Elsevier

Geomorphology

Volume 410, 1 August 2022, 108291
Geomorphology

Changes in ice-surface debris, surface elevation and mass through the active phase of selected Karakoram glacier surges

https://doi.org/10.1016/j.geomorph.2022.108291Get rights and content

Highlights

  • We describe glacier-surface debris and surface elevation changes through surges on ten different Karakoram glaciers.

  • Surface debris transport through the glacier surges leads to folding and concentration of debris near glacier termini, confluences and margins

  • Ice and debris mass redistribution for the ten glaciers varies between 0.11 and 0.94 Gt per event

Abstract

Surge-type glaciers switch between phases of rapid and slow flow on timescales of a few years to decades. During the active surge phase, large volumes of ice are transported downglacier, creating distinct geomorphological signatures that reflect these dynamic events but ice and sediment transport remain poorly quantified. The impact of surge events, in comparison to non-surge activity, is also unclear. Here, we describe glacier-surface debris and elevation changes through surges on ten different Karakoram glaciers (Khurdopin, Drenmang, Kunyang, Braldu, Chong Kumdan II, Qiaogeli, Saxinitulu, Shakesiga, Skamri and North Crown). We use these data to characterise the surface geomorphological changes during the surges. We also calculate the mass redistribution during each of the surges and compare this to an estimate of overall glacier mass. Repeat geomorphological mapping shows that surface debris transport through the surges leads to widespread rearrangement of surface features, folding and the concentration of debris near glacier termini, confluences and margins. Ice and debris mass redistribution varies between 0.11 and 0.94 Gt per event and shows moderate correlation with total glacier volume (r2 = 0.57) and glacier length (r2 = 0.40). Mass change as a proportion of total glacier volume ranges from about 0.5 to 5% and is moderately correlated with total glacier volume (r2 = 0.43) but not glacier length (r2 = 0.10). The overall conclusion is that surge events in this region account for only a small proportion of overall mass transport, but have an important role in surface debris transport and redistribution, as well as glacier mass balance.

Introduction

Surge-type glaciers undergo cyclical non-steady flow consisting of two distinct phases (Meier and Post, 1969). The active phase, typically lasting a few months to a few years, is a dynamic period during which glacier velocity increases by at least an order of magnitude. The quiescent phase, typically lasting tens to a few hundreds of years, is a period of relative stagnation during which the lower portion of the glacier thins and mass builds up in an upper, reservoir area. During surges, mass is transferred down-glacier to the receiving area, sometimes (but not always), resulting in advance of the glacier terminus. The distribution of surge-type glaciers is non-random and they are concentrated in a few areas including the Canadian and Russian High Arctic, Svalbard, Iceland, Greenland, Alaska and across the majority of High Mountain Asia (Murray et al., 2003; Sevestre and Benn, 2015; Guillet et al., 2021). While surges in some areas may be predominantly controlled by changes in hydrology (Kamb et al., 1985; Björnsson, 1998), changes in basal thermal conditions are also important (Clarke et al., 1984; Murray et al., 2000). In the former case, observed examples of surge onset and termination were often rapid and the active phase short, whereas in the latter, observations of more gradual surge onset and cessation were made, with the entire surge taking up to a decade to complete in some cases (Murray et al., 2003). Benn et al. (2019) have argued that a broad spectrum of dynamic behaviour associated with glacier surges exists, which can be best understood in terms of basal enthalpy balance - the difference between enthalpy (thermal energy and water) inputs and outputs at the glacier bed. The highest densities of surge-type glaciers occur within an optimal climatic envelope bounded by temperature and precipitation thresholds (Sevestre and Benn, 2015).

