Cambrian enigmas     

SIMON CONWAY MORRIS 《Geology Today》1987,3(3-4):88-92

Bizarre soft-bodied animals from the Cambrian, principally the Burgess Shale of British Columbia, are throwing new light on the major diversification of early metazoans. A distinctive range of new body-plans hint at explosive rates of evolution, but the underlying mechanisms are still a matter for conjecture. Whether these unusual fossils suffered extinction because of bad design or bad luck is uncertain, but some evidence suggests that chance factors played an important role.  相似文献   

A linear scattering problem in magnetohydrodynamics: Transmission resonances in a magnetic slab     

John A. Adam 《Astrophysics and Space Science》1987,133(2):317-337

The reduced linearized equations of ideal magnetohydrodynamics which are highly nonlinear in the eigenvalue parameter, are linearized about a prescribed value of that parameter, enabling the equation to be expressed as a Schrödinger equation with piecewise uniform coefficients. Reflection and transmission coefficients are obtained using standard techniques, and in addition to the possibility of total transmission of an incident wave occurring (together with complex-valued resonance energies), the magnetic field introduces other total transmission energy levels which have no counterpart in the absence of a magnetic field.  相似文献   

Simultaneous inversion for 3-D variations in shear and bulk velocity in the mantle   总被引：7，自引：0，他引：7  

Wei-jia Su  Adam M. Dziewonski 《Physics of the Earth and Planetary Interiors》1997,100(1-4):135-156

Until recently, most of the seismic tomographic modeling has been addressing the question of lateral heterogeneity either in P- or S-wave velocities. The S-wave velocity variations are larger and hence provide stronger signal on long-period waveforms. The direct P travel times, being the first arrivals, on the other hand, are most frequently reported in the International Seismological Centre (ISC) Bulletins. In mineral physics experiments, the variation in bulk velocity is more often measured. To better understand the differences between δv_P and δv_P patterns and better link the results from mineral physics to those of seismic tomography, we formulate the inverse problem in terms of relative perturbations in the shear velocity v_S = (μ/)^1/2 and bulk sound velocity v_Φ = (K/)^1/2. We use a large data set which consists of waveforms, waveform-derived travel times and travel times from the ISC Bulletins. The earthquakes are relocated using corrections for lateral heterogeneity. The events which cannot be reliably determined are discarded. The model is defined as spherical harmonics to Degree 12 horizontally and as Chebyshev polynomials to order 13 radially, for both shear and bulk sound velocity. The inversion is performed under smoothness constraint. The resolution tests and bootstrapping analysis indicate that the model is well recovered, particularly at long wavelength.

The results indicate a much larger variability of shear than bulk sound velocity. The model explains observations well. The most intriguing result obtained in this study is that the variations in shear velocity and bulk sound velocity are negatively correlated in the lowermost mantle. The explanation is not very clear. From the mineral physics point of view, it is not unlikely that this could be explained in terms of thermal variation, even though we are unwilling to rule out the possibility of large wavelength compositional variations.  相似文献   

A note on σ-stability in hydromagnetics     

J. A. Adam 《Astrophysics and Space Science》1982,82(1):115-121

The concept of -stability is reviewed and applied to the case of aligned magnetoatmospheric flow. A sufficient condition for -stability is derived, and the upper bound arising from this condition is investigated as a function of .  相似文献   

Comparison of wind-stress algorithms and their influence on wind-stress curl using buoy measurements over the shelf off Bodega Bay, California     

Adam Kochanski  Darko Kora in  Clive E. Dorman 《Deep Sea Research Part II: Topical Studies in Oceanography》2006,53(25-26):2865

