Chapter 15. Periglacial Processes in Glacial Environments

Home , Cryoturbation, Frost heaving, Gelifluction, Ice cap, Ice house (building), Ice lens

CHAPTER PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS 15 W. Pollard McGill University, Montreal, QC, Canada

15.1 INTRODUCTION The geologic record contains evidence of a planetary history that has seen the Earth’s climate fluc- tuate on geological time scales between conditions that were much warmer than present and other periods when it was considerably colder. These fluctuations between ‘greenhouse’ and ‘icehouse’ conditions are the result of complex global and astronomical processes. During an icehouse interval a combination of astronomical, tectonic, and geochemical events lead to global cooling and a subsequent accumulation of ice on land and in the oceans at higher latitudes and altitudes. These factors and their feedback systems exist in a complex cause and effect relationship that remains poorly understood. As highlighted throughout this volume an icehouse Earth is characterized by continental-scale ice sheets, ice caps, valley glaciers, and ice shelves that advance and retreat in cycles known as glacial and interglacial periods. However, during an icehouse regime cold temperatures extend well beyond the geographic limits of glaciation to create a periglacial zone dominated by frost action, frozen ground, snow, and various forms of nonglacial ice. Given that many of the criteria used to define ice house conditions are currently present, such as large ice sheets and widespread glacial activity, together with the fact that the Earth recently experienced full glacial conditions it can be concluded that the Earth is still under the influence of an ice house regime. In addition to glaciers and ice sheets, other conditions that define an icehouse regime such as widespread sea ice, seasonal/perennial snow cover, and frozen ground (permafrost and ground ice) are also currently active. The area of the Earth’s surface where water persists in a frozen state is the called the cryosphere (Barry and Yew Gan, 2011). The cryosphere includes all aspects of Earth’s environment dominated by cold climate (both seasonal and perennial) and include places where snow, lake and river ice, sea ice, glaciers, ice caps, ice sheets, ice shelves, and icebergs as well as the various types of cryotic ground (Fig. 15.1). The cryosphere concept provides a useful perspective because it includes both glacial and periglacial (cold nonglacial) systems. This chapter focuses on geomorphic processes and features that characterize areas dominated by periglacial conditions and their relationship with glacial environments. Fig. 15.1 shows the current extent of the Earth’s cryosphere and highlights relationships between areas dominated by ice sheets/glaciers and periglacial processes linked to permafrost, frost action, and snow cover. The following discussion focuses on geomorphic features associated with the periglacial zone with an emphasis on their relationship to past and present glacial systems.

FIGURE 15.1 Map of the global cryosphere showing the distribution of glaciers and ice sheets, ice shelves, sea ice, permafrost, and snow. Based on information from the World Meteorological Organization.

15.2 COLD NONGLACIAL ENVIRONMENTS The cryosphere is not only distinguished by the presence of various forms of ice and snow but also by processes and landforms related to cold subfreezing (cryotic) temperatures. The global pattern of climate tends to reflect a strong zonal bias driven by the latitudinal variation in insolation. The negative radiation balance of the Earth’s higher latitudes helps sustain air temperatures conducive to the freezing of free water (frost action). Since geomorphology is concerned with the study of landforms, landscapes, and their genetic processes, it follows that cryospheric geomorphology can be divided into two broad categories; glacial and periglacial geomorphology. As discussed throughout this volume the geomorphology of glacial and glaciated landscapes reflects the processes and landforms directly related to the action of ice sheets and glaciers. Glacial geomorphology is a complex science focusing on the dynamics of flowing ice masses and their ability to erode, transport, and deposit rock and sediments as well as processes and landforms not directly related to the action of glaciers like glacial fluvial and glacial lacustrine activity. Some cryospheric landscapes owe their 15.2 COLD NONGLACIAL ENVIRONMENTS 539

origin to cold nonglacial conditions and processes; for example, frozen ground and frost action linked to cryotic temperatures and freezing soil moisture and groundwater. Under full glacial conditions a proglacial belt of cold nonglacial conditions will parallel the limit of active glaciation. In some cases these ice-free areas may be completely surrounded by ice (e.g., Beringia); however, most of the time it forms a broad zone that transitions from intense cold and frozen ground to progressively more temperate environments where frost action and permafrost are replaced by seasonal frost and snow. In addition to subfreezing (cryotic) conditions the ‘periglacial zone’ may also be extremely dry and prone to desert conditions that subsequently influence patterns of vegetation and geomorphic processes. Areas adjacent to large ice sheets experience dry gravity (katabatic) winds flowing off the ice sheet that drive various aeolian processes and erosion. As glacier ice retreats the belt of cold periglacial conditions tends to shift with it. As Laurentide Ice disappeared from continental North America it was replaced by a zone of widespread permafrost. Fresh glacial sediments and drift-covered landscapes are susceptible to rapid change linked to slope process, glacial meltwater, frost action, and wind erosion. As ice sheets retreated from their last maximum position large amounts of glacigenic material were and continue to be reworked and redeposited. This area of accelerated geomorphic activity is termed ‘paraglacial’ (Ryder, 1971a). Paraglacial and proglacial are complementary terms that refer to areas adjacent to active and retreating glacial ice, whereas the term periglacial refers specifically to cold nonglacial conditions (French, 2007). Hence the presence of glaciers is not a prerequisite for periglacial conditions although periglacial conditions occur adjacent to glacial systems due to the pervasive cold that drives both systems.

15.2.1 PERIGLACIAL ENVIRONMENTS The term ‘periglacial’ describes terrain conditions, geomorphic processes and landforms that result from climates subject to prolonged and intense freezing conditions irrespective of proximity to glaciers. Periglacial environments are areas where landforms and geomorphic processes reflect the cumulative effects of cold subfreezing temperatures, cyclic freezing and thawing of sediments, and the volumetric expansion of soil moisture as it freezes. The defining criteria for periglacial environments include: (1) intense frost action, and/or (2) the presence of permafrost. Today, periglacial conditions affect up to 35% of the Earth’s land area and have its greatest presence in the northern hemisphere (French, 2007; Williams and Smith, 1989). The term periglacial was proposed by the Polish geologist Walery von Lozinski in 1909 to describe weathering processes responsible for the widespread shattered rock surfaces in the Carpathian Mountains (French, 1996). Lozinski also introduced the concept of a ‘periglacial zone’ to describe climatic and geomorphic regimes peripheral to the Pleistocene ice sheets (Washburn, 1973). The periglacial concept has gone through a series of contextual changes; originally the periglacial concept was firmly rooted in climatic geomorphology and was constrained by geographic proximity to glaciers and ice sheets. Climatic geomorphology was popular in the 1930s and 1940s and equated landscapes with climate. It was predicated on the idea that climate regime, mainly seasonal patterns and extremes in temperature and precipitation, control the geomorphic nature and intensity of process, which in turn control landform development. Even though climate remains a defining variable, the current focus of periglacial geomorphology is on the mechanics of heat flow, ice formation, the properties of ice, freezing and thawing, and the dynamic interaction between these processes and various distinctive landforms. Thorn (1992) unsuccessfully tried to 540 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

equate the term periglacial with processes and landforms associated specifically with ground ice. However, the periglacial environment remains synonymous with areas characterized by frost action and/or permafrost (French, 2007). Subtle differences in the meaning of the terms frozen (i.e., the solid phase of water) and cryotic (temperatures below 0C) have led to the popular use of the term geocryology (Washburn, 1979; Williams and Smith, 1989; Yershov, 1998). There is considerable conceptual overlap between the terms permafrost and geocryology in that both focus on the frozen and cryotic condition of earth materials.

