Article révisé par les pairs
Résumé : The EPICA Dome C ice core (EDC) reached a final depth of 3260m, at a maximum height of about 15m above the ice-bedrock interface in December 2004. We present here data gained from a detailed investigation of selected samples of the deeper part of the core located below 3200m and referred to as bottom ice. This part of the core has been poorly investigated so far mainly because there are significant challenges in interpreting paleo-records that were very likely modified by long term in situ processes. Our study combines high resolution ion chromatography, high resolution synchrotron X-Ray micro-fluorescence (micro XRF), scanning, and transmission electron microscopy. Our aim was to identify the long term physico-chemical processes at work close to the bedrock, to determine how they have altered the initial registers, and, ultimately to extract information on the very ancient Antarctic environment.The ubiquitous presence of nanometer iron oxide crystals at the surface of wind-borne dust aggregates containing also large amount of organic matter raises the possibility that the consolidation of windborne dust clusters formed during ice recrystallization could be related to microbial iron reduction and, thus, to the progressive reactivation of dormant bacterial activity in warming ice. Inclusions of size and number density increasing with depth observed in the 12 last meters (3248-3260m) contain liquid and solid species, among them marine biogenic acids, numerous wind-borne dust aggregates and clusters of large reversible calcium carbonate particles precipitated once the inclusion was formed and often covered by secondary gypsum. The refreezing of slush lenses is discussed as a potential cause of the formation of such heterogeneous and complex mixtures. In addition to the very fine micrometer size minerals windborne from extra-Antarctic continental sources and often accreted in large aggregates, single medium size particles (a few to ca 20μm and among them organic debris) are commonly encountered. Their size, surface shape, and mineralogy suggest that aerosol transport from Antarctic ice-free areas played a significant role at the time EDC bottom ice was formed. Concentrations and concentration ratios of biogenic sulfur species also advocate for the strengthening of peri-Antarctic meteorological patterns that favor the inland penetration of disturbed flow carrying local material. Very large well preserved mineral particles several tens of micrometers in diameter, and biotope relics in deeper ice close to 3260m likely come from the sub-glacial environment.© 2013 Elsevier Ltd.