Climatic information and chronological constraints inferred from the air trapped in polar ice cores

Polar regions are particularly sensitive to the present climate change because of amplification mechanisms at play. Ice cores from Antarctica and Greenland provide precious archives of paleo-climatic variations at these high latitudes but also provide information on environmental changes at lower latitudes. This work takes advantage of information inferred from the air trapped in ice: the elemental and isotopic composition of air (N₂, O₂, Ar and Kr) and the methane concentration. Our results provide new insights on three different issues: (i) Past dynamic of Antarctic firn. Measurements of krypton isotopes provide direct evidences for the presence of a deep convective zone (20-50 m) in the firn at Dome C during glacial periods. Based on nitrogen isotopic profiles (δ¹⁵N) from EPICA Dronning Maud Land (EDML) and TALDICE ice cores, we suggest that the evolution of the firn structure is mainly controlled by accumulation rate only below a threshold in accumulation defined at 5 ± 1.5 cm ice equivalent yr^-1. (ii) Ice core dating. We present the first record of δO₂/N₂ over the EDC ice core covering the period between 300 and 800 ka. Even if we call for cautiousness in the use of this parameter as an orbital dating tool during periods characterized by a low eccentricity, the δO₂/N₂ is a useful tool to test the current EDC3 chronology. By coupling the isotopic composition of atmospheric oxygen and methane concentration, we synchronise the EDML (Antarctica) and NorthGRIP (Greenland) ice cores between 75000 and 123000 years with an associated uncertainty smaller than 400 years. (iii) This common timescale enables us to depict the abrupt climatic variability at the beginning of the last glacial period. We evidence sub-millennial scale climatic variations superimposed to the classical Dansgaard-Oeschger (DO) event succession, i.e. precursor-type peak events preceding some DO events and rebound-type climatic events at the end of the interstadial phase. We demonstrate that the bipolar seesaw pattern between Antarctica and Greenland is at play at a sub-millennial scale. A multi-parametric study on the first abrupt event (DO 25) identified in the NorthGRIP ice core shows that this event characterises the transition between the last interglacial and the onset of the abrupt climatic variability associated with a strong coupling between low and high latitudes. Our results highlight an intimate interplay between millennial-scale variations and long term evolution of the mean climate sate (orbital configuration, ice sheet volume). (author)