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. This study makes use of a highly instrumented activelandslide observatory (9 years of data) in the French Alps, theSéchilienne slope. Here, we use a combination of major element chemistryand isotopes ratios (87Sr / 86Sr, δ34S) measured indifferent water types of the stable and unstable part of the Séchilienneinstability to assess the contribution of the different lithologies of theslope and the chemical weathering mechanisms. Chemical and isotopic ratiosare used to characterize weathering processes and the origin of waters andtheir flow paths through the massif. A mixing model allows us to allocate thedifferent major elements to different sources, to identify secondarycarbonate formation as a major process affecting solutes in the subsurfacewaters of the instability, and to quantify the involvement of sulfuric andcarbonic acids as a source of protons. We show that the instability creates favorable and sustained conditions forthe production of sulfuric acid by pyrite oxidation, by opening newfractures and supplying fresh reactive surfaces. We clearly identify thecontribution of the dissolution of each mineral phase to the chemistry ofthe waters, with a clear role of remote gypsum dissolution to the sulfatebudget in the sampled waters. We are also able to refine the preexistinghydrogeological views on the local water circulation and water flow paths inthe instability by showing the hydrological connectivity of the differentzones. Overall, our results show that the Séchilienne landslide, despiteits role in accelerating rock chemical and physical weathering, acts as ageological source of CO2 to the atmosphere. If generalizable to otherlarge instabilities in mountain ranges, this study illustrates the complexcoupling between physical and chemical erosion and their impact on thecarbon cycle and global climate. The study also highlights the importance ofdistinguishing between sulfite oxidation and gypsum dissolution as a sourceof sulfate ions to rivers, particularly in mountain ranges.

earth surface dynamics

Landslides as geological hotspots of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; emission: clues from the instrumented Séchilienne landslide, western European Alps