Box 126 Numerical Modelling Of Former Ice Sheets The British Isles Ice Sheet

High-resolution numerical computer models are an important tool in glacial geology because they can be used to make quantitative palaeoglaciological predictions about Quaternary ice sheets. Models can be used to predict parameters such as the thickness, distribution and age of tills, the distribution of indicator erratics, the distribution of eskers and tunnel valleys, relative sea-level change and the glacial sedimentary and isotopic flux to the oceans. Boulton and Hagdorn (2006) used a thermo-mechanically coupled numerical ice-sheet model, driven by a proxy climate, to explore the key properties of the last British Isles Ice Sheet - an ice sheet that had a relatively low surface profile, low summit elevation and extensive, elongated lobes at its margin. Their approach was to determine the boundary conditions that permit the simulated ice sheet to mimic the evolution of the real ice sheet through the last glacial cycle. Their simulations show how a British Isles Ice Sheet may have been confluent with a Scandinavian Ice Sheet during some parts of its history and how unforced periodic and asynchronous oscillations might occur in different parts of the ice sheet margins. Marginal ice lobes are a reflection of ice-stream development within the ice sheet. Such ice streams can be ephemeral, with 'dynamic ice streams' located because of ice sheet properties, or 'fixed ice streams', with their location determined by subglacial bed properties or topography. Boulton and Hagdorn (2006) concluded that the simulations that best satisfy the glacial geological constraints of ice-sheet extent, ice-surface elevation and relative sea levels are those with major fixed ice streams that strongly draw down the ice-sheet surface. In their simulations the core upland areas of the ice sheet were cold-based at the Last Glacial Maximum, ice-stream velocities were between 500 and 1000 m yr_1 (compared with velocities of 10-50 m yr_1 in interstream zones), shear stresses were as low as 15-25 kPa under the ice streams (compared with 70110 kPa in upland areas) and 60-84% of the ice flux was delivered to the margin via ice streams. Shown below is output from the computer model Boulton and Hagdorn (2006).

Output is for the Last Glacial Maximum (LGM) around 17000 years ago. c1 shows the surface form of the ice sheet, c2 shows areas of basal melting (in red) and basal freezing (in blue), and c3 shows basal ice velocities, with red areas indicating high velocities and white areas indicating frozen-bed areas with zero velocities.

Output is for the Last Glacial Maximum (LGM) around 17000 years ago. c1 shows the surface form of the ice sheet, c2 shows areas of basal melting (in red) and basal freezing (in blue), and c3 shows basal ice velocities, with red areas indicating high velocities and white areas indicating frozen-bed areas with zero velocities.

Source: Boulton, G.S. and Hagdorn, M. (2006) Glaciology of the British Isles ice sheet during the last glacial cycle: Form, flow, streams and lobes. Quaternary Science Reviews, 25, 3359-90. [Modified from: Boulton and Hagdorn (2006) Quaternary Science Reviews, 25, figure 14, p. 3382]

dynamics of large ice masses is still not fully understood but it is possible that the migration of tributaries is partly responsible for the switching on and off of ice streams.

4. Lateral shear zones develop at the contact between fast-flowing ice in ice streams or ice-stream tributaries and slow ice in interstream ridges. In the landform record, the boundaries of palaeo-ice streams, tens to hundreds of kilometres in length, have been interpreted as the sites of lateral shear zones. Sometimes, but not always, their location is marked by an ice stream shear-margin moraine (Figure 12.8). These features range in length from 11 to 22 km, with constant widths of around 500 m, and heights between 10 and 50 m. Examples exist of location control by topography and also of palaeoshear zones without any topographic or substratum control (Figure 12.6C). On the contemporary Antarctic Ice Sheet, lateral shear zones appear as linear and often intensely crevassed structures up to several hundred kilometres in length (Figure 12.6D). The occurrence of long, fully continuous shear zones without any obvious external control on location strongly suggests that internal differences

Figure 12.8 Landsat TM satellite image showing the location of ice stream shear-margin moraines (arrowed) on Storkerson Peninsula, Canada. [Image courtesy of: C.R. Stokes]

in material properties, for example shear-softened, warmer and more deform-able ice in the shear margins, play an important role in determining the upstream, and possibly also downstream, propagation of fast ice flow.

Reconstructions of the basic arrangement of these ice sheet components for the Laurentide and Fennoscandian ice sheets show that these are similar in configuration to the contemporary Antarctic ice sheet. Both the Laurentide and Fennoscandian ice sheets had relatively slow-flowing interiors and were drained by large peripheral ice streams. Ice streams were arranged in a basically radial pattern, with individual ice streams separated from each other by topographically guided frozen-bed patches on intervening ridges (Figure 12.7).

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