• A Multi-Counter System for High Precision Carbon-14 Measurements

      Schoch, Hildegard; Bruns, Michael; Münnich, Karl Otto; Münnich, Marianne (American Journal of Science, 1980-01-01)
      A new 14C detector system containing nine, independently working, CO2 proportional counters is described. The system is designed for a sufficient measuring capacity at a precision level better than sigma = +/- 2 per mil, which requires a counting time of about one week per sample. The size of the installation requires a simple and economic design of counters and electronics. A single anticoincidence shield for all counters consists of five newly developed flat counters. The modern counting rate (52 cpm) is sensitively checked by running Heidelberg sodium carbonate standard samples wth a counting rate of about 10 times modern. A microcomputer (DEC PDP-11/03) is used for data acquisition. Recent developments in laboratory techniques (preparation and gaschromatographic purification of samples) are also reported.
    • Accuracy of the Radiocarbon Time-Scale Beyond 15,000 BP

      Vogel, J. C. (American Journal of Science, 1980-01-01)
      Ionium dates for the Upper Lisan Formation in the Dead Sea valley average (10 +/- 3) percent higher than a set of radiocarbon dates from the same profiles. No analytical explanation can be found so that the discrepancy may be real for the period 15,000 BP to 35,000 BP (conventional radiocarbon years). This would have implications for the chronology of the Upper Pleistocene.
    • A Possible Source of Error in 14C Dates: Volcanic Emanations (Examples from the Monte Amiata District, Provinces of Grosseto and Sienna, Italy)

      Saupé, Francois; Strappa, Osvaldo; Coppens, René; Guillet, Bernard; Jaegy, Robert (American Journal of Science, 1980-01-01)
      Wood from regular timbering of a shallow seated mine in Tuscany gave 14C ages of 5730 +/- 100 years BP, much too old to be attributed to one of the known civilizations of Italy. This mine is located in a region of declining volcanic activity, noticeable especially through numerous emanations (natural or incidentally induced by drillings). It was suspected that the analyzed wood had grown in an environment where the normal atmosphere had been diluted by volcanic emanations. To check this hypothesis, living plants (trees, bushes and reeds) and volcanic emanations have been sampled and their 14C content measured. All present day plants are depleted in 14C, giving a fictitious age different from 0 (1805, 1820, 2540, 4350 years BP). Of the gaseous emanations sampled, two have a high pressure and show virtually no 14C (>41,000 years BP). Two others have a pressure close to atmospheric, and small amounts of 14C were introduced by atmospheric contamination (22,570 and 30,580 years BP). Conclusion: plants grown in the vicinity of volcanic emanations have 14C activities that are too low because of a natural 14C dilution and yield anomalously high ages. The delta-13C values obtained for two of these plants (-27.4 per mil and -23.7 per mil) are close to the average for plants in general (-25 per mil), whereas the CO2 of mofettes is heavier than atmospheric CO2.
    • Comparison of 14C Dates and Other Age Estimations Between 2000 BC and AD 1000

      Willkomm, Horst (American Journal of Science, 1980-01-01)
      The 14C age of charcoal and wood from Lebanon and central Europe is compared partly with the dendrochronologically determined age of the samples and partly with the archaeologically expected value. While the dendrologic values approximately confirm the correction curve of Ralph, Michael, and Han (1973), charcoal of burned layers seemes to be generally 2 to 3 centuries older than expected from contemporary archaeologically retrieved materials.
    • 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.
    • 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.
    • Applications of Resonance Ionization Spectroscopy to Ultralow-Level Counting and Mass Spectroscopy

      Kramer, S. D.; Hurst, G. S.; Young, J. P.; Payne, M. G.; Kopp, M. K.; Callcott, T. A.; Arakawa, E. T.; Beekman, D. W. (American Journal of Science, 1980-01-01)
      In this paper it is shown that the ability to directly detect a daughter atom, using resonance ionization spectroscopy, in delayed time coincidence with the decay of a parent species promises to drastically reduce the background in low-level counting experiments. In addition, resonance ionization can also be used as an ion source for a mass spectrometer system that is capable of discriminating between isobars.
    • Radiocarbon, Volume 22, Number 2 (1980)

      American Journal of Science, 1980-01-01
    • An Analysis of Shielding Efficiency for 14C Counters

      Nydal, Reidar; Gulliksen, Steinar; Lövseth, Knut (American Journal of Science, 1980-01-01)
      The "shielding" efficiency of the guard counters has been a main scope of the present investigation. Our special guard counters consist of closed shells (ca 3cm thick) filled with propane at 1.2 atmospheres pressure. These guard counters are nearly 100 percent efficient against charged particles, and 1 to 2 percent against gamma and neutrons. The efficiency has now been studied more in detail in an arrangement with four guard shells around a 14C counter. For each extra guard shell, the cosmic fraction of the counter background was reduced by ca 13 percent. The reduction does not involve penetrating high energy charged particles, but is related to ray showers penetrating the guards. A thicker old lead shield between 14C counter and the guard counters also reduces the background and serves the same purpose. In order to approach underground conditions for the 1.5 liter counter background (0.32 +/- 0.01 c/min), most of the shielding material has to be put inside guard shells. An ordinary guard counter combined with an extra guard on top of the iron shield is very efficient. A background of 0.48 +/- 0.01 c/min has already been obtained.
    • 14C Variations Caused by Changes in the Global Carbon Cycle

