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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-2020-264
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2020-264
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  28 Sep 2020

28 Sep 2020

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This preprint is currently under review for the journal TC.

Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide deep ice core site

Tun Jan Young1, Carlos Martín2, Poul Christoffersen1, Dustin M. Schroeder3,4, Slawek M. Tulaczyk5, and Eliza J. Dawson3 Tun Jan Young et al.
  • 1Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, United Kingdom
  • 2British Antarctic Survey, Natural Environment Research Council, Cambridge CB3 0ET, United Kingdom
  • 3Department of Geophysics, Stanford University, Stanford, CA 94305, USA
  • 4Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA
  • 5Department of Earth and Planetary Sciences, University of California, Santa Cruz, CA 95064, USA

Abstract. The Crystal Orientation Fabric (COF) of ice sheets records the past history of ice sheet deformation and influences present-day ice flow dynamics. Though not widely implemented, coherent ice-penetrating radar is able to detect anisotropic COF patterns by exploiting the birefringence of ice crystals at radar frequencies. Most previous radar studies quantify COF at a coarse azimuthal resolution limited by the number of observations made with a pair of antennas along an acquisition plane that rotates around an azimuth centre. In this study, we instead conduct a suite of quad-polarimetric measurements consisting of four orthogonal antenna orientation combinations at the Western Antarctic Ice Sheet (WAIS) Divide Deep Ice Core site. From these measurements, we are able to quantify COF at this site to a depth of 1500 m at azimuthal and depth resolutions of up to 1° and 15 m. Our estimates of fabric asymmetry closely match corresponding fabric estimates directly measured from the WAIS Divide Deep Ice Core. While ice core studies are often unable to determine the absolute fabric orientation due to core rotation during extraction, we are able to unambiguously identify and conclude that the fabric orientation is depth-invariant to at least 1500 m, equivalent to 7400 years BP (years before 1950), and coincides exactly with the modern surface strain direction at WAIS Divide. Our results support the claim that the deformation regime at WAIS Divide has not changed substantially through the majority of the Holocene. Rapid polarimetric determination of bulk COF compares well with much more laborious sample-based COF measurements from thin ice sections. Because it is the former that ultimately influences ice flow, these polarimetric radar methods provide an opportunity for accurate and widespread mapping of bulk COF and its incorporation into ice flow models.

Tun Jan Young et al.

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Latest update: 25 Oct 2020
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Short summary
If the molecules that make up ice are oriented in specific ways, the ice becomes softer and is more easily deformed. We use radar to measure the orientation of ice molecules in the top 1500 m of the ice sheet at the Western Antarctic Ice Sheet divide. Our results match those from an ice core extracted 10 years ago, and concludes that the ice flow has not changed direction for the last 7400 years. Our methods are easy and accurate, and can be applied in many places across ice sheets.
If the molecules that make up ice are oriented in specific ways, the ice becomes softer and is...
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