The Methods Used In Palaeoglaciology

A major impetus for the science of palaeoglaciology was the advent of optical satellite imagery in the late 1980s and early 1990s. Satellite images are useful because a single image covers a large area (100s of km2) of the Earth's surface, making it possible to gain a coherent view of regional landform distributions. Mapping of the spatial distribution of landforms visible on satellite images is

Glacial Geology: Ice Sheets and Landforms Second Edition Matthew R. Bennett and Neil F. Glasser © 2009 John Wiley & Sons, Ltd

_OUTPUTS_

Former spatial extent Former vertical extent Former patterns of ice discharge Palaeo-ice streams Former ice divides and basal thermal regime History of growth and decay Contribution to global sea level

Figure 12.1 The genetic and inversion problems in glacial geomorphology. [Modified from: Kleman etal. (2006), in Glacier Science and Environmental Change (ed P.G. Knight), Blackwell, plate 38.4, p. 197]

fundamental to palaeoglaciology. A basic outline of the steps required for a palaeo-glaciological reconstruction follows.

1. Obtain satellite image coverage of the area in question. Satellite images need to be acquired in the visible spectrum (optical images) and at a spatial resolution (say 30 m or less) so that landforms can be identified and mapped. Satellite images are often draped over a digital terrain model (DTM) to aid landform interpretation in areas of complex terrain. Aerial photographs might also be used to provide specific information on small areas where finer resolution is required.

2. Landform mapping. Mapped landforms are identified using established identification criteria (Table 12.1). Their location and boundaries are digitised onscreen and stored in a Geographical Information System (GIS) for later retrieval. If possible, maps should be checked in the field to ensure that landforms have been correctly identified. If it is available, information about striae and till stratigraphy can also be added at this stage. The most commonly used landforms in palaeoglaciological reconstructions are ice-moulded subglacial landforms (see Section 9.2.1), sometimes referred to as streamlined subglacial landforms (e.g., drumlins, flutes, megaflutes, megascale glacial lineations). To these information on ice-marginal positions and subglacial hydrology can be added by mapping terminal moraines, eskers and meltwater channels, although other landforms can also be used (see Section 12.2).

Table 12.1 Palaeoglaciological variables and the primary landforms that can be used to reconstruct them

Palaeoglaciological reconstruction

Landforms

Information provided

Former spatial extent of an ice sheet and its variation through time (i.e. location of the ice margin and how this fluctuated)

Former vertical extent of an ice sheet and its variation through time (i.e. former ice-surface elevation, ice thickness and how this changed through time)

Former patterns of ice discharge in an ice sheet (i.e. locations of major outlet glaciers and ice streams, former ice-flow trajectories)

Terminal moraines Ice-marginal meltwater channels Eskers

Trimlines (see Box 12.5) Erratic boulders and erratics Ice-marginal meltwater channels

Raised shorelines and raised beaches

Glacial lineations Striations

Subglacial meltwater channels

Eskers

Erratic dispersal

Terminal moraines mark maximum former extent. Can be dated with 14C, OSL or cosmogenic isotopes.

Marginal meltwater channels indicate position of former ice margin and its recessional history.

Eskers generally form at or close to the former ice margin, parallel or subparallel to ice-flow direction.

Trimlines can be used to infer former vertical height.

Erratic boulders deposited on mountains around the ice sheet indicate former debris transport paths. Elevation of erratic boulders on mountains provides an estimate of former vertical ice-sheet height.

Marginal meltwater channels indicate former vertical dimensions of an ice sheet.

Former ice thickness can be calculated from pattern of isostatic rebound following deglaciation.

Ice flow always parallel to orientation of glacial lineations. Can be used to identify location of former ice streams (see Box 12.4).

Ice flow always parallel to orientation of striations. Subglacial meltwater channels generally parallel or subparallel to former ice-flow direction.

Eskers generally form parallel or subparallel to former ice-flow direction. Patterns of erratic dispersal indicate former ice-flow trajectories.

Table 12.1 Continued.

Palaeoglaciological reconstruction

Landforms

Information provided

Former locations of ice divides and frozen-bed areas

Preglacial and periglacial landforms, e.g., rounded summits, fluvial valleys, cryoplanation terraces and pediments, and the presence of tors, blockfields and saprolites

Overall history of ice-sheet growth and decay through time

Spatial distribution of all the above landforms and their relative ages

Landform preservation provides information on former subglacial thermal regimes, that is, location of former frozen-bed areas and their spatial extent.

Patterns of ice-divide migration can be traced through deglaciation. Long-lived former ice-divide locations may be indicated by areas of maximum isotatic rebound.

Landforms can be combined to make a full palaeoglaciological reconstruction. Can incorporate or make comparisons with results of numerical ice-sheet models or isostatic rebound models (see Box 12.6).

Can incorporate or make comparisons with other proxy measures of global ice volume (e.g., oxygen isotope records).

3. Data reduction. The pattern of mapped landforms is simplified into a number of coherent landform systems using a glacial inversion model (Box 12.1). This step is required because across an ice sheet bed there may be many hundreds or even thousands of individual landforms, as well as striae and other point data (e.g., till-fabric analyses, dated landform surfaces). The most commonly used method is to convert landforms into a number of coherent 'flow sets' (also called 'flow packages', 'fans' or 'swarms'), where each flow set reflects an ice-flow system that is spatially and temporally distinct (Figure 12.2). Established criteria are used to group landforms into these flow sets, including parallel concordance (i.e. similar orientation); proximity to one another; and morphometry (e.g., similar length, width, height, elongation ratios for streamlined features such as drumlins). It is also important to ensure that these flow sets, once defined, form a glaciologically plausible pattern.

4. The final step is the palaeoglaciological reconstruction itself. This involves making an overall interpretation of the patterns of events and their relative timing based on the patterns of mapped landforms and flow sets. It is also possible to combine

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