15.2.2 PARAGLACIAL ENVIRONMENTS Paraglacial geomorphology is ‘the study of earth-surface processes, sediments, landforms, landsystems and landscapes that are directly conditioned by former glaciation and deglaciation’ (Ballantyne, 2002, p. 1935).The term paraglacial was formally introduced by Ryder (1971a, 1971b) to characterize the rapid changes that took place in glacial sediments as the Wisconsin ice sheet retreated. Church and Ryder (1972, p. 3059) formalized the paraglacial concept by defining it as ‘non-glacial processes that are directly conditioned by glaciation’ and ‘refers to both proglacial processes and to those occurring around and within the margins of a former glacier that are the direct result of the former presence of ice’. Ryder and Church focused mainly on the reworking of glacial drift by fluvial processes following deglaciation and its contribution to alluvial fan structure and morphology. Ballantyne’s (2002) comprehensive review identifies six key ‘paraglacial landsystems’, including: rock slopes, drift-covered slopes, glacial forelands, and alluvial, lacustrine, and coastal systems. Ballantyne’s review highlights the complex nature of paraglacial systems where changing energetics and levels of instability lead to geomorphic disequilibrium and adjustment through erosion and deposition. The nature and significance of periglacial activity on proglacial and paraglacial geomorphic systems depends of several variables, however the role of climate as a driver of both deglaciation and ground freezing processes is most important. Climatic drivers such as the seasonal patterns and extremes in temperature, maritime versus continental influences, and climate change vary in both space and time. This chapter discusses several periglacial processes that contribute to the evolution of both proglacial and paraglacial landscapes.

15.3 FROST ACTION Frost action refers to the direct effects of freezing and thawing on earth materials; it is a collective term that includes frost heave, frost wedging, frost sorting, and frost shatter. An essential component of frost action is the change in phase of water in soils and rock from a liquid to a solid and the corresponding 9% increase in volume as water turns to ice or the corresponding volume loss as ice melts. The movement of water and the development of cryostatic pressure during freezing also play a dynamic role in heave-based types of frost action. Frost action is the most widespread periglacial process and can occur any place that experiences freezing soil temperatures (note that in this case freezing refers to the temperature required for the formation of ice which may occur below 0C). The effects of frost action range from the formation of needle ice at or near the ground surface in areas that experience occasional surface freezing to frost heave in areas of deep-seated freezing and permafrost in areas where mean annual ground surface temperatures remain below 15.3 FROST ACTION 541

0C. In areas characterized by bedrock and boulder-rich sediments (till-like) frost action leads to the mechanical disintegration and fracture of intact rock. The frequency and magnitude of frost processes are a function of the air temperature, soil texture, depth of frost penetration, and the number of freezeÀthaw cycles. Frequent freezing and thawing, sometimes on a diurnal scale, can lead to intense frost action. It follows that areas characterized by seasonal frost processes tend to experience the greatest frost action in the fall and late spring; whereas areas underlain by permafrost may experience frost action anytime during the summer. Technically, frost action has two stages linked to the freezing of soil water and melting of soil water (including water in rock). In periglacial environments frost action becomes critical when either (1) the freezing phase is accompanied by noticeable heaving of the ground surface and large clasts, or (2) the thawing phase is accompanied by a noticeable softening of the ground and sometimes liquefaction. The formation of ice during freezing produces heave roughly equal to 9% of the volume of water. Different soil textures and the uneven distribution of soil water as well as the movement of soil water as soils freeze (cryostatic pressure, capillary processes) cause differential frost heaving and the preferential heaving of objects imbedded in the soil. Fine-grained soils may also move during the thawing phase, which together with the heave effects during freezing produces a convection-like system that contributes to the formation of various forms of patterned ground, as well as the downslope creep of sediments on slopes. In bedrock frost action can expand existing joints and fractures, fracture intact rock (frost shatter), and vertically heave rock fragments, a process referred to as ‘frost jacking’. Even though frost action is an important component of the periglacial activity in soils proximal to active glaciers and ice sheets, periglacial and glacial activity are not mutually inclusive, thus periglacial processes like frost action also occur in the absence of glaciers. Fine-grained sediments like those associated with glacial drift, marine and lacustrine deposition, and loess are particularly susceptible to ice lens formation and frost heave. It therefore should not be a surprise that frost action plays a dynamic role in reshaping paraglacial and proglacial areas.

15.3.1 FROST HEAVING AND FROST SORTING Frost heave is the upward displacement of soil and rock due to frost action; it occurs where the climate is sufficiently cold to allow freezing temperatures to propagate below the ground surface, where there is an adequate supply of soil water to feed ice lens formation and where the soils are frost-susceptible (Washburn, 1956, 1973). Soils that are considered frost-susceptible have fine- grained textures with high silt content (e.g., silty-clays, silts, and silty-fine sand). Frost heaving occurs due to the formation of ice lenses in the soil column and selectively below larger clasts. The nature and size of an ice lens vary with two variables; first, the quantity of free water available in the soil matrix; and second, the rate of freezing. As frost penetrates the soil profile free water in pore spaces turns to ice and expands, if the soil is saturated frost heave will be proportional to 9% of the volume of pore space. If freezing is rapid the pore water forms ice crystals in the pore spaces and ice coating on individual particles, this is called pore ice. If the rate of freezing is slow soil water is drawn to the freezing front and a horizontal lens of ice will tend to form. Once initiated an ice lens will continue to grow as long as a supply of free water is available. Water migrates from adjacent unfrozen soil toward the ice lens by capillary action. The distance water migrates can be significant; under ideal conditions water may migrate up to 1À2 m. This type of enhanced ice formation is called ice segregation and can result in the formation of not just ice lenses but thick 542 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

layers of nearly pure ice. In this case the volume of ice can be several times the volume of the available pore space; the volume of ice in excess of saturation is called excess ice. This pattern of ice segregation can result in tremendous frost heave. Silty soils are frost-susceptible because their small particle size and high porosity support high moisture contents, while their permeable nature and high hydraulic conductivity encourage capillary action. Thus soils with high silt content pro- mote the formation of segregated ice lenses and frost heaving. Till tends to be frost-susceptible because of its high silt and silty-clay content. Frost-jacking refers to a frost heave process involving the upward displacement of an object embedded in a freezing soil. When heaving occurs as described above an object or structure imbedded in the ground that is not properly anchored will be forced upward when an ice lens preferen- tially forms beneath the object. In most cases the structure does not return to its original position when the frozen soil seasonally thaws. The net upward movement is called ‘jacking’. This phenomenon can occur wherever there is seasonal freezing and thawing of a surficial layer and is not limited to permafrost areas. Wherever frost heave occurs settlement occurs during the thawing phase of the cycle. The ground surface as well as objects or structures built on ‘thaw-unstable’ permafrost will settle if excess ice is melted. Melting is typically caused by heat from the structure or changes to the natural thermal conditions. The cyclic preferential heave during freezing of larger clasts in a heterogeneous soil and the fluidization of the fine-grained matrix results in sorting of fine and coarse materials. The mixing and sorting of soils by frost action is a very dynamic process and widely observed in periglacial environments as patterned ground.

