DENSIFICATION OF POLAR ICE AFTER THE CLOSE-OFF

Polar ice sheet flow is hindered by limited knowledge of the rheological law for polycrystalline ice. Theoretical considerations, laboratory experiments and inclinometer data support a value lower than 2 for the exponent of the flow law relating stress and strain rate in polar ice at low stresses (P. PIMIENTA and P. DUVAL : J. Phys. (Paris), 48,243,1987). Another way of identifying deformation processes of ice at low stresses is to analyze the densification of the upper layers in polar ice sheets. After the close-off the densification is determined by the creep of the thick spherical shell surrounding each bubble. The densification rates by power law creep and by diffusion can be calculated by using the models presented by E. ARTZ et al. (Metall. Trans., 14,211,1983). The power law creep model was used with n=3 and n=1. A good fit to the experimental results given by A. J. GOW (J. Glaciol., 7,167,1968) was obtained taking successively n=3 and n=1 (the effective pressure ΔP decreases when depth increases; at Byrd Station the transition takes place when ΔP=0.4MPa). On the other hand, the densification rates deduced from the diffusion model are about one order of magnitude lower than experimental values. The quasi-Newtonian behavior of polar ice deduced from laboratory tests and inclinometer data is therefore supported by this analysis. The simulation of polar ice densification from the power law creep model leads us to some interesting results. The overall polar ice density profile is dependent on temperature and accumulation rate. The effective stresses around bubbles in regions as cold as Vostok Station are more important than shear stresses even at depths as deep as 500m. Therefore they can have an important role in the polar ice sheet flow, as dislocation sources for example