Box 124 Identifying Former Ice Streams Palaeoice Streams

Ice streams are critical components of both contemporary and palaeo-ice sheets. Using the characteristics of contemporary ice streams such as those in Antarctica, Stokes and Clark (1999, 2001) have proposed a set of criteria that can be used to identify the locations of ice streams on former ice-sheet beds as set out in the table below. In all, eight geomorphological criteria are identified as indicative of former ice-stream behaviour. It is of course highly unlikely that all eight of these criteria will be met in any one location, but collectively they provide a good set of objective criteria for identifying palaeo-ice streams.

Geomorphological criteria for identifying former ice streams.

Contemporary ice-stream Geomorphological signature characteristic

Characteristic shape and dimensions

1. Characteristic shape and dimensions (>20 km wide and >150 km long)

2. Highly convergent flow patterns

3. Highly attenuated subglacial landforms (length : width ratio > 10:1)

4. Boothia-type erratic dispersal trains Sharply delineated margins 5. Abrupt lateral margins

6. Ice stream shear margin moraines

7. Glaciotectonic deformation of sediments or bedrock

8. Submarine sediment accumulations (e.g., 'trough-mouth fans' or 'till deltas' where marine-terminating)

Rapid velocity

Deformable bed conditions Focused sediment delivery

Sources: Stokes, C.R. and Clark, C.D. (1999) Geomorphological criteria for identifying Pleistocene ice streams. Annals of Glaciology, 28, 67-74. Stokes, C.R. and Clark, C.D. (2001) Palaeo-ice streams. Quaternary Science Reviews, 20, 1437-57. [Table modified from: Stokes and Clark, (2001) Quarternary Science Reviews, Table 6, p. 145.]

3. Ice-stream tributaries flow at velocities intermediate between full ice-stream and sheet flow (Figure 12.6A). Tributaries link the large interstream areas (cold-based ice), to the major ice streams of the contemporary Antarctic Ice Sheet (Figure 12.6B). The role of these ice-stream tributaries in determining the overall

Figure 12.7 Reconstruction of former subglacial thermal organisation at the Last Glacial Maximum (LGM) or near-LGM for (A) the North American Ice Sheet and (B) the Fennoscandian Ice Sheet. In both cases the patterns have been reconstructed from the distribution of landforms mapped in remotely sensed images. [Modified from: Kleman and Glasser (2007), Quaternary Science Reviews, 26, figure 13, p. 594]

Figure 12.7 Reconstruction of former subglacial thermal organisation at the Last Glacial Maximum (LGM) or near-LGM for (A) the North American Ice Sheet and (B) the Fennoscandian Ice Sheet. In both cases the patterns have been reconstructed from the distribution of landforms mapped in remotely sensed images. [Modified from: Kleman and Glasser (2007), Quaternary Science Reviews, 26, figure 13, p. 594]

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