The Onset of Recrystallization in Polar Firn, Summit, Greenland, June 2017

Constraining the onset of dynamic recrystallization (DRX) and its effects on the mechanical properties of firn is crucial for firn densification modeling. To that end, samples from a depth of 13 meter (m) in a Summit, Greenland (72°35'North [N], 38°25'West [W]) firn core were subjected to creep tests at -14º Celsius (C) and 0.21 megapascals (MPa) compressive stress to strains of 7, 12, 18, and 29 percent (%). Microstructural analyses using thin-section imaging and microcomputed x-ray tomography (micro-CT) revealed smaller grain sizes, reduced specific surface area and connectivity, and increased density in relation to reduced porosity as the strain increases. These results show that DRX occurs in firn under creep, with strain-induced boundary migration (SIBM) and nucleation and growth starting at ~7%. DRX leads to elongated grains, reduced grain size, and the development of a preferred crystallographic orientation, indicating that DRX occurs by both SIBM and nucleation and growth.