8:30am - 8:45am
LONG-TERM ELEMENTAL TRENDS IN DRIP WATERS FROM MONITORING BUNKER CAVE: NEW INSIGHTS FOR PAST PRECIPITATION VARIABILITY
1Institute for Geosciences, Johannes Gutenberg University Mainz, Mainz, 55128, Germany; 2Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Bochum, 44801, Germany; 3Institute of Environmental Physics, Ruprechts-Karls-University Heidelberg, Heidelberg, 69120, Germany
Long-term cave monitoring studies are important to detect changes in proxies on time scales, which are predominantly preserved in speleothems. In this study, we present data from a seven-year-long monitoring in Bunker Cave (northwest Germany) from 2006 to 2013. Five drip sites in Bunker Cave were monitored for their drip rates and drip and soil waters were analyzed for their cation and anion concentrations. Furthermore, the amount of rainwater and its cation and anion concentrations were monitored as well. Additionally, the concentration of Ca, Mg, and Sr were analyzed from the two host rock components limestone and dolomite.
The drip water results distinguish the origin of the cations and anions from anthropogenic sources, soil, and host rock. The source of Ca2+, Mg2+, Sr2+, HCO3-, and SO42- is the host rock and in particular for SO42- the pyrite in the limestone. Chlorite and NO3- are from anthropogenic sources such as aerosols from road salt, fertilizers, and industry. Drip water at site TS 1 is strongly influenced by a seasonal fracture flow, providing water with a high load of ions during winter and spring. Drip water of site TS 5 shows low and constant Mg2+ and Sr2+ concentrations indicating that it is fed by dissolved prior calcite precipitation (PCP). The other three drip sites (TS 2, 3, and 8) are strongly influenced by PCP and drip sites TS 2 and 8 in addition by incongruent dolomite dissolution, indicated by increasing Mg2+ and Sr2+ concentrations. Furthermore, these drip sites show increasing Cl- and decreasing SO42- concentrations with time.
The decrease in precipitation/infiltration from 2000 to 2013 is not very pronounced, however, the continuous draining of the aquifer is visible in the continuously decreasing drip rates in the cave. This induces the increasing Mg/Ca and Sr/Ca ratios, due to PCP, increasing Cl- concentrations and decreasing SO42- concentrations at the drip sites TS 2, 3, and 8. Therefore, these four proxies are related to precipitation/infiltration. Mg/Ca and Sr/Ca are the strongest proxies for reconstructing past precipitation/infiltration from speleothems. Furthermore, the sulphur concentration may be a proxy for this as well, in case of a geogenic source from the host rock.
Riechelmann, D.F.C., Riechelmann, S., Schröder-Ritzrau, A., 2022. Long-term elemental trends in drip waters from monitoring Bunker Cave: New insights for past precipitation variability. Chemical Geology 590, 120704.
8:45am - 9:00am
Characterizing the hydrological dynamics of the karstic unsaturated zone above Villars Cave, SW-France, using Time-lapse electric resistivity tomography
1Environnements et Paléoenvironnements Océaniques et Continentaux (EPOC), UMR CNRS, 5805, Université de Bordeaux, 33615 Pessac Cedex, France; 2CNRS, Arts et Metiers Institute of Technology, Bordeaux INP, INRAE, I2M Bordeaux, Université de Bordeaux, 33400 Talence, France; 3School of Civil Engineering and Mechanics, Yanshan University, Qinhuangdao, PR China
Subsurface geophysics, especially 2D time-lapse electrical resistivity tomography (TL-ERT), is considered an effective method for studying the links between hydrological and geological features in karst research. This study presents results from the Villars Cave site, South-West France, where we compared the long-term hydrological time series and the images of TL-ERT made above the cave covering the karst critical zone (KCZ) during a 2-year monitoring program. Between February 2020 and January 2022, the 71 m long profile (with probe spacing of 1m) was monitored at the same place above the cave, by the concatenation of Pole-Dipole and Gradient arrays. It provided information of electric resistivity spatial and seasonal changes, which can be summarized as follows: (1) TL-ERT allows to visualize cave galleries and water storage, wetting fronts, and potential preferential flow path changes at the seasonal inter-annual scales. (2) To visualize the spatial resistivity variability, 12 interpreted resistivity models were combined into one composite model using a multi-dimensional statistical approach (hierarchical agglomerative clustering, HAC), dividing the ERT image into seven clusters. The individual blocks gathered into 5 clusters from the synthetic model show a similar resistivity seasonal variability (Classes 2-6), which may be obviously related to water excess data (rainfall - evapotranspiration > 0, also known as efficient rainfall) However, clusters 1 and 7 only exhibits high resistivity without obvious seasonal variability. (3) The difference in the time delay (several days to months) between long-term drip water monitoring and water excess reveals the discrepancy of multiple infiltration routes and karst reservoirs dynamics. This was confirmed by our ERT images on the seasonal as well as year-to-year changes which show significant resistivity changes in the water storage zone. By comparing drip-rate monitoring inside the Villars cave, meteorological data, and TL-ERT profiles during 2 years, we studied the complex links between water content, electrical conductivity, and hydrological variability in the infiltration zone above cave galleries. Results revealed heterogeneity of the karst hydrological system, which is meant not only for understanding the hydrological processes in the karst unsaturated zone but also to improve our understanding of paleoclimate records from inside the cave stalagmites.
