全文获取类型
收费全文 | 36977篇 |
免费 | 592篇 |
国内免费 | 579篇 |
专业分类
测绘学 | 933篇 |
大气科学 | 3066篇 |
地球物理 | 7395篇 |
地质学 | 12196篇 |
海洋学 | 3358篇 |
天文学 | 8693篇 |
综合类 | 120篇 |
自然地理 | 2387篇 |
出版年
2021年 | 198篇 |
2020年 | 257篇 |
2019年 | 255篇 |
2018年 | 611篇 |
2017年 | 563篇 |
2016年 | 878篇 |
2015年 | 619篇 |
2014年 | 846篇 |
2013年 | 1882篇 |
2012年 | 1032篇 |
2011年 | 1448篇 |
2010年 | 1190篇 |
2009年 | 1770篇 |
2008年 | 1568篇 |
2007年 | 1490篇 |
2006年 | 1404篇 |
2005年 | 1285篇 |
2004年 | 1206篇 |
2003年 | 1166篇 |
2002年 | 1090篇 |
2001年 | 975篇 |
2000年 | 983篇 |
1999年 | 917篇 |
1998年 | 834篇 |
1997年 | 841篇 |
1996年 | 720篇 |
1995年 | 645篇 |
1994年 | 561篇 |
1993年 | 514篇 |
1992年 | 510篇 |
1991年 | 480篇 |
1990年 | 476篇 |
1989年 | 418篇 |
1988年 | 402篇 |
1987年 | 449篇 |
1986年 | 433篇 |
1985年 | 522篇 |
1984年 | 582篇 |
1983年 | 558篇 |
1982年 | 519篇 |
1981年 | 462篇 |
1980年 | 435篇 |
1979年 | 397篇 |
1978年 | 412篇 |
1977年 | 358篇 |
1976年 | 323篇 |
1975年 | 336篇 |
1974年 | 336篇 |
1973年 | 338篇 |
1972年 | 201篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
D R Livingstone C L Mitchelmore S C O'Hara P Lemaire J Sturve L F?rlin 《Marine environmental research》2000,50(1-5):57-60
The present study investigated the proposed involvement of contaminant-stimulated reactive oxygen species (ROS) production in disease processes in fish. NAD(P)H-dependent ROS production of subcellular fractions was determined by the iron/EDTA-mediated oxidation of 2-keto-4-methiolbutyric acid. Hepatic cytosolic NADPH-dependent and microsomal NAD(P)H-dependent ROS production were increased 51-160% (P < 0.05) in rainbow trout (Oncorhynchus mykiss) 15 weeks after a single i.p. injection of polychlorobiphenyl (PCB) (100 mg Clophen A50 kg-1 wet wt.). Hepatic microsomal NADH-dependent ROS production was 114% higher in perch (Perca fluviatilis) from PCB-contaminated Lake J?rnsj?n compared to clean Lake V?nern, Sweden. Hepatic mitochondrial NADH-dependent, cytosolic NADH-dependent and microsomal NADPH-dependent ROS production were variously elevated up to 160% in flounder (Platichthys flesus) at various sites along two pollution transects near to the ports of Rotterdam and Amsterdam, Netherlands. Overall the data indicate increased potential for ROS production in liver of fish exposed to field pollution, and support the hypothesis of oxidative stress as a mechanism of contaminant-mediated disease in fish. 相似文献
952.
Evaluating fishery impacts using metrics of community structure 总被引:8,自引:14,他引:8
953.
Interpolation of wave heights 总被引:1,自引:0,他引:1
Remote sensing of waves often necessitates presentation of data in the form of wave height values grouped over large time intervals. This restricts their use to long-term applications only. This paper describes how such data can be made suitable for short-term usage in the field. Weekly mean significant wave heights were derived from their monthly mean observations with the help of different alternative techniques. These include model-free neural network schemes as well as model-based statistical and numerical methods. Superiority of neural networks was noted when the estimations were compared with corresponding observations. The network was trained using three different training algorithms, viz., error back propagation, conjugate gradient and cascade correlation. The technique of cascade correlation took minimum training time and showed better coefficient of correlation between observations and network output. 相似文献
954.
