• Acknowledgments

      Department of Geosciences, The University of Arizona, 1992-01-01
    • AMS Radiocarbon Dating of Ice: Validity of the Technique and the Problem of Cosmogenic In-Situ Production in Polar Ice Cores

      Wilson, A. T.; Donahue, D. J. (Department of Geosciences, The University of Arizona, 1992-01-01)
      In the "sublimation technique," carbon dioxide entrapped in ice is recovered by sublimation, converted to graphite and ratio of 14C/13C in the CO2 determined by AMS measurements. We describe here several experiments performed to check the validity of such measurements and to study the effect of cosmogenically produced in-situ 14C on the measurements.
    • Announcement of a Further International Intercomparison Exercise

      Scott, E. M; Harkness, D. D; Miller, B. F; Cook, G. T; Baxter, M. S. (Department of Geosciences, The University of Arizona, 1992-01-01)
      Following recommendations of the Glasgow International Workshop on Intercomparison of Radiocarbon Laboratories (Scott, Long & Kra 1990), a further international intercomparison is planned. This new intercomparison is complementary to the existing IAEA intercalibration, and will make use of natural samples whose ages will be unknown to the participants. The study has been funded by the UK Research Councils (SERC and NERC), and samples will be free to all participants. We anticipate that this intercomparison will be ongoing, with distribution of samples in 1992, and presentation of the results at a later meeting. We present here details of the samples available and the time scale of the study. Briefly, we envisage that the new study will be more focused than the ICS (Scott et al. 1986), and will include natural samples in both pretreated and unpretreated forms.
    • Anthropogenic Influence on the 14C Activity and Other Constituents of Recent Lake Sediments: A Case Study

      Srdoč, Dušan; Horvatinčić, Nada; Ahel, Marijan; Giger, Walter; Schaffner, Christian; Krajcar Bronić, Ines; Petricioli, Donat; Pezdič, Jože; Marčenko, Elena; Plenković-Moraj, Anđelka (Department of Geosciences, The University of Arizona, 1992-01-01)
      Anthropogenic activities that introduce an excess of nutrients and other pollutants into rivers and lakes are causing significant changes in their aquatic environment. Excessive nutrients greatly accelerate eutrophication, and lake marl formed during eutrophication differs from that formed in oligotrophic water. We analyzed recent sediment cores from Prošće and Kozjak lakes located in Plitvice National Park, central Croatia. Analyses consisted of 14C activity of calcareous lake marl, the ratio of stable isotopes (delta-13C, delta-18O), organic compounds in the sediment and the distribution of diatoms. Previous 14C activity measurements helped to determine the sedimentation rate and thus the time period of increased input of nutrients into lakes. We determined the increased 14C activity in lake sediments caused by nuclear bomb effect in recent depth profiles. We attributed the sudden increase in diatom species, Cyclotella operculata unipuctata and Achnanthes clevei rostrata, in the uppermost 5-cm layer, to eutrophication of the lake water. We performed a molecular characterization of hydrocarbons isolated from the sediments, and applied computer-assisted high-resolution gas chromatography/mass spectrometry to estimate contributions of biogenic, fossil and pyrolytic hydrocarbons.
    • Anthropogenic Radiocarbon in the Eastern Irish Sea and Scottish Coastal Waters

      Begg, F. H.; Coop, G. T.; Baxter, M. S.; Scott, E. M.; McCartney, Martin (Department of Geosciences, The University of Arizona, 1992-01-01)
      14C is produced as an activation product in nuclear reactors, and may be discharged to both the atmosphere and the marine environment during nuclear fuel reprocessing. In the UK, 14C is discharged, under license, into the Eastern Irish Sea by the British Nuclear Fuels plc (BNFL) reprocessing plant at Sellafield, Cumbria, northwest England, and is then transported into Scottish coastal waters. We analyzed intertidal biota samples to determine the effect of these discharges. The specific activities of 14C found in these samples indicate that the uptake and bioaccumulation of 14C is dependent on the type of organism and its feeding behavior. Measured 14C concentrations in mussels (Mytilus edulis) were higher than those in winkles (Littorina littorea), which were greater than those found in seaweed (Fucus spp.); maximum observed activities were ca. 7, 5 and 3.5 times the accepted current ambient level of 260 Bq kg-1 C, respectively. Annual Nori (Porphyra umbilicalis) samples were analyzed for their 137Cs, 241Am and 14C contents; both the 137Cs and 241Am results correlated well with published Sellafield discharge data (r = 0.877 and 0.918, respectively), whereas there was no significant correlation between measured 14C activities and the discharge record, indicating increased complexity in the chemical and biological behavior of 14C or some discrepancy in the estimated discharge records.
    • Assessment of 0.3-ML Minivials for Radiocarbon Dating by Liquid Scintillation Counting of Benzene

