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221.
Ryan S. King William V. Deluca Dennis F. Whigham Peter P. Marra 《Estuaries and Coasts》2007,30(3):469-481
The invasion of North American tidal marshes byPhragmites australis, or common reed, is a large-scale ecological problem that has been primarily studied at small spatial scales. Previous local-scale
studies have provided evidence that the expansion ofPhragmites is facilitated by disturbance and increased nitrogen (N) associated with agricultural and urban-suburban (developed) land
uses along wetland-upland borders. We tested the generality of previous findings across a larger spatial scale and wider range
of environmental conditions in Chesapeake Bay, the largest estuarine ecosystem in the USA. We sampled 90 tidal wetlands nested
within 30 distinct subestuarine watersheds and examined the relationship between land use andPhragmites abundance and foliar N, an indicator of nitrogen availability. We estimated land use adjacent to wetland borders and within
subestuary watersheds and explored the importance of spatial proximity by weighting land use by its distance from the wetland
border or subestuary shoreline, respectively. Regression tree and changepoint analyses revealed thatPhragmites abundance sharply increased in almost every wetland where development adjacent to borders exceeded 15%. Where development
was <15% but natural land cover at the near the subestuary shoreline was low (<∼35%),Phragmites was abundant, suggesting that wetlands in highly modified watersheds also were susceptible to invasion, regardless of land
use adjacent to wetlands.Phragmites foliar N was markedly elevated in watersheds with >14–22% shoreline development, the same level of development that corresponded
to high levels of invasion. Our results suggest that development near wetlands is at least partially responsible for patterns
of invasion across Chesapeake Bay. Larger-scale phenomena, such as nitrogen pollution at the watershed-subestuary scale, also
may be facilitating invasion. Urbanization near coastlines appears to play an important role in the invasion success ofPhragmites in coastal wetlands of Chesapeake Bay and probably much of eastern North America. 相似文献
222.
Stratigraphic constraints on late Pleistocene glacial erosion and deglaciation of the Chukchi margin, Arctic Ocean 总被引:2,自引:0,他引:2
At least two episodes of glacial erosion of the Chukchi margin at water depths to ∼ 450 m and 750 m have been indicated by geophysical seafloor data. We examine sediment stratigraphy in these areas to verify the inferred erosion and to understand its nature and timing. Our data within the eroded areas show the presence of glaciogenic diamictons composed mostly of reworked local bedrock. The diamictons are estimated to form during the last glacial maximum (LGM) and an earlier glacial event, possibly between OIS 4 to 5d. Both erosional events were presumably caused by the grounding of ice shelves originating from the Laurentide ice sheet. Broader glaciological settings differed between these events as indicated by different orientations of flutes on eroded seafloor. Postglacial sedimentation evolved from iceberg-dominated environments to those controlled by sea-ice rafting and marine processes in the Holocene. A prominent minimum in planktonic foraminiferal δ18O is identified in deglacial sediments at an estimated age near 13,000 cal yr BP. This δ18O minimum, also reported elsewhere in the Amerasia Basin, is probably related to a major Laurentide meltwater pulse at the Younger Dryas onset. The Bering Strait opening is also marked in the composition of late deglacial Chukchi sediments. 相似文献
223.
Salt marsh intertidal creeks are important habitats for dozens of species of nekton, but few studies have attempted to quantify
patterns of tidal movement. We used the sweep flume, a new sampling device, to investigate relationships between depth and
movements of nekton inside the mouths of intertidal creeks. Sweep flumes located in three creek beds were used to collect
nekton at 10 cm increments (10–100 cm of water depth) during flood and ebb tides in the North Inlet, South Carolina, salt
marsh. Of the 37 taxa collected, 13 comprised>99.5% of the total catch and were the focus of the analysis. A nonlinear mixed
modeling procedure was used to determine, the depth at which each major taxon reached peak abundance during flood tides. With
high degrees of spatial and temporal consistency, resident taxa entered early on the rpsing tide and transient taxa entered
during mid to late tide. Depths of peak migrations varied among taxa and were consistent between creeks, days (within months),
and years. As summer progressed, depths of peak migration increased for young-of-the-yearLeiostomus xanthurus, Lagodon rhomboides, Mugil curema, Eucinostomus argenteus, andLitopenaeus setiferus as their median sizes increased. Within tides, depths of migration increased as a function of size forL. xanthurus andM. curema. Comparisons between flood and ebb tides indicated that most taxa exited the creeks at approximately the same depths at which
they entered. Relationships between major taxa pairs suggested that biotic interactions may have contributed to the structure
of the migrations observed in this study. Our results are the first to demonstrate quantitatively that the migrations of nektonic
taxa into intertidal creeks are structured and related to depth. 相似文献
224.
The primary scientific objectives of the Hard X-Ray Burst Spectrometer (HXRBS) to be flown on the Solar Maximum Mission are as follows: (1) To determine the nature of the mechanisms which accelerate electrons to 20–100 keV in the first stage of a solar flare and to > 1 MeV in the second stage of many flares; and (2) to characterize the spatial and temporal relation between electron acceleration, storage and energy loss throughout a solar flare.Measurements of the spectrum of solar X-rays will be made in the energy range from 20 to 260 keV using an actively-shielded CsI(Na) scintillator with a thickness of 0.635 cm and a sensitive area of 71 cm2. Continuous measurements with a time resolution of 0.128 s will be made of the 15-channel energy-loss spectrum of events in this scintillator in anticoincidence with events in the CsI(Na) shield. Counting-rate data with a time resolution as short as 1 ms will also be available from a limited period each orbit using a 32K-word circulating memory triggered by a high event rate.In the first year after launch, it is expected that approximately 1000 flares will be observed above the instrument sensitivity threshold, which corresponds to a 20–200 keV X-ray flux of 2 × 10–1 photons (cm2 s)–1 lasting for at least one second. 相似文献
225.
Lightcurves of 433 Eros are reported for 11 bandpasses ranging from 0.65 to 2.2 μm in wavelength. The relative spectral reflectance, R(λ), was not seen to vary during our observations. Eros has R(1.6 μm) = 1.5 ± 0.1 and R(2.2 μm) = 1.7 ± 0.1, where R(λ) is the spectral reflectance scaled to unity at λ = 0.56 μm. This spectral reflectance is suggestive of a mixture of silicates and material with high infrared reflectance, perhaps a metallic phase such as meteoritic “iron”. 相似文献
226.
227.
The purpose of this paper is to expand debate about the future landscapes of the upper Midwest of the United States. The paper addresses options that could reinvent the agricultural systems of the Corn Belt, which coincides with the Upper Mississippi River Basin. The changes would move this region from one dependent on a grain economy, with low economic returns and high nutrient and sediment losses, to a more ecologically-based landscape emphasizing nutrient sinks, especially for nitrogen, and a legume base for supplementing fertilizer nitrogen. The reinvented systems require a higher level of management to lessen nitrogen and phosphorus losses while supporting family farms and strong rural communities. This reinvented agriculture would ultimately benefit the Gulf of Mexico by significantly lowering the amount of nitrate exported to the Gulf. The paper is not intended to be a comprehensive review of the literature, nor one that offers the full range of options to address the problems facing the watershed and the owners and operators of the land. Rather, I hope to facilitate discussion of the goals of midwestern U.S. agriculture in relation to ecosystem protection. 相似文献
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