• 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.
    • Experimental Determination of the 14C Initial Activity of Calcareous Deposits

      Krajcar Bronić, Ines; Horvatinčić, Nada; Srdoč, Dušan; Obelić, Bogomil (Department of Geosciences, The University of Arizona, 1992-01-01)
      We have reconstructed the initial activity of calcareous sediments by 1) measuring the 14C activity of recent pre-bomb-test calcareous deposits, 2) calculating the ratio of the 14C activity of the calcareous sediment to that of the adjacent coeval wood, and 3) extrapolating 14C activity of the deep lacustrine sediment layers to the surface, assuming a constant sedimentation rate. We show here that the uppermost sediment is affected by the global increase of atmospheric 14C activity due to thermonuclear bomb tests in the early 1960s. 14C activity of submerged aquatic plants showed values similar to 14C activity of DIC. Thus, organic residue of the sediment cannot be used for reliable 14C dating because of the unknown fraction of aquatic plant detritus. The initial activity, obtained in a case study of the geochronology of carbonates in the Plitvice Lakes area, ranged from 70.5 to 72.2 pMC for Lake Prošće sediment, and from 75.9 to 76.7 pMC for Lake Kozjak sediment. These values also agree with calculated values based on the downstream increase of 14C activity of DIC in freshwater open to the atmosphere.
    • Experiments on 14C Dating of Contaminated Bone Using Peptides Resulting from Enzymatic Cleavage of Collagen

      Van Klinken, Gert Jaap; Hedges, Robert E. M. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We describe the use of collagenase to digest ancient collagen samples to obtain a peptide mixture, and the use of reversed-phase chromatography to purify these peptides. The isolation of the tripeptides glycine-prolinehydroxyproline and glycine-proline-alanine is especially encouraging.
    • Factorial Design Techniques Applied to Optimization of AMS Graphite Target Preparation

      Verkouteren, R. M.; Klouda, G. A. (Department of Geosciences, The University of Arizona, 1992-01-01)
      Many factors influence the preparation and quality of graphite targets for 14C accelerator mass spectrometry (AMS). We identified four factors (sample size, HZ pressure, catalyst temperature and pretreatment time) as potentially critical, and investigated their effects on two particular characteristics; the integrated rates of CO2 reduction (to graphite) and methane production. We used a 2-level fractional factorial experimental design and determined chemical reduction yield rates through manometry and partial pressure monitoring of residual gases by mass spectrometry. Chemical reduction yield rates ranged from 0.2% to 6.2% per hour. With respect to their influence on percent yield rate, the factors we studied were ordered as; sample size > level of hydrogen > pretreatment of the catalyst. The temperature of the catalyst, and the sample size x hydrogen (2-factor) interaction, were only marginally influential. Other interactions did not appear to be significantly important. We estimated uncertainty in the order of influence and magnitudes of the effects by the Monte Carlo method of error propagation. We observed significant methane production in only one experiment, which suggests that methane originates from indigenous carbon in untreated iron catalyst only in the presence of hydrogen and only at thermodynamically favorable temperatures. This exploratory investigation indicates that factorial design techniques are a useful means to investigate multivariate effects on the preparation and quality of AMS graphite targets.
    • Foreword

      Kra, Renee; Long, Austin (Department of Geosciences, The University of Arizona, 1992-01-01)
    • Further Analysis of the International Intercomparison Study (ICS)

      Scott, E. M.; Cook, G. T.; Harkness, D. D.; Miller, B. F.; Baxter, M. F. (Department of Geosciences, The University of Arizona, 1992-01-01)
      The major findings of the Intercomparison Study (ICS) have already been published (Scott, Long & Kra 1990), but a number of questions remain unresolved. We address here some issues of user and technical relevance, which include; 1) further investigation of the quoted errors and their relation to the perceived precision and accuracy, which is of interest to users of 14C dates; 2) the analysis of the known-age wood samples provided in Stages 2 and 3 of the ICS; 3) an investigation of the corresponding delta-13C data base, of more technical relevance to laboratories.
    • Gamma Flux in 14C Laboratories

      Theodórsson, Páll; Kaihola, Lauri; Loosli, H. H.; Rodríguez, José M. (Department of Geosciences, The University of Arizona, 1992-01-01)
      An informal collaborative group of radiocarbon dating laboratories, the Low-Level Club, has been established to measure the gamma radiation flux and to test the efficiency of the anticoincidence counting system in laboratories with a NaI detector unit. The detector will record gamma radiation from cosmogenic nuclides, muons and secondary y radiation formed in the passive shield by charged cosmic-ray particles. We present here the first phase of this work.
    • Geochronologic and Paleoclimatic Characterization of Quaternary Sediments in the Great Hungarian Plan

