Chapter 6. Supraglacial Environments

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CHAPTER SUPRAGLACIAL ENVIRONMENTS 6 A. Schomacker1 and´ I.O¨ . Benediktsson2 1UiT The Arctic University of Norway, Tromsø, Norway, 2University of Iceland, Reykjav´ık, Iceland

6.1 INTRODUCTION The supraglacial environment comprises the surface of active glaciers and dead-ice (Fig. 6.1). It is directly accessible for observations of glacial processes, sediments, and landforms in contrast to the subglacial and englacial environments where direct assess is limited. A very wide range of glacial and sedimentary processes occur on the surface of glaciers and in dead-ice environments (e.g., Benn and Evans, 2010). On glacier surfaces, debris may experience entrainment, transport, and deposition. In sediment-covered dead-ice environments, the main process is resedimentation by gravitational, fluvial, and lacustrine processes before final melt-out and deposition (e.g., Eyles, 1979; Lawson, 1979; Schomacker, 2008; Schomacker and Kjær, 2007, 2008; Ewertowski and Tomczyk, 2015). The properties of supraglacial debris depend both on its source and the modification it has been exposed to on the ice surface. Six main processes cause sediment to accumulate on the surface of glaciers: (1) rockfall and avalanches (e.g., Andre,´ 1990; Hambrey et al., 1999; Glasser and Hambrey, 2002; Dunning et al., 2015), (2) thrusting or squeezing of subglacial material (e.g., Sharp, 1985; Huddart and Hambrey, 1996; Kru¨ger and Aber, 1999; Glasser and Hambrey, 2002; Benediksson et al., 2008; Schomacker and Kjær, 2008; Rea and Evans, 2011), (3) melt-out of englacial debris bands (e.g., Sharp, 1949; Boulton, 1970; Hambrey et al., 1999; Swift et al., 2006; Larson et al., 2015), (4) deposition from meltwater emerging on the ice surface (Kru¨ger and Aber, 1999; Russell and Knudsen, 2002; Dunning et al., 2013), (5) deposition of dust or other aeolian sediment, such as tephra, on the ice surface (e.g., Larsen et al., 1998; Adhikary et al., 2000; Kjær et al., 2004; Casey and Ka¨a¨b, 2012), and (6) deposition of extraterrestrial material, as seen on blue- ice areas in Antarctica, where meteorites accumulate as supraglacial debris (Corti et al., 2003; Harvey, 2003). The sediment type, reworking processes, and climate govern the final product left in the geological record. Characteristic landforms formed in the supraglacial environment by the melting of dead-ice are hummocky moraines, kames, and eskers (Fig. 6.2). The aim of many investigations of modern supraglacial environments is to provide analogues to past glacial environments and understand what processes formed ancient supraglacial sediments and landforms. Pleistocene glacial landscapes reveal that widespread dead-ice areas existed, e.g., along the margins of the Laurentide and Scandinavian Ice Sheets where hummocky moraines, rim ridges, and kames formed in the supraglacial environment. The Pleistocene hummocky moraines in

FIGURE 6.1 Examples of supraglacial environments. (A) Englacial debris bands melt out at the surface of Moore Glacier, northernmost Greenland, August 2006. (B) Englacial debris bands and sediment-filled thrusts deliver debris to the surface of Ko¨tlujo¨kull, south Iceland, August 2001. (C) The debris-covered surface of a large dead-ice area at Holmstro¨mbreen, Svalbard, July 2004. (D) A near-vertical section in ice-cored moraines with englacial debris bands at Holmstro¨mbreen, Svalbard, July 2004. Person for scale. (E) The completely debris-covered snout of Tasman Glacier, New Zealand, December 2008. (F) Dirt cone on the surface of Bru´arjo¨kull, Iceland, July 2005. Person for scale. (E) Photograph by Meer, J.J.M van der 6.1 INTRODUCTION 161

FIGURE 6.2 A sequential model for supraglacial landscape formation along the debris-charged lowland glacier margin of Ko¨tlujo¨kull, south Iceland. (A) The glacier advance phase with production of minor ice-contact fans fed by supraglacial streams and formation of an end-moraine. (B) The ice-stagnation and initial, or young, dead-ice phase with continued development of hochsander fans (supraglacially fed alluvial fans) and beginning decrease in altitude of the frontal part of the glacier because of large-scale backwasting of the major free-faces of ice related to the steep frontal ice slope. (C) The mature phase of dead-ice melting where the terminal part of the glacier has decreased significantly in altitude due to ice-disintegration. Cave enlargement and collapse of tunnel roofs have (Continued) 162 CHAPTER 6 SUPRAGLACIAL ENVIRONMENTS

