全文获取类型
收费全文 | 11969篇 |
免费 | 3011篇 |
国内免费 | 2724篇 |
专业分类
测绘学 | 243篇 |
大气科学 | 299篇 |
地球物理 | 4861篇 |
地质学 | 9113篇 |
海洋学 | 1208篇 |
天文学 | 18篇 |
综合类 | 925篇 |
自然地理 | 1037篇 |
出版年
2024年 | 86篇 |
2023年 | 239篇 |
2022年 | 388篇 |
2021年 | 493篇 |
2020年 | 471篇 |
2019年 | 572篇 |
2018年 | 495篇 |
2017年 | 518篇 |
2016年 | 597篇 |
2015年 | 575篇 |
2014年 | 710篇 |
2013年 | 742篇 |
2012年 | 712篇 |
2011年 | 682篇 |
2010年 | 638篇 |
2009年 | 801篇 |
2008年 | 812篇 |
2007年 | 786篇 |
2006年 | 775篇 |
2005年 | 661篇 |
2004年 | 700篇 |
2003年 | 578篇 |
2002年 | 552篇 |
2001年 | 492篇 |
2000年 | 496篇 |
1999年 | 395篇 |
1998年 | 393篇 |
1997年 | 340篇 |
1996年 | 332篇 |
1995年 | 331篇 |
1994年 | 296篇 |
1993年 | 252篇 |
1992年 | 197篇 |
1991年 | 129篇 |
1990年 | 106篇 |
1989年 | 116篇 |
1988年 | 79篇 |
1987年 | 60篇 |
1986年 | 26篇 |
1985年 | 18篇 |
1984年 | 15篇 |
1983年 | 7篇 |
1980年 | 4篇 |
1979年 | 22篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1954年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Atmospherically-promoted photosynthetic activity in a well-mixed ecosystem: Significance of wet deposition events of nitrogen compounds 总被引:1,自引:0,他引:1
C. Boulart P. Flament V. Gentilhomme K. Deboudt C. Migon F. Lizon M. Schapira A. Lefebvre 《Estuarine, Coastal and Shelf Science》2006,69(3-4):449
Wet atmospheric deposition of dissolved N, P and Si species is studied in well-mixed coastal ecosystem to evaluate its potential to stimulate photosynthetic activities in nutrient-depleted conditions. Our results show that, during spring, seawater is greatly depleted in major nutrients: Dissolved Inorganic Nitrogen (DIN), Dissolved Inorganic Phosphorus (DIP) and Silicic acid (Si), in parallel with an increase of phytoplanktonic biomass. In spring (March–May) and summer (June–September), wet atmospheric deposition is the predominant source (>60%, relative to riverine contribution) for nitrates and ammonium inputs to this N-limited coastal ecosystem. During winter (October–February), riverine inputs of DIN predominate (>80%) and are annually the most important source of DIP (>90%). This situation allows us to calculate the possibility for a significant contribution to primary production in May 2003, from atmospheric deposition (total input for DIN ≈300 kg km−2 month−1). Based on usual Redfield ratios and assuming that all of the atmospheric-derived N (AD-N) in rainwater is bioavailable for phytoplankton growth, we can estimate new production due to AD-N of 950 mg C m−2 month−1, during this period of depletion in the water column. During the same episode (May 2003), photosynthetic activity rate, considered as gross primary production, was estimated to approximately 30 300 mg C m−2 month−1. Calculation indicates that new photosynthetic activity due to wet atmospheric inputs of nitrogen could be up to 3%. 相似文献
73.
A model for the depth-limited distribution of the highest wave in a sea state is presented. The distribution for the extreme wave height is based on a probability density function (pdf) for depth-limited wave height distribution for individual waves [Méndez, F.J., Losada, I.J., Medina, R. 2004. Transformation model of wave height distribution. Coastal Eng, Vol. 50, 97:115.] and considers the correlation between consecutive waves. The model is validated using field data showing a good representation of the extreme wave heights in the surf zone. Some important statistical wave heights are parameterized obtaining useful expressions that can be used in further calculations. 相似文献
74.
本文提出在小偏心受压构件正截面强度计算中,取消弯曲抗压强度fcm(Rw),采用轴心抗压强度fc,这与构件的实际破坏形态相符。对小偏心受压构件正截面强度计算提出了设计建议。特别是对工程上大量应用的矩形截面对称配筋小偏压构件提示了简捷的设计方法,与以往的近似计算方法相比具有物理概念明确,精度高,方法简捷的特点。 相似文献
75.
Seasonal DOC accumulation in the Black Sea: a regional explanation for a general mechanism 总被引:2,自引:0,他引:2
Gustave Cauwet Gaëlle Dliat Anton Krastev Galina Shtereva Sylvie Becquevort Christiane Lancelot Andr Momzikoff Alain Saliot Adriana Cociasu Lucia Popa 《Marine Chemistry》2002,79(3-4)
During three cruises in the Black Sea, organised in July 1995 and April–May 1997, biological and chemical parameters that can influence the carbon budget were measured in the water column on the NW shelf, particularly in the mixing zone with Danube River waters. We observed in early spring (end of April–May) conditions an important input of freshwater organisms that enhanced the microbial activity in the low salinity range. High bacterial activity regenerates nitrogen in the form of nitrates, but is also responsible for an important consumption of ammonium and phosphate, leading to a high N/P ratio and a strong deficit in phosphorus. The consequence is a limitation of phytoplankton development but also a production of carbohydrates that accumulate all along the salinity gradient. These mechanisms are responsible for a seasonal accumulation of dissolved organic carbon (DOC) that increases from 210 μM in winter to about 280 μM in summer. All this excess DOC disappears during winter, probably degraded by bacterial activity. The degradation of carbon-rich organic matter increases the phosphorus demand by bacteria bringing limitation to phytoplankton primary production. 相似文献
76.
