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

Abstract

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.

Inferring Ice Fabric From Birefringence Loss in Airborne Radargrams: Application to the Eastern Shear Margin of Thwaites Glacier, West Antarctica

“…The resulting phase shift rotates the electric field and can cause polarization misalignment with linearly polarized antennas, resulting in power loss (Doake, 1981). Previous radar studies have utilized multi‐ and quadrature‐polarization setups to observe and quantify COF, and have shown good agreement with measurements from thin section analyses at coincident ice core sites (Fujita et al., 2006; Eisen et al., 2007; Ershadi et al., 2021; Dall, 2010, 2021; K. Matsuoka et al., 2003, 2009; Li et al., 2018; Jordan et al., 2019; Jordan, Besson, et al., 2020; Young et al., 2020) as well as provided evidence of more complex fabric at sites with more dynamic flow regimes (K. Matsuoka et al., 2012; Brisbourne et al., 2019; Jordan, Schroeder, et al., 2020; Jordan, Martín, et al., 2020). In‐situ observations of ice fabric have been crucial to understanding the stress patterns and behaviors of ice sheets over time and over large areas (e.g., Alley, 1988; Budd, 1972) and have provided constraints on the influence of crystal fabric on ice sheet flow (Azuma, 1994; Martín et al., 2009; Thorsteinsson et al., 2003).…”

“…(2006). Because the radar antennas were co‐linearly aligned (Table 1), we did not apply azimuthal averaging to the resulting backscatter model results, as was the case in previous power‐based analyses (K. Matsuoka et al., 2012; Brisbourne et al., 2019; Young et al., 2020). For our calculations, we used f c = 750 MHz (Arnold et al., 2020), and at 3.169 and 3.134, respectively, with Δ ϵ ′ = 0.035 (T. Matsuoka et al., 1997), and a model depth and azimuthal step of 1 m and 1°, respectively.…”

“…Code used to calculate fabric asymmetry from consecutive minima was published through Young et al. (2020).…”

“…In lieu of this constraint, ice-penetrating radar has provided an alternative method to quantify bulk anisotropic COF patterns by exploiting the birefringence of polar ice, without the practical limitations of drilling (e.g., Hargreaves, 1977;Fujita et al, 2006;K. Matsuoka et al, 2012;Jordan et al, 2019;Young et al, 2020).…”

Comments on tc-2020-370 by Reza Ershadi et al

“…The theory of radar birefringence in glaciology has long been known (Hargreaves, 1978;Woodruff and Doake, 1979;Matsuoka et al, 1997;Fujita et al, 1999), and has recently been significantly extended to exploit the capacity of phase information from newer radar systems that were previously not available (Dall, 2010;Jordan et al, 2019Jordan et al, , 2020. Examples for applications of radar polarimetry exist near ice domes in Greenland (Gillet-Chaulet et al, 2011;Jordan et al, 2019) and Antarctica (Fujita et al, 1999;Brisbourne et al, 2019), on ice rises (Drews et al, 2015;Matsuoka et al, 2015;Brisbourne et al, 2019), in flank-flow regimes (Eisen et al, 2007), divides (Young et al, 2020), and for ice streams (Robert et al, 1993;Joughin et al, 1999;Jordan et al, 2020). However, there is not yet a clear observation-based picture of how ice fabric develops across the different flow regimes.…”