Glacial geological studies of surge-type glaciers in Iceland — Research status and future challenges

Abstract

Surging glaciers are potential analogues for land-terminating palaeo-ice streams and surging ice sheet lobes, and research on surge-type glaciers is important for understanding the causal mechanisms of modern and past ice sheet instabilities. The geomorphic signatures left by the Icelandic surge-type glaciers vary and range from glaciotectonic end moraines formed by folding and thrusting, crevasse-squeeze ridges, concertina eskers, drumlins and fluted forefields, to extensive dead-ice fields and even drift sheets where fast ice-flow indicators are largely missing. We outline some outstanding research questions and review case studies from the surge-type outlets of Brúarjökull, Eyjabakkajökull and Tungnaárjökull (Vatnajökull ice cap), Múlajökull and Sátujökull (Hofsjökull ice cap), Hagafellsjökull and Suðurjökull (Langjökull ice cap), Kaldalónsjökull, Leirufjarðarjökull and Reykjarfjarðarjökull (Drangajökull ice cap), as well as the surge-type cirque glaciers in northern Iceland. We review the current understanding of how rapid ice flow is sustained throughout the surge, the processes that control the development of the surge-type glacier landsystem and the geological evidence of surges found in sediments and landforms. We also examine if it is possible to reconstruct past surge flow rates from glacial landforms and sediments and scale-up present-day surge processes, landforms and landsystems as modern analogues to past ice streams. Finally, we also examine if there is a climate/mass-balance control on surge initiation, duration and frequency.

Introduction

A basic definition of a surge-type glacier identifies it as an outlet glacier that periodically has major fluctuations in velocity over timescales that range from a few years to several decades or centuries (Benn and Evans, 2010). Surge-type glaciers experience distinctive changes in geometry and activity over a surge cycle, where the phase of rapid motion lasting a few months to several years and typically results in rapid frontal advance is described as the surge or active phase, and the period of slow flow and/or stagnation for tens to hundreds of years between surges is described as the quiescent phase (Kamb et al., 1985, Raymond, 1987, Harrison and Post, 2003). It has been estimated that less than 1% of Earth's glaciers surge (Jiskoot et al., 1998), and they have been shown to be unevenly distributed around the world's glacierized regions and cluster in certain areas, notably Alaska (Kamb et al., 1985), Arctic Canada (Copland et al., 2003), Greenland (Murray et al., 2002), Iceland (Björnsson et al., 2003), Svalbard (Dowdeswell et al., 1991, Hagen et al., 1993), Novaya Zemlya (Grant et al., 2009), as well as in the Caucasus, Karakoram, Pamir and Tien Shan mountain ranges (Hewitt, 2007, Benn and Evans, 2010, Quincey et al., 2011). This clustering implies that certain environmental factors control the location of surge-type glaciers, but despite a number of studies that have investigated the possible constraints at a regional scale the reasons for this remain poorly understood (Meier and Post, 1969, Clarke et al., 1986, Clarke, 1991, Hamilton and Dowdeswell, 1996, Jiskoot et al., 1998, Jiskoot et al., 2000, Murray et al., 2003) and as yet no unifying theory to explain the surge mechanism exists (Rea and Evans, 2011). While both temperate and polythermal glaciers exhibit surging behaviour, it has been suggested the highest densities of surge-type glaciers occur in a relatively narrow climatic band bounded by mean annual temperatures of ca. 0 to − 10 °C and mean annual precipitation of ca. 200–2000 mm (Sevestre and Benn, 2015). With its maritime cold-temperate to low-arctic climate and numerous temperate glaciers, Iceland largely lies within this climatic band.

All major ice caps in Iceland have surge-type outlet glaciers, and glaciological studies and historical records have revealed at least 26 surge-type outlet glaciers in Iceland (Fig. 1, Fig. 2) (Björnsson, 1998, Björnsson et al., 2003, Björnsson and Pálsson, 2008, Thorarinsson, 1964a, Thorarinsson, 1969). Glacial geological studies have confirmed at least 2 additional surge-type glaciers (Evans, 2011, Larsen et al., 2015). Over 80 surge advances have been recorded, ranging from tens of metres up to 10 km, and systematic observations over the last several decades have allowed for a detailed description of several surges (Björnsson, 1998, Björnsson, 2009, Björnsson et al., 2003).

Combining the historical records of ice-front variations and glaciological field research, Björnsson et al. (2003) and Björnsson and Pálsson (2008) summarized the geographic distribution of surge-type glaciers, their subglacial topography, the frequency and duration of surges, changes in glacier surface geometry during the surge cycle, and measured velocity changes compared to calculated balance velocities. They also recorded the indicators of surge onset and described changes in ice, meltwater and suspended sediment fluxes during a surge. They show that surge-type glaciers in Iceland are characterized by gently sloping surfaces and that they move too slowly to remain in balance given their accumulation rate, and that surge frequency was neither regular nor clearly related to glacier size or mass balance. Aðalgeirsdóttir et al. (2005) found that the mass transport during surges of Vatnajökull outlets could be up to 25% of the total ice flux of individual glaciers, and that this could affect the location of the ice divides, the flow field and the size and shape of the ice cap. Fischer et al. (2003) suggested that a surge cycle on Sylgjujökull and Dyngjujökull, outlets of the Vatnajökull ice cap (Fig. 1), spans several years, characterized by a progressive acceleration in motion over an extended area in the beginning, and more pronounced velocity changes during the active surge phase lasting 1–2 years. They further suggested that the most active surge phase lasted for about one year for these glaciers.

