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101.
Joel Kronfeld 《Earth and Planetary Science Letters》1974,21(3):327-330
234Th alpha-recoil appears to be a valid mechanism for explaining the generation of extreme234U/238U disequilibria found in the waters of the Trinity aquifer. 相似文献
102.
Adam J.R. Kent Benjamin Jacobsen David W. Peate Tod E. Waight Joel A. Baker 《Geostandards and Geoanalytical Research》2004,28(3):417-429
We present data for the concentrations of eleven rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Yb, Lu) in eleven international geochemical reference materials obtained by isotope dilution multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). We have analysed both rock powders and synthetic silicate glasses, and the latter provide precise data to support the use of these as reference materials for in situ trace element determination techniques. Our data also provide precise measurements of the abundance of mono-isotopic Pr in both glasses and powders, which allows more accurate constraints on the anomalous redox-related behaviour of Ce during geochemical processes. All materials were analysed in replicate providing data that typically reproduce to better than one percent. Sm/Nd ratios in all these materials also reproduce to better than 0.2% and are accurate to < 0.2% and can thus be used as calibrants for Sm-Nd geochronology. Our analyses agree well with existing data on these reference materials. In particular, for NIST SRM 610, USGS BHVO-2, AGV-1 and AGV-2, our measured REE abundances are typically within < 2% (and mostly 1%) of REE concentrations previously determined by isotope dilution analysis and thermal ionisation mass spectrometry, consistent with the higher degree of precision and accuracy obtained from isotope dilution techniques. Close agreement of results between basaltic glass reference materials USGS BHVO-2G and BCR-2G and the BHVO-2 and BCR-2 powders from which they were created suggests that little fractionation, concentration or dilution of REE contents occurred during glass manufacture. 相似文献
103.
Joel C. Creed 《Estuaries and Coasts》2004,27(2):197-200
This note reports an unknown trophic interaction between a mammalian herbivore, the capybara (Hydrochaeris hydrochaeris), and the seagrassRuppia maritima (wigeongrass) and compares the feeding behavior of capybaras to other seagrass grazers. Observations were made in Spring
2002 in the Barra Grande, a small, shallow, moderately stratified, bar-built estuary at Ilha Grande, Rio de Janeiro State,
southeast Brazil. The activities of the capybaras were investigated and grazing impacts were quantified in situ. The capybaras
were observed feeding onR. maritima during the day and aquatic feeding alternated with periods of feeding on land.R. maritima was the only submerged aquatic vegetation to be consumed by the capybaras. The feeding activity of the capybaras on wige
ongrass consisted of alternately diving down to theR. maritima then surfacing; the capybaras spent a significantly greater amount of time under water. In the area where the capybaras foraged
18.1% of the seagrass meadow showed recent grazing scars. Vegetation of recently and not recently grazed areas were compared.
Grazing scars, which were slightly elongated, were not completely devoid ofR. maritima but presented reduced standing crop: canopy height, shoot density, and shoot, rhizome, and root biomass were reduced in grazed
areas. The grazing patterns observed in capybaras resembled those previously reported in the sirenia, mammals that include
two seagrass-eating species. 相似文献
104.
105.
The distribution of epicenters of both historic earthquakes and recent seismic events in southeastern Ghana, compiled from local and teleseismic networks, show strong correlation with the Pan-African structures onshore and indicate an alignment with disruptions on seismic sections offshore. The seismic reflection sections reveal basement structures of the external zone of the Pan-African Dahomeyide orogen and these structures can be traced to offsets of shelf strata and seabottom reflectors, providing direct evidence, for the first time, for neotectonic activity that may be responsible for seismicity in the area. The deep structure of the external zone consists of moderately-dipping reflectors inferred to represent high-strain zones in the variably deformed margin of the West African craton. Taken together, the available data suggest that active tectonics in this intraplate environment may involve inversion of the Pan-African thrust structures but that this activity is apparently not related to reactivation of the nearby Romanche Fracture Zone. 相似文献
106.
Joel S. Steward Robert W. Virnstein Lori J. Morris Edgar F. Lowe 《Estuaries and Coasts》2005,28(6):923-935
Seagrass protection and restoration in Florida’s Indian River Lagoon system (IRLS) is a mutual goal of state and federal programs.
These programs require, the establishment of management targets indicative of seagrass recovery and health. We used three
metrics related to seagrass distribution: areal coverage, depth limit, and light requirement. In order to account for the
IRLS’s spatial heterogeneity and temporal variability, we developed coverage and depth limit targets for each of its 19 segments.
Our method consisted of two steps: mapping the union of seagrass coverages from all availabe mapping years (1943, 1986, 1989,
1992, 1994, 1996, and 1999) to delineate wherever seagrass had been mapped and determining the distribution of depth limits
based on 5,615 depth measurements collected on or very near the deep-edge boundary of the union coverage. The frequency distribution
of depth limits derived from the union coverage, along with the median (50th percentile) and maximum (95th percentile) depth
limits, serve as the seagrass depth targets for each segment. The median and maximum depth targets for the IRLS vary among
segments from 0.8 to 1.8 and 1.2 to 2.8 m, respectively.Halodule wrightii is typically the dominant seagrass species at the deep-edge of IRLS grass beds. We set light requirement targets by using
a 10-yr record of light data (1990–1999) and the union coverage depth limit distributions from the most temporally stable
seagrass segments. The average annual light requirement, based on the medians of the depth limit distributions, is 33 ± 17%
of the subsurface light. The minimum annual light requirement, based on of the 95th percentile of the depth distributions,
is 20 ± 14%; the minimum growing season light requirement (March to mid September) is essentially the same (20 ± 13%). Variation
in depth limits and light requirements, is probably due to factors other than light that influence the depth limit of seagrasses
(e.g., competition, physical disturbance). The methods used in this study are robust when applied to large or long-term data
sets and can be applied to other estuaries where grass beds are routinely monitored and mapped. 相似文献
107.
Joel B. Smith 《Climatic change》1998,40(3-4):705-707
108.
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