a study to probe the depths of the Orx marsh, a potential carbon sink

A small blood sample right in the heart of the north ventricle of the Orx Swamp. From October 28 to 30, 2024, a small cluster of humans gathered around a drilling programmed as part of a study of paleoecology, the science which links fossil organisms and their environment.

Tuesday October 29, near the two Spanish employees of the Basque group Alios, specialist in soil engineering, the site's naturalist guard, Yohann Montané, and the scientist Hervé Cubizolle wait, to the rhythm of the giant hammer blows of the corer, their next piece of marsh. There are ten in total for as many meters.

Shared knowledge

This operation in wet terrain is a step, the preamble to long hours in the office and laboratory. “The least fun begins, I'm going to bring the carrots back to Saint-Étienne and we're going to divide the work with Jean-Nicolas Haas (paleoecologist at the University of Innsbruck, in Austria, Editor's note),” plans Hervé Cubizolle, adjusting his leather hat.

This peatland specialist will also share his samples with geochemists and archaeologists from Bordeaux, because research is an area conducive to solidarity and in this sense, coring is made possible by the credits (financing from the Region) available to “a colleague medievalist (François Boutoulle, Editor’s note), interested in the project.”

The answers contained in the bowels of the Orx swamp are numerous and embrace a range of disciplines. The first corings, carried out two years ago, yielded some. “We were a little surprised because by digging four meters down, we thought we were going back a long way into the past. » This was not really the case, but it “has an advantage because it means that the sequence is very dilated”, relishes Hervé Cubizolle. The time frame is shorter, but there is therefore more precise data to exploit.

Analyzes of the first cores are still in progress. Each segment is carefully peeled, to the millimeter, from the macroremainder (large animal or plant remains) to the pollen grain. “We are still waiting for radiocarbon dating results on wood and plants. You have to sift, observe, it takes a long time. »

With this second phase of drilling, the study goes further into the soil and the past of the marsh. It touches on its sedimentary history and should, ultimately, provide information on the evolution of places over the millennia; and why not solve endemic enigmas, like the original bed of the Adour. But more than this part of the history of the marsh, the dozen cores taken raise an issue that transcends the ages. And that’s where peat, Hervé Cubizolle’s first love, comes in.

Carbon trap

“From minus 4 meters, we have proof of marine incursions, in any case a bunch of creatures that belong to the marine world. Then around 4,000 years ago (higher in the ground, Editor's note), we fell into a peat bog. For what ? We don’t know,” says the academic. However, because it is its nature, peat, which allows carbon emissions to be trapped, would have stored a certain quantity under the spongy soil of the marsh (the equivalent of 3.6 million euros, at 82 euros per tonne of CO2, according to a student of Hervé Cubizolle).

The professor explains the process: “The place, quickly saturated with water, sees the water table constantly rising, this prevents any gaseous oxygen. The micro-organisms (bacteria, fungi) which decompose organic matter cannot work. The carbon, which makes up organic matter, is not released.”

A saving function on a planetary scale, yet far too neglected: “There are only 100,000 hectares of peatlands left in France, whereas in 1850, there were ten times more”. The marsh, artificialized over the course of contemporary human uses, has not cut into it, but the path taken over nearly two centuries (1) is by no means a dead end.

Contained by drainages and a belt canal, Orx empties and fills perpetually. “In the perspective of a restoration, it will be necessary, for example, to ask ourselves which ditches should be filled as a priority. Originally, the marsh was larger than it is today; the risk is that 300 to 400 hectares around it will become marsh again. We would then enter the domain of local politics,” Hervé Cubizolle weighs.

A crazy project

“It’s a bit like science fiction,” for Yohann Montané, passionate about the carbon stock restoration aspect of the project, but aware of the effort it involves. The guide nevertheless knows that his jewel will have to adapt to climate change. “We could concentrate on the northern marsh, but in the current state, we have not done enough prospecting work to be certain,” says the rural police officer.

The idea remains attractive and the presence on Tuesday October 29, on the drilling site, of Béatrice Ducout, member of CPIE Seignanx Adour (an association certified for environmental awareness, education and training), attests to this. “It’s very interesting, particularly for our Sainte-Marie-de-Gosse peat complex. I know that the Etang Noir Nature Reserve is also interested in it. We interfere, before perhaps reproducing this work. »

These are only in the embryonic stage. Funding for the studies that will follow, and do not only focus on this carbon aspect, must still be found. “We have budgetary constraints and before thinking about the past and the future of the marsh, we have to take care of the present,” adds Yohann Montané. The site guard knows that in the swamp cores, the three times are linked.

Another chapter

The Orx marsh, property of the Conservatoire du Littoral, gives its 1,000 hectares, including 775 classified as a National Nature Reserve, a large part of its biodiversity and in particular its almost 300 listed bird species. But the current paleoecological study could give new educational impetus to the site, which is also engaged in a management plan until 2031.
“We have an idea of ​​the Napoleonic era but we can go back much further,” explains Yohann Montané. We will be able to describe the environment over several periods and propose an evolution of the landscape, with a body of knowledge where there was a void. It's a great opportunity. »

(1) A Napoleonic decree of 1808 ordered the draining of the marsh at state expense.