Landslides as geological hotspots of CO₂ to the atmosphere: clues from the instrumented Séchilienne landslide, Western European Alps

Abstract. This study makes use of a highly instrumented active landslide observatory (9 years of data) in the French Alps, the Séchilienne slope. Using a combination of major element chemistry and isotopes ratios (⁸⁷Sr / ⁸⁶Sr, δ³⁴S) measured in different water types of the stable and unstable part of the Séchilienne instability to assess the contribution of the different lithologies of the slope and the chemical weathering mechanisms. Chemical and isotopic ratios appear useful to characterize weathering processes and the origin of waters and their flowpaths through the massif. A mixing model allows us to allocate the different major elements to different sources and quantify the involvement sulfuric and carbonic acids as a source of protons.

As a consequence of the model, we are able to show that the instability creates favorable and sustained conditions for the production of sulfuric acid by pyrite oxidation by supplying reactive surfaces. We clearly identify the contribution of gypsum dissolution to the sulfate budget in the landslide. We are also able to refine the pre-existing hydrogeological views on the local water circulation and water flow paths in the instability but showing the hydrological connectivity of the different zones. Overall, our results show that the Séchilienne landslide, despite its role in accelerating rock chemical and physical weathering, acts, at a geological time scale (i.e. at timescales longer that carbonate precipitation in the ocean) as a source of CO₂ to the atmosphere. If generalizable to other instable zones in mountain ranges, this study illustrates the complex coupling between physical and chemical erosion and climate. The study also highlights the importance of deciphering between sulfite oxidation and gypsum dissolution as a source of sulfate ions to rivers, particularly in mountain ranges.