Now showing items 21-28 of 28

    • How Smooth Should Curves Be for Calibrating Radiocarbon Ages?

      Törnqvist, Törbjorn E.; Bierkens, Marc F. P. (Department of Geosciences, The University of Arizona, 1994-01-01)
      We show that smoothed versions of the high-resolution calibration curve should be used when 14C ages are calibrated with large (> ~30 14C yr) measurement errors (represented by standard deviation sigma-m) or are mixtures of elements of variable age (natural sample error with standard deviation sigma-n). The degree of smoothing should agree with the standard deviation of total sample error, sigma-1 the square root of the quadratic sum of sigma-m and sigma-n. However, in most cases, sigma-1 is not well known, especially due to difficulties in quantifying sigma-n. We present an inverse method that gives a measure of mean sigma-1 for different materials that are widely used in (conventional) 14C dating. Calculations with large (>100) data sets of wood, charcoal, ombrotrophic peat and minerotrophic peat/gyttja samples indicate that sigma-1 of such materials is generally much larger than previously assumed, mainly because of large values of sigma-n. This means that particularly in organic deposits, strongly smoothed calibration curves should be used where medium-term 14C variations (wiggles) are completely straightened. This has especially major consequences for calibrating 14C histograms for natural 14C variations. We conclude that 14C histograms consisting of samples of organic deposits do not require correction for medium-term 14C variations and that uncalibrated 14C histograms need not be as suspect as is usually believed.
    • Department of Earth Sciences at the University of Rome Radiocarbon Dates II

      Calderoni, Gilberto (Department of Geosciences, The University of Arizona, 1994-01-01)
    • Department of Earth Sciences at the University of Rome Radiocarbon Dates III

      Calderoni, Gilberto; Caneva, Isabella; Cazzella, Alberto; Frangipane, Marcella; Petrone, Vincenzo (Department of Geosciences, The University of Arizona, 1994-01-01)
    • delta-13C Measurements from the Irish Oak Chronology

      McCormac, F. G.; Baillie, M. G. L.; Pilcher, J. R.; Brown, D. M.; Hoper, S. T. (Department of Geosciences, The University of Arizona, 1994-01-01)
      Measurements of the stable isotope ratio 13C/12C, relative to PDB, were made for fractionation correction purposes on all oak samples used in the Irish oak 14C calibration curve. Stable isotope data have not been published previously. We have collated the stable isotope data from the calibration work, carried out some further measurements to investigate anomalies in the original results, and generated tables of data that include site and tree information pertaining to both stable isotopes and source material for 14C calibration measurements. The data suggest that land-grown trees tend to be isotopically lighter than bog-grown wood, and that the Irish trees used in the calibration exercise tend to. be isotopically heavier than those from Scotland and England. Preliminary analysis of the data is given.
    • Calibration of 14C Histograms: A Comparison of Methods

      Stolk, Ad; Törnqvist, Törbjorn E.; Hekhuis, Kilian V.; Berendsen, Henk A.; van der Plicht, Johannes (Department of Geosciences, The University of Arizona, 1994-01-01)
      The interpretation of 14C histograms is complicated by the non-linearity of the 14C time scale in terms of calendar years, which may result in clustering of 14C ages in certain time intervals unrelated to the (geologic or archaeologic) phenomenon of interest. One can calibrate 14C histograms for such distortions using two basic approaches. The KORHIS method constructs a 14C histogram before calibration is performed by means of a correction factor. We present the CALHIS method based on the Groningen calibration program for individual 14C ages. CALHIS first calibrates single 14C ages and then sums the resulting calibration distributions, thus yielding a calibrated 14C histogram. The individual calibration distributions are normalized to a standard Gaussian distribution before superposition, thus allowing direct comparison among various 14C histograms. Several experiments with test data sets demonstrate that CALHIS produces significantly better results than KORHIS. Although some problems remain (part of the distortions due to 14C variations cannot be eliminated), we show that CALHIS offers good prospects for using 14C histograms, particularly with highly precise and accurate 14C ages.
    • British Museum Natural Radiocarbon Measurements: XXIII

      Ambers, Janet; Bowman, Sheridan (Department of Geosciences, The University of Arizona, 1994-01-01)
    • Apparent Ages of Marine Shells: Implications for Archaeological Dating in Hawai'i

      Dye, Tom (Department of Geosciences, The University of Arizona, 1994-01-01)
      The conventional 14C ages of 8 marine shells of known age and 11 marine shells stratigraphically associated with dated wood charcoal show considerable variation from expected ages. One source of this variation is seashore geology; comparison of 6 AMS dates on 3 species of shallow-water, herbivorous gastropod shells from Pleistocene limestone and Holocene volcanic coasts shows that shells from Pleistocene limestone coasts can have apparent, or reservoir, 14C ages up to 620 yr greater than shells of the same species from volcanic coasts. The relatively great variation in apparent ages of Hawaiian marine shells poses problems for their use in dating archaeological sites. For best results, an archaeological marine shell should be sourced to a particular local environment, and the apparent age of shells in that environment determined by dating well-provenienced shells of known age.
    • ANU Radiocarbon Date: List XI, Radiocarbon Dates from Lakes Barrine and Eacham, Atherton Tableland, North Queensland, Australia

      Head, M. J.; Taylor, L. J.; Walker, D. (Department of Geosciences, The University of Arizona, 1994-01-01)