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   1719篇
  免费   67篇
  国内免费   18篇
测绘学   40篇
大气科学   123篇
地球物理   497篇
地质学   566篇
海洋学   165篇
天文学   281篇
综合类   5篇
自然地理   127篇
  2021年   18篇
  2020年   23篇
  2019年   24篇
  2018年   42篇
  2017年   32篇
  2016年   56篇
  2015年   36篇
  2014年   42篇
  2013年   83篇
  2012年   45篇
  2011年   62篇
  2010年   78篇
  2009年   75篇
  2008年   66篇
  2007年   83篇
  2006年   61篇
  2005年   55篇
  2004年   51篇
  2003年   52篇
  2002年   49篇
  2001年   26篇
  2000年   33篇
  1999年   28篇
  1998年   27篇
  1997年   29篇
  1996年   26篇
  1995年   29篇
  1994年   31篇
  1993年   14篇
  1992年   31篇
  1991年   29篇
  1990年   38篇
  1989年   25篇
  1988年   21篇
  1987年   17篇
  1986年   16篇
  1985年   28篇
  1984年   35篇
  1983年   26篇
  1982年   19篇
  1981年   30篇
  1980年   23篇
  1979年   21篇
  1978年   21篇
  1977年   15篇
  1976年   12篇
  1975年   12篇
  1974年   14篇
  1973年   15篇
  1971年   11篇
排序方式: 共有1804条查询结果,搜索用时 203 毫秒
121.
Radiocarbon dates indicate a Farmdalian through Woodfordian age for fossil white spruce (Picea glauca) in the Tunica Hills, a greater time span than previously documented. These dates, furthermore, require revision of the ages of fluvial terrace deposits in the region. Spruce-bearing Terrace 1 sediments, previously regarded as late Woodfordian to Holocene age, are of Farmdalian to late Holocene age. Terrace 2 sediments, previously assigned a Sangamonian or Farmdalian age, are of probable Altonian age. Plant fossils in Terrace 1 sediments represent two climatically and temporally distinct assemblages: a cool-temperate, Farmdalian-Woodfordian assemblage dominated by white spruce but including deciduous hardwoods suggestive of an oak-hickory association; and a warm-temperate Holocene assemblage composed of southern hardwoods and nonarboreal species representative of the modern flora of the Tunica Hills. Locally, fossils of white spruce have been reworked into younger Terrace 1 sediments containing the warm-temperate plant assemblage. The transition from the cool-temperate flora to the modern warm-temperate flora in the Tunica Hills probably occurred around 12,000 yr B.P.  相似文献   
122.
123.
There are many data base management systems now available as commercially marketed software packages. Although most of these packages were initially aimed at bussiness or administrative data processing applications, they may frequently also be the right tool for a scientific data processing task. This becomes more apparent as we notice that scientific computer programmers are spending more and more time on data management requirements rather than the coding of mathematical algorithms. In a scientific environment, a generalized data base management package is best viewed as a tool for programmers, rather than as a tool for direct, independent use by end users or by agency management. To the end user, the most attractive feature of a commerical DBMS is usually the interactive retrieval and update language. To the programmer, the most attractive feature is more likely to be the strong support for various types of keyed access. All of the manipulations necessary to build and maintain indices and other tables can be treated as procedural abstractions. Coupled with a procedural language, a DBMS offers the programmer a higher level (in the sense of more abstract) language. The most important geodetic requirement on a commercial DBMS is therefore that the package contain a strong Data Manipulation Language, with strong support for the algorithmic language used for scientific processing. Presented at International Symposium on Management of Geodetic Data, Copenhagen, August 24–26, 1981.  相似文献   
124.
Seasonal occurrence and vertical distribution of larvae of two genera of brachyuran crab were studied in a secondary estuary flowing into Delaware Bay. Spawning in the xanthid crabRhithropanopeus harrisii occurred earlier with peak abundance of larvae in June and with a distinct decline in abundance in August. In contrast,Uca spp. larvae reached peak abundance in August. All zoeal stages ofR. harrisii were collected in the river suggesting that larvae of this species are retained in secondary estuaries near areas of prime adult habitat. Only zoea stage I larvae and megalopa ofUca spp. were collected in the river indicating that larvae of these speies may be flushed into the Delaware Bay and may not return to secondary estuaries near areas suitable for adult habitat until the megalopa stage is reached. It is not clear if this dispersal pattern is an active or passive process. *** DIRECT SUPPORT *** A01BY019 00004  相似文献   
125.
