• Chemical Removal of Conservation Substances By 'Soxhlet'-Type Extraction

      Bruhn, Frank; Duhr, Alexander; Grootes, Pieter M.; Mintrop, Annette; Nadeau, Marie-Josée (Department of Geosciences, The University of Arizona, 2001-01-01)
      At the Leibniz radiocarbon lab, art and archaeological objects, often chemically conserved and thus potentially contaminated with respect to their 14C content, are treated using a computer-controlled “Soxhlet”-type series extractor. This device uses a continuous procedure of boiling and condensation of different solvents for extraction and vacuum filtration under constant process conditions. An elutrope sequence of five solvents that dissolve most customary conservation chemicals was selected. A study of these different contaminants applied to reference wood samples with subsequent accelerator mass spectrometry (AMS) measurements demonstrates that their effective removal is dependent on the use of adequate solvents. For many contaminants (e.g. Wood glue, methyl cellulose, Klucel(R), sugar, and polyethylene glycol), routine acid-alkali-acid (AAA) treatment already yields satisfactory results, whereas for Caparol(R) and beeswax a relatively “mild” treatment with acetone, methanol, water, and subsequent standard AAA extraction is sufficient. Complete removal of rubber glue, epoxyresin, and paraffin can only be accomplished with our full set of solvents. The latter procedure is also appropriate when no or only incomplete information about the type of conservation material is available. For epoxy resin the contamination appears to be enriched in the alkali residue, and the easily soluble “humic acid” fraction, even after standard AAA treatment, gives satisfactory results. Two case studies on the application of the extraction procedures are presented.
    • Carbonate 14C Background: Does It Have Multiple Personalities?

      Nadeau, Marie-Josée; Grootes, Pieter M.; Voelker, Antje; Bruhn, Frank; Oriwall, Alexander (Department of Geosciences, The University of Arizona, 2001-01-01)
      Measurements of the radiocarbon concentration of several carbonate background materials, either mineral (IAEA C1 Carrara marble and Icelandic double spar) or biogenic (foraminifera and molluscs), show that the apparent ages of diverse materials can be quite different. Using 0.07 pMC obtained from mineral samples as a processing blank, the results from foraminifera and mollusc background samples, varying from 0.12 to 0.58 pMC (54.0–41.4 ka), show a species-specific contamination that reproduces over several individual shells and foraminifera from several sediment cores. Different cleaning attempts have proven ineffective, and even stronger measures such as progressive hydrolization or leaching of the samples prior to routine preparation, did not give any indication of the source of the contamination. In light of these results, the use of mineral background material in the evaluation of the age of older unknown samples of biogenic carbonate (>30 ka) proves inadequate. The use of background samples of the same species and provenance as the unknown samples is essential, and if such material is unavailable, generic biogenic samples such as mixed foraminifera samples should be used. The description of our new modular carbonate sample preparation system is also introduced.
    • Automatic AMS Sample Combustion and CO2 Collection

      Aerts-Bijma, A. T.; van der Plicht, J.; Meijer, H. A. J. (Department of Geosciences, The University of Arizona, 2001-01-01)
      In Groningen, all organic samples for accelerator mass spectrometry (AMS) are combusted in an automatic Elemental Analyzer, coupled to an Isotope Ratio Mass Spectrometer and Cryogenic Trapping System. The Gas Chromatographic (GC) column, part of the Elemental Analyzer system, appeared to be the main cause for memory effects. Therefore we modified the Elemental Analyzer, such that the trapped CO2 no longer passed the GC column. Our system modification reduced the memory effect significantly, as shown by lower radiocarbon concentration values for anthracite backgrounds, and a much smaller spread in these values. Our modified system can perform up to 40 combustions unattended in about 6 hr.
    • An Improvement in Preparation of Mortar for Radiocarbon Dating

      Sonninen, E.; Jungner, H. (Department of Geosciences, The University of Arizona, 2001-01-01)
      An error source in radiocarbon dating of ancient mortar is dead carbon of limestone mixed in the matrix. To eliminate the influence of limestone the difference in feasibility to react with acid between mortar and limestone is used. Since the rate of reaction depends on grain size use of a well-defined grain size can give a better separation between mortar and limestone. We present results for the grain size dependence of reaction rates for several mortar and limestone samples and discuss the application for dating.
    • An Extraction System to Measure Carbon-14 Terrestrial Ages of Meteorites with a Tandetron AMS at Nagoya University

