全文获取类型
收费全文 | 1992篇 |
免费 | 58篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 35篇 |
大气科学 | 139篇 |
地球物理 | 501篇 |
地质学 | 709篇 |
海洋学 | 155篇 |
天文学 | 343篇 |
综合类 | 12篇 |
自然地理 | 162篇 |
出版年
2022年 | 12篇 |
2021年 | 14篇 |
2020年 | 16篇 |
2019年 | 17篇 |
2018年 | 41篇 |
2017年 | 22篇 |
2016年 | 57篇 |
2015年 | 21篇 |
2014年 | 50篇 |
2013年 | 92篇 |
2012年 | 56篇 |
2011年 | 68篇 |
2010年 | 79篇 |
2009年 | 97篇 |
2008年 | 92篇 |
2007年 | 85篇 |
2006年 | 89篇 |
2005年 | 62篇 |
2004年 | 67篇 |
2003年 | 54篇 |
2002年 | 38篇 |
2001年 | 36篇 |
2000年 | 36篇 |
1999年 | 32篇 |
1998年 | 32篇 |
1997年 | 24篇 |
1996年 | 48篇 |
1995年 | 24篇 |
1994年 | 20篇 |
1993年 | 30篇 |
1992年 | 20篇 |
1991年 | 25篇 |
1990年 | 20篇 |
1989年 | 24篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 18篇 |
1985年 | 39篇 |
1984年 | 27篇 |
1983年 | 50篇 |
1982年 | 51篇 |
1981年 | 30篇 |
1980年 | 23篇 |
1979年 | 45篇 |
1978年 | 35篇 |
1977年 | 22篇 |
1976年 | 22篇 |
1975年 | 24篇 |
1974年 | 31篇 |
1973年 | 35篇 |
排序方式: 共有2056条查询结果,搜索用时 343 毫秒
191.
Inversely graded stratification, generated by the migration of wind ripples, and adhesion structures permit unequivocal identification of Precambrian eolianites. These criteria, in combination with scale of cross-beds, angle of inclination of foresets, geometry of depositional units, and associated non-eolian facies, are used to discriminate between Precambrian dune/draa, dune-plinth, sand-sheet, and interdune deposits that formed in inland and coastal settings. Based on an analysis of published literature, fundamental conclusions can be drawn on the spatial and temporal distribution of Precambrian eolianites. The oldest reported eolianites are from the ca. 2.1 Ga Deweras Group in Zimbabwe and Hurwitz Group in Canada and numerous examples of eolianites are reported from the 1.8 Ga and younger rock record. Lack of Archean and early Paleoproterozoic eolianites and their widespread development after 1.8 Ga are examined with respect to: absence of vegetation, crustal growth and tectonic setting, relative sea-level fluctuations, unfavorable atmospheric and/or climatic change, and non-recognition. The lack of pre-2.2 Ga eolianites may be related to reworking by braided rivers combing across non-vegetated floodplains, reworking of coastal eolianites during transgression or their non-recognition in the Early Precambrian record. The temporal concentration of eolianites at 1.8 Ga may best be related to the early stages of breakup and the assembly phases of supercontinents. 相似文献
192.
Humic Ion-Binding Model VI: An Improved Description of the Interactions of Protons and Metal Ions with Humic Substances 总被引:1,自引:0,他引:1
Edward Tipping 《Aquatic Geochemistry》1998,4(1):3-47
Humic Ion-Binding Model VI, a discrete site/electrostatic model of the interactions of protons and metals with fulvic and humic acids, is applied to 19 sets of published data for proton binding, and 110 sets for metal binding. Proton binding is described with a site density, two median intrinsic equilibrium constants, two parameters defining the spread of equilibrium constants around the medians, and an electrostatic constant. Intrinsic equilibrium constants for metal binding are defined by two median constants, log KMA and log KMB, which refer to carboxyl and weaker-acid sites respectively, together with a parameter, LK1, defining the spreads of values around the medians. A further parameter, LK2, takes account of small numbers of strong binding sites. By considering results from many data sets, a universal average value of LK1 is obtained, and a correlation established between log KMB and log KMA. In addition, a relation between LK2 and the equilibrium constant for metal-NH3 complexation is tentatively suggested. As a result, metal-binding data can be fitted by the adjustment of a single parameter, log KMA. Values of log KMA are derived for 22 metal species. Model VI accounts for competition and ionic strength effects, and for proton-metal exchange. 相似文献
193.