      Hogg, A. G. (Department of Geosciences, The University of Arizona, 1992-01-01)
      I have made an evaluation of 0.3-ml minivials for 14C dating of small samples by liquid scintillation counting of benzene. A calibrated wood standard was diluted by varying amounts of ancient CO2, with synthesized benzene counted in both conventional 3.0-ml vials and 0.3-ml minivials in a 1220 Quantulus. The accuracy and precision of results are compared for samples ranging in weight from 50 to 240 mg of carbon. I examined two significant potential problems associated with handling small samples, namely, memory effects within the vacuum system, and signal within the dilution gas. Although accurate radiocarbon dates can be obtained using either standard vials or minivials, minivials are more suitable for dating small samples because they are less influenced by these sources of error.
    • Bomb-Produced 14C in Tree Rings

      Kaimei, Dai; Youneng, Qian; Fan, C. V. (Department of Geosciences, The University of Arizona, 1992-01-01)
      The 14C content in 1961-1967 and 1970-1983 tree rings of a spruce grown in Dailing, China (47 degrees N, 129 degrees E) was measured by liquid scintillation. As a result of USSR bomb tests at Novaya Zemlya (72 degrees N, 53 degrees E), Delta-14C values rose dramatically from 250 per mil in 1961 to a maximum 909 per mil in 1964, and then gradually decreased to 238 per mil in 1983. We compared Delta-14C values in the rings of an oak tree grown at 43 degrees N, 74 degrees W and that of a pine grown at 49 degrees N, 9 degrees E, and atmospheric Delta-14C values in both northern and southern hemispheres. We observe that: 1) annual tree rings grown in the same latitude zone have the same Delta-14C values, reflecting rapid longitudinal mixing of the atmosphere; 2) atmospheric 14C concentrations reached a global equilibrium distribution at the end of 1968, and tree ring 14C content reflects atmospheric 14C concentration; 3) 1976-1982 rings of the Dailing spruce show excessive 14C, likely due to the effect of 1976 and 1980 Chinese bomb tests; 4) Delta-14C decreases exponentially, halving every 17 yr.
    • CalibETH: An Interactive Computer Program for the Calibration of Radiocarbon Dates

      Niklaus, Thomas R.; Bonani, Georges; Simonius, Markus; Suter, Martin; Wölfli, Willy (Department of Geosciences, The University of Arizona, 1992-01-01)
      A computer program for convenient calibration of radiocarbon dates has been developed. The program has a simple user interface, which includes pull-down menus, windows and mouse support. All important information, such as calibration curves, probability density function and results, in text form, are displayed on the screen and easily can be rearranged by the user. Two versions of CalibETH, one for an IBM-PC and one for the Macintosh, are available. CalibETH runs under the graphics interface, GEM, from Digital Research, on an IBM PC.
    • Chronology of the Baxie Loess Profile and the History of Monsoon Climates in China Between 17,000 and 6000 Years BP

      Zhou, Weijian; An, Zhisheng; Lin, Benhai; Xiao, Jule; Zhang, Jinzhao; Xie, Jun; Zhou, Mingfu; Porter, S. C.; Head, M. J.; Donahue, D. J. (Department of Geosciences, The University of Arizona, 1992-01-01)
      The valley of the Baxie River, situated in the western region of the Loess Plateau in central China, contains a loess profile 15 m thick, which can be considered representative of loess-paleosol sequences formed over the last 17 ka. Both thermoluminescence (TL) determinations on fine-grained sediment (4-11 micrometers) and 14C determinations on various organic fractions of paleosols from the profile have provided an extremely useful chronological framework for these sequences. These sequences indicate a weakened summer monsoon during the last glacial maximum followed by a strengthening of the summer monsoon, beginning ca. 13 ka cal BP. An abrupt change to a weakened summer monsoon regime lasted from ca. 10.9 to 10.2 ka cal BP. The Asian summer monsoon circulation, recording the Holocene optimum, then increased and lasted from ca. 10.2 to 6 ka cal BP. The organic component of samples taken down the profile has delta-13C values ranging from -21 to -24 per mil with respect to the PDB standard. The more positive delta-13C values suggest that the proportion of C4-type plants in river valleys of the Loess Plateau increased as Asian summer monsoon influence weakened, and C3-type vegetation increased as the summer monsoon influence strengthened. Magnetic susceptibility and organic content were low during loess deposition, also reflecting weakening of summer monsoon. Two 14C determinations on the humin fraction of the organic component near the top of the lower paleosol and the base of the upper paleosol complex gave ages of 10.2 and 10.9 ka cal BP, respectively. These ages mark the beginning and termination of a brief event involving increased dust influx under weakened summer monsoon conditions.
    • Common Spectral Features in the 5500-Year Record of Total Carbonate in Sea Sediments and Radiocarbon in Tree Rings

