Transfer of environmental signals from the surface to the underground at Ascunsă Cave, Romania

• Published in: Hydrology and earth system sciences Vol. 21; no. 10; pp. 5357 - 5373
• Main Authors: Dragusin, Virgil, Balan, Sorin, Blamart, Dominique, Forray, Ferenc Lazar, Marin, Constantin, Mirea, Ionut, Nagavciuc, Viorica, Oraseanu, Iancu, Persoiu, Aurel, Tirla, Laura, Tudorache, Alin, Vlaicu, Marius
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 26-10-2017; European Geosciences Union; Copernicus Publications
• Subjects: Air temperature; Annual variations; Carbon dioxide; Carbon dioxide concentration; Cave air; Caves; Continental interfaces, environment; Data logging; Deuterium; Drainage; Environmental aspects; Environmental monitoring; Groundwater; Headspace; Henrys law; Humidity; Isotopes; Karst; Logging; Maxima; Meteoric water; Monthly rainfall; Natural history; Ocean, Atmosphere; Organic matter; Paleoclimate science; Precipitation; Rain; Rainfall; Reservoirs; Sciences of the Universe; Seasonal variations; Stable isotopes; Summer; Surface temperature; Underground caverns; Vapor sources; Ventilation; Water; Winter; Romania
• ISSN: 1607-7938; 1027-5606; 1607-7938
• DOI: 10.5194/hess-21-5357-2017

We present here the results of a 4-year environmental monitoring program at Ascunsă Cave (southwestern Romania) designed to help us understand how climate information is transferred through the karst system and archived by speleothems. The air temperature inside the cave is around 7 °C, with slight differences between the upper and lower parts of the main passage. CO2 concentrations in cave air have a seasonal signal, with summer minima and winter maxima. These might indicate the existence of an organic matter reservoir deep within the epikarst that continues to decompose over the winter, and CO2 concentrations are possibly modulated by seasonal differences in cave ventilation. The maximum values of CO2 show a rise after the summer of 2014, from around 2000 to about 3500 ppm, following a rise in surface temperature. Using two newly designed types of water–air equilibrators, we were able to determine the concentration of CO2 dissolved in drip water by measuring its concentration in the equilibrator headspace and then using Henry's law to calculate its concentration in water. This method opens the possibility of continuous data logging using infrared technology, without the need for costly and less reliable chemical determinations. The local meteoric water line (δ2H  =  7.7 δ18O + 10.1), constructed using monthly aggregated rainfall samples, is similar to the global one, revealing the Atlantic as the strongly dominant vapor source. The deuterium excess values, as high as 17 ‰, indicate that precipitation has an important evaporative component, possibly given by moisture recycling over the European continent. The variability of stable isotopes in drip water is similar at all points inside the cave, suggesting that the monitored drip sites are draining a homogenous reservoir. Drip rates, as well as stable isotopes, indicate that the transfer time of water from the surface is on the order of a few days.