Spatiotemporal distribution of δ¹³C-CO₂ in a shallow cave and its potential use as indicator of anthropic pressure

Abstract

This study deals with the spatiotemporal dynamics of CO₂ and its isotopic composition (δ¹³C-CO₂) 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 δ¹³C-CO₂ ( 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 δ¹³C-CO₂ (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 δ¹³C 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 δ¹³C-CO₂ 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 60-85 min; i.e., 4 people for 20 min) significantly affected δ¹³C-CO₂ (up to Δδ¹³C~-2‰) 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 δ¹³C-CO₂ is sensitive to perturbations produced by visitors during certain periods.