• 13C Variation in Limestone on an Aquifer-Wide Scale and Its Effects on Groundwater 14C Dating Models

      Muller, A. B.; Mayo, A. L. (American Journal of Science, 1986-01-01)
      In modeling the initial 14C activity of ground waters, the delta-13C of marine limestone is taken conventionally to vary little about 0 per mil PDB. This variation was found to be 6.28 per mil in samples taken over intervals from 10^-2 to 1^5 meters in the Mooney Falls Member of the Redwall Limestone in northern Arizona. Such a variation will cause appreciable variability in the results of all four initial activity models tested. The variability, due primarily to a numerical instability in the models dependent on this parameter, can introduce significant uncertainty into groundwater "age" calculations.
    • 13CO2 and 14CO2 Measurements on Soil Atmosphere Sampled in the Western Great Plains of the US

      Haas, Herbert; Fisher, D. W.; Thorstenson, D. C.; Weeks, E. P. (American Journal of Science, 1983-01-01)
      Soil gas samples were obtained from the unsaturated zone at eight sites in the Great Plains. Three of these sites were sampled extensively for gas composition and carbon isotopes. Sampling equipment consisted of a nest of gas probes vertically spaced by roughly 3m at most sites, generally approaching the water table. Water wells, 10cm in diameter, were screened in the topmost layer groundwater. Inverted cattle tanks were used to collect CO2 samples from the soil surface. The major gas components were analyzed with emphasis on CO2, Delta--13C, and 14C measurements. The same components were studied in groundwater samples. Higher than atmospheric CO2 concentrations were found in all soil samples. Root respiration and oxidation of organic matter were sources for the additional CO2. When lignite was present in the unsaturated zone, gaseous oxygen reacted almost completely, and CO2 levels rose to 19%. Near the surface, annual cycles in total CO2, Delta--13C, and 14C were observed. 14C activities were close to present post-bomb levels at the surface and generally declined with depth. At some sites, oxidation of lignite caused decline of 14C levels to 1 or 2% of their surface value at 8m depth. Without lignite, the 14C activity remained above 50% at all depths. Concentrations of total carbon and its isotopes in ground water remained very stable throughout the study. This implies that geochemical processes in the aquifer vary on time scales longer than the seasonal effects observed in the near-surface unsaturated zone.
    • 13th International Radiocarbon Conference

      American Journal of Science, 1988-01-01
    • 13th International Radiocarbon Conference

      American Journal of Science, 1988-01-01
    • 14C Ages and Magnetic Stratigraphy in Three Australian Maars

      Barton, C. E.; Potach, H. A. (American Journal of Science, 1980-01-01)
      Detailed radiocarbon chronologies from three volcanic crater lakes (maars) in southeast Australia are examined in relationship to the magnetic mineral stratigraphies within lakes, and the magnetic secular variation stratigraphy between lakes. Some implications for magnetic dating are considered.
    • 14C and Other Parameters During the Younger Dryas Cold Phase

      Oeschger, Hans; Welten, Max; Eicher, Ulrich; Möll, Markus; Riesen, Trudi; Siegenthaler, Ulrich; Wegmüller, Samuel (American Journal of Science, 1980-01-01)
      Pollen analysis as well as 18O/16O results on lake marl show that the Younger Dryas climatic period, between about 11,000 and 10,300 BP, was the last vigorous cold phase of the Wurm Glacial. Detailed 14C analyses from a peat bog near Wachseldorn (Switzerland) point to a 14C anomaly in this period. Further indication of a 14C anomaly is given by the observation that, during the Younger Dryas period, the sedimentation rates in several lakes apparently were higher than in adjacent periods; an explanation might be that the 14C time scale was compressed between 11,000 and 10,000 BP, ie, the atmospheric 14C/C ratio varied. If real, this suggested 14C variation would probably be connected to the climatic events during this transition period from Later Glacial to Postglacial.
    • 14C Background Levels in an Accelerator Mass Spectrometry System

      Vogel, J. S.; Nelson, D. E.; Southon, J. R. (American Journal of Science, 1987-01-01)
      The levels and sources of the measurement background in an AMS 14C dating system have been studied in detail. The relative contributions to the total background from combustion, graphitization, storage, handling, and from the accelerator were determined by measuring the 14C concentrations in samples of anthracite coal ranging in size from 15 micrograms to 20mg. The results show that, for the present system, the uncertainty in the background is greater than that due to measurement precision alone for very old or for very small samples. While samples containing 100 micrograms of carbon can yield useful 14C dates throughout the Holocene, 200 to 500 micrograms are required for dating late Pleistocene materials. With the identification of the procedures that introduce contamination, the level and uncertainty of the total system background should both be reducible to the point that 100 micrograms of carbon would be sufficient for dating most materials.
    • 14C Dating and Magnetostratigraphy

