Search

What controls short-term variations in CO2 and 222Rn concentrations in Modrič Cave atmosphere? Radon (222Rn), as a naturally occurring radioactive geogenic gas, presents potential health hazard in confined, poorly ventilated areas such as caves. Thus, in order to ensure safe work environment for the guides and cave scientists, it is necessary to determine precise temporal and spatial variations of radon concentrations. In Modrič Cave (littoral Croatia), small (829 m) two-branch horizontal show cave, we conducted 6.5-years and 5-years monthly measurements of CO2 and Rn concentrations, respectively, which revealed basic ventilation pattern ; it is seasonal density-driven air circulation controlled by the difference between outside and cave air temperature on which cave morphology and bedrock architecture are superimposed. Aiming to decipher short-term variation with respect to meteorological conditions (precipitation, wind), additional continuous measurements of cave-air concentration of Rn and CO2 began in 6/2020 and 10/2021, respectively. Bearing in mind that the highest gas concentrations (~24, 000 Bq/m3 and ~12, 000 ppm) were recorded in summer during the peak tourist season, sudden gases concentration increase/decrease governed by intensive precipitation/bora wind should be respected in terms of cave management. Since the radon underground pathways could be affected by the phenomena that precede or accompany the earthquakes we considered 13 earthquakes – 8 nearby (7-56 km) with magnitude M 2.1-4.7 and 5 distant earthquakes (115-175 km) with greater magnitude M 4.8-6.2, but no significant radon flux has been recorded. However, it could have been masked by other environmental features, especially during the warm periods when both radon and CO2 as its carrier gas, are intensively produced in the soil horizon by plant respiration, microbial activity and limestone dissolution that enhances radon release.

Several speleothem studies from Croatian karst caves proved to be valuable archives of local to regional palaeoenvironmental changes on various time scales. Variations of oxygen and carbon stable isotope ratios (δ18O and δ13C) from eight stalagmites recovered from six caves demonstrated diverse responses to hydroclimate changes throughout the Holocene. The main reason for this diversity stems from their geographical distribution and specific locations: Nova Grgosova Cave (239 m a.s.l.) is in the continental part of Croatia, mid-Adriatic region is represented by the coastal Modrič Cave (32 m a.s.l.), mountainous Manita peć Cave (570 m a.s.l.) and Strašna peć Cave (74 m a.s.l.) on the island of Dugi otok, while the records from the southernmost part of Croatia are kept in Mala špilja (60 m a.s.l.) and Velika špilja (90 m a.s.l.) caves on Mljet Island. According to the long term continuous or periodical monitoring of cave environmental settings and in accordance with other regional records, spelean carbonate δ13C is interpreted as a proxy for spring/summer environmental conditions. It is proposed that this proxy mainly reflects bioproductivity (i.e. microbial activity and vegetation density) which is at its highest in spring and summer. δ18O is interpreted as a proxy for autumn/winter conditions held to be linked to the rainfall amount variability. Starting from possible record of Younger Dryas, signals of some major hydroclimate events are more (e.g. 4.2 ka) or less (e.g. 8.2 ka) pronounced when compared to some other European records. Within Holocene Climate Optimum, around 7.4 ka, continental region experienced sudden drought, which is not clearly expressed in speleothem records from South Croatian coastal region. Additionally, human intervention into the natural landscape left anthropogenic signal, sometimes hiding or modifying the latest short-term (centennial) climatic variability throughout the last millennium.

The topic of climate change is one of the fronts of scientific research in the 21st century. A special focus of speleological research is the discovery of records of climate change in caves. However, the question arises as to the impact of climate change on the conditions of speleological research and on the hydrological dynamics in karst. Extreme weather conditions around the world have become more frequent. That is why we need to define new standards of speleological research. New recommendations to the speleological community should include methods for monitoring, collecting and analyzing climate data for risk assessment purposes. Analyzes of hydrological extremes and other climate-related processes in karst and caves are of interest to the wider community. We will highlight examples from the Dinaric Karst in Croatia (deep pits, hydrologically active caves, caves with ice).

