engleski

Do recent soda-straw speleothems in Postojna Cave precipitate under isotopic equilibrium conditions?

Recent studies of environmental cave conditions resulted in a modern approach to speleothem studies that comprises comprehensive and simultaneous monitoring of environmental, geochemical and isotopic parameters within each cave and exploring the existence of equilibrium isotope effects in modern speleothem deposition. Observation that cave environments reflect the mean annual temperature of the region is the basic assumption for a possible use of isotopic composition (d18O and d13C) of speleothems for reconstructions of paleoclimate conditions providing equilibrium isotopic conditions during speleothem formation. 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 (d13Cc and d18Oc) 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 (d18Ow) and of dissolved inorganic carbon (DIC, d13CDIC): measured difference between d13CDIC and d13Cc was compared with the theoretical fractionation epsilon value ; fractionation factor (alpha) calculated from the measured d18Ow and d18Oc values was compared with the equilibrium fractionation factor alpha-eq 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. d13C and d18O values of soda straws varied from -10.6 to -7.8 ‰ VPDB and from -7.8 to -4.8 ‰ VPDB, respectively. The difference D = d13Cc – d13CDIC was larger than epsilon for most modern samples, and only a few soda-straw samples gave D comparable to epsilon indicating isotopic equilibrium. The temperature calculated from d18Ow and d18Oc by using several paleotemperature equations was to low compared to the present-day air temperature. Only at locations 06 – Stebrišče and 09 – Zgornji Tartar the calculated and measured temperatures were comparable. Similar conclusion was obtained after comparison of fractionation factors alpha with the equilibrium alpha-eq values. The purpose of this study was to determine areas where future paleoclimate research may be conducted. The measured parameters (temperature, humidity, air circulation, drip water d18O) pointed to the fulfillment of the environmental preconditions for isotopic equilibrium precipitation of carbonates, specially at the inner locations. However, isotopic composition of carbonates, both d13C and d18O, did not show equilibrium precipitation at most of the studied locations. Locations 06 – Stebrišče and 09 – Zgornji Tartar are the only ones that have potential for future paleoclimatic studies. Mandić M., 2013. Determination of equilibrium conditions of carbonate precipitation in Postojna Cave with possible application to paleoclimatology. Ph.D. Thesis, Zagreb, University of Zagreb, 205 p.

speleothem; Postojna Cave; isotopic equilibrium; 13C; 18O

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