全文获取类型
收费全文 | 7649篇 |
免费 | 1116篇 |
国内免费 | 961篇 |
专业分类
测绘学 | 651篇 |
大气科学 | 967篇 |
地球物理 | 2380篇 |
地质学 | 3635篇 |
海洋学 | 631篇 |
天文学 | 380篇 |
综合类 | 387篇 |
自然地理 | 695篇 |
出版年
2024年 | 24篇 |
2023年 | 77篇 |
2022年 | 178篇 |
2021年 | 184篇 |
2020年 | 134篇 |
2019年 | 157篇 |
2018年 | 588篇 |
2017年 | 499篇 |
2016年 | 367篇 |
2015年 | 307篇 |
2014年 | 240篇 |
2013年 | 297篇 |
2012年 | 869篇 |
2011年 | 664篇 |
2010年 | 337篇 |
2009年 | 345篇 |
2008年 | 285篇 |
2007年 | 341篇 |
2006年 | 347篇 |
2005年 | 1008篇 |
2004年 | 1001篇 |
2003年 | 752篇 |
2002年 | 271篇 |
2001年 | 159篇 |
2000年 | 94篇 |
1999年 | 40篇 |
1998年 | 20篇 |
1997年 | 25篇 |
1996年 | 15篇 |
1992年 | 5篇 |
1991年 | 10篇 |
1990年 | 12篇 |
1989年 | 6篇 |
1987年 | 5篇 |
1985年 | 3篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1973年 | 3篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1965年 | 3篇 |
1963年 | 2篇 |
1961年 | 2篇 |
1959年 | 2篇 |
1955年 | 2篇 |
1954年 | 3篇 |
1951年 | 2篇 |
1948年 | 2篇 |
排序方式: 共有9726条查询结果,搜索用时 31 毫秒
991.
Franti?ek?Pato?ka Petr??torchEmail author 《International Journal of Earth Sciences》2004,93(5):728-741
Cambrian and Ordovician-Middle Devonian sequences of two successive Early Palaeozoic basins of the Barrandian unconformably overlie Cadomian basement in the Bohemian Massif NW interior (Teplá-Barrandian unit) which is the easternmost peri-Gondwanan remnant within the Variscides. Correlation of stratigraphy and geochemistry of the Early Palaeozoic siliciclastic rocks elucidated sediment provenances. Sandstones of the Middle Cambrian Píbram-Jince Basin were derived from a Cadomian Neoproterozoic island arc. The source area of the Ordovician shallow-marine siliciclastics of the successor Prague Basin is a dissected Cadomian orogen. Late Cambrian acid volcanics of the Barrandian and Cambrian (meta)granitoids emplaced in the W part of the Teplá-Barrandian Cadomian basement are also discernible in these sediments. Old sedimentary component increased during the Ordovician. Early Llandovery siliciclastic rocks show characteristics of an abruptly weakened supply of terrigenous material and an elevated proportion of synsedimentary basic volcanics as a result of Silurian transgression. Emsian siliciclastics (intercalated in the Late Silurian to Early Devonian limestone suite) presumably comprise an addition of coeval basic/ultrabasic volcaniclastics. Middle Devonian flysch-like siliciclastics indicate reappearance of Cadomian source near the Barrandian during early Variscan convergences of Armorican microplates that preceeded accretion of the Teplá-Barrandian unit within the Bohemian Massif terrane mosaic.Dr. Patoka deceased in July 2004. 相似文献
992.
Lower to Middle Devonian carbonates of the Prague Syncline, the Carnic Alps, the Montagne Noire, and the Cantabrian Mountains were investigated for δ13Ccarb and δ13Corg. These values were measured on bulk rocks, selected components and cements. Many carbonates exhibit primary marine values, but some are altered by diagenesis. A δ13C curve can be presented for the latest Pridolian to Emsian time interval. Several sharp or broad positive excursions are obvious in the woschmidti-postwoschmidti, sulcatus, kitabicus, Late serotinus, and kockelianus conodont zones. The excursion at the Silurian–Devonian boundary is known worldwide and therefore considered global in nature. Some of the others are described for the first time from central and southern Europe, and their global nature has to be verified by further investigations in other regions. Most excursions relate to and/or started during major regressions whereas sea-level highstands correspond to minimal δ13C values. Similar relationships between sea-level changes and δ13C have been observed from other early Palaeozoic intervals. The transgressive Chote? (?) and Ka?ák events are marked by positive isotope excursions, this type of combination is usually observed in late Palaeozoic to Cenozoic black shale events. 相似文献
993.
Iva frutescens is a common shrub at the upland fringe of salt marshes throughout the East and Gulf Coasts of North America. Within a marsh, its location and relative size are governed largely by the degree of flooding by seawater.Iva’s wide distribution and restricted location within salt marshes may make it a useful indicator of overall conditions of the marshes. This work was designed to provide basic information on the age and growth ofI. frutescens, especially as they relate to the degree of flooding that is needed in order to investigateIva’s potential as an indicator. Cross-sections of older stems (living and standing dead) from salt marshes in Rhode Island, United States, were examined in order to age stems and estimate their growth rate from cumulative increase in woody tissue. Most stems were six yr old or less, suggesting that aboveground structures live for only a few years. Stem diameter correlated with growth rate and aboveground biomass. Elevation at the root zone was used to estimate the duration that plants were flooded, which was negatively correlated with stem diameter. The most robust plants came from sites that were flooded only up to 6–7% of the total time during the growing season. No plants were found in areas flooded more than 30% of the time. 相似文献
994.