Surge-type glaciers are of great interest in glaciology because they can shed light on dynamic instabilities and threshold behaviour in glacier systems. Glaciological research in Iceland in general has focused on glacier distribution as an effect of climatic and topographical conditions, glacio-meteorology, glacier geometry (including extensive mapping of subglacial topography), as well as glacier mass balance, glacio-hydrology, jökulhlaups and modelling glacier responses to climate change, whereas research on surge-type glaciers has focused on dynamic behaviour of the glacier during their most active surge phase, surge periodicity and meltwater production associated with surges (Aðalgeirsdóttir et al., 2005, Aðalgeirsdóttir et al., 2006, Aðalgeirsdóttir et al., 2011, Björnsson, 1982, Björnsson, 1998, Björnsson, 2009, Björnsson and Pálsson, 2008, Björnsson et al., 2003, Flowers et al., 2003, Guðmundsson et al., 2011, Magnússon et al., 2005, Marshall et al., 2005, Pálsson et al., 1991, Pálsson et al., 2012).

Glacial geological studies of surge-type glaciers in Iceland have typically had a different focus from the glaciological research (e.g. Sharp, 1985a, Sharp, 1985b, Croot, 1987, Bennett et al., 2004a, Benediktsson et al., 2009, Evans, 2011, Evans and Rea, 1999, Evans et al., 1999, Kjær et al., 2006). The motivation has been that ice streams and surge-type glaciers are dynamic constituents of the glacial system that influence and control form, flow, discharge and stability of present and former ice sheets and ice caps, and a better understanding of basal processes is particularly important for fast-flowing ice streams because of their crucial role in ice sheet dynamics (Evans and Rea, 2003, Dowdeswell et al., 2004, Nelson et al., 2005, Cofaigh and Stokes, 2008, King et al., 2009, Boulton, 2010). Surge-type glaciers are potential analogue for land-terminating palaeo-ice streams and surging ice sheet lobes, and research on surge-type glaciers is important for understanding the causal mechanisms of modern and past ice sheet instabilities and their contribution to sea level rise, and to better interpret evidence for past glacier activity and its climatic implications (Dowdeswell et al., 1995, Domack et al., 2005, Evans and Rea, 2003, Evans et al., 2008, Ottesen et al., 2008). Surge-type glaciers provide an opportunity to address important questions about the basal boundary conditions of fast flowing ice, in particular the significance of sediment deformation and sliding/subglacial decoupling (Murray, 1997, Clarke, 2005, Kjær et al., 2006). Icelandic surge-type glaciers have been intensely studied to better understand glacier-induced stresses and ice-flow mechanisms, basal temperature and hydrology, as well as the processes at work in the sub-marginal and ice-marginal zones (Croot, 1988b, Fuller and Murray, 2000, Fuller and Murray, 2002, Bennett, 2001, Russell et al., 2001, Andrzejewski, 2002, Bennett et al., 2004a, Bennett et al., 2004b, Nelson et al., 2005, Evans et al., 2007, Schomacker and Kjær, 2007, Benediktsson et al., 2008, Benediktsson et al., 2009, Benediktsson et al., 2010, Benediktsson, 2012). Outstanding research questions include if there is a mass balance or climatic control on surges (Dowdeswell et al., 1995, Benn and Evans, 2010, Copland et al., 2011, Striberger et al., 2011), and how is rapid ice flow sustained through a surge (Alley et al., 1987, Engelhardt and Kamb, 1998, Benediktsson et al., 2008, Kjær et al., 2006)? During the quiescent phase, the glaciers retreat and landform–sediment assemblages are exposed imprinted with information on sub-glacial and ice-marginal driving processes (Sharp, 1985b, Sharp and Dugmore, 1985, Bennett et al., 2000a, Evans and Rea, 2003, Benediktsson et al., 2008, Kjær et al., 2006, Kjær et al., 2008, Schomacker et al., 2006, Schomacker et al., 2014, Waller et al., 2008). Outstanding research questions thus also concern the actual geological fingerprinting of surges, if different types of surge-type glaciers produce different sediment–landform assemblages, and what is the impact of surges on sediment distribution (Evans and Rea, 2003, Benn and Evans, 2010, Brynjólfsson et al., 2012, Schomacker et al., 2014)? A question raised by Bennett (2001) is what palaeoglaciological and environmental inferences, if any, can be made from the occurrence of large, often multi-crested push moraines in the geological record, and if they are a characteristic landform in front of surge-type glaciers (Croot, 1988a, Hart and Watts, 1997: Boulton et al., 1999)? Studies of surge-type glacier landsystems (Evans and Rea, 1999) have highlighted research questions related to the genesis of e.g. crevasse-squeeze ridges, concertina eskers (Knudsen, 1995) and streamlined landforms, such as drumlins (Johnson et al., 2010, Jónsson et al., 2014), and if they are intrinsically linked to fast flow during surges? This review aims to give an overview of advances in our understanding of the surge-type glacier landsystem, processes and products that have emerged from the research in Iceland, as well as outlining future research challenges.