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111.
Abstract— Isotopic compositions and abundances of boron were measured in sixteen chondrules from seven chondrites by ion microprobe mass spectrometry. The chondrules are of the porphyritic, barred, and radial type and host meteorites include carbonaceous, ordinary, and enstatite chondrites. Boron abundances are generally low with average boron concentrations of between 80 and 500 ppb. These abundances are lower than those of bulk chondrites (0.35 to 1.2 ppm; Zhai et al., 1996), confirming earlier suggestions that boron is mostly contained in the matrix. No significant variation in the 11B/10B ratio is observed among these chondrules, outside our experimental error limits of several permil, and B‐isotopic compositions agree with those reported for bulk chondrites. The lack of a significant isotope fractionation between chondrules and matrix implies that the low boron abundances are not the result of a Rayleigh fractionation during chondrule formation. Isotopic heterogeneities within individual chondrules are constrained to be < ±20%0 at > 95% confidence level at a spatial scale of 20–30 μm, significantly lower than the value of about ±40%0 previously reported for chondrules from carbonaceous and ordinary chondrites (Chaussidon and Robert, 1995, 1998). The observed B‐isotopic homogeneity does not conflict with the presence of decay products from extinct 10Be, with (10Be/9Be)0 ? 10?3, as was inferred for calcium‐aluminum‐rich inclusions. Extinct 10Be in chondrules would shift the abundance ratio 11B/10B at best by several permil because of their commonly observed low Be/B ratios (<2). The results show that potential B‐isotopic heterogeneities in the solar nebula due to the presence of components with different B‐isotopic signatures, such as boron produced by high‐energy galactic cosmic rays (11B/10B ? 2.5), or by the hypothetical low‐energy particle irradiation (11B/10B ? 3.5–11) or boron from type II supernovae (11B/10B >> 1), did not survive the chondrule formation processes to a measurable extent. 相似文献
112.
Magnesium, potassium and calcium isotope compositions in terrestrial samples and refractory phases from primitive meteorites
are determined using an ion microprobe. A thorough investigation of the different instrument parameters is carried out to
ensure that conditions necessary for high mass resolution and high precision isotopic studies are adequately satisfied. The
instrument can be tuned to achieve mass resolution (M/ΔM) of up to 10,000 (M≤60); it has a very good dynamic stability (ΔB/B≤10
ppm over durations of ≤40 minutes) and the counting system has an effective dead-time of ≤25 nsec and a dynamic background
of ≤0·01 c/s. Reproducibility and precision of isotopic measurements are checked by analyzing magnesium and titanium isotopic
compositions in terrestrial standards and isotopically doped silicate glasses. A precision of 2‰ (2σ
m
) was achieved during magnesium isotopic analysis in samples with low Mg content (200 ppm). Results from studies of magnesium
and potassium isotopic compositions in several Ca−Al-rich refractory inclusions (CAIs) from the primitive meteorites Efremovka
and Grosnaja, representing some of the early solar system objects, are presented. The well-behaved Mg−Al isotopic systematics
confirm the pristine nature of the Efremovka CAIs inferred earlier from petrographic and trace element studies. The Grosnaja
CAIs that have experienced secondary alterations show disturbed magnesium isotopic systematics. Observation of excess26Mg in several of the analyzed CAIs confirms the presence of the now extinct26Al (t
1/2=7×105 years) in the solar nebula at the time of CAI formation. Our data also suggest a relatively uniform distribution of26Al in the solar nebula. Several Efremovka CAIs with excess26Mg also have excess41K resulting from the decay of41Ca (t
1/2≃105 years). This observation constrains the time interval between cessation of nucleosynthetic input to the solar nebula and
the formation of some of the first solar system solids (CAIs) to less than a million years. 相似文献
113.
