全文获取类型
收费全文 | 70719篇 |
免费 | 624篇 |
国内免费 | 552篇 |
专业分类
测绘学 | 1770篇 |
大气科学 | 4210篇 |
地球物理 | 12848篇 |
地质学 | 28350篇 |
海洋学 | 5945篇 |
天文学 | 15874篇 |
综合类 | 289篇 |
自然地理 | 2609篇 |
出版年
2022年 | 600篇 |
2021年 | 897篇 |
2020年 | 1013篇 |
2019年 | 1078篇 |
2018年 | 4170篇 |
2017年 | 3713篇 |
2016年 | 3313篇 |
2015年 | 1076篇 |
2014年 | 2049篇 |
2013年 | 3131篇 |
2012年 | 2891篇 |
2011年 | 4513篇 |
2010年 | 3923篇 |
2009年 | 4528篇 |
2008年 | 3838篇 |
2007年 | 4410篇 |
2006年 | 2581篇 |
2005年 | 1674篇 |
2004年 | 1601篇 |
2003年 | 1574篇 |
2002年 | 1473篇 |
2001年 | 1236篇 |
2000年 | 1106篇 |
1999年 | 805篇 |
1998年 | 848篇 |
1997年 | 836篇 |
1996年 | 667篇 |
1995年 | 654篇 |
1994年 | 656篇 |
1993年 | 527篇 |
1992年 | 525篇 |
1991年 | 530篇 |
1990年 | 603篇 |
1989年 | 455篇 |
1988年 | 443篇 |
1987年 | 497篇 |
1986年 | 374篇 |
1985年 | 569篇 |
1984年 | 547篇 |
1983年 | 506篇 |
1982年 | 519篇 |
1981年 | 437篇 |
1980年 | 481篇 |
1979年 | 375篇 |
1978年 | 425篇 |
1977年 | 341篇 |
1976年 | 327篇 |
1975年 | 336篇 |
1974年 | 314篇 |
1973年 | 341篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
Birger Rasmussen Ian R. Fletcher Janet R. Muhling 《Contributions to Mineralogy and Petrology》2011,162(6):1259-1277
Xenotime is a widespread accessory mineral in lower greenschist to upper amphibolite facies metasedimentary rocks from the
Palaeoproterozoic Mount Barren Group, southwestern Australia. Xenotime is closely associated with detrital zircon, commonly
forming syntaxial outgrowths, in samples of sandstone, micaceous quartzite, slate, phyllite, garnet-bearing semi-pelites,
and in kyanite-, garnet-, and staurolite-bearing mica schists. In situ geochronology of xenotime from lower greenschist sandstones
has previously yielded multiple U–Pb ages with peaks at ~2.0, ~1.7, and ~1.65 Ga, interpreted to represent the age of detritus,
early diagenesis, and a later thermal event, respectively. New U–Pb dating of xenotime in slate yields a major population
at ~1.7 Ga with a minor population at ~1.2 Ga, reflecting diagenetic and metamorphic growth, respectively, whereas xenotime
in phyllite forms a minor age population at ~1.7 Ga and a main peak at ~1.2 Ga. Mid-greenschist facies semi-pelitic schists
(quartz-muscovite-garnet) contain xenotime that formed before 1.8 Ga and at 1.2 Ga, representing detrital and peak metamorphic
ages, respectively. Xenotime in samples of amphibolite facies schist (650°C and ~8 kbars) yields U–Pb ages of ~1.2 Ga, coinciding
with the time of peak metamorphism. A single analysis of a xenotime core from an amphibolite facies schist gave an age of
~1.8 Ga, consistent with the presence of detrital xenotime. Our results suggest that detrital xenotime may be preserved under
greenschist facies conditions, but is largely replaced during upper amphibolite facies conditions. Detrital xenotime is replaced
through dissolution–reprecipitation reactions forming compositionally distinct rims during greenschist and amphibolite facies
metamorphism at 1.2 Ga. Diagenetic xenotime is present in lower greenschist facies samples, but was not observed in metasedimentary
rocks that had experienced temperatures above mid-greenschist facies metamorphism (450°C). The apparent disappearance of detrital
and diagenetic xenotime and appearance of metamorphic xenotime during prograde metamorphism indicates that some of the yttrium,
heavy rare earth elements, and phosphorus needed for metamorphic xenotime growth are probably derived from the replacement
of detrital and diagenetic xenotime. 相似文献
952.
