Variations of isotopic composition of carbon in the karst environment from southern Poland, present and past.

Abstract

We describe a comprehensive study of carbon isotopes in several karst springs and their environs in a contemporary karst environment in the region of the Cracow-Wielun Upland and Western Tatra Mountains, Southern Poland. We collected samples of water, plants and carbonate deposited on aquatic plants, and obtained (super 13) C values and (super 14) C concentrations. We also investigated a group of the youngest calcium carbonates from caves where deposition is still being observed or ceased no more than a few hundred years ago. The determination of a (super 14) C dilution factor (q) in these carbonates allows us to determine the "true" radiocarbon ages of old speleothems from caves in the area under investigation and enables the use of old speleothems as suitable material for extending the (super 14) C calibration time scale, the "Absolute" age having been determined by U/Th or amino acid racemization (AAR) dating methods. Measurements of delta (super 13) C and (super 14) C concentrations were made on dissolved inorganic carbon (DIC) extracted from water samples. Calculated values of q range from 0.55 to 0.68 and delta (super 13) C values range from -10 per mil to -13 per mil versus VPDB with mean values equal to 0.65 and -12 per mil, respectively. Results indicate that the dissolution process of limestone bedrock is a closed system with the dominating contributor being biogenic carbon dioxide. Isotopic composition of carbon in contemporary plants collected at the karstic springs at 3 localities is highly diverse, with different species distinctly varying in both q and delta (super 13) C values. Extremely light values of (super 13) C (under -40 per mil), observed in Algae and Hyloconium splendens, are correlated with (super 14) C concentrations that are much lower than 100 pMC. Small systematic changes of isotopic composition were found in plants of the same species collected along streams at various distances from the spring. The youngest calcium carbonates from different caves show a relatively high scatter of both delta (super 13) C values and (super 14) C concentration. The lower reservoir effect for (super 14) C is observed in samples with higher value of delta (super 13) C, indicating equilibrium conditions in the sedimentation of carbonate. Pazdur et al. (1995b) presented (super 14) C dating results and paleoclimatic interpretation of 170 (super 14) C analyses of 89 speleothems from 41 caves obtained through 1994. Investigations continued until early 1997, during which time a speleothem, JWi2, was dated by (super 14) C, U/Th and AAR dating methods, and its stable isotope composition (delta (super 13) C and delta (super 18) O) analyzed in detail (reported here). Carbon isotope analyses indicate very large differences among results obtained by U/Th, AAR, and (super 14) C dating methods.