M. Sankar-Rao Prashant Goswami H. V. Nagaraj Upadhyaya 《Journal of Earth System Science》1991,100(2):127-144
The experiments reported here emphasize the importance of observations in the prediction of tropical cyclones. Towards this
end a symmetric numerical model in which convection, parametrized by the Arakawa-Schubert (AS) scheme, was adopted. A mean
thermodynamical state, which represents the monsoon conditions over the Bay of Bengal, with constant moist static energy for
the mixed layer was adopted. Experiments were then done with different initial conditions.
We found that tropical cyclone development measured by the central pressure was very sensitive to the initial convergence
field. In the present state of satellite technology, it was impossible to predict even a gross parameter like the central
pressure with an accuracy better than 6 mb for 12 hours. However, it was seen that under a variety of initial conditions the
final state characterized by the magnitude of the central low pressure remained practically unaltered. We suggest that, given
the necessary conditions for genesis, the final state of the cyclone acts as an attractor (regarding its central pressure)
and the diverse initial conditions, under the influence of thermodynamic forcing, will lead to the same final state. 相似文献
114.
Using shallow water equations on an equatorial beta plane, the nonlinear dynamics of the equatorial waves is investigated. A general mathematical procedure to study the nonlinear dynamics of these waves is developed using the asymptotic method of multiple scales. On faster temporal and spatial scales the equations describe the equatorial wavesviz, the Rossby waves, Rossby gravity waves, the inertia gravity waves and the Kelvin waves. Assuming that the amplitude of these waves are functions of slower time and space scales, it is shown that the evolution of the amplitude of these waves is governed by the nonlinear Schrodinger equation. It is then shown that for the dispersive waves like Rossby waves and Rossby-gravity waves, the envelope of the amplitude of the waves has a ‘soliton’ structure. 相似文献
115.
The suitability of the European Centre for Medium Range Weather Forecasting (ECMWF) operational wind analysis for the period
1980–1991 for studying interannual variability is examined. The changes in the model and the analysis procedure are shown
to give rise to a systematic and significant trend in the large scale circulation features. A new method of removing the systematic
errors at all levels is presented using multivariate EOF analysis. Objectively detrended analysis of the three-dimensional
wind field agrees well with independent Florida State University (FSU) wind analysis at the surface. It is shown that the
interannual variations in the detrended surface analysis agree well in amplitude as well as spatial patterns with those of
the FSU analysis. Therefore, the detrended analyses at other levels as well are expected to be useful for studies of variability
and predictability at interannual time scales. It is demonstrated that this trend in the wind field is due to the shift in
the climatologies from the period 1980–1985 to the period 1986–1991. 相似文献
116.
Linking subaerial erosion with submarine geomorphology in the western Ionian Sea (south of the Messina Strait), Italy
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Rajasmita Goswami Neil C. Mitchell Simon H. Brocklehurst Andrea Argnani 《Basin Research》2017,29(Z1):641-658
Sediment supplied by continental sources is commonly suspected to have exerted a strong influence on the development of canyons and other morphological features on the continental slopes, but rarely is the sediment supply known sufficiently quantitatively to test this link. Here, we outline an area where offshore morphology, in the western Ionian Sea, may be linked to estimated sediment fluxes produced by subaerial erosion in NE Sicily and SW Calabria. Shelves in this area are narrow (<1 km), and the bathymetry shows that rivers and adjacent submarine channels are almost directly connected with each other. Integrated topographic analyses were performed on a merged digital elevation model (DEM) of ASTER data for subaerial topography and multibeam sonar data for submarine bathymetry. Spatial variations in sediment fluxes from onshore erosion were assessed using a variety of methods, namely: long‐term sediment flux from Pleistocene uplift rates, decadal sediment flux from landslide occurrences and published long‐term exhumation rates from 10Be cosmogenic nuclide concentrations. Submarine channels associated with rivers delivering larger sediment fluxes have broad channels, high relief and smooth concave‐upward longitudinal profiles. Conversely, submarine channels that lie offshore small‐flux rivers have straight longitudinal profiles, low relief and steep gradients. Where river catchments supply a greater sediment flux offshore, shelves tend to be wider (ca. 400 m) and submarine channels have gentler gradients. In contrast, where catchments supply less sediment flux, shelves are narrow (250–300 m) and offshore channel gradients are steeper. The variation of submarine morphology with tectonic uplift rate was also studied, but we find that, unlike onshore terrains where tectonics is commonly an important factor influencing channel morphology, in the submarine landscapes, sediment flux appears to dominate here. 相似文献
117.