North America have a relief up to 60 m, which suggests the supraglacial environment at the southern margin of the last Laurentide Ice Sheet must have been characterized by a very thick sediment cover (Ham and Attig, 1996). Hummocky moraines have been observed forming in modern supraglacial environments from the melt-out of debris-covered dead-ice but only reach a few metres in thickness. It is therefore challenging to understand the supraglacial environment at Pleistocene ice sheet margins and not straightforward to use small, present-day glacier margins as a modern analogue. Here, we review the processes, sediments, and landforms that characterize modern supraglacial environments. Furthermore, we briefly describe Pleistocene supraglacial deposits and demonstrate how the depositional processes are interpreted from the sedimentology and geomorphology of such deposits.

6.2 SOURCES AND CHARACTERISTICS OF SUPRAGLACIAL DEBRIS Supraglacial debris can enter the glacier system from various sources, depending on the glacier type and setting. Debris that enters the glacier surface in the accumulation zone gets incorporated into the glacier through progressive burial by snow and ice from snowfall or avalanches, or by fall- ing into crevasses and moulins on the glacier surface. This debris may then be subject to either supraglacial or englacial passive transport or subglacial active transport before reentering the ice surface in the ablation zone due to compressive flow and surface melting (Fig. 6.3). In contrast, debris entering the glacier surface in the ablation zone, where mass balance is negative, will remain in supraglacial position until it is released by gravitational processes or melting of the underlying ice (Boulton, 1978; Benn and Evans, 2010).

6.2.1 SOURCES OF SUPRAGLACIAL DEBRIS In every glacier setting, the distribution, size, and gradient of debris source areas are determined by the catchment topography, which consequently has an effect on the input of debris to glacier surfaces by gravitational processes (Benn et al., 2003). Supraglacial debris is usually most abundant on glacier, ice sheet, and ice cap surfaces that are towered by steep valley walls or nunataks, which can deliver sediments by mass movement processes (Fig. 6.1C and E). Valley and cirque glaciers commonly contain substantial supraglacial debris that originates from rockfalls and avalanches, but

L produced numerous mounds, ice-degradation niches, collapsing edges, steep ice walls, and sinkholes. The production of hochsander fans has stopped. (D) The final, or old, phase of dead-ice melting where the ice has thinned considerably, so the ice mass is disintegrated into isolated dead-ice blocks capped by a thick cover of multiple resedimented deposits. (E) The postmelt landscape with a characteristic series of glacial and glaciofluvial landforms. After Kru¨ger, J., Kjær, K.H., Schomacker, A., 2010. Chapter 7: Dead-ice environments: a landsystems model for a debris-charged stagnant lowland glacier margin, Ko¨tlujo¨kull. In: Schomacker, A., Kru¨ger, J., Kjær, K.H. (Eds.), The My´rdalsjo¨kull Ice Cap, Iceland: Glacial Processes, Sediments and Landforms on an Active Volcano. Developments in Quaternary Science 13. Elsevier, Amsterdam, pp. 105À126. 6.2 SOURCES AND CHARACTERISTICS OF SUPRAGLACIAL DEBRIS 163

FIGURE 6.3 Transport pathways through glaciers, with supraglacial, englacial, and subglacial routes. This model is applicable to valley and cirque glaciers as well as outlet glaciers and ice streams that drain ice caps and ice sheets with protruding nunataks. Modified after Boulton, G.S., 1978. Boulder shapes and grain-size distributions of debris as indicators of transport paths through a glacier and till genesis. Sedimentology 25, 773À799.

