Palaeoice sheet volume and weathering zones of the Long Range Mountains

In eastern Canada, the extent of glaciation is not easily interpreted from a routine application of glacial geomorphology. This is

Figure 89.1 Location of the two weathering zone type localities in western Newfoundland of Grant (1977) and the TCN sampling localities (numbers are last three digits of sample IDs in Table 89.1, erratic boulders in italics).

because non-erosive ice may have covered much of the highland summits leaving little or no erosional or depositional evidence of its presence. The few boulders (even perched in places) that have been discovered on bedrock knobs on these summits have been rejected as erratics and considered core stones by previous geo-morphologists because their lithology is similar to the underlying bedrock.

An important basis of the 'limited ice cover' paradigm in eastern Canada is the occurrence of altitudinally distinct zones of differential weathering along the coastal highlands (see Marquette etal. (2004) for a more complete summary). Like most regions in Eastern Canada, the weathering zones in western Newfoundland have been distinguished according to (i) general appearance of maturity, (ii) presence or preservation of glacial erosional and depositional landforms of various scales, (iii) degree of bedrock weathering, including weathering rinds, rounding of edges, protrusion of relatively resistant veins, and abundance and size of gnammas and rillen, and (iv) geometry of stream cross-sectional profiles. The lowest zones generally have till and ample evidence of glaciation. The upper zones have no evidence of recent glaciation and often comprise felsenmeer (which is a 0.5 to >3m thick surface layer of porous angular boulders and regolith, particularly in the discontinuous permafrost zones in eastern Canada) and relatively more developed soils. In the 1970s, D. Grant and I. Brookes independently mapped three weathering zones in all high relief regions (800 m) of western Newfoundland (e.g. Brookes, 1977; Grant, 1977, 1986, 1989). They interpreted the boundaries between the zones to represent limits of glaciation. The boundary between the lowest ('A') and intermediate ('B') zone was interpreted to represent the trim-line of the last glaciation (letter designations from Grant, 1977). The boundary between 'B' and 'C' zones was the penultimate trimline and zone 'C' areas were either never glaciated (i.e. the summits were nunataks) or were glaciated in the mid- or early Pleistocene. Corroborating the notion that zone boundaries are trimlines is the observation that the boundaries in eastern Canada tend to dip seaward, just as ice in the fjord valleys drained ice from farther inland and spilled onto the coastal piedmont. Early in the development of the nunatak hypothesis were a series of ecological studies that identified over a dozen rare beetle, fish and bryophyte species isolated in the uppermost weathering zones in eastern Canada and Greenland. These disjunct species were believed to have survived in ice-free enclaves along the coastal highlands during the last glaciation (e.g. Fernald, 1911; Belland & Brassard, 1988) (Fig. 89.2a).

An alternative to the nunatak hypothesis is that the weathering zones simply represent regions exhibiting different degrees of glacial erosion, as discussed above and observed by many others elsewhere (Sugden, 1968; Sugden & Watts, 1977; Dyke, 1993; Kleman & Hattestrand, 1999; Dredge, 2000; Stroeven et al., 2002a,b; Marquette et al., 2004; Staiger et al., 2005;). In fjord valleys we should expect the greatest amount of erosion where glacial ice is converging and the ice flux is greatest, with additional sliding due to lubrication from frictional melting. The felsen-meer-capped summits are conducive to frozen-based non-erosive ice cover, particularly where the ice cover is thin due to drawdown into the bounding fjords (Fig. 89.2b).

