Low-Calcium Cave Dripwaters in a High CO2 Environment: Formation and Development of Corrosion Cups in Postojna Cave, Slovenia

Kukuljan L., Gabrovsek F., Covington M.

Density-driven chimney effect airflow is the most common form of cave ventilation, allowing gas exchange between the outside and the karst subsurface. However, cave ventilation can also be driven by other mechanisms, such as barometric changes or pressure differences induced by the outside winds. We discuss the mechanism and dynamics of wind-driven ventilation using observations in Postojna Cave, Slovenia. We show how seasonal airflow patterns driven by the chimney effect are substantially modified by outside winds. Wind flow over irregular topography forms near-surface air pressure variations and thus pressure differences between cave entrances at different locations. These pressure differences depend on wind speed and direction and their relationship to surface topography and the location of cave entrances. Winds can act in the same or opposite direction as the chimney effect and can either enhance, diminish or even reverse the direction of the density-driven airflows. To examine the possibility of wind-driven flow, we used a computational fluid dynamics model to calculate the wind pressure field over Postojna Cave and the pressure differences between selected points for different configurations of wind speed and direction. We compared these values with those obtained from airflow measurements in the cave and from simple theoretical considerations. Despite the simplicity of the approach and the complexity of the cave system, the comparisons showed satisfactory agreement. This allowed a more general assessment of the relative importance of wind pressure for subsurface ventilation. We are certain that this example is not unique and that the wind-driven effect needs to be considered elsewhere to provide better insights into the dynamics of cave climate, air composition or dripwater geochemistry.

Kukuljan L., Gabrovšek F., Covington M.D., Johnston V.E.

Understanding the dynamics and distribution of CO2 in the subsurface atmosphere of carbonate karst massifs provides important insights into dissolution and precipitation processes, the role of karst systems in the global carbon cycle, and the use of speleothems for paleoclimate reconstructions. We discuss long-term microclimatic observations in a passage of Postojna Cave, Slovenia, focusing on high spatial and temporal variations of pCO2. We show (1) that the airflow through the massif is determined by the combined action of the chimney effect and external winds and (2) that the relationship between the direction of the airflow, the geometry of the airflow pathways, and the position of the observation point explains the observed variations of pCO2. Namely, in the terminal chamber of the passage, the pCO2 is low and uniform during updraft, when outside air flows to the site through a system of large open galleries. When the airflow reverses direction to downdraft, the chamber is fed by inlets with diverse flow rates and pCO2, which enter via small conduits and fractures embedded in a CO2-rich vadose zone. If the spatial distribution of inlets and outlets produces minimal mixing between low and high pCO2 inflows, high and persistent gradients in pCO2 are formed. Such is the case in the chamber, where vertical gradients of up to 1000 ppm/m are observed during downdraft. The results presented in this work provide new insights into the dynamics and composition of the subsurface atmosphere and demonstrate the importance of long-term and spatially distributed observations.

Johnston V., Martín-Pérez A., Skok S., Mulec J.

Subterranean calcite dissolution and precipitation are often considered as strictly geochemical processes. The active involvement of microbes in these processes is commonly underestimated in the literature due to general oligotrophic conditions in caves, except in particular cave conditions, such as sulfidic caves and moonmilk deposits, where the presence of microbes likely plays a key role in mineral deposition. Here, we study the possible involvement of microbes from Postojna Cave, Slovenia, in carbonate dissolution (litholysis) and precipitation (lithogenesis). Microbes were sampled from small pools below hydrologically diverse drip sites and incubated on polished limestone tablets at 10 and 20°C for 2 and 14 weeks under cave-analogue conditions. The tablets were then observed under scanning electron microscope to investigate microbe–rock interactions. Our experiments showed the presence of various microbial morphotypes, often associated with extracellular polymeric substances, firmly attached on the surfaces. Unfortunately, our surface sterilization method using 96% and 70% ethanol could not establish the complete aseptic conditions in deep natural cracks in the experimental limestone tablets. Nonetheless, our results emulate the interaction of environmental microbes with limestone rock. Conspicuous calcite dissolution and precipitation were observed in association with these microbes. Furthermore, we show evidence of entombment of microbes in a Si-rich precipitate during nutrient-depleted growth conditions and we suggest that microbial involvement in silica mobilization under ambient conditions may be a widespread and often overlooked phenomenon. Our findings have important implications for microbial-mediation of cave carbonate dissolution and precipitation, including the preservation of past climate proxy signals in speleothems and prehistoric cave art. Improvements to the methodology and further work are suggested to enable more robust ex-situ cultivation experiments in the future, facilitating better and more detailed research into this topic.

