• Study of the 14C-Contamination Potential of C-Impurities in CuO and Fe

      Vandeputte, Kurt; Moens, Luc; Dams, Richard; van der Plicht, Johannes (Department of Geosciences, The University of Arizona, 1998-01-01)
      The carbon concentration in CuO and iron was determined by isolating C. The values were in agreement with results reported in other studies. Contaminating carbon from CuO and Fe was transformed to AMS targets and measured for 14C. C-traces in CuO were shown to be the major contribution to the 14C sample processing blank. In addition, there is a significant variability in the 14C content of CuO observed between different production batches. The combined contamination potential of CuO and Fe was found to be 4.47-8.92 micrograms recent carbon, whereas the more realistic estimate for AMS-target preparation conditions ranged between 1.63 and 3.24 micrograms recent carbon, depending on the 14C level in CuO.
    • Segments of Atmospheric 14C Change as Derived from Late Glacial and Early Holocene Floating Tree-Ring Series

      Kromer, Bernd; Spurk, Marco; Remmele, Sabine; Barbetti, Mike; Toniello, Vladimiro (Department of Geosciences, The University of Arizona, 1998-01-01)
      We present results of 14C dating of several tree-ring series from the Late Glacial and Early Holocene, analyzed at the Heidelberg University radiocarbon laboratory. Although these are floating series, they contribute high-resolution information about the variability of atmospheric 14C during those periods.
    • Secular Variation of Delta-14C During the Medieval Solar Maximum: A Progress Report

      Damon, P. E.; Eastoe, C. J.; Hughes, M. K.; Kalin, R. M.; Long, A.; Peristykh, A. N. (Department of Geosciences, The University of Arizona, 1998-01-01)
      The Earth is within the Contemporaneous Solar Maximum (CSM), analogous to the Medieval Solar Maximum (MSM). If this analogy is valid, solar activity will continue to increase well into the 21st century. We have completed 75 single-ring and 10 double-ring measurements from AD 1065 to AD 1150 to obtain information about solar activity during this postulated analog to solar activity during the MSM. Delta-14C decreases steadily during the period AD 1065 to AD 1150 but with cyclical oscillations around the decreasing trend. These oscillations can be successfully modeled by four cycles. These four frequencies are 1/52 yr-1, 1/22 yr-1, 1/11 yr-1, and 1/5.5 yr, i.e., the 4th harmonic of the Suess cycle, the Hale and Schwabe cycles and the 2nd harmonic of the Schwabe cycle.
    • Sample Throughput and Data Quality at the Leibniz-Labor AMS Facility

      Nadeau, M.-J.; Grootes, P. M.; Schleicher, Markus; Hasselberg, Peter; Rieck, Anke; Bitterling, Malte (Department of Geosciences, The University of Arizona, 1998-01-01)
      Since our first report on the performance of the Kiel accelerator mass spectrometry (AMS) system and our early work on sample preparation, systems have been built to improve the sample quality and throughput of the laboratory. Minor modifications were also made on the AMS system, mainly in order to reduce the amount of work and time needed to maintain the system in optimal condition. The design and performance of a 20-port reduction system, a pneumatic target press, and a remote alarm unit for the AMS system are discussed, along with an overview of the results obtained during the last year and the procedure used to obtain them. Statistical analysis shows that the contribution of the AMS system to the measuring uncertainty at our current level (0.3% for a modern sample) is negligible.
    • Reproducibility of Seawater, Inorganic and Organic Carbon 14C Results at NOSAMS