      Minami, Masayo; Nakamura, Toshio (Department of Geosciences, The University of Arizona, 2001-01-01)
      We have constructed a system to extract carbon from meteorites using a vacuum-tight RF melting method in order to study radiocarbon activities in meteorites. The extraction system was examined using iron standards of known carbon content. The carbon extraction efficiencies and 14C ages of the iron standards by this method were compared with the results obtained previously by our older melting system and a wet oxidation method. Higher collection efficiencies of about 90% for the iron samples of relatively high carbon content were achieved by the new system. The efficiency of extracting a small amount of carbon is also near 90% after improving the extraction procedure. The 14C ages of the iron standards were compared to the ages by the wet method. The results indicate that contamination by modern carbon is negligible in the system. Furthermore, terrestrial 14C ages of two Antarctic meteorites, Y-75102 and ALH-77294, from the Yamato and Allan Hills ice fields, respectively, were determined. The age of Y-75102 is estimated 4.0 +/1.0 ka, and the age of ALH-77294 is 19.5 +/1.2 ka. The 14C ages on the meteorites roughly agree with the literature value. However, further study is needed in improvement on reducing a background value and of complete fusion of a meteorite in the extraction system.
    • AMS Radiocarbon Dating of Ancient Iron Artifacts: A New Carbon Extraction Method in Use at LLNL

      Cook, Andrea C.; Wadsworth, Jeffrey; Southon, John R. (Department of Geosciences, The University of Arizona, 2001-01-01)
      A new sealed double tube combustion method was developed at Lawrence Livermore National Laboratory (LLNL) to extract carbon from modern steels and ancient iron artifacts. Iron samples were chemically pretreated with 10% nitric acid, vacuum sealed in 6 mm quartz tubes with CuO, vacuum sealed again inside 9 mm quartz tubes, and combusted at 1000 degrees C for a minimum of 10 hr. The resulting CO2 was graphitized routinely using hydrogen reduction (Vogel et al. 1989). After the initial phase of development, carbon yields of 100% were consistently obtained. The activities of two modern high carbon steels (treated as process blanks, manufactured using only coal as the carbon source) were determined to be 0.0077 +/0.0009 (n = 12, +/1 sigma) for a 1.3% C steel and 0.0090 +/0.0038 (n = 12, +/1 sigma) for a 1.9% C steel, indicating that very little contamination is introduced during the sample preparation process. Since the Iron Age began less than 5000 years ago, these background uncertainties should introduce errors of no more than +/30 years to the radiocarbon ages of actual artifacts. Two ancient iron artifacts of known date were analyzed and demonstrate that the new methodology can be used to obtain the correct date of manufacture for iron objects, provided that they are made exclusively using charcoal that was contemporaneous with the manufacture of the artifact. Since only 1 mg of carbon is required for accelerator mass spectrometry (AMS), very small iron samples can now be analyzed (50 mg of a 2.0% C iron or 1 g of a 0.1% C iron). We anticipate that this methodology will be particularly useful to archeologists who currently have to rely on context to date iron artifacts.
    • Accelerator Mass Spectrometry at High Voltage Engineering Europa (HVEE)

      Gottdang, A.; Klein, M.; Mous, D. J. W. (Department of Geosciences, The University of Arizona, 2001-01-01)
      In recent years, High Voltage Engineering Europa (HVEE) has demonstrated its capability of developing and installing turnkey accelerator mass spectrometry (AMS) equipment for the analysis of, among others, 14C and 129I. Five 3MV systems using sequential and/or simultaneous injection are operational in the field today, and they have shown excellent long-term stability providing high-precision analyses on a routine basis. Another three AMS systems are in production. It is projected that for the time being, AMS will continue to broaden its field of applications; a saturation of the market of AMS facilities is not expected in the near future. To meet the specific demands of the biomedical research community, we have developed an extreme compact 14C AMS system comprising a hybrid ion source capable of handling both graphite as well as CO2 samples. The source is optimized for easy maintenance, accommodates up to 200 samples, and can be implemented in any other AMS system. More recently, HVEE has initiated the development of an AMS system capable of detecting, among others, 36Cl and 41Ca. The design will include a HVEE 5MV Tandetron(TM). The accelerator is currently under construction as part of an ion beam analysis system for the Universidad Autonoma de Madrid (Spain).
    • A Marine Reservoir Data Correction Database and On-Line Interface

      Reimer, Paula J.; Reimer, Ron W. (Department of Geosciences, The University of Arizona, 2001-01-01)
      Calibration is essential for interpretation of radiocarbon dates, especially when the 14C dates are compared to historical or climatic records with a different chronological basis. 14C ages of samples from the marine environment, such as shells or fish bones, or samples with a marine component, such as human bone in coastal regions, require an additional consideration because of the reservoir age of the ocean. While the pre-industrial global mean reservoir correction, R(t), is about 400 years, local variations (Delta-R) can be several hundred years or more. Delta-R compilations on a global scale have been undertaken previously (Stuiver et al. 1986; Stuiver and Braziunas 1993), but have not been updated recently. Here we describe an on-line reservoir correction database accessed via mapping software. Rather than publishing a static Delta-R compilation, new data will be incorporated when it becomes available. The on-line marine reservoir correction database can be accessed at the website http://www.calib.org/.
    • A Fresh Water Diet-Derived 14C Reservoir Effect at the Stone Age Sites in the Iron Gates Gorge