Baldini J.U., Lechleitner F.A., Breitenbach S.F., van Hunen J., Baldini L.M., Wynn P.M., Jamieson R.A., Ridley H.E., Baker A.J., Walczak I.W., Fohlmeister J.

Stalagmites are an extraordinarily powerful resource for the reconstruction of climatological palaeoseasonality. Here, we provide a comprehensive review of different types of seasonality preserved by stalagmites and methods for extracting this information. A new drip classification scheme is introduced, which facilitates the identification of stalagmites fed by seasonally responsive drips and which highlights the wide variability in drip types feeding stalagmites. This hydrological variability, combined with seasonality in Earth atmospheric processes, meteoric precipitation, biological processes within the soil, and cave atmosphere composition means that every stalagmite retains a different and distinct (but correct) record of environmental conditions. Replication of a record is extremely useful but should not be expected unless comparing stalagmites affected by the same processes in the same proportion. A short overview of common microanalytical techniques is presented, and suggested best practice discussed. In addition to geochemical methods, a new modelling technique for extracting meteoric precipitation and temperature palaeoseasonality from stalagmite d18O data is discussed and tested with both synthetic and real-world datasets. Finally, world maps of temperature, meteoric precipitation amount, and meteoric precipitation oxygen isotope ratio seasonality are presented and discussed, with an aim of helping to identify regions most sensitive to shifts in seasonality.

White J.H., Domínguez-Villar D., Hartland A.

• Condensation corrosion is detected using stable isotope analysis of calcite surfaces. • Dissolution and recrystallization cause distinct changes in c and o isotope ratios. • Negative isotope correlation from degassing (higher δ 13 C) and condensation (lower δ 18 O). • Results are consistent with high-resolution studies of speleothem hiatuses. • A new approach to the detection of condensation corrosion in managed caves. Condensation corrosion is a natural process which enhances the chemical weathering of limestone cave chambers and speleothems. We evaluated the use of carbonate tablets for detecting condensation corrosion in Glowworm Cave, New Zealand, using local limestone and speleothem as experimental substrates (herein tablets ). Evidence for condensation corrosion was assessed via three methods: gravimetric (mass wasting), microscopic (surface pitting, recrystallization) and isotopic (δ 13 C and δ 18 O changes). Our results show little evidence of tablet mass loss throughout a 6-month deployment period. However, SEM imaging and isotope analysis (δ 13 C and δ 18 O) of the upper ~50 μm layer of the tablets, suggest that condensation corrosion operates in the cave, especially in sectors affected by large diurnal microclimate variations. Most notably, condensation water altered the tablet surface δ 13 C and δ 18 O values. Small, positive shifts in surface δ 13 C and δ 18 O values are considered to reflect pure dissolution (where dissolution favours the removal of lighter isotopologues). In contrast, tablets that exhibited large positive shifts in δ 13 C in tandem with large negative shifts in δ 18 O values, are interpreted as showing calcite recrystallization and the inheritance of higher DIC δ 13 C values ( 13 C fractionation by CO 2 degassing), lighter water δ 18 O values and/or kinetic fractionation of δ 18 O. This study therefore demonstrates that stable isotopes could be applied to detect paleoclimatic episodes of condensation corrosion in speleothems.

Surić M., Czuppon G., Lončarić R., Bočić N., Lončar N., Bajo P., Drysdale R.N.

Speleothems deposited from cave drip waters retain, in their calcite lattice, isotopic records of past environmental changes. Among other proxies, δ18O is recognized as very useful for this purpose, but its accurate interpretation depends on understanding the relationship between precipitation and drip water δ18O, a relationship controlled by climatic settings. We analyzed water isotope data of 17 caves from different latitudes and altitudes in relatively small but diverse Croatian karst regions in order to distinguish the dominant influences. Drip water δ18O in colder caves generally shows a greater resemblance to the amount-weighted mean of precipitation δ18O compared to warmer sites, where evaporation plays an important role. However, during glacial periods, today’s ‘warm’ sites were cold, changing the cave characteristics and precipitation δ18O transmission patterns. Superimposed on these settings, each cave has site-specific features, such as morphology (descending or ascending passages), altitude and infiltration elevation, (micro) location (rain shadow or seaward orientation), aquifer architecture (responsible for the drip water homogenization) and cave atmosphere (governing equilibrium or kinetic fractionation). This necessitates an individual approach and thorough monitoring for best comprehension.

