• Radiocarbon Dating of Calcareous Tufa: How Reliable Data Can We Expect?

      Srdoč, Dušan; Obelić, Bogomil; Hovatinčic, Nada; Sliepčević, Adela (American Journal of Science, 1980-01-01)
      Systematic dating of tufa samples has been undertaken to establish achronology of tufa deposits in Plitvice National Park, Yugoslavia. We conclude thattufa samples give reproducible data within the time span of 40,000 years up torecent. The 14C/12C ratio of carbon in the ambient biosphere and hydrosphere gives adetailed picture of the distribution of carbon isotopes in the investigated system.Despite the susceptibility of calcareous material to ambient conditions in the hydrosphere,the original 14C composition of tufa has not been substantially changed. Avertical profile which was cut in tufa during pathway construction showed that thetufa isochrones run vertically in the investigated area. The vertical isochrones and the14C gradient were horizontal due to tufa build-up in Plitvice National Park whichoccurs on vertical escarpments that have been flooded. The relatively uniform radiocarboncontent of most tufa deposits indicates redistribution of radiocarbon duringthe formation period. While detailed stratigraphy is often obscured, a general chronologyof tufa deposits in the investigated area has been established.
    • Radiocarbon Dating of Earthquakes

      Berger, Rainer; Kaufman, T. S. (American Journal of Science, 1980-01-01)
    • Radiocarbon Dating by Ion Counting: Proposals and Progress

      Hedges, R. E. M.; White, N. R.; Wand, J. O.; Hall, E. T. (American Journal of Science, 1980-01-01)
      In the two years since the first successful measurements of 14C by high energy mass spectrometry, the advantages and the basic techniques of the method have been sufficiently reported. They will not be repeated here. This paper describes the work being done at Oxford to fill the gap between demonstrating the effectiveness of the approach and the creation of a facility dedicated to the carbon dating of milligram samples.
    • Radiocarbon Dating of Ceramic Materials: Progress and Prospects

      De Atley, Suzanne P. (American Journal of Science, 1980-01-01)
      Although initial studies in the 1960's established the feasibility of applying the radiocarbon technique directly to ceramics, subsequent analyses have revealed this category of materials to be more complex than previously believed. Consideration of the points at which various types of organic carbon are introduced into ceramic materials by natural or human agencies allows more effective screening of potential sherd samples. It also suggest ways to overcome some of the limiting factors involved in dating ceramics.
    • Radiocarbon Concentration in the Atmosphere: 8000-Year Record of Variations in Tree Rings: First Results of a USA Workshop

      Klein, Jeffrey; Lerman, Juan Carlos; Damon, Paul E.; Linick, Timothy (American Journal of Science, 1980-01-01)
    • Radiocarbon Activity Measurements of Oolitic Sediments from the Persian Gulf

      Šilar, Jan (American Journal of Science, 1980-01-01)
      Radiocarbon activity of successive parts of Pleistocene and Holocene ooids and mollusk shells from the Persian Gulf, Kuwait, was measured. The inner part of the ooids showed the lowest activity and the cement between grains the highest. Radiocarbon activities correspond to the general stratigraphy and to the position of the sediments. Radiocarbon ages of Pleistocene sediments seem to be very low due to recrystallization of aragonite. Higher radiocarbon activity of cement indicates that atmospheric carbon dioxide was involved in the subaerial diagenetic process. The radiocarbon age of well-preserved mollusk shells seems to be lower than their allegedly Pleistocene geologic age.
    • Progress in Radiocarbon Dating with the Chalk River MP Tandem Accelerator

      Andrews, H. R.; Ball, G. C.; Brown, R. M.; Davies, W. G.; Imahori, Yoshio; Milton, J. D. (American Journal of Science, 1980-01-01)
      The evolution of a tandem accelerator 14C dating system at Chalk River is recounted. Background problems and sources of instability are discussed and solutions are described. Details of sample chemistry and source preparation are presented.
    • Paleoclimatic Evidence in Apparent 14C Ages of Saharian Groundwaters

      Sonntag, Christian; Thorweihe, Ulf; Rudolph, Jochen; Löhnert, E. P.; Junghans, Christel; Münnich, K. O.; Klitzsch, Eberhard; El Shazly, E. M.; Swailem, F. M. (American Journal of Science, 1980-01-01)
      Frequency distributions of more than 300 14C groundwater ages from various regions in northern and southern Sahara reflect the alternating sequence of humid and arid periods in the Sahara during the Holocene and late Pleistocene. A broad frequency maximum between 20,000 and 50,000 years BP indicates a long humid period. During this time span, the northern Sahara received rain from the Western Drift, which is concluded from a west-east decrease of deuterium and oxygen 18 of these groundwaters (continental effect). In the time-slice between 14,000 and 20,000 years BP, groundwater formation was significantly lower due to a cool and (semi-)arid period. In the Holocene, the Saharian climate is characterized by a sequence of dry and wet periods.
    • On the 14C to Tritium Relationship in the North Atlantic Ocean