medial moraines and transverse englacial debris bands are also an important source of supraglacial debris, particularly in the ablation zone (Swift et al., 2006; Kirkbride and Deline, 2013; Deline et al., 2015). Rockfalls occur when debris is released from rock slopes due to destabilizing stresses and preexisting weaknesses that are exploited in particular during frost weathering, heavy rainfalls, or earthquakes (McColl, 2015). Water in cracks, joints, holes, and at stratigraphic contacts can force the rock apart as it volumetrically expands by 9% upon freezing. This most commonly occurs during rapid freezing down to temperatures around 25C, whereas expanding lenses of segregation ice wedge apart the rock mainly in sustained temperatures below 24C(Benn and Evans, 2010). In periglacial environments, rockfalls tend to happen most commonly during daytime, when cliffs become exposed to sunlight and loose rock is no longer cemented by ice (Andre,´ 1997; McColl, 2015). Although less common, rockslope failure can also occur when water pressure in joints, cracks, and at stratigraphic interfaces is raised due to excess water during periods of heavy rainfall and snowmelt (Sosio, 2015). In tectonically active areas such as New Zealand and Alaska, earthquakes are commonly responsible for large-scale rock avalanches onto glaciers (Shulmeister et al., 2009; Shugar and Clague, 2011). As an example, Black Rapids Glacier in Alaska received 25À37 3 106 m3 of debris in three landslides in connection to the M7.9 Denali Earthquake in 2002, covering about 11 km2 of the glacier (Jibson et al., 2006; Shugar and Clague, 2011). Large 164 CHAPTER 6 SUPRAGLACIAL ENVIRONMENTS

rockslope failures are also promoted by glacier retreat due to the release of support by the glacier to oversteepened slopes, changing glacier morphology due to thinning, and undercutting by thinner ice and running meltwater (McColl, 2015; Purdie et al., 2015). Thinning of glaciers increases the difference in height between the ice surface and the source of rock failure, which can cause larger run-out distances of the debris on the glacier surface. This introduces supraglacial debris to areas of the glacier surface that have previously been largely free of debris but also exacerbates the rockfall hazard to glacier scientists and tourists (Deline et al., 2015; Purdie et al., 2015). During current and future climate warming, increased rockfall and rock avalanche activity may thus be expected due to glacier retreat and thinning, meltwater erosion in the lateral trough between the glacier and the slope, more rainfall instead of snowfall, and melting of permafrost on periglacial slopes surrounding valley glaciers (Purdie et al., 2015). This is exemplified by a recent large rock avalanche at Morsarjo´ ¨kull, SE-Iceland, on March 20, 2007 when 4 3 106 m3 fell onto the glacier, attributed to slope instability caused by recent glacier retreat and climate warming (Fig. 6.4). The rock avalanche deposits are now being transported downglacier as supraglacial debris by 80À90 m year21 (Decaulne et al., 2010; Sæmundsson et al., 2011). Due to the flow of ice, such single-event rock avalanche deposits eventually reach the glacier terminus and become emplaced through the melting of the underlying ice as either sharp-crested or hummocky terminal moraines. Such moraines do not represent glacier advances and must be cautiously interpreted in palaeoglaciological reconstruc- tions (Reznichenko et al., 2016). Medial moraines are striking surface features on many glaciers and an important source of supraglacial debris, particularly in glacier ablation zones where they tend to increase in relief and widen toward the snout due to differential melting and the melt-out and outward migration of debris from debris-rich septa (Small and Clark, 1974; Small et al., 1979; Anderson, 2000; Kirkbride and Deline, 2013). The most common medial moraines form downstream from glacier confluences when debris that has been eroded from nunataks and valley walls is expressed on the glacier surface (Fig. 6.5). Debris concentrations and morphological evolution of the moraines is thus largely influenced by the erosion rates of the overlooking slopes. Where several ice streams coalesce into one main valley glacier, medial moraines are marked surface features that illustrate the flow of ice and transport of sediment (Eyles and Rogerson, 1978). Medial moraines have not received much attention in glacial geological research and the three categories defined by Eyles and Rogerson (1978) can still be considered as the most useful framework. They divided medial moraines into: (1) ice-stream interaction moraines (Fig. 6.5A and C), which form where debris is expressed on the surface below or close to the equilibrium line where confluent valley glaciers intersect and their lateral moraines merge; (2) ablation-dominant moraines that form through the melt-out of englacial debris either above or below the equilibrium line, or where converging ice flow elevates debris from the bed in the lee of subglacial bedrock knobs (Fig. 6.5B); and (3) avalanche-type moraines that are the result of significant rockfall events onto glacier surfaces and consist of isolated debris concentrations passing down-ice. Vere and Benn (1989) argued that these models were best regarded as fundamental classes that would contribute to specific medial moraine aggregations in the field. The debris concentration in medial moraines results in differential ablation and a gradual increase in moraine relief downglacier, causing the moraines to stand above the surrounding clean ice (Anderson, 2000; Kirkbride and Deline, 2013; Schomacker et al., 2014; Lønne, 2016). Once the ice melts, subtle traces of the medial moraines can be found in con- temporary glacier forelands and formerly glaciated areas in the form of thin and diffuse concentrations of debris that bear the characteristics of supraglacial deposits (Sharp, 1958; Danson, 1979; 6.2 SOURCES AND CHARACTERISTICS OF SUPRAGLACIAL DEBRIS 165

