Estimation of carbon dioxide flux degassing from percolating waters in a karst cave: Case study from Bijambare cave, Bosnia and Herzegovina

Drip water at three sites appears to have been originated from a parent solution with an equilibrium carbon dioxide partial pressure ranging between 15,000 and 26,000 ppm. A large part of the variability in drip water composition observed in the cave can be explained by different stage of degassing. Water composition at several cave pools confirms that drip waters rapidly achieve equilibrium with the cave atmosphere after impact on the stalagmite apex, while oversaturation is retained longer. Water leaving the cave environment towards the phreatic zone has been estimated to be in equilibrium both with cave atmosphere and calcite during summer, while it retains some calcite oversaturation during winter. The difference of DIC between the solution entering and leaving the cave represents the total inorganic carbon lost by degassing into cave atmosphere and by precipitation of calcite. Separating these last two terms can be accomplished using the difference of calcium content of the two solutions. Once these concentrations have been defined, they were converted into fluxes by unit of surface using an average effective infiltration of 497 mm year−1 or 1.6 × 10−5 L m2 s−1. The resulting flux of carbon dioxide degassing from drip water is in the range of 0.03–0.06 µmol m−2 s−1. These values are similar to the results estimated by modelling of carbon dioxide variations in the cave atmosphere.

Understanding the hydrological processes of dissolved organic matter (DOM) in the surface karst zone is crucial for the utilization and safety of groundwater resources. However, research on DOM in drip water from karst caves is limited. In this study, continuous monitoring was conducted for four years at four drip water monitoring sites (MP1, MP2, MP3, and MP9) in Furong Cave, Southwest China. The three-dimensional fluorescence excitation–emission spectroscopy (3D-EEM) and fluorescence region integration (FRI) methods were employed, along with correlation analysis, to investigate the spectral characteristics, composition, sources, and influencing factors of the DOM in the drip water of Furong Cave. The results indicated that (1) the three-dimensional fluorescence peaks in the drip water were classified into six categories: A, B, C, T, Ti, and M. (2) The dominant source of the drip water DOM is endogenous organic matter. (3) The FRI analysis indicates a relatively high proportion of Type II substances in the drip water, predominantly composed of tryptophan-like substances. (4) The DOM in the drip water of Furong Cave was influenced by various factors, including the mixing effect of “new” and “old” water, water residence time in karst systems, and water–rock interactions (WRI), resulting in the complex responses of drip rates and DOM to surface precipitation and temperature at different drip water sites. This study provides a reference for comparative research on DOM in cave drip water in karst regions, which contributes to a better understanding of the migration mechanism of DOM in karst aquifers under different climate and karst ecological conditions.

