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  • 1
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    A Time History of Pre- and Post-Bomb Radiocarbon in the Barents Sea Derived from Arcto-Norwegian Cod Otoliths (2001)
    Cambridge University Press
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    Publication Date: 2001-01-01
    Description: Radiocarbon measured in seawater dissolved inorganic carbon (DIC) can be used to investigate ocean circulation, atmosphere/ocean carbon flux, and provide powerful constraints for the fine-tuning of general circulation models (GCMs). Time series of 14C in seawater are derived most frequently from annual bands of hermatypic corals. However, this proxy is unavailable in temperate and polar oceans. Fish otoliths, calcium carbonate auditory, and gravity receptors in the membranous labyrinths of teleost fishes, can act as proxies for 14C in most oceans and at most depths. Arcto-Norwegian cod otoliths are suited to this application due to the well-defined distribution of this species in the Barents Sea, the ability to determine ages of individual Arcto-Norwegian cod with a high level of accuracy, and the availability of archived otoliths collected for fisheries research over the past 60 years. Using measurements of 14C derived from Arcto-Norwegian cod otoliths, we present the first pre- and post-bomb time series (1919–1992) of 14C from polar seas and consider the significance of these data in relation to ocean circulation and atmosphere/ocean flux of 14C. The data provide evidence for a minor Suess effect of only 0.2‰ per year between 1919 and 1950. Bomb 14C was evident in the Barents Sea as early as 1957 and the highest 14C value was measured in an otolith core from a cod with a birth date of 1967. The otolith 14C data display key features common to records of 14C obtained from a Georges Bank mollusc and corals from the tropical and subtropical North Atlantic.
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
    Published by Cambridge University Press
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  • 2
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    Radiocarbon Beyond this World (2000)
    Cambridge University Press
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    Publication Date: 2000-01-01
    Description: In this paper, we review the production of radiocarbon and other radionuclides in extraterrestrial materials. This radioactivity can be produced by the effects of solar and galactic cosmic rays on solid material in space. In addition, direct implantation at the lunar surface of 14C and other radionuclides can occur. The level of 14C and other radionuclides in a meteorite can be used to determine its residence time on the Earth's surface, or “terrestrial age”. 14C provides the best tool for estimating terrestrial ages of meteorites collected in desert environments. Age control allows us to understand the time constraints on processes by which meteorites are weathered, as well as mean storage times. Third, we discuss the use of the difference in 14C/12C ratio of organic material and carbonates produced on other planetary objects and terrestrial material. These differences can be used to assess the importance of distinguishing primary material formed on the parent body from secondary alteration of meteoritic material after it lands on the earth.
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
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  • 3
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    Radiocarbon in the Water Column of the Southwestern North Pacific Ocean—24 Years After Geosecs (2004)
    Cambridge University Press
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    Publication Date: 2004-01-01
    Description: In the framework of the Worldwide Marine Radioactivity Studies (WOMARS) project, water profile samples for radiocarbon measurements were taken during the IAEA'97 cruise at 10 stations in the southwestern North Pacific Ocean. While 14C concentrations were rapidly decreasing from the surface (Δ14C about 100‰) down to about 800 m at all visited stations (Δ14C about −200‰), the concentrations below 1000 m were almost constant. Some stations were in proximity to the GEOSECS stations sampled in 1973; thus, 14C profiles could be compared after a 24-yr interval. Generally, 14C concentrations had decreased in surface waters (by 50–80‰) and increased (by about the same amount) in intermediate waters when compared with GEOSECS data. In deep waters (below 1000 m), the observed 14C concentrations were similar to GEOSECS values. The bomb-produced 14C inventory had increased by more than 20% over the 24 yr from 1973 to 1997 and was estimated to be about (32 ± 5) 1012 atom m-2, with an annual 14C flux of (1.3 ± 0.3) 1012 atom m-2 yr-1. The results suggest that bomb-produced 14C has been advected northwards by the Kuroshio Current and the Kuroshio Extension and stored in the intermediate layer as North Pacific Intermediate Water.
