• Accuracy and Precision in Dating Microgram Carbon Samples

      Vogel, J. S.; Nelson, D. E.; Southon, J. R. (Department of Geosciences, The University of Arizona, 1989-01-01)
      The accuracy of AMS radiocarbon determinations on very small samples has been tested by measuring a suite of microgram-sized samples of a known-age material. The total measurement precision for the smallest sample (50 micrograms) was found to be +/- 3% and the precision improved with larger sample size. The accuracies of the measurements were found to be within the measurement precisions
    • Anomalous AMS Radiocarbon Ages for Foraminifera from High-Deposition-Rate Ocean Sediments

      Broecker, W. S.; Trumbore, Susan; Bonani, Georges; Wölfli, Willy; Klas, Millie (Department of Geosciences, The University of Arizona, 1989-01-01)
      Radiocarbon ages on handpicked foraminifera from deep-sea cores are revealing that areas of rapid sediment accumulation are in some cases subject to hiatuses, reworking and perhaps secondary calcite deposition. We present here an extreme example of the impacts of such disturbances. The message is that if precise chronologies or meaningful benthic planktic age differences are to be obtained, then it is essential to document the reliability of radiocarbon ages by making both comparisons between coexisting species of planktomc foraminifera and detailed down-core sequences of measurements.
    • Correcting 14C Histograms for the Non-Linearity of the 14C Time Scale

      Stolk, A.; Hogervorst, Koos; Berendsen, Henk (Department of Geosciences, The University of Arizona, 1989-01-01)
      Large numbers of 14C dates of the base and top of Holocene peat layers may be plotted in 14C histograms in order to establish statistically a chronology of periods of essentially elastic sedimentation and peat formation. Due to the non-linearity of the 14C time scale in terms of calendar years, clustering of 14C dates on random peat growth may occur. This seriously hampers the interpretation of histograms. A quantitative method and computer program were developed to correct the histograms for this effect. The correction factor that has to be applied depends on the calibration curve and the interval width of the correction parameter dy. For peat samples, an interval width of 100 14C yr and a calibration curve based on a 100-yr moving average seems to be a reasonable choice.
    • High-Resolution 14C Dating of Organic Deposits Using Natural Atmospheric 14C Variations

      Van Geel, Bas; Mook, Willem G. (Department of Geosciences, The University of Arizona, 1989-01-01)
      The occurrence of atmospheric 14C variations complicates calibration, ie, the translation of 14C ages into real calendar ages. The procedure of wiggle matching, however, allows very precise calibration, by matching known 14C variations with wiggles in the floating chronology. In principle, wiggle matching can also be applied to a series of 14C dates from organic (peat) deposits. Where, in general, 14C ages are required at short distances and on small samples, dating by 14C accelerator mass spectrometry (AMS) is required.
    • Problems Associated with the Use of Coal as a Source of 14C-Free Background Material

      Lowe, David C. (Department of Geosciences, The University of Arizona, 1989-01-01)
      Many 14C dating laboratories have established that coal samples exhibit a finite 14C age, apparently caused by contamination of the specimens before any laboratory preparation is undertaken. In this work, the possibility that the contamination is due to microbial and fungal activity in the coal substrate is considered and some suggestions are made for alternative sources of background test materials for 14C dating laboratories. Initial results indicate that geologically formed graphites contain little 14C and are likely to be good background test materials, especially in 14C AMS laboratories.
    • Radiocarbon Dating of Deep-Sea Sediments: A Comparison of Accelerator Mass Spectrometer and Beta-Decay Methods

      Jones, G. A.; Jull, A. J. Timothy; Linick, T. W.; Donahue, Douglas J. (Department of Geosciences, The University of Arizona, 1989-01-01)
    • Temporal 10Be and 14C Variations: A Tool for Paleomagnetic Research

      Kocharov, G. E.; Blinov, A. V.; Konstantinov, A. N.; Levchenko, V. A. (Department of Geosciences, The University of Arizona, 1989-01-01)
      Temporal variations of cosmogenic radionuclide atmospheric concentrations can be caused by such global phenomena as solar activity and geomagnetic field changes as well as atmospheric circulation processes. These causes can be distinguished by the comparison of several isotope records corresponding to the same time period. We discuss a possibility for reconstructing the geomagnetic moment during the last 30,000 years from the comparison of 10Be and 14C concentrations in terrestrial archives. The results agree with conventional paleomagnetic data and promise to enrich our knowledge of geomagnetic field variations and reversals.