全文获取类型
收费全文 | 46523篇 |
免费 | 1360篇 |
国内免费 | 413篇 |
专业分类
测绘学 | 1049篇 |
大气科学 | 3417篇 |
地球物理 | 10213篇 |
地质学 | 17114篇 |
海洋学 | 4027篇 |
天文学 | 9468篇 |
综合类 | 276篇 |
自然地理 | 2732篇 |
出版年
2022年 | 265篇 |
2021年 | 480篇 |
2020年 | 526篇 |
2019年 | 609篇 |
2018年 | 1675篇 |
2017年 | 1584篇 |
2016年 | 1442篇 |
2015年 | 792篇 |
2014年 | 1213篇 |
2013年 | 2072篇 |
2012年 | 1927篇 |
2011年 | 2144篇 |
2010年 | 1616篇 |
2009年 | 2027篇 |
2008年 | 1837篇 |
2007年 | 1872篇 |
2006年 | 1743篇 |
2005年 | 2114篇 |
2004年 | 2279篇 |
2003年 | 1962篇 |
2002年 | 1224篇 |
2001年 | 974篇 |
2000年 | 872篇 |
1999年 | 768篇 |
1998年 | 750篇 |
1997年 | 757篇 |
1996年 | 627篇 |
1995年 | 593篇 |
1994年 | 519篇 |
1993年 | 469篇 |
1992年 | 427篇 |
1991年 | 439篇 |
1990年 | 456篇 |
1989年 | 407篇 |
1988年 | 383篇 |
1987年 | 410篇 |
1986年 | 420篇 |
1985年 | 522篇 |
1984年 | 561篇 |
1983年 | 559篇 |
1982年 | 513篇 |
1981年 | 468篇 |
1980年 | 438篇 |
1979年 | 427篇 |
1978年 | 392篇 |
1977年 | 399篇 |
1976年 | 351篇 |
1975年 | 363篇 |
1974年 | 346篇 |
1973年 | 380篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Andrew S. Cohen Kiram E. Lezzar Julia Cole David Dettman Geoffrey S. Ellis Meagan Eagle Gonneea Pierre-Denis Plisnier Victor Langenberg Maarten Blaauw Derrick Zilifi 《Journal of Paleolimnology》2006,36(2):189-209
Microlaminated sediment cores from the Kalya slope region of Lake Tanganyika provide a near-annually resolved paleoclimate record between ∼∼2,840 and 1,420 cal. yr B.P. demonstrating strong linkages between climate variability and lacustrine productivity. Laminae couplets comprise dark, terrigenous-dominated half couplets, interpreted as low density underflows deposited from riverine sources during the rainy season, alternating with light, planktonic diatomaceous ooze, with little terrigenous component, interpreted as windy/dry season deposits. Laminated portions of the studied cores consist of conspicuous dark and light colored bundles of laminae couplets. Light and dark bundles alternate at decadal time scales. Within dark bundles, both light and dark half couplets are significantly thinner than within light bundles, implying slower sediment accumulation rates during both seasons over those intervals.Time series analyses of laminae thickness patterns demonstrate significant periodicities at interannual–centennial time scales. Longer time scale periodicities (multidecadal to centennial scale) of light and dark half couplet thicknesses are coherent and in some cases are similar to solar cycle periods on these time scales. Although laminae thickness cycles do not strongly covary with the actual Δ14C record for this same time period, two large Δ14C anomalies are associated with substantial decreases in both light and dark laminae thickness. In contrast to the multidecadal– centennial time scale, significant annual to decadal periodicities, which are broadly consistent with ENSO/PDO forcing and their impact on East African climate, are not coherent between light and dark half couplets. The coherency of light–dark couplets at decadal–centennial time scales, but not at shorter time scales, is consistent with a model of a long-term relationship between precipitation (recorded in wet season dark laminae thickness) and productivity (light laminae thickness), which is not manifest at shorter time scales. We hypothesize that this coupling results from long-term recharging of internal nutrient loading during wet periods (higher erosion of soil P) and reduced loading during drought intervals. The relationship is not expressed on short time scales during which the dominant control on productivity is wind-driven, dry season upwelling, which is uncorrelated with wet-season precipitation. Our record greatly extends the temporal record of this quasi-periodic behavior throughout the late Holocene and provides the first evidence linking decade- to century-scale episodes of enhanced productivity to enhanced precipitation levels and nutrient recharge in a productive tropical lake. 相似文献
993.