Prelovšek M., Šebela S., Turk J.

The CO2 concentration of the air in Postojna Cave (400–7900 ppm) is found to be induced by CO2 sources (human respiration contributing ~ 20,000–58,000 ppm per breath, outgassing of dripwater and water seeping from the vadose zone/epikarst with a pCO2 values of 5000–29,000 ppm, and underground Pivka River having pCO2 at 2344–4266 ppm) and CO2 dilution (inflow of outside air with a CO2 concentration of ~ 400 ppm). Measurements show that sinking Pivka River has the lowest CO2 concentration among plausible CO2 sources but still continuously exceeds the surrounding cave air CO2 concentration. During the winter months, intensive ventilation reduces the cave air CO2 concentration to outside levels (~ 400 ppm), even in the centre of the cave system. CO2 dilution is less pronounced in summer (CO2(min) ≈ 800 ppm), since the ventilation rate is not as strong as in winter and the outside air that enters the cave through breathing holes and fractures is enriched with soil CO2. During spring and autumn, the daily alternation of the ventilation regime with a smaller rate of air exchange results in yearly cave air CO2 peaks of up to ~ 2400 ppm. Some dead-end passages can be much less affected by ventilation, resulting in a cave air CO2 concentration of up to 7900 ppm. The strongest diurnal CO2 peaks due to human respiration were recorded during the spring holidays (increase of up to 1300 ppm day−1), compared to considerably smaller summer peaks despite peak visits (increase of ~ 600 ppm day−1).

Bergel S.J., Carlson P.E., Larson T.E., Wood C.T., Johnson K.R., Banner J.L., Breecker D.O.

Canonical models for speleothem formation and the subsurface carbon cycle invoke soil respiration as the dominant carbon source. However, evidence from some karst regions suggests that belowground CO 2 originates from a deeper, older source. We therefore investigated the carbon sources to central Texas caves. Drip-water chemistry of two caves in central Texas implies equilibration with calcite at CO 2 concentrations (P CO2_sat ) higher than the maximum CO 2 concentrations observed in overlying soils. This observation suggests that CO 2 is added to waters after they percolate through the soils, which requires a subsoil carbon source. We directly evaluate the carbon isotope composition of the subsoil carbon source using δ 13 C measurements on cave-air CO 2 , which we independently demonstrate has little to no contribution from host rock carbon. We do so using the oxidative ratio, OR, defined as the number of moles of O 2 consumed per mole of CO 2 produced during respiration. However, additional belowground processes that affect O 2 and CO 2 concentrations, such as gas-water exchange and/or diffusion, may also influence the measured oxidative ratio, yielding an apparent OR (OR apparent ). Cave air in Natural Bridge South Cavern has OR apparent values (1.09 ± 0.06) indistinguishable from those expected for respiration alone (1.08 ± 0.06). Pore space gases from soils above the cave have lower values (OR apparent  = 0.67 ± 0.05) consistent with respiration and gas transport by diffusion. The simplest explanation for these observations is that cave air in NB South is influenced by respiration in open-system bedrock fractures such that neither diffusion nor exchange with water influence the composition of the cave air. The radiocarbon activities of NB South cave-air CO 2 suggest the subsoil carbon source is hundreds of years old. The calculated δ 13 C values of the subsoil carbon source are consistent with tree-sourced carbon (perhaps decomposing root matter), the δ 13 C values of which have shifted during industrialization due to changes in the δ 13 C values and concentrations of atmospheric CO 2 . Seasonal variations in P CO2_sat in most of the drip waters suggest that these waters exchange with ventilated bedrock fractures in the epikarst, implying that the subsoil CO 2 source contributes carbon to speleothems.

Hartmann A., Baker A.

