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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/tc-2020-25
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2020-25
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  03 Feb 2020

03 Feb 2020

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A revised version of this preprint was accepted for the journal TC and is expected to appear here in due course.

Revealing the former bed of Thwaites Glacier using sea-floor bathymetry

Kelly A. Hogan1, Robert D. Larter1, Alastair G. C. Graham2, Robert Arthern1, James D. Kirkham1,3, Rebecca Totten Minzoni4, Tom A. Jordan1, Rachel Clark5, Victoria Fitzgerald4, John B. Anderson6, Claus-Dieter Hillenbrand1, Frank O. Nitsche7, Lauren Simkins8, James A. Smith1, Karsten Gohl9, Jan Erik Arndt9, Jongkuk Hong10, and Julia Wellner5 Kelly A. Hogan et al.
  • 1British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge, CB3 0ET, UK
  • 2College of Marine Science,Universityof South Florida, Saint Petersburg, FL 33701, USA
  • 3Scott Polar Research Institute, University of Cambridge, Lensfield Road, Cambridge, CB2 1ER, UK
  • 4Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487, USA
  • 5Departmentof Earth and Atmospheric Sciences,University of Houston, Houston, TX77204, USA
  • 6Department of Earth Science, Rice University,Houston, TX 77005, USA
  • 7Lamont‐Doherty Earth Observatory, Columbia University, Palisades, New York, USA
  • 8Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA
  • 9Alfred Wegener Institute Helmholtz-Centre for Polar and Marine Research, 27568 Bremerhaven, Germany
  • 10Korea Polar Research Institute (KOPRI), Incheon 21990, Republic of Korea

Abstract. The geometry of the sea floor beyond Thwaites Glacier (TG) is a major control on the routing of warm ocean waters towards the ice stream’s grounding zone, which has led to increased mass loss through sub-ice-shelf melting and resulting accelerated ice flow. Nearshore topographic highs act as pinning points for the Thwaites Ice Shelf and potentially provide barriers to warm water incursions. To date, few vessels have been able to access this area due to persistent sea-ice and iceberg cover. This critical data gap was addressed in 2019 during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project, with more than 2000 km2 of new multibeam echo-sounder data (MBES) were acquired offshore TG. Here, these data along with legacy MBES datasets are compiled to produce a set of standalone bathymetric grids for the inner Amundsen Sea shelf beyond both Pine Island and Thwaites glaciers. At TG, the bathymetry is dominated by a > 1200 m deep, structurally-controlled trough and discontinuous ridge, on which the Eastern Ice Shelf is pinned. The geometry and composition of the ridge varies spatially with some parts having distinctive flat-topped morphologies produced as their tops were planed-off by erosion at the base of the seaward-moving Thwaites Ice Shelf, suggesting a positive feedback mechanism for ice-shelf ungrounding. Knowing that this offshore area is a former bed for TG, we applied a novel spectral approach to investigate bed roughness and find that derived power spectra can be approximated using an inverse-square law, a result that is consistent with spectra for bed profiles from the modern TG. Using existing ice-flow theory, we also make a first assessment of the form drag (basal drag contribution) for ice flow over this topography. Ice flowing over the sea-floor troughs and ridges would have been affected by similarly high basal drag to that acting in the grounding zone today. We show that the sea-floor bathymetry is an analogue for extant bed areas of TG and that more can be gleaned from these 3D bathymetric datasets regarding the likely spatial variability of bed roughness and bed composition types underneath TG. Comparisons with existing regional bathymetric compilations for the area show that high-frequency (finer than 5 km) bathymetric variability beyond Antarctic ice shelves can only be resolved by observations such as MBES and that without these data calculations of the capacity of bathymetric troughs, and thus oceanic heat flux, may be significantly underestimated. This work meets the requirements of recent numerical ice-sheet and ocean modelling studies that have recognised the need for accurate and high-resolution bathymetry to determine warm water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour.

Kelly A. Hogan et al.

Kelly A. Hogan et al.

Kelly A. Hogan et al.

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Short summary
The sea floor geometry around the rapidly changing Thwaites Glacier is a key control on warm ocean waters reaching the ice shelf and grounding zone beyond. This area was un-surveyed due to icebergs and sea-ice cover. The International Thwaites Glacier Collaboration mapped this area for the first time. The new data reveals troughs over 1200 m deep, and as this region is thought to have only un-ground recently, provides key insights into the morphology beneath the grounded ice sheet.
The sea floor geometry around the rapidly changing Thwaites Glacier is a key control on warm...
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