Erosion by the LIS

In many ways our concept of glacial erosion is governed by data from present-day, often small, mountain glaciers (Hallet et al., 1996). Thus when considering glacial erosion these data usually are considered as more efficient per unit area than fluvial erosion. However, we need to consider present-day rates in the longer Quaternary context of the LIS. Thus a rate of 0.5mmyr-1 indicates the removal of 500 m of bedrock in 1 Myr or nearly 1500 m since the generally accepted date of Northern Hemisphere general glaciation ca. 2.6Ma.

Flint (1971) attempted to compute the volume of glacial sediments from various papers and reports and suggested an average lowering of 10 m (or 0.001 mm yr-1) but the topic of how much erosion had occurred under a 12 x 106km2 ice sheet did not attract much attention until 1972 when White (1972) published an article that claimed evidence for 1000 m or so of erosion in the centre of the ice sheet, that is Hudson Bay. The paper provoked a series of sharp exchanges, most of which were, rightly so, extremely critical (Gravenor, 1975; Sugden, 1976), but one observation that White made is important and that is he noted that along the contact between the Canadian shield (mainly granites and gneisses) and the onlapping younger and less resistant bedrock lay 'the arc of exhumation' or a series of large lake basins, assumed to be the result of glacial erosion. The location of these basins, fringing the Precambrian Shield, can be carried seaward along the Labrador margin where a deep, coast-parallel trough lies along the contact between the shield rocks and the softer sediments of the continental shelf (Holtedahl, 1958) (Fig. 40.3). The contrast between the 'hard bed' of the Canadian Shield and the 'soft bed' that lies toward the margins of the former ice sheet is now considered to be fundamental in terms of glaciological processes.

Erosional products from terrestrial sites, given enough time, will eventually be transported seaward and their final point of deposition will be the deep-sea basins. Bell and Laine (Laine, 1980; Bell & Laine, 1985) examined the offshore evidence for the amount of sediment stored in the basins flanking the eastern sector of the LIS. The evidence consisted of a series of deep-sea drilling sites, the lithostratigraphy of the cores, and the interpretation of seismic profiles. With these data they were able to compute the volume of sediments associated with the eastern sector of the LIS. They arrived at a value of around 100 m of erosion in the past 2.6Myr or so, or a long-term average erosion rate of ca. 0.04 mmyr-1.

The next issue then is how this average rate might apply to the bed of the ice sheet. How variable was erosion and what were its spatial characteristics?

0 0

Post a comment