions have been recognized in presentday temperate areas. Many of these landforms were initially interpreted as the remains of pingos formed during Quaternary cold periods. Wisely, however, some researchers were more cautious (Sparks et al., 1972), and many landforms once interpreted as pingo ‘scars’ have since been reinterpreted as the remains of other types of frost mounds (see Pissart, 2000, 2003 and references therein), or even as landforms formed by glacial processes (Iannicelli, 2003). This article describes (i) the growth and collapse of active frost mounds in present-day periglacial environments, and (ii) Quaternary landforms (‘ramparted depressions’) interpreted as relict frost mounds. It focuses on those contemporary frost mounds that are likely to have a long-term geomorphological signature (i.e., pingos, mineral palsas, and lithalsas).
Modern-Day Frost Mounds Pingos Pingos are perennial frost mounds found in the continuous and discontinuous permafrost zones (Figures 1 and 2). They are typically produced by the injection and subsequent freezing of water within near-surface sediments or bedrock, resulting in the formation of a massive ground-ice body and the heave of overlying materials.
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The injection of water into near-surface permafrost to form a pingo ice core capable of deforming units of overlying frozen sediments (often several meters thick) and dome the land surface requires high subsurface water pressures. These pressures can develop in two ways: either (i) in a closed system, as a result of water expulsion from saturated coarse-grained sediments during refreezing of a talik (a zone of unfrozen sediment within continuous permafrost); or (ii) in an open system, because of artesian water pressures within a subpermafrost aquifer (Holmes et al., 1968; Mackay, 1998; Mu¨ller, 1959). In permafrost environments, both processes lead to the upward injection and freezing of water. Pingos are categorized on the basis of their process of formation: the first type is referred to as ‘closed-system pingos’ (or hydrostatic pingos) and the second type as ‘open-system pingos’ (or hydraulic pingos). The former occur in lowland settings within the continuous permafrost zone, generally in localities where surface lake drainage has occurred, while the latter are more common in valley-bottom and footslope localities in both discontinuous and continuous permafrost.
Closed-system pingo formation Much of our understanding of the nature and origin of closedsystem pingos derives from the seminal studies of Mackay in the area northeast of the Mackenzie Delta, Canada (Mackay, 1998). Here, permafrost is developed in a thick sequence of deltaic sands, often mantled by clays and silts. In these lowland areas, surface water accumulates in summer to form pools and lakes. If the lake depth is greater than the winter ice thickness, then the lower parts of the lake will remain liquid throughout the year and the mean annual lake bottom temperature will be above zero. This will have a strong thermal influence on the underlying permafrost, and a thaw bulb or talik will develop below the lake (Figure 3). The lakes of this region are typically impounded by perennially frozen but unconsolidated sediments. As a result, the lakes are highly susceptible to sudden drainage events generated by thermal erosion around the lake shore and by masswasting processes. When lakes drain, the underlying sediments are no longer insulated by the overlying water body, and so the talik begins to refreeze. Permafrost aggrades both from the surface downwards and from the lateral margins of the talik inwards (Figure 3). When the unfrozen saturated sands in the talik freeze, the 9% volume increase associated with the phase change from liquid water to ice results in expulsion of approximately 9% of the porewater ahead of the advancing freezing front. This expelled water migrates, under hydrostatic pressure, to the zone of least confining stress, usually below a residual pond within the drained lake basin (Figure 3). Freezing from the surface downwards then initiates the formation of the pingo ice core.
PERMAFROST AND PERIGLACIAL FEATURES | Frost Mounds: Active and Relict Forms
Figure 1 View of a closed-system pingo, Mackenzie Delta, Canada. Photograph by Charles Harris.
Figure 2 Open-system pingo, Adventdalen, Svalbard. The pingo has a height of 30 m and a diameter of around 200 m. Photograph by Neil Ross.