Understanding past climatic changes allows us to better understand how our planet will evolve in the future. One important source of information on paleoclimate is the analysis of speleothems that develop in karst caves and conduits due to the dissolution and precipitation of calcite. However, there are many uncertainties in paleoclimatic reconstruction with speleothems; one of them being hydrological variability. Up to now only few studies have considered the impact of hydrological variability on speleothem formation and composition. This review paper will provide an introduction to hydrological processes that have the potential to affect speleothem composition and the hydrological modelling approaches that are able to account for them. It presents the current state of knowledge on paleoclimatic reconstruction using speleothems and shows that many important flow and transport processes have not yet been included in the interpretation of these archives, mostly due to a lack of field information to parametrize them. Possible directions of future research efforts therefore include a better exploration of karst vadose zone processes and new approaches to incorporate this information into simulation models. Finally, we foresee the exciting advances in reconstructing paleohydrology using karst hydrology models combined with speleothem growth rate and geochemical composition to understand how past climate changes affected the hydrological cycle and water availability.

Noronha A.L., Hardt B.F., Banner J.L., Jenson J.W., Partin J.W., James E.W., Lander M.A., Bautista K.K.

Carbon dioxide concentrations in caves are a primary driver of rates of carbonate dissolution and precipitation, exerting strong control on speleothem growth rate and geochemistry. Long-term cave monitoring studies in mid-latitude caves have observed seasonal variability in cave pCO2, whereby airflow is driven by temperature contrasts between the surface and subsurface. In tropical settings, where diurnal temperature cycles are larger than seasonal temperature cycles, it is has been proposed caves will ventilate on daily timescales, preventing cave pCO2 from increasing substantially above atmospheric pCO2. By contrast, the relatively small temperature difference between the surface and subsurface may be insufficient to drive complete ventilation of tropical caves. Here we present results of an 8-year cave monitoring study, including observations of cave pCO2 and carbonate chemistry, at Jinapsan Cave, Guam (13.4°N, 144.5°E). We find that cave pCO2 in Jinapsan Cave is both relatively high and strongly seasonal, with cave pCO2 ranging from 500 - 5000 ppm. The seasonality of cave pCO2 cannot be explained by temperature contrasts, instead we find evidence that seasonal trade winds drive cave ventilation and modulate cave pCO2. Calcite deposition rates at seven drip sites in Jinapsan Cave are shown to be seasonally variable, demonstrating that speleothem growth rates in Jinapsan Cave are strongly affected by seasonal variations in cave pCO2. These results highlight the importance that advection can have on cave ventilation processes and carbonate chemistry. Seasonality in carbonate chemistry and calcite deposition in this cave effect the interpretation of speleothem-based paleoclimate records. This article is protected by copyright. All rights reserved.

Krajnc B., Ferlan M., Ogrinc N.

The objective of this research is to detect abiotic sources of soil CO2 above a subterranean cave in the Slovenian karst region. The research was performed in the forest above Pisani rov (Postojna Cave) near the town of Postojna (SW Slovenia) and also in the cave. Soil gas, atmospheric air and cave air carbon stable isotope composition (δ13CCO2) and CO2 concentration were measured. Sampling and measurements were performed bi-monthly at the test and control sites above the cave. The abiotic source of soil CO2 was estimated using a stable isotope mass balance calculation. Similar seasonal patterns of soil CO2 and δ13CCO2 values were observed at both the test and control sites until spring, with higher levels of CO2 observed in summer and lower in winter. The δ13CCO2 showed the opposite trend, i.e. lower values (−26 to −20 ‰) in summer and higher values (up to −17 ‰) in winter and early spring. In spring, the soil CO2 concentration decreases and the δ13CCO2 value increases only at the control site. A time series of a modelled “isotopically light” endmember revealed large shifts in the data values, due to the presence of an abiotic CO2 source. Results suggest that the subterranean CO2 pool and its ventilation is the main source of soil CO2, accounting for up to 80 % of the soil gas during cold periods. Ventilation from subterranean cavities is an important source of soil CO2 in karstic areas and should be taken into account during carbon cycling studies.

Baker A., Flemons I., Andersen M.S., Coleborn K., Treble P.C.

Cave drip water calcium ion concentration is a primary determinant of speleothem deposition and growth rate. The factors that determine drip water calcium ion concentrations are the soil and vadose zone CO 2 concentrations, and the hydrogeochemical evolution of the water from soil to cave. Here, we use a systematic literature review of cave drip water calcium concentrations, combined with PHREEQC equilibrium modelling, to investigate the global relationship between calcium concentration and surface climate. Our results are discussed in the context of understanding the climatic and environmental controls on drip water calcium concentration, speleothem growth rates and proxies of past climate and environmental change. We use an empirical, global soil CO 2 concentration–temperature relationship to derive PHREEQC modelled cave drip water calcium concentrations. The global mean modelled drip water calcium concentration is close to that observed, but it over-predicts at high and low temperatures, and significantly under-predicts at temperate conditions. We hypothesise that closed system hydrochemical evolution due to water saturation is an important control on carbonate dissolution at colder temperatures. Under warmer conditions, for example temperate climates with a dry and hot or warm summer, seasonally-limited water availability can lead to: 2 concentrations than modelled from soil values are necessary to explain the observed drip water calcium values, which we propose is from an additional source of CO 2 from microbial activity and root respiration in the vadose zone during open system hydrochemical evolution.

