全文获取类型
收费全文 | 72517篇 |
免费 | 2665篇 |
国内免费 | 3192篇 |
专业分类
测绘学 | 2283篇 |
大气科学 | 6301篇 |
地球物理 | 13677篇 |
地质学 | 30524篇 |
海洋学 | 6674篇 |
天文学 | 13283篇 |
综合类 | 1224篇 |
自然地理 | 4408篇 |
出版年
2022年 | 903篇 |
2021年 | 1256篇 |
2020年 | 1234篇 |
2019年 | 1360篇 |
2018年 | 4495篇 |
2017年 | 4157篇 |
2016年 | 3308篇 |
2015年 | 1397篇 |
2014年 | 1937篇 |
2013年 | 2784篇 |
2012年 | 2986篇 |
2011年 | 4942篇 |
2010年 | 4410篇 |
2009年 | 4831篇 |
2008年 | 4098篇 |
2007年 | 4649篇 |
2006年 | 2326篇 |
2005年 | 1857篇 |
2004年 | 1758篇 |
2003年 | 1915篇 |
2002年 | 1690篇 |
2001年 | 1330篇 |
2000年 | 1158篇 |
1999年 | 1082篇 |
1998年 | 991篇 |
1997年 | 1018篇 |
1996年 | 838篇 |
1995年 | 818篇 |
1994年 | 722篇 |
1993年 | 628篇 |
1992年 | 568篇 |
1991年 | 525篇 |
1990年 | 523篇 |
1989年 | 497篇 |
1988年 | 443篇 |
1987年 | 460篇 |
1986年 | 470篇 |
1985年 | 547篇 |
1984年 | 587篇 |
1983年 | 569篇 |
1982年 | 524篇 |
1981年 | 481篇 |
1980年 | 468篇 |
1979年 | 426篇 |
1978年 | 388篇 |
1977年 | 391篇 |
1976年 | 353篇 |
1975年 | 368篇 |
1974年 | 349篇 |
1973年 | 381篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
121.
K.A. MAASCH P.A. MAYEWSKI E.J. ROHLING J.C. STAGER W. KARLEN L.D. MEEKER E.A. MEYERSON 《Geografiska Annaler: Series A, Physical Geography》2005,87(1):7-15
ABSTRACT. Although considerable attention has been paid to the record of temperature change over the last few centuries, the range and rate of change of atmospheric circulation and hydrology remain elusive. Here, eight latitudinally well-distributed (pole-equator-pole), highly resolved (annual to decadal) climate proxy records are presented that demonstrate major changes in these variables over the last 2000 years. A comparison between atmospheric 14 C and these changes in climate demonstrates a first-order relationship between a variable Sun and climate. The relationship is seen on a global scale. 相似文献
122.
Progress in the introduction of coulometry for the analysis of total carbon dioxide (TCO2) in marine waters is described. An extractor—stripper removes CO2 that is measured coulometrically by the quantity of electricity (coulombs) used to electrogenerate OH? ions for the titration of the acid formed by the reaction of CO2 and ethanolamine. The equivalence point is detected photometrically with thymolphthalein as the indicator, and Faraday's Law relates coulombs to equivalents of titrant generated and CO2 determined so that there are no standard curves needed or titrants to standardize or store. Accuracy was determined by adding gelatin capsules containing 100–1500 μg C of pure CaCO3 into the stripper, and accuracies of better than ± 1 μg C were achieved. The best precision for natural seawater (± 1 standard error) of ± 0.5 μmol l?1 was found for 17 samples of Bermuda coastal waters having a mean TCO2 of 2007.2 μmol l?1 (0.05% CV). Sources of error and precautions are discussed. This method, which has been used successfully at sea, can be used to study a variety of marine phenomena involving TCO2. 相似文献
123.
