全文获取类型
收费全文 | 2057篇 |
免费 | 105篇 |
国内免费 | 506篇 |
专业分类
测绘学 | 104篇 |
大气科学 | 247篇 |
地球物理 | 824篇 |
地质学 | 749篇 |
海洋学 | 205篇 |
天文学 | 78篇 |
综合类 | 303篇 |
自然地理 | 158篇 |
出版年
2023年 | 15篇 |
2022年 | 7篇 |
2021年 | 18篇 |
2020年 | 11篇 |
2019年 | 21篇 |
2018年 | 22篇 |
2016年 | 20篇 |
2015年 | 17篇 |
2014年 | 32篇 |
2013年 | 28篇 |
2012年 | 79篇 |
2011年 | 94篇 |
2010年 | 28篇 |
2009年 | 60篇 |
2008年 | 100篇 |
2007年 | 119篇 |
2006年 | 119篇 |
2005年 | 129篇 |
2004年 | 124篇 |
2003年 | 111篇 |
2002年 | 107篇 |
2001年 | 105篇 |
2000年 | 118篇 |
1999年 | 51篇 |
1998年 | 55篇 |
1997年 | 38篇 |
1996年 | 48篇 |
1995年 | 63篇 |
1994年 | 67篇 |
1993年 | 76篇 |
1992年 | 70篇 |
1991年 | 94篇 |
1990年 | 74篇 |
1989年 | 83篇 |
1988年 | 76篇 |
1987年 | 62篇 |
1986年 | 47篇 |
1985年 | 65篇 |
1984年 | 52篇 |
1983年 | 29篇 |
1982年 | 25篇 |
1981年 | 18篇 |
1980年 | 44篇 |
1979年 | 4篇 |
1978年 | 10篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1974年 | 8篇 |
1963年 | 3篇 |
1957年 | 2篇 |
排序方式: 共有2668条查询结果,搜索用时 15 毫秒
921.
Stewart KR Keller JM Templeton R Kucklick JR Johnson C 《Marine pollution bulletin》2011,62(7):1396-1409
To assess threats to endangered species, it is critical to establish baselines for contaminant concentrations that may have detrimental consequences to individuals or populations. We measured contaminants in blubber and fat from dead leatherback turtles and established baselines in blood and eggs in nesting turtles. In fat, blubber, blood and eggs, the predominant PCBs were 153 + 132, 187 + 182, 138 + 163, 118, and 180 + 193. Total PCBs, 4,4′-DDE, total PBDEs and total chlordanes were significantly and positively correlated between blood and eggs, suggesting maternal transfer. Significant positive relationships also existed between fat and blubber in stranded leatherbacks. Less lipophilic PCBs appeared to more readily transfer from females to their eggs. PBDE profiles in the four tissues were similar to other wildlife populations but different from some turtle studies. Concentrations were lower than those shown to have acute toxic effects in other aquatic reptiles, but may have sub-lethal effects on hatchling body condition and health. 相似文献
922.
Human pharmaceuticals, like the lipid lowering agent gemfibrozil and the non-steroidal anti-inflammatory drug diclofenac are causing environmental concern. In this study, the marine mussel (Mytilus spp.) was exposed by injection to environmentally relevant and elevated (1 μg/L and 1000 μg/L) concentrations of both compounds and biomarker expression was observed. Gemfibrozil exposure induced biomarkers of stress (glutathione S-transferase and metallothionein) at both concentrations 24 h and 96 h after exposure, respectively. Biomarkers of damage (lipid peroxidation (LPO) and DNA damage) were significantly affected, as well as the biomarker for reproduction, alkali-labile phosphate assay, indicating the potential oxidative stress and endocrine disrupting effect of gemfibrozil. Diclofenac significantly induced LPO after 96 h indicating tissue damage. Additionally standard toxicity tests using the marine species Vibrio fischeri, Skeletonema costatum and Tisbe battagliai showed differences in sensitivity to both drugs in the mg/L range. Results indicate a suite of tests should be used to give accurate information for regulation. 相似文献
923.
924.
Galatius A Dietz R Rigét FF Sonne C Kinze CC Lockyer C Bossi R 《Marine pollution bulletin》2011,62(7):1476-1483
Eighty-five stranded or bycaught harbor porpoises collected from the Danish North Sea between 1980 and 2005 were analyzed for perfluorochemicals in the liver. PFOS was the predominant compound, making up on average 88.9% of the ∑PFC, followed by PFOSA (7.8%). PFUnA (1.9%) and PFDA (1.2%) were detected in most samples. PFHxS, PFNA and PFOA were only found in a minority of the samples. We found substantial differences in PFC concentrations among life history stages, the highest concentrations were found in neonates, suckling juveniles and lactating females. Such differences should be considered when PFC levels in wildlife are evaluated. The high concentrations found in young porpoises are of concern as PFCs have known toxic effects on the development of the central nervous system and reproductive organs. Despite efforts to reduce PFC emissions, a decreasing temporal trend of concentrations was not detected for any compound. PFCA concentrations were found to be increasing. 相似文献
925.