15.3.2 PATTERNED GROUND Patterned ground is a periglacial phenomenon often found in proglacial and paraglacial settings (Washburn, 1979; 1997). The Glossary of Permafrost and Related Ground-Ice Terms defines patterned ground as ‘any ground surface exhibiting a discernibly ordered, more-or-less symmetrical, morphological patterns of ground and, where present, vegetation’ (Permafrost Subcommittee, 1988, p. 61). Technically this also includes ground patterning in nonperiglacial environments. Periglacial patterned ground can be grouped into two categories based on the mechanics of formation and scale of patterning. The first includes small-scale geometric patterns produced by cryoturbation. Cryoturbation is the vertical and horizontal displacement of soils, sediments, and large clasts within the active layer associated with the complex interaction of soil water and soil particles during repeated freezing and thawing. The genetic use of the term patterned ground is reserved for this category of landforms. The second category includes macroscale geometric patterns produced by thermal contraction cracking in continuous permafrost and the formation of networks of intersecting vertically oriented ice veins called ice wedges (see Section 15.6). Patterned ground associated with cryoturbation is common in regions of intensive frost action and occurs in both permafrost and areas of deep seasonal frost. In permafrost regions it is limited to the active layer. Patterns include sorted and nonsorted circles, polygons, nets, stripes, and steps. The shape of the pattern occurs either as patches of frost-heaved bare soil surrounded by vegetation, or as sorted arrangements of coarse and fine mineral sediment. Patterns may occur as single cells or as groups of cells. Cryoturbation involves several processes, including: frost heaving and up-freezing, frost jacking, cryostatic pressure, and thaw settlement. Soil water and its redistribution during freezing and thawing, the formation of ice lenses, and the tendency for different particle sizes to heave at 15.3 FROST ACTION 543

FIGURE 15.2 Examples of a frost heaving and frost sorting; (top left) shows a frost heaved cobble, note the sediment draping the top of the cobble; (top right) is a mud boil or nonsorted circle and (bottom left and right) are sorted circles. different rates underlies most of these processes (Hallet, 1990). The presence of frost-susceptible soils plays an important role in the redistribution of soil moisture during freezing and segregated ice lens formation. Sorted and nonsorted circles, polygons, nets occur predominantly on flat surfaces (,3 degrees slope) and typically have diameters from 0.5 to 3.0 m. Cell shapes tend to become elongated at gradients between 2 and 7 degrees (Washburn, 1956); on progressively steeper slopes patterns trend toward sorted and nonsorted steps and stripes. Frost hummocks and mud boils are forms of nonsorted circles. Several contemporary patterned ground models invoke a convection cell mechanics as part of the formational process. Patterned ground is common in most periglacial environments, including areas of present and past glacial activity. The heterogeneous nature of glacial till, including the presence of coarse clasts in a fine-grained matrix is susceptible to processes leading to patterned ground formation (Fig. 15.2).

15.3.3 SOLIFLUCTION AND GELIFLUCTION In addition to the linear forms of patterned ground (e.g., sorted and nonsorted stripes) frost action on slopes also results in lobate forms that slowly migrate downslope, a process called solifluction 544 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

FIGURE 15.3 An example of two forms of frost heave processes on slopes; including a solifluction lobe in a glacial foreland on Axel Heiberg island (left) and fossil stone stripes in the Richardson Mountains, Yukon Territory (right). The latter occur in an unglaciated area and reflect cold nonglacial conditions.

(Fig. 15.3). Solifluction is a form of slow, creep-based mass movement of waterlogged soils that reflects the combined effect of gravity and freezeÀthaw activity on gentle to moderate slopes in periglacial environments. Repeated freezeÀthaw of slope sediments results in the incremental downslope displacement of materials by a process called frost creep. Solifluction is a common creep-based mass wasting process; the role of freezeÀthaw or the presence of frozen soil was not part of the original solifluction definition (van Everdingen, 1998). Accordingly, Washburn (1979) proposed that the term gelifluction be used to distinguish forms of solifluction unique to areas of frozen ground. In both cases soil moisture and soil texture are deemed to be important controlling variables in that creep will occur when moisture contents equal or exceed the Atterberg Liquid Limit of the affected sediments. Solifluction occurs widely on slopes in periglacial environments, including areas adjacent to active glacial activity.

15.3.4 WEATHERING IN COLD CLIMATES The term weathering refers to the in situ disintegration and decomposition of rock material and sediments. It is widely suggested that weathering regimes in areas dominated by cold climate are predominantly mechanical in nature and that the cold temperatures which drive mechanical processes related to frost action effectively limit chemical weathering (Hall et al., 2002). This assumption is supported by minimal soil development and the prevalence of angular weathered debris, block fields, and shattered bedrock and boulders that are often found in periglacial environments. However, Hall et al. (2002) argue that moisture availability rather than cold temperature is the limiting condition governing chemical weathering; although the distinction may be semantics given that extreme cold limits the availability of liquid water. The paucity of liquid water also restricts biological weathering processes. The impact of weathering processes in periglacial environments is largely a reflection of geology, time, event frequency, and magnitude, and the synergis- tic relationship between mechanical, chemical, and biological weathering processes. 15.4 PERMAFROST 545

FIGURE 15.4 Examples of frost-weathered bedrock and boulders. The photo on the left shows a block field in the northern Yukon Territory consisting of frost-weathered bedrock. The photo on the right shows a frost-weathered (shattered) boulder from the Untersee area of Queen Maud Land, Antarctica.

Frost-wedging is a form of mechanical weathering caused by repeated freezeÀthaw of water contained in cracks in large boulders and bedrock. Most rocks have microfractures and small cracks called joints. Water seeps into these microfractures and joints and as temperatures fall below freezing, the water inside the joints exerts significant pressure on the rock walls through a combination of ice crystal growth and hydraulic action. When the pressure exceeds the tensile strength of the rock material the joint or crack expands. In some cases, the rock will split, though this usually requires repeated freeze and thaw. With each cycle of frost wedging the joints are spread further apart. Ice segregation also contributes to this process. Another frost action process referred to as frost cracking involves thermal contraction cracking due to high tensile stress associated with thermal expansion and contraction of frozen soil and bedrock. In bedrock and boulders rapid changes in temperature may produce fractures and joint expansion through thermal shock. It was once thought that the cyclic freezing and thawing of solid rock resulted in a weathering process called frost shatter. The extensive rock rubble surfaces of many periglacial regions were deemed as evidence of such a process. Experimental evidence has subsequently shown that pure frost shatter is probably not as common as initially thought and that a combination of hydration shatter, thermal shock, and thermal fatigue is possibly more likely. However, the rapid cooling of the ground may cause thermal stresses that may result in some level of frost cracking (thermal contraction cracking) of frozen ground (Fig. 15.4).

