A brief history of changes in the concept of the LIS

By the 1890s a good deal of research had been carried out in the glaciated area of the USA and in southern Canada. State and Provincial Geological Surveys undertook much of this research. Of particular note were the long journeys (over several months) undertaken by Canadian geologists, often members of the Geological Survey of Canada. In retrospect the work of individuals such as Low, Tyrrell, Bell and others was amazing in its perception (Bell, 1884, 1898; Tyrell, 1898; Low, 1893; and see review in

Prest, 1990), given the fact that much of the area north of 50°N latitude was scientifically largely unexplored. In 1898 Tyrrell produced a map of the former North America ice sheets (Fig. 40.2). This map is remarkably similar in many ways to present-day concepts of the LIS, that is, an ice sheet that like the Antarctic Ice Sheet today had several dispersal centres. Even 110yr or so ago he showed that the ice sheet consisted of ice divides over Labrador, Foxe Basin, Keewatin and the Patrician Centre off southwest Hudson Bay. This notion prevailed until the late Professor Flint (1943) published a seminal paper in which he argued for the growth of an ice sheet from the mountains of the eastern Canadian Arctic to finally form a massive single-domed (~ ridge) ice sheet centred over Hudson Bay. Flint's model was not based per se on any specific field evidence but rather on a model for the growth and development of the ice sheet. This model continued to have adherents into the 1970s and 1980s (Denton & Hughes, 1981), although it is important to note that there is no erratic evidence on the east side of Hudson Bay for a flow from a supposed Hudson Bay centre (Figs 40.1 & 40.3).

An alternative model to that proposed by Flint was developed in the 1960s and onward. It was based initially more on a different view of how the onset of glacierization occurred and to a large degree it was offered on the basis both of field work and, as importantly, the first topographic maps (1:250,000 to

Figure 40.3 Map of the extent of the Precambrian shield rocks under the Laurentide Ice Sheet and the arc of exhumation (White, 1972) that lies along the contact between the 'hard' shield rocks and the onlapping sedimentary rocks. This extends along the Labrador coast as a coast-parallel trough, but this is not noted along the Baffin Island coast. Arrows indicate areas of erosion and transport of Palaeozoic carbonates (calcite and dolomites) onto land or to marine sites. The distribution of Palaeozoic carbonates in the High Canadian Arctic and North West Greenland is too complicated to show on this map but basically crop out on the floors of the major channels. Some of the major transport directions for the carbonates are shown (note the lack of carbonates from Hudson Bay on the Labrador shield region).

Figure 40.3 Map of the extent of the Precambrian shield rocks under the Laurentide Ice Sheet and the arc of exhumation (White, 1972) that lies along the contact between the 'hard' shield rocks and the onlapping sedimentary rocks. This extends along the Labrador coast as a coast-parallel trough, but this is not noted along the Baffin Island coast. Arrows indicate areas of erosion and transport of Palaeozoic carbonates (calcite and dolomites) onto land or to marine sites. The distribution of Palaeozoic carbonates in the High Canadian Arctic and North West Greenland is too complicated to show on this map but basically crop out on the floors of the major channels. Some of the major transport directions for the carbonates are shown (note the lack of carbonates from Hudson Bay on the Labrador shield region).

1: 500,000) of large areas of the Canadian Arctic. What struck Ives (1957, 1962) and earlier writers (Brooks, 1926) was that the eastern Canadian Arctic consisted of the uplifted rim of the Precambrian shield, so that there are vast areas of high, rolling uplands >500m a.s.l. Based on field and imagery observations in Labrador and Baffin Island (Ives, 1962; Andrews et al., 1976) the concept on 'instanteous glacierization' was proposed and examined by some of the first energy-balance and three-dimensional glaciological models (Andrews & Mahaffy, 1976; Williams, 1978a). Coeval with this conceptual work on the distribution of raised marine beaches, now datable by radiocarbon dating, came arguments that the glacial rebound of the LIS also suggested the presence of several rebound centres (Andrews, 1970). The inception of LIS glaciation on the unscoured uplands of the Eastern Canadian Arctic, first examined rigorously in the 1970s, is supported by later, more sophisticated three-dimensional gla-ciological reconstructions (Kleman et al., 2002; Marshall & Clark, 2002).

The debate between a single-domed ice sheet and a mul-tidomed LIS then became involved with the issue of the extent of the ice sheet, especially along its eastern and northern margins, and the debate between 'big and little ice or minimum and maximum ice sheets' was born. The two sides of the debate were effectively discussed by Denton & Hughes (1981) in their book The Last Great Ice Sheets. I was one of the 'minimum ice sheet' crowd primarily for two reasons:

1 radiocarbon dates from the margin of the Eastern Canadian Arctic invariably (and still to this day) gave a series of results <9000 14C yr BP and often associated with a massive moraine complex called the Cockburn Moraines, located near the fiord heads, whereas on the outer coast Late-glacial/Holocene marine limits were often low and there were complex strati-graphical exposures with the uppermost sediments dating >35,000 to >54,000 14C yr BP (Andrews, 1980);

2 the evidence for the maximum model was frequently based on geomorphological arguments with no temporal control, e.g. shelf troughs were cut by outlet glaciers of the ice sheet, therefore the ice sheet extended along the trough during the Last Glacial Maximum (LGM).

The nail in the coffin of the absolute minimum model was delivered by the work of Anne Jennings in Cumberland Sound, Baffin Island (Jennings, 1993), who showed unequivocally that the Sound, shown as largely unglaciated in Dyke & Prest (1987a), had been filled with a major ice stream that had not retreated from the outer basin until ca. 10,200 14C yr BP. Since her work there has been a substantial revision of the glacial history based in large measure on research into the stratigraphical record on adjacent shelves, slopes and deep-sea basins (Jennings et al., 1996; Andrews et al., 1998b), and on the application of cosmogenic exposure age dating (Steig et al., 1998; Marsella et al., 2000; Briner et al., 2003), the results of which have tended to confirm some of the earlier suspicions articulated by Sugden & Watts (1977) and has resulted in 'just the right ice' model for the LGM (Miller et al., 2002), with ice reaching the coastline in some areas but extending well seaward in other areas (Fig. 40.1), such as Cumberland Sound (Jennings et al., 1996) and Hudson Strait (Hesse, 1995; Hesse & Khodabakhsh, 1998).

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