全文获取类型
收费全文 | 62938篇 |
免费 | 1322篇 |
国内免费 | 985篇 |
专业分类
测绘学 | 1766篇 |
大气科学 | 4685篇 |
地球物理 | 12850篇 |
地质学 | 25472篇 |
海洋学 | 4868篇 |
天文学 | 12088篇 |
综合类 | 395篇 |
自然地理 | 3121篇 |
出版年
2022年 | 309篇 |
2021年 | 531篇 |
2020年 | 514篇 |
2019年 | 544篇 |
2018年 | 3941篇 |
2017年 | 3727篇 |
2016年 | 2826篇 |
2015年 | 1018篇 |
2014年 | 1280篇 |
2013年 | 2166篇 |
2012年 | 2596篇 |
2011年 | 4498篇 |
2010年 | 4121篇 |
2009年 | 4590篇 |
2008年 | 3672篇 |
2007年 | 4322篇 |
2006年 | 1852篇 |
2005年 | 1768篇 |
2004年 | 1584篇 |
2003年 | 1571篇 |
2002年 | 1316篇 |
2001年 | 951篇 |
2000年 | 941篇 |
1999年 | 720篇 |
1998年 | 790篇 |
1997年 | 747篇 |
1996年 | 608篇 |
1995年 | 620篇 |
1994年 | 549篇 |
1993年 | 464篇 |
1992年 | 458篇 |
1991年 | 423篇 |
1990年 | 498篇 |
1989年 | 404篇 |
1988年 | 377篇 |
1987年 | 456篇 |
1986年 | 380篇 |
1985年 | 469篇 |
1984年 | 557篇 |
1983年 | 487篇 |
1982年 | 490篇 |
1981年 | 446篇 |
1980年 | 473篇 |
1979年 | 389篇 |
1978年 | 386篇 |
1977年 | 378篇 |
1976年 | 336篇 |
1975年 | 332篇 |
1974年 | 336篇 |
1973年 | 375篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
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
992.
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
993.
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
994.
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
995.
Schinus terebinthifolius (Brazilian pepper), a native of South America, is currently naturalized in 20 countries worldwide and can alter native systems
by displacing flora and forming monotypic stands. The primary described mechanism of seed dispersal is through consumption
of fruits by birds and mammals. We evaluated an alternative dispersal method by evaluating the potential for S. terebinthifolius growing in freshwater and estuarine environments to disperse via water currents. Specifically, we: (1) determined the duration
fruits remained buoyant in three salinities, (2) estimated the viability of seeds after 7 days in water, (3) estimated the
dispersal rate of floating solitary fruits, and (4) examined the role of boat wakes in moving seeds above mean high water
at the shoreline. The length of time fruits floated in 0 ppt water (4.9 days) was significantly less than 15 ppt saltwater
(6.2 days), and 30 ppt saltwater (6.9 days). After 7 days, over 13% of seeds remained viable in 0 ppt, 15 ppt, and 30 ppt
water. By combining mean dispersal rates and the mean number of days fruits floated, we calculated individual fruits could
be transported 16.9 km in 0 ppt and over 22 km in 15 and 30 ppt water. To increase germination, seeds must be stranded above
the intertidal zone. Wind wakes alone never achieved this result; however, boat wakes plus wind wakes significantly increased
the movement of fruits above the intertidal region into drier soils. The use of both vertebrate dispersal vectors and water
dispersal may potentially increase the rate of invasion, establishment, and survival of S. terebinthifolius in freshwater and estuarine environments. 相似文献
996.
Large-scale ancient landslides of the area of more than 5 km2 and volume exceeding 200 × 106 m3 are characteristic features of the valleys incised in the northern periphery of the Crimean Mountains (Ukraine). The largely
affected area is located in the outermost cuesta range of the Crimean Mountains which consists of rigid Sarmatian limestones
overlying weak Middle Miocene and Upper Palaeogene deposits. A giant landslide arose in the Alma water gap as a reflection
of several coincident preparatory factors such as suitable bedrock stratification, smectite-rich bedrock exposed to swelling
activity, presence of faults parallel to the valley trend, and river capture event which preceded the landslide event. The
occurrence of such ancient megaslides is particularly interesting in the area which is characterized by low precipitation
(<500 mm/year) and weak contemporary seismicity. It probably reflects a more dynamic environment in humid phases of the Holocene;
however, seismic triggering along the Mesozoic suture zone cannot be rejected. Compressional features such as gravitational
folds in the central and distal parts of the landslide, which probably correlate with the whole landslide genesis or its significant
reactivation, arose, according to the radiocarbon dating, during the Holocene climatic optimum in the Atlantic period. The
slope deformation has been relatively quiescent since that time, except minor historic reactivization which took place in
the frontal part of the landslide. We suppose that the studied landslide could be classified as a transitional type of slope
deformation with some signs of spreading and translational block slides. 相似文献
997.
