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| Titel |
Constraining the timing of last glacial plucking of tors on Cumberland Peninsula, Baffin Island, Eastern Canadian Arctic |
| VerfasserIn |
Annina Margreth, John Gosse, Arthur Dyke |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093881
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| Publikation (Nr.) |
 EGU/EGU2014-9043.pdf |
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| Zusammenfassung |
| Highly-weathered rock outcrops (tors) often occur on regolith-covered, low-relief upland
plateaus in formerly glaciated polar landscapes. Owing to their advanced weathering degree
and lack of glacial erosional or depositional features, they have traditionally been
interpreted to have escaped ice sheet coverage as nunatak refugia for flora and fauna.
However, in many places terrestrial cosmogenic nuclide (TCN) exposure dated erratic
blocks deposited on the regolith and the asymmetric streamlining of tor outcrops
allude to past ice coverage. Complex cosmic ray exposure histories of ice cover
have been deciphered using two radiogenic nuclides with dissimilar decay rates.
However, while 26Al/10Be ratios can indicate that the rock had been previously
buried by ice, the ratios alone cannot determine when the cover occurred. Thus,
interpretation that ice cover occurred during the last glacial maximum (LGM) may be
flawed.
We have developed a novel approach to interpret ratios of TCN in the context of complex
exposure histories accounting for recurring burial by cold-based ice and address the problem
of episodic glacial plucking. First, we establish the average exposure:cover ratio for the tor
sites we visited. Assuming orbital pacing of glacial-interglacial cycles, we model plausible
exposure histories of periodic exposure and burial intervals. The majority of the 26 samples
collected from highly-weathered tors on Cumberland Peninsula interfjord plateaus require
average relative exposure durations of 20% within a glacial-interglacial cycle (i.e., 20 ka of
exposure and 80 ka of ice coverage). Three samples located along narrow, highly-weathered
coastal ridges indicate ice-free conditions throughout their entire exposure history. Minimum
total exposure durations range from 320 ka up to 1.8 Ma, which are approximately
twice as long as previous estimates of total exposure histories. This model assumes
ice coverage during LGM, but a Monte Carlo simulation has shown that several
summits could also have been ice free since oxygen isotope stage 3 (i.e., the last 60
ka).
Second, we determine when the last glacial plucking event occurred. TCN concentrations
decrease exponentially below surface, but the 26Al/10Be increase with depth due to increased
muogenic production of 26Al relative to 10Be with depth. By modelling the variation in TCN
ratio and concentrations with depth and time, we are able to constrain the timing of last
plucking of any sampled surface for different thicknesses of overlying tor blocks. We can
show that the samples with the longest total exposure histories likely have never been
plucked. Assuming reasonable thicknesses of bedrock blocks of 50 – 120 cm (based on
horizontal joint spacing), we calculate that most sampled surfaces were plucked either
between 200 – 350 ka and 500 - 800 ka. This timing can further be constrained
if the thickness of the plucked bedrock block can be determined independently. |
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