Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide ice core site
Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide ice core site
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DOI：
10.5194/tc-15-4117-2021
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发表时间：
2021-08
期刊：
影响因子：
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通讯作者：
T. J. Young;C. Martín;P. Christoffersen;D. Schroeder;S. Tulaczyk;E. Dawson
中科院分区：
文献类型：
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作者：
T. J. Young;C. Martín;P. Christoffersen;D. Schroeder;S. Tulaczyk;E. Dawson
抽象的。冰盖的晶体取向组构(COF)记录了过去冰盖变形的历史，并影响着当今的冰流动力学。虽然相干探冰雷达没有得到广泛应用，但它能够利用冰晶在雷达频率上的双折射，在假设其中一个结晶轴在垂直方向上对齐的情况下，检测出大块的各向异性组构图案。在这项研究中，我们在南极西部冰盖(WAIS)划分冰核位置附近进行了一套由四个正交天线方向组合组成的四极化测量。通过这些测量，我们能够以高达15米的整体平均分辨率将该站点的方位向组构不对称性量化到1400米的深度。我们对组构不对称性的估计与从WAIS分离冰芯直接测量的相应组构估计值非常接近。虽然冰芯研究往往无法确定由于提取过程中岩心旋转而导致的绝对组构取向，但我们能够识别并得出结论，组构取向至少在1400米处是深度不变的，相当于6700年前的BP(1950年前)，并与WAIS分水岭的现代表面应变方向密切一致。我们的结果支持了WAIS分水岭的变形制度在全新世的大部分时间里没有发生实质性变化的说法。快速测定散装织物的不对称性和取向的旋光法，与从薄冰切片进行更费力的基于样品的COF测量相媲美。由于最终影响冰流的是体积平均的织物，偏振雷达方法为其准确和广泛的测绘以及将其纳入冰流模型提供了机会。
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.