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  • Artikel  (574)
  • Cambridge University Press  (574)
  • 1995-1999  (574)
  • Geologie und Paläontologie  (574)
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  • Artikel  (574)
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  • 1
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    Target Preparation for Continuous Flow Accelerator Mass Spectrometry (1997)
    Cambridge University Press
    Details
    Publikationsdatum: 1997-01-01
    Beschreibung: For very small samples, it is difficult to prepare graphitic targets that will yield a useful and steady sputtered ion beam. Working with materials separated by preparative capillary gas chromatography, we have succeeded with amounts as small as 20 μg C. This seems to be a practical limit, as it involves 1) multiple chromatographic runs with trapping of effluent fractions, 2) recovery and combustion of the fractions, 3) graphitization and 4) compression of the resultant graphite/cobalt matrix into a good sputter target. Through such slow and intricate work, radiocarbon ages of lignin derivatives and hydrocarbons from coastal sediments have been determined. If this could be accomplished as an “online” measurement by flowing the analytes directly into a microwave gas ion source, with a carrier gas, then the number of processing steps could be minimized. Such a system would be useful not just for chromatographic effluents, but for any gaseous material, such as CO2 produced from carbonates. We describe tests using such an ion source.
    Print ISSN: 0033-8222
    Digitale ISSN: 1945-5755
    Thema: Klassische Archäologie , Energietechnik , Geologie und Paläontologie
    Publiziert von Cambridge University Press
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  • 2
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    Radiocarbon Dating Tephra Layers in Britain and Iceland (1995)
    Cambridge University Press
    Details
    Publikationsdatum: 1995-01-01
    Beschreibung: Layers of volcanic ash, or tephra form widespread chronostratigraphic marker horizons which are important because of their distinctive characteristics and rapid deposition over large areas. Absolute dating of prehistoric layers effectively depends upon 14C analysis. We focus here on Icelandic tephra layers at both proximal and distal sites and consider three strategies to obtain age estimates: 1) the conventional dating of individual profiles; 2) high-precision multisample techniques or “wiggle-matching” using stratigraphic sequences of peat; and 3) a combination of routine analyses from multiple sites. The first approach is illustrated by the dating of a peat profile in Scotland containing tephra from the ad 1510 eruption of Hekla. This produced a 14C age compatible with ad 1510, independently derived by geochemical correlation with historically dated Icelandic deposits. In addition, the ca. 2100 bp date for the Glen Garry tephra in Scotland, determined by a series of dates on a peat profile in Caithness, is supported by its stratigraphic position within 14C dated profiles in Sutherland, and may be applied over a very large area of Scotland. More precise dates for individual tephras may be produced by “wiggle-matching”, although this approach could be biased by changes in peat-bog stratigraphy close to the position of the tephra fall. As appropriate sites for “wiggle-match” exercises may be found only for a few Icelandic tephras, we also consider the results of a spatial approach to 14C dating tephra layers. We combined dates on peat underlying the same layer at several sites to estimate the age of the tephra: 3826 ± 12 bp for the Hekla-4 tephra and 2879 ± 34 bp for the Hekla-3 tephra. This approach is effective in terms of cost, the need for widespread applicability to Icelandic tephra stratigraphy and the production of ages of a useful resolution. We stress the need for accurate identification of tephra deposits without which the conclusions drawn from subsequent 14C dating will be fundamentally flawed.
    Print ISSN: 0033-8222
    Digitale ISSN: 1945-5755
    Thema: Klassische Archäologie , Energietechnik , Geologie und Paläontologie
    Publiziert von Cambridge University Press
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  • 3
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    Low-Level Liquid Scintillation Counter Array with Computerized Data Acquisition and Age Calculation Capabilities for 14C Dating (1995)
    Cambridge University Press
    Details
    Publikationsdatum: 1995-01-01
    Beschreibung: We describe a two-phase study directed toward background reduction of a manual liquid scintillation counter and the interfacing of electronics for counting to a computer data acquisition system. Counter background reduction is achieved with afterpulse electronics, a high-performance cocktail, an auxiliary detector/guard and a special sample vial holder. The data acquisition system is comprised of an electronic signal processor and sorter for operating up to eight counters simultaneously and interfacing to a computer with software for data storage, acquisitions and age dating calculations. We discuss low-background counter modifications, electronic signal processing and computer software for 14C age dating.
