DOME A

Scientific background and objectives Ice cores drilled in polar regions give access to climate and environmental-relevant data as well as to the atmospheric greenhouse gas mixing ratios. They allow one to evaluate the climate model sensitivity to a change of greenhouse gas concentrations, and the model ability to reproduce the sequence of processes when the climate system is disturbed by changing forcings. After nearly two decades of progress in ice core studies, some key data relevant to the natural climate dynamics and the climate / carbon cycle coupling still need to be improved or acquired. Two of them provide the main background of our project : (1) what is the precise timing between greenhouse gas concentrations and Antarctic temperature changes ? (2) is CO2 partly responsible for the change of climate pacing about 1 million years ago ? Question (1) depends on the accuracy of estimates of the difference of age (Δage) between the trapped bubbles (providing the atmospheric composition) and the surrounding ice (providing the climatic signal) under glacial conditions. Based on current Δage estimates on the East Antarctic plateau, it was suggested that CO2 lagged the Antarctic warming at the start of the last deglaciation. Thus insolation changes would have provoked oceanic changes around Antarctica, which then drove the CO2 increase and its radiative feedback on the climate system. Recently, we re-investigated the EPICA/ Dome C (EDC) Δage and we suggested that firn densification models overestimated it by up to 35%. Thus the CO2 lag on Antarctic temperature would be overestimated and could have been a lead. Consequences of such finding are considerable. It requires to be confirmed or infirmed by re-evaluating the firn densification process (and thus Δage) under more extreme conditions than today at the deep drilling sites of the Antarctic plateau. There are probably no exact analog of last glacial conditions at the surface of Antarctica today. But one site gets very close to them : Dome Argus (Dome A). Question (2) is the first priority of the IPY project IPICS sponsored by IGBP/PAGES. To determine the role of CO2 in the change of climate pacing about 1 million years ago means to extract a new ice core reaching back to 1.5 million years. Current information suggests that the sector of Dome A could well be a primary target. Description of project, methodology, results Dome A is the highest site of the Antarctic ice sheet. During the season 2004/05, the Chinese glaciologists have reached the site for the first time in history. With the coldest mean annual temperature in the world (-58.5°C) and an accumulation rate of less than 1.5 cm H2O/yr, it offers a very close analog to EDC glacial conditions, thus giving the unique possibility to investigate the densification process of firn under colder and drier conditions than today at EDC. In the frame of the International Polar Year (IPY), we will collaborate with Chinese, Japanese and British glaciologists to drill a firn core at Dome A in 2008/09. We will retrograde firn samples to Dome Concordia (which has a facility for firn microstructure studies), using airborne logistics, to avoid any thermal or mechanical stress on the samples. In 2009/10, physical properties of the samples will be measured at Concordia. In 2008/09, we will also conduct a firn air sampling down to the close-off depth at Dome A, and measure the permanent gas isotopes on the air samples. It will provide us with key constraints on the densification process under extreme conditions, which has been modelled for the past but not yet tested against such observations. To confirm the potential of Dome A for accessing to the oldest ice in Antarctica, we will carry on an intermediate drilling down to ~600 m during the same season 2008/09. Covering the last deglaciation and Marine Isotopic Stage 3, it will document the amplitude of glacial-interglacial accumulation change in this Antarctic region and allow extrapolation of the possible bottom age in the vicinity of Dome A. In addition, environmental changes in this sector of Antarctica will be investigated for the first time through a multi-parametric analysis of the ice core over the 3 years following the drilling. The DOME A project will build on the bilateral collaboration that we developed with Chinese glaciologists over the last 20 years ; it will provide an unique opportunity for French and European glaciologists to collaborate with Chinese scientists for the search for the oldest ice in Antarctica. It will make an optimal glaciological use of the Dome Concordia station. The success of this project through a support from the ANR is particularly important for the long-term French glaciological activities in Antarctica.