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. IPCC 2007 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 component 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. The mechanical properties of the ice are furthermore modified by recrystallisation processes induced mainly by deformation energy and temperature changes. 

The deformation and recrystallisation processes leave behind traces in the grain morphologies and orientations (microstructures) which can be used to identify and under certain assumptions to quantify 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.

We study deformation microstructures in ice core samples from Greenland and Antarctica with the following experimental methods:

Microstructure Mapping:The ice sample surface is polished and exposed to controlled sublimation, which acts as thermal grooving and reveals grain and subgrain features in very high resolution. These features are imaged by optical light microscopy (~3µm/pix) or Large Area Scanning Macroscope (LASM, ~5µm/pix).

Automated fabric analyzer:The classical polarisation microscopy principles are used to determine c-axes orientations. Automatic control of polarizers, lambda-quarter plate and viewing direction by a computer software, as well as automatic samples stage scanning enables fast acquisition of data.