    Print ISSN: 0033-8222
    Digitale ISSN: 1945-5755
    Thema: Klassische Archäologie , Energietechnik , Geologie und Paläontologie
    Publiziert von Cambridge University Press
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  • 4
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    Reappraisal of Chinese Loess Plateau Stratigraphic Sequences Over the Last 30,000 Years: Precursors of an Important Holocene Monsoon Climatic Event (1997)
    Cambridge University Press
    Details
    Publikationsdatum: 1997-01-01
    Beschreibung: Through the establishment of radiocarbon chronozones relating common geological events within lacustrine and eolian sediments from five profiles representative of loess yuan (tablelands), river valley and northwest margin features of the Loess Plateau, we propose a series of stratigraphic divisions within the last 30 ka. The focus of this detailed study involves stratigraphic relationships contributing to evidence of Younger Dryas events, with the recognition of cold-dry, cool-wet and cold-dry periods represented within the Heiheze silt, Midiwan peat and Liushuwan eolian sand. The stratigraphic profiles reflect century-scale fluctuations of the East Asian monsoons. The precursor events enable us to place the Pleistocene/Holocene boundary at 10,000 bp.
    Print ISSN: 0033-8222
    Digitale ISSN: 1945-5755
    Thema: Klassische Archäologie , Energietechnik , Geologie und Paläontologie
    Publiziert von Cambridge University Press
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  • 5
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    Radiocarbon Dispersion around Canadian Nuclear Facilities (1995)
    Cambridge University Press
    Details
    Publikationsdatum: 1995-01-01
    Beschreibung: Canadian deuterium uranium (CANDU) pressurized heavy-water reactors produce 14C by neutron activation of trace quantities of nitrogen in annular gas and reactor components (14N(n,p)14C), and from 17O in the heavy water moderator by (17O(n,α)14C). The radiocarbon produced in the moderator is removed on ion exchange resins incorporated in the water purification systems; however, a much smaller gaseous portion is vented from reactor stacks at activity levels considerably below 1% of permissible derived emission limits. Early measurements of the carbon speciation indicated that 〉90% of the 14C emitted was in the form of CO2. We conducted surveys of the atmospheric dispersion of 14CO2 at the Chalk River Laboratories and at the Pickering Nuclear Generating Station. We analyzed air, vegetation, soils and tree rings to add to the historical record of 14C emissions at these sites, and to gain an understanding of the relative importance of the various carbon pools that act as sources/sinks within the total 14C budget. Better model parameters than those currently available for calculating the dose to the critical group can be obtained in this manner. Global dose estimates may require the development of techniques for estimating emissions occurring outside the growing season.
    Print ISSN: 0033-8222
    Digitale ISSN: 1945-5755
    Thema: Klassische Archäologie , Energietechnik , Geologie und Paläontologie
    Publiziert von Cambridge University Press
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  • 6
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    Historic Measurements of Radiocarbon in New Zealand Soils (1996)
    Cambridge University Press
    Details
    Publikationsdatum: 1996-01-01
    Beschreibung: Extensive measurements of radiocarbon have been used in New Zealand since the mid-1960s to follow carbon (C) movement and turnover in soils. We present here unpublished radiocarbon (14C) measurements on a range of eight New Zealand soils with details of the sites, ecosystems, climates, soil descriptions and associated analytical data. An overview is also given of published 14C measurements on soils, and the use of these measurements to model soil C turnover.
    Print ISSN: 0033-8222
    Digitale ISSN: 1945-5755
    Thema: Klassische Archäologie , Energietechnik , Geologie und Paläontologie
    Publiziert von Cambridge University Press
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  • 7
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    Problems and Possible Solutions Concerning Radiocarbon Dating of Surface Marine Sediments, Ross Sea, Antarctica (1999)
    Cambridge University Press
    Details
    Publikationsdatum: 1999-09-01
    Beschreibung: Radiocarbon accelerator mass spectrometric (AMS) dates on the acid-insoluble fraction from 38 core tops from the western Ross Sea, Antarctica, are used to address these questions: (1) What are the apparent ages of sediments at or close to the present sediment/water interface? (2) Is there a statistically significant pattern to the spatial distribution of core top ages? and (3) Is there a “correction factor” that can be applied to these age determinations to obtain the best possible Holocene (downcore) chronologies? Ages of core top sediments range from 2000 to 21,000 14C yr B.P. Some “old” core top dates are from piston cores and probably represent the loss of sediment during the coring process, but some core top samples 〉6000 14C yr B.P. may represent little or no Holocene deposition. Four possible sources of variability in dates ≤6000 14C yr B.P. (n = 28) are associated with (1) different sample preparation methods, (2) different sediment recovery systems, (3) different geographic regions, and (4) within-sample lateral age variability. Statistical analysis on an a posteriori design indicates that geographic area is the major cause of variability; there is a difference in mean surface sediment age of nearly 2000 yr between sites in the western Ross Sea and sites east of Ross Bank in south-central Ross Sea. The systematic variability in surface age between areas may be attributed to: (a) variable sediment accumulation rates (SAR) (surface age is inversely related to SAR), (b) differences in the percentage of reworked (dead) carbon between each area, and/or (c) differences in the CO2 exchange between the ocean and the atmosphere.
