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Characterisation of infiltration in the cave of Villars (Dordogne) by geophysical and isotopic methods: Importance for paleo-climatic studies based on speleothems and for the conservation of heritage sites

Résumé de thèse
Caractérisation de l’infiltration dans la grotte de Villars (Dordogne) par des méthodes géophysiques et isotopiques : importance pour les études paléo-climatiques basées sur les spléléothèmes ainsi que pour la conservation des sites patrimoniaux
Jian Zhang

Notes de l’auteur

PhD co-directed by Dominique GENTY and Colette SIRIEX, defended on 26 June 2023, available online at https://theses.hal.science/tel-04210988

Texte intégral

1The geometry of the surroundings of caves and water infiltration zones, as well as the processes of rainfall infiltration in karst areas, are complex and not yet fully understood. However, this knowledge is crucial for interpreting climate reconstructions based on speleothems, for studying water recharge in karst areas, and for the conservation of prehistoric cave heritage. To deepen our understanding of this subject, we chose a typical shallow cave, the Villars Cave in southwestern France, which have been monitored for decades, and numerous studies have been conducted on the current environment and speleothems.

2The first question we addressed was the input signal of precipitation above the Villars Cave. What is the source of precipitation above the cave, and how does it vary? To investigate this, we used an atmospheric retro-trajectory model (HYSPLIT) to better understand the isotopic sources of oxygen in precipitation (δ18O) and infiltration water. The results revealed a pronounced seasonality of rainfall sources (expressed here as a percentage of initial moisture sources) related to the distance between the Villars Cave and moisture source regions, as well as long-term trends.

3The second question focused on the characterization of the infiltration and its temporal evolution from the isotopic composition of rain to the water that feeds stalagmites. We utilized a long-term isotopic dataset (>20 years) combined with a conceptual model (KarstFor) that blends water fluxes and isotopes to better understand the karstic infiltration process. Results showed that the modeled variations in water storage quantity and infiltration rate coincide with the observed drip rates beneath stalactites at two different levels within the cave. The fact that the model strikingly simulates the low but constant isotopic difference between the upper and lower galleries of the Villars Cave not only reinforces the model itself but also reveals the impact of multiple infiltration pathways and karst reservoir dynamics on the δ18O of drip-water.

4The third question focused on the geometry of the infiltration zone: water reservoirs and preferential flow paths. To address this, we applied 2D Electrical Resistivity Tomography (ERT) above the Villars Cave at intervals of one to two months over three years (February 2020 to January 2023). Using a multidimensional statistical approach (Hierarchical Agglomerative Classification, HAC), the ERT images highlighted a pronounced seasonality in resistivity variations in different infiltration areas. This allowed us to visualize specific karstic areas characterized by their resistivity and their variations, leading to the identification of cave galleries, water storage reservoirs, infiltration fronts, soil layers, and potential preferential flow paths down to 30 meters in depth.

5By combining electrical tomography, hydrology, and isotopic methods (fig. 1), we have significantly improved our understanding of water flow and variability in water flow above the Villars Cave, as well as the mixing processes in the reservoirs that supply stalactites. This knowledge is of first importance for the preservation of cave environments and for paleoclimatological research based on the study of speleothems.

Fig. 1: A figure that illustrates the interest of the combination of ERT, long-term monitoring, and modeling for paleo-climatic studies based on speleothems as well as for the conservation of heritage sites. Fig. 1 : Une figure qui illustre l’intérêt d’associer la tomographie de résistivité électrique en 2D (TRE), la surveillance à long terme des débits dans les grottes et la modélisation pour les études paléoclimatiques basées sur les spéléothèmes, ainsi que pour la conservation des sites patrimoniaux.

Fig. 1: A figure that illustrates the interest of the combination of ERT, long-term monitoring, and modeling for paleo-climatic studies based on speleothems as well as for the conservation of heritage sites. Fig. 1 : Une figure qui illustre l’intérêt d’associer la tomographie de résistivité électrique en 2D (TRE), la surveillance à long terme des débits dans les grottes et la modélisation pour les études paléoclimatiques basées sur les spéléothèmes, ainsi que pour la conservation des sites patrimoniaux.

Jian Zhang

6ZHANG J., 2023 - Caractérisation de l’infiltration dans la grotte de Villars (Dordogne) par des méthodes géophysiques et isotopiques: Importance pour les études paléo-climatiques basées sur les spéléothèmes ainsi que pour la conservation des sites patrimoniaux. Thèse de doctorat, Université de Bordeaux, 227 p.

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Titre Fig. 1: A figure that illustrates the interest of the combination of ERT, long-term monitoring, and modeling for paleo-climatic studies based on speleothems as well as for the conservation of heritage sites. Fig. 1 : Une figure qui illustre l’intérêt d’associer la tomographie de résistivité électrique en 2D (TRE), la surveillance à long terme des débits dans les grottes et la modélisation pour les études paléoclimatiques basées sur les spéléothèmes, ainsi que pour la conservation des sites patrimoniaux.
Crédits Jian Zhang
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Référence électronique

Jian Zhang, « Characterisation of infiltration in the cave of Villars (Dordogne) by geophysical and isotopic methods: Importance for paleo-climatic studies based on speleothems and for the conservation of heritage sites »Quaternaire [En ligne], vol. 35/1 | 2024, mis en ligne le 31 janvier 2024, consulté le 25 mai 2024. URL : http://journals.openedition.org/quaternaire/19045 ; DOI : https://doi.org/10.4000/quaternaire.19045

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Auteur

Jian Zhang

Environnement Paléoenvironnements Océaniques Continentaux, Université de Bordeaux, Allée Geoffroy Saint-Hilaire, Bât. B18, CS 50023 FR-33615 TALENCE, cedex. Emails: jian.zhang@u-bordeaux.fr, zj136888@email.swu.edu.cn

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