Abstract: This study deals with the spatiotemporal dynamics of CO2 and its isotopic composition (d13CeCO2) in the atmosphere of Altamira Cave (northern Spain) over two annual cycles. In general terms, the cavity shows two distinct ventilation modes, acting as a CO2 reservoir from October to May (recharge stage), while actively exchanging gases with the outside atmosphere between July and September (discharge stage). In recharge mode, the in-cave air shows higher and relatively homogeneous CO2 values (3332 ± 521 ppm) with lower d13CeCO2 (
23.2 ± 0.4?). In contrast, during the discharge stage, the CO2 concentrations are lower and relatively more variable (1383 ± 435 ppm) and accompanied by higher d13CeCO2 (up to
12?). This seasonal pattern is controlled by the distinct rates of exchange of air masses with the external atmosphere through the annual cycle, as well as by changes in the production of CO2 in the soil and natural fluctuations in the concentration of dissolved inorganic carbon transported by drip water into the cave. In contrast to the interpretations of previous studies in Altamira Cave, no local air intakes into the deepest cave sections were flagged by our d13C measurements. This finding is also supported by analyses of CO2 and 222Rn in air, density of airborne particles and air temperature. In addition, preliminary experiments examining the visitor-produced disturbances on d13CeCO2 were conducted during the various cave ventilation stages to explore the potential use of this parameter as an indicator of anthropic pressure in caves. Our data show that visits (overall stay of 60e85 min; i.e., 4 people for 20 min) significantly affected d13CeCO2 (up to Dd13C ~ -2x100) in the Polychrome Hall of Altamira Cave under conditions of low natural CO2 (discharge stage), whereas it remained almost unaltered under circumstances of high CO2 concentration (recharge stage). This demonstrates that d13CeCO2 is sensitive to perturbations produced by visitors during certain periods.

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