Determining sources of CO2 in Pisani rov (Postojna cave) using carbon isotopes (12C, 13C and 14C)

Karstic caves can represent subterranean reservoirs of CO2, which can originate from different sources. Certain environmental conditions can cause transport of cave CO2 to the open atmosphere. In order to investigate local carbon cycle it is important to determine the sources of cave CO2 and mechanisms causing accumulation and ventilation of this gas. Beside its importance for the carbon balance assessment, which is one of the important global topics due to the global increase of CO2 in the atmosphere, is this topic also important from the technical point of view in relation to the determination of soil or ecosystem CO2 fluxes in karstic areas. Influence of cave ventilation on CO2 flux measurements and its interpretations should be taken into account at such areas. Measurements and sampling of soil and cave air were performed in the last hall of Pisani rov and soil above it. Pisani rov is a dead end passage of the Postojna cave system and is closed for the tourist. CO2 concentration and stable isotopic composition were measured twice a month from October 2014 to September 2015. In addition two samplings for Δ14C in soil CO2 were performed in winter 2015 (with and without snow cover) and three samplings for Δ14C in cave air (two in winter at the same day as soil air sampling and one in late summer 2015. Our results suggest that CO2 represents a mixture between atmospheric CO2 and CO2 coming from decomposition of soil organic matter. It indicates that the CO2 concentration is not directly related to the rate of soil respiration, but it is mostly controlled by the ventilation. Sources such as CO2 from deep geological origin, decomposition of organic matter in cave sediment and CO2 coming from tourists or cave animals were found to be not important. Further, results suggest that sources contributing to cave CO2 (beside atmospheric CO2) are decomposition of organic matter at depths where outside low temperatures do not limit the decomposition of organic matter and CO2 coming to the cave dissolved in percolating water. The main parameter affecting cave ventilation is the difference in temperatures between outside and cave air. Since the temperature in the cave is relatively constant the main driver is the change of the outside temperature. Two ventilation regimes in the cave can be observed, the summer regime when CO2 is accumulating and winter regime when cave ventilates. Exceptions to this pattern happened during the winter when temperatures are below 0˚C with the snow cower above the cave. At this occasion the rise of CO2 concentrations in the cave was observed, which indicates that possible exits for cave air, enabling ventilation were closed accumulating of CO2 in the cave. The study was supported by the Innovative scheme for co-financing of doctoral studies financed by the European Union through the European Social Fund and by the scholarship granted by the World Federation of Scientists.