The effect of deformation mechanisms for ice sheet dynamics
The huge ice masses stored in the polar ice sheets are the main fresh water reservoirs and thus have potentially huge effects on sea level evolution. Especially the role and development of ice streams, such as the NEGIS, are still under debate. IPCC specified the estimates of the dynamic flow of polar ice sheets and the insufficient understanding of ice sheet physics as major error sources on predictions of sea level change. One of two main components governing the dynamic flow of ice is the internal deformation of the ice body.
Any large-scale plastic flow of even huge bodies of any material is actually conducted by deformation on smaller scales; down to crystallite and subgrain as well as atomic scales. As these deformation mechanisms significantly control the rheology of the material, knowledge on these processes is essential to understand and predict ice dynamics seriously. Depending on the conditions (e.g. depth in the ice sheet, speed of deformation, temperature) different mechanisms mobilize smallest volumes of the ice (e.g. molecule groups, crystal lattice planes, crystal boundaries) whose movements in total lead to the shape change of the ice sheet.
These small scale deformation mechanisms leave behind traces in the grain morphologies and orientations (microstructures) which can be used to identify the relevant mechanisms. These traces are e.g. grain sizes, grain shapes, crystal orientation distributions (fabrics), subgrain boundary occurrence, subgrain boundary misorientations and types, dislocation densities and types.
The mechanical properties of the ice are furthermore modified by recrystallisation processes induced mainly by deformation energy and temperature changes, which again can be traced in the microstructure.
We study deformation microstructures in ice core samples from Greenland and Antarctica with the following experimental methods:
Electron Backscatter Diffraction (EBSD, in cooperation with Prof. Dr. Martyn Drury, Utrecht University, The Netherlands)
Diffraction of backscattered electrons in an Scanning Electron Microscope (SEM) enables the determination of full crystal orientations (c- and a-axes) of ice core samples in a high spatial (~3µm) and angular resolution (relative orientation ~0.5°). We use EBSD to characterize and quantify subgrain boundaries and the dislocation types they are composed of.
Microstructure modelling (in cooperation with Prof. Dr. Paul Bons, Eberhard Karls Universität Tübingen & Dr. Albert Griera, Universitat Autònoma de Barcelona)
The microstructure modeling platform ELLE as one of the main numerical modeling codes used in the Earth sciences simulates microstructures with a front tracking approach. It was recently extended with full-field viscoplastic formulation based on the Fast Fourier Transform. FFT/ELLE is able to model microstructure evolution of ice during plastic deformation and recrystallization. Different mechanisms can be switched on and off to varying intensities in order to study their effect on the microstructure. At one hand plastic deformation modeling is needed which respects the high anisotropy of ice, on the other hand recrystallization plays an important role in the hot material ice (homologous temperatures > 0.8 in natural settings).
Jun.-Prof. Dr. Ilka Weikusat
Dr. Tobias Binder
Dr. Jens Rößiger
Dr. Christian Weikusat
Dr. Maddalena Bayer
Felicitas Mundel (Tübingen)
Anneke Tammen (Mainz)
Eric Gleiß (Heidelberg)
Julien Westhoff (Tübingen)
Sophie Ehrhardt (Leipzig)
Lißbeth Langhammer (Berlin)
Yasuyuki Oishi (Nagaoka, Japan)
Jakub Surma (Köln)
Wataru Shigeyama (Nagaoka, Japan)
Prof. Dr. Sergio H. Faria BC3, Bilbao
Prof. Dr. Paul D. Bons Universität Tübingen
Prof. Dr. Martyn R. Drury Utrecht University
Dr. Albert Griera University Barcelona