Mechanisms of fast flow in Jakobshavns Isbræ, West Greenland: Part I. Measurements of temperature and water level in deep boreholes

Abstract

Several holes were drilled to depths of 1500–1630 m along a profile across Jakobshavns Isbræ, 50 km upstream from the calving front. Drilling was by hot water and required approximately 20 h. The holes were rapidly closed by refreezing, but it was possible to instrument them with thermistors and tilt sensors before this occurred.Near the margins of the ice stream the holes reached the bed and connected with the subglacial drainage system. Water-level changes recorded in these holes are discussed in terms of the basal hydraulic system. The temperature measurements show that the glacier is temperate-based. Moreover, extrapolation of a measured temperature profile and its curvature suggests that a temperate layer of substantial thickness may exist at the bed near the center of the ice stream. There is a striking difference in the shapes of temperature profiles measured at different locations: beneath the center line the temperature minimum is at a considerably smaller relative depth than near the margins, but it is nearly the same in magnitude (−22.1°C). This may indicate a disproportionately large vertical stretching of the basal ice in the center of the ice stream. Since the basal ice is warmer and much less viscous than the ice above, a thickening of that layer would cause a corresponding increase of surface velocity. We presume that this mechanism contributes to the fast flow of Jakobshavns Isbræ.

Flow of the West Antarctic Ice Sheet on the continental margin of the Bellingshausen Sea at the Last Glacial Maximum

“…Streaming would be enhanced by the presence of a topographic trough, which would facilitate strain heating and an increase in velocity [cf. Iken et al , 1993]. However, the relationship of the MSGL to the area of the trough underlain by a soft bed implies that subglacial geology also acted as a major control on the development of streaming flow.…”

Streamlined bedrock terrain and fast ice flow, Jakobshavns Isbrae, West Greenland: implications for ice stream and ice sheet dynamics

“…Thus, Flow Direction 2 represents a flow deviation from Flow Direction 1 during an advance cycle, and relates to increased ice thickness and diffluent ice flow. Iken et al (1993) report an ice depth of 2500 m in the centre of the current ice stream. If this is analogous to ice depths during full glacial conditions, gravitational spreading of the Isbrae would have been inevitable given that water depths shallow to between 200 and 600 m at the sill of the Isfjord, and the walls of the Isfjord only attain altitudes of 400 m a.s.l.…”

“…The Isbrae starts up to c. 100 km inland of the current ice margin, which has a floating terminus c. 10-14 km long. The ice tongue is c. 600-1100 m thick but grounded ice may approach 2500 m inland (Iken et al 1993;Clarke & Echelmeyer 1996). The Isbrae has an ice discharge of 34-40 km 3 a À1 and an ice velocity of 6-7 km a À1 (Bindschadler 1984).…”

“…The steep, up-ice surface gradient of the Isbrae drives large shear stresses within it. As there is no deformable bed, initial research suggested that internal ice deformation was the main mechanism of ice motion (Echelmeyer & Harrison 1990;Iken et al 1993;Echelmeyer et al 1994). More recent work, however, suggests that large driving stresses (c. 200-300 kPa) cause basal melt rates that account for up to 60% of surface movement through basal motion (Truffer & Echelmeyer 2003), while deformation of the lowest 1700 m of 'softer' Wisconsin-age ice is thought to account for the rest of the Isbrae's high surface velocity (Luthi et al 2002).…”

Surface melt‐driven seasonal behaviour (englacial and subglacial) from a soft‐bedded temperate glacier recorded by in situ wireless probes

“…The pattern of water pressure was very different to that seen at Skálafellsjökull. During spring, warming temperatures caused the meltwater input to be higher than drainage capacity, leading to water pressure rising higher than overburden pressure and resultant basal sliding ('spring event') (Iken et al, 1993;Bartholomew et al, 2011;Cowton et al, 2013). With the arrival of spring, water pressures initially rose to over 100% and then there was an overall decline in velocity over the summer, with some small velocity increases due to strong melt events or lake drainage (Hoffman et al, 2011;Das et al, 2008) before slowly rising as the summer ended.…”