Mechanisms of fast flow in Jakobshavn Isbræ, West Greenland: Part III. Measurements of ice deformation, temperature and cross-borehole conductivity in boreholes to the bedrock

Abstract

At a site on the ice sheet adjacent to the Jakobshavn ice stream in West Greenland, ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830 m depth. Enhanced deformation rates were recorded just below the Holocene^Wisconsin transition at 680 m depth. A 31m layer of temperate ice and the temperature 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 temperature 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 compare 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.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 2500 m. 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 convergent 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. GOALS OF THE STUDYA 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

Sliding of temperate basal ice on a rough, hard bed: creep mechanisms, pressure melting, and implications for ice streaming

“…These palaeo-ice stream zones are surrounded by areas also subjected to less intense, but still warm-based, ice erosion suggesting intermediate ice velocities (e.g. Bradwell, 2013), consistent with ice velocity analysis and borehole observations from the Greenland Ice Sheet that show significant warm-based sliding (10-100 m yr −1 ) outside ice-streams (Lüthi et al, 2002;Ryser et al, 2014;Joughin et al, 2010). Thus, fast ice flow appears to be possible on hard, rough beds and cannot be explained by a simple cold/warm thermal boundary (cf.…”

“…Drilling in Greenland Ice Sheet adjacent to the Jakobshavn Isbrae has recorded a basal layer of temperate ice below cold ice of some 30 m thickness (Lüthi et al, 2002), equal to or greater in height than typical bedrock obstacles (roches moutonnées, whalebacks) in most crystalline gneiss terrains (Krabbendam and Bradwell, 2014). Temperate ice has also been modelled to occur beneath other parts of the Greenland Ice Sheet (e.g.…”

Force Balance along Isunnguata Sermia, West Greenland

“…Liquid waterinduced softening through growth of a temperate basal layer is not physically viable, considering that a thick temperate layer is absent in the measured profile 50 km from the observed low driving stress (Harrington et al, 2015). Other factors influencing ice rheology (e.g., related to impurity content or crystal orientation) can cause E to change by a factor of two or more with ice depth (e.g., Shoji and Langway, 1984;Lüthi et al, 2002), but changes of a similar magnitude over the necessary horizontal length scales (∼10 km) lack observational or conceptual basis.…”

Testing hypotheses of the cause of peripheral thinning of the Greenland Ice Sheet: is land-terminating ice thinning at anomalously high rates?

“…This lack of seasonal velocity variations is thought to be due to the majority of ice motion, at least for JI, being supplied by deformation of a thick (∼300 m) layer of softer, temperate ice at the glacier bed, rather than by basal sliding or a deforming sediment layer (Clarke and Echelmeyer, 1996;Funk et al, 1994;Luthi et al, 2002). Also, current summer accelerations of marine-terminating outlet glaciers in west Greenland represent speed-ups of less than 15% of their annual mean velocities (Joughin et al, 2008a).…”