Sir,
An unseasonal release of meltwater from the western margin of the Greenland ice sheet near Sondre Stromfjord was noted by residents of Sondre Stremfjord during January and February 1990 (Reference RussellRussell, 1990). Reference RussellRussell (1990) proposed a possible release of subglacial meltwater to account for the unusual flows into the Orkendalen and Sandflugtdalcn rivers. Since this event, further observations pertinent to the observed release of meltwater have come to light. Two circular depressions, located at a distance of 20–30 km from the ice-sheet margin, were observed on the ice surface from aircraft flying into Sondre Stromfjord (Fig. 1). The nature, origin and significance of these unusual features are briefly considered.
Fig. 1. Two circular crater-like depressions on the ice-sheet surface c. 30 km from the ice-sheet margin. Depressions are 1.5–2 km in diameter and are characterized by a series of concentric circular fractures. (Photograph by courtesy of U. Larsen.)
Photographs of these features were taken late in the winter of 1990 with a hand-held video camera from an aircraft cockpit by Captain U. Larsen. Frames were “grabbed” using an ERDAS image-processing system, enabling photographs to be taken. Each circular depression is in excess of 1.5–2 km diameter, the boundary of which is marked by a series of concentric fractures (Fig. 1). The interior of both structures is characterized by a more chaotic, hummocky appearance, with secondary fracture lines (Figs. 2 and 3). Relief within the circular depressions is irregular as indicated by shadows behind fracture edges and hummocky zones of ice blocks (Figs. 2 and 3). In at least one location, the ice surface is littered with individual blocks (Fig. 3). Although detailed topographic information is not available, total relief amplitude produced by these depressions was estimated at 10 m. The location of these depressions corresponds with that of large supraglacial lakes familiar for many years to pilots as “permanent” features on the ice sheet. Observations of similar-sized supraglacial lakes made during the summer months indicates that they form to considerable depth and contain rafts of seasonal lake ice.
Fig. 3. Close-up of fracture patterns and hummocky topography. Note large numbers of ice blocks scattered across the snow surface, indicating considerable disruption and possible water upwelling during lake drainage. (Photograph by courtesy of U. Larsen.)
Supraglacial depressions of similar morphology to those described here have been noted on Antarctic ice shelves (Reference MellorMellor, 1960; Reference Mellor and McKinnonMellor and McKinnon, 1960; Reference SwithinbankSwithinbank, 1988). The features described from the Antarctic ice shelves are oval in shape, reaching dimensions of 1.5 km × 3 km with depths as great as 80 m (Reference Mellor and McKinnonMellor and McKinnon, 1960). The Antarctic depressions also have an irregular bottom topography, indicative of ice collapse. Mechanisms for ice-surface collapse include the emptying of an englacial water reservoir or an ice-covered supraglacial lake (Reference Mellor and McKinnonMellor and McKinnon, 1960). The latter explanation appears to be most acceptable for the collapse structures observed in the Antarctic (Reference Mellor and McKinnonMellor and McKinnon, 1960; Reference SwithinbankSwithinbank, 1988). Reference Mellor and McKinnonMellor and McKinnon (1960) suggested that meltwater contained in these lakes periodically drained through crevasses into the sea below the ice shelves. The term “ice doline” has been suggested for such depressions showing signs of cavity collapse (Reference MellorMellor, 1960). The term “ice doline” seems appropriate to describe the Greenland depressions because of their similarity to those described from the Antarctic.
The fact that these unusual depressions were observed simultaneously with extraordinary water flow from the ice margin near Søndre Strømfjord would suggest that these phenomena are linked. Two water-filled ice dolines 1.5 km in diameter with an average depth of 10 m would contain about 35 × 106 m3 of water. This is the same order of magnitude as the volume of water released from the ice margin, as estimated by Reference RussellRussell (1990). If the ice dolines are deeper, their water-storage capacity would be closer to the suggested 90 × 106 m3 of water released during the drainage event (Reference RussellRussell, 1990). As there was no sign of water flow over the ice surface and as water was observed emanating from the ice-sheet margin it is suggested that these lakes drained either sub- or englacially.
The drainage of supraglacial lake water into two major outwash systems supports the existence of well-established drainage routeways which are likely to survive from year to year. Supraglacial lake drainage, although rarely observed during the winter months, may take place more regularly in the summer months. Supraglacial storage of meltwater is likely to delay the run-off of a significant proportion of the annual melt. The release of supraglacially stored water is also likely to be important for downstream river-channel morphology and sedimentology. Supraglacial lake drainage may be as important as the drainage of ice-marginal lakes when large ice sheets are considered. The drainage of supraglacial lakes should also be recognized in relation to the meltwater-flow regimes of proglacial rivers emerging from former ice sheets in Europe and North America.
I should like to thank K. Swanson and A. Reenberg for keeping me up to date with the above events. Thanks also go to R. Gard, Department of Geography, University of Aberdeen, for operating the ERDAS system and obtaining the photographs.
24 May 1992
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