15.4 PERMAFROST The presence of permafrost is the second criterion used to define periglacial conditions (French, 2007). A combination of extremely cold winter temperature and mean annual ground surface temperature below 0C are essential for the formation and preservation of permafrost. The term 546 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

permafrost refers only to the perennially cryotic (thermal) condition of the ground and is distinguished from seasonal frost by the fact that it persists for periods longer than a single winter season. Permafrost has its greatest vertical and spatial expression in areas where it has persisted for long periods of time, thousands of years and longer. The cold climate that promotes glacial activity is also responsible for the formation of permafrost. At the last glacial maximum a band of proglacial permafrost extended considerable distances south of the glacial limit; it also existed in the ice-free areas called refugia (e.g., the Bering refugium) and exposed continental shelf areas of the Arctic Ocean due to lower sea levels. Since both glacial and periglacial activity are closely linked to cold climate conditions the spatial pattern of Pleistocene permafrost generally paralleled glacial limits. The relationship between permafrost and glaciers and ice sheets is complicated and is largely a function of the thermal regime of the glacier ice cover and the dynamic nature of glacial limits. In areas of cold-based glacial ice, subglacial permafrost may exist as well as advancing glaciers may override existing permafrost. The Mackenzie Delta areas of Pleistocene permafrost were overridden and deformed by Laurentide Ice (Murton et al., 2004). Fig. 15.5 shows the current extent of permafrost conditions in the northern hemisphere.

FIGURE 15.5 Simplified permafrost map of the northern hemisphere showing the distribution of continuous, discontinuous, and subsea permafrost. Based on the International Permafrost Association permafrost map. 15.4 PERMAFROST 547

BOX 15.1 WHAT IS PERMAFROST? Permafrost is defined ‘ground that remains at or below 0C for at least 2 years’ (Permafrost Subcommittee, 1988, p. 63). The term permafrost was introduced in 1943 by S.W. Muller as a contraction for permanently frozen ground (Muller, 1945). The permafrost concept however dates back to observations on frozen ground by early polar explorers; for example in the mid-1800s John Richardson (Franklin Expedition) described frozen ground conditions in the Mackenzie Delta (Mackay, 1979). Around the same time the Russian zoologist Alexander von Middendorf made detailed permafrost temperature measurements in an abandoned well in Yakutsk, Siberia (Yershov, 1998). The nature of permafrost reflects the thermodynamic balance between ground surface temperature and heat flow from the Earth’s interior. Although closely linked to the latitudinal and altitudinal pattern of climate (i.e., mean annual air temperature) the spatial distribution and depth of permafrost also reflects other factors such as snow depth, slope aspect, surface water bodies, and vegetation. Locally, the nature of the ground surface defines the boundary layer conditions that determine the degree to which air temperatures influence ground thermal regimes. Geologic, tectonic, and subsurface hydrologic conditions further influence permafrost conditions. Permafrost is estimated to affect approximately 26% of the Earth’s land surface (Williams and Smith, 1989) and occurs extensively in Arctic and Subarctic regions, including up to 80% of Alaska, 50% of Canada, and 60% of Russia. Ancient permafrost .700,000 years old has been documented in central Yukon, Canada (Froese et al., 2008), .1 million years old in Siberia, Russia (Yershov, 1998), and .2.3 million years old in the upper McMurdo Dry Valleys, Antarctica (Shafer et al., 2000). In North America, permafrost is broadly divided into continuous and discontinuous zones based on its areal extent. The latitudinal zonation of permafrost is mirrored attitudinally with alpine permafrost grading from discontinuous to continuous with increasing elevation. Permafrost depths range from .1000 m (B1450 m in Siberia) to only a few metres near its southern limit. During the summer, ground surface temperatures rise above 0C producing a thin thawed layer above permafrost called the active layer. Active layer depths range from a few decimetres in the high Arctic to more than 2 m in parts of the discontinuous permafrost zone. The seasonal freezing and thawing of the active layer and the seasonal pattern of temperature change in the upper part of permafrost produces a number of very distinctive features unique to Arctic tundra, like patterned ground, gelifluction lobes, active layer detachments, seasonal frost mounds, frost cracks, and tundra tussocks. In areas of continuous permafrost rapid changes in near-surface ground temperatures lead to thermal contraction cracking and the formation of ice wedges. Polygonal patterns of ice wedge troughs are one of the most iconic images of Arctic tundra and continuous permafrost. Although not part of the definition of permafrost the inclusion of ice (ground ice) in its structure is an important component of many permafrost landforms.

15.4.1 THE THERMAL REGIME OF PERMAFROST The thermal regime of the upper part of the lithosphere is controlled by exchanges of heat and moisture between the atmosphere and the ground surface, the thermal properties of ground materials, and heat flow from inside the Earth. The geothermal gradient is the rate of temperature change with increasing depth inside the Earth. It is usually marked by an increase in temperature with depth and is determined by heat flow from the Earth’s interior and the thermal conductivity of crustal materials. The temperature at the ground surface undergoes periodic fluctuations (diurnal, seasonal, annual, and long-term) in response to changes in energy transfers determined by boundary conditions, and the progression of seasons. The propagation of these temperature fluctuations downward depends on the period and amplitude of the temperature wave and the thermal conductivity of ground materials. Diurnal fluctuations in ground surface temperature propagate only a few decimetres, longer-term fluctuations associated with migrating weather systems (cold and warm events) may propagate up to a metre or more, while the annual cycle of temperature produces a seasonal fluctuation about the mean temperature to a depth of 10À20 m. The depth at which the annual temperature wave is completely attenuated is called the depth of zero annual temperature variation; below this depth any 548 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

perturbation in ground temperature is a reflection of either a shift in climate or a change in ground surface conditions. The temperature profile is anchored to the mean annual temperature of the ground surface and gradually warms with depth at a rate determined by the geothermal gradient. The presence of permafrost requires a negative mean annual surface temperature; the depth of permafrost is therefore a function of the severity of the mean surface temperature and the steepness of the geothermal gradient. The upper part of this profile reflects the annual temperature wave which fluctuates about the mean temperature marked by decreasing amplitude. Below the depth of zero annual ground temperature variation permafrost temperatures follow the geothermal gradient (Fig. 15.6). An inter- esting feature of the annual pattern in near-surface temperature is the depth to which warm summer temperatures penetrate. Positive summer temperatures produce thawing of a thin surface layer called the active layer. The active layer experiences an annual cycle of freezing and thawing and thus buffers permafrost from warm summer temperatures. Increasing summer temperatures and/or increased length of the thaw season associated with climate change are resulting in increased active layer depths and destabilization of near-surface permafrost. On the local scale the nature of the ground surface defines the boundary layer conditions that determine the degree to which air temperatures influence ground thermal regimes; this includes slope angle and aspect, surface water (e.g., lakes, rivers, wetlands), vegetation (e.g., tundra, boreal forest), and snow cover. Geologic, tectonic, and subsurface hydrologic conditions further influence permafrost conditions at depth. This complex pattern of energy exchange can result in a highly variable pattern in permafrost temperature, depth, and distribution over relatively short distances, even where climate conditions are relatively uniform. Fig. 15.6 illustrates the pattern of permafrost temperature with depth; because of the shape of the upper part of the pattern this diagram is sometimes referred to as a trumpet diagram.

