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401.
402.
Doreen M. C. Walker 《Geophysical Journal International》1981,67(1):1-18
Summary. The Upper Mesozoic section from Northern Tunisia provided an Upper Jurassic palaeomagnetic pole of 65.2°S 20.3°E α95 = 6.1 calculated from the means of normal and reversely magnetized samples from the uppermost Callovian, Oxfordian, Kimmeridgian and Portlandian rocks. In general the only Cretaceous rocks to yield acceptable results were the few samples collected from fresh outcrops.
A polarity sequence can be established for the Upper Jurassic which can be correlated with the oceanic Keathley anomaly sequence. One consequence of the proposed correlation of the oceanic anomaly with the terrestrial palaeomagnetic sequence is to suggest a slightly different age for the Oxfordian-Kimmeridgian boundary. One interpretation of the frequent intermediate directions of magnetization in the Cretaceous sequence is that there may be a number of unrecognized short period reversals within the Cretaceous and, more particularly, during the so-called Cretaceous normal period. 相似文献
A polarity sequence can be established for the Upper Jurassic which can be correlated with the oceanic Keathley anomaly sequence. One consequence of the proposed correlation of the oceanic anomaly with the terrestrial palaeomagnetic sequence is to suggest a slightly different age for the Oxfordian-Kimmeridgian boundary. One interpretation of the frequent intermediate directions of magnetization in the Cretaceous sequence is that there may be a number of unrecognized short period reversals within the Cretaceous and, more particularly, during the so-called Cretaceous normal period. 相似文献
403.
A. B. C. Walker Jr. C. E. Deforest Richard B. Hoover Troy W. Barbee Jr. 《Solar physics》1993,148(2):239-252
Normal incidence multilayer coated EUV/XUV optical systems provide a powerful technique for the study of the structure of the solar corona. Such systems permit the imaging of the full solar disk and corona with high angular resolution in narrow wavelength bands that are dominated by a single line or a line multiplet excited over a well defined range of temperatures. We have photometrically analysed, and derived temperature and density information from, images of polar plumes obtained with a multilayer Cassegrain telescope operating in the wavelength interval = 171 to 175 , which is dominated by FeIX and FeX emission. This observation was obtained in October 1987, and is the first high resolution observation of an astronomical object obtained with normal incidence multilayer optics techniques. We find that photometric data taken from this observation, applied to a simple, semi-empirical model of supersonic solar wind flow, are consistent with the idea that polar plumes are a source of the solar wind. However, we are not able to uniquely trace high speed streams to polar plumes. The temperatures that we observed are typically 1 500 000 K for both the plumes and the interplume regions, with the plume temperatures slightly higher than those of the surrounding atmosphere. Typical electron densities of the plume and interplume regions, respectively, are 5 × 109 cm–3 and 1 × 108 cm–3 at the limb of the Sun. 相似文献
404.
R. J. Walker L. M. Echeverria S. B. Shirey M. F. Horan 《Contributions to Mineralogy and Petrology》1991,107(2):150-162
The Re — Os isotopic systematics of komatiites and spatially associated basalts from Gorgona Island, Colombia, indicate that they were produced at 155±43 Ma. Subsequent episodes of volcanism produced basalts at 88.1±3.8 Ma and picritic and basaltic lavas at ca. 58 Ma. The age for the ultramafic rocks is important because it coincides with the late-Jurassic, early-Cretaceous disassembly of Pangea, when the North- and South-American plates began to pull apart. Deep-seated mantle upwelling possibly precipitated the break-up of these continental plates and caused a tear in the subducting slab west of Gorgona, providing a rare, late-Phanerozoic conduit for the komatiitic melts.Mantle sources for the komatiites were heterogeneous with respect to Os and Pb isotopic compositions, but had homogeneous Nd isotopic compositions (Nd+9±1). Initial 187Os/186Os normalized to carbonaceous chondrites at 155 Ma (Os) ranged from 0 to +22, and model-initial values ranged from 8.17 to 8.39. The excess radiogenic Os, compared with an assumed bulk-mantle evolution similar to carbonaceous chondrites, was likely produced in portions of the mantle with long-term elevated Re concentrations. The Os, Pb and Nd isotopic compositions, together with major-element constraints, suggest that the sources of the komatiites were enriched more than 1 Ga ago by low (<20%) and variable amounts of a basalt or komatiite component. This component was added as either subducted oceanic crust or melt derived from greater depths in the mantle. These results suggest that the Re — Os isotope system may be a highly sensitive indicator of the presence of ancient subducted oceanic crust in mantle-source regions. 相似文献
405.
The Earth's gravitational potential is usually expressed as an infinite series of tesseral harmonics, and it is possible to evaluate “lumped harmonics” of a particular order m by analyses of resonant satellite orbits—orbits with tracks over the Earth that repeat after m revolutions. In this paper we review results on 30th-order harmonics from analyses of 15th-order resonance, and results on 29th- and 31st-order harmonics from 29:2 and 31:2 resonance.The values available for 30th-order lumped harmonics of even degree are numerous enough to allow a solution for individual coefficients of degree up to 40. The best-determined coefficients are those of degree 30, namely The standard deviations here are equivalent to 1 cm in geoid height.For the 29th- and 31st-order harmonics, and for the 30th-order harmonics of odd degree, there are not enough values to determine individual coefficients, but the lumped values from particular satellites can be used for “resonance testing” of gravity field models, particularly the Goddard Earth Model 10B (up to degree 36) and 10C (for degree greater than 36). The results of applying these tests are mixed. GEM 10B/C emerges well for order 30, with s.d. about 3×10?9; for order 31, the GEM 10B values are probably good but the GEM 10C values are probably not; for order 29, the test is indecisive. 相似文献
406.
