Recrystallization and deformation mechanisms in the NEEM deep ice core, Greenland
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Abstract:
An overview of the deformation and recrystallization mechanisms that are active in the North Greenland Eemian Ice Drilling (NEEM) ice core is given, based on microscale models, light microscopy and cryogenic electron backscatter diffraction (cryo-EBSD). The Holocene ice (0-1419 m depth) deforms by dislocation creep with basal slip accommodated by non-basal slip. The amount of non-basal slip is controlled by the extent of strain induced boundary migration (SIBM). The most important recrystallization mechanisms and processes in the Holocene ice are grain dissection, strain induced boundary migration (SIBM), and bulging nucleation. In the glacial ice (1419-2207 m of depth) basal slip is accommodated by both non-basal slip and grain boundary sliding (GBS). Rotation recrystallization is more important, while SIBM is less important in the glacial ice compared to the Holocene ice. In the Eemian ice (2207-2540 m depth), which is at high temperature, different microstructures occur depending on the impurity content of the ice. The difference in microstructure and deformation mechanisms, between interglacial and glacial ice can have important consequences for ice rheology and ice sheet dynamics.Keywords:
Recrystallization (geology)
Ice core
Eemian
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Crystallographic investigations have been conducted of cold (−17°C) debris‐bearing ice from the base of an Antarctic outlet glacier (Taylor Glacier). The 4‐m‐thick sequence studied has been retrieved from a 20‐m‐long tunnel dug from the glacier snout and has been analyzed with an automatic ice fabric analyzer (AIFA). The top and bottom of the sequence consists of clean meteoric ice (englacial facies), whereas alternating debris‐rich and clean bubbly ice layers are found in the middle part (stratified facies). Ice from the englacial facies displays a polygonal texture and a strong c‐axis clustering toward the vertical, denoting recrystallization through “subgrain rotation” (SGR). In contrast, clean ice from the stratified facies shows SGR fabrics which are delimited at the contact with debris‐rich layers by large, interlocking grains organized in ribbons. These two distinct textures within the stratified facies are associated with looser c‐axis patterns at the scale of single thin sections, which is interpreted as resulting from “migration recrystallization” (MR). The change from SGR to MR trends marks a clear increase in grain boundary and nucleation kinetics (hence the term “discontinuous recrystallization”) and may be associated with strain localization at rheological interfaces during basal ice genesis. Analogies with bottom ice from deep polar ice sheets, where temperature is commonly higher than at the studied site, are highlighted. Two recrystallization scenarios are proposed, accounting for the development of both types of fabrics. It is shown that by controlling the repartition of stress and strain energy within basal ice, the rheology of debris‐bearing ice layers plays a decisive role in recrystallization dynamics at structural interfaces. We also demonstrate how the same recrystallization regimes may occur in cold glaciers and temperate ice sheets, provided that strain accumulation has been high enough in the former. This challenges the common belief that migration fabrics observed in bottom ice from deep ice sheets are exclusive to warm, stagnant, annealed ice.
Recrystallization (geology)
Dynamic Recrystallization
Ice wedge
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Firn
Ice core
Ice divide
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Recrystallization (geology)
Dynamic Recrystallization
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Greenland ice sheet
Ice-sheet model
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Pancake ice
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Dynamics
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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
Ice core
Recrystallization (geology)
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