全文获取类型
收费全文 | 1271篇 |
免费 | 112篇 |
国内免费 | 245篇 |
专业分类
测绘学 | 29篇 |
大气科学 | 285篇 |
地球物理 | 337篇 |
地质学 | 575篇 |
海洋学 | 240篇 |
天文学 | 17篇 |
综合类 | 24篇 |
自然地理 | 121篇 |
出版年
2024年 | 3篇 |
2023年 | 18篇 |
2022年 | 55篇 |
2021年 | 47篇 |
2020年 | 35篇 |
2019年 | 47篇 |
2018年 | 48篇 |
2017年 | 63篇 |
2016年 | 74篇 |
2015年 | 70篇 |
2014年 | 94篇 |
2013年 | 108篇 |
2012年 | 46篇 |
2011年 | 77篇 |
2010年 | 54篇 |
2009年 | 82篇 |
2008年 | 103篇 |
2007年 | 85篇 |
2006年 | 70篇 |
2005年 | 66篇 |
2004年 | 55篇 |
2003年 | 39篇 |
2002年 | 39篇 |
2001年 | 37篇 |
2000年 | 41篇 |
1999年 | 27篇 |
1998年 | 24篇 |
1997年 | 26篇 |
1996年 | 18篇 |
1995年 | 10篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1981年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有1628条查询结果,搜索用时 15 毫秒
11.
A theoretical and an experimental investigation was carried out, where a carbon fibre corrugated circular cylinder was tested to destuction under external hydrostatic pressure. The theoretical investigation was via the finite element method, where the structure was modelled with several orthotropic axisymmetric thin-walled shell elements. The experimental observations were aided with strategically placed strain gauges. Comparison between theory and experiment showed that the experimentally observed buckling pressure was a little lower than the theoretical prediction. This may have been due to the fact that the model had slight initial geometrical imperfections in the circumferenential direction. 相似文献
12.
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
13.
14.
Size distribution of colloidal trace metals and organic carbon during a coastal bloom in the Baltic Sea 总被引:1,自引:0,他引:1
Johan Ingri Susanna Nordling Jenny Larsson Jenny Rnnegrd Nina Nilsson Ilia Rodushkin Ralf Dahlqvist Per Andersson
rjan Gustafsson 《Marine Chemistry》2004,91(1-4):117-130
The physico-chemical speciation of organic carbon and selected metals was measured during a coastal bloom in Ekhagen Bay, Baltic Sea, using ultrafiltration.One important objective with the study was to see if any depletion of trace metals could be measured in the directly bioavailable fraction (<1000 Da, the soluble low molecular weight fraction, LMW) during a plankton bloom. Filters with five different cut-offs were used (1 kD (1000 Da), 5 kD, 10 kD, 100 kD and 0.22 μm) in order to delineate the size distribution of colloidal organic carbon (COC) and trace metals.During the bloom in May, LMW Al, Co, Cu, Mn and Ni concentrations decreased although the colloidal and particulate concentrations were relatively high. Data show that desorption of colloidal and particulate bound trace metals to the LMW fraction was slower than the process depleting the LMW fraction.Estimates of the maximum active uptake of Cu, Ni and Mn by the phytoplankton, and the loss of non-bioactive Al from the LMW fraction, indicate that processes other than active uptake by phytoplankton must contribute to the observed depletion of trace metals in the LMW fraction. Hence, in order to estimate the bioavailable pool of trace metals for plankton during bloom conditions, these other processes must be understood and quantified.Transparent Exopolymeric Particles (TEP, reflecting sugar-rich phytoplankton exudates) increased around eight times during the plankton bloom. We hypothesize that the formation of TEP is a process that might be important for the transfer of trace metals from the LMW to the particulate fraction during the phytoplankton bloom, but the significance of TEP for this depletion in Baltic Sea surface water remains to be shown. 相似文献
15.
