Arctic Polythermal Glaciers

Glaciers are common on many of the landmasses surrounding the Arctic Ocean. Excluding the Greenland Ice Sheet, glaciers and ice caps cover about 275 000 km2 of the archipelagos of the Canadian, Norwegian and Russian High Arctic and the area north of about 60°N in Alaska, Iceland and Scandinavia. Although there is of course huge variability in their behaviour it appears that since the 1940s most Arctic glaciers have experienced a predominantly negative net surface mass balance. Indeed, there is evidence in the form of Arctic ice shelf break-up that recent environmental change is accelerating. Until recently, a large ice shelf (the Ward Hunt Ice Shelf: 443 km2 in area) existed along the northern coast of Ellesmere Island in Canada's Nunavut territory. After a 30 year decline in extent, the ice shelf suddenly broke-up between 2000 and 2002. The ice shelf fragmented into two main parts with many additional fissures and calved a number of ice islands. This marked the disappearance of the largest ice shelf in the Arctic.

One of the most intensely studied areas of the Arctic is the Svalbard archipelago (77° to 80°N), at the northern extent of the mild Norwegian Current, a branch of the Gulf Stream. The archipelago enjoys a relatively mild climate for its northern latitude.

On the west coast, the average annual temperature is -6°C. The average temperature on the west coast in the warmest month (July) is 5°C, whereas in the coldest month (January) it is -15°C. Although there are contrasts between the maritime west coast and the more continental interior, precipitation in Svalbard is generally low. Typical values at sea level are 400-600 mm annually, falling to half these values inland. Precipitation in the more mountainous regions is increased by orographic effects, but even on the glaciers snowfall of more than 2-4 m is rare. Ice-free land areas are underlain by permafrost to depths of between 100 and 400 m. In this sense Svalbard is typical of the Arctic, in that precipitation is generally low and declines linearly with elevation.

There are of course local variations in glacier type and morphology in the Arctic, but it is possible to make the following generalisations based on Svalbard glaciers.

1. Many Arctic glaciers are polythermal; that is the snout, lateral margins and surface layer of the glacier are below the pressure-melting point, whereas thicker, higherlevel ice in the accumulation area is often warm-based (see Section 3.4). This mix of thermal regimes makes the dynamics of polythermal glaciers complex. The typical polythermal glacier moves by sliding on its bed or by subsole deformation where warm-based ice dominates in the accumulation zone, and moves only by internal deformation where it is cold-based ice at the snout and lateral margins. Meltwater tends to follow supra- and englacial routes and well-developed basal hydrological networks are rare. As a result, polythermal glaciers tend to carry a high basal debris load, with debris-rich basal ice zones between 1 and 3 m thick, and debris concentrations of up to 50%. Their surfaces rarely have a substantial cover of debris, although medial moraines are commonly observed in their lower ablation areas (Figure 2.11).

2. Svalbard is famous for its high proportion of surge-type glaciers (Box 2.4). An estimated 35% of the glaciers on Svalbard are surge-type. These glaciers are prone to dramatic increases in velocity and rapid frontal advances, followed by periods of quiescence during which velocities are generally low. Surge-type glaciers in Svalbard typically have relatively long quiescent phases (10-200 years) between short-lived surge events (1-5 years).

3. Superimposed ice formed by the refreezing of melted snow is common on the surface of these glaciers. This commonly forms on the glacier surface if there are short periods of positive air temperatures in early winter, often coinciding with rainfall, which cause rapid glacier-wide melting. Percolating water then refreezes to form superimposed ice on the lower half of the glacier, and wetted-refrozen snow and ice lenses at higher altitudes. The formation of superimposed ice has significant implications for glacier mass balance, because it locally comprises up to 20% of winter balances and accounts for between 16 and 25% of the annual accumulation.

4. There is evidence that the sediments and landforms produced by Svalbard glaciers are strongly influenced by structural glaciological controls on debris entrainment and transport (Figure 2.12). Moraine-mound complexes in front of receding Svalbard glaciers are formed by proglacial and englacial thrusting, folding and deformation of the bed during surges. On a smaller scale, a range of glacier structures arrange debris into foliation-parallel ridges, supraglacial debris stripes and geometrical ridge networks (see Section 7.5).

Figure 2.11 Aerial photograph of the glacier Kongsvegen in Svalbard. [Image courtesy of: Norsk


Figure 2.11 Aerial photograph of the glacier Kongsvegen in Svalbard. [Image courtesy of: Norsk


0 0

Post a comment