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171.
172.
Nicolas C.M. Marty Jordi Cama Daisuke Chino Angelina Razafitianamaharavo Jocelyne Brendlé Josep M. Soler Christophe Tournassat 《Geochimica et cosmochimica acta》2011,75(20):5849-5864
The effect of pH and Gibbs energy on the dissolution rate of a synthetic Na-montmorillonite was investigated by means of flow-through experiments at 25 and 80 °C at pH of 7 and 9. The dissolution reaction took place stoichiometrically at 80 °C, whereas at 25 °C preferential release of Mg over Si and Al was observed. The TEM-EDX analyses (transmission electronic microscopy with quantitative chemical analysis) of the dissolved synthetic phase at 25 °C showed the presence of newly formed Si-rich phases, which accounts for the Si deficit. At low temperature, depletion of Si concentration was attributed to incongruent clay dissolution with the formation of detached Si tetrahedral sheets (i.e., alteration product) whereas the Al behaviour remains uncertain (e.g., possible incorporation into Al-rich phases). Hence, steady-state rates were based on the release of Mg. Ex situ AFM measurements were used to investigate the variations in reactive surface area. Accordingly, steady-state rates were normalized to the initial edge surface area (11.2 m2 g−1) and used to propose the dissolution rate law for the dissolution reactions as a function of ΔGr at 25 °C and pH∼9:
173.
Climate change involves alterations in seasonality as well as shifts in mean annual temperature. Cold temperate lakes show
strong seasonality, with winter ice cover and alternating mixing and stratification periods during the ice-free season. These
physical changes are ultimately related to seasonal weather variation and also drive annual phytoplankton succession. Therefore,
phytoplankton remains in lake sediment records are potentially useful for reconstructing past seasonal climate signals. With
the exception of investigations on varved sediments, however, little research has been carried out on the subject. Here we
present two lines of evidence demonstrating that chrysophyte stomatocysts can be useful for inferring past climatic seasonality.
First, we show that marked seasonal stomatocyst replacement is related to periods of the main physical processes in the lake.
Second, using instrumental climate data and microfossils in sediment cores, we show that two main components of stomatocyst
variability over the last ~150 years are related to seasonal fluctuations. The first of these components is related to stomatocysts
present during summer stratification that respond to general warming trends, particularly in July and August. The second component
relates to stomatocysts typically present in the spring and autumn mixing periods. Numbers of these two stomatocyst types
vary inversely in response to variation in spring (April–May) temperatures. The number of stomatocyst types affected by spring
temperatures is much greater than those related to summer temperatures. We provide evidence that chrysophyte stomatocysts
are an excellent proxy for spring air temperature reconstructions, with little influence from summer or general annual trends.
We also show that with relatively little effort, it is possible to categorise the chrysophyte stomatocyst assemblage of a
lake to carry out detailed investigation of the record of seasonal changes preserved within the sediment. 相似文献
174.
AbstractThe Roc de Frausa Massif, located at the Eastern Pyrenees, is formed by a stratoid Pre-Hercynian deformed granite (orthogneiss) interbedded with metasedimentary series. Hercynian granitoids (St. Llorenç — La Jonquera pluton) surround the southern and eastern part of the massif and Hercynian basic igneous rocks (Ceret stock) occupy the central part of it. The Pre-Hercynian granite and the sedimentary series were involved, during the Hercynian orogeny, in complex polyphasic tectonics and metamorphism. As a result, an ubiquitous penetrative foliation was developed during the earlier stages. This foliation was subsequently folded into a complex antiformal structural formed by a double dome : Roc de Frausa dome and Mas Blanc dome. Main lithological boundaries (gneiss — metasediments and metasediments — granitoids) are broadly parallel to the regional foliation, and they all display the dome geometry. Interference fold pattern between two late phases, an ealier one with NE-SW trending folds and a younger one with NW-SE trending folds is responsible for the dome geometry. Mylonitic deformation, with W-E to NW-SE orientations has been attributed to the last folding phase. Regional metamorphic climax and contact metamorphism, the last one resulting from Hercynian granitoid emplacement, preceeded the above mentioned late folding event, which developed under retrograde metamorphic conditions. Regional peak metamorphism is recognized by the static crystallization of cordierite + potassium feldspar. This paragenesis indicates pressure — temperature conditions of about 3.1 Kbar and 660 °C maximum. Contact metamorphism overprints the earlier regional metamorphism. Parageneses and thermal gradient of contact metamorphism around La Jonquera pluton are very similar to those related to regional metamorphism, whereas parageneses produced around Ceret stock present garnet + potassium feldspar. Geothermometry indicates metamorphic conditions locally higher for this paragenesis (around 700 °C). 相似文献
175.
