Subglacial groundwater in past and modern environments

As ice sheets grow and expand over permeable rocks, groundwa-ter flow evolves from a subaerial, precipitation-fed system con-

Figure 9.2 Modelled groundwater flow pattern under the margin of the Weichselian ice sheet at its maximum extent at the Main Stationary Line in Denmark (Bovbjerg). Large glaciotectonic folding was facilitated by high porewater pressure in the low-transmissivity bed, partly due to thin aquifers wedging out in the direction of groundwater flow. Note that the transition from groundwater recharge to discharge still occurs under the ice sheet (b), caused by the low hydraulic conductance of the bed not capable of evacuating all meltwater to the ice foreground (Piotrowski et al., 2004).

Figure 9.2 Modelled groundwater flow pattern under the margin of the Weichselian ice sheet at its maximum extent at the Main Stationary Line in Denmark (Bovbjerg). Large glaciotectonic folding was facilitated by high porewater pressure in the low-transmissivity bed, partly due to thin aquifers wedging out in the direction of groundwater flow. Note that the transition from groundwater recharge to discharge still occurs under the ice sheet (b), caused by the low hydraulic conductance of the bed not capable of evacuating all meltwater to the ice foreground (Piotrowski et al., 2004).

Table 9.1 Hydraulic conductivities of some subglacial tills derived from in situ and laboratory measurements

Location

Hydraulic conductivity (ms1)

Reference

Bakaninbreen

3 x 10-7 to 8 x 10-8

Porter & Murray (2001)

Bakaninbreen

8 x 10-3

Kulessa & Murray (2003)

Midre Lovenbreen

2 x 10-5

Kulessa & Murray (2003)

Trapridge Glacier

5 x 10-4

Stone et al. (1997)

Trapridge Glacier

1-2 x 10-9

Waddington & Clarke (1995)

Trapridge Glacier

2 x 10-8

Murray & Clarke (1995)

Trapridge Glacier

1 x 10-8

Flowers & Clarke (2002a)

South Cascade Glacier

10-7 to 10-4

Fountain (1994)

Haut Glacier d'Arolla

10-7 to 10-4

Hubbard et al. (1995)

Haut Glacier d'Arolla

8 x 10-12 to 9 x 10-9

Hubbard & Maltman (2000)

Breidamerkurjokull

1-2 x 10-6

Boulton et al. (1974)

Breidamerkurjokull

6 x 10-7 to 4 x 10-4

Boulton & Dent (1974)

Storglaciaren

10-7 to 10-6

Iverson et al. (1994)

Storglaciaren

10-9 to 10-8

Fischer et al. (1998)

Storglaciaren

10-7 to 10-6

Baker & Hooyer (1996)

Gornegletscher

2 x 10-2

Iken et al. (1996)

Ice Stream B

2 x 10-9

Engelhardt et al. (1990b)

Ice Stream B

10-9

Tulaczyk et al. (2001a)

trolled by landscape topography, to a system pressurized by and recharged from the overlying ice. Studies reconstructing water flow in subglacial sediments are very scarce, but they all show that flow pattern and velocity differed substantially from the interglacial situation in the same areas. Owing to the complexity of the glacial and groundwater systems, numerical methods have been applied following the pioneering study of Boulton & Dobbie (1993). They have demonstrated that under glaciers resting on aquifers with high hydraulic transmissivity, all basal meltwater can be evacuated through the bed and no other drainage systems at the ice base need to form. If, on the other hand, a glacier is underlain by fine-grained sediment, its conductance may be insufficient triggering formation of more efficient drainage pathways, such as channels. This conclusion is important for inferences on ice movement mechanisms and for interpreting the origin of eskers and tunnel valleys, which, accordingly, should indicate areas with excess of basal meltwater.

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