Modélisation de la transformation de la neige en glace à la surface des calottes polaires : étude du transport des gaz dans ces milieux poreux

When ice is formed in central area of ice sheets, it is surmounted by more than 100 m of snow and firn. Ice can be older than 1000 years when the air is trapped in bubbles. This work contributes to precise stages of the snow/firn transformation using data on the structure of this porous medium, to validate physical model. The 2D structure has been caracterized with a new method based on the observation of sublimated samples using coaxial reflected light. With these experimental data, and using theory developed for ceramics and metals, the normal grain growth in polar firn and its influence on microstructure have been studied. Densification of firn has been modelled using physical processes described for the hot isostatic pressing of engineering materials. Our physical densification model enables to simulate ail experimental density profiles. The observed decrease with temperature of the density at the snow/firn transition (packing density) seems to result from the competition between grain boundary sliding and power-Iaw creep. Thanks to the grain growth study and the densification model, an original model describing the air trapping (close-off) during the firn/ice transition has been constructed. This geometrical model enables to simulate the observed evolution with density of closed porosity and allows to give an explanation for the geographical variation in gas content measurements. Finally, densification and close-off models have been used to simulate thesnow/firn/ice transformation in glacial climatic conditions at Vostok. Based on these models of the evolution of this porous medium, a gas transport model from the atmosphere to the bubbles in the ice has been developed. A particular emphasis has been made to study the influence of the open porosity on the diffusion coefficients of gases through the porous firn. They have been measured as a function of open porosity on two different cores. The model quantifies the smoothing effect of the firn diffusion and bubble trapping on atmospheric signal. It enables to reproduce the gaz mixing ratio versus depth in the fun from an atmospheric scenario. This gas transport model represents a first stage with objective deconvolution of the atmospheric signal from gas measurments in firn and ice.