A number of glaciological and geomorphological features indicate surge-type behaviour (Bhambri et al., 2017, Bhambri et al., 2022; Copland et al., 2003; Copland et al., 2011). These include: (1) looped or bulbous moraines and deformed ice structures formed as fast-flowing, active-phase surge-type glaciers flow past less active or stagnant neighbours and deform the medial moraines between them; (2) heavily crevassed ice surfaces, indicative of a surge-type glacier in its active phase; (3) potholes on the glacier surface during the quiescent phase; (4) a rapid advance of a glacier terminus compared to surrounding glaciers; (5) shear margins on the glacier surface, formed at the boundary between fast-moving surging ice and slower non-surging ice; (6) surface velocities that are typically an order of magnitude or so higher than during the quiescent phase; (7) a strandline of ice on surrounding bedrock, formed when the glacier surface lowers rapidly as mass is transferred down-ice during surge initiation. Surge-type glaciers have come under increased scrutiny in recent years due to their association with glacier hazards (Haemmig et al., 2014; Steiner et al., 2018; Bhambri et al., 2017) and their impact on glacier mass balance (Bhattacharya et al., 2021; King et al., 2021). Particular attention has been focussed on surge-type glaciers in the Karakoram, as the region has been identified as one of the few globally that has until recently been in equilibrium or even slightly positive mass balance on a broad scale (Hugonnet et al., 2021; Bolch et al., 2017). Other studies have focussed on surges to provide insight into glacier basal processes (Quincey et al., 2011) and landscape evolution (Diolaiuti et al., 2003; Seong et al., 2009).

High-magnitude velocity perturbations during surges have the potential to alter the ‘normal’ (quiescent phase) glacier surface debris transport patterns (Hambrey and Dowdeswell, 1997; Lawson et al., 2000; Roberts, 2009; Rashid et al., 2018), although surprisingly this has never been studied in a systematic manner. Nowhere is this more important than in high-relief, tectonically active areas such as the Karakoram, where glaciers typically transport large quantities of supraglacial debris (Owen et al., 2003; Herreid and Pellicciotti, 2020). Debris is delivered to the ice surface from degrading lateral moraines, avalanches, debris flows and rockfall on surrounding slopes, sometimes covering the ice completely to a depth of several metres in the ablation zone (Hambrey et al., 2008). In the accumulation zone, surface debris is continually buried by new snowfall and it is usually only exposed lower on the glaciers, where it is concentrated into flow-parallel ridges, including lateral and medial moraines (Hambrey et al., 2008). These features typically merge down-ice to form a more continuous debris cover in the lower ablation zone. Surface ablation concentrates debris into mounds and ridges, and as slope angles increase by differential melting, surface debris is prone to movement by sliding and slumping. Sediment thus accumulates in hollows.

When sufficiently thick, debris further dampens ablation and these hollows gradually become new topographic high points creating topographic inversion (Mölg et al., 2019). Glacier surges also produce distinct sediment-landform assemblages including prominent lateral-terminal moraine complexes, many of which retain an internal ice core. Debris cover is important because it can both inhibit and enhance ablation, depending on its thickness, therefore directly impacting glacier ablation gradients (Adhikari et al., 2000; Kayastha et al., 2000; Nakawo and Rana, 1999; Singh et al., 2000; Rowan et al., 2015). The presence of supraglacial debris, together with enhanced energy absorption at the ice cliffs and supraglacial ponds that are typically scattered across the debris-covered surface, can delay adjustment of the glacier to changing climatic conditions. Thus the behaviour of debris-covered glaciers is not necessarily a useful indicators of current climate change.

The majority of glaciers in the High Mountain Asia have lost mass since at least 1970 and are receding or thinning at an increasing rate today (Bolch et al., 2017; Shean et al., 2020; Bhattacharya et al., 2021; Hugonnet et al., 2021). The pattern in the neighbouring Karakoram is distinctly different though (Rankl and Braun, 2016; Bolch et al., 2017; Zhou et al., 2017). Using Landsat imagery from 1976 to 2012, Rankl et al. (2014) analysed 1219 Karakoram glaciers and concluded that the vast majority (969) showed a stable terminus position during the observation period. Fifty-six glaciers advanced, 93 glaciers retreated, and 101 surge-type glaciers were identified, of which 10 were new observations. Scherler et al. (2011) showed similar results, concluding that 58% of Karakoram glaciers had stable or advancing terminus positions between 2000 and 2008 with the remaining 42% retreating. A range of studies using multiple elevation datasets to calculate mass changes have previously reported a positive or balanced mass regime in the region since the start of the century (Brun et al., 2017; Shean et al., 2020), although recent data suggest this anomalous pattern is weakening or has ended (Hugonnet et al., 2021). There appears to be no clear difference in the mass balance of surge and non-surge type glaciers when calculated at the regional level in the Karakoram (Gardelle et al., 2012; Guillet et al., 2021).

