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In Situ X-Ray Tomography Densification of Firn: Decoupling the Role of Mechanics and Diffusion Processes

One of the most efficient proxy method for paleoclimatology consists of obtaining data previously preserved within polar ice cores. Models for past climate reconstruction are based in particular on the characterization of entrapped gases in ice closed pores. Improving the temporal accuracy of these models requires a better understanding of firn densification mechanisms. In particular, the interplay between viscoplastic deformation and diffusion processes for pore closure is not well understood. In this work, we propose the first in situ densification experiments on polar firn retrieved from Antarctica with live characterization by X-ray tomography. Our in situ tests allow pore closure, which take thousands of years to occur in Antarctica, to be visualized and quantitatively characterized in a few hours in the laboratory. Pore separation and closure parameters with the accompanying microstructural changes were followed. We show that densification of polar firn and pore closure could be replicated at high strain rate and temperature. Experiments allow the viscoplastic part of the firn deformation to be decoupled from the diffusion mechanisms that occur at high temperature. Our results show that density alone is not sufficient to predict the close-off density at which gases get entrapped. More generally, the method laid out here may find useful application in the domain of high temperature powder compaction, for which pore closure and grain growth are significant process parameters.