Large aspect ratio cells in two-dimensional thermal convection     

J.M. Hewitt  D.P. McKenzie  N.O. Weiss 《Earth and Planetary Science Letters》1980,51(2):370-380

Numerical experiments have been carried out on two-dimensional thermal convection, in a Boussinesq fluid with infinite Prandtl number, at high Rayleigh numbers. With stress free boundary conditions and fixed heat flux on upper and lower boundaries, convection cells develop with aspect ratios (width/depth) λ? 5, if heat is supplied either entirely from within or entirely from below the fluid layer. The preferred aspect ratio is affected by the lateral boundary conditions. If the temperature, rather than the heat flux, is fixed on the upper boundary the cells haveλ ≈ 1. At Rayleigh numbers of 2.4 × 10⁵ and greater, small sinking sheets are superimposed on the large aspect ratio cells, though they do not disrupt the circulation. Similar two-scale flows have been proposed for convection in the earth's mantle. The existence of two scales of flow in two-dimensional numerical experiments when the viscosity is constant will allow a variety of geophysically important effects to be investigated.  相似文献   

Al-rich calcic pyroxene in pseudotachylite: An indicator of high pressure and high temperature?     

H.R. Wenk  L.E. Weiss 《Tectonophysics》1982,84(2-4)

An aluminous pyroxene with an approximate formula (Ca_0.60Na_0.05□_0.20) (Mg_0.58Fe_0.11²⁺ Al_0.60^VI)[Si_1.68A1_0.32O₆] or {Jd_0.05Ts_0.32Es_0.20Wo_0.10En_0.27Fs_0.06} has been found in a pseudotachylite vein from gabbros in the Musgrave ranges, Central Australia. Textural evidence indicates that it formed by breakdown of calcic plagioclase, augite, and olivine which constitute the isochemical country rock. There is a large amount of M(2) vacancy compared to most pyroxenes. The vein mineral is most similar in composition to natural pyroxenes in kimberlites and eclogites and especially to synthetic pyroxenes which formed at pressures in excess of 3 GPa (= 30 kb). We suggest that these high pressures were not lithostatic but represent dynamic stresses released during catastrophic brittle rupture either at tips of propagating fractures or spallation of unsupported crackwalls. Such transient shear stresses may reach the shock range. We propose such a fracture mechanism as an alternative to frictional melting for the productoin of pseudotachylite veins.  相似文献   

Depth profiles of chlorofluoromethanes in the Norwegian sea     

A. Hahne  A. Volz  D. H. Ehhalt  H. Cosatto  W. Roether  W. Weiss  B. Kromer 《Pure and Applied Geophysics》1978,116(2-3):575-582

Depth profiles of the chlorofluoromethanes (CFM), CFCl₃ and CF₂Cl₂, have been obtained together with tritium profiles from water samples collected in the Norwegian Sea between surface and 2800 m depth. CFM analysis was performed by vacuum extraction of the dissolved gases from 500 ml samples of seawater and subsequent gaschromatographic measurement. The CFM concentration decreases with depth to about 10 percent of surface concentration at depths below 2000 m. The same behaviour is found for the tritium content. From a correlation of the CFM and tritium concentration the upper limit of the preindustrial atmospheric CFM levels can be estimated to 5 percent of the present day concentrations.  相似文献   

Characterization of sediments in an abandoned mining area; a case study of Mansfeld region,Germany   总被引：1，自引：0，他引：1  

Rainer?WennrichEmail author  Jürgen?Mattusch  Peter?Morgenstern  Klaus?Freyer  Hanns-Christian?Treutler  Hans-Joachim?St?rk  Lutz?Brüggemann  Albrecht?Paschke  Birgit?Daus  Holger?Weiss 《Environmental Geology》2004,45(6):818-833

The distribution of selected heavy metals, including some radionuclides, metalloids and non-metals was determined in stream sediments in a region influenced by abandoned copper mining and ore processing activities. A considerable amount of the ore processing waste with a very complex composition and highly elevated concentrations of zinc, sulfur, lead, copper, arsenic, and a lot of other elements in the range between 100 and 1,000 mg/kg (Sb, Mn, Ni, Cr, Cd, Hg, and Ag) was piled up on mine dumps. The dispersion of the pollutants originating from this source and their environmental impact were investigated. Both, sediments and original waste material were studied to indicate the pathways and the mobilization behavior of different pollutants. For this purpose, the process of the elution of pollutants by application of different fractionation schemes was studied. The capabilities of different analytical techniques are shown for the analysis of solid samples (X-ray fluorescence spectrometry, Gamma-spectrometry) and liquid ones (ICP-atomic emission spectrometry, ICP-mass spectrometry and different techniques of atomic absorption). Additionally, the coupling of ion chromatography and ICP-MS detection was used to study the distribution of arsenic species in the sediment cores of a lake which acts as a natural sink for the region.  相似文献   

