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81.
82.
83.
Toxicological and chemical studies were performed with a silty and a sandy marine sediment spiked with 2,6-dinitrotoluene (2,6-DNT), 2,4,6-trinitrophenylmethylnitramine (tetryl), or 2,4,6-trinitrophenol (picric acid). Whole sediment toxicity was analyzed by the 10-day survival test with the amphipod Ampelisca abdita, and porewater toxicity tests assessed macro-algae (Ulva fasciata) zoospore germination and germling growth, sea urchin (Arbacia punctulata) embryological development, and polychaete (Dinophilus gyrociliatus) survival and reproduction. Whole sediments spiked with 2,6-DNT were not toxic to amphipods. The fine-grained sediment spiked with tetryl was also not acutely toxic. The tetryl and picric acid LC50 values in the sandy sediment were 3.24 and 144 mg/kg dry weight, respectively. The fine-grained sediment spiked with picric acid generated a U-shaped concentration-response curve in the amphipod test, with increased survival both in the lowest and highest concentration. Grain-size distribution and organic carbon content strongly influenced the behavior of ordnance compounds in spiked sediments. Very low concentrations were measured in some of the treatments and irreversible binding and biodegradation are suggested as the processes responsible for the low measurements. Porewater toxicity varied with its sedimentary origin and with ordnance compound. The sea urchin embryological development test tended to be the least sensitive. Tetryl was the most toxic chemical in all porewater tests, and picric acid the least toxic. Samples spiked with 2,6-DNT contained a degradation product identified as 2-methyl-3-nitroaniline (also known as 2-amino-6-nitrotoluene), and unidentified peaks, possibly degradation products, were also seen in some of the picric acid- and tetryl-spiked samples. Degradation products may have played a role in observed toxicity. 相似文献
84.
Local and regional variations in Central American arc lavas controlled by variations in subducted sediment input 总被引:9,自引:0,他引:9
Lina C. Patino Michael J. Carr Mark D. Feigenson 《Contributions to Mineralogy and Petrology》2000,138(3):265-283
The sedimentary section (at DSDP Site 495) on the subducting Cocos Plate has large stratigraphic changes in incompatible
elements and element ratios, the result of early carbonate deposition followed by late hemipelagic deposition. Lavas from
Central America define both local and regional geochemical trends that reflect the strong influence of the two Cocos Plate
sediment units. Element ratios with large stratigraphic variations on the Cocos Plate (e.g. Ba/Th, U/La) define local variations
within individual volcanic centers in Central America, indicating that marine stratigraphy controls some geochemical characteristics
of the lavas. These local trends can be explained by changing the proportions of hemipelagic sediment input into the magma
generation process. These local trends are observed in all the segments of the arc, regardless of the intensity of the slab
signature. Regional variations are most clearly seen in element ratios that are nearly constant through the Cocos Plate sediment
stratigraphy (e.g. Ba/La, U/Th), suggesting that regional variations reflect differences in the intensity of the flux from
the subducting slab. The slab signal is strongest in Nicaragua and along the volcanic front. The signal decreases to the northwest
and southeast of Nicaragua and toward the back arc. The large slab signature in the lavas from western Nicaragua occurs in
the area with the thinnest continental crust and steepest dip of the slab. The mass flux of incompatible elements into the
system is easily estimated, except for elements, like Pb, that have high and variable abundances in the basaltic oceanic crust
section. The mass flux of elements out of the system depends on eruption rates, which are variable along the arc and only
approximately known. Comparison of input and output fluxes for five different segments of the arc reveals that some elements
(K, B, Cs, and Rb) are very efficiently delivered to the volcanoes from the subducted slab. Other elements (Sr, Ba, and U)
are returned to the surface with moderate efficiency, whereas some elements (REEs) may come mostly from the mantle wedge with
minor slab contribution. The relative order of recycling efficiencies of incompatible elements implies that a hydrous fluid
dominates the transfer of material from the slab to mantle.
Received: 7 September 1998 / Accepted: 30 September 1999 相似文献
85.
Mathematical Geosciences - 相似文献
86.
