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

Pierre Nevers,Catherine Bertrand,Jérôme Gaillardet,Delphine Charpentier,Laëticia Faure,Julien Bouchez,Christophe Thomazo

doi:10.5194/esurf-9-487-2021

Abstract

Abstract. This study makes use of a highly instrumented active landslide observatory (9 years of data) in the French Alps, the Séchilienne slope. Here, we use 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 are used to characterize weathering processes and the origin of waters and their flow paths through the massif. A mixing model allows us to allocate the different major elements to different sources, to identify secondary carbonate formation as a major process affecting solutes in the subsurface waters of the instability, and to quantify the involvement of sulfuric and carbonic acids as a source of protons. We show that the instability creates favorable and sustained conditions for the production of sulfuric acid by pyrite oxidation, by opening new fractures and supplying fresh reactive surfaces. We clearly identify the contribution of the dissolution of each mineral phase to the chemistry of the waters, with a clear role of remote gypsum dissolution to the sulfate budget in the sampled waters. We are also able to refine the preexisting hydrogeological views on the local water circulation and water flow paths in the instability by 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 as a geological source of CO2 to the atmosphere. If generalizable to other large instabilities in mountain ranges, this study illustrates the complex coupling between physical and chemical erosion and their impact on the carbon cycle and global climate. The study also highlights the importance of distinguishing between sulfite oxidation and gypsum dissolution as a source of sulfate ions to rivers, particularly in mountain ranges.

Highlights

The weathering of rocks plays a key role in the chemical and climatic evolution of the Earth surface and is one of the geological processes that impacts atmospheric CO2 concentration
Because weathering by sulfuric acid is mainly limited by the supply of sulfide minerals to the Earth surface, it is prominent in active mountain belts characterized by high erosion rates (Calmels et al, 2007; Torres et al, 2016; Blattmann et al, 2019)
The two limestone samples show a distinctive response to the leaching procedure, with most of the Ca of the “Laffrey” limestone located in the acetic acid leachate (67 %) and the rest in the HCl fraction (11 %), which is indicative of the calcitic nature of this rock sample (Table 1)

Summary

Introduction

The weathering of rocks plays a key role in the chemical and climatic evolution of the Earth surface and is one of the geological processes that impacts atmospheric CO2 concentration. Because weathering by sulfuric acid is mainly limited by the supply of sulfide minerals to the Earth surface, it is prominent in active mountain belts characterized by high erosion rates (Calmels et al, 2007; Torres et al, 2016; Blattmann et al, 2019). Within these tectonically active environments, landslides are likely to be hotspots of sulfuric acid production, carbonate weathering and CO2 release (Emberson et al, 2015, 2018). Slope instability leads to sustained grain comminution and fractures opening, thereby providing a continuous supply of contact surfaces between water, air and minerals that can, in particular, allow for sulfuric acid production and carbonate mineral weathering (Binet et al, 2009; Bertrand et al, 2015)

Results

Discussion

Conclusion