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Volume 10, issue 1
The Cryosphere, 10, 359-370, 2016
https://doi.org/10.5194/tc-10-359-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
The Cryosphere, 10, 359-370, 2016
https://doi.org/10.5194/tc-10-359-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 12 Feb 2016

Research article | 12 Feb 2016

Small-scale disturbances in the stratigraphy of the NEEM ice core: observations and numerical model simulations

D. Jansen1, M.-G. Llorens2,1, J. Westhoff2, F. Steinbach2,1, S. Kipfstuhl1, P. D. Bons2, A. Griera3, and I. Weikusat1,2 D. Jansen et al.
  • 1Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
  • 2Department of Geosciences, Eberhard Karls University Tübingen, Tübingen, Germany
  • 3Departament de Geologia, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, Spain

Abstract. Disturbances on the centimetre scale in the stratigraphy of the North Greenland Eemian Ice Drilling (NEEM) ice core (North Greenland) can be mapped by an optical line scanner as long as the ice has visual layering, such as, for example, cloudy bands. Different focal depths allow, to a certain extent, a three-dimensional view of the structures. In this study we present a detailed analysis of the visible folds, discuss their characteristics and frequency, and present examples of typical fold structures. We also analyse the structures with regard to the deformation boundary conditions under which they formed. The structures evolve from gentle waves at about 1500m to overturned z folds with increasing depth. Occasionally, the folding causes significant thickening of layers. Their similar fold shape indicates that they are passive features and are probably not initiated by rheology differences between alternating layers. Layering is heavily disturbed and tracing of single layers is no longer possible below a depth of 2160m. C axes orientation distributions for the corresponding core sections were analysed, where available, in addition to visual stratigraphy. The data show axial-plane parallel strings of grains with c axis orientations that deviate from that of the matrix, which shows a single maximum fabric at the depth where the folding occurs.

Numerical modelling of crystal viscoplastic deformation and dynamic recrystallisation was used to improve the understanding of the formation of the observed structures during deformation. The modelling reproduces the development of bands of grains with a tilted-lattice orientation relative to the single maximum fabric of the matrix, and also the associated local deformation. We conclude from these results that the observed folding can be explained by formation of these tilted-lattice bands.

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In this study we present examples of typical small-scale folds observed in the NEEM ice core, North Greenland, and discuss their characteristics. Numerical modelling of viscoplastic deformation and dynamic recrystallisation was used to improve the understanding of the formation of the observed structures under simple shear boundary conditions. We conclude that the folds originate from bands of grains with a tilted lattice relative to the strong lattice preferred orientation below 1500 m depth.
In this study we present examples of typical small-scale folds observed in the NEEM ice core,...
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