Sund.indd 115Sund 2006: Polar Research 25(2), 115–122 A surge of Skobreen, Svalbard Monica Sund Surging glaciers are common in Svalbard yet relatively few glaciers have been observed during a surge. This paper presents observations of the currently surging glacier Skobreen, in southern Spitsbergen. The study is based on examinations of new and archival photographs and maps. Sko- breen, an 18 km2 valley glacier terminating into the lower part of the gla- cier Paulabreen, has not been registered previously as a surging glacier. Skobreen experienced a build-up in its upper part, while there has been a lowering of the surface in the terminal region. Photographs from 1990 show incipient crevassing in the upper part. Photographs from 2003 show a slight advance of the terminus and marginal crevassing, indicating an initiation period of about 15 years for a surge of this glacier. In June 2005 transverse crevassing appeared in the upper part of the glacier, while the middle section moved as a block with strong shear margins and a pro- nounced drawdown of the ice surface. No traces of a surge front could be seen in the crevasse pattern. However, the crevasse pattern indicates an initiation area in the transition zone between the transverse crevassing in the upper part and the block of ice in the middle region. M. Sund, Svalbard Science Forum, Box 506, NO-9171 Longyearbyen, Norway; sund.monica@gmail.com. Glacier surges are characterized by cyclical advances related to internal changes in the gla- cier system rather than to climate change (Meier & Post 1969). The cause of these instabilities is still debated. Thermal trigger mechanism have been proposed (e.g. Schytt 1969; Clarke 1976), and changes in the subglacial drainage system are also thought to be important to the triggering of surges (Clarke et al. 1984; Kamb et al. 1985). The trigger mechanism has yet to be identifi ed adequately but identifying the trigger zone will contribute to our understanding of the elements involved in a surge (Lawson 1996). Surge-type glaciers represent only a small per- centage of all glaciers and the phenomenon is highly concentrated in some glaciated regions (Raymond 1987). Surges constitute a common form of glacier advance in Svalbard (Liestøl 1969). From 13 % (Jiskoot et al. 1998) up to 90 % (Liestøl 1988; Lefauconnier & Hagen 1991) of the glaciers here are assumed to be surge-type. Nevertheless, relatively few glaciers have been observed during the active surge phase (Rol stad et al. 1997). The identifi cation of surge activity is essential in the study of the duration of the active and quiescent phases of surging glaciers (Dowdes- well et al. 1995). The initiation of a surge is normally seen as surface crevassing of the glacier in areas where crevasses have not been present earlier and/or by an advancing glacier tongue (Hagen et al. 1993). Crevasses are easily mapped on glaciers, for example, from aerial photographs. They are fun- damentally important to the understanding and interpretation of glacier fl ow (Meier 1958; Whil- lans et al. 1993). Crevasses form in a perpendic- ular direction to the principal extending strain rate if the strain rate exceeds a threshold value (Nye 1952; Meier et al. 1974). Valuable informa- tion can be acquired from observations of the cre- vassing in an early stage of a surge. Later during the surge, the initial crevasse pattern will become 116 A surge of Skobreen, Svalbard more obscured. The location on the glacier at which the surge motion is initiated is one of the basic spatial properties of a surge. Determination of this location is based on an understanding of the relationship between surge behaviour and cre- vassing (Hodgkins & Dowdeswell 1994; Lawson 1997). The objective of this paper is to document Sko- breen’s ongoing surge. Photographs of Skobreen during a surge—at a stage which makes it possi- ble to interpret its surge dynamics—are present- ed. An additional objective is to pinpoint an ini- tiation area through the crevasse pattern and to describe the recent build-up phase using various available sources. Skobreen Located at 77° 42' N, 17° 03' E, Skobreen is a 8.2 km long valley glacier with a drainage basin of 18.2 km2 (Hagen et al. 1993) in Heer Land in southern Spitsbergen (Fig. 1). The glacier fl ows north-eastward, towards the lower part of the gla- cier Paulabreen, a 