Controls on grain growth in polar ice

Abstract

This thesis provides a comprehensive study of the controls on grain growth, specifically in the North Greenland Eemian Ice Drilling (NEEM) ice core. It is very important to understand which processes control the grain size in polar ice, because grain size can influence the strength of the ice structure and the deformation mechanisms in the ice sheets. These processes in turn effect the behaviour and movement of the massive ice sheets that have a large impact on global climate and sea level. The controls on grain size in the ice sheets can vary with depth, age, impurity content, temperature and stress, and the relations between these various processes are not yet completely understood. To improve the understanding of grain growth relations, one of the main aims of this thesis is to see whether the gradual increase of grain-size at the deepest glacial deposits in the NEEM ice core can be explained by impurity controlled grain growth. Another main aim of this thesis is to see if the ice core grain-size with age relations can be used to constrain grain size evolution models, with the ultimate goal to contribute to the grain size sensitive ice rheology models for large-scale sealevel-forecasting ice sheet models. The available data of the ice core are compiled and compared with each other. Moreover, a grain size evolution model is made to include advection processes to grain growth laws. Several correlations and trends are found between grain size and age, grain size and particle content and grain size and stress. The grain size evolution model turns out to provide a good estimate for grain size in the Holocene ice of the core, using the best fitting grain growth law parameters of Diebold (nd). Moreover, the logarithmic trend between grain size and dust content is a useful tool to estimate the grain size in glacial ice by impurity controlled growth. The grain size in the Eemian ice is harder to constrain as it seems to be more dynamically controlled, but the paleopiezometer of Jacka and Jun (1994) can provide a rough estimate.