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If the rate at which expelled porewater accumulating beneath the bottom of the pingo ice core exceeds the downward rate of freezing of the water, then a subpingo water lens develops (Figure 3). In this instance, the hydrostatic pressure is able to support the overburden stress and cause further uplift and deformation of the overlying ice core and frozen overburden (Mackay, 1998). Monitoring of closed-system pingos indicates that high hydrostatic pressures in subpingo water lenses can mechanically fracture the overlying materials, leading to (i) rupturing of the pingo sides, (ii) high-angle faulting of overlying frozen sediments, (iii) formation of surface springs, and (iv) development of winter icings. Closed-system pingos in northwest Canada may exceed 40 m in height, though the vast majority are <20 m high (Mackay, 1998). Basal diameters up to about 250 m have also been recognized. The diameter and plan form of closedsystem pingos depend on the size and shape of the original talik, which is often circular or elliptical beneath thaw ponds, but can be markedly linear beneath elongated large water bodies, leading to the formation of linear, ridge-like pingos (Pissart and French, 1976). Importantly, closed-system pingos have the potential to grow extremely rapidly. Data collected from a pingo in northwest Canada, which began growing soon after 1950, showed rapid initial growth that then slowed (Mackay, 1998). However, Mackay was careful to warn that generalizations on pingo Lake
Permafrost
Unfrozen saturated sands
Permafrost
(a)
Lake drains Residual pond Pore water under hydrostatic pressure due to porewater expulsion
Permafrost aggradation
Permafrost aggradation
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Dilation cracks Subpingo water lens at <0 ⬚
Pingo ice
(c)
Collapsing pingo
Pond
Thawing pingo ice
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Figure 3 Formation of closed-system pingos. (a) Development of a talik beneath a thaw lake. (b) Lake drainage leading to refreezing of the talik and pore water expulsion as the permafrost table advances inwards. (c) Progressive freezing of a pressurized subpingo water lens leading to thickening of the pingo ice core and pingo growth. (d) Pingo collapse due to partial thaw of pingo ice beneath a central pond. Reproduced from Mackay JR (1998) Pingo growth and collapse, Tuktoyaktuk Peninsula area, Western Arctic Coast, Canada: A long term field study. Ge´ographie Physique et Quaternaire 52(3): 271–323, with permission.
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growth rates based on this sole example were probably inappropriate. Only some of the pingos monitored in his studies showed a progressive uplift: some pingos ruptured, causing water loss from subpingo water lenses and subsequent subsidence, while others showed up-and-down oscillations (Mackay, 1998).
Open-system pingo formation In contrast to the closed-system type, open-system pingos are most common in areas of marked relief, often in lower valley sides or in valley bottoms (Figure 2). The first detailed description of open-system pingos was by Mu¨ller in East Greenland (Mu¨ller, 1959), but long-term studies of open-system pingo growth and decay have not been undertaken. Much less is therefore known, from direct observations, about their origins and internal structure than for closed-system forms. Many open-system pingos occur in areas with thin and/or discontinuous permafrost. They have been described in central Alaska (Holmes et al., 1968), the Yukon Territory, Canada, the Tibetan Plateau, and central Yakutia, Russia. However, they are also found in areas of continuous permafrost, and have been recorded in Greenland (Mu¨ller, 1959; Worsley and Gurney, 1996) and Svalbard (Liestøl, 1977). Open-system pingos develop within a range of superficial sediments, including fluvial and slope deposits, and some even develop in weathered bedrock (Holmes et al., 1968; Liestøl, 1977; Mu¨ller, 1959). Open-system pingos are often somewhat smaller than their closed-system counterparts and are reported as single, isolated features and in clusters or groups of mutually interfering landforms of different ages. The most widely cited model of open-system pingo formation is that of Holmes et al. (1968), who observed that nearly all pingos of Central Alaska lie on gentle to moderate slopes near the slope base in minor valleys and in gently sloping valley-bottom locations. Their preferential development on south- and southeast-facing slopes was attributed to thinner permafrost on slopes with these aspects. Holmes et al. (1968) suggested that local groundwater flow was crucial in determining the location and likelihood of open-system pingo development (Figure 4). Pressurized groundwater at intermediate depths beneath the valley sides has a tendency to flow toward the ground surface within subpermafrost unfrozen sediments or through bedrock fractures. Groundwater flow is restricted, however, by the base of the overlying permafrost, which acts like an aquiclude, confining upward flow (Figure 4). But near the base of the slopes – where the artesian pressure of groundwater sourced from the more elevated parts of the topography is greatest, and where thicker permafrost below the valley floor blocks the lateral flow of subpermafrost groundwater – water is forced upwards toward the ground surface. This water will either freeze in the near-surface to form a pingo, or – if it is too warm or the permafrost is locally absent – it will issue from the ground surface as a spring. Whether one or the other develops appears to be a very delicate balance of local thermal and geological conditions: Holmes et al. (1968) reported active and apparently perennial springs in association with some open-system pingos, a feature also characteristic of opensystem pingos in Greenland and Svalbard. In Eastern Greenland, open-system pingos are formed in glaciogenic materials, alluvial sediments, and bedrock in the
