全文获取类型
收费全文 | 8598篇 |
免费 | 275篇 |
国内免费 | 129篇 |
专业分类
测绘学 | 165篇 |
大气科学 | 711篇 |
地球物理 | 2005篇 |
地质学 | 3020篇 |
海洋学 | 797篇 |
天文学 | 1236篇 |
综合类 | 37篇 |
自然地理 | 1031篇 |
出版年
2022年 | 33篇 |
2021年 | 115篇 |
2020年 | 153篇 |
2019年 | 156篇 |
2018年 | 202篇 |
2017年 | 196篇 |
2016年 | 241篇 |
2015年 | 201篇 |
2014年 | 260篇 |
2013年 | 475篇 |
2012年 | 283篇 |
2011年 | 410篇 |
2010年 | 375篇 |
2009年 | 475篇 |
2008年 | 397篇 |
2007年 | 407篇 |
2006年 | 340篇 |
2005年 | 292篇 |
2004年 | 283篇 |
2003年 | 300篇 |
2002年 | 239篇 |
2001年 | 200篇 |
2000年 | 219篇 |
1999年 | 164篇 |
1998年 | 149篇 |
1997年 | 132篇 |
1996年 | 144篇 |
1995年 | 131篇 |
1994年 | 124篇 |
1993年 | 101篇 |
1992年 | 100篇 |
1991年 | 68篇 |
1990年 | 97篇 |
1989年 | 78篇 |
1988年 | 84篇 |
1987年 | 94篇 |
1986年 | 84篇 |
1985年 | 110篇 |
1984年 | 131篇 |
1983年 | 122篇 |
1982年 | 105篇 |
1981年 | 77篇 |
1980年 | 57篇 |
1979年 | 71篇 |
1978年 | 67篇 |
1977年 | 61篇 |
1976年 | 61篇 |
1975年 | 69篇 |
1974年 | 55篇 |
1973年 | 68篇 |
排序方式: 共有9002条查询结果,搜索用时 0 毫秒
201.
Trace metal-binding proteins in marine molluscs and crustaceans 总被引:1,自引:0,他引:1
Some marine invertebrates, such as the American oyster, Crassostrea virginica, the blue crab, callinectes sapidus and the American lobster, Homarusamericanus, concentrate trace metals in their tissues. The occurrence of metallothionein, a low molecular weight, sulfur-containing metal-binding protein, has been correlated with elevated levels of trace metals in these organisms. It is our hypothesis that, whilst metallothioneins are involved in the sequestration of elevated levels of trace metals, they primarily function in regulating normal metal metabolism. In this paper we describe recent field and laboratory experiments designed to examine how oysters, blue crabs and lobsters manage accumulated cadmium, copper and zinc. The possible rôles of metallothionein, as well as the tissue distribution of metals, are emphasized. Metallothioneins were found in all animals we examined; concentrations of metallothioneins and the the amount of bound trace metals are related to time of exposure. We have also demonstrated that the laboratory developed metallothionein models for blue crabs and lobsters may be useful in predicting the partitioning of trace metals in animals living in environments polluted with trace metals. 相似文献
202.
203.
204.
Experiments were conducted in an oscillatory water tunnel to investigate what effects temporal inertia has on the resistance of a granular medium. The flow law governing stationary porous media flow is reasonably well accepted and understood but the effects of unsteady flow have commonly been neglected. The present research was designed to assess the magnitude of the acceleration effects on media of uniformly packed spheres of equal diameter and on one sample of randomly placed stone.Oscillatory flow tests were made in a large oscillatory flume tunnel with periods varying from 3 to 12 seconds. The influence of properties of the medium (grain size and porosity) were tested by using spheres with two different diameters and packing each size sphere in different geometric arrangements. Tests made on a stone sample provided a qualitative assessment of the effects of more random material properties.For the experiments described in this paper, the Forchheimer unsteady-stationary flow law described the oscillatory measurements well when velocities and energy losses were maximum. Empirical coefficients determined from steady-stationary flow were generally found to apply to the unsteady flow, however some evidence of dependency on the period of oscillation was noted. 相似文献
205.
An ecosystem-based approach for Alaska groundfish fisheries 总被引:2,自引:2,他引:2
Witherell David; Pautzke Clarence; Fluharty David 《ICES Journal of Marine Science》2000,57(3):771-777
206.
