Mechanisms of fast flow inJakobshavn Isbr×,West Greenland: Part III. Measurements of ice deformation, temperature and cross-borehole conductivity in boreholes to the bedrock Martin LU « THI 1 Martin FUNK, 1 Almut IKEN, 1 Shivaprasad GOGINENI, 2 Martin TRUFFER 1, 3 1 Versuchsanstaltfu«rWasserbau,HydrologieundGlaziologie,Eidgeno«ssischeTechnischeHochschule,ETH-Zentrum,CH-8092Zu«rich,Switzerland E-mail:funk@vaw.baug.ethz.ch 2 Radar Systemsand Remote Sensing Laboratory, Universityof Kansas 2335, Irving Hill Road, Lawrence, Kansas 66045-7612, U.S.A. 3 GeophysicalInstitute, Universityof Alaska Fairbanks, 903 Koyukuk Drive, P.O. Box 757320, Fairbanks, Alaska 99775-7320, U.S.A. ABSTRACT . At a site on the ice sheet adjacent to theJakobshavn ice stream inWest Greenland, ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830m depth. Enhanced deformation rates were recorded just below the Holocene^Wisconsin transition at 680 m depth. A 31m layer of temperate ice and the tem- perature minimum of ^22³C at 520 m depth were detected. The good agreement of these data with results of a two-dimensional thermomechanically coupled flow model implies that the model input is adequate. Discrepancies between modelled and measured tem- perature profiles on a flowline at the ice-stream centre have been attributed to effects not accounted for by the model. We have suggested that the convergent three-dimensional flow leads to a vertical extension of the basal ice entering the stream. A thick basal layer of temperate and Wisconsin ice would explain the fast flow of this ice stream. As a test of this hypothesis, the new core-borehole conductivity (CBC) method has been used to com- pare conductivity sequences from the ice stream to those of the adjacent ice sheet. The correlation thus inferred suggests that the lowest 270 m of the ice sheet correspond to the lowermost 1700 m of the stream, and, consequently, that the lower part of the ice stream has experienced a very large vertical extension. 1. INTRODUCTION The western coastal region of the Greenland ice sheet exhibits large areas with temperate bed conditions where fast ice flow occurs. Especially in theJakobshavn basin, draining 6.5% of the Greenland ice sheet into Disko Bay, high mass fluxes have been observed (Carbonell and Bauer, 1968; Fastook and others,1995). Ice flow is occasionally concentrated in regions of narrow lateral extent where very fast ice streams form.The fastest of these is Jakobshavn Isbr× with a velocity of about 7 Km a ^1 at the calving front and an estimated calving flux of 25^28 km 3 a ^1 (Carbonell and Bauer, 1968; Lingle and others, 1981; Echelmeyer and Harrison, 1990; Echelmeyer and others, 1992). Jakobshavn Isbr× flows through a deeply eroded bedrock trough which extends about 80 km inland of the calving front, as indicated in Figure 1 (Clarke and Echelmeyer, 1996). Thermomechanically coupled flow models in two and three dimensions have revealed that a basal layer of temperate ice would form in the coastal region as an effect of the geometry, the surface temperature and the mass-balance distribution alone (Budd and others, 1982; Letre¨ guilly and others,1991; Funk and others,1994; Greve, 1997). An interesting implication is that ice streams are secondary features of the high mass fluxes and temperate bed conditions. 2. PREVIOUS BOREHOLE INVESTIGATIONS ON THE ICE STREAM Englacial temperatures have been recorded in boreholes on Jakobshavn Isbr×, 50 km upstream from the calving front (Iken and others,1993). The holes, drilled with a hot-water system, reached the bed near the ice-stream margins at approximately 1600 m, but stopped far above the bed in the ice-stream centre where the ice thickness is 2500m. At the centre line, the shape of the measured temperature profile differed substantially from that of a modelling study (Funk and others, 1994). This discrepancy was attributed to conver- gent three-dimensional flow into the bedrock channel, not accounted for by the two-dimensional model. It was suggested that the convergent flow causes enhanced vertical extension of the basal ice at the centre line and thereby a thickening of the temperate (``soft'') basal layer.This could explain the high flow velocity of this ice stream. 3. GOALS OF THE STUDY A better understanding of the specific dynamics of the ice stream is obtained by investigating the conditions in the adjacent ice sheet where the ice is of the same origin, but where flow conditions are less complex. In this study we Journalof Glaciology , Vol. 48, No . 162, 2002 369 Downloaded from https://www.cambridge.org/core. 08 Nov 2021 at 12:23:34, subject to the Cambridge Core terms of use.