continuous permafrost zone, where mean annual air temperature is around 10 C and permafrost is recorded to a depth of 220 m. Here, the hydraulic system leading to pingo growth is probably fed, to some degree, by deep intra- or subpermafrost groundwater (Mu¨ller, 1959; Worsley and Gurney, 1996), rather than by significant amounts of recharge from seasonal surface melt. The postulated deep origins for these waters is supported by measurements of warm (>0 C) and highly mineralized water discharging from perennial groundwater springs close to pingos in northeast Greenland; in winter, the springs form large surface icings. These characteristics suggest geothermal warming of ground waters, perhaps as they percolate along structurally controlled seepage zones and through taliks determined by zones of high geothermal heat flow (Worsley and Gurney, 1996). Interestingly, some Greenland pingos occur in localized clusters. This tendency has been attributed not only to the importance of regional hydrogeology in determining localities suitable for pingo development, but also to the evolution of very localized groundwater pathways through time. On Svalbard, open-system pingos are also common in valleys. A simple but elegant model that suggested a close link between the pingos and polythermal glaciers on Svalbard was proposed by Liestøl (1977). Although permafrost is thick (100–200 m) and continuous beneath the inland parts of Svalbard, Liestøl (1977) recognized that it thins rapidly beneath the margins of the polythermal glaciers, and is probably entirely absent beneath the thicker warm-based ice beneath glacier accumulation areas. Subglacial meltwaters are therefore able to recharge groundwater where permeable strata underlie the warm-based parts of these glaciers. This provides a major source of meltwater to maintain (and pressurize) the groundwater throughout a significant part of the year, despite the continuous nature of the surrounding permafrost. Beyond the glacier margins – in a system akin to that responsible for the development of Alaskan open-system pingos (Figure 4) – this glacier-sourced groundwater is able to migrate through confined subpermafrost aquifers, within which considerable artesian pressures can be generated. In large, broad valleys such as Reindalen (Figure 5), this artesian pressure is sufficient for groundwaters to reach the near-surface, where freezing can generate open-system pingo growth. As suggested for Greenland, groundwater flow paths in Svalbard may also be influenced by the geological structure and geothermal heat. Deep percolation and geothermal heating of groundwater are indicated by the warm surface springs in parts of western Svalbard (Liestøl, 1977). Many pingos in Svalbard also show surface discharge of water (Figure 2) whose elevated salt content significantly lowers the freezing point. Other similarities may exist between the open-system pingos of Greenland and Svalbard, inasmuch as deep groundwaters in Greenland are also likely to have their initial origins in glacial meltwaters, probably from the Greenland Ice Sheet. A subtype of open-system pingo has been identified in lowlying areas below the maximum Holocene sea level in Svalbard and Greenland. These landforms presumably date from the late to mid-Holocene and formed where, following relative sea-level fall, permafrost has aggraded in newly exposed and highly frost-susceptible fine-grained marine muds that favor
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Permafrost Injection ice
Active layer
Subpermafrost groundwater flow (a)
Tension cracks Mass movement on pingo sides
Pingo ice core
(b)
Degrading pingo
Pond
(c)
Figure 4 Cycle of formation and decay of open-system pingos. (a) Percolation of subpermafrost groundwater within a confined aquifer and injection into thin permafrost, initiating formation of a pingo ice core. (b) Formation of cracks due to tensional stresses developed as the surface is stretched by updoming of pingo center. (c) Development of central crater with pond as the ice core decays. Modified from Holmes GW, Hopkins DM, and Foster HL (1968) Pingos in central Alaska. United States Geological Survey Bulletin 1241-H: H1–H40; Ballantyne CK and Harris C (1994) The Periglaciation of Great Britain. Cambridge, UK: Cambridge University Press.
ice segregation and frost heave (Ross et al., 2007). As a consequence, these pingos may be intermediate between closed- and open-system types. Such near-shore pingos, however, also share many characteristics with lithalsas.
Pingo collapse
Figure 5 Ross.
