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   192篇
  免费   11篇
  国内免费   4篇
大气科学   22篇
地球物理   53篇
地质学   57篇
海洋学   47篇
天文学   16篇
综合类   1篇
自然地理   11篇
  2021年   3篇
  2020年   3篇
  2019年   2篇
  2018年   4篇
  2017年   5篇
  2016年   7篇
  2015年   2篇
  2014年   10篇
  2013年   9篇
  2012年   2篇
  2011年   5篇
  2010年   12篇
  2009年   5篇
  2008年   6篇
  2007年   11篇
  2006年   6篇
  2005年   6篇
  2004年   4篇
  2003年   8篇
  2002年   12篇
  2001年   8篇
  2000年   8篇
  1999年   5篇
  1998年   3篇
  1997年   1篇
  1996年   3篇
  1995年   2篇
  1994年   4篇
  1993年   5篇
  1992年   2篇
  1991年   1篇
  1990年   2篇
  1989年   1篇
  1988年   1篇
  1987年   1篇
  1986年   6篇
  1985年   3篇
  1984年   3篇
  1983年   2篇
  1982年   4篇
  1981年   2篇
  1980年   2篇
  1979年   4篇
  1978年   4篇
  1977年   3篇
  1974年   3篇
  1973年   1篇
  1970年   1篇
排序方式: 共有207条查询结果,搜索用时 93 毫秒
141.
142.
The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life, represented by prominent biological evolution from the first appearance of soft-bodied animals from the late Neoproterozoic to the sudden diversification of animals with mineralized skeletons in the Cambrian. In South China several areas contain many fossils and are well exposed, suitable for the investigation of PC/C boundary. However, geochronological relationships are still poorly known because of lack of combined detailed investigations of internal structures of zircons and in-situ U–Pb dating.We focus on the internal structure of zircons from a tuff layer within Bed 5 in the Meishucun section on which we undertook cathodoluminescence (CL) imaging and in-situ U–Pb dating with LA-ICP-MS and nano-SIMS. Over 600 zircons from the tuff layer were classified into three types based on their CL images: oscillatory rims, inherited cores and dull structures. U–Pb dating of the internal structure of the zircons by LA-ICP-MS clearly shows a distinct unimodal age population dependent on the structure: 531 ± 17 Ma for the oscillatory rims and 515 Ma for the dull structures. The clear oscillatory zonation, the prismatic morphology, and their occurrence indicate that the oscillatory rims were formed from felsic magmatism, and that the U–Pb nano-SIMS age of 536.5 ± 2.5 Ma records the depositional age of the tuff. Our results indicate that the PC/C boundary is situated below Bed 5, and therefore the bottom of Zone 1 (Marker A) is more appropriate for the PC/C boundary than is the top of Zone 1 (Marker B). The age of a positive anomaly (P2) in the early Cambrian is estimated to be ca. 536 Ma.  相似文献   
143.
Standard deviations for vertical velocity and scalar quantities, such as temperature, T, and specific humidity, q, were analyzed on the basis of Monin-Obukhov (M-O) similarity theory in the atmospheric surface layer. The correlation coefficient between scalar quantities T and q, RTq, was derived from the similarity functions and can be expressed as the ratio of BT/Bq (BT≤ Bq), where parameter B is the value of the normalized standard deviation of any scalar quantity at neutral conditions.  相似文献   
144.
145.
Velocity as well as attenuation factorQ –1 ofP-wave in a dry granitic rock sample under uniaxial compressions were measured in the range of frequency between 100 kHz and 710 kHz by using the pulse transmission technique. Above the stress of 0.5 f , where f is the fracture stress, theP-wave velocity decreases with increasing axial stress, whereasQ –1 increases. Particularly, the change ofQ –1 is greater for high frequency than for low frequency. At a given stress level, the higher the frequency, the higher theP-wave velocity and the largerQ –1. This result means that the velocity decrease with increasing stress is smaller for higher frequency. Because of this frequency-dependence of velocity decrease, theP-wave in the rock under dilatant state shows dispersion. The body wave dispersion is more remarkable at higher stress, and is not found in a homogeneous material with no cracks. Thus the disperison is attributed to the generation of cracks. When the frequency-dependence ofQ –1 is approximated asf n in the present frequency range, the exponentn takes a value from 0.63 to 0.77.  相似文献   
146.
A field survey was undertaken to reveal the structure of the tidal front in Iyo-nada. An obvious tidal front was found between the mixed region around Hayasui Straits and the stratified region in Iyo-Nada. Its structure was typical in the eastern part and was influenced by the river discharge in the western part. An intense chlorophylla maximum was found in the subsurface layer of the eastern typical front. Analysis using TS diagram suggested that, around the eastern front, there was an intrusion from the mixed water to the middle layer of the stratified water. This intrusion was supposed to be an important process supplying nutrients from the mixed water to the subsurface of the frontal region and causing the intense chlorophylla maximum.  相似文献   
147.
