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| Titel |
Headwall erosion rates from cosmogenic 10Be in supraglacial debris, Chhota Shigri Glacier, Indian Himalaya |
| VerfasserIn |
Dirk Scherler, David Egholm |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250129265
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| Publikation (Nr.) |
 EGU/EGU2016-9351.pdf |
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| Zusammenfassung |
| Debris-covered glaciers are widespread within the Himalaya and other steep mountain
ranges. They testify to active erosion of ice-free bedrock hillslopes that tower above valley
glaciers, sometimes more than 1 km high. It is long known that debris cover significantly
reduces surface ablation rates and thereby influences glacial mass balances; but its dynamic
evolution along with climatic and topographic changes is poorly studied. Better
understanding the coupling of ice-free bedrock hillslopes and glaciers in steep mountains
requires means to assess headwall erosion rates. Here, we present headwall erosion rates
derived from 10Be concentrations in the ablation-dominated medial moraine of the Chhota
Shigri Glacier, Indian Himalaya. We combine our empirical, field-based approach with a
numerical model of headwall erosion and glacial debris transport to assess permissible
patterns of headwall erosion on the ice-free bedrock hillslopes surrounding the Chhota Shigri
Glacier.
Our five samples, each separated by approximately 500 m along the glacier, consist of an
amalgamation of >1000 surface clasts with grain sizes between ∼1 and ∼30 mm that were
taken from the medial moraine. Our results show that 10Be concentrations increase
downglacier from ∼3×104 to ∼6×104 atoms g−1, yielding headwall erosion rates of
∼1.3-0.6 mm yr−1. The accumulation of 10Be during debris residence on the ice surface can
only account for a small fraction (<20%) of the downglacier increase. Other potential
explanations include (1) heterogeneous source areas with differences average productions
rates, and (2) homogeneous source areas but temporally variable headwall erosion
rates.
We use the 10Be-derived headwall erosion rates to define debris supply rates
from ice-free bedrock hillslopes in the numerical ice model iSOSIA. Headwall
debris that is deposited in the ablation zone of the ice surface becomes englacial, is
passively advected with the ice and emerges in the ablation zone where it forms
supraglacial debris cover that influences surface melting. Preliminary results show that the
model reproduces the actual medial moraine of the Chhota Shogri Glacier quite
well.
We conclude that the observed 10Be concentrations in the medial moraine of the Chhota
Shigri Glacier yield reasonable headwall erosion rates, and that the systematic downglacier
change in the concentration may reflect a changing erosion rates through time. Combining the
10Be results with the numerical model we are presently exploring new avenues to test simple
models of debris production by frost cracking, e.g., spatially uniform versus temperature
dependent. |
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