• Trends of 13C/12C Ratios in Pinyon Tree Rings of the American Southwest and the Global Carbon Cycle

      Leavitt, S. W.; Long, Austin (American Journal of Science, 1986-01-01)
      An accurate atmospheric 13C/12C chronology can provide important constraints to models of the global carbon cycle. Trees accumulate carbon from atmospheric CO2 into growth rings and offer potential for 13C/12C reconstructions, but results have not been reproducible. This paper presents 5 degree C curves from 5 sites, representing 20 pinyon (Pinus edulis) trees, where cores of 4 trees from each site have been pooled into a composite sample. Isotopic analysis of cellulose in 5-yr ring groups produces curves with a general trend of decreasing 5 degree C after 1800, but with pronounced short-term fluctuations superimposed upon the trend. Evidence indicates the fluctuations are strongly related to moisture availability (drought). A mean curve of the 5 delta-13C chronologies from which the fossil-fuel component is subtracted suggests a substantial biospheric CO2 contribution to the atmosphere since 1800.
    • Toward a Thesaurus of Radiocarbon Dating and Related Terms

      Polach, Dilette (American Journal of Science, 1986-01-01)
      The development of a Thesaurus of Radiocarbon Dating and Related Terms was undertaken to provide a basic common vocabulary of terms for the purpose of indexing an Annotated Bibliography of Radiocarbon Dating (1948-68) and form the basis of a language for information storage and retrieval in 14C dating and related term areas. The interdisciplinary nature of 14C dating dictates that selected terms have meanings in the context of archaeology, geology, earth sciences, oceanography, and environmental sciences, to name but a few, as well as chemistry and nuclear instrumentation on which the technique of 14C determinations is based. This paper describes the method used for selection of terms and explains the structure of the proposed thesaurus. A limited edition Draft Thesaurus was distributed to a selected panel of colleagues for critical evaluation.
    • The Value of 210Pb in Dating Scandinavian Aquatic and Peat Deposits

      El-Daoushy, Farid (American Journal of Science, 1986-01-01)
      Sediment and peat chronologies have been further improved allowing alternative radiometric methods to complement 14C dating. Lacustrine and coastal marine sediments as well as peat deposits in various parts in Scandinavia are studied using 137Cs, 210Pb, 14C and other methods primarily to evaluate the 210Pb but also to extend the 14C chronology. The sampling sites have various sources of input and are characterized by different geochemical, depositional, and post-depositional conditions.
    • Thin Layer delta-13C and Delta-14C Monitoring of "Lessive" Soil Profiles

      Becker-Heidmann, Peter; Scharpenseel, Hans-Wilhelm (American Journal of Science, 1986-01-01)
      The natural 14C and 13C content of soil organic matter and their dependence on depth for two Alfisols are presented. This soil type which covers a large area of the earth's surface is characterized by clay migration processes ("Lessivé"). The samples were taken as successive horizontal layers of 2cm depth from an area of ca 1 m2 size as deep as the C content allows 14C analysis. The minima of the D14C distribution decrease with depth, while the maxima increase in the upper, leached horizon (A1) due to bomb 14C and decrease in the lower, clay illuviated (Bt). delta-13C indicates proceeding decomposition in Al and protection of carbon, probably due to the formation of clay humus complexes in Bt. delta-13C values were also used for age correction of the 14C data due to isotopic fractionation. The D14C and delta-3C depth distributions are characterized by sharp peaks at the boundaries of the horizons, probably caused by the influence of textural changes on the transport of C with percolating water.
    • The Temporal Distribution of 'Bomb' 14C in a Forest Soil