Mathieu Duval Christophe Falguères Jean-Jacques Bahain Rainer Grün Qingfeng Shao Maxime Aubert Jean-Michel Dolo Jordi Agustí Bienvenido Martínez-Navarro Paul Palmqvist Isidro Toro-Moyano 《Quaternary Research》2012,77(3):482-491
The combined U-series/electron spin resonance (ESR) dating method was applied to nine teeth from two Early Pleistocene archaeological sites located in the Orce area (Guadix-Baza Basin, Southern Spain): Fuente Nueva-3 (FN-3) and Barranco León (BL). The combination of biostratigraphy and magnetostratigraphy places both sites between the Olduvai and Jaramillo subchrons (1.78–1.07 Ma).Our results highlight the difficulty of dating such old sites and point out the limits of the combined U-series/ESR dating method based on the US model. We identified several sources of uncertainties that may lead to inaccurate age estimates. Seven samples could not be dated because the dental tissues had (230Th/234U) activity ratios higher than equilibrium, indicating that uranium had probably leached from these tissues. It was however possible to calculate numerical estimates for two of the teeth, both from FN-3. One yielded a Middle Pleistocene age that seems to be strongly underestimated; the other provided an age of 1.19 ± 0.21 Ma, in agreement with data obtained from independent methods. The latter result gives encouragement that there are samples that can be used for routine dating of old sites. 相似文献
176.
177.
178.
The objective of this study was to investigate the removal mechanism of U(VI) from groundwater by magnetite as the main product of anoxic steel corrosion. For this purpose, a systematic sequence of batch experiments was conducted to focus the active role of magnetite in the reduction of U under different conditions. Results indicated that under anoxic conditions U(VI) was sorbed at the magnetite surface, whereas under reducing conditions at different H2(g) pressures, U was present in tetravalent form as amorphous UO2. 相似文献
179.
Leandro G. Althaus Alejandro H. Córsico Jordi Isern Enrique García-Berro 《Astronomy and Astrophysics Review》2010,18(4):471-566
White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. Since the coolest white
dwarfs are very old objects, the present population of white dwarfs contains a wealth of information on the evolution of stars
from birth to death, and on the star formation rate throughout the history of our Galaxy. Thus, the study of white dwarfs
has potential applications in different fields of astrophysics. In particular, white dwarfs can be used as independent reliable
cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations,
such as our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs
allow these stars to be used as cosmic laboratories for studying physical processes under extreme conditions that cannot be
achieved in terrestrial laboratories. Last but not least, since many white dwarf stars undergo pulsational instabilities,
the study of their properties constitutes a powerful tool for applications beyond stellar astrophysics. In particular, white
dwarfs can be used to constrain fundamental properties of elementary particles such as axions and neutrinos and to study problems
related to the variation of fundamental constants. These potential applications of white dwarfs have led to renewed interest
in the calculation of very detailed evolutionary and pulsational models for these stars. In this work, we review the essentials
of the physics of white dwarf stars. We enumerate the reasons that make these stars excellent chronometers, and we describe
why white dwarfs provide tools for a wide variety of applications. Special emphasis is placed on the physical processes that
lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance
changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational
instability strips. The existence of these instability strips provides astronomers with a unique opportunity to peer into
their internal structure that would otherwise remain hidden from observers. We will show that this allows one to measure stellar
masses with unprecedented precision and to infer their envelope thicknesses, to probe the core chemical stratification, and
to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white
dwarfs and the most recent applications of white dwarf asteroseismology. 相似文献
180.
A procedure is presented for investigating the response of reinforced concrete buildings to rockfall impact. The method considers
a single rock hit on the basement columns, and it includes four steps: (a) calculation of the probability of a rock impact
on a member of the load-bearing system, taking into account the block size and the design of the structure; (b) evaluation
of the response of one or more structural elements to the hit based on element capacity; (c) in the case of structural element
failure, assessment of the robustness of the whole structural system, calculating the potential for progressive collapse;
and (d) calculation of a damage index (DI), which is the ratio of structural elements that fail to the total number of structural
elements. The proposed method is applied to a reinforced concrete building for a range of rockfall paths and intensities.
The analysis has been carried out for a 2-m-diameter block and velocities < 3.5 m/s. The possible damage range is found to
be highly variable, with DI values ranging from 0.01 to 1 depending on the impact location and block velocity. 相似文献