全文获取类型
收费全文 | 35691篇 |
免费 | 464篇 |
国内免费 | 383篇 |
专业分类
测绘学 | 804篇 |
大气科学 | 3663篇 |
地球物理 | 7481篇 |
地质学 | 13473篇 |
海洋学 | 2399篇 |
天文学 | 6769篇 |
综合类 | 73篇 |
自然地理 | 1876篇 |
出版年
2018年 | 492篇 |
2017年 | 481篇 |
2016年 | 691篇 |
2015年 | 449篇 |
2014年 | 675篇 |
2013年 | 1449篇 |
2012年 | 750篇 |
2011年 | 1031篇 |
2010年 | 878篇 |
2009年 | 1251篇 |
2008年 | 1061篇 |
2007年 | 948篇 |
2006年 | 1063篇 |
2005年 | 890篇 |
2004年 | 861篇 |
2003年 | 876篇 |
2002年 | 875篇 |
2001年 | 751篇 |
2000年 | 798篇 |
1999年 | 672篇 |
1998年 | 650篇 |
1997年 | 695篇 |
1996年 | 639篇 |
1995年 | 578篇 |
1994年 | 516篇 |
1993年 | 477篇 |
1992年 | 555篇 |
1991年 | 555篇 |
1990年 | 547篇 |
1989年 | 521篇 |
1988年 | 496篇 |
1987年 | 618篇 |
1986年 | 554篇 |
1985年 | 567篇 |
1984年 | 688篇 |
1983年 | 704篇 |
1982年 | 662篇 |
1981年 | 646篇 |
1980年 | 572篇 |
1979年 | 592篇 |
1978年 | 591篇 |
1977年 | 528篇 |
1976年 | 456篇 |
1975年 | 481篇 |
1974年 | 516篇 |
1973年 | 509篇 |
1972年 | 357篇 |
1971年 | 343篇 |
1970年 | 277篇 |
1969年 | 220篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
881.
882.
Ohne Zusammenfassung 相似文献
883.
884.
This study is devoted to oceanographic features of the semi-enclosed Gulf of Aqaba, Red Sea. The data were recorded in winter—spring
1999 on the R/V Meteor cruise leg 44/2. Temperature and salinity profiles were measured at six positions (I—VI). The shipboard
NarrowBand Acoustic Doppler Current Profiler (NB ADCP) 150 kHz continuously recorded current profiles down to 350 m en route.
The research revealed that the current near the Strait of Tiran front (position VI) represents a semidiurnal signal of an
internal tide wave (∼12 h period; 0.2 ms−1 amplitude) that might be generated by the barotropic tide at the sill of the Strait. A sequence of cyclonic and anticyclonic
eddy pairs is found along the axis of the Gulf of Aqaba during winter-spring seasons. These sub-mesoscale signals are dominant
above the main thermocline and might be caused by wind forcing and the narrowness of the Gulf; it might remain in other seasons
with different dimensions in relation to the depth of thermocline. The total diameter of each pair was twice the baroclinic
Rossby radius (R ≈ 10 km). A single anti-cyclonic eddy was observed in the upper 300 m in the northern tip of the Gulf with a diameter of
about 5–8 km. 相似文献
885.
A. H. Ruffell G. D. Price J. Mutterlose K. Kessels E. Baraboshkin D. R. Grcke 《Geological Journal》2002,37(1):17-33
A study of clay mineral and calcareous nannofossil abundances in late Jurassic–early Cretaceous sediments from the Volga Basin, SE Russia, is presented. From these results, we are able to compare some general patterns of mineralogical and palaeontological change for the Volga Basin to the palaeoclimate models developed for northern Europe and beyond. The two successions examined comprise calcareous mudstones with black organic‐rich shale horizons, overlain by a series of phosphatic silty sands. Clay mineralogical results show a progressive decrease in kaolinite and the concomitant increase of smectite and illite through the middle Volgian, followed by an abrupt increase in kaolinite in the late Volgian. The clay mineral evidence suggests increasing aridity at the end of the Jurassic, similar, in part, to many western European successions. Because of differential settling of clay minerals, superimposed upon this possible climatic signature is likely to be the effect of relative sea‐level change. Calcareous nannofossil analysis from a single section reveals a shift through the middle Volgian from low nutrient, warm water assemblages dominated by Watznaueria to cooler surface water and high nutrient assemblages dominated by Biscutum constans. These observations suggest that increased aridity is also associated with climatic cooling. Black shales are associated with increased productivity, higher sea levels and increases in smectite content. Hence, periods of low (chemical) hinterland weathering during semi‐arid conditions are paradoxically associated with relatively nutrient‐rich waters, and organic‐rich shales. Comparison of published carbon and oxygen stable isotope results from this and other sections to the clay mineral and nannofossil data confirms the palaeoclimatic interpretation. This study significantly improves the published biostratigraphically constrained clay mineral database for this time period, because other European and North American successions are either non‐marine (and thus poorly dated), absent (through penecontemporaneous erosion) or condensed. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
886.
K. Garofalo F. Tassi O. Vaselli A. Delgado-Huertas D. Tedesco M. Frische T. H. Hansteen R. J. Poreda W. Strauch 《Bulletin of Volcanology》2007,69(7):785-795
Mombacho is a deeply dissected volcano belonging to the Quaternary volcanic chain of Nicaragua. The southern, historic collapse
crater (El Crater) currently hosts a fumarolic field with a maximum temperature of 121°C. Chemical and isotopic data from
five gas-sampling field campaigns carried out in 2002, 2003 and 2005 highlight the presence of high-temperature gas components
(e.g. SO2, HCl and HF), which indicate a significant contribution of juvenile magmatic fluids to the hydrothermal system feeding the
gas discharges. This is strongly supported by the mantle-derived helium and carbon isotopic signatures, although the latter
is partly masked by either a sedimentary subduction-related or a shallow carbonate component. The observed chemical and isotopic
composition of the Mombacho fluids seems to indicate that this volcanic system, although it has not experienced eruptive events
during the last centuries, can be considered active and possibly dangerous, in agreement with the geophysical data recorded
in the region. Systematic geochemical monitoring of the fumarolic gas discharges, coupled with a seismic and ground deformation
network, is highly recommended in order to monitor a possible new eruptive phase. 相似文献
887.
