Deformation and recrystallisation at the firn-ice transition - Micro structural simulations
Final Report Abstract
Ice sheets and glaciers flow under their own weight and their flow of ice is a major contributor to both global sea-level and climate changes. The macroscopic flow of ice is affected by the properties of the microstructure, which is formed by a small aggregate of individual ice crystals. The deformation of ice is accompanied by recrystallisation, a term which describes mechanisms causing re-orientations of the crystalline lattice, the formation of new crystals or the migration of their boundaries. The ice crystal is marked by a significant viscoplastic anisotropy, which causes a distinctly higher resistance to flow, if the crystalline lattice is unfavourably oriented. With deformation, the ice grains align and develop a crystallographic-preferred orientation within the iceaggregate, which induces a macroscopic anisotropy. Knowledge of the micro-dynamic deformation and recrystallisation mechanisms and how they affect the properties of the ice aggregate is a key to understand ice sheet dynamics. The objective of this project was to investigate the deformation and recrystallisation mechanisms in ice and the involved changes in the microstructures of ice- and ice-air aggregates. This was done by means of two-dimensional numerical simulations using the modelling platform Elle, which is optimised for modelling interacting micro-dynamic processes. The simulations couple a numerical model for viscoplastic deformation of anisotropic polycrystalline aggregates to implementations of recrystallisation mechanisms in Elle. In particular, an explicit numerical approach to consider secondary phases such as air inclusions was developed, implemented and used for the first time. Resulting "virtual" ice microstructures were compared with natural ice microstructures, as observed in drill cores. The results of this project show that the deformation and microstructures of ice are generally more heterogeneous than previously thought. Strain localisation is common in ice and related to viscoplastic anisotropy and intensified by the presence of air inclusions. Strain localisation probably occurs over a range of scales and has implications for the large-scale flow of ice. The project demonstrated that deformation-induced recrystallisation is in ice, even at shallow depths. This is partly due to the heterogeneity of strain rate that is enhanced by the presence of bubbles. deformation can thus already occur locally, even when bulk conditions to activate recrystallisation are not yet achieved. The project further revealed that the dissection of grains by migrating grain boundaries may be an additional grain-size-reducing process in polar ice that was not considered so far. This project confirmed that the activation of deformation and recrystallisation mechanisms is a function of the deformation conditions such as strain rate, temperature, but also bubble content and probably the concentration of impurities and dust particles. The numerical method allowed simulation of the evolution of the ice microstructure to high finite strain, achieving steady state. The resulting numerical-microstructures reflect the imposed deformation conditions, but appear largely independent from the initial microstructures. These results of this study indicate a high rate of change in crystallographic-preferred orientation and other microstructural properties. Furthermore, the thesis confirms that the development of crystallographic-preferred orientation is a function of strain rather than time or stress.
Publications
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Converging flow and anisotropy cause large-scale folding in Greenland's ice sheet Nat Commun, 2016, 7, 11427
Bons, P. D.; Jansen, D.; Mundel, F.; Bauer, C. C.; Binder, T.; Eisen, O.; Jessell, M. W.; Llorens, M.-G.; Steinbach, F.; Steinhage, D. & Weikusat, I.
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Small-scale disturbances in the stratigraphy of the NEEM ice core: observations and numerical model simulations The Cryosphere, 2016, 10, 359-370
Jansen, D.; Llorens, M.-G.; Westhoff, J.; Steinbach, F.; Kipfstuhl, S.; Bons, P. D.; Griera, A. & Weikusat, I.
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Strain localization and dynamic recrystallization in the ice-air aggregate: a numerical study The Cryosphere, 2016, 10, 3071-3089
Steinbach, F.; Bons, P. D.; Griera, A.; Jansen, D.; Llorens, M.-G.; Roessiger, J. & Weikusat, I.
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Dynamic recrystallization during deformation of polycrystalline ice: insights from numerical simulations Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, The Royal Society, 2017, 375, 20150346 (1-24)
Llorens, M.-G.; Griera, A.; Steinbach, F.; Bons, P. D.; Gomez-Rivas, E.; Jansen, D.; Roessiger, J.; Lebensohn, R. A. & Weikusat, I.
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Numerical modelling of deformation and recrystallisation mechanics in ice and ice-air aggregates PhD-thesis, Mathematisch-Naturwissenschaftlichen Fakultät der Eberhard Karls Universität Tübingen, 2017
Steinbach, F.
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Numerical modelling on deformation and recrystallisation in ice Frontiers in Earth Science - Figshare, 2017
Steinbach, F.; Kuiper, E.-J. N.; Eichler, J.; Bons, P. D.; Drury, M. R.; Griera, A.; Pennock, G. M. & Weikusat, I.
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The Relevance of Grain Dissection for Grain Size Reduction in Polar Ice: Insights from Numerical Models and Ice Core Microstructure Analysis Frontiers in Earth Science, 2017, 5, 66
Steinbach, F.; Kuiper, E.-J. N.; Eichler, J.; Bons, P. D.; Drury, M. R.; Griera, A.; Pennock, G. M. & Weikusat, I.
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High-strain deformation of conglomerates: numerical modelling, strain analysis, and an example from the Wutai Mountains, North China Craton. Journal of Structural Geology, 2018, 114, 222-234
Ran, H.; Bons, P.D.; Wang, G.; Steinbach, F.; Finch, M.; Griera, A.; Gomez-Rivas, E.; Llorens, M.-G.; Ran, S.; Liang, X.; Zhou, J.