      Harkness, D. D.; Harrison, A. F.; Bacon, P. J. (American Journal of Science, 1986-01-01)
      Patterns of 14C enrichment in the superficial plant debris and mineral soil horizons of an established woodland have been monitored at regular intervals during the past 15 years. These data are compared with a model evaluation of carbon turnover based on the recorded changes in atmospheric 14C concentration since AD 1900. Leaf litter and decomposing plant debris are characterized by steady-state turnover values of ca 2 and ca 8 years, respectively. A two-component system of `fast' (less than or equal to 20 yr) and `slow' (ca 350 yr) cycling carbon is indicated for the surface (0-5cm) soil humus; below 10cm, the `fast' component is rare (<5%). Selective microbal humification of leaf litter, branch, and root debris is proposed to explain a delay of several years in the peak transfer of `bomb' 14C to the soil carbon pool.
    • The Use of Natural and Anthropogenic 14C to Investigate the Dynamics of Soil Organic Carbon

      O'Brien, Bernard John (American Journal of Science, 1986-01-01)
      Radiocarbon has been measured in two soil profiles, one of which has been covered by a building since 1956. A comparison of the Delta-14C values in horizons of each profile gives an estimate of the total input of atom bomb 14C into the soil profile. From the Delta-14C and carbon density profile data, the carbon input rates, respiration rates, and diffusivity are calculated. The lack of vegetation on one soil affects the mobility and the respiration rate of the soil carbon in that soil. The data from this soil profile are also used to check the assumption, used in previous analyses, that there is a uniform distribution of "old" carbon down the soil profile. The input rate, turnover time, and diffusivity parameters determined from the Delta-14C profiles in these soils are compared with other published data on pasture and forest soils.
    • The Power of 14C Measurements Combined with Chemical Characterization for Tracing Urban Aerosol in Norway

      Currie, L. A.; Klouda, G. A.; Schjoldager, Jorgen; Ramdahl, Thomas (American Journal of Science, 1986-01-01)
      Changing fuel patterns and increased awareness of health effects from combustion aerosols have generated considerable interest in the use of 14C as a biogenic-fossil aerosol source discriminator. Prior studies in the US demonstrated the importance of 14C measurement for estimating the wood-burning contribution to urban aerosols. The present work treats a specific air-pollution problem in the town of Elverum, Norway where large wintertime concentrations of aerosol carbon and polycyclic aromatic hydrocarbons (PAH) were suspected to come from residential woodl combustion (RWC). The problem was significant in that up to 50 micrograms/m3[C] and 490 micrograms/m3[PAH] were found during pollution episodes. Samples collected during two winters were analyzed for C, C, PAH, and several elements in the fine fraction (<3 micrometers) aerosol. Source apportionment based on these species indicated an average of ca 65% RWC-carbon (14C), ca 5% fine particle mass from motor vehicles (Pb), but negligible contributions from heavy fuel oil (Ni, V). Patterns of 14C and total C, examined as a function of temperature and PAH, indicated large increases in RWC aerosol on the coldest days, and a major RWC contribution to the PAH fraction. Patterns with inorganic species implied multiple tracer sources, and one important case of long-range transport.
    • The Potential of the London Underground for Liquid Scintillation Counting

      Bowman, Sheridan (American Journal of Science, 1986-01-01)
      A portable gamma spectrometer has been used to survey three locations that are part of the London Underground Transport System (the "tube"). Up to an order of magnitude reduction in the cosmic ray flux was observed relative to the laboratory level. The likely reduction in background count rate achievable by underground siting of currently used Packard and LKB liquid scintillation counters is considered. It is noted that in the present, surface usage for radiocarbon dating, the background count rate of low-potassium glass vials in the LKB is not substanially higher than that of PTFE vials.
    • The Prehistoric Expansion of Farming Into "Arctic" Norway: A Chronology Based on 14C Dating

      Johansen, Olav Sverre; Vorren, Karl-Dag (American Journal of Science, 1986-01-01)
      Palynologic and archaeologic studies using 14C dating indicate that elements of farming were introduced even further north than the Arctic Circle during the Neolithic period, ca 4000 BP. A second stage with heavier reliance on farming and with probable establishment of permanent farmsteads is dated to 2000-2500 BP.
    • The Effects of Contamination of Calcareous Sediments on Their Radiocarbon Ages

