• Discussion: Recovery from Tracer Contamination in AMS Sample Preparation

      Jull, A. J. T.; Donahue, Douglas J.; Toolin, L. J. (Department of Geosciences, The University of Arizona, 1990-01-01)
    • 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.
    • Memory Effects in an AMS System: Catastrophe and Recovery

      Vogel, J. S.; Southon, J. R.; Nelson, D. E. (Department of Geosciences, The University of Arizona, 1990-01-01)
      A sample with a 14C concentration estimated to be greater than 30,000 Modern was inadvertently graphitized and measured in an AMS system. No measurable contamination of the cesium sputter ion source was observed. Simple cleaning procedures removed the contamination from the sample preparation system, with the exception of the reaction vessel in which the sample was graphitized. Sample cross-contamination factors were estimated for all of the preparation and measurement procedures.
    • Operation of the Harwell UK 14C Data Base and Its Expansion Through Data Exchange with Other Laboratories

      Walker, A. J.; Otlet, R. L.; Housley, R. A.; van der Plicht, Johannes (Department of Geosciences, The University of Arizona, 1990-01-01)
      The use of computer data bases for storage and retrieval of 14C data is a logical application for the rapidly expanding numbers of 14C determinations. Harwell has established a data base for all samples originating from sites in the United Kingdom and Eire. The core of the data is the Council for British Archaeology's published Index of Radiocarbon Dates which we are expanding to include all Harwell UK dates released for publication by the submitters plus dates from other laboratories both within and outside the UK. As a demonstration of the feasibility of direct database-to-database communication, cooperation has been sought from Groningen and Oxford to transfer computer files containing 14C results for UK sites. Neither of these laboratories use the same system as Harwell for their in-house data base and this exercise highlights the importance of a transfer language for both the national and international schemes as it is no longer practical to carry out such procedures through keyboard typing.
    • Re-Evaluation of British Museum Radiocarbon Dates Issued Between 1980 and 1984

      Bowman, S. E.; Ambers, J. C.; Leese, M. N. (Department of Geosciences, The University of Arizona, 1990-01-01)
      Dates issued by the British Museum radiocarbon laboratory between 1980 and 1984 are known to have been in error. This paper outlines the cause of the problem and the procedures adopted to revise the results affected. Where revision has been possible, on average this has given dates older by 200 to 300 radiocarbon years. The individual revised results are tabulated.
    • 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.
    • Superconducting Mini-Cyclotrons as AMS Instruments

      Subotic, K. M.; Milinkovic, L. S.; Zupancic, M. T.; Novkovic, D. N.; Stojanovic, M. S. (Department of Geosciences, The University of Arizona, 1990-01-01)
      We have studied the limitations of conventional mass spectrometry and have examined accelerator based methods which could help circumvent these limitations. In particular, cyclotron-based accelerator mass spectrometric (AMS) techniques are discussed with an emphasis on evaluating performances of superconducting mini-cyclotrons designed for use as AMS instruments. We discussed the design of superconducting mini-cyclotrons dedicated to radioisotope dating research.
    • Survey of the Dispersion of 14C in the Vicinity of the UK Reprocessing Site at Sellafield

      Otlet, R. L.; Walker, A. J.; Fulker, M. J. (Department of Geosciences, The University of Arizona, 1990-01-01)
      We have been measuring 14C in natural, biological materials growing in the vicinity of Sellafield, Cumbria as a continuing project with BNFL to understand the dispersion of releases from the site over several seasons. We have measured locally grown foodstuffs for monitoring purposes, individual tree rings to establish a chronology of releases, hawthorn berries for spatial investigations and are now carrying out controlled plot uptake experiments. We have been attempting to validate a current dispersion model (Clarke 1979) over a six-year period, and although we observe general agreement in most years, certain anomalies, which probably relate to topographical features, are leading to more detailed correlation with local meteorological data.
    • 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.