Browsing by Author "Li, YK"

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    Cosmogenic nuclide evidence for minimal erosion across two subglacial sliding boundaries of the late glacial Fennoscandian ice sheet
    (Elsevier, 2006-04) Harbor, J; Stroeven, AP; Fabel, D; Clarhäll, A; Kleman, J; Li, YK; Elmore, D; Fink, D
    The existence of sliding and frozen bed areas under ice sheets is significant in understanding basal thermal regimes, patterns of erosion and landform development, and in constraining boundary conditions for the reconstructions of ice sheets. Recognition of subglacial boundaries between sliding and frozen-bed areas for former ice sheets is typically based on distinct morphological contrasts between areas with glacial landform assemblages and relict areas showing little alteration of pre-existing features. Some of these boundaries, especially on continental shield areas, however, are clearly visible from air photos but have minimal topographic expression. Understanding the chronology and erosional development of such boundaries is important to provide insight into the pattern and persistence of basal conditions under ice sheets. Geomorphic evidence and cosmogenic radionuclide concentrations of bedrock outcrops on either side of two sliding boundaries on Ultevis and Arvestuottar, low-relief upland plateaus in northern Sweden, are consistent with negligible erosion in relict landscape (frozen bed) areas due to the last glaciation, but also indicate insignificant erosion in the sliding areas. Such a pattern and magnitude of landscape modification indicates that sliding was short lived in these areas, likely as a transient phase during deglaciation. These sites demonstrate that short periods of sliding are in some cases sufficient to produce landscapes that are recognized as ‘glacial’ from air photos. Thus, regions of sliding identified on shield areas must be viewed as the cumulative total area that has experienced sliding at any time during a glaciation. The actual extent of sliding areas during any single ice sheet phase is presumably considerably less than this cumulative total, which has important implications for establishing appropriate basal boundary conditions for ice sheet reconstructions.© 2005 Elsevier B.V.
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    Ice sheet erosion patterns in valley systems in northern Sweden investigated using cosmogenic nuclides
    (Wiley, 2005-08-30) Li, YK; Harbor, J; Stroeven, AP; Fabel, D; Kleman, J; Fink, D; Caffee, M; Elmore, D
    Erosion patterns associated with glaciation of trunk and hanging valley systems in northern Sweden were investigated using cosmogenic nuclide 10Be apparent exposure ages and inferred nuclide inheritance. Sequences of samples taken across valleys known to have been covered repeatedly by the Fennoscandian ice sheet revealed two primary patterns of erosion. In Vávlávágge the exposure age pattern is consistent with >2 m of glacial erosion during the last glacial cycle along the entire profile. At Rávtasvággi, Dievssavággi and Alisvággi, exposure ages in the valley bottom contrast with apparent exposure ages two to four times older on the valley sides. The older ages on the valley sides reflect cosmogenic nuclide inheritance due to limited (<2 m) bedrock erosion of the valley sides during the last glacial cycle. The pattern and scale of erosion in these valleys indicates that glacial valley formation is a result of multiple glacial cycles rather than the result of topographic modification during a single glacial cycle. Initial data comparing hanging valley and trunk valley sites do not show distinct differences in apparent exposure ages. Slightly older ages for samples from hanging valley bottoms may suggest nuclide inheritance indicating lower erosion than in trunk valley bottoms, as would be expected given the marked topographic step between hanging and trunk valleys. Although quantifying the amount of erosion depends on the assumed cosmogenic nuclide inheritance prior to the onset of erosion, the pattern of erosion is independent of this. Hence the observed pattern of cosmogenic nuclide concentrations provides constraints on spatial patterns of erosion and helps to refine understanding of the timing and extent of landform modification by glaciation. Copyright © 2005 John Wiley & Sons, Ltd.