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

摘要

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 delta(18)O (SMOW) values of 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 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 parts per thousand heavier than the rainfall above the cave, which is taken to reflect the mixing of fresh with residual evaporated water in the epikarst zone. delta(13)C (PDB) values of dissolved 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 waters and their supersaturation with respect to calcium carbonate indicates 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 calcite (LMC) speleothems range from -6.5 to -4.3 parts per thousand and delta(13)C values from -13 to -5.5 parts per thousand and are not correlated with speleothem type. An analysis of delta(18)O values of present-day calcite rafts and pool waters shows that they form in oxygen isotope equilibrium. Similarly, the measured ranges of delta(13)C and delta(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 temperatures and isotopic compositions of the present-day cave waters. The 10 parts per thousand variation in the delta(13)C 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 C-3 to C-4-type plants. This consideration emphasizes that it is necessary to characterize the full range of delta(13)C 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 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. 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 delta(18)O-delta(13)C range representative of any;given climatic period. [References: 54]