      Elder, Kathryn L.; McNichol, Ann P.; Gagnon, Alan R. (Department of Geosciences, The University of Arizona, 1998-01-01)
      The majority of samples processed at the National Ocean Sciences AMS Facility (NOSAMS) thus far were collected as part of the World Ocean Circulation Experiment (WOCE). Due to the long storage time (2-3 yr) required to analyze thousands of samples on the accelerator mass spectrometer (AMS), a test was designed and implemented to determine the effects, if any, of storage time on 14C concentration. We find no systematic offsets in AMS measurements made over a 5-yr period between a total of 16 replicate sets from surface and deep water collected at the same locality. Furthermore, the average Delta-14C value from the deepwater AMS replicates (-213.1 per mil, std. dev. 7.3) agrees very closely with the conventional 14C results published for GEOSECS (-212.7 per mil) from station 320 taken 20 yr earlier. A total of 73 WOCE shipboard replicate sets (162 AMS measurements) were analyzed with a mean precision of 4.3 per mil. AMS results from 20 more shipboard replicate sets (44 AMS measurements) submitted as CO2 from the Stable Isotope Laboratory (SIL) at the University of Washington were analyzed with a mean precision of 3.4 per mil. These results suggests no significant difference between water stripping methods used in each preparation lab. To assess reproducibility, we calculate a pooled estimate of sigma for replicates called s, which we use as an approximation of sigma (sub TOT) for a given sample type. The s for WOCE seawater replicates is 4.9 per mil and 5.8 per mil for SIL gas replicates. These numbers demonstrate an overall reproducibility of seawater AMS results at NOSAMS that is in line with reported errors. We take the difference between total error s and machine error as the overall standard deviation of combined uncertainties associated with preparation of samples and with AMS. For seawater samples processed at NOSAMS, sigma (sub SPL) is calculated to be 2.4 per mil, and for the SIL gas replicates it is 4.8 per mil. Reproducibility of samples prepared with an acid hydrolysis technique is demonstrated using 24 coral samples submitted in triplicate by Dr. R. G. Fairbanks of Lamont Doherty Earth Observatory. Seventy-two replicates were prepared and analyzed at NOSAMS with a mean reported precision of 1.2 per mil. The pooled estimate s for the Fairbanks triplicates is 2.6 per mil. We calculate a laboratory reproducibility uncertainty for coral hydrolysis samples of 2.2 per mil. In 1993, NOSAMS participated in the Third International Radiocarbon Intercomparison (TIRI) Study. We report here 60 AMS analyses of the six TIRI test materials, five of which are organic carbon samples, to validate sample-processing methods for organic carbon sample AMS analyses at NOSAMS.
    • Reduction of the Error Multiplier by a Long-Term Analysis of the Characteristic Behaviors of Proportional Counters

      Olsson, Ingrid U. (Department of Geosciences, The University of Arizona, 1998-01-01)
      A significant reduction in cosmic-ray activity and backgrounds of the gas-filled proportional counters, as measured in a heavy iron shield, was observed when the Uppsala Conventional 14C laboratory was moved in 1984. The new site was better shielded from cosmic rays because of additional concrete layers above the laboratory. A study that lasted over one year yielded a figure for the muon reduction. The backgrounds were reduced approximately to the extent expected from the soft-component contribution at the old Laboratory as judged from barometric-pressure dependence. After a few years, new electronics enforced, and enabled, the revision and improvement of the standard values for the activity and age calculations. A careful analysis of the results for the counters has increased the accuracy of the small corrections needed to yield internal error-multiplication factors mostly between 1 and 1.5 for the background for short periods of up to 12 months, and <1.1 for the oxalic acid samples combined for the last few years of measurements in the laboratory. Similar results were obtained for two counters.
    • Recent Developments in the Procedures Used at the SSCER Laboratory for the Routine Preparation of Lithium Carbide

      Skripkin, Vadim V.; Kovaliukh, Nikolai N. (Department of Geosciences, The University of Arizona, 1998-01-01)
      In this paper we describe and discuss the advantages from improvements in equipment design and operating procedures developed at the State Scientific Centre of Environmental Radiogeochemistry (SSCER) laboratory in Kiev. Two experimental areas are considered, viz. 1. The direct chemisorption into a lithium alloy of carbonaceous gases produced by the controlled thermal degradation (pyrolysis) of organic materials under vacuum. This approach offers the advantage of a single stage, highly efficient and economical procedure for the production of lithium carbide. It is applicable for most types of sample material encountered in routine dating work and including organic detritus dispersed in a highly (up to 95% by weight) mineral matrix and/or carbonates. Bone collagen can also be processed without the need for its prior extraction and purification. 2. A conical thin walled reaction vessel for achieving improved and reproducible recoveries in the production of lithium carbide from CO2 gas. This apparatus allows a much improved control over the surface dependent reaction in instances where there is no option other than the direct interaction of molten lithium with prepared CO2 gas.
    • Radiometric 14C Dating: New Background Analysis, Basis of Improved Systems

