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

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 bulk
anisotropic fabric patterns by exploiting the birefringence of ice crystals at
radar frequencies, with the assumption that one of the crystallographic axes
is aligned in the vertical direction. In this study, we conduct a suite of
quad-polarimetric measurements consisting of four orthogonal antenna
orientation combinations near the Western Antarctic Ice Sheet (WAIS) Divide
ice core site. From these measurements, we are able to quantify the azimuthal
fabric asymmetry at this site to a depth of 1400 m at a
bulk-averaged resolution of up to 15 m. Our estimates of fabric
asymmetry closely match corresponding fabric estimates directly measured from
the WAIS Divide 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 identify and conclude that the fabric orientation is depth-invariant
to at least 1400 m, equivalent to 6700 years BP (years before
1950) and aligns closely 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 fabric asymmetry and
orientation compares well with much more laborious sample-based COF
measurements from thin ice sections. Because it is the bulk-averaged fabric
that ultimately influences ice flow, polarimetric radar methods provide an
opportunity for its accurate and widespread mapping and its incorporation into
ice flow models.