Argon and CO₂ on the race track in silicate melts: A tool for the development of a CO₂ speciation and diffusion model     

Marcus Nowak  Dominik Schreen 《Geochimica et cosmochimica acta》2004,68(24):5127-5138

We have analysed the kinetics of Argon and CO₂ diffusion in simplified iron free rhyolitic to hawaiitic melts using the diffusion couple technique. The concentration distance profiles of Ar and CO₂ were measured with electron microprobe analysis and Fourier Transform Infrared Spectroscopy, respectively. Error functions were fitted to the symmetrical concentration distance profiles to extract the diffusion coefficients.In the temperature range 1373 to 1773 K the activation energies for Ar diffusion range from 169 ± 20 to 257 ± 62 kJ mol⁻¹. Ar diffusivity increases exponentially with the degree of depolymerisation. In contrast, the mobility of total CO₂, that is identical to Ar mobility in rhyolitic melt, keeps constant with changing bulk composition from rhyolite to hawaiite. CO₂ speciation at 1623 K and 500 MPa was modeled for the range of compositions studied using the diffusion data of Ar and total CO₂ in combination with network former diffusion calculated from viscosity data. Within error this model is in excellent agreement with CO₂ speciation data extrapolated from temperatures near the glass transition temperature for dacitic melt composition. This model shows that even in highly depolymerised hawaiitic and tholeiitic melts molecular CO₂ is a stable species and contributes 70 to 80% to the total CO₂ diffusion, respectively.  相似文献   

Magnetohydrodynamics of stellar interiors     

David Hughes  Robert Rosner  Nigel Weiss 《Astronomy& Geophysics》2005,46(4):4.35-4.36

David Hughes, Robert Rosner and Nigel Weiss describe what was achieved during a programme on stellar magnetic fields at the Isaac Newton Institute in Cambridge. Over a four-month period more than 90 participants visited the Institute for a mixture of structured workshops and informal collaboration.  相似文献   

Efficient and Responsible Use of Prior Information in Inverse Methods     

Richard Weiss  Leslie Smith 《Ground water》1998,36(1):151-163

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Flux expulsion by inhomogeneous turbulence     

L. Tao  M. R. E. Proctor  & N. O. Weiss 《Monthly notices of the Royal Astronomical Society》1998,300(3):907-914

Flux expulsion is an important consequence of the interaction of magnetic fields with fluid convection and has been well studied for particular cases of steady, single-cell flows. Here we examine a related phenomenon in inhomogeneous turbulence using direct numerical simulations. To understand our numerical results, we analyse average properties of our model, and obtain mean transport coefficients which can be used to describe the approach of the system to its final state. For the kinematic problem these transport coefficients give an excellent prediction of the expulsion process; however, the enhanced transport is suppressed by dynamical back-reaction of the Lorentz force. Finally, we discuss the astrophysical implications for magnetic fields in stellar convection zones. Segregation of magnetic fields from turbulent motion not only allows strong toroidal fields to accumulate in regions of convective overshoot but also permits significant poloidal fields to be maintained by dynamo action in stars like the Sun.  相似文献   

Transport of the environmental radionuclides in an alpine watershed     

J. Dominik  D. Burrus  J.-P. Vernet 《Earth and Planetary Science Letters》1987,84(2-3)

The radionuclides¹³⁷Cs,²¹⁰Pb and⁷Be have been examined in the alpine Rhoˆne watershed (Switzerland) during a period of two years in order to evaluate their usefulness as tracers of the removal and transport rate of top-soil particles and particle-reactive contaminants of atmospheric origin. The specific activities of the radionuclides in fluvial suspension show a distinct seasonal pattern which depends on the hydrologic regime of the stream and the sources of the suspended matter. Input-output budgets based on the atmospheric deposition and fluvial removal of three radionuclides in the alpine Rhoˆne watershed are used to estimate their erosional residence times. The simplest one box model yields mean residence times of about 800 and 1400 years for¹³⁷Cs and²¹⁰Pb, respectively. The removal rate of short-lived⁷Be suggests that a part of the watershed (0.6–2.3% of the total surface) is exposed to a rapid erosion, in which the mean residence time of the radionuclides is in the range of 1–220 days. This has little influence on the calculated residence time of¹³⁷Cs but increases the estimated residence time of²¹⁰Pb in soil to over 1800 years. The use of²¹⁰Pb-⁷Be pair in fluvial output may be very helpful in the assessment of the impact of atmospheric pollutants on the water quality in rivers and lakes.  相似文献   

Simulation of Ground-Water Flow in a Mined Watershed in Eastern Ohioa     

Jonathan S. Weiss  Allan C. Razem 《Ground water》1984,22(5):549-560

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