• A Comparison of Radiocarbon and U/Th Ages on Continental Carbonates

      Fontes, J.-C.; Andrews, J. N.; Causse, Christiane; Gibert, Elisabeth. (Department of Geosciences, The University of Arizona, 1992-01-01)
      Authigenic micrite from a playa in the northern Sahara has been dated by both the U/Th and 14C methods. The U/Th ages can be a few thousand years greater or less than the 14C ages obtained on the smallest crystals of micrite. The latter are considered to form a reasonable chronology for the Holocene deposits. The deviations of the U/Th ages are explained by quantifiable losses and gains of uranium under changing redox conditions. Under conditions where U is conservative (reduced sediments with low permeability), the U/Th method can provide good chronologies for lacustrine deposits.
    • A Day in the Life of... Or, My Thirty-Year War with the Background

      Rubin, Meyer (Department of Geosciences, The University of Arizona, 1992-01-01)
    • A High Throughput 14C Accelerator Mass Spectrometer

      Purser, Kenneth H. (Department of Geosciences, The University of Arizona, 1992-01-01)
      I present design details of a tandem accelerator mass spectrometer, which has been installed at the National Ocean Sciences AMS Facility at Woods Hole, Massachusetts, to provide precision 14C/13C/12C isotopic ratios for submilligram-size samples of graphite with throughputs of >4000 samples per year. A unique feature is the capability for simultaneous measurement of all three isotopes after acceleration, to avoid differential transmission effects and to allow on-line fractionation corrections and diagnosis of instrument health. Using filamentous graphite fabricated from a recent sample, we have established the counting rate of 14C ions at between 60-120 s-1.
    • A Minivial for Small-Sample 14C Dating

      Kaihola, Lauri; Kojola, Hannu; Heinonen, Aarne (Department of Geosciences, The University of Arizona, 1992-01-01)
      We have designed a 0.3-m1 Teflon minivial for 14C dating of small samples in a liquid scintillation counter. We use a special adapter of standard vial size to optimize the position of the vial with respect to the phototubes and to intercept the light path between them, thus reducing optical cross-talk. Better performance can be achieved by using customized vials than by diluting small samples for counting in large vials. We have achieved counting efficiencies up to 80% in 0.3-ml vials typically with 0.05 cpm background.
    • A New Data Acquisition System for the Groningen Counters

      van der Plicht, Johannes; Streurman, H. J.; Schreuder, G. R. (Department of Geosciences, The University of Arizona, 1992-01-01)
      A new GPIB/IEEE-488 based data acquisition system has been built for the Groningen proportional counter setup, consisting of 11 counters. The IEEE bus is connected to an XT-compatible host PC. A versatile computer program controls the data entry; the same program can be used offline for final calculations.
    • A New Tree-Ring Width, delta-13C and 14C Investigation of the Two Creeks Site

      Leavitt, S. W.; Kahn, R. M. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We have made isotopic and dendrochronologic measurements on material collected from the Two Creeks site. Radiocarbon dating of outside wood of four logs yielded an average age of 11,760 +/100 BP, in good agreement with results of Broecker and Farrand (1963) over 25 years ago. The range of 11,640 +/160 to 11,900 +/160 BP suggests a period of forest growth of 200-300 years, consistent with a ring-width chronology established by Kaiser (1987). Ring counting of five specimens gave a range of individual tree ages from 110 to 182 years, and width measurements indicate very low year-to-year variation in ring size. However, preliminary cross-dating of five samples produced a 202-year floating chronology. Stable-carbon isotope chronologies on cellulose from five-year ring groups show delta-13C scatter among trees typical of that found within modern sites, with some matches of isotopic maxima and minima. Some downward delta-13C trends may result from physiological response to rising lake levels (and/or cooling temperatures) at the site, which also produced very narrow rings in the outer ca. 50 +/20 years.
    • A Radiocarbon Dating Protocol for Use with Packard Scintillation Counters Employing Burst-Counting Circuitry