FIGURE 6.4 (A) An overview of the rock avalanche deposits on Morsarjo´ ¨kull, Iceland. This avalanche occurred March 20, 2007. Inset photograph shows a large angular block that travelled nearly 1 km with the avalanche. Note persons for scale. (B) Overview of the rock avalanche front in July 2014. Note the B50 m height difference between the surface of the rock avalanche and the clean glacier ice to the right. This difference is caused by differential melting; the avalanche deposits significantly hamper the melting of the underlying ice. Red symbols indicate two persons for scale. (A) Overview photograph by M.J. Roberts, July 9, 2011. Inset photograph by Þ. Sæmundsson, May 15, 2007. (B) Photograph by Þ. Sæmundsson.

Smiraglia, 1989; Evans and Twigg, 2002; Schomacker et al., 2014; Lønne, 2016). Thus the preser- vation potential of medial moraines is generally low. Transverse englacial debris bands or debris septa are an important source of supraglacial debris in glacier ablation zones. They are most common close to the glacier snout where surface melting 166 CHAPTER 6 SUPRAGLACIAL ENVIRONMENTS

FIGURE 6.5 (A) A composite 2002À07 infrared Spot Image of the southern part of the Vatnajo¨kull ice cap, Iceland. G, Gr´ımsvo¨tn volcano; B, Breiðamerkurjo¨kull; S´ı,S´ıðujo¨kull; Sk, Skeiðararjo´ ¨kull; H, Heinabergsjo¨kull; K, Kv´ıarjo´ ¨kull. Note the prominent ice-stream interaction medial moraines at Breiðamerkurjo¨kull and Heinabergsjo¨kull, the avalanche-type and ablation-dominant medial moraine on Kv´ıarjo´ ¨kull, and the ablation- dominant medial moraine at Skeiðararjo´ ¨kull. Note also the striking tephra bands on Skeiðararjo´ ¨kull and S´ıðujo¨kull that demonstrate the glacier flow. (B) A view of the Kv´ıarjo´ ¨kull medial moraine and associated supraglacial debris at the ice margin in 2010. (C) The Heinabergsjo¨kull medial moraine (2008) originating at the confluence of two ice streams downstream from the nunatak in the upper right. rates are high and the ice flow is compressive, causing the elevation of basal and englacial debris to the surface chiefly through folding of and thrusting along steep, transverse englacial foliae. Primary dispersal of the debris is controlled by spatial patterns that are predetermined by the struc- tural glaciology, while differential ablation and topographic inversion promote various gravitational processes (fall, flow, rolling, sliding), which govern the secondary dispersal of debris by locally distributing it down slopes. At glacier termini that are obstructed by large moraine dams, rock- 6.2 SOURCES AND CHARACTERISTICS OF SUPRAGLACIAL DEBRIS 167

glacier-like lobes, or rheological barriers of cold-based ice, debris cover tends to thicken rapidly and stabilize downstream. This alters both rates and patterns of surface melting so that mass loss and glacier thinning is focused in the mid-parts of ablation zones, causing a reduction in surface gradient, driving stress, and ice-flow velocity. Thick and stable debris cover may thus eventually cause glacier termini to stagnate but also provides a negative feedback between surface melting and glacier thinning that preserves debris-covered glacier tongues and promotes the formation of large ice-marginal moraine ridges (Benn et al., 2003, 2012; Kirkbride and Deline, 2013). Outside valley and cirque glaciers and on ice cap and ice sheet surfaces far from nunataks, supraglacial debris is normally negligible. In volcanic areas like Iceland, tephra from explosive eruptions periodically provides a major source of supraglacial sediment on glaciers and ice caps (Larsen et al., 1998; Kru¨ger and Kjær, 2000; Schomacker et al., 2010; Jude-Eton et al., 2012; Figs. 6.5A and 6.6). Tephra that falls on the glacier surface in the accumulation area usually gets quickly buried by new snow and carried downglacier by ice flow. Such tephra deposits end up forming dark marker horizons in the ice that represent specific events. Once the tephra horizons melt out in the ablation zone, they are clear indicators of the flow of ice and an important source of supraglacial debris (Figs. 6.5A and 6.6B). Due to differential ablation, tephra plays a major role in the formation of ice-cored moraines that eventually develop into ice-free hummocky moraines in the marginal and proglacial zones of many Icelandic glaciers (Kru¨ger and Kjær, 2000; Fig. 6.1B). On vast ice sheet surfaces like in Antarctica or Greenland, meteorites can constitute the most important or even the only source of supraglacial debris. This extraterrestrial material is embedded within the ice and transported downstream of snow accumulation zones by ice flow toward blue-ice fields that typically occur on the upglacier side of submerged bedrock obstacles or nunataks where the ice decelerates. The meteorite-bearing, blue ice is uplifted and exhumed by wind ablation

