• Measurement of 14C Directly from CO2 Using a Tandetron Accelerator Mass Spectrometer Facility

      Raisbeck, Grant M.; Yiou, Françoise; Arnold, Maurice; Duplessy, Jean Claude (American Journal of Science, 1986-01-01)
      Tests have been carried out on the measurement of 14C directly from CO2, using a source designed by Middleton, and a Tandetron accelerator mass spectrometer system. These tests were very promising from the point of view of minimum quantity of carbon necessary (<50 micrograms), but suffered from a background (apparently not memory effect) giving an equivalent age of 25,000 years. As a demonstration of the technique, a single thread from the mummy wrapping of the Egyptian king Ramses II was dated.
    • Multiple Dating of a Long Flowstone Profile

      Geyh, Mebus A.; Hennig, G. J. (American Journal of Science, 1986-01-01)
      Dense speleothem samples are considered as closed systems and are, therefore, possibilities for any dating method. Four dating methods (14C, U/Th, paleomagnetism, and electron spin resonance = ESR) were used for samples up to 1,000,000 yr old and taken along a vertical flowstone profile in the Heggen cave in West Germany. Also delta-18O and delta-13C analyses were carried out. The reliability of the results of each method is dependent on the diagenetic processes that took place during the complex growth history of the flowstone. Speleothem growth was interrupted during glacial periods. During interglacial periods, at least the stalagmite growth rate was greater by one order of magnitude than during interstadial periods. During the periods of low interstadial growth rate various processes might have changed the 14C, 18O, and 13C concentrations, leaching might have removed uranium, recrystallization might have moved thorium several centimeters, and increased content of radon in the cave might have exaggerated the accumulated dose (AD) at the speleothem surface. As a result, 14C ages may be too small and U/Th as well as ESR data may be too large.
    • On the Initial 14C Activity of Karst Aquifers with Short Mean Residence Time

      Krajcar-Bronić, Ines; Horvatinčić, Nada; Srdoč, Dušan; Obelić, Bogomil (American Journal of Science, 1986-01-01)
      The 14C activity of total dissolved inorganic carbon (DIC) as well as tritium activity and stable isotope content (13C, 2H, 18O) of spring water were measured for 3 consecutive years at 3 karst springs that feed Plitvice Lakes, NW Yugoslavia. The constant chemical and stable isotope composition and temperature indicated well mixed aquifers. A very short mean residence time of underground water (2-5 yr) was determined by tritium activity measurements. Measured 14C activities of 3 karst springs varied from 60.0% to 82.5%. The measured 14C activities were compared with calculated 14C activities based on various models using measured values of 14C activity of soil at different depths and 13C content of limestone, plants, and soils from the recharge area. Our measurements indicate that recharge systems in karst aquifers are open to the atmosphere. The atmospheric CO2 contributes to the 13C content and the 14C activity of ground water to an estimated extent which varies between 10 and 40% of DIC.
    • Operation of the NSF-Arizona Accelerator Facility for Radioisotope Analysis and Results from Selected Collaborative Research Projects

      Linick, T. W.; Jull, A. J. T.; Toolin, L. J.; Donahue, D. J. (American Journal of Science, 1986-01-01)
      Radiocarbon dating at the Arizona accelerator facility has improved substantially in the last three years. Since starting to use graphite targets (see Jull et al, 1986), we have been able to obtain routinely a precision of ca +/- 1 % (ca 80 yr) for relatively modern material. Our routine technique of tuning and operating the tandem accelerator mass spectrometer (TAMS) and our method of calculating 14C results are discussed in detail. Data on activity ratios of oxalic acid-II/oxalic-I are presented. Examples of the wide variety of projects on which we have collaborated are given. Brief discussions of three such projects are presented for our colleagues who were unable to attend this conference: an Arizona Indian archaeologic project, a study of megafaunal extinctions, and a study of the growth of phosphorite nodules on the sea floor off the Peruvian coast.
    • Optimization of Liquid Scintillation Counting for 14C Dating

      Angiolini, Fernando E.; Albero, Miguel C. (American Journal of Science, 1986-01-01)
      The effects of different measurement conditions and sample preparation in liquid scintillation counting of benzene for 14C dating were tested. The goal of this work was to find the highest figure of merit through the variation of scintillation cocktail to benzene ratio, and measurement windows. Measurements were performed on 5m1 of benzene plus increasing quantities of scintillation cocktail (7g of PPO and 0.5g of Me2POPOP in IL of scintillation-grade toluene). Spectra of both background and standard were obtained and the counting windows selected for the highest figure of merit (Q). The best condition was 5m1 of benzene plus 0.75ml of scintillation cocktail. This spectrum is compared with that of I) pure toluene-14C plus cocktail, and 2) pure benzene-14C in which the equivalent amount of PPO and Me2POPOP present in 0.75ml of cocktail was dissolved. A correlation between the increase of the aniount of cocktail and the broadening of the spectrum was observed. The effect of wrapping the upper part of the counting vial with aluminum foil to reduce background was also tested. A drastic background increment was observed in the low energy range, for unwrapped vials; the standard spectrum showed a broadening and a lower counting efficiency which reduced the figure of merit.
    • Origins of Carbon in Potsherds

