Il pleistocene medio-superiore nella carota di ghiaccio di Talos Dome (antartide orientale): riscostruzione ad alta risoluzione del record delle polveri fini atmosferiche e loro significato paleoclimatico

The study of the past global climatic variations plays a very important role in the comprehension of present climate change and in the assessment of its future development. In particular, it is important to understand the natural variability of the climatic system and in which way human activity is responsible for these variations. Paleoclimatic studies are based on information preserved in Natural Archives, sites around the world where it is possible to obtain indirect data, the so called proxy data, about climate-dependant parameters. Polar Ice Caps are one of the better natural archives both in terms of preservation and completeness. During the last two decades many drillings were performed in both Antarctic and Greenlandic ice caps, and in particular three ice cores from East Antarctic plateau have allowed to investigate a long period of past climate. These are Vostok ice core (Petit et al, 1999), Dome Fuji ice core (Goto-Azuma, 2006), and EPICA (European project for Ice Coring in Antarctica) Dome C ice core (EPICA community members, 2004). The last one covers 800,000 years BP (Before Present), and contains the oldest ice ever drilled. It is possible to collect many information from ice cores, and this work is focused on mineral dust concentration variability in the East Antarctic ice sheet, with the aim to create a very high resolution dust record. Atmospheric dust that has reached and deposited on Antarctic ice sheet from remote areas, are considered one of the best-preserved and detailed proxy data. Dust concentration, size distribution, isotopic signature and mineralogical studies give us information about climate variability and potential source areas (PSA), moreover it is possible to reconstruct paleoatmospheric circulation and back trajectories through models. In this way it is possible to contribute in increasing knowledge about coupled ocean-atmospheric system dynamics and about the important and not completely understood climate forcing role of the dust. In this work dust concentration was investigated through a new laser particle counter technique. The Laser Sensor Abakus Particle Counter (LS) operates through a 670 nm wavelength laser beam. The analysis were performed melting ice samples, from the new deep ice core drilling in Talos Dome (East Antarctica), in continuous through the Continuous Flow Analysis (CFA) system. The work was carried out in Alfred Wegner Institute for Polar and Marine Research (AWI – Bremerhaven, Germany) in the contest of a collaboration between Germany, Switzerland and Italian groups. This work is part of the TALDICE (TALos Dome ICE core) project, leaded by Italy, that involved also Germany, France, Switzerland and UK. Talos Dome (TD: 159°11’ E, 72°49’ S; 2315 m a.s.l.) is a peripheral dome at the edge of the East Antarctic plateau, 250 km from the Ross Sea and 300 km from the Southern Ocean. Dust concentration and size distribution analysis were performed also with the Beckmann Coulter Multisizer 3 (CC), a device which works through an electric principle, in a clean room located in the Environmental Science Department of Milano Bicocca University. The LS device has provided two kinds of outputs: bag mean values and high resolution data. Both signals were processed and have generated two dust records (with low and high resolution). In this work it was discussed part of the dust record that covered a period of about 60,000 years between the end of the last deglaciation (about 12,000 years BP) and Marine Isotopic Stage 4 (MIS 4, 70,000 years BP). The LS dust record was compared at first with the CC one and the good agreement between the two devices confirmed that LS is a suitable tool for this kind of studies. Than it was compared with other records from different ice cores drilled in East Antarctic plateau and on the Greenland ice sheet. In particular, the EPICA project ice cores drilled in Dome C (EDC) and Dronning Maud Land (EDML) were considered, together with Northern Hemisphere NGRIP (North Greenland Ice core project) ice core. In addition, dust records and stable isotope records (δ18O, δD) from Antarctic ice core (TD, EDC, EDML) and only δ18O record for NGRIP ice core were considered too. The comparison between TD dust record and the above-mentioned records provided interesting information about climatic condition in the TD area, atmospheric circulation and the link between climate and PSA during late Quaternary. It was noticed that TD dust record shows a good agreement with EDC and EDML ones, both for