      Siegenthaler, Ulrich; Heimann, Martin; Oeschger, Hans (American Journal of Science, 1980-01-01)
      A box-diffusion model for the carbon cycle is used to estimate the magnitude of 14C variations caused by changes of reservoir sizes and exchange fluxes in the global carbon system. The influence of changes in atmospheric CO2 concentration, biomass, CO2 exchange rate between atmosphere and ocean, and ocean mixing is considered. Steady-state 14C concentrations as well as the transients are calculated. For changing biomass, atmospheric CO2 levels and 13C/12C ratios are also calculated. Carbon-cycle-induced 14C variations may have been significant in the transition period from Glacial to Postglacial when drastic changes in environmental conditions took place within short time periods, while they were probably less important during the climatically more stable Postglacial. Changes of the oceanic circulation, as supposedly occurred, are considered the most important factor, besides variations of the production rate, affecting the global distribution of 14C. 14C variations due to changes of the atmospheric CO2 level or the air-sea exchange probably did not exceed one to a few percent. Fluctuations of the forest biomass, which may have occurred between Glacial and Postglacial, hardly affected the 14C concentration over a long term. Responses of the atmospheric 14C concentration are also calculated for variations of the 14C production rate by cosmic radiation. The following cases are considered: a step change, square-wave changes producing "wiggles", and sinusoidal variations.
    • 14C in Extractives from Wood

      Olsson, I. U. (American Journal of Science, 1980-01-01)
      Two Pinus aristata samples submitted by C W Ferguson were separated in different fractions, as was done earlier with Pinus silvestris L from Sweden, to yield different fractions for studies of the pretreatment. One sample in this new series consisted of heartwood and the second of sapwood. The treatment performed in the radiocarbon laboratory involved an acid treatment by boiling, washings, an hydroxide treatment at 80 degrees C, washings and, finally, another acid treatment before being dried before the combustion. The sodiumhydroxide treatment was repeated to yield at least two soluble and two insoluble fractions. The treatment performed by the wood chemists involved extractions with ethanolbenzene and water. The remaining wood was dated but was also used for the production of holocellulose. The extractives were partitioned between ethyl ether and water and that from the older wood was used for the isolation of neutrals, acids, and phenols. In all, 19 fractions of these two wood samples were dated. It is confirmed that a treatment for 30 min in sodium hydroxide at 80 degrees C is not sufficient to remove the extractives from the heartwood or the sapwood. A treatment at 80 degrees C overnight with 1 percent NaOH yielded a sample from the older wood with a 14C content in good agreement with the results predicted from the curve presented by Stuiver (1978). The final statistical uncertainty in the present investigation was ca 4 per mil. The younger wood yielded results indicating a lower activity than that given by Stuiver.
    • Solar Modulation Effects in Terrestrial Production of Carbon-14

      Castagnoli, Giuliana; Lal, Devendra (American Journal of Science, 1980-01-01)
      This paper is concerned with the expected deviations in the production rate of natural 14C on the earth due to changes in solar activity. We review the published estimates of the global production rates of 14C due to galactic and solar cosmic ray particles, and present new estimates of the expected secular variations in 14C production, taking into account the latest information available on galactic cosmic ray modulation and long-term variations in solar activity. Estimated secular variations are related to data on atmospheric 14C/12C ratios based on tree rings. It is concluded that the observed higher frequency wiggles in atmospheric 14C/12C ratios occurring with time scales of about two hundred years and of 1 to 2 percent in magnitude (de Vries, 1958; Suess, 1970a,b; 1979; Damon, Lerman, and Long, 1978), are largely due to solar activity dependent modulation of the galactic cosmic ray flux by solar plasma. These variations override a slowly varying sinusoidal change of about 10 percent in magnitude during the last approximately 8000 years, which is believed to be primarily due to changes in the geomagnetic field. The high frequency modulation effect in 14C production is substantial, about 20 percent, which, considering the response function of the atmosphere (cf Houtermans, Suess, and Oeschger, 1973), is adequate to explain the observed 14C wiggles (also named Suess wiggles or de Vries oscillations) if in the past, periods of large modulation effects and also periods of weak modulation persisted, ie, the sun remained both active and inactive over long periods of time, of the order of several decades to centuries. The pioneering investigations of Eddy (1976; 1977) of the ancient records of solar activity make it plausible that the 14C wiggles in the ancient 14C/12C ratios are primarily due to modulation of galactic cosmic ray flux by a varying sun. Thus, the 14C wiggles are good indicators of solar activity in the past. We also present revised estimates of the production rates of 14C on the earth due to solar flare accelerated cosmic rays and limits of direct accretion of 14C on the earth.
    • Temperature-Dependent Seasonal Variation of the Background in Counters Used for Radiocarbon Dating