Phaeocystis material contains polysaccharides that are built from at least eight different monosaccharides. Differences have been reported between the carbohydrate composition of different Phaeocystis species, and also between samples taken from Phaeocystis globosa blooms in different areas. In order to elucidate factors that could play a role in determining variation in carbohydrate composition and production, a number of Phaeocystis globosa strains were studied under laboratory conditions. Although there was a clear distinction of a northern and a southern cluster in the Phaeocystis globosa strains based on RAPD analysis, the differences in the composition of the mucopolysaccharides were relatively small. The contribution of glucose, however, ranged from 7–85% of total sugars. A strain that was cultured in seawaters of diverse origin produced polysaccharides of a different composition, suggesting the effect of environmental factors. The presence of bacteria affected neither the amount, nor the composition of the carbohydrates that were produced by Phaeocystis globosa. Glucose is part of both the intracellular polysaccharide pool and of the mucopolysaccharides in the colony matrix. Using specific digestion of the intracellular chrysolaminaran by laminarinase, the distribution of polysaccharides over different pools could be assessed. During growth of an axenic, mucus-producing strain, the portion of glucose present as chrysolaminaran appeared to increase. The polyglucose that was not digested by laminarinase remains unidentified. This study shows that environmental factors rather than strain differences determine differences in the sugar composition of Phaeocystis globosa, especially with respect to the glucose content of the material. A difference in the contribution of glucose could be correlated to the portion of cells in the culture that are not in the colonies. Our study emphasises that for studying polysaccharide dynamics in Phaeocystis globosa it is important to be able to discriminate between the different polysaccharide pools. Preliminary results of an enzymatic approach were promising 相似文献
955.
A detailed marine survey made to the South West of Mount Eratosthenes is described. The results show the existence of a narrow zone of WNW trending deformed sediment west of the mount surrounded by an evaporite sequence in different stages of deformation.This work was intended to settle the conflict between the results of three previous interpretations of the tectonic structure of this area. We see the area being characterised by five areas with different degrees of tectonic deformation yet showing little causal relationship to overburden of Nile Cone sediments. A fault controlled cliff coincides with the previously described salt wall and strike slip fault yet we see no evidence for its diapiric origin nor positive evidence for strike slip motion along it. We concur with the previously observed area of higher free-air gravity anomalies termed the Eratosthenes tectonic zone, although it is traversed by several boundaries between areas of different deformation. Our results thus show some agreements and some re-interpretations of the observations. This complicated region is not yet fully understood however. 相似文献
956.
Core 7710, recovered on the George Bligh/Rockall Channel, is composed of fine grained ocher-colored sediments rich in ferriferous smectite (montmorillonite, beidellite). These sediments were deposited during early Eocene in a shallow marine environment. Their mineralogic, chemical and isotopic constituents indicate subaerial weathering of lower Eocene intermediate igneous rocks (ferrodiorite, marscoiite) eroded from the still-emergent George Bligh and Rockall Banks. Lack of any post Eocene Sediments may be due to major morphologic and hydrologic changes forming sedimentation gaps. 相似文献
957.
This paper discusses the use of Hermite polynomial in the derivation of statistical properties of waves, wave field kinematics and dynamics and wave forces under various conditions. Specifically, covariance functions and approximate spectra are obtained for (1) wave force on vertical cylinder according to Morison's formula, (2) horizontal fluid particle velocity considering the effects of free surface fluctuations, and (3) elevation of breaking waves. 相似文献
958.
The seismic expression of a salt-filled channel which cuts across the Mid North Sea High in Quadrant 37 is described, with features interpreted as being produced by salt-edge dissolution forming both eastern and western margins of the channel. The apparent half-graben nature of the channel is shown to be only superficial, and due to complex faulting associated with, but not defining, its western margin. The shallower faulting here is a Mesozoic to early Tertiary growth fault related to local dissolution of Zechstein salt. The dissolution effect appears in turn to have been localized by the presence of a deeper fault that was already downthrown to the east in Zechstein times, when it seems to have limited the eastward progradation of Zechstein shelf carbonates and anhydrites, and had probably ceased to move significantly before the onset of the Late Cimmerian erosional phase. The origin of this arcuate fault is tentatively ascribed to subsidence around a granite batholith. Zechstein salt originally spread some distance to the east and west of the channel; it was dissolved from the edges inwards, mainly before the Late Cretaceous, giving rise to a thicker Mesozoic sequence on parts of the flanks of the channel than in the middle. Besides providing an interesting structural case history, the features described have implications regarding Zechstein sedimentation, reservoir potential, the tectonic history of the North Sea, and the nature of the Mid North Sea High itself. 相似文献
959.
960.
Late Holocene back-barrier sediments have been studied in a former lagoon reclaimed during the last century. Several shallow boreholes were cored, from which nine organic-rich samples were selected for radiocarbon dating, and others for palynological and palaeontological analysis. This enabled the reconstruction of the evolution of a back-barrier coastal basin during the late Holocene coastline progradation. Since late Roman times, the rate of relative sea-level rise (RSLR) has increased from 1.1 mm/yr to over 2 mm/yr, even after removal of the main human impacts during of the last century. This change is ascribed to sediment compaction related to changes in the hydrological regime, and possibly to an isostatic subsidence wave accompanying the northward shift of the Po Delta during the last 2000 years. 相似文献