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

摘要

In order to better understand the densification of polar firn, firn cores from the three sites within similar to 10 km of Dome Fuji, Antarctica, were investigated using surrogates of density: dielectric permittivities epsilon(v) and epsilon(h) at microwave frequencies with electrical fields in the vertical and horizontal planes respectively. Dielectric anisotropy Delta epsilon (=epsilon(v) - epsilon(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.