      Håkansson, Sören (American Journal of Science, 1980-01-01)
      All background values measured in one of our 14C counters during 1977 were analyzed together with corresponding values for atmospheric pressure, outdoor temperature, and coincidence counting rate for investigation of the possibilities to separate the temperature dependency of the background from the better known atmospheric pressure dependency. The background values were normalized to a common atmospheric pressure and plotted in a time diagram. A seasonal trend with higher background values in winter than in summer shown by this diagram was confirmed by regression analysis. The regression line and the "Standard error of estimate" was calculated for the background values as a function of the atomspheric pressure, on the one hand and as a function of the coincidence counting rate, on the other hand. A comparison showed that the coincidence counting rate in this case was a better parameter than the atmospheric pressure for calculation of the actual background. The approximate consistency of the temperature effects on the background was checked for the years 1976 to 1978.
    • Possibility of Climatically Induced Variations in the 14C and 13C Enrichment Patterns as Recorded by a 3000-Year-Old Norwegian Pine

      Harkness, D. D.; Miller, B. T. (American Journal of Science, 1980-01-01)
      The secular patterns of Deltaand delta-13C measured in wood cellulose are compared with a palaeotemperature index published for the immediate growth region. 14C enrichment shows an overall decrease of ca 20 per mil (Delta) during the 18th and 19th centuries with shorter term (decade) variations superimposed on the general trend. Measured Delta values correlate significantly with the average mean summer temperatures and indicate a linear coefficient of ca -10.2 per mil C-1. Short-term variations in the natural concentration of 14C which are closely related to climatic change may reflect the latitudinal dependence for atmospheric mixing. The secular trend in delta-13C covers a range of 2 per mil but variations of this magnitude reflect influences other than those directly associated with climatic change, eg, possible assimilation of recycled biospheric CO2 during early life and post AD 1850 anthropogenic effects. No evidence was found to suggest a significant correlation between delta-13C and the mean atmospheric temperature during summer growth periods.
    • Radiocarbon in Annual Coral Rings of Belize and Florida

      Druffel, Ellen M. (American Journal of Science, 1980-01-01)
      Radiocarbon measurements on a 109-year-old (1868-1977) core of Montastrea annularis coral from Glover Reef, Belize, in the Gulf of Honduras, reveal uptake of fossil fuel CO2 and bomb 14C by surface ocean waters. The history of Delta-14C values revealed by this Belize growth agree remarkably well with results for coral growth from the Florida Straits. It is concluded that these corals are reliable recorders of 14C concentrations of dissolved inorganic carbon (DIOC) in surface waters representative of the Gulf Stream.
    • Radiocarbon Variations in Consecutive Single Rings of a 4000-Year-Old Pine from the British Isles

      Hewson, A. D.; Burleigh, Richard (American Journal of Science, 1980-01-01)
      At the Ninth International Radiocarbon Conference a paper was presented concerning possible short-term 14C variations in 4000-year-old red deer antlers (Genius elaphus) found in Neolithic flint mines in Norfolk, England (Burleigh and Hewson, 1980). It was argued, on archaeologic grounds, that the true age of the samples varied by a few years at most. Their radiocarbon ages, however, varied by a considerably greater amount than could be explained by the errors in the measurements. Duplication of the measurements confirmed this unexpected variation. Farmer and Baxter (1972) claimed a significant correlation of atmospheric 14C levels in the northern hemisphere with sunspot number based on radiocarbon assay of single tree rings for the period 1829 to 1865. In contrast, Stuiver (1978) stated that a series of single-year Douglas Fir measurements did not show a statistically significant periodicity. This paper reports measurements made on a series of single-year dendrochronologic samples of approximately the same age as the red deer antlers, which have been carried out at the British Museum Research Laboratory. The results show that the variation in 14C between rings is not statistically significant; some other explanation must be sought for the anomalous antler measurements.
    • Radiocarbon Dating of Boise Apatite Using Thermal Release of CO2

      Haas, Herbert; Banewiez, John (American Journal of Science, 1980-01-01)
      Extraction of carbon from bone hydroxy apatite as CO2 by heating in an oxygen atmosphere is an alternative method to hydrolysis of the bone. Heating in specific steps allows separation of CO2 fractions from different sources, including weakened or sound bone material and secondary deposits. Pretreatments to remove most secondary carbonate and much of the collagen are necessary. Thermogravimetric (weight loss) curves and CO2 release patterns during heating show that the temperature interval for collection of the most reliable CO2 sample for dating purposes lies between 800 and 950 degrees C. Age dates run on such samples support this conclusion.
    • 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 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.