9:00am - 9:15am
New monitoring data from Ascunsă Cave, Romania
1Emil Racovita Institute of Speleology, Bucharest, Romania; 2Research Institute of the University of Bucharest, Romania; 3Department of Geography and Environmental Sciences, Northumbria University, Newcastle-upon-Tyne, United Kingdom; 4Bjerknes Centre for Climate Research and Department of Earth Science, University of Bergen, Norway; 5Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA; 6National Research and Development Institute for Cryogenics and Isotopic Technologies (ICSI), Râmnicu Vâlcea, Romania; 7Faculty of Geography, University of Bucharest, Romania;
We present here the latest results from the long term monitoring program at Ascunsă Cave, Romania. For the past eight years we measured physical and chemical parameters in cave air and drip water, such as stable isotopes, CO2, drip rates, temperature, or pH, in an effort to better understand speleothem paleoenvironmental proxies from this cave.
A substantial variation of the measured values was observed between 2019 and 2021. The temperature in the cave increased by 3°C, and large shifts in drip water and farmed calcite δ18O took place at the same time. This offered the rare opportunity to observe isotopic changes over a wide and otherwise uncommon temperature range for Ascunsă cave.
Apart from already published data, we present δ13C values of DIC, in order to gain a better understanding of carbon isotope dynamics during calcite precipitation and its relationship with CO2 outgassing and reaction time. As well, we completed the measurement of farmed calcite clumped isotopes, covering the period between 2016 and 2021. The results show that clumped isotopes did not respond to temperature change and we will discuss here the implications of this observations for speleothem clumped isotope paleothermometry.
9:15am - 9:30am
Cave temperature in Australasian caves
The ACKMA Cave Climate Team
Average cave air temperature approximates to the average external air temperature. But to what extent? Cave air temperature is a function of conduction of heat from the surface; advection of heat by air and water; the geomorphology of the cave, and surface conditions (shaded or unshaded, aspect). With the continued search for a precise and accurate speleothem paleothermometer, improving our understanding of the relationship between cave temperatures and external temperature is timely.
Here, we report the results of the first two years of the Australasian Cave and Karst Management Association (ACKMA) initiative to baseline monitor the cave climate of Australasian show caves during and after COVID-19 lockdown. Fifty QP 6013 temperature loggers were cross-calibrated, including temperature calibration against a Fluke reference standard thermocouple. Temperature precision was < 0.1 °C and accuracy < 0.2 °C. Since June 2020, forty-four loggers have been collecting climate data at 10-minute intervals in 27 caves: one logger is placed outside the cave or caves to collect local external climate data, and other loggers placed inside each cave to assess natural cave climate variability and the potential effect of tour groups.
Our results show that in general, visitor impacts on cave temperature are negligible so far, due to professionally managed visitor numbers, and that natural processes control cave climate. Initial analyses (June 2020-April 2022) have been made at 14 caves that have more than one year of data. Cave air temperatures range between median values of 8.3 and 20.5 °C. The caves vary between weakly and strongly ventilated systems, as demonstrated by the cave air temperature variability of individual caves between 0.1 to 2.4 °C (1s). Where multiple caves have been monitored at one location, the differences between median cave air temperature range between 0.0 to 1.7 °C. Within-site differences in median cave air temperature are observed for both well ventilated and less ventilated caves, indicating that differences in surface land cover (controlling direct surface heating), aspect and surface microclimate are determining the within-site variability. For all caves, preliminary comparison with median external air temperatures demonstrates a correlation of 0.91 (Spearman’s Rho) and that cave air temperature can be estimated from external air temperature with a ±2.4 °C confidence (95% prediction bands).
