• 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.
    • Instrumentation and Software for Low-Level Liquid Scintillation Counting Radiocarbon Dating

      Sanyuan, Guan; Yuanming, Xie (Department of Geosciences, The University of Arizona, 1992-01-01)
      In China, the development of 14C dating is closely related to that of liquid scintillation counting (LSC). New demands are continuously made on LSC by 14C dating, and at the same time, the development of LSC promotes 14C dating. Benzene synthesis for 14C sample preparation was first developed in China in 1974 by the 14C Laboratory of the Department of History, Peking University. This has laid the foundation for 14C dating by counting with LS counters. Initially, we used a modified conventional LS counter. Despite its poor detection efficiency and high background, it did make a great contribution to 14C dating and LSC in China. Additional 14C laboratories were established for the fields of archaeology, geology, geography, vertebrate paleontology, marine geology and seismology, for which 14C dating is becoming an important research tool. At present, over 5Delta-14C laboratories have been established in China; 90% of the labs use LS counters for counting, most of which are manufactured in China. 14C dating in China has been primarily concerned with sample synthesis techniques. Since 1979, we have developed three types of LS counters. One is of conventional design; the others are with anticoincidence shielding. We describe here an anticoincidence-shielded LS counter.
    • Lake Sediments from the Kashmir Himalayas: Inverted 14C Chronology and Its Implications

      Kusumgar, Sheela; Agrawal, D. P.; Bhandari, Narendra; Deshpande, R. D.; Raina, Alok; Sharma, Chhemendra; Yadava, M. G. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We have measured 14C, 210Pb and 137Cs profiles in two representative cores from Manasbal Lake, Kashmir, India. The sedimentation rate derived from 210Pb and 137Cs in the upper part of the core is in the range of 3.4 to 5.5 mm yr-1. In contrast, 14C ages show an inversion at depths >20 cm. These results are attributed to the erosion of the ubiquitous 10-20-m-thick loess mantle, based on the similarity of 14C ages of the inversion layer in the sediments and the paleosols present in the catchment area. Frequency-dependent mineral magnetic susceptibility (Xfd), carbon to nitrogen ratios and pigment concentrations in the profile show a significant amount of allochthonous component in the lake deposits and support the conclusion that the 14C dates do not reflect the chronology of the in-situ lake sedimentation but episodic deposition of the surrounding loess. Thus, 14C serves as a useful tracer to understand source components of the sediments.
    • Mass Spectrometric 14C and U-Th Measurements in Coral

      Burr, G. S.; Edwards, R. L.; Donahue, D. J.; Druffel, E. R. M.; Taylor, F. W. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We discuss U-Th and 14C measurements in coral. Samples with U-Th dates in excess of 50 ka BP were chosen for study. Some bulk samples from this group have measurable 14C dates, which range from 30 ka to 43 ka BP. These can be explained by 0.5-2.5% contamination by modern carbon. This small amount of contamination can produce significant offsets in 14C dates of coral samples older than -10 ka. It may be undetectable in X-ray powder diffraction patterns. We describe a sample pretreatment that removes the modern carbon by selective dissolution and produces accurate 14C dates.
    • Measurement of 14C Concentrations of Stratospheric CO2 by Accelerator Mass Spectrometry

      Nakamura, Toshio; Nakazawa, Takakiyo; Nakai, Nobuyuki; Kitagawa, Hiroyuki; Honda, Hideyuki; Itoh, Tomozio; Machida, Toshinobu; Matsumoto, Eiji (Department of Geosciences, The University of Arizona, 1992-01-01)
      In order to measure the concentrations of anthropogenically influenced gases in the stratosphere, we have collected air samples from the lower stratosphere since 1985, by a balloon-borne cryogenic sampling method, developed at the Institute of Space and Astronautical Science (ISAS). Air samples of ~16 liters at STP were collected in the stratosphere at altitudes from 18.6 to 30.4 km, over the northeastern part of Japan (39.5 degrees N, 139-142 degrees E), on 1 September 1989. We conducted 14C analyses to study the vertical and horizontal air-mass movement in the stratosphere, and to investigate the air transport mechanism between troposphere and stratosphere. Carbon dioxide (containing a few mg carbon) was separated cryogenically from the air samples, and the 14C concentration of the CO2 was measured by a Tandetron accelerator mass spectrometer, using Fe-graphite targets prepared by reducing CO2 on Fe-powder with hydrogen in a Vycor tube at 650 degrees C. The 14C concentrations, expressed as Delta-14C, of CO2 were 267-309 per mil at altitudes of 21-30 km, and 134 per mil at 19-20 km. The Delta-14C values at 21-30 km were higher than those of the current tropospheric CO2, of around 80-200 per mil. The observed 14C concentrations, higher in the stratosphere than the troposphere, seem to be explained by large bomb-produced 14C inventories and/or high 14C production by cosmic rays, as well as weak vertical mixing of air masses in the stratosphere.
    • Measurements of Cosmogenic 14C Produced by Spallation in High-Altitude Rocks

