Surge-type Uisu glacier and its undisturbed forefield relief, Eastern Pamir, Tajikistan
, und | 31. Okt. 2022
Online veröffentlicht: 31. Okt. 2022
Seitenbereich: 227 - 236
Eingereicht: 20. Apr. 2022
Akzeptiert: 23. Mai 2022
DOI: https://doi.org/10.2478/mgrsd-2022-0009
Schlüsselwörter
© 2022 Bogdan Gądek et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.
Surging glaciers are distinguished by internally triggered cyclical instability. In the quiescent phase, they do not reach balance velocity, which leads to the build-up of ice in the accumulation zone. This ice is released rapidly in the active phase, during which – by contrast – the balance velocity is exceeded. The movement of the surge front is associated with changes in the longitudinal profile of glaciers and the intensive formation of crevasses (Meier & Post 1969; Baranowski 1977; Raymond 1987; Harrison & Post 2003). The surge can propagate as far as the snout, and can sometimes lead to its far-reaching advance (e.g. Dolgushin & Osipova 1975; Clarke 1987; Braun et al. 2011; Mansell et al. 2012; Ingólfsson et al. 2016). The durations of the surge and quiescent phases are expressed in months–years and decades–centuries, respectively (Ingólfsson et al. 2016).
Even though glaciers of all types and in almost all climatic conditions surge (Meier & Post 1969), during the timespan over which glaciers have been monitored the phenomenon was observed in only about 1% of the Earth's glaciers, mostly in the Arctic and Central Asia (Jiskoot et al. 1998). However, taking into account the controlling factors and surge models (Sevestre & Benn 2015; Benn et al. 2019), as well as changes in environmental conditions over time, it can be presumed that the distribution of surging glaciers has changed and that glaciers in areas where the phenomenon is not observed at present have also surged in the past (e.g. Serrano & Martín-Moreno 2018). The maximum extents of some Pleistocene glacier lobes and glaciers in the mountains of North America and Europe could also have been associated with surging (Evans & Rea 1999; Benn 2021).
The characteristics of glaciers in a surge phase include (Copland et al. 2011): 1) acceleration of surface velocities/extensive crevassing, 2) rapid terminus advance, 3) looped/folded medial moraines and surface foliation, and 4) strandlines on valley side walls. In turn, former surges are reflected in the diagnostic components of the surging glacier landsystem (Evans & Rea 1999, 2003), wherein three overlapping geomorphic zones may exist: (1) subglacial till, flutes, drumlins, crevasse-squeeze ridges, zig-zag eskers (proximal zone), 2) hummocky moraine (intermediate zone), and 3) thrust block, composite thrust moraines (external zone).
Therefore, we were surprised not to find any landforms evidencing a surge of the Pamir Uisu glacier, which, following a noticeable advance, has been in the quiescent phase for at least several decades (Osipova et al. 1998). The purpose of this paper is to present the Uisu glacier morphodynamics and its forefield geomorphology, focussing on the glacier–landforms relationships. The results may be useful for interpreting the relief of areas glaciated in the past, when establishing a regional chronology of former glaciations and conducting palaeogeographical studies.
The Uisu glacier (39°19′25″N; 73°08′49″E) lies in the upper section of the Koksoy valley (the Markansu catchment area) at the confluence of the Zaalayski and the Zulumart Ranges in the Tajik part of Eastern Pamir (Gorno-Badakhshan Autonomous Region, Tajikistan). It is a surging valley glacier fed by six tributary glaciers, which are assigned the numbers 44 to 49 according to the USSR glacier inventory (Katalog Lednikov SSSR 1973). In the 1980s, the length and surface area of the Uisu glacier system were 12.9 km and 51.5 km2, respectively, and the equilibrium line lay at 5190 m a.s.l. (Osipova et al. 1998). Currently, this glacier extends from about 6200 m a.s.l. to 4400 m a.s.l., with the frontal zone formed by dead ice covered by an extensive supraglacial moraine with numerous meltwater lakes (Fig. 1).
