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

Abstract

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 1500 m 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 2160 m. 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.

Dynamic recrystallization during deformation of polycrystalline ice: insights from numerical simulations

“…The ELLE software is open source and can be downloaded from the online repository (http://www.elle.ws). ELLE processes reproduce the evolution of microstructures during deformation and metamorphism, such as grain growth [30][31][32], DRX [8,33,34], folding [12,27,35,36] and rotation of rigid objects in anisotropic materials [37,38].…”

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

“…A particular rotation axis is not expected for these boundaries, although the occurrence of a strong CPO will tend to produce preferred rotation axes (Mainprice et al, 1993). At deeper depth levels in polar ice, where a characteristically strong CPO development with ice sheet depth is generally observed (Weikusat et al, 2017b;Jansen et al, 2016;, Montagnat et al, 2014;Fitzpatrick et al, 2014;Faria et al, 2014a, and references therein), we suggest that type III SGBs might occur. SGBs that develop from GBs may be recognizable as such because they are part of the grain boundary network.…”

Physical analysis of an Antarctic ice core—towards an integration of micro- and macrodynamics of polar ice

“…Owing to the high homologous temperature for natural ice in general () and the slow creep flow in most deep ice coring sites, such effects of deformation modes are very moderate and difficult to discern, because recrystallization strongly overprints deformation effects and masks typical deformation microstructures [17–20]. Furthermore, deformation mechanisms, such as small-scale folding initiated by grain kinking, provide new insights into the microdynamic processes with consequences for mesoscale [21] and possibly also macroscale structures [22]. The meso- to macroscale impact of changing deformation regimes with different deformation modes and mechanisms with depth can be monitored by repeatedly logging the borehole geometry after certain time intervals, which also reveals the onset of shear deformation dominance with depth.…”