Abstract
The crystal orientation fabric (COF) of a polar ice sheet has a significant effect on the rheology of the sheet. With the aim of better understanding the deformation regime of ice sheets, the present work investigated the COF in the upper 80 % of the depth within the 3035 m long Dome Fuji Station ice core drilled at one of the dome summits in East Antarctica. Dielectric anisotropy (âε) data were acquired as a novel indicator of the vertical clustering of COF resulting from vertical compressional strain within the dome, at which the ice cover has an age of approximately 300 kyrs BP. The âε values were found to exhibit a general increase moving in the depth direction, but with fluctuations over distances on the order of 10–102 m. In addition, significant decreases in âε were found to be associated with depths corresponding to three major glacial to interglacial transitions. These changes in âε are ascribed to variations in the deformational history caused by dislocation motion occurring from near-surface depths to deeper layers. Fluctuations in âε over distances of less than 0.5 m exhibited a strong inverse correlation with at depths greater than approximately 1200 m, indicating that they were enhanced during the glacial/interglacial transitions. The âε data also exhibited a positive correlation with the concentration of chloride ions together with an inverse correlation with the amount of dust particles in the ice core at greater depths corresponding to decreases in the degree of c-axis clustering. Finally, we found that fluctuations in âε persisted to approximately 80 % of the total depth of the ice sheet. These data suggest that the factors determining the deformation of ice include the concentration of chloride ions and amount of dust particles, and that the layered contrast associated with the COF is preserved all the way from the near-surface to a depth corresponding to approximately 80 % of the thickness of the ice sheet. These findings provide important implications regarding further development of the COF under the various stress-strain configurations that the ice will experience in the deepest region, approximately 20 % of the total depth from the ice/bed interface.
Highlights
The crystal orientation fabric (COF) is one of the most important factors determining the physical properties of polar ice sheets, 35 as both the deformation and flow of ice sheets are highly dependent on the COF
With the aim of obtaining a better understanding of the deformation regime in ice sheets, we assessed the dielectric anisotropy, Dielectric anisotropy (De), as a new indicator of crystal orientation fabric (COF) using ice core samples taken from Dome Fuji in East Antarctica
This method is a useful means of determining the degree of COF vertical clustering resulting from vertical compressional strain at the dome
Summary
The crystal orientation fabric (COF) is one of the most important factors determining the physical properties of polar ice sheets, 35 as both the deformation and flow of ice sheets are highly dependent on the COF. In the dome summit regions of ice sheets, the vertical compressional stress imparted by the mass of the ice is the primary deformation stress In such cases, the c-axes of the ice crystal grains rotate toward the compression direction and the COF becomes more concentrated toward the core axis (that is, in the vertical direction) with increasing depth (e.g., 40 Thorsteinsson et al, 1997; Azuma et al, 2000; Wang et al, 2003; Durand et al, 2007, 2009; Montagnat et al, 2014). It has far been challenging to examine small fluctuations in the COF or to compare COF data generated using different algorithms (e.g., Wang and Azuma, 70 1999; Wilen et al, 2003; Wilson et al, 2003) To overcome these limitations, Saruya et al (2021) proposed a technique that permits the continuous non-destructive and rapid assessment of the COF in thick ice sections, based on measuring the tensorial components of the relative permittivity, e, using microwave open resonators. Though precise thicknesses of thin sections were not provided in Wang et al (2003) and Durand et al (2007, 2009), we assume that it must be ~0.5 mm or less as thin sections for optically-based COF measurements
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