Historical data of total dissolved inorganic carbon (CT), together with nitrate and phosphate, have been used to model the evolution of these constituents over the year in the Atlantic water of the Norwegian Sea. Changes in nutrient concentration in the upper layer of the ocean are largely related to biological activity, but vertical mixing with the underlying water will also have an impact. A mixing factor is estimated and used to compute the entrainment of these constituents into the surface water from below. After taking the mixing contribution into account, the resulting nutrient concentration changes are attributed to biological production or decay. The results of the model show that the change in CT by vertical mixing and by biological activity based on nutrient equivalents needs another sink to balance the carbon budget. It cannot be the atmosphere as the surface water is undersaturated with respect to carbon dioxide and is, thus, a source of CT in this region. Inasmuch as the peak deficit of carbon is more than a month later than for the nutrients, the most plausible explanation is that other nitrogen and phosphate sources than the inorganic salts are used together with dissolved inorganic carbon during this period. As nitrate and phosphate show a similar trend, it is unlikely that the explanation is the use of ammonia or nitrogen fixation but rather dissolved organic nitrogen and phosphate, while dissolved organic carbon is accumulating in the water. 相似文献
16.
南海南部约30 ka来沉积有机质的生物输入特征 总被引:7,自引:4,他引:7
对位于南沙海区的1962柱状样中的有机质进行了热解色谱分析,估算了沉积有机质中水生生物输入和陆源生物输入的变化情况,得出了两种输入的高分辨率的堆积速率曲线,并依此探讨了有关的古海洋事件。发现Younger Dryas、Heinrich及Bond周期事件在本海区皆有表现,说明“西太平洋暖池”在末次冰期是不稳定的。 相似文献
17.
Quantum yield for the photochemical production of dissolved inorganic carbon in seawater 总被引:2,自引:0,他引:2
The direct photooxidation of coloured dissolved organic matter (CDOM) to dissolved inorganic carbon (DIC) may provide a significant sink for organic carbon in the ocean. To calculate the rate of this reaction on a global scale, it is essential to know its quantum yield, or photochemical efficiency. We have determined quantum yield spectra, φ(λ), (moles DIC/mole photons absorbed) for 14 samples of seawater from environments ranging from a turbid, eutrophic bay to the Gulf Stream. The spectra vary among locations, but can be represented quite well by three pooled spectra for zones defined by location and salinity: inshore φ(λ)=e−(6.66+0.0285(λ−290)); coastal φ(λ)=e−(6.36+0.0140(λ−290)); and open ocean φ(λ)=e−(5.53+0.00914(λ−290)). Production efficiency increases offshore, which suggests that the most highly absorbing and quickly faded terrestrial chromophores are not those directly responsible for DIC photoproduction. 相似文献
18.
A preliminary study of carbon system in the East China Sea 总被引:1,自引:0,他引:1
Shizuo Tsunogai Shuichi Watanabe Junya Nakamura Tsuneo Ono Tetsuro Sato 《Journal of Oceanography》1997,53(1):9-17
In the central part of the East China Sea, the activity of CO2 in the surface water and total carbonate, pH and alkalinity in the water column were determined in winter and autumn of 1993.
The activity of CO2 in the continental shelf water was about 50 ppm lower than that of surface air. This decrease corresponds to the absorption
of about 40 gC/m2/yr of atmospheric CO2 in the coastal zone or 1 GtC/yr in the global continental shelf, if this rate is applicable to entire coastal seas. The normalized
total carbonate contents were higher in the water near the coast and near the bottom. This increase toward the bottom may
be due to the organic matter deposited on the bottom. This conclusion is supported by the distribution of pH. The normalized
alkalinity distribution also showed higher values in the near-coast water, but in the surface water, indicating the supply
of bicarbonate from river water. The residence time of the East China Sea water, including the Yellow Sea water, has been
calculated to be about 0.8 yr from the excess alkalinity and the alkalinity input. Using this residence time and the excess
carbonate, we can estimate that the amount of dissolved carbonate transported from the coastal zone to the oceanic basin is
about 70 gC/m2/yr or 2 GtC/yr/area-of-global-continental-shelf. This also means that the rivers transport carbon to the oceans at a rate
of 30 gC/m2/yr of the coastal sea or 0.8 GtC/yr/ area-of-global shelf, the carbon consisting of dissolved inorganic carbonate and terrestrial
organic carbon decomposed on the continental shelf. 相似文献
19.
Thermohaline Anomalies in the Spring and Early Summer of 2000 in the Gulf of Trieste 总被引:1,自引:0,他引:1
Abstract. In the framework of the Interreg II Project (July 1998 - June 2001), hydro-logical, chemical and biological data were collected in the Gulf of Trieste.
