La dynamique du carbone inorganique dans le continuum sol-épikarst-cavité du site de la Grotte de Lascaux (Dordogne, France) : apports des monitorings hydrogéochimique et microclimatique continus pour l’étude de l’aérologie et le développement d’une méthode de simulation des processus calco-carboniques aux parois

Abstract : Since its invention in 1940 but especially as a result of its closure to the public in 1963, the preservation of the Cave of Lascaux bases itself among others on the understanding of its interactions with the surrounding karstic massif in particular the processes sitting in the épikarst and the zone of superficial transmission. That is why this thesis research attempted to understand the dynamics of the CO2 in the continuum soil-epikarst-cave to estimate the potential impacts on the preservation of walls. We benefit in Lascaux of an observation window on the flows resulting from the epikarst known emergence in the SAS 1 of the cavity, but also the consequent instrumentation. It allows the acquisition of numerous time series data of the microclimatic parameters, carbon dioxide partial pressures at different points of the cave and the discharge of the epikarstic emergence since the beginning of the century.A first part of the study is dedicated to the characterization of the dynamics of the CO2 in the context of an epikarst under soil cover. To this end, an experimental parcel is instrumented to follow the hydroclimatic parameters and the contents in PCO2 at various depths. Periods of recharge (accumulation) and draining (emanations towards the atmosphere) of the superficial epikarst CO2 are highlighted when the constitution of a low variable CO2 stock is observed in the subsuperficiel epikarst. The understanding of these various mechanisms ends in a conceptual scheme of the CO2 dynamics in the epikarst.In a second part, this dynamic is studied in the Cave of Lascaux from the temporal series of the microclimatic parameters and the contents in CO2, but also the δ13CCO2. It is then demonstrated that the flows of CO2 entering the cavity result from three different origins: the atmosphere (entrance), the superficial epikarst (Mondmilch Gallery and Silted-up Rooms) and the massif (screw of the Shaft of the Sorcerer). At the same time, two aerological regimes responsible for the spatiotemporal distribution of the PCO2 in the cavity are observed: stratification and thermoconvection. They are the main mechanisms responsible for the dynamics of the CO2 in the Cave of Lascaux because of the low exchanges with the atmosphere. Finally, the impact of the pumping of the air on the aerology and the dynamics of the CO2. The comparison of these dynamics with and without extraction of the air of the cavity leads to the creation of conceptual schemes of the dynamics of the CO2 in the Cave Lascaux.The study of the flowing conditions in the epikarst of the Cave of Lascaux, the third part of these works, was made from a continuous monitoring of the discharge, physico-chemical parameters and the natural fluorescence of the water. The analysis of the temporal series of these natural tracers leads to characterize in a detailed way the flowing conditions and the importance of the moisture content of the epikarst on the size of the recharge area and the types of water arriving at the emergence. In parallel, the impact of these conditions on the calco-carbonic balances of waters arriving in the cavity is analyzed.Finally, the acquired knowledge are applied to determine the potential continuous impact of the waters (condensation and exfiltration) present at the adorned walls of the cave. To this end, a methodology of estimation of the mass of calcite potentially precipitated by exfiltration and dissolved by condensation based on hydrogeochemical simulations is developed. Its application to the left wall of the Hall of the Bulls with and without pumping leads to the evaluation of the potential impact of this device but also the aerology of the cavity on the preservation of walls.
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Nicolas Houillon. La dynamique du carbone inorganique dans le continuum sol-épikarst-cavité du site de la Grotte de Lascaux (Dordogne, France) : apports des monitorings hydrogéochimique et microclimatique continus pour l’étude de l’aérologie et le développement d’une méthode de simulation des processus calco-carboniques aux parois. Sciences de la Terre. Université de Bordeaux, 2016. Français. ⟨NNT : 2016BORD0441⟩. ⟨tel-01467170⟩

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