      Theodórsson, Páll (Department of Geosciences, The University of Arizona, 1998-01-01)
      A recent broad study of the background of all types of low-level beta and gamma detectors has now made it possible to analyze its components more reliably and with greater detail. This general analysis is developed further here for gas proportional and liquid scintillation counters used in radiocarbon dating. The background of gas counters, which is dominated by secondary cosmic gamma radiation, is now well understood and can be described quantitatively. The background of liquid counters is less well understood and can only be described semiquantitatively, its analysis resting partly on estimates. Methods to reduce the background of both types of systems are described and their effectiveness discussed. This analysis may help in evaluating the quality of existing systems as well as in designing better ones.
    • Probability and Dating

      Bronk Ramsey, Christopher (Department of Geosciences, The University of Arizona, 1998-01-01)
      Statistical analysis is becoming much more widely used in conjunction with radiocarbon dating. In this paper I discuss the impact of Bayesian analysis (using computer programs such as OxCal) on archaeological research. In addition to simple analysis, the method has implications for the planning of dating projects and the assessment of the reliability of dates in their context. A new formalism for describing chronological models is introduced here: the Chronological Query Language (CQL), an extension of the model definitions found in the program OxCal. New methods of Bayesian analysis can be used to overcome some of the inherent biases in the uncertainty estimates of scientific dating methods. Most of these methods, including 14C, uranium series and thermoluminescence (TL), tend to favor some calendar dates over others. 14C calibration overcomes the problem where this is possible, but a Bayesian approach can be used more generally.
    • Preparation of Inorganic and Organic Carbon for 14 C Analysis from a Single Marine Sample

      Griffin, Sheila; Druffel, Ellen R. M. (Department of Geosciences, The University of Arizona, 1998-01-01)
      We have developed a technique using a single apparatus to recover the inorganic and organic carbon from a small (few milligrams) aliquot of dried marine material for radiocarbon analysis. The main advantages of using a single apparatus are: 1) less sample is required, 2) decreased handling reduces contamination, and 3) less time and materials are used. Blank values of approximately 5 micrograms and 19-44 micrograms are obtained for the inorganic and organic carbon extractions, respectively. Delta-14C results from sinking particulate organic and inorganic carbon are presented for samples collected in deep-sea sediment traps deployed for 10-30 day periods at 650 and 100 m above bottom (mab) in the northeast Pacific Ocean.
    • Performance of the Packard Tri-Carb(R) 2770TR/SL Liquid Scintillation Analyzer for 14C Dating

      Passo, C. J.; Anderson, Robert; Roberts, David; Cook, G. T. (Department of Geosciences, The University of Arizona, 1998-01-01)
      We present results that demonstrate the potential of the Packard Tri-Carb(R) Model 2770TR/SL for radiocarbon dating. For 2 g of sample benzene, a stable background count rate of 0.307 cpm and a stable counting efficiency of 64.78% were determined using a 13-75 keV counting window. Changes to the mathematical routines for t-SIE (quench indicating parameter) calculation and a reduction in the activity of the external standard have enabled stability of the t-SIE to be achieved, and combined with the use of a suitable balance point counting window; all of these factors give the stability of performance required for 14C dating. Calculations based on the above parameters indicate that the limit of detection for 2 g samples, counted for 5000 min, is <48,900 yr BP. The great advantage of this system is that these data were acquired using inexpensive standard 7-mL low potassium borosilicate glass vials. Vial holders manufactured from BGO reduced the background to 0.15 cpm with a minimum effect on efficiency (64.46% for 13-75 keV). A similar calculation of the limit of detection gave >51,700 BP. The use of the BGO vial holders in other instruments employing time-resolved liquid scintillation counting (TR-LSC) (Models 2250CA and 2260XL) also brought about significant improvements in detection limits.
    • On the Validity of the Poisson Hypothesis for Low-Level Counting: Investigation of the Distributional Characteristics of Background Radiation with the NIST Individual Pulse Counting System