During spring and summer 2000, some particular thermohaline anomalies were observed in the Gulf of Trieste. Especially in May and June the water body showed: a very strong thermohaline stratification, an increase of advective salt water coming from the south and the presence of sharp pycnoclines. In July the temperature was higher than usual in the whole water column. Moreover, in late May and in June, massive mucilaginous aggregates were observed along the water column and at the surface.
In order to highlight these particular thermohaline features the hydrological data of 16 stations were analysed (Fig. 1). Two stations, in particular, were considered: one offshore (St. AA1, average depth 20 m) and one close to the coast (St. C1, average depth 17 m). For these two stations a best-fit analysis, computed over 11 and 7 years, respectively, was performed on temperature, salinity and density excess data.
Moreover, the hydrological features were compared with the rainfall, air temperature, wind speed data (Istituto Sperimentale Talassografico di Trieste - ISTT) and the Isonzo River's flow rate (Direzione Regionale dell'Ambiente - Regione F. V. G.) collected from January 1998 to December 2000. 相似文献
During spring and summer 2000, some particular thermohaline anomalies were observed in the Gulf of Trieste. Especially in May and June the water body showed: a very strong thermohaline stratification, an increase of advective salt water coming from the south and the presence of sharp pycnoclines. In July the temperature was higher than usual in the whole water column. Moreover, in late May and in June, massive mucilaginous aggregates were observed along the water column and at the surface.
In order to highlight these particular thermohaline features the hydrological data of 16 stations were analysed (Fig. 1). Two stations, in particular, were considered: one offshore (St. AA1, average depth 20 m) and one close to the coast (St. C1, average depth 17 m). For these two stations a best-fit analysis, computed over 11 and 7 years, respectively, was performed on temperature, salinity and density excess data.
Moreover, the hydrological features were compared with the rainfall, air temperature, wind speed data (Istituto Sperimentale Talassografico di Trieste - ISTT) and the Isonzo River's flow rate (Direzione Regionale dell'Ambiente - Regione F. V. G.) collected from January 1998 to December 2000. 相似文献
20.
Intense studies of upper and deep ocean processes were carried out in the Northwestern Indian Ocean (Arabian Sea) within the framework of JGOFS and related projects in order to improve our understanding of the marine carbon cycle and the ocean’s role as a reservoir for atmospheric CO2. The results show a pronounced monsoon-driven seasonality with enhanced organic carbon fluxes into the deep-sea during the SW Monsoon and during the early and late NE Monsoon north of 10°N. The productivity is mainly regulated by inputs of nutrients from subsurface waters into the euphotic zone via upwelling and mixed layer-deepening. Deep mixing introduces light limitation by carrying photoautotrophic organisms below the euphotic zone during the peak of the NE Monsoon. Nevertheless, deep mixing and strong upwelling during the SW Monsoon provide an ecological advantage for diatoms over other photoautotrophic organisms by increasing the silica concentrations in the euphotic zone. When silica concentrations fall below 2 μmol l−1, diatoms lose their dominance in the plankton community. During diatom-dominated blooms, the biological pathway of uptake of CO2 (the biological pump) appears to be more efficient than during blooms of other organisms, as indicated by organic carbon to carbonate carbon (rain) ratios. Due to the seasonal alternation of diatom and non-diatom dominated exports, spatial variations of the annual mean rain ratios are hardly discernible along the main JGOFS transect.Data-based estimates of the annual mean impact of the biological pump on the fCO2 in the surface water suggest that the biological pump reduces the increase of fCO2 in the surface water caused by intrusion of CO2-enriched subsurface water by 50–70%. The remaining 30 to 50% are attributed to CO2 emissions into the atmosphere. Rain ratios up to 60% higher in river-influenced areas off Pakistan and in the Bay of Bengal than in the open Arabian Sea imply that riverine silica inputs can further enhance the impact of the biological pump on the fCO2 in the surface water by supporting diatom blooms. Consequently, it is assumed that reduced river discharges caused by the damming of major rivers increase CO2 emission by lowering silica inputs to the Arabian Sea; this mechanism probably operates in other regions of the world ocean also. 相似文献