      Cook, G. T.; Anderson, Robert (Department of Geosciences, The University of Arizona, 1992-01-01)
      Considerable research has been carried out in this laboratory on the use of Packard scintillation counters employing burst-counting circuitry. The nature of the pulse-shape discrimination circuit has led to redefining our sample vialing philosophy and scintillation cocktail optimization. Results presented here demonstrate; 1) the enhancements in efficiency that are achievable using a two-component cocktail compared with the use of a primary fluor only; 2) the development of a cocktail in which efficiency and quenching are relatively unaffected by moderate variations in fluor concentrations and ratio; 3) the rationale behind the use of screw-cap vials rather than sealable ampules; 4) the advantage of nickel/chromium-plated brass caps over standard plastic caps.
    • A Review of Current Approaches in the Pretreatment of Bone for Radiocarbon Dating by AMS

      Hedges, R. E. M.; Van Klinken, G. J. (Department of Geosciences, The University of Arizona, 1992-01-01)
      Although the reliability of 14C dates of bone has increased greatly since AMS methods permitted better pretreatment on smaller samples, most old, badly contaminated or severely weathered bone still give serious problems. Several groups have recently proposed improvements to sample purification methods, often supported by a number of 14C measurements. We present here an overview of these improvements. The issue is complicated by the following: 1. Different problems are presented depending on age, preservation and degree of contamination of bone. 2. Methods may or may not be developed with routine application in mind. 3. Determining the conditions for which any method can be regarded as reliable is not at all straightforward.
    • A Simple Technique for Converting CO2 to AMS Target Graphite

      Wilson, A. T. (Department of Geosciences, The University of Arizona, 1992-01-01)
      I describe a simple, rapid and inexpensive method for converting CO2 samples into AMS target graphite. The technique is applicable for both smalland large-scale production facilities. With some modification, the method is suitable for use with very small samples.
    • Accelerator Mass Spectrometry at the Lund Pelletron Accelerator

      Skog, Göran; Hellborg, Ragnar; Erlandsson, Bengt (Department of Geosciences, The University of Arizona, 1992-01-01)
      Three years ago, funds were raised to equip the 3 MV Pelletron accelerator at the Department of Physics, Lund University for accelerator mass spectroscopy (AMS). We have modified the accelerator for mass spectroscopy by relocating focusing devices on both the lowand high-energy side of the accelerator and installing a Wien velocity filter and detectors for measuring the particle energy (E) and energy loss (DE). We have been working exclusively with 14C during the initial period. About 40 samples of elemental carbon have been produced, using Fe or Co as catalyst, during the last two years. The 12C- current from these samples is about 3-51 micro-A, using an ANTS sputtering source. We are now planning 14C analyses in the fields of archaeology, Quaternary geology and radioecology.
    • Accelerator Mass Spectrometry of 14C at the Australian National University

      Fifield, L. K.; Allan, G. L.; Ophel, T. R.; Head, M. J. (Department of Geosciences, The University of Arizona, 1992-01-01)
      A 14C measurement capability has been developed on the 14UD accelerator at the Australian National University. At present, this system operates on a medium-precision, low-throughput basis with slow cycling between isotopes. We describe unusual features of the system, and review preliminary experience with this mode of operation, in sample preparation, and with a recently installed injection system.
    • Account of the Business Meeting, 24 May 1991

      Mook, Willem G. (Department of Geosciences, The University of Arizona, 1992-01-01)
    • Accurate Dating of Organic Deposits by AMS 14C Measurement of Macrofossils

      Törnqvist, Törbjorn E.; De Jong, Arie F. M.; Oosterbaan, W. A.; Van Der Borg, Klaas (Department of Geosciences, The University of Arizona, 1992-01-01)
      We made a comparative study of AMS 14C ages of organic deposits (minerotrophic peats and gyttjas) and macrofossils in order to evaluate the magnitude of a number of sources of error that may be present in bulk sediment samples. The consistency of 14C ages found for coexisting macrofossils suggests that they are unlikely to record disturbances. Some of our gyttja samples yielded an age 0.2-0.6 ka 14C years too old due to hardwater effect. We also found an aging effect in several bulk samples with a high admixture of siliciclastic material; this is attributed to fluvial input of reworked, older organic debris. Rejuvenation of bulk material as a result of root contamination occurs mainly in samples overlain by slowly accumulated deposits, and particularly in samples affected by (sub)recent roots.
    • 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.