• Oceanic Radiocarbon Between Antarctica and South Africa Along WOCE Section 16 at 30 Degrees E

      Leboucher, Viviane; Orr, James; Jean-Baptiste, Philippe; Arnold, Maurice; Monfray, Patrick; Tisnérat-Laborde Nadine; Poisson, Alain; Duplessy, Jean-Claude (Department of Geosciences, The University of Arizona, 1999-01-01)
      Accelerator mass spectrometry (AMS) radiocarbon measurements were made on 120 samples collected between Antarctica and South Africa along 30 degrees E during the WOCE-France CIVA1 campaign in February 1993. Our principal objective was to complement the Southern Ocean's sparse existing data set in order to improve the 14C benchmark used for validating ocean carbon-cycle models, which disagree considerably in this region. Measured 14C is consistent with the theta -S characteristics of CIVA1. Antarctic Intermediate Water (AAIW) forming north of the Polar Front (PF) is rich in 14C, whereas surface waters south of the PF are depleted in 14C. A distinct old 14C signal was found for the contribution of the Pacific Deep Water (PDW) to the return flow of Circumpolar Deep Waters (CDW). Comparison to previous measurements shows a 14C decrease in surface waters, consistent with northward displacement of surface waters, replacement by old deep waters upwelled at the Antarctic Divergence, and atmospheric decline in 14C. Conversely, an increase was found in deeper layers, in the AAIW. Large uncertainties, associated with previous methods for separating natural and bomb 14C when in the Southern Ocean south of 45 degrees S, motivated us to develop a new approach that relies on a simple mixing model and on chlorofluorocarbon (CFC) measurements also taken during CIVA1. This approach leads to inventories for CIVA1 that are equal to or higher than those calculated with previous methods. Differences between old and new methods are especially high south of approximately 55 degrees S, where bomb 14C inventories are relatively modest.
    • A Simple Method to Separate Pollen for AMS Radiocarbon Dating and Its Application to Lacustrine and Marine Sediments

      Mensing, Scott A.; Southon, John R. (Department of Geosciences, The University of Arizona, 1999-01-01)
      We present a simple method for manually separating pollen concentrates for radiocarbon accelerator mass spectrometry (AMS) dating using a mouth pipetting system. The required equipment is readily available from scientific equipment supply houses at minimal cost. Pollen samples from lake sediments required about 4 h of hand picking, whereas samples from marine sediments required about 8 h labor. Pollen dates from marine sediments were much older than expected. We are attempting to resolve whether this is due to contamination of the pollen or the presence of significant quantities of old reworked pollen. Pollen dates from lake sediments associated with Mazama Ash were consistent with other published ages; however, replicate dates on pollen samples from above the ash were consistently older than the surrounding sediment. Our results suggest that caution must be used when interpreting pollen dates if the potential for sediment reworking is present.
    • Chronology of Vegetation and Paleoclimatic Stages of Northwestern Russia During the Late Glacial and Holocene

      Arslanov, Kh A.; Saveljeva, L. A.; Gey, N. A.; Klimanov, V. A.; Chernov, S. B.; Chernova, G. M.; Kuzmin, G. F.; Tertychnaya, T. V.; Subetto, D. A.; Denisenkov, V. P. (Department of Geosciences, The University of Arizona, 1999-01-01)
      We have studied 6 reference sections of bog and lake sediments in the Leningrad and Novgorod provinces to develop a geochronological scale for vegetational and paleoclimatic changes in northwestern Russia during the Late Glacial and Holocene. Every 10-cm layer along the peat and gyttja sections (4-8.5 m thick) was investigated palynologically and the great majority of them were radiocarbon dated. Using the data obtained, standard palynological diagrams were plotted and vegetation history reconstructed. The palynozones indicated on the diagrams were related to the climatic periods and subperiods (phases) of the Blytt-Sernander scheme. On the basis of 230 14C dates obtained, we derived the geochronology of climatic periods and phases, as well as the chronology for the appearance and areal distribution of forest-forming tree species. The uppermost peat layers were dated by using the "bomb effect". We compared the stages of Holocene vegetation and paleoclimatic changes discovered for the Leningrad and Novgorod provinces with the those obtained for Karelia, which we had studied earlier using the same methodology.
    • The Maunder Minimum: An Interlaboratory Comparison of Delta-14C from AD 1688 to AD 1710

      Damon, Paul E.; Eastoe, Christopher J.; Mikheeva, Irina B. (Department of Geosciences, The University of Arizona, 1999-01-01)
      Measurements on same-age tree-ring samples from proximal Ural Mountain trees by the Ioffe Institute research group and at the University of Arizona demonstrate a variance corresponding to a standard deviation of +/5.1 per mil for Ioffe compared to +/2.1 per mil for Tucson. There is also a calibration difference of 4.3 +/1.2 sigma per mil. Comparison of the same years measured in Seattle on wood from the Pacific Northwest shows an offset of 2.2 +/0.5 sigma per mil. This is not a calibration error, but rather is expected from the well-documented evidence for divergence and upwelling of 14C-depleted CO2 along the west coast of North America.
    • Variations of Isotopic Composition of Carbon in the Karst Environment from Southern Poland, Present and Past

