Fabric along the NEEM ice core, Greenland, and its comparison with GRIP and NGRIP ice cores

M Montagnat,S Fujita,D Samyn,S Kipfstuhl,F Gillet-Chaulet,A Svensson,N Azuma,J Eichler,D Dahl-Jensen,I Weikusat

doi:10.5194/tc-8-1129-2014

Abstract

Abstract. Fabric (distribution of crystallographic orientations) along the full NEEM ice core, Greenland was measured in the field by an automatic ice texture analyzer every 10 m, from 33 m down to 2461 m depth. The fabric evolves from a slightly anisotropic fabric at the top, toward a strong single maximum at about 2300 m, which is typical of a deformation pattern mostly driven by uniaxial compression and simple shearing. A sharp increase in the fabric strengthening rate is observed at the Holocene to Wisconsin (HW) climatic transition. From a simple model we estimate that this depth is located at a transition from a state dominated by vertical compression to a state dominated by vertical shear. Comparisons are made to two others ice cores drilled along the same ridge; the GRIP ice core, drilled at the summit of the ice sheet, and the NGRIP ice core, drilled 325 km to the NNW of the summit along the ridge, and 365 km upstream from NEEM. This comparison tends to demonstrate that the ice viscosity change with the HW climatic transition must be associated with the shear-dominated state to induce the abrupt fabric strengthening observed at NEEM. This comparison therefore reflects the increasing role of shear deformation on the coring site when moving NW along the ridge from GRIP to NGRIP and NEEM. The difference in fabric profiles between NEEM and NGRIP also evidences a stronger lateral extension associated with a sharper ridge at NGRIP. Further along the core, centimeter scale abrupt texture (fabric and microstructure) variations are observed in the bottom part of the core. Their positions are in good agreement with the observed folding layers in Dahl-Jensen et al. (2013).

Highlights

Crystal preferred orientation, or fabric, analyses along ice cores drilled in polar ice sheets are highly necessary to provide information about conditions of deformation along ice cores
Grain size increases and fabric progressively clusters with depth until close to the Holocene to Wisconsin (HW) climatic transition, where grain size is strongly reduced and fabric abruptly strengthens to a strong single maximum
This observation supports those from the EPICA Dome C ice core (Durand et al, 2009), the GRIP and NGRIP ice cores (Svensson et al, 2009) and the Talos Dome ice core (Montagnat et al, 2012), and contradicts the results obtained by Lomonaco et al (2011) from sparse fabric measurements performed in the top 120 m of a firn core drilled at Summit, with the electron backscattering technique

Summary

Introduction

Fabric, analyses along ice cores drilled in polar ice sheets are highly necessary to provide information about conditions of deformation along ice cores. M. Montagnat et al.: Fabric along the NEEM ice core, Greenland tension axis (Gow and Williamson, 1976; Alley, 1988; Paterson, 1994). Fabric concentrates toward the compression direction with increasing compressive strain with depth. This is qualitatively confirmed by the fabrics measured along cores, such as those of Greenland GRIP (Thorsteinsson et al, 1997), and Antarctic Dome Fuji (Azuma et al, 1999), EPICA Dome C (Wang et al, 2003; Durand et al, 2009), and Talos Dome (Montagnat et al, 2012), which are all located close to topographic domes. At NGRIP, Greenland (Wang et al, 2002), Vostok (Lipenkov et al, 1989), and EDML (Bargmann et al, 2011; Faria et al, 2014), Antarctica, a vertical girdle-type fabric is observed, which characterizes regions with a component of horizontal tension associated with an asymmetry in the two lateral extensions

Results

Discussion

Conclusion