FIGURE 6.6 (A) Tephra effusion during the explosive 2010 eruption of Eyjafjallajo¨kull, Iceland, providing a major source of supraglacial debris. Note the dirty appearance of the small G´ıgjo¨kull outlet glacier (below the eruption plume) and black ice cap completely covered with tephra. May 11, 2010. (B) Tephra horizon from the Katla 1918 eruption melting out in the ablation zone of the southern branch of the Sandfellsjo¨kull glacier, My´rdalsjo¨kull ice cap, southern Iceland. Note the contrast between the clean and dirty ice above and below the tephra horizon, respectively. August 2001. 168 CHAPTER 6 SUPRAGLACIAL ENVIRONMENTS

resulting in high concentration of meteorites on the surface (Corti et al., 2003; Harvey, 2003). The flow of ice around major bedrock barriers, coupled to high ablation, may also lead to the formation of meteorite traps on the lee side of major topographic obstacles (Corti et al., 2003). The summer ablation in Greenland, however, makes it less likely that small meteorites accumulate on blue-ice surfaces than in Antarctica (Haack et al., 2007).

6.2.2 CHARACTERISTICS OF SUPRAGLACIAL DEBRIS In general, supraglacial debris tends to retain the characteristics of the parent material. For example, debris derived from talus slopes usually contains angular and platy frost-weathered clasts whereas rock avalanche debris contains clasts that are also angular but display a wide range of shapes from blocky to blade- and rod-shaped (e.g., Benn and Ballantyne, 1994; Lukas et al., 2013; Deline et al., 2015; Reznichenko et al., 2016; Fig. 6.4A). Once supraglacial debris is entrained and transported subglacially, the clasts quite rapidly acquire the shape characteristics of subglacially modified debris, i.e., they become more rounded and blocky in form, provided that sufficient subglacial transport has occurred (Swift et al., 2006; Lukas et al., 2013). Therefore debris that melts out in the marginal zone of glaciers commonly derives from the subglacial traction zone and thus shows characteristics of subglacial debris with mainly blocky and subangular to subrounded clasts. It must be considered, however, that clast shape is primarily determined by the lithology as different lithologies respond differently to erosional, transportational, and depositional processes, even within the same catchment. Reworking within the glacial system may thus not be effective enough to modify particles that generally originate from supraglacial and extraglacial sources. Likewise, bedrock lithology can introduce contrasts in supraglacial debris supply between otherwise similar catchments. This is exemplified by high rates of debris delivery from uplifted sedimentary rocks overlooking some glaciers in Nepal despite relatively low precipitation, while glaciers surrounded by crystalline rocks in Norway, where precipitation is much higher, receive comparatively little supraglacial debris (Vere and Benn, 1989; Benn et al., 2003). Although clast shape analysis has proved to be a valuable tool in yielding information on debris source, transport mechanism, and even transport distances (Humlum, 1985; Benn and Ballantyne, 1994; Kjær, 1999), a rigorous strat- egy for sampling and analyses is advised where control samples are collected from, e.g., talus slopes and unequivocal exposures of subglacial traction till and different lithologies are analyzed separately (cf. Lukas et al., 2013). This is important for advancing our understanding of debris cas- cades in glacial environments.

