Interpreting and modelling the daily extreme sediment events in karst mountain watersheds.

Increasingly frequent extreme rainfall as a result of climate change is strongly damaging the global soil and water environment. However, few studies have focused on daily extreme sediment events (DESE) in heterogeneous karst watersheds based on long-term in-situ observations. This study quantitatively assessed the time effect of DESE on rainfall response, decoupled the impact of environmental factors on DESE by using structural equation modelling, and finally explored the modelling scheme of DESE based on the hybrid model. The results showed that DESE had the highest frequency of occurrence in May-July, with dispersed distribution in the value domain. Rainfall with a time lag of 1 day and a time accumulation of 2 or 3 days was an important contribution to DESE (P < 0.01, R = 0.47-0.68). Combined effects of environmental factors explained 53.6 %-64.1 % of the variation in DESE. Runoff and vegetation exerted the strongest direct and indirect effects on DESE, respectively (ß = 0.66/-0.727). Vegetation was the dominant driver of DESE in Dabanghe and Yejihe (ß = -0.725/-0.758), while the dominant driver in Tongzhihe was climate (ß = 0.743). In the future, the risk of extreme sediments should be prevented and resolved through the comprehensive regulation of multiple paths, such as runoff and vegetation. Hybrid models significantly improved the modelling performance of machine learning models. Generalized additive model-Extreme gradient boost had the best performance, while Partial least squares regression-Extreme gradient boost was the most valuable when considering performance and input data cost. Two methods can be used as recommended solutions for DESE modelling. This study provides new and in-depth insights into DESE in karst watersheds and helps the region develop forward-looking soil and water management models to cope with future extreme erosion hazards.

Sources and transport of CO2 in the karst system of Jiguan Cave, Funiu Mountains, China.

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.

A Multimodal Research Approach to Assessing the Karst Structural Conditions of the Ceiling of a Cave with Palaeolithic Cave Art Paintings: Polychrome Hall at Altamira Cave (Spain).

Integrating geomatics remote sensing technologies, including 3D terrestrial laser scanning, unmanned aerial vehicles, and ground penetrating radar enables the generation of comprehensive 2D, 2.5D, and 3D documentation for caves and their surroundings. This study focuses on the Altamira Cave's karst system in Spain, resulting in a thorough 3D mapping encompassing both cave interior and exterior topography along with significant discontinuities and karst features in the vicinity. Crucially, GPR mapping confirms that primary vertical discontinuities extend from the near-surface (Upper Layer) to the base of the Polychrome layer housing prehistoric paintings. This discovery signifies direct interconnections helping with fluid exchange between the cave's interior and exterior, a groundbreaking revelation. Such fluid movement has profound implications for site conservation. The utilization of various GPR antennas corroborates the initial hypothesis regarding fluid exchanges and provides concrete proof of their occurrence. This study underscores the indispensability of integrated 3D mapping and GPR techniques for monitoring fluid dynamics within the cave. These tools are vital for safeguarding Altamira, a site of exceptional significance due to its invaluable prehistoric cave paintings.

Microplastics in caves: A new threat in the most famous geo-heritage in the world. Analysis and comparison of Italian show caves deposits.

Microplastic pollution represent a worldwide concern, however, in karst areas is still largely unknown, especially in underground environments. Caves are the most important geological heritage worldwide, rich in speleothems, unique ecosystems custodians of important drinking water reserves, and a significant economic resource. Thank to their relatively stable environmental conditions, they can preserve information for a long time such as paleontological/archaeological remains, however, these characteristics make caves vulnerable environments too, easily damaged by climate variations and pollution. To increase the current knowledge of microplastic pollution, the deposits of different Italian show caves were investigated, improving the method for microplastic separation. Microplastic were identified and characterised using MUPL automated software, observed with and without UV light under a microscope, and verified under µFTIR-ATR, highlighting the importance of combine different methods. Microplastics were present in sediments of all examined caves, and were always greater along the tourist route (an average of 4300 items/kg) than the speleological areas (an average of 2570 items/kg). Microplastics less than 1 mm dominated the samples and the amount increased with the decrease in the size considered. Fibre-shaped dominated the samples and 74% particles was fluorescent under UV light. Analysed sediment samples contained especially polyesters and polyolefins. Our results highlight the presence of microplastic pollution in show caves, giving useful information to assess risks posed by microplastics in show caves and emphasizing the importance of pollutants monitoring in underground environments to define strategies for the conservation and management of caves and natural resources.

