Modelling of deformtion and recrystallisation microstructures in polar ice
Final Report Abstract
The flow of ice in polar ice sheets is a primary factor in controlling the amount of fresh water that is stored in these sheets, which is directly linked to sea level. Ice is a crystalline material that can flow by crystal plastic processes, such as dislocation creep. To predict the behaviour of ice sheets as a result of climate change, intimate knowledge of the flow behaviour of ice is imperative. This study used numerical simulations to address two main questions: • What are the effects of different processes and boundary conditions on the (grain-scale) microstructure? • How can we deduct boundary conditions and the operation of processes from observations on natural ice microstructures in deep drill cores? A large part of the project involved the development and implementation of a number of algorithms that significantly increased the sophistication of the numerical simulation of microstructures in ice and other minerals. The microstructure is an important, but often neglected or underestimated state variable. The simulations, for example, showed how the microstructure affects the rate of grain growth in ice, both in pure ice and in the presence of bubbles, as is the case in the upper few hundred meters of ice sheets. Simulations of grain growth in combination with polygonisation or the presence of bubbles indicate that polygonisation and strain-induced grain boundary migration are already significant at shallow levels in ice sheets, thus challenging the paradigmatic "three regime model", which assumes that only grain growth is active at these levels. Flow in ice sheets leads to extreme deformation. For the first time, this project was able to simulate the microstructural and mechanical evolution in deforming ice up to high strains, by coupling viscoplastic code with recrystallisation routines. These simulations show a very large impact of recrystallisation on the ice microstructure, which through future studies may significantly improve the interpretation of natural microstructures. The simulations also showed that the flow in ice is expected to be very heterogeneous. However, continuous reworking of the microstructure makes this localisation of deformation virtually impossible to see in natural microstructures. First results from this project thus suggest that heterogeneity of flow, or strain localisation, should be included in ice sheet flow models that currently tend to assume that flow is mostly homogeneous. The simulations also indicated that the lattice-preferred orientation (LPO) is hardly affected by recrystallisation. This is a surprising outcome and will need further investigation, as this LPO is usually regarded as an important indicator of the operation of various microstructural processes. Apart from the tangible scientific results, published in several journals, the project delivered powerful numerical code for the simulation of microstructural processes in deforming ice, but also minerals, such as olivine or quartz. This code is now in use for ongoing projects in Germany and Australia.
Publications
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2011. Competition between grain growth and grain size reduction in polar ice. Journal of Glaciology 57, 942-948
Roessiger, J., Bons, P.D., Griera, A., Jessell, M.W., Evans, L. Montagnat, M., Kipfstuhl, S., Faria, S.H., Weikusat, I.
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2013. What happens to deformed rocks after deformation? A refined model for recovery based on numerical simulations. In: "Deformation Structures and Processes within the Continental Crust", Special Volume 394, Geol. Soc. London
Borthwick, V., Piazolo, S., Evans, L., Griera, A., Bons, P.D.
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2014. Influence of bubbles on grain growth in ice. Journal of Structural Geology. Journal of Structural geology 61, 123-132
Roessiger, J., Bons, P.D., Faria, S.H.
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2014. Microdynamics of Ice (editorial). Journal of Structural Geology 61, 1
Koehn, D., Bons, P.D.
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2014. Multiscale modeling of ice deformation behavior. Journal of Structural Geology 61, 78-108
Montagnat, M., Castelnau, O., Bons, P.D., Faria, S.H., Gagliardini, O., Gillet-Chaulet, F., Grennerat, F., Griera, A., Lebensohn, R.A., Moulinec, H., Roessiger, J., Suquet, P.