FIGURE 15.6 A simplified representation of the ground temperature regime in permafrost. Modified from Brown, R.J., 1970. Permafrost in Canada: Its Influences on Northern Development. University of Toronto Press, Toronto, ON, 234 pp (Brown, 1970). 15.4 PERMAFROST 549

15.4.2 PERMAFROST DISTRIBUTION AND ZONATION Permafrost conditions reflect a strong zonal bias characterized by decreasing mean annual air temperatures as a result of increasing latitude (Fig. 15.5) or increasing elevation. The limiting threshold for its occurrence is highly transient such that small changes in either the mean annual air and ground surface temperatures and surface conditions or soil properties can result in its disappearance or its increase in distribution and thickness. The spatial distribution and depth of permafrost on a global scale is a function of regional and global climate. On the local scale, however, the nature of the ground surface, including slope angle and aspect, surface water (e.g., lakes, rivers, wetlands), vegetation (e.g., tundra, boreal forest) and snow cover, define boundary layer conditions that determine the degree to which air temperatures influence the ground thermal regimes. Geologic, tectonic, and subsurface hydrologic conditions may further influence permafrost conditions. As a result, the temperature, depth, and distribution of permafrost can be highly variable over relatively short distances, even where climate conditions are relatively uniform. Permafrost can occur anywhere mean annual air temperatures and surface conditions support subsurface cryotic conditions (van Everdingen, 1998). The closer mean ground surface temperature is to 0C, the less stable are the permafrost conditions. Permafrost is classified based on the three-dimensional extent of cryotic conditions from a shallow patchy distribution near its southern limit to progressively more widespread and deeper conditions with increasing latitude (or elevation), to a situation where permafrost underlies more than 90% of the land surface, sometimes exceeding 500À600 m in depth. Some of the deepest permafrost occurs in central Siberia where it is in excess of a kilometre to the base of permafrost. Continental permafrost is classified as either continuous or discontinuous with the latter subdivided into sporadic and widespread. Other types of permafrost include subsea and relict permafrost. The relationship between permafrost and glacial activity and climate change is complex. Fig. 15.7 is a diagrammatic representation showing the spatial pattern of permafrost in profile.

15.4.2.1 Continuous permafrost The continuous permafrost zone is where cryotic conditions underlie more than 90% of the land surface. The only places where permafrost does not exist is beneath large lakes .2 m deep and large rivers that flow year round. In both cases the positive temperature at the base of the water column extends to a depth proportional to the size and depth of the water body. These patches of unfrozen sediments are called taliks. Taliks may either be limited to a few metres below the water body (known as an open talik) or may penetrate through the permafrost (known as a through talik). Continuous permafrost tends to reflect extremely cold winter temperatures and relatively cool, short summers with a mean annual air temperature below 27C. Continuous permafrost extends to depths of 700 m in North America and 1500 m in Eurasia. The time required for equilibrium continuous permafrost conditions to form is on the order of thousands of years; as such most continuous permafrost is quite old.

15.4.2.2 Discontinuous permafrost Discontinuous permafrost refers to areas that are partially underlain by cryotic ground. Since near its southern limit permafrost is relatively warm and unstable it exists mainly in discrete patches that are tens to hundreds of square metres in extent and only a few to tens of metres 550 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

FIGURE 15.7 A diagrammatic cross-section through the permafrost zone showing the change in pattern from discontinuous to continuous permafrost with increasing latitude and corresponding change in ecozone. Modified from Brown, R.J., 1970. Permafrost in Canada: Its Influences on Northern Development. University of Toronto Press, Toronto, ON, 234 pp. deep. When ,50% of the ground surface is underlain by permafrost it is referred to as sporadic discontinuous permafrost. If cryotic conditions affect between 50% and 90% of the landscape it is termed widespread discontinuous permafrost. In some cases discontinuous permafrost occurs as islands of cryotic soil associated with thick accumulations of peat. The insulating properties of peat help preserve patches of frozen soil which over time may form the core of a peat mound called a palsa. Palsas are one of the few landforms indicative of discontinuous permafrost. Sporadic discontinuous permafrost can be transient in nature; lasting only a few decades and vulnerable to small changes in surface conditions like snow depth and vegetation. Warmer conditions associated with climate change, increased frequency of forest fires, and the northward shift of the tree and shrub line are all expected to push the southern limit of permafrost farther north 15.4 PERMAFROST 551

and transform some continuous permafrost into discontinuous. The end result is expected to be a reduction in the total extent of permafrost and probably periglacial conditions.

15.4.2.3 Subsea permafrost Subsea permafrost exists beneath many shallow continental shelf areas surrounding the Arctic Ocean, including parts of the Eastern Siberian Shelf, Laptev Shelf, Chukchi Shelf, and Beaufort Shelf. Most subsea permafrost is actually terrestrial permafrost that formed during the Pleistocene that has been submerged by rising sea levels. As such it is closely linked to both the glacial and sea-level histories. As rising sea levels gradually shifted conditions from terrestrial to submarine the affected permafrost systems experienced a dramatic shift in thermal and physical conditions that subsequently change its distribution and temperature profile. Areas where mean seabed temperatures remain at or near 0C effectively preserve cryotic conditions despite a general warming of the permafrost. With marine transgression the surface sediments both warm and become saline, the presence of brines results in cryotic but unfrozen conditions for seabed materials.

15.4.3 RELICT PERMAFROST Not all active periglacial systems are in equilibrium with present-day climates, and as climates warm many more areas will be characterized by permafrost temperatures and depths inconsistent with ambient conditions. In some settings (e.g., deep continuous permafrost) it takes several thou- sand years for an equilibrium permafrost temperature profile to stabilize. It follows therefore that any significant change in surface conditions will take a similar amount of time for the new thermal regime to stabilize. Permafrost conditions that reflect different past climatic conditions are generally referred to as relict permafrost. At the last glacial maximum vast stretches of Arctic coastal shelf and ice-free refugia developed deep permafrost and in some cases well-developed ice wedges and extensive ground ice. Current seabed temperatures on the Beaufort Shelf are sufficiently cold to preserve extensive subsea permafrost that could not form under the current conditions. Similarly permafrost in the Dawson city area of central Yukon is much deeper and contains more ground ice than can form under current climate conditions.

