engleski

Determination of equilibrium conditions of carbonate precipitation in Postojna Cave with application to paleoclimatology

The contents of stable carbon and oxygen isotopes, 13C and 18O, in secondary precipitated carbonates (speleothems) of caves in karst areas can provide important information about paleoclimatic conditions of the area if the speleothems are formed at equilibrium conditions, i.e., if the isotopic composition of speleothems depends only on the deposition temperature. The purpose of this study was to determine areas within Postojna Cave with isotopic equilibrium conditions of carbonate precipitation where future paleoclimate research may be conducted. Environmental factors (temperature, concentration of atmospheric CO2), chemical composition of drip water, isotopic composition of water, CO2 and deposited carbonates were monitored during one year at nine locations. Difference between d13C in modern carbonates and in DIC was compared with the equilibrium fractionation factor e. Fractionation factor a was determined from measured d18O values of drip water and carbonates and compared to its equilibrium value aeq. Although environmetal and chemical parameters pointed to the fulfillment of preconditions for isotopic equilibrium precipitation of carbonates, specially at inner locations, isotopic composition of carbonates, both d13C and d18O, did not show equilibrium precipitation at most of the studied locations. Only locations 06 – Stebrišče and 09 – Zgornji Tartar have the potential for future paleoclimatic studies. It was also shown that recent carbonate precipitated on watch glasses did not reflect the isotopic composition of speleothems at the same location. Process of prior calcite precipitation was identified at location Podrti kapnik.

Stable isotopes; mass spectrometry; paleoclimatology; Postojna Cave

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