engleski

Moonmilk in Postojna Cave precipitates under isotopic equilibrium conditions

Here we present a part of a comprehensive multidisciplinary study of Postojna Cave, the largest known cave in Europe and most visited cave in Slovenia, related to modern speleothem formation conditions (Mandić, 2013). One year monitoring was conducted at nine locations within Postojna Cave in order to determine environmental conditions of carbonate deposition. Modern carbonates, mostly soda- straw samples, were collected and their stable isotope composition (13Cc and 18Oc) was determined. The assessment of process (equilibrium or kinetic) of precipitation for modern carbonates was done by comparing these values with isotopic composition of water (18Ow) and of dissolved inorganic carbon (DIC, 13CDIC): measured difference between 13CDIC and 13Cc was compared with the theoretical fractionation ε value ; fractionation factor (α) calculated from the measured 18Ow and 18Oc values was compared with the equilibrium fractionation factor αeq for the measured temperature ; temperature of carbonate precipitation was calculated from the isotopic 18O composition of carbonate and water and compared with the measured temperature. Even thou the temperatures of calcite precipitation calculated by using different equations generally do not agree with the measured temperatures at given locations, research has resulted in finding locations and samples which are promising candidates for paleoclimate investigations. One of such samples is moonmilk from location Zgornji Tartar. Monitored environmental parameters of location microclimate and requirements for carbonate deposition under isotopic equilibrium on this location are fulfilled. All these parameters indicate that the moonmilk is a promising candidate for paleotemperature reconstruction. Because this is the first time that moonmilk is described in Postojna Cave, the investigation of its origin has started. Petrographicaly, the moonmilk is composed of calcite needles approximately 100 µm long, and 10 µm wide. SEM micrographs of moonmilk show evidence of microbial biomediation or bioprecipitation. Also, attenuated total reflectance Fourier transform infrared (ATR- FTIR) analysis pertains to presence of organic compounds. Since Postojna Cave is tourist cave it has to be distinguished whether the organic compounds found in moonmilk sample are results of cave microorganisms (native) of they are allochthonous species. To rule out this two possibilities sample is given for RNA sequencing.

moonmilk ; postojna cave ; stable isotopes ; isotopic ekvilibrium ; paleotemperature ; microorganisms ; SEM

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