2023-10-16 04:00:10
Study of radiocarbon levels in ancient trees located on the eroded banks of the Drouzet, near Gap (Hautes-Alpes). CÉCILE MIRAMONT/COVER IMAGESCOV/SIPA
Trees keep the chronicle of the past. And researchers can read it in two ways. The first, known as “dendrochronology”, consists of counting and studying the rings, these rings of wood which form each year at the periphery of the tree. We can thus admire, in the botanical gallery of the National Museum of Natural History, in Paris, the section of a giant sequoia some two thousand years old, on which labels go back in time to the birth of Christ.
The second way of traveling through the past through wood relies on the carbon 14 present in each ring. It is by combining the two methods on the remains of very old trees that a Franco-British team has just made an astonishing discovery, published on October 9 in review Philosophical Transactions of the Royal Society A : an exceptional solar storm, previously undetected, occurred 14,300 years ago.
To understand the link between multi-millennial terrestrial strains and the bloodshed of our star, located 150 million kilometers away, we must delve into carbon 14. This radioactive isotope of carbon is present in an almost stable manner in the atmosphere: in fact, its progressive disintegration (which explains why we cannot use it to date archaeological objects beyond 55,000 years) is constantly compensated by the formation of new atoms, thanks to cosmic rays which reach the Earth constantly. At the end of a cascade of reactions, nitrogen atoms see a neutron take the place of a proton in their nucleus, which transmutes them into carbon-14 atoms.
A colossal quantity of ejected material
So much for the daily life of 14C, as atomic physics specialists write. However, it happens that the Sun enters the dance. During one of its bursts – eruption, coronal mass ejection – myriads of electrically charged particles are propelled into space. When they reach the Earth, they form a surplus of carbon 14 in the atmosphere. And if the quantity of material ejected is colossal, it is possible that this excess 14C is so abundant that it can be detected in wood. A phenomenon highlighted in 2012 by Japanese researcher Fusa Miyake, who, by studying Japanese cedars, discovered a carbon-14 peak for the year 774, attributed to a solar event.
History repeats itself today, but with incomparably older Scots pines, since they are even described as“subfossil trees”. As explained by Edouard Bard, professor at the Collège de France and first author of the study published in Philosophical Transactions of the Royal Society A, “We call them “subfossils” because, without being rock, they have begun a small transformation.” For the study, the European Center for Research and Teaching in Environmental Geosciences, where Edouard Bard works, relied on the collection of these strains carried out for a quarter of a century, in the south of the Alps between Grenoble and Sisteron, the Mediterranean Institute of Biodiversity and Marine and Continental Ecology. Buried in thick alluvium of the Drouzet, a local river, 172 subfossil trees were recovered and studied.
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