      Srdoč, Dušan; Horvatinčić, Nada; Obelić, Bogomil; Krajcar-Bronić, Ines; O, Malley Peg (American Journal of Science, 1986-01-01)
      Two principal reasons for the inherent uncertainty in 14C dating of calcareous sediments such as tufa or those of lacustrine origin are the unknown initial 14C activity (A0) of the sediment, mainly affecting younger (Holocene) samples, and contamination of older sediments with recent carbonate, causing 14C ages to be excessively young. To assess the contamination effect, samples of old tufa from the Riss/Würm interglacial were examined. These sediments contain essentially no 14C except that contributed by surface contamination. Tufa samples were crushed and grains ranging in size from <1 mm, 1 to 2mm, up to 4 to 5mm were separated for analysis; 2M HCl was then used to dissolve the samples in successive steps. 14C measurements indicated that each subsequent soluble fraction obtained from porous tufa gave a successively older age, indicating that the surface of the sample was contaminated by younger carbonates. No consistent effect of grain size on the 14C age was observed. Compact tufa proved to be less subject to contamination. 14C ages obtained on this material were also too young, yet older than the age obtained from porous tufa samples. C ages of interglacial tufa were cross-checked with the 230Th/234U dating method, using samples of very clean calcite which overlies the tufa blocks. Inferred 230Th/234U ages of the interglacial tufa (which had yielded 14C dates ranging from 25,000 to 37,000 yr) coincided with the last interglacial (Riss/Wurm, Stage 5). Samples of Holocene tufa, in which contributions of recent 14C from surface contamination would pose less of a problem, yielded 14C and 230Th/234U dates which were in excellent agreement.
    • The ETH/SIN Dating Facility: A Status Report

      Bonani, Georges; Hofmann, Hans-Jakob; Morenzoni, Elvezio; Nessi, Marzio; Suter, Martin; Wölfli, Willy (American Journal of Science, 1986-01-01)
      The ETH/SIN AMS dating facility is routinely used for 10Be,14C, 26Al, and 36Cl measurements. The present status and performance of this facility are reviewed and some of its major applications summarized.
    • The Influence of Contaminating (Fossil) Carbonate and the Variations of delta-13C in Mortar Dating

      Van Strydonck, Mark; Dupas, Michel; Dauchot-Dehon, Michele; Pachiaudi, Christiane; Marechal, Joelle (American Journal of Science, 1986-01-01)
      The influence of the aggregate in mortar dating is examined. Sample activity as well as isotopic fractionation approach the expected values at lower yields of the preparation reaction of the counting gas. Good results are obtained at low fossil carbonate concentration. delta-13C cannot give information about this concentration but preliminary visual and chemical analysis of the mortar makes prediction of sample validity possible.
    • The AMS Dating of Separate Fractions in Archaeology

      Batten, R. J.; Gillespie, Richard; Gowlett, J. A. J.; Hedges, Robert E. M. (American Journal of Science, 1986-01-01)
    • The Chemical Preparation of AgCl for Measuring 36Cl in Polar Ice with Accelerator Mass Spectrometry

      Conard, N. J.; Elmore, David; Kubik, P. W.; Gove, H. E.; Tubbs, L. E.; Chrunyk, B. A.; Wahlen, Martin (American Journal of Science, 1986-01-01)
      A method of chemical separation and purification of chloride from relatively small samples (500 to 2100g) of glacial ice is presented. With this procedure the first successful measurements of pre-bomb levels of 36Cl in Greenland ice have been made. Emphasis is placed on methods of reducing sulfur, which causes interference in the accelerator mass spectrometry, and in maximizing the yield. Data regarding the selection of materials for sample holders and the use of metal powders for extending the lifetime of the sample are also presented.
    • The Development of Practical Systems for 14C Measurement in Small Samples Using Miniature Counters

