全文获取类型
收费全文 | 69530篇 |
免费 | 990篇 |
国内免费 | 1256篇 |
专业分类
测绘学 | 2464篇 |
大气科学 | 5323篇 |
地球物理 | 13319篇 |
地质学 | 28931篇 |
海洋学 | 4873篇 |
天文学 | 11643篇 |
综合类 | 2230篇 |
自然地理 | 2993篇 |
出版年
2022年 | 146篇 |
2021年 | 318篇 |
2020年 | 379篇 |
2019年 | 403篇 |
2018年 | 7206篇 |
2017年 | 6522篇 |
2016年 | 4551篇 |
2015年 | 977篇 |
2014年 | 1132篇 |
2013年 | 1838篇 |
2012年 | 2822篇 |
2011年 | 5649篇 |
2010年 | 5025篇 |
2009年 | 5509篇 |
2008年 | 4470篇 |
2007年 | 5353篇 |
2006年 | 1402篇 |
2005年 | 1599篇 |
2004年 | 1681篇 |
2003年 | 1611篇 |
2002年 | 1262篇 |
2001年 | 783篇 |
2000年 | 796篇 |
1999年 | 643篇 |
1998年 | 640篇 |
1997年 | 613篇 |
1996年 | 450篇 |
1995年 | 462篇 |
1994年 | 476篇 |
1993年 | 351篇 |
1992年 | 332篇 |
1991年 | 292篇 |
1990年 | 354篇 |
1989年 | 294篇 |
1988年 | 265篇 |
1987年 | 317篇 |
1986年 | 258篇 |
1985年 | 347篇 |
1984年 | 375篇 |
1983年 | 358篇 |
1982年 | 347篇 |
1981年 | 309篇 |
1980年 | 310篇 |
1979年 | 241篇 |
1978年 | 227篇 |
1977年 | 238篇 |
1976年 | 198篇 |
1975年 | 208篇 |
1974年 | 197篇 |
1973年 | 191篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Stéphanie Brichau Jean-Patrick Respaut Patrick Monié 《International Journal of Earth Sciences》2008,97(4):725-738
During the development of the Variscan orogeny, large amounts of granitic melt were produced, giving rise to the intrusion
of granitoids at different structural levels. Despite numerous studies, ages available from previous work on the Cévennes
granites remain largely imprecise. In order to better constrain the age and emplacement mode of these granites, we have combined
U–Pb dating on monazites and zircons and 40Ar/39Ar dating on biotites with petrological observations, major element chemical analysis and SEM zircon imaging on five samples
from the Aigoual–St Guiral–Liron and Mont Lozère granitic massifs. The results revealed that granitic intrusions and cooling
in Southern Cévennes occurred in a short time span at ∼306 Ma after the main episode of regional metamorphism. Petrological
and chemical data suggest that they result from a mixing between mantle-derived basic magmas (lamprophyres) and lower crust
acid magmas. At a regional scale the production of these melts occurred at the end of crustal thickening induced by nappe
stacking, at the same time as the late anatectic events recorded further north in the Velay dome and the granulite facies
metamorphism recorded in metasedimentary granulite enclaves brought up by Tertiary volcanoes of the Velay area (Bournac). 相似文献
992.
Mauro Cacace Ulf Bayer Anna Maria Marotta 《International Journal of Earth Sciences》2008,97(5):899-913
The large-scale crustal deformations observed in the Central European Basin System (CEBS) are the result of the interplay
between several controlling factors, among which lateral rheological heterogeneities play a key role. We present a finite-element
integral thin sheet model of stress and strain distribution within the CEBS. Unlike many previous models, this study is based
on thermo-mechanical data to quantify the impact of lateral contrasts on the tectonic deformation. Elasto-plastic material
behaviour is used for both the mantle and the crust, and the effects of the sedimentary fill are also investigated. The consistency
of model results is ensured through comparisons with observed data. The results resemble the present-day dynamics and kinematics
when: (1) a weak granite-like lower crust below the Elbe Fault System is modelled in contrast to a stronger lower crust in
the area extending north of the Elbe Line throughout the Baltic region; and (2) a transition domain in the upper mantle is
considered between the shallow mantle of the Variscan domain and the deep mantle beneath the East European Craton (EEC), extending
from the Elbe Line in the south till the Tornquist Zone. The strain localizations observed along these structural contrasts
strongly enhance the dominant role played by large structural domains in stiffening the propagation of tectonic deformation
and in controlling the basin formation and the evolution in the CEBS. 相似文献
993.
H. Zöllner K. Reicherter P. Schikowsky 《International Journal of Earth Sciences》2008,97(5):1013-1027
The pre-Alpine structural and geological evolution in the northern part of the North German Basin have been revealed on the
basis of a very dense reflection seismic profile grid. The study area is situated in the coastal Mecklenburg Bay (Germany),
part of the southwestern Baltic Sea. From the central part of the North German Basin to the northern basin margin in the Grimmen
High area a series of high-resolution maps show the evolution from the base Zechstein to the Lower Jurassic. We present a
map of basement faults affecting the pre-Zechstein. The pre-Alpine structural evolution of the region has been determined
from digital mapping of post-Permian key horizons traced on the processed seismic time sections. The geological evolution
of the North German Basin can be separated into four distinct periods in the Rerik study area. During Late Permian and Early
Triassic evaporites and clastics were deposited. Salt movement was initiated after the deposition of the Middle Triassic Muschelkalk.
