Densification of layered firn of the ice sheet at NEEM, Greenland

Abstract

Densification of firn at the North Greenland Eemian Ice Drilling (NEEM) camp is investigated using density surrogates: dielectric permittivities ∊v and ∊h at microwave frequencies with electrical fields in the vertical and horizontal planes, respectively. Dielectric anisotropy Δ∊ (= ∊v − ∊h) is then examined as a surrogate for the anisotropic geometry of firn. Its size, fluctuations and mutual correlations are investigated in samples taken at depths from the surface to ~90 m. The initial Δ∊ of ~0.06 appears within the uppermost 0.2 m. After that, Δ∊ decreases rapidly until 21–26 m depth. Below this, Δɛ decreases slowly. Layers with more ions of fluorine, chlorine and some cations deposited between the autumn and the subsequent summer deform preferentially during all these stages. This layered deformation is explained partly by the textural effects initially formed by the seasonal variation of metamorphism, and partly by ions such as fluorine, chlorine and ammonium, which are known to modulate dislocation movement in the ice crystal lattice. Insolation-sensitive microstructure appears to be preserved all the way to the pore close-off, within layers of the summer-to-autumn metamorphism. Like previous authors, we hypothesize that calcium is not the active agent in the reported deformation– calcium correlations.

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

“…This is small compared to the measurements shown in Fig. 11 and confirms the statement of Fujita et al (2014). It is worth noting here that the dielectric anisotropy due to the vertical crystal orientation of fresh snow has the opposite sign as the dielectric anisotropy due to the horizontal structural orientation of fresh snow.…”

“…For firn (with ρ = 0.4 g cm −3 , ε = −0.05), as observed by Fujita et al (2014) for the upper 5 m of the ice sheet at NEEM (Northwest Greenland), we would expect a negative anisotropy A = −0.25 (A = 1.3) due to vertical struc-tures. Similar firn conditions (ρ = 0.4 g cm −3 , ε = −0.05) have been found by Fujita et al (2009) at Dome Fuji in Antarctica, who determined a slightly lower structural anisotropy (A ≈ 1.15) by means of X-ray microtomography.…”

“…We note that ζ vanishes only for isotropic (polycrystalline) ice. This is not generally the case as for ice on glaciers and ice sheets the crystal axis of ice (c axis) can have a preferential orientation (e.g., Matsuoka et al, 1997;Fujita et al, 2014, and Appendix A).…”

“…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.…”

“…Fujita et al (2014) reported that the dielectric anisotropy due to oriented crystal fabrics is often much lower than dielectric anisotropy expected from a structural anisotropy. In Appendix A we used measurements of the crystal orientation fabric (COF) in snow from Riche et al (2013) to estimate a maximum dielectric anisotropy of ε = −0.002, corresponding to a structural anisotropy of A = −0.02.…”