Treble P.C., Fairchild I.J., Baker A., Meredith K.T., Andersen M.S., Salmon S.U., Bradley C., Wynn P.M., Hankin S.I., Wood A., McGuire E.

Forest biomass has the potential to significantly impact the chemistry and volume of diffuse recharge to cave dripwater via the processes of nutrient uptake, transpiration and forest fire. Yet to-date, this role has been under-appreciated in the interpretation of speleothem trace element records from forested catchments. In this study, the impact of vegetation is examined and quantified in a long-term monitoring program from Golgotha Cave, SW Australia. The contribution of salts from rain and dry-deposition of aerosols and dissolved elements from soil mineral and bedrock dissolution to dripwater chemistry are also examined. This study is an essential pre-requisite for the future interpretation of trace element data from SW Australian stalagmite records, whose record of past environmental change will include alterations in these biogeochemical fluxes. Solute concentrations in dripwater vary spatially, supporting the existence of distinct flow paths governed by varying amounts of transpiration as well as nutrient uptake by deeply-rooted biomass. Applying principal components analysis, we identify a common pattern of variation in dripwater Cl, Mg, K, Ca, Sr and Si, interpreted as reflecting increasing transpiration, due to forest growth. Mass-balance calculations show that increasing elemental sequestration into biomass has the largest impact on SO4, providing an explanation for the overall falling dripwater SO4 concentrations through time, in contrast to the transpiration-driven rising trend dominating other ions. The long-term rise in transpiration and nutrient uptake driven by increased forest bioproductivity and its impact on our dripwater chemistry is attributed to (i) the post-fire recovery of the forest understorey after fire impacted the site in 2006 CE; (ii) and/or increased water and nutrient demand as trees in the overlying forest mature. The impact of climate-driven changes on the water balance is also examined. Finally, the implications for interpreting SW Australian speleothem trace element records are discussed.

Mattey D.P., Atkinson T.C., Barker J.A., Fisher R., Latin J.-., Durrell R., Ainsworth M.

We put forward a general conceptual model of CO2 behaviour in the vadose zone of karst aquifers, based on physical principles of air flow through porous media and caves, combined with a geochemical interpretation of cave monitoring data. This ‘Gibraltar model’ links fluxes of water, air and carbon through the soil with the porosity of the vadose zone, the circulation of ground air and the ventilation of caves. Gibraltar hosts many natural caves whose locations span the full length and vertical range of the Rock. We report results of an 8-year monitoring study of carbon in soil organic matter and bedrock carbonate, dissolved inorganic carbon in vadose waters, and gaseous CO2 in soil, cave and ground air. Results show that the regime of cave air CO2 results from the interaction of cave ventilation with a reservoir of CO2-enriched ground air held within the smaller voids of the bedrock. The pCO2 of ground air, and of vadose waters that have been in close contact with it, are determined by multiple factors that include recharge patterns, vegetation productivity and root respiration, and conversion of organic matter to CO2 within the soil, the epikarst and the whole vadose zone. Mathematical modelling and field observations show that ground air is subject to a density-driven circulation that reverses seasonally, as the difference between surface and underground temperatures reverses in sign. The Gibraltar model suggests that cave air pCO2 is not directly related to CO2 generated in the soil or the epikarstic zone, as is often assumed. Ground air CO2 formed by the decay of organic matter (OM) washed down into the deeper unsaturated zone is an important additional source of pCO2. In Gibraltar the addition of OM-derived CO2 is the dominant control on the pCO2 of ground air and the Ca-hardness of waters within the deep vadose zone. The seasonal regime of CO2 in cave air depends on the position of a cave in relation to the density-driven ground air circulation pattern which is itself determined by the topography, as well as by the high-permeability conduits for air movement provided by caves themselves. In the steep topography of Gibraltar, caves in the lower part of the Rock act as outflow conduits for descending ground air in summer, and so have higher pCO2 in that season. Caves in the upper Rock have high pCO2 in winter, when they act as outflow conduits for rising currents of CO2-enriched ground air. Understanding seasonal flows of ground air in the vadose zone, together with the origins and seasonal regimes of CO2 in cave air underpins robust interpretation of speleothem-based climate proxy records.

