Regional variability of polar snow densification during large climatic transitions

The phasing between increases in temperature and greenhouse gas concentrations during large climatic variations in the past is classically estimated using analyses in polar ice cores. This phasing is still insufficiently constrained and solving this problem requires a better understanding of the mechanical process of snow to ice. In the absence of melting, the transformation of snow into ice depends essentially to temperature and accumulation rate. A thermo-mechanical model of snow densification has been developed at LGGE. Measurements of isotopes of inert gases (δ¹⁵N et δ⁴⁰Ar) in the air trapped in ice cores provide direct information about past variations of firn depth. Large differences between firn densification model outputs and gas isotopic data are obtained in Antarctica, and imply a large uncertainty on past climatic reconstructions. As part of this thesis work, we took into account the temperature dependence of activation energies and impurities on the firn densification speed. It allowed to reconcile the model results with available data. We also analyze new high resolution measurements of δ⁴⁰Ar over Terminations 2 (129-138 ka) and 3 (243-251 ka) on the Dome C and Vostok ice cores. We have shown that the different evolutions of δ¹⁵N between different sites and different deglaciations are largely explained by differences in accumulation rates that control the snow/ice transition depth. We also showed that the use of air isotopes was an important complement to the use of water isotopes to constrain local climatic dynamics in eastern Antarctica during deglaciations. (author)