ABSTRACT: Mapping thermal and hydrological conditions beneath a polythermal glacier with radio-echo sounding.

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Abstract

Using ground-based radio-echo sounding, spatial patterns in residual bed reflection power (BRPr) were recorded and used to determine the likely thermal and hydrological conditions at the base of polythermal John Evans Glacier, Ellesmere Island. The radar power returned from a basal reflection is primarily a function of: (a) the dielectric loss with depth in the glacier, and (b) the dielectric properties of the glacier bed. To determine the rate of dielectric loss with depth, we plotted the relationship between measured bed reflection power and ice thickness, and found a generally good fit between the two. By analysing the residuals from this best-fit line (i.e., BRPr), together with the presence/absence of an internal reflecting horizon which is interpreted as the boundary between warm and cold ice, we identified three types of thermal structure: (a) the occurrence of positive BRPr and an internal reflecting horizon over the glacier terminus suggests a warm basal layer, (b) positive BRPr and the absence of an internal reflector over the upper ablation area suggests that the pressure melting point is only reached at the bed, and (c) negative BRPr and the absence of an internal reflector in all other regions suggests that the basal ice is cold. In areas where the glacier bed is identified as being at the pressure melting point, variations in positive BRPr are similar to patterns of subglacial water flow predicted from the subglacial hydraulic equipotential surface. This suggests that basal hydrological conditions provide a dominant control on BRPr, which is most likely because the dielectric contrast between ice and water is higher than between ice and any other naturally occurring subglacial material

Bibliography of Canadian Geomorphology