Sean Fitzsimons,1 Nicola Webb,1 Sarah Mager,1 Shelley MacDonell,1 Regi Lorrain,2 and Denis Samyn2
1 Department of Geography, University of Otago, Dunedin, New Zealand.
2 Laboratoire de Glaciologie, Departement des Sciences de la Terre et de l’Environnement, Universite Libre de Bruxelles, Bruxelles, Belgium.
Previous studies of polar glaciers have argued that basal ice can form when these glaciers override and entrain ice marginal aprons that accumulate adjacent to steep ice cliffs. To test this idea, we have studied the morphology, structure, composition, and deformation of the apron and basal ice at the terminus of Victoria Upper Glacier in the McMurdo dry valleys, which are located on the western coast of the Ross Sea at 77°S in southern Victoria Land, Antarctica. Our results show that the apron has two structural elements: an inner element that consists of strongly foliated ice that has a steep up-glacier dip, and an outer element that lacks a consistent foliation and has a down-glacier, slope-parallel dip. Although strain measurements show that the entire apron is deforming, the inner element is characterized by high strain rates, whereas relatively low rates of strain characterize the outer part of the apron. Co-isotopic analyses of the ice, together with analysis of solute chemistry and sedimentary characteristics, show that the apron is compositionally different from the basal ice. Our observations show that aprons may become deformed and partially entrained by advancing glaciers. However, such an ice marginal process does not provide a satisfactory explanation for the origin of basal ice observed at the ice margin. Our interpretation of the origin of basal ice is that it is formed by subglacial processes, which are likely to include deformation and entrainment of subglacial permafrost.
Fitzsimons, S., Webb, N., Mager,S., MacDonell, S., Lorrain, R. and Samyn, D. 2008. Mechanisms of basal ice formation in polar glaciers: An evaluation of the apron entrainment model. Journal of Geophysical Research, 113, F02010, doi:10.1029/2006JF000698. [pdf 405Kb]
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