16 km long surge-type tide- water glacier (Hagen et al. 1993) feeding into the bay Rindersbukta at the head of Van Mijen- fjorden. The upper part of Skobreen can be divid- ed into one main basin, followed by three sub- basins along the south-eastern side. The elevation ranges from approximately 200 to 800 m a.s.l., and the ELA is estimated at 400 m a.s.l. (Simões 1990). Mountains from 500 to 890 m a.s.l. sur- round the glacier, which overlies approximately horizontal Tertiary and Cretaceous sedimentary rocks (Salvigsen et al. 1989). Skobreen was surveyed by radio-echo sound- ing (RES) in 1985 and 1986 by a team from the Scott Polar Research Institute (Drewry 1985, 1987). A rock sill was found near the glacier ter- minus (Drewry 1987) and the mean ice thick- ness of the glacier was determined to be 135 m (Simões 1990). Most Svalbard glaciers are clas- sifi ed as polythermal (Schytt 1969; Liestøl 1988) and Skobreen has been assumed to have a poly- thermal regime (Drewry 1985; Simões 1990). Skobreen has not previously been registered as a surge-type glacier (Simões 1990; Hagen et al. 1993). However, the glacier was found to be extremely sensitive to small variations in air tem- perature. Simões (1990) concluded, based on this and the RES, that the possibility of surge behav- iour, with a long quiescent phase, could not be ruled out. The moraine in front of the snout of Skobreen is classifi ed as a lateral moraine of Pau- labreen that has been pushed by Skobreen some Fig. 1. The location of Skobreen. (A) marks the develop- ment of new crevasses in the bergs chrund area; (B) marks the site marginal crevasses; (C) indicates the location of trans- verse crevasses; (D) is the site of a slight bulge in 2003; and (E) is the possible initiation area of the surge. Paulabreen terminates in Rindersbukta, Van Mijenfjorden. Svalbard 117Sund 2006: Polar Research 25(2), 115–122 time in the past, and not as a terminal moraine of Skobreen (Simões 1990). Data sources and methods Two sets of oblique aerial photographs covering the entire Skobreen were obtained during com- mercial fl ights on 14 June 2005 and 20 July 2005. A digital camera with 4 megapixel resolution was used. The photographs were taken from an alti- tude of approximately 1500 - 2000 m a.s.l. Slide photographs taken during a fl ight on 9 Septem- ber 2003 have also been examined—these images cover only the terminus of Skobreen and Paula- breen on account of fog. M. Jochmann from Store Norske Spitsbergen Kullkompani (SNSK) pro- vided a photograph from 25 August 2003. The Norwegian Polar Institute’s (NPI) archive of ver- tical aerial photographs from various years was also studied (some photographs only cover part of the glacier). The contrast in the snow in the 1970 images is very good and is also rather good in the 1990 photographs. All photographs are listed in Table 1. It is also worth mentioning that a camera set up by The University Centre in Svalbard, in cooperation with SNSK, documented an advance (not “surge”, as stated on the website at www. unis.no/research/geology/projects.htm) of Paula- breen’s front from 21 April to 15 August 2005. The NPI’s 1 : 100 000 topographic map C10 Braganzavågen (NPI 1983), on which Skobreen appears, is based on a photogrammetric compila- tion of oblique aerial photographs from 1936 (and 1938). A new unpublished map made on the basis of 1990 aerial photographs was also obtained. Longitudinal profi les of the glacier centreline were drawn on the two maps (Figs. 1, 2). The qual- ity of the older maps based on the oblique aerial photographs is dependent on the object’s distance from the camera. In this case, Skobreen is rather close and the typical error in height is ± 2.5 m, while the horizontal error is 20 m (T. Eiken, pers. comm. September 2005). Comparison of the con- tour lines on bedrock on the 1936 and 1990 maps indicates that the error in the old map is no more than 10 m. Results Quiescent phase A notable feature of the Skobreen–Paulabreen interaction is the rather stable position of the ter- minus and moraine in front of Skobreen during the studied period, 1936–1990. The moraine is more pronounced in the 1990 photographs, indicating a slight lowering of the glacier terminus. Sever- al supraglacial channels on Skobreen are visible on the various aerial photographs of the glacier: three main channels