首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   274篇
  免费   45篇
  国内免费   49篇
测绘学   6篇
大气科学   56篇
地球物理   137篇
地质学   88篇
海洋学   16篇
天文学   5篇
综合类   27篇
自然地理   33篇
  2024年   1篇
  2023年   4篇
  2022年   3篇
  2021年   9篇
  2020年   9篇
  2019年   8篇
  2018年   20篇
  2017年   16篇
  2016年   19篇
  2015年   8篇
  2014年   11篇
  2013年   12篇
  2012年   17篇
  2011年   10篇
  2010年   20篇
  2009年   14篇
  2008年   16篇
  2007年   11篇
  2006年   15篇
  2005年   10篇
  2004年   7篇
  2003年   12篇
  2002年   9篇
  2001年   8篇
  2000年   11篇
  1999年   14篇
  1998年   13篇
  1997年   18篇
  1996年   9篇
  1995年   10篇
  1994年   5篇
  1993年   7篇
  1992年   4篇
  1991年   1篇
  1990年   2篇
  1989年   1篇
  1988年   2篇
  1986年   1篇
  1985年   1篇
排序方式: 共有368条查询结果,搜索用时 31 毫秒
271.
To improve the burial flux calculations of bioavailable phosphorus(P) and study opal-associated P(Opal-P) in the East China Sea(ECS), surface and core sediments were collected in the Changjiang Estuary(CE) and the south of the Cheju Island. In this study, sedimentary P was operationally divided into seven different forms using modified sedimentary extraction(SEDEX) technique: LSor-P(exchangeable or loosely sorbed P), Fe-P(easily reducible or reactive ferric Fe-bound P), CFA-P(authigenic carbonate fluorapatite and biogenic apatite and CaCO_3-bound P), Detr-P(detrital apatite), Org-P(organic P), Opal-P and Ref-P(refractory P). The data revealed that the concentrations of the seven different P forms rank as Detr-P CFA-P Org-P Ref-P Opal-P Fe-P LSor-P in surface sediments and CFA-P Detr-P Org-P Ref-P Fe-P Opal-P LSor-P in core sediments. The distributions of the total phosphorus(TP), TIP, CFA-P, Detr-P are similar and decrease from the CE to the south of the Cheju Island. Meanwhile, Org-P and Opal-P exhibit different distribution trends; this may be affected by the grain size and TOM. The concentrations of potentially bioavailable P are 9.6-13.0 μmol g~(-1) and 10.0-13.6 μmol g~(-1), representing 61%-70% and 41%-64% of the TP in surface and core sediments, respectively. The concentrations of Opal-P are 0.6-2.3 μmol g~(-1) and 0.6-1.4 μmol g~(-1) in surface and core sediments, accounting for 5.3%-19.8% and 4.2%-10.6% of bioavailable P, respectively. The total burial fluxes of Opal-P and bioavailable P are 1.4×10~9 mol yr~(-1) and 1.1×10~10 mol yr~(-1) in the ECS, respectively. Opal-P represents about 12.7% of potentially bioavailable P, which should be recognized when studying P cycling in marine ecosystems.  相似文献   
272.
科尔占地区位于滨里海盆地阿斯特拉罕-阿克纠宾斯克隆起带东南部的比伊克扎尔次级隆起上,该地区广泛发育下二叠统孔谷组盐岩,以盐岩沉积为界,它将整个剖面划分为盐上层系(以碎屑岩为主的含油气层系)和盐下层系(以碳酸盐岩为主的含油气层系)两个大的含油气层系,石炭系为盐下层系中主要的含油气层系之一。通过对研究区唯一的盐下井K井岩屑及其薄片的详细观察,并结合该井测井资料,分析了石炭系的沉积特征,并对其沉积相进行了划分与描述。K井石炭系主要发育3种类型沉积相,分别为深水浊积扇相、斜坡相和台地相,其中深水浊积扇相可进一步划分为内扇、中扇和外扇3个亚相,台地相划分为滩间洼地和台内滩2个亚相。利用K井钻探结果,结合区域资料,初步预测出科尔占地区早石炭世和中石炭世末期沉积相带的平面分布特征。  相似文献   
273.
