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81.
Intensive recreational SCUBA diving threatens coral reef ecosystems. The reefs at Dahab, South Sinai, Egypt, are among the world’s most dived (>30,000 dives y−1). We compared frequently dived sites to sites with no or little diving. Benthic communities and condition of corals were examined by the point intercept sampling method in the reef crest zone (3 m) and reef slope zone (12 m). Additionally, the abundance of corallivorous and herbivorous fish was estimated based on the visual census method. Sediments traps recorded the sedimentation rates caused by SCUBA divers. Zones subject to intensive SCUBA diving showed a significantly higher number of broken and damaged corals and significantly lower coral cover. Reef crest coral communities were significantly more affected than those of the reef slope: 95% of the broken colonies were branching ones. No effect of diving on the abundance of corallivorous and herbivorous fish was evident. At heavily used dive sites, diver-related sedimentation rates significantly decreased with increasing distance from the entrance, indicating poor buoyancy regulation at the initial phase of the dive. The results show a high negative impact of current SCUBA diving intensities on coral communities and coral condition. Corallivorous and herbivorous fishes are apparently not yet affected, but are endangered if coral cover decline continues. Reducing the number of dives per year, ecologically sustainable dive plans for individual sites, and reinforcing the environmental education of both dive guides and recreational divers are essential to conserve the ecological and the aesthetic qualities of these dive sites. 相似文献
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Ekaterina V. Korochantseva Alexei I. Buikin Jens Hopp Cyrill A. Lorenz Alexander V. Korochantsev Ulrich Ott Mario Trieloff 《Meteoritics & planetary science》2016,51(12):2334-2346
Dhofar 280 recorded a complex history on the Moon revealed by high‐resolution 40Ar‐39Ar dating. Thermal resetting occurred less than 1 Ga ago, and the rock was exposed to several impact events before and afterwards. The cosmic ray exposure (CRE) age spectrum indicates a 400 ± 40 Ma CRE on the lunar surface. A unique feature of this lunar sample is a partial loss of cosmogenic 38Ar, resulting in a (low‐temperature) CRE age plateau of about 1 Ma. This was likely caused by the same recent impact event that reset the (low‐temperature) 40Ar‐39Ar age spectrum and preceded the short transit phase to Earth of ≤1 Ma. Dhofar 280 may be derived from KREEP‐rich lunar frontside terrains, possibly associated with the Copernicus crater or with a recent impact event on the deposits of the South Pole–Aitken basin. Although Dhofar 280 is paired with Dhofar 081, their irradiation and thermal histories on the Moon were different. An important trapped Ar component in Dhofar 280 is “orphan” Ar with a low 40Ar/36Ar ratio. It is apparently a mixture of two components, one endmember with 40Ar/36Ar = 17.5 ± 0.2 and a second less well‐constrained endmember with 40Ar/36Ar ≤10. The presence of two endmembers of trapped Ar, their compositions, and the breccia ages seem to be incompatible with a previously suggested correlation between age or antiquity and the (40Ar/36Ar)trapped ratio (Eugster et al. 2001; Joy et al. 2011a). Alternatively, “orphan” Ar of this impact melt breccia may have an impact origin. 相似文献
87.
U. Ott M. Altmaier U. Herpers J. Kuhnhenn S. Merchel R. Michel R. K. Mohapatra 《Meteoritics & planetary science》2005,40(11):1635-1652
Abstract— We have determined the recoil range of spallation xenon produced by irradiation of Ba glass targets with ?1190 and ?268 MeV protons, using a catcher technique, where spallation products are measured in target and catcher foils. The inferred range for 126Xe produced in silicon carbide is ?0.19 μm, which implies retention of ?70% for 126Xe produced in “typical” presolar silicon carbide grains of 1 μm size. Recoil loss of spallation xenon poses a significantly smaller problem than loss of the spallation neon from SiC grains. Ranges differ for the various Xe isotopes and scale approximately linearly as function of the mass difference between the target element, Ba, and the product. As a consequence, SiC grains of various sizes will have differences in spallation Xe composition. In an additional experiment at ?66 MeV, where the recoil ranges of 22Na and 127Xe produced on Ba glass were determined using γ‐spectrometry, we found no evidence for recoil ranges being systematically different at this lower energy. We have used the new data to put constraints on the possible presolar age of the SiC grains analyzed for Xe by Lewis et al. (1994). Uncertainties in the composition of the approximately normal Xe component in SiC (Xe‐N) constitute the most serious problem in determining an age, surpassing remaining uncertainties in Xe retention and production rate. A possible interpretation is that spallation contributions are negligible and that trapped 124Xe/126Xe is ?5% lower in Xe‐N than in Q‐Xe. But also for other reasonable assumptions for the 124Xe/126Xe ratio in Xe‐N (e.g., as in Q‐Xe), inferred exposure ages are considerably shorter than theoretically expected lifetimes for interstellar grains. A short presolar age is in line with observations by others (appearance, grain size distribution) that indicate little processing in the interstellar medium (ISM) of surviving (crystalline) SiC. This may be due to amorphization of SiC in the ISM on a much shorter time scale than destruction, with amorphous SiC not surviving processing in the early solar system. A large supply of relatively young grains may be connected to the proposed starburst origin (Clayton 2003) for the parent stars of the mainstream SiC grains. 相似文献
88.