draining from the upper part and two channels that cross the moraine to Pau- labreen. The location of these channels is very much the same in the different years, indicating stable conditions. The stable termini of Skobreen and Paulabreen, together with the integrity of the meltwater channels, suggest little activity in the lower parts of both glaciers prior to the present surge. The stable termini of Skobreen and Paulabreen, together with the semicircular shape of the Sko- breen moraine and the integrity of the meltwater channels, suggest low velocities and little activity Table 1. Photographs used in this study. Year Date Source Angle or scale Number Remarks 1936 – NPI oblique S36 2340 - 2344 3000 - 3500 m a.s.l. 1956 – NPI oblique S56 0536 - 0539 covers only the lower part 1961 15 Aug NPI 1 : 40 000 S61 2995 - 2996 covers only the upper part 1970 23 - 25 Aug NPI 1 : 17 000 S70 4617 - 4618 1977 16 Aug NPI 1 : 20 000 S77 0892 covers only a part 1990 25 Aug NPI 1 : 50 000 S90 6826 - 6827 2003 25 Aug M. Jochmann covers only the terminus 2003 9 Sep M. Sund covers only the terminus 2005 14 Jun M. Sund 2005 20 Jul M. Sund 118 A surge of Skobreen, Svalbard in lower parts of both glaciers prior to the present surge. It is not clear when Paulabreen’s last surge took place, but several previous surges of the Paulabreen system (including the tributary gla- ciers) have been described (e.g. Hald et al. 2001). In 1898 the terminus of the Paulabreen system stood near the outlet of Rindesbukta (Murray et al. 1998). Photographs from 1936 show that a few decades later the terminus had retreated some 4 km, and it continued to retreat until very recent- ly. In total this retreat measures about 14.5 km. This rapid retreat could indicate that the glacier system surged not too long before 1898. An inter- val of 30 (Lefauconnier & Hagen 1991) to 500 years (Dowdes well et al. 1991) has been suggest- ed as the duration period of the quiescent phase for Svalbard glaciers, depending in part on the rate of snow accumulation (Dowdes well et al. 1995). As no indications of a previous surge have been found during the last century (Simões 1990; Hagen et al. 1993) there is reason to believe that the Skobreen has a build-up phase of 100 years or more. The longitudinal profi les of Skobreen from 1936 and 1990 (Fig. 2) show a decreasing ice thickness below 380 m a.s.l. Above this, there has been a build-up during the same period. Data (not shown) from the uppermost part are more uncer- tain as the glacier ends in a steep hillside. The average change in thickness above 350 m a.s.l. is about 24 m. Given the area of the glacier above this elevation (6.34 km2), this implies a build- up of about 0.155 km3 of ice in the upper part of the glacier. Simões (1990) found that the mean annual accumulation rate during the years 1967– 1986 was 0.33 m a–1 water equivalent. The dif- ference between the profi les illustrates a typical pattern of a Svalbard surge-type glacier during a build-up phase. The 1936 profi le reveals the com- bination of a convex feature in the lower part at 350 m a.s.l. followed by a fl at surface 2.5 km up the glacier to 400 m a.s.l. Another bulge is found at the 450 m level. Initiation Generally, the 1936, 1961 and 1970 photographs show very little crevassing. There are only a very few bergschrunds in the upper areas. From 1970 to 1990 there is a difference in the crevasse pat- tern, but only a few of the crevasses can be attrib- uted to variations in the snow cover. The major areas of crevassing are distinct in the 1990 photo- graphs and totally absent in the 1970 photographs, indicating that they are new. The development of additional crevasses in the bergschrund area Fig. 2. Longitudinal surface profi les drawn from the 1936 and 1990 maps. Bedrock is drawn after Drewry (1985, 1987). 