      Otlet, R. L.; Huxtable, George; Sanderson, D. W. (American Journal of Science, 1986-01-01)
      Miniature gas counters have been in use since the early 1960s for the measurement of 14C but were for a long time seen as suitable for providing approximate indications of activity rather than measurements for more precise dates. In recent years the need for better measurements of small samples has posed a continuing challenge for the 14C laboratories. This paper examines how the challenge has been met across the world using conventional beta decay counting techniques and proportional gas counters of 50ml volume or less. A survey is made of the rise of these techniques and attention paid to the solution through modern technology of earlier problems. Some practical systems, now in routine use, are described and consideration is given to the future for miniature counter measurements. Such systems have several attractive features that will guarantee their usefulness in 14C measurements for the future.
    • Standardizing Procedures for Collecting, Submitting, Recording, and Reporting Radiocarbon Samples

      Kra, Renee (American Journal of Science, 1986-01-01)
      A new radiocarbon sample record form has been compiled for consensus among radiocarbon daters and users. The form is designed to obtain comprehensive standardized documentation of radiocarbon dates for reporting and disseminating valuable data. This method of reporting 14C dates will help resolve problems which include: 1) publication of all dated samples, 2) selection of significant samples, 3) interpretation of data, 4) recognition of problematic dates, 5) maintenance of an active archive of 4C dates, and 6) implementation of an international retrieval system for dates.
    • Studies of Non-Marine Mollusks for the Selection of Shell Samples for Radiocarbon Dating

      Yates, Timothy (American Journal of Science, 1986-01-01)
      14C dating of shells from terrestrial and freshwater mollusks is prone to distortion by post-depositional diagenesis as well as incorporation of material depleted in 14C while the mollusk was alive. Three types of diagenetic change can result: etching, the development of surface crusts, and replacement of aragonite by calcite. Inspection under the light microscope, x-ray diffraction, and scanning electron microscope make it possible to assess the relative importance of the changes. When they are corrfined to the surface, mechanical cleaning combined with judicious leaching can reduce them to <1% of the total sample. The corresponding errors, which can now be specified, are often no greater than those associated with the statistics of counting.
    • Sample Credentials Necessary for Meaningful High-Precision 14C Dating

      Jope, E. M. (American Journal of Science, 1986-01-01)
      Samples presented for high-precision 14C dating must satisfy stringent requirements if the 14C determinations are to yield meaningful sharp calendric dates, such as are now possible with the bidecadal high-precision calibration curve. The total carbon content should come from a confined time range 10-20 years (10-20 tree rings in wood or charcoal) appropriate for the bidecadal calibration curve. For accurate calendric dating the relation of these rings to the outer growth rings must be known. Application of the high-precision calibration curve to some archaeologic examples is discussed. It is now up to archaeologists and geoscientists to use this refined chronometric instrument to fullest advantage.
    • Secular Variations of Cosmogenic 14C on Earth: Their Discovery and Interpretation

      Suess, Hans E. (American Journal of Science, 1986-01-01)
      Measurement of 14C in samples of wood of precisely known age have shown that the cosmogenic 14C content of the CO2 in the atmosphere has not remained constant during the period of time covered by radiocarbon dating. As the terrestrial atmosphere mixes with a time constant of less than 3 years, these variations must be essentially independent of geographic location. The 14C in atmospheric CO2 must be a quantity that, at a given time, pertains to the terrestrial atmosphere as a whole. Not only is its knowledge necessary for deriving accurate radiocarbon dates, but it is also valuable in connection with many geophysical and astrophysical problems. Unfortunately, progress in our knowledge of 14C variations in the terrestrial atmosphere has been delayed by hidden experimental errors in results obtained by many laboratories. By rigorous statistical analysis of the La Jolla results, it is now possible to show that the 14C variations are not simple random fluctuations but show distinct regularities. Similar patterns of variations have been found in the growth rate of trees during the last 5000 years. Measurements of radiogenic 10Be currently being done by European workers promise to conclusively elucidate the 14C findings.