Figure 1
Location of the study area on continent (a), region (b), and valley (c). The crosshair, and the geographical coordinates assigned to it, indicate the central part of the Uisu glacier system, numbers 44–68 identify the tributary glaciers according to the USSR glacier inventory (Katalog Lednikov SSSR 1973), the white frame covers the area of detailed geomorphological mapping, the elevations at the spots marked with black dots are expressed in metres above sea level
Source: own study based on the satellite imagery provided by Google Maps: Image, © 2022 TerraMetrics
The study area is built of crystalline and sedimentary rocks of the Carboniferous-Lower Permian age (Lozev 1968). The area, which was uplifted intensively during the Hercynian and Alpine orogenesis (Zaharov et al. 1968), is still characterised by considerable tectonic activity (Strecker et al. 1995; Havenith & Bourdeau 2010). In the Pleistocene, the entire Badakhshan part of the Markansu valley was glaciated (Trofimov 1968).
The contemporary climate is cold and arid with high insolation and strong winds. The average annual air temperature at the Karakul meteorological station (3930 m a.s.l.; 45 km from the terminus of the Uisu glacier) is about −4°C, and the annual precipitation is close to 80 mm (Komatsu & Tsukamoto 2015; Heinecke et al. 2017). The climatic and topographic conditions are conducive to permafrost, which is common in the area (Gruber 2012). Although current global warming has resulted in both the retreat of glaciers (Khromova et al. 2006; Kayumov 2010) and the degradation of permafrost (Mętrak et al. 2019), the changes in Eastern Pamir are proceeding much more slowly (e.g. Lv et al. 2019) than in other areas of the world (Zemp et al. 2015; Oliva & Fritz 2018).
Eight cloudless Landsat satellite scenes acquired during the summer and early autumn of 1977–2019 were used (Table 1). True colour composites with georeferencing (UTM map projection; WGS 84 datum) were downloaded from the USGS Earth Explorer website (
List of Landsat satellite scenes used. MSS: MultiSpectral Scanner (Landsat 1), TM: Thematic Mapper (Landsat 5), ETM+: Enhanced Thematic Mapper + (Landsat 7), and OLI: Operational Land Imager (Landsat 8). “Date” shows day.month, and “P-R” shows path-row. Source: USGS Earth Explorer
| 1 | MSS | 22.08 | 1977 | 162-033 |
| 2 | TM | 30.09 | 1993 | 151-033 |
| 3 | TM | 28.09 | 1998 | 151-033 |
| 4 | ETM+ | 28.09 | 2001 | 151-033 |
| 5 | ETM+ | 15.09 | 2008 | 151-033 |
| 6 | OLI | 05.09 | 2013 | 151-033 |
| 7 | OLI | 15.08 | 2017 | 151-033 |
| 8 | OLI | 22.09 | 2019 | 151-033 |
Satellite images were primarily used to control the location of the terminus and measure the surface area of the Uisu glacier (including the supraglacial moraine) over the past 42 years. In addition, images with a resolution of 30 m were used to determine the approximate displacement speeds of characteristic areas of its surface in the vicinity of the central flow line (e.g. Kotlyakov et al. 2008). In the process, the slope of the glacier surface, calculated on the basis of the SRTM-C (3”) model, was taken into account. In addition, changes in the surface area of identified supraglacial lakes were determined. All measurements were made manually in the QGIS software.
Detailed landform mapping was completed for an area of 280 ha. The area covered the terminal section of the Uisu glacier, its forefield to a distance of 700 m, and lower sections of the valley slopes (Fig. 1). The fieldwork involved identifying all landforms and drawing their contours on an orthophotomap with a resolution of 0.5 m. The orthophotomap dates back to September 2011 and was imported from Microsoft Bing Maps. The outlines of the forms located in the forefield of the Uisu glacier within the bottom of the Koksoy valley were made using a GNSS telephone receiver and the QField v.1.0 application (OPENGIS.ch). The other forms were contoured by hand through their simultaneous identification in the field and on the orthophotomap. In order to correlate the fluvioglacial terraces with each other, measurements were taken of their heights in the valley cross-section. In addition, the heights of the termini of the Uisu glacier and neighbouring rock glaciers were measured by means of a TruPulse 360 B laser rangefinder (Laser Technology Inc.). The instrument's factory-set accuracy is within the 0.3–1 m range.
