Creep of Polar Firn

In contrast, to ice, little effort has been devoted to experimentally determining the flow rate of natural firn, which eventually turns into ice in polar ice sheets and entraps dissolved gases. In this work, we performed constant-load creep tests at various stresses at -10^oC on specimens taken every 10 m along a firn core extracted at Summit, Greenland in June, 2017. A modified Andrade equation ε=βt^k could be used to describe the primary creep strain, ε, as a function of time, t, where k was generally found to be greater (up to 0.84) than the 0.33 that has been observed for fully-dense ice. A strain rate minimum was only reached in a few specimens at higher stresses from depths >50 m, and at larger strains (4-10%) than that ( ~1%) normally observed for fully-dense ice. A strain rate minimum was not observed in any shallow specimens even though the total creep strain in some tests was greater than 30%. Fitting the 80 m firn data, where a strain rate minimum was observed, to έ=Aσⁿ, where έ is the strain rate, A is the temperature-dependent constant, and σ is the compressive stress, yielded a value for the stress exponent, n, of 4.2. Analysis of the microstructures before and after creep testing using X-ray microcomputed tomography showed that the density and structure thickness, a measure of the feature size, had increased while the total porosity and specific surface area had decreased after creep testing. This work was sponsored by the National Science Foundation Arctic Natural Science grant 1743106.