Landsystem Model for Palaeo Ice Streams

It would be highly unlikely that all of the geomorphological criteria should be found in one location, produced by a single ice stream. This is because not all ice streams will leave a complete geomorphological signature and because preservation and modification often obscure the complete picture. However, the criteria outlined above can be thought of as comprising a characteristic 'landsystem' produced by a former ice stream. This illustrates the perfect, or unaltered, geomorphological signature of ice-stream activity. The more criteria we can find the more certain we can be of the existence of a palaeo-ice stream. We now bring together these criteria to assemble a series of 'fantasy' landsystem models.

9.7.1 Marine and Terrestrially Terminating Ice Streams

Ice streams can be broadly categorized as either terrestrial or marine-based, depending upon the environment in which they terminate. All contemporary ice streams are marine in nature but former ice sheets are likely to have been drained by terrestrial ice streams. In Fig. 9.8, terrestrial and marine-terminating ice streams are illustrated, and an important consideration is the means by which they can ablate large ice fluxes from their termini. For the marine ice streams, this is by iceberg calving directly into the ocean, or via floating ice shelves. Terrestrial examples may calve ice into large proglacial lakes, or if these are not present, we presume that the terminus must comprise a large splayed lobe extending beyond the adjacent ice-sheet margin. These lobes would present a large surface area below the equilibrium line altitude and thus facilitate efficient surface melting and mass loss. It is only by such a configuration that sufficiently high ablation losses could balance the high ice flux being delivered to the margin. Note that diverging flow within the lobe implies much lower ice velocities than in the trunk of the ice stream.

9.7.2 Bedform Signature

Given the distinct velocity field that ice streams possess, then we might expect to find an expression of this in the subglacial bedform signature. From the earlier discussion about the relationship that seems to exist between ice velocity (cumulative strain) and elongation ratio of

Figure 9.8 Simplified configurations of terrestrial and marine-based ice streams, and contemporary and palaeo examples of each. (a) For ice streams that terminate on land, there must be a method for rapidly removing ice mass. For this reason we presume that the margin must advance, producing a large splayed ice lobe which further lowers surface elevation below the equilibrium line, enhancing ablation losses. (b) Other terrestrial ice streams may drain into locally impounded glacial lakes and thus discharge their ice flux in the form of icebergs. All contemporary ice streams are marine-based and either drain directly into open water (c) or feed floating ice shelves (d).

subglacial bedforms, then we propose that, under ideal circumstances, bedform elongation ratios should vary in accordance with Figure 9.9.

9.7.3 'Rubber Stamped' Versus 'Smudged' Ice-Stream Imprints

Ice streams must initiate at some point in time, remain active and then cease to function. During all stages of their activity, they are likely to do geomorphological work, eroding, transporting and depositing sediment, and erasing and generating landforms. The imprint left may relate to a single phase or a complex combination of activity during the ice-stream life cycle. We hypothesize two end-member landsystems, shown in Figure 9.10.

The whole ice-stream imprint may have been produced isochronously leaving behind a snapshot of ice-stream activity at a point in time. We can think of this as a 'rubber-stamped' imprint. This may be produced by an ice stream that operates, then switches off, 'freezing' the geomorphology

Figure 9.9 Given the distinctive velocity field that ice streams possess, and that elongation ratio of subglacial bedforms approximates (cumulative strain) velocity, then we propose that bedforms (drumlins and mega-scale glacial lineations) should, under ideal circumstances, conform to the above patterns. A) For a marine-terminating ice stream, velocity increases towards the grounding line and so it should be expected that lineations should progressively increase downstream. Short drumlins in the slower-moving convergence zone should grade into larger forms or mega-scale glacial lineations downstream. B) Terrestrially terminating ice streams will show a similar pattern, except that where ice diverges into a lobate terminus, the slower velocities should produce shorter lineations. For both cases, lateral variation in bedform elongation is likely to be slight, with longer forms along the central axis and shortening towards the shear margins, reflecting slower velocities here.

Figure 9.9 Given the distinctive velocity field that ice streams possess, and that elongation ratio of subglacial bedforms approximates (cumulative strain) velocity, then we propose that bedforms (drumlins and mega-scale glacial lineations) should, under ideal circumstances, conform to the above patterns. A) For a marine-terminating ice stream, velocity increases towards the grounding line and so it should be expected that lineations should progressively increase downstream. Short drumlins in the slower-moving convergence zone should grade into larger forms or mega-scale glacial lineations downstream. B) Terrestrially terminating ice streams will show a similar pattern, except that where ice diverges into a lobate terminus, the slower velocities should produce shorter lineations. For both cases, lateral variation in bedform elongation is likely to be slight, with longer forms along the central axis and shortening towards the shear margins, reflecting slower velocities here.

at that time, with the imprint remaining preserved during deglaciation. Preservation of landforms during deglaciation is not as problematic as once thought and is now known to be widespread (cf. Kleman and Borgström, 1994; Clark, 1999). On the other hand, an ice stream may operate throughout several cycles of advance and retreat, or be continuously operating during margin retreat, whereby the earlier imprints are modified and overprinted by the younger ice flow patterns. The other extreme therefore, is a 'time-transgressive' imprint that is continuously reorganized over time and may appear disjointed and complex (a 'smudged' imprint). Figure 9.10 illustrates the different patterns that we might expect. Clark (1999) specifically addresses how the geomorphological record may be used to distinguish between the two cases, and this is outlined in Figure 9.11.

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