Now showing items 21-40 of 52

    • Patterns of Atmospheric 14C Changes

      Stuiver, Minze; Quay, Paul D. (American Journal of Science, 1980-01-01)
      Natural atmospheric 14C changes are caused by fluctuations in upper atmospheric 14C production rates (Q) that are related to earth geomagnetic field variations and changes in solar wind magnetic shielding properties. Climate variability may also be responsible for some of the changes because it influences exchange rates of 14C between the various terrestrial carbon reservoirs. Upper atmospheric 14C production rates QM, in at/sec cm2 (earth), were calculated for the past 1200 years from the atmospheric 14C record and a carbon reservoir model. The changes in are compared in detail with the predicted Q variability derived from an Aa solar modulation mechanism and 20th century neutron flux observations. The influence of earth geomagnetic field changes on the magnitude of the solar wind modulation is discussed, and it is shown that the variations in this magnitude agree with the known differences in earth magnetic field intensity during the past 1200 years. The larger calculated QM oscillations during the sixth millennium BP also agree with this concept. Solar wind magnetic as well as geomagnetic forces modulate the incoming cosmic ray flux and explain the main features of the atmospheric 14C record. It is argued that climatic fluctuation is not a dominant cause. The oscillations between 3200 and 3700 BP, as measured by de Jong, Mook, and Becker, differ in rise time from those found for the current millennium.
    • On the Climate-Radiocarbon Relationship: Nitric Oxide and Ozone as Connecting Links Between Radiation and the Earth's Surface Temperatures

      Roedel, Walter (American Journal of Science, 1980-01-01)
      The correlations between atmospheric radiocarbon cencentrations, on the one hand, and solar activity and the earth's magnetic field, on the other, as well as possible relations between surface temperatures and solar activity and the earth's magnetic field are considered. To draw a consistent picture of these relations, the following mechanisms for climatic changes are proposed: The earth's magnetic field and solar activity both influence the fluxes of charged solar particles into the upper stratosphere; higher surface temperatures are positively correlated with higher particles fluxes. Charged particles produce nitric oxide, which controls, to some extent, the ozone destruction and, thus, the ozone inventory in the upper layers of the stratosphere (above the 10 mbar level) in latitudes greater than about 60 degrees. The varying ozone contents in the upper stratosphere affect the radiation balance and the temperatures on the earth's surface. It has been estimated that a reduction, or increase, respectively, of ozone in the layers above 10 mbar warms, or cools, respectively, the earth's surface. A change of ozone in the lower layers of the stratosphere has the opposite effect. The maximum net effect is in the order of Delta-T is approximately equal to 0.3 1 K, in both directions. This hypothesis might be in accordance with an orbital origin of long-term climatic changes.
    • OH Radicals Via Atmospheric 14CO: An Extended Summary

      Volz, Andreas; Tönnissen, Alfred; Ehhalt, D. H.; Khedim, Ahmed (American Journal of Science, 1980-01-01)
      Measurements of 14CO in the lower troposphere show a well-defined seasonal variation between 11 +/- 1 molecules cm-3 in summer and 25 +/- molecules cm-3 in winter at 51 degrees N. The concentration at 27 degrees N in summer is found to be 4.2 +/- 0.7 molecules cm-3. From these data and published 12C O measurements, the average concentration of OH radicals in the troposphere is calculated to be 6.5 +/- 2.5 X 10^5 molecules cm-3 using a 2-D time-dependent model. The corresponding 12C O lifetime is two months; the 14CO lifetime is five months.
    • Natural Radiocarbon Variations from AD 200 to 800

      Bruns, Michael; Münnich, K. O.; Becker, Bernd (American Journal of Science, 1980-01-01)
      High precision radiocarbon measurements (+/- 2.5 per mil STD) were carried out on absolutely dated European oak material. These showed that significant shortterm atmospheric 14C variations of up to 2 percent occurred between AD 200 and 800. These changes, reflecting a 150-to 180-year periodicity, seem to correlate with the average sunspot activity around that time. In comparison with results obtained by other authors, a very regular pattern of natural atmospheric 14C variations is exhibited.
    • New Measurements of the 81Kr Atmospheric Abundance

