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131.
Max Richter 《International Journal of Earth Sciences》1937,28(3-4):243-249
Ohne Zusammenfassung 相似文献
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Abstract. Microplankton (silicoflagellates, dinoflagellates, tintinnids, pteropods, crustaceans, and fecal pellets) was analysed in 71 0–95 m samples from the Greenland Sea collected from May to June 1989. Abundances were clearly associated with the ice edge, highest concentrations occurring in ice-free waters. The 15 to 30 m stratum was generally richer in microplankton than deeper waters, with the exception of microcrustaceans, which often peaked below 30 m. Six tintinnid species were identified. The mean size of the tintinnid Parafavella denticulata from the Greenland Sea was consistently greater than that of Barents Sea specimens. It is suggested that colder waters and scarcer food are responsible for these morphometric differences. 相似文献
135.
Seismic reflection profiles from the northern end of Juan de Fuca Ridge reveal three axial valleys having a basement relief of as much as 2 sec (two-way travel time). A thick sequence, presumably of turbidites, mainly less than 0.7 m.y. old, covers much of the area. The oldest turbidites form the upper part of the fill of a possible Tertiary trench between the ridge and North America. The second turbidite unit extends beyond the trench and once formed an abyssal plain over most of northern Juan de Fuca Ridge and the area west to Explorer Ridge. Following formation of the plain, vertical movements began that broadly uplifted the crest of Juan de Fuca Ridge, block-faulted its northern end, produced faulting along Sovanco Fracture Zone, and upwarped the basement north of the ridge. Younger turbidites have filled the lowlands created by the vertical movements. The present sea floor topography and seismic activity show evidence of continued movements. 相似文献
136.
The fractional porosity of marine sediments is one of the fundamental index properties of rocks. For the determination of porosity a nondestructive, inductive method was used on unsplit sediment cores. The results were compared with galvanically measured resistivities using a miniature Wenner array on split cores. The measurements agree well except for cores with high clay content where measurement frequency related effects cause a resistivity difference of about 10%. Porosities were derived from resistivities using a published resistivity-porosity relationship by Boyce (1968) and compared with sample porosities. A comparison of both data sets shows good agreement. 相似文献
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The Ter Vell (NE Iberian Peninsula) is a eutrophic coastal lagoon which has been flooded by the excess irrigation water and the agricultural runoff during the last decades. Between 1999 and 2003, restoration measures were applied to improve its water quality. At the same time, but independently, agricultural water management drastically reduced the freshwater inflow. The short-term effects of these management actions on the limnological characteristics of the lagoon were analysed by comparing two hydrological cycles, one before (1999/2000) and the other one after (2002/2003) the actions. The two cycles are illustrative of opposite situations in the hydrological functioning of coastal wetlands. In the first, the lagoon was exorheic, with prolonged flooding periods and a low residence time; in the second, it had a more endorheic character, with scarce water inputs and prolonged periods of confinement. Consequently, nitrogen inputs diminished and organic load and salinity increased as the internal loading and the accumulation effects became more relevant. These effects were actually caused by the drastic reduction in the freshwater inflow which prevented, in turn, the success of the restoration measures. The zooplankton community of the Ter Vell lagoon was not significantly altered by the hydrological change, at least in the short-term, and rotifers and cladocerans, mainly those species indicative of eutrophy, dominated the community. 相似文献
139.
Ruslan A. Mendybaev Frank M. Richter Andrew M. Davis 《Geochimica et cosmochimica acta》2006,70(10):2622-2642
Type B CAIs are subdivided into B1s, with well-developed melilite mantles, and B2s, with randomly distributed melilite. Despite intensive study, the origin of the characteristic melilite mantle of the B1s remains unclear. Recently, we proposed that formation of the melilite mantle is caused by depletion of the droplet surface in volatile magnesium and silicon due to higher evaporation rates of volatile species compared to their slow diffusion rates in the melt, thus making possible crystallization of melilite at the edge of the CAI first, followed by its crystallization in the central parts at lower temperatures. Here, we present the results of an experimental study that aimed to reproduce the texture observed in natural Type B CAIs. First, we experimentally determined crystallization temperatures of melilite for three melt compositions, which, combined with literature data, allowed us to find a simple relationship between the melt composition, crystallization temperature, and composition of first crystallizing melilite. Second, we conducted a series of evaporation and cooling experiments exposing CAI-like melts to gas mixtures with different oxygen fugacities (fO2). Cooling of the molten droplets in gases with logfO2?IW-4 resulted in crystallization of randomly distributed melilite, while under more reducing conditions, melilite mantles have been formed. Chemical profiles through samples quenched right before melilite started to crystallize showed no chemical gradients in samples exposed to relatively oxidizing gases (logfO2?IW-4), while the near-surface parts of the samples exposed to very reducing gases (logfO2?IW-7) were depleted in volatile MgO and SiO2, and enriched in refractory Al2O3. Using these experimental results and the fact that the evaporation rate of magnesium and silicon from CAI-like melts is proportional to , we estimate that Type B1 CAIs could be formed by evaporation of a partially molten precursor in a gas of solar composition with . Type B2 CAIs could form by slower evaporation of the same precursors in the same gas with . 相似文献
140.
Frank M. Richter Ruslan A. Mendybaev Ian D. Hutcheon Neil C. Sturchio 《Geochimica et cosmochimica acta》2006,70(2):277-289
Experiments specifically designed to measure the ratio of the diffusivities of ions dissolved in water were used to determine . The measured ratio of the diffusion coefficients for Li and K in water (DLi/DK = 0.6) is in good agreement with published data, providing evidence that the experimental design being used resolves the relative mobility of ions with adequate precision to also be used for determining the fractionation of isotopes by diffusion in water. In the case of Li, we found measurable isotopic fractionation associated with the diffusion of dissolved LiCl (D7Li/D6Li=0.99772±0.00026). This difference in the diffusion coefficient of 7Li compared to 6Li is significantly less than that reported in an earlier study, a difference we attribute to the fact that in the earlier study Li diffused through a membrane separating the water reservoirs. Our experiments involving Mg diffusing in water found no measurable isotopic fractionation (D25Mg/D24Mg=1.00003±0.00006). Cl isotopes were fractionated during diffusion in water (D37Cl/D35Cl=0.99857±0.00080) whether or not the co-diffuser (Li or Mg) was isotopically fractionated. The isotopic fractionation associated with the diffusion of ions in water is much smaller than values we found previously for the isotopic fractionation of Li and Ca isotopes by diffusion in molten silicate liquids. A major distinction between water and silicate liquids is that water surrounds dissolved ions with hydration shells, which very likely play an important but still poorly understood role in limiting the isotopic fractionation associated with diffusion. 相似文献