      Gabasio, Martine; Evin, Jacques; Arnal, Gaston Bernard; Andrieux, Philippe (American Journal of Science, 1986-01-01)
    • Palaeosols Within Loess: Dating Palaeoclimatic Events in Kashmir

      Kusumgar, Sheela; Agrawal, D. P.; Juyal, Navin; Sharma, Prabhakar (American Journal of Science, 1986-01-01)
      The 14C dates of Kashmir loess-palaeosols form five clusters. The dates, mineral magnetic, stable isotopic, and pollen data help decipher major climatic oscillations as distinct from the minor ones.
    • Precise Calendrical Dating of Known Growth-Period Samples Using a 'Curve Fitting' Technique

      Pearson, G. W. (American Journal of Science, 1986-01-01)
      Absolute high-precision radiocarbon time-scale calibration is now available and is shown by Stuiver and Pearson (1986) and Pearson and Stuiver (1986) to be internationally valid. The technique of curve fitting presented in this paper shows how certain samples may be dated calendrically to within a very narrow band width ca +/- 20 years, at time periods when single sample analysis would give ambiguous calendrical conversion. Samples of known deposition rate covering a time span of >50 years are subdivided to produce a 'floating curve' and this curve is then `wiggle matched' with the high-precision calibration to give a precise calendrical age.
    • Production of Graphite Targets by Deposition from Co/H2 for Precision Accelerator 14C Measurements

      Jull, A. J. T.; Donahue, D. J.; Hatheway, A. L.; Linick, T. W.; Toolin, L. J. (American Journal of Science, 1986-01-01)
      A method has been developed at the Arizona AMS facility for production of graphite directly from CO2 by catalytic reduction of CO/H2 mixtures. The method is a modification of procedures discussed by Vogel et al (1984). Currents of 12C- from these targets vary considerably, but have a median value which is 70% of that achieved from spectroscopic graphite. Precisions of +/- 60 to 100 yr are routinely achieved for 14C ages of 5000-yr-old samples containing one milligram of carbon. The yield of 14C from "blank" targets is 0.6 +/- 0.1% of that from modern material.
    • Progress at the IsoTrace Radiocarbon Facility

      Beukens, Roelf P.; Gurfinkel, Debbie M.; Lee, Henry W. (American Journal of Science, 1986-01-01)
      Since August 1984 over 100 samples have been analyzed. 12C, 13C, and 14C isotopes were measured after acceleration and all dates were corrected for natural and preparation-induced fractionation. A precision of better than 1.0% was obtained for samples 10,000 yr BP or less and these results were shown to be reproducible at precision levels as low as 0.3%. An accuracy of better than 1.0% was demonstrated for the same age range by analyzing samples which were previously dated with the conventional technique. The machine background, measured on blank sample holders in a clean ion source, yielded a count rate equivalent of 0.015 +/- 0.007% modern.
    • Radiocarbon Activity Variation in Dated Tree Rings Grown in Mackenzie Delta

      Fan, C. Y.; Tie-Mei, Chen; Si-Xun, Y.; Kai-Mei, Dai (American Journal of Science, 1986-01-01)
      We measured the Delta-14C values in 57 rings (from AD 1824 to 1880) of a white spruce grown in Mackenzie Delta (68 degrees N, 130 degrees W), as part of our continuing study of the Delta-14C variation related to solar activities. The values exhibit a 10 per mil fluctuation with an 11-year periodicity anti-correlated with the solar activity cycle. We also measured the Delta-14C values in 6 rings (from AD 1940 to 1945). The abnormally high value in the 1943 ring may be due to two large solar flares occurring in 1942.
    • Radiocarbon Concentration of California Aerosols

      Berger, Rainer; McJunkin, David; Johnson, Roberta (American Journal of Science, 1986-01-01)
      In this study the origin of the carbonaceous fraction of total suspended particles (TSP) in air was analyzed. While the summer data show increasing carbon concentrations in the Los Angeles air basin from west to east, in the winter high levels of carbon particles can be found over the coast. The smallest and most dangerous particle fraction is principally composed of fossil carbon.
    • Radiocarbon Dating of Lake Sediment from Two Karst Lakes in Yugoslavia

      Srdoč, Dušan; Obelić, Bogomil; Horvatinčić, Nada; Krajcar-Bronić, Ines; Marčenko, Elena; Merkt, Joseph; Wong, How Kin; Sliepčević, Adela (American Journal of Science, 1986-01-01)
      Samples of sediment cores from two lakes in the karst area of northwest Yugoslavia were analyzed. Both Lakes Kozjak and Prošće are in the Plitvice National Park, Central Croatia. 14C dating, sedimentologic, seismic, and isotopic studies, and distribution of diatoms are presented. 14C dating of lake marl revealed a uniforn sedimentation rate in Lake Prošće as opposed to Lake Kozjak. Both lake sediments belong to the Holocene period. 14C dating of lake sediment is in agreement with seismic profiles, sedimentologic analysis, and diatom frequency measurements both in an undisturbed as well as in a disturbed lake sediment.
    • Radiocarbon Dating of Sediments

