Firn air pumping campaigns for improving deep ice core dating constraints (ICORDA project) and identifying signature of past katabatic wind activity in noble gases trapped in ice cores (KATABATIC project)

The study envisaged is the field mission associated with three funded projects (ANR JCJC KATABATIC, PI A. Orsi ; ERC CoG ICORDA, PI A. Landais ; ITN DEEPICE, PI A. Landais). It has strong links with the European project Beyond EPICA Oldest Ice Core.

The interpretation of deep ice cores relies on the establishment of a robust chronology. For long time scales, ice core chronologies are based on the measurement and interpretation of orbital dating tracers such as the elemental ratio O2/N2 or the air content in air bubbles trapped in the ice. Empirical measurements in the ice show that these two tracers are influenced by the insolation received by the snow on the surface of the ice cap. However, the exact associated process linking the metamorphism of snow grains at the surface and the trapping of air bubbles at a depth of about 100 m is not understood. In particular, a significant role of temperature and accumulation rate is expected. In order to understand this mechanism, it is essential to measure the evolution of the physical properties of snow and the O2/N2 and air content parameters at two very different sites in terms of temperature and accumulation rate. This is the main objective of WP1 of the ICORDA project. In parallel, the KATABATIC project aims to study the influence of katabatic winds on surface temperatures in East Antarctica. For this purpose, measurements of noble gases in the air trapped in the ice should make it possible to quantify the strength of katabatic winds in the past. In order to calibrate this effect, it is essential to study it at one of the sites with extremely strong katabatic winds, D47. We also wish to better understand the isotopic signature in water vapour of katabatic winds (link with the ADELISE project).

In this project, to address these two issues, we therefore propose to carry out two boreholes and firn air pumping for analysis of the elemental and isotopic composition in the open and closed porosity at the sites of D47 (strong katabatic wind, high accumulation, high temperature) and Little Dome C (low accumulation, low temperature). The Little Dome C site has another advantage in connection with the Beyond EPICA project. Indeed, this European project does not include firn analysis, which is essential for the interpretation of the deep ice core that will be drilled there. This analysis has therefore been planned within the framework of the DEEPICE associated project with 2 PhD students who will work on the results of this field mission. Our project will therefore also greatly improve not only the dating but also the interpretation of the greenhouse gases records on the 1.5 million year old Beyond EPICA ice core. Finally, we also wish to deploy during the 3 weeks at D47 and Little Dome C the measurement of water isotopes in water vapor in parallel with the long-term monitoring carried out at Dumont d’Urville and Concordia.