Salt pillows, which were previously unmapped in the study area, are responsible for the creation of smaller subsidence centers
and angular unconformities in the Late Triassic Keuper, especially in the vicinity of the fault-bounded Grimmen High. In this
area, partly Lower Jurassic sediments overlie the Keuper unconformably. The change from extension to compression in the regional
stress field remobilized the salt, leading to a major unconformity marked at the base of the Late Cretaceous. 相似文献
994.
U. Altenberger R. Schmid R. Oberhänsli 《International Journal of Earth Sciences》2008,97(6):1301-1314
As part of the Yangtze plate, segments of the Dabie Shan terrane of Central China underwent ultra-high pressure metamorphism
during Triassic subduction. We studied the geochemistry of the abundant eclogites to evaluate the nature of the protoliths
and their geodynamic setting. Although some previous geochemical work exists, the analyses and interpretation herein are based
on a new subdivision of the ultra-high pressure sequence into basement and cover units (Changpu and Ganghe Unit), revealing
new and important results. In addition, eclogites of the so-called HP Unit south of the UHP units were studied. Whereas the
large ion lithophile elements indicate postmagmatic, metasomatic changes of some samples, the high-field strength elements
and the rare earth elements display original magmatic trends. The geochemical characteristics of the eclogites of the ultra-high
pressure areas display a strong dependence on their “structural” and geographic position. The eclogites of the basement and
the Changpu Unit indicate melt intrusion and extrusion in a continental rift system, i.e. during extensional tectonics. In
contrast, the Ganghe Unit is characterized by a pronounced chemical homogeneity. The composition of the eclogites indicates
generation from a mantle source highly influenced by slab-derived fluids. Those of the HP Unit show similar characteristics.
Magmatism of the Ganghe and HP Unit probably occurred in a continental arc setting. A similar age for both units, geographically
and/or tectonically separated, is possible. The geodynamic interpretation based on the geochemistry of the four units points
to a Neoproterozoic scenario in which the protoliths of the HP and the cover units could have been of similar age and deposited
in one evolving geological system. A rift-related larger-scale basin might have formed, e.g. a continental back-arc basin
behind a magmatic arc after or simultaneous to sedimentation and magmatism in the magmatic arc. Alternatively, magmatism occurred
in independent geodynamic settings, distinct in time and space. The units were juxtaposed during exhumation, after subduction
to varying depths. 相似文献
995.
Irina N. Plotnikova 《International Journal of Earth Sciences》2008,97(5):1131-1142
The study area is the South Tatarstan Arch located in the Volgo-Ural Region, which is an enigmatic crustal segment occupying
one third of the East European Platform. Monitoring studies have shown that fluid discharge processes are still active and
time-dependent. This paper presents an integrated review of the geological, geophysical, hydrochemical and geochemical studies
of the crystalline basement of Tatarstan. These studies are based on the stratigraphic and compositional schemes within the
crystalline basement, the drilling of deep wells, the geodynamic activity of the fractured zones of the crystalline basement
and the presence of fluids therein. Furthermore, the changes in the chemical composition of the basement waters are taken
into account. 相似文献
996.
Autumn Oczkowski Scott Nixon Kelly Henry Peter DiMilla Michael Pilson Stephen Granger Betty Buckley Carol Thornber Richard McKinney Joaquin Chaves 《Estuaries and Coasts》2008,31(1):53-69
Narragansett Bay has been heavily influenced by human activities for more than 200 years. In recent decades, it has been one
of the more intensively fertilized estuaries in the USA, with most of the anthropogenic nutrient load originating from sewage
treatment plants (STP). This will soon change as tertiary treatment upgrades reduce nitrogen (N) loads by about one third
or more during the summer. Before these reductions take place, we sought to characterize the sewage N signature in primary
(macroalgae) and secondary (the hard clam, Mercenaria mercenaria) producers in the bay using stable isotopes of N (δ15N) and carbon (δ13C). The δ15N signatures of the macroalgae show a clear gradient of approximately 4‰ from north to south, i.e., high to low point source
loading. There is also evidence of a west to east gradient of heavy to light values of δ15N in the bay consistent with circulation patterns and residual flows. The Providence River Estuary, just north of Narragansett
Bay proper, receives 85% of STP inputs to Narragansett Bay, and lower δ15N values in macroalgae there reflected preferential uptake of 14N in this heavily fertilized area. Differences in pH from N stimulated photosynthesis and related shifts in predominance of
dissolved C species may control the observed δ13C signatures. Unlike the macroalgae, the clams were remarkably uniform in both δ15N (13.2 ± 0.54‰ SD) and δ13C (−16.76 ± 0.61‰ SD) throughout the bay, and the δ15N values were 2–5‰ heavier than in clams collected outside the bay. We suggest that this remarkable uniformity reflects a
food source of anthropogenically heavy phytoplankton formed in the upper bay and supported by sewage derived N. We estimate
that approximately half of the N in the clams throughout Narragansett Bay may be from anthropogenic sources. 相似文献
997.
