Isotropic ice samples containing measured concentrations of solutes and debris similar to basal material found in several cold-based glaciers in the McMurdo Dry Valleys, Antarctica, were manufactured in a laboratory and tested for peak shear strength at constant strain rates with a direct-shear device. The shear tests show that differences in rheology and shear strength appear to be related to impurity content and concentration. Debris-laden ice becomes more ductile with greater concentrations of solutes, whereas, low solute-concentrations and high debris-concentrations are associated with increases in shear strength and brittle behaviour. Stress exponents from Glen�s flow law calculated for isotropic solute and debris-laden ice ranged between 4 and 5, leading to the conclusion that higher rates of deformation may be expected in dirty basal ice than predicted for glacial ice models that use stress exponents where, n = 3. Observations of both natural and synthetic samples tested over a range of temperatures between -25�C and -5�C showed that natural basal ice samples containing high solute and debris concentrations were highly sensitive to temperature change. These tests showed an approximate 10 % loss in shear strength for every 1�C increase in temperature between -25�C and -10�C. In addition, contrasts in rheology and rates of deformation within basal ice are responsible for the development of debris-laden ice structures in the basal zones of cold-based glaciers that flow over unconsolidated substrates. As layered sedimentary bedding was preserved in frozen blocks within the deforming basal ice of several of these glaciers, the evidence suggests that at some point each glacier has interacted with its bed and entrained portions of the substrate material. Empirical shear strength data and observations of rheological changes attributed to composition together with evidence acquired during fieldwork in Antarctica help to support the argument that cold-based glaciers flowing over unconsolidated sediment are capable of affecting geomorphic change. Hence, isotropic ice models that exclude basal processes may need to be adjusted, especially where small increases in the temperature of the basal zones of cold glaciers may occur. In conclusion, palaeo-climate inferences based purely upon small amounts of geomorphic evidence, which suggest warmer climate conditions, may need to be re-evaluated in order to portray more accurate renditions of formerly glaciated landscapes.
| Identifer | oai:union.ndltd.org:ADTP/208005 |
| Date | January 2008 |
| Creators | Sirota, Paul, n/a |
| Publisher | University of Otago. Department of Geography |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Paul Sirota |
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