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Chapter 6. Supraglacial Environments

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Chapter 6. Supraglacial Environments 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 eng- lacial 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 Past Glacial Environments. DOI: http://dx.doi.org/10.1016/B978-0-08-100524-8.00005-1 © 2018 Elsevier Ltd. All rights reserved. 159 160 CHAPTER 6 SUPRAGLACIAL ENVIRONMENTS 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 south- ern 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 supra- glacial 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 sur- faces 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
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