Open-system pingo, Reindalen, Svalbard. Photograph by Neil
Pingo degradation is normally initiated by the development of radial dilation cracks and concentric cracks caused by tensional stresses arising from bending and extension of the frozen overburden during pingo growth (Figures 4 and 6; Mackay, 1988). Sediment covering the ice core is lost because of slumping and solifluction of the active layer on the steep pingo sides (Babinski, 1982; Mackay, 1988), leading to the exposure of the pingo ice core, which is then highly susceptible to melting in summer months.
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Figure 6 Closed-system pingo in the Mackenzie Delta, Canada, showing development of radial dilation cracks caused by tensional stress generated by upward growth. The exposed ice core is susceptible to thawing, initiating pingo collapse. Photograph by Charles Harris.
Figure 7 Frozen lake within the crater of an open-system pingo, Reindalen, Svalbard. Photograph by Neil Ross.
Once melting of the ice core begins, the landform can be susceptible to an important positive feedback. As the ice core melts, a summit crater is generally formed, surrounded by a complex arrangement of ridges and mounds. These ridges may impound meltwater from the melting ice core, to form a lake. Crater lakes, many with perennial groundwater springs and outflow streams, have been reported from active pingos in Canada (Mackay, 1988), Greenland (Mu¨ller, 1959), Svalbard (Liestøl, 1977), Mongolia (Babinski, 1982), and Alaska (Holmes et al., 1968) (Figure 7). Because of the high thermal conductivity of water, melting of the underlying ice core can occur rapidly beneath these water bodies. The lake will also generally increase the rate at which lateral (thermal) erosion of the surrounding frozen ridge occurs. As degradation progresses, the lake increases in diameter, further increasing (i) the surface area of the ice core subject to melt and (ii) the length of crater-lake shoreline subject to thermal erosion. Some processes, however, slow the rate of degradation. Intermittent, sometimes seasonal, breaching of crater lakes will result in outflow from, and possibly drainage of, the enclosed basin. If a crater-lake drains completely, then the rate of landform degradation may reduce significantly. Inward movement of thawing sediment from the crater rim is also
Figure 8 A collapsed closed-system pingo, Mackenzie Delta area, Canada. The pingo has an external diameter of approximately 250 m. Photograph by Ross Mackay.
possible once degradation is well advanced, and this may also slow the degradation of the ice core by insulating it from warm air temperatures. Complete landform degradation often leaves a pond surrounded by a circular, sometimes ice-cored, ridge of sediment. Although mature collapsed pingos with ramparts have been described from the contemporary permafrost zone (Figure 8; Babinski, 1982; Holmes et al., 1968; Mackay, 1988; Mu¨ller, 1959; Worsley and Gurney, 1996), there have been few investigations of their internal structures. Solifluction and slumping are clearly important processes in their development, as they result in a net transfer of sediment to the base of the mound where it accumulates to form a circular ridge. In addition to mass-movement processes, however, short-lived streamflow events may also play an important role in the redistribution of sediment necessary to form rampart ridges around collapsing pingos. Small-scale outlet channel forms, associated with small alluvial fans, have been observed around the periphery of several collapsing pingos (Mackay, 1988; Mu¨ller, 1959).
Other Frost Mounds In addition to closed- and open-system pingos, arrays of smaller seasonal and perennial ground-ice mounds have been recognized. This section describes seasonal frost mounds, palsas, mineral palsas, and lithalsas. Because seasonal frost mounds and palsas have limited long-term preservation potential, emphasis is given here to mineral palsas and lithalsas, which are more likely to be represented in the Quaternary record.
Seasonal frost mounds Ephemeral frost and icing mounds or blisters (Pollard, 1988), usually less than 5 m high, form in permafrost regions as a result of winter freeze-back of the active layer, trapping suprapermafrost groundwater fed by springs. Mounds are mantled with a thin layer of peat and soil, underlain by mineral soils, and contain bubble-rich or clear ice cores. These are sometimes layered and sometimes arched over a water-filled chamber within which sufficiently high hydrostatic pressures are developed in winter to uplift and deform the frozen overburden. Such landforms have a low preservation potential in the
PERMAFROST AND PERIGLACIAL FEATURES | Frost Mounds: Active and Relict Forms
Figure 9 Palsa mire, northern Finland, with low palsa mounds in the background and a decayed and collapsed mound in the foreground. Photograph by Matti Seppa¨la¨.
geological record and are unlikely to be found, or easily identified, in areas formerly affected by permafrost.