15.4.4 THE ACTIVE LAYER The active layer is a thin layer above the permafrost that annually freezes and thaws. Generally, the active layer depth varies very little from 1 year to the next. However, any change in the ground surface conditions (natural or anthropogenic) or in summer climate patterns will change the surface energy balance and the depth of the active layer. Any number of things may produce an increase in active layer depth, most notably: (1) warmer summer temperatures, (2) change in surface moisture conditions, (3) change in or removal of surface vegetation, and (4) removal of a layer of sediment. If the subsequent increase in thaw depth results in melting of ground ice then thermokarst may result. The active layer plays a key role in permafrost systems in that it effectively buffers permafrost warm summer temperatures; since it goes through annual cycles of freezing and thawing it tends to be relatively stable. 552 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

15.5 PLEISTOCENE PERMAFROST During periods of glaciation periglacial activity and permafrost play an important geomorphic role helping to shape landscapes adjacent to large ice sheets. Since their distribution is closely linked to many of the same climatic drivers it follows that the periglacial zone, including permafrost, waxes and wanes in lock step with changing glacial extent. This scenario will have played out during all past glaciations. In addition to the periglacial zone adjacent to major ice sheets, areas of cold-based glacier ice will develop subglacial permafrost (Fig. 15.8). Ice sheet thermal regimes play a significant role in shaping glaciated landscapes and paraglacial characteristics. Debris-rich basal ice is often preserved as ice-rich permafrost following deglaciation. Evidence illustrating the interaction between glacial and periglacial activity can be seen in the periglacial sedimentary record preserved in landforms and structures that formed during the Pleistocene epoch beyond the limit of glaciation. Most areas of relict and subsea permafrost also owe their origin to Pleistocene periglacial and glacial activity. To some degree the distribution of modern-day permafrost is also linked to the Pleistocene conditions in that much of it began forming in the late Pleistocene as Weichselian and Wisconsinan ice retreated. Under full glacial conditions ice sheets profoundly affected the Earth’s climate by causing drought, desertification, and a dramatic drop in sea level. Hence Pleistocene permafrost conditions adjacent to large ice sheets probably were quite different from the permafrost conditions we observe today. Under the colder dryer

FIGURE 15.8 Diagrammatic representation of subglacial permafrost showing the thermal regime of (A) cold- and (B) warm- based glaciers and corresponding proglacial settings. 15.6 GROUND ICE 553

conditions typical of full glacial conditions wind-blown sand and silt (loess) was deposited over vast areas adjacent to continental ice sheets. In some cases filling thermal contraction cracks forming sand wedges or developing unique syngenetic permafrost systems termed yedoma. Murton et al. (2004) describe Pleistocene permafrost that was overridden and deformed by Laurentide Ice. In this study the authors describe how blocks of ground ice are sheared and deformed by an overriding ice sheet. In this case the Pleistocene permafrost is preserved beneath a basal till containing Holocene permafrost and ground ice. There is little information about the Pleistocene permafrost conditions that existed at its more southerly extent. However, it is likely that the greater insolation associated with the longer daylight and longer summer season ameliorated permafrost and periglacial conditions somewhat.

15.5.1 PLEISTOCENE PERMAFROST DISTRIBUTION The extent of Pleistocene periglacial conditions is directly related to the position of Pleistocene ice sheets. In addition to the sedimentary record of glacial deposits, lower sea levels and loess deposits associated with the last glacial maximum, the cold climates that extended well beyond the limits of Weichselian and Wisconsinan ice developed a distinct periglacial landscape that helped form the basis of palaeoenvironmental reconstruction. Fossilized periglacial and permafrost structures and landforms occur throughout continental areas of central Europe and central North American well beyond the limit of Holocene permafrost and periglacial activity. Bradley (1999) and French (2007) summarize the geo- graphical evidence from studies describing mid-latitude periglacial activity. By mapping fossil permafrost landforms (e.g., ice wedge casts, sand wedges, thermokarst, collapsed pingos, etc.) and various fossil features related to frost action (e.g., patterned ground, frost cracks, cryoturbation, weathered bedrock, tors, and boulder streams) researchers have been able to piece together not just the limits of Pleistocene permafrost activity but spatial patterns of periglacial conditions. These data are greatly enhanced by complementary records from lacustrine and organic deposits. As illustrated in Fig. 15.9 periglacial processes occurred in some cases more than a hundred kilometres beyond the limits of glaciation.

15.6 GROUND ICE An important geomorphic aspect of permafrost, although technically not part of its definition, is the presence of ground ice. The term ground ice refers to ‘all types of ice formed in freezing and frozen ground’ (Permafrost Subcommittee, 1988, p. 46). It ranges from disseminated ice crystals in a soil matrix (termed pore ice) to thick (5À10 m), horizontally layered bodies of nearly pure massive ice that extends for several square kilometre (massive ice). Many processes and landforms unique to permafrost regions are directly related to the aggradation and degradation of ground ice. The study of its structure, chemistry, and stratigraphic characteristics is termed cryostratigraphy. Ground ice can be either epigenetic (formed in situ) or syngenetic (formed in sediments as they are being deposited) in origin; the mechanism of formation and placement imparts a distinct cryogenic structure (Murton and French, 1994). Normally soil moisture content is constrained by porosity; however the ice content in permafrost can readily exceed the saturation limit of its host sediments. The fact that ground ice is capable of existing in volumes far greater than saturation (a phenomenon called excess ice) makes 554 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

FIGURE 15.9 Map showing the extent of permafrost at the last glacial maximum. Modified from Mercier, D., 2008. Paraglacial and paraperiglacial landsystems: concepts, temporal scales and spatial distribution. Geomorphologie´ 14 (4), 223À233 (Mercier, 2008). periglacial landscapes highly susceptible to thaw-related terrain instability. Ground ice remains one of the most problematic aspects of permafrost and a major obstacle to development in Arctic regions. There is widespread concern that warming associated with global climate change will not only cause a shift in the distribution of permafrost but also widespread thermokarst. From an applied perspective ground ice is important for two reasons; first, because its formation is responsible for significant pressure changes and redistribution of materials (e.g., frost heaving), and second, its presence alters the rheological properties of its host materials. Ground ice is of particular significance in areas underlain by continuous permafrost. Although many types of ground ice are recognized, pore ice, wedge ice, segregated ice, and buried ice are significant in terms of their potential volume and widespread occurrence (Harry, 1988). Massive ice together with wedge ice may possibly represent the most vulnerable types of ground ice with climate change. Knowledge about the distribution, origin, and nature of ground ice and ice-rich sediments is necessary to assess potential impacts of thermokarst in response to natural and anthropogenic disturbance of permafrost. Ground ice studies also provide useful proxy information on Arctic palaeoclimate and palaeogeomor- phology and thus may play a valuable role in reconstruction glacial histories. 15.6 GROUND ICE 555

BOX 15.2 GROUND ICE CLASSIFICATION One of the most widely cited ground ice classifications was proposed by J.R. Mackay (1972); this genetic classification identifies four basic forms of ground ice (thermal contraction, tension rupture, segregated, and intrusive) that are subdivided into 10 specific ice types (open cavity, single vein, ice wedge, tension crack, closed cavity, epigenetic, aggradational, sill, pingo, and pore). Mackay identifies four primary sources of water for ground ice formation (atmosphere, surface, subsurface, and expelled). The functionality of this classification is limited for two reasons; first, because it fails to reflect the relative importance of specific types of ice, and second because it excludes buried ice. Mackay did however propose a simple classification for massive ice (presented in French, 2007, p. 182) that includes buried ice. A descriptive classification by Pihlainen and Johnston (1963) includes three categories of ground ice based on appearance in a permafrost core. In this classification ground ice is divided into ‘not visible’, ‘visible but ,1 in. thick’, or ‘visible and .1 in. thick’. Each category is further subdivided based on easily described characteristics. For example, the not visible category is described in terms of its bonding properties and whether or not it contained excess ice. The visible ice categories focus on the appearance of ice in core (e.g., individual crystals, coatings on sediment grains, ice lens pattern, and whether ice occurred with or without soil inclusions). Pollard (1990) developed a petrographic classification for massive ground ice that differentiated between epigenetic and buried ice types based on appearance,

FIGURE 15.10 A morpho-genetic ground ice classification based on geologic setting, hydrology, and transfer process. This classification incorporates terminology and concepts used by Mackay (1972), Pollard (1990), and Vasil’chuk (2012).

