Granular decoherence precedes ice mélange failure and glacier calving at Jakobshavn Isbræ

Abstract

The stability of the world’s largest glaciers and ice sheets depends on mechanical and thermodynamic processes occurring at the glacier–ocean boundary. A buoyant agglomeration of icebergs and sea ice, referred to as ice melange, often forms along this boundary and has been postulated to affect ice-sheet mass losses by inhibiting iceberg calving. Here, we use terrestrial radar data sampled every 3 min to show that calving events at Jakobshavn Isbrae, Greenland, are preceded by a loss of flow coherence in the proglacial ice melange by up to an hour, wherein individual icebergs flowing in unison undergo random displacements. A particle dynamics model indicates that these fluctuations are likely due to buckling and rearrangements of the quasi-two-dimensional material. Our results directly implicate ice melange as a mechanical inhibitor of iceberg calving and further demonstrate the potential for real-time detection of failure in other geophysical granular materials. Calving of an outlet glacier in Greenland is consistently preceded by distinctive flow patterns in the melange of sea ice and icebergs in front of the terminus, according to terrestrial radar observations and particle dynamic modelling of the Jakobshavn Isbrae system.