      Thompson, Roy (American Journal of Science, 1983-01-01)
      The history of the earth's magnetic field is preserved in the fossil magnetism of archaeologic specimens, natural rocks and sediments. Samples such as lava flows and baked sherds that acquired a thermoremanent magnetization on cooling can be used to estimate ancient geomagnetic field intensities and directions. Paleofield directions can also be obtained from fine-grained sediments that acquired detrital magnetic remanence when deposited. Study of the earth's magnetic field over the last few tens of thousands of years yields information on geomagnetic dynamo theories, causes of fluctuations in cosmic-ray activity, and the formulation of a new regional chronologic tool.
    • 14C Dating of Calcareous Tufa from Different Environments

      Pazdur, Anna; Pazdur, Mieczysław F. (American Journal of Science, 1986-01-01)
      14C dates of carbonate and organic matter fractions are compared with a series of calcareous tufa samples from selected sites representing different geochemical environments and associated with different bedrocks. Results obtained in this study indicate values of apparent age ranging from 940 +/- 110 yr for calcareous tufas associated with Pleistocene sediments to 2000 +/- 110 yr and even ca 4000 yr for tufas from sites associated with Jurassic or Cretaceous limestone. It was found also that within each of the investigated sites the value of apparent age does not change significantly with the age of the tufa layer.
    • 14C Dating of Plant Macrofossils in Lake Sediment

      Andree, Michael; Oeschger, Hans; Siegenthaler, Ulrich; Riesen, Trudi; Moell, Markus; Ammann, Brigitta; Tobolski, Kazimierz (American Journal of Science, 1986-01-01)
      Macrofossils of terrestrial plants have been picked from a sediment core taken in Lake Lobsigen, a small lake on the Western Swiss Plateau. The sediments were previously analyzed for pollen composition, plant and animal macrofossils, and stable isotopes. Plant macrofossils were selected near pollen zone boundaries in Late Glacial and early Postglacial sediment for 14C dating by AMS. In the same lake carbonate and gyttja (aquatic plant) samples were dated by decay counting. The dates on terrestrial material are generally younger than those on carbonate and gyttja, ie, material reflecting the 14C/C ratio of dissolved bicarbonate in lake water. This is probably due to a contribution of dissolved limestone carbonate and thus a somewhat reduced 14C/C, ratio in the lake's water (hard water effect).
    • 14C Dating of Recent Crustal Movements in the Persian Gulf and the Iranian Makran

      Vita-Finzi, Claudio (American Journal of Science, 1980-01-01)
      Radiocarbon dating of mollusks and barnacles from fossil shoreline deposits in the Persian Gulf and on the coast of Iranian Makran is being used to assess the extent and rate of recent crustal deformation in the area. Samples are selected with the help of x-ray diffraction and of light and scanning electron microscopy; whenever possible, two or more ages are determined for each locality on monospecific samples. Age/height values have been used to compute local uplift rates by allowing for sea-level fluctuations, but eustatic controversy can be avoided by limiting the analysis to fault chronology and to relative vertical movements between dated sections. Short counting times on large, carefully pretreated samples would supply the numerous, cheap, low-resolution ages required to follow up the preliminary results obtained by the survey.
    • 14C Depth Profiles as Indicators of Trends of Climate and 14C/12C Ratio

      Brown, Robert H. (American Journal of Science, 1986-01-01)
      Composite curvature averages for 14C age depth profiles of deep ocean sediment, continental sediment, and soil each indicate a global trend for 14C age increment per cm depth to increase with 14C age over the range for which a definitive statistical sample is available. The global trend indicated for peat profiles is constant 14C age increment per cm depth over the past 10,000 14C yr. Correlation coefficients between changes in 14C yr/cm and maximum profile thickness contradict compaction as an adequate explanation for the global trend indicated by sediment and soil profiles. This trend must be explained by additional factors such as progressively decreasing contamination from older carbon, increasing cosmic ray intensity, decreasing geomagnetic intensity, diminishing 12C in the active biosphere during profile accumulation, and climate factors affecting the rate of accumulation. The diverse trend of peat profiles may indicate climatic conditions more favorable to peat growth during the earlier portion of the past 10,000 yr.
    • 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.
    • 14C in the Deep Water of the East Atlantic

