Densification of layered firn in the ice sheet at Dome Fuji, Antarctica

Abstract

ABSTRACT. In order to better understand the densification of polar firn, firn cores from the three sites within ∼10 km of Dome Fuji, Antarctica, were investigated using surrogates of density: dielectric permittivities ε v and ε h at microwave frequencies with electrical fields in the vertical and horizontal planes respectively. Dielectric anisotropy Δε (=ε v − ε h ) was then examined as a surrogate of the anisotropic geometry of firn. We find that layered densification is explained as a result of complex effects of two phenomena that commonly occur at the three sites. Basically, layers with initially smaller density and smaller geometrical anisotropy deform preferentially throughout the densification process due to textural effects. Second, layers having a higher concentration of Cl − ions deform preferentially during a limited period from the near surface depths until smoothing out of layered Cl − ions by diffusion. We hypothesize that Cl − ions dissociated from sea salts soften firn due to modulation of dislocation movement. Moreover, firn differs markedly across the three sites in terms of strength of geometrical anisotropy, mean rate of densification and density fluctuation. We hypothesize that these differences are caused by textural effects resulting from differences in depositional conditions within various spatial scales.

Anisotropy of seasonal snow measured by polarimetric phase differences in radar time series

“…. −0.05) were used as a surrogate for a geometrically anisotropic microstructure (Fujita et al, 2016). The density and dielectric measurements of the studies of Fujita indicate that a CPD of φ CPD = −80 • per meter would have been measured for the radar parameters of the satellite TSX as used in the following study on seasonal snow: in Leinss et al (2014b) a CPD of 60-150 • m −1 was measured for fresh snow (ρ = 0.2) in Finland at θ 0 = 32.7 • and 9.65 GHz, corresponding to elongated horizontal structures with an anisotropy between A = +0.2 and +0.5 (A −1 = 1.2 and 1.7).…”

“…On macroscopic scales, the anisotropy of snow can be characterized by measuring the anisotropy of the thermal conductivity (e.g., Izumi and Huzioka, 1975) or from the anisotropy of the dielectric permittivity (e.g., Fujita et al, 2016). The anisotropy of the thermal conductivity is much stronger compared to the anisotropy of the dielectric permittivity because the contrast in thermal conductivity k between ice and air k ice /k air is ≈ 100 compared to the contrast in permittivity between ice and air, which is ε ice /ε air ≈ 3 (Löwe et al, 2013).…”

“…Using open microwave resonators of the design of Jones (1976), Matsuoka et al (1996Matsuoka et al ( , 1997 measured the dielectric anisotropy of ice, which is caused by the c-axis orientation of the crystal fabric. With the same method, the dielectric anisotropy in snow, caused by a structural anisotropy of the ice matrix, has been measured and higher permittivities have been found in the vertical than the horizontal direction in multiyear firn on both the Greenland ice sheet (Fujita et al, 2014) and on the Antarctic ice sheet (Fujita et al, 2009(Fujita et al, , 2016; Fujita et al (2009) did the analysis in conjunction with a CT analysis. Using a method of microwave propagation, Lytle and Jezek (1994) also detected a higher vertical than horizontal permittivity in multiyear firn on the Greenland ice sheet combined with a photographic analysis.…”

Analytical constraints on layered gas trapping and smoothing of atmospheric variability in ice under low-accumulation conditions

“…Even if the bulk behavior in firn is the increase in density and decrease in open porosity with depth, local physical heterogeneities affect firn densification and gas trapping Martinerie et al, 1992;Hörhold et al, 2011;Fujita et al, 2016). Working on ice cores and firn from highaccumulation sites, Etheridge et al (1992), Mitchell et al (2015), and Rhodes et al (2016) have discussed the influence of centimeter-scale physical variability in firn on recorded gas concentrations.…”

Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities

“…Arnaud et al, 2000;Salamatin et al, 2009). The model developed over the past 30 years at Laboratoire de Glaciologie et Géo-physique de l'Environnement (LGGE) (Arnaud et al, 2000;Barnola et al, 1991;Goujon et al, 2003;Pimienta, 1987) aims at using a physical approach that remains sufficiently simple to be used on very long timescales (covering the ice core record length). More complex models, explicitly representing the material microstructure, have been developed but require a lot more computing time (Hagenmuller et al, 2015;Miller et al, 2003).…”