      Fowler, Alison J.; Gillespie, Richard; Hedges, Robert M. (American Journal of Science, 1986-01-01)
    • Radiocarbon Dating with the University of Washington Accelerator Mass Spectrometry System

      Grootes, Pieter M.; Stuiver, Minze; Farwell, George W.; Leach, Donald D.; Schmidt, Fred H. (American Journal of Science, 1986-01-01)
      The University of Washington FN tandem accelerator mass spectrometry (AMS) system has been used in a series of 14C studies. 1) The 14C concentrations in annual growth rings for 1962, 1963, and 1964 of a Sitka spruce, each divided into ten sequential segments, were measured; a full and rapid response of tree-ring cellulose to atmospheric changes in 14CO2 is indicated, with a delay, if any, of not more than three weeks. 2) The 14C concentrations in two chemical fractions of dissolved organic carbon and in two fractions (by size) of particulate organic carbon were measured for Amazon River samples from several locations. All contain bomb carbon, but the amounts differ significantly. 3) Algae samples from lakes in the dry valleys of Antarctica were dated in order to assist in the reconstruction of the climatic history of Antarctica. 4) Background studies indicate that the contribution of the AMS system itself to the observed 14C concentrations is equivalent to an age of ca 60,000 14C yr BP; for a prepared sample of 5mg of carbon the background corresponds to ca 50,000 years.
    • Radiocarbon in Particulate Matter from the Eastern Sub-Arctic Pacific Ocean: Evidence of a Source of Terrestrial Carbon to the Deep Sea

      Druffel, Ellen R. M.; Honjo, Susumu; Griffin, Sheila; Wong, C. S. (American Journal of Science, 1986-01-01)
      Carbon isotope ratios were measured in organic and inorganic carbon of settling particulate matter collected with a sediment trap at Ocean Station "P" in the Gulf of Alaska from March to October, 1983. Dissolved inorganic carbon (DIG) in surface sea water collected during two different seasons in 1984 were analyzed using large gas proportional counters and revealed a minimum seasonal Delta-14C variation of 14 per mil. Results show that the 14C of calcium carbonate sedimenting to the deep sea is the same as that measured in surface water DIG. In contrast, particulate organic carbon (POC) had significantly higher Delta-14C values (by 25-70 per mil) than that in surface water DIG. Also, the delta-13C of the POC was markedly lower than previously reported values from other trap stations and marine particulate matter in general. Results from this study suggest that a significant amount of the POC settling to the deep sea at this pelagic station is of terrestrial origin, not strictly of marine origin as had previously been believed.
    • Regional Suess Effect in the Upper Silesia Urban Area

      Awsiuk, Romuald; Pazdur, Mieczysław F. (American Journal of Science, 1986-01-01)
      The study of a regional Suess effect is based on three sets of samples of atmospheric CO2: 1) a series of samples collected at the same site in Gliwice from 1980 to 1984, 2) samples collected simultaneously at different sites within the limits of an urban and industrial region of Upper Silesia, and 3) samples collected simultaneously outside this region along an eastern direction. Results of 14C concentration measurements show systematic decrease of Delta-14C with the rate close to the corresponding value for clean air. Depletion of 14C concentration was found to be virtually the same in the whole urban area. Analysis of regional synoptic data reveals correlation of individual Delta-14C values with wind direction, frequency of calm, and vertical stability of the atmosphere.
    • Revealing Histories of Exposure Using In Situ Produced 26Al and 10Be in Libyan Desert Glass

      Klein, Jeffrey; Giegengack, Robert; Middleton, Roy; Sharma, Pankaj; Underwood, J.; Weeks, R. A. (American Journal of Science, 1986-01-01)
      We present the results of measurements of 26Al and 10Be produced in situ in 12 samples of Libyan Desert Glass by cosmic rays during the last ten million years. Based on the variability of the concentrations of 10Be and of the 26Al/10Be ratios we measured, we conclude that individual fragments of glass have experienced different exposure histories, implying several major redistributions of the glass within the past 106 years. The 26Al and 10Be concentrations are inconsistent with the theoretical estimates of the rates of in situ production. We estimate minimum production rates of 70 atoms g-1 yr-1 and 10 atoms g-1 yr-1 for 26Al and10Be, respectively, produced in quartz at sea level between 60-90 degree latitude. Despite the present uncertainty in the rates of production, we feel that these results show clearly the effectiveness of in situ produced 26Al and10Be in studying earth-surface processes.
    • Routine AMS Dating of Bone and Shell Proteins

      Gillespie, Richard; Hedges, Robert E. M.; Humm, M. J. (American Journal of Science, 1986-01-01)