      Cook, Gordon T.; Bonsall, C.; Hedges, Robert E. M.; McSweeney, K.; Boronean, V.; Pettitt, Paul B. (Department of Geosciences, The University of Arizona, 2001-01-01)
      Human bones from single inhumation burials and artifacts made from terrestrial mammal (ungulate) bone found in direct association with the skeletons were obtained from the Stone Age site of Schela Cladovei situated just below the Iron Gates Gorge of the River Danube. The results of stable isotope analyses of the human bone collagen are consistent with a heavy dependence on aquatic protein while radiocarbon dating of the samples reveals an offset of 300-500 years between the two sample types, indicating a freshwater reservoir effect in the human bone samples. Since protein consumption is by far the major source of nitrogen in the human diet we have assumed a linear relationship between delta-15N and the level of aquatic protein in each individual's diet and derived a calibration for 14C age offset versus delta-15N which has been applied to a series of results from the site at Lepenski Vir within the gorge. The corrected 14C ages (7310-6720 BP) are now consistent with the previous 14C age measurements made on charcoal from related contexts (7360-6560 BP). In addition, the data indicate a change from a primarily aquatic to a mixed terrestrial/aquatic diet around 7100 BP and this may be argued as supporting a shift from Mesolithic to Neolithic. This study also has wider implications for the accurate dating of human bone samples when the possibility exists of an aquatic component in the dietary protein and strongly implies that delta-15N analysis should be undertaken routinely when dating human bones.
    • A Database System for Geochemical, Isotope Hydrological, and Geochronological Laboratories

      Suckow, Axel; Dumke, Ingolf (Department of Geosciences, The University of Arizona, 2001-01-01)
      We present a data model designed for laboratories in which many different methods are used. The main feature of the model is the recursive relationships of data records in the subsample table. This makes it possible to model all steps from the preparation of the sample to the final value and enables the raw data to be stored together with the final values, even if many preparation steps or many subsamples are involved. We show ways to adopt this basic model to laboratories with many years of data. The data model is extended to include laboratory records and attribute data, e.g. The geographical coordinates of the sampling site or links to the various projects for which the samples are used.
    • 14C Wiggle Matching of the 'Floating' Tree-Ring Chronology from the Altai Mountains, Southern Siberia: The Ulkandryk-4 Case Study

      Slusarenko, I. Y.; Christen, J. A.; Orlova, L. A.; Kuzmin, Yaroslav V.; Burr, George S. (Department of Geosciences, The University of Arizona, 2001-01-01)
      The Bayesian approach to calibration of radiocarbon dates was used to wiggle-match the "floating" tree-ring chronology from a Pazyryk culture (Scythian-type complex from Sayan-Altai Mountain system, southern Siberia) burial ground in order to estimate the calendar age of its construction. Seventeen bidecadal tree-ring samples were 14C dated with high precision (+/20-30 yr). The results of wiggle-matching show that the Pazyryk-type burial mounds in the southern Altai Mountains were created in the first part of 3rd century BC.
    • 14C Wiggle-Match Dating in High-Resolution Sea-Level Research

      van de Plassche, O.; Edwards, E. J.; van der Borg, K.; de Jong, A. F. M. (Department of Geosciences, The University of Arizona, 2001-01-01)
      Comparison of two sets of marsh-accumulation records from each of three Connecticut (USA) salt marshes, one based on individually calibrated dates and the other on wiggle-match dating of the same series of dates, shows that wiggle-match dating results in more precise and objective reconstructions of longer-term (10^2-10^3 yr) changes in accumulation rate. On (sub-)century time scales, wiggle-match dating can reveal steps in the calibrated marsh-accumulation envelope as artefacts of the calibration curve, but may also leave real short-term changes in accumulation rate undetected. Wiggle-matches are non-unique, being dependent on the number, quality and distribution of radiocarbon dates in a sequence, how a series of dates is subdivided into groups (representing intervals of uniform accumulation rate), and what is considered a "best match". Samples from the studied salt-marsh deposits required no correction for reservoir effects prior to calibration.
    • 'Wiggle Matching’ Radiocarbon Dates

      Bronk Ramsey, Christopher; van der Plicht, Johannes; Weninger, B. (Department of Geosciences, The University of Arizona, 2001-01-01)
      This paper covers three different methods of matching radiocarbon dates to the "wiggles" of the calibration curve in those situations where the age difference between the 14C dates is known. These methods are most often applied to tree-ring sequences. The simplest approach is to use a classical Chi-squared fit of the 14C data to the 14C curve. This gives the calendar date where the data fit best and allows tests of how good the fit is. The only drawback of this method is that it is difficult to ascertain the uncertainty in the date found in this way. An extension of this technique uses a Monte-Carlo simulation to sample possible 14C concentrations consistent with the measurement made and for each of these possibilities performs a Chi-squared fit. This method yields a distribution of values in the calendrical time-scale, from which the overall dating uncertainty can be derived. A third, rather different approach, based on Bayesian statistics, calculates the relative likelihood of each possible calendar year fit. This can then be used to calculate a range of most likely dates in a similar way to the probability method of 14C calibration. The theories underlying all three methods are discussed in this paper and a comparison made for the fitting of specific model sequences. All three methods are found to give consistent results and the application of any one of them depends on the nature of the scientific question being addressed.