Velocity structure, flow instability and mass flux on a large Arctic ice cap from satellite radar interferometry

Satellite radar interferometry allows calculation of the ice-surface velocity distribution throughout the largest ice cap in the Eurasian Arctic: Austfonna in eastern Svalbard. Data on ice-cap velocity structure show strong spatial variations observed in unprecedented detail. Ice-cap drainage basins have clearly defined fast-flowing units with marked shear margins at the surface, associated with troughs in the subglacial bedrock identified from 60 MHz icepenetrating radar. Interferometric measurements show that the velocity structure in most icecap drainage basins remained stable over a 2-year period. Unexpectedly, however, in the basin where surface velocities were the highest on the ice cap, at 140 m yr-1 in winter 1992, the flow rate had halved by winter 1994. A time series of visible-band Landsat imagery shows that this basin was relatively inactive between 1973 and 1991, but had a distinctly disturbed surface in 1993. This flow instability was initiated after summer 1991 and is likely to be a glacier surge or mini-surge, demonstrating the increasing diversity of motion events revealed by radar interferometry. Mass flux through this basin, calculated using ice velocity and thickness data, was 0.1 km3 yr-1 in 1992, an increase of up to two orders of magnitude over that prior to flow instability. Calculated balance velocity is an order of magnitude less than the interferometrically derived velocity, implying that the flow rates observed for the fast-flowing unit cannot be sustained under the present climate.