(Continued ) 556 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

BOX 15.2 GROUND ICE CLASSIFICATION (CONTINUED) crystal morphology, and fabric. French (2007, p. 156) presents a useful classification that incorporates the strengths of both the Mackay and Pihlainen and Johnston systems. There are numerous Russian ground ice classifications; but they tend to be complex and reflect a strong geologic emphasis and in some cases combine both surface and subsurface ice (e.g., Shumskii, 1959). Ground ice terminology is complicated by the widespread use of various semiquantitative terms like massive ice and excess ice or hybrid genetic terms like intrasedimental ice. The incorporation of multilingual or cross-disciplinary terms to describe ground ice setting and processes also adds a degree of confusion. Fig. 15.10 presents a new classification loosely based on Mackay’s (1972) genetic classification, a petrographic classification by Pollard (1990) and a geochemistry-based classification of massive ice by Vasil’chuk (2012). This classification uses basic geological (cryostratigraphic) context and water source as the defining basis and seeks to reduce complexity by grouping ground ice into six previously defined morpho-types based on hydrodynamics. The goal of this classification is to simplify ground ice description.

15.6.1 ICE WEDGES Ice wedges represent a significant source of ground ice volumes. Ice wedge polygons are one of the largest and most distinctive patterned ground phenomena. Ice wedge formation involves the repetition of two processes: first, thermal contraction cracking of the ground, and second, infiltra- tion and freezing of melt water resulting in the formation of a vertical ice vein. Cold winter air temperatures cause intense cooling of the surface and near-surface sediments causing thermal contraction of the ground. Thermal contraction generates tensile stresses sufficient to trigger cracking of the ground. Thermal contraction cracks 1À2 cm in width and ranging from a few metres up to 30À40 m in length propagate several metres into the permafrost (Lachenbruch, 1962). The resulting cracks remain open through the winter but at the onset of spring melt water infiltrates into the crack and freezes, forming a vertically oriented vein of ice. Since thermal contraction cracking tends to occur in the same place in subsequent years (but not every year) the incremental addition of ice veins eventually form large vertically foliated V-shaped bodies of ground ice called ice wedges. The top of the ice wedge is often at or close to the maximum depth of seasonal thaw. Fluctuations in seasonal thaw depth of the active layer contribute to the formation of shallow troughs over the top of the ice wedge. Networks of troughs form distinctive polygon patters called ice wedge polygons. Any increase in active layer depth can lead to increased trough depths and in extreme cases complete melt out of the ice wedge (Fig. 15.11).

15.6.2 MASSIVE GROUND ICE Within the ground ice literature the term massive ice is used to describe thick tabular bodies of nearly pure ice irrespective of its origin. It is defined as a ‘large mass of ground ice having a gravi- metric ice content .250%’ (Mackay, 1989, p. 6). It is a descriptive term designed to highlight ground ice bodies characterized by high excess ice contents. Implicit in its description is the potential for thaw subsidence.Two types of massive ice are recognized, buried surface ice and intrasedimental ice. The former refers to any type of surface (allochthonous/lithogenic) ice mass that is subsequently incorporated into and preserved by permafrost. Buried glacier ice is potentially the most significant source of buried ice and is a common constituent of modern and Pleistocene 15.6 GROUND ICE 557

FIGURE 15.11 Examples of ice wedge morphology and ice wedge polygon geometry. On the left is a large ice wedge exposed in a thaw slump on Herschel Island, northern Yukon Territory. This ice wedge is nearly 3 m wide and penetrates to a depth of 10 m. The photo on the right shows a polygonal network of ice wedges on Ellesmere Island, Nunavut. Polygon diameters are between 8 and 12 m. moraines preserved by permafrost. Buried snowbank ice has also been identified in areas of reacti- vated thermokarst. By comparison intrasedimental ice forms in situ (autochthonous/authigenic)by freezing of groundwater and can include any combination of ice segregation and intrusive ice (Mackay and Dallimore, 1992). One of the greatest sources of debate in the ground ice literature is the differentiation between buried and intrasedimental ice. In some cases a buried origin is readily determined based on the discordant relationship between the internal structure of the ice mass and the enclosing sediment. Unconformable upper and lower contacts, truncated structures, contrasting textures, and mineralogy between the sediments contained within the ice and those enclosing it, and hydrochemistry are just a few ways buried ice can be differentiated. By contrast intrasedimental ice usually displays some degree of stratigraphic continuity. Glaciers and glacial processes may both directly or indirectly influence massive ice formation. The inclusion of glacier and basal ice in a moraine system is an obvious direct impact. Less obvious would be the burial of blocks of glacier ice in fluvial glacial deposits or the overriding of river (or icing) ice by an advancing ice sheet. Indirect impacts can be related to the formation of intrasedimental ice from glacial meltwater. This mechanism has been suggested for massive ice in the Mackenzie Delta Tuktoyuktuk Peninsula area. Another example relates to falling sea levels and the formation of intrasedimental ice as permafrost aggrades into emergent surfaces (Fig. 15.12).

15.6.3 BURIED GLACIER ICE The dynamic relationship between ice sheet and glacier movement together with glacial deposition make terminal and recessional moraine systems areas where glacier ice is readily buried. Similarly the downwasting of a debris-rich ice sheet or glacier is also another way glacier and basal ice is buried. In both cases the preservation of the ice core relies on the aggradation of permafrost into moraine deposits before melt completely removes the enclosed ice. The ablation of debris-rich ice 558 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

FIGURE 15.12 Examples of massive ground ice of intrasedimental origin. On the left is a 17 m exposure of massive ground ice on Herschel Island, northern Yukon Territory. The photo on the right shows massive ice in the headwall of a thaw slump on Ellesmere Island. accumulates a drift blanket that will eventually be thick enough to buffer the buried ice from seasonal changes and thereby effectively develop an active layer. Rock glaciers and debris-covered glaciers in some cases are examples of buried glacier ice retaining some glacial flow characteristics (Fig. 15.13).