The Atlantic menhaden,Brevoortia tyrannus, is an abundant plankton-feeding fish that undertakes extensive seasonal migrations, moving from overwintering locations offshore south of Cape Hatteras to the mid-Atlantic Bight and New England inshore waters during spring and summer. A bioenergetic model, based on field and laboratory studies, shows that when large numbers of menhaden enter Narragansett Bay, Rhode Island, during spring and early summer, they significantly influence plankton populations through size-selective grazing and nutrient regeneration. A population biomass of 9.1×106 kg of menhaden feeding for 12 h each day in the upper bay would result, in a substantial reduction of the instantaneous growth rate of the >20-μm phytoplankton. Instantaneous growth rates of zooplankton would be negative if the same population of menhaden was present, resulting in a reduction in the biomass of zooplankton. Given the ambient phytoplankton and zooplankton populations, menhaden could achieve the seasonal growth measured in Narragansett Bay during 1976 by feeding on average about 5 h d?1. Daily nitrogen excretion rates of the 9.1×106 kg menhaden population were 56.4% of the mean standing stock of ammonia-N in the upper bay. Because menhaden travel in schools their effects are likely to be intense but strongly localized, increasing spatial heterogeneity in the ecosystem. When the fish migrate southward in the fall they transfer between 3.3% and 6.2% of the nitrogen exported annually from the bay. 相似文献
194.
The property of sphericity is used to identify some anthropogenic particles in a Lake Michigan sediment. Three classes of products from high temperature processes, primarily fossil fuel burning, have been isolated: charcoals, iron oxides and aluminosilicates. The morphologies, structures and surface characteristics are indicative of the combustion process. 相似文献
195.
Edward R.D. Scott 《Earth and Planetary Science Letters》1978,39(3):363-370
Tungsten concentrations have been determined by instrumental neutron activation in 104 iron meteorites, and range from 0.07 to 5 μg/g. In individual groups, concentrations vary by factors of between 1.5 and 8, but there are negative W-Ni correlations in 8 groups: IAB, IC, IIAB, IID, IIE, IIIAB, IIICD, and IIIF. The lowest W concentrations are found in groups IAB and IIICD, which also have the smallest slopes on a W-Ni plot. Eighteen anomalous irons have W concentrations between 5 μg/g (Butler) and 0.11 μg/g (Rafrüti). The distribution of W in irons shows similarities to that of other refractory siderophilic elements (except Mo), but is closest to the distribution of Ru and Pt.Assuming that chemical trends in group IIIAB were produced by fractional crystallization, a value of 1.6 can be deduced for the distribution coefficient of W between solid and liquid metal, cf. 0.89 for Mo. Experimental evidence in support of these values is tenuous. 相似文献
196.
197.
The Ibitira eucrite is remarkable both for its vesicles and its unbrecciated nature. It consists of ~63 vol. % pyroxene (Wo14En38Fs48), 31% plagioclase (An95–96), ~2% of nickel-iron, troilite, ilmenite, titanian chromite, and 4% of a silica polymorph. It has a mean track density of 1.8 ± 0.3 × 106 cm?2, mainly due to cosmic rays. Its pre-atmospheric radius must have been at least 10 cm.The absence of complex radiation effects and presence of vesicles place constraints on the thickness of the Ibitira basalt flow. From the freezing time calculations of Provost and Bottinga, it appears that Ibitira came from a flow no less than 2.5 m and probably no more than 10 or 20 m thick. However, this estimate depends strongly on the viscosity of the melt, which is not well known. 相似文献
198.
Herbert Weiss Kathe Bertine Minoru Koide Edward D. Goldberg 《Geochimica et cosmochimica acta》1975,39(1):1-10
Chemical analyses on water from dated strata of a south Greenland permanent ice sheet revealed that there is a larger amount of sulfate in samples accumulated during the past decade than in those 60 or more years older. This increase is attributed to combustion of fossil fuel. With the exception of mercury, cadmium and possibly copper, the heavy metal distributions in the glacial waters are similar to those in atmospheric dusts. Previously reported higher mercury values in recently deposited strata were not confirmed. 相似文献
199.
Kenneth W. Bruland Minoru Koide Carl Bowser Louis J. Maher Edward D. Goldberg 《Quaternary Research》1975,5(1):89-98
Rates of sedimentation in two Lake Superior deposits were determined by both ragweed pollen and 210Pb geochronologies. The former yields an average rate over the time since the first appearance of enhanced concentrations of the pollen as a consequence of human settlement. Sedimentation rates derived on these two bases can be brought into accord if the first appearance of ragweed pollen in the sediments was around 1830 and if the sedimentation rates have been uniform over the past century. 相似文献
200.