Euglena gracilis makes two small Cd-binding proteins/peptides (Cd-BP I and Cd-BP II) in response to exposure to Cd2+. These proteins migrate slower than mammalian Cd-metallothionein (Cd-Mt) through Sephadex G-75, but like Cd-Mt also contain Zn and are separated into two species when chromatographed over a DEAE column. Both BPs are much less stable than Cd-Mt under acidic conditions. One source of this instability is that at least Cd-BP II contains acid-labile sulfide co-ordinated to the metals. It has been quantitated in Cd-BP II and shown to have a value of 1·25 ± 0·10 S2−/Cd. The reactivity of the Cd-binding sites in ligand substitution and thioldisulfide exchange reactions has been assessed. The reactions of Cd-BP I and II with pyridylazoresorcinol (PAR) were slow and displayed complex kinetic behavior. Similarly, both Cd-BPs react slowly and with complicated kinetics with 5,5′-dithiobis-(2-nitrobenzoate). 相似文献
207.
208.
A simple shaker table for seismometer calibration 总被引:1,自引:0,他引:1
Frederick K. Duennebier George H. Sutton David Harris David A. Byrne 《Marine Geophysical Researches》1984,6(3):311-328
A unique and simple shaker table (shake table or shaking table), designed, constructed, and installed at the Hawaii Institute of Geophysics, has proven to be a valuable aid in testing and calibrating short period seismometers, as well as ocean bottom and ocean sub-bottom seismometer/tilt meter packages. It consists of a platform suspended in a stairwell by a single elastic cord (10 m extended length) driven by GeoSpace HS-10 geophones. Platform motion is monitored by orthogonal reference geophones and tilt meters. The relatively low natural periods of the platform, about 1.9 sec vertical and 6.5 sec horizontal, provide sufficient isolation from local vibrations that calibration can be made near operational amplitudes. Vertical or horizontal driver geophones can be driven by a commercial signal generator or white noise generator, or from magnetic tape output. The table can also be tilted with respect to the drivers to determine tilt tolerances and to calibrate tilt meters. A Hewlett-Packard 3582-A spectrum analyzer, used to analyze both reference and output signals, provides near real-time system cabibration and is an efficient means for investigating parasitic system resonances. The analyzer can also provide a white noise signal source to the driver geophones.Hawaii Institute of Geophysics Contribution 1443. 相似文献
209.
The accumulation of cadmium from seawater by the blue crab, Callinectes sapidus, was studied as a function of metal concentration and exposure time, with special emphasis on cadmium-binding proteins. Cadmium was found, in decreasing order of magnitude, in gills, digestive gland and hemolymph. When exposed to 0·5 ppm cadmium for 2–24h, virtually all of the cadmium in the cytosolic fraction of the gill was associated with a low molecular weight (LMW) cadmium-binding protein (MW 8000). However, after 48h of exposure only 50% of the cadmium in the cytosol was bound to this protein. The rest wasfound to be associated with proteins of a molecular weight of 300 000 and 60 000. This pattern of cadmium distribution did not change over a 12-day depuration period. Similar results were obtained upon exposure to 0·1 ppm cadmium. The pattern of cadmium accumulation in the cytosolic fraction of the digestive gland was in marked contrast to that observed for the gill. Initially, the cadmium was distributed over three low molecular weight fractions. During depuration the distribution of cadmium changed and all of the metal became bound to a low molecular weight protein (MW 9000). The cadmium concentrations in the gill and digestive gland remained essentially constant during depuration (12 days). The LMW cadmium-binding proteins were purified by a combination of gelpermeation and ion-exchange chromatography. Their molecular weight, spectral properties and amino acid composition are characteristic of the vertebrate metallothioneins. During exposure to cadmium the metal rapidly appeared in the hemolymph, mainly associated with hemocyanin. During depuration cadmium was transferred from the hemolymph to the digestive gland, demonstrating that hemocyanin acts as a carrier in trace metal transport. 相似文献
210.
Framvaren, a super-anoxic fjord in southern Norway, contains 7–8 mmoll−1 of sulphide and a total carbonate concentration of 18.5 mmol kg−1 in the bottom water. The chemistry of calcium has been studied, considering sources, biogenic and chemical processes and sedimentary sinks. Calcium associated with the bacteria biomass at the redox interface (18m depth) appears to be the primary source of dissolved calcium in the deep, anoxic water. Excess calcium and high total carbonate cause supersaturation of calcite, which is precipitated chemogenically. Calcite (and presumably some aragonite) is identified both in sediment trap material and the bottom sediments below the depth of supersaturation. 相似文献