Dynamic Observations of the Densification of Polar Firn Under Compression Using a Micro‐Computed Tomograph

Abstract

AbstractThe present work is the first to undertake systematicin situobservations of the microstructural changes on samples taken at ∼10‐m intervals along the length of a 80‐m firn core, extracted at Summit, Greenland (72°35’ N, 38°25’ W) in June, 2017, under interrupted load and at a strain rate of ∼8 × 10−5s−1at −10°C, using a X‐ray micro‐computed tomograph. Several noteworthy features of the densification were found: the ice particle size increases, while the specific surface area, the total porosity, the pore size, and the structure model index (a measure of convexity/concavity of ice surface) decreases. The results were used to formulate semi‐empirical models (valid in the stress range of ∼0.05–2.15 MPa) that can be used to quantitatively assess the relative contributions of lattice diffusion (LD) and grain boundary diffusion (GBD) to the densification of polar firn. We found that 0.28 and 2.15 MPa are two critical stresses, which represent the start and end of LD as the dominant deformation mechanism to the densification of polar firn under the interrupted increasing loads. This bimodality when LD dominates implies that stress is not the only factor governing the densification of polar firn. On the other hand, GBD dominates the densification of polar firn both for stresses lower than 0.28 MPa and greater than 2.15 MPa. At stresses greater than 2.41 MPa, the firn specimens either fractured or other deformation mechanisms dominated, e.g., grain boundary sliding or power‐law dislocation creep.