Mechanisms of Fast Flow in Jakobshavn Isbrae, Greenland

The flow behaviour of a fast, polythermal polar ice stream is addressed. Eight boreholes to the bedrock have been drilled in the ice sheet adjacent to Jakobshavns Isbræ. Borehole temperature measurements and complementary modeling studies revealed the existence of a substantial temperate layer (some 200-400 m thick) at the base of the ice stream. To investigate the causes of the fast flow of Jakobshavn Isbrae further, additional borehole experiments were accomplished at a centerline of the ice stream and on the ice sheet 5 km north of it. They provided the following results:

• A substantial temperate layer (30m thick or 3.7% of the ice thickness) was detected at the base of the ice sheet bordering the ice stream.
• Borehole tilt measurements confirmed the depth of the Holocene/Wisconsin transition predicted by flow modeling. Beneath this depth (approximately 80% of the total ice thickness), a decrease in ice viscosity by an order of magnitude was detected. The tiltmeter data also suggest that basal sliding contributes about 60% to the total velocity measured at the surface.
• Measurements of electrical current between electrodes in adjacent boreholes revealed distinct layers of enhanced conductivity. A comparison of the depth arrangement of theses layers in the ice sheet and in Jakobshavn Isbrae seems to corroborate the special vertical stretching in the ice stream, proposed in a previous study.
• Borehole water level measurements proved that a hydraulically active subglacial drainage system exists at the drill site.

These experimental facts are interpreted with help of a thermo-mechanically coupled flow model. The study contributes important evidence on the rheology of ice and the dynamics of a deeply eroded ice stream in the Greenland IceSheet.