These initial results from a systematic, regional cave climate monitoring program suggest there is merit in an expanded cave climate monitoring program that focuses on paired measurements of cave air temperature and external air temperatures at a cave locality to quantify the relative importance of the various local controls on land surface heating above a cave. Our observations of within-site variability of median cave air temperatures of up to 1.7 °C due to differences in land-cover agree with previously published data (e.g. a cooling of 2 °C over time due to afforestation, Domińquez-Villar et al. 2013, doi:10.1016/j.epsl.2013.03.017). Caution is warranted in the interpretation of speleothem paleotemperature proxies at locations where past land cover changes have occurred.
9:30am - 9:45am
Numerical Modeling of a ventilated Cave by Coupling Heat and Mass Transfer between Rock and Air
1Swiss Institute for Speleology and Karst Studies- SISKA, Switzerland; 2Center for Hydrogeology and Geothermics (CHYN), University of Neuchâtel, 2000 Neuchâtel, Switzerland; 3Université Paris-Saclay, CNRS, FAST, 91405, Orsay, Rue André Rivière, France; 4Sorbonne Université, UFR 919, 4 place Jussieu, F-75252, Paris Cedex 05, France
The thermal response and temperature signals in ventilated cave systems subject to chimney effects due to local climate variations can provide important information about the structural, hydrogeological and geothermal features of underground processes. In this research, different mechanisms of heat transfer associated with air circulations, including free convection of air, conduction in the rock and latent heat exchange by evaporation or condensation are being modeled by a consistent coupled system of partial differential equations and proper boundary conditions. The effect of each mechanism of heat exchange on the total heat transfer are determined individually under different hydro-thermo-geological settings leading to a comprehensive insight into the dominant heat transfer mechanism and the pertinent variables. The model aims to quantify the air and rock temperature profile along the conduit length for a one-year period to be compared with the field data measured in different parts of the cave. The effect of cave diameter, cave closure by a temporary lake and air humidity are examined under variation of external atmosphere temperature at cave entrances. The proposed numerical model can predict the thermal penetration length along the conduits, which is controlled by the heat exchanges between the rock and the air flow. The model calculates the relative humidity as well as water-vapor exchange rate along the cave and also can indicate rock temperature distribution surrounding the cave wall. Our preliminary results contribute to a better understanding of the long-term cave dynamics and may support a quantitative interpretation of speleothem records.
9:45am - 10:00am
A 114 000-year-old convection current accounts for the growth of aragonite needles in the Esparros cave
1University of Toulouse, France; 2Géologie Environnement Conseil, France; 3Université Paris Cité, IPGP, France; 4Université de Bordeaux, France; 5Xi'an Jiaotong University, China; 6University of Lorraine, France; 7Gouffre d'Esparros, France
The Esparros cave, renowned for its gorgeous clusters of aragonite needles, was discovered in 1938 by Norbert Casteret (1897-1987). Aragonite growth on both walls and speleothems, is arranged below a well-defined boundary that can be traced using 3D laser scanning for several hundred meters along the gallery. Previous interpretations considered this upper limit of aragonite crystallization as the imprint of a fossil flow, air or water. The recent growth of additional aragonite needles along pathways built to accommodate tourist visits, suggests instead an active system.
The combined installation of autonomous temperature sensors and Piche evaporimeters reveal the presence of two air masses permanently superimposed in the gallery: the lower air mass is undersaturated and slightly cooler than the upper saturated air mass. The boundary between the two air masses follows the upper limit of aragonite growth. A fog machine was used in combination with two particle counters placed in both air masses at different locations to confirm the presence of a very slow air circulation that dries out the bottom part of the gallery, thus allowing the formation of aragonite concretions. At the end of the gallery, the moist air flows back at the gallery roof, and condenses as it encounters cooler regions of the cavity. Drip rate measurements show a very regular pattern completely independent of those measured on active speleothems. Unlike the bottom part of the gallery coated with aragonite needles, the upper part is strongly corroded with the presence of a significant microbial life. Continuous measurements of 222Rn indicate a permanent ventilation of the gallery even when the outside temperature is higher than the cavity temperature. Evidence of air circulation was also directly acquired using a flag sensor made from a mylar sheet and a position sensor, that detects air velocities less than 1 cm/s. U-Th dating of aragonite needles suggests that the climatic system has been established for at least 114 000 years. Those observations relate a unique example of coupled processes competing in a natural site with a remarkable application of out-of-equilibrium thermodynamics for the preservation of precious underground heritage in the context of climate change and anthropogenic disturbances.