climatic variation at millennial and multimillennial scale. During Last Glacial Maximum (LGM) the dust flux in the three drilling site is comparable, but this is not the case for MIS 4, where TD shows an average lower flux value compared to LGM. This result is shown for the first time by the TD ice core. Consequently, now it is possible to suppose that dust transport mechanisms towards TD, EDC and EDML were the same during the last glacial period. Anyway, dust signal in TD is ambiguous, probably because it is the result of interaction of many factors. Likely, during LGM, very cool air masses extended above the Antarctic plateau, even in TD area, have created a subsidence condition which haven’t allowed the cyclonic perturbation to penetrate the interior of Antarctica continent. In fact, the Polar Front has moved towards lower latitude in that period, maintaining far away these “disturbances”. It is possible to hypothesize that this condition was weaker during MIS 4 and that the cyclonic perturbation have interfered with air masses coming from the plateau, and prevented dust flux towards TD. So it could be said that TD site shows a global climatic signal and it is sensitive to Southern Ocean conditions, in particular in the Indo-Pacific sector (like EDC), during LGM, instead it shows a mixed signal during MIS 4. In this period the TD area probably is sensitive partly to Southern Ocean Indo-Pacific sector, and partly to a more local Ross Sea sector condition. This work even highlights the significance of the PSA sensitivity to climate variations and to small temperatures changes during the glacial period. Many studies have proved that the main dust source area in East Antarctic plateau is the Southern South America, more precisely Patagonia (Delmonte et al., 2008a & 2004b; Basile et al., 1997), and a new work (Delmonte et al., 2009) shows that this is substantial even for TD. In fact It was noticed in TD, EDC and EDML, three clear oscillation over LGM, two of these likely due to periglacial activity related to the development of the Patagonian ice cap (Sugden et al., 2009). Through comparison with EDML δ18O, it has been possible to identify some Antarctic Isotopic Maximum (AIM, isotope peaks along the record) that correspond to dust minimum in TD dust record. AIMs have a correspondence with another kind of fast oscillation in the Northern Hemisphere, called Dansgaard/Oeschger events. These are abrupt warm events, more or less 2,000 years long and followed by less rapid cooling. EDML δ18O profile shows that AIMs are in antiphase with the D/O event, and data from TD ice core confirm this behaviour. This difference between Northern and Southern Hemisphere is explained by a recent theory called “Bipolar Seesaw”, that tries to understand the mechanisms with which the two Hemisphere were alternatively warmed up and cooled down. Although some works (Stenni et al., submitted; Barker et al., 2009; Lamy et al., 2007; EPICA community members, 2006; Stocker et al., 2003; Blunier et al., 2001 e 1998) tried to understand the dynamics involved in the Bipolar Seesaw theory, many uncertainties and doubts must be solved and the new results obtained by TD ice core have given a very important contribution for a better comprehension of this climatic trend. At least it is possible to assert that the new Talos Dome ice core results obtained in this work provided: - a confirmation of dust validity as climatic variability indicator; - an important contribution on next models studies and on a better understanding of coupled ocean-atmospheric circulation dynamics and their climatic pattern; - a confirmation of previous studies obtained by other ice cores from different drilling sites; - new information that will give an important motivation to improve these kind of studies, in particular on new coastal deep ice cores. The future work and perspective will be the conclusion of high resolution data processing, focusing mainly on the Holocene period. In particular analysis on the first 70 m of TD ice core will be carry out, with the aim to reconstruct the last 2,000 years of climate history. With the same goal, some shallow firn cores that were drilled along a traverse from TD to Dome C and from TD to Dumont d’Urville (East Antarctic coastal site on Pacific Sector and where is located the French Antarctic Station) in the frame of ITASE (International Trans Antarctic Scientific Expedition) project during 1998 and 2001 drilling campaign, will be analyse. This studies represent a real challenge for the future because, in the contest of a proved global warming, it will be essential to understand how and in which proportion natural variability and human activities are responsible of this change.