The origin and environmental dependencies of lamination in stalagmites from Katerloch, common in speleothems from other cave sites, are examined in detail. Petrographic observations and chemical analyses (including isotopes) of stalagmites and modern calcite were combined with multi‐annual cave monitoring. All investigated stalagmites are composed of low‐Mg calcite and show white, porous laminae and typically thinner, translucent dense laminae. The binary lamination pattern results from changes in the calcite fabric: white, porous laminae are characterized by a high porosity and abundant fluid inclusions and also by enhanced vertical growth and thinning towards the flanks. Translucent, dense laminae exhibit a compact fabric and constant thickness of individual growth layers. U‐Th dating supports an annual origin of the lamination and the seasonally changing intensity of cave ventilation provides a robust explanation for the observed relationships between lamination, stable C isotopic compositions and trace elements (Mg, Sr and Ba). The seasonally variable air exchange, driven by temperature contrasts between the cave interior and outside atmosphere, modulates the rate and amount of CO2 degassing from the drip water and affects the hydrochemistry and consequently the fabric of the precipitating calcite. Although cave air composition and drip rate are both major variables in controlling CO2 degassing from the drip water, the seasonally changing ventilation in Katerloch exerts the primary control and the results suggest a secondary (amplifying/attenuating) influence of the drip rate. Drip rate, however, might be the controlling parameter for lamina development at cave sites experiencing only small seasonal cave air exchange. Importantly, the seasonally variable composition of drip water does not reflect the seasonal cycle of processes in the soil zone, but results from exchange with the cave atmosphere. The alternating porous and dense calcite fabric is the expression of a variable degree of lateral coalescence of smaller crystallites forming large columnar crystals. The white, porous laminae represent partial coalescence and form during the warm season: low calcite δ13C values are linked to low δ13C values of cave air and drip water during that time. This observation corresponds to times of reduced cave ventilation, high pCO2 of cave air, low drip water pH, lower calcite supersaturation and typically high drip rates. In contrast, the translucent, dense laminae represent more or less complete lateral coalescence (inclusion‐free) during the cold season (high calcite, drip water and cave air δ13C values), i.e. times of enhanced cave ventilation, low cave air pCO2, increased drip water pH, relatively high calcite supersaturation and typically low drip rates. In essence, the relative development of the two lamina types reflects changes in the seasonality of external air temperature and precipitation, with a strong control of the winter air temperature on the intensity of cave‐air exchange. Thick translucent, dense laminae are favoured by long, cold and wet winters and such conditions may be related closely to the North Atlantic Oscillation mode (weak westerlies) and enhanced Mediterranean cyclone activity during the cold season. Studies of speleothem lamination can thus help to better understand (and quantify) the role of seasonality changes, for example, during rapid climate events.

Caves are natural laboratories to study the subsurface deep biosphere, where drip water connect the caves with outside environments. Drip water may bring microorganisms and DOC from the outside environments into caves which subsequently play a fundamental role in sustaining the oligotrophic subsurface ecosystem. To understand the seasonal dynamics of bacterial communities, their potential metabolic functions, and their association with environmental factors such as precipitation, temperature, drip rate, pH, NO₃⁻, SO₄²⁻, Ca²⁺, Mg²⁺, TOC and FI, drip water samples from Heshang Cave were collected across a two years’ sampling period and subject to 16S rRNA Illumina sequencing and the Biolog-ECO plates analysis. Our results show that the richness and diversity of bacterial communities in the drip water revealed no significant seasonal difference, whereas the compositions of bacterial communities showed a clear seasonal variation, especially <italic>Acinetobacter</italic> and <italic>Pseudomonas</italic>, the dominant groups. In details, the relative abundance of <italic>Acinetobacter </italic>was higher in spring and summer seasons, while lower in autumn and winter seasons. In contrast, the relative abundance of <italic>Pseudomonas</italic> showed an opposite variation to that of <italic>Acinetobacter</italic>. Precipitation, temperature inside and outside the cave, drip rate and conductivity showed strong seasonal variations, whereas other environmental factors exhibited moderate variation within the two years’ period. Among all the environmental factors investigated, precipitation significantly shaped the bacterial community structure, explaining about 27.6% of their seasonal variation. <italic>Pseudomonas</italic> significantly negatively correlated with multiple environmental factors such as precipitation, drip rate and temperature, whereas <italic>Acinetobacter</italic> showed a positive correlation only with the drip rate. Furthermore, potential metabolic functions of bacterial communities in drip water also show clear seasonal variation as indicated both by 16S rRNA PICRUSt and Biolog-ECO plate analysis. The redundancy analysis (RDA) results showed that the relative abundance of <italic>Acinetobacter</italic> demonstrated a significant relationship with the seasonal dynamics of metabolic ability. In addition, <italic>Acinetobacter</italic> revealed a remarkable positive correlation with amino acid metabolism, lipid metabolism, metabolism of terpenoids and polyketid, xenobiotics biodegradation and metabolism, and carboxylic acids metabolism. <italic>Pseudomonas</italic> showed a significant negative correlation with lipid metabolism, metabolism of terpenoids and polyketid, xenobiotics biodegradation and metabolism, and amine metabiolism. Overall, our results showed that precipitation significantly controls the seasonal variation of bacterial communities in drip water and <italic>Acinetobacter</italic> plays an important role in the variation of bacterial functions. This the first report about the bacterial communities and their carbon utilization variation over a 2 years’ period which greatly enhances our understanding about the seasonal dynamics of microbial communities and their correlation with environmental conditions.