The state and future of Mars polar science and exploration. 总被引:1,自引:0,他引:1
S M Clifford D Crisp D A Fisher K E Herkenhoff S E Smrekar P C Thomas D D Wynn-Williams R W Zurek J R Barnes B G Bills E W Blake W M Calvin J M Cameron M H Carr P R Christensen B C Clark G D Clow J A Cutts D Dahl-Jensen W B Durham F P Fanale J D Farmer F Forget K Gotto-Azuma H J Zwally 《Icarus》2000,144(2):210-242
As the planet's principal cold traps, the martian polar regions have accumulated extensive mantles of ice and dust that cover individual areas of approximately 10(6) km2 and total as much as 3-4 km thick. From the scarcity of superposed craters on their surface, these layered deposits are thought to be comparatively young--preserving a record of the seasonal and climatic cycling of atmospheric CO2, H2O, and dust over the past approximately 10(5)-10(8) years. For this reason, the martian polar deposits may serve as a Rosetta Stone for understanding the geologic and climatic history of the planet--documenting variations in insolation (due to quasiperiodic oscillations in the planet's obliquity and orbital elements), volatile mass balance, atmospheric composition, dust storm activity, volcanic eruptions, large impacts, catastrophic floods, solar luminosity, supernovae, and perhaps even a record of microbial life. Beyond their scientific value, the polar regions may soon prove important for another reason--providing a valuable and accessible reservoir of water to support the long-term human exploration of Mars. In this paper we assess the current state of Mars polar research, identify the key questions that motivate the exploration of the polar regions, discuss the extent to which current missions will address these questions, and speculate about what additional capabilities and investigations may be required to address the issues that remain outstanding. 相似文献
87.
Projected 21st century climate change on snow conditions over Shasta Dam watershed by means of dynamical downscaling 下载免费PDF全文
Snow is an important component of the Earth's climate system and is particularly vulnerable to global warming. It has been suggested that warmer temperatures may cause significant declines in snow water content and snow cover duration. In this study, snowfall and snowmelt were projected by means of a regional climate model that was coupled to a physically based snow model over Shasta Dam watershed to assess changes in snow water content and snow cover duration during the 21st century. This physically based snow model requires both physical data and future climate projections. These physical data include topography, soils, vegetation, and land use/land cover, which were collected from associated organizations. The future climate projections were dynamically downscaled by means of the regional climate model under 4 emission scenarios simulated by 2 general circulation models (fifth‐generation of the ECHAM general circulation model and the third‐generation atmospheric general circulation model). The downscaled future projections were bias corrected before projecting snowfall and snowmelt processes over Shasta Dam watershed during 2010–2099. This study's results agree with those of previous studies that projected snow water equivalent is decreasing by 50–80% whereas the fraction of precipitation falling as snowfall is decreasing by 15% to 20%. The obtained projection results show that future snow water content will change in both time and space. Furthermore, the results confirm that physical data such as topography, land cover, and atmospheric–hydrologic data are instrumental in the studies on the impact of climate change on the water resources of a region. 相似文献
88.
The wave-induced velocity and pressure fields beneath a large amplitude internal solitary wave of depression propagating over
a smooth, flat, horizontal, and rigid boundary in a shallow two-layer fluid are computed numerically. A numerical ocean model
is utilised, the set-up of which is designed and tuned to replicate the previously published experimental results of Carr
and Davies (Phys Fluids 18(1):016,601–1–016,601–10, 2006). Excellent agreement is found between the two data sets and, in particular, the numerical simulation replicates the finding
of a reverse flow along the bed aft of the wave. The numerically computed velocity and pressure gradients confirm that the
occurrence of the reverse flow is a consequence of boundary layer separation in the adverse pressure gradient region. In addition,
vortices associated with the reverse flow are seen to form near the bed. 相似文献
89.
90.
Instrumentation built to record seeing data automatically via image motion measurements of bright stars in small telescopes is described. The centroid of the star image is found 256 times s-1 in one dimension and is analyzed on-line. The device works over a range of FWHM values as would be seen through a large telescope between <0.1 and 3.0 arcsec. The first results for two identical instruments set up at two locations near the duPont Telescope at Las Campanas Observatory are reported. For a total of 61 nights of data (450 h at each site), the median seeing is 0.6 arcsec, with quartiles at 0.4 and 0.8 arcsec. These are FWHM values referred to 5000 Å at the zenith. So far, the two sites are indistinguishable on average. 相似文献