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81.
Ralph D. Lorenz Elizabeth P. Turtle Alice Le Gall Oded Aharonson Ellen Stofan 《Icarus》2011,211(1):699-706
Cassini RADAR topography data are used to evaluate Titan’s hypsometric profile, and to make comparisons with other planetary bodies. Titan’s hypsogram is unimodal and strikingly narrow compared with the terrestrial planets. To investigate topographic extremes, a novel variant on the classic hypsogram is introduced, with a logarithmic abscissa to highlight mountainous terrain. In such a plot, the top of the terrestrial hypsogram is quite distinct from those of Mars and Venus due to the ‘glacial buzz-saw’ that clips terrestrial topography above the snowline. In contrast to the positive skew seen in other hypsograms, with a long tail of positive relief due to mountains, there is an indication (weak, given the limited data for Titan so far) that the Titan hypsogram appears slightly negatively skewed, suggesting a significant population of unfilled depressions. Limited data permit only a simplistic comparison of Titan topography with other icy satellites but we find that the standard deviation of terrain height (albeit at different scales) is similar to those of Ganymede and Europa. 相似文献
82.
Daniel H. Sandweiss Alice R. Kelley Daniel F. Belknap Joseph T. Kelley Kurt Rademaker David A. Reid 《Quaternary Research》2010,73(3):439-448
Los Morteros (8?39′54″S, 78?42 ′00″W) is located in coastal, northern Peru, one of the six original centers of world civilization. The site consists of a large, sand-covered, isolated prominence situated on a Mid-Holocene shoreline, ~ 5 km from the present coast. Preceramic archaeological deposits (4040 ± 75 to 4656 ± 60 14C yr BP or ~ 3600–5500 cal yr BP) cap this feature, which has been identified by prior researchers as a sand-draped, bedrock-cored landform or a relict dune deposit. Because neither explanation is geomorphologically probable, we used ground-penetrating radar (GPR) and high-resolution mapping to assess the mound's interior structure. Our results indicate an anthropogenic origin for Los Morteros, potentially placing it among the earliest monumental structures in prehistoric South America. The extremely arid setting raises new questions about the purpose and the logistics of early mound construction in this region. This work demonstrates the value of an integrated Quaternary sciences approach to assess long-term landscape change and to understand the interaction between humans and the environment. 相似文献
83.
The frost sensitivity of a rock is resulting from the combined action of processes linked to porous network characteristics (state parameters) and to the way water flows into this porous network (transfer parameters), our study was thus about the influence of these parameters on frost weathering of rocks. Sedimentary rocks often found on buildings (limestone and sandstone) and consequently submitted to temperature variations have been selected. State and transfer parameters have been measured for sample characterization and the follow-up of weathering during freeze–thaw cycles. The coupled influence of a state parameter (dynamic modulus of elasticity) and a transfer parameter (water permeability), i.e. between rock skeletal strength and voids connection, has thus been discussed. 相似文献
84.
85.
Alice Toppani Guy Libourel Ccile Engrand Michel Maurette 《Meteoritics & planetary science》2001,36(10):1377-1396
Abstract— Depending on their velocity, entry angle and mass, micrometeorites suffer different degrees of heating during their deceleration in the Earth's atmosphere, leading, in most cases, to significant textural, mineralogical and chemical modifications. One of these modifications is the formation of a magnetite shell around most micrometeorites, which until now could not be reproduced, neither theoretically nor experimentally. The present study was designed to better understand the entry heating effects on micrometeorites and especially the formation of the magnetite shell. Fragments of the Murchison and Orgueil meteorites were used as analogue material in flash‐heating experiments performed in a high‐temperature furnace; effects of temperature, heating duration, and oxygen fugacity were investigated. These experiments were able to reproduce most of the micrometeorites textures, from the vesicular fine‐grained micrometeorites to the totally melted cosmic spherules. For the first time, the formation of a magnetite shell could be observed on micrometeorite analogues. We suggest that the most plausible mechanism for the formation of this shell is a peripheral partial melting with subsequent magnetite crystallization at the surface of the micrometeorite. Furthermore, with this study, it is possible to estimate the atmospheric entry conditions of micrometeorites, such as the peak temperature and the duration of flash‐heating. 相似文献
86.
Andrew Jordan Tim Rayner Heike Schroeder Neil Adger Kevin Anderson Alice Bows 《Climate Policy》2013,13(6):751-769
Since the mid-1990s, the aim of keeping climate change within 2?°C has become firmly entrenched in policy discourses. In the past few years, the likelihood of achieving it has been increasingly called into question. The debate around what to do with a target that seems less and less achievable is, however, only just beginning. As the UN commences a two-year review of the 2?°C target, this article moves beyond the somewhat binary debates about whether or not it should or will be met, in order to analyse more fully some of the alternative options that have been identified but not fully explored in the existing literature. For the first time, uncertainties, risks, and opportunities associated with four such options are identified and synthesized from the literature. The analysis finds that the significant risks and uncertainties associated with some options may encourage decision makers to recommit to the 2?°C target as the least unattractive course of action. 相似文献
87.
