全文获取类型
收费全文 | 97408篇 |
免费 | 2283篇 |
国内免费 | 1638篇 |
专业分类
测绘学 | 2485篇 |
大气科学 | 7139篇 |
地球物理 | 19763篇 |
地质学 | 35278篇 |
海洋学 | 8602篇 |
天文学 | 21613篇 |
综合类 | 434篇 |
自然地理 | 6015篇 |
出版年
2022年 | 628篇 |
2021年 | 1058篇 |
2020年 | 1107篇 |
2019年 | 1178篇 |
2018年 | 2502篇 |
2017年 | 2360篇 |
2016年 | 2936篇 |
2015年 | 1807篇 |
2014年 | 2885篇 |
2013年 | 5167篇 |
2012年 | 3232篇 |
2011年 | 4357篇 |
2010年 | 3734篇 |
2009年 | 4868篇 |
2008年 | 4419篇 |
2007年 | 4236篇 |
2006年 | 4036篇 |
2005年 | 3157篇 |
2004年 | 3039篇 |
2003年 | 2814篇 |
2002年 | 2583篇 |
2001年 | 2378篇 |
2000年 | 2289篇 |
1999年 | 1842篇 |
1998年 | 1967篇 |
1997年 | 1820篇 |
1996年 | 1524篇 |
1995年 | 1533篇 |
1994年 | 1296篇 |
1993年 | 1192篇 |
1992年 | 1166篇 |
1991年 | 1066篇 |
1990年 | 1193篇 |
1989年 | 1001篇 |
1988年 | 912篇 |
1987年 | 1135篇 |
1986年 | 928篇 |
1985年 | 1209篇 |
1984年 | 1356篇 |
1983年 | 1223篇 |
1982年 | 1199篇 |
1981年 | 1036篇 |
1980年 | 991篇 |
1979年 | 889篇 |
1978年 | 878篇 |
1977年 | 834篇 |
1976年 | 798篇 |
1975年 | 747篇 |
1974年 | 759篇 |
1973年 | 766篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The conditional acid dissociation constants (pKa′) of two sulfonephthalein dyes, thymol blue (TB) and m-cresol purple (mCP), were assessed throughout the estuarine salinity range (0<S<40) using a tris/tris–HCl buffer and spectrophotometric measurement. The salinity dependence of the pKa′ of both dyes was fitted to the equations (25 °C, total proton pH scale, mol kg soln−1):The estimated accuracy of pH measurements using these calculated pKa′ values is considered to be comparable to that possible with careful use of a glass electrode (±0.01 pH unit) but spectrophotometric measurements in an estuary have the significant advantage that it is not necessary to calibrate an electrode at different salinities. pH was measured in an estuary over a tidal cycle with a precision of ±0.0005 pH unit at high (S>30) salinity, and ±0.002 pH unit at low (S<5) salinity. The pH increased rapidly in the lower salinity ranges (0<S<15) but less rapidly at higher salinities. 相似文献
992.
Amvrakikos Gulf is a Neogene basin, formed during a late extensional tectonic phase within the Plio-Quaternary period. It is a semienclosed embayment, separated from the Ionian Sea by a shallow (< 10 m) channel. The analysis of 3.5-kHz seismic reflection profiles shows that, during the last (Würm) glacial period, the parts of the Gulf that lie at water depths >41 m (below present sea level) were a paleo-lake while the rest were exposed to subaerial erosion. Subsequent offshore depositional sequences accumulated at rates of 1.2–2.3 m/ka. 相似文献
993.
