• 14C Activity in Different Sections and Chemical Fractions of Oak Tree Rings, AD 1938-1981

      Olsson, I. U.; Possnert, Göran (Department of Geosciences, The University of Arizona, 1992-01-01)
      The 14C activity in rings from an oak tree grown in a suburb of Uppsala, Sweden has been studied for the period, AD 1938 to 1981. We compare the results with the atmospheric carbon dioxide records from Abisko, northern Sweden, where local or regional contamination from fossil-fuel combustion can be disregarded. We assess the influence from different chemical pretreatment procedures in use and compare HCl-NaOH-HCl treatment with cellulose extraction. We split each ring into two samples corresponding to early (spring) and late wood. A more refined partitioning has been applied to the years 1963 and 1964.
    • 14C Dating Mortar in Ireland

      Berger, Rainer (Department of Geosciences, The University of Arizona, 1992-01-01)
      I have developed a method of dating early medieval Irish buildings using charcoal encased in mortar. Due to the inclement weather over centuries, timbers do not preserve well in these structures, leaving little suitable material for 14C dating. Initially, several buildings of known age were analyzed to verify the mortar charcoal technique. Then, a series of buildings for which no definite architectural-historical dates existed, e.g., churches, houses, oratories and round towers, was successfully tested. I discuss here the results of this dating approach, and provide architectural historians with a firmer understanding of the origin and antiquity of early Irish buildings.
    • 14C Dating of Groundwater Containing Microbial CH4

      Hackley, K. C.; Liu, C. L.; Coleman, D. D. (Department of Geosciences, The University of Arizona, 1992-01-01)
      Groundwater samples were collected from several different depths in Illinois glacial deposits at a site in eastcentral Illinois. Dissolved gases were extracted from many of the water samples, measured volumetrically, and analyzed by gas chromatography. The DIC was precipitated as barium carbonate and analyzed for both delta-13C and 14C. Due to the formation of microbial CH4, some of the DIC had unusually heavy delta-13C values of -3 to -1 per mil. The standard groundwater 14C-age correction models were developed to account for carbon dissolution as the primary carbon input beneath the soil zone. If the heavy delta-13C values observed in this study are used in readily available groundwater dating models without accounting for the effects of microbial methane formation, many of the resultant calculated ages are negative (future ages) or mathematically unsolvable. Isotopic and analytical results show a positive correlation (r2 = 0.90) between the delta-13C of the DIC and the concentration of methane in the groundwater. With this correlation, we were able to correct the delta-13C values of the DIC which were altered due to microbial CH4 formation. This adjustment of delta-13C values, along with estimations of dead carbon input from the redox processes, allowed us to calculate 14C ages using standard groundwater age correction models.
    • 14C Gas Counting: is There Still a Future?

      Povinec, Pavel (Department of Geosciences, The University of Arizona, 1992-01-01)
      Reviewed here are developments in low background proportional counters and chambers used for low-level internal gas counting of 14C. I have studied the influence of counter geometry and the nature of the gas filling on counter characteristics and describe in detail operational characteristics of multi-element proportional chambers. I also discuss detector developments for ultra-low-level gas counting based on a time projection chamber. My bservations show that proportional and drift chambers, together with multichannel electronics operating on-line with a computer, open new possibilities in ultra-low-level gas counting.
    • 14C in Austrian Wine and Vinegar

      Schonhöfer, Franz (Department of Geosciences, The University of Arizona, 1992-01-01)
      I analyzed wine from the vicinity of Vienna for 14C and 3H, using an ultra-low-level liquid scintillation counter. A series of certified samples from 1955 to 1987 was used to establish a calibration curve, which was applied successfully to a sample of apricot brandy. I used a simple extraction method to isolate acetic acid from vinegar, which I measured directly after mixture with a scintillation cocktail. A sample labeled as manufactured from wine contained too high a concentration of 14C, and thus may have been adulterated. Analyticaland technical-grade acetic acid also showed different concentrations of 14C, although it is unlikely that they are produced from organic material.
    • 14C Profiles in the Norwegian and Greenland Seas by Conventional and AMS Measurements

