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

Abstract

Abstract. We present here the results of a 4-year environmental monitoring program at Ascunsȃ 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. CO 2 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 CO 2 concentrations are possibly 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 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 (δ 2 H = 7.7 δ 18 O + 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.

Kinetic processes and stable isotopes in cave dripwaters as indicators of winter severity

“…The average values of δ 2 H (−63 ‰) and δ 18 O (−9 ‰) in precipitation are slightly lower than those in SW Romania (−60.5 ‰ and −8.8 ‰ for δ 2 H and δ 18 O, respectively, at Dumbrava [Bojar, Halas, Bojar, & Chmiel, ] and −53 ‰ and −8.3 ‰, respectively, at Drobeta Turnu‐Severin [Drăguşin et al, ]) but higher than those in the central Bihor Mountains at Scărișoara (−88 ‰ and −12 ‰) and the Hațeg Basin (−76 ‰ and −10.8 ‰; Bojar, Ottner, Bojar, Grigorescu, & Perşoiu, ). However, in SW Romania, a higher proportion of precipitation is sourced from the Mediterranean Sea, and mean annual air temperatures are higher by approximately 0.5 °C than at Urșilor Cave, thus accounting for the higher stable isotope values in precipitation there.…”

“…Romania has a very diverse karst landscape, offering a great potential for reconstructing past climate based on cave deposits (Constantin, Bojar, Lauritzen, & Lundberg, 2007;Drăguşin et al, 2014;Onac et al, 2015;Onac, Constantin, Lundberg, & Lauritzen, 2002;Tămaş, Onac, & Bojar, 2005). However, there are only a few studies available which documented the rainwater δ 18 O variability and corresponding cave dripwater changes in the region (Drăguşin et al, 2017;Perşoiu, Onac, Wynn, Bojar, & Holmgren, 2011), making the climatic interpretation of speleothem δ 18 O from this region ambiguous.…”

Speleothem Records from the Eastern Part of Europe and Turkey—Discussion on Stable Oxygen and Carbon Isotopes

“…Monitoring of key in-cave variables (e.g., cave temperature, ventilation regime) influencing dripwater hydrology and carbonate saturation conditions provides the basis for a process-based understanding of modern environmental controls on stable isotope composition of speleothem and their growth conditions (e.g., [86][87][88]. In the lack of site-specific information (e.g., infiltration processes, calcite precipitation conditions), most of the paleoclimatic inferences of the early studies, e.g., [17,45], were not able to provide more than simplistic explanations.…”

Tracing the Relationship between Precipitation and River Water in the Northern Carpathians Base on the Evaluation of Water Isotope Data

“…Reduced amplitudes of stable isotope values in river water compared to precipitation water variability suggest that rivers are recharged via subsurface flow, with water from individual rain events being stored in the phreatic zone before recharging the rivers [17,43]. Low d-excess in river water occurs in summer, suggesting that Combined, the d-excess and δ values suggest that, similar to southwestern and western Romania [33,41], Mediterranean cyclones have a strong imprint on winter precipitation in eastern Romania. Similar influences have been found also further to the east [42], showing that moisture originating from the Mediterranean and Black Seas reaches into the central East European plain.…”