Assessing cave internal aerology in understanding carbon dioxide (CO2) dynamics: implications on calcite mass variation on the wall of Lascaux Cave (France)
Résumé
Carbon dioxide gas is a key component in dissolution and precipitation of carbonates in karst and cave systems. Therefore, characterizing the internal aerology of a cave is essential to obtain the spatiotemporal distribution of temperature and CO2 level. In this research, Lascaux Cave (France), an important adorned cavity, was studied. First, the spatiotemporal distribution of CO2 and temperatures were examined using continuous monitoring at a per minute basis. High-resolution spatial measurements (14 PCO2 locations and 27 locations for temperature) were carried out for a year in the epikarst and the cave (February 2015 to February 2016). The spatiotemporal analysis presents that air and rock temperatures vary for less than a degree Celsius (12.4–12.9 °C). These are controlled by the conduction of the external thermal waves through the overlying calcarenite massif. As a consequence, two seasonal internal aerologic regimes were identified: stratification and convection. These regimes govern the spatiotemporal distribution of the CO2 levels (1.1–3.7%), showing that this parameter is a good natural marker of the internal air movements. Second, a method was proposed to estimate the calcite mass potentially affected by condensation water (dissolution process) and exfiltration water (precipitation process). This method, based on numerical simulations, relies on CO2 and air and rock temperature spatiotemporal distributions in the cave. Third, the method was applied using the case of the left wall of the Hall of the Bulls (one of the most adorned part of the cave). Results showed that the calcite mass, possibly dissolved, varies from 0.0002 to 0.006 g when the mass potentially precipitated is higher (from 0.013 to 0.067 g) depending on the aerologic conditions. This method allows determining which alteration process (e.g., precipitation or dissolution) could eventually lead to the largest variation of calcite on the wall. The results can serve as useful data to the cave experts of the French Ministry of Culture and Communication in terms of Lascaux Cave management policies.