| Summary: | 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 mélange, 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 Isbræ, Greenland, are preceded by a loss of flow coherence in the proglacial ice mélange 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 mélange as a mechanical inhibitor of iceberg calving and further demonstrate the potential for real-time detection of failure in other geophysical granular materials. We thank A. Robel and T. Snow for stimulating conversations. We gratefully acknowledge CH2MHill Polar Service and Air Greenland for logistics support, NASA NNX08AN74G (M.A.F. and M.T.) for funding the field work, financial support from NASA Earth and Space Fellowship NNX14AL29H (R.K.C.), the National Science Foundation grant nos. DMR-1506446 (J.C.B.) and DMR-1506307 (J.M.A. and R.K.C.), and the Gordon and Betty Moore Foundation grants nos. GBMF2626 (M.A.F.) and GBMF2627 (M.T.) for the purchase of the TRIs. Yes
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