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排序方式: 共有649条查询结果,搜索用时 31 毫秒
621.
In Hong Kong, discharges of domestic and industrial wastewaters and disposal of contaminated muds into coastal waters have resulted in high levels of persistent organic contaminants in the water column, biota and bottom sediments. Although the acute toxicity of effluents has been tested by government regulators using mortality-based bioassays, the sublethal effects of many of the pollutants, including environmental carcinogens, have not been well studied. The objective of the present work was to examine the efficiency and efficacy of using: (1) DNA strand breaks (comet assay and alkaline agarose gel electrophoresis assay); (2) oxidative DNA damage (levels of 8-hydroxy-2-deoxyguanosine in cellular DNA) and (3) micronuclei (MN) formation as early warning toxicological indicators of potential environmental carcinogen exposure. Green-lipped mussels (Perna viridis) were transplanted to study sites and retrieved after 4, 8, 12, 16 and 30 days. The bioaccumulation of organic pollutants, including the polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OC pesticides) and polychlorinated biphenyls (PCBs) was observed, and compared with biomarker assays. Some significant dose- and time-dependent inductions of the genotoxic biomarkers, including 8-OHdG, DNA strand breaks and micronuclei were observed. In particular, several very strong (r>0.9) and significantly positive relationships were found at all sites during days 0-12. In addition, significant strong relationships between certain organic contaminants (carcinogenic PAHs and 4 common OC pesticides) and micronuclei were detected at the polluted sites, Tsim Sha Tsui and Sai Wan Ho. The results suggest that the use of micronuclei as a biomarker is potentially effective, especially at the more polluted sites. Nonetheless, further research is required to validate their effectiveness under varying field conditions, especially to elucidate responses under lower contaminant loads, interactions between the environmental and physiological factors affecting organisms in differing coastal environments, and changes in biotransformation and DNA repair enzymes in test organisms. 相似文献
622.
One of the greatest challenges to the U.S. Department of Defense is maintaining the sustainability of military lands while maximizing their use for training activities. Restoration of disturbed lands in the Mojave Desert has proven to be a challenge, particularly revegetation from seed. This study presents soil moisture dynamics from three restoration sites at the National Training Center, Ft. Irwin, CA. The soil microclimate (matric potential, water balance, and temperature) was monitored beneath various surface treatments to better assess their effects on seed germination and establishment. Results show that under the typically hot and dry climatic conditions that prevail, the soil microclimate is a hostile environment. Irrigation had the most positive effect on the seedbed. Surface mulches including gravel, straw, emulsion, and bark had limited benefits often detrimental. Although seedlings germinated, survival after three months was negligible. Results presented here suggest that successful seeding with broadcast sprinkler irrigation in the central Mojave Desert is not possible unless the seedbed area is limited or advantageous environmental conditions prevail. Improved knowledge of the germination requirements and applied numerical modeling would result in more successful seedbed management. 相似文献
623.
Aleš Farda Michel Déué Samuel Somot András Horányi Valery Spiridonov Helga Tóth 《Studia Geophysica et Geodaetica》2010,54(2):313-332
Results obtained with two versions of the Limited Area Model (LAM) ALADIN over differently sized integration domains (large,
intermediate and small) in the European area are presented in order to investigate both the general model performance and
the influence of domain choice on the quality of obtained results. The aim is also to illustrate the issues related to the
strategy of selection of the optimal integration domain. Each of these studies has been performed with two versions of the
ALADIN model: the first one is ALADIN-CLIMATE developed at CNRM/Météo-France, the second one is ALADIN-CLIMATE/CZ prepared
at the Czech Hydrometeorological Institute (CHMI). This leaves us with total of six experiments forced by the European Centre
for Medium-Range Weather Forecasts (ECMWF) ERA-40 reanalysis data. The west Balkan domain covering Bulgaria is used as an
evaluation region for investigation of the temporal and spatial properties of simulated precipitation and temperature fields.
This region has been selected for its challenging orography making the results obtained here a valuable source for studies
leading to further developments in climate modeling. It was found that size of the domain strongly affects the quality of
obtained results. We have found that the largest domain reproduces the spatial characteristics of climate (such as bias) very
well, but its use results in a poor representation of temporal aspects, which are however captured very well in experiments
over both smaller domains. Our findings suggest that there is no optimal choice of domain size, securing the best results
for both spatial and temporal evaluation. 相似文献
624.
This paper presents initial results from a study of factors that inhibit or enhance hard coral recovery in rubble fields created by blast fishing in Komodo National Park and Bunaken National Park, Indonesia. Within nine sites monitored since 1998, there was no significant natural recovery. Levels of potential source coral larvae were assessed with settlement tiles in the rubble fields and in nearby high coral cover sites. Rubble movement was measured and shown to be detrimental to small scleractinians, especially in high current areas. In shallow water (2-6 m deep), rubble is often overgrown by soft corals and corallimorpharians, which inhibit hard coral survival. There is increased scleractinian recruitment in quadrats cleared of soft coral, and Acropora nubbins transplanted into soft coral fields suffer greater mortality than those transplanted above the soft coral canopy. Gaining an understanding of the prognosis for coral recovery is essential not only in order to assess the long-term impacts of blast fishing, but also to improve management decisions about protection of intact reefs and potential restoration of damaged areas. 相似文献
625.