      Hertelendi, Ede; Sümegi, Pål; Szöör, Gyula (Department of Geosciences, The University of Arizona, 1992-01-01)
      We reconstructed the climate of the Great Hungarian Plain between the years, 7-32 ka BP using a malacothermometer method. The reconstruction is based on seven Gastropoda taxa, for which optimal temperature and tolerance ranges have been determined. The temporal scales of the malacofaunal levels were calibrated with radiocarbon data. We compared our paleotemperature values with the temperature values of existing climatic curves and found the same climatic periods.
    • Half-Life Determination of 41Ca and Some Other Radioisotopes

      Kutschera, Walter; Ahmad, Irshad; Paul, Michael (Department of Geosciences, The University of Arizona, 1992-01-01)
      We have performed a new determination of the half-life of 41Ca by measuring the specific activity of an enriched Ca material with known 41Ca abundance. We measured the activity via the 3.3-keV X-rays emitted in the electron capture decay of 41Ca, and the 41Ca abundance was measured by low-energy mass spectrometry. The result, t1/2 = (1.01 +/- 0.10) x 10^5yr, agrees with the recent 'geological' half-life of Klein et al., (1991), t1/2 = (1.03 +/- 0.07) x 10^5 yr, and with the corrected value of Mabuchi et al. (1974), t1/2 = (1.13 +/- 0.12) x 10^5yr. We recommend the weighted mean of these three measurements, t1/2 = (1.04 +/- 0.05) x 10^5yr, as the most probable half-life of 41Ca. We also discuss the situation of the radioisotopes, 32Si, 44Ti, 79Se and 126Sn, whose half-lives, though still uncertain, are potentially interesting for future AMS studies and other applications.
    • High-Performance 14C Gas-Proportional Counting System Applying Pulse-Shape Discrimination

      Äikää, Osmo; Mäntynen, Pertti; Kankainen, Tuovi (Department of Geosciences, The University of Arizona, 1992-01-01)
      The new 14C dating system of the Geological Survey of Finland consists of three CO2 proportional counters of conventional size, a liquid scintillation anticoincidence guard, and a passive shield of about 90-year-old iron. The counting room is an air-conditioned Faraday cage constructed of concrete with aggregate of crystalline Precambrian limestone. We apply pulse-shape discrimination that greatly improves the precision of our 14C analyses the reduction of background is over 70%, while the modern count rate is reduced only by 15 to 20%. The purity of the counting gas is monitored by recording the cosmic muon pulse-height spectrum. At present, the dating limit for a two-day measurement is 57 ka. We summarize here the most significant results and aspects of pulse-shape discrimination
    • High-Sensitivity Radiocarbon Dating in the 50,000 to 70,000 BP Range Without Isotopic Enrichment

      Long, Austin; Kahn, R. M. (Department of Geosciences, The University of Arizona, 1992-01-01)
      Calculations show the possibility of detecting 14C remaining after 10 or 11 decay half-lives in natural materials, such as wood, using commercially available liquid scintillation (LS) detectors. Assuming in-situ 14C production has contributed insignificantly to the measured 14C, one can calculate finite ages approaching 70 ka. In practice, defensible finite age determinations involve careful considerations of several critical elements in the procedure. These critical elements are: 1) the integrity of the sample itself, in terms of younger contaminants and of in situ-produced 14C; 2) the availability of "dead" background material; 3) chemical blank in laboratory preparation of, in this case, benzene; and 4) stability of background and efficiency of the LS counting system. High-sensitivity 14C dating reveals a low level of memory effect 14C in benzene synthesized in the laboratory from anthracite or marble. This level of 14C activity is equivalent to that found in 53 ka old wood, and thus, is not distinguishable from petrochemical benzene used in routine dating. If careful control of laboratory conditions can maintain this 14C blank constant, reproducible dating beyond 53 ka would be possible. Although we have not completed a systematic analysis of the origins of memory effect, lithium reactors used in acetylene production and organic solvents in wood pretreatment are likely sources.
    • Illumination of a Black Box: Analysis of Gas Composition During Graphite Target Preparation

      McNichol, A. P.; Gagnon, A. R.; Jones, G. A.; Osborne, E. A. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We conducted a study of relative gas composition changes of CO2, CO and CH4 during the formation of graphite targets using different temperatures, catalysts and methods. Reduction with H2 increases the reaction rate without compromising the quality of the AMS target produced. Methane is produced at virtually any temperature, and the amount produced is greater at very low temperatures. The reduction of CO to graphite is very slow when H2 is not included in the reaction.