Data from the Pulkovo spectrophotometric data base on the absolute quasimonochromatic fluxes from oCet in the 320–1080 nm range are used to determine the physical parameters of this star in different phases of its light curve. The continuum emission layer is found to expand between the phases of the cycle corresponding to the rising and falling branches of the light curve. The average expansion velocity is 32 km/s. By the time the star’s brightness has fallen by roughly three magnitudes, its radius has increased by almost a factor of three. Over this same time the temperature of the layer has fallen from 3000 K to 2200 K. For this expansion velocity, the calculated mass rate loss is ⊙ M /year.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 175–189 (May 2005). 相似文献
124.
125.
Silicon limitation on primary production and its destiny in Jiaozhou Bay, China——Ⅳ:Study on cross-bay transect from estuary to ocean 总被引:1,自引:0,他引:1
The authors analyzed the data collected in the Ecological Station Jiaozhou Bay from May 1991 to November 1994, including 12
seasonal investigations, to determine the characteristics, dynamic cycles and variation trends of the silicate in the bay.
The results indicated that the rivers around Jiaozhou Bay provided abundant supply of silicate to the bay. The silicate concentration
there depended on river flow variation. The horizontal variation of silicate concentration on the transect showed that the
silicate concentration decreased with distance from shorelines. The vertical variation of it showed that silicate sank and
deposited on the sea bottom by phytoplankton uptake and death, and zooplankton excretion. In this way, silicon would endlessly
be transferred from terrestrial sources to the sea bottom. The silicon took up by phytoplankton and by other biogeochemical
processes led to insufficient silicon supply for phytoplankton growth. In this paper, a 2D dynamic model of river flow versus
silicate concentration was established by which silicate concentrations of 0.028–0.062 μmol/L in seawater was yielded by inputting
certain seasonal unit river flows (m3/s), or in other words, the silicate supply rate; and when the unit river flow was set to zero, meaning no river input, the
silicate concentrations were between 0.05–0.69 μmol/L in the bay. In terms of the silicate supply rate, Jiaozhou Bay was divided
into three parts. The division shows a given river flow could generate several different silicon levels in corresponding regions,
so as to the silicon-limitation levels to the phytoplankton in these regions. Another dynamic model of river flow versus primary
production was set up by which the phytoplankton primary production of 5.21–15.55 (mgC/m2·d)/(m3/s) were obtained in our case at unit river flow values via silicate concentration or primary production conversion rate.
Similarly, the values of primary production of 121.98–195.33 (mgC/m2·d) were achieved at zero unit river flow condition. A primary production conversion rate reflects the sensitivity to silicon
depletion so as to different phytoplankton primary production and silicon requirements by different phytoplankton assemblages
in different marine areas. In addition, the authors differentiated two equations (Eqs. 1 and 2) in the models to obtain the
river flow variation that determines the silicate concentration variation, and in turn, the variation of primary production.
These results proved further that nutrient silicon is a limiting factor for phytoplankton growth.
This study was funded by NSFC (No. 40036010), and the Director's Fund of the Beihai Sea Monitoring Center, the State Oceanic
Administration. 相似文献
126.
127.
The existence of prolonged periods of abnormally low solar activity (such as the Maunder minimum) is explained within the framework of Leighton's model of a solar cycle with a hypothetical internal magnetic field of the Sun taken into account. 相似文献
128.
129.
130.
We performed polarization observations of giant radio pulses from the millisecond pulsar B1937+21. The observations were carried out in July 2002 with the 64-m Kalyazin radio telescope at a frequency of 600 MHz in two polarization channels with left-and right-hand circular polarizations (RCP and LCP). We used the S2 data acquisition system with a time resolution of 125 ns. The duration of an observing session was 20 min. We detected twelve giant radio pulses with peak flux densities higher than 1000 Jy; five and seven of these pulses appeared in the RCP and LCP channels, respectively. We found no event that exceeded the established detection threshold simultaneously in the two polarization channels. Thus, we may conclude that the detected giant pulses have a high degree of circular polarization, with the frequency of occurrence of RCP and LCP pulses being the same. 相似文献