119Sund 2006: Polar Research 25(2), 115–122 (marked A on Fig. 1) and small incipient margin- al crevassing (B) approximately 2 km from the head along the western margin and in the eastern margin in the main basin, as well as the devel- opment of transverse crevassing (C) in the two uppermost sub-basins, can be seen. This is best observed by comparison with the 1970 photo- graphs. The presence of these marginal crevasses is probably the fi rst sign of the glacier decoupling from the bedrock. The incipient crevassing of the sub-basins (C) in 1990 support these indications. A slight advance of the moraine in front of Skobreen can be seen on the 9 September 2003 photographs in comparison to the 1990 photo- graphs. The 25 August 2003 photograph confi rms this advance, showing traces of marginal shear against the mountain walls in the lower part of the glacier. However, there are no visible crevass- es in the terminus area. From these photographs the advance is estimated to 200 to 400 m. A slight bulge (marked D on Fig. 1) can also be seen. The use of Paulabreen as a snowmobile track is clear evidence that the advance at this stage was still limited as no changes were reported during the 2003 or 2004 seasons. Skobreen is not commonly visited or particularly noticeable when passing by via Paulabreen and changes of Skobreen’s upper reaches would not necessarily be observed. Surge phase The fi rst recent glaciological change noted in the Rindersbukta area was the advancing glacier front of Paulabreen, observed by a tourist guide in March 2005 (Hanne C. Christiansen, pers. comm. June 2005). Independently, observations were made and photographs were taken during a fl ight to Svalbard on 14 June 2005 (Fig. 3a), revealing that the advancing front of Paulabreen originated from the progressing terminus of Skobreen and that there was extensive crevassing along this gla- cier. These signs indicate an active surge (Liestøl 1969; Meier & Post 1969). No crevasses could be seen on Paulabreen further up from the Sko- breen–Paulabreen interaction. Transverse crevasses on Skobreen can be seen from the upper area and down to the middle sec- tion, partly appearing as huge chasms as the ice from the basins fl ows into the main ice body in area E (Fig. 1). The middle section has a rather even surface, quite undisturbed by crevassing at this stage. The fi rst set of photographs was taken at the beginning of the melt season; crevasses in this area could therefore have been hidden under the snow cover. However, the second set of pic- tures (Fig. 3b), taken more than a month later, still does not reveal crevassing of the same magnitude as further up or down this section. The western side of the glacier has no basins and shows a pro- nounced draw down of the ice. From the photo- graphs, the general lowering of the surface at this stage is estimated to be 40 - 50 m. Approximately 6.5 km from the head of the glacier, on the eastern side, a pronounced shear margin is found where debris is squeezed up along the mountainside. All the differential movement occurs in a narrow zone at the glacier margins. The presence of a sharp debris-laden shear margin indicates that a uni- form block fl ow has occurred across glacier. Fur- ther downwards of the block the crevassing again intensifi es, partly also into a more mixed pattern, as the tongue curves 90º around the mountain ridge Sokken and drains further down and west into Paulabreen. The shear margin is also visible at the confl uence with Paulabreen. However, the moraine in front of Skobreen is pushed forward almost undisturbed (Fig. 3c). Discussion The bulges in the 1936 profi le could be a part of the build-up pattern of Skobreen during its qui- escent phase. On Hessbreen, 55 km west–south- west of Skobreen, there were indications of a build-up as a result of smaller bulges propagating slowly down the glacier (Sund & Eiken 2004). The initiation of surges in Svalbard can be a slow process developing through several years (Murray et al. 1998; Murray et al. 2003; Sund & Eiken 2004) and is often followed by a longer surge period (Dowdeswell et al. 1991; Murray et al. 2003), unlike glaciers with a faster initia- tion, for example, in Alaska (Harrison et al. 1994; Lawson 1997). Hessbreen’s surge showed that even when crevasses began to open in the upper area of the glacier, the tongue remained unaffect- ed (Sund & Eiken 2004). Much the same progress is found at Skobreen. This suggests that the ini- tiation phase of this surge of Skobreen lasted for about 15 years. For several glaciers the active phase of the surge cycle has been characterized by a surge front propagating down glacier (e.g. Echelmeyer et al. 1987; Clarke & Blake 1991; Lawson 1997; Murray et al. 1998). From the 1990 profi le, no 120 A surge of Skobreen, Svalbard Fig. 3. The effects of the surge of Skobreen. (a) View of Skobreen toward the