全文获取类型
收费全文 | 8641篇 |
免费 | 343篇 |
国内免费 | 128篇 |
专业分类
测绘学 | 167篇 |
大气科学 | 717篇 |
地球物理 | 2027篇 |
地质学 | 3036篇 |
海洋学 | 822篇 |
天文学 | 1266篇 |
综合类 | 37篇 |
自然地理 | 1040篇 |
出版年
2021年 | 115篇 |
2020年 | 157篇 |
2019年 | 159篇 |
2018年 | 205篇 |
2017年 | 198篇 |
2016年 | 242篇 |
2015年 | 200篇 |
2014年 | 263篇 |
2013年 | 476篇 |
2012年 | 286篇 |
2011年 | 413篇 |
2010年 | 376篇 |
2009年 | 481篇 |
2008年 | 398篇 |
2007年 | 409篇 |
2006年 | 343篇 |
2005年 | 296篇 |
2004年 | 286篇 |
2003年 | 304篇 |
2002年 | 246篇 |
2001年 | 202篇 |
2000年 | 222篇 |
1999年 | 165篇 |
1998年 | 150篇 |
1997年 | 135篇 |
1996年 | 152篇 |
1995年 | 140篇 |
1994年 | 124篇 |
1993年 | 102篇 |
1992年 | 103篇 |
1991年 | 69篇 |
1990年 | 98篇 |
1989年 | 79篇 |
1988年 | 85篇 |
1987年 | 94篇 |
1986年 | 84篇 |
1985年 | 112篇 |
1984年 | 132篇 |
1983年 | 123篇 |
1982年 | 107篇 |
1981年 | 80篇 |
1980年 | 60篇 |
1979年 | 71篇 |
1978年 | 68篇 |
1977年 | 62篇 |
1976年 | 61篇 |
1975年 | 72篇 |
1974年 | 58篇 |
1973年 | 70篇 |
1972年 | 32篇 |
排序方式: 共有9112条查询结果,搜索用时 250 毫秒
581.
A deposit of Fe-rich, Al-poor, hydrothermal nontronite was recovered from the Juan de Fuca Ridge. Analyses show the deposit to be mineralogically and chemically similar to nontronite described at other oceanic localities. The deposit is located near the tip of a propagating segment of the Juan de Fuca Ridge. Rare earth elements and Sr isotopes indicate that the nontronite precipitated from seawater. A formation temperature of 57°C is suggested by oxygen isotopic composition. The low-temperature nontronite deposits apparently form from newly established hydrothermal systems associated with the propagating rift segment. More mature hydrothermal systems that deposit sulfide on the seafloor may develop from these low-temperature systems. 相似文献
582.
583.
584.
Richard W Gossett Harold W Puffer Robert H Arthur David R Young 《Marine pollution bulletin》1983,14(2):60-65
In 1978 the California Department of Fish and Game carried out a major, independent sportfishing survey in southern California. The results of that survey indicated that approximately one million angler-trip hours per year were spent fishing and that one in three fish caught was a white croaker (Genyonemus lineatus). There was no attempt by the Department of Fish and Game to determine the disposition of caught fish. We conducted a subsequent survey in 1980 which included 12 popular sportfishing sites in the Los Angeles area. Our results also indicated that one in three fish caught was a white croaker. In addition our survey showed that 82% of the sportfishermen ate the white croaker they caught and the median consumption was 14.8 g day?1 person?1. White croaker were then sampled by hook-and-line from 11 of the 12 survey sites plus two control sites and analysed for PCB, DDT and BaP. Median PCB levels ranged from 0.014 to 0.41 mg wet kg?1; median DDT from 0.059 to 7.5 mg wet kg?1 and BaP was below detectable limits of 1 ng for all sites. 相似文献
585.
David B Webber 《Marine pollution bulletin》1983,14(11):416-421
Bulk hydrocarbon deposition rates have been measured over a 15 month period at four stations in south-eastern Virginia surrounding the lower Chesapeake Bay. A nearly linear trend of atmospheric particulate deposition was recorded. Deposition rates at the urban station (195 μg m?2 day?1) were aproximately three times greater than those recorded for nonurban and coastal locations (mean value 69 μg m?2 day?1). The increased levels at the urban location were attributed to localized source inputs. Anthropogenic hydrocarbons accounted for approximately 50% of the total deposition at all stations. Significant biogenic inputs were indicated by the odd/even n-alkane distribution. A minimum flux to the water surface, based on mean nonurban deposition rates (24 mg yr?1), indicated an annual particulate hydrocarbon flux of +275 metric tons. Little information is available for the comparison of additional source inputs; however, the data reported here indicate that the aerial deposition of hydrocarbons is of the same order of magnitude as the input from municipal wastewater facilities and accidental discharge and is a potentially significant source of hydrocarbon pollution to the Chesapeake Bay Estuary. 相似文献
586.
