Many ice shelves fringing Antarctica have experienced increased mass loss from iceberg calving in recent years (Liu et al., 2015). Ice shelves provide significant resistance to the flow of ice from inland (backstress), so their loss can lead to increased rates of ice discharge from the interior, and potentially destabilise marine-based parts of the ice sheet. This is of particular concern in the case of Pine Island and Thwaites Glaciers and adjacent glaciers in the Amundsen Sea sector of the West Antarctic Ice Sheet.
There is a unique record of terminus positions of Pine Island Glacier, spanning many large iceberg calving events extending back to 1947 (MacGregor et al., 2012). Despite well-documented thinning of the glacier’s ice shelf by many metres per year since the early 1990s, it exhibited a very consistent calving behaviour up to 2013. The glacier front typically advanced at 2.5 to 4 km yr-1 for more than five years and then calved a single tabular iceberg with an area of >600 km2, with each new calving front in approximately the same location.
After 2013, however, calving events have occurred every 1 to 2 years. Although individual calved icebergs have been smaller than previously, retreat and realignment of the calving front accompanied this change in behaviour (Arndt et al., 2018).
Integration of sequences of satellite images with recently acquired bathymetry data suggested that interaction between the ice shelf and bathymetric features (pinning points) was important in triggering the 2007 and 2013 calving events (Arndt et al., 2018). In 2007 a curved rift propagated from the point where the northern corner of the ice front made contact with a pinning point on the sea floor. Similarly, in 2013 another curved rift propagated from an area of the ice shelf that was compressing a dense ice mÃ©lange against another bathymetric high. Thwaites Glacier has a history of growing a long ‘ice tongue’, a floating extension of the glacier that is laterally unconstrained (Rabus et al., 2003). In general, such a configuration is not conducive to ice shelf stability, but in the case of Thwaites Glacier it has become clear that pinning points and high concentrations of sea ice or mÃ©lange contributed to stability of the formerly extensive ice tongue. Recent retreat of the Thwaites ice tongue appears to be associated with ice thinning and detachment from pinning points combined with low concentrations of sea ice.
The long records of calving events and detailed knowledge of the sea-floor and subglacial topography at Pine Island and Thwaites Glaciers provide a unique opportunity to investigate the controls on the stability of ice shelves and ice tongues, and the factors that lead to calving and disintegration.
There is increasing concern that the loss of ice shelves in West Antarctica could lead to runaway ice sheet retreat through marine ice cliff instability (MICI) or catastrophic mechanical failure of deep ice fronts (Bassis and Walker, 2012; DeConto and Pollard, 2016). This process remains hypothetical, but potential confirmation is provided by the size and depth distributions of iceberg ploughmarks in Pine Island Trough, which have been interpreted as indicating that MICI occurred during post-glacial retreat of the glacier (Wise et al., 2017). Importantly, Wise et al. suggested that retreat of the calving front onto a transverse ridge arrested this episode of marine ice cliff instability.
Until recently, the controls on iceberg calving have been poorly understood because of the difficulty in representing fracture processes in numerical models. However, newly developed modelling techniques have removed this difficulty, allowing calving and MICI processes to be investigated in unprecedented detail (Benn and Ã…strÃ¶m, 2018).
This PhD project aims to use remote sensing data and new numerical models to:
• determine combinations of parameters (e.g. ice shelf strength, crevasse penetration depth, shear margin drag, coupling to pinning points) that enable the simulation of the calving history of Pine Island Glacier;
• establish the key factors that determined growth and breakup of the Thwaites Glacier ice tongue; and
• assess plausibility of the hypothesis that retreat of a calving front onto a transverse ridge could arrest retreat driven by marine ice cliff instability.
