全文获取类型
收费全文 | 76039篇 |
免费 | 907篇 |
国内免费 | 1266篇 |
专业分类
测绘学 | 2610篇 |
大气科学 | 5338篇 |
地球物理 | 14808篇 |
地质学 | 29690篇 |
海洋学 | 5640篇 |
天文学 | 14121篇 |
综合类 | 2280篇 |
自然地理 | 3725篇 |
出版年
2022年 | 327篇 |
2021年 | 532篇 |
2020年 | 581篇 |
2019年 | 613篇 |
2018年 | 5975篇 |
2017年 | 5160篇 |
2016年 | 3991篇 |
2015年 | 1013篇 |
2014年 | 1529篇 |
2013年 | 2650篇 |
2012年 | 2560篇 |
2011年 | 4710篇 |
2010年 | 3874篇 |
2009年 | 4826篇 |
2008年 | 4037篇 |
2007年 | 4470篇 |
2006年 | 2108篇 |
2005年 | 1705篇 |
2004年 | 1884篇 |
2003年 | 1791篇 |
2002年 | 1570篇 |
2001年 | 1230篇 |
2000年 | 1212篇 |
1999年 | 972篇 |
1998年 | 1021篇 |
1997年 | 955篇 |
1996年 | 783篇 |
1995年 | 801篇 |
1994年 | 724篇 |
1993年 | 620篇 |
1992年 | 558篇 |
1991年 | 585篇 |
1990年 | 663篇 |
1989年 | 555篇 |
1988年 | 519篇 |
1987年 | 666篇 |
1986年 | 552篇 |
1985年 | 706篇 |
1984年 | 768篇 |
1983年 | 747篇 |
1982年 | 645篇 |
1981年 | 687篇 |
1980年 | 598篇 |
1979年 | 550篇 |
1978年 | 546篇 |
1977年 | 496篇 |
1976年 | 474篇 |
1975年 | 469篇 |
1974年 | 457篇 |
1973年 | 481篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
991.
The origin and biogeochemistry of organic matter in surface sediments of Lake Shihwa and Lake Hwaong
To understand the origin and biogeochemistry of the organic matter in surface sediments of Lake Shihwa and Lake Hwaong, organic
nitrogen, inorganic nitrogen, labile organic carbon, and residual organic carbon contents as well as stable isotope ratios
for carbon and nitrogen were determined by KOBr-KOH treatment. Ratios of organic carbon to organic nitrogen (Corg/Norg) (mean = 24) were much higher than ratios of organic carbon to total nitrogen (Corg/Ntot) (mean=12), indicating the presence of significant amounts of inorganic nitrogen in the surface sediments of both lakes.
Stable isotope ratios for organic nitrogen were, on average, 5.2‰ heavier than ratios of inorganic nitrogen in Lake Shihwa,
but those same ratios were comparable in Lake Hwaong. This might be due to differences in the origin or the degree of degradation
of sedimentary organic matter between the two lakes. In addition, stable isotope ratios for labile organic carbon were, on
average, 1.4‰ heavier than those for residual organic carbon, reflecting the preferential oxidation of13C-enriched organic matter. The present study demonstrates that KOBr-KOH treatment of sedimentary organic matter can provide
valuable information for understanding the origin and degradation state of organic matter in marine and brackish sediments.
This also suggests that the ratio of Corg/Norg and stable isotope ratios for organic nitrogen can be used as indexes of the degree of degradation of organic matter. 相似文献
992.
Shane M. Ohline Malcolm R. Reid Shamus L.G. Husheer Kim I. Currie Keith A. Hunter 《Marine Chemistry》2007
Accurate measurement of seawater pH has long been sought by marine chemists (for example: [Dickson, A.G. 1993a. The measurement of sea water pH. Marine Chemistry, 44, 131–142, Dickson, A.G. 1993b. pH buffers for sea water media based on the total hydrogen ion concentration scale. Deep-Sea Research, 40, 107–118; Zhang, 1996; Tapp, M., Hunter, K.A., Currie, K. and Macaskill, B. 2000. Apparatus of continuous-flow underway spectrophotometric measurement of surface water pH. Marine Chemistry 72(2–4), 193–202; Friis, K., Koetzinger, A., Wallace, D.W.R. 2004. Spectrophotometric pH measurement in the ocean: Requirements, design and testing of an autonomous charge-coupled device detector system. Limnology and Oceanography: Methods 2, 126–136]. Recently, such attempts have taken on greater significance as anthropogenic carbon dioxide emissions may create rapidly changing oceanic pH. Spectrophotometric techniques have been accepted generally as the best for determination of seawater pH. Here we report a new technique using thymol blue as the indicator dye and fitting the entire spectrum from 400 to 900 nm rather than measuring the absorbance values at only two or three points in the spectrum. This full-spectrum modelling enables a reduction in signal to noise over other techniques. In the laboratory, we find with seawater samples a pH precision increase of five-fold “within” a sample and seven-fold “between” samples when comparing the full spectrum to the three-point method of analysis [Zhang, H., Byrne, R.H. 1996. Spectrophotometric pH measurements of surface seawater at in-situ conditions: absorbance and protonation behaviour of thymol blue. Marine Chemistry 52, 17–25]. 相似文献
993.
994.
