Hydrol. Earth Syst. Sci., 21, 5357–5373, 2017 https://doi.org/10.5194/hess-21-5357-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Transfer of environmental signals from the surface to the underground at Ascuns˘ a Cave, Romania Virgil Dr˘ agu¸ sin 1 , Sorin Balan 2 , Dominique Blamart 3 , Ferenc Lázár Forray 4 , Constantin Marin 1 ,Ionu¸t Mirea 1,4 , Viorica Nagavciuc 5 , Iancu Or ˘ a¸ seanu 6 , Aurel Per¸ soiu 5,7 , Laura Tîrl ˘ a 8 , Alin Tudorache 1 , and Marius Vlaicu 1 1 Emil Racovi¸ t˘ a Institute of Speleology, Frumoas˘ a 31, 010986, Romania 2 National Research and Development Institute for Marine Geology and Geoecology, Mamaia 304, Constan¸ ta, 900581, Romania 3 Laboratoire des Sciences du Climat et de l’Environnement LSCE-IPSL CEA-CNRS-UVSQ, Paris-Saclay, Avenue de la Terrasse, Bât. 12, 91198 Gif-sur-Yvette CEDEX, France 4 Department of Geology, Babe¸ s-Bolyai University, Kog˘ alniceanu 1, 400084 Cluj-Napoca, Romania 5 Stable Isotope Laboratory, ¸ Stefan cel Mare University, Universit˘ a¸ tii 13, Suceava 720229, Romania 6 Romanian Association of Hydrogeologists, Traian Vuia 6, Bucharest 020956, Romania 7 Emil Racovi¸ t˘ a Institute of Speleology, Clinicilor 5, Cluj Napoca 400006, Romania 8 Faculty of Geography, University of Bucharest, N. B˘ alcescu 1, Romania Correspondence to: Virgil Dr˘ agu¸ sin (virgil.dragusin@iser.ro) Received: 25 November 2016 – Discussion started: 4 January 2017 Revised: 14 September 2017 – Accepted: 20 September 2017 – Published: 26 October 2017 Abstract. We present here the results of a 4-year environ- mental monitoring program at Ascuns˘ a Cave (southwestern Romania) designed to help us understand how climate infor- mation 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. CO 2 concentrations in cave air have a seasonal signal, with summer minima and winter max- ima. These might indicate the existence of an organic mat- ter reservoir deep within the epikarst that continues to de- compose over the winter, and CO 2 concentrations are pos- sibly modulated by seasonal differences in cave ventilation. The maximum values of CO 2 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 CO 2 dissolved in drip water by measuring its concentration in the equilibrator headspace and then us- ing Henry’s law to calculate its concentration in water. This method opens the possibility of continuous data logging us- ing infrared technology, without the need for costly and less reliable chemical determinations. The local meteoric water line (δ 2 H = 7.7 δ 18 O + 10.1), constructed using monthly ag- gregated rainfall samples, is similar to the global one, re- vealing the Atlantic as the strongly dominant vapor source. The deuterium excess values, as high as 17 ‰, indicate that precipitation has an important evaporative component, pos- sibly given by moisture recycling over the European conti- nent. The variability of stable isotopes in drip water is simi- lar 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. 1 Introduction The large-scale monitoring of karst systems is mainly under- taken from the perspective of water resource management or conservation (e.g., White, 1988; Ford and Williams, 2013). However, with the use of speleothems as tools for climate re- construction came the need for monitoring karst systems at a much finer spatial and temporal scale, where water–rock and water–atmosphere interactions have to be taken into account (Fairchild and Baker, 2012). In speleothem paleoclimatol- ogy, cave monitoring studies are used as tools for local, case- Published by Copernicus Publications on behalf of the European Geosciences Union.