全文获取类型
收费全文 | 26182篇 |
免费 | 369篇 |
国内免费 | 218篇 |
专业分类
测绘学 | 522篇 |
大气科学 | 1981篇 |
地球物理 | 5244篇 |
地质学 | 9108篇 |
海洋学 | 2121篇 |
天文学 | 6033篇 |
综合类 | 47篇 |
自然地理 | 1713篇 |
出版年
2021年 | 162篇 |
2020年 | 199篇 |
2019年 | 212篇 |
2018年 | 500篇 |
2017年 | 468篇 |
2016年 | 570篇 |
2015年 | 439篇 |
2014年 | 613篇 |
2013年 | 1218篇 |
2012年 | 735篇 |
2011年 | 1026篇 |
2010年 | 883篇 |
2009年 | 1229篇 |
2008年 | 1062篇 |
2007年 | 1072篇 |
2006年 | 987篇 |
2005年 | 831篇 |
2004年 | 847篇 |
2003年 | 803篇 |
2002年 | 746篇 |
2001年 | 693篇 |
2000年 | 641篇 |
1999年 | 573篇 |
1998年 | 579篇 |
1997年 | 579篇 |
1996年 | 443篇 |
1995年 | 423篇 |
1994年 | 381篇 |
1993年 | 339篇 |
1992年 | 315篇 |
1991年 | 278篇 |
1990年 | 307篇 |
1989年 | 282篇 |
1988年 | 236篇 |
1987年 | 313篇 |
1986年 | 257篇 |
1985年 | 350篇 |
1984年 | 386篇 |
1983年 | 372篇 |
1982年 | 336篇 |
1981年 | 312篇 |
1980年 | 313篇 |
1979年 | 284篇 |
1978年 | 313篇 |
1977年 | 263篇 |
1976年 | 267篇 |
1975年 | 276篇 |
1974年 | 235篇 |
1973年 | 238篇 |
1972年 | 158篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Climatic response of thick leaf spruce (Picea crassifolia) tree-ring width at different elevations over Qilian Mountains, northwestern China 总被引:3,自引:0,他引:3
Tree-ring cores of thick leaf spruce (Picea crassifolia) taken from four sites at different elevations, in the middle of the Qilian Mountains, in the arid and semi-arid region of northwestern China, were used to develop four tree-ring width chronologies using standard dendrochronological methods. Results indicate that with increasing altitude the chronologies’ year-to-year variations decreased. Hence, the sensitivity of the tree-ring chronologies to climate decreases with altitude. Further analysis showed that the significant limiting factor on tree growth is spring precipitation. Measurements of stomata density and leaf dry weight suggest the species’ ecological adaptation strategy changes with elevation. At high elevation the metabolic rate of thick leaf spruce decreases, thus showing the effect of the climate. 相似文献
112.
An expanded surface-water palaeotemperature inference model for use with fossil midges from eastern Canada 总被引:4,自引:0,他引:4
Ian R. Walker André J. Levesque Les C. Cwynar André F. Lotter 《Journal of Paleolimnology》1997,18(2):165-178
Using an expanded surface sample data set, representing lakes distributed across a transect from southernmost Canada to the Canadian High Arctic, a revised midge-palaeotemperature inference model was developed for eastern Canada. Modelling trials with weighted averaging (with classical and inverse deshrinking; with and without tolerance downweighting) and weighted averaging partial least squares (WA-PLS) regression, with and without square-root transformation of the species data, were used to identify the best model. Comparison of measured and predicted temperatures revealed that a 2 component WA-PLS model for square-root transformed percentage species data provided the model with the highest explained variance (r
=0.88) and the lowest error estimate (RMSEP
jack
=2.26 °C). Comparison of temperature inferences based on the new and old models indicates that the original model may have seriously under-estimated the magnitude of late-glacial temperature oscillations in Atlantic Canada. The new inferences suggest that summer surface water temperatures in Splan Pond, New Brunswick were approximately 10 to 12 °C immediately following deglaciation and during the Younger Dryas. During the Allerod and early Holocene, surface water temperatures of 20 to 24 °C were attained. The new model thus provides the basis for more accurate palaeotemperature reconstructions throughout easternmost Canada. 相似文献
113.
114.
115.
116.