The geomorphological map was made on the basis of the WGS 84 coordinate system using the QGIS software. Depending on their genesis, the individual landforms were assigned colours and some symbols, proposed by Gustavsson et al. (2006), following the rule that the older the form, the lighter the colour (Gilewska 1968). In addition, the SRTM-C (3”) model was used to generate the contour lines. Complementary morphometric measurements were also made using GIS tools (measure line, area, angle).
From the Landsat satellite imagery, it could be determined that the front of the Uisu glacier did not change its position over the 1977–2019 period (Fig. 2). However, although the glacier continued to be approximately 12.9 km in length, its surface area decreased by 12.8%. The ice surface area shrank by 13.7%, while the supraglacial moraine area increased by 8.1%. These changes progressed consistently from year to year. In 2019, the areas of the active and inactive parts of the glacier were 45.8 km2 and 2.6 km2, respectively (Fig. 3). The above signs of the glacier's shrinkage resulted from the gradual decrease in its thickness. The decreasing supply of ice from all the lateral valleys is also reflected in the redirection of the movement of the termini of glaciers 47 and 45 towards the centre of the valley (Fig. 2).
Figure 2
The Uisu glacier system in: a) 1993, and b) 2019. Numbers 45 and 47 stand for two tributary glaciers according to the USSR glacier inventory (for explanation, see text)
Source: own study based on the Landsat satellite imagery provided by the USGS Earth Explorer
Figure 3
Surface area of the Uisu glacier system in 1977–2019. Light grey bars: total area, white bars: area of the part without supraglacial cover, dark grey bars: area of supraglacial cover
Source: own study
Surface velocity was measured on the tongue of Glacier No. 47 and in the terminal part of Glacier No. 45. In the former case, the tongue of the glacier lying below the equilibrium line is densely crevassed and carries packages of debris delivered from the rocky slopes situated above the accumulation zone as well as from Glacier No. 49. By contrast, in the latter case, medial moraines bend characteristically under the pressure of the mainstream Uisu glacier (Fig. 1).
The surface velocity of Glacier No. 47 (along the central line) increased with distance from the terminus – from a dozen or so m a−1 to >100 m a−1 (Fig. 4). The speed at which this part of the medial moraine of Glacier No. 45 travelled was also a dozen or so m a−1.
Figure 4
Surface velocity along the central line of the tongue of the Uisu glacier and Glacier No. 47 in the 1993–2019 period
Source: own study
The part of the Uisu glacier system covered with supraglacial moraine did not show any movement over a zone of about 2100 m in length. The gradual increase in the total surface area of the meltwater lakes within this zone may be indicative of its progressive degradation. The surface area of the supraglacial lakes identified in the Landsat imagery over the past 26 years has increased by 89% (Fig. 5).
Figure 5
Surface area of the supraglacial lakes in 1993–2019
Source: own study
The study area features landforms created by glacial, fluvioglacial, paraglacial, periglacial and slope processes (Fig. 6).
Figure 6
Geomorphological map of Uisu glacier forefield
Source: own study
The front of the Uisu glacier showed no movement during the period of field observations. Its ice cliffs, with an average height close to 50 m, were covered with light grey mineral sediments of the silty and sandy fractions, together with a small proportion of thicker deposits (Fig. 7a). The previous activity of this part of the glacier is reflected in the relief of the supraglacial moraine, which consists of forms arranged in flow-banded arcs. The present-day degradation of these forms is manifested in numerous kettles (Fig. 7b), the area of the largest ones exceeding 5000 m2. The width of the part of the supraglacial moraine examined ranges from 400 m to 1200 m. The glacier, which transports this moraine, has moved onto well-developed fluvioglacial terraces, squeezing into a narrowing valley between two lateral rock glaciers. At times of ablation, the mineral material slides off the melting ice walls or flows down them with melt water, creating small colluvial and silt loam deposits at their base. Glacial ice with clear oblique foliation (Fig. 7c) is revealed from the side of the valley slopes, proving that during times of activity of this part of the glacier, the trajectories of movement of the ice were directed towards its surface (compressive flow).
