全文获取类型
收费全文 | 50573篇 |
免费 | 4575篇 |
国内免费 | 6405篇 |
专业分类
测绘学 | 2741篇 |
大气科学 | 6736篇 |
地球物理 | 10872篇 |
地质学 | 25361篇 |
海洋学 | 4145篇 |
天文学 | 3156篇 |
综合类 | 4466篇 |
自然地理 | 4076篇 |
出版年
2024年 | 115篇 |
2023年 | 504篇 |
2022年 | 1118篇 |
2021年 | 1350篇 |
2020年 | 1165篇 |
2019年 | 1275篇 |
2018年 | 5889篇 |
2017年 | 5125篇 |
2016年 | 3979篇 |
2015年 | 1448篇 |
2014年 | 1786篇 |
2013年 | 1767篇 |
2012年 | 2588篇 |
2011年 | 4305篇 |
2010年 | 3558篇 |
2009年 | 3771篇 |
2008年 | 3147篇 |
2007年 | 3560篇 |
2006年 | 1154篇 |
2005年 | 1171篇 |
2004年 | 1047篇 |
2003年 | 1102篇 |
2002年 | 1022篇 |
2001年 | 760篇 |
2000年 | 850篇 |
1999年 | 1115篇 |
1998年 | 898篇 |
1997年 | 957篇 |
1996年 | 871篇 |
1995年 | 720篇 |
1994年 | 609篇 |
1993年 | 559篇 |
1992年 | 426篇 |
1991年 | 323篇 |
1990年 | 248篇 |
1989年 | 201篇 |
1988年 | 198篇 |
1987年 | 148篇 |
1986年 | 121篇 |
1985年 | 92篇 |
1984年 | 64篇 |
1983年 | 51篇 |
1982年 | 48篇 |
1981年 | 53篇 |
1980年 | 66篇 |
1979年 | 39篇 |
1978年 | 20篇 |
1976年 | 22篇 |
1975年 | 17篇 |
1958年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Yu-long Luo Liang Qiao Xing-xing Liu Mei-li Zhan Jin-chang Sheng 《Natural Hazards》2013,65(3):1361-1377
The long-term large hydraulic head in flood season is the main cause of suffusion failure or dam break, while the suffusion failure mechanism in this case has not been specially studied. First, a newly designed hydro-mechanical coupling suffusion apparatus was introduced. Second, two short-term suffusion experiments under different confining pressures were carried out to determine the suitable large hydraulic heads. Finally, four experiments on suffusion under long-term large hydraulic heads and different confining pressures were performed to investigate the influences of the long-term large hydraulic heads and the confining pressures on the evolution of suffusion. The results indicate that: (1) The signs of suffusion failure in the short-term experiments are the sudden decrease in hydraulic gradient and the sharp increase in eroded mass. While the flow rate in the long-term experiments appearing suffusion failures increases rapidly with the decrease in hydraulic gradient. (2) The suffusion failure in the long-term large hydraulic head experiment is more likely to happen and much more serious than that in the short-term experiment. The long-term large hydraulic head can reduce the suffusion failure hydraulic gradient significantly and increase the eroded mass dramatically. The results gained here provide a deeper understanding on the suffusion failure caused by the rapid increase in the high water level upstream the hydraulic earth structures in flood season. 相似文献
992.
The Beijing storm of 21 July attracted public and social attention widely. Recently, some scientists expressed their opinion that urbanization has exacerbated the storm. However, our analysis suggests that while urbanization might have played some role, it is mainly the topographic effect that made the storm intense. Our conclusion is that the Beijing storm of 21 July is generated due to natural climatic factors in a changing climate system. Moreover, we think that the factor that contributes to the tremendous flooding disaster of 21 July is the low standards for mountain torrents control for medium and small rivers in the affected region. Therefore, the mountain torrents disasters control and medium and small rivers harnessing should be the foremost task in China’s water conservancy construction in the future, and effective adaptation strategies should also be developed and implemented to cope with the climate change impacts. 相似文献
993.