P. F. L. Maxted C. K. J. Moran T. R. Marsh A. A. Gatti 《Monthly notices of the Royal Astronomical Society》2000,311(4):877-880
We have used the radial velocity variations of two sdB stars previously reported to be binaries to establish their orbital periods. They are PG 0940+068 ( P =8.33 d) and PG 1247+554 ( P =0.599 d). The minimum masses of the unseen companions, assuming a mass of 0.5 M⊙ for the sdB stars, are 0.090±0.003 M. for PG 1247+554 and 0.63±0.02 M⊙ for PG 0940+068. The nature of the companions is not constrained further by our data. 相似文献
117.
H.R.M. Magee P.L. Dufton F.P. Keenan W.R.J. Rolleston D. Kilkenny D. O'Donoghue C. Koen R.S. Stobie 《Monthly notices of the Royal Astronomical Society》2001,324(3):747-756
We present high-resolution spectroscopic observations of 21 B-type stars, selected from the Edinburgh–Cape Blue Object Survey. Model atmosphere analyses confirm that 14 of these stars are young, main-sequence B-type objects with Population I chemical compositions. The remaining seven are found to be evolved objects, including subdwarfs, horizontal branch and post-AGB objects. A kinematical analysis shows that all 14 young main-sequence stars could have formed in the disc and subsequently been ejected into the halo. These results are combined with the analysis of a previous subsample of stars taken from the Survey. Of the complete sample, 31 have been found to be young, main-sequence objects, with formation in the disc, and subsequent ejection into the halo, again being found to be a plausible scenario. 相似文献
118.
A. Carraro P. Fabbri A. Giaretta L. Peruzzo F. Tateo F. Tellini 《Environmental Earth Sciences》2013,70(7):3067-3084
A pilot area within the Venetian Plain was selected to assess the arsenic (As) contamination of groundwater. The area represents a typical residential, industrial and agricultural organization representative of most western countries, and is also devoid of lithologies with high or anomalous As content. Hydrogeological and chemical data have been collected, the latter spatialized by a geostatistical approach. The unconfined aquifer reservoir varies from a predominantly gravel composition in the north to a sandy and silt–clay composition further south, including peat layers. The hydrochemical features of the waters are rather homogeneous, featuring low mineral content and a Ca-bicarbonate signature. In contrast, the redox state is highly variable; oxidizing conditions are predominant in the northern and coarse parts of the aquifer, whereas reducing potentials prevail in the southern and silt–clay parts. Several well waters contain arsenic in excess of drinkable limits (=10 ppb), and most of these wells are located in the southern area. A large portion of the studied area has a high probability of containing non-potable water (up to 150 ppb As). Remarkably, As “hot spots” (As > 300 ppb, up to 431 ppb) were identified at the transition from gravel to silt–clay sediments. No industrial or agricultural source of As has been found. 相似文献
119.
Mulamba Teddy Bacopoulos Peter Kubatko Ethan J. Pinto Gerard F. 《Climatic change》2019,152(3-4):533-550
Climatic Change - Salinity response to sea-level rise is evaluated for a low-gradient, tidally active estuary, the lower St. Johns River, Florida. A high-resolution numerical model is forced by... 相似文献
120.
Late-glacial lake sediments containing the Laacher See Tephra (LST, 11 000 yr B.P.) have been analyzed for their pollen and diatom content at three sites at varying distances from the volcano and on different bedrock geologies. The aim was to test the null hypothesis that this major volcanic eruption had no effect on terrestrial pollen or aquatic diatom assemblages. The pollen spectra at all sites show a short-lived increase in grass pollen following the LST. Partial redundancy analysis and associated Monte Carlo permutation tests suggest, however, that the LST had no statistically significant effect at two sites but it had a statistically significant impact on the pollen assemblages at the site nearest (60 km) to the volcano.The diatom assemblages at the three sites changed individualistically after the LST deposition, with increases inAchnanthes minutissima at one site, an expansion ofAulacoseira species at another, and an increase ofAsterionella formosa andFragilaria brevistriata at the third site. Partial redundancy analysis and associated permutation tests suggest a statistically significant change in diatoms in relation to the LST and associated changes in sediment lithology at the one site situated on acidic bedrock. No significant impacts were found at the sites on volcanic or calcareous rocks. Due to the interaction between tephra and sediment lithology, it is not possible to conclude if the statistically significant diatom changes were a direct result of the LST deposition or an indirect result of lithological changes following LST deposition.This is the first paper in a series of papers published in this issue on high-resolution paleolimnology. These papers were presented at the Sixth International Paeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. F. Lotter and Dr. M. Sturm served as guest editors for these papers. 相似文献