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91.
New insights into geochemical behaviour in ancient marine carbonates (Upper Jurassic Ammonitico Rosso): Novel proxies for interpreting sea‐level dynamics and palaeoceanography 下载免费PDF全文
Rute Coimbra Adrian Immenhauser Federico Olóriz Víctor Rodríguez‐Galiano Mario Chica‐Olmo 《Sedimentology》2015,62(1):266-302
Elemental concentrations in Phanerozoic sea water are known to fluctuate both in time and space. With regard to carbonates precipitated from marine fluids, elemental concentrations in the carbonate crystal lattice are affected by a complex array of equilibrium and non‐equilibrium as well as post‐depositional alteration processes. To assess the potential of carbonate elemental chemostratigraphy, seven Upper Jurassic sections were investigated along a proximal to distal transect across the south‐east Iberian palaeomargin. The aim was to explore stratigraphic and spatial variations in calcium, strontium, magnesium, iron and manganese elemental abundances. The epicontinental geochemical record is influenced by the combination of continental runoff and a significant diagenetic overprint. In contrast, the epioceanic geochemical record agrees with reconstructed open marine sea water values, reflecting a moderate degree of syn‐depositional to early marine pore water diagenesis. Establishing a fair degree of preservation of matrix micrite, a thorough statistical approach was applied and elemental associations tested for their environmental significance. Principal component and hierarchical cluster analyses revealed a persistent relation between carbonate magnesium, iron and strontium abundances. Processes related to early diagenetic nodulation in Ammonitico Rosso facies most probably account for the incorporation of these elements in the calcium carbonate lattice. The clear decoupling of carbonate manganese abundance with respect to the remaining elements is documented and related to high sea floor spreading rates and hydrothermal activity during the Late Jurassic. The investigation of potential time‐fluctuation of geochemical patterns was approached through variogram computation. The observed temporal behaviour is most likely to be forced by relative sea‐level dynamics, reflecting Late Jurassic palaeoceanographic conditions and potential planetary interactions. The data obtained in this study highlight the utility of elemental data from carbonate matrix micrites as geochemical proxies for studying the influence of remote trigger factors. 相似文献
92.
Coupling between subtropical cloud feedback and the local hydrological cycle in a climate model 总被引:2,自引:1,他引:1
In HadGEM2-A, AMIP experiments forced with observed sea surface temperatures respond to uniform and patterned +4 K SST perturbations with strong positive cloud feedbacks in the subtropical stratocumulus/trade cumulus transition regions. Over the subtropical Northeast Pacific at 137°W/26°N, the boundary layer cloud fraction reduces considerably in the AMIP +4 K patterned SST experiment. The near-surface wind speed and the air-sea temperature difference reduces, while the near-surface relative humidity increases. These changes limit the local increase in surface evaporation to just 3 W/m2 or 0.6 %/K. Previous studies have suggested that increases in surface evaporation may be required to maintain maritime boundary layer cloud in a warmer climate. This suggests that the supply of water vapour from surface evaporation may not be increasing enough to maintain the low level cloud fraction in the warmer climate in HadGEM2-A. Sensitivity tests which force the surface evaporation to increase substantially in the +4 K patterned SST experiment result in smaller changes in boundary layer cloud and a weaker cloud feedback in HadGEM2-A, supporting this idea. Although global mean surface evaporation in climate models increases robustly with global temperature (and the resulting increase in atmospheric radiative cooling), local values may increase much less, having a significant impact on cloud feedback. These results suggest a coupling between cloud feedback and the hydrological cycle via changes in the patterns of surface evaporation. A better understanding of both the factors controlling local changes in surface evaporation and the sensitivity of clouds to such changes may be required to understand the reasons for inter-model differences in subtropical cloud feedback. 相似文献
93.
Abstract— Studies of two separate stones of the CV3 chondrite Vigarano have revealed the presence of previously unreported occurrences of calcite. In the first stone, calcite occurs as thin veins in a type B CAI. In contrast, observations of the second stone, which was recovered one month after its fall, show three calcite occurrences: networks of veins, vesicle fillings in the fusion crust, and pseudomorphic replacement of augite associated with a porphyritic olivine chondrule. The most common occurrence is as veins ranging in thickness from <1 μm to 25 μm and extending for more than several hundred μm. Some veins crosscut the fusion crust and are connected to a carbonate coating on the exterior of the meteorite. Extensive minor element zoning occurs in carbonate masses, indicating variations in the fluid composition and/or redox potential during carbonate growth. Based on the textural evidence and a comparative study with carbonate veins in the CV3 chondrite Leoville, we conclude that the veins are terrestrial in origin. We propose a model for rapid carbonate formation in which calcite precipitation is driven by hydrolysis and oxidation in the meteorite interior that move the fluid composition to alkaline values. In addition, both stones also contain minor occurrences of carbonate that are not readily explained by terrestrial alteration. Minor carbonate in a type B CAI occurs in the first stone and calcite occurs as pseudomorphic replacement of augite in the second stone. Both of these occurrences appear to be preterrestrial, probably asteroidal in origin. 相似文献
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We investigate the possibility of gravitational capture of planetesimals as temporary or permanent satellites of Uranus and Neptune during the process of planetary growth. The capture mechanism is based in the enhancement of the Hill's sphere of action not only due to the mass acquired by the planet, but also by the variation of the planet-Sun distance as a consequence of the scattering of planetesimals by the planets of the outer solar system. Our calculations indicate that satellite capture was very important, specially during the first stages of the accretion process, contributing in a significant way to the planetary growth. 相似文献
97.
