Abstract. The crystal orientation fabric (COF) of ice sheets records the past history ofice sheet deformation and influences present-day ice flow dynamics. Though notwidely implemented, coherent ice-penetrating radar is able to detect bulkanisotropic fabric patterns by exploiting the birefringence of ice crystals atradar frequencies, with the assumption that one of the crystallographic axesis aligned in the vertical direction. In this study, we conduct a suite ofquad-polarimetric measurements consisting of four orthogonal antennaorientation combinations near the Western Antarctic Ice Sheet (WAIS) Divideice core site. From these measurements, we are able to quantify the azimuthalfabric asymmetry at this site to a depth of 1400 m at abulk-averaged resolution of up to 15 m. Our estimates of fabricasymmetry closely match corresponding fabric estimates directly measured fromthe WAIS Divide ice core. While ice core studies are often unable to determinethe absolute fabric orientation due to core rotation during extraction, we areable to identify and conclude that the fabric orientation is depth-invariantto at least 1400 m, equivalent to 6700 years BP (years before1950) and aligns closely with the modern surface strain direction at WAISDivide. Our results support the claim that the deformation regime at WAISDivide has not changed substantially through the majority of theHolocene. Rapid polarimetric determination of bulk fabric asymmetry andorientation compares well with much more laborious sample-based COFmeasurements from thin ice sections. Because it is the bulk-averaged fabricthat ultimately influences ice flow, polarimetric radar methods provide anopportunity for its accurate and widespread mapping and its incorporation intoice flow models.
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Estimation of ice fabric within Whillans Ice Stream using polarimetric phase-sensitive radar sounding
Abstract Here we use polarimetric measurements from an Autonomous phase-sensitive Radio-Echo Sounder (ApRES) to investigate ice fabric within Whillans Ice Stream, West Antarctica. The survey traverse is bounded at one end by the suture zone with the Mercer Ice Stream and at the other end by a basal ‘sticky spot’. Our data analysis employs a phase-based polarimetric coherence method to estimate horizontal ice fabric properties: the fabric orientation and the magnitude of the horizontal fabric asymmetry. We infer an azimuthal rotation in the prevailing horizontal c -axis between the near-surface ( z ≈ 10–50 m) and deeper ice ( z ≈ 170–360 m), with the near-surface orientated closer to perpendicular to flow and deeper ice closer to parallel. In the near-surface, the fabric asymmetry increases toward the center of Whillans Ice Stream which is consistent with the surface compression direction. By contrast, the fabric orientation in deeper ice is not aligned with the surface compression direction but is consistent with englacial ice reacting to longitudinal compression associated with basal resistance from the nearby sticky spot.
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- Award ID(s):
- 1745137
- NSF-PAR ID:
- 10143103
- Date Published:
- Journal Name:
- Annals of Glaciology
- ISSN:
- 0260-3055
- Page Range / eLocation ID:
- 1 to 10
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/aog.2020.6