Factors affecting Kepone uptake and lipid content of the clam,Rangia cuneata, were tested over a 12 month study. Clams obtained from the Rappahannock and James Rivers were held in submersible liver boxes at two sites in the James River estuary and were sampled monthly from September 1978 through August 1979. Clams held in the freshwater zone near the source of Kepone contamination (Hopewell, Virginia) generally had higher Kepone and lipid content than those held downstream in the oligohaline zone. Significant differences in Kepone content between test sites and months are largely, but not entirely, a function of ambient water temperature, dissolved oxygen, amount of lipid in the clam, turbidity, Kepone content of the water column and duration of exposure. Lipid content of clams varied significantly between test sites, river of origin and months and is significantly related to salinity, ambient water temperature, pH and duration of exposure. Kepone content was more closely correlated with total lipid stores of clams than any other real variable. This association may be due to lipid reserves acting as a storage site for Kepone, but may also be interpreted as the result of selection against clams lacking lipid stores that might act as reservoirs for Kepone thus protecting more delicate tissues.  相似文献   
126.
127.
Using the solubility model developed by Harvie and Weare (1980), the stable mineral-solution assemblages for the six-component system Na-K-Mg-Ca-SO4-Cl-H2O and its constituent 5-, 4- and 3-component systems at 25°C have been defined. Invariant point maps have been constructed showing the connections by univariant lines. The solubility volumes for all 20 minerals considered are also illustrated. Of the 37 invariant points, only 3 have solutions which are Ca-rich; the remaining 34 can be plotted in the reciprocal system Na-K-Mg-SO4-Cl, which is similar to the seawater system, except that the restriction of halite saturation has been removed. Application of these results and implications for the evolution of major brine types are briefly discussed.  相似文献   
128.
This review is intended to provide an introduction for the nonspecialist to concepts and techniques which are useful for analysing palaeomagnetic time series. Emphasis is placed on analysis in the frequency domain, particularly the periodogram and maximum entropy methods. The review consists of two parts dealing with techniques and applications respectively.  相似文献   
129.
Throughout the aerial radiometric reconnaissance survey portion of the U.S. Department of Energy's National Uranium Resource Evaluation (NURE) program, the identification of outliers (anomalies) was an important approach to locating regions with radio-element concentrations that are either higher or lower than expected. The method introduced herein to locate such regions involves three steps: selection of a high (or low) threshold for the variate of interest; use of the sample percentile to identify all points of interest; and movement of a window over the selected data to locate significant clusters of observations. These steps, applied to aerial radiometric 214Bi (equivalent uranium) data collected over the Copper Mountain area in Wyoming, resulted in the identification of areas enriched in that variate.  相似文献   
130.
In a reconnaissance study, trace amounts of Cd, Cu, Pb, and Zn were determined in “A-zone” soil from 22 locations in the Dayton, Ohio, area. Soil samples were collected at high-volume air monitoring sites in urban, suburban, and rural areas. Measurable amounts of the elements were found in all of the samples. Positive correlations occur between each metal and particulate matter, with correlation coefficients of aboutr=0.70, at the 99% confidence level. Natural background values for the elements were measured in soil from rural areas. Theoretical background values, which are very close to those measured, were calculated from the individual regression equations. Natural background levels do not exceed 1.00 ppm Cd, 15 ppm Cu, 25 ppm Pb, and 55 ppm Zn. The heavy metal contents of most soils in the area exceed background by factors of up to 3.0 for Cd, up to 4.5 for Cu, up to 11 for Pb, and up to 4.5 for Zn. Significant positive correlations among the metals suggest a common source (or sources) for at least some, if not most, of the heavy metals. For the most part, the highest metal values are found in soils near coal-burning plants. Fly ash from a local plant contains substantial amounts of the elements. The high lead values are largely due to vehicular exhaust. However, there is evidence that the metals can also come from the normal deterioration of vehicles. It appears that airborne pollution is an important source of heavy metals in Dayton area soils.  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