Abstract
AbstractConstant-load creep tests were performed at −10°C at various compressive stresses from 0.05 to 0.75 MPa on specimens taken every 10 m along a firn core extracted at Summit, Greenland in June 2017. The microstructures before and after creep testing were examined using both X-ray microtomography (micro-CT) and optical images from thin sections. An Andrade-like equation was used to describe the primary creep behavior and yielded the time exponentkof 0.17–0.76. The onset of secondary creep occurred at strains of ~0.5–3% but was sometimes not observed at all in shallow firn specimens and at stresses ⩽0.43 MPa even for strain up to 32%. For the 50–80 m firn crept at stresses ⩾0.55 MPa, secondary creep occurred at strains of 2.6 ± 0.28%, and the stress exponent,n, in Glen's law, was found to range from 4.1 to 4.6, similar to those observed for fully dense ice. Micro-CT observations of crept specimens showed that in most cases, the specific surface area, the total porosity and the structure model index decreased, while the structure thickness increased with increasing density. These microstructural characteristics are consistent with the densification of the firn. Optical images from thin sections showed that recrystallization occurred in some specimens that had undergone secondary creep.
Highlights
The evolution of the density of firn with depth is usually related to mean annual air temperature, accumulation rate and surface density (Herron and Langway, 1980; Maeno and Ebinuma, 1983; Martinerie and others, 1992), overburden pressure (Kameda and Naruse, 1994) or surface winds (Craven and Allison, 1998)
It is evident that the polar firn densifies with depth mainly by the sintering-pressure mechanism forced by the accumulated snow load
A threshold stress occurs ∼0.43–055 MPa that divides secondary creep behavior into two categories: (1) for stresses of ⩾0.55 MPa, which was used only in the 80 m core, the secondary creep occurs at a fixed strain (2.6 ± 0.28%); and (2) for stresses ⩽0.43 MPa, the secondary creep occurred over a wide range of strains and was sometimes not observed at all
Summary
The evolution of the density of firn with depth is usually related to mean annual air temperature, accumulation rate and surface density (Herron and Langway, 1980; Maeno and Ebinuma, 1983; Martinerie and others, 1992), overburden pressure (Kameda and Naruse, 1994) or surface winds (Craven and Allison, 1998). Wang and Baker (2013) used both continuous and interrupted compression tests in situ in a micro-CT on snow densities ranging from 100 to 350 kg m−3 at −6°C. Wang and Baker (2014) investigated the structural evolution of low-density snow under high-temperature gradients (100–500°C m−1) using a micro-CT. Gregory and others (2014) examined the nature of pore closure in firn cores from both the West Antarctic Ice Sheet Divide and Megadunes in East Antarctica using a micro-CT They found that the open pore structure played a more important role than the density in predicting gas transport properties. Burr and others (2019) using a micro-CT found an abnormal decrease of connectivity with increasing density in Antarctic firn samples at −10°C They supposed that densification of firn was related to dislocation glide and diffusional processes and not to viscoplastic deformation. We characterized the firn microstructures before and after creep testing using both X-ray microcomputed tomography (micro-CT) and thin sections viewed between crossed polarizers
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