19. Decomposition of CO₂ Signal in Lascaux Cave

Air CO₂ partial pressure (Pco₂) in one of the monitored parts in Lascaux cave (Hall of the Bulls, near the entrance) is variable. For the year 2009, three major components of Pco₂ were identified: a seasonal component, a low-frequency component and a high-frequency component. As a first step to understand the presence of Pco₂ in caves, this study aims to separate these components and quantify their contributions to the Pco₂ signal. Seasonal component is calculated with a sinusoid curve fitted to the measured Pco₂. This component is subtracted from the measured Pco₂. Then, from the remaining value of Pco₂, low- and high-frequency components were separated using Fourier transform, threshold filtering of the frequency and Inverse Fourier transform. Seasonal component is the main contributor (82 % on the average); high-frequency component participates 12 % on the average. Low-frequency component is not present all the year; the average participation is 6 %. Finally, to link these three components with other measured parameters such as temperature, water flow and atmospheric pressure, hypotheses were proposed.