Results And Discussion

Table 1 shows a list of the surge-type glaciers on Disko Island classified in this study. Eight glaciers have experienced surge events since 1953, three glaciers advanced between 1931 and 1953, which are likely to have been surge events, and 64 glaciers are inferred as surge-type glaciers due to one or more significant surge characteristics. Out of the 247 glaciers larger than 1 km2, 30% are classified as surge-type glaciers. Of the previous 24 glaciers classified as surge-type, six (glaciers 1HB15018, 1HB15039, 1HD06026, 1HE01011, 1HE01015, 1HE01010) are now reclassified as normal flow glaciers. There is no evidence of glaciers, which have experienced two or more surge events since 1931. This indicates that the surge cycle is relatively long compared to other surge clusters in the world. The quiescent phase is likely to be 100 years or longer, whereas the active surge phase is about 2-3 years. The number of surge events per decade seems to have declined through the second half of the 20th-century. This is in concordance with observations from Svalbard (Dowdeswell et al., 1995), and may be a consequence of climatic amelioration resulting in decrease in glacier extent, higher equilibrium line altitude, and small glaciers may change their thermal regime from warm-based or polythermal to entirely cold-based due to decreased ice thickness.

Yde and Knudsen (2007) compared physical parameters of surge-type and normal flow glaciers larger than 1 km2 on Disko Island. The results showed that surge-type glaciers are larger (12.0 km2) than normal flow glaciers (3.6 km2), indicating that surge-type glaciers comprise 59% of glacierized area. Their mean terminus altitude is nearly similar (620 m a.s.l.) as for normal flow glaciers (670 m a.s.l.). Between 1953 and 2005, the mean recession rate of surge-type glaciers was about 16 m yr-1, and as much as 20 m yr-1 if only quiescent glaciers are included. In comparison, normal flow glaciers experienced a mean recession rate of about 8 m yr-1 during the second half of the 20th-century. This has implications when recession rates are related to climate change. The current positions of the terminus of surge-type glaciers reflect the magnitude of their last surge events and the time elapsed since the last surge events rather than climatic responses.

All surge-type glaciers on Disko Island are located on basaltic rocks. Although basaltic rocks constitute the dominant lithology, it is likely that basalts increase the potential for glacier surging as both Nuussuaq peninsula and the East Greenland surge cluster are situated on basalts. More information on enhanced geothermal heat flux or subglacial groundwater drainage within cracks in the basalt beds or in the intercalated layers of laterite soil residue will elucidate the significance of the presence of a basalt substratum on glacier surging.

Most surge-type glaciers are found in the large central valleys. Complex moraine systems, old moraine ridges and dead ice areas are often found on the valley floors, and in most cases they can be associated with one or more surge-type glaciers. In Kuganguaq (catchment 1HE09), 13 glaciers show signs of past surge activity, but none have surged since 1953. Twelve surge-type glaciers are identified in Stordalen/Kussuaq (catchment 1HD06) of which two have surged since 1953. Ten glaciers are classified as surge-type in Kuannersuit Kuussuat (catchment 1HB15). There seems to be no surge-type glaciers among the small maritime glaciers on the western- and southern-most parts of the island. Below is a description of four of the largest surge-type glaciers on Disko Island, and their informal names are applied.

Kuannersuit Glacier (1HB15017)

Between 1995 and 1998 this outlet glacier, which descends from Sermersuaq ice cap into Kuannersuit Kuussuat, experienced a 10.5 km surge advance with maximum velocities of more than 70 m d-1 (Figure 2). This is one of the longest terrestrial frontal advances ever recorded, and several aspects of the consequences of the surge event have been described (Gilbert et al., 2002; Yde et al., 2003, 2005a,b; Yde and Knudsen 2005a,b; Knudsen et al., 2007; Roberts et al., 2009; Yde, Chapter 5). About 3 km3 of ice was moved from the reservoir area to the newly formed glacier tongue, significantly increasing the extent of the ablation area to constitute about 80-90% of the total area. The glacier surface became heavily crevassed and undulated, and a looped medial moraine was moved several kilometres down-glacier, but survived the surge event. The glacier overrode a pingo and proglacial naled assemblages, and subglacial sediment was entrained within several shear planes. During the initial quiescent phase rapid thinning lowered the glacier surface behind the debris-covered glacier terminus. Lateral recession commenced immediately after the termination of the surge events with about 10 m yr-1, and chasms formed along the margins above the main subglacial channels. In 2005 when the glacier was last visited, a series of evolving chasms caused subsidence across the glacier tongue up-glacier of the uppermost shear plane. This will eventually lead to detachment of the entire debris-covered glacier terminus.