物源研究是理解黄土高原风成沉积物中古环境记录的关键一环.众多证据表明,黄土高原西北部的阿拉善高原及其周边广袤干旱地区是黄土高原黄土的主要风尘源区.但是,基于锆石U-Pb同位素地质年代学的证据对此提出了质疑.黄土高原黄土与青藏高原北部物质中的锆石具有相似的年龄分布,预示黄土高原风尘主要以西风搬运的方式来自青藏高原北部,特别是柴达木盆地.本研究通过分析西宁黄土的锆石年龄特征验证此假说.西宁处于在从青藏高原北部到黄土高原的假想搬运路线上.西宁黄土主要来自青藏高原北部的干旱区.西宁黄土中锆石的主要年龄分布与黄土高原黄土相似,反映青藏高原北部确实为黄土高原黄土的主要物质来源.但是,与黄土高原黄土相比,西宁黄土缺少年龄在360Ma左右和年龄小于100Ma的锆石.这两组年龄的锆石可能来自其他地区,例如处于中亚造山带.由于阿拉善于旱区也通过河流搬运接收来自青藏高原北部的物质,锆石U-Pb年龄证据并不能排除阿拉善干旱区作为主要风尘物源的可能.相反,阿拉善干旱区还接受来自其北部中亚造山带和其本身华北克拉通的物质,来自阿拉善干旱区的风尘可能更好地解释黄土高原黄土中的锆石年龄特征.  相似文献   
274.
Intermsoftheharmfulmattersincoalandcoalminingwastes,thefirstisabouthazardouselements ,theotherin cludessulfurandhazardousorganicmatter.Muchresearchontracetoxicelementsandtheirimpactsonenvironmenthasbeeninvestigatedinmanycountries (Goodarzi,1995 ;Finkelman ,1994 ;Chouetal.,1982 ) .Althoughthere searchconcerningthisaspectwasstartedmuchlaterinChi na ,manystudieshavebeenstrengthenedsince 1980 ,andmuchprogresshasbeenmade ,alargenumberofdatahavebeencollected ,andthecomparisonshavebeenmadeforthedata…  相似文献   
275.
通过对出露于小兴安岭西北部白石砬子地区中生代花岗岩的研究,查明该地区存在一套由英云闪长岩-花岗闪长岩-石英闪长岩和少量闪长岩为组合的花岗岩,锆石U-Pb LA-ICP-MS测年分别获得(171.1±1.3)、(173.9±3.9)、(178.3±1.5)和(169.1±6.2)Ma的岩体侵位年龄,证明其形成于早—中侏罗世。通过岩石地球化学方法判别该花岗岩组合具有高Al_2O_3型T_1T_2G_1花岗岩特征,整体具有富硅、富铝,钠质,贫铁、镁、钛,属于偏铝质-弱过铝质钙碱性系列,富集轻稀土,相对亏损重稀土,富集Rb、Ba、Th、U、Sr等大离子亲石元素,相对亏损Nb、Ta、Zr、Hf等高场强元素,Eu具有轻微负异常的特点,与安第斯I型花岗岩具有相似性。其低Sr/Y值、(La/Yb)N值,弱或无负δEu异常等特征反映源区残留相矿物以角闪石、辉石为主,有少量斜长石,石榴石可能有少量残留。岩体露头有闪长质岩浆与英云(花岗)闪长质岩浆两种不完全机械混合的现象,反映可能存在两个以上的不同源区。另外,岩体不规则侵入元古宙地层(兴华渡口群和落马湖群)和岩浆中残留有大量早期捕获锆石,说明在岩浆上升侵位过程中与围岩发生同化混染作用。综合研究认为,该早—中侏罗世TTG岩浆形成与蒙古—鄂霍茨克洋的闭合作用密切相关,反映侏罗纪蒙古—鄂霍茨克洋存在向南侧微陆块的俯冲作用。  相似文献   
276.
中国地表太阳辐射再分析数据与观测的比较   总被引:2,自引:1,他引:2       下载免费PDF全文
利用我国地表太阳辐射台站资料和海上观测资料与同期的NCEP/NCAR, NCEP/CFSR再分析资料进行比较,检验再分析资料是否能够反映中国地区的太阳辐射特征。结果表明:1979年之前NCEP/NCAR太阳辐射资料的可信度较低,存在虚假的明显上升趋势,1979年之后两套再分析资料的可信度均较高,在我国东部和低纬度地区的可信度好于西部和高纬度地区;由逐6 h再分析数据直接计算得到的逐日太阳辐射比实际观测偏低,剔除太阳辐射为零的情况计算逐日资料更合理。在大陆地区,NCEP/NCAR,NCEP/CFSR再分析资料与台站太阳辐射资料的1979—2009年共31年平均误差分别为10.37 W·m-2和-42.68 W·m-2,误差的标准差分别为12.31 W·m-2和4.19 W·m-2;在海洋区域,NCEP/NCAR,NCEP/CFSR再分析资料与海上观测太阳辐射资料的平均误差分别为-161.19 W·m-2和-179.66 W·m-2,误差的标准差分别为37.07 W·m-2和35.36 W·m-2。与大陆台站资料相比,海上观测与再分析资料的误差偏大,这可能与海上观测资料较少,限制了NCEP模式的评估和改进有关。  相似文献   
277.