The overall conceptual model of the inorganic carbon transport within a karst massif starts from decomposition of organic matter in the soil and epikarst which produces carbon dioxide. The biological activity gives rise to a gas phase in the porous soil with high CO2 partial pressure, up to several tens of thousands ppm. Most of the carbon dioxide is released to the atmosphere, but a fraction is dissolved in the percolating water. When water enriched with soil CO2 comes into contact with limestone in the soil (pebbles) or the epikarst, the calcium carbonate starts to be dissolved. In this process, carbon dioxide in water reacts with carbonate ions to form bicarbonate ions. Two end-members can be considered: (a) dissolution of carbonates occurs without presence of a gas phase (closed system); (b) dissolution process occurs in the presence of a carbon dioxide rich gas phase (open system). In this last case additional CO2 is dissolved in the water enhancing the amount of calcium carbonate that can be kept into solution (and therefore the total amount of inorganic carbon). Water percolating through the vadose zone may reach the cave atmosphere with lower carbon dioxide content and therefore excess of CO2 is released until a new equilibrium condition is achieved. The decrease of carbon dioxide content in the solution creates an oversaturation with respect to calcium carbonate, and consequently calcite precipitates. Direct diffusion from soil and epikarst voids and direct human release may be considered as supplementary sources of carbon dioxide in the cave atmosphere. Other potential sources such as decomposition of large quantity of organic matter in the cave and from deep seated sources have not been considered in this conceptual model. Carbon dioxide in cave air is transported between underground passages by advection and then released to the external atmosphere. Part of the inorganic carbon stays in the water and eventually reaches the phreatic zone, where it is stored until it ultimately exits at springs.

The study of cave waters is important for the preservation of the Castle Cave, which lies underneath the Buda Castle Hill. The objectives of this research were to carry out quantitative measurements of drip water and to identify the changes from previous observations. This study also aimed to investigate the relationship between drip water locations and drip water and precipitation data. The present study employed self-made tipping bucket gauges and ad hoc drip water measurements. Correlation analysis was carried out on the values measured at the two drip locations and on the drip water and precipitation time series. Annual drip water volumes were calculated, and the results were compared with past measurements. The results indicate that there is a strong correlation between the two drip locations, and both have a weak connection to precipitation. The annual drip water volume has possibly increased compared with past data, based on the study period. Further research with longer measured data and water chemistry tests is needed to determine the origin of the cave waters.

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.

3H–3He measurements constitute a well-established method for the determination of the residence time of young groundwater. However, this method has rarely been applied to karstified aquifers and in particular to drip water in caves, despite the importance of the information which may be obtained. Besides the determination of transfer times of climate signals from the atmosphere through the epikarst to speleothems as climate archives, 3H–3He together with Ne, Ar, Kr, Xe data may also help to give new insights into the local hydrogeology, e.g. the possible existence of a perched aquifer above a cave. In order to check the applicability of 3H–3He dating to cave drips, we collected drip water samples from three adjacent caves in northwestern Germany during several campaigns. The noble gas data were evaluated by inverse modelling to obtain recharge temperature and excess air, supporting the calculation of the tritiogenic 3He and hence the 3H–3He age. Although atmospheric noble gases were often found to be close to equilibrium with the cave atmosphere, several drip water samples yielded an elevated 3He/4He ratio, providing evidence for the accumulation of 3He from the decay of 3H. No significant contribution of radiogenic 4He was found, corresponding to the low residence times mostly in the range of one to three years. Despite complications during sampling, conditions of a perched aquifer could be confirmed by replicate samples at one drip site. Here, the excess air indicator ΔNe was about 10 %, comparable to typical values found in aquifers in mid-latitudes. The mean 3H–3He age of 2.1 years at this site presumably refers to the residence time in the perched aquifer and is lower than the entire transit time of 3.4 years estimated from the tritium data.