      Currie, L. A.; Eijgenhuijsen, E. M.; Klouda, G. A. (Department of Geosciences, The University of Arizona, 1998-01-01)
      Does radioactive decay follow the Poisson distribution?—a fundamental question, to which the theoretical answer seems to be, Yes. On the practical side, the answer to this question impacts the best achievable precision in well-controlled counting experiments. There have been some noteworthy experimental tests of the Poisson assumption, using systems carefully designed for the analysis of individual pulses from stable radioactive sources; thus far, experiment supports theory. For low-level counting, the nature of the background distribution can be of profound practical importance, especially for very long counting experiments where validation by an adequate number of full replicates may be impracticable. One is tempted in such cases to assume that the variance is equal to the mean, in order to estimate the measurement uncertainty. Background radiation, however, has multiple components, only some of which are governed by the laws of radioactive decay. A specially designed low-level gas counting system at NIST for interactive, retrospective individual pulse shape and time series analysis makes possible the investigation of the empirical distribution function of the background radiation, in a manner similar to the previous empirical distribution studies of radioactive decay. Benefits of individual pulse analysis are that there is no information loss due to averaging and that two independent tests of the Poisson hypothesis can be performed using data from a single, extended measurement period without the need for replication; namely, tests of the distribution of arrival times, expected to be uniform, and the distribution of inter-arrival times, expected to be exponential. For low-level counting the second test has a very interesting and very informative complement: the distribution of coincidence-anticoincidence inter-arrival times. Key outcomes from the study were that: 1) nonstationarity in the mean background rate over extended periods of time could be compensated by an on-line paired counter technique, which is far preferable to the questionable practice of using an "error-multiplier" that presumes the wandering (nonstationary) background to be random; and 2) individual empirical pulse distributions differed from the ideal GM and Poisson processes by exhibiting giant pulses, a continuum of small pulses, afterpulses, and in certain circumstances bursts of pulses and transient relaxation processes. The afterpulses constituted ca. 8% of the anti-coincidence background events, yet they escaped detection by the conventional distributional tests.
    • New 14C Reference Materials with Activities of 15 and 50 pMC

      Le Clercq, Martijn; van der Plicht, Johannes; Gröning, Manfred (Department of Geosciences, The University of Arizona, 1998-01-01)
      Two new 14C reference materials have been developed for international use, filling a gap in the present C1-C6 series available from the IAEA. By mixing a modern and a synthetic substance, 150 kg of C7 (ca. 50 pMC activity) and C8 (ca. 15 pMC activity), respectively, were obtained.
    • Microscale AMS 14C Measurement at NOSAMS

      Pearson, Ann; McNichol, Ann P.; Schneider, Robert J.; Von Reden, K. F.; Zheng, Yan (Department of Geosciences, The University of Arizona, 1998-01-01)
      Techniques for making precise and accurate radiocarbon accelerator mass spectrometry (AMS) measurements on samples containing less than a few hundred micrograms of carbon are being developed at the NOSAMS facility. A detailed examination of all aspects of the sample preparation and data analysis process shows encouraging results. Small quantities of CO2 are reduced to graphite over cobalt catalyst at an optimal temperature of 605 degrees C. Measured 14C/12C ratios of the resulting targets are affected by machine-induced isotopic fractionation, which appears directly related to the decrease in ion current generated by the smaller sample sizes. It is possible to compensate effectively for this fractionation by measuring samples relative to small standards of identical size. Examination of the various potential sources of background 14C contamination indicates that the sample combustion process is the largest contributor, adding ca. 1 micrograms of carbon with a less-than-modern 14C concentration.
    • Methodological Issues in the 14C Dating of Rock Paintings