      Kuzminov, V. V.; Pomansky, A. A. (American Journal of Science, 1980-01-01)
      We have determined the 81Kr activity to be (0.067 +/- 0.003) decay/min. 1 krypton. Using this activity in conjunction with our new measurement of the 80 KR(n,y)81Kr reaction cross-section of (12 +/- 4)b, we infer that the 81Kr activity is 1.5-2.0 times that which would be calculated calculated using the current cosmic ray flux. This implies that the average cosmic ray intensity in the atmosphere during the 81Kr life-time was greater than the current intensity, a fact that has implications for the 14C time scale.
    • Natural Processes and Time Fluctuations in the Radiocarbon Concentration of the Atmosphere

      Dergachev, V. E.; Kocharov, G. E. (American Journal of Science, 1980-01-01)
      The concentration of radiocarbon in the earth's atmosphere is used to analyze the complex of problems associated with the study of various astrophysical, geophysical, and geochemical phenomena within the framework of the All-Union problem of Astrophysical Phenomena and Radiocarbon. Effects of the supernovae explosions, solar activity, geomagnetic field and climatic changes in the 14C level in tree-rings are considered.
    • Mini-Radiocarbon Measurements Chemical Selectivity, and the Impact of Man on Environmental Pollution and Climate

      Currie, Lloyd A.; Klouda, George A.; Cooper, John A. (American Journal of Science, 1980-01-01)
      Underlying principles and results are presented for our program to use isotopic and chemical methods to quantify anthropogenic and natural sources of carbonaceous pollutants. Radiocarbon data have been obtained with a specially. developed miniature low-level gas counting system which has permitted us to assay samples containing as little as 5mg carbon. Measurements of carbonaceous particles, using chemical selectivity and size fractionation to supplement the radiocarbon data, have revealed major impact from both fossil fuel and vegetative (contemporary) sources on urban aerosols. Residential wood-burning has been specifically identified as an important source of respirable particles. Current investigations are directed toward the carbonaceous gases and the application of the accelerator technique for the assay of radiocarbon in individual chemical fractions containing microgram quantities of carbon.
    • Memory Effects in the Production of Benzene for Radiocarbon Dating

      Radnell, C. J.; Muller, A. B. (American Journal of Science, 1980-01-01)
      The use of materials having high levels of 14C activity (up to 113 times the activity of modern carbon) enabled a quantitative analysis of the magnitude and sites of memory occurring in the routine synthesis of benzene, via lithium carbide, for radiocarbon dating. Memory may be expressed as the percentage or fractional contribution of carbon from sources other than the original sample in this synthesis. Although tritium and radon contamination have also been found, the major site of memory was the inner surface of the stainless steel reaction vessel used for lithium carbide production. Up to 1 percent memory has been found there under extreme conditions in the routine dating system at Harwell. Values of half to one-third this size were more usual, but even etching and scouring the inner surface of the reaction vessel reduced the memory only by a factor of four. This lower limit is believed to exist because of the carburization and decarburization of steel which occurs at the temperature required for the production of lithium carbide. The levels of memory found are of the same order as the levels of significance associated with present radiocarbon techniques. With the accuracy and extended chronologies expected from direct 14C measurement by accelerator techniques, these levels of memory become increasingly important in the preparation of acetylene and pyrolitic graphite (via lithium carbide) used as target materials. This effect, however, can be limited by lowering the temperature of the carbide reaction stage or by lining or impregnating the lithium carbide reaction vessel with some carbon-impermeable alloy or material.
    • Medium-Term Atmospheric 14C Variations

      De Jong, A. F. M.; Mook, W. G. (American Journal of Science, 1980-01-01)
      High-precision 14C measurements are presented, carried out on single tree rings from a section of the floating South German Neolithic tree-ring chronology. They confirm the existence of pronounced medium-term variations in the order of 2 percent during the 33rd to 38th centuries BC. These variations turn out to be very regular while the precision of 1.5 per mil allows a comparison with a geochemical model calculation. Good agreement is acquired for an input function with a periodicity of about 150 years and an amplitude of approximately 30 percent in the 14C production rate.
    • Low-Level Gas Proportional Counting in an Underground Laboratory