      Jull, A. J. T.; Wilson, Amy E.; Burr, G. S.; Toolin, L. J.; Donahue, D. J. (Department of Geosciences, The University of Arizona, 1992-01-01)
      The production of radioisotopes at the Earth's surface by cosmic-ray effects has been discussed for many years. Only in the past few years, with the higher sensitivity provided by accelerator mass spectrometry (AMS) in detecting 10Be, 26Al and 36Cl, have the radioisotopes produced in this way been measured. We report here our measurements of cosmogenic 14C in terrestrial rocks at high altitude, and comparisons to other exposure-dating methods.
    • Participants

      Department of Geosciences, The University of Arizona, 1992-01-01
    • Possibilities for Reconstructing Radiocarbon Level Changes During the Late Glacial by Using a Laminated Sequence of Lake

      Goslar, Tomasz; Kuc, Tadeusz; Pazdur, Mieczysław F.; Ralska-Jasiewiczowa, Magdalena; Różański, Kazimierz; Szeroczynska, Krystyna; Walanus, Adam; Wicik, Bogumił; Więckowski, Kazimierz; Arnold, Maurice; et al. (Department of Geosciences, The University of Arizona, 1992-01-01)
      Laminated sediments of Gościąż Lake can be used as an independent source of material for calibrating the radiocarbon time scale. The varve chronology is based on three long cores from the deepest part of the lake, with one additional core from the second deepest part. From pollen and Cladocera spectra and stable isotope and chemical content sequences, we have determined the Allerød(AL)/Younger Dryas(YD) and Younger Dryas/Preboreal(PB) boundaries in the three long cores with relatively good accuracy, and have tentatively defined the AL/YD boundary in the fourth core. The Younger Dryas period contains at least 1520 varves, with 980 varves in fragments well replicated in all four cores. The duration of the Younger Dryas as recorded in sediments of Gościąż Lake corresponds well to the duration derived from 230Th/234U and 14C dates on Barbados corals, but disagrees with estimates from Soppensee, Lake Holzmaar and Swedish varves. Two AMS dates of terrestrial macrofossils from the PB and YD periods seem to fit both the data obtained for Swiss lake sediments and Barbados corals.
    • Possible Effects of Ozone Depletion on the Global Carbon Cycle

      Tsung-Hung, Peng (Department of Geosciences, The University of Arizona, 1992-01-01)
      The increase of UV-B radiation resulting from ozone depletion is considered to have damaging effects on marine ecosystems. A cutback of marine productivity would tend to reduce the oceanic uptake of atmospheric CO2. Box models of the global oceans based on the distribution of bomb-produced 14C are used to evaluate the possible effects of ozone depletion on the atmospheric CO2 concentration. The maximum effect presumably takes place if the ozone hole reduces the marine productivity to zero in the Antarctic Ocean. In a business-as-usual scenario of future CO2 emissions, the atmospheric CO2 partial pressure (pCO2) would increase by an additional 37 micro-atm over the course of the next century. This increase corresponds to 4.6% of the projected atmospheric pCO2 in the year 2090. However, if the damaging effect caused by the destruction of the stratospheric ozone layer is assumed to lower the productivity over the Antarctic Ocean by 10%, the atmospheric pCO2 would rise by less than 3 micro-atm over the expected atmospheric level in the next century.
    • PRIME Lab: A Dedicated AMS Facility at Purdue University

      Elmore, David; Rickey, F. A.; Simms, P. C.; Lipschutz, M. E.; Mueller, K. A.; Miller, T. E. (Department of Geosciences, The University of Arizona, 1992-01-01)
      A new facility for accelerator mass spectrometry has been established at Purdue University. First results have been obtained for 10Be and 36Cl, and several internal research projects have been initiated. Plans are to become a national AMS facility to serve the Earth and planetary science communities for the full range of cosmogenic radionuclides.
    • Progress in AMS Measurements at the LLNL Spectrometer

      Southon, J. R.; Vogel, J. S.; Trumbore, S. E.; Davis, J. C.; Roberts, M. L.; Caffee, M. W.; Rinkel, R. C.; Proctor, I. D.; Heikkinen, D. W.; Berno, A. J.; et al. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We report on the present status of the Lawrence Livermore AMS spectrometer, including sample throughput and progress towards routine 1% measurement capability for 14C, first results on other isotopes and experience with a multisample high-intensity ion source.
    • Quantifying Background Components of Low-Level Gas Proportional Counters

      Theodórsson, Páll (Department of Geosciences, The University of Arizona, 1992-01-01)
      I discuss background components of low-level gas proportional counters and show how each component can be estimated based on available data. For more reliable background analysis, further studies are suggested. Based on new information, a generation of low-level gas proportional counting systems for radiocarbon dating may emerge with lower and more predictable background.