Accumulation of heavy metals in the lacustrine sediment of Longgan Lake, middle reaches of Yangtze River, China 总被引:1,自引:0,他引:1
Haijian Bing Yanhong Wu Wook-Hyun Nahm Enfeng Liu 《Environmental Earth Sciences》2013,69(8):2679-2689
Spatial and temporal distribution of heavy metals (Chromium, Copper, Nickel, Lead, and Zinc) in the sediment of Longgan Lake, middle reaches of Yangtze River, China were analyzed to discuss their enrichment characteristics and history in combination with geochronological data and to identify anthropogenic effects. The results showed that the enrichment state of heavy metals was higher in the western versus eastern lake area, although their concentrations behave in the opposite case, which demonstrated that stronger human activities existed in the western lake catchment. The enrichment history of heavy metals pointed out that prior to 1950, the enrichment state was lower in the ambient lake areas illustrating the natural effect on the lake; thereafter the human activities in the catchment enhanced markedly causing an increasing heavy metal enrichment. The increase of heavy metal enrichment in the 1970s occurred in the central lake areas. The temporal difference between the ambient and central areas elucidated that the heavy metal accumulation might be buffered by the lake before they entered the sediment. Correlation analysis revealed that there was a significant correlation between heavy metals in the sediment suggesting their similar sources. Heavy metals were distinctly related to aluminum, calcium, lithium, iron, and organic matters, which could denote their forms in the sediment. 相似文献
994.
995.
Because of the confining compression of the Pacific,Eurasian and Indian plates,the Chinese mainland is frequently stricken with earthquakes,especially in the Qinghai-Tibet Plateau and surrounding areas and along the NS-trending tectonic zone from Yinchuan to Lanzhou,Chengdu,and finally to the Kunming tectonic belt (Fig.1).Historical records show that there are 14 earthquakes of>Ms 8 occurring in the two regions,eight of which occurred along the latter tectonic belt.The 2008 Wenchuan earthquake located on both the NS tectonic zone and the Longmenshan fault zone along the eastern margin of the Qinghai-Tibet Plateau. 相似文献
996.
过去一般认为西准噶尔西南部的侵入岩均为海西中晚期,但近年在玛依勒及巴尔鲁克等构造带发现了一些早古生代侵入岩体,部分岩体显示有铜(金)矿化。本文对其中的占贝勒成矿斑岩体,利用LA-ICP-MS法进行锆石U-Pb同位素测年,结果获得425.3±2.6 Ma的年龄,揭示西准噶尔西南部地区存在有加里东期中酸性侵入岩,进而说明西准噶尔西南部地区存在加里东期斑岩型铜金成矿事件。该研究成果不仅对分析本地区地质构造演化具有重要意义,且对西准地区成矿规律的总结和进一步找矿预测具有重要意义。 相似文献
997.
T. A. Myskova N. M. Ivanov M. A. Korsakova R. I. Mil’kevich N. G. Berezhnaya S. L. Presnyakov 《Stratigraphy and Geological Correlation》2013,21(4):337-358
The geological, geochemical, and isotopic-geochronological data obtained for Sumian moderate-basic metavolcanites of Shombozero and Lekhta structures of the Panayarvi-Vygozero belt shows that the Tunguda Formation is confined to the Paleoproterozoic structural and material complex. This formation is represented by the complex of weakly differentiated andesitobasalts and andesites of calc-alkaline series with higher contents of MgO and moderate contents of Al2O3 and rare earth elements. The rocks of the Tunguda Formation are different from the Late Archean basic rocks of the Hiziyarvi Formation represented mainly by tholeiitic basalts with low REE contents and undifferentiated spectrum of REE distribution. The age of volcanites of the Tunguda Formation was determined to be 2439 ± 21 Ma. The xenogenic zircons from metaandesites of the Tunguda Formation have Neoarchean age according to the 207Pb/206Pb ratio (from 2536 ± 4 to 2825 ± 7 Ma). The Neoarchean zircons, a negative value of ?Nd (?3.8), and indicative geochemical parameters are evidence that the crustal component took part in formation of the protolith of the studied rocks. 相似文献
998.
