Polar Ice Marginal Environments

Polar continental glaciers constitute the bulk of ice on earth (Table 5.1). Polar glaciers constitute over 95 per cent of the glacier-covered area and over 97 per cent of ice volume (excluding the Ross and Ronne-Filchner ice shelves). Although polar continental glaciers dominate the earth's glacial systems, sediment-landform associations produced by these glaciers are the least well known and understood. The primary reason for this is that there is little land area beyond current ice margins and the glaciers are largely inaccessible. Consequently there is little terrestrial evidence of the growth and decay of the glaciers.

Our knowledge of the geomorphology and sedimentology of large polar continental ice masses is mainly derived from studies of small land areas that fringe Antarctica and Greenland. These areas

Area (km2)

%

Volume (km3)

%

Ice caps, ice fields, valley glaciers

H 680,000

4.24

■ 180,000

0.55

Greenland

1,784,694

1 1.06

2,620,000

7.96

Antarctica

East Antarctic Ice Sheet

10,153,170

63.27

26,039,200

79.11

West Antarctic Ice Sheet

1,918,170

11.96

3,262,000

9.90

Antarctic Peninsula

H 446,690

2.79

■ 227,100

0.69

Ross Ice Shelf

H 536,070

3.34

■ 229,600

0.70

Ronne-Filchner Ice Shelf

H 532,200

3.33

| 351,900

1.07

Total

16,051,094

100

32,909,800

100

Table 5.1 Estimated areas and volumes of glaciers (From Williams and Ferrigno, 1993)

Table 5.1 Estimated areas and volumes of glaciers (From Williams and Ferrigno, 1993)

provide limited access to glacier margins and to landscapes that have experienced the advance and retreat of glaciers. These areas, often called oases, are cold deserts characterized by low mean annual temperatures (—10 to —20 °C), light precipitation the vast bulk of which falls as snow, and strong winds (typically mean monthly wind speeds of 2—9 m.s-1).

The main controls on the nature and location of glacial deposition are glacier mass balance, thermal regime, bed configuration, the properties of the material being deposited and the climate near the ice margin (Andrews, 1975; Lawson, 1979). Studies of glacial deposits forming at the margin of glaciers have stressed the role of thermal regime in determining the processes involved in their deposition (Boulton, 1972b, 1975). Three different thermal boundary conditions have been recognized in glaciers on the basis of englacial temperature gradients (Weertman, 1961):

1. a temperature gradient that is sufficient to conduct all heat from the glacier bed, in which case there is no melting and the ice remains frozen to the bed

2. a temperature gradient that is just sufficient to conduct heat from the bed, in which case there is an approximate balance between melting and freezing

3. a temperature gradient that is insufficient to conduct heat from the bed, in which case there is melting and sliding.

These boundary conditions define two types of ice that are often called 'temperate' and 'cold' ice and recognizes the possibility that the state of the ice may change in space and time. When applied to whole glaciers the scheme has yielded a three-part classification of thermal or glaci-dynamic basal regimes that can be identified at modern glacier margins: 'temperate' glaciers, 'subpolar' or polythermal glaciers and 'polar' glaciers. The geographic terminology is regrettable because the distribution of glacier types is not simply determined by latitude. Consequently the terms 'wet-based', 'polythermal' and 'dry-based' or 'cold-based' are preferred and are used in this chapter. Most polar continental ice masses are of the polythermal type: where the ice is thin, such as the ice margins, they are dry-based and where the ice is thick or flowing rapidly the base of the ice is at pressure melting point and therefore wet-based. Thin glaciers in particularly cold environments may be entirely dry-based.

It has been argued that each thermal regime can be associated with diagnostic landform and sediment assemblages (Boulton, 1972b; Boulton and Paul, 1976; Eyles et al., 1983b). Eyles et al. recognized a polar arid sediment assemblage based primarily on the work of Shaw (1977a, b) who examined depositional processes in the McMurdo dry valleys. The title of this sediment assemblage 'polar arid' encapsulates the problem of a thermal regime-based classification because this sediment assemblage is differentiated by the climate of the terminus area rather than basal thermal regime. Glacier thermal regime exerts a fundamental control on glacier behaviour by determining ice motion and erosion processes. Thermal regime is determined by the englacial temperature gradient, which is influenced by climate and the generation of heat close to the glacier bed. The indirect role of climate in controlling thermal regime contrasts strongly with the direct influence of climate on depositional processes at glacier margins. The use of both thermal regime and climate to distinguish sediment raises several interesting questions including:

• Can the roles of thermal regime and climate in glacier sedimentation at the terminus area be differentiated?

• If the roles of thermal regime and climate can be differentiated, which is the higher-order control in polar continental environments?

• Is glacier thermal regime a satisfactory basis for defining landform and sediment assemblages?

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