Idael Francisco Blanco-Quintero C. Lázaro A. García-Casco J. A. Proenza Y. Rojas-Agramonte 《Contributions to Mineralogy and Petrology》2011,162(2):395-413
Whole-rock compositions of muscovite-bearing amphibolite, trondhjemite, pegmatite and quartz-muscovite rocks from Sierra del
Convento and La Corea mélanges (eastern Cuba), as well as mineralogy, record complex circulation of Ba-rich fluids and melts
in the subduction environment. Partial melting of fluid-fluxed, MORB-derived amphibolite produced trondhjemite magmas that
crystallized at depth, in some cases evolving into pegmatites. Qtz-Ms rocks probably crystallized from primary fluids derived
from subducted sediments. All these rocks have elevated concentrations of large-ion lithophile elements, especially Ba (up
to 11,810 ppm), presumably released from slab sediments by fluids and/or melts. Fluid–rock interaction produced crystallization
of phengite in parental amphibolites. The phengite crystallized in all types of rocks is rich in Ba, with concentric zoning,
characterized by Ba-rich cores and Ba-poor rims, indicating a compatible behaviour of Ba in the studied systems. Zoning in
phengite is governed primarily by the celadonite (tschermak) exchange vector ((Mg,Fe)Si-(Al)-2), with more moderate contributions of the celsian (BaAl-(KSi)-1) and paragonite (NaK-1) exchange vectors. Late remobilization of Ba at relatively low temperature formed retrograde celsian. The compatible behaviour
of Ba in the studied rocks strengthens the importance of the stability of phengite for the transfer of LIL elements from the
subduction to the volcanic arc environments. 相似文献
953.
954.
955.
A. M. Nikishin 《Moscow University Geology Bulletin》2011,66(4):225-241
The Earth formed through a hot accretion process. Almost simultaneously, the core and the mantle were separated from each
other. At the final stages of the accretion process, the outer layer approximately 2000 km thick was molten, thus representing
a magma ocean. This magma ocean produced the primary crust of the Earth. Surface waters were precipitated from the atmosphere
and released from the crystallizing magma ocean. The plate tectonic processes started at around 4.3 to 4 Ga BP. In the Archean,
the overall tectonic mechanism was quite specific, due to substantially higher mantle temperature and thicker oceanic crust.
The normal plate tectonics acted during the Proterozoic and Phanerozoic with the periodic assembly of continents, which are
known as supercontinent cycles. 相似文献
956.
Redox problems in the “metallogenic specialization” of magmatic rocks and the genesis of hydrothermal ore mineralization 总被引:1,自引:0,他引:1
I. N. Kigai 《Petrology》2011,19(3):303-321
Considering the history and current state of the problem of the so-called metallogenic specialization of magmatic rocks, the
paper places emphasis onto various aspects of the genesis of ore mineralization depending on the redox state of magmas (as
a logical continuation of S. Ishihara’s works), fluids, and host rocks. These problems were inadequately poorly explored and
discussed by researchers dealing with ore deposits. Various possible variants of ore-forming redox processes for different
types of mineral deposits, with ore mineralization affiliated to granites (Ta, Sn, W, Mo, and Be) and mafic magmas (Au, Ag,
U, Cu, Zn, Pb, As, Sb, and Hg) and, accordingly, to crustal and mantle origin, are discussed. On the basis of analyzed geological
data, including those published over the past three decades, it is shown that the redox state of ore-producing magmas commonly
significantly impacted not only the ore potential of magmatic complexes but also the genetic type of the ore mineralization.
The redox state of the fluids predetermined the transport and precipitation speciation of metals. Influence mechanisms of
hydrocarbons from sedimentary country rocks and gaseous products of their pyrolysis on ore deposition of various metals are
considered. Understanding these mechanisms can be helpful for predicting the possible precipitation sites of ore mineralization
of noble, radioactive, and chalcophile metals. 相似文献
957.
Catarina Vinagre João Salgado Henrique N. Cabral Maria J. Costa 《Estuaries and Coasts》2011,34(4):663-674
River flow variability is known to influence estuarine production, yet knowledge on its effect upon estuarine food webs dynamics
is still scarce. Stable carbon and nitrogen isotopes were used to assess the effect of river flow in the connectivity and
food web interactions between the two main fish nursery areas of the Tagus estuary. The aims of the present work were to investigate
the seasonal variation in food web structure and the exchange rate of individuals of marine juvenile fish among estuarine
nurseries, to compare the spring of a rainy year (2001) with that of an average year (2000), and to investigate the impact
of the winter floods of 2001. A low level of connectivity was observed for the fish species that use these areas as nurseries.