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
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  • 4
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    Turbulent transport of momentum and heat in magnetohydrodynamic rectangular duct flow with strong sidewall jets (2000)
    Cambridge University Press
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    Publication Date: 2000-03-10
    Description: This paper presents an experimental study of the momentum and heat transport in a turbulent magnetohydrodynamic duct flow with strong wall jets at the walls parallel to the magnetic field. Local turbulent flow quantities are measured by a traversable combined temperature-potential-difference probe. The simultaneous measurements of time-dependent velocity and temperature signals facilitates the evaluation of Reynolds stresses and turbulent heat fluxes. Integral quantities such as pressure drop and temperature at the heated wall are evaluated and compared with results from conservative design correlations. At strong enough magnetic fields the destabilizing effect of strong shear generated at the sidewalls wins the competition with the damping effect by Joule's dissipation and turbulent side layers are created. Due to the strong nonisotropic character of the electromagnetic forces, the turbulence structure is characterized by large-scale two-dimensional vortices with their axis aligned in the direction of the magnetic field. As most of the turbulent kinetic energy is concentrated in the near-wall turbulent side layers, the temperatures at the heated wall are governed by the development of the thermal boundary layer in the turbulent flow.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
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  • 5
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    Rayleigh–Bénard convection in liquid metal layers under the influence of a horizontal magnetic field (2002)
    Cambridge University Press
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    Publication Date: 2002-02-25
    Description: This article presents an analytical and experimental study of magnetohydrodynamic Rayleigh-Bénard convection in a large aspect ratio, 20 : 10 : 1, rectangular box. The test fluid is a eutectic sodium potassium Na22K78 alloy with a small Prandtl number of Pr ≈ 0.02. The experimental setup covers Rayleigh numbers in the range 103 〈 Ra 〈 8 × 104 and Chandrasekhar numbers 0 ≤ Q ≤ 1.44 × 106 or Hartmann numbers 0 ≤ M ≤ 1200, respectively. When a horizontal magnetic field is imposed on a heated liquid metal layer, the electromagnetic forces give rise to a transition of the three-dimensional convective roll pattern into a quasi-two-dimensional flow pattern in such a way that convective rolls become more and more aligned with the magnetic field. A linear stability analysis based on two-dimensional model equations shows that the critical Rayleigh number for the onset of convection of quasi-two-dimensional flow is shifted to significantly higher values due to Hartmann braking at walls perpendicular to the magnetic field. This finding is experimentally confirmed by measured Nusselt numbers. Moreover, the experiments show that the convective heat transport at supercritical conditions is clearly diminished. Adjacent to the onset of convection there is a significant region of stationary convection with significant convective heat transfer before the flow proceeds to time-dependent convection. However, in spite of the Joule dissipation effect there is a certain range of magnetic field intensities where an enhanced heat transfer is observed. Estimates of the local isotropy properties of the flow by a four-element temperature probe demonstrate that the increase in convective heat transport is accompanied by the formation of strong non-isotropic time-dependent flow in the form of large-scale convective rolls aligned with the magnetic field which exhibit a simpler temporal structure compared to ordinary hydrodynamic flow and which are very effective for the convective heat transport.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
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  • 6
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    Magnetohydrodynamic convection in a vertical slot with horizontal magnetic field (2003)
    Cambridge University Press
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    Publication Date: 2003-01-25
    Description: This article presents an experimental study of magnetohydrodynamic convection in a tall vertical slot under the influence of a horizontal magnetic field. The test fluid is an eutectic sodium potassium Na22 K78 alloy with a small Prandtl number of Pr ≈ 0:02. The experimental setup covers Rayleigh numbers in the range 103 ≲ Ra ≲ 8 × 104 and Hartmann numbers 0 〈 M 〈 1600. The effect of the magnetic field on the convective heat transport is determined not only by damping as expected from Joule dissipation but also, for magnetic fields not too strong, the convective heat transfer may be considerably enhanced compared to ordinary hydrodynamic (OHD) flow. Estimates of the isotropy properties of the flow by a four-element temperature probe demonstrate that the increase in convective heat transport accompanies the formation of strong local anisotropy of the turbulent eddies in the sense of an alignment of the main direction of vorticity with the magnetic field. The reduced three-dimensional nonlinearities in non-isotropic flow favour the formation of largescale vortex structures compared to OHD flow, which are more effective for convective heat transport. Along with the formation of quasi-two-dimensional vortex structures, temperature fluctuations may be considerably enhanced in a magnetic field that is not too strong. However, above Hartmann numbers M ≳ 400 the formerly strongly time-dependent flow suddenly becomes stationary with an extended region of high convective heat transport at stationary flow. Finally, for very high Hartmann numbers the convective motion is strongly suppressed and the heat transport is reduced to a state close to pure heat conduction.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
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  • 7
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    High-resolution evidence from southern China of an early holocene optimum and a mid-Holocene dry event during the Past 18,000 Years (2004)
    Cambridge University Press
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    Publication Date: 2004-07-01
    Description: Computer models suggest that the Holocene Optimum for East Asian summer monsoon precipitation occurred at different times in different regions of China. Previous studies indicate that this time-transgressive Holocene Optimum should have been experienced about 3000 yr ago in southern China. In this study we describe a section which allows us to test this timing directly. We have closely examined high-resolution eutrophic peat/mud sequences covering the past 18,000 cal yr at Dahu, Jiangxi, on the southern boundary of the mid subtropical zone in China. Late Pleistocene successions in the Dahu record indicate cooler and much wetter conditions relative to synchronous events in north-central China. Our results indicate that the Holocene Optimum occurred between ca. 10,000 and 6000 cal yr ago in southern China, consistent with the global pattern. Conditions were relatively dry and cold from 6000 to 4000 cal yr ago. Our data also support the conclusion that the last deglaciation to early Holocene in the south was much wetter, resulting in the formation of dense broad-leaved forests, which could have acted to moderate land temperature ∼10,000 to 6000 cal yr ago, yielding a stable early-Holocene climate. After 6000 cal yr, forest reduction led to unstable land temperatures, and possibly to a northerly shift of the subtropical high-pressure system. Whatever the mechanism, these changes resulted in decreased precipitation between 6000 and 4000 cal yr B.P. in southern China.