15.7 THERMOKARST Thermokarst is an erosional process unique to permafrost with excess ice; it is defined as ‘the process by which characteristic landforms result from the thawing of ice-rich permafrost and/or melting of massive ice’ (van Everdingen, 1998). Its significance is frequently discussed in terms of anthropogenic disturbance and its potential impact on infrastructure. However, thermokarst should also be viewed as a naturally occurring erosional process. The term thermokarst was introduced to describe the dissected topography in northern Siberia caused by the melting ground ice (Shumskii, 1964). The current usage of the term applies to processes and landforms associated with the thaw of all forms of ground ice (French, 2007). It therefore includes both the thaw degradation of epigenetic ground ice as well as the formation of dead ice topography caused by melting buried glacier ice in disintegration moraines. This has not always been the case, as thaw subsidence in glacial sediments has also been called ‘glacier karst’ (Sugden and John, 1976). The importance of this pertains to the interaction between glaciers and permafrost as well as the palaeoenvironmental reconstruction of glacial landscapes. Thermokarst has been widely reported throughout the Arctic and more recently the Antarctic. It occurs when the thermal stability of the upper part of ice-rich permafrost is disrupted by an increase in active layer depth. Thermokarst can occur as either thaw subsidence or thermal erosion. Thaw subsidence is primarily vertical in direction and involves downwasting while thermal erosion involves lateral planation (French, 2007). Thermokarst subsidence tends to occur in flat to low relief areas and often produces 15.8 HYDROLOGY IN PERIGLACIAL ENVIRONMENTS 559

FIGURE 15.13 In this picture the exposed massive ground ice is buried glacier ice from western Siberia. surface ponding and shallow depressions. By comparison, thermal erosion occurs on slopes where melting of exposed ground ice produces a laterally retreating face of ice-rich permafrost. In this case large pools of supernatant water and liquefied sediment collect at the base of the exposure and then gradually flow away keeping fresh ice exposed and sustaining the process. Very distinctive ‘C-shaped’ depressions called retrogressive thaw slumps are a common expression of thermal erosion (Fig. 15.14).

15.8 HYDROLOGY IN PERIGLACIAL ENVIRONMENTS Low winter air temperature and cold permafrost temperature conditions in periglacial regions have a profound impact on surface and subsurface hydrologic systems. The development of thick ice covers on rivers and lakes, as well as the formation of icings and frost mounds, reflect the interaction between winter freezing and hydrologic conditions. In many cases the main source of runoff and ground water is from glacial meltwater. The hydrologic relationship between runoff and groundwater of any origin and periglacial environments is complex and complicated by the seasonal pattern of freezing and the largely impermeable nature of permafrost. The following discussion focuses on two uniquely periglacial hydrologic systems. 560 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

FIGURE 15.14 A retrogressive thaw slump on Herschel Island, northern Yukon Territory. This slump is approximately 80 m across with a 5 m high headwall and has the classic parabolic shape typical of this type of thermokarst.

15.8.1 ICINGS Icings are sheet-like masses of layered ice formed on either ground surfaces or river ice surfaces by repeated overflow events (Pollard and Van Everdingen, 1992). In addition to the term icing other terms occur in the literature to describe the same phenomenon, including the German term ‘aufeis’ and the Russian term ‘naled’ (Carey, 1973). Three types of icings are differentiated on the basis of location and hydrologic system, including ground, spring, and river icings. Ground and spring icings are produced by the freezing of successive flows of groundwater forced to the ground surface by freeze back of the active layer, or constriction of surface seeps and perennial spring outlets. River icings form when the seasonal ice cover becomes sufficiently thick that it restricts the cross- sectional area of the channel and forces overflow and flooding of riverbanks. Icings are relatively common in areas of discontinuous and thin continuous permafrost as well as periglacial settings where permafrost is absent. In areas of thick permafrost ground and spring icings are rare due to the limited availability of groundwater. In this environment the most common form of icing is associated with glaciers and late-season discharge of subglacial meltwater. Many Arctic glaciers develop proglacial icing accumulations that range in size depending on the volume of water stored in the glacial system. Fig. 15.15 shows a proglacial icing on Axel Heiberg Island. The main driver for icing development is extremely cold winter air temperatures and rapid freezing of surface water. As icing ice forms it tends to obstruct flow, thus forcing discharge to find another path. This tends 15.8 HYDROLOGY IN PERIGLACIAL ENVIRONMENTS 561

FIGURE 15.15 These pictures show examples of a proglacial hydrologic activity. The upper left photo is an example of a proglacial icing and the upper right shows a frost blister that formed in a proglacial icing. The lower right and left photos are of proglacial pingos. All of these features are fed by subglacial meltwater that either discharges onto the surface of the outwash plain or through flood fluvial glacial deposits where they interact with aggrading permafrost. to result in overflow events where thin layers of water fill lower parts of the icing surface in a self- leveling manner. Depending on the size of the icing and the volume of discharge the overflow process can be active on many parts of the icing at the same time. The icing thickness accumulates gradually through sequential overflow events. Several icing layers with different thicknesses may be produced and deposited at different locations during a single overflow event. River icings can be extremely large, accumulating several metres thick and extending for tens of kilometres along river channel and flood plain. Large river icings will block the main river channel during spring melt, causing flooding and erosion. Icings are a significant problem for many northern highways.

15.8.2 FROST MOUNDS Frost mounds are another hydrologic feature related to the interaction between frozen (and freezing) ground and groundwater. The term frost mound refers to the family of mound phenomena produced in periglacial environments by the hydraulic and hydrostatic pressure of groundwater, volumetric 562 CHAPTER 15 PERIGLACIAL PROCESSES IN GLACIAL ENVIRONMENTS

expansion of water during freezing, and the force of crystallization during freezing (Pollard, 1988). Frost mounds and icing often share the same water source and therefore frequently occur together. Like icings, frost mounds often occur in a proglacial setting as a result of glacial meltwater. Various frost mounds, including frost blisters, icing mounds, icing blisters, and open system pingos frequently develop in a proglacial setting through the interaction between subglacial discharge and permafrost. Fig. 15.15 includes two examples of proglacial open-system pingos and a frost blister, also from Axel Heiberg Island in the Canadian High Arctic.

15.9 THE CHANGING PERIGLACIAL REALM As the Earth’s climate becomes progressively warmer the fates of both glacial and periglacial systems are intimately linked and will experience similar changes. The latitudinal zonation of permafrost is expected to shift northward; for example, in North America continuous permafrost will likely be limited to the Arctic Archipelago while the continental area currently underlain by continuous permafrost is expected to gradually become discontinuous. The southern limit of permafrost will shift northward; and given the inherently unstable nature of discontinuous permafrost its disappearance is likely to progress rapidly. The shift in periglacial activity will probably be less dramatic in extent simply because the seasonal nature of climates in mid and high latitudes will still include winter conditions marked by freezing and frost action, although warmer winters will reduce its intensity. Of particular significance will be the formation of new permafrost and increased periglacial activity in areas vacated by retreating glaciers, ice cap, and ice sheets. The Antarctic continent in the Southern Hemisphere and Greenland and the Arctic island of both North America and Eurasia in the Northern Hemisphere will become the new periglacial realm.

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