Deformation and recrystallisation at the firn-ice transition - Micro structural simulations

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The firn-ice transition is probably one of the most important and, at the same time, least understood regions in polar ice sheets. Under pressure of overlying layers, snow compacts to firn and then to ice at about 80-100 m depth. At this transition connectivity is lost between enclosed air, which becomes trapped in isolated bubbles. Trapped air provides an invaluable paleoclimate record, measurable in polar deep-drilling projects. Before entrapment of air in bubbles, air is in open communication with air at higher levels in the column and can thus exchange and equilibrate its climate signature. There is thus a difference in age between ice and trapped air. Correct understanding of the physical process of porosity closure and, hence, the firn-ice transition is imperative for the correct interpretation of the climate record in polar ice sheets.By developing advanced microstructural analysis tools, researchers of the Alfred Wegener Institute (AWI) have been able to show that the compaction process at the firn-ice transition is more complex than hitherto assumed, as dynamic recrystallisation is a first-order process. In the meantime, workers in Tübingen and Barcelona have developed numerical code to simulate the complex interplay of recrystallisation and highly heterogeneous crystal-plastic deformation, using the full-field approach. The proposed project combines the expertise (in both analytical and modelling tools) of the AWI, Tübingen and Barcelona to numerically model deformation and recrystallisation of the complex composite ice with air. The numerical simulations, in combination with analyses on natural ice sample, will make it possible to constrain the relative contributions of the numerous operating processes and their impact on microstructure and mechanical properties of ice and firn. This, in turn, will provide better insight in the entrapment of air in ice, which is where palaeo-atmosphere is recorded in polar ice sheets.