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331.
The kinetics of hydrogen diffusion in enstatite was studied by hydrogen–deuterium exchange experiments in the range of 1–5,000 bar and 700–850°C using synthetic single crystals of pure and Cr-doped enstatites. The OH- and OD-content in the samples was quantified after each thermal treatment with Fourier transformed infrared spectroscopy. H–D-exchange rates were measured parallel to the three crystallographic axes. In addition, in order to visualize diffusion profiles, OH and OD were mapped for some samples, utilizing synchrotron IR micro-spectroscopy. Hydrogen self-diffusivities derived from D/H exchange experiments at one atmosphere are very similar to the chemical diffusivity of hydrogen in natural Fe-bearing orthopyroxene, which was reported previously (Stalder and Skogby 2003) to exhibit a small, but significant anisotropy (D[001] > D[100] > D[010]). Activation energies are estimated to be 211 (±31) kJ/mol for diffusion parallel [100] and 185 (±28) kJ/mol for diffusion parallel [010]. Lattice diffusion of hydrogen is decelerated by more than one order of magnitude when Cr is dissolved in enstatite. In comparison to the chemical composition, pressure seems to have only a minor influence on hydrogen diffusion. Compared to other minerals in the Earth’s upper mantle, enstatite exhibits the highest activation energy for hydrogen diffusion, suggesting faster diffusion than in other mafic minerals at mantle temperatures, but slower diffusion at crustal conditions. Thus under upper mantle conditions, physical properties that are expected to be influenced by hydrogen mobility, such as electrical conductivity, may in enstatite be more intensely affected by the presence of hydrogen than in other upper mantle minerals. 相似文献
332.
Acidification in European mountain lake districts: A regional assessment of critical load exceedance 总被引:1,自引:0,他引:1
333.
John Grattan Roland Rabartin Stephen Self Thorvaldur Thordarson 《Comptes Rendus Geoscience》2005,337(7):641-651
The impact that volcanic eruptions may have upon environments far from the volcanic source is conventionally assumed to depend on climatic modification by emitted gases. However, recent research has suggested that the damage caused by the direct impact of volcanic gases, mainly H2SO4, may be profound. This paper highlights the severity of this mechanism by reference to human sickness and death in France and contiguous with the eruption of the Laki fissure in Iceland in 1783. This work demonstrates the gains which may be made by interdisciplinary teams of researchers and illustrates the valuable knowledge that remains to be revealed by further research in the French historical record. To cite this article: J. Grattan et al., C. R. Geoscience 337 (2005). 相似文献
334.
335.
AMENITY MIGRATION IN THE U.S. SIERRA NEVADA* 总被引:1,自引:0,他引:1
ABSTRACT. Since 1960 California's Sierra Nevada counties have ranked among the regions with the strongest relative population growth in the state. Reassessment of peripheral areas has been the main force driving population and settlement growth in the central Sierra Nevada of California and Nevada, termed “amenity migration” or “counterurbanization.” This study analyzes the impacts of amenity migrants—“urban refugees”—on socioeconomic conditions in high‐mountain regions. We define these regions as the “High Sierra,” comprising zones at elevations more than 1,800 meters above sea level. People who migrate to the High Sierra tend to be white and well educated, with considerable household earnings. Unlike the population in the foothills, these migrants are not senior citizens. Their demand for periodic or permanent residences has caused housing prices to increase enormously. As a result, a majority of homes are now priced well beyond the reach of local salaries, which may lead to potential conflict between locals and newcomers. The massive settlement expansion in high‐mountain areas requires a new approach to land‐use planning, one that takes functional regions into account. Therefore, it is expedient to reassess existing jurisdictional boundaries. 相似文献
336.
Thomas Brühwiler Peter A. Hochuli Roland Mundil Wolfgang Schatz Peter Brack 《Swiss Journal of Geoscience》2007,100(3):443-455
New finds of fossils including bivalves, ammonoids, brachiopods and palynomorphs from the Middle Triassic Reifling Formation
significantly improve the age assignment for this unit in Liechtenstein and Vorarlberg. The lower part of the Reifling Formation
is tentatively referred to the Late Anisian Paraceratites trinodosus Zone and somewhat older levels, whereas the uppermost part reaches the Ladinian Protrachyceras archelaus Zone (ammonoid zonation). The Middle Triassic successions of the study area are correlated with the coeval South Alpine reference
section at Bagolino (Brescian Prealps), which also bears the Ladinian GSSP. The comparison shows that the Reifling Formation
in the study area is age-equivalent with the South Alpine Prezzo Limestone and the Buchenstein Formation. A volcanoclastic
layer in the upper part of the Reifling Formation at Flexenpass yields a U-Pb zircon age of 239.3 +/- 0.2 Ma. This value is
slightly older than previously published minimum ages from equivalent horizons in the Southern Alps; the difference is thought
to be mainly due to improved pre-treatment of zircons (annealing/chemical abrasion), which significantly reduces the effects
of Pb loss. The new radio-isotope age further constrains the stratigraphical age of the Reifling Formation and supports the
proposed biostratigraphy-based correlation of Middle Triassic successions in the Eastern and Southern Alps.