Figure 7
Terminal zone of Uisu glacier: a) glacier front, b) supraglacial moraine with meltwater lakes and kettle holes, c) supraglacial material and outcrop of glacial ice with oblique foliation – view from the northern side of the valley
Source: own study; aerial photos: Marcin Zegarek (a and b), terrestrial photo: Bogdan Gądek (c)
The ice-cored lateral moraine of the neighbouring glacier is located on the southern side of the terminus of the Uisu glacier. This glacier is in retreat, but it has receded only 250 m. The distal slopes of the moraine are modelled primarily by slope processes associated with the undercutting of the bank by the Koksoy proglacial river (Fig. 6).
Melt waters from the Uisu glacier are drained through narrow channels between the dead ice, with its supraglacial moraine, and the valley slopes. The main watercourse runs along the slopes on the south side of the valley. Outside the area where the glacier comes into contact with rock glaciers, proglacial waters flow in many, often redirected, channels (braided type). The channels join at a distance of a few hundred metres from the glacier terminus. Along this section their width ranges from 200 m to 400 m (Fig. 8a).
Figure 8
Fluvioglacial forefield of Uisu glacier: a) multi-channel bed of the Koksoy river, b) terrace in front of the glacier terminus. T1 – terrace 1, T2 – terrace 2
Source: own study; aerial photos: Marcin Zegarek
There are two levels of fluvioglacial terraces within the glacier forefield under study. The height of the lower level (terrace 1) is 1.0–1.5 m. On the northern side of the Koksoy valley, only two fragments of the terrace have survived: 40 m and 80 m wide, respectively. They are located at the mouth of a lateral valley, at the base of the old levels of the alluvial fan. Terrace 1 is better preserved on the south side of the main valley, where its maximum width is 240 m. The height of the higher level terrace (terrace 2) exceeds 4 m. Fragments of this have been preserved in front of the glacier itself, ahead of one of the older levels of the above-mentioned alluvial fan and at the base of a rock glacier on the southern side of the valley (Fig. 6). Terrace 2, in front of the Uisu glacier, has a triangular shape (Fig. 8b), with sides of close to 200 m in length. Only the northern escarpment is well preserved. The south-east sector of the terrace is being degraded by the main proglacial stream. Meanwhile, the melting ice cliff is a source of light grey silt loam and small-sized debris colluvia deposited on the fluvioglacial surface. No glacitectonic deformation of the glacier forefield was found. However, traces of ancient riverbeds (braided type) and sorted polygons are clearly discernible on all the fragments of the level 2 terrace.
The alluvial fan mentioned above was chiefly formed by proglacial waters. Currently, they flow out of a debris-covered residual glacier a few hundred metres away from the base of the alluvial fan. The fan consists of five alluvial levels varying in age (Fig. 9a). The contemporary level is modelled in spring – during the thaw, and in summer – mainly on warm days during the hours of maximum insolation. Deposition of rock material carried by the stream has resulted in the covering of two older fan levels (1 and 2). In its south-eastern part, river terraces 1 and 2 are the continuation of alluvial levels 1 and 2, respectively. Above these there are two older alluvial levels (3 and 4) with well-developed sorted polygons. The surfaces of their western fragments are located 23 m and 33 m above river terrace 1, respectively. In the upper part of the Koksoy valley, on the eastern side of the outlet of the adjacent lateral valley, the three oldest alluvial levels survive with a rock-glacierised moraine creeping onto them (Fig. 9b). The water draining the glacier and the contemporary alluvial fan currently lie on its western side (Fig. 6).
Figure 9
Alluvial fans at the mouths of valleys located on the northern side of the terminal zone of the Uisu glacier: a) mouth of the eastern valley, b) mouth of the western valley (see Fig. 6). AF – alluvial fan, 1–4 old alluvial levels, RGm – rock glacier (rock-glacierised moraine), Gl – lateral edge of Uisu glacier
Source: own study; aerial photo: Marcin Zegarek (a), terrestrial photo: Bogdan Gądek (b)
The rock glacier in the northern part of the Koksoy valley emerges from a push moraine closing a lateral valley, and has the shape of a wide fan. It is 670 m long, 630 m wide, and its area exceeds 0.27 km2. The debris material forms semi-circular mounds on the surface, with small lakes developed in between as a result of the degradation of buried glacial ice. The relief and geometry of the rock glacier indicate that its southward creep had proceeded uninterrupted for a long time, and was only stopped by the advance of the Uisu glacier, which has blocked the creep and resulted in small lobes of ice-debris flowing eastwards.
