Can we retrieve a clear paleoclimatic signal from the deeper part of the EPICA Dome C ice core?

An important share of paleoclimatic information is buried within the lowermost layers of deep ice cores. Because improving our records further back in time is one of the main challenges in the near future, it is essential to judge how deep these records remain unaltered, since the proximity of the bedrock is likely to interfere both with the recorded temporal sequence and the ice properties. In this paper, we present a multiparametric study (δD-δ18Oice, δ18Oatm, total air content, CO2, CH4, N2O, dust, high resolution chemistry, ice texture) of the bottom 60m of the EPICA Dome C ice core from central Antarctica. These bottom layers have been subdivided in two sections: the lower 12m showing visible solid inclusions (basal ice) and the 48m above which we refer to as “deep ice”. Some of the data are consistent with a pristine paleoclimatic signal, others show clear anomalies. It is demonstrated that neither large scale bottom refreezing of subglacial water, nor mixing (be it internal or with a local basal end-term from a previous/ initial ice sheet configuration) can explain the observed bottom ice properties. We focus on the high-resolution chemical profiles and on the available remote sensing data on the subglacial topography of the site to propose a mechanism by which relative stretching of the bottom ice sheet layers is made possible, due to the progressively confining effect of subglacial valley sides. This stress field change, combined with bottom ice temperature close to the pressure melting point, induces accelerated migration recrystallization, which results in spatial chemical sorting of the impurities, depending on their state (dissolved vs. solid) and if they are involved or not in salt formation. This chemical sorting effect is responsible for the progressive build-up of the visible solid aggregates that therefore mainly originate “from within”, and not from incorporation processes of allochtone material at the ice–bedrock interface. We also discuss how th