Isotopic (13C) signature of CO2 sources in the vadose zone of a mediterranean karst (Nerja Cave site, Southern Spain)

Abstract

This study is based on in situ measurements of the soil and the vadose zone (<60 m) in a Mediterranean karst experimental site near Nerja Cave (a show cave in dolomite marbles in South Spain). CO2 concentrations in depth were registered in boreholes drilled in experimental site. The CO2 content generally increases with depth. Measurements indicate average vadose air CO2 concentrations of nearly 40,000 ppm, with a maximum of nearly 60,000 ppm. In this context, the cave itself appears to be a vadose subsystem above the groundwater level, with significantly lower CO2 concentrations (a few thousands of ppm maximum) due to its ventilation. The δ13C-CO2 data of the vadose air point to an origin of the gas mainly related to microbiological processes associated to the consumption of dissolved organic matter in the groundwater surface. This gas can diffuse or flow laterally, upward or downward through karst conduits. Interactions between air masses of surface origin (relatively dry, with variable temperature and low CO2 content) and typical vadose attributes (relatively high CO2 content, near-saturated humidity and 21 °C temperature) produce clear ascendant or descendant air fluxes inside the boreholes, especially those that cross significant karst voids.