• Development and Operation of the Harwell Small Counter Facility for the Measurement of 14C in Very Small Samples

      Otlet, M. L.; Huxtable, G.; Evans, C. V.; Humphreys, D. C.; Short, D. C.; Conchie, S. J. (American Journal of Science, 1983-01-01)
      The Harwell system for measuring milligram size samples using Brookhaven miniature gas counters is fully operative. It comprises 12 counters of different sizes which operate simultaneously within a single NaI crystal (300mm diameter x 300mm long) acting as an anti-coincidence guard counter. Brief details are given of the construction and commissioning of the system, including counter assembly, shield design, electronics, data capture, data analysis, and chemical processing and filling procedures. The performance of the system and an overall view of the fields of application for which the counters have important applications are discussed.
    • Direct Bone Dating in a Small CO2 Counter

      Berger, Rainer (American Journal of Science, 1983-01-01)
      A small 200ml capacity CO2 proportional counting system has been developed which uses only 100mg of carbon for complete filling. Thus, with respect to the small quantities needed, it compares favorably to dedicated accelerators at significantly lower cost. The performance of this equipment is demonstrated using a variety of samples including some human bone fragments from La Jolla which had been estimated to be 28,000 years old by aspartic acid racemization analysis.
    • Distribution of Gaseous 12CO2, 13CO2, and 14CO2 in the Sub-Soil Unsaturated Zone of the Western US Great Plains

      Thorstenson, D. C.; Weeks, E. P.; Haas, Herbert; Fisher, D. W. (American Journal of Science, 1983-01-01)
      Data on the depth distribution of the major atmospheric gases and the abundance of gaseous 12CO2, 13CO2, and 14CO2 in the subsoil unsaturated zone have been obtained from several sites in the western Great Plains of the United States. Sample profiles range from land surface to depths of 50m. Although each site must be considered on an individual basis, several general statements can be made regarding the profiles. 1) Diffusion of these gaseous molecules through the unsaturated zone is an important transport mechanism. 2) As predicted by diffusion theory, depth profiles of the various isotopic species of CO2 differ substantially from one another, depending on individual sources and sinks such as root respiration and oxidation of organic carbon at depth. 3) In general, post-bomb (> 100% modern) 14C activities are not observed in the deep unsaturated zone, in contrast to diffusion model predictions. 4) In spite of generally decreasing 14C activities with depth, absolute partial pressures of 14CO2 in the subsoil unsaturated zone are 1-2 orders of magnitude higher than the partial pressure of 14CO2 in the atmosphere.
    • Evaluation of Direct-Precipitation and Gas-Evolution Methods for Radiocarbon Dating of Ground Water

      Yang, In Che (American Journal of Science, 1983-01-01)
      The extraction of dissolved carbonate species for age dating from a 100L water sample by the direct-precipitation method (DPM) and by the gas-evolution method (GEM) has been investigated. Stable carbon-isotope fractionation between initial and final carbon dioxide evolved was ca 11 per mil by GEM and 1 per mil by DPM. GEM will produce isotopically lighter carbon dioxide compared with DPM if carbonate recovery is low. Extraction efficiency of > 95% can be achieved by GEM in 3 hours using nitrogen gas at a sweeping rate of 2000cc per minute. DPM requires precipitates to settle overnight to assure > 95% recovery. GEM is little affected by a high concentration of sulfate ions, whereas DPM is greatly affected by sulfate resulting in less yield.
    • Foreword

      Stuiver, Minze (American Journal of Science, 1983-01-01)
    • Ground-Water Circulation in the Meade Thrust Allochthon Evaluated by Radiocarbon Techniques

