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301.
Morris RV Golden DC Bell JF rd Lauer HV Adams JB 《Geochimica et cosmochimica acta》1993,57(19):4597-4609
We have examined a Hawaiian palagonitic tephra sample (PN-9) that has spectroscopic similarities to Martian bright regions using a number of analytical techniques, including Mossbauer and reflectance spectroscopy, X-ray diffraction, instrumental neutron activation analysis, electron probe microanalysis, transmission electron microscopy, and dithionite-citrate-bicarbonate extraction. Chemically, PN-9 has a Hawaiitic composition with alkali (and presumably silica) loss resulting from leaching by meteoric water during palagonitization; no Ce anomaly is present in the REE pattern. Mineralogically, our results show that nanophase ferric oxide (np-Ox) particles (either nanophase hematite (np-Hm) or a mixture of ferrihydrite and np-Hm) are responsible for the distinctive ferric doublet and visible-wavelength ferric absorption edge observed in Mossbauer and reflectivity spectra, respectively, for this and other spectrally similar palagonitic samples. The np-Ox particles appear to be imbedded in a hydrated aluminosilicate matrix material; no evidence was found for phyllosilicates. Other iron-bearing phases observed are titanomagnetite, which accounts for the magnetic nature of the sample; olivine; pyroxene; and glass. By analogy, np-Ox is likely the primary pigmenting agent of the bright soils and dust of Mars. 相似文献
302.
"This paper describes an emerging field of demographic enquiry focused on the population mobility of indigenous minorities in Australasia and North America. Political, scientific and policy rationales for research on the spatial dynamics of such groups are advanced as a prelude to a review of the Australian literature. This review is structured around four complementary perspectives on mobility: the propensity to move; spatial redistribution; flows and networks; and migration careers. Comparison of the relative strengths of mobility research on the indigenous and non-indigenous populations against these four perspectives assists in identifying outstanding research needs and priorities. Commonalities in the patterns and processes of indigenous mobility in Australia, New Zealand, Canada and the U.S. suggest the value of more detailed cross-national comparisons, and also provide a basis for contemplating an indigenous variant of Zelinsky's hypothesised mobility transition." 相似文献
303.
304.
Benthic resource utilization by, red drum (Sciaenops ocellatus) and spotted seatrout (Cynoscion nebulosus) was studied in a restored, mangrove-rimmed impoundment (Cabbagehead Bayou) of Upper Tampa Bay, Florida, and in a nearby, natural site of unaltered tidal regime (Double Branch Bay). Diets of fish captured from August 1990 to May 1992 were determined from stomach content analysis. Simultaneously, food availability was evaluated by sampling benthic macroinvertebrates, mobile decapods, and small fish. Red drum and spotted seatrout utilized the restored habitat 1 yr after it was opened to tidal influence. Both species also were collected in the natural mangrove. Although there were noted differences in benthic assemblages between the two sites, red drum and spotted seatrout exhibited flexibility in diet, feeding on abundant and accessible prey. The high abundance of microcrustacea, such as amphipods, on detritus accumulated in the restored habitat constituted a main food resource for both fish species. Major food items in the diet of small (<200 mm) red drum were amphipods, mysids, and nereid and arenicolid polychaetes. Large (200–590 mm) red drum fed on polychaetes, xanthid crabs, palaemonid shrimp, and small fishes. Spotted seatrout preyed primarily upon mysids, shrimp, and small fishes, and to a lesser extent, upon a nereid polychaete. Our findings on fish feeding in a restored mangrove impoundment indicated that the detrital-associated benthic community is utilized by reinvading fish within a short time period, suggesting that not only habitat but food resources were augmented by the reopening of this wetland. 相似文献
305.
306.
R. N. Ogley S. J. Bell Burnell R. P. Fender G. G. Pooley E. B. Waltman M. van der Klis 《New Astronomy Reviews》1998,42(9-10)
Multi-wavelength photometric observations of Cygnus X-3 were carried out at 18 cm through to 450 μm, complemented by X-ray (2–10 keV) observations. The system was mildly active with cm fluxes at 150–250 mJy. We find the spectrum to be flat with a spectral index of zero. Using a modified Wolf-Rayet wind model, and assuming emission is generated in synchrotron emitting jets from the source, we find an upper-limit to the magnetic field of 20 G at a distance 5×1012 cm is required. 相似文献
307.
Paul W. Layer Margarita Lopez-Martinez Alfred Kröner Derek York & Michael McWilliams 《Geophysical Journal International》1998,135(1):129-145
The pluton has a well-defined palaeomagnetic pole which is dated at 3179±18 (2 σ ) Ma by
308.
309.
J. Fleming A. Hibbert K. L. Bell & N. Vaeck 《Monthly notices of the Royal Astronomical Society》1998,300(3):767-772
The superposition of configurations (SOC) method has been used to calculate f -values for the Mg ii doublet at 1240 Å, which has been observed in the interstellar medium near stars such as ζ Ophiuchi. Our best value for the multiplet oscillator strength is 0.00083. SOC calculations have also been undertaken for the stronger (3s−3p) doublet at 2800 Å giving a multiplet f -value of 0.92. 相似文献
310.
Alfred Gautschy Hideyuki Saio & Housi Harzenmoser 《Monthly notices of the Royal Astronomical Society》1998,301(1):31-41
Rapidly oscillating Ap stars constitute a unique class of pulsators with which to study non-radial oscillations under some — even for stars — unusual physical conditions. These stars are chemically peculiar, they have strong magnetic fields and they often pulsate in several high-order acoustic modes simultaneously. We discuss here an excitation mechanism for short-period oscillation modes based on the classical κ mechanism. We particularly stress the conditions that must be fulfilled for successful driving. Specifically, we discuss the roles of the chemical peculiarity and strong magnetic field on the oscillation modes and what separates these pulsators from δ Scuti and Am-type stars. 相似文献