全文获取类型
收费全文 | 75113篇 |
免费 | 1326篇 |
国内免费 | 636篇 |
专业分类
测绘学 | 2067篇 |
大气科学 | 5368篇 |
地球物理 | 15274篇 |
地质学 | 25435篇 |
海洋学 | 6704篇 |
天文学 | 17658篇 |
综合类 | 180篇 |
自然地理 | 4389篇 |
出版年
2021年 | 661篇 |
2020年 | 678篇 |
2019年 | 825篇 |
2018年 | 1716篇 |
2017年 | 1569篇 |
2016年 | 2043篇 |
2015年 | 1146篇 |
2014年 | 1896篇 |
2013年 | 3723篇 |
2012年 | 2110篇 |
2011年 | 2921篇 |
2010年 | 2679篇 |
2009年 | 3584篇 |
2008年 | 3051篇 |
2007年 | 3112篇 |
2006年 | 2902篇 |
2005年 | 2273篇 |
2004年 | 2298篇 |
2003年 | 2172篇 |
2002年 | 2174篇 |
2001年 | 1906篇 |
2000年 | 1780篇 |
1999年 | 1521篇 |
1998年 | 1521篇 |
1997年 | 1518篇 |
1996年 | 1258篇 |
1995年 | 1208篇 |
1994年 | 1117篇 |
1993年 | 1001篇 |
1992年 | 907篇 |
1991年 | 901篇 |
1990年 | 902篇 |
1989年 | 918篇 |
1988年 | 842篇 |
1987年 | 974篇 |
1986年 | 880篇 |
1985年 | 1050篇 |
1984年 | 1198篇 |
1983年 | 1071篇 |
1982年 | 1014篇 |
1981年 | 1008篇 |
1980年 | 853篇 |
1979年 | 841篇 |
1978年 | 824篇 |
1977年 | 765篇 |
1976年 | 688篇 |
1975年 | 681篇 |
1974年 | 650篇 |
1973年 | 711篇 |
1972年 | 448篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
591.
592.
Within the framework of the linear theory taking into account the action of the Coriolis force, we study waves induced in a continuously stratified fluid with one thermocline by periodic variations of pressure on the free surface. The fluid fills a bounded basin of constant depth. We establish the dependences of the wave amplitudes and wave velocities on the period of disturbing pressure. The sequence of transformations of generated waves from the barotropic character of motion to baroclinic depending on changes in the period of disturbing pressure is studied in detail. 相似文献
593.
Fertilisation and larval metamorphosis of reef-building corals are important life history events leading to recruitment of juvenile corals to reef populations. Little is known of the sensitivity of these early life phases to pollution, or their relative susceptibility to certain toxicants compared with established coral colonies. Inhibition of fertilisation and larval metamorphosis of the coral Acropora millepora (Ehrenberg, 1834) was assessed in response to solutions of the antifoulants tributyltin (TBT) and copper (Cu) using laboratory-based bioassays. Nominal concentrations that inhibited 50% fertilisation and metamorphosis (IC50) were calculated from 4 h fertilisation and 24 h metamorphosis assays and were based on introduced dose. Cu was most potent towards fertilisation with an IC50 of 17.4 micrograms/l. TBT however, proved more toxic to larval metamorphosis having an IC50 of 2.0 micrograms/l. Inert surfaces coated with either Cu- or TBT-based antifouling paint also inhibited fertilisation and metamorphosis. The degree of inhibition was correlated with surface area of the paint coating. These results indicate fertilisation and metamorphosis of coral can be sensitive to active components of antifouling paints. 相似文献
594.
Phytoplankton variability on the Faroe Shelf 总被引:2,自引:2,他引:2
595.
The paper considers the application of self-organizing models, specifically, the method of grouped arguments consideration
(MGAC), to forecast short and non-stationary time series of observations in the ocean. A sequence of operations for the treatment
of observational series is suggested. To assess its efficiency, we have used mean monthly oxygen concentration data collected
in the surface and near-bottom layers of the Taganrog Bay. It is shown that the application of the MGAC model allows one to
reduce by two times the root-mean-square error of that of the series prediction by five points, in comparison with the Jenkins-Box
regressional model. It has been concluded that the predictors' non-linear functions may be effectively used in the treatment
of short samplings.
Translated by Vladimir A. Puchkin. 相似文献
596.
597.
598.
599.
Preservation conditions and the use of sediment pigments as a tool for recent ecological reconstruction in four Northern European estuaries 总被引:4,自引:0,他引:4
Down-core sediment pigment concentrations from four Northern European estuaries were measured using high-performance liquid chromatography (HPLC) to investigate phytoplankton community structure and preservation conditions over the last ca. 100 years where all sites have experienced different levels of eutrophication. Phytoplankton pigments have been shown to be useful biomarkers for phytoplankton community structure and abundance due to their taxonomic specificity. The pigment concentrations and sediment pigment inventory showed large variation between the four sites. Concentrations ranged from more than 6000 nmol/g OC to less than 100 nmol/g OC and the inventory integrated over the top 10 cm from more than 300 nmol/cm2 to less than 30 nmol/cm2 for total identified pigments. Good pigment preservation in Mariager Fjord (Denmark) reflected the almost permanently anoxic conditions. Pigments in Laajalahti (Finland) showed peak concentrations around the time of highest nitrogen loading events known from historical and modelled records over the past 100 years. In contrast, poor down-core preservation of pigments (especially carotenoids) was observed in the Ems-Dollard (The Netherlands) and Himmerfjärden (Sweden) estuaries. The Ems-Dollard site is an intertidal mudflat that experiences daily exposure to light and air, which enhances pigment degradation. In Himmerfjärden, resuspension is an important process affecting both the sedimentation rate and degradation properties. The different preservation conditions at the four sites were supported by the differences in two degradation indicators; the ratio of pheopigment-a to chlorophyll-a and total carotenoids to total pigments. Class-specific carotenoid pigments represented the dominant algal groups reported from each site, however, no distinct down-core changes in the pigment composition were observed at any of the four sites. This indicated that changes in plankton community structure on the group level have been limited over this time period or masked by low preservation of pigments. 相似文献
600.
Reservoir pressures within the Bullwinkle minibasin (Green Canyon 65, Gulf of Mexico continental slope) increase at a hydrostatic gradient whereas pressures predicted from porosity within mudstones bounding these reservoirs increase at a lithostatic gradient: they are equal at a depth 1/3 of the way down from the crest of the structure. Two- and three-dimensional steady-state flow models demonstrate that bowl-shaped structures will have lower pressures than equivalent two-dimensional structures and that if a low permeability salt layer underlies the basin, the pressure is reduced. We conclude that at Bullwinkle, pressure is reduced due to an underlying salt body and the bowl-shape of the basin. A geometric approach to predict sandstone pressure is to assume that the reservoir pressure equals the area-weighted average of the mudstone pressure. When the mudstone pressure gradient is constant, as at Bullwinkle, the reservoir pressure equals the mudstone pressure at the average depth (centroid) of the reservoir. 相似文献