Click on an image to expand
Figure_1.pdf: Calving events at Pine Island Glacier. (From Arndt et al. 2018)
1. Remote sensing
A range of archive and newly acquired satellite imagery (ASTER, Landsat 8, SENTINEL-1, ICESat-2) will be used to document calving from the Pine Island ice shelf and Thwaites Glacier tongue. Particular attention will be paid to the magnitude, timing and style of calving events, as well as ice velocity and patterns of strain. Data on ice-surface elevation and subglacial and sea-floor topography will be collated and used to constrain evolving ice geometry, and the changing importance of local ‘pinning points’.
The controls on calving behaviour will be investigated using two state-of-the-art numerical models: ELMER/Ice and the Helsinki Discrete Element Model (HiDEM). ELMER/Ice is a continuum model that solves for full stress and velocity fields in 3-D, and is ideal for analysing the way pinning points affect patterns of stress and strain within the ice. HiDEM is a radically new kind of model that represents ice as arrays of particles linked by breakable bonds. This means that unlike traditional models, HiDEM can explicitly simulate fracture and calving processes with results that are strikingly similar to observations.
Taken together, ELMER/Ice and HiDEM provide a uniquely powerful toolkit for investigating iceberg calving and its controls (Benn et al., 2017; Benn and Ã…strÃ¶m, 2018). Guidance in initializing and running ELMER/Ice simulations will be provided in-house in St Andrews by Post Doctoral Researcher Joe Todd, and additional training will be given during a 3-day workshop at the IT-Center for Science, Helsinki by core developers of ELMER/Ice.
Literature review, research design, training in remote sensing data analysis, ELMER/Ice and HiDEM. Analysis of remote sensing and model setup.
Analysis of remote sensing data, diagnostic simulations with ELMER/Ice and HiDEM.
Data analysis and synthesis, prognostic simulations with ELMER/Ice and HiDEM. Writing up thesis.
Writing up thesis.
The candidate will be trained in analysis of satellite imagery, and in the glacier models Elmer/Ice and HiDEM in-house at the University of St Andrews and the British Antarctic Survey, and at the IT-Center for Science in Helsinki.
References & further reading
Arndt, J.E., Larter, R.D., Friedl, P., Gohl, K. and HÃ¶ppner, K. 2018. Bathymetric controls on calving processes at Pine Island Glacier. The Cryosphere 12: 2039-2050.
Bassis, J.N. and Walker, C.C., 2012. Upper and lower limits on the stability of calving glaciers from the yield strength envelope of ice. Proc. R. Soc. A 468: 913-931.
Benn, D.I. and 6 others 2017. Melt-undercutting and buoyancy-driven calving from tidewater glaciers: new insights from discrete element and continuum model simulations. J. Glaciol. 63: 691-702.
Benn, D.I. and Ã…strÃ¶m, J.A. 2018. Calving glaciers and ice shelves. Adv. Phys. X 3:1, 1513819.
DeConto, R.M. and Pollard, D. 2016. Contribution of Antarctica to past and future sea-level rise. Nature 531: 591.
Liu and 7 others. 2015. Ocean-driven thinning enhances iceberg calving and retreat of Antarctic ice shelves. PNAS 112 (11) 3263-3268.
MacGregor, J.A., Catania, G.A., Markowski, M.S. and Andrews, A.G. 2012. Widespread rifting and retreat of ice-shelf margins in the eastern Amundsen Sea Embayment between 1972 and 2011. J. Glaciol. 58: 458-466.
Rabus, B.T., Lang, O. and Adolphs, U. 2003. Interannual velocity variations and recent calving of Thwaites GlacierTongue, West Antarctica. Ann. Glaciol. 36: 215-224.
Rignot, E., Jacobs, S. Mouginot, J. and Scheuch, B. 2013. Ice-shelf melting around Antarctica. Science 341: 266-270.
Wise, M.G., Dowdeswell, J.A., Jakobsson, M. and Larter R.D. 2017. Evidence of marine ice-cliff instability in Pine Island Bay from iceberg-keel plough marks. Nature 550: 506-510.
For further information, please contact
Prof. Doug Benn or Dr Anna Crawford
School of Geography and Sustainable Development
University of St Andrews
+44 (0)1334 463853