Field experiments were conducted to study the short-time response in growth and sediment properties of Zizania latifolia to four levels of water depth: 10, 50, 90, and 130 cm. The results showed that Z. latifolia was sensitive to high water depth stress in terms of the significantly decreased basal stem diameter, leaf width, root length, total biomass, and root to shoot ratio with increasing water depth. It was found suitable to grow in shallow water less than 50 cm in depth. The growth of Z. latifolia significantly increased sediment moisture content and porosity, while reduced wet bulk density in sediment and NH4–N concentration in interstitial water. Along the water depth gradient, the growth of Z. latifolia significantly impacted sediment wet bulk weight and loss on ignition, both NH4–N and PO4–P concentrations in interstitial water. However, no obvious regularities were observed in the sediment vertical profiles. NH4–N and PO4–P concentrations in interstitial water were much higher than in overlying water, indicating that they could diffuse from sediment to overlying water. NH4–N concentration was also higher in deep sediment. Growth properties of Z. latifolia (except for leaf length) are significantly correlated to wet bulk density, loss on ignition, NH4–N concentration in sediment and NH4–N, PO4–P concentrations in interstitial water. The results indicate that water depth less than 50 cm is favorable for the growth of Z. latifolia, where it can exert its ecological function effectively. This research suggests a possibility to promote the growth of Z. latifolia and exert its ecological function by rational water depth management. 相似文献
999.
随着全球冰川正在越来越多的地区融化, 冰川微生物资源很可能会由于冰川退缩而未被人类所发现就已受到生存的威胁而濒危.以祁连山老虎沟12号冰川消融区和末端雪样及末端土样为研究对象, 采用培养方法、分子鉴定, 研究冰川雪样优势菌群在冰川末端土样的分布状况及生理生化指标变化情况, 分析冰川细菌优势菌群在冰川退缩后适应非雪环境的能力.结果表明: 冰川末端雪样优势菌为1BW1和1BW2所代表的Pedobacter, 该属在冰川消融区雪样和冰川末端土样中未分离到; 冰川消融区雪样优势菌为2BW所代表的Acinetobacter, 该属在冰川末端雪样中的数量较少, 在冰川末端土样中的数量更少.不同采样位点16S rRNA序列相似性高的菌株其生理生化特征比相同采样位点的大.因此, 冰川冰退缩可能会引发冰川雪样中的优势种群不能适应新环境而灭绝. 应加强冰川细菌资源利用和保护的研究基础. 相似文献
1000.
http://www.sciencedirect.com/science/article/pii/S1674987113000595 总被引:11,自引:0,他引:11
Mei-Fu Zhou Wei Terry Chen Christina Yan Wang Stephen A.Prevec Patricia Pingping Liu Geoffrey H.Howarth 《地学前缘(英文版)》2013,4(5):481-502
Magmatic oxide deposits in the~260 Ma Emeishan Large Igneous Province(ELIP),SW China and northern Vietnam,are important sources of Fe,Ti and V.Some giant magmatic Fe-Ti-V oxide deposits, such as the Panzhihua,Hongge,and Baima deposits,are well described in the literature and are hosted in layered mafic-ultramafic intrusions in the Panxi region,the central ELIP.The same type of ELIP- related deposits also occur far to the south and include the Anyi deposit,about 130 km south of Panzhihua,and the Mianhuadi deposit in the Red River fault zone.The Anyi deposit is relatively small but is similarly hosted in a layered mafic intrusion.The Mianhuadi deposit has a zircon U-Pb age of~260 Ma and is thus contemporaneous with the ELIP.This deposit was variably metamorphosed during the Indosinian orogeny and Red River faulting.Compositionally,magnetite of the Mianhuadi deposit contains smaller amounts of Ti and V than that of the other deposits,possibly attributable to the later metamorphism.The distribution of the oxide ore deposits is not related to the domal structure of the ELIP.One major feature of all the oxide deposits in the ELIP is the spatial association of oxide-bearing gabbroic intrusions,syenitic plutons and high-Ti flood basalts.Thus,we propose that magmas from a mantle plume were emplaced into a shallow magma chamber where they were evolved into a field of liquid immiscibility to form two silicate liquids,one with an extremely Fe-Ti-rich gabbroic composition and the other syenitic.An immiscible Fe-Ti-(P) oxide melt may then separate from the mafic magmas to form oxide deposits.The parental magmas from which these deposits formed were likely Fe-Ti-rich picritic in composition and were derived from enriched asthenospheric mantle at a greater depth than the magmas that produced sulfide-bearing intrusions of the ELIP. 相似文献