      Schlitzer, Reiner (American Journal of Science, 1986-01-01)
      The renewal of east Atlantic deep water and its large-scale circulation and mixing have been studied in observed distributions of temperature, silicate, Sigma-CO2, and 14C. 14C variations in northeast Atlantic deep water below 3500m depth are small. Delta-14C values range from -100 per mil to -125 per mil.14C bottom water concentrations decrease from &4C = -117 per mil in the Sierra Leone Basin to Delta-14C = -123 per mil in the Iberian Basin and are consistent with a mean northward bottom water flow. The characteristic of the water that flows from the west Atlantic through the Romanche Trench into the east Atlantic was determined by inspection of theta/Delta-14C and theta/Si02 diagrams. A mean potential temperature of theta = 1.50 +/- .05 degrees C was found for the inflowing water. A multi-box model including circulation, mixing, and chemical source terms in the deep water has been formulated. Linear programing and least-squares techniques have been used to obtain the transport and source parameters of the model from the observed tracer fields. Model calculations reveal an inflow through the Romanche Trench from the west Atlantic, which predominates over any other inflow, of (5 +/- 2) Sv (potential temperature 1.50 degrees C), a convective turnover of (150 +/- 50) years and a vertical apparent diffusivity of (4 +/- 1) cm2/s. Chemical source terms are in the expected ranges.
    • 14C in the Southern Indian Ocean

      Geyh, M. A. (American Journal of Science, 1980-01-01)
      14C measurements were carried out on sea water samples collected in 1973, in the Indian ocean. The results obtained for 9 vertical profiles between 27 degrees S and 48 degrees S are presented. In surface water, the bomb 14C content is maximum at middle latitudes. A time lag relative to the north hemisphere bomb 14C delivery is apparent. In the more southern latitudes, 14C content remains very low.
    • 14C Interlaboratory Comparison in the UK: Experiment Design, Preparation, and Preliminary Results

      Otlet, R. L.; Walker, A. J.; Hewson, A. D.; Burleigh, R. (American Journal of Science, 1980-01-01)
      An interlaboratory comparison experiment for 14C measurements has been organized jointly by the Harwell and British Museum laboratories to include the working radiocarbon laboratories of the United Kingdom. The experiment has been run along the lines of that organized by the IAEA for tritium and has explored the problems of sample preparation, verification of equivalent levels, and presentation of results. Samples of benzene representing 5 age equivalent levels between twice modern and 20,000 years old have been prepared and distributed for measurement either by direct counting (liquid scintillation) or full process tests. Preliminary results received show excellent agreement both between laboratories and in comparison with the known relative activities of the prepared solutions. The possibility of extending the experiment to cover different sample types and a wider distribution of testing laboratories is briefly discussed.
    • 14C Traced in Kraków After the Chernobyl Accident

      Kuc, Tadeusz (American Journal of Science, 1987-01-01)
      Results of the 14C measurements in atmospheric CO2 in the first half of 1986 are presented. CO2 samples were systematically collected in Krakow in two-week cycles and, after conversion to benzene, measured in a liquid scintillation spectrometer. 14C activity and 13C/12C ratio are reported as delta-14C and delta-13CPDg, respectively. For about three weeks after April 26, 1986 (the Chernobyl accident) an increase of ~9% above the normal level for Krakow was observed. A rough estimate of the 14C release to the lower atmosphere during the accident gave a value 900 Ci, which is ~1.8 x 10^5 of the total activity released to the atmosphere.
    • 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 Variations During the Upper Pleistocene

      Vogel, J. C. (American Journal of Science, 1983-01-01)
      Parallel determinations of 14C and ionium dates on a stalagmite from the Cango Caves provide evidence of variations in the 14C content of the atmosphere beyond the range of the California tree-ring sequence. During the Holocene growth period the 230Th dates are compatible with the tree-ring calibrated 14C dates. At 18,000 BP and between 30,000 and 40,000 BP the 14C ages are markedly younger than the 230Th ages, suggesting that the 14C level of the atmosphere was considerably higher at these times. Between the 230Th ages of 35,000 and 29,000 BP the 14C ages remain nearly constant at 29,500 BP, indicating that 14C production must have been drastically reduced during this period. The 14C fluctuation is greater than that predicted by Barbetti (1980) but it may be explained by postulating a substantial increase in the geomagnetic dipole field, for which there is mounting evidence.
    • 14C Variations from Tasmanian Trees—Preliminary Results

      McPhail, Steve; Barbetti, Mike; Francey, Roger; Bird, Trevor; Dolezal, Jiri (American Journal of Science, 1983-01-01)
      Huon pine is endemic to Tasmania. It has well-defined annual rings, may live for over 2000 years, and is particularly resistant to decay. Celery-top pine has similar characteristics and may live for 800 years. As part of a multi-disciplinary study of these trees and their habitat, a simple wood pretreatment method for isotope analysis is described. The solvent-acid-alkali-acid sequence yields a value of Delta-14C = -16 +/- 6% for AD 1941-45 Huon pine heartwood; Delta-4C for extracts containing various proportions of post-AD 1955 carbon are also presented. Delta-14C measurements on super-canopy and subcanopy leaves from Celery-top pines are compared and used to place an upper limit of 10% on the amount of sub-canopy CO2 assimilated by sapling leaves, originating from decaying litter-mass. 14C ages from well-preserved logs illustrate the potential for a continuous Holocene chronology from 7400 years BP to the present. A 12,000-year-old Celery-top log has also been found.