    Print ISSN: 0033-5894
    Electronic ISSN: 1096-0287
    Topics: Geography , Geosciences
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  • 8
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    Stepped-Combustion 14C Dating of Bomb Carbon in Lake Sediment (2004)
    Cambridge University Press
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    Publication Date: 2004-01-01
    Description: In this study, we applied a stepped-combustion approach to dating post-bomb lake sediment from north-central Mississippi. Samples were combusted at a low temperature (400 °) and then at 900 °. The CO2 was collected separately for both combustions and analyzed. The goal of this work was to develop a methodology to improve the accuracy of 14C dating of sediment by combusting at a lower temperature and reducing the amount of reworked carbon bound to clay minerals in the sample material. The 14C fraction modern results for the low and high temperature fractions of these sediments were compared with well-defined 137Cs determinations made on sediment taken from the same cores. Comparison of “bomb curves” for 14C and 137Cs indicate that low temperature combustion of sediment improved the accuracy of 14C dating of the sediment. However, fraction modern results for the low temperature fractions were depressed compared to atmospheric values for the same time frame, possibly the result of carbon mixing and the low sedimentation rate in the lake system.
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
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  • 9
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    Marine04 Marine Radiocarbon Age Calibration, 0–26 Cal Kyr Bp (2004)
    Cambridge University Press
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    Publication Date: 2004-01-01
    Description: New radiocarbon calibration curves, IntCal04 and Marine04, have been constructed and internationally ratified to replace the terrestrial and marine components of IntCal98. The new calibration data sets extend an additional 2000 yr, from 0–26 cal kyr BP (Before Present, 0 cal BP = AD 1950), and provide much higher resolution, greater precision, and more detailed structure than IntCal98. For the Marine04 curve, dendrochronologically-dated tree-ring samples, converted with a box diffusion model to marine mixed-layer ages, cover the period from 0–10.5 cal kyr BP. Beyond 10.5 cal kyr BP, high-resolution marine data become available from foraminifera in varved sediments and U/Th-dated corals. The marine records are corrected with site-specific 14C reservoir age information to provide a single global marine mixed-layer calibration from 10.5–26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve (Buck and Blackwell, this issue). The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed here. The tree-ring data sets, sources of uncertainty, and regional offsets are presented in detail in a companion paper by Reimer et al. (this issue).
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
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  • 10
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    Radiocarbon in Seawater at Radioactive Waste Dumping Sites in the Northeast Atlantic and Northwest Pacific (2001)
    Cambridge University Press
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    Publication Date: 2001-01-01
    Description: Radiocarbon has been an important constituent of radioactive wastes dumped in the open ocean and marginal seas as well as wastes released from nuclear reprocessing plants. Therefore, in some regions these sources could have a greater impact on 14C concentration in seawater than global fallout. The high analytical sensitivity of 14C measurement by accelerator mass spectrometry (AMS) allows even tracer amounts of 14C to be found in seawater at radioactive waste dumping sites and their comparison with the global distribution of this radionuclide. Data on measurements of 14C in samples taken at former dumping sites in the northeast Atlantic and northwest Pacific Oceans and in the open ocean are discussed and compared with other anthropogenic radionuclides, namely 3H. Small increases in 14C concentrations observed in some bottom and surface seawater samples collected at the Northwest Pacific Ocean dumping sites require further 14C analyses before final conclusions can be made on possible leakages from dumped radioactive wastes.
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
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