Diffusive boundary layers (DBL), sediment oxygen flux, and natural passive flow through model macrofauna burrows were compared
in two benthic chambers: one with a conventional rotational stirrer and the other with a two-dimensional flow diffuser system.
Oxygen microprobe profiles showed that at similar velocities the mean diffusive boundary layer (DBL) thickness induced by
a conventional rotor stirrer (453±118 μm) was not significantly different to that produced by the diffuser system (403±53
μm). The rotor produced twice as much DBL spatial variability (coefficient of variation 27%) as the diffuser (CV 11%). Variability
between the rotor system’s DBL transect replicates was also two times greater (average CV of 22%) compared to the diffuser
(CV 14%). At equivalent stirring speeds over experimental sediments, mean O2 consumption rates were also not significantly different between the two systems. The diffuser induced consistently greater
(16–37%) passive Bernoullian flow through model macrofauna borrows irrespective of the position of inhalant-exhalent openings.
The rotor stimulated anomalous burrow flow regimes over a greater area of the chamber floor (36%) compared to the diffuser
(23%). Depending on vent orientation the rotor was shown to reverse (exhalent to inhalant) burrow flow regimes in the central
9% of the chamber floor. This artifact of radial pressure and velocity differentials may have severe implications for tube
dwelling infauna that rely on unidirectional flow. The diffuser system more closely mimicked natural two-dimensional water
flow over the sediment surface and structures therein and is likely to give more representative results when measuring benthic
processes within incubation devices. 相似文献
995.
Spis Castle, a monument included in the UNESCO World Heritage Site list (Eastern Slovakia) is built on a travertine mound overlying soft Paleogene rocks. Lateral spreading resulting from the subsidence of strong upper travertine into soft claystone strata has fractured and separated the castle rock into several cliffs. The differential movement of individual cliff faces is the primary influence on the stability of the monument. In order to monitor and quantify the movement, two techniques were adopted: a mechanical-optical TM-71 crack gauge and a demec gauge (demountable mechanical—SOMET type crack gauge). This paper presents the results and interpretation of medium and short-term monitoring at the site. 相似文献
996.
Fa-Wu?WangEmail author Ye-Ming?Zhang Zhi-Tao?Huo Tatsunori?Matsumoto Bo-Lin?Huang 《Landslides》2004,1(2):157-162
The Qianjiangping landslide occurred after the first impoundment of the Three Gorges Reservoir in July 2003. Field investigation revealed that failure occurred when the reservoir reached 135 m, but the stability of the affected slope was already reduced by pre-existing bedding-plane shears, quarrying of mudstone from the landslide toe, and previous heavy rain. A possible explanation of the rapid and long runout mechanism of the landslide is that movement on a bedding-plane shear ruptured the calcite cement and rapidly reduced the sandstone strength to residual shear strength. 相似文献
997.
998.
Instability and geotechnical problems of the Buddha niches and surrounding cliff in Bamiyan Valley,central Afghanistan 总被引:2,自引:0,他引:2
This paper describes the slope-instability processes and geological hazards affecting the historical site of Bamiyan, central Afghanistan, a major world-cultural heritage site. Here, two standing Buddhas carved in rock during the 2nd–4th centuries A.D. were destroyed by the Taliban in March 2001. There are very evident slope instabilities resulting from both explosions and natural processes: rock slides and rock falls had already occurred in the past and most areas are prone to collapse. Under the coordination of UNESCO, field data were collected and laboratory tests were performed to determine mechanisms for potential evolution of the cliff and niches. Areas of greatest instability, based on the field survey and data interpretation for both the small and great Buddha niches and surrounding cliff, were determined. This information helps to identify the most suitable method for restoration, in accord with the high cultural value of this site. 相似文献
999.
The International Consortium on Landslides 总被引:7,自引:6,他引:1
1000.
Picophytoplankton and bacterioplankton in the Mississippi River plume and its adjacent waters 总被引:3,自引:0,他引:3
Hongbin?LiuEmail author Michael?Dagg Lisa?Campbell Juanita?Urban-Rich 《Estuaries and Coasts》2004,27(1):147-156
Picoplankton abundance and distribution in the Mississippi River plume and its adjacent waters were studied during two cruises
in April (high discharge) and October (low discharge) 2000 using flow cytometry. Concentrations of photosynthetic picoplankton,Synechococcus and picoeukaryotes were low in the turbid plume water but high in the coastal waters—i.e., the green waters resulting from
mixing of river and oceanic waters. In this region, three types ofSynechococcus, characterized by their phycoerythrin chromophore composition, were found:Synechococcus cells with a low phycourobilin to phycoerythrobilin ratio (PUB:PEB) occurred throughout the region and dominated the totalSymechococcus abundance during both seasons; high PUB:PEB cells, which are the dominant strains in the open or blue ocean, occurred only
at the outer shelf stations; and PEB-onlySynechococcus were abundant in most of the surveyed area during april, but were not observed during October.Prochlorococcus cyanobacteria only occurred at the oceanic stations, but extended farther inshore in October compared to April. This was
a consequence of the reduced discharge and plume size during October. Picophytoplankton were a less important component of
total phytoplankton biomass in the turbid river water and more important in the oligotrophic Gulf water. Seasonally, the contribution
of picophytoplankton to total phytoplankton biomass in the surveyed area was higher during low discharge in October than during
high discharge in April, even though the spring 2000 river discharge was unusually low and might not present a typical high
discharge scenario. The abundance of heterotrophic bacteria was weakly correlated to chlorophylla (chla) concentration, but better correlated to picophytoplankton biomass. A higher proportion of High DNA bacteria occurred in
the river-impacted regions during both seasons, with the ratio of High DNA bacteria to Low DNA bacteria significantly higher
in April. 相似文献