Lutz Nasdala Wolfgang Hofmeister Nicholas Norberg James M. Martinson Fernando Corfu Wolfgang Dörr Sandra L. Kamo Allen K. Kennedy Andreas Kronz Peter W. Reiners Dirk Frei Jan Kosler Yusheng Wan Jens Götze Tobias Häger Alfred Kröner John W. Valley 《Geostandards and Geoanalytical Research》2008,32(3):247-265
We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206pb/238U and 0.7392 ± 0.0003 for 207pb/235U. Its 206pb/238U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ~ 840 μg g?1 U (Th/U ~ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 1018 g?1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn. 相似文献
998.
Tamás Mikes Dominik Christ Rüdiger Petri István Dunkl Dirk Frei Mária Báldi-Beke Joachim Reitner Klaus Wemmer Hazim Hrvatović Hilmar von Eynatten 《Swiss Journal of Geoscience》2008,101(1):31-54
Sandwiched between the Adriatic Carbonate Platform and the Dinaride Ophiolite Zone, the Bosnian Flysch forms a c. 3000 m thick, intensely folded stack of Upper Jurassic to Cretaceous mixed carbonate and siliciclastic sediments in the Dinarides. New petrographic, heavy mineral, zircon U/Pb and fission-track data as well as biostratigraphic evidence allow us to reconstruct the palaeogeology of the source areas of the Bosnian Flysch basin in late Mesozoic times. Middle Jurassic intraoceanic subduction of the Neotethys was shortly followed by exhumation of the overriding oceanic plate. Trench sedimentation was controlled by a dual sediment supply from the sub-ophiolitic high-grade metamorphic soles and from the distal continental margin of the Adriatic plate. Following obduction onto Adria, from the Jurassic–Cretaceous transition onwards a vast clastic wedge (Vranduk Formation) was developed in front of the leading edge, fed by continental basement units of Adria that experienced Early Cretaceous synsedimentary cooling, by the overlying ophiolitic thrust sheets and by redeposited elements of coeval Urgonian facies reefs grown on the thrust wedge complex. Following mid-Cretaceous deformation and thermal overprint of the Vranduk Formation, the depozone migrated further towards SW and received increasing amounts of redeposited carbonate detritus released from the Adriatic Carbonate Platform margin (Ugar Formation). Subordinate siliciclastic source components indicate changing source rocks on the upper plate, with ophiolites becoming subordinate. The zone of the continental basement previously affected by the Late Jurassic–Early Cretaceous thermal imprint has been removed; instead, the basement mostly supplied detritus with a wide range of pre-Jurassic cooling ages. However, a c. 80 Ma, largely synsedimentary cooling event is also recorded by the Ugar Formation, that contrasts the predominantly Early Cretaceous cooling of the Adriatic basement and suggests, at least locally, a fast exhumation. 相似文献
999.
1000.
Jeremy M. Testa W. Michael Kemp Walter R. Boynton James D. Hagy III 《Estuaries and Coasts》2008,31(6):1021-1037
We conducted a quantitative assessment of estuarine ecosystem responses to reduced phosphorus and nitrogen loading from sewage
treatment facilities and to variability in freshwater flow and nonpoint nutrient inputs to the Patuxent River estuary. We
analyzed a 19-year dataset of water quality conditions, nutrient loading, and climatic forcing for three estuarine regions
and also computed monthly rates of net production of dissolved O2 and physical transport of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) using a salt- and water-balance model.
Point-source loading of DIN and DIP to the estuary declined by 40–60% following upgrades to sewage treatment plants and correlated
with parallel decreases in DIN and DIP concentrations throughout the Patuxent. Reduced point-source nutrient loading and concentration
resulted in declines in phytoplankton chlorophyll-a (chl-a) and light-saturated carbon fixation, as well as in bottom-layer O2 consumption for upper regions of the estuary. Despite significant reductions in seaward N transport from the middle to lower
estuary, chl-a, turbidity, and surface-layer net O2 production increased in the lower estuary, especially during summer. This degradation of water quality in the lower estuary
appears to be linked to a trend of increasing net inputs of DIN into the estuary from Chesapeake Bay and to above-average
river flow during the mid-1990s. In addition, increased abundance of Mnemiopsis leidyi significantly reduced copepod abundance during summer from 1990 to 2002, which favored increases in chl-a and allowed a shift in total N partitioning from DIN to particulate organic nitrogen. These analyses illustrate (1) the value
of long-term monitoring data, (2) the need for regional scale nutrient management that includes integrated estuarine systems,
and (3) the potential water quality impacts of altered coastal food webs. 相似文献