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

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).

Full-field predictions of ice dynamic recrystallisation under simple shear conditions

“…This evolution is typically characterised by a strong single maximum almost perpendicular to the shear plane (Bouchez and Duval, 1982), as predicted by torsion experiments (Bouchez and Duval, 1982) and inferred from ice core data (S.H. Montagnat et al, 2014;Fitzpatrick et al, 2014). A strong LPO develops rapidly with increasing strain in all simulations, independent of whether DRX is active or not, and regardless of the applied strain rate (Fig.…”

“…Our results suggest that the LPO observed in these cores can be considered as being mainly deformation-induced, rather than significantly altered by recrystallisation. In the deepest part of some cores however, multiple maxima have been reported (Gow et al, 1997;Montagnat et al, 2014). Some aspects have to be considered in this context: (1) the ice is close to the pressure melting point close to the bedrock, (2) strain rates are generally highest in deepest parts and (3) grain sizes become very large, typically larger than available sample sizes from drill cores.…”

“…Near the flow divide of ice sheets, ice deforms under vertical compression and horizontal extension (Montagnat et al, 2014). On the contrary, simple shear boundary conditions dominate in zones away from the divide and close to the bedrock (e.g.…”

“…Consequently, ice crystals tend to be oriented with their basal planes close to perpendicular to the maximum compression or shortening direction (e.g. Alley, 1988;Montagnat et al, 2014). Ice polycrystals in ice sheets and glaciers thus form a lattice preferred orientation (LPO) and develop a bulk intrinsic anisotropy with deformation (S.H.…”

“…NEEM, 2013). For a correct interpretation of these records, it is crucial to determine to what extent deformation processes, such as dynamic recrystallisation and heterogeneous flow, may affect or distort ice stratigraphy, especially in the oldest and, thus, deepest ice, which can undergo strong deformation (Cuffey and Paterson, 2010;Montagnat et al, 2014).…”