Ice Shelves And Other Marine Glaciers

Glaciers in contact with the sea can be given a general name of marine glaciers. Such glaciers are significant because their ice discharge is sufficient to bring them into contact with the sea and its powerful eroding forces. Most Antarctic glaciers reach the sea and some extend many hundreds of kilometres offshore. The Pacific Sector of the Antarctic contains a representative selection of different types of marine glaciers, from ice shelves to fjord glaciers to those that terminate on beaches below the high tide mark. Examples and explanations of these are given below.

Ice shelves appear as quite flat bodies of generally snow-covered glacier ice floating over most of their area but grounded along coastlines and over other shallow parts of the seafloor. They are formed from glaciers, particularly ice streams flowing off grounded ice sheets to merge offshore, and by thickening of old sea ice in bays near such glaciers, particularly where this sea ice is anchored by islands or grounded icebergs. Ice shelves spread outwards over the sea driven by the flow of the feeder glaciers and by thinning under their own weight. Thicknesses range from as much as 1,300 m at their inland margins to less than 10 m at free floating seaward fronts. Where the ice runs aground or ice streams merge, the ice tends to thicken and may buckle as it deforms to produce pressure rollers (ridges) with wavelengths of a few hundred metres or less and amplitudes up to 15 m. Other undulations or depressions with wavelengths of 1-10 km and amplitudes up to 5 m are widespread due to complex but incompletely understood dynamics (Swithinbank and Zumberge, 1965; Thomas, 1979b; Collins and McCrae, 1985). In general, however, an ice shelf appears quite flat and covered with snow. Ross Ice Shelf, for instance, has a slope mostly between 1 in 103 and 1 in 104- Snow-free surfaces and exposures of sediment are rare. The western portion of the McMurdo Ice Shelf which is covered with seafloor debris through freeze on below and ablation above (Swithinbank, 1970), and the Nansen Ice Shelf in Terra Nova Bay, seem quite exceptional in this respect.

Ice shelves are nourished mainly by influx of glacier ice and by accumulation of snow and rime on their top surface. Seasonal stratification in the consolidating snow pack is normally conspicuous along ice fronts and crevasse walls. In circumstances where thermal conduction (e.g., western McMurdo Ice Shelf) or cold-water circulation allows it, water freezes on their underside.

Wastage is mainly by calving from seaward edges at rates averaging probably some hundreds of metres per year to produce icebergs. Bottom melting is also significant, especially near the ice front, with rates estimated to be up to 10 m yr1 (Robin, 1979). Loss by surface ablation, particularly wind erosion, is relatively small but looks spectacular when blowing snow pours over the ice front.

Glacier (or ice) tongues are narrower protrusions of floating glacier ice usually formed by single or converging ice streams or glaciers laterally confined at the coast and discharging into the sea. These tongues extend seaward because discharge of ice across their landward grounding line (the junction between the grounded feeder glacier and the floating ice) is normally faster than the rate at which icebergs break off. Thinning proceeds seaward of the grounding line and towards the sides of the tongue. Serrations or "teeth", with horizontal amplitudes and wavelengths of some hundred metres, are common along the sides of many tongues (e.g., Erebus Glacier Tongue, Ross Island); their formation may be due to lateral deformation and expansion (due to relaxation of pressure) of the glacier as it moves out of the constraining valley causing periodic variations in ice discharge. Wastage is again mainly by calving and bottom melting but melting and sublimation from the surface of glacier tongues is probably much more widepread than on ice shelves as shown by the relative extents of snow-free firn (old snow that has been transformed into a denser form), ice and meltwater features. Some glacier tongues are actually partly grounded on the seafloor, often becoming quite crevassed and even breaking up to form an iceberg tongue (e.g., Thwaites Iceberg Tongue, Marie Byrd Land).

Other glaciers that reach the sea do not extend out past their valley sides or, in extreme cases, past the high-tide level because their rate of ablation in contact with the sea is relatively high compared to ice discharge. The combined rates of wave erosion, minor calving of ice, iceberg production, submarine melting and surface ablation equal the forward motion of the ice much closer to the grounding line than in ice shelves or glacier tongues. Marine glaciers flowing down valleys may terminate at the valley mouth (e.g., Pine Island Glacier, Marie Byrd Land) or part way up a fjord (e.g., Ferrar Glacier, South Victoria Land). Unconfined glacial ice (such as slow moving parts of a grounded ice sheet, glaciers which mantle low-

lying coastal terrain, or lobe-shaped glaciers ending in the sea below coastal mountains) ends just seaward of the grounding line, or closer inshore.