We present the preliminary results of mineralogical analyses of submerged speleothem collected within the project SEALeveL (HRZZ-IP- 2019-04-9445) funded by Croatian Science Foundation. The ML-12 stalagmite was collected 12 m below the water table in a submerged coastal cave on Lošinj Island. Several orange layers are visible on the longitudinal cross section of the stalagmite. A thin section was prepared for investigation by petrographic and scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS) system. Prior to SEM analysis, the freshly-cut surface was also analysed by X-ray fluorescence (XRF) analysis. Oil was used for thin section preparation to avoid dissolution of highly soluble minerals. Optical inspection showed reduction in crystal size and relatively high porosity within the orange-coloured layers indicating a change in depositional environment.. XRF analysis show the presence of chlorine, sulphur and silicon bearing minerals in these orange layers but not in the clean calcitic parts. SEM inspection of these layers show presence of several minerals in the pores within this orange layer. Chemical composition obtained by EDS analysis suggests that these minerals are quartz, halite, gypsum, muscovite, hematite, and an unidentified phosphate. Especially halite, but also gypsum are highly soluble minerals that are abundant in solute form in the sea. This indicates that the sea might have played a key role in the speleothem formation environment and deposition of the minerals found in the orange layers. The formation of the orange layers might have been as well controlled by sea- level oscillations. Further analysis is necessary to confirm such hypothesis.

Phreatic overgrowths on speleothems (POS), a secondary depositional structure (carbonate phreatic encrustation) which precipitates at the water table around pre-existing vadose speleothems if favourable geochemical conditions are acquired. Since the POS precipitate at the very top of the water column, i.e. at the water-air interface, their occurrence is limited to a very narrow band of the tide-induced groundwater level fluctuation. POS can be precisely dated with U-Th as well as with radiocarbon method, and their occurrence in caves guarantees protection from weathering. The combination of their narrow occurrence, datability, and preservation makes POS excellent sea-level indicators. So far, it has been used in sea-level research in only several locations in the world while most of the investigated sites are located in Mallorca. Recently, within the project SEALeveL (HRZZ-IP- 2019-04-9445) funded by Croatian Science Foundation, phreatic overgrowths on speleothems (POS) have been discovered in Croatian coastal caves. Since the POS investigation is the first of that kind in the Adriatic, information on their morphology, conditions and time of deposition, will contribute to a better understanding of the POS as sea-level indicator and will allow comparison with results from other parts of the Mediterranean basin. We present the preliminary results of U-Th dating and XRD, XRF analyses of POS samples from Šipun Cave (Cavtat, southern Dalmatia) and Medvjeđa špilja Cave (Lošinj island, Kvarner region) which resulted in first relative sea-level index point 3000 years, indicating a relative sea-level stability for a few hundred years 1 m below modern sea level.

Caves are located within the Earth’s Critical Zone (ECZ), which is marked by intensive interactions between atmosphere, biosphere and hydrosphere. Temporal changes in these interactions can be recorded in cave sediments (i.e. speleothems) which led to caves and cave sediments being extensively used in various palaeoenvrionmental and palaeoclimatical studies. Within those researches, the need for long-term environment monitoring emerged as a necessity. Furthermore, monitoring programmes have also provided valuable data for show caves’ sustainable management. We present the results of ongoing continuous measurement of CO2 and 222Rn concentrations in the Modrič Cave (Croatia), a relatively small show cave located at the foothills of the Velebit Mt. near Adriatic coast at 32 m a.s.l. It is a simple cave with two mainly horizontal passages with total length of 829 m. Previous monitoring campaigns in the Modrič Cave with monthly and seasonal resolution showed strong seasonal changes in CO2 concentrations with two distinctive seasons: summer/autumn with the highest, and winter/spring with the lowest values. The 222Rn concentration in the cave air follows spatial and temporal patterns similar to the CO2 concentrations. Monitoring data also showed significant differences in cave air CO2 and 222Rn concentrations between the two main passages. The diverse features of the two passages can explain these differences as the wider left passage is well ventilated in comparison to the more constrained right passage. The continuous logging of cave air CO2 and 222Rn launched in 2021 provided high-resolution data, which revealed sudden variations in both CO2 and 222Rn concentrations that had not been detected in the previous campaign as they occur on smaller time scales. Apparently, these peaks concur with intensive rain events indicating more dynamic cave atmosphere system than previously thought, which will affect future (palaeo)environmental studies in this cave.