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41.
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Sébastien Owona Marion Tichomirowa Lothar Ratschbacher Joseph Mvondo Ondoa Dieudonné Youmen J?rg Pf?nder Félix M. Tchoua Pascal Affaton Georges Emmanuel Ekodeck 《International Journal of Earth Sciences》2012,101(7):1689-1703
Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and 207Pb/206Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624?±?2?Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The 87Rb/86Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. 87Rb/86Sr isochron ages differ from sample to sample (599?±?3, 572?±?4, 554?±?5, 540?±?5?Ma) yielding the oldest Neoproterozoic age (~600?Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (~540?Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The 87Rb/86Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (~540?Ma) more closely to the border of the Congo Craton. 相似文献
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Sebastien Owona Bernhard Schulz Lothar Ratschbacher Joseph Mvondo Ondoa Georges E. Ekodeck Félix M. Tchoua Pascal Affaton 《Journal of African Earth Sciences》2011,59(1):125-139
Garnet-bearing micaschists and paragneisses of the Yaounde Group in the Pan-African Central African Orogenic Belt in Cameroon underwent a polyphase structural evolution with the deformation stages D1–D2, D3 and D4. The garnet-bearing assemblages crystallized in course of the deformation stage D1–D2 which led to the formation of the regional main foliation S2. In XCa–XMg coordinates one can distinguish several zonation trends in the garnet porphyroblasts. Zonation trends with increasing XMg and variably decreasing XCa signalize a garnet growth during prograde metamorphism. Intermineral microstructures provided criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The syndeformational P–T path sections calculated from cores and rims of garnets in individual samples partly overlap and align along clockwise P–T trends. The P–T evolution started at ~450 °C/7 kbar, passed high-pressure conditions at 11–12 kbar at variable temperatures (600–700 °C) and involved a marked decompression toward 6–7 kbar at high temperatures (700–750 °C). Th–U–Pb dating of metamorphic monazite by electron microprobe (EMP-CHIME method) in eight samples revealed a single period of crystallization between 613 ± 33 Ma and 586 ± 15 Ma. The EMP-monazite age populations between 613 ± 33 Ma enclosed in garnet and 605 ± 12 Ma in the matrix apparently bracket the high temperature–intermediate pressure stage at the end of the prograde P–T path. The younger monazites crystallized still at amphibolite-facies conditions during subsequent retrogression. The Pan-African overall clockwise P–T evolution in the Yaounde Group with its syndeformational high pressure stages and marked pressure variations is typical of the parts of orogens which underwent contractional crustal thickening by stacking of nappe units during continental collision and/or during subduction-related accretionary processes. 相似文献
45.
Lothar Noethe 《Experimental Astronomy》2009,26(1-3):1-18
Since the invention of the telescope the mirrors in reflecting telescopes have evolved from small pieces of polished speculum to sophisticated, computer-controlled systems. This review describes the major problems with the casting, figuring and support of these mirrors, and how fundamental inventions like depositing a silver layer on glass or electronic devices like computers opened the path to new solutions for fabricating larger mirrors with improved performance. 相似文献
46.
Labrador Sea convection and subpolar North Atlantic Deep Water export in the SODA assimilation model
Friedrich A. Schott Lothar Stramma Benjamin S. Giese Rainer Zantopp 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(6):926-938
Labrador Sea convection was most intense and reached the greatest depths in the early 1990s, followed by weaker, shallower, and more variable convection after 1995. The Simple Ocean Data Assimilation (SODA) version 2.0.2/2.0.4 assimilation model is used to explore convective activity in the North Atlantic Ocean for the period from 1992 to 2007. Hydrographic conditions, which are relatively well observed during this period, are used to compare modeled and observed winter mixed-layer depths and water mass anomalies in relation to Deep Western Boundary Current transports and meridional overturning circulation (MOC) changes at the exit of the subpolar basin. The assimilation differs markedly from local observations in the March mixed-layer depth, which represents deep convection and water mass transformation. However, mean MOC rates at the exit of the subpolar gyre, forced by stratification in the mid-latitudes, are similar to estimates based on observations and show no significant decrease during the 1992–2007 period. SODA reproduces the deep Labrador Sea Water formation in the western North Atlantic without any clear indication of significant formation in the Irminger Sea while the lighter upper Labrador Sea Water density range is reached in the Irminger Sea in the 1990s, in agreement with existing assumptions of deep convection in the Irminger Sea and also supported by computed lag correlations with the Labrador Sea. Deep Water transformation mainly takes place in the eastern North Atlantic. The introduction of CFC-11 into the SODA model as a tracer reproduces the mean and multiyear variations of observed distributions. 相似文献
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A Dominic Fortes, Ian G Wood and Lothar Oberauer think through the possibilities for an entirely new – and currently technologically impossible – means of examining the core of the Earth. 相似文献
49.
Prof. Dr. Lothar Geitler 《Aquatic Sciences - Research Across Boundaries》1960,22(1):131-135
Zusammenfassung Die grosse Formenfülle der Flechtenalgen l?sst sich durch einige neue F?lle belegen: inBiatorella pruinosa trittMyrmecia Biatorellae auf, die bisher nur aus verh?ltnism?ssig wenigen Flechten bekannt ist; eineVerrucaria sp. enth?lt eine noch unbeschriebene Protococcale, deren Chromatophor eine eigentümliche Zwischenstellung zwischen zentraler
und parietaler Lage zeigt und in dieser Hinsicht der GattungJaagia ?hnelt;Verrucaria acrotella undacrotella f.subathallina sowie zweiThelidium-Arten enthalten einen schon von anderen Flechten mehrfach bekannten oder ihm sehr ?hnlichen Protococcus. 相似文献
50.
The Solar Maximum Mission satellite did not record any γ-ray counts in excess of the background for a time interval of 223 s after the arrival of the first
e's from the supernova 1987A. On the basis of the original data we derive a new 3σ upper limit on the γ fluence for this period and derive improved bounds on the νi → νjγ and νT → νee−e+γ radiative decay channels for neutrino masses up to the experimentally allowed value of around 30 MeV. 相似文献