Carbon and oxygen isotope systematics in cave environments: Lessons from an artificial cave “McMaster Cave”

Soil loss through fissures and its responses to rainfall based on drip water monitoring in karst caves

Seagrass meadows are one of the most productive ecosystems on the planet, but their photosynthesis rate may be limited by carbon dioxide but mitigated by exploiting the high concentration of bicarbonate in the ocean using different active processes. Seagrasses are declining worldwide at an accelerating rate because of numerous anthropogenic pressures. However, rising ocean concentrations of dissolved inorganic carbon, caused by increases in atmospheric carbon dioxide, may benefit seagrass photosynthesis. Here we compare the ability of two seagrass from the Mediterranean Sea, Posidonia oceanica (L.) Delile and Zostera marina L., to use carbon dioxide and bicarbonate at light saturation, and model how increasing concentrations of inorganic carbon affect their photosynthesis rate. pH-drift measurements confirmed that both species were able to use bicarbonate in addition to carbon dioxide, but that Z. marina was more effective than P. oceanica. Kinetic experiments showed that, compared to Z. marina, P. oceanica had a seven-fold higher affinity for carbon dioxide and a 1.6-fold higher affinity for bicarbonate. However, the maximal rate of bicarbonate uptake in Z. marina was 2.1-fold higher than in P. oceanica. In equilibrium with 410 ppm carbon dioxide in the atmosphere, the modelled rates of photosynthesis by Z. marina were slightly higher than P. oceanica, less carbon limited and depended on bicarbonate to a greater extent. This greater reliance by Z. marina is consistent with its less depleted 13C content compared to P. oceanica. Modelled photosynthesis suggests that both species would depend on bicarbonate alone at an atmospheric carbon dioxide partial pressure of 280 ppm. P. oceanica was projected to benefit more than Z. marina with increasing atmospheric carbon dioxide partial pressures, and at the highest carbon dioxide scenario of 1135 ppm, would have higher rates of photosynthesis and be more saturated by inorganic carbon than Z. marina. In both species, the proportional reliance on bicarbonate declined markedly as carbon dioxide concentrations increased and in P. oceanica carbon dioxide would become the major source of inorganic carbon.

Variations and significance of Mg/Sr and 87Sr/86Sr in a karst cave system in southwest China

When calcite precipitates in caves, its carbon stable isotope signature can be modified by the CO2 outgassing gradient between drip water and cave atmosphere. This effect is modulated by the water residence time in the cave, from its emergence in the cave until the deposition of calcite. Moreover, CO2 solubility, calcite precipitation rate, and isotopic fractionation are controlled by temperature. Here, we present up to date results of an ongoing monitoring study at Ascunsa Cave (Romania), exploring the relationship between farmed calcite δ13C, drip rate, and CO2 outgassing. In addition to measuring CO2 concentration in cave air, we also measured the CO2 concentration in the headspace of a water-air equilibrator that collects drip water without exposing it to cave atmosphere, preventing outgassing. δ13C from calcite farmed at two neighboring stalagmites with different drip rates was also measured. Although caves have generally stable temperatures, we show here that temperature inside Ascunsa and Isverna caves has risen by more than 2°C over the course of a year, bearing important implications for stable isotopic fractionation equations and CO2 dynamics. Our results show that δ13C of farmed calcite has a strong relationship with drip rate at the slow dripping site, but no correlation at the faster dripping site. These two sites are also different when δ13C is compared to the outgassing gradient. At the slower drip site, δ13C and the outgassing gradient are directly correlated, whereas at the faster drip site their correlation is inverse. Our study brings new light onto speleothem δ13C behavior in general, and at Ascunsa Cave in particular, which is crucial for understanding the paleoclimate information captured by speleothems from this cave or elsewhere.

Microbiology and geochemistry in a hydrogen-sulphide-rich karst environment

Long-term monitoring of drip water and groundwater stable isotopic variability in the Yucatán Peninsula: Implications for recharge and speleothem rainfall reconstruction