      Roether, Wolfgang; Münnich, Karl-Otto; Schoch, Hildegard (American Journal of Science, 1980-01-01)
      Nuclear-weapon produced 14C (or bomb 14C) in the ocean can be traced by simultaneous tritium observations. Data are presented on the general relationship of bomb 14C and tritium in the North Atlantic. For the period 1965 to 1973, the excess 14C to tritium ratios in the surface water vary, systematically, over a factor of 10: the ratios monotonically increase with time, and decrease with latitude, particularly so for the later observations. The sub-surface water ratios show that the midand low-latitude water below about the 15 degrees C isothermal horizon (~500m depth) originates from higher northern latitudes, rather than being renewed by local vertical mixing. It is further shown that in the North Atlantic, bomb 14C did not penetrate beyond the horizon where the presently observed 14C concentration is Delta-14C = -75 per mil. Observed concentrations up to about -40 per mil can be corrected for a bomb contribution if the tritium concentration is known because the bomb 14C to tritium concentration ratio is rather uniform in this range. A surface water 14C concentration versus time curve is presented for the period since 1957. This curve is based on a North Atlantic mixing model and is fitted to the 14C observations. Making use of a previously published tritium versus time curve obtained by the same model, a time curve for the average excess 14C to tritium ratio in North Atlantic surface water is given. This curve reproduces the observations well. The presented data and theoretical curves show the usefulness of simultaneous 14C and tritium observations for mixing studies and to provide corrections for bomb 14C in sub-surface 14C data in the North Atlantic.
    • New National Bureau of Standards Contemporary Carbon-14 Standards

      Cavallo, L. M.; Mann, W. B. (American Journal of Science, 1980-01-01)
    • Modeling the Carbon System

      Broecker, Wallace S.; Peng, Tsung-Hung; Engh, Richard (American Journal of Science, 1980-01-01)
      Claims that forest cutting (luring the last few decades has contributed significantly to the buildup in atmospheric CO2 have cast doubt on the validity of models used to estimate CO2 uptake by the ocean. In this paper we review the existing models and conclude that the box-diffusion model of Oeschger and his co-workers provides an excellent fit to the average distributions of natural and bomb-produced radiocarbon. We also take the first steps toward a more detailed ocean model which takes into account upwelling in the equatorial zone and deep water formation in the polar zone. The model is calibrated using the distribution of bomb-produced and cosmic ray-produced radiocarbon in the ocean. Preliminary calculations indicate that the fossil fuel CO2 uptake by this model will be greater than that by the box-diffusion model of Oeschger and others (1975) but not great enough to accommodate a significant decline in the mass of the terrestrial biosphere over the past two decades.
    • MACS: An Accelerator-Based Radioisotope Measuring System

      Purser, Kenneth H.; Liebert, Reuel B.; Russo, Carl J. (American Journal of Science, 1980-01-01)
      A description is given of an accelerator-based radioisotope measuring system, being supplied to the Universities of Arizona and Toronto and in part to the University of Oxford. This system will be capable of measuring 14C/12C and 14C/12C ratios in milligram samples of cracked acetylene. At present 200 micrograms of carbon obtained from cracked acetylene appears to be adequate for an isotopic ratio determination. Previous performance and new calculations indicate that a precision better than 1 percent will be achieved in a half-hour period only limited by counting statistics. A precision of 0.2 percent will be obtained in a ten-hour period. Using a carbon sample with an age greater than 60,000 years, the measured background 14C/12C ratio will be less than 0.07 percent of modern.
    • Isotopic Fractionation of Norwegian Materials for Radiocarbon Dating

      Gulliksen, Steinar (American Journal of Science, 1980-01-01)
      To improve reporting of radiocarbon dates, Stuiver and Polach (1977) recommend that reported standard errors should include the error in the applied delta-13C value, and suggest estimated mean values for delta-13C to be applied when not measured. Based on delta-13C data for ca 250 samples measured during 1975-1979, mean values for different materials dated by the Trondheim radiocarbon laboratory have been compiled. All material is from Norway and Svalbard (marine bone collagen). For peat, gyttja, and terrestrial bone material, delta-13C should be measured to obtain optimal precision in the dates. For shell, wood, charcoal, and marine bones, the standard error in an estimated delta-13C value will only increase uncertainty of a date from +/- 50 years to ca +/- 55 years.
    • Hydrological Implications from 14C Profiling of UK Tufa

