Australia: The Land Where Time Began

A biography of the Australian continent 

Antarctica - Outlet, Valley and Piedmont Glaciers

Small ice caps, outlet glaciers and valley glaciers are the smallest elements of the Antarctic glacial regime. Valley glaciers flow through mountain valleys, and outlet glaciers are constrained by exposed rock to channels as they pass through mountain valleys. Valley glaciers and outlet glaciers can coalesce at the base of mountains to form thick, continuous ice sheets known as piedmont glaciers. The western Ross Sea on the flank of the Transantarctic Mountains (TAM), and seas adjacent to the Antarctic Peninsula are examples of marine settings in which these types of glacial systems are present. Valley glaciers and outlet glaciers, that directly into the sea (tidewater glaciers) are found in these settings, which often form glacier tongues and piedmont glaciers that extend seawards as fringing ice shelves.

The relatively rapid response of outlet, valley and piedmont glaciers' response to short-term climate change, of decadal to century time scales, is of particular interest to marine geologists. As a result of their drainage basins being relatively small, rates of accumulation that are high and their thickness that is small compared to that of ice caps and ice sheets, these smaller glacial systems are the sensitive to short-term climate changes. High resolution sedimentary records of the recent climate of Antarctica are apparently present in that bays and fjords into which the glaciers drain (Griffith & Anderson, 1989; Domack, 1990; Domack & Williams, 1990; Domack & Ishman, 1993). An understanding of the factors regulating the advance and retreat of glaciers in Antarctica, outlet, valley and piedmonts glaciers, is required to properly interpret this record contained in the sediment.

Direct gain from precipitation is the almost exclusive way in which accumulation occurs in the case of valley, outlet and piedmont glaciers. Variations in precipitation along the mountain front in the Transantarctic mountains result in differences in the glacial setting that are spectacular. In the northern part of the mountain belt glaciers receive relatively high amounts of precipitation, extending to the sea as tidewater glaciers. In the McMurdo Dry Valley region glaciers are not well fed and most terminate long before they reach the coast. Precipitation rates on the Antarctic Peninsula are 500-1,000 mm/yr, which are the highest accumulation rate in Antarctica (Drewry & Morris, 1993). Direct gain by precipitation is the most important form of accumulation for glaciers in this region (Koerner, 1964; Sadler, 1968, Rundle, 1974; Curl, 1980, Orheim & Govorukha, 1982; Wager & Jamieson, 1983).

There are other ways in which accumulation can occur, including rime formation (Koerner, 1964; Mercer, 1967), accumulation of snow that is blowing and drifting (Sadler, 1968), and direct advection from ice fields and ice caps (Griffith, 1988). As patterns of precipitation on the mass balance of glaciers, the author¹ suggests it follows that recent observations of ice shelves that are retreating that were fed by valley and outlet glaciers in the region of the Antarctica Peninsula may reflect precipitation pattern changes as well as the warming that has been observed.

Calving is the almost the only process by which ablation occurs in the valley, outlet and piedmont glaciers of Antarctica (Koerner, 1964; Rundle, 1974;  Wager & Jamieson, 1983). Among minor form of ablation are direct sublimation, wind advection and melting, therefore the for most tide-water glaciers the equilibrium line  is at the calving line. The South Shetland Islands is the only place where exceptions are found, where temperatures that are relatively warm promote melting and in the region of the McMurdo Dry Valleys wind ablation and evaporation are important.

Calving does not vary systematically along climatic trends as calving is not climate-dependent. Factors of importance in calving regulation:

1. Rate of glacial advance.
2. Glacier exposure to waves and tidal influences.
3. The relative amount of dampening of sea waves by sea ice.
4. The geometry of the valleys confining the glaciers.

The calving line position is mostly controlled by sea level and bathymetry of the bays and fjords.

The role of the bed deformation  is considered to be minimal in valley glaciers, as sediment cover is virtually absent from fjords in Antarctica. Valley glaciers of the South Shetland Islands where there is relatively thick sediment accumulations in the fjords are suggested by the author¹ to possibly be the only exceptions.

Sources & Further reading

1. Anderson, John B., 1999, Antarctic Marine Geology, Cambridge University Press