Grounded ice cliffs are termed ice walls and rest on beaches or on the seafloor in water up to at least 400 m deep (Robin, 1979). Ice walls terminating on pebbly beaches exposed at low tide are typical of the western coast of the Antarctic Peninsula, from its northern tip to 69°S at the northern edge of the Wordie Ice Shelf.

The state of equilibrium, mass balance and seaward extent of floating glaciers are influenced by a variety of glacial and non-glacial processes acting over periods of a few hours (e.g., storm surges and tides) to many tens of thousands of years. Changes in the position and geometry of the grounding line, in ice thickness and in the position of the seaward margin, all reflect a complex inter-related set of processes. For instance, an ice shelf that is growing thicker may be due to : an increase in drainage from the ice sheet because of accelerated thinning or increased snowfall; a reduction in creep-rates and/or velocities in the ice shelf (caused by cooling temperatures or development of grounding areas offshore as sea level falls or the seabed rises); an increase in snowfall or a decreased rate of bottom melting on the ice shelf; and a decrease in the rate of iceberg production leading to growth in the ice shelf area and a greater mass for the ice shelf to push past its margins (Thomas, 1979b). Similar factors apply to glacier tongues.

Islands, headlands or shoal areas seem particularly important for pinning or protecting the seaward margins of some ice shelves and fjord glaciers and hence influencing their seaward extent (Fig. 4.1). Lateral stretching and thinning of the ice beyond these anchoring points leads to ice failure. A calving front is established near the pinning points along a relatively narrow zone across which outward ice flow is in approximate equilibrium with the rates of iceberg production and bottom melting. However, iceberg production is not regular in space or time and therefore the position of the calving front (or seaward end of an ice tongue) may vary (e.g., Fig. 4.2) over a cycle of several years or even decades, irrespective of more fundamental trends (if any) in mass balance. The length of ice tongues may vary even more noticeably due to their smaller size and perhaps a greater vulnerability to iceberg formation, for instance (Swithinbank et al., 1977) by impact of colliding icebergs.

A sustained retreat of an ice shelf is signficant because it produces an increased number of icebergs and possibly speeds up the drainage of the inland ice. Recent extreme retreats of possibly three ice shelves in the Pacific Sector illustrate that ice shelves are not perennial features. George VI Ice Shelf in the Antarctic Peninsula may have disappeared about seven thousand years ago (Clapperton and Sugden, 1982). Microfossils in seafloor sediments in Pine Island Bay, Marie Byrd Land, suggest that an expanded ice shelf was present a few hundred years ago (Kellogg and Kellogg, 1986) where only ice tongues and small ice shelves exist today. An ice shelf in Lady Newnes Bay, North Victoria Land, may have broken up and disappeared between 1912 and 1960 (Anonymous, 1966b) leaving a series of glacier tongues. The last Ice Age saw major advances and thickening of Antarctic ice shelves (Drewry, 1979; Stuiver et al., 1981).

Ice shelves are especially significant in the Pacific Sector of the Antarctic.

Fig. 4.2. The Ross Ice Shelf showing the position of the ice front in 1902,1962 and 1985, as well as ice thickness contours and glacial flow-lines (adapted from Bentley et al., 1979; Neal, 1979; Jacobs et al., 1986).

Between Cape Adare in North Victoria Land and Marguerite Bay in the Antarctic Peninsula about 40% of the coastline is occupied by ice shelves. Ice shelves are barriers to southern movement of ships and are the major sources of Antarctic icebergs. The 750 km wide Ross Ice Shelf and the 70 km long Drygalski Ice Tongue (western Ross Sea) affect sea currents and sea ice distribution in parts of the Ross Sea.

Ice shelves, particularly large ones, are important because of their role in restricting the discharge of the Antarctic ice sheet. The Ross Ice Shelf is especially so because it buttresses the West Antarctic Ice Sheet.

Knowledge of ice shelves, including modelling of their flow and dynamics, has been reviewed by Swithinbank and Zumberge (1965), Thomas (1979b) and Barkov (1985). The next three sections look at the Ross Ice Shelf and some other marine glaciers in the Pacific Sector emphasizing their seaward margins and recent behaviour as these relate closely to the coastline of the Pacific Sector.

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