Šebela S., Čar J.

Faimon J., Baldík V., Štelcl J., Všianský D., Rez J., Pracný P., Novotný R., Lang M., Roubal Z., Szabó Z., Hadacz R.

AbstractCorrosion processes were studied in epigenic caves of the Moravian Karst (MK). Samples of naturally corroded speleothems and carbonate rocks collected in various MK caves were compared with limestone and calcite standards that were corroded under well-controlled conditions in the laboratory. Two stages of corrosion were distinguished: (1) an early stage of pitting and linear corrosion predisposed by point and line defects in the standards, and (2) an advanced stage characterized by deep corrosion of the entire standard surface. During the second stage, the internal structure of the corroded substrate remained partially preserved, however, if the water was extremely aggressive, the corrosion progressed to depth en-bloc regardless of the structure. In this paper, we (1) proposed a new corrosion mechanism based on the binding of aqueous calcium by clays and tested it experimentally in the cave, (2) characterized conditions that generally favor the formation of aggressive waters (water undersaturated with calcite) into seven categories incorporating them into a conceptual model of the karst vertical profile, and (3) discussed anthropogenic influences on these processes.

Li Y., Yang Y., Wang X., Luo W., Zhao J., Sun Z., Ye Z., Chen X., Shi X., Xu Y., Baker J.L.

Conveyance and modification of carbon-isotope signals within the karst system remain difficult to constrain, due to the complexity of interactions between multiple components, including precipitation, bedrock, soil, atmosphere, and biota. Cave monitoring is thus critical to understanding both their transport in the karst system and dependence on local hydroclimatic conditions. Jiguan Cave, located in Funiu Mountain in central China, is representative of karst tourist caves with relatively thin epikarst zone. We conducted a comprehensive monitoring program of cave climate from 2018 to 2021 and measured δ13C during 2021 in monthly and heavy-rainfall samples of soil CO2, cave CO2, cave water (drip water and underground river), and underground river outlet. Our results demonstrate synchronous variations between CO2 concentration and δ13CCO2 in both soil and cave air on seasonal time scales. Cave pCO2 and carbon-isotope composition further exhibited a high sensitivity to human respiration with fluctuations of ~2000-3000 ppm within 4 days during the cave closure period in July 2021 without tourists. 13C-depleted isotopic signal of cave air in summer is the mixture of human respiration and soil CO2 which varies as a function of regional hydrological conditions of the summer monsoon during the rainy season with high temperatures and humidity. However, respired CO2 from the overlying soil was expected to be the only principal source of the cave CO2 when the anthropogenic CO2 source was removed. The high seasonal amplitude of cave air δ13CCO2 reflects ventilation dynamics, which leads to a prominent contribution from the external atmosphere during winter. Intriguingly, although the δ13C signal reflects complex vertical processes in the vertical karst profile, a heavy summer rainfall event was related to anomalously high δ13C values of cave water that can be utilized to interpret rainfall intensity and regional hydroclimate.

Liñán C., de Cisneros C.J., Benavente J., Vadillo I., del Rosal Y., Ojeda L.

Following the declaration of the COVID-19 pandemic, the Spanish Government restricted non-essential movements of all citizens and closed all public spaces, such as the Nerja Cave, until May 31, 2020. This particular condition of the closure of the cave provided a unique opportunity to study the micro-climate conditions and carbonate precipitation in this tourist cave without the presence of visitors. Our results show the significant effect of visitors on the air isotopic signature of the cave and on the genesis of the extensive dissolution features affecting the carbonate crystals formed in the tourist sector of the cave, alerting us to the possible corrosion of the speleothems located there. The movement of visitors within the cave also favours the mobilisation of aerial fungi and bacterial spores and their subsequent sedimentation simultaneously with the abiotic precipitation of carbonates from the drip water. The traces of these biotic elements could be the origin of the micro-perforations previously described in the carbonate crystals formed in the tourist galleries of the cave, but they are subsequently enlarged due to abiotic dissolution of the carbonates through these weaker zones.