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891.
Fairfax Lake is a small, oligotrophic to mesotrophic headwater lake situated in the Foothills of the Rocky Mountains of west-central Alberta (Latitude 52° 58 N; Longitude 116° 34 W). Through acquisition of a sediment core, and analyses of the diatoms, chrysophyte stomatocysts, pollen and sedimentary pigments, including myxoxanthophyll and oscillaxanthin, a palaeoenvironmental history of the lake has been determined. The sedimentary record spans ca. 13 200 years. An open tree-less vegetation existed in the region ca. 13 200–ca. 11 600 years BP. Maximum oscillaxanthin and myxoxanthophyll concentrations, hence the largest blue-green algal populations, occurred during the same interval. With increasing temperature pioneering parkland vegetation appeared ca. 11 600 years BP but was replaced ca. 10 100 years BP by spruce forest. Pine appeared ca. 7800 years BP and this marked the development of the present day montane boreal forest. Diatoms were not found until ca. 11 255 years BP. Benthic taxa dominated but by ca. 10 100 years BP planktonic taxa had become more prominent. Lake levels are interpreted as having risen, and the lake water was probably more transparent. Maximum chlorophyll and total carotenoid concentrations occur ca. 11 255 to ca. 7000 years BP corresponding to the warm early to mid-Holocene period. Lake nutrient levels appear to have been higher prior to ca. 7000 years BP, and the lake has changed from being eutrophic during the early Holocene to its present status as an oligotrophic to mesotrophic lake. Subtle hydrological changes have also occurred in the catchment as water levels do not appear to have remained constant. 相似文献
892.
Lianyang Zhang 《Soil Dynamics and Earthquake Engineering》1998,17(4):219-226
The feasibility of using the optimum seeking method to assess the seismic liquefaction potential of sands has been investigated. Optimization theory is a very important branch of applied mathematics and has a wide application in the practical world. Using the available field sand liquefaction data, the influence of various factors is quantified using the optimum seeking method. The factors considered are: the earthquake magnitude M, the distance of the site from the source of the earthquake L, the depth of the water table Dw, the depth of the sand deposit Ds, and the standard penetration test (SPT) blow count N. The most important factors have been identified as the earthquake magnitude and the SPT blow count. Prediction results show that the proposed method is effective and feasible. Since neither normalization of the SPT blow count nor calculation of the seismic shear-stress ratio are required, the proposed method is simpler and more direct than the conventional methods of evaluating liquefaction potential. 相似文献
893.
Comparative Tertiary stratigraphy of the Rhine Graben, Bresse Graben and Molasse Basin: correlation of Alpine foreland events 总被引:1,自引:0,他引:1
W. Sissingh 《Tectonophysics》1998,300(1-4):249-284
Comparative tectonostratigraphic analysis of the Tertiary (Middle Eocene–Pliocene) of the Rhine–Bresse graben system and the western Molasse Basin demonstrates the occurrence of eleven correlative sequences (CRF I–XI). These show a close relationship between intra-basinal tectonics and depositional history. Their punctuated sediment accumulation can be related to phases of extra-Alpine taphrogenesis and Alpine orogenesis, and to coeval eustatic changes in sea-level. Apparent simultaneity in rift and foredeep sequence development, structural deformation and global change in sea-level suggests a common causal control in which the development of the Alps played an important role. 相似文献
894.
895.
896.
Ternary (triangular) diagrams show the compositional variation of three end-members, recalculated to 100%, and represent the projection into two dimensions of three or more components (since an end-member may represent the sum of more than one original component), and hence multiple space. During the present century they have been of particular importance in chemistry, the earth sciences, colour-mixing and the studies of colour that underlie the development of geochemical colour maps. Such concepts can be traced back to Newton's Opticks although he did not use an explicit ternary theory of colour mixing. Development of the trichromatic theory of colour deficiency began with Mariotte in the 17th century and was definitively established by Maxwell in the 1850s. These colour theorems underlie the development in recent years of the colour maps now in widespread use in geochemistry. The progress of regional geochemical mapping in the British Isles has gone hand-in-hand with the development of computer software and hardware. The use of multi-element colour-combined geochemical maps can be traced from early work at Imperial College by Webb and his co-workers (1964–79) to its later application in the British Geological Survey's Geochemical Atlas series (from 1983 onwards). Between 1955 and 1975 the use of ternary diagrams came to be commonly used in studies of mineralogical composition, including economic minerals, petrology, classification, and phase relationships. In igneous petrology, the quartz–alkali-feldspar–plagioclase-feldspathoid double-triangle has achieved increasing international acceptance. We illustrate these applications with examples pertinent to the Northern Highland Caledonides, with particular emphasis on the lamprophyres and associated rocks. Two distinct multivariate trends of variation are shown to be present which may warrant further investigation to elucidate their bearing on Caledonian mineralisation. 相似文献
897.