The second rock-glacierised moraine, covering an area of 0.38 km2, is located in the forefield of a lateral glacier on the southern side of the Koksoy valley (Fig. 10b). The height of its steep terminus exceeds 40 m. It originated from a supraglacial moraine and its interior is still formed of glacial ice. In the western part, the rock glacier comes into contact with the above-mentioned lateral-terminal moraine, and is separated from the snout of the Uisu glacier by a proglacial riverbed. Meanwhile, it creeps into the Koksoy valley from the east onto fluvioglacial terrace 1 (Fig. 6). Its left side, looking downstream, is more distant from the contemporary glacier and is more degraded.
Figure 10
Geomorphology of the terminal zone of the Uisu glacier: a) slopes on the northern side; b) slopes on the southern side and forefield (Gl – front of the Uisu glacier, RGm – rock-glacierised moraine, RG – periglacial rock glacier, S – talus slope with solifluction lobes, AF – alluvial fan, 2–4 – old alluvial levels, LM – lateral moraine, T2 – fluvioglacial terrace 2; c) sorted polygons on the surface of terrace T2 (Fig. 10b)
Source: own study; photos: Bogdan Gądek
There is also a periglacial rock glacier in the immediate vicinity of the Uisu glacier. It is located on the north side of the Koksoy valley, at the foot of a shallow rocky niche with southern exposure (Fig. 6). It has the shape of a convex lobe consisting of many flow-banded ridges (Fig. 10a). Its area is close to 0.065 km2, and the height of the terminus is 10 m. Most probably, the creeping talus material is bonded with pore ice below the active layer. By contrast, all that can be seen on the surface of the adjacent talus slopes are small solifluction swells (Fig. 10a).
The periglacial environment is also evidenced by sorted polygons, which occur in large numbers on the river-glacial terraces and old alluvial fans. On flat surfaces, their diameter ranges from 0.15 m to about 1 m (Fig. 10c).
The remote sensing data from the 1977–2019 period and the data contained in the inventory of surging Pamir glaciers (Osipova et al. 1998) show that the position of the terminus of the Uisu glacier has not changed since at least 1946. However, the Uisu glacier is losing mass, although the changes are proceeding slowly even when compared to other slowly transforming Pamir glaciers (Osipova & Tsvetkov 2002; Khromova et al. 2006; Kayumov 2010; Lv et al. 2019). This recession is mainly manifested in a gradual decrease in thickness and, consequently, in the surface area of the glaciers, and in the degradation of the supraglacial moraine. This weakening of the response to climate warming is typical of glaciers whose tongues are covered with debris (Mayer et al. 2006; Lambrecht et al. 2014; Schomacker & Benediktsson 2018; Tielidze et al. 2020).
In the years 1993–2019, the Uisu glacier did not surge. Osipova et al. (1998) classified it as a surging glacier on the basis of changes in the width of the component glaciers, enhanced ice flow, and the displacement of activation fronts in the years 1966–1990. All these changes took place inside the glacial system. The photo texture of the final supraglacial cover makes the Uisu glacier resemble many surge-type glaciers in the mountains of Central Asia (e.g. Yao et al. 2016), as is shown particularly clearly by an animation of the sequence of satellite images of the Karakoram glaciers (Paul 2015). This suggests that the contemporary location of the terminus of the Uisu glacier may be the result of its former surge.