6.3 PROCESSES IN THE SUPRAGLACIAL ENVIRONMENT Two main types of surface processes characterize the supraglacial environment: ice melt and resedimentation of the supraglacial debris cover. Both types of processes vary greatly with the climate, topographic setting, and the nature and thickness of the debris cover. We focus on supraglacial processes in debris-covered dead-ice rather than ablation of clean glacier ice. Dead-ice melts by backwasting of ice cliffs and by downwasting from the top and/or the bottom of the ice body (Fig. 6.7)(Pickard, 1983, 1984; Kru¨ger and Kjær, 2000; Kjær and Kru¨ger, 2001; 6.3 PROCESSES IN THE SUPRAGLACIAL ENVIRONMENT 169

Kru¨ger et al., 2010). Backwasting is the lateral retreat of steep ice walls (Eyles, 1979; Kru¨ger and Kjær, 2000; Schomacker and Kjær, 2008). It can be directly observed in the field and measured by the distance between a stable point and the retreating ice cliff edge. Downwasting is not as easily observed and is defined as the surface lowering and thinning of dead-ice by melting on the top and/or bottom surfaces. In permafrost environments, there is no melting on the bottom surface. The total downwasting can be measured as the elevation difference between the surface and a stable benchmark (Schomacker, 2008). The total surface lowering of dead-ice areas might also be measured remotely by differencing time series of Digital Elevation Models (Irvine-Fynn et al., 2011; Korsgaard et al., 2015). The melt rate of debris-covered dead-ice surfaces depends on the climate and the thickness and thermal properties of the sediment cover (e.g., Lukas et al., 2005; Schomacker and Kjær, 2007, 2008; Nicholson and Benn, 2006; Schomacker, 2008). A thin sediment cover of up to c. 1 cm will decrease the albedo and enhance melting, whereas a thicker sediment cover will have an insulating effect and lower the melt rate. This was documented by Østrem (1959) by field measurements of ablation of surfaces with different debris cover thickness at Isfallsglacia¨ren, Sweden, in JulyÀAugust 1956 (Fig. 6.8). The insulating effect is mainly seen on downwasting rates because

FIGURE 6.7 Melting processes in dead-ice. Horizontal surfaces melt by downwasting (D) from the top and/or bottom. Bottom melt only occurs in nonpermafrost environments. Ice cliffs and ice-cored slopes melt by backwasting (B). Modified after Kjær, K.H., Kru¨ger, J., 2001. The final phase of dead-ice moraine development: processes and sediment architecture, Ko¨tlujo¨kull, Iceland. Sedimentology 48, 935À952 and Schomacker, A., 2008. What controls dead-ice melting under different climate conditions? A discussion. Earth-Sci. Rev. 90, 103À113. 170 CHAPTER 6 SUPRAGLACIAL ENVIRONMENTS

FIGURE 6.8 (A) Ablation rates at Isfallsglacia¨ren, Sweden, as a function of debris cover thickness. Ablation peaks at a debris cover thickness of 0.5À1 cm. With thicker debris cover, the insulation effect exceeds the effect of the lower albedo and the ablation rate decreases. (B) Differential ablation forms a ‘glacier table’. The thin, dark debris cover around the boulder lowers the albedo and increases the ablation, whereas the boulder itself insulates the ice. Stick for scale. Kverkjo¨kull, central Iceland, August 2000. Modified after Østrem, G., 1959. Ice melting under a thin layer of moraine and the existence of ice cores in moraine ridges. Geogr. Ann. 41, 228À230. ice walls or steep ice-core slopes are unstable and rarely develop a thick sediment cover. Dirt cones also form due to differential ablation (Fig. 6.1F). Schomacker (2008) reviewed the backwasting rates of ice walls in dead-ice areas at 14 glaciers in order to study the climatic effect on dead-ice melting. One of the main findings was that the backwasting rate is almost the same in the mild, humid climate of south Iceland as on Svalbard glaciers in a dry and cold climate. This is most likely because the permafrost of Svalbard prevents percolation of surface water which maintains a liquefied debris cover with exposed ice walls. The best correlation between a number of climatic parameters and backwasting rate was found between mean annual air temperature and backwasting. In the dry and cold climate of Antarctica, backwasting rates range from 0.3À1.7 m year21 (Pickard, 1983, 1984; Chinn, 1987; Lewis et al., 1999). In contrast, they reach 15À24 m s21 in milder settings in Yukon, Himalaya, and New Zealand (Johnson, 1971, 1992; Sakai et al., 1998, 2002; Purdie and Fitzharris, 1999; Benn et al., 2001). Lawson (1979) studied supraglacial resedimentation at the snout of the Matanuska Glacier, Alaska. He concluded that c. 95% of the deposits had undergone resedimentation by gravitational and fluvial processes. This is consistent with observations from many other modern glacial environments documenting ubiquitous resedimentation of the supraglacial debris cover (e.g., Kjær and Kru¨ger, 2001; Schomacker and Kjær, 2008; Ewertowski and Tomczyk, 2015). Kjær and Kru¨ger (2001) also noted that the frequent gravity sorting of boulders at ice-cored slopes produce clusters of boulders that might survive final deicing. Lawson (1979) observed that the sediment gravity flows occurred as four distinct types, mainly depending on the degree of liquefaction, leading to flow lobes of different morphology and sedimentary facies. Flow type I has water content of 8À14% and is nonchannelized with a ‘plug’-like flow. It forms individual, thick flow lobes. Flow 6.4 SUPRAGLACIAL SEDIMENTS AND LANDFORMS 171