Microplastic pollution in show cave sediments: First evidence and detection technique.

Microplastic particles are a global problem, which has been widely found in marine and terrestrial environments. However, microplastic pollution in caves and karst aquifers is still poorly studied. To improve the current knowledge of microplastic pollution, we investigated the sediments of a show cave in Italy. We developed a methodology based on a cave-adapted version of the methods used in several studies to detect microplastics from sediments of different environments and with various laboratory tests. The microplastics were extracted from sediments via density separation and subjected to organic matter removal. Filters were observed with and without UV light under a microscope, before and after organic matter removal, and the microplastics were characterised according to shape, colour, and size, with visual identification. About 55% of the fibres observed under the microscope on filters were removed via organic matter removal. An average of 4390 items/kg dry weight was calculated for the touristic zone and 1600 items/kg dry weight for the speleological/research section. Fibre (84.9%) was the most abundant shape, and most microplastics were smaller than 1 mm, accounting for 85.4%, of which 58.4% were shorter than 0.5 mm. The highest microplastic abundance was fluorescent under UV light (87.7%); however, 12.3% of the microplastics observed on filters were not fluorescent. Most fluorescent fibres were transparent (84%), whereas blue (46.1%) and black (22.4%) fibres were more common for the non-fluorescent ones. Our results highlight the presence of microplastics in show caves, and we provide a valid non-invasive and non-expensive analytical technique for the preparation and isolation of microplastics from cave sediments, giving useful information for evaluating the environmental risks posed by microplastics in show caves.

Evaluation of natural background levels of high mountain karst aquifers in complex hydrogeological settings. A Gaussian mixture model approach in the Port del Comte (SE, Pyrenees) case study.

The hydrogeological processes driving the hydrochemical composition of groundwater in the alpine pristine aquifer system of the Port del Comte Massif (PCM) are characterized through the multivariate statistical techniques Principal Component Analysis (PCA) and Gaussian Mixture Models (GMM) in the framework of Compositional Data (CoDa) analysis. Also, the groundwater Natural Background Levels (NBLs) for NO3 and SO4 and Cl are evaluated, which are specially important for indicating the occurrence of groundwater contamination derived from the anthropic activities conducted in the PCM. The different hydrogeochemical facies found in the aquifer system of the PCM comprises low mineralized Ca-HCO3 water for the main Eocene karst aquifer, and Ca-SO4 and highly mineralized NaCl water types in the minor aquifers discharging from the PCM. The NBL values of SO4, Cl and NO3 obtained for the main karst aquifer are 14.33, 4.06 and 6.55 mg/L, respectively. These values are 35, 3 and 1.2 times lower than the respective official NBLs values that were determined by the water administration to be compared with in the case of conducting a pollution assessment characterization in the main karst aquifer. Official overestimation of NBLs can put important groundwater resources in the PCM at risk.

'Henicorhynchus' thaitui, a new species of cavefish from Central Vietnam (Teleostei, Cyprinidae).

'Henicorhynchus' thaitui sp. nov. is described from a subterranean stream in a karst cave in Phong Nha Ke Bang National Park, Quang Binh Province, Central Vietnam. It differs from all congeners in having a pale pink body in life, smaller eyes with diameter less than the maxillary barbel length, and two pairs of barbels, the maxillary barbel being much longer than the rostral barbel.

Biofilm forming bacteria and archaea in thermal karst springs of Gellért Hill discharge area (Hungary).

