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.
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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.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Douglas Jerolmack and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: James Super.
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Extended Data Fig. 1 The relationship between iceberg calving, ocean tides, line-of-sight ice mélange speeds, and glacier speeds.
The relationship between iceberg calving (black horizontal lines), (a) ocean tides, (b) line-of-sight ice mélange speeds and (c) glacier speeds with time ascending down along the y-axis in all panels. Most of the calving occurred during a spring tide when tidal amplitudes (mean difference between two high and two low tides each day) were high. Mélange speeds were similar in magnitude but more variable than glacier speeds, indicating proglacial mechanisms affect ice mélange flow. For nearly all calving events, an increase in mélange speeds occurred without coincident increase in glacier speeds; the sole exception was Aug 9 when a small calving event was precipitated by a partial loss of mélange flow coherence located downfjord of the sampled time-series.
Extended Data Fig. 2 Tidal oscillations in mélange flow for two different time periods in the early record.
(a) Tidal height measured ~5 km from the calving front and (b) Mélange 2D-derived speeds sampled along a centerline profile between Aug 1 19:02 and Aug 2 15:25. (c,d) same as (a,b) but for Aug 3 14:13 – Aug 5 9:43. Time ascends downward along the y-axis for all plots. The location of the profiles is shown in Extended Data Fig. 4.
Extended Data Fig. 3 Tidal oscillations in mélange flow for the time period Aug 6 20:52 – Aug 9 20:28.
(a) Tidal height measured ~5 km from the calving front. (b) Mélange 2D-derived speeds sampled along a center line profile shown in Extended Data Fig. 4. Time ascends downward along the y-axis for both plots.
TRI backscatter reference image showing the locations of the mélange 1D line-of-sight (maroon) and glacier speed (green; from Cassotto et al.29) time-series shown in Extended Data Fig. 1. The location of a center profile (orange) used to sample mélange 2D speeds in Extended Data Figs. 2 and 3 is also shown; blue points indicate 1-km distances along the profile.
Mean speeds between Aug 6 20:51 and Aug 8 19:21, 2012 derived from (a) PyCORR and (b) phase-based values. The difference between each method (phase-based minus PyCORR) shown in (c) map view and as a (d) histogram. The phase-based values are 1.7 m d−1 lower than PyCORR derived values with a standard deviation of 2.8 m d−1; we adopt the latter as the error in phase derived velocity fields.
Legends for four Supplementary Videos and a brief discussion.
2D speed and velocity anomalies.
Divergence of velocity fields.
Time-lapse of 3-min TRI backscatter observations.
Time-lapse of 15-min camera observations.
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Cassotto, R.K., Burton, J.C., Amundson, J.M. et al. Granular decoherence precedes ice mélange failure and glacier calving at Jakobshavn Isbræ. Nat. Geosci. 14, 417–422 (2021). https://doi.org/10.1038/s41561-021-00754-9
Nature Geoscience (2021)