The genesis, distribution, and shape of the landforms in the forefield of the Uisu glacier, and its steep and high terminus, also imply that its current range is the result of rapid advance. The lack of moraine material at the bottom of the valley, the well-developed fluvioglacial levels with sorted polygons, and rock glaciers at the interface with the Uisu glacier prove that the glacier was smaller than today throughout most of the Holocene. According to Kabala et al. (2021), terrace 2, overlapped by the Uisu glacier, may have developed between 5000 cal BP and 650/700 cal AD. Given the ranges of Central Asian glaciers during the Little Ice Age (LIA), and their geomorphological evolution (Savoskul 1997; Aizen 2011; Li et al. 2016), it can be assumed that moraines were formed at the outlets of the valleys terminating in the Koksoy valley during the LIA. Based on the glaciated valley landsystem model (Benn et al. 2003), it can be expected that these moraines were formed by glaciers with low ice supply and moderate debris supply, while the ice advection was balanced by frontal melt (steady-state conditions). The tongue of the Uisu glacier, intensively supplied by ice and debris, moved into the narrowing of the valley between the rock glaciers creeping out of these moraines. Therefore the advance could have occurred sometime in the period between the Little Ice Age and 1947. The lack of a moraine in front of the Uisu glacier terminus, and the progressive degradation of its terminal zone, prove that the advance has been the result of the loss of stability of the Uisu glacier.
Glacier surges are commonly characterised by rapid sliding, high rates of bed abrasion, redistribution of substratum sediments towards the marginal zone covered by fluvioglacial gravels, and sediments elevated to the glacier tongue surface, where they can form an extensive mantle (Benn & Evans 2010). According to Krüger et al. (2010), the supraglacial landscape formation along the Uisu glacier is in the initial phase, despite the fact that the formation has already been underway for several decades. The lack of any glaciotectonic substrata deformations and/or marginal landforms, which would indicate a surge (Evans & Rea 2003; Ingólfsson et al. 2016), are a specific feature of the present-day forefield of the Uisu glacier. On the other hand, in genetic terms, the forefield of the Uisu glacier is similar to the forefields of the neighbouring creeping rock glaciers. It might be assumed that the terminal section of the glacier's tongue, which is covered with supraglacial moraine, is also creeping, and has covered the marginal glacitectonic landforms during the time that has elapsed since its advance. However, the flow-banded ridges on the supraglacial moraine in line with the direction of the glacier axis, the high ice cliff, and the undeformed ice foliation rule this out. Thus, there must be other reasons for the lack of surge-diagnostic landforms on the Uisu glacier forefield. They may include the small thickness of deformable, fine-grained subglacial sediments (Benn & Evans 2010) and/or the overtaking of these sediments by the rapid moving glacier terminus (Benediktsson et al. 2009) – especially in winter, when the active layer of permafrost in the glacier's forefield is frozen (Bennett et al. 2003). Based on the geomorphological evidence, it can only be concluded that, when surging, the Uisu glacier did not form any glacial landforms in its present-day forefield. It can also be expected that in the future, after the ice has completely melted in the terminal area with the supraglacial cover, the current extent of the Uisu glacier will be revealed by the spread of the hummocky moraine and by other glacial landforms formed in this zone (Benn 2021), if the latter are not buried or washed out during deglaciation. Therefore, the presence of hummocky moraine in the glaciated areas in the past and, at the same time, the lack of marginal glacial landforms, may indicate not only the long-term stagnation of the glacial terminal zone, but also a past surge.
The results of the glaciological and geomorphological observations in the Koksoy valley presented above may be useful in interpreting the glacial relief of Pamir and other areas. This is exemplified by the hummocky moraines surrounded by fluvioglacial fans in the forefield of the neighbouring Oktyabr'skiy glacier (Osipova & Tsvetkov 2002), which reached as far as Lake Karakul during the Late Glacial Maximum (Mischke et al. 2010).
The current location of the Uisu glacier terminus is the result of a surge that occurred before 1947. Currently, the glacier is losing mass, which is evidenced by the progressive decrease in its thickness and active surface and the increasing surface area of the supraglacial moraine and kettle holes. There are visible symptoms of dendritic disintegration of the glacier into the main component glaciers (shortening of their active parts). However, the front of the zone with supraglacial cover has been stable for several decades.
The lack of landforms created by the Uisu glacier in its outer zone indicates that advances of surging glaciers may not always produce surge-diagnostic features in their forefields. Thus, in glaciated areas, a glacier surge can also be evidenced by the presence of hummocky moraine (former supraglacial moraine) with no other glacial landforms on its edges (such as terminal and lateral moraines).
The genesis and interactions of landforms can reveal the glacier dynamics, even in the absence of glacial landforms.