      Loosli, H. H.; Heimann, Martin; Oeschger, Hans (American Journal of Science, 1980-01-01)
      Results of measurements in an underground laboratory, 70m of water equivalent. below surface, are given. Background values of proportional gas counters with volumes between 16cc and 1.5L are lower by a factor of 2-4.5 compared to the values in the previously used laboratory (7m w.e.). High counting gas pressures, up to 5 at CH, and up to 36 at P-10, enable the use of relatively small counters with correspondingly small background contributions from the walls. A separation of the residual background into different components is attempted, distinguishing a pressure dependent volume effect and surface correlated contributions. It can be shown that the selection of radioactively pure counter construction material is very important for a good low :level counting system.
    • Large-Volume Liquid Scintillation Counting of Carbon-14

      Eichinger, Lorenz; Rauert, Werner; Salvamoser, Josef; Wolf, Manfred (American Journal of Science, 1980-01-01)
      Efforts have been undertaken to further improve the relatively simple technique of low-level liquid scintillation counting of 14C. Two different approaches have been made. By synthesizing more benzene for 14C measurement than usual (with up to 19.5g of carbon) an experimental detection limit of about 0.1 percent modern has been achieved (97.5% confidence level, 1000 min). Absorption of CO2 with up to 5.3g of carbon in 160ml of an absorbent-scintillation solution and counting in a special measuring chamber resulted in an experimental detection limit of about 1 percent modern, with the sample preparation taking only 1 hour. The detection limits achieved by the two techniques correspond to 14C ages of about 55,000 and 35,000 years BP, respectively.
    • Low-Level Liquid Scintillation Counting in an Underground Laboratory

      Schotterer, Ulrich; Oeschger, Hans (American Journal of Science, 1980-01-01)
      A commercial liquid scintillation counter (LSC) was tested for its ability of low level counting (LLC) in two underground laboratories (70m of water equivalent cover). Reduction of total background and background components compared to the normal counting laboratory (5m we. cover) up to a factor of 9 depending on the energy range and the shielding of counting cell and laboratory walls were obtained. First experiments with a large volume cell (50cc) led to a modern 14C net effect of 425cpm and a corresponding background of 1.6cpm.
    • Isotopic Fractionation of NBS Oxalic 14C Standard and Its Effect on Calculated Age of Materials

      Nehmi, Victor A. (American Journal of Science, 1980-01-01)
      There is much controversy about the significance to radiocarbon dating of isotopic fractionation during the oxidation to CO2 of NBS oxalic acid, the carbon-14 dating standard. To check the effect of fractionation, 30 routine potassium permanganate oxidations of oxalic acid were performed. The isotopic composition of resulting CO2 was determined mass-spectrometrically. delta-13C results ranged between -17.7 and -21.2, with an average of -18.9 per mil wrt PDB. The effect of not applying an isotopic fractionation correction to NBS oxalic acid when performing age calculations is evaluated.
    • Information on the CO2 Cycle from Ice Core Studies

      Berner, Werner; Oeschger, Hans; Stauffer, Bernhard (American Journal of Science, 1980-01-01)
      Information on the history of the atmospheric CO2 content and the 12C /12 and 14C/C ratios is recorded in natural ice. Measurements on samples from very cold accumulation regions show that CO2 is occluded not only in air bubbles, but also in the ice lattice. The two CO2 components are of similar size. It is very difficult to measure CO2 in the bubbles and CO2 in the ice lattice separately. By melting the samples and extracting the evolving gases in two fractions, it is possible to estimate CO2 concentration in the bubbles and the ice lattice. Enrichment or depletion of CO2 in the bubbles by exchange with the ice is difficult to estimate. Information about this effect is expected from 12C /C analysis on the extracted CO2 fractions. To investigate whether atmospheric CO2 content was different during the last glaciation than during the present one, sets of 16 and 20 samples distributed over the last 40,000 years from the two deep ice cores from Camp Century (North Greenland) and Byrd Station (West Antarctica) were measured. The time scales for the two cores are based on a rheological model. Results and conclusions are: — The data series from both cores show similar trends correlated to a certain degre to the delta-18O profiles. — For both cores, the values for the CO2 concentration of the first fraction, considered to best represent the atmospheric composition, show lower values during glaciation than in the Holocene, with a minimum before the end of glaciation. — For both cores, the values for the CO2 concentration of the first fraction, considered to best represent the atmospheric composition, show lower values during glaciation than in the Holocene, with a minimum before the end of glaciation.
    • High Precision Radiocarbon Dating by Liquid. Scintillation Counting Applied to Radiocarbon Timescale Calibration

