Box 121 Glacial Geological Inversion Models

In a series of papers, Johan Kleman and his co-workers have outlined the procedures used in glacial geological inversion models (i.e. a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets) to reconstruct past ice sheets from the glacial geomorpholo-gical record (Kleman and Borgstrom, 1996; Kleman et al., 1997, 2006). Their inversion model comprises a classification system for glacial landform assemblages and uses the following 'rules':

1. The basic control on landform creation, preservation and destruction is the location of the phase boundary between water and ice at or under the ice sheet base, that is the basal temperature, because this separates frozen from thawed material.

2. Basal sliding requires a thawed bed.

3. Glacial lineations can form only if basal sliding occurs.

4. Glacial lineations are formed parallel to local ice-flow directions and perpendicular to the ice sheet surface contours at the time of creation.

5. Frozen-bed conditions inhibit the reshaping of the subglacial landscape.

6. Regional deglaciation is always accompanied by the creation of a spatially coherent but metachronous system of meltwater features, such as meltwater channels, eskers and glacial lake traces.

7. Eskers are formed in an inward-transgressive fashion inside a retreating ice front.

8. Meltwater channels will form the major landform record during frozen-bed deglaciations, whereas eskers are typically lacking under these conditions.

Using these assumptions, Kleman et al. (1997) mapped the landforms on the bed of the former Fennoscandian Ice Sheet and its change through time. They successfully combined their palaeoglaciological reconstructions with stratigra-phical records (e.g., till stratigraphy, cross-cutting striations) and dating chronologies (e.g., 14C dates, to make time-dependent reconstructions of the Fennoscandian Ice Sheet through its growth and decay. The figure below shows their reconstructions of the Fennoscandian Ice Sheet at six time periods during the last glacial cycle. The reconstructions show the ice sheet extent, dispersal-centre locations (D) and inferred ice-flow patterns (arrows). Reconstructed parameters include the configuration of the ice sheet and its change over time, including the location of the main ice divides and the main ice-flow directions. Based on the preservation of areas of pre-Late Weichselian landscapes Kleman et al. (1997) inferred that the ice sheet had a frozen-bed core, which was only partly diminished in size by inward-transgressive wet-bed zones during the decay phase.

Sources: Kleman, J. and Borgstrom, I. (1996) Reconstruction of palaeo-ice sheets: The use of geomorphological data. Earth Surface Processes and Landforms, 21, 893-909. Kleman, J., Hattestrand, C., Borgstrom, I. and Stroeven, A. (1997) Fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model. Journal ofGlaciology, 144, 283-99. Kleman, J., Hattestrand, C., Stroeven, A.P. et al. (2006) Reconstruction of palaeo-ice sheets - inversion of their glacial geomorphological record, in Glacier Science and Environmental Change (ed. P.G. Knight), Blackwell, Oxford, pp. 192-8. [Modified from: Kleman et al. (1997) Journal of Glaciology, 144, figure 11, p. 297]

Figure 12.2 Example of how ice-flow systems can be reconstructed from landform evidence. In this example, two events are recognised on the basis of the flow traces identified. The landforms relating to Event 2 are interpreted as the youngest because they overprint those of

Event 1.

these data with existing stratigraphical records and dating techniques to produce a more comprehensive picture of ice-sheet growth and decay (Box 12.1). The mapped landforms and the flow sets they define are then used to reconstruct glaciological parameters such as the location of major outlet glaciers and ice streams, the directions of former ice flow, the location of ice divides, the position of glacier termini, changes in basal thermal regime and the patterns of ice recession.

A number of different landforms are used in palaeoglaciological reconstructions (Table 12.1). The following eight groups of landforms provide the most information about former subglacial conditions and ice-sheet dynamics (see Chapters 9-11). The most powerful palaeoglaciological reconstructions, however, are those that use a combination of many different landforms.

1. Ice-moulded or streamlined subglacial landforms (Box 12.2) cut in bedrock (grooves and mega-grooves) or made of sediment (e.g., glacial lineations, flutes and

Figure 12.2 Example of how ice-flow systems can be reconstructed from landform evidence. In this example, two events are recognised on the basis of the flow traces identified. The landforms relating to Event 2 are interpreted as the youngest because they overprint those of

Event 1.

Flow traces —■» Interpreted pattern —Reconstructed fans

Flow traces —■» Interpreted pattern —Reconstructed fans

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