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481.
482.
Caroline P. Slomp John ThomsonGert J. de Lange 《Geochimica et cosmochimica acta》2002,66(7):1171-1184
Phosphorus regeneration and burial fluxes during and after formation of the most recent sapropel S1 were determined for two deep-basin, low-sedimentation sites in the eastern Mediterranean Sea. Organic C/P ratios and burial fluxes indicate enhanced regeneration of P relative to C during deposition of sapropel S1. This is largely due to the enhanced release of P from organic matter during sulfate reduction. Release of P from Fe-bound P also increased, but this was only a relatively minor source of dissolved P. Pore-water HPO42− concentrations remained too low for carbonate fluorapatite formation. An increased burial of biogenic Ca-P (i.e., fish debris) was observed for one site. Estimated benthic fluxes of P during sapropel formation were elevated relative to the present day (∼900 to 2800 vs. ∼70 to 120 μmol m−2 yr−1). The present-day sedimentary P cycle in the deep-basin sediments is characterized by two major zones of reaction: (1) the zone near the sediment-water interface where substantial release of HPO42− from organic matter takes place, and (2) the oxidation front at the top of the S1 where upward-diffusing HPO42− from below the sapropel is sorbed to Fe-oxides. The efficiency of aerobic organisms in retaining P is reflected in the low organic C/P ratios in the oxidized part of the sapropel. Burial efficiencies for reactive P were significantly lower during S1 times compared with the present day (∼7 to 15% vs. 64 to 77%). Budget calculations for the eastern Mediterranean Sea demonstrate that the weakening of the antiestuarine circulation and the enhanced regeneration of P both contributed to a significant increase in deep-water HPO42− concentrations during sapropel S1 times. Provided that sufficient vertical mixing occurred, enhanced regeneration of P at the seafloor may have played a key role in maintaining increased productivity during sapropel S1 formation. 相似文献
483.
484.
The transition to democratic governments in Central America over the past decade, and the subsequent emergence of institutional
mechanisms for regional cooperation have greatly increased the frequency and productivity of efforts for preservation of cultural
patrimony. Both the archaeological and colonial data bases overlap modern political boundaries and regional collaboration
in training, pubic education, curation and research is essential. Natural forces continually impact, and human economic forces
(especially international tourism) increasingly impact cultural resources. Regional cooperation in the utilization of human
resources is necessary to confront the management and salvage demands of major economic development projects, and rescue efforts
following natural disasters. The development of policies and programs will be effective only if matched by the growth of cultural
preservation professionals in each of the Central American republics. Finally, other art consuming nations in addition to
the United States must begin to respect the cultural heritage of the Central American countries and develop policies to deter
abuses by diplomatic staff and employees of NGO’s (nongovernment organizations) 相似文献
485.
The thermal evolution of the Earth is controlled by radioactive elements whose heat production rate decays with time and whose spatial distribution depends on chemical segregation processes.
We present a 2-D and finite-difference Boussinesq convection model with temperature-dependent viscosity and time- and space-dependent radioactive heat sources. We used Newtonian rheology, boxes of aspect ratio 3, and heating from within. Starting from the geochemical results of Hofmann (1988), it is assumed that the radioactive heat sources of the mantle were initially distributed homogeneously. In a number of calculations, however, higher starting abundances of radioactive sources were assumed in the upper mantle. For the present geological situation, this also results in a depleted upper mantle. It was assumed that, if the viscosity falls below a certain critical value, chemical segregation will take place. In this way, model continental crust develops, leaving behind areas of a depleted mantle. We obtained the heat source, flow line, temperature, viscosity and heat-flow distribution as a function of time with realistic values, especially for the present time. The present viscosity of the upper mantle is approximately at the standard value obtained for postglacial uplift modelling; the deeper-mantle viscosity is considerably higher. The time dependence of the computed mean of the kinetic energy of mantle convection bears a resemblance to that of the magmatic and orogenetic activity of the Earth. We assumed that the 670 km discontinuity cannot be penetrated by the flow. 相似文献
We present a 2-D and finite-difference Boussinesq convection model with temperature-dependent viscosity and time- and space-dependent radioactive heat sources. We used Newtonian rheology, boxes of aspect ratio 3, and heating from within. Starting from the geochemical results of Hofmann (1988), it is assumed that the radioactive heat sources of the mantle were initially distributed homogeneously. In a number of calculations, however, higher starting abundances of radioactive sources were assumed in the upper mantle. For the present geological situation, this also results in a depleted upper mantle. It was assumed that, if the viscosity falls below a certain critical value, chemical segregation will take place. In this way, model continental crust develops, leaving behind areas of a depleted mantle. We obtained the heat source, flow line, temperature, viscosity and heat-flow distribution as a function of time with realistic values, especially for the present time. The present viscosity of the upper mantle is approximately at the standard value obtained for postglacial uplift modelling; the deeper-mantle viscosity is considerably higher. The time dependence of the computed mean of the kinetic energy of mantle convection bears a resemblance to that of the magmatic and orogenetic activity of the Earth. We assumed that the 670 km discontinuity cannot be penetrated by the flow. 相似文献
486.
An ir spectrum of asteroid Vesta, the first of any asteroid, has been recorded at a spectral resolution of 44 cm?1 with a Fourier spectrometer. An electronic absorption band is observed that is assigned to an iron-rich pyroxene (pigeonite) spectroscopically similar to that found in certain eucrites. Other important rock-forming minerals such as olivine and plagioclase feldspar are not observed. There is no evidence for compositional variation with rotational phase angle. This spectroscopic picture of Vesta suggests considerable evolution including the melting and differentiation of silicates. 相似文献
487.