Darío R. Minkoff Mauricio Escapa Flix E. Ferramola Silvio D. Maraschín Jorge O. Pierini Gerardo M.E. Perillo Claudio Delrieux 《Estuarine, Coastal and Shelf Science》2006,69(3-4):403
The Bahía Blanca Estuary (38° 50′ S, and 62° 30′ W) presents salt marshes where interactions between the local flora (Sarcocornia perennis) and fauna (Chasmagnathus granulatus) generate some kind of salt pans that alter the normal water circulation and condition its flow and course towards tidal creeks. The crab–vegetation dynamics in the salt marsh presents variations that cannot be quantified in a reasonable period of time. The interaction between S. perennis plant and C. granulatus crab is based on simple laws, but its result is a complex biological mechanism that causes an erosive process on the salt marsh and favors the formation of tidal creeks. To study it, a Cellular Automata model is proposed, based on the laws deduced from the observation of these phenomena in the field, and then verified with measurable data within macroscale time units. Therefore, the objective of this article is to model how the interaction between C. granulatus and S. perennis modifies the landscape of the salt marsh and influences the path of tidal creeks. The model copies the basic laws that rule the problem based on purely biological factors.The Cellular Automata model proved capable of reproducing the effects of the interaction between plants and crabs in the salt marsh. A study of the water drainage of the basins showed that this interaction does indeed modify the development of tidal creeks. Model dynamics would likewise follow different laws, which would provide a different formula for the probability of patch dilation. The patch shape can be obtained changing the pattern that dilates. 相似文献
995.
996.
Monitoring offshore platforms, long span bridges, high rise buildings, TV towers and other similar structures is essential for ensuring their safety in service. Continuous monitoring assumes even greater significance in the case of offshore platforms, which are highly susceptible to damage due to the corrosive environment and the continuous action of waves. Also, since a major part of the structure is under water and covered by marine growth, even a trained diver cannot easily detect damage in the structure. In the present work, vibration criterion is adopted for structural monitoring of jacket platforms. Artificial excitation of these structures is not always practicable and ambient excitation due to wind and waves may not be sufficient for collecting the required vibration data. Alternate methods can be adopted for the same purpose, for example, the application of an impact or a sudden relaxation of an applied force for exciting the structure. For jacket platforms, impact can be applied by gently pushing the structure at the fender while relaxation can be accomplished by pulling the structure and then suddenly releasing it using a tug or a supply vessel in both cases. The present study is an experimental investigation on a laboratory model of a jacket platform, for exploring the feasibility of adapting vibration responses due to impulse and relaxation, for structural monitoring. Effects of damage in six members of the platform as well as changes in deck masses were studied. A finite element model of the structure was used to analyze all the cases for comparison of the results as well as system identification. A data acquisition and analysis procedure for obtaining the response signatures of the platform due to the impulse and relaxation procedure was also developed for possible adoption in on-line monitoring of offshore platforms. From the study, it has been concluded that both impulse and relaxation responses are useful tools for monitoring offshore jacket platforms. The present work forms the basis for the development of an automated, on-line monitoring system for offshore platforms, using neural networks. 相似文献
997.
A minor generalization of the theory of random walk is used as a basis for a model of ocean current flow. The model is then applied in a computer simulation of drifter motion. The results of simulation indicate that the geometry of a coastline can have significant impact on the distribution of drifter landings. 相似文献
998.
Rivers draining into the Gulf of Papua (GOP) from the Papua New Guinea mainland deliver approximately 340 × 106 t yr–1 of sediment to the marine environment. The terrestrially derived sediment contains 1.1 ± 0.2% particulate organic carbon with a carbon-isotope composition of –26.5 ± 0.2, and amounts to 3.7 ± 0.7 × 106 t yr–1. The carbon-isotope composition of sediments in the Gulf of Papua indicates that 40% of the sediment cover contains 75% or more terrestrially derived carbon. Suspended sediments that are transported beyond the delta complex of the Fly River are transported north and northwest, augmented by sediments from other rivers along the coast of the GOP. The carbon-isotope results suggest that a significant quantity of terrestrially derived sediment escapes from the GOP, either along the coastlines to east and west or into the deep ocean via the Moresby and Pandora troughs. Little sediment travels south onto the Great Barrier Reef shelf. Extrapolating the results from this study to the region of Oceania suggests a total flux of particulate organic carbon to the world's oceans from the islands of Oceania of ~ 90 × 106 t yr–1 or twice the flux of riverine POC from the major rivers of North America, South America, and Africa combined. While such a calculation must be considered illustrative only, the similar tectonic, geomorphologic, and climatic features of the islands of Oceania suggest that the calculation is unlikely to be grossly in error and that the rivers of Oceania therefore represent a major but poorly documented source of sediment and organic carbon to the global ocean. 相似文献
999.
A large submarine slide on the southern flank of the Bear Island Trough Mouth Fan, southwestern Barents Sea continental slope, has a run-out distance of about 400 km, a total volume of about 1100 km3, and is younger than 330 ka. Three seismic units, comprising mainly hemipelagic sediments has partly filled the slide scar. An increased sedimentation rate on the Bear Island Trough Mouth Fan from Late Pliocene time, probably in combination with abundant earthquakes, is the most likely cause of the slide. Based on these and previous studies, we suggest that large-scale slides were important sediment transport processes during Plio-Pleistocene. 相似文献
1000.
Iapetus (S8) is unique in our solar system in that the albedo of its leading hemisphere is only 0.05 while that of the trailing side is 0.5. Several existing hypotheses are examined and found inadequate. Photometric studies of the dark side are compared to comet nuclei and class D asteroids. It is hypothesized that in the last 106–108 yrs the leading side suffered a high-velocity collision with a cometary body of mass 1013–1015 kg and traveling at a speed of 20 km s–1. About 5–16% of the excavated material was ejected into space, where the vaporized ices dissipated while the dark carbonaceous/silicate material was reaccreted on the leading side. The collision, although not sufficient to break Iapetus' tidal lock, resulted in a period of oscillation of about 5 yr. Until tidal friction reasserted a lock, the oscillation gave rise to the longitude effect, viz., the observed fact that the dark material covers more than 220 of longitude but only 110 of latitude. 相似文献