Dowty RA Shaffer GP Hester MW Childers GW Campo FM Greene MC 《Marine environmental research》2001,52(3):195-211
Research was conducted to assess the impact of oiling on fresh-marsh plant communities and to test the efficacy of techniques that may be used to enhance the bioremediation of crude oil spills in these environments while minimizing secondary anthropogenic impacts. To emulate field conditions, a mesocosm facility was used that houses 120 mesocosm vessels, each of 200-1 capacity. A five-way factorial treatment arrangement was used that included two substrates (inorganic, organic), two nutrient regimes (fertilized, not fertilized), two aeration levels (substrate aeration, no aeration), three oiling concentrations (0-, 5-, 10-1 m(-2) of South Louisiana Sweet Crude oil), and four vascular plant species (Alternanthera philoxeroides, Panicum hemitomon, Phragmites australis, Sagittaria lancifolia, and an unplanted control). Under the 5- and 10-1 m(-2) oiling concentrations, S. lancifolia displayed a short-term response of increased productivity, whereas P. hemitomon had the highest biomass production and photosynthetic rates at the end of the 18-month experiment. Overall plant growth and productivity, as well as oil degradation, were significantly higher in the inorganic substrate, indicating that biodegradation of oil spills in organic substrates may require a longer time period. Time-released fertilizer also stimulated plant productivity and resulted in higher soil respiratory quotients, suggestive of greater microbial activity, particularly in aerated mesocosms. The amount of oil remaining after 18 months was lowest in aerated and fertilized mesocosms containing either P. hemitomon or S. lancifolia and a substrate of low organic matter content. 相似文献
994.
Video assessment of environmental impacts of salmon farms 总被引:1,自引:0,他引:1
995.
996.
Measurements of 226Ra and 210Pb in barite samples separated from different depths of a piston core from the eastern equatorial Pacific are reported. 226Ra is enriched in the barite as would be expected from the chemistries of Ra and Ba; the ratio centers around 1.5 · 1014 atoms/mole. The results suggest that barite is concentrating Ra at a rate comparable to its decay rate, viz. 5.78 · 10?3 yr?1. The gross 210Pb depletion with respect to 226Ra is indicative of 222Rn loss due to the smallness of the barite crystals. 相似文献
997.
998.
A stochastic prognostic model of the atmospheric precipitation in the tropical area of the Atlantic Ocean is developed on the basis of a large data array. The data represent a series of monthly precipitation rates for 31 towns in the Republic of Guinea covering a period of 35–64 years as well as satellite data on the meridional displacement of the Intertropical Convergence Zone during 11 years. The model takes into account major regularities of the tropical precipitation: their impulsive periodic regime, trends, and the modulation of the stochastic component by the determined variability.Translated by Mikhai M. Trufanov. 相似文献
999.
Rodney L. Stevens 《Geo-Marine Letters》1991,11(3-4):162-165
Quartz and feldspar extracts, obtained by chemical dissolution of the clay minerals, provide insight into the inorganic flocculation
and mineralogical influences upon the texture of fine-grained glaciomarine deposits. Glacial comminution in higher latitudes
explains the relatively high contents of quartz and feldspars. Quartz and feldspar are better sorted and were apparently less
influenced by flocculation than were phyllosilicates, except for particles less than approximately 16 μm, which are poorly
sorted due to their inclusion in flocs. The 16-μm boundary is suggested to be useful for approximately separating fines that
are usually involved in flocculation and coarse silt and larger grains that are less influenced. 相似文献
1000.
J. Christopher Rutherford Roger G. Young John M. Quinn Robert J. Wilcock 《新西兰海洋与淡水研究杂志》2020,54(3):410-430
ABSTRACT Instream processes alter the concentration and bioavailability of nutrients as they are transported downstream. By relating primary production and periphyton composition to changes in nutrient concentration in a gravel-bed river this study made inferences about recycling and attenuation. Where dissolved inorganic nitrogen (DIN) was abundant, concentrations decreased linearly with distance but by less than required to meet the nitrogen demand of primary production. Where DIN was barely measurable photosynthesis was reduced but only by 50%. We infer that recycling sustained primary production even when DIN concentrations were negligibly small. One implication is that DIN removal underestimates attenuation. Further experimental research on recycling and improved modelling is required to better quantify the length of streams adversely affected by nutrients. 相似文献