Carbon and oxygen isotope study of the active water-carbonate system in a karstic Mediterranean cave: Implications for paleoclimate research in semiarid regions |
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| Institution: | 1. Geological Survey of Israel, 30, Malkhei Israel St., Jerusalem 95501, Israel;2. Institute of Earth Sciences, Hebrew University of Jerusalem 91904, Israel;1. Geological Survey of Israel, Yisha''ayahu Leibowitz St. 32, Jerusalem, Israel;2. Institute of Earth Sciences, The Hebrew University, Jerusalem 91904, Israel;3. Cave Research Center, The Hebrew University, Jerusalem 90627, Israel;1. Geological Survey of Israel, 32 Yesha''ayahu Leibowitz Street, Jerusalem, 9692100, Israel;2. Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem, 91904, Israel;3. Department of Bible, Archaeology and the Ancient Near East, Ben-Gurion University of the Negev, PO Box 653, Beer-Sheva, 8410501, Israel;4. Archaeological Research Department, Israel Antiquities Authority, POB 586, Jerusalem, 91004, Israel;5. D-REAMS Radiocarbon Laboratory, Scientific Archaeology Unit, Weizmann Institute of Science, 7610001, Rehovot, Israel;6. Department of Anatomy and Anthropology, Dan David Center for Human Evolution and Biohistory Research, Shmunis Family Anthropology Institute, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel;7. The Steinhardt Museum of Natural History, Tel Aviv University, PO Box 39040, Tel Aviv, 6997801, Israel;1. Faculty of Physics and Electrical Engineering, University of Bremen, Germany;2. Institute for Geosciences, University of Mainz, Germany;1. Fredy and Nadine Herrmann Institute of Earth Sciences, Hebrew University of Jerusalem, Edmond J. Safra, Givat Ram Campus, Jerusalem, 91904, Israel;2. Geological Survey of Israel, Yisha''ayahu Leibowitz St., 32, Jerusalem, 9692100, Israel;3. Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081HV, Amsterdam, the Netherlands;4. Permanent Address: Max Planck Institute for Chemistry, Climate Geochemistry Department, Hahn-Meitnerweg 1, 55128, Mainz, Germany;1. Department of Geological Sciences, The University of Texas at Austin, Austin, TX, United States;2. Department of Physics, Astronomy, and Geosciences, Valdosta State University, Valdosta, GA, United States;1. School of Environmental and Life Sciences, University of Newcastle, Callaghan 2308, NSW, Australia;2. Museo delle Scienze, Corso del Lavoro e della Scienza, 3, 38122 Trento, Italy;3. Dipartimento Valorizzazione delle Risorse Naturali, Istituto Agrario di San Michele all’Adige, Via Mach 1, 38010 San Michele all’Adige (TN), Italy |
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| Abstract: | In a semiarid climatic zone, such as the Eastern Mediterranean region, annual rainfall variations and fractionation processes in the epikarst zone exert a profound influence on the isotopic compositions of waters seeping into a cave. Consequently, the isotopic compositions of speleothems depositing from cave waters may show complex variations that need to be understood if they are to be exploited for paleoclimate studies. This is confirmed by a four-year study of the active carbonate-water system in the Soreq cave (Israel). The δ18O (SMOW) values of cave waters range from −6.3 to −3.5%.. The highest δ18O values occur at the end of the dry season in waters dripping from stalactites, and reflect evaporation processes in the epikarst zone, whereas the lowest values occur in rapidly dripping (fast-drip) waters at the peak of the rainy seasons. However, even fast-drip waters are about 1.5%. heavier than the rainfall above the cave, which is taken to reflect the mixing of fresh with residual evaporated water in the epikarst zone. δ13C (PDB) values of dissolved inorganic carbon (DIC) vary from −15.6 to −5.4%., with fast-drip waters having lower δ13C values (mostly −15.6 to −12%.) and higher DIC concentrations relative to pool and stalactite-drip water. The low δ13C values of fast-drip waters and their supersaturation with respect to calcium carbonate indicates that the seepage waters have dissolved both soil-CO2 derived from overlying C3-type vegetation and marine dolomite host rock.The δ18O (PDB) values of various types of present-day low-magnesium calcite (LMC) speleothems range from −6.5 to −4.3%. and δ13C values from −13 to −5.5%. and are not correlated with speleothem type. An analysis of δ18O values of present-day calcite rafts and pool waters shows that they form in oxygen isotope equilibrium. Similarly, the measured ranges of δ13C and δ18O values for all types of present-day speleothems are consistent with equilibrium deposition at cave temperatures. The δ13C–δ18O range of contemporary LMC thus reflects the variations in temperatures and isotopic compositions of the presentday cave waters. The 10%. variation in the δ13C values in waters can be modeled by a simple Rayleigh calculation of the carbon isotope fractionation accompanying CO2-degassing and carbonate precipitation. These variations may obscure the differences in the carbon isotopic composition of speleothems that could arise when vegetation cover changes from C3 to C4-type plants. This consideration emphasizes that it is necessary to characterize the full range of δ13C values associated with contemporaneous speleothems in order to clarify the effects of degassing from those due to differing vegetation types.Isotopic studies of a number of different types of fossil LMC speleothems show many of them to exhibit isotopic trends that are similar to those of present-day LMC, but others show both higher and lower δ18O ranges. In particular, the higher δ18O range has been shown by independent age-measurements to be associated with a period of drier conditions. The results of the study thus indicate that it is necessary to work on a well calibrated cave system in semiarid climates and that the fossil speleothem record should be obtained from different types of contemporaneous deposit in order to fully characterize the δ18O–δ13C range representative of any given climatic period. |
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