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Pierre Nevers thesis defense

Pierre Nevers is pleased to invite you to his thesis defense entitled, Hydrogeological functioning and transfer processes in the unstable fissured massif of Séchilienne (French Alps): contributions of the coupling of natural and isotopic tracers. The defense will take place in restricted committee, on Friday 28 May at 2 pm, in the conference room of the Observatoire des Sciences de l’Univers (OSU THETA), 41 bis avenue de l’Observatoire, 25000 Besançon.

It will however be broadcasted by videoconference on Zoom to allow as many people as possible to attend.
Connection link: https://bit.ly/3p3HkAN
Meeting ID : 984 6172 3151
Secret code : 558848

Jury composition
Rapporteur : Christelle Guilhe-Batiot, University of Montpellier
Rapporteur: Christian France-Lanord, DR CRPG-CNRS Nancy
Examiner: Hélène Celle-Jeanton, Prof. University of Franche-Comté
Examiner: Luc Aquilina, Prof. University of Rennes 1
Dissertation Director : Catherine Bertrand, University of Franche-Comté
Co-supervisor : Jérôme Gaillardet, Institut de Physique du Globe de Paris

Summary
Gravity hazards are at the origin of major disasters that can cause material and human damage, they are also one of the main mechanisms of denudation of mountain ranges. It is now clearly established that there is a link between gravity instability and the water it receives. Understanding the functioning of aquifers in unstable environments is of fundamental importance in the study of these complex structures whose destabilisation involves considerable stakes. It is in this spirit that this thesis focuses on the relationship between the unstable environment and the underground flows of the Séchilienne slope (Isère, France). This work aims to characterise the hydrogeological functioning of the unstable environment aquifer through the analysis of two outlets within the instability. The use of hydrogeological data collected over the long term allows a temporal analysis providing good information on the functioning, dynamism and recharge of the aquifer on several scales ranging from the hydrological cycle to the year. Correlational analyses are proving to be a good tool for understanding the hydrodynamic functioning of the aquifer. The contribution of water chemistry is essential for identifying the hydrochemical signatures characterising the slope. It also provides information on the state of alteration of the slope, as the chemical composition of the water depends on water-rock interactions. The comparative analysis of the two outlets within the instability coupled with the analysis of an outlet in the stable zone reflects the heterogeneity of unstable and fractured environments. The second part of this work focuses on the coupling between isotopic tracers and the chemistry of major elements, in order to specify the amplitude of the chemical alteration reactions of the mineral phases induced by the water-rock interactions within the slope. The use of a mixing model allows to attribute the different major elements to different sources and to quantify the involvement of sulphuric and carbonic acids as a source of protons. The results show that instability creates favourable and sustainable conditions within the fracture, by opening new fractures providing fresh and reactive surfaces allowing the production of sulphuric acid by oxidation of pyrite. The results obtained also make it possible to refine the pre-existing hydrogeological model by highlighting the contribution of gypsum dissolution to the sulphate budget of the waters. The originality of this work also lies in our results which show the behaviour of the Séchilienne landslide which, despite its role in accelerating the chemical and physical alteration of rocks, acts on a geological time scale (i.e. on time scales longer than carbonate precipitation in the ocean) as a source of CO2 in the atmosphere.

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