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| Titel |
Evaluating the importance of grain size sensitive creep in terrestrial ice sheet rheology |
| VerfasserIn |
C. N. P. J. Maaijwee, J. H. P. de Bresser |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029479
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| Zusammenfassung |
| The rheology of ice in terrestrial ice sheets is generally considered to be independent of the
size of the grains (crystals), and appears well described by Glen’s flow law. In recent years,
however, new laboratory deformation experiments on ice as well as analysis of in situ
measurements of deformation at glaciers suggested that grain size and variations therein
should not be discarded as important parameters in the deformation of ice in nature. Ice, just
like crystalline rock materials, exhibits distributed grain sizes. Taking now that not only
grain size insensitive (GSI; dislocation) mechanisms, but also grain size sensitive
(GSS; diffusion and/or grain boundary sliding) mechanisms may be operative in ice,
variations in the shape of the distribution (e.g. the width) can be expected to affect
the rheological behaviour. To evaluate this effect, we have derived a composite
GSI+GSS flow law and combined this with full grain size distributions. The constitutive
flow equations for end-member GSI and GSS creep of ice were taken from the
work of Goldsby and Kohlstedt (2001, J.Geophys.Res., vol. 106). We used their
description of grain boundary sliding controlled creep as representative of GSS
creep. The grain size data largely came from published measurements from the top
800-1000 m of two Greenland ice cores (NorthGRIP and GRIP) and one Antarctic ice
core (Epica, Dome Concordia). Temperature profiles were available for both core
settings. The grain size data show a close to lognormal distribution in all three
settings, with the median grain size increasing with depth. We constructed a synthetic
grain size profile up to a depth of 3100 m (cf. GRIP) by allowing the median grain
size and standard deviation of the distribution to linearly increase with depth. The
percentage GSS creep contributing to the total strain rate has been calculated for
a range of strain rates that were assumed constant along the ice core axes. The
results of our calculations show that at realistic strain rates in the order of 10-11 to
10-12 s-1, GSS mechanisms can be expected to dominate creep in the parts of
the ice sheets investigated (i.e. the top ~1000 m). In the synthetic core, the GSS
contribution decreases if going to greater depth (~2500 m), but increases again close
to the contact with the bedrock (at 3100 m). Although many assumptions have
been made in our approach, the results confirm the important role that grain size
might play in ice sheet rheology. The application of full grain size distributions in
composite flow equations helps to come to reliable extrapolation of lab data to nature. |
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