The Buda Thermal Karst System (BTKS) is an extensive active hypogenic cave system located beneath the residential area of the Hungarian capital. At the river Danube, several thermal springs discharge forming spring caves. To reveal and compare the morphological structure and prokaryotic diversity of reddish-brown biofilms developed on the carbonate rock surfaces of the springs, scanning electron microscopy (SEM), and molecular cloning were applied. Microbial networks formed by filamentous bacteria and other cells with mineral crystals embedded in extracellular polymeric substances were observed in the SEM images. Biofilms were dominated by prokaryotes belonging to phyla Proteobacteria, Chloroflexi and Nitrospirae (Bacteria) and Thaumarchaeota (Archaea) but their abundance showed differences according to the type of the host rock, geographic distance, and different water exchange. In addition, representatives of phyla Acidobacteria, Actinobacteria, Caldithrix, Cyanobacteria, Firmicutes Gemmatimonadetes, and several candidate divisions of Bacteria as well as Crenarchaeota and Euryarchaeota were detected in sample-dependent higher abundance. The results indicate that thermophilic, anaerobic sulfur-, sulfate-, nitrate-, and iron(III)-reducing chemoorganotrophic as well as sulfur-, ammonia-, and nitrite-oxidizing chemolithotrophic prokaryotes can interact in the studied biofilms adapted to the unique and extreme circumstances (e.g., aphotic and nearly anoxic conditions, oligotrophy, and radionuclide accumulation) in the thermal karst springs.

Persistent organic pollutants in shallow percolated water of the Alps Karst system (Zugspitze summit, Germany).

In the German Calcareous Alps at the Zugspitze, percolated water close to a permafrost bedrock in a tunnel system was monitored long-term for polychlorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDF), polychlorinated biphenyls (PCB), polycyclic aromatic hydrocarbons (PAH), and 28 organochlorine pesticides (OCP). Semi-permeable membrane devices (SPMD) were deployed in a temporary surface water system at the Zugspitze plateau and analysed for PCB, PAH, and OCP. The high-volume water sampling was successfully implemented and all compounds were identified in the water percolated through the Karst system. However, the percentage distribution of contaminants in the percolated water differed significantly from that found in surface waters. The highest chlorinated PCDD homologues were the predominant compounds of the PCDD/F family, whereas percentages of PCB #52 increased in percolated water. Toxic equivalent values (TEQ) of samples ranged from 2.0 to 4.2pgTEQ/m3 and from 0.017 to 0.069pgTEQ/m3 for PCDD/F and PCB, respectively. Low and intermediate molecular weight PAH were the prevailing compounds in the samples. Endosulfan sulfate, endrin, and cis-heptachlor epoxide were enhanced after water percolation through the Karst system in comparison with the surface waters (wet deposition). The relative enrichment on these pesticides was related to the environmental bedrock conditions and glacier melting sources. In summary, the Karst system highly influenced the fate of organic persistent pollutants generating different chemical patterns in their percolated waters than those found at the surface systems.

In-stream metabolism and atmospheric carbon sequestration in a groundwater-fed karst stream.

Atmospheric carbon sequestered in karst systems through dissolution of carbonate minerals is considered to have no net effect on long-term regional and global carbon budgets because precipitation of dissolved carbonate minerals emits CO2 back to the atmosphere. Even though recent studies have implied that rapid kinetics of carbonate dissolution coupled with the aquatic photosynthetic uptake of dissolve inorganic carbon (DIC) could facilitate a stable atmospheric C sink in karst rivers and streams, little is known about the magnitudes and long-term stability of this C sink. To assess in-stream biogeochemical processes and their role on stream C cycling, we measured diel cycles of water characteristics and chemical composition (temperature, pH, DO, SpC, DIC, Ca2+, δ13CDIC) in a groundwater-fed karst stream in southwest China. Our results show no diel variations at the groundwater discharge point (CK site) due to the absence of a sub-aquatic community (SAC). However, all hydrochemical parameters show significant diel cycle 1.3km downstream (LY site). Diel variations in pH, DO, and δ13CDIC were inversely related to diel changes in SpC, DIC, Ca2+ and pCO2. This result indicates that in-stream metabolism (photosynthesis and respiration) of SAC controls diel variations in stream water chemistry. Significant diel cycles of net ecosystem production (NEP) influences in-stream diel fluctuation of pH, DO, SIc, DIC, pCO2, Ca2+ and δ13CDIC, with gross primary production (GPP) dominating in day and ecosystem respiration (ER) dominating at the night. Absence of in-stream metabolism at CK enhances CO2 degassing from stream to the atmosphere, which is estimated to be 3-5 times higher than at LY. We estimate the carbon sink through in-stream metabolism of SAC to be 73tCkm-2a-1, which is around half the rate of the oceanic biological pump. These results imply in-stream photosynthesis sequesters DIC originating from karst weathering and controls CO2 evasion.