The main objectives of this study were to compare three wind-stress algorithms of varying intricacy and estimate the extent to which each method altered computed wind-stress curl. The algorithms included (1) a simple bulk formula for neutral conditions that is dependent only on wind velocity components; (2) a formula that in addition to dependence on wind components includes a simplified effect of thermal stability through differences in air and sea temperatures; and (3) an algorithm that includes full treatment of dynamics and atmospheric stability. Data for the analysis were from a field program that used a special buoy network off Bodega Bay during 28 June–4 August 2001.A diamond-shaped setup of five closely separated buoys in Bodega Bay allowed for one of the first attempts to compute wind-stress curl over the ocean using buoy measurements. Based on an analysis of the available dataset, the marine layer over Bodega Bay is characterized by positive wind-stress curl with a median value around 0.2 Pa (100 km)⁻¹ and maximum values reaching 2.5 Pa (100 km)⁻¹. Positive wind-stress curl was observed for all wind speed conditions, whereas negative wind-stress curl episodes were associated mostly with low-wind conditions.Comparison of wind-stress curl computed using the three algorithms showed that differences among them can be significant. The first and third algorithms indicated similar stress curl (difference around 10%), but the differences between these two and the second algorithm were much higher (approximately 40%). The reason for the difference is the stability correction, which in the third algorithm strongly decreases with an increase in wind speeds, but stays at a similar level for all wind speeds in the second algorithm. Consequently, for higher wind speeds the variability of wind stress calculated using the second algorithm is greater than for the other two algorithms, causing significant differences in computed wind-stress curl (root mean-square error equal to 0.19 Pa (100 km)⁻¹).Despite the apparent biases in computed wind stress and wind-stress curl among the algorithms, all of them show a significant trend of decreasing sea-surface temperature (SST) with increasing wind-stress curl. The bootstrapping analysis has revealed that both the along-shore wind stress and wind-stress curl have noticeable correlation with the changes in the sea-surface temperature as an indirect indication of the upwelling. An additional analysis, based on the low-pass filtered data, showed also significant agreement between the measured divergence in the cross-shore surface transport and the wind-stress curl computed for all three algorithms.  相似文献   

湘西沃溪钨锑金矿床黄铁矿中发现Au-Sb矿物相   总被引：1，自引：0，他引：1  

彭渤  Adam  Piestrzynski  刘升友  Jadwiga  Pieczonka 《地质通报》2007,26(5):553-559

利用扫描电镜和电子探针等方法,对沃溪钨锑金矿床中产于(块状白钨矿-石英脉中的)黄铁矿微裂隙中的自然金和Au-Sb矿物相进行了较详细的观察和研究。结果表明,黄铁矿中Au-Sb矿物相除了有前人已经鉴定出的方锑金矿(AuSb2.13)外,还新发现有锑金矿(Au1.02Sb)矿物相。二者以显微颗粒的形式存在于黄铁矿显微裂隙中。黄铁矿微裂隙中Au-Sb矿物相的存在,指示矿床成矿在黄铁矿形成之后存在Au-Sb混溶的成矿流体。自然金、方锑金矿、锑金矿等矿物的沉淀析出可能是Au-Sb混溶流体从高温(高于350℃)向低温(250℃)演变的结果。此外,Au-Sb矿物相的发现对改进矿石冶炼中Au、Sb的分离技术有参考价值。  相似文献   

Magnetite solubility and iron transport in magmatic-hydrothermal environments   总被引：2，自引：0，他引：2  

Adam C. Simon  Thomas Pettke  Philip M. Piccoli 《Geochimica et cosmochimica acta》2004,68(23):4905-4914

We have examined the effect of pressure on the apparent equilibrium constant, K′, for magnetite solubility (Fe₃O₄^mt + 6HCl^fluid + H₂^fluid = 3FeCl₂^fluid + 4H₂O^fluid) and the relative iron-carrying capacities of magmatic vapor and brine by conducting experiments in a rhyolite melt-vapor-brine-magnetite system at 800°C, f_O2 = NNO and pressures ranging from 100 to 145 MPa. Iron concentrations in synthetic vapor and brine fluid inclusions were quantified by using laser-ablation inductively-coupled-plasma-mass-spectrometry (LA-ICPMS). Hydrogen chloride (HCl) concentrations in magmatic vapor were inferred by potentiometric measurements of H⁺ in quenched run product fluids. These data yield calculated values for log K′, assuming a_H2O = X_H2O, of 1.7, 4.9, 6.2, 6.8 and 9.1 at 100, 110, 130, 140 and 145 MPa, respectively. The concentration of iron in magmatic vapor increases by an order of magnitude, whereas the concentration of iron in magmatic brine remains constant (within 1σ) with increasing pressure as the 800°C critical pressure is approached along the vapor-brine solvus. The concentrations of iron in vapor and brine fluid inclusions yield calculated partition coefficients (D_Fe^v/b) of 0.05, 0.14, 0.27 and 0.56 at 110, 130, 140 and 145 MPa, respectively. Our data reveal that pressure fluctuations may significantly affect the value of log K′. More importantly, the data demonstrate conclusively that a significant amount of iron can be transported by a low-density aqueous vapor in the magmatic-hydrothermal environment.  相似文献   