north-east, photograph taken on 14 June 2005. Notice the draw down along most of the glacier. (b) View of Skobreen toward the north-east; picture taken on 20 July 2005. (c) The terminus of Paulabreen and Skobreen; view is toward the south-east, photo- graph taken on 20 July 2005. (a) (b) (c) 121Sund 2006: Polar Research 25(2), 115–122 surge front or bulge could be identifi ed. Surge fronts developing in the accumulation area and propagating down glacier leave a chaotic cre- vasse pattern in both longitudinal and transver- sal directions, due to compressing and extending forces (Clarke & Blake 1991; Lawson 1996). The appearance of transverse crevasses indicates there is no propagation of a surge front (Lawson 1996; Murray et al. 2003). The shear zone observed during the surge of Skobreen indicates a predom- inant contribution of basal sliding to surge motion (Echelmeyer et al. 1987). This feature is also found in a number of other actively surging gla- ciers in Svalbard (Pillewizer 1939; Meier & Post 1969; Rolstad et al. 1997; Sund & Eiken 2004). The presence of transversal crevasses followed by a shear zone and block fl ow down glacier implies there no evidence of a propagating surge front on Skobreen. The slight bulge feature (marked D on Fig. 1) found in Skobreen is likely to have been formed by the progressing block of ice decelerat- ing and aggregating on to slower ice at the termi- nus of Skobreen and Paulabreen. Thus, formation is a result of the steadily proceeding mass of acti- vated ice, which is different to a surge front prop- agating down glacier having a causal relationship to the origin of the surge. On Variegated Glacier, in Alaska, transverse crevassing was found further up from where the surge was believed have been initiated (Lawson 1996). Hessbreen experienced heavy transverse crevassing in its upper part followed by a block movement further down glacier. The surge was probably initiated in the upper region of this block (Sund & Eiken 2004). If the behaviour of Skobreen is consistent with the results from Var- iegated Glacier and Hessbreen, the surge of Sko- breen was initiated around the location marked E on Fig. 1. Summary From 1936 to 1990, Skobreen experienced a build- up in the upper part, while there has been a reduc- tion of ice thickness in the lower area. The 1936 profi le shows bulges which could be a part of the build-up pattern of the glacier. The smooth 1990 profi le has no indication of such bulges. The qui- escent phase is assumed to have lasted for more than 100 years. Comparisons of the 1970 and 1990 vertical aerial photographs reveal incipi- ent crevassing in the upper area in 1990, proba- bly consistent with the initiation phase of a surge. Photographs from 2003 show a slightly advanc- ing moraine and marginal crevassing in the lower part of the glacier. Observations during the summer of 2005 showed that Skobreen, not previ- ously registered as a surge-type glacier (Hagen et al. 1993), was actively surging. There were large crevasses in the upper part followed by a block of ice with strong shear margins advancing into and pushing the terminal part of Paulabreen. The surge is therefore categorized as a block surge (Pillewizer 1939). The transition between the cre- vassed upper part and the lower block is proposed to be the initiation area. Acknowledgements.—Thanks to the SAS–Braathens pilots who contributed to the best possible position for photo- graphing the glacier on the fl ight from Svalbard to mainland Norway on 20 July 2005. Thanks to an anonymous review- er and M. Bennett for constructive comments and to C. Rol- stad for comments on an earlier draft of the manuscript. Dis- cussions and help with fi gures from T. Eiken were very much appreciated. One of the photographs was kindly provided by M. Jochmann. References Clarke, G. K. C. 1976: Thermal regulation of glacier surging. J. Glaciol. 16, 231–250. Clarke, G. K. C. & Blake, E. W. 1991: Geometric and ther- mal evolution of a surge-type glacier in its quiescent state: Trapridge Glacier, Yukon Territory, Canada, 1969–89. J. Glaciol. 37, 158–169. Clarke, G. K. C., Collins S. G. & Thompson, D. E. 1984: Flow, thermal structure and subglacial conditions of a surge-type glacier. Can. J. Earth Sci. 21, 232–240. Dowdeswell, J. A., Hamilton, G. S. & Hagen, J. 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