A regional geochemical and mineralogical study aimed at investigating the mineralization in the western-part of the Walawe Ganga (river) Basin in Sri Lanka is represented in this paper. The river basin is the 3rd largest in the country and has within it a boundary zone between two geologically different crustal blocks, which are marked by granulitic grade rocks and amphibolite grade rocks. Size fractions of stream sediments (< 63 μm; 63–125 μm; 125–177 μm and 177–250 μm) developed on the granulite-grade metamorphic terrain have been analysed at their source for their mineralogical and selected element compositions. Thirty-eight (38) sediment samples and 15 representative probable parent rock samples were chemically analysed giving special emphasis to the High Field Strength trace Elements (HFSE) including the Rare Earth Elements (REE). The granulite grade rocks in the study area is geochemically similar to that of post Archean upper crust. However the stream sediments developed from the high-grade rocks during the intense weathering, are markedly enriched with HFSE and REE. The enrichment of HFSE and LREE is accounted for by the presence of HFSE- and REE- rich accessory mineral phases such as zircon, monazite, apatite, garnet and rutile in the sediments. In some samples, the content of heavy minerals contributes as much as 50 wt. %. These minerals act as a source of elements in the sediments. However, extreme hydraulic sorting of HFSE- and REE-bearing minerals during the sediment deposition cannot be expected within a short distance from near the sources except from a mineralized occurrence. Therefore, the higher enrichment of these elements presumably indicates occurrences of scattered mineral sources such as highly differentiated granites and associated pegmatites within the Walawe Ganga drainage basin. These granitic pegmatites are probably intruded during or soon after the main granulite-facies metamorphic event and similar events are seen in other terrains of East-Gondwana. 相似文献
888.
Seismic data collected at four volcanoes in Central America during 1973 and 1974 indicate three sources of seismicity: regional
earthquakes with hypocentral distances greater than 80 km, earthquakes within 40 km of each volcano, and seismic activity
originating at the volcanoes due to eruptive processes. Regional earthquakes generated by the underthrusting and subduction
of the Cocos Plate beneath the Caribbean Plate are the most prominent seismic feature in Central America. Earthquakes in the
vicinity of the volcanoes occur on faults that appear to be related to volcano formation. Faulting near Fuego and Pacaya volcanoes
in Guatemala is more complex due to motion on a major E-W striking transform plate boundary 40 km north of the volcanoes.
Volcanic activity produces different kinds of seismic signatures. Shallow tectonic or A-type events originate on nearby faults
and occur both singly and in swarms. There are typically from 0 to 6 A-type events per day withb value of about 1.3. At very shallow depths beneath Pacaya, Izalco, and San Cristobal large numbers of low-frequency or B-type
events are recorded with predominant frequencies between 2.5 and 4.5 Hz and withb values of 1.7 to 2.9. The relative number of B-type events appears to be related to the eruptive states of the volcanoes;
the more active volcanoes have higher levels of seismicity. At Fuego Volcano, however, low-frequency events have unusually
long codas and appear to be similar to tremor. High-amplitude volcanic tremor is recorded at Fuego, Pacaya, and San Cristobal
during eruptive periods. Large explosion earthquakes at Fuego are well recorded at five stations and yield information on
near-surface seismic wave velocities (α=3.0±0.2 km/sec.). 相似文献
889.
An intensive approach to Barrow, Alaska’s adaptations to climate change and variability during recent decades suggests reconsideration
of the interconnected roles of science, policy, and decision-making structures. First, profound uncertainties are inherent
in unique interactions among the many natural and human factors affecting Barrow’s vulnerability. Science cannot significantly
reduce these uncertainties through extensive approaches, but intensive approaches can reconstruct and update local trends,
clarify the underlying dynamics, and harvest experience for policy purposes. Second, sound policies to reduce Barrow’s vulnerability
to coastal erosion and flooding must incorporate these profound uncertainties and the multiple values of the community. Minimizing
vulnerability to climate change is only one of the community’s interests, and must compete with other interests for limited
time, attention, funds and other resources. Third, the community itself is in the best position to understand its own context,
to decide on sound policies, and to take responsibility for those decisions. In short, local context matters in science, policy,
and decision-making structures for adaptation to climate change and variability. Overall, cognitive constraints may be the
most important human dimension of climate change. Factoring the global problem into more tractable local problems would make
the most of our cognitive capacity. 相似文献
890.
P. R. Dawes R. H. Smithies J. Centofanti D. C. Podmore 《Australian Journal of Earth Sciences》2013,60(6):635-639
Contacts between Archaean granites and greenstones in the northeastern part of the Pilbara Craton have been described as intrusive and tectonic. New field observations in the Shay Gap region demonstrate that greenstones of the Gorge Creek Group unconformably overlie the Muccan and Warrawagine batholiths. Regionally, the unconformity is marked by a persistent but relatively thin basal clastic sequence, locally with a granite boulder conglomerate, overlain by ore‐bearing banded iron‐formation, fine‐grained clastic rocks and chert. The granite basement is dated at 3443 ± 6 Ma. The precise age of the hiatus is unknown but its maximum effect might have been the removal of a substantial thickness of Early to Middle Archaean strata. 相似文献