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

Densification of firn at the North Greenland Eemian Ice Drilling (NEEM) camp is investigated using density surrogates: 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) 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 Δϵ of ~0.06 appears within the uppermost 0.2 m. After that, Δϵ decreases rapidly until 21-26 m depth. Below this, Δϵ 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. [ABSTRACT FROM AUTHOR]