engleski

Characterization of Modrič Cave (Croatia) hydrological and geochemical settings as prerequisite for the palaeoenvironmental studies

Uniqueness of each particular cave system, in terms of depth, morphology, aquifer characteristics, climate region etc., requires establishment of cave monitoring program as prerequisite for reliable and confident interpretation of speleothem palaeoclimate records. In order to reconstruct Quaternary changes in the littoral Croatia, Modrič Cave has been chosen as one of the potential sites. We have selected three drip sites in deep cave interior, collected associated speleothems for U- Th dating and the stable isotope analyses, and commenced data logging and water sampling. After the one-year monitoring (Nov 2014 - Nov 2015), we collected preliminary data on cave and surface air temperature and relative humidity, stable isotopic composition of the precipitation and dripwater, and response of cave drip intensity to rain events, which provided insight into the cave microclimate settings, karst aquifer architecture and geochemical processes. Stable mean annual cave air temperature (16.4 °C) that closely reflects mean annual surface air temperature (16.1 °C), and relative humidity of 100% year- round, appear to be appropriate conditions for the calcite precipitating at or near isotopic equilibrium with drip water, which is essential for the reliable interpretation of speleothem isotopic signals. However, additional analyses on modern calcite, as well as Handy tests on fossil spelean carbonate should approve these assumptions. Drip sites with uniform and constant drip intensities have been practically unresponsive to rain events i.e. independent of major showers or prolonged dry periods, displaying seepage flow. Such hydrological regime already point to the good homogenization of the water within the aquifer, and additional proof lies in the isotopic composition of rain and drip water. Seasonal variations of rainwater δ18O and δ2H values with amplitude of 6.8‰ and 45.2‰, respectively, are attenuated to dripwater range of only 0.2‰ and 0.9‰, respectively. Apparently, the residence time of the water in the epikarst and karst aquifer is long enough to practically completely diminish atmospheric seasonal variations and detect multi-annual climate trends. Isotopic composition of the meteoric precipitation (in particular, δ18O and δ2H values and their relationship) reveals the vapour source region. Modrič local meteoric waterline (δ2H = 6.8 × δ18O + 5.5) lies above the global meteoric waterline (δ2H = 8 × δ18O + 10) with lower slope and intercept, indicating the primary evaporation in the vapour source region and secondary evaporation during the rainfall, as expected in this Mediterranean coastal site. However, mean d- excess of 12.5‰ has shown mutual influences of the Atlantic and Mediterranean air masses, but it has been unexpectedly low. Prolonged monitoring campaign should verify these findings, but based on these preliminary data, it is likely that Modrič Cave stalagmites, with the U-Th ages between 340 ka and the Recent, would enable trustworthy palaeoenvironmental reconstruction.

cave monitoring; stable isotopes; palaeoenvironmental changes; Quaternary; Croatia

Projekt HRZZ-a: Reconstruction of Quaternary Environments in Croatia using Isotope Methods (2014- 2018)