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Who is threatened by cave-air CO2? Modrič and Manita peć caves (Croatia) case study

Although just a trace gas in atmosphere (412 ppm in April 2019, NOAA), CO2 plays an important role in overall Earth’s system, especially when misbalanced from the natural state. On incomparably smaller scale, CO2 in cave air is equally interesting in underground environment for its various sources, sinks and effects with and without human interaction. Generally, it governs both limestone dissolution and speleothem deposition (MILANOLO & GABROVŠEK, 2009). Cave-air CO2 has several sources such as: i) diffusion within epikarst air generated by root respiration and organic matter decay, ii) degassing from the cave water which was enriched by CO2 on its way through the soil and epikarst, iii) biological productivity (micro-organisms feeding on organic matter, usually guano) and iv) deep- seated (thermal) sources (FAIRCHILD & BAKER, 2012). In addition to natural sources, show caves receive extra anthropogenic CO2-flux from breathing, which is sometimes recognized as a threat to the vulnerable cave decoration. Namely, elevated pCO2 in cave air can hamper degassing CO2 from the dripping groundwater resulting with ceased calcite deposition. Additionally, CO2 dissolved in water condensed in cave environment may be destructive for already crystalized spelean calcite (FAIMON et al., 2006). On the other hand, impact of the cave air CO2 on human health is not negligible at certain concentrations, which must be considered both for the visitors and guides in show caves. The concentration and variation of cave-air pCO2 is a function both of production and cave ventilation (FAIRCHILD & BAKER, 2012), so for the understanding and reliable reconstruction of natural processes and eventual show-cave management it is necessary to conduct multi- year monitoring of the cave environment. Two relatively small show caves Modrič and Manita Peć have been subjected to the research from 2017 on, in order to estimate interaction and mutual influence between natural cave atmosphere and human presence. Different overlaying bedrock, soil and vegetation cover, morphology and microclimate result with different ventilation pattern and intensity. Manita peć Cave consists of one spacious descending channel, relatively thick but significantly fractured roof, and modest soil and vegetation cover (SURIĆ et al., 2017), so its air circulation keeps CO2 values bellow 1400 ppm year-round, even during the summer pCO2 peak. Visitors periodically and temporary increase the pCO2 for ca. 100 ppm. Modrič cave is shallow horizontal cave developed in two branches. Bushes and recently planted trees, along with thin overburden (SURIĆ et al., 2018), associated with narrow channels produce summer CO2 concentrations >10000 ppm. Even during the winter time, the innermost parts retain pCO2 around 1500 ppm. Given such settings, anthropogenic input of 100-150 ppm of small visitor groups (up to 20 cavers) is insignificant. The main concern in Modrič Cave should be radon activity which profile matches the pCO2 variations (FAIRCHILD & BAKER, 2012) due to the same ventilation issue, and which is currently under the monitoring. References: FAIMON, J., ŠTELCL, J., SAS, D. (2006): Anthropogenic CO2-flux into cave atmosphere and its environmental impact: A case study in the Císarská Cave (Moravian Karst, Czech Republic), Science of the Total Environment, 369, 231-245. FAIRCHILD, I.J., BAKER, A. (2012): Speleothem Science: From Process to Past Environments. Wiley-Blackwell, Chichester, 432 p. MILANOLO, S., GABROVŠEK, F. (2009): Analysis of Carbon Dioxide Variations in the Atmosphere of Srednja Bijambarska Cave, Bosnia and Herzegovina, Boundary-Layer Meteorology, 131/3, 479–493. NOAA. https://www.esrl.noaa.gov/gmd/ccgg/trends/ (accessed 30 April 2019) SURIĆ, M., LONČARIĆ, R., LONČAR, N., BUZJAK, N., BAJO, P., DRYSDALE, R.N. (2017): Isotopic characterization of cave environments at varying altitudes on the eastern Adriatic coast (Croatia) – Implications for future speleothem- based studies. Journal of Hydrology 545, 367- 380. SURIĆ, M., LONČARIĆ, R., BOČIĆ, N., LONČAR, N., BUZJAK, N. (2018): Monitoring of selected caves as a prerequisite for the speleothem-based reconstruction of the Quaternary environment in Croatia. Quaternary International, 494, 263-274.

cave monitoring, CO2, Modrič Cave, Manita peć Cave, Croatia

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