Browsing by Author "Harbor, J"

Now showing 1 - 7 of 7
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    Characteristic cosmogenic nuclide concentrations in relict surfaces of formerly glaciated regions
    (Wiley, 2006-08-21) Stroeven, AP; Harbor, J; Fabel, D; Kleman, J; Hättestrand, C; Elmore, D; Fink, D
    This chapter contains sections titled: Significance of relict surfaces Characteristic cosmogenic nuclide concentrations in relict surfaces Conclusions © 2006 by Blackwell Science Ltd
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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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    Exposure ages from relict lateral moraines overridden by the Fennoscandian ice sheet
    (Cambridge University Press, 2006-01) Fabel, D; Fink, D; Fredin, O; Harbor, J; Land, M; Stroeven, AP
    Lateral moraines constructed along west to east sloping outlet glaciers from mountain centred, pre-last glacial maximum (LGM) ice fields of limited extent remain largely preserved in the northern Swedish landscape despite overriding by continental ice sheets, most recently during the last glacial. From field evidence, including geomorphological relationships and a detailed weathering profile including a buried soil, we have identified seven such lateral moraines that were overridden by the expansion and growth of the Fennoscandian ice sheet. Cosmogenic 10Be and 26Al exposure ages of 19 boulders from the crests of these moraines, combined with the field evidence, are correlated to episodes of moraine stabilisation, Pleistocene surface weathering, and glacial overriding. The last deglaciation event dominates the exposure ages, with 10Be and 26Al data derived from 15 moraine boulders indicating regional deglaciation 9600 ± 200 yr ago. This is the most robust numerical age for the final deglaciation of the Fennoscandian ice sheet. The older apparent exposure ages of the remaining boulders (14,600–26,400 yr) can be explained by cosmogenic nuclide inheritance from previous exposure of the moraine crests during the last glacial cycle. Their potential exposure history, based on local glacial chronologies, indicates that the current moraine morphologies formed at the latest during marine oxygen isotope stage 5. Although numerous deglaciation ages were obtained, this study demonstrates that numerical ages need to be treated with caution and assessed in light of the geomorphological evidence indicating moraines are not necessarily formed by the event that dominates the cosmogenic nuclide data. © University of Washington
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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.
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    Importance of sampling across an assemblage of glacial landforms for interpreting cosmogenic ages of deglaciation
    (Academic Press INC Elsevier, 2011-07-01) Stroeven, AP; Fabel, D; Harbor, J; Fink, D; Caffee, M; Dahlgren, T
    Deglaciation chronologies for some sectors of former ice sheets are relatively poorly constrained because of the paucity of features or materials traditionally used to constrain the timing of deglaciation. In areas without good deglaciation varve chronologies and/or without widespread occurrence of material that indicates the start of earliest organic radiocarbon accumulations suitable for radiocarbon dating, typically only general patterns and chronologies of deglaciation have been deduced. However, mid-latitude ice sheets that had warm-based conditions close to their margins often produced distinctive deglaciation landform assemblages, including eskers, deltas, meltwater channels and aligned lineation systems. Because these features were formed or significantly altered during the last glaciation, boulder or bedrock samples from them have the potential to yield reliable deglaciation ages using terrestrial cosmogenic nuclides (TCN) for exposure age dating. Here we present the results of a methodological study designed to examine the consistency of TCN-based deglaciation ages from a range of deglaciation landforms at a site in northern Norway. The strong coherence between exposure ages across several landforms indicates great potential for using TCN techniques on features such as eskers, deltas and meltwater channels to enhance the temporal resolution of ice-sheet deglaciation chronologies over a range of spatial scales. (C) 2011 University of Washington.
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    Landscape preservation under Fennoscandian ice sheets determined from in situ produced 10Be and 26Al
    (Elsevier, 2002-07-30) Fabel, D; Stroeven, AP; Harbor, J; Kleman, J; Elmore, D; Fink, D
    Some areas within ice sheet boundaries retain pre-existing landforms and thus either remained as ice free islands (nunataks) during glaciation, or were preserved under ice. Differentiating between these alternatives has significant implications for paleoenvironment, ice sheet surface elevation, and ice volume reconstructions. In the northern Swedish mountains, in situ cosmogenic 10Be and 26Al concentrations from glacial erratics on relict surfaces as well as glacially eroded bedrock adjacent to these surfaces, provide consistent last deglaciation exposure ages (∼8–13 kyr), confirming ice sheet overriding as opposed to ice free conditions. However, these ages contrast with exposure ages of 34–61 kyr on bedrock surfaces in these same relict areas, demonstrating that relict areas were preserved with little erosion through multiple glacial cycles. Based on the difference in radioactive decay between 26Al and 10Be, the measured nuclide concentration in one of these bedrock surfaces suggests that it remained largely unmodified for a minimum period of 845−418+461 kyr. These results indicate that relict areas need to be accounted for as frozen bed patches in basal boundary conditions for ice sheet models, and in landscape development models. Subglacial preservation also implies that source areas for glacial sediments in ocean cores are considerably smaller than the total area covered by ice sheets. These relict areas also have significance as potential long-term subglacial biologic refugia. © 2002 Elsevier Science B.V.
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    Slow, patchy landscape evolution in northern Sweden despite repeated ice sheet glaciation
    (Geological Society of America, 2006-01-01) Stroeven, AP; Harbor, J; Fabel, D; Kleman, J; Hättestrand, C; Elmore, D; Fink, D; Fredin, O
    The conventional assumption that erosion by ice sheets is pervasive and effective in landscape evolution is tested in northern Sweden using geomorphic mapping and cosmogenic nuclide analyses of formerly glaciated surfaces. The following evidence indicates that recent glaciations in this region have produced only slow and patchy landscape evolution: (1) Geomorphic mapping shows that at least 20% of the repeatedly glaciated study region in the northern Swedish mountains has landforms that are relict, i.e., clearly nonglacial in origin. (2) The contrast between cosmogenic apparent exposure ages from relict landforms in the northern Swedish mountains and from overlying glacial erratics and juxtaposed glacially eroded bedrock surfaces, which are consistent with last deglaciation, implies that the relict landforms have been preserved through multiple glacial cycles. (3) Apparent 10Be and 26Al exposure ages for tor summit bedrock surfaces in the northern Swedish lowlands reveal that these relict landforms have survived at least eleven exposure and ten burial events with little or no erosion over the past ∼1 m.y. (4) The northern Swedish lowland and mountains are primarily covered by glacial landforms. However geomorphic mapping suggests that even these landforms may have undergone limited erosion during the last glacial cycle. Cosmogenic 10Be and 36Cl data from what appear to be heavily scoured areas in one glacial corridor indicate erosion of only ∼2 ± 0.4 m of bedrock during the last glaciation. These results suggest that in some areas the overall modification produced by ice sheets may be more restricted than previously thought, or it has occurred preferentially during earlier Quaternary glacial periods. © 2020 GeoScienceWorld