• Formation of 13C/12C Isotope Ratios in Speleothems: A Semi-Dynamic Model

      Dulinski, Marek; Rozanski, Kazimierz (Department of Geosciences, The University of Arizona, 1990-01-01)
      A theoretical approach towards predicting the carbon isotope composition of carbonate cave deposits is presented. The proposed model simulates time variations of both the chemical and isotopic composition of the carbonate solution and deposited calcite in the course of CaCO3 precipitation. Two phases of the precipitation process are distinguished and treated separately: initial outgassing of the solution until a certain degree of supersaturation of CO3^2- ions is reached and subsequent precipitation of CaCO3 related to further outgassing. Precipitation rates of CaCO3 predicted by the model agree fairly well with literature data. The model predicts Delta-13C values of deposited calcite within a range of ca −16 to +3 per mil depending on temperature, chemical and isotope parameters of the initial solution and actual degree of the precipitation process.
    • Further Investigations on 14C Dating of Calcareous Tufa

      Pazdur, Anna; Pazdur, Mieczysław F. (Department of Geosciences, The University of Arizona, 1990-01-01)
      Systematic studies on 14C dating of tufa profiles in southern Poland have resulted in developing a simple phenomenological model which enables us to estimate the reservoir correction factor of 14C dates of individual carbonate samples. We made further studies to test model assumptions and to verify relationships between the value of reservoir correction factor TR and 13C content of tufa carbonate. Two new study sites, Rudawa and Szklarka, are close to previously studied sites. Four tufa samples with associated organic fraction from ca 2000m asl in South America (Peruvian Andes) were measured to test the possible application of the developed model to tufas in different geochemical and climatic environments. Finally, a series of calcareous tufa samples from the Villers-devant-Orval profile (Belgium) were dated, although no direct comparison with organic matter dates is available for this profile.
    • Some Radiocarbon Dates for Tufas of the Craven District of Yorkshire

      Pentecost, Allan; Thorpe, P. M.; Harkness, D. D.; Lord, T. C. (Department of Geosciences, The University of Arizona, 1990-01-01)
      14C dates of relict tufa deposits at Gordale indicated a Subboreal age when the carbonate age was corrected with empirical bedrock dilution factors ‘q’ of 0.79 or 0.85. Estimates of ‘apparent age,’ based on extrapolated Delta-13C values were about twice those obtained with q, and the 1 sigma error was large. The Delta--13C values of tufa samples were not correlated with carbonate age and were close to −10. Application of q values in this district requires caution as they appear to be site-specific. We recommend that wherever possible, levels of 13C and 14C are measured in the associated tufa-depositing water, and an empirical dilution factor employed.
    • The Use of Radiocarbon Measurements in Atmospheric Studies

      Manning, M. R.; Lowe, D. C.; Melhuish, W. H.; Sparks, R. J.; Wallace, G.; Brenninkmeijer, C. M.; McGill, R. C. (Department of Geosciences, The University of Arizona, 1990-01-01)
      14C measured in trace gases in clean air helps to determine the sources of such gases, their long-range transport in the atmosphere, and their exchange with other carbon cycle reservoirs. In order to separate sources, transport and exchange, it is necessary to interpret measurements using models of these processes. We present atmospheric 14CO2 measurements made in New Zealand since 1954 and at various Pacific Ocean sites for shorter periods. We analyze these for latitudinal and seasonal variation, the latter being consistent with a seasonally varying exchange rate between the stratosphere and troposphere. The observed seasonal cycle does not agree with that predicted by a zonally averaged global circulation model. We discuss recent accelerator mass spectrometry measurements of atmospheric 14CH4 and the problems involved in determining the fossil fuel methane source. Current data imply a fossil carbon contribution of ca 25%, and the major sources of uncertainty in this number are the uncertainty in the nuclear power source of 14CH4, and in the measured value for delta 14C in atmospheric methane.