Under a metal platform allowing tourists to admire 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 rainwater infiltrated, accumulated and dissolved underground to form, over the millennia, limestone concretions, and the carbon, resulting from the succession of plants above the cave.
By fixing the two elements, these stalagmites "recorded" the climate of the past.
Paleoclimatologist Dominique Genty in the underground galleries of Villars, May 23, 2023 in Dordogne © MEHDI FEDOUACH / AFP
"Their variation is related to the abundance or not of vegetation above the cave, and as the presence of vegetation on the surface is directly related to climate, these elements inform us about its evolution," summarizes this CNRS research director.
With Ludovic Devaux, a former French Navy diver who is now an assistant engineer, the researcher explores European and North African caves to collect stalagmites, veritable "climate archives".
Only the concretions already broken are cut with a diamond saw so as not to "destroy the aesthetics" of the place, close to that observed by the first homo sapiens.
In his Bordeaux laboratory, armed with a dentist's strawberry, the scientist then "samples" the calcite dust on the harvested stalagmites.
He inserts them into a mass spectrometer to measure the abundance of carbon isotopes and decipher "the climate signal".
Paleoclimatologist Dominique Genty in the underground galleries of Villars, May 23, 2023 in Dordogne © MEHDI FEDOUACH / AFP
A similar tool, measuring uranium and thorium, can date the sample back to 500,000 years.
In China, a researcher has even traced the evolution of local monsoons over 640,000 years.
Nuclear testing
In Villars, the chronological analysis of the contents of carbon-14 (C14) - a radioactive isotope of carbon - of stalagmites made it possible to detect the impact of the peak of nuclear tests conducted around the world during the Cold War.
"The tests conducted at that time released a lot of C14 into the atmosphere," which then infiltrated the living, then, via rainwater, into underground stalagmites, according to the researchers.
Paleoclimatologist Dominique Genty in the underground galleries of Villars, May 23, 2023 in Dordogne © MEHDI FEDOUACH / AFP
The peak of C14 measured in other caves in France, Slovenia and Belgium, occurs each time several years behind after 1963, date of the Moscow Treaty that put an end to nuclear tests in the atmosphere.
This discovery "proves" that most of the carbon from stalagmites was that previously present in the atmosphere and vegetation, and serves as a "tracer" to better understand the infiltration time of water and carbon between the surface and the cave.
It has made it possible to accredit the discipline, now booming with dozens of laboratories in Austria, Germany, France, the United Kingdom, Australia, the United States and China.
With longer dating, localized data, and low shipping costs, research on cave "speleothems" complements the analysis of ice or marine cores, other memories of climate, taken from the poles and oceans.
Paleoclimatologist Dominique Genty in the underground galleries of Villars, May 23, 2023 in Dordogne © MEHDI FEDOUACH / AFP
Abrupt changes
It reconstructs the major cycles of climate history, between ice ages and interglacials, generated by the evolution of the parameters of the Earth's orbit, and detects abrupt variations within these cycles.
"Technological progress" will also soon make it possible to "estimate the average temperatures" of distant eras, bets Mr. Genty, by modeling in 3D a stalagmite of the cave, with a consumer application of his smartphone.
Paleoclimatologist Dominique Genty in the underground galleries of Villars, May 23, 2023 in Dordogne © MEHDI FEDOUACH / AFP
To assess the current warming linked to human activity, the researcher has installed underground sensors since 1993 to measure changes in temperatures, water flow or CO2 content.
35 meters underground, in an ultra-stable environment, the duo of scientists updates, on a laptop, the temperatures recorded: 12.2 ° C against 11.1 ° C thirty years ago. A "huge" increase in such a short time.
"We have already experienced abrupt changes" in past cycles but "never such rapid warming in an interglacial period," as now, Genty said.
© 2023 AFP