The cave contains invaluable information, and constitutes a true “climate archive”, enabling environmental changes to be tracked over thousands of years.
Since 1992, the researcher Dominique Genti, wearing a lamp on his head several times a year, has been visiting caves that extend for kilometers underground in Villars in the French Dordogne region, to decipher climate changes.
At the bottom of a metal platform that allows tourists to witness the shadows of horses painted 20,000 years ago, or the magical view of thousands of cave stalagmites and stalagmites, the Périgord paleontologist shows two holes drilled while searching for mineral deposits from the subterranean environment.
The cave contains invaluable information: the oxygen present in the leaking rainwater accumulated and dissolved underground to form, over thousands of years, layers of limestone and carbon, resulting from the succession of plants above the cave.
By identifying these two elements, these stalagmites “recorded” the climate of the past.
This research director at the National Center for Scientific Research in France indicates that “its changes are related to the abundance of vegetation over the cave or its absence, and since the presence of vegetation on the surface is directly related to the climate, these elements tell us about its development.”
With Ludovic Defoe, a former diver in the French Navy and now an assistant engineer, the researcher explores caves in Europe and North Africa to collect stalagmites that form veritable “climate archives”.
Only pieces of concrete already broken are cut with a diamond saw so as not to “ruin the aesthetics” of the place, near sites where the first Homo sapiens lived.
In his laboratory in Bordeaux, the researcher then “samples” calcite dust collected on the stalagmites, then feeds it into a mass spectrometer to measure the abundance of carbon isotopes and decode the “climate signal”.
A similar tool for measuring uranium and thorium can date the sample up to 500,000 years old. In China, a researcher has successfully tracked the evolution of the local monsoons over 640,000 years.
Climate memory preservation
At Villar, chronological analysis of the contents of carbon-14, a radioactive isotope of carbon, in stalagmites made it possible to detect the influence of the heyday of nuclear testing conducted in the world during the Cold War.
According to the researchers, “the tests conducted at that time released large amounts of carbon-14 into the atmosphere,” which then infiltrated living organisms, and then through rainwater, into the underground stalagmites.
The carbon-14 peaks, detected in other caves in France, Slovenia and Belgium, occur every year after 1963, the date of the Moscow treaty that ended nuclear testing in the atmosphere.
This discovery “proves” that most of the carbon from the stalagmites was already present in the atmosphere and vegetation, and serves as a “tracker” to better see when water and carbon seeped between the surface and the cave.
It has made it possible to accredit this specialty, which is now flourishing, with dozens of laboratories in Austria, Germany, France, the United Kingdom, Australia, the United States or China.
With longer dating capabilities, spot data, and lower shipping costs, this cave research complements analysis of sea ice or cores, which are other memory-memory components of climate drawn from the poles and oceans.
Big temperature changes
This research re-shows the major cycles in the history of climate, between glacial and interglacial periods, resulting from the evolution of Earth’s orbital parameters, and also reveals abrupt changes within these cycles.
“Technological progress” will soon also allow “estimating average temperatures” in distant times, according to Jannati, by modeling cave stalagmites in three dimensions, using an application on a smartphone.
To assess the current warming associated with human activity, the researcher has installed underground sensors since 1993 to measure changes in temperature, water flow, or carbon dioxide content.
At a depth of 35 meters underground, in an ultra-stable environment, the two researchers update the temperatures recorded on a laptop computer: 12.2°C versus 11.1°C thirty years ago, a “huge” increase in such a short time.
“We’ve already seen strong changes” in past cycles, but “no such rapid warming has occurred in the interglacial period,” says Jannati.