Figure 2. The central part of Kuannersuit Glacier (1HB15017) in July 2002, looking north towards the surge reservoir area. The glacier surface is pitted with numerous potholes, and the lateral margins are crevassed. The glacier is moving towards the right.

Jost (1940) visited Kuannersuit Glacier on an expedition in May 1913. At that time the glacier was located at about 1 km up-valley from its 1998 post-surge position. Jost (1940) was very intrigued by the glacier recession of more than 5 km between his observations in 1913 and surveys for the first local topographic map in 1931-33 (GID, 1941), suggesting a recession rate of at least 250 m yr-1. He described an at least 1.5 km long ice-gorge crossing the glacier, which could indicate that a detachment of the glacier terminus had occurred. He also observed transverse crevasse ridges, looped medial moraines and supraglacial "craters" (i.e. potholes or chasms), and compared his observations with descriptions of glaciers in Karakoram Himalaya, making his observations some of the first scientific observations of surge-type glacier glaciomorphology.

Based on the observations of Jost (1940) and what is known about the evolution of glacier surface features after the 1995-98 surge event, it is suggested that the previous surge event occurred in year 1900 ± 5.

Sorte Hak Glaciers (1HB15029 and 1HB15031)

These glaciers are most likely identical to the one observed by Steenstrup in 1898 (Steenstrup, 1901) and described by Jost in 1913 (Jost, 1940) occupying the central part of Kuannersuit Valley. Based on field observations of end-moraines and the topographic map (GID, 1941) the glaciers maintained their largest extent until at least 1931. Since then, the glaciers have receded about 10.5 km from their outermost moraine through a steep, narrow gorge called Sorte Hak. The recession was very rapid between 1931 and 1953 with rates of about 200 m yr"1 and resulted in detachment of the debris-covered glacier snout, forming an extensive dead ice area with many circular dead ice lakes. Parts of the debris-cover constitute of glaciofluvial outwash sediments rather than supraglacial meltout till, indicating that bulk meltwater emanating from a glacier portal located in the Sorte Hak Gorge covered the lower part of the glaciers. In the 1970s, the confluent glacier tongue split into a northern (1HB15029) and southern branch (1HB15031), which means that the next surge event of either branch will not affect the other branch. The last surge event must have triggered while both branches were together, or both branches must have surged separately within a short period. Hence, it is suggested that the last surge event occurred during the second half of the 19th-century, when the Little Ice Age peaked.

Avdlaugissat Glacier (1HB11029)

This outlet glacier, which flows southward from Brapasset ice cap into Avdlaugissat Valley, has the longest tongue of all glaciers on Disko Island. Its surface is covered by numerous potholes (Figure 3), which have existed for more than 50 years. One of the most interesting characteristics of Avdlaugissat Glacier is that the glacier has only receded about 2.2 km since 1931, where the terminus was at the outermost moraine. Most of this recession has occurred in recent decades, where the glacier tongue has thinned significantly. This slow recession from the maximum position suggests that the last surge event of Avdlaugissat Glacier must have been very dramatic, moving many km3 of ice down-valley in order to make the glacier tongue thick enough to resist the enhanced ablation at low altitudes. The current recession is witnessed in the field by a marked trimline, stranded ice on the valley walls hundred meters above the current glacier surface, and mature chasms along the lateral margins. It is very likely that the recession rate will accelerate in the near future.

Figure 3. The central part of Avdlaugissat Glacier (1HB11029) in July 2003, looking west. Several potholes are found on the glacier surface. The glacier is moving from right to left.

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