Palsas Palsas are low permafrost mounds, up to around 7 m high and commonly up to 30 m in diameter (Figure 9). Pissart (2002) described palsas as “perennial mounds covered in peat, situated in the discontinuous permafrost zone and due chiefly to segregation ice fed by cryosuction.” Palsas develop in peatland mires in the circumpolar discontinuous or sporadic permafrost zones, and are normally observed in groups or ‘fields’ (Gurney, 2001). They form as a result of differential winter frost penetration. Deeper frost penetration in areas with thinner snow cover leads to frost heave due to ice segregation. The more deeply frozen peat is frost-heaved above the surrounding landscape, and in summer its surface layers dry, forming an effective insulating cover that limits the depth of thaw and preserves a frozen core beneath the palsa. During succeeding winters, snow is blown from the upper surface of the growing palsa but accumulates in the low-lying wetter areas, causing further differential frost penetration, ice segregation, and palsa growth (Gurney, 2001). Palsas eventually decay when erosion of their sides and loss of peat cover from their top cause the frozen core to melt and collapse (Gurney, 2001). This often forms ponds (Figure 9) that are occasionally surrounded by a low ridge. Such ponds, however, rapidly fill with peat and their preservation potential is extremely low. All stages of cyclic palsa evolution (growth, maturity, and collapse) can commonly be observed in the same mire, though there is recent evidence for climatically driven degradation of palsa mires in Scandinavia and North America.
Mineral palsas and lithalsas The frozen core of a palsa may extend through the peat cover into underlying frost-susceptible sediments. These sediments are heaved upwards by the formation of segregation ice, fed by cryosuction, to form a domed mineral core (Gurney, 2001; Pissart, 2002). The peat cover of such ‘mineral palsas’ may be <10 cm thick and they appear to form part of a continuum of landforms caused by local redistribution of winter snow, differential frost penetration, and associated differential heave. At one end member are palsas formed entirely in peat, and at the
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Figure 10 Decaying mineral palsas or lithalsas with central ponds and raised rims, 20 km inland from the eastern coast of Hudson Bay, near Umiujaq, northern Quebec, Canada. The features are developed in marine silts. Photograph by Michel Allard.
other are low (<10 m high) perennial frost mounds developed in sediments with little or no peat cover (Gurney, 2001; Pissart, 2002). To distinguish them from other types of palsa, circular to oval mounds formed in sediments with no peat cover have been named ‘lithalsas’ (Pissart et al., 1998). Mineral palsas and lithalsas occur in extensive contiguous fields, with younger features commonly disturbing older ramparts and depressions (Figure 10). They are particularly common in Scandinavia and North America. Although normally developed in fine-grained lacustrine and marine sediments (Allard et al., 1987; Pissart et al., 1998), they can also form in areas with marine sands, in gravel, till, and stony glaciomarine diamicton (Allard et al., 1987). Besides being associated with low-lying zones of latitudinal permafrost, lithalsas have also been recognized in high-mountain environments. Wu¨nnemann et al. (2008) mapped lithalsas at elevations >4500 m in the Himalayas. Unlike palsas developed entirely in peat, mineral palsas and lithalsas are more likely to generate enclosed rimridges during their collapse (Gurney, 2001; Pissart, 2002). Because they are composed of mineral sediments, these landforms, like collapsed pingos, have much greater preservation potential than palsas.
Mineral palsa and lithalsa collapse The growth of radial and concentric cracks, small-scale landsliding, thaw consolidation, and solifluction have all been reported from mineral palsas and lithalsas (Allard et al., 1987; Calmels et al., 2008a,b). Like the collapse of pingos, the collapse of these smaller landforms results in the development of thermokarst depressions surrounded by ring-ridge ramparts (Figure 10). These are composed of mineral sediments accumulated by mass movement, as well as through lateral compression due to the growth of segregation ice (Calmels and Allard, 2008). There is some evidence for a general trend of mineral palsa and lithalsa degradation in response to twentieth-century warming, although some studies emphasize the influence of local hydrology on their growth or decay. For example, Lewkowicz and Coultish (2004) showed that mineral palsa formation was controlled by the influence of beaver on the
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local environment: frost-mound degradation followed flooding due to dam construction, and growth occurred after dams were breached and the ground surface was exposed. This unusual example clearly shows the influence that local hydrology has on frost-mound development, particularly in the sporadic or discontinuous permafrost zones.
