• 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.
    • Radiocarbon Anomalies Observed for Plants Growing in Icelandic Geothermal Waters

      Andersen, G. J.; Heinemeier, Jan; Nielsen, H. L.; Rud, Niels; Thomsen, M. S.; Johnsen, Sigfús; Sveinbjörnsdóttir, Árný; Hjartarson, Arni (Department of Geosciences, The University of Arizona, 1992-01-01)
      We have studied plant remains in thick beds of lacustrine sediments in the upper part of the Markarfljót drainage area in southern Iceland. We collected small samples of plant species from the same horizon and 14C dated them at the Aarhus AMS Dating Laboratory. Terrestrial plants yielded an age of 9 ka BP, whereas aquatic moss (Fontinalis antipyretica Hedw.) yielded the surprisingly old 14C age of 16 ka BP. We believe the age of the terrestrial plants reflects the true age of the sediment. The anomalously old 14C age of the aquatic moss may be an effect of geothermal water on the moss, as the area is known to be geothermally active today. Modern aquatic moss growing in geothermal water showed a similar 14C anomaly, with measured ages ranging from 6 to 8 ka BP, which may be explained by the equally old ages measured for the corresponding water samples. The 14C content of geothermal springs and neighboring rivers in the area ranges from 9 to 50 pMC, equivalent to an apparent age of 20-5.5 ka BP.
    • 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 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.
    • 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.
    • Radiocarbon AMS Dating of Pollen Extracted from Peat Samples

      Brown, T. A.; Farwell, G. W.; Grootes, P. M.; Schmidt, F. H. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We present 14C AMS measurements and discuss the extraction procedure used on pollen extracted from peat samples. Microscopic examination of the extracts shows that the procedure is sufficient to remove most non-pollen materials and results in an extract that is composed predominantly of pollen. The 14C dates that we obtained for pollen extracts from peat samples associated with the Mazama Ash layer are consistent with the range of bulk-sample dates obtained by others in previous studies. The limited measurement time and resulting precision (+/100 yr) of these initial measurements restrict our ability to draw firm conclusions from a comparison of the pollen extract dates with previous bulk-sample dates. We intend to adjust our procedure to improve the rejection of non-pollen materials and to increase the precision of our 14C measurements on pollen extracts from peat samples in future studies.
    • Radiocarbon Chronology of Late Neolithic Settlements in the Tisza-Maros Region, Hungary

      Hertelendi, Ede; Horváth, Ferenc (Department of Geosciences, The University of Arizona, 1992-01-01)
      We investigated chronological questions of five Late Neolithic settlements in the Hungarian Tisza-Maros region. Fifty new radiocarbon dates provide an internal chronology for the developmental phases of the tell settlements, and place them into the wider framework of the southeastern European Neolithic. An example is presented of how a unique type of stratigraphic excavation helps the interpretation of radiocarbon data, which are in contradiction with the stratigraphic position of the samples.
    • 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.
    • 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.
    • 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 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 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.
    • 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.
    • Radiocarbon Dating of Anodonta in the Mojave River Basin

      Berger, Rainer; Meek, Norman (Department of Geosciences, The University of Arizona, 1992-01-01)
      A 450-year correction is required to make Anodonta 14C dates comparable to 14C dates on other materials in the Mojave River basin. The internal stratigraphic consistency of 34 conventional 14C dates on Anodonta in this drainage basin indicates that such dates are usually reliable. The validity of most conventional 14C dates in the Mojave River basin may be a product of the basin's crystalline bedrock in a region usually typified by thick Paleozoic carbonate sections.
    • 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.
    • 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.
    • Radiocarbon Dating of Travertines Precipitated from Freshwater

      Mas-Pla, Joseph; Trilla, Josep; Valls, M. L. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We have studied the isotopic composition of recently precipitated travertines to determine the initial 14C activity of ancient travertine deposits. We found that the 14C activity of recent travertines of northeastern Spain was a function of the distance from the spring, resulting in variations in the initial 14C activity of the ancient travertine formations. We calculated the ancient travertine radiocarbon ages by using the ratio between the 14C activity of recent travertines and that of present atmospheric CO2 as the initial 14C activity.
    • 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.