Master Thesis Defense by Yannick Elias Heiser

Title: The role of grain size on ice properties and deformation - Assessing new grain size data from the NEEM ice core generated by microstructure mapping. 

The significance of polar ice sheet behavior has grown in climate research as a result of global warming. A comprehensive understanding of the ice dynamics is vital for modeling past and future ice flow, which affect global climate and sea level. Ice deformation comes along with microstructural changes in the polycrystalline ice making the grain size an important parameter for the strain rate. Mapping the microstructure, i.e. grain boundary networks, allows to experimentally extract the grain size and other microstructural deformation features from ice cores. A set of high resolution sublimation groove images is produced by the Extra Large Area Scanning Microscope (xLASM) for a section in the deep ice of the North Greenland Eemian Ice Drilling (NEEM) ice core. A digital image processing approach extracts the grain sizes, which are compared to the existing CFA impurity concentrations from NEEM. The effect of the grain sizes on the strain rate is investigated assuming basal glide, rate- limited by grain boundary sliding (GBS), and comparing it to Glen’s flow law. The strongest correlations of the grain sizes were found to the dust, calcium and sodium concentrations. However, the mechanism linking the grain size to the impurity concentrations remains unclear. GBS-limited creep showed twice as large strain rates in the stadial than in the interstadial period. Glen’s flow law yields similar strain rates for the stadial period, while it overestimates the strain rates in the interstadial by a factor of 2. The results imply, that the grain size is an important parameter in some creep regimes, that needs to be accounted for by ice flow models. 

Supervisors: Anders Svensson and Nicholas Rathmann
Censor: Nanna B. Karlsson