Implications of daily thermal variations in the ventilation and indoor CO2 levels in the karstic system of the Altamira Cave

Abstract

Within the Preventive Conservation Plan framework established for the Cave of Altamira, the continuous monitoring of environmental variables constitutes one of the focal points for preserving the parietal art contained therein. This paper presents new aspects of the ventilation of the Cave of Altamira derived from detecting short-period thermal fluctuations observed in different areas inside the cave. For 2021 and 2022, there are periods with daily air temperature oscillations, mainly in the summer and early autumn, corresponding to day/night thermal variations. The concentration of CO₂ was considered as a passive tracer to investigate air exchange dynamics. We observed temporally correlated fluctuations in CO₂ levels across three interior rooms (Hall, Crossing, and Polychromes Room). This pattern suggests the influence of a shared ventilation pathway or a common driver of gas exchange with the exterior. The correlations between these fluctuations and the variations of the indoor/outdoor air thermal gradient in each area indicate that these gradients could explain, to a large extent, the variations of CO₂ concentration during the period analysed. These results allow us to know more precisely the behaviour of the concentration of this gas inside the cave and to better characterise the cave's ventilation.