Algunes dades sobre els gasos de l’atmosfera de la Cova dets Ases (Felanitx, Mallorca). Avaluació de les concentracions de CO i les seves signatures isotòpiques

Abstract

[eng] The terrestrial passages and air chambers that are interspersed in the underwater caves of Mallorca frequently present high concentrations of CO2, which in many cases exceed 50,000 ppm (5 %). During the years 2019 and 2020, a sampling campaign of gases in the atmosphere of Cova dets Ases (Felanitx, Mallorca) and its underwater sectors was carried out, including also the neighboring Cova de ses Figueres. The CO2 and CH4 contents in the atmosphere of the caves have been analyzed, together with the carbon isotopic signature of both gases. The CO2 concentrations measured in some of the chambers of Cova dets Ases are extraordinarily high, with maximum values greater than 83,000 ppm, and CH4 concentration almost null (lower than 0.25 ppm). Both gases do not show a seasonal pattern due to the almost fully restricted ventilation of some of the air chambers. The atmospheric dynamics of the investigated caves are outlined, taking the opportunity to carry out a detailed discussion on the origin of the very high levels of CO2 observed. The results obtained would show the presence of air within the vadose zone (ground air), which exhibits specific characteristics linked to very confined environments –high CO2 content, with a differentiated isotopic signature–, and whose source is relatively disconnected from the epikarst and its edaphic horizon. In addition, there are some evidences on the likely onsite production of CH4 and its coeval and intense consumption in some air chambers and sectors above the coastal water table.