      Pazdur, Anna; Goslar, Tomasz; Pawlyta, Mirosława; Hercman, Helena; Gradziński, Michał (Department of Geosciences, The University of Arizona, 1999-01-01)
      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 13C values and 14C 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 14C 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 14C calibration time scale, the "Absolute" age having been determined by U/Th or amino acid racemization (AAR) dating methods. Measurements of delta-13C and 14C 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-13C 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-13C values. Extremely light values of 13C (under -40 per mil), observed in Algae and Hyloconium splendens, are correlated with 14C 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-13C values and 14C concentration. The lower reservoir effect for 14C is observed in samples with higher value of delta-13C, indicating equilibrium conditions in the sedimentation of carbonate. Pazdur et al. (1995b) presented 14C dating results and paleoclimatic interpretation of 170 14C analyses of 89 speleothems from 41 caves obtained through 1994. Investigations continued until early 1997, during which time a speleothem, JWi2, was dated by 14C, U/Th and AAR dating methods, and its stable isotope composition (delta-13C and delta-18O) analyzed in detail (reported here). Carbon isotope analyses indicate very large differences among results obtained by U/Th, AAR, and 14C dating methods.
    • From the Managing Editor

      Sewell, David R. (Department of Geosciences, The University of Arizona, 1999-01-01)
    • The Reliability of AMS Radiocarbon Dating of Shells from China

      Zhou, Weijian; Head, M. J.; Wang, Fubao; Donahue, D. J.; Jull, A. J. T. (Department of Geosciences, The University of Arizona, 1999-01-01)
      We tested the feasibility of dating freshwater and terrestrial molluscs from the semiarid and arid zone in China, since these types of shell material deposit only aragonite to form their shell structure, and shell integrity can be easily observed using X-ray diffraction. We also tested the possibility of estimating microenvironmental changes from shell delta-13C values, but variations within shell populations preclude the use of these values as a reliable indicator. Reservoir ages were calculated for living shells of the same species as fossil shells by using their measured 14C ages, which were recalculated using an average value of atmospheric 14C activity for the years spanning their time of collection as the modern standard. The results indicate that freshwater and terrestrial shells are potentially useful as dating material, provided extreme care is taken in their collection and other datable material (in this case wood and pollen) is within the profile to act as a comparison.
    • A Delta-R Correction Value for Samoa from Known-Age Marine Shells

      Phelan, Matthew B. (Department of Geosciences, The University of Arizona, 1999-01-01)
      A first-order Delta-R correction value for marine samples is presented based on 3 radiocarbon determinations of known-age marine shells from Samoa.
    • Associate Editors

      Department of Geosciences, The University of Arizona, 1999-01-01
    • A 23-Year Retrospective Blind Check of Accuracy of the Copenhagen Radiocarbon Dating System

      Rasmussen, Kaare L.; Tauber, Henrik; Bonde, Niels; Christensen, Kjeld; Theodórsson, Páll (Department of Geosciences, The University of Arizona, 1999-01-01)
      A 23-yr record of the measuring accuracy of the Copenhagen radiocarbon dating laboratory has retrospectively been provided through a true blind test. A total of 92 samples of oak from old tree trunks were dated in the period 1971 to 1993 and their dendrochronological age determined independently. The 14C activity of the dendrochronological samples measured in the Copenhagen radiocarbon laboratory was compared to the activity of the tree rings of the same age measured by Stuiver and Pearson (1993) for calibration purposes. The average difference was found to be 54 +/72 14C yr. The results further indicate that the actual standard deviation is only 7% higher than that quoted by the laboratory. The investigation has shown a long-term stability of laboratory accuracy with no systematic laboratory variations either with respect to sample age or to the time of measurement from 1971 to 1993.
    • Radiocarbon, Volume 41, Number 1 (1999)

      Department of Geosciences, The University of Arizona, 1999-01-01
    • Radiocarbon Dates from Northern Mongolia

      Hall, Mark; Batsaikhan, Zagd; Honeychurch, William (Department of Geosciences, The University of Arizona, 1999-01-01)
      Since 1996, the Mongolian-American Expedition to Northern Mongolia has been excavating in the Egiin Gol Valley. The goal of this research has been to examine the competing hypotheses explaining the emergence of pastoral nomadism and the evolution of nomadic complexity. The chronological placement of burials and sites in the survey area has been a key facet of this research. At present, these investigations have generated 10 radiocarbon dates from archaeological contexts. Presented here are the previously unpublished 14C dates and some comments on their significance.
    • Radiocarbon Distribution in Northwest Belarus Near the Ignalina Nuclear Power Plant

      Mikhajlov, Nikolaj D.; Kolkovsky, Vladimir M.; Pavlova, Iren D. (Department of Geosciences, The University of Arizona, 1999-01-01)
      Since 1994, the Institute of Geological Sciences has undertaken an environmental monitoring program to measure radiocarbon levels in territory adjacent to active nuclear power plants (NPP). We determined 14C concentrations in natural objects from areas contiguous to Ignalina NPP as well as 14C background concentration in areas remote from the NPP. In the environs of the Ignalina station comparatively elevated levels of 14C were observed in vegetation and waters of Lake Drisvyaty. This appears to be a consequence of release of carbon radioisotope into the atmosphere and probably into waters of the lake during operation of the nuclear reactor.