Ice melange dynamics and implications for terminus stability, Jakobshavn Isbrae Greenland

We used time-lapse imagery, seismic and audio recordings, iceberg and glacier velocities, ocean wave measurements, and simple theoretical considerations to investigate the interactions between Jakobshavn Isbrae and its proglacial ice melange. The melange behaves as a weak, granular ice shelf whose rheology varies seasonally. Sea ice growth in winter stiffens the melange matrix by binding iceberg clasts together, ultimately preventing the calving of full-glacier-thickness icebergs ( the dominant style of calving) and enabling a several kilometer terminus advance. Each summer the melange weakens and the terminus retreats. The melange remains strong enough, however, to be largely unaffected by ocean currents ( except during calving events) and to influence the timing and sequence of calving events. Furthermore, motion of the melange is highly episodic: between calving events, including the entire winter, it is pushed down fjord by the advancing terminus ( at similar to 40 m d(-1)), whereas during calving events it can move in excess of 50 x 10(3) m d(-1) for more than 10 min. By influencing the timing of calving events, the melange contributes to the glacier's several kilometer seasonal advance and retreat; the associated geometric changes of the terminus area affect glacier flow. Furthermore, a force balance analysis shows that large-scale calving is only possible from a terminus that is near floatation, especially in the presence of a resistive ice melange. The net annual retreat of the glacier is therefore limited by its proximity to floatation, potentially providing a physical mechanism for a previously described near-floatation criterion for calving.