Significance of relict surfaces

Many formerly glaciated regions include some areas dominated by glacial landforms (Fig. 90.1A) and other areas with distinct non-glacial features (Fig. 90.1B & C). In addition to a suite of characteristic non-glacial geomorphological features (e.g. tors, mountains with approximate radial symmetry, convex-concave slopes, and well-developed blockfields and other periglacial features), areas with non-glacial features, termed relict areas, are particularly conspicuous in formerly glaciated areas because they often lack features indicative of glacial erosion. Because many relict landscapes in formerly glaciated areas of northern mid-latitude ice sheets occur on highlands close to coasts, and because they often flank deep fjords, in some cases these areas might not have been covered by ice at all (i.e. they were nunataks), and may then have served as glacial refugia for vegetation and animals. Although this contention appears logically plausible along the coasts, where fast flowing ice in fjord systems managed to lower the regional ice surface below the elevation of the inter-fluves and highest summits, it becomes more problematic when these surfaces occur inland from the current mountain divide. This is the case in many locations in central (Kleman & Borgstrom, 1990) and northern Sweden (Kleman & Stroeven, 1997). Here we know that ice sheets formerly covered these areas because they include glacial deposits or features from meltwater erosion. In this case the relict areas potentially provide important information about subglacial conditions under these ice sheets. Differentiating between nunataks and areas preserved under ice sheets is important for understanding former ice sheets. The subglacial temperature beneath ice sheets is a key ingredient of the inversion model described in Kleman et al. (see this volume, Chapter 38) used for reconstructing the dynamics of former mid-latitude ice sheets.

It remains unclear exactly when relict surfaces were originally formed, and, hence, for how long they were preserved essentially intact despite possible multiple episodes of ice sheet overriding. It is clear that post-glacial Holocene processes were ineffective in some locations and, if this is true of other interglacial periods, then some relict areas may have remained largely intact since the Tertiary.

Figure 90.1 Apparent cosmogenic isotope ages and ratios of bedrock samples from (A) the bottom of a hanging glacial valley, (B) a relict surface and (C) a lowland tor in northern Sweden. See map insert for approximate locations and table for isotope concentrations and apparent exposure ages. (Data in (B) and (C) are modified from Fabel etal. (2002) and Stroeven etal. (2002b), respectively.) (See www.blackwellpublishing.com/knight for colour version.)

Figure 90.1 Apparent cosmogenic isotope ages and ratios of bedrock samples from (A) the bottom of a hanging glacial valley, (B) a relict surface and (C) a lowland tor in northern Sweden. See map insert for approximate locations and table for isotope concentrations and apparent exposure ages. (Data in (B) and (C) are modified from Fabel etal. (2002) and Stroeven etal. (2002b), respectively.) (See www.blackwellpublishing.com/knight for colour version.)

90.2 Characteristic cosmogenic nuclide concentrations in relict surfaces

The concentrations of cosmogenic nuclides are different in rock surfaces that have been continuously exposed to the cosmic-ray flux, and surfaces that have been shielded by ice for long periods of time. For example the two radionuclides 10Be and 26Al are produced at a fixed ratio in quartz, but have different decay constants; thus if a surface that has been exposed for a time is shielded and nuclide production is halted, 26Al decays faster than 10Be and their ratio diverges from the production ratio of 6.1.

Two issues pertinent to ice-sheet reconstruction were revitalized by recent investigations that have used terrestrial cosmogenic nuclide techniques to examine relict areas within glaciated regions: (i) can these areas help constrain the surface elevation of former ice sheets, i.e. did they survive as nunataks, and (ii) can these areas be used to constrain the subglacial temperature regime, i.e. did they survive underneath cold-based ice?

The ability of the cosmogenic nuclide technique to differentiate between nunataks and areas preserved under ice can be illus trated using data from relict landscapes in northern Sweden. This area has been covered repeatedly by Fennoscandian ice sheets, and includes both relict areas and areas with substantial glacial modification. For example, apparent exposure ages from bedrock samples in U-shaped valleys (Fig. 90.1A) are sometimes consistent with expected deglaciation ages and hence consistent with substantial glacial erosion. Such glacial valleys occur adjacent to relict uplands, where apparent exposure ages from bedrock samples are older than deglaciation (Fig. 90.1B), proving that glacial erosion rates over these surfaces were lower (Fabel et al., 2002). In fact, the presence of tors on many of these uplands and in the northern Swedish lowlands (Fig. 90.1C), and their apparent exposure ages, are consistent with the interpretation of minimal erosion and landscape modification by ice sheets that covered them. In such relict areas the cosmogenic nuclide apparent exposure ages of erratics, erosional scarps and meltwa-ter channels incised into relict surfaces all yield deglaciation ages, reinforcing the conclusion derived from geomorphological mapping that these landscapes survived subglacially (e.g. Fabel et al., 2002, Stroeven et al., 2002a, b). A significant feature of cosmogenic nuclide concentrations in subglacially preserved relict landscapes is that the ratio between cosmogenically produced 26Al and 10Be in relict surfaces is lower (at 1 standard error) than the exposure ratio of 6.1 (Fig. 90.1). Similar results have derived for some relict areas in North America (e.g. Bierman et al., 1999). Thus, in these cases the cosmogenic data allow us to conclude that certain areas were covered repeatedly by ice sheets, and that these sites represent areas where subglacial conditions were not conducive to erosion during growth, maximum and decay phases of multiple glaciations. However, it is important to note that exposure ratios within 1 standard deviation of 6.1 do not prove that a relict area was a nunatak. Unfortunately, the analytical uncertainty in 26Al and 10Be measurements is such that surfaces that have been shielded for up to ca. 150 kyr cannot be confidently distinguished from surfaces that have not been shielded.

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