Andrzej Baran John Telting Roy Østensen Maciej Winiarski Marek Drożdż Dorota Kozieł Mike Reed Raquel Oreiro Roberto Silvotti Michał Siwak Uli Heber Peter Papics 《Astrophysics and Space Science》2010,329(1-2):199-203
We present early results of the application of a method which uses multicolor photometry and spectroscopy for ? discrimination. This method has been successfully applied to the pulsating hot subdwarf Balloon?090100001. Here we apply the method to QQ?Vir (PG1325+101). This star was observed spectroscopically and photometrically in 2008. Details on spectroscopy can be found in Telting et al. (Astrophys. Space Sci. 2010, this volume), while photometry and preliminary results on ? discrimination are provided here. The main aim of this work was to compare the value of the ? parameter derived for the main mode in QQ?Vir to previously published values derived by using different methods. 相似文献
89.
Das et al. (2012) claim that in several cases nominal cosmic ray exposure ages derived from concentrations of cosmogenic Ne in individual olivine grains separated from chondrules substantially exceed exposure ages of matrix samples. Some grains were also reported to show larger apparent exposure ages than other grains from the same chondrule. The authors conclude that the excesses were caused by an exposure of chondrules to high fluences of solar energetic particles and suggest that their data provide direct evidence for a highly active phase of the early Sun, similar to what is observed in X‐ray emissions of recent naked T‐Tauri stars. Here, we show that the production rates of cosmogenic Ne used by Das et al. (2012) to derive nominal cosmic ray exposure ages of their olivine grains are often much too low, as the reported major element concentrations in many cases sum up to considerably less than 100% even if converted to oxides. In contrast, adopted element concentrations for matrix samples are basically self‐consistent. A precompaction exposure of chondrules to a very high flux of solar energetic particles is thus not supported by the data presented by Das et al. (2012). Das et al. (2012) claim that in several cases nominal cosmic ray exposure ages derived from concentrations of cosmogenic Ne in individual olivine grains separated from chondrules substantially exceed exposure ages of matrix samples. Some grains were also reported to show larger apparent exposure ages than other grains from the same chondrule. The authors conclude that the excesses were caused by an exposure of chondrules to high fluences of solar energetic particles and suggest that their data provide direct evidence for a highly active phase of the early Sun, similar to what is observed in X‐ray emissions of recent naked T‐Tauri stars. Here, we show that the production rates of cosmogenic Ne used by Das et al. (2012) to derive nominal cosmic ray exposure ages of their olivine grains are often much too low, as the reported major element concentrations in many cases sum up to considerably less than 100% even if converted to oxides. In contrast, adopted element concentrations for matrix samples are basically self‐consistent. A precompaction exposure of chondrules to a very high flux of solar energetic particles is thus not supported by the data presented by Das et al. (2012). 相似文献
90.
A linear model for neutral surface-layer flow over orography is presented. The Reynolds-Averaged Navier-Stokes and E– turbulence closure equations are expressed in a terrain-following coordinate system created from a simple analytical expression
in the Fourier domain. The perturbation equations are solved spectrally horizontally and by numerical integration vertically.
Non-dimensional solutions are stored in look-up tables for quick re-use. Model results are compared to measurements, as well
as other authors’ flow models in three test cases. The model is implemented and tested in two-dimensional space; the equations
for a full three-dimensional version are presented. 相似文献