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831.
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834.
835.
Valerie A. Barber Glenn Patrick Juday Bruce P. Finney Martin Wilmking 《Climatic change》2004,63(1-2):91-120
Maximum latewood density and δ 13C discrimination of Interior Alaska white spruce were used to reconstruct summer (May through August) temperature at Fairbanks for the period 1800–1996, one of the first high-resolution reconstructions for this region. This combination of latewood density and δ 13C discrimination explains 59.9% of the variance in summer temperature during the period of record 1906–1996. The 200-yr. reconstruction is characterized by 7 decadal-scale regimes. Regime changes are indicated at 1816, 1834, 1879, 1916, 1937, and 1974, are abrupt, and appear to be the result of synoptic scale climate changes. The mean of summer temperature for the period of reconstruction (1800–1996) was 13.49 °C. During the period of instrument record (1903–1996) the mean of summer temperature was 13.31 °C for both the reconstruction and the recorded data. The coldest interval was 1916–1937 (12.62 ° C) and the warmest was 1974–1996 (14.23 °C) for the recorded data. The reconstruction differs from records of northern hemisphere temperatures over this period, especially because of Interior Alaska warm periods reconstructed from 1834 to 1851 (14.24 °C) and from 1862 to 1879 (14.19 °C) and because of the cool period in the early part of the 20th century (1917–1974). We show additional tree ring data that support our reconstruction of these warm periods. Alternate hypotheses involving autogenic effect of tree growth on the site, altered tree sensitivity, or novel combinations of temperature and precipitation were explored and while they cannot be ruled out as contributors to the anomalously warm 19th century reconstruction, they were not supported by available data. White spruce radial growth is highly correlated with reconstructed summer temperature, and temperature appears to be a reliable index of carbon uptake in this system. 相似文献
836.
The weathering of a rhyolitic lava flow of the Paraná Basin (Brazil) developed, on few centimeters only, simplified petrographic features. The initial meteoric fluid pass-way is marked by celadonite-to-nontronite transformation. The second step is the complete dissolution of plagioclases, and the last one is the massive precipitation of halloysite. The geochemical mass balances according to the alteration features demonstrate the progressive leaching of major and trace elements fractionated by plagioclases, glass and opaques, and the sorption of Cu>As>Sb in the halloysite matrix. To cite this article: M. Bernard et al., C. R. Geoscience 336 (2004). 相似文献
837.
Anne S. Marsh Daniel P. Rasse Bert G. Drake J. Patrick Megonigal 《Estuaries and Coasts》2005,28(5):694-704
The effects of long-term exposure to elevated atmospheric CO2 (ambient + 340 ppmv) on carbon cycling were investigated for two plant communities in a Chesapeake Bay brackish marsh, one
dominated by the C3 sedgeSchoenplectus americanus and the other by the C4 grassSpartina patens. Elevated CO2 resulted in a significant increase in porewater concentrations of DIC at 30 cm depth (p < 0.1). The CO2 treatment also yielded increases in DOC (15 to 27%) and dissolved CH4 (12–18%) in the C3 marsh (means for several depths over the period of June 1998 and June 1999), but not at a significant level. Elevated CO2 increased mean ecosystem emissions of CO2 (34–393 g C m−2 yr−1) and CH4 (0.21–0.40 g C m−2 yr−1) in the C3 community, but the effects were only significant on certain dates. For example, CO2 enrichment increased C export to the atmosphere in the C3 community during one of two winter seasons measured (p = 0.09). In the C4 community, gross photosynthesis responded relatively weakly to elevated CO2 (18% increase, p > 0.1), and the concomitant effects on dissolved carbon concentrations, respiration, and CH4 emissions were small or absent. We concluded that elevated CO2 has the potential to increase dissolved inorganic carbon export to estuaries. 相似文献
838.
839.
Diopside-rich, skarn-hosted, copper–gold ore derived primarily from carbonaceous metapelites at Mount Elliott forms a distinctive
member of the spectrum of Cu–Au–(Fe oxide) deposit styles in the Cloncurry district of the Paleoproterozoic to Mesoproterozoic
Mount Isa Block. The mine sequence is a package of carbonaceous metapelites and metagreywackes containing amphibolites derived
from tholeiitic basic rocks. A 40Ar–39Ar age spectrum with an extensive plateau-like segment at 1,510 ± 3 Ma from an actinolite associated with sulfides is taken
to represent the age of mineralization and is identical within error to the ages of most of the nearby batholithic granitoids.
The mine sequence is locally intruded by 1- to 10-m-thick late- to post-tectonic trachyandesite dykes, which were emplaced
during the hydrothermal activity that created the orebodies and have affinities with the regional high potassium “Eureka”
supersuite granitoids. Stable isotope data are consistent with dominantly magmatic fluids during mineralization and the regionally
distinctive skarn (Ca–Mg) and Cu–Au–Ni–Co–Te–Se (low Pb–Zn–Ag–Sb) chalcophile element associations may reflect a primitive
magmatic fluid source and/or leaching of these elements from country rocks. Mount Elliott is an unusual skarn deposit characterized
by pronounced early albitization (K–Fe–Mg depletion) of the host rocks succeeded by predominantly open-space deposition of
sodic diopside ± actinolite ± scapolite ± andradite ± magnetite ± sulfides ± apatite ± allanite ± tourmaline ± calcite. The
Ca–Fe–Mg(–Na)-rich (manganese-poor) chemistry was imposed from the fluid phase in the absence of carbonate-rich protoliths.
Immobile trace element (Ti, Zr, Nb) geochemistry shows that Mount Elliott skarns formed in both metasedimentary and mafic
metavolcanic host rocks, but the former are the main hosts of ore in upper and lower ore zones that represent most of the
resource. Banded skarns derived from a distinct calc-silicate/marble package at the nearby SWAN prospect have higher Nb/TiO2 and Zr/TiO2 ratios than the Mount Elliott metasediment-derived skarns, consistent with different provenance of the detrital components
in the two sequences. Medium- to coarse-grained massive skarn and skarn breccia in the Mount Elliott lower ore zone formed
in pelites and the trachyandesite dykes are the only intrusive rocks that could be genetically related to the mineralization
in the immediate vicinity of the orebodies.
Received: 1 September 1999 / Accepted: 28 September 2000 相似文献
840.
Patrick Campbell 《Journal of Arid Environments》1997,36(4):705-709
Perennial plants eaten by mountain gazelle (Gazella gazella) and Nubian ibex (Capra ibex nubiana) were examined during the 1994 growing season (May and June) in the Ibex Reserve, Saudi Arabia. Individual plants were assigned a browse rating based on extent of vegetative tissue removal. Gazelles and ibex were found to have a diverse diet, but were primarily browsers of woody plants. 相似文献