Evidence of marine ice-cliff instability in Pine Island Bay from iceberg-keel plough marks

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 lost. 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 slopes. 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 accelerates. It is thought 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 study, driven by MICI. Iceberg-keel plough marks on the sea-floor provide geological evidence of past and present iceberg morphology, keel depth and drift direction. 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 today, which would produce wide, flat-based plough marks or toothcomb-like multi-keeled plough marks. 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 MICI. 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.

[1]  C. C. Walker,et al.  Upper and lower limits on the stability of calving glaciers from the yield strength envelope of ice , 2012, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences.

[2]  John B. Anderson,et al.  Ice sheet retreat dynamics inferred from glacial morphology of the central Pine Island Bay Trough, West Antarctica , 2012 .

[3]  J. Dowdeswell,et al.  Geomorphic signature of Antarctic submarine gullies: Implications for continental slope processes , 2013 .

[4]  John B. Anderson,et al.  Reconstruction of changes in the Amundsen Sea and Bellingshausen Sea sector of the West Antarctic Ice Sheet since the Last Glacial Maximum , 2014 .

[5]  John B. Anderson,et al.  Paleo ice flow and subglacial meltwater dynamics in Pine Island Bay, West Antarctica , 2012 .

[6]  A. Jenkins,et al.  Submarine glacial-landform distribution across the West Antarctic margin, from grounding line to slope: the Pine Island–Thwaites ice-stream system , 2016, memoirs.

[7]  B. Scheuchl,et al.  Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers, West Antarctica, from 1992 to 2011 , 2014 .

[8]  C. Ó. Cofaigh,et al.  Mechanisms of Holocene palaeoenvironmental change in the Antarctic Peninsula region , 2009 .

[9]  Frank Pattyn,et al.  Power law or power flaw , 1998 .

[10]  Eric Rignot,et al.  Basal terraces on melting ice shelves , 2014 .

[11]  Bo Sun,et al.  Bedmap2: improved ice bed, surface and thickness datasets for Antarctica , 2012 .

[12]  D. Macayeal,et al.  Breakup of the Larsen B Ice Shelf triggered by chain reaction drainage of supraglacial lakes , 2013 .

[13]  Fernando S. Paolo,et al.  Volume loss from Antarctic ice shelves is accelerating , 2015, Science.

[14]  John B. Anderson,et al.  Geological record of ice shelf break-up and grounding line retreat, Pine Island Bay, West Antarctica , 2011 .

[15]  R. DeConto,et al.  Contribution of Antarctica to past and future sea-level rise , 2016, Nature.

[16]  D. Vaughan,et al.  Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier , 2016, Nature.

[17]  J. Dowdeswell,et al.  Possible iceberg-produced submarine terraces in Hambergbukta, Spitsbergen , 2016, memoirs.

[18]  Joseph A. MacGregor,et al.  Widespread rifting and retreat of ice-shelf margins in the eastern Amundsen Sea Embayment between 1972 and 2011 , 2012, Journal of Glaciology.

[19]  D. Abbot,et al.  Laboratory investigations of iceberg capsize dynamics, energy dissipation and tsunamigenesis , 2012 .

[20]  Eric Rignot,et al.  Ice-shelf changes in Pine Island Bay, Antarctica, 1947-2000 , 2002, Journal of Glaciology.

[21]  James A. Smith,et al.  West Antarctic Ice Sheet retreat driven by Holocene warm water incursions , 2017, Nature.

[22]  Richard B. Alley,et al.  Potential Antarctic Ice Sheet retreat driven by hydrofracturing and ice cliff failure , 2015 .

[23]  J. Dowdeswell,et al.  A subglacial landform assemblage on the outer shelf of M'Clure Strait, Canadian Arctic, ploughed by iceberg keels , 2016, memoirs.

[24]  Eric Rignot,et al.  Changes in West Antarctic ice stream dynamics observed with ALOS PALSAR data , 2008 .

[25]  J. Goff,et al.  Seismic and bathymetric evidence for four different episodes of iceberg scouring on the New Jersey outer shelf: Possible correlation to Heinrich events , 2008 .

[26]  T. Hughes,et al.  On the Disintegration of Ice Shelves: The Role of Fracture , 1983, Journal of Glaciology.

[27]  J. Dowdeswell,et al.  Relict sea-floor ploughmarks record deep-keeled Antarctic icebergs to 45°S on the Argentine margin , 2011 .

[28]  L. M. Cathles,et al.  The role of cooperative iceberg capsize in ice-shelf disintegration , 2013, Annals of Glaciology.

[29]  D. Benn,et al.  Calving processes and the dynamics of calving glaciers , 2007 .

[30]  B. Smith,et al.  Has a marine ice sheet collapse begun in the Thwaites Glacier basin, West Antarctica? , 2014 .

[31]  R. Finkel,et al.  Rapid Thinning of Pine Island Glacier in the Early Holocene , 2014, Science.

[32]  W. T. Pfeffer,et al.  Kinematic Constraints on Glacier Contributions to 21st-Century Sea-Level Rise , 2008, Science.

[33]  O. Gagliardini,et al.  The stability of grounding lines on retrograde slopes , 2012 .

[34]  A. Payne,et al.  Retreat of Pine Island Glacier controlled by marine ice-sheet instability , 2014 .

[35]  Matt A. King,et al.  A new glacial isostatic adjustment model for Antarctica: calibrated and tested using observations of relative sea‐level change and present‐day uplift rates , 2012 .

[36]  T. Hughes.  West Antarctic Ice Streams , 1977 .

[37]  Y. Kristoffersen,et al.  Seabed erosion on the Lomonosov Ridge, central Arctic Ocean: A tale of deep draft icebergs in the Eurasia Basin and the influence of Atlantic water inflow on iceberg motion? , 2004 .

[38]  Projecting Antarctic ice discharge using response functions from SeaRISE ice-sheet models , 2013 .

[39]  J. Barrie,et al.  The physical processes of seabed disturbance during iceberg grounding and scouring , 1991 .

[40]  Jonathan L. Bamber,et al.  Keel depths of modern Antarctic icebergs and implications for sea-floor scouring in the geological record , 2007 .

[41]  J. Goff,et al.  Relict iceberg keel marks on the New Jersey outer shelf, southern Hudson apron , 2001 .

[42]  B. Todd,et al.  Comparison of trends of iceberg scour marks with iceberg trajectories and evidence of paleocurrent trends on Saglek Bank, northern Labrador Shelf , 1988 .