ANN application for prediction of atmospheric nitrogen deposition to aquatic ecosystems 总被引:1,自引:0,他引:1
The occurrences of increased atmospheric nitrogen deposition (ADN) in Southeast Asia during smoke haze episodes have undesired consequences on receiving aquatic ecosystems. A successful prediction of episodic ADN will allow a quantitative understanding of its possible impacts. In this study, an artificial neural network (ANN) model is used to estimate atmospheric deposition of total nitrogen (TN) and organic nitrogen (ON) concentrations to coastal aquatic ecosystems. The selected model input variables were nitrogen species from atmospheric deposition, Total Suspended Particulates, Pollutant Standards Index and meteorological parameters. ANN models predictions were also compared with multiple linear regression model having the same inputs and output. ANN model performance was found relatively more accurate in its predictions and adequate even for high-concentration events with acceptable minimum error. The developed ANN model can be used as a forecasting tool to complement the current TN and ON analysis within the atmospheric deposition-monitoring program in the region. 相似文献
926.
This study is concerned with the radioactivity and mineralogy of the younger granites and pegmatites in the Wadi Haleifiya area, southeastern Sinai Peninsula, Egypt. The area is occupied by metasediments, migmatites, older and younger granites. Most of these rocks, especially granites, are dissected by mafic and felsic dykes as well as pegmatites. The younger granites are represented by three main varieties: monzogranites, syenogranites and alkali feldspar granites. The monzogranite consists essentially of quartz, plagioclase, potash feldspar and biotite with minor musco-vite. Iron oxide, titanite, zircon and allanite are the main accessory minerals. Syenogranite is massive, medium- to coarse-grained and commonly exhibits equigranular and hypidiomorphic textures. It is made up essentially of potash feldspar, quartz, plagioclase and biotite. Iron oxides, allanite, epidote, titanite, and zircon are accessory minerals. The alkali feldspar granite consists mainly of perthite, quartz, alkali amphibole (arfvedsonite and riebekite), biotite, sub-ordinate plagioclase and aegirine. Iron oxide, zircon and apatite are accessory minerals, whereas chlorite and sas-surite are secondary minerals. The altered monzogranite and pegmatite recorded high radioelement contents. The eU reaches up to 120 (av.=82×10-6) in the altered monzogranite and up to 55 (av.=27×10-6) in the pegmatites. The high radioactivity in the altered monzogranite is due to the presence of thorite, uranothorite and metamict zircon. In the pegmatites, it is re-lated to the presence of uranophane, uranothorite, thorite, zircon, samarskite, monazite, xenotime, magnetite, ilmen-ite, hematite and rutile. 相似文献
927.
A study on the mixing proportion in groundwater samples by using Piper diagram and Phreeqc model 总被引:1,自引:0,他引:1
Karmegam U. Chidambaram S. Prasanna M.V. Sasidhar P. Manikandan S. Johnsonbabu G. Dheivanayaki V. Paramaguru P. Manivannan R. Srinivasamoorthy K. Anandhan P 《中国地球化学学报》2011,30(4):490-495
Piper (1944) diagram has been the basis for several important interpretations of the hydrogeochemical data. As seen in this diagram, most natural waters contain relatively few dissolved constituents, with cations (metals or bases) and anions (acid radicles) in chemical equilibrium with one another. Apart from the facies representation, the composition of the mixed sample can be identified in terms of the composition of the parental solution. To bring out this advantage of the Piper diagram, a study was conducted in the Kalpakkam region of Tamilnadu, South India. By taking the geology and water table into consideration, two sample locations were selected as parent solution and third one as the mixture sample. All three samples were analyzed for calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), chloride (Cl), sulphate (SO4) and phosphate (PO4) by Ion Chromatograph (Metrohm IC 861). HCO3 was determined by volumetric titration. The Piper diagram shows that parent solutions clustered towards Na-Mg-Ca-HCO3-Cl and Na-HCO3 facies, and the mixing sample belongs to Na-Mg-HCO3 facies. Phreeqc interac-tive (Ver 2.8) along with the original composition of the mixture sample was used to correlate the mixing proportion identified by the Piper diagram. 相似文献
928.