      Nydal, Reidar; Gislefoss, Jorunn; Skjelvan, Ingunn; Skogseth, Fred; Jull, A. J. T. (Department of Geosciences, The University of Arizona, 1992-01-01)
      CO2 in the atmosphere is an important climate gas because of its absorption of infrared radiation. More knowledge about CO2 uptake in the ocean is of critical significance in predicting future climate development. For a period of approximately 30 years, radioactive carbon from nuclear tests has been a very useful tracer in CO2 exchange studies. Up to now, the measurements have been based mainly on the conventional counting technique with large CO2 samples (ca. 5 liters). Accelerator mass spectrometry (AMS) with small CO2 samples (1-2 ml) has made sampling much easier, and has especially stimulated the use of 14C as a tracer in the ocean. At higher latitudes, the ocean acts as a sink for CO2. In addition to Delta-14C measurements, we are concerned here with dissolved inorganic carbon (DIC) and delta-13C in the Norwegian and Greenland Seas. During cruises in 1989 and 1990, we obtained several Delta-14C profiles, and also repeated a few GEOSECS profiles taken in 1972. The shape of these profiles changes with time, and provides information about the mixing rate and the age of the deep water. From changes in the profiles, it appears that the deep water in the Greenland Sea has obtained about 25% of the 14C concentration in the ocean surface over a period of 25 years. The Norwegian Sea deepwater is estimated to be 50-100 years older than that of the Greenland Sea.
    • 1993 Price List

      Department of Geosciences, The University of Arizona, 1992-01-01
    • 1993 Price List

      Department of Geosciences, The University of Arizona, 1992-01-01
    • 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.
    • A Supernova Shock Ensemble Model Using Vostok 10Be Radioactivity

      Sonett, C. P. (Department of Geosciences, The University of Arizona, 1992-01-01)
      Analysis of the Vostok ice-core record of 10Be (Raisbeck et al. 1987) suggests that the sharply resolved increases in 10Be at 35 ka (kyr) and 60 ka are due to cosmic-ray (CR) increases. As an alternate to long-term solar modulation or strong decreases in the Earth's magnetic field, supernova (SN) forcing is qualitatively consistent with the generation of a forward-reverse shock ensemble from a spherical blast wave of age very approximately at 75 ka. This age agrees with Davelaar, Bleeker and Deerenberg's (1980) identification of 75 ka for the age of a North Polar Spur SN remnant. Confirmation would be the first geochemical detection of supernova forcing of spallogenic and perhaps cosmogenic isotope production in the atmosphere. The three 10Be increases can be satisfied by a modification of the Sonett, Morfill, and Jokipii (1987) model. This consists of 2 or 3 shock waves from a single SN event, which includes the first stage in the expansion, leading to a forward shock, S1+, and a pair of reverse waves, S1− and S2−. One reverse wave arises from the spherical expansion, itself, and the other is a reflected wave from a remnant precursor shell boundary from a more ancient SN. The model requires the solar system to be immersed in the ‘bubble’ of the earlier post-SN evolution, possibly affecting estimates of heliospheric boundary distance. However more recent analysis of Camp Century ice core data discloses only the 35 ka 10Be peak. This recent result compounds the difficulty of constructing a completely consistent model for the source of the Vostok spikes. This paper is written in the spirit of suggesting only one of possibly several different models, even within the subclass of SN models.
    • A Tandem Mass-Spectrometric Method of Cosmogenic Isotope Analysis

      Pavlov, A. K.; Kogan, V. T.; Gladkov, G. Y. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We propose an original method for analysis of low-concentrations of stable and long-lived radioactive nuclides. We discuss the parameters of the main features of the “Trace” spectrometer (a multicharged-ion laser source, a highly sensitive time-of-flight mass spectrometer, a charge-exchange chamber and a mass spectrometer for positive and negative single-charged ion analysis). We also compare these features with conventional AMS devices.
    • 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.