      Hedges, R. M.; Bronk Ramsey, Christopher; Van Klinken, G. J.; Pettitt, P. B.; Nielsen-Marsh Christina; Etchegoyen, Alberto; Fernandez Niello, J. O.; Boschin, M. T.; Llamazares, A. M. (Department of Geosciences, The University of Arizona, 1998-01-01)
      Chemical and isotopic analyses have been made of pigment samples from two separate rock art sites in Argentina. The purpose of the study has been to establish the feasibility of extracting carbonaceous material from the samples which will permit reliable radiocarbon dates for the time of painting. The two sites, Catamarca and Rio Negro, present quite different problems. Most of the paper is concerned with Catamarca, and here we have shown that the paint pigments contain very little or no organic binder; but they do contain calcium oxalate derived from local cacti, and calcium carbonate derived probably from local plant ash. We describe a method to purify carbon extracted from the calcium oxalate, and present the dates obtained on both components. We show that, though rare, natural deposits containing both calcium oxalate and calcite do occur, but that they are very distinct in both 13C and 14C compositions; and we argue that they are very unlikely to contaminate the pigments to such an extent that the 14C dates are altered. For the Rio Negro site we show that the ground for the paint pigments contains carbon derived from fires burnt inside the cave, and discuss how analytical methods provide information to develop a strategy for extracting material, from both ground and pigment, for more reliable dating.
    • Interlaboratory Comparisons: Lessons Learned

      Scott, E. M.; Harkness, D. D.; Cook, G. T. (Department of Geosciences, The University of Arizona, 1998-01-01)
      Interlaboratory comparisons have been widely used in analytical chemistry and radiochemistry as an important part of ongoing quality assurance programs. The 14C community has been no exception in this respect, and in just under 20 years, there have been a number of significant and very extensive interlaboratory trials organized by individual laboratories and the International Atomic Energy Agency (IAEA) to the benefit of the 14C community (both labs and users) (Otlet et al. 1980; ISG 1982; Scott et al. 1990; Rozanski et al. 1992; Scott et al. 1992; Gulliksen and Scott 1995). The comparisons have varied widely in terms of sample type and preparation, but all have had as their primary goal the investigation of the comparability of results produced under possibly quite different laboratory protocols. As techniques have been developed and new labs formed, the reference materials created as a result of the intercomparisons have presented an opportunity for checking procedures and results. Users have been reassured by the existence of regular comparisons as one sign of the concern that laboratories have to ensure highest quality results, but also confused about how to make use of the results from past exercises in the interpretation of sets of dates from many laboratories. The laboratories have also learned valuable lessons from participation in such studies. These have included identification of systematic offsets and additional sources of variation and in studies which have used realistic samples requiring pretreatment, chemical synthesis and counting, it has been possible to identify the stage at which such problems have arisen and to quantify the relative contributions to the overall variation. In this paper, we will briefly review the comparisons so far, draw some conclusions from their findings, and make proposals for the future organization of intercomparisons.
    • Interlaboratory Comparisons of 14C Measurements in Milk and Vegetation

      Milton, G. M.; Kramer, S. J.; Milton, J. C. D. (Department of Geosciences, The University of Arizona, 1998-01-01)
      The need for increased quality assurance for radiocarbon measurements performed by the monitoring laboratories at nuclear stations has spurred the introduction of a number of interlaboratory comparisons. We organized two such intercomparisons: the first set, circulated in 1994, consisted of two milk samples, one containing current global levels of 14C, the other containing an added spike of 14C-methylated casein. The second set, circulated in 1995, consisted of two samples of natural vegetation growing on the Chalk River Laboratories (CRL), containing two different levels of 14C, both well above global background. The response to our invitation to participate in these studies was very encouraging; six laboratories took part in the first intercomparison, eleven in the second. The list included both monitoring laboratories and those whose main function is 14C dating. Understandably, some of the latter preferred not to analyze the higher-activity samples. The results in 3 of the 4 data sets were consistent with a statistical distribution based on the reported errors. This report provides details of two intercomparisons, including the preparation of the samples, which may now be considered potential secondary reference materials, the range of analytical techniques in use at the participating laboratories, and a statistical analysis of the results returned to us.
    • Gas Counting System for 14C Dating of Small Samples in the Kraków Laboratory

