• 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.
    • 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.
    • 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 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.
    • 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.
    • Variations in Radiocarbon Production in the Earth's Atmosphere

      Korf, Serge A.; Mendell, Rosalind B. (American Journal of Science, 1980-01-01)
      We have investigated solar phenomena associated with unusual changes in the production rates of 14C in the atmosphere. 14C is produced in interactions of cosmic ray neutrons with nitrogen in the atmosphere. Intensity of the neutrons varies globally and fluctuates with time as a result of interactions of galactic cosmic rays which generate neutrons with plasma and magnetic fields of the solar wind. We estimate the total mean production rate of 14C for solar cycle 20, specifically 1965 to 1975, to be 2.25 +/- 0.1 nuclei-cm-2sec-1 from galactic cosmic rays alone, with negligible integrated contribution from solar particle events. Annual averages of Rz, the Zurich sunspot number, and the production rate of 14C, n(14C), were related by n(14C) = 2.60 5.53 x 10^(-3) Rz, +/- 3 percent. The contribution of solar flare particles and the zero sunspot limit are discussed with relation to major fluctuations that appear in the radiocarbon versus dendrochronology over short (~100 years) integration times.
    • Variations of 14C in Oats Grown from 1957 to 1978 in Quebec

      Barrette, Louis; La Salle, Pierre; Martel, Yvon; Samson, Claude (American Journal of Science, 1980-01-01)
      Annual atmospheric radiocarbon concentrations for the year 1957 to 1978 are measured through oat seeds grown in the rural region of La Pocatière, Quebec (70 degrees W, 47 degrees N). Results follow the general pattern of other curves obtained from grains elsewhere in the northern hemisphere. Some disagreements suggest a non-uniform mixing process with faster response to stratospheric contamination in definite regions.
    • 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.
    • Some Possibilities for Development of 14C Measurements by Liquid Scintillation Counting

      Rajamäe, R.; Punning, J.-M. (American Journal of Science, 1980-01-01)
      The possibilities of widening the ranges of usage of scintillation counters are studied. As a result of the introduction of the active guard, the background in 14C channel reduces 30 to 75 percent and the stability of background increases. The application of quartz vials and a plastic scintillator as an active guard establishes 14C activity in small amounts of scintillation cocktail (up to 0.1g C) and dates samples with older ages (up to 57,000 years). The parameters of devices are given in case of different amounts of scintillation cocktail.
    • Radiocarbon Ages of Shells in Holocene Marine Deposits

      Donner, Joakim; Jungner, Hogne (American Journal of Science, 1980-01-01)
      Radiocarbon dates of marine shells from Central West Greenland, Finnmark, in North Norway, and Dublin Bay, in Ireland, were used in dating relative sea-level changes. When fossil assemblages and formation of marine deposits and their relationship to sea-level were taken into account, the constructed curves of relative sea-level changes agreed with the shell dates. The origin of the shells in the deposits studied varied from site to site, but the dates gave additional information of the formation of marine deposits which could not have been obtained from the study of sediments alone.
    • 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.
    • Participants

      American Journal of Science, 1980-01-01