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

Tun Jan Young,Carlos Martín,Slawek M Tulaczyk,Dustin M Schroeder,Poul Christoffersen,Eliza J Dawson

doi:10.5194/tc-15-4117-2021

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 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.

Highlights

There is a growing need to understand the dynamics of ice sheets and how they will respond to future climate change (IPCC, 2013)
Using a phase-sensitive radar and two open-structure antennas, we conducted a suite of quadrature-polarimetric measurements within the proximity of the Western Antarctic Ice Sheet (WAIS) Divide ice core
(ii) unambiguously identify the fabric asymmetry to be depth invariant with the E1 eigenvector oriented at 19◦ ± 8◦ to the same depth of 1400 m

Summary

Introduction

There is a growing need to understand the dynamics of ice sheets and how they will respond to future climate change (IPCC, 2013). The flow of ice sheets is governed by the balance between the gravitational driving stress, basal resistance to sliding, and the internal deformation of ice (Cuffey and Paterson, 2010). Past flow history influences the ice crystal orientation fabric (COF), which, in turn, influences the presentday anisotropic ice viscosity and flow field. Because ice crystals effectively re-orient themselves to minimize resistance when subjected to stress, the COF of ice is reflective of longterm strain at timescales proportional to the depth–age relationship The COF of ice is known to be influenced by perturbations in climate on a yearly timescale (Kennedy et al, 2013).

Results

Discussion

Conclusion