Editorial handling: E. Erba & J.-P. Billon-Bruyat 相似文献
337.
Roland Mundil Rainer Zühlke Thilo Bechstädt Arndt Peterhänsel Sven O. Egenhoff Felix Oberli Martin Meier Peter Brack Hans Rieber 《地学学报》2003,15(2):81-87
ABSTRACT Sedimentary cycles recorded in young sediments are often attributed to fluctuations of the Earth's climate on a 104 −106 -year scale which in turn is governed by periodic variations in solar insolation linked to orbital (Milankovitch) parameters. A spectacular example of cyclic stratal patterns in ancient deposits is the Middle Triassic Latemar carbonate platform (W Dolomites, N Italy). Based on spectral analyses from previous studies, a superimposition of precession (∼20 ka) and eccentricity (∼100 ka) controlled sea-level fluctuations has been suggested to account for the stacking hierarchy at Latemar, with ∼20 ka being assigned to each highest-order depositional cycle. Zircon U–Pb isotopic ages from volcanic-ash layers within the cyclic succession, corroborated by biostratigraphic constraints, suggest that the average time interval for every individual cycle is significantly smaller than the shortest Milankovitch period and therefore challenge previously published interpretations relating distinct spectral peaks to the above mentioned hierarchy. However, our new spectral data indicate that cyclicities resembling Milankovitch characteristics might exist, but on an entirely different scale. Our findings show that frequency spectra should only be interpreted in combination with robust age control. They also encourage the search for complementary mechanisms controlling carbonate deposition. 相似文献
338.
339.
The title reaction was studied by photolyzing mixtures of Cl2 and SO2 with and without O2 present in an atmosphere of N2, using Fourier transform infrared spectrophotometry to monitor reactants and products. In the absence of oxygen, sulfur dioxide is quantitatively converted to sulfuryl chloride. With 10 to 150 Torr O2 present H2SO4 is produced as well as SO2Cl2. When a number of speculative reactions inferred from these experiments are added to a published model for Venus stratospheric chemistry, it emerges that SO2Cl2 is a key reservoir species for chlorine and that the reaction between Cl and So2 provides an important cycle for destroying O2 and converting SO2 to H2So4. The modified model could provide a possible solution to the photochemistry of the Venus stratosphere if the mixing ratio of chlorine on Venus were as high as 8 ppm. 相似文献
340.
The vertical distribution of foraminifera, testate amoebae and diatoms was investigated in saltmarshes in the Taf estuary (south Wales), the Erme estuary (south Devon) and the Brancaster marshes (north Norfolk), to assess the use of multiproxy indicators in sea‐level reconstructions. A total of 116 samples were subjected to regression analyses, using the program calibrate, with duration of tidal flooding as the dependent variable. We found that the relationship between flooding duration and taxa was strongest for diatoms and testate amoebae and weakest for foraminifera. The vertical range of testate amoebae in saltmarshes is small. Their lower tolerance limit in present‐day saltmarshes occurs where tides cover the marsh less than a combined total of 7 days (1.9%) in a year. However, they are important sea‐level indicators because information for sea‐level reconstruction is best derived from sediments that originate in the highest part of the intertidal zone. Diatoms span the entire sampled range in intertidal and supratidal areas, whereas the upper limit of foraminifera is found very close to the highest astronomical tide level. Local training sets provide reconstructions with higher accuracy and precision than combined training sets, but their use is limited if they do not represent adequate modern analogues for fossil assemblages. Although analyses are time consuming, a regional training set of all three groups of micro‐organisms yields highly accurate (r2 = 0.80) and precise (low value of root mean square error) predictions of tidal level. This approach therefore could improve the accuracy and precision of Holocene sea‐level reconstructions. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献