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   86篇
  免费   2篇
  国内免费   1篇
测绘学   2篇
大气科学   5篇
地球物理   13篇
地质学   28篇
海洋学   4篇
天文学   34篇
自然地理   3篇
  2020年   1篇
  2018年   2篇
  2017年   1篇
  2015年   1篇
  2013年   3篇
  2012年   1篇
  2011年   3篇
  2010年   3篇
  2009年   3篇
  2008年   1篇
  2007年   3篇
  2006年   1篇
  2005年   2篇
  2004年   8篇
  2003年   5篇
  2002年   5篇
  2001年   3篇
  2000年   1篇
  1999年   3篇
  1998年   2篇
  1997年   1篇
  1996年   1篇
  1995年   1篇
  1994年   2篇
  1990年   2篇
  1988年   1篇
  1986年   1篇
  1985年   1篇
  1984年   6篇
  1982年   2篇
  1981年   1篇
  1980年   3篇
  1979年   4篇
  1978年   2篇
  1977年   4篇
  1976年   2篇
  1973年   2篇
  1957年   1篇
排序方式: 共有89条查询结果,搜索用时 31 毫秒
61.
62.
Carbon and oxygen isotopic determinations have been made of 29 species of Recent Indian Ocean planktonic foraminifera. Fourteen core-top samples were used and as many as 18 species were chosen from a single core-top sample. The δ13C of the foraminifera was compared with that of total dissolved CO2 (ΣCO2) and of calcite precipitated in isotopic equilibrium with ΣCO2. The foraminiferal calcite is always at least 1.2‰ less than the value estimated for equilibrium calcite. This carbon isotopic disequilibrium suggests the partial utilization of13C-depleted metabolic CO2. The calcite tests of several species, however, have δ13C values which are similar to the δ13C of ΣCO2 in seawater. This relationship suggests that important paleohydrographic information may be obtained from carbon isotope records based on analyses of several foraminiferal species from single deep-sea sediment samples.  相似文献   
63.
Magnesium/calcium, Sr/Ca, and Na/Ca atom ratios were determined in the calcite and aragonite regions of Mytilus edulis shells which were grown in semi-artificial ‘seawater’ solutions having varying Mg/Ca, Sr/Ca, and Na/Ca ratios. These ratios were measured by instrumental neutron activation, atomic absorption, and electron microprobe analytical techniques. Strontium/calcium ratios in both calcite and aragonite were linearly proportional to solution Sr/Ca ratios. Magnesium/calcium ratios in calcite increased exponentially when solution Mg/Ca ratios were raised above the normal seawater ratio; whereas in aragonite, Mg/Ca ratios increased linearly with increases in solution Mg/Ca ratios. Sodium/calcium and sulfur/calcium ratios in calcite covaried with Mg/Ga solution ratios. Conversely, in aragonite, Na/Ca ratios varied linearly with solution Na/Ca ratios.Magnesium is known to inhibit calcite precipitation at its normal seawater concentration. We infer from the results of the work reported here that Mytilus edulis controls the Mg activity of the outer extrapallial fluid, thus facilitating the precipitation of calcitic shell. Increases in sulfur content suggest that changes in shell organic matrix content occur as a result of environmental stress. Certain increases in Mg content may also be correlated to stress. Sodium/calcium variations, and their absolute amounts in calcite and aragonite, are best explained by assuming that a substantial amount of Na is adsorbed on the calcium carbonate crystal surface. Strontium/calcium ratios show more promise than either Mg/Ca or Na/Ca ratios as seawater paleochemistry indicators, because the Sr/Ca distribution coefficients for both aragonite and calcite are independent of seawater Ca and Sr concentrations.  相似文献   
64.
65.
66.
Voltaite is a mineral of fumaroles, solfatares, coal-fire gas vents, and acid-mine drainage systems. The nominal composition is K2Fe5 2+Fe3 3+Al(SO4)12·18H2O and the nominal symmetry is cubic, $Fd\overline{3}c$ . The tetragonal (I41/acd) superstructure of voltaite is known as the mineral pertlikite. In this study, we investigated 22 synthetic voltaite samples in which Fe2+ was partially or completely replaced by Mg, Zn, Mn, or Cd, by single-crystal and powder X-ray diffraction (both in-house and synchrotron). Two samples contained NH4 + instead of K+. The structure of voltaite is based on a framework defined by kröhnkite-like heteropolyhedral chains which host both M3+ and M2+ in octahedral coordination. Unit cell dimensions of the end-members scale almost linearly with the size of M2+. In the Fe2+-Mg-Zn solid solutions, the Fe2+-Mg and Fe2+-Zn solutions are linear (ideal) in terms of their lattice-parameter variations. The Mg-Zn solid solution, however, is strongly non-ideal. A detailed analysis of the topology of the chains showed that this behavior originates in expansion and contraction of individual M2+-O bonds within the chains. In the Mg-Zn solid solution, some of the M2+-O bonds expand while none contract. In the other solid solutions, expansion of some M2+-O bonds is always compensated by contraction of the other ones. Parts of the nominally cubic crystals are optically anisotropic and their symmetry is found to be tetragonal by single crystal X-ray diffraction measurements. The coexistence of cubic and tetragonal sectors within a single crystal without any detectable difference in their chemical composition is difficult to explain in terms of growth of such composite crystals. Mössbauer and infrared spectra collected on our synthetic crystals conform with previously published data.  相似文献   
67.
68.
69.
70.
The Galicia Interior Basin (GIB; NW Iberian Peninsula) is located near a critical transition between the subtropical (temperate) and subpolar (cold) gyres of the North Atlantic. It therefore witnesses oceanographic changes driven by global climatic events. This study reports on the recent (latest Pleistocene) sedimentary, palaeoceanographic and palaeoclimatic history of the basin. We integrated analysis of deep‐sea sediment cores retrieved from an E–W transect across the GIB. The analysis indicated three types of sedimentary processes recording glacial (Marine Isotope Stage 2–4) and deglacial events: along‐slope bottom currents (forming contourite deposits), pelagic and hemipelagic sedimentation, and gravitational dislocation. Variation in depositional patterns and sedimentation rates indicate distinctive transport (along‐slope and down‐slope) and depositional processes. These in turn reflect climatic and oceanographic drivers. We interpret changes in sea level from core evidence showing changes in sediment supply. The cores exhibited conspicuous sedimentary evidence of Heinrich events (HEs). The stratigraphic intervals associated with HEs showed significant lateral variation. We suggest that the lateral variation may result from the development of an oceanographic boundary between surface water masses with different temperature and salinity parameters or changes in surface currents which may have introduced relatively warmer water into the GIB during the last glacial period.  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