J362D28: a new joint model of compressional and shear velocity in the Earth's mantle     

Michael Antolik  Yu J. Gu  Göran Ekström  Adam M. Dziewonski 《Geophysical Journal International》2003,153(2):443-466

  相似文献   

Paleozoic northward drift of the North Tien Shan (Central Asia) as revealed by Ordovician and Carboniferous paleomagnetism   总被引：7，自引：0，他引：7  

Mikhail L. Bazhenov  Adam Q. Collins  Kirill E. Degtyarev  Natalia M. Levashova  Alexander V. Mikolaichuk  Vladimir E. Pavlov  Rob Van der Voo   《Tectonophysics》2003,366(1-2):113-141

Three new Middle–Late Ordovician and two new Early Carboniferous paleomagnetic poles have been obtained from the North Tien Shan Zone (NTZ) of the Ural–Mongol belt in Kyrgyzstan and Kazakhstan. Paleolatitudes for the Carboniferous are unambiguously northerly and average 15.5°N, whereas the Ordovician paleolatitudes (6°, 9°, and 9°) are inferred to be southerly, given that a very large (180°) rotation of the NTZ would be necessary during the middle Paleozoic if the other polarity option was chosen. Thus, the NTZ drifted northward during much of the Paleozoic; east–west drift cannot be determined, as is well known, from paleomagnetic data. In addition, detailed thermal demagnetization analysis reveals two overprints, one of recent age and the other of Permian age, which is a time of strong deformation in the NTZ. The paleolatitude of the combined Permian overprint is 30.5+2°N. The paleolatitudes collectively track those predicted for the area by extrapolation from Baltica very well, but are different from those of Siberia for Ordovician times. This finding is compatible with Sengör and Natal'in's [Sengör, A.M.C., Natal'in, B.A., 1996. Paleotectonics of Asia: fragments of a synthesis. In: Yin A., Harrison, M. (Eds.), The Tectonic Evolution of Asia. Cambridge Univ. Press, Cambridge, pp. 486–640] model of tectonic evolution of the Ural–Mongol belt and disagrees with the models of other researchers. Declinations of the Ordovician and Early Carboniferous results range from northwesterly to northeasterly, and are clearly affected by local relative rotations, which seem characteristic for the entire NTZ, because the Permian overprint declinations also show such a spread. Apparently, the important latest Paleozoic–Triassic deformation involved shear zone-related rotations as well as folding and significant granitic intrusions.  相似文献   

Evaluation of the AMIP soil moisture simulations     

Alan Robock  C. Adam Schlosser  Konstantin Ya. Vinnikov  Nina A. Speranskaya  Jared K. Entin  Shuang Qiu 《Global and Planetary Change》1998,19(1-4)

The Atmospheric Model Intercomparison Project (AMIP) conducted simulations by 30 different atmospheric general circulation models forced by observed sea surface temperatures for the 10-year period, 1979–1988. These models include a variety of different soil moisture parameterizations which influence their simulations of the entire land surface hydrology, including evaporation, soil moisture, and runoff, and their simulations of the energy balance at the surface. Here we compare these parameterizations, and evaluate their simulations of soil moisture by comparing them with actual observations of soil moisture, literally ground truth. We compared model-generated ‘data sets' and simulations of soil moisture with observations from 150 stations in the former Soviet Union for 1979–1985 and Illinois for 1981–1988. The spatial patterns, mean annual cycles, and interannual variations were compared to plant-available soil moisture in the upper 1 m of soil. The model-generated ‘data sets' are quite different from the observations, and from each other in many regions, even though they use the same bucket model calculation method. The AMIP model simulations are also quite different from each other, especially in the tropics. Models with 15-cm field capacities do not capture the observed large high latitude values of soil moisture. In addition, none of the models properly simulate winter soil moisture variations in high latitudes, keeping soil moisture constant, while observations show that soil moisture varies in the winter as much as in other seasons. The observed interannual variations of soil moisture were not captured by any of the AMIP models. Several models have large soil moisture trends during the first year or two of the AMIP simulations, with potentially large impacts on global hydrological cycle trends and on other climate elements. This is because the simulations were begun without spinning up the soil moisture to the model climatology. The length of time it took for each to reach equilibrium depended on the particular parameterization. Although observed temporal autocorrelation time scales are a few months, some models had much longer time scales than that. In particular, the three parameterizations based on the Simple Biosphere model (SiB) had trends in some regions for virtually the entire AMIP simulation period.  相似文献