Preprints
https://doi.org/10.5194/tc-2021-173
https://doi.org/10.5194/tc-2021-173

  11 Jun 2021

11 Jun 2021

Review status: a revised version of this preprint is currently under review for the journal TC.

Nunataks as barriers to ice flow: implications for palaeo ice-sheet reconstructions

Martim Mas e Braga1,2, Richard Selwyn Jones3,4, Jennifer C. H. Newall1,2, Irina Rogozhina5, Jane L. Andersen6, Nathaniel A. Lifton7,8, and Arjen P. Stroeven1,2 Martim Mas e Braga et al.
  • 1Geomorphology & Glaciology, Department of Physical Geography, Stockholm University, Stockholm, Sweden
  • 2Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
  • 3Department of Geography, Durham University, Durham, UK
  • 4School of Earth, Atmosphere and Environment, Monash University, Melbourne, Australia
  • 5Department of Geography, Norwegian University of Science and Technology, Trondheim, Norway
  • 6Department of Geoscience, Aarhus University, Aarhus, Denmark
  • 7Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, USA
  • 8Department of Physics and Astronomy, Purdue University, West Lafayette, USA

Abstract. Numerical models predict that discharge from the polar ice sheets will become the largest contributor to sea level rise over the coming centuries. However, the predicted amount of ice discharge and associated thinning depends on how well ice sheet models reproduce glaciological processes, such as ice flow in regions of large topographic relief, where ice flows around bedrock summits (i.e. nunataks) and through outlet glaciers. The ability of ice sheet models to capture long-term ice loss is best tested by comparing model simulations against geological data. A benchmark for such models is ice surface elevation change, which has been constrained empirically at nunataks and along margins of outlet glaciers using cosmogenic exposure dating. However, the usefulness of this approach in quantifying ice sheet thinning relies on how well such records represent changes in regional ice surface elevation. Here we examine how ice surface elevations respond to the presence of obstacles that create large topographic relief by modeling ice flow around and between idealised nunataks during periods of imposed ice sheet thinning. We found that, for realistic Antarctic conditions, a single nunatak could exert an impact on ice thickness over 20 km away from its summit, with its most prominent effect being a local increase (decrease) of the ice surface elevation of hundreds of metres upstream (downstream) of the obstacle. A direct consequence of this differential surface response for cosmogenic exposure dating was a delay in the time of bedrock exposure upstream relative to downstream of a nunatak. A nunatak elongated transverse to ice flow, with a wide subglacial continuation, was able to increase ice retention and therefore impose steeper ice surface gradients, while efficient ice drainage through outlet glaciers alleviated the differential response. Such differences, however, are not typically captured by continent-wide ice sheet models due to their coarse grid resolutions. This appears to be a key reason why models overestimate ice-sheet surface elevations and underestimate the pace of ice sheet melt contributing to sea level rise compared to empirical reconstructions. We conclude that a model grid refinement over complex topography and information about sample position relative to ice flow near the nunatak are necessary to improve data-model comparisons of ice surface elevation, and therefore the ability of models to simulate ice discharge in regions of large topographic relief.

Martim Mas e Braga et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on tc-2021-173', Anonymous Referee #1, 09 Jul 2021
    • AC1: 'Reply on RC1', Martim Mas e Braga, 28 Aug 2021
  • RC2: 'Review of Mas e Braga and others, "Nunataks as barriers to ice flow: implications for palaeo ice-sheet reconstructions"', Anonymous Referee #2, 19 Jul 2021
    • AC2: 'Reply on RC2', Martim Mas e Braga, 28 Aug 2021
  • RC3: 'Comment on tc-2021-173', Anonymous Referee #3, 22 Jul 2021
    • AC3: 'Reply on RC3', Martim Mas e Braga, 28 Aug 2021

Martim Mas e Braga et al.

Martim Mas e Braga et al.

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Short summary
Mountains higher than the ice surface are sampled to know when the ice reached the sampled elevation, which can be used to guide numerical models. This is important to understand how much ice will be lost by ice sheets in the future. We use a simple model to understand how ice flow around mountains affects the ice surface topography, and show how much this influences results from field samples. We also show that models need a finer resolution over mountainous areas to better match field samples.