Enhancement factor in the LGM ice

Analyses of the deformation behaviour of polar ice clearly show that ice deposited during the Last Glacial Maximum (LGM)

deforms more readily than Holocene ice (cf. reviews of Budd & Jacka, 1989; Paterson, 1991). The enhanced flow is believed to be due to the high concentration of solid impurities (Dahl-Jensen, 1985; Fisher & Koerner, 1986; Dahl-Jensen & Gundestrup, 1987). Data on the concentration of solid impurities in Holocene and LGM ice in the Antarctic and Greenland ice sheets are given in Table 59.1. There is, indeed, a good correlation between ice viscosity and impurity content.

In order to express the relative fluidity of LGM ice, the flow law for isotropic ice is given by e = EB tn—1a(1)

where e,y and c',y are the strain rate and stress deviator components, t is the effective shear stress, B is a parameter which depends on temperature and E is the enhancement factor. From borehole tilting measurements at Dye 3, this factor is equal to unity in the Holocene ice and has a value close to 3 in the LGM ice (Dahl-Jensen, 1985).A similar value was found in other Arctic stations (Paterson, 1991). By using an anisotropic flow model, Azuma & Goto-Azuma (1996) attributed the enhancement factor at Dye 3 to fabric anisotropy. From Thorsteinsson et al. (1999), the enhancement factor at Dye 3 is larger than expected from the effect of fabrics alone. These authors assume that impurities and crystal size should be invoked to explain the relative softness of

Figure 59.3 Photographs of thin-sections in polarized light and ice fabrics from the Greenland Ice Core at 2806 (top) and 2860m (bottom) (from De la Chapelle et al., 1998).
Table 59.1 Mean concentration of solid impurities in the Antarctic and Greenland ice sheets during the Holocene and the Last Glacial Maximum (LGM)

Station

Climatic

Typical

Source

period

concentration

(ngg-1)

Byrd

Holocene

10

Paterson (1991)

LGM

100

Vostok

Holocene

15

Petit et al. (1999)

LGM

750

Camp Century

Holocene

80

Paterson (1991)

LGM

1200

Dye 3

Holocene

50

Hammer

LGM

1300

et al. (1985)

GRIP

Holocene

50

Steffensen

LGM

5000

et al. (1997)

LGM ice. It is important to note that the high concentration of dust and other impurities is highly correlated to crystal size, which makes it difficult to assess their respective effect on ice viscosity (Dahl-Jensen & Gundestrup, 1987). Using data on both borehole tilt and closure rate in the Agassiz Ice Cap, Fisher & Koerner (1986) concluded that solid impurities associated with small crystal size are at the origin of the enhancement factor in glacial ice. Cuffey et al. (2000c) suggest the enhancement factor in the LGM ice at Dye 3 is totally explained by fabric and crystal size. Based on the analysis developed above on the deformation modes of polar ice at low stresses, the enhancement factor in glacial ice is probably due to the effect of crystal size and crystal orientation. Impurities, by affecting the crystal growth, would indirectly be at the origin of the relative low viscosity of LGM ice. It is, however, worth noting that solid impurities found in basal glacier ice with a much higher concentration can harden ice by impeding dislocation slip (Ashby, 1966, Fitzsimons, this volume, Chapter 65).

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