    Print ISSN: 0033-5894
    Digitale ISSN: 1096-0287
    Thema: Geographie , Geologie und Paläontologie
    Publiziert von Cambridge University Press
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  • 8
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    Temperature history and accumulation timing for the snowpack at GISP2, central Greenland (1998)
    Cambridge University Press
    Details
    Publikationsdatum: 1998-01-01
    Beschreibung: Previous research has documented a close association between high-resolution snow-pit profiles of hydrogen and oxygen stable-isotope ratios and multi-year Special Sensor Microwave/lmager (SSM/I) 37 GHz brightness temperature data in central Greenland. Comparison of the SSM/I data to profiles obtained during the 1989-91 field seasons indicated thatδD andδ18O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy. To test this new technique further, additional stable-isotope data were obtained from a 2 m snow pit constructed during late-June 1995 near the GISP2 site.This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures. The sub-annual variation of theδDrecord at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time-constrained hoar layers. The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year. This is also independently confirmed by analysis of H2O2data. The slope of the multi-yearTvsδcorrelation was evaluated along with the sub-annual variation in the amount, rate and timing of accumulation. These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub-annual resolution. This encourages confident interpretation of the paleoclimatic signal variations in the GISP2 and GRIP deep cores.
    Print ISSN: 0022-1430
    Digitale ISSN: 1727-5652
    Thema: Geographie , Geologie und Paläontologie
    Publiziert von Cambridge University Press im Namen von International Glaciological Society.
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  • 9
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    14C Calibration in the Southern Hemisphere and the Date of the Last Taupo Eruption: Evidence from Tree-Ring Sequences (1995)
    Cambridge University Press
    Details
    Publikationsdatum: 1995-01-01
    Beschreibung: Tree rings from a section of Prumnopitys taxifolia (matai) covering the period ad 1335–1745 have been radiocarbon dated and used to generate a 14C calibration curve for southern hemisphere wood. Comparison of this curve with calibration data for northern hemisphere wood does not show a systematic difference between 14C ages measured in the northern and southern hemispheres. A floating chronology covering 270 yr and terminating at the last Taupo (New Zealand) eruption, derived from a sequence of 10-yr samples of tree rings from Phyllocladus trichomanoides (celery pine, or tanekaha), is also consistent with the absence of a systematic north-south difference, and together with the matai data, fixes the date of the Taupo eruption at ad 232 ± 15.
    Print ISSN: 0033-8222
    Digitale ISSN: 1945-5755
    Thema: Klassische Archäologie , Energietechnik , Geologie und Paläontologie
    Publiziert von Cambridge University Press
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  • 10
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    The Reliability of AMS Radiocarbon Dating of Shells from China (1999)
    Cambridge University Press
    Details
    Publikationsdatum: 1999-01-01
    Beschreibung: We tested the feasibility of dating freshwater and terrestrial molluscs from the semiarid and arid zone in China, since these types of shell material deposit only aragonite to form their shell structure, and shell integrity can be easily observed using X-ray diffraction. We also tested the possibility of estimating microenvironmental changes from shell δ13C values, but variations within shell populations preclude the use of these values as a reliable indicator. Reservoir ages were calculated for living shells of the same species as fossil shells by using their measured 14C ages, which were recalculated using an average value of atmospheric 14C activity for the years spanning their time of collection as the modern standard. The results indicate that freshwater and terrestrial shells are potentially useful as dating material, provided extreme care is taken in their collection and other datable material (in this case wood and pollen) is within the profile to act as a comparison.
    Print ISSN: 0033-8222
    Digitale ISSN: 1945-5755
    Thema: Klassische Archäologie , Energietechnik , Geologie und Paläontologie
    Publiziert von Cambridge University Press
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