      Cini Castagnoli, G.; Bonino, Giuseppe; Serio, Marina; Sonett, Charles P. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We analyze here the time record of total carbonate carried as CaCO3 in a sea-bottom core from the Ionian Sea. Several major periods appear, most strongly at 1100, 690, 500, 340, 250 and 90 yr, confirmed both in the periodogram and maximum entropy estimates. The Gleissberg period appears in common with the 14C and other records, such as the sunspot index and aurorae. The manifestation of this period in what we surmise to be a climatic record is further evidence that the Gleissberg period has correlated bolometric and electrodynamic aspects.
    • Comparing Continental Carbonates with Other Materials in Dating a Paleolake

      Garcia, J. F.; Mestres, J. F.; Rauret, Gemma (Department of Geosciences, The University of Arizona, 1992-01-01)
      We have studied the application of carbonates and organic matter to the radiocarbon dating of a paleolake. The results show a systematic apparent age shift of these materials with respect to contemporary wood. The apparent age of carbonates is evidently due to the hard-water effect, whereas the apparent age of organic matter, systematically younger than carbonates, is attributed to aquatic plants, which metabolize dissolved CO2. Terrestrial plants that deposit organic matter also cause apparent age discrepancies between carbonates and organic matter.
    • Dating Pre-Columbian Museum Objects

      Van Strydonck, Mark J. Y.; van der Borg, Klaas; De Jong, Arie F. M. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We have radiocarbon dated some Pre-Columbian artifacts. We have used both conventional beta counting and AMS to date textiles, bamboo from weaving looms, a feather carpet and straw from a clay mask. We discuss the particular problems in sample pretreatment.
    • delta-13C Variations in C3 Plants Over the Past 50,000 Years

      Leavitt, S. W.; Danzer, S. R. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We examined three sets of data to determine if there are consistent changes in delta-13C of C3 plants through time, under the hypothesis that environmental changes from glacial to postglacial may have caused such isotopic changes over the last 50 ka. The records of delta-13C change in all types of plant data from RADIOCARBON and from the University of Arizona Radiocarbon Laboratory archives both reveal significant decline of 0.8-1.0 per mil in delta-13C from preto post-10 ka BP averages. The delta-13C of wood data alone from RADIOCARBON shows a larger significant decline of 3.0 per mil, and twigs, leaves and Juniperus categories from the Arizona data individually show declines of 0.4-1.44 per mil. Peat and charcoal from both data sets show no significant mean delta-13C differences. A highly constrained set of wood samples from the Great Lakes region spanning the last 12 ka show isotopic changes of ca. 3 per mil, but most of that variation apparently does not reflect global environmental changes.
    • Developments in Sample Combustion to Carbon Dioxide, and in the Oxford AMS Carbon Dioxide Ion Source System

      Hedges, R. E. M.; Humm, M. J.; Foreman, John; Van Klinken, G. J.; Bronk, C. R. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We describe the operation of a commercial system as modified for preparation of CO2 for the CO2 source. AMS samples are automatically combusted in a CHN analyzer, and stable isotope measurements are made on line. We describe the performance of this equipment, with reference to yield, sample contamination, memory effect, accuracy of isotope measurement, convenience and cost. We discuss the current status of dating using the CO2 source. This is the only source in operation at Oxford, and has been in routine dating since September 1989. We assess the practicalities of operation, including the latest measurements on background, memory, sample-size requirements and operating schedules. We also describe modifications to the sputter beam optics and to the gas handling systems.
    • Direct Radiocarbon Dating of Rock Art

      Russ, Jon; Hyman, Marian; Rowe, Marvin (Department of Geosciences, The University of Arizona, 1992-01-01)
      In 14C dating of pictographs, we use a low-temperature oxygen plasma coupled with high-vacuum techniques to selectively remove carbon-containing material in the paint without contamination from the rock substrate, even if limestone (CaCO3). In addition to one previously published measurement, we analyzed two more pictograph samples, which are in accord with archaeological inference. A sample of known age charcoal, also processed by our method, matched the control. This technique produces little mass fractionation, the maximum delta-13C being 0.16 per mil from the untreated sample. Limestone decomposition does not occur during our procedure. Although the technique development is in its infancy, these new results demonstrate that our non-destructive technique has great potential for producing accurate 14C ages.
    • Dissolved Organic and Inorganic 14C Concentrations and Ages for Coastal Plain Aquifers

