Earth tide signal on tracer gases fluctuations from a huge cave (Torca del Carlista cave, Biscay, Spain)

This study presents the results of monitoring air temperature, atmospheric pressure, relative humidity, air radon (222Rn) concentration and CO2 concentration in a huge cave, Torca del Carlista (–353 m). The cave ends in one of the largest halls in Europe, the Grupo de Espeleología Vizcaino Hall, at –158 m. This hall has a volume of 2.14 Mm3 and is connected to the outside by a narrow vertical tube 56 m long and 2 m wide. The objective was to determine the processes that control the airflow of this huge hall during a short time period (two weeks) with marked changes in the meteorological conditions (rapid shifts from cold and rainy to hot and dry days). The results show that the 12.5 h, 23.5 h and 23.6 h oscillations of CO2 and 222Rn are a response to Earth tidal forcing. We suggest that K1-O1 and M2-S2 Earth tides are detectable in the radon and CO2 concentrations of a natural cave. The poroelastic deformation under the tidal forcing could explain this signal. In consequence, the airflow in Torca del Carlista is a process that depends on thermo-barometric constraints, thermo-hygrometric conditions and Earth tides.