Scott W. White 《Natural Resources Research》2006,15(4):271-281
This paper updates a life-cycle net energy analysis and carbon dioxide emissions analysis of three Midwestern utility-scale
wind systems. Both the Energy Payback Ratio (EPR) and CO2 analysis results provide useful data for policy discussions regarding an efficient and low-carbon energy mix. The EPR is
the amount of electrical energy produced for the lifetime of the power plant divided by the total amount of energy required
to procure and transport the materials, build, operate, and decommission the power plants. The CO2 analysis for each power plant was calculated from the life-cycle energy input data.
A previous study also analyzed coal and nuclear fission power plants. At the time of that study, two of the three wind systems
had less than a full year of generation data to project the life-cycle energy production. This study updates the analysis
of three wind systems with an additional four to eight years of operating data.
The EPR for the utility-scale wind systems ranges from a low of 11 for a two-turbine system in Wisconsin to 28 for a 143-turbine
system in southwestern Minnesota. The EPR is 11 for coal, 25 for fission with gas centrifuge enriched uranium and 7 for gaseous
diffusion enriched uranium. The normalized CO2 emissions, in tonnes of CO2 per GWeh, ranges from 14 to 33 for the wind systems, 974 for coal, and 10 and 34 for nuclear fission using gas centrifuge and gaseous
diffusion enriched uranium, respectively. 相似文献
994.
995.
996.
Alejandro R. Carlini Gustavo A. Daneri Ricardo Casaux María E. I. Márquez 《Polar research》2006,25(2):139-144
The seasonal haul-out pattern of itinerant male Antarctic fur seals ( Arctocephalus gazella ) was determined by regular counts at Mossman Peninsula, Laurie Island, South Orkney Islands, from 1996 to 2005. Small numbers of animals began to arrive at the beach in late December/early January (mean date 28 December Ø 15 days, n= 10). Peak numbers of animals ashore changed considerably between seasons. In 1996, 1998 and 2001, peak numbers were registered in March (6/3, 18/3 and 6/3, respectively). Numbers peaked in 1997, 1999 and 2005 at the end of January/beginning of February (26/1,2/2 and 28/1, respectively). In 2000, 2002, 2003 and 2004 peaks were registered in the third week of February (15/2, 22/2, 14/2 and 20/2, respectively). Peaks in numbers of seals ashore also varied between years, being minimum during 2001 (2531 individuals) and maximum during 2006 (16 610 individuals). In March 1998 the coasts of Laurie Island were surveyed by navigating inflatable boats near the shoreline; 18 haul-out places were identified. The big differences in peak numbers, as well as in the dates of peak events among years, suggest that local conditions could have an effect on the numbers of animals hauled out in a given year. It may therefore be difficult to predict trends from summer censuses in non-breeding places. 相似文献
997.
The Dahuiyeh (Zarand) earthquake of 2005 February 22 in central Iran: reactivation of an intramountain reverse fault 总被引:3,自引:0,他引:3
998.
V. T. Doroshenko S. G. Sergeev C. M. Gaskell Yu. V. Golubinskii E. S. Klimek E. A. Sergeeva 《Astronomy Reports》2006,50(9):708-713
We present the results of our U BV R c I c photoelectric and CCD photometry of the NLS1 galaxy Ark 564 in 1987–2004. Between late 1997 and 2002, a slight increase in brightness was observed followed by a small but systematic decline until 2004. Ark 564 has a low variability amplitude, within 0.2m in the U BV R c I c bands. The galaxy’s spectral energy distribution at 3600–9000 Å is unlike those for Seyfert 1 (Syl) galaxies. A delay of the Ic flux relative to the B flux of about three days was found, which is in agreement with the standard disk model. This delay probably reflects the geometric size of the region that emits the observed continuum. 相似文献
999.
1000.