118.
J. N. Goswami 《Journal of Earth System Science》1998,107(4):401-411
Isotopic records in meteorites provide evidence for the presence of several short-lived nuclides in the early solar system
with half-lives varying from 105 to ∼8x107 years. Most of the nuclides with longer half-life (> 107 years) are considered to be products of stellar nucleosynthesis taking place over long time scales in our galaxy. However,
for the relatively shorter-lived nuclides, two possibilities exist; they could be products of energetic particle interactions
taking place in a presolar or early solar environment, or, they could have been produced in a stellar source and injected
into the protosolar molecular cloud just prior to its collapse. The presently available data appear to support the latter
case and put a stringent constraint of less than a million years for the time scale for the collapse of the protosolar molecular
cloud to form the Sun and some of the first solar system solids. This short time scale also suggests the possibility of a
triggered origin for the solar system with the very process of injection of the short-lived nuclides acting as the trigger
for the collapse of the protosolar molecular cloud. Fossil records of the short-lived nuclides in meteorites also provide
very useful chronological information on the early solar system processes like the time scale for nebular processing, the
time scales for differentiation and for metal/silicate fractionation within planetesimals. The currently available data suggest
a time scale of a few million years for nebular processing and a relatively short time scale of about ten million years within
which differentiation, melting and recrystallization in some of the planetesimals took place. 相似文献
119.
Fossil track analyses of a ~ 3 cm section of boulder fragment 72255, collected at the base of the South Massif, yield a surface exposure age for this boulder in its present location of ~ 40 m.y. This age is in good agreement with the81Kr-Kr exposure age (Leichet al., 1975), suggesting that the boulder was either never exposed to cosmic radiation prior to its emplacement at the foot of the South Massif or that it was heavily shielded during any previous irradiation. High-voltage electron microscope observations reveal no evidence of solar flare irradiation prior to breccia compaction, indicating that the breccia components were never part of a pre-Serenitatis near-surface regolith. The fission track record of a whitlockite crystal from 72255 yields a fission track age of 3.96 ?0.07 +0.04 g.y. Comparison with the40Ar-39 Ar age of 4.00±0.03 g.y. suggests that this age represents the compaction age of the parent boulder. 相似文献
120.
Potential predictability of the Asian summer monsoon on monthly and seasonal time scales 总被引:1,自引:0,他引:1
Summary ¶The potential predictability of the monthly and seasonal means during the Northern Hemisphere summer and winter is studied by estimating the signal-to-noise ratio. Based on 33 years of daily low-level wind observations and 24 years of satellite observations of outgoing long wave radiation, the predictability of the Asian summer monsoon region is contrasted with that over other tropical regions. A method of separating the contributions from slowly varying boundary forcing and internal dynamics (e.g., intraseasonal oscillations) that determine the predictability of the monthly mean tropical climate is proposed. We show that the Indian monsoon climate is only marginally predictable in monthly time scales as the contribution of the boundary forcing in this region is relatively low and that of the internal dynamics is relatively large. It is shown that excluding the Indian monsoon region, the predictable region is larger and predictability is higher in the tropics during northern summer. Even though the boundary forced variance is large during northern winter, the predictable region is smaller as the internal variance is larger and covers a larger region during northern winter (due to stronger intraseasonal activity). Consistent with the estimates of predictability of monthly means, estimates of potential predictability on seasonal time scales also indicate that predictability of seasonal mean Indian monsoon is limited.Received December 6, 2002; accepted March 16, 2003
Published online: June 12, 2003 相似文献