      Purdy, C. B.; Burr, G. S.; Rubin, Meyer; Helz, G. R.; Mignerey, A. C. (Department of Geosciences, The University of Arizona, 1992-01-01)
      The Aquia (Paleocene) and Magothy (Late Cretaceous) Formations of the Atlantic Coastal Plain represent two well-characterized (hydrodynamically and geochemically) aquifers in southern Maryland. 14C measurements of the dissolved organic (DOC) and inorganic carbon (DIC) of Aquia and Magothy groundwaters have been made using accelerator mass spectrometry (AMS). Both DI14C and DO14C concentrations in the initial flow path are unexpectedly low. As the water progresses farther from the recharge area, the DI14C percent modern carbon (pMC) is consistently lower than the DO14C pMC; this difference stays constant for all samples. The 14C-derived ages for an Aquia water sample downgradient at Site 4 are 17 ka and 12 ka for DI14C and DO14C, respectively. Radiocarbon ages have been compared to ages determined by two other independent dating methods: computer-simulated hydrodynamic modeling and age estimates based on changes in Cl-,18O and 2H distributions, which are interpreted to be influenced by sea level and climate.
    • Evaluating Dissolved Inorganic Carbon Cycling in a Forested Lake Watershed Using Carbon Isotopes

      Aravena, Ramon; Schiff, S. L.; Trumbore, S. E.; Dillon, P. I.; Elgood, Richard (Department of Geosciences, The University of Arizona, 1992-01-01)
      Dissolved inorganic carbon (DIC) is the main acid buffer in forested lake watersheds in Canada. We used carbon isotopes (13C, 14C) to evaluate the production and cycling of DIC in an acid-sensitive lake watershed of the Precambrian Shield. Soil CO2, groundwater and stream DIC were characterized chemically and isotopically. Soil CO2 concentration profiles reflect both changes in production and in losses due to diffusion. delta-13C soil CO2 profiles (delta-13C values of -23 per mil in summer, slightly enriched during the fall and -25 per mil during the winter) are a reflection of the isotopic composition of the sources and changes in isotopic fractionation due to diffusion. Carbon isotopic composition (13C, 14C) of the groundwater and stream DIC clearly indicate that weathering of silicates by soil CO2 is the main source of DIC in these watersheds. 14C data show that, in addition to recent groundwater, an older groundwater component with depleted 14C activity is also present in the bedrock. The carbon isotope pattern in the groundwater also implies that, besides the main springtime recharge events, contributions to the groundwater may also occur during late winter/early spring.
    • Evaluation of a Prototype Low-Level Liquid Scintillation Multisample Counter

      Einarsson, S A. (Department of Geosciences, The University of Arizona, 1992-01-01)
      A new, improved photomultiplier has been incorporated and a more efficient active guard installed in a single photomultiplier liquid scintillation counter. A special quartz vial was designed to improve the photoelectron cathode index (photoelectrons/keV), and thus, to reduce background. Four photomultipliers were installed so that four samples can be counted simultaneously. The measurements presented here cover background spectra and counting efficiency obtained under various conditions in the laboratory. These measurements and derived parameters, such as the figure of merit (E2/B) and the factor of merit (S/VB), are used to evaluate the system performance. Also presented are the results of measurements on the cathode index and further background spectra, obtained at an underground site.
    • Evaluation of High-Purity Synthetic Silica Vials in Active and Passive Vial Holders for Liquid Scintillation Counting of Benzene

      Hogg, A. G.; Noakes, J. E. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We evaluate high-purity synthetic silica vials in both delrin and active plastic holders (Pico adaptersTM) for 14C dating, using liquid scintillation counting of benzene. We have designed synthetic silica vials in the form of simple cylinder-cells based on the standard 7-ml vial. We have also designed a delrin holder for supporting 7-ml silica or glass vials. We compare the counting efficiency and background of the silica vials with Teflon, plastic and low-K glass vials for both delrin holders and Pico adapters in the 1220 Quantulus and Packard Tri-Carb 2660 XL, fitted with a plastic detector guard. In the 1220 Quantulus, synthetic silica vials in Pico adapters have the highest figure of merit (FM), closely followed by silica in delrin holders and Teflon. In the Packard Tri-Carb 2660 XL, plastic vials in Pico adapters give the highest FM.
    • Examination of Background Contamination Levels for Gas Counting and AMS Target Preparation in Trondheim

      Gulliksen, Steinar; Thomsen, M. S. (Department of Geosciences, The University of Arizona, 1992-01-01)
      The Radiological Dating Laboratory in Trondheim relatively often dates samples with ages >30 ka BP. Contaminated background materials are known to affect the accuracy of very old dates. We have found, by measurements of different materials, that such contamination is small when using our conventional gas proportional counting (GPC) system. We have also studied contamination levels of our target preparation for 14C accelerator mass spectrometry (AMS) dating in Uppsala. A significant lower background is obtained for Icelandic double spar than for marbles, probably due to a crystal structure of the double spar that is more insensitive to contaminating processes. The background for combusted samples is at the same level as for samples of double spar, indicating that additional 14C contamination due to combustion is negligible. Dates obtained on interstadial samples (T >30 ka BP) by both GPC and AMS agree well.