The “anomalous” layer in the lowermost mantle, identified as D″ in the notation of K.E. Bullen, appears in the PREM Earth model as a 150 km-thick zone in which the gradient of incompressibility with pressure, , is almost 1.6, instead of 3.2 as in the overlying mantle. Since PREM shows no accompanying change in density or density gradient, we identify D″ as a thermal boundary layer and not as a chemically distinct zone. The anomaly in is related to the temperature gradient by the temperature dependence of Ks, for which we present a thermodynamic identity in terms of accessible quantities. This gives the numerical result (?Ks/?T)P=?1.6×107 Pa K?1 for D″ material. The corresponding temperature increment over the D″ range is 840 K. Such a layer cannot be a static feature, but must be maintained by a downward motion of the lower mantle toward the core-mantle boundary with a strong horizontal flow near the base of D″. Assuming a core heat flux of 1.6 × 1012 W, the downward speed is 0.07 mm y?1 and the temperature profile in D″, scaled to match PREM data, is approximately exponential with a scale height of 73 km. The inferred thermal conductivity is 1.2 W m?1 K?1. Using these values we develop a new analytical model of D″ which is dynamically and thermally consistent. In this model, the lower-mantle material is heated and softened as it moves down into D″ where the strong temperature dependence of viscosity concentrates the horizontal flow in a layer ~ 12 km thick and similarly ensures its removal via narrow plumes. 相似文献
587.
Conservation laws of wave action and potential enstrophy for rossby waves in a stratified atmosphere
David M. Straus 《Pure and Applied Geophysics》1983,121(5-6):917-946
The purpose of this article is to discuss the evolution of wave energy, enstrophy and action for atmospheric Rossby waves
in a variable mean flow. The presentation is theoretical, but does not represent original research; rather, it is pedagogic
in nature. The work of a number of people has been drawn together into a unified account, with much of the algebra implicit
in previous work made explicit here. The central results are that wave energy is conserved only when there are no spatial
variations in the mean flow, and wave action is conserved even in the presence of such variations as long as they are not
in the longitudinal direction. Finally, wave enstrophy is conserved in the presence of arbitrary (slow) mean flow variations. 相似文献
588.
David H. Zeuch 《Tectonophysics》1983,93(1-2)
The recent theoretical results of Twiss (1976) and Goetze (1978) suggest that at high temperatures and sufficiently high stresses the creep behavior of dry olivine should be dominated by either nonlinear diffusion accommodated grain-boundary sliding or nonlinear Coble creep mechanisms. This would result following the production of a fine grain-size by dynamic recrystallization. For the high-temperature experimental work performed by Karato et al. (1982) dry single crystals of olivine were almost totally recrystallized during creep, and temperature changing experiments were performed on the resulting dynamically recrystallizing polycrystalline aggregates. However, the activation energy for creep determined by Karato et al. (1982) was far higher than that predicted by the models of Twiss (1976) or Goetze (1978), although the conditions required for operation of at least the model of Twiss (1976) apparently were satisfied. The data for the highly recrystallized specimens from the higher stress, lower temperature experiments of Zeuch and Green (1979) and Zeuch (1980) are in good agreement with the results of Karato et al. (1982). These latter experiments were conducted under conditions where either the model of Twiss (1976) or Goetze (1978) should have been applicable. I tentatively conclude that although fine grain sizes were produced in the experiments of Karato et al. (1982), Zeuch and Green (1979) and Zeuch (1980) by dynamic recrystallization, there is no evidence for a transition to grain-boundary diffusional creep mechanisms at high or low stresses despite the predictions of Twiss (1976) or Goetze (1978). Instead, deformation is dominated by dislocation movement with recovery by dynamic recrystallization. 相似文献
589.
The sulfide minerals exhibit a rich diversity in sturctural chemistry and in electrical, magnetic and other physical properties. Models based on molecular orbital theory and incorporating some elements of band theory can be developed to describe the diverse valence electron behavior in these minerals. Qualitative models can be proposed on the basis of observed properties, and the models can be tested and refined using experimental data from X-ray emission and X-ray photoelectron spectroscopy and quantum mechanical calculations performed on cluster units which form the basic building blocks of the crystals. This approach to chemical bonding in sulfide minerals is illustrated for binary non-transition metal sulfides (ZnS, CdS, HgS, PbS), binary transition metal sulfides (FeS2, CoS2, NiS2, CuS2 ZnS2) and more complex sulfides (CuFeS2, Cu2S, Ag2S, CuS, Co3S4, CuCo2S4, Fe3S4). The relationship between qualitative and quantitative theories is reviewed with reference to the pyrite-marcasite-arsenopyrite-loellingite series of minerals. Application of the models to understanding structure-determining principles, relative stabilities, solid solution limits and properties such as color, reflectance and hardness are discussed. 相似文献
590.