      Pearson, G. W. (American Journal of Science, 1980-01-01)
      The precision quoted on a radiocarbon date is often misleading. Improvement in the accuracy of measurement is discussed together with the use of appropriate precisions. The effect of high accuracy in 14C dating and meaningful precisions are illustrated and applied to radiocarbon time-scale calibration. Intercomparison of such calibrations reveals a significant continuous saw-tooth pattern of short-term periodicity.
    • Geomagnetic Strength Over the Last 50,000 Years and Changes in Atmospheric 14C Concentration: Emerging Trends

      Barbetti, Mike (American Journal of Science, 1980-01-01)
      Palaeomagnetic field strength measurements for the last 50,000 years are summarized. The period before 12,000 yr bp** is characterized by low dipole moments, but high values are associated with the Lake Mungo polarity excursion between ~32,000 and 28,000 yr bp. The variation since 12,000 yr bp, based on new results from Australia and published data from the Northern Hemisphere has a quasi-cyclic appearance with maxima at ~10,000 and ~3500 yr bp. The geomagnetic record is used to predict variations in atmospheric 14C concentration, and the results are compared with independent comparisons between 14C and other dating methods. Long-term variations in the 14C time-scale are readily explained by known geomagnetic changes.
    • Further Developments in On-Line Computing and Radiocarbon Dating at the British Museum

      Hewson, A. D.; Hall, J. A. (American Journal of Science, 1980-01-01)
      In November 1973 the British Museum Research Laboratory acquired a Hewlett Packard 2100A mini-computer for the storage, calculation and retrieval of scientific measurements made on museum objects. A part of the computer's work is the calculation of radiocarbon dates based on the liquid scintillation counting of 14C activities. A system of programs and files has been developed and has been in daily use since August 1974 (Hall and Hewson, 1977). This paper describes changes and improvements to the system to make it more flexible so that it now provides the full range of facilities required by an active 14C laboratory. The reporting procedures in particular have been restructured in the light of experience. The paper will be of interest to all laboratories that have, or hope to have, access to similar minior micro-computers.
    • Enrichment of 14C and Sample Preparation for Beta and Ion Counting

      Grootes, P. Al; Stuiver, Minze; Farwell, G. W.; Schaad, T. P.; Schmidt, F. H. (American Journal of Science, 1980-01-01)
      We here report on two technical research projects of the Quaternary Isotope Laboratory (QL) vis, (1) the use of thermal diffusion isotopic enrichment to extend the technical range of 14C dating, (2) the preparation of samples for ion counting using a Van de Graaff tandem accelerator. The second project is carried out in cooperation with, and partly at, the Nuclear Physics Laboratory. A gain in dating range of 3 to 4 half-lives can routinely be obtained with the QL and the Groningen enrichment systems. The same gain in age range can be obtained for ion counting with a simplified system that requires only 0.5 to 2g of carbon and 3 to 7 days enrichment time. A method to convert CO2 quantitatively via CO into carbon is described. For short intervals the carbon deposit yields good 12C beams. We also give a different procedure to make graphite-like carbon samples. The preparation of beryllium metal samples is given last.
    • Experimental and Theoretical Data on Radiocarbon Variation in the Earth's Atmosphere in the Past

      Dergachev, V. A. (American Journal of Science, 1980-01-01)
      Some questions in the study of cosmic ray intensity (solar magnetic field, irregular solar activity, 14CO2 and 14CO in radiocarbon models, etc) on the basis of 14C concentration in the earth's atmosphere are considered.
    • Contemporary 14C Levels and Their Significance to Sedimentary History of Bega Swamp, New South Wales

      Polach, H. A.; Singh, Gurdip (American Journal of Science, 1980-01-01)
      Atmospheric 14C variations in nature, as previously documented for the Southern Hemisphere by studies carried out in South Africa and New Zealand, were supplemented by 14C concentration measurements of wheat-grain samples collected in southeastern New South Wales. Our measurements cover the critical period of 1945/46 up to 1956/57, and span the transition of Suess and atom-bomb effects. The observed variations can be followed quite precisely in the peat deposits of the Bega Swamp, New South Wales, and indicate that vertical mixing of organic components within the peat is negligible. Pollen analytical data covering the last 400 years also show that the peats act as efficient traps; thus, time-precise zonations can be identified, and historically documented man-induced changes in pollen assemblages can be correlated with 14C ages in recent times.