Adrian C. Armstrong 《水文研究》1988,2(4):383-389
Linear system theory can be used to model and predict watertable responses to precipitation inputs in an artificially drained field. The response function is mathematically equivalent to the Unit Hydrograph concept familiar to hydrologists. This paper shows that it is possible to derive such response functions, and comments on the problems encountered in their derivation. Response functions for two contrasted sites are presented, and the possibilities for their use discussed. 相似文献
98.
Adrian J. Brearley 《Meteoritics & planetary science》1993,28(4):590-595
Abstract— Rare, ultrafine-grained Ti oxides (Ti3O5 and the Magnéli phases, Ti5O9 and Ti8O15) have been identified by transmission electron microscopy in the CM2 carbonaceous chondrite, Bells and a carbonaceous chondrite matrix clast from the Nilpena polymict ureilite. In both meteorites the Ti oxides occur in the matrix as isolated grains and clusters of two or more grains. They are euhedral in shape and have grain sizes of 0.05–0.3 μm. Magnéli phases have been recently shown to be a common component in some interplanetary dust particles, but this is the first reported occurrence in a meteorite. The morphological properties and grain size of the Ti oxides are consistent with formation by vapor phase condensation either within the solar nebula or possibly in a presolar environment. 相似文献
99.
Adrian Gilli Flavio S. Anselmetti Daniel Ariztegui† Milan Beres† Judith A. McKenzie Vera Markgraf‡ 《Sedimentology》2005,52(1):1-23
The results of a seismic stratigraphic analysis of a closed lake basin, Lago Cardiel, in southernmost South America are reported. Very few high-resolution, continental records spanning the Late Quaternary have been obtained from this region. Seismic sequence stratigraphic analysis allows a reconstruction of lake level variations. Two major hiatuses of unknown age occurred during the early evolution of the basin with the deposition of an alluvial fan in a restricted area in the intervening time period. Following the development of a relatively shallow lake during the late Pleistocene and a short desiccation pulse around 11 220 14C yr BP, a transgression of over 135 m occurred at the beginning of the Holocene. The transgression was associated with the formation of beach ridges preserved in the lake stratigraphy on the floor of the modern Lago Cardiel at four different elevations. The preservation of largely unreworked beach ridges indicates a stepwise rise in the lake level. There is no seismic evidence of a major lowering of the lake below modern level during the entire Holocene. Deposition since the mid-Holocene is marked by strong lateral differences in sediment accumulation with a depocentre slightly to the north of the basin midpoint and a pronounced mounded distribution. Seismic reflection geometries, as well as sedimentological characteristics indicate a lacustrine contourite drift covering an area of 80–100 km2. As Lago Cardiel is under the influence of westerly winds, these most likely drove lake circulation. The identification of drowned beach ridges and of contourite drifts illustrates that high-resolution seismic stratigraphy is not only a powerful tool in reconstructing past lake level elevations for closed lake basins, but it can also provide information about the rate of lake level changes and the presence and strength of lake currents. 相似文献
100.
The TerraSAR-X (TSX) synthetic aperture radar (SAR) marks the recent emergence of a new generation of spaceborne radar sensors
that can for the first time lay claim to localization accuracies in the sub-meter range. The TSX platform’s extremely high
orbital stability and the sensor’s hardware timing accuracy combine to enable direct measurements of atmospheric refraction
and solid Earth movements. By modeling these effects for individual TSX acquisitions, absolute pixel geolocation accuracy
on the order of several centimeters can be achieved without need for even a single tiepoint. A 16-month time series of images
was obtained over a fixed test site, making it possible to validate both an atmospheric refraction and a solid Earth tide
model, while at the same time establishing the instrument’s long-term stability. These related goals were achieved by placing
trihedral corner reflectors (CRs) at the test site and estimating their phase centers with centimeter-level accuracy using
differential GPS (DGPS). Oriented in pairs toward a given satellite track, the CRs could be seen as bright “points” in the
images, providing a geometric reference set. SAR images from the high-resolution spotlight (HS) mode were obtained in alternating
ascending and descending orbit configurations. The highest-resolution products were selected for their small sample dimensions,
as positions can be more precisely determined. Based on the delivered product annotations, the CR image positions were predicted,
and these predictions were compared with their measured image positions both before and after compensation for atmospheric refraction and systematic solid Earth deviations. It was possible to show that when the atmospheric
distortion and Earth tides are taken into account, the TSX HS products have geolocation accuracies far exceeding the specified
requirements. Furthermore, this accuracy was maintained for the duration of the 16-month test period. It could be demonstrated
that with a correctly calibrated sensor, and after accounting for atmospheric and tidal effects, tiepoint-free geolocation
is possible with TSX with an absolute product accuracy of about 5 cm. 相似文献