CARBON AND OXYGEN-ISOTOPE STUDY OF THE ACTIVE WATER-CARBONATE SYSTEM IN A KARSTIC MEDITERRANEAN CAVE - IMPLICATIONS FOR PALEOCLIMATE RESEARCH IN SEMIARID REGIONS

In a semiarid climatic zone, such as the Eastern Mediterranean region, annual rainfall variations and fractionation processes in the epikars t zone exert a profound influence on the isotopic compositions of wate rs seeping into a cave. Consequently, the isotopic compositions of spe leothems depositing from cave waters may show complex variations that need to be understood if they are to be exploited for paleoclimate stu dies. This is confirmed by a four-year study of the active carbonate-w ater system in the Soreq cave (Israel). The delta(18)O (SMOW) values o f cave waters range from -6.3 to -3.5 parts per thousand. The highest delta(18)O values occur at the end of the dry season in waters drippin g 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 water s are about 1.5 parts per thousand heavier than the rainfall above the cave, which is taken to reflect the mixing of fresh with residual eva porated water in the epikarst zone. delta(13)C (PDB) values of dissolv ed inorganic carbon (DIG) vary from -15.6 to -5.4 parts per thousand, with fast-drip waters having lower delta(13)C values (mostly -15.6 to -12 parts per thousand) and higher DIC concentrations relative to pool and stalactite-drip water. The low delta(13)C values of fast-drip wat ers and their supersaturation with respect to calcium carbonate indica tes that the seepage waters have dissolved both soil-CO2 derived from overlying C-3-type vegetation and marine dolomite host rock. The delta (18)O (PDB) values of various types of present-day low-magnesium calci te (LMC) speleothems range from -6.5 to -4.3 parts per thousand and de lta(13)C values from -13 to -5.5 parts per thousand and are not correl ated with speleothem type. An analysis of delta(18)O values of present -day calcite rafts and pool waters shows that they form in oxygen isot ope equilibrium. Similarly, the measured ranges of delta(13)C and delt a(18)O values for all types of present-day speleothems are consistent with equilibrium deposition at cave temperatures. The delta(13)C-delta (18)O range of contemporary LMC thus reflects the variations in temper atures and isotopic compositions of the present-day cave waters. The 1 0 parts per thousand variation in the delta(13)C values in waters can be modeled by a simple Rayleigh calculation of the carbon isotope frac tionation accompanying CO2-degassing and carbonate precipitation. Thes e variations may obscure the differences in the carbon isotopic compos ition of speleothems that could arise when vegetation cover changes fr om C-3 to C-4-type plants. This consideration emphasizes that it is ne cessary to characterize the full range of delta(13)C values associated with contemporaneous speleothems in order to clarify the effects of d egassing from those due to differing vegetation types. Isotopic studie s of a number of different types of fossil LMC speleothems show many o f them to exhibit isotopic trends that are similar to those of present -day LMC, but others show both higher and lower delta(18)O ranges. In particular, the higher delta(18)O range has been shown by independent age-measurements to be associated with a period of drier conditions. T he 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 contempo raneous deposit in order to fully characterize the delta(18)O-delta(13 )C range representative of any;given climatic period.