      Thorpe, P. M.; Otlet, R. L.; Sweeting, M. M. (American Journal of Science, 1980-01-01)
      Tufa is a superficial, secondary deposit of calcium carbonate which accumulates on precipitation from emergent spring waters. It occurs as discrete, localized masses in regions of calcareous country rock. In the United Kingdom, deposits vary widely in structure and thickness but little is known of the rate of deposition. Some deposits contain laminae assumed to represent annual growth increments and which should contain a 14C and stable isotope record related to the original water from which it was precipitated. Investigations are reported on tufa from three areas of different limestones in the United Kingdom (Northwest Yorkshire, South Derbyshire and North Oxfordshire). Hydrologically, the dating of tufa by 14C involves the same problems as the dating of groundwater. In the case of actively forming tufa, however, it is possible to derive a sequence of measurements, beginning with present day deposition, which clearly demonstrates the applications of age corrections. At Gordale Scar, Northwest Yorkshire, a profiling study of laminated tufa appears to show bomb trial 14C to a depth of 18mm below the surface, with almost constant values around 50 percent modern (raw data) from 18 to 48mm below the surface. The 14C content of surface tufa lies within the seasonal range of 14C measurements from the parent stream waters. Results of 50 percent modern in the sequence are consistent with the simplest correction procedures based on delta-13C balance and the observed delta-13C change on tufa precipitation is a practical demonstration of the fractionation factor e13. However, the application of corrections to active, surface tufa and parent waters collected monthly over a period of study (14 months) from all three sites, produce results higher than would be expected from published world 14C levels.
    • Holocene Sea-Level History: Case Study of the Statistical Evaluation of 14C Dates

      Geyh, M. A. (American Journal of Science, 1980-01-01)
      The limits of the statistical evaluation of 14C dates by histograms are discussed. The method of the attempt to establish a global chronology of the shortterm eustatic sea-level changes during the Holocene is examined as well as its correlation with the precisely determined Suess wiggles.
    • Gas Exchange Rate Measurements in Natural Systems

      Broecker, W. S.; Peng, T.-H.; Mathieu, G.; Hesslein, R.; Torgersen, T. (American Journal of Science, 1980-01-01)
      Rates of CO, exchange across the air-water interface in oceans and lakes measured to date by the L-DGO group are summarized. They range from 3 to 38 moles/m2/yr. The possible causes for this range include the differences in salinity, mean wind speed, and pH. Wind tunnel studies comparing fresh water and sea water are required before a satisfactory explanation can be found.
    • Exploratory Analysis of the International Radiocarbon Cross-Calibration Data: Consensus Values and Interlaboratory Error: Preliminary Note

      Currie, L. A.; Polach, H. A. (American Journal of Science, 1980-01-01)
      A preliminary review is given of the evaluation of the International Cross-Calibration Radiocarbon Data. Consensus values and standard errors are D14C = 508.1 +/- 2.0 per mil (ANU Sucrose) and Delta = -2.5 +/- 1.5 per mil, (1850 Wood). Reporting format follows recornmendations of Stuiver and Polach (1977).
    • Direct Detection of 14C at the Harwell Tandem

      Shea, J. H.; Conlon, T. W.; Asher, James; Read, P. M. (American Journal of Science, 1980-01-01)
      Direct detection of 14C using the Harwell 6MV Tandem accelerator has been achieved using a gaseous ion source. The implications of our results for the use of such sources in this role and for the machine, in general, are discussed.
    • Content of 14C in Marine Mammals from Northern Europe

      Olsson, Ingrid U. (American Journal of Science, 1980-01-01)
      The reservoir effect of Scandinavian sea water has been determined by dating seals and whales killed well before man's impact on the natural 14C concentration became significant. The samples were collected at different places along the Swedish coasts and in the Gulf of Finland. They derive from AD 1657 or 1658, 1868, 1875, 1894, and 1906. The EDTA treatment of bones was used to obtain collagen free from contaminants. An elk, originating from AD 1881 was selected for comparison. A seal from AD 1899 from the Caspian Sea was also included in the investigation. All results have been normalized to delta-13C = -25 per mil vs PDB. The determinations yield values of the reservoir effect in agreement with earlier results obtained from shells and mammals. The final results are discussed in light of previous variations of 14C content in the atmosphere. Using a smoother curve, the reservoir effect is slightly smaller than was hitherto believed. An event thought to be of cosmic origin caused the count rate of both proportional and Geiger counters to increase significantly around December 4, 1978. The correction for this has been studied. The statistics for background, oxalic acid, and unknown samples, measured repeatedly after this correction, were as good as usual.
    • Climate Periods in Trees and a Sea Sediment Core

      Pandolfi, L. J.; Kalil, E. L.; Doose, Paul Robin; Levine, L. H.; Libby, L. M. (American Journal of Science, 1980-01-01)
      Chemical components in a sea sediment core from the Santa Barbara Basin show the same periodic variations as do stable isotope variations in a Japanese cedar.