By using Acoustic Doppler Current Profiler (ADCP) measurements with the four round-trips method to remove diurnal/semidiurnal tidal currents, the detailed current structure and volume transport of the Tsushima Warm Current (TWC) along the northwestern Japanese coast in the northeastern Japan Sea were examined in the period September–October 2000. The volume transport of the First Branch of the TWC (FBTWC) east of the Noto Peninsula was estimated as approximately 1.0 Sv (106 m3/s), and the FBTWC continued to flow along the Honshu Island to the south of the Oga Peninsula. To the north of the Oga Peninsula, the Second Branch of Tsushima Warm Current and the eastward current established by the subarctic front were recombined with the FBTWC and the total volume transport increased to 1.9 Sv. The water properties at each ADCP line strongly suggested that most of the upper portion of the TWC with high temperature and low salinity flowed out to the North Pacific as the Tsugaru Warm Current. In the north of the Tsugaru Strait, the volume transport of the northward current was observed to be as almost 1 Sv. However, the component of the TWC water was small (approximately 0.3 Sv).  相似文献   
148.
Barotolerant bacterium was isolated from sediment sample which was obtained from the depth of 4033 m in the Izu-Ogasawara Trench. The physiological property, growth characteristics and fatty acid composition were examined. The strain was a psychrotrophic and barotolerant bacterium, and was identified as species in the genusAlteromonas. The fatty acids of the strain were from C12 to C18. As the growth pressure increased, the portion of unsaturated fatty acid in membrane fraction increased due to an increase in the portion of C171 and C181, while the relative portion of C160 and C161 decreased. On the other hand, as the growth temperature decreased, the portion of unsaturated fatty acid increased due to the increase in the portion of C161 and C181.  相似文献   
149.
Recent statistical analyses on the isotopic compositions of oceanic, arc, and continental basalts have revealed that the Earth's mantle is broadly divided into eastern and western hemispheres. The present study aimed to characterize the isotopically defined east–west geochemical hemispheres using trace-element concentrations. Basalt data with Rb, Sr, Nd, Sm, Pb, Th, and U in addition to the isotopic ratios 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb were selected mostly from the GEOROC and PetDB databases. A total of 4787 samples were used to investigate the global geochemical variations. The results show that the wide trace-element variations are broadly explained by the melting of melt-metasomatized and fluid-metasomatized mantle sources. The larger amount of the fluid component derived from subducted plates in the eastern hemisphere than that in the western hemisphere is inferred from the basalts. These characteristics support the hypothesis that focused subduction towards the supercontinent created the mantle geochemical hemispheres.  相似文献   
150.
Koryu矿山位于北海道南西部,为一浅成低温热液型Au-Ag石英脉型矿床.矿床产于中新世黑色泥岩中的东西向剪切带内.矿床含有8个主要脉体,总体呈东西走向,在水平、垂直方向上均发生结构、构造的变化.1号脉和3号脉中冰长石K-Ar年龄为0.8~1.2 Ma,表明矿化时代为更新世.根据矿脉体的穿插关系和脉内矿物共生关系得出矿区有2期矿化,早期矿化可分3个阶段(即E-Ⅰ,E-Ⅱ,E-Ⅲ),晚期矿化可分成7个阶段(即L-Ⅰ,L-Ⅱ,L-Ⅲ,L-Ⅳ,L-Ⅴ,L-Ⅵ,L-Ⅶ).宽矿脉体由多阶段矿化组成,Au-Ag矿化主要与L-Ⅲ阶段矿化密切相关.早期矿化形成的特征矿物为钙菱镁矿、钙锰辉石及少量矿石矿物;晚期矿化形成大量矿石矿物.含AuAg的矿物主要为银金矿、螺状硫银矿-辉硒银矿、硫锑铜银矿-硫砷铜银矿、浓红银矿-淡红银矿、辉铜银矿、马硫铜银矿、碲银矿和黝铜矿.而脉石矿物为绿泥石、蒙脱石、石英.区内许多成矿阶段均含富气相流体包裹体,这表明沸腾作用在整个过程中间歇发生.早期矿化温度(263~283℃)稍高于晚期矿化温度(246~260℃).成矿溶液盐度为0.5%~6.0% NaCl当量,尽管CO2含量在晚期矿化阶段达0.4%(质量分数).这些特征表明成矿流体最大压力为3.1~6.8 M Pa,相当于古潜水面下430~850 m深度.石英形态学结合流体包裹体研究证实流体沸腾作用反复发生,导致SiO2过饱和,形成结构各异的二氧化硅矿物.在脉体形成过程中,SiO2重结晶形成石英.稳定同位素资料、矿物共生组合特征、石英结构特征和流体包裹体特征综合研究得出Koryu金-银矿床成矿模式如下:矿床矿化分为明显的两期,即早期和晚期,分别对应类型1和类型2两种热液流体.早期流体具相对高的W18O值(-5.3‰~-4.7‰),温度不低于260℃;晚期流体W18O值相对偏低(-9.3‰~-6.0‰),温度250℃以上.类型1流体发生深部循环,淋滤Ca和M n元素,在早期矿化过程中形成钙菱镁矿、钙锰辉石.与浅成水混合的类型2流体沿新通道上升,富含Au、Ag元素.在晚期矿化过程中,流体上升至沸腾带( < 850 m),金、银在250℃沉淀成矿.  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