Radar characterization of ice crystal orientation fabric and anisotropic rheology within Rutford Ice Stream, 2017-2019

• Main Authors: Jordan, Tom, Martin, Carlos, Brisbourne, Alex, Schroeder, Dustin, Smith, Andrew
• Format: Dataset
• Language: English
• Published: UK Polar Data Centre, Natural Environment Research Council, UK Research & Innovation 2020
• Subjects: "EARTH SCIENCE","CRYOSPHERE","GLACIERS/ICE SHEETS","GLACIER MOTION/ICE SHEET MOTION"; "EARTH SCIENCE","CRYOSPHERE","GLACIERS/ICE SHEETS"; "EARTH SCIENCE","SPECTRAL/ENGINEERING","RADAR"; anisotropy; fabric; ice flow; polarimetry; radar; rheology; Antarctic; Rutford; Rutford Ice Stream; West Antarctica; Antarc*; Antarctica; Ice Sheet
• Online Access: https://dx.doi.org/10.5285/d5b7e5a1-b04d-48d8-a440-c010658ec146; https://data.bas.ac.uk/full-record.php?id=GB/NERC/BAS/PDC/01428

We use polarimetric radar sounding to investigate variation in ice crystal orientation fabric within the near-surface (top 40-300 m) of Rutford Ice Stream, West Antarctica. To assess the influence of the fabric on ice flow, we use an analytical model to derive anisotropic enhancements of the flow law from the fabric measurements. In the shallowest ice (40-100 m) the azimuthal fabric orientation is consistent with flow-induced development and correlates with the surface strain field. Notably, toward the ice-stream margins, both the horizontal compression angle and fabric orientation tend toward 45 degrees relative to ice flow. This result is consistent with theoretical predictions of flow-induced fabric under simple shear, but to our knowledge has never been observed. The fabric orientation in deeper ice (100-300 m) is significantly misaligned with shallower ice in some locations, and therefore inconsistent with the local surface strain field. This result represents a new challenge for ice flow models which typically infer basal properties from the surface conditions assuming simplified vertical variation of ice flow. Our technique retrieves azimuthal variations in fabric but is insensitive to vertical variation, and we therefore constrain the fabric and rheology within two end-members: a vertical girdle or a horizontal pole. Our hypotheses are that fabric near the center of the ice-stream tends to a vertical girdle that enhances horizontal compression, and near the ice-stream margins tends to a horizontal pole that enhances lateral shear. ApRES radar data were collected as part of the BEAMISH Project (NERC AFI award numbers NE/G014159/1 and NE/G013187/1). Tom Jordan would like to acknowledge support from EU Horizon 2020 grant 747336-BRISRES-H2020-MSCA-IF-2016. : Polarimetric data acquisition follows Brisbourne et al. 2019. This paper describes the quad-polarized measurement technique that was applied in the field. ApRES signal processing follows Brennan et al. 2014. This paper describes the processing steps applied to the raw data to generate ice-depth profiles for the phase and amplitude of radar wave. Polarimetric data processing follows Jordan et al. 2019. This paper describes the coherence methodology that is used to determine the quality of the radar data for fabric estimation. : Radar: ApRES (British Antarctic Survey's Autonomous phase-sensitive Radio-Echo Sounder) - chirps of 200 to 400 MHz in time intervals of 1 s. : Polarimetric data quality is established from the coherence magnitude, which provides a metric for sections of the ice column where fabric can be reliably estimated. Fabric estimates could be made in the top 100-200 m of the ice column at 18 of the 20 measurement sites (maximum depth ~ 100 m near the shear-margin, and ~200 m in the ice-stream centre).