Uwe Fink Bradford A. Smith D. Chris Benner James R. Johnson H.J. Reitsema James A. Westphal 《Icarus》1980,44(1):62-71
Using a low-resolution spectrograph and a CCD array, a spectrum of Pluto from 0.58 to 1.06 μm was obtained. The spectrum had a resolution of and a signal-to-noise ratio of ~300. It showed CH4 absorption bands at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 Å. The strongest of these bands was at 8900 Å with an absorption depth of 0.23. This band was heavily saturated, compared to the weaker bands, providing proof for the gaseous origin of the observed absorptions. By applying CH4 band model parameters to our data, a total CH4 abundance of 80 ± 20 m-am was derived. This translates into a one-way abundance of 27 ± 7 m-am and a CH4 surface pressure of 1.5 × 10?4 atm. An upper limit to the total pressure of ~0.05 atm could be set. First-order calculations on atmospheric escape showed that this methane atmosphere would be stable if the mass of Pluto is increased 50% over its current value and its radius is 1400 km. Alternatively a heavier gas mixed with the CH4 atmosphere would aid its stability. The relatively large amount of gaseous CH4 observed implies that the absorption bands recently reported at 1.7 and 2.3 μm are likely due to atmospheric CH4 absorptions rather than surface frost as interpreted earlier. 相似文献
488.
Birgit Lange Fritz Scholz Hans-Joachim Bautsch Ferdinand Damaschun Gert Wappler 《Physics and Chemistry of Minerals》1993,19(7):486-491
The electrochemical reduction of xanthoconite, proustite, pyrostilpnite, and pyrargyrite was studied by abrasive stripping voltammetry, a technique which is based upon a preliminary mechanical transfer of trace amounts of the mineral onto the surface of a paraffin impregnated graphite electrode. Because the electrochemical reduction proceeds near to reversibility and is very similar for each pair of minerals, the peak potentials in differential pulse voltammetry can be used to calculate the standard enthalpy of phase transformation of xanthoconite to proustite and of pyrostilpnite to pyrargyrite: T
H
(xanth proust)
O = 35.46 ± 14.15 kJ/mol and T
H
(pyrostilp pyrarg)
O = 38.85 ± 6.60 kJ/mol. These values are not accessible otherwise until now. 相似文献
489.
Abstract The 9 km diameter Red Wing Creek structure, North Dakota, is located within the oil-rich Williston Basin at 47°36′N and 103°33′W. Earlier geophysical studies indicated that this subsurface structure has a central uplift, surrounded by an annular crater moat, and a raised rim. Breccias were encountered during drilling between ~2000 and 2800 m depth in the central uplift area, and the presence of shatter cone fragments in drill core samples was suggested to indicate an impact origin of the Red Wing Creek structure. We studied the petrographic and geochemical characteristics of samples of well cuttings from two boreholes at the center of the structure: the True Oil 22–27 Burlington Northern and True Oil 11–27 Burlington Northern wells. We found planar deformation features (PDFs) in quartz with up to three sets of different crystallographic orientations in sandstone- and siltstone-dominated samples from the True Oil 11–27 borehole. U-stage measurements of the crystallographic orientations of the PDFs showed the occurrence of the shock-characteristic (0001), and orientations, with a dominance of (0001) and orientations. The relative frequencies of the orientations indicate a shock pressure of at least 12–20 GPa. These results provide unambiguous evidence for shock metamorphism at Red Wing Creek and confirm that the structure was formed by impact. 相似文献
490.
Mirko Dreßler Thomas Hübener Solvig Görs Petra Werner Uwe Selig 《Journal of Paleolimnology》2007,37(2):205-219
During monthly investigations from 1996 to 2000, a hypolimnetic layer of phototrophic sulphur bacteria (Chromatium spp.) were observed in Lake Dudinghausen, a small dimictic lake in northern Germany. This paleolimnological study was initiated
to detect if the occurrence of sulphur bacteria was related to cultural eutrophication or reflected natural conditions. Therefore,
diatoms, algal pigments, okenone, geochemical proxies, and 210Pb and 137Cs were used in four sediment cores to investigate historical changes in trophic development, hypolimnetic redox conditions,
anoxia and phototrophic sulphur bacteria abundances. Fossil diatoms, pigments, the ratio of chlorophyll derivatives to total
carotenoids and the ratio of chlorophyll a to its derivatives suggest two phases of eutrophication coupled with hypolimnetic anoxia over the last ~80 years: a first
phase from about 1923–1932 and a second from 1952 to 1982. In the first phase the ratios of Fe–Mn as well as Fe–Ca increased,
suggesting seasonal anoxia. However, hypolimnetic anoxia was only weak because low levels of okenone suggest no mass development
of sulphur bacteria. In contrast, sulphur bacteria increased during the early stages of the second eutrophication phase, suggesting
increased temporal and spatial hypolimnetic anoxia. Surprisingly, the ratios of Fe–Mn as well as Fe–Ca decreased during this
time. Possibly Fe, Mn and Ca were equally reduced through the intense anoxia. In the final stage, sulphur bacteria decreased
again. As these bacteria need both anoxic conditions and a certain amount of light, the increased nutrient load probably led
to low Secchi depth and therefore insufficient light conditions. In more recent years, diatoms and pigments suggest a decrease
in nutrient levels. A second mass development of sulphur bacteria occurred, probably due to improved light conditions and
continued anoxia in the upper hypolimnion. We conclude that the recent development of phototrophic sulphur bacteria do not
represent natural conditions in Lake Dudinghausen. Furthermore, the upper sediments contain a completely new diatom flora
that never occurred in older sediments of Lake Dudinghausen. Therefore, nutrient levels may eventually reach natural conditions,
however they may not represent biological background reference conditions. 相似文献