Granite-hosted,Nb-,Ta-,Sn-,U-,Th-,and Zr(Hf)-bearing mineralization from the Abu Rusheid shear zones occurs about 97 km southwest of the town of Marsa Alam,South Eastern Desert,Egypt.The SSE-trending brittle-ductile Abu Rusheid shear zones crosscut the peralkalic granitic gneisses and cataclastic to mylonitic rocks(mylonite,protomlyonite,and ultramylonite).The northern shear zone varies in width from 1 to 3 m with a strike length of >500 m,and the southern shear zone is 0.5 to 8 m wide and >1 km long.These shear zones locally host less altered lamprophyre and locally sheared granitic aplite-pegmatite dykes.The rare-metal minerals,identified from the peralkalic granitic gneisses and cataclastic to mylonitic rocks are associated with muscovite,chlorite,quartz,fluorite,pyrite,magnetite,and rare biotite that are restricted to the Abu Rusheid shear zones;these are columbite-tantalite and pyrochlore(var.betafite) in the northern shear zone and ferrocolumbite in the southern shear zone.Cassiterite occurs as inclusions in the columbite-tantalite minerals.U-and Th-minerals(uraninite,thorite,uranothorite,ishikawaite,and cheralite) and Hf-rich zircon coexist.Magmatic(?) zircon contains numerous inclusions of rutile,fluorite,U-Th and REE minerals,such as uranothorite,cheralite,monazite,and xenotime.Compositional variations in Ta/(Ta+Nb) and Mn/(Mn+Fe) in columbite range from 0.07-0.42 and 0.04-0.33,respectively,and Hf contents in zircon from 1.92-6.46 of the two mineralized shear zones reflect the extreme degree of magmatic fractionation.Four samples of peralkalic granitic gneisses and cataclastic to mylonitic rocks from the southern shear zone have very low TiO2(0.02 wt%-0.04 wt%),Sr [(15-20)×10-6],and Ba [(47-78)×10-6],with high Fe2O3T(0.94 wt%-1.99 wt%),CaO(0.14 wt%-1.16 wt%),alkalis(9.2 wt%-10.1 wt%),Rb [(369-805)×10-6],Zr [(1033-2261)×10-6],Nb [(371-913)×10-6],U [(51-108)×10-6],Th [(36-110)×10-6],Ta [(38-108)×10-6],Pb [(39-364)×10-6],Zn [(21-424)×10-6],Y [(8-304)×10-6],Hf [(29-157)×10-6],and ∑REE [(64-304)×10-6],especially HREE [(46-167)×10-6].Three samples from the northern shear zone also have very low TiO2(0.03 wt%),Sr [(11-16)×10-6],and Ba [(38-47)×10-6],with high Fe2O3T(1.97 wt%-2.91 wt%),CaO(0.49 wt%-1.01 wt%),alkalis(7.2 wt%-8.3 wt%),Rb [(932-978)×10-6],Zr [(1707-1953)×10-6],Nb [(853-981)×10-6],Ta [(100-112)×10-6],U [(120-752)×10-6],Th [(121-164)×10-6],Pb [(260-2198)×10-6],Zn [(483-1140)×10-6],Y [(8-304)×10-6],Hf [(67-106)×10-6],and ∑REE [(110-231)×10-6],especially HREE [(91-177)×10-6].The very high Rb/Sr(57.5-88.9),and low Zr/Hf(16.9-25.6),Nb/Ta(7.7-9.8),and Th/U(0.21-1.01) are consistent with very frac-tionated fluorine-bearing granitic rocks that were altered and sheared.The field evidence,textural relations,and compositions of the ore minerals suggest that the main mineralizing event was magmatic(629+/-5 Ma,CHIME monazite),with later hydrothermal alteration and local remobilization of the high-field-strength elements. 相似文献
929.
Geochemistry of groundwater,Markandeya River Basin,Belgaum district,Karnataka State,India 总被引:1,自引:0,他引:1
The Markandeya River Basin stretches geographically from 15o56′ to 16o08′ N latitude and 74o37′ to 74o58′ E longitude, positioned in the midst of Belgaum district, in the northern part of Karnataka. The groundwater quality of 54 pre-monsoon samples in the Markandeya River Basin was evaluated for its suitability for drinking and irrigation purposes by estimating pH, EC, TDS, hardness and alkalinity besides major cations (Na+, K+, Ca2+, Mg2+) and anions (HCO3–, Cl–, SO42–, PO43-, F-, NO3–), boron, SAR, % Na, RSC, RSBC, chlorinity index, SSP, non-carbonate hardness, Potential Salinity, Permeability Index, Kelley’s ratio, Magnesium hazard and Index of Base Exchange. Negative Index of Base Exchange indicates the chloro-alkaline disequilibrium in the study area and the majority of water samples fall in the rock dominance field based on Gibbs’ ratio. Permeability indices of classes I and II suggest suitability of groundwater for irrigation. Based on Cl, SO4, HCO3 concentrations, water samples can be classified as normal chloride (96.3%) and normal sulfate (94.4%) and normal bicarbonate (44.4%) water types. 相似文献
930.
Sebastiscus marmoratus larvae were exposed to waterborne polycyclic aromatic hydrocarbons (PAHs) containing 3-5 rings, benzo[a]pyrene (BaP), pyrene (Py) or phenanthrene (Phe), respectively at 0.01, 0.1 and 1 μg L?1. Cumulative mortality, frequency of dorsal curvature and rate of pericardial and yolk sac edema in larvae treated for 8 days were significantly increased in a dose-dependent manner. All three PAHs resulted in reduction of the lower jaw, and the extent of reduction increased with increasing concentrations of the PAHs. Na+/K+-ATPase and Ca2+-ATPase activity in larvae exposed to the three PAHs were all significantly inhibited in a dose-dependent manner. Analysis using the Pearson correlation coefficient indicated a significant correlation between the rate of the dorsal curvature and edema and the inhibition of Na+/K+-ATPase and Ca2+-ATPase activity, suggesting that the developmental defects caused by PAHs were related to their inhibition of Na+/K+-ATPase and Ca2+-ATPase activity. 相似文献