FIGURE 6.9 (A) Sediment gravity flow attacking the supraglacial debris cover on ice-cored slopes at Holmstro¨mbreen, Svalbard. Note spade for scale (circled). July 2004. The slopes are also shown in Fig. 6.1C. (B) Grain size distributions for diamict samples along the ice-cored slopes. The content of fines (clay and silt) decreases downslope. Modified after Schomacker, A., Kjær, K.H., 2008. Quantification of dead-ice melting in ice-cored moraines at the high-Arctic glacier Holmstro¨mbreen, Svalbard. Boreas 37, 211À225.

type II has a water content of 14À19% and, hence, the sediment is more liquefied and might experience local fluidization in channels. The flow lobes are stacked and coalesced. Flow type III has a water content of 18À25% and is channelized. The flow lobes are coalesced fans. Flow type IV has a water content of .25% and is therefore liquefied and the flow is channelized. The flow lobes are fan-shaped and larger clasts will concentrate at the bottom of the deposit. Due to the high water content of flow types III and IV, they will normally be longer and thinner. At the Matanuska Glacier they reach up to c. 400 m in length from the backwall to the snout of the flow (Lawson, 1979). The repeated resedimentation cycles of supraglacial debris cover can lead to depletion of fines from the sediment (Lawson, 1979; Lønne and Lysa˚, 2005; Lukas et al., 2005). Schomacker and Kjær (2008) showed that the content of fines in the diamict cover on ice-cored slopes at Holmstro¨mbreen, Svalbard, was c. 60% on the highest, undisturbed parts of a slope transect and 37% at the base of the transect where the sediment had been exposed to many resedimentation cycles (Fig. 6.9). Accordingly, hummocky moraines formed by melt-out of stagnant debris-covered ice often consist of coarse-grained crudely stratified, stacked diamicts (e.g., Benn, 1992).

6.4 SUPRAGLACIAL SEDIMENTS AND LANDFORMS IN THE PLEISTOCENE RECORD Circular moraines or ‘rim ridges’ on parts of the great plains of North America have been interpreted as evidence for large-scale stagnant ice melting (Fig. 6.10; Johnson and Clayton, 2003). They occur in clusters with hundreds or thousands of individual low-relief landforms, each up to 172 CHAPTER 6 SUPRAGLACIAL ENVIRONMENTS

FIGURE 6.10 Circular moraine ridges in Saskatchewan, Canada, formed during the last deglaciation of the Laurentide Ice Sheet. Each landform is up to c. 200 m across and they are located in a low-relief glacial landscape. Landsat 7ETM 1 imagery recorded on May 24, 2001. Coordinates are in metres. After Schomacker, A., 2011. Moraine. In: Singh, V.P., Singh, P., Haritashya, U.K. (Eds), Encyclopedia of Snow, Ice and Glaciers. Springer, Dordrecht, pp. 747À756.

200 m across. A landform consists of an outer rim that surrounds a lower depression. It has been suggested that they formed supraglacially and experienced a topographical inversion in the final phase of deicing (Fig. 6.11; Clayton and Moran, 1974; Ham and Attig, 1996; Johnson and Clayton, 2003). Some contain glacial clay in the middle, suggesting that they originated as ice-walled lake plains. In southeast Sweden, circular landforms with glacial clay in the centre also indicate large- scale melting of stagnant ice during the last deglaciation of the Scandinavian Ice Sheet (Lidmar- Bergstro¨m et al., 1991). The last phase of the Weichselian glaciation of eastern Denmark was characterized by a series of glacier readvances that produced large areas of debris-covered dead-ice (Fig. 6.12; Houmark-Nielsen and Kjær, 2003; Houmark-Nielsen et al., 2005). Fig. 6.12A shows the loca- tion of the ice sheet margin c. 17À16 ka BP. The dead-ice areas in front of the ice margin melted during the final deglaciation and formed hummocky moraines (Fig. 6.12B). Kames con- sisting of glacial clay occur frequently in this landscape and indicate that ice-walled or ice- dammed lakes existed during the deglaciation. The kames formed by topographical inversion 6.4 SUPRAGLACIAL SEDIMENTS AND LANDFORMS 173