韩宗珠 《中国海洋大学学报(自然科学版)》1994,(1)
以微量元素、稀土元素、Sr和Nd同位素变异特征为依据,确定鲁苏榴辉岩为多成因、多来源和多阶段,指出主要是在印支期扬子陆块与华北陆块碰撞造山作用过程中,挤入的上地幔碎片以及不同原岩类型的壳内高压变质岩碎块。燕山晚期的区域构造热事件使得某些榴辉岩的同位素体系再平衡。 相似文献
77.
D. C. Mason I. J. Davenport R. A. Flather C. Gurney G. J. Robinson J. A. Smith 《Estuarine, Coastal and Shelf Science》2001,53(6):759
A sensitivity analysis of the waterline method of constructing a Digital Elevation Model (DEM) of an intertidal zone using remote sensing and hydrodynamic modelling is described. Variation in vertical height accuracy as a function of beach slope is investigated using a set of nine ERS Synthetic Aperture Radar (SAR) images of the Humber/Wash area on the English east coast acquired between 1992 and 1994. Waterlines from these images are heighted using a hydrodynamic tide-surge model and interpolated using block kriging. On 1:500 slope beaches, an average block height estimation standard deviation of 18–22 cm is achieved. This rises to 27 cm on 1:100 slope beaches, and 32 cm on 1:30 slope beaches. The average heighting error at different slopes is decomposed into components due to waterline heighting error, inadequate sensor resolution and interpolation inaccuracy. It is shown that, at 1:500 slope, waterline heighting error and interpolation inaccuracy are the main error sources, whilst at 1:30 slope, errors due to inadequate sensor resolution become dominant. The ability of the technique to generate intertidal DEMs for almost the entire coastal zone in a complete ERS SAR scene covering 100×100 km is demonstrated. 相似文献
78.
The objective of this study was to examine the redox reactions and other transformations of mercury (Hg) species in surface waters, and the factors determining the rates of these reactions. For the redox studies completed at the Chesapeake Biological Laboratory (CBL), two isotopes (199HgII and 202Hg0) were added into different types of filtered water (fresh to seawater) to examine the oxidation and reduction reactions. Further studies of both the redox reactions and methylation/demethylation reactions of Hg were conducted with unfiltered water on board research vessels during cruises in May and July 2005 on the Chesapeake Bay and shelf. While CH3199HgII was added to allow the examination of demethylation, 201HgII was used to examine both reduction and methylation, and 202Hg0 was used to examine oxidation. Overall, the results showed that both Hg oxidation and reduction were simultaneously occurring and were photochemically mediated in the waters investigated. In contrast to the previously assumed “unreactive” nature of Hg0, the studies found that the magnitude of the rate constant for Hg0 oxidation was greater than that for reduction, indicating its importance in estuarine and coastal waters. In addition, both experiments at CBL and on board ship showed that HgII reduction was similar in magnitude, suggesting that biotic processes were relatively unimportant. While no measurable methylation occurred during the incubation period during the on board studies, concentration of CH3199HgII decreased over the time during the experiments. It appeared that the demethylation processes were not dominantly photochemically driven, but could be microbially mediated. Further studies are needed in order to help better understand Hg redox and transformations in natural water systems. 相似文献
79.
Ceramicola S. Rebesco M. De Batist M. Khlystov O. 《Marine Geophysical Researches》2001,22(5-6):445-464
High resolution, single-channel seismic sparker profiles across the Akademichesky Ridge, an intra-basin structural high in Lake Baikal (Russia), reveal the presence of small sediment mounds and intervening moats in the upper part of the sedimentary cover. Such features interrupt the generally uniform and even acoustic facies and are not consistent with the hemipelagic sedimentation, which is expected on such an isolated high and which would produce a uniform sediment drape over bottom irregularities. The influence of turbidity currents is excluded since the ridge is an isolated high elevated more than 600-1000 m above adjacent basins. The mounded seismic facies, including migrating sediment waves and non-depositional/erosional incisions, strongly suggest that sediment accumulation was controlled by bottom-current activity. We interpret the mounds as small-scale (< few tens of km2 in area) lacustrine drifts. Four basic types of geometry are identified: 1) slope-plastered patch sheets; 2) patch drifts; 3) confined drifts; 4) fault-controlled drifts. The general asymmetry in the sedimentary cover of the ridge, showing thicker deposits on the NW flank, and the common location of patch drifts on the northeast side of small basement knolls indicate that deposition took preferentially place at the lee sides of obstacles in a current flowing northward or sub-parallel to the main contours. Deep-water circulation in the ridge area is not known in detail, but there are indications that relatively cold saline water masses are presently flowing out of the Central Basin and plunging into the deep parts of the North Basin across the ridge, a process that appears to be driven mainly by small differences in salinity. We infer that the process responsible for the observed bottom-current-controlled sedimentary features has to be sought in these large-scale water-mass movements and their past equivalents. The age of the onset of the bottom-current-controlled sedimentation, based on an average sedimentation rate of 4.0 cm/ky, is roughly estimated to be as least as old as 3.5 Ma, which is generally regarded as the age of the onset of the last major tectonic pulse of rift basin development in the Baikal region. 相似文献
80.
Abstract. Six macrobenthic assemblages of the circalittoral zone found in Strymonikos Gulf, North Aegean Sea, are described and compared with the corresponding ones from other Mediterranean and Atlantic areas. For the delimination of these assemblages and the evaluation of the major environmental gradients governing their distribution, several numerical techniques were used. 相似文献