      Gorczyca, Zbigniew; Jeleń, Kazimierz; Kuc, Tadeusz (Department of Geosciences, The University of Arizona, 1998-01-01)
      The application of traditional gas or liquid scintillation counting (LSC) is necessary for assessing radionuclide activity in countries without operating accelerator mass spectrometry (AMS) facilities. A simple and relatively inexpensive system of mini gas counters for measurement of radiocarbon in archaeological and environmental samples has been set up recently in the Kraków laboratory (Department of Environmental Physics, University of Mining and Metallurgy). The system is composed of a gas purification and counter filling line, three identical 15-mL copper/quartz counters, active and passive shielding, and an electronic unit with data acquisition. One counter measures 22 mg of carbon as CO2 with efficiency >95% at a background reduced to 0.044 cpm by a NaJ(T1) guard counter and lead shield. The detection limit (1 sigma) for a two-week measurement of 48 mL of CO2 is 0.52 pMC. The corresponding counting error of a 100 pMC environmental sample is 1.3 pMC for 22 mgC (one counter) and 0.75 pMC for 66 mgC (three counters filled with the sample).
    • First 14C Results from Archaeological and Forensic Studies at the Vienna Environmental Research Accelerator

      Wild, Eva; Golser, Robin; Hille, Peter; Kutschera, Walter; Priller, Alfred; Puchegger, Stephan; Rom, Werner; Steier, Peter; Vycudilik, W. (Department of Geosciences, The University of Arizona, 1998-01-01)
      14C dating with the new Vienna Environmental Research Accelerator (VERA) began with the age determination of a mummified marmot found in the Austrian Alpine region. Soft tissue and bones of the marmot were used for the investigation. For comparison, bone material from known-age samples was also processed and measured. These exercises showed that 14C dating with VERA is reliable, and since that time various samples from archaeological context have been dated. We also studied the applicability of the 14C method in forensic sciences to determine the time of death of human individuals. 14C/ 12C measurements of samples from different organic human material (bone collagen, lipids from bone and bone marrow, hair) were performed and compared with the tropospheric "bomb peak" values to transform the measured ratios into "calibrated ages". For specific substances with rapid turnover rates, this gives an estimate for the time of death of the individual. In our study, lipids and hair yield reasonable times of death, whereas the collagen fraction from bones, which has a relatively long turnover time, is not suitable for this purpose.
    • Extension of the 14C Calibration Curve to ca. 40,000 cal BC by Synchronizing Greenland 18O/16O Ice Core Records and North Atlantic Foraminifera Profiles: A Comparison with U/Th Coral Data

      Jöris, Olaf; Weninger, Bernhard (Department of Geosciences, The University of Arizona, 1998-01-01)
      For a better understanding of pre-Holocene cultural history, archaeologists are in need of an absolute time scale that can be confirmed and duplicated by different dating methods. Proxy data available from archaeological sites do not, in themselves, allow much reflection on absolute age. Even when founded on supporting radiocarbon data, Paleolithic chronologies that are beyond the actual limits of 14C calibration still remain relative ones, and thus are often quite tentative. Lacking the possibility of calibration for the Paleolithic, archaeologists often attempt to correlate their data with different time scales from different archives that are thought to be absolute or calendric. The main result of this paper is that the GISP2 and U/Th chronologies duplicate each other over their entire range of data overlap, while other time scales (i.e., GRIP, most varve sites) differ significantly. The context-derived 14C calibration curve provides a large potential to correlate the various climate archives as recorded in ice cores and deep ocean drillings with terrestrial sequences.