Observations of creep of polar firn at different temperatures

Abstract. To improve our understanding of firn compaction and deformation processes, constant-load compressive creep tests were performed on specimens from a Summit, Greenland (72°35′ N, 38°25′ W) firn core that was extracted in June 2017. Cylindrical specimens were tested at temperatures of −5, −18 and −30 °C from depths of 20, 40 and 60 m at stresses of 0.21, 0.32 and 0.43 MPa, respectively. The microstructures were characterized before and after creep using both X-ray micro-computed tomography (micro-CT) and thin sections viewed between optical crossed polarizers. The results of these experiments comprise a novel data set on the creep of firn at three depths of a firn column at three different temperatures, providing useful calibration data for firn model development. Examining the resulting strain vs. time and strain vs. strain rate curves from the creep tests revealed the following notable features. First, the time exponent k was found to be 0.34–0.69 during transient creep, which is greater than the 0.33 usually observed in fully-dense ice. Second, the strain rate minimum (SRmin) in secondary creep occurred at a greater strain from specimens with lower density and at higher temperatures. Third, tertiary creep occurred more easily for the lower-density specimens at greater effective stresses and higher temperatures, where strain softening is primarily due to recrystallization. Fourth, the SRmin is a function of the temperature for a given firn density. Lastly, we developed empirical equations for inferring the SRmin, as it is difficult to measure during creep at low temperatures. The creep behaviors of polar firn, being essentially different from full-density ice, imply that firn densification is an indispensable process within the snow-to-ice transition, particularly firn deformation at different temperatures connected to a changing climate.