The CO₂ Dynamics in the Continuum Atmosphere-Soil-Epikarst and Its Impact on the Karstification Potential of Water: A Case Study of the Lascaux Cave Site (Montignac, France)

Abstract

The aim of this work was to understand the dynamics of CO₂ in the atmosphere-soil-epikarst continuum of the Lascaux Cave site and to evaluate its impact on the hydrogeochemical signal of epikarstic waters. For these purposes, PCO₂, $\updelta^{13} {\text{C}}_{{{\text{CO}}_{2} }}$ and associated hydroclimatic parameters were recorded at different depths in soil and epikarst. In parallel, hydrogeochemistry of an epikarstic emergence (SAS1 spring) located in the entrance of the cave (6 m depth) was studied. PCO₂ recorded in the soils underlines the seasonality and different levels of biogenic production according to the pedological facies and the physicochemical properties (higher in the calcisol than in the brunisol). CO₂ efflux measurements show a seasonal evolution with maximum value (25 g m⁻² d⁻¹) in summer and minimum in winter (6 g m⁻² d⁻¹). It decreases during rain events because of the soil moisture increases which limits CO₂ diffusivity. In the same way, seasonal processes of CO₂ accumulation (winter and spring) and CO₂ emission to the atmosphere (summer and autumn) are observed under the effect of soil moisture seasonal variations. These processes explain the very variable CO₂ concentrations (0.6–6%) and $\updelta^{13} {\text{C}}_{{{\text{CO}}_{2} }}$ values (−22.1 to −24.5‰) in the superficial epikarst (0–3 m depth). This CO₂ accumulation explains the presence of higher CO₂ contents in the epikarst than in the main production area: the soil. At deeper level exists a differentiated CO₂ stock slightly variable (4–6%) with stable $\updelta^{13} {\text{C}}_{{{\text{CO}}_{2} }}$ values from −22.7 to −21.7‰. Inter-annual variability of CO₂ contents is influenced by the synchronicity or not of intense production and high soil moisture. A conceptual scheme of CO₂ cycle is produced as a conclusion. The comparison with the evolution of water chemistry at the epikarstic emergence brings elements for understanding the acquisition of the karstification potential. Indeed, the seasonal evolution of the calcic carbonated mineralization of water is very well explained by the evolution of the CO₂ contents in the epikarst (PCO₂ at saturation with respect to calcite similar to the PCO₂ of the air of the epikarst). In addition, the presence of two different CO₂ compartments in the epikarst makes it possible to explain the variability of the chemical marking of water at the outlet depending on the flow conditions (transmissible and capacitive zones).