Headlamp on the head, the researcher Dominique Genty rushes several times a year, since 1992, in the kilometers of underground galleries of Villars, in the Dordogne, in the south-west of France to decipher the evolution of the climate. .Under a metal platform allowing tourists to admire the silhouettes of horses drawn 20,000 years ago or the magical spectacle of thousands of stalagmites, stalactites and other limestone veil flows, the Perigord paleoclimatologist shows two holes drilled during his research on “speleothems”, these mineral deposits of the underground environment. The cave contains invaluable information: the oxygen present in the infiltrated rainwater, accumulated and dissolved underground to form, over the millennia, limestone concretions , and carbon, from the succession of plants above the cave. By fixing the two elements, these stalagmites have “recorded” the climate of the past. “Their variation is linked to the abundance or not of vegetation at the above the cave, and as the presence of vegetation on the surface is directly linked to the climate, these elements inform us about its evolution”, summarizes this research director at the CNRS. engineer, the researcher thus explores European and North African caves to collect stalagmites, veritable “climatic archives”. Only the already broken concretions are cut with a diamond saw so as not to “destroy the aesthetics” of the places, close that observed by the first h**o sapiens.In his laboratory in Bordeaux, armed with a dental drill, the scientist then “samples” the calcite dust on the stalagmites collected. He inserts it into a mass spectrometer to measure the abundance of carbon isotopes and decipher “the climatic signal”. A similar tool, measuring uranium and thorium, makes it possible to date the sample going back up to 500,000 years. evolution of the local monsoons for 640,000 years.- Nuclear tests -In Villars, the chronological analysis of the carbon 14 (C14) content – a radioactive isotope of carbon – of the stalagmites made it possible to detect the impact of the peak of the nuclear tests carried out in the world during the Cold War.”The tests carried out at that time released a lot of C14 into the atmosphere”, which then infiltrated into living things, then, via rainwater, into underground stalagmites, according to the researchers. The peak of C14 measured in other caves of France, Slovenia and Belgium, intervenes each time in shift of several years after 1963, date of the treaty of Moscow which put an end to the nuclear tests in the atmosphere. This discovery “proves” that most of the carbon taken from the stalagmites was indeed the one previously present in the atmosphere and the vegetation, and serves as a “tracer” to better know the time of infiltration of water and carbon between the surface and the cave. It has made it possible to accredit the discipline, now in full swing with dozens of laboratories in Austria, Germany, France, United Kingdom, Australia, United States or China. With a longer dating, localized data, and a low shipping cost, research on cave “speleothems” completes the analysis of ice or marine cores, other memories of the climate, taken from the poles and the oceans.- Brutal changes -It reconstructs the major cycles in the history of the climate, between the glacial and interglacial periods, generated by the evolution of the parameters of the Earth’s orbit, and detects the abrupt variations within these cycles. “Technological progress” will allow also soon to “estimate the average temperatures” of distant times, bets Mr. Genty, by modeling a stalagmite in the cave in 3D, with a consumer application on his smartphone. To assess the current warming linked to human activity , the researcher has installed underground sensors since 1993, in order to measure the evolution of temperatures, water flow or CO2 content.35 meters underground, in an ultra-stable environment, the scientific duo updates the temperatures recorded on a laptop computer: 12.2°C against 11.1°C thirty years ago. An “enormous” increase in such a short time.”We have already experienced brutal changes” in the cycles of the past but “never such a rapid warming in an interglacial period”, as currently, observes Mr.