Spring response to precipitation events using δ(18)O and δ(2)H in the Tanour catchment, NW Jordan.

The Tanour spring is one of the several karst springs located in the northern part of Jordan. Water samples from the Tanour spring and precipitation were collected in the area of Ajloun in NW Jordan for the analysis of stable oxygen and hydrogen isotopes to evaluate the spring response to precipitation events. Rainwater and snow samples were collected from different elevations during winters of 2013-2014 and 2014-2015. In addition, spring samples were collected between December 2014 and March 2015. δ(18)O values in rainwater vary from -3.26 to -17.34 ‰ (average: -7.84 ± 3.23 ‰), while δ(2)H values range between -4.4 and -110.4 ‰ (average: -35.7 ± 25.0 ‰). Deuterium excess ranges from 17.8 to 34.1 ‰ (average: 27.1 ± 4.0 ‰). The Local Meteoric Water Line for the study area was calculated to be δ(2)H = 7.66*δ(18)O + 24.43 (R(2) = 0.98). Pre-event spring discharge showed variation in δ(18)O (range -6.29 to -7.17 ‰; average -6.58 ± 0.19 ‰) and δ(2)H values (range -28.8 to -32.7 ‰; average: -30.5 ± 1.0 ‰). In contrast, δ(18)O and δ(2)H rapidly changed to more negative values during rainfall and snowmelt events and persisted for several days before returning to background values. Spring water temperature, spring discharge, and turbidity followed the trend in isotopic composition during and after the precipitation events. The rapid change in the isotopic composition, spring discharge, water temperature, and turbidity in response to recharge events is related to fast water travel times and low storage capacity in the conduit system of the karst aquifer. Based on the changes in the isotopic composition of spring water after the precipitation events, the water travel time in the aquifer is in the order of 5-11 days.

Comparative Study of Lead Concentration in Feathers of Urban and Rural Passerines in Merida, Mexico.

Lead is a commonly monitored heavy metal because of potential health effects on exposed organisms. We quantified lead in secondary feathers of two passerine bird species, clay-colored thrushes (Turdus grayi) and great-tailed grackles (Quiscalus mexicanus), from an urban and a rural site in the municipality of Merida, Yucatan. Urban lead concentration was significantly higher than its rural counterpart for both species (p < 0.05). In the urban site, lead concentration was similar in both species (p = 0.14). However, data from the rural site showed that lead concentration was significantly higher in thrush feathers (p < 0.05). Lead levels herein presented are among the lowest ever reported suggesting that either lead accumulation or absorption is limited. Finally, our data seem to support the hypothesis that species feeding ecology plays a major role in lead accumulation.

Diversity and morphological structure of bacterial communities inhabiting the Diana-Hygieia Thermal Spring (Budapest, Hungary).

The Buda Thermal Karst System is an active hypogenic karst area that offers possibility for the analysis of biogenic cave formation. The aim of the present study was to gain information about morphological structure and genetic diversity of bacterial communities inhabiting the Diana-Hygieia Thermal Spring (DHTS). Using scanning electron microscopy, metal accumulating and unusual reticulated filaments were detected in large numbers in the DHTS biofilm samples. The phyla Actinobacteria, Firmicutes and Proteobacteria were represented by both bacterial strains and molecular clones but phyla Acidobacteria, Chlorobi, Chlorofexi, Gemmatimonadetes, Nitrospirae and Thermotogae only by molecular clones which showed the highest similarity to uncultured clone sequences originating from different environmental sources. The biofilm bacterial community proved to be somewhat more diverse than that of the water sample and the distribution of the dominant bacterial clones was different between biofilm and water samples. The majority of biofilm clones was affiliated with Deltaproteobacteria and Nitrospirae while the largest group of water clones was related to Betaproteobacteria. Considering the metabolic properties of known species related to the strains and molecular clones from DHTS, it can be assumed that these bacterial communities may participate in the local sulphur and iron cycles, and contribute to biogenic cave formation.