Marine ice-cliff instability (MICI) processes could accelerate future retreat of the Antarctic Ice Sheet if ice shelves that buttress grounding lines more than 800 metres below sea level are lost1,2. The present-day grounding zones of the Pine Island and Thwaites glaciers in West Antarctica need to retreat only short distances before they reach extensive retrograde slopes3,4. When grounding zones of glaciers retreat onto such slopes, theoretical considerations and modelling results indicate that the retreat becomes unstable (marine ice-sheet instability) and thus accelerates5. It is thought1,2 that MICI is triggered when this retreat produces ice cliffs above the water line with heights approaching about 90 metres. However, observational evidence confirming the action of MICI has not previously been reported. Here we present observational evidence that rapid deglacial ice-sheet retreat into Pine Island Bay proceeded in a similar manner to that simulated in a recent modelling study1, driven by MICI. Iceberg-keel plough marks on the sea-floor provide geological evidence of past and present iceberg morphology, keel depth6 and drift direction7. From the planform shape and cross-sectional morphologies of iceberg-keel plough marks, we find that iceberg calving during the most recent deglaciation was not characterized by small numbers of large, tabular icebergs as is observed today8,9, which would produce wide, flat-based plough marks10 or toothcomb-like multi-keeled plough marks11,12. Instead, it was characterized by large numbers of smaller icebergs with V-shaped keels. Geological evidence of the form and water-depth distribution of the plough marks indicates calving-margin thicknesses equivalent to the threshold that is predicted to trigger ice-cliff structural collapse as a result of MICI13. We infer rapid and sustained ice-sheet retreat driven by MICI, commencing around 12,300 years ago and terminating before about 11,200 years ago, which produced large numbers of icebergs smaller than the typical tabular icebergs produced today. Our findings demonstrate the effective operation of MICI in the past, and highlight its potential contribution to accelerated future retreat of the Antarctic Ice Sheet.
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M.G.W. is funded by a UK Natural Environment Research Council (NERC) PhD studentship (LCAG/247 RG72013) held at the Scott Polar Research Institute, University of Cambridge. The OSO0910 expedition with Swedish icebreaker Oden was carried out as collaboration between the Swedish Polar Research Secretariat, the Swedish Research Council, and the US National Science Foundation (NSF). Part of the data shown in Extended Data Fig. 2 was collected on UK NERC-funded cruise JR179.
The authors declare no competing financial interests.
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Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Map of iceberg-keel plough marks observed in the study area (n = 10,831). The location of iceberg-keel plough marks in Fig. 2a–c are highlighted in pink, with figure bounds shown by a labelled pink outline. Iceberg-keel plough marks produced during an earlier period of iceberg activity, seaward of a large GZW (the seaward limit of which is shown by a labelled dashed red line)28, are highlighted in blue (n = 30). Observations are limited by data extent and cross-cutting relationships. The inset shows the location of the study area. We note the observation of distinct sets of mega-scale glacial lineations directly seaward and landward of a large GZW, some of which are shown in green. The general area of corrugation ridges (CR) is shown by the dashed yellow outline.
a, Bathymetry42 of inner Pine Island Bay (grid-cell size, 35 m). The Bedmap2-defined calving line43 is shown in yellow. b, Elevation profile of a prominent across-trough bathymetric ridge (red line in a), located about 108 km from the Bedmap2-defined calving line43. Arrows show the location of isolated breaches, interpreted as cross-cutting channels. The blue line shows the estimated ice freeboards, assuming ice was just grounded (in hydrostatic equilibrium) on the ridge surface and was meteoric in density (917 kg m−3). Dashed lines above breaches show the interpolated ice freeboard, if an absence of grounding in the breach itself is assumed. Source data
a–c, The location of the IceBridge transect from flight DC8-100124 (7 November 2009; green line labelled X–X′; analysed in Fig. 3b) is overlaid on NASA MODIS imagery from 7 November 2009 (a), 3 November 2013 (b) and 18 November 2013 (c) available from the NSIDC31 (https://nsidc.org/data/iceshelves_images/index_modis.html). We note the positioning of this transect directly over tabular iceberg B31, which calved from Pine Island Ice Shelf on 13 November 2013. In a and b, the black dashed line shows the landward-most extent of iceberg B31. The floating extent of Pine Island Ice Shelf as defined by Bedmap243 is shown in red.
Extended Data Figure 4 Schematic of iceberg-keel plough marks expected to be produced by tabular icebergs.
Wide, flat-bottomed iceberg-keel plough marks (a) will be produced if iceberg drift is approximately perpendicular to the rugged, ridge-channel underside morphology identified in Fig. 3b. By contrast, if iceberg drift is approximately parallel to the ridge-channel underside morphology, then wide, multi-keeled iceberg plough marks with toothcomb-like morphologies (b) will be produced.
Extended Data Figure 5 Schematic illustration of the cross-sectional dimensions of an iceberg needed to calculate its gravitational potential energy and establish metastable conditions.
H, total thickness of the iceberg; W, width of the iceberg; CM, centre of mass; θ, angle of rotation from an upright (U) to a rotated position (R). The longest axis extends into the page and is not shown. Image adapted from ref. 37, American Geophysical Union.
Extended Data Figure 6 The angle an iceberg 949 m in width (or length) and variable thickness H must be rotated by an external agent to overcome an energy barrier and capsize due to instability (blue curve).
By capsizing, an iceberg width (or length) of 949 m could be transformed into a thickness of equal size. As iceberg thickness decreases, the iceberg becomes increasingly stable and requires a greater angle of rotation about its centre of mass to develop unstable conditions that will cause it to capsize. Calculated from the computations of ref. 37. Source data
Extended Data Figure 7 NASA satellite image of the Pine Island Ice Shelf calving line on 26 January 2017.
The image illustrates the typical planform morphology of icebergs (arrowed) that were calved during the period between the production of large tabular icebergs. With surface areas of 240,000–5,200,000 m2, these icebergs are at most 1% of the size of iceberg B31, which calved from the Pine Island Ice Shelf in November 2013. The largest iceberg (labelled ‘X’ and shaded pink) measures 4,800 m and 1,050 m, in the across- and along-ice-flow directions, respectively. The dashed line overlying a prominent across-ice-flow rift shows the likely calving position of the next large, tabular iceberg. Image adapted from ref. 44, NASA.
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Wise, M., Dowdeswell, J., Jakobsson, M. et al. Evidence of marine ice-cliff instability in Pine Island Bay from iceberg-keel plough marks. Nature 550, 506–510 (2017). https://doi.org/10.1038/nature24458
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