论伊犁古裂谷   总被引:52,自引:27,他引:52  
车自成  刘良 《岩石学报》1996,12(3):478-490
伊犁石炭纪裂谷叠加在中天山造山带之上.后者是在加里东旋回中发育的造山隆起带,除发育下古生界优地槽沉积外,前寒武系变质基底广泛出露,石炭纪沉积以大小不筹的上叠盆地不整合在这样一个基底之上。由于基底性质的差异,西段盆地范围大,沉积厚度大;向东到巴仑台直至托克逊地区,则呈断续延伸的小型断用盆地,厚度薄,而且底砾岩层的厚度大.现有资料说明,石炭纪火山岩主要是玄武岩一玄武安山岩-粗安岩和粗面岩-流纹岩组合.以碱性系列的岩石为主,多为高钾系列,次为钠质;主要为低Al型,也有高Al型;轻稀土和不相容元素从低度富集到中等富集(La/Yb为1.8~6.7和5.3~14.9),Nb、Ta从微亏损到强亏损(Th/Ta≈1.3~15);Nd(t)值偏低(≈+2.3~+4.6),而ISr(~0.7058)和208Pb偏高(208Pb/204Pb≈38.452,206Pb/204Pb≈18.378),反映其岩浆来自弱富集或异常地幔源区.由于裂谷发育程度的不同,岩浆或者直接来出,或者在下地壳环境下受到混染或发生分异,从而导致岩石组合在走向上的差异。  相似文献   
278.
Summary Forty years of daily global and diffuse radiation data have been used to characterize the atmospheric optical conditions at 14 premium radiation stations in China. The clearness index (K T ), diffuse ratio (K D ), and diffuse coefficient (K C ) have been analyzed for long-term and seasonal variations, correlations of relative sunshine (R S ) versus clearness index and diffuse coefficient, respectively. Two polynomial models have been developed to simulate clearness index and diffuse ratio by using relative sunshine data and have been shown to be reasonable at most of the 14 stations. The results show that there are significant declining trends in clearness index and increasing trends in diffuse ratio at most of the 14 premium radiation stations, but there are no significant trends for diffuse coefficient. The seasonal variations of clearness index are determined by the regional climate and changes with relative sunshine.  相似文献   
279.
This study analyzes and compares aerosol properties and meteorological conditions during two air pollution episodes in 19–22 (E1) and 25–26 (E2) December 2016 in Northeast China. The visibility, particulate matter (PM) mass concentration, and surface meteorological observations were examined, together with the planetary boundary layer (PBL) properties and vertical profiles of aerosol extinction coefficient and volume depolarization ratio that were measured by a ground-based lidar in Shenyang of Liaoning Province, China during December 2016–January 2017. Results suggest that the low PBL height led to poor pollution dilution in E1, while the high PBL accompanied by low visibility in E2 might have been due to cross-regional and vertical air transmission. The PM mass concentration decreased as the PBL height increased in E1 while these two variables were positively correlated in E2. The enhanced winds in E2 diffused the pollutants and contributed largely to the aerosol transport. Strong temperature inversion in E1 resulted in increased PM2.5 and PM10 concentrations, and the winds in E2 favoured the southwesterly transport of aerosols from the North China Plain into the region surrounding Shenyang. The large extinction coefficient was partially attributed to the local pollution under the low PBL with high ground-surface PM mass concentrations in E1, whereas the cross-regional transport of aerosols within a high PBL and the low PM mass concentration near the ground in E2 were associated with severe aerosol extinction at high altitudes. These results may facilitate better understanding of the vertical distribution of aerosol properties during winter pollution events in Northeast China.  相似文献   
280.
An extensive field experiment for measurement of physical and chemical properties of aerosols was conducted at an urban site in the Chinese Academy of Meteorological Sciences (CAMS) in Beijing and at a rural site in Gucheng (GC), Hebei Province in December 2016. This paper compares the number size distribution of submicron particle matter (PM1, diameter < 1 μm) between the two sites. The results show that the mean PM1 number concentration at GC was twice that at CAMS, and the mass concentration was three times the amount at CAMS. It is found that the accumulation mode (100–850 nm) particles constituted the largest fraction of PM1 at GC, which was significantly correlated with the local coal combustion, as confirmed by a significant relationship between the accumulation mode and the absorption coefficient of soot particles. The high PM1 concentration at GC prevented the occurrence of new particle formation (NPF) events, while eight such events were observed at CAMS. During the NPF events, the mass fraction of sulfate increased significantly, indicating that sulfate played an important role in NPF. The contribution of regional transport to PM1 mass concentration was approximately 50% at both sites, same as that of the local emission. However, during the red-alert period when emission control took place, the contribution of regional transport was notably higher.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号