Reginald?L.?HermannsEmail author Samuel?Niedermann Susan?Ivy-Ochs Peter?W.?Kubik 《Landslides》2004,1(2):113-122
Generally landslide dams which exist for several hundreds to thousands of years are considered as stable. We show with an example from the Argentine Andes that such dams can exist for several thousands of years but still may fail catastrophically. Multiple rock avalanches impounded two lakes with surface areas of ~8 km2 and ~600 km2, respectively, in Las Conchas valley, NW Argentina. Surface exposure dating (SED) by 10Be of the rock-avalanche deposits or landslide scars indicates that these landslides occurred at 15,300±2,000 yr and 13,550±900 yr. The dams were stable during a strong earthquake, as suggested by seismites within related lake sediments and by multiple coeval landslides in this region, which occurred at ~7.5 kyr. However, when a further rock-avalanche fell into the lower, smaller lake at 4,800±500 yr the dam downriver was destroyed, presumably by the resulting tsunami wave. The resulting flood also destroyed an additional rock-fall dam which had formed at ~5,630 yr 14C cal BP 30 km downriver. The new dam formed by the second rock avalanche was eroded prior to 3,630 yr 14C cal BP. This dam erosion coincides with an important climatic shift towards more humid conditions in the Central Andes. Our results show that instead of direct effects of strong seismicity on landslide dams, (1) landsliding into a landslide-dammed lake, (2) abrupt hydrological changes, and (3) climate change towards conditions related to enhanced run-off are processes which can produce failures of quasi-stable natural dams. 相似文献
626.
Samuel C. Vila 《Earth, Moon, and Planets》1984,31(3):313-315
The question of the survival of the planet Earth as the Sun becomes a red giant and possibly a planetary nebula depends on the maximum extent of the Sun's envelope. This study shows that if the Earth is reached by this envelope, it will be destroyed; but if it is not, it will survive. Present stellar evolution calculations imply that the Earth will be destroyed. 相似文献
627.
We present 20-μm photometry of Uranus and Neptune which confirms the presence of a temperature inversion in the lower stratospheres in both planets. We find the brightness temperature difference between 17.8 and 19.6 μm to be 0.8 ± 0.5°K for Uranus and 1.8 ± 0.6°K for Neptune. These results indicate that the temperature inversions on both planets are weaker than previously thought. Comparison to model atmospheres by J. Appleby [Ph.D. thesis, SUNY at Stony Brook 1980] indicates that the temperature inversions can be understood as arising from heating by the absorption of sunlight by CH4 and aerosols. However, the stratospheric CH4 mixing ratio on Neptune must be higher than that at the temperature minimum. 相似文献
628.
629.
630.
The mode of formation of gullies on Mars, very young erosional–depositional landforms consisting of an alcove, channel, and fan, is one of the most enigmatic problems in martian geomorphology. Major questions center on their ages, geographic and stratigraphic associations, relation to recent ice ages, and, if formed by flowing water, the sources of the water to cause the observed erosion/deposition. Gasa (35.72°S, 129.45°E), a very fresh 7-km diameter impact crater and its environment, offer a unique opportunity to explore these questions. We show that Gasa crater formed during the most recent glacial epoch (2.1–0.4 Ma), producing secondary crater clusters on top of the latitude-dependent mantle (LDM), interpreted to be a layered ice-dust-rich deposit emplaced during this glacial epoch. High-resolution images of a pre-Gasa impact crater ~100 km northeast of Gasa reveal that portions of the secondary-crater-covered LDM have been removed from pole-facing slopes in crater interiors near Gasa; gullies are preferentially located in these areas and channels feeding alcoves and fans can be seen to emerge from the eroding LDM layers to produce multiple generations of channel incision and fan lobes. We interpret these data to mean that these gullies formed extremely recently in the post-Gasa-impact time-period by melting of the ice-rich LDM. Stratigraphic and topographic relationships are interpreted to mean that under favorable illumination geometry (steep pole-facing slopes) and insolation conditions, melting of the debris-covered ice-rich mantle took place in multiple stages, most likely related to variations in spin-axis/orbital conditions. Closer to Gasa, in the interior of the ~18 km diameter LDM-covered host crater in which Gasa formed, the pole-facing slopes display two generations of gullies. Early, somewhat degraded gullies, have been modified by proximity to Gasa ejecta emplacement, and later, fresh appearing gullies are clearly superposed, cross-cut the earlier phase, and show multiple channels and fans, interpreted to be derived from continued melting of the LDM on steep pole-facing slopes. Thus, we conclude that melting of the ice-rich LDM is a major source of gully activity both pre-Gasa crater and post-Gasa crater formation. The lack of obscuration of Gasa secondary clusters formed on top of the LDM is interpreted to mean that the Gasa impact occurred following emplacement of the last significant LDM layers at these low latitudes, and thus near the end of the ice ages. This interpretation is corroborated by the lack of LDM within Gasa. However, Gasa crater contains a robustly developed set of gullies on its steep, pole-facing slopes, unlike other very young post-LDM craters in the region. How can the gullies inside Gasa form in the absence of an ice-rich LDM that is interpreted to be the source of water for the other adjacent and partly contemporaneous gullies? Analysis of the interior (floor and walls) of the host crater suggest that prior to the Gasa impact, the pole-facing walls and floor were occupied by remnant debris-covered glaciers formed earlier in the Amazonian, which are relatively common in crater interiors in this latitude band. We suggest that the Gasa impact cratering event penetrated into the southern portion of this debris-covered glacier, emplaced ejecta on top of the debris layer covering the ice, and caused extensive melting of the buried ice and flow of water and debris slurries on the host crater floor. Inside Gasa, the impact crater rim crest and wall intersected the debris-covered glacier deposits around the northern, pole-facing part of the Gasa interior. We interpret this exposure of ice-rich debris-covered glacial material in the crater wall to be the source of meltwater that formed the very well-developed gullies along the northern, pole-facing slopes of Gasa crater. 相似文献