Nathan S. Hall R. Wayne Litaker Elizabeth Fensin Jason E. Adolf Holly A. Bowers Allen R. Place Hans W. Paerl 《Estuaries and Coasts》2008,31(2):402-418
A dense bloom of the ichthyotoxic dinoflagellate Karlodinium veneficum was discovered in the Neuse River Estuary, North Carolina, on 19 October 2006 and was associated with four subsequent fish
kills. Microscopic, photopigment, DNA, and toxicological techniques confirmed bloom identity and toxicity. High-resolution
spatio-temporal data from ship-board and fixed automated sampling stations provided a unique opportunity to investigate the
environmental conditions that initiated, maintained, and terminated the K. veneficum bloom. Bloom initiation and growth were favored by high nutrient availability and reduced dispersal during the period of
declining riverine discharge after Tropical Storm Ernesto. K. veneficum out-competed other co-occurring dinoflagellates, perhaps because of the production of karlotoxins that are known to act as
grazing deterrents and to facilitate mixotrophic feeding. Once the bloom was established, small-scale hydrodynamic processes,
coupled with vertical migration, concentrated cells along a frontal convergence to high densities (>200,000 cells per milliliter).
By 26 October 2006, wind mixing and possible nutrient stress disrupted the bloom. Release of cell-bound toxins during the
bloom collapse likely accounted for the associated fish kill events where fish were reported as exhibiting typical symptoms
of karlotoxin poisoning. The dynamics of this bloom underscore the tight control of harmful algal blooms by meteorological
forcing, hydrology, and sediment nutrient input in this shallow lagoonal estuary. 相似文献
998.
Quantitative basin characterisation to refine debris-flow triggering criteria and processes: an example from the Italian Western Alps 总被引:1,自引:1,他引:0
This paper investigates rapid channelized debris flow related to rainfalls in small alpine basins. Its goal is to evaluate
and correlate different geological and technical aspects with predisposing and triggering factors that can control these phenomena.
The study area is the upper part of the Susa Valley where 12 small basins were selected. For each of them, lithological, geomorphological,
climatic and technical information were mapped and analysed. Debris-flow triggering conditions, flow and depositional processes
were related to physical characteristics of the basin that can be easily measured and quantified. At least three different
groups of basins were found: G1) basins with one event each 4–6 years, characterised by massive or blocky calcareous rocks,
G2) basins with more than one event per year that show an abundance of layered or sheared fine-grained rocks and G3) basins
with recurrence levels exceeding 10 years, activated only by heavy and prolonged rainfalls, marked by massive or blocky coarse-grained
igneous rocks. Furthermore, important morphometric differences were found. These considerations are useful in terms of hazard
zonation and risk mitigation. 相似文献
999.
Three debris-flow simulation model software have been applied to the back analysis of a typical alpine debris flow that caused
significant deposition on an urbanized alluvial fan. Parameters used in the models were at first retrieved from the literature
and then adjusted to fit field evidence. In the case where different codes adopted the same parameters, the same input values
were used, and comparable outputs were obtained. Results of the constitutive laws used (Bingham rheology, Voellmy fluid rheology
and a quadratic rheology formulation which adds collisional and turbulent stresses to the Bingham law) indicate that no single
rheological model appears to be valid for all debris flows. The three applied models appear to be capable of reasonable reproduction
of debris-flow events, although with different levels of detail. The study shows how different software can be used to predict
the debris-flow motion for various purposes from a first screening, to predict the runout distance and deposition of the solid
material and to the different behaviour of the mixtures of flows with variation of maximum solid concentration. 相似文献
1000.
Recently, strong top-down (consumer) control of cordgrass (Spartina alterniflora) has been demonstrated. Here, we manipulated the densities of cordgrass consumers, acridid grasshoppers (Melanoplus bivittatus and Melanoplus femurrubrum), to examine their impact on cordgrass in the Plum Island Estuary (PIE), MA, USA. After 1 month, there was no detectable
effect of grasshopper density on S. alterniflora biomass and grasshoppers at the highest densities (34 individuals per square meter) consumed only ~14% of the standing stock
biomass. However, significant impacts of grasshopper density on grazing damage were seen. For example, plant damage and scarring
length increased by 160% and 6,156%, respectively, at the highest grasshopper densities relative to exclusion (zero grasshoppers)
densities. Plant height was significantly reduced with increasing grasshopper densities, although this may be a function of
leaf tip removal instead of reduced plant growth. No other strong consumers of cordgrass (e.g., Littoraria irrorata, Prokelisia marginata) were observed in PIE and we suggest that consumer regulation of cordgrass is weak in this system. 相似文献