Alexandria B. Boehm Kevan M. Yamahara Sarah P. Walters Blythe A. Layton Daniel P. Keymer Rachelle S. Thompson Karen L. Knee Matt Rosener 《Estuaries and Coasts》2011,34(5):925-936
This study quantifies dissolved inorganic nitrogen (DIN), soluble reactive phosphorous (SRP), and microbial pollutant inputs
to a tropical embayment, Hanalei Bay, Kaua'i, Hawai'i from rural watersheds during two field excursions during non-storm conditions.
We employ land cover analysis and a suite of nucleic acid fecal source tracking markers (host-specific Bacteroidales and human enterovirus) to identify sources of pollutants to the bay. The highest concentrations of DIN and SRP are in streams
draining watersheds with large areas of cultivated land, suggesting fertilizer is a source of these nutrients to the streams
and coastal waters. Pollutant areal loading correlates with the fractions of urban and cultivated land cover. Microbial source
tracking indicates the presence of human, pig, and ruminant feces in the streams. This work provides preliminary evidence
that human development affects loading of DIN, SRP, and microbial pollutants to tropical coastal waters; further study is
needed to confirm this. Additionally, results point to a mix of microbial pollutant sources. 相似文献
88.
In this contribution we present our preliminary investigation on pulsar sensitivity of the Large Area Telescope, the main
instrument aboard the GLAST mission. In particular we concentrated our attention to pulsars located at low galactic latitudes.
We created a set of simulated pulsars having different fluxes in an array of galactic coordinates separated by a distance
greater than the LAT Point Spread Function in order to avoid confusion between adjacent sources. Galactic gamma-ray sky background
as used during the second LAT Data Challenge (DC2) is also included. We then run an automatic routine for testing periodicity for all the pulsars considering an opportune
timing solution. In this way we can obtain a map of the sensitivity of the periodic searches for different fluxes and for
various Galactic latitudes. Some assumptions have been made by simulating the pulsar sources, but this study is a first step
toward an estimate for pulsed emission sensitivity of the GLAST LAT. The pulsed flux sensitivity profile we generate could
also be input to a population synthesis code of Galactic pulsars in order to obtain more accurate predictions of the number
of expected pulsar detections by GLAST.
On behalf of the GLAST LAT Collaboration. 相似文献
89.
Gerald Herrling Marius Becker Alice Lefebvre Anna Zorndt Knut Krämer Christian Winter 《地球表面变化过程与地形》2021,46(11):2211-2228
The bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large-scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large-scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood- and ebb-oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune-induced directional bed roughness in numerical models of estuarine and tidal environments. 相似文献
90.
Irene Schimmelpfennig Alice Williams Raphaël Pik Pete Burnard Samuel Niedermann Robert Finkel Björn Schneider Lucilla Benedetti 《Quaternary Geochronology》2011,6(5):425-436
Because the intensity and energy spectrum of the cosmic ray flux are affected by atmospheric depth and geomagnetic-field strength, cosmogenic nuclide production rates increase considerably with altitude and to a lesser degree with latitude. The scaling methods used to account for spatial variability in production rates assume that all cosmogenic nuclides have the same altitude dependence. In this study we evaluate whether the production rates of cosmogenic 36Cl, 3He and 21Ne change differently with altitude, which is plausible due to the different threshold energies of their production reactions. If so, nuclide-specific scaling factors would be required.Concentrations of the three cosmogenic nuclides were determined in mafic phenocrysts over an altitude transect between 1000 and 4300 m at Kilimanjaro volcano (3°S). Altitude dependence of relative production rates was assessed in two ways: by determination of concentration ratios and by calculation of apparent exposure age ratios for all nuclide pairs. The latter accounts for characteristics of 36Cl that the stable nuclides 3He and 21Ne do not possess (radioactive decay, high sensitivity to mineral composition and significant contributions from production reactions other than spallation). All ratios overlap within error over the entire transect, and altitudinal variation in relative production rates is not therefore evident. This suggests that nuclide-specific scaling factors are not required for the studied nuclides at this low-latitude location. However, because previous studies have documented anomalous altitude-dependent variations in 3He production at mid-latitude sites, the effect of latitude on cross-calibrations should be further evaluated.We determined cosmogenic 21Ne/3He concentration ratios of 0.1864 ± 0.0085 in pyroxenes and 0.377 ± 0.018 in olivines, agreeing with those reported in previous studies.Despite the absence of independently determined ages for the studied lava surfaces, the consistency in the dataset should enable progress to be made in the determination of the production rates of all three nuclides as soon as the production rate of one of the nuclides has been accurately defined.To our knowledge this is the first time that 36Cl has been measured in pyroxene. The Cl extraction method was validated by measuring 36Cl in co-existing plagioclase phenocrysts in one of the samples. 相似文献