      Muller, A. B.; Mayo, A. L. (American Journal of Science, 1983-01-01)
      The Meade thrust, in southeastern Idaho, is a major element of the Western Overthrust Belt. The allochthon is of geo-economic importance both as a potential hydrothermal area and as the principal mining area within the Western Phosphate Field. To assist in the development of these two resources, an understanding of the regional ground-water circulation was sought. Geologic and hydrologic data from boreholes in this area are virtually nonexistent. Waterwell development in the area has not occurred because of the abundance of springs and only a few hydrocarbon exploration boreholes have been drilled. Thus, the problem lends itself to evaluation by isotope hydrologic and geochemical methods. Ten springs from within the thrust block and around its periphery were sampled for major ions, 2H/18O, and 14C/13C analysis. Data from these analyses and from field geologic evidence have identified two distinct flow regimes within the Meade thrust allochthon. Shallow flow systems lie above the impermeable Phosphoria Formation, usually within a few hundred meters of the surface. Most of the spring waters from this system are recent and cool. In all cases, they have mean subsurface residence times of less than a few hundred years. The deeper flow systems which lie below the Phosphoria formation are hydraulically isolated from the shallow system. Warm waters from these springs have 14C contents suggesting mean ground-water residence times on the order of 15,000 years. Although these waters could have circulated to as deep as 1900m, 2H/18O results show that high temperatures were never reached. There is no evidence to suggest that water from beneath the Meade thrust has contributed to the circulation in the allochthon.
    • High-Precision 14C Measurement of Irish Oaks to Show the Natural 14C Variations from 200 BC to 4000 BC

      Pearson, G. W.; Pilcher, J. R.; Baillie, M. L. (American Journal of Science, 1983-01-01)
      Bi-decade samples of dendrochronologically matched Irish Oak, measured with a precision of ca +/20 years, covering the period 200 to 4000 BC are presented. The data are compared with the published data of Suess, de Jong, and Mook to provide a general calibration of the 14C time scale for this period. Although the dendrochronologic sequences presented are not absolutely tied to present, the best fit (based on 14C evidence) of the Belfast data to absolute chronologies, the error and evidence associated with such positioning is given. The intervals chosen for analysis were 20 years, reducing slightly the resolution of short-term variations when compared to 10-year intervals, which are sometimes measured. However, this calibration would suffice for most scientific purposes and certainly for the calendrical conversion of 14C dates derived from archaeologic samples.
    • High-Precision 14C Measurement of Irish Oaks to Show the Natural Atmospheric 14C Variations of the AD Time Period

      Pearson, G. W.; Baillie, M. G. L. (American Journal of Science, 1983-01-01)
      The high-precision 14C measurement of bi-decade and decade samples of Irish Oak for the time period AD 50 to 1830 is presented. The samples were taken from dendrochronologically dated Irish Oak providing an absolute chronology for this period. While the natural atmospheric 14C concentration shows cyclic deviations from a constant value, the amplitudes of such deviations vary considerably. Repeated measurement and interlaboratory checks justify a claim to accuracy in the data given. Comparison of a large number of data sets between Belfast and Seattle show that the different techniques of scintillation counting of 14C6H6 and gas counting of 14CO2 derived from different wood species grown in different continents give no significant bias. Thus, it is justifiable to use this data set as a high-precision calibration curve for the AD period. Over most of this period bi-decade samples were measured giving a slightly reduced resolution of shortterm variation when compared to the measurement of decade intervals; however, any difference is not apparent when such curves are compared. It is of much greater importance that a comparison of data sets is without bias if a combined calibration curve is to have sufficient integrity for general use. Averaging the data from Belfast and Seattle would improve the validity of such an AD 14C calibration curve since it would tend to smooth out slight local variations and become internationally more acceptable.
    • International Agreements and the Use of the New Oxalic Acid Standard

      Stuiver, Minze (American Journal of Science, 1983-01-01)
    • Long-Term Variability of Temperature and 14C in the Gulf Stream: Oceanographic Implications

      Druffel, Ellen M. (American Journal of Science, 1983-01-01)
      Variability in temperature and 14C levels are recorded in coralline aragonite that grew in the Gulf Stream during the past four centuries. In particular, 18O/16O ratios reflect a decrease of ca 1 degree C in surface water temperature during the latter part of the Little Ice age. 14C levels also rose in the surface waters of the Gulf Stream and in atmospheric CO2 during the Maunder minimum. These observations indicate that ocean circulation may have been significantly different in the North Atlantic around the beginning of the 18th century.
    • Mathematical Modeling of the Distribution of Natural 14C, 234U, and 238U in a Regional Ground-Water System

      Pearson, F. J.; Noronha, C. J.; Andrews, R. W. (American Journal of Science, 1983-01-01)
    • Measurement of Carbon Fixation and Allocation Using 11C-Labeled Carbon Dioxide