89.3 Methods

For a recent review of the principles and applications of TCN exposure dating the reader is referred to Gosse & Phillips (2001). Secondary cosmic rays penetrate the upper metres of rock and sediment cover on Earth's surface and interact with nuclei of exposed minerals. For instance, cosmogenic 10Be is produced when a secondary particle breaks apart an oxygen or silicon atom in exposed quartz during an interaction referred to as spallation. Similarly 26Al is produced from 28Si. We used standard sample preparation procedures (e.g. Kohl & Nishiizumi, 1991) and a constant source for Be-carrier (shielded beryl crystal digested by Jeff Klein and Barbara Lawn, University of Pennsylvania) and Al-carrier (commercial inductively coupled plasma mass spectrometry (ICP-MS) standard). The accelerator mass spectrometry (AMS) facilities at University of Pennsylvania (1992 samples) and Lawrence Livermore National Laboratory (2002 samples) were used for the 10Be/9Be and 26Al/27Al measurements, and the inductively coupled plasma atomic emission spectrophotometry (ICP-AES) at EES-1, Los Alamos National Laboratory was used for Al and Be concentrations.

Felsenmeer Landform Manual Diagram

Figure 89.2 Block diagrams of the two opposing hypotheses for the glacial history and ice volume during the last glaciation for western Newfoundland. (a) Nunatak hypothesis, showing extent of Long Range Ice Cap during the Last Glacial Maximum (LGM), after Grant (1977). (b) Complete cover hypothesis, requring thin ice cover on summits and possibly exposed cliffs during the LGM. If the ice cover is from Laurentide Ice Sheet from the west, the cliffs will not be exposed during the LGM.

Figure 89.2 Block diagrams of the two opposing hypotheses for the glacial history and ice volume during the last glaciation for western Newfoundland. (a) Nunatak hypothesis, showing extent of Long Range Ice Cap during the Last Glacial Maximum (LGM), after Grant (1977). (b) Complete cover hypothesis, requring thin ice cover on summits and possibly exposed cliffs during the LGM. If the ice cover is from Laurentide Ice Sheet from the west, the cliffs will not be exposed during the LGM.

Our strategy for using TCN to test the weathering zone hypothesis was developed over a decade ago (Gosse et al., 1993). The isotope approach consists of three steps.

1 Using a single TCN, document the timing of last deglaciation in a region by dating erratics strewn throughout a summit plateau or on a moraine. This is a critical step, because it can demonstrate the systematic retreat of ice margins on summits which otherwise appear never to have been glaciated. Unfortunately there are not many large boulders on the highest weathering zones, and the boulders are often similar in lithol-ogy to the underlying bedrock because of their short glacial transportation distance. We choose boulders that are perched on bedrock or are on the highest summit to preclude their deposition by mass wasting. If the highest zones were never glaciated, the boulders should have accumulated TCN unhindered by ice cover and therefore should be much older than the timing of the last glacial maximum. Determine the concentration of a single TCN in bedrock surfaces adjacent to the boulders. If the concentrations in the boulders and adjacent bedrock surfaces (typically tor-like features) are equivalent, then the glaciers must have eroded the bedrock more than 2 m to remove any TCN produced in the bedrock prior to glaciation. It is common to have this parity in glaciated valleys where glacial erosion is concentrated (e.g. Labrador and Baffin Island—Marquette et al., 2004, Staiger et al., 2005; elsewhere—Stroeven et al., 2002a,b). If the TCN concentration in bedrock is greater (>2 o) than that in the adjacent boulders, the glacier that deposited the boulders must have eroded less than 2 m of rock. To substantiate the notion that the bedrock surfaces were actually covered by non-erosive ice (one or more times in the past few million years), a second isotope is measured. The ratio of their concentrations (e.g. 26Al/10Be) is used to test whether the bedrock surface experienced interruptions in its long exposure history (i.e. due to ephemeral cover by glacier ice, till, ash, water, etc.). A surface that has been eroded more than 2 m deep has very little record of its history of exposure prior to glaciation, so the isotopic ratio will be close to the theoretical production ratio of the isotopes on a continuously exposed surface (upper curve, Fig. 89.3). However, if the surface was buried but not eroded, the measured concentration of TCNs in the rock will record the concentration prior to ice cover (minus any that may have decayed during the ice cover) plus any TCN that has been produced since deglaciation. The ratio of the rapidly:slowly decaying radioisotopes will decrease during periods of burial (below the shaded field, Fig. 89.3).

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