Ternary (triangular) diagrams show the compositional variation of three end-members, recalculated to 100%, and represent the projection into two dimensions of three or more components (since an end-member may represent the sum of more than one original component), and hence multiple space. During the present century they have been of particular importance in chemistry, the earth sciences, colour-mixing and the studies of colour that underlie the development of geochemical colour maps. Such concepts can be traced back to Newton's Opticks although he did not use an explicit ternary theory of colour mixing. Development of the trichromatic theory of colour deficiency began with Mariotte in the 17th century and was definitively established by Maxwell in the 1850s. These colour theorems underlie the development in recent years of the colour maps now in widespread use in geochemistry. The progress of regional geochemical mapping in the British Isles has gone hand-in-hand with the development of computer software and hardware. The use of multi-element colour-combined geochemical maps can be traced from early work at Imperial College by Webb and his co-workers (1964–79) to its later application in the British Geological Survey's Geochemical Atlas series (from 1983 onwards). Between 1955 and 1975 the use of ternary diagrams came to be commonly used in studies of mineralogical composition, including economic minerals, petrology, classification, and phase relationships. In igneous petrology, the quartz–alkali-feldspar–plagioclase-feldspathoid double-triangle has achieved increasing international acceptance. We illustrate these applications with examples pertinent to the Northern Highland Caledonides, with particular emphasis on the lamprophyres and associated rocks. Two distinct multivariate trends of variation are shown to be present which may warrant further investigation to elucidate their bearing on Caledonian mineralisation. 相似文献
898.
899.
R. McGrath 《The Australian geographer》1998,29(2):205-221
Few studies have been conducted into the environmental history of the North Coast region of New South Wales (NSW). To address this, two sedimentary sequences located in the coastal Bundjalung National Park and at Bungawalbin Creek, just north of the Park (both near Evans Head, NSW) have been investigated. Palynological and geochemical analyses were used to reconstruct the history of the sites. Fossil pollen evidence representing much of the Holocene period — basal radiocarbon dates of 8700 ± 60 years BP and 6600 ± 60 years BP are available from the Bundjalung National Park and Bungawalbin Creek sites, respectively — indicates a dynamic past environment, with an overall tendency towards drier conditions. This evidence is supported, in part, by charcoal analyses indicating an increase in the occurrence of fire over time. Chemical analyses of the sediments indicate that the Bungawalbin site was strongly influenced by estuarine conditions, whereas the Bundjalung National Park site was only influenced indirectly by estuarine conditions. 相似文献
900.
R.J. Flower 《Journal of Paleolimnology》1998,20(2):107-117
Recent environmental change research in Lake Baikal is introduced together with an overview of several interrelated papers published concurrently in this issue of Journal of Paleolimnology. Five themes are tackled by analysis of recent Baikal sediment cores, dating, geochemistry, particulate pollutants, magnetism and diatoms. The concurrent papers focus on the first four themes in some detail and summary results of diatom analysis (from Mackay et al., 1998) are given here. Taken together these studies provide a time-space framework for recent environmental change in Lake Baikal not previously available.There are significant shifts in species composition of the endemic planktonic diatom assemblages in uppermost sediments collected from throughout the lake. However, these changes usually precede the sediment record of low level but widespread contamination by industrial products. The most clear sign of industrial contamination is the presence of particles from fossil fuel combustion in sediment post dating the 1930s.Although evidence for widespread biostratigraphic changes by pollution is lacking, radionuclide, diatom, lithostratigraphic and magnetic stratigraphies indicate two main features, (i) it is possible to make stratigraphic correlations within and between basins using recent sediment cores, (ii) that turbidite deposits, from several to tens of cm thick, are frequently encountered in recent sediments.Turbidite deposits occur in 210Pb dated and pre-210Pb sediment core sections and are undoubtedly a major macro-disturbance feature in many deep water locations in Lake Baikal. If profiles are to be used as direct proxy records of climate variability, then screening of cores for turbidites is a pre-requisite for quality assurance in future paleoenvironmental studies.On-going international research including Swiss, Russian and British joint paleoenvironmental studies on the distribution and biological formation of recent sediments will hopefully lead to better interpretation of Holocene and pre-Holocene sediment records in Lake Baikal. 相似文献