A stratigraphy-based method for reconstructing ice core orientation

Abstract

Ever since the first deep ice cores were drilled, it has been a challenge to determine their original, in-situ orientation. In general, the orientation of an ice core is lost as the drill is free to rotate during transport to the surface. For shallow ice cores, it is usually possible to match the adjacent core breaks, which preserves the orientation of the ice column. However, this method fails for deep ice cores, such as the EastGRIP ice core in Northeast Greenland. We provide a method to reconstruct ice core orientation using visual stratigraphy and borehole geometry. As the EastGRIP ice core is drilled through the Northeast Greenland Ice Stream, we use information about the directional structures to perform a full geographical re-orientation. We compared the core orientation with logging data from core break matching and the pattern of the stereographic projections of the crystals’ c-axis orientations. Both comparisons agree very well with the proposed orientation method. The method works well for 441 out of 451 samples from a depth of 1375–2120 m in the EastGRIP ice core. It can also be applied to other ice cores, providing a better foundation for interpreting physical properties and understanding the flow of ice.

Microstructure, Micro-inclusions and Mineralogy along the EGRIP ice core – Part 1: Localisation of inclusions and deformation patterns

“…With depth, crystals rotate and basal planes shift towards the direction of extension and produce vertical girdle CPOs (Thorsteinsson et al, 1997;Wang et al, 2002). This agrees with the observed surface flow pattern of NEGIS (Fahnestock et al, 2001;Joughin et al, 2017;Hvidberg et al, 2020), visual stratigraphy and CPOs measured in EGRIP thin sections from the Last Glacial (Westhoff et al, 2020). Fabric images show a diversity in c-axis orientations and enhance the girdle formation.…”

“…3) indicates dynamic recrystallization and thus active deformation by dislocation activity throughout the upper 1340 m of the EGRIP ice core. Active deformation probably continues with depth due to similar deformation mechanisms taking place below 1340 m as indicated by vertical girdle CPOs (Westhoff et al, 2020). Dynamic recrystallization does not act only at specific depth regimes, but throughout the ice column as was observed in other deep ice cores from less dynamic sites (e.g., Durand et al, 2008;Weikusat et al, 2009;Faria et al, 2014b microstructure throughout the EGRIP ice core, analysing a larger amount of samples, is however needed to investigate the internal mechanisms taking place in detail.…”

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

“…The azimuthal constraints that radar polarimetry provides can, in general, not be validated by ice-core measurements with few exceptions (e.g., Westhoff et al, 2020). However, the alignment of the ice-fabric principal axes with the surface-flow direction detailed below adds credibility to our inferences and shows advantages of this approach over previous attempts focusing on the power anomalies only (Fujita et al, 2006;Matsuoka et al, 2012).…”

Melt in the Greenland EastGRIP ice core reveals Holocene warming events

“…The line scanner is a well-established and powerful tool for high resolution analysis of ice stratigraphy, making use of contrast enhancement by the optical dark-field method. Different devices with similar setups have been used on every deep ice core since the NorthGRIP drilling in 1995 (Svensson et al, 2005;McGwire et al, 2008;Jansen et al, 2016;Faria et al, 2018;Morcillo et al, 2020;Westhoff et al, 2020). The device used at EastGRIP is the second generation Alfred-Wegener-Institute (AWI) line scanner.…”