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where W is the bulk water concentration, M is the bulk ionic concentration, 0 is temperature depression of the ice below its melting point and K is a constant (Hubbard etal., 2004). The bulk water content affects ice-flow rate, even at relatively small water contents. The presence of water, even in small amounts in interfacial films nanometres thick, also has important effects on sliding (Cuffey et al., 1999), so water content is a key variable in terms of glacier dynamic behaviour. Duval (1977) found that strain rate increased rapidly with small water contents, from 5yr—1 for 0.1% water content to 20yr—1 for 1% water content. He found a linear relationship between ice-flow rate and water content (Duval, 1977), such that

where A is relative hardness of the ice. Given the enhanced soluble impurity content of basal ice facies, it is likely that basal ice generally has relatively high bulk water contents compared with englacial ice facies, although there is little research in this area reported in the literature. There is evidence, however, that water is squeezed out of basal ice during metamorphic type processes (Souchez & Tison, 1981). Hubbard etal. (2004) used the two relationships above, in conjunction with measurements of the ionic content of basal ice layers, to estimate that the basal dispersed-facies ice at Tsanfleuron Glacier had an effective viscosity more than an order of magnitude greater than the overlying englacial ice, as a result of differences in bulk water content caused by differences in soluble impurity content.

It is clear that the water content of ice decreases at temperatures further from the melting point (see relationship (3) above), and it is often implicitly assumed that it is only in temperate ice and close to the melting point that liquid water is significant in flow rate enhancement (e.g. Budd & Jacka, 1989). However, the small-scale concentration of stress at asperities on solid impurities can cause melting even at temperatures many degrees below the melting point: for example, in uniaxial compression tests on ice samples from Taylor Glacier at -25°C, meltwater was observed to form (Lawson, 1996). Similarly, Cuffey etal. (1999) found that liquid water in interfacial films at the base of Meserve Glacier at —17°C could account for rapid sliding and clean ice lenses within debris-laden ice. They attributed the presence of this water in thicker interfacial films than predicted theoretically to the high solute concentration in the ice.

As indicated above, temperature has a major impact on ice deformation, through its impact on flow parameter A, such that colder ice deforms much less readily. In terms of differential thermal effects on different ice facies, the characteristics of ice facies are such that different facies are likely to have different thermal sensitivities, although this is currently relatively poorly

Figure 63.5 Temperature dependence of uniaxial compressive strength for (a) debris-laden basal stratified-ice facies and (b) englacial-ice facies from Taylor Glacier, Antarctica. Note that the envelope of strength defined by the maximum value of strength at each increases systematically as temperature decreases for the debris-laden basal stratified-ice facies.

Temperature (°C)

Figure 63.5 Temperature dependence of uniaxial compressive strength for (a) debris-laden basal stratified-ice facies and (b) englacial-ice facies from Taylor Glacier, Antarctica. Note that the envelope of strength defined by the maximum value of strength at each increases systematically as temperature decreases for the debris-laden basal stratified-ice facies.

understood. In rapid strain rate tests, Lawson (1996) found that the strength of debris-laden basal ice facies from Taylor Glacier increased with decreasing temperature, such that the ice was more than twice as strong at —25°C than at —5°C, whereas the strength of the overlying englacial ice did not vary systematically with temperature (Fig. 63.5)

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