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Geochemical and stable isotope characterisation of drip water and modern carbonate from Postojna Cave, Slovenia

Postojna Cave (Slovenia) is one of the most famous karst caves in South Eastern Europe visited by numerous tourists. There is evidence of vegetation change during the past and recently also of land use change, as well as of modification of a cave structure itself. Together with climate changes these alterations may affect the cave environment. Comprehensive investigation of speleothems and cave environment has been undertaken with the aim to map recent geochemical and stable isotope characteristics of the Postojna Cave which could help in providing insights into climate and environmental changes. Here we present a part of this study: the geochemical and stable isotope characterisation of drip water and modern carbonate. One-year monitoring of the Postojna Cave was performed from March 2010 till April 2011. Sampling of drip water was done at 9 locations within the cave. Sampling locations have been selected to cover various conditions of cave environment and thus different conditions under which formation of speleothems is occurring. Additionally, the Pivka River was also sampled at two locations: at the entrance to the cave and 2.5 km inside the cave. Parameters measured on the field were air and water temperature, pH, conductivity, drip rate and air CO2 concentration. Laboratory chemical analyses of water samples included regular measurements of Ca2+, Mg2+, HCO3 concentrations and twice a year measurement of Na+, K+, Cl-, SO42- and NO3- concentrations. Saturation index with respect to calcite and dissolved CO2 concentration in water were calculated from the measured data. The following stable isotope measurements were performed: 18O in drip water and river, 13C in DIC (Dissolved Inorganic Carbon, under measured conditions mostly HCO3 ) and in air CO2 and 13C and 18O in modern carbonates. The seasonal and spatial (within the cave in relation to the distance from the entrance) variations of these parameters will be shown as well as relationships between various parameters. Air temperature within the Cave varies slightly around 10 ºC, which is close to the mean air temperature measured at the meteorological station during last decade. Variations around the mean are larger at locations closer to the entrance to the Cave. Drip water temperature also slightly varies around 10 ºC, while the river water follows more closely the outside air temperature. Average drip rate at studied locations varies from 10 drips per minute to 150 drips per minute, while after abundant precipitation it can reach 500 drips per minute. delta18O values of drip water vary between -10‰ and -8‰ VSMOW, with practically no seasonal variations. pH values of drip water vary between 7.1 and 8.4 and show seasonal fluctuation – lowest pH values are measured in August and September, and highest during winter months (November – March). Mean HCO3 concentrations range between 200 and 273 mg/l at all locations except for "Podrti kapnik" site where it is 152 mg/l. Similarly, calcium concentrations range between 74 and 89 mg/l at all locations except for "Podrti kapnik" site (65 mg/l). Consequently, conductivity at this site is lower (277 microS/cm) than at other sites (312 – 454 microS/cm). At all locations, magnesium concentrations are lower than 1.5 mg/l, and concentrations of other minor ions are negligible. CO2 concentration in cave air is always higher than the atmospheric CO2 concentration (398 ppm) and the average values range from 770 ppm (close to the entrance) to 1580 ppm deeper in the cave ("Čarobni vrt"), where a maximal value of 3000 ppm was measured in September 2010. Consequently, average delta13C of CO2 in air range from the values for open atmosphere (-7.7‰ VPDB) to -18‰ at location "Čarobni vrt". A good correlation between the CO2 concentration and its delta13C value has been obtained showing that the excess CO2 in the cave is a result of degassing of soil CO2 that was transported by the drip water. Aquatic CO2 concentration (partial pressure of aquatic CO2) in drip waters ranges from 550 microatm to 26200 microatm. Saturation index with respect to carbonate ranges from from 0.57 to 12.82 and it is generally higher when concentration of air CO2 is lower. Average delta13C values of DIC range from -12.4‰ VPDB to -13.3‰, except for "Podrti kapnik" site (-9.9‰). delta13C and delta18O of modern carbonates show the variations from -10.58 to -7.81‰ VPDB (with an exception of -3.94‰) and from -7.84‰ to -4.84‰, respectively. Variations of stable isotope composition of modern speleothems within the same locations are lower than 2‰ in both isotopes. The 18O values will be used to estimate the speleothem formation temperature, which can then be compared with the actually measured temperature and other environmental parameters at the location. The final aim of this study is to determine possible locations within the Postojna Cave where speleothems could be used fore palaeoclimate investigations.

Postojna cave; speleothem; stable isotopes; geochemistry

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