Active frost mounds: a summary Frost-mound growth depends on a complex interplay of processes and environmental conditions. Growth is strongly controlled not just by the presence or absence of permafrost (and hence the climatic regime), but also by the regional and local hydrogeological conditions, glacier meltwaters, geological structure, and the geothermal heat flux. Such complexity must be considered when interpreting Quaternary landforms that have all the hallmarks of relict frost mounds.
Relict Frost Mounds Enclosed depressions surrounded by annular ring-ridges or ramparts have been interpreted as remnant landforms that mark the former location of Quaternary frost mounds. They are frequently found in clusters, and, in Western Europe, are sited on valley bottoms, at slope–valley interfaces, and in some plateau locations. Various terms have been introduced to describe these relict features. Given the complexities of interpretation, however, terms such as ‘relict frost mound’ or ‘ramparted depression’ are preferred to those with genetic connotations such as ‘pingo scar.’ Useful reviews of ramparted ground-ice depression sites of Pleistocene age from northwest Europe and North America are given by Flemal (1976) and Pissart (2003). In Quaternary ramparted depressions, the ramparts may contain overturned, deformed, and tilted stratified sediments overlain by mass-movement deposits showing crude radially dipping stratification (Pissart, 2000; Ross et al., 2011). The presence of these structures often provides important evidence for an origin related to the decay of ground-ice mounds. The classic site where Pleistocene ramparted depressions were first detailed is on the Hautes Fagnes Plateau, Belgium. Here, enclosed, peat-filled depressions surrounded by distinct ramparts are widespread. The ramparts are commonly less than 1 m high, but some reach a height of 5 m (Pissart, 2000, 2003), and the thickness of basin infill varies between 1 and 7.5 m, with diameters of up to several hundred meters. These features were first interpreted as the remains of open-system pingos of the Younger Dryas age, dated by radiocarbon, tephra, and pollen stratigraphy methods (Pissart, 2000, 2003). The absence of an obvious aquifer to supply groundwater subsequently led Pissart to conclude that segregation ice rather than injection ice was responsible for the development of these landforms, and that they represent former lithalsas rather than open-system pingos. Their ramparts are thought to have formed by lateral compression and accumulation of slumped sediment (Pissart, 2000). In Scandinavia and the British Isles, relict ramparted depressions (Figure 11) formerly interpreted as open-system pingos have also been reinterpreted as lithalsa remnants (Pissart, 2000, 2003). Pissart (2002) argued that the known spatial characteristics of contemporary pingos (widely spaced,
Figure 11 Relict ramparted depression thought to mark the former location of a Quaternary frost mound, Llanpumsaint, Wales, UK. Photograph by Neil Ross.
low number per unit area) and lithalsas (groups of closely spaced landforms, often very numerous, with a high concentration per unit area) allow simple interpretation of relict Pleistocene landforms. Interpretation of ramparted depressions becomes more complicated, however, when it is accepted that in past and present glaciated regions burial and subsequent melting of marginal glacier ice can form landforms topographically similar to those produced by the collapse of frost mounds. Extensive areas with ramparted depressions in the northern North American Prairies have long been interpreted not as ground-ice-related phenomena but as the product of subglacial and supraglacial processes (Flemal, 1976; Iannicelli, 2003; Mollard, 2000). Similar interpretations have been proposed in Scandinavia. Icebergs, grounded by lake drainage, have also been used to explain ring-ridges (Mollard, 2000). Therefore, while many European researchers have favored a periglacial origin for ramparted depressions, the North American literature tends to favor a glacial origin (Flemal, 1976). Contemporary studies in permafrost regions aimed at assessing the processes of pingo decay and collapse are therefore critical for the accurate identification and classification of relict Pleistocene ramparted ground-ice depressions. Recent studies of ramparted depression in Europe have been more aware of the risks associated with assuming that ramparted depressions represent a relict permafrost landform and have also assessed potential glacial origins (Ross et al., 2011). Even when detailed sedimentological and geophysical data are available from particular sites, however, precise interpretation is frequently difficult, given (i) the range of frost-mound types identified in modern cold environments and (ii) the array of glacial processes that can lead to the burial and melt-out of glacier ice, and therefore the formation of topographic depressions with enclosing ridges or ramparts.