FIGURE 6.11 Conceptual model for topographical development by melt-out of dead-ice with a supraglacial debris cover. Cycles of resedimentation and topographical inversion (AÀD) lead to either hummocky moraines (E and F) or circular disintegration ridges (G and H). Modified after Clayton, L., Moran, S.R., 1974. A glacial process-form model. In: Coates, D.R. (Ed.), Glacial Geomorphology. State University of New York, Binghamton, NY, pp. 89À119. 174 CHAPTER 6 SUPRAGLACIAL ENVIRONMENTS

FIGURE 6.12 (A) Large areas of dead-ice were left during the retreat from the last advance of the Scandinavian Ice Sheet in southeast Scandinavia 17À16 ka BP. Hummocky moraines formed in a supraglacial environment characterize the landscape of eastern Denmark. (B) Hummocky dead-ice moraine east of Hillerød, eastern Denmark, February 2004. Irregular hummocks, small lakes, and peat bogs dominate the supraglacial landscape. Modified after Houmark-Nielsen, M., Kru¨ger, J., Kjær, K.H., 2005. De seneste 150.000 a˚r i Danmark. Istidslandskabet og naturens udvikling (in Danish). Geoviden 2/2005. Accessible at: ,http://geocenter.dk/publikationer/geoviden/..

where ice-cored depressions with meltwater lakes were drained and the lake sediments were left as the final landform. During Early and Mid-Weichselian glaciations, c. 80 and 65 ka BP, respectively, the Taymyr Peninsula in Arctic Siberia was inundated by an ice sheet advancing from the Kara Sea Shelf, forming the so-called North Taymyr ice-marginal zone. A thin Late Weichselian ice sheet margin also occupied part of this zone between 20 and 12 ka BP. The North Taymyr ice-marginal zone consists of ice-marginal and supraglacial sediment and landform associations and is characterized by an out- ermost zone of reworked sediments, an intermediate zone of buried, relict glacier ice with a thin till on top, and an innermost zone corresponding to debris-poor ice that melted relatively quickly leav- ing few traces in the present landscape (Alexanderson et al., 2002). Large thrust-block moraines and smaller push moraines are covered by a thin, silty, supraglacial melt out till and their surface is scattered with relatively large ( . 0.5 km in diameter) and irregularly shaped ice-walled (glacio- karst) lakes situated in basins dominated by gelifluction processes. Debris flows and small meltwater streams supply sediments to the lakes. This infilling along with the melting of the surrounding buried ice will lead to topographic inversion and the formation of ice-walled lake plains similar to those observed in North America and Sweden. Similar processes, sediments, and landforms to those described above have been observed from many modern glacial environments. They provide suitable analogues to supraglacial REFERENCES 175

sedimentÀlandform associations in the Pleistocene record (e.g., Bennett et al., 2000; Spedding and Evans, 2002; Schomacker and Kjær, 2008).

6.5 SUMMARY AND CONCLUSIONS Supraglacial sediments on valley and cirque glaciers, or on glacier or ice sheet surfaces below nunataks, are most commonly derived from rockfalls and avalanches, medial moraines, and englacial debris bands. Rockfalls and avalanches effectively deliver supraglacial debris in the accumulation zone while debris from medial moraines and englacial debris bands more commonly melt out in the ablation zone. Tephra and meteorites may constitute the main source of supraglacial debris in volcanic regions or areas on glacier and ice sheet surfaces far from valley walls or protruding nunataks. Surface processes in the supraglacial environment are characterized by ice melting by downwasting and backwasting, and the debris-cover is exposed to resedimentation mainly by gravitational and fluvial processes. These processes operate in a wide range of climates and lead to the same end productÀhummocky moraines. The palaeoclimatic significance of hummocky moraines is therefore limited, but glaciodynamically, they indicate the melting of debris-covered stagnant ice. Supraglacial sediments and landforms are widespread in Pleistocene landscapes, such as in the marginal zones of the Laurentide, Scandinavian, and the Kara Sea ice sheets. In periglacial environments, relict glacier ice may still be buried by till and postglacial sediments but subject to ablation leading to gelifluction processes, resedimentation by meltwater, and topographic inversion.

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