Search

Understanding the flow of ice is essential to predict the contribution of the polar ice sheets to global mean sea level rise in the next decades and centuries. During this PhD project, the recrystallization and deformation mechanisms that govern the flow of ice were studied along the length of the North Greenland Eemian Ice Drilling (NEEM) ice core in northwest Greenland. Two methods were used during this study: (i) cryogenic electron backscatter diffraction (cryo-EBSD) in combination with (polarized) light microscopy and (ii) flow law modelling using two different flow laws for ice constrained by the actual temperature and grain size data from the NEEM ice core. The NEEM ice core was divided up into three depth intervals originating from different climatic stages that differ strongly in terms of impurity content, microstructure, deformation mode and temperature: the Holocene ice (01419 m of depth), the glacial ice (1419-2207 m of depth) and the Eemian-glacial facies (2207-2540 m of depth). Microstructures indicate that the Holocene ice deforms by the easy slip system (crystallographic basal slip) accommodated by the harder slip systems (non-basal slip), also known as dislocation creep, and by recovery via strain induced boundary migration (SIBM), which removes dislocations and stress concentrations and allows further deformation to occur. The amount of non-basal slip that is activated is controlled by the extent of SIBM. The dominant recrystallization mechanisms in the Holocene ice are SIBM, bulging recrystallization and grain dissection in total leading to dynamic grain growth, with a contribution from normal grain growth in the upper 250 m. The strain rate variability with depth in the Holocene ice, estimated by flow law modelling, is low. In contrast, the strain rate variability is relatively high in the glacial ice as a result of variability in grain boundary sliding (GBS) with depth that accommodates basal slip (GBS-limited creep). Grain boundary sliding in the glacial ice is particularly strong in fine grained sub-horizontal bands which contain many aligned grain boundaries. Subgrain boundaries (SGBs) form ahead of the aligned grain boundaries in these fine grained subhorizontal bands and when a misorientation angle of 5.0°-6.0° is reached, the SGB has rotated into a sliding boundary. Rotation recrystallization is more prominent, while SIBM is less important in the glacial ice compared to the Holocene ice. The ice in the Eemian-glacial facies, which is affected by premelting along the grain boundaries, alternates between relatively fine grained glacial ice with a single maximum crystallographic preferred orientation (CPO) and very coarse grained Eemian ice with a partial girdle CPO. Due to the difference in grain size and CPO, it is argued that the glacial ice in the Eemian-glacial facies deforms almost entirely by GBS-limited creep in simple shear and at high strain rates, while the Eemian ice in the Eemian-glacial facies deforms at much lower strain rates in coaxial deformation with a roughly equal contribution of GBS-limited creep and dislocation creep to bulk strain rate. The large difference in microstructure, and consequently viscosity, between impurity-rich glacial ice and impuritydepleted interglacial ice in the premelting layer (262K

The flow of ice depends on the properties of the aggregate of individual ice crystals, such as grain size or lattice orientation distributions. Therefore, an understanding of the processes controlling ice micro-dynamics is needed to ultimately develop a physically based macroscopic ice flow law. We investigated the relevance of the process of grain dissection as a grain-size-modifying process in natural ice. For that purpose, we performed numerical multi-process microstructure modeling and analyzed microstructure and crystallographic orientation maps from natural deep ice-core samples from the North Greenland Eemian Ice Drilling (NEEM) project. Full crystallographic orientations measured by electron backscatter diffraction (EBSD) have been used together with c-axis orientations using an optical technique (Fabric Analyser). Grain dissection is a feature of strain-induced grain boundary migration. During grain dissection, grain boundaries bulge into a neighboring grain in an area of high dislocation energy and merge with the opposite grain boundary. This splits the high dislocation-energy grain into two parts, effectively decreasing the local grain size. Currently, grain size reduction in ice is thought to be achieved by either the progressive transformation from dislocation walls into new high-angle grain boundaries, called subgrain rotation or polygonisation, or bulging nucleation that is assisted by subgrain rotation. Both our time-resolved numerical modeling and NEEM ice core samples show that grain dissection is a common mechanism during ice deformation and can provide an efficient process to reduce grain sizes and counter-act dynamic grain-growth in addition to polygonisation or bulging nucleation. Thus, our results show that solely strain-induced boundary migration, in absence of subgrain rotation, can reduce grain sizes in polar ice, in particular if strain energy gradients are high. We describe the microstructural characteristics that can be used to identify grain dissection in natural microstructures.