Collaborative Research: Towards a physics-based firn model: understanding the relationship between microstructure and creep

Polar firn is multi-year snow that has survived more than one season and is a transition phase in the formation of glacial ice. A comprehensive understanding of how firn densifies into solid ice is important for several reasons: (1) to interpret ice sheet mass balance changes from remote-sensing observations; (2) to determine how the microstructural evolution of firn contributes to the resulting ice sheet microstructure and its ice flow rates; and (3) most importantly for better interpretation of ice-core paleoclimate records by understanding how air becomes entrapped in the firn, and ultimately in the ice. Currently, firn densification is not fully understood, and a physics-based model based on experimentally-observed deformation mechanisms is needed to improve our firn estimates for the important applications described above. This research aims to develop a comprehensive understanding of the compaction and microstructural evolution of polar firn by performing mechanical testing, and characterization of the resulting microstructure on firn cores from four different locations at Taylor Dome, Antarctica. Results from each of these firn cores will provide valuable information about how firn microstructure, and its impact on densification, evolves under varying environmental conditions outside the range of existing datasets. The research will involve a PhD student and several undergraduates. Densification of polar firn involves several different mechanisms including pressure sintering, plastic deformation, grain rearrangement, and, near the surface, temperature gradient metamorphism due to water vapor transport. A key step towards a physics-based firn model is understanding how the microstructure impacts the dominant deformation mechanism(s) at different depths in the firn column. This project will provide valuable data needed to answer this question by performing creep tests and characterizing the microstructural evolution under creep loading on firn cores drilled to pore close-off at four different locations at Taylor Dome, Antarctica. Each of the four firn-core sites vary in their annual accumulation rate (2 cm.a-1, 4 cm.a-1, 8 cm.a-1, 25 cm.a-1), creating different microstructure-depth profiles. We will use a unique combination of X-ray micro-CT imaging coupled with high-resolution imaging of the evolved microstructure using a scanning electron microscope equipped with electron backscatter diffraction, that will provide 3-D orientation information. In addition, we will examine the local microchemistry, particularly around bonds between grains, using energy-dispersive X-ray spectroscopy in the SEM and Raman spectroscopy in an optical imaging system. These results will allow for a full analysis of the role firn microstructure has on densification. This work will ultimately be used to improve the Community Firn Model. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.