The aim of this paper is to present data on changes in the surface characteristics of ten selected Karakoram glaciers (Khurdopin, Drenmang, Kunyang, Braldu, Chong Kumdan II, Qiogeli, Saxintulu, Shakesiga, Skamri and North Crown) through the active phase of their recent surge cycles (Fig. 1). Measurements of glacier surface elevation pre- and post-surge, combined with observed changes in their distinct geomorphology in each case, provide the opportunity to make explicit links between surface morphological change and mass (ice and debris) transport for the first time, as well as allowing us to quantify the importance of surge events for mass transfer within the broader glacier system.

Section snippets

Study area

Surge-type glaciers may represent ~45% of glacier area in the Karakoram (223 out of a total of 11,586 glaciers, covering 9736 km2 out of a total of 21,475 km2 according to Guillet et al., 2021), as they tend to be longer, larger and cover a greater elevation range than non-surge-type glaciers (Hewitt, 1969). Surges in the Karakoram may develop quickly (Kick, 1958; Gardner and Hewitt, 1990), or more gradually over several years (Paul, 2015; Quincey et al., 2011). Surge phase velocities increase

Geomorphological mapping

ASTER, Landsat TM, Landsat ETM+ and Landsat OLI satellite imagery provided coverage of each of the ten surges (Supplementary Table 1) at appropriate spatial and temporal resolution to construct a timeline of surge events (Fig. 2). The 15 m spatial resolution of the panchromatic band was sufficient to be able to delineate the large surface structures and debris patterns that characterise the surface morphology of Karakoram glaciers. Cloud-free and snow-free images were required for the

Geomorphological and geodetic observations for individual glaciers

In this section we describe the main changes in glacier surface elevation and ice-surface morphology through each surge event (Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9).

Shakesiga Glacier (35° 46′ 51.93″ N, 76° 49′ 08.92″ E) is a long (~27 km) glacier confined to a narrow valley (Fig. 1). The majority of the glacier's total area comprises a single tongue into which several small tributaries flow in the higher part of the valley. The most recent surge of the glacier occurred between

Changes in surface debris and surface elevation

The net result of each of the ten glacier surges was to increase debris supply towards the terminus, although each of the surges had different effects on the surface debris distribution. In some cases (e.g. Skamri Glacier, Braldu Glacier, Chong Kumdan II Glacier, Khurdopin Glacier) longitudinal surface structures became folded with debris on the glacier surface and there is evidence of surface deformation during rapid transport of debris towards the terminus (Fig. 5, Fig. 6, Fig. 9). In other

Conclusion

We have combined observations of changes in glacier surface morphology with a quantitative analysis of surface elevation and mass change through the active phase of ten different Karakoram glacier surges (Khurdopin, Drenmang, Kunyang, Braldu, Chong Kumdan II, Qiaogeli, Saxintulu, Shakesiga, Skamri and North Crown). Glacier surface debris transport through the surges includes widespread rearrangement of surface debris features, folding and the concentration of debris near glacier termini,

Declaration of competing interest

The authors declare no financial interests.

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      These flow regimes produce many patterns of deformation that have motivated the study of glacier structure, both as an analogue for crustal deformation (e.g. Ramberg, 1964; Hudleston, 1977; Sharp et al., 1988; Herbst and Neubauer, 2000) and as a way of better characterising the underlying dynamic controls on glacier change (e.g. Allen et al., 1960; Lawson et al., 1994; Hambrey and Glasser, 2003; Hambrey et al., 2005; Jiskoot et al., 2017; Trantow and Herzfeld, 2018; Monz et al., 2021; Jennings and Hambrey, 2021). Among the most iconic glacier structures are spectacular glacier-scale fold trains, often spanning tens of kilometres downvalley on surge-type glaciers and highlighted by medial moraines (supraglacial debris trails) inherited from upstream dendritic valley networks (e.g. Gripp, 1929; Post, 1969, 1972; Driscoll, 1980; Lawson et al., 1994; Herreid and Truffer, 2016; Glasser et al., 2022) (Fig. 1). These large fold trains record the flux histories of glaciers that experience surges: dramatic and transient shifts in the partitioning of ice flow between internal ductile polycrystalline ice creep (Nye, 1951; Glen, 1955; Duval et al., 2010) and frictionally-controlled sliding at the glacier bed (Kamb and LaChapelle, 1964; Kamb, 1970).

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