Recent Developments in Frost-Mound Research Several key factors have limited our understanding of frostmound-related processes. These are (i) a lack of natural exposure (of both the internal structures of frost mounds and the surrounding host material), (ii) the cost and logistical challenges associated with creating artificial exposures (e.g., by drilling) in remote high-latitude permafrost environments, (iii) too great a focus on site-specific studies, and (iv) a lack of long-term monitoring. This section outlines a series of recent studies that have begun to address these research shortcomings. To overcome the lack of natural exposure of frost-mound internal structure, and the costs associated with drilling, several recent studies have applied geophysical tools (ground penetrating radar, electrical resistivity, seismic, and electromagnetic
PERMAFROST AND PERIGLACIAL FEATURES | Frost Mounds: Active and Relict Forms
techniques) to image the internal structures of frost mounds (Fortier et al., 2008; Ross et al., 2007; Yoshikawa et al., 2006). Because of the contrasting physical properties of unfrozen water, ice, and host geological materials, these techniques offer significant potential for characterizing the internal structures, ice content, and host materials of frost mounds, which can then be used to infer the likely mechanisms of formation. The most comprehensive of these studies is that of Yoshikawa et al. (2006), who integrated a suite of ground and airborne geophysical data with borehole data to constrain the internal structures of open-system pingos in Alaska. These authors showed that two-dimensional (2D) electrical resistivity tomography seems to be the most successful single technique for imaging pingo internal structures (although it is best combined with others). Because pure ice is highly resistive, electrical resistivity is normally extremely useful for delineating the massive ice core in pingos; values of >10 000 O m are common for pingo cores (Yoshikawa et al., 2006). Ross et al. (2007) have shown, however, that the response of the pingo core can be highly dependent on the host material and the type of ice (injection or segregation ice). Near-shore open-system pingos in Svalbard are characterized by very low values of resistivity (<2000 O m) for permanently frozen ground. These results were attributed to pingo growth by segregation-ice development within fine-grained, saline marine clays (Ross et al., 2007), a mechanism not dissimilar to that by which lithalsas form. Research into present-day frost mounds has tended to be rather focused on site-specific investigations and has, arguably, ignored the ‘bigger picture.’ Small-scale Geographic Information System (GIS) and remote-sensing studies of high-latitude environments show considerable potential for addressing this shortcoming and for contributing toward our understanding of frost-mound development. By far, the best example of this is the study of Grosse and Jones (2011), who compiled a geodatabase of over 6000 pingos from openly accessible Russian map and remote-sensing datasets, and used GIS tools to analyze the spatial distribution of these landforms across Northern Asia. By integrating open-access permafrost, geological, climatological, hydrological, and elevation datasets, they were able to (i) quantify the key factors for pingo development across northern Asia (near-surface geology and hydrology), (ii) delimit the range of mean annual ground and air temperatures that the pingos in the region of study were found in, (iii) propose that 34% of the pingos studied are located in areas where climate models predict permafrost thaw by 2100, and which may initiate pingo degradation, and (iv) provide an upto-date figure for the number of pingos on Earth (>11 000). Thanks to the work of Mackay (1998), closed-system pingos have been the subject of long-term monitoring studies, revealing key data concerning both internal structures and the way in which this type of frost mound forms and degrades. Until recently, however, comparable monitoring studies had not been undertaken on other types of frost mound. In 2000, a long-term monitoring and investigative study of a lithalsa was initiated in Quebec (Calmels et al., 2008a,b and references therein). During the period of monitoring (2000–2005), the lithalsa showed a pattern of long-term degradation, including the initial stages of rim-ridge formation (Calmels et al., 2008a). Tomodensitometric scanning of cores from the monitored lithalsa revealed unparalleled information regarding the ice
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and gas content within the landform, while ground temperature measurements were used to infer the processes of groundwater migration and ice-lens growth (Calmels et al., 2008b). Longterm monitoring studies such as this provide critical information, and if we are ever to fully understand the mechanisms that drive the evolution of other frost mounds (e.g., open-system pingos), it will only be achieved through systematic investigations similar to those undertaken by Calmels et al. (2008a,b) on lithalsas. The quantitative studies outlined above demonstrate that frost-mound research has clearly explored new and exciting avenues in recent years, not just by embracing and applying modern and novel techniques (geophysics and GIS) but also by doing the ‘basics’ correctly (through the setting up of longterm monitoring programs). These studies have provided important information regarding the processes responsible for the growth and collapse of modern-day frost mounds, information that is critical for making accurate interpretations of the likely mechanisms responsible for the formation of Quaternary ramparted depressions.
See also: Permafrost and Periglacial Features: Permafrost; Thermokarst Topography.
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