In low river flow conditions, two isotopically distinct food webs were established in each nursery area. These food webs were
very sensitive to small variations in the freshwater input. Winter floods seem to disrupt the localized food webs that are
established in low river flow periods, leading to the re-establishment of a wider food web. While in rainy years this wide
food web is maintained until spring, in average years the food web undergoes fragmentation into two localized and isotopically
distinctive food webs. The increase in frequency of droughts due to climate change should lower the connectivity of the estuarine
fish nurseries food webs, causing habitat fragmentation and consequent loss in complexity and resilience. 相似文献
958.
Understanding the relationships between speleothem stable isotopes (δ13C δ18O) and in situ cave forcing mechanisms is important to interpreting ancient stalagmite paleoclimate records. Cave studies have demonstrated that the δ18O of inorganically precipitated (low temperature) speleothem calcite is systematically heavier than the δ18O of laboratory-grown calcite for a given temperature. To understand this apparent offset, rainwater, cave drip water, groundwater, and modern naturally precipitated calcite (farmed in situ) were grown at multiple locations inside Hollow Ridge Cave in Marianna, Florida. High resolution micrometeorological, air chemistry time series and ventilation regimes were also monitored continuously at two locations inside the cave, supplemented with periodic bi-monthly air gas grab sample transects throughout the cave.Cave air chemistry and isotope monitoring reveal density-driven airflow pathways through Hollow Ridge Cave at velocities of up to 1.2 m s−1 in winter and 0.4 m s−1 in summer. Hollow Ridge Cave displays a strong ventilation gradient in the front of the cave near the entrances, resulting in cave air that is a mixture of soil gas and atmospheric CO2. A clear relationship is found between calcite δ13C and cave air ventilation rates estimated by proxies pCO2 and 222Rn. Calcite δ13C decreased linearly with distance from the front entrance to the interior of the cave during all seasons, with a maximum entrance-to-interior gradient of Δδ13CCaCO3 = −7‰. A whole-cave “Hendy test” at multiple contemporaneous farming sites reveals that ventilation induces a +1.9 ± 0.96‰ δ13C offset between calcite precipitated in a ventilation flow path and calcite precipitated on the edge or out of flow paths. This interpretation of the “Hendy test” has implications for interpreting δ13C records in ancient speleothems. Calcite δ13CCaCO3 may be a proxy not only for atmospheric CO2 or overlying vegetation shifts but also for changes in cave ventilation due to dissolution fissures and ceiling collapse creating and plugging ventilation windows.Farmed calcite δ18O was found to exhibit a +0.82 ± 0.24‰ offset from values predicted by both theoretical calculations and laboratory-grown inorganic calcite. Unlike δ13CCaCO3, oxygen isotopes showed no ventilation effects, i.e. Δδ18OCaCO3 appears to be a function of growth temperature only although we cannot rule out a small effect of (unmeasured) gradients in relative humidity (evaporation) accompanying ventilation. Our results support the findings of other cave investigators that water-calcite fractionation factors observed in speleothem calcite are higher that those measured in laboratory experiments. Cave and laboratory calcite precipitates may differ mainly in the complex effects of kinetic isotope fractionation. Combining our data with other recent speleothem studies, we find a new empirical relationship for cave-specific water-calcite oxygen isotope fractionation across a range of temperatures and cave environments:
1000lnα=16.1(103T-1)-24.6 相似文献
959.
A geophysical study, based on 96 electrical resistivity measurements with a line length up to 4 km, was performed in the southern and southwestern parts of the Meknes Plateau, Morocco, which is a part of the Saiss Basin, located between the Rif Range to the north and the Middle Atlas Range to the south. This basin, whose maximum depth is 1.5km in the north, is filled with Triassic to Quaternary deposits overlying the Palæozoic basement and includes two main aquifers. The interpretation of the resistivity measurements, calibrated from deep boreholes, made it possible to obtain a new hydrogeological model for the Saiss Basin. The understanding of the basin structure is of primary importance for the water supply of this area, which has been affected by severe droughts in recent years. 相似文献
960.
The problem of calculating equivalent grid block permeability tensors for heterogeneous porous media is addressed. The homogenization method used involves solving Darcy's equation subject to linear boundary conditions with flux conservation in subregions of the reservoir and can be readily applied to unstructured grids. The resulting equivalent permeability tensor is stable as defined relative to G-convergence. It is proposed to use both conforming and mixed finite elements to solve the local problems and compute approximations from above and below of the equivalent permeability, respectively. Comparisons with results obtained using periodic, pressure and no-flux boundary conditions and the renormalization method are presented. A series of numerical examples demonstrates the effectiveness of the methodology for two-phase flow in heterogeneous reservoirs. 相似文献