The magnetoconvection problem under the magnetostrophic approximation is investigated as the nonlinear regime is entered. The model consists of a fluid filled sphere, internally heated, and rapidly rotating in the presence of a prescribed, axisymmetric, toroidal magnetic field. For simplicity only a dipole parity and a single azimuthal wavenumber (m = 2) is considered here. The leading order nonlinearity at small amplitude is the geostrophic flow U
g
which is introduced to the previously linear model (Walker and Barenghi, 1997a, b). Walker and Barenghi (1997c) considered parameter space above critical and found that U
g
acts as an equilibration mechanism for moderately supercritical solutions. However, for solutions well above critical a Taylor state is approached and the system can no longer equilibrate. More importantly though, in the context of this paper, is that subcritical solutions were found.
Here subcritical solutions are considered in more detail. It was found that, at
is strongly dependent on . (
is the critical value of the modified Rayleigh number
is a measure of the maximum amplitude of the generated geostrophic flow while , the Elsasser number, defines the strength of the prescribed toroidal field.) Rm at
proves to be the key measure in determining how far into the subcritical regime the system can advance. 相似文献
407.
P. J. Henderson I. McMartin G. E. Hall J. B. Percival D. A. Walker 《Environmental Geology》1998,34(1):39-58
Trace element geochemistry of humus (<0.425 mm) and till (<0.002 mm) collected in the Flin Flon-Snow Lake area, northern
Manitoba and Saskatchewan, provides a regional context for assessing smelter contamination in the environment. The area includes
a Cu-Zn smelter known to discharge As, Cd, Cu, Fe, Hg, Pb, and Zn. In this study, sequential extraction analyses, scanning
electron microscopy and x-ray diffraction analyses were used on a suite of samples to determine: (1) the chemical and physical
characteristics of heavy metals in surficial sediments related to distance from the smelter, (2) criteria for assessing the
relative contribution of these metals from natural and anthropogenic sources, and (3) the potential of these metals for remobilization
in the environment.
Humus geochemistry reflects the anthropogenic and natural component of heavy metal concentrations. Smelter-related elements
show anomalously high values adjacent to the smelter, decreasing with distance until background values are reached at 70–104 km,
depending on the element. In humus, Zn is associated primarily with labile phases; Hg with non-labile phases. Adjacent to
the smelter, high proportions and concentrations of Zn and Hg in non-labile phases, indicative of smelter-derived particulates,
are confirmed by SEM examination. The particles occur as spheres, irregular grains, and with organics. With increasing distance
from the smelter, the geochemical response to bedrock composition is more obvious than the anthropogenic input.
Till geochemistry reflects the natural variation imposed by bedrock composition. At highly contaminated sites (<3 km from
the smelter), increased percentages of smelter-related elements in labile phases suggests heavy metals are leached from humus
to the underlying sediment.
Received: 5 November 1996 · Accepted: 31 March 1997 相似文献
408.
Sediments from Tugulnuit Lake in the Okanagan Valley of British Columbia, Canada, were examined for chironomid assemblages. The chironomid stratigraphy obtained encompasses the last 4000 to 5000 years and suggests a warm and fairly stable climate typical for a temperate lake at low- to mid-elevation. This is indicated by the even distribution of warm-water taxa, such as Cladopelma, Dicrotendipes, Polypedilum, Pentaneurini, Stempellina, Stempellinella/Zavrelia and Pseudochironomus throughout the core. Very few cold-water taxa occurred in the sediments. However, stream inputs have had a major impact on Tugulnuit Lake. Sandy sediments and the appearance of Simuliidae and stream-inhabiting chironomid taxa (e.g., Brillia/Euryhapsis, Eukiefferiella/Tvetenia, Rheocricotopus) indicate that a stream intruded into the current lake's basin ca. 3800 yr Before Present (BP). Sediments deposited prior to, and after, the stream's intrusion show a distinctly different chironomid assemblage exhibiting chironomid taxa more typical for lentic habitats. This result indicates that chironomids can serve to detect past stream influences on lake environments. Thus, rheophilic chironomids preserved in lake cores provide a new alternative for reconstructing stream palaeoenvironmental records. 相似文献
409.
Gavin Ramsay David A. H. Buckley & Mark Cropper 《Monthly notices of the Royal Astronomical Society》1998,294(1):187-192
We present optical and X-ray data of the cataclysmic variable RX 0744−52 discovered using ROSAT by Motch et al. High-resolution spectroscopy centred on the Hα line indicates a probable orbital period of 3.60 h. From its distance (obtained using polarimetry), its X-ray luminosity, its X-ray colour and its X-ray/UV+optical ratio, we suggest that RX 0744−52 is a new intermediate polar. The absence of a significant coherent modulation in the X-ray light curve suggests either that RX 0744−52 has a low inclination or that the rotational and magnetic axes must be closely aligned. This is consistent with its small radial velocity amplitude. 相似文献
410.
Gavin Ramsay Mark Cropper Coel Hellier & Kinwah Wu 《Monthly notices of the Royal Astronomical Society》1998,297(4):1269-1273
We determine the mass of the white dwarf in the eclipsing intermediate polar XY Ari following the method given in Cropper, Ramsay &38; Wu using a multitemperature bremsstrahlung model. By fitting X-ray spectra from Ginga RXTE and ASCA we find that the mean of the best fits to the data taken using different detectors is M wd = 1.28 ± 0.04 M⊙. This figure is too high to be consistent with the mass of the white dwarf found by Hellier from X-ray eclipse timings. There are also small systematic differences between the masses derived using different X-ray satellites. 相似文献