      Strain, B. R.; Goeschl, J. D.; Jaeger, C. H.; Fares, Youhanna; Magnuson, C. E.; Nelson, C. E. (American Journal of Science, 1983-01-01)
    • Mid-Wisconsinan Radiocarbon Dates from Mastodon- and Mammoth-Bearing Springs, Ozark Highland, Missouri

      Haynes, C. V.; Stuiver, Minze; Haas, Herbert; King, J. E.; King, F. B.; Saunders, J. J. (American Journal of Science, 1983-01-01)
    • Miniature Signals and Miniature Counters: Accuracy Assurance Via Microprocessors and Multiparameter Control Techniques

      Currie, L. A.; Gerlach, R. W.; Klouda, G. A.; Ruegg, F. C.; Tompkins, G. B. (American Journal of Science, 1983-01-01)
      When 14C signals approach background levels, the validity of assumptions concerning Poisson counting statistics and measurement system stability becomes crucial in interpreting the resultant low-level counting observations. This has been demonstrated in our previous work on detection limits for non-Poisson error and it is critical in our current studies of carbonaceous pollutants, where the 14C signal from just 5 mg C is comparable to that of the background for our miniature gas proportional counters. To assure data quality, our multi-detector system is designed for the on-line monitoring of critical parameters that reflect both the (statistical) nature of the non-Poisson errors and the underlying (physical) causes. It sends >60 bits of information/pulse to a microprocessor which automatically generates, for each counting period, two-dimensional spectra and multiparameter correlation and control charts. To evaluate the validity of long-term counting of 1–10 mg C we use robust (statistical) estimators, optimal counting interval subdivision, and time series analysis of the individual pulses. New opportunities for selective sampling and chemical fractionation which come with the small sample measurement capability have led us to give special attention also to higher control levels, involving e g, isotonic heterogeneity and representative standard materials.
    • Modeling of Atmospheric Radiocarbon Fluctuations for the Past Three Centuries

      Damon, P. E.; Sternberg, R. S.; Radnell, C. J. (American Journal of Science, 1983-01-01)
    • Non-Concordance of Radiocarbon and Amino Acid Racemization: Deduced Age Estimates on Human Bone: Implications for the Dating of the Earliest Homo Sapiens in the New World

      Taylor, R. E. (American Journal of Science, 1983-01-01)
      Radiocarbon determinations, employing both decay and direct counting, were obtained on various organic fractions of four human skeletal samples previously assigned ages ranging from 28,000 to 70,000 years on the basis of their D/L aspartic acid racemization values. In all four cases, the 14C values require an order of magnitude reduction in age.
    • Notice to Readers and Contributors

      American Journal of Science, 1983-01-01
    • On a 50-Year "Climate-Free" delta-13C Record from Juniper Tree Rings

      Leavitt, Steven W.; Long, Austin (American Journal of Science, 1983-01-01)
    • On the Origin of Carbonaceous Particles in American Cities: Results of Radiocarbon "Dating" and Chemical Characterization

      Currie, L. A.; Klouda, G. A.; Continetti, R. E.; Kaplan, I. R.; Wong, W. W. W.; Dubay, G. W.; Stevens, R. K. (American Journal of Science, 1983-01-01)
      During the past three years radiocarbon assay has emerged as a primary tool in the quantitative assignment of sources of urban and rural particulate pollution. Its use in several major field studies has come about because of its excellent (fossil/biogenic) discriminating power, because of advances in 14C measurements of small samples, and because of the increased significance of carbonaceous particles in the atmosphere. The problem is especially important in the cities, where increased concentrations of fine particles lead to pollution episodes characterized by poor visibility and changes in the radiation balance (absorption, scattering), and immediate and possibly long-term health effects. Efforts in source apportionment in such affected areas have been based on emissions inventories, dispersion modeling, and receptor modeling – ie, chemical and physical (and statistical) characterization of particles collected at designated receptor sites. It is in the last category that 14C has become quite effective in helping to resolve particle sources. Results are presented for studies carried out in Los Angeles, Denver, and Houston which incorporated 14C measurements, inorganic and organic chemical characterization, and receptor modeling. The 14C data indicated wide ranging contributions of biogenic and fossil carbon sources – eg, <10% to 60% contemporary (biogenic) in Houston – depending on meteorological, biological, and anthropological activity. The combined (chemical, isotopic, statistical) data point to sources such as vehicles, wood combustion, power plants, and vegetation.