In the capital of Hungary, Budapest, which Ottokar Kadic`´ called 'the capital of the caves', beneath the Rózsadomb (Rose Hill) district of the city, many thermal-karstic caves were discovered based on the lucky coincidence of geologic-hydrologic-speleological fundamentals. At present more than a hundred caves and cave indications are known in this 5-6 km2 area. The hills are made up of Triassic and Eocene carbonate sequences. Five caves are km-size. The corridors are sometimes longer than 100 m, and their walls are often adorned by spherical niches. The total length of the caves exceeds 35 km at present. The galleries of these caves are situated in the Eocene Szépvölgy Limestone. Some galleries and most of the cave indications can be found in the Eocene Buda Marl. The lowest galleries of some caves extend into the Triassic carbonate sequences. Although many paleokarstic cavities and caverns exist in the area the age of the largest caves - according to the preliminary results of investigations still in progress - is some hundreds of thousands of years only, based on the radiometric age of the syngenetic minerals. The exceptional value of the caves are the more than dozen species of minerals (especially the variety and mass of carbonates and sulfates are surprising). Minerals precipitated from warm water, minerals of aerosol formation, as well as recent, still developing minerals of cold water origin can also be found. The caves can be regarded as the fossil source levels of the present-day thermal springs at the banks of the Danube. Their genesis is interpreted as a result of mixing corrosion along tectonic fractures at the level of karst water.

Abstract

In this study, already published and new monitoring data are compiled from the Baradla and Béke caves in the Aggtelek Karst, from the Vacska Cave in the Pilis Mountains as well as from the Szemlőhegy and Pálvölgy caves in the Buda Hills. Recent investigations (2019–2020) include monitoring of climatological parameters (e.g., temperature, CO₂) measured inside and outside the caves, and the chemical, trace element and stable isotopic compositions of drip waters. In the Baradla Cave, the main focus of the investigation was on the stable isotope composition and the temperature measurements of drip water. In the Vacska Cave, which belongs to the Ajándék-Ariadne cave system, CO₂ measurements and drip water collection were conducted in order to perform chemical and stable isotope measurements. In the Szemlőhegy and Pálvölgy caves, the chemical and stable isotope compositions of drip waters at six sites were determined. These datasets were used to characterize the studied caves and the hydrological processes taking place in the karst, and to trace anthropogenic influences. Climatological investigation revealed seasonality in CO₂ concentration related to outside temperature variation, indicating a variable ventilation regime in the caves. In addition, the contributions of the winter and summer precipitation to the drip water were also estimated, in order to evaluate the main infiltration period. The knowledge of these parameters plays a crucial role in constraining the carbonate precipitation within the cave. Thus, the dataset compiled in this study can provide a basis for the interpretation of speleothem-based proxies.

Determination of the long-term behavior of cave systems and their response to changing environmental conditions is essential for further paleoclimate analyses of cave-hosted carbonate deposits. For this purpose, four actively forming stalagmites were collected in the Baradla Cave where a three-year monitoring campaign was also conducted. Based on textural characteristics and radiocarbon analyses, the stalagmites are composed of annual laminae, whose counting was used to establish age–depth relationships. Fast and slowly growing stalagmites have different stable carbon and oxygen isotope compositions as well as trace element contents that could be attributed to differences in drip water migration pathways. The stable isotope compositions were compared with meteorological data of the last ∼100 years indicating that carbon isotope compositions of the stalagmites may reflect changes in precipitation amount, while oxygen isotope compositions are more related to temperature variations. The combined textural–geochemical–meteorological interpretation lead us to select the isotope record that can best reflect variations in environmental conditions and can be used for further evaluation of the climate–proxy relationships.