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81.
Yen-Hong Shau Melanie E. Feather Eric J. Essene Donald R. Peacor 《Contributions to Mineralogy and Petrology》1991,106(3):367-378
Electron microbeam techniques have been used to examine submicroscopically intergrown paragonite, phengite and chlorite from
the South Fork Mountain Schist of the Franciscan Terrane of northern California, which was subjected to blueschist facies
metamorphism. The sample also contains quartz, albite, lawsonite, and rutile. The subassemblage albite-lawsonite-rutile requires
metamorphic conditions on the low-temperature side of the equilibrium albite+lawsonite+rutile=paragonite+sphene+quartz+H2O (T<200° C and P<7.4 kbars based on thermodynamic data of Holland and Powell 1990). The white micas appear to be optically
homogeneous, but back-scattered electron images can distinguish two different micas by their slight difference in contrast.
Electron microprobe analyses (EMPA) of micas show Na/(Na+K) ranging from 0.2 to 0.8. The two micas are resolved by transmission
electron microscopy (TEM) as packets of phengite and paragonite that range from 20 to several hundred nm in thickness. The
compositions, determined by analytical electron microscopy (AEM), constrain the limbs of the phengite-paragonite solvus to
values of Na/(Na+K)=<0.02 and 0.97, representing less mutual solid solution than ever reported by EMPA. The textural relations
imply that the sheet silicates were derived from reactions between fluids and detrital clays and that they are in an intermediate
stage of textural development. We caution that microprobe analyses of apparently homogeneous sheet silicates may yield erroneous
data and lead to faulty conclusions using phengite barometry and paragonite-muscovite thermometry, especially in fine-grained
rocks that formed at relatively low temperatures.
Contribution no. 473 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann
Arbor, Michigan, USA 相似文献
82.
The effects of source size on plume behaviour have been examined in a 1.2 m wind tunnel boundary layer for isokinetic sources with diameters from 3 to 35 mm at source heights of 230 mm and at ground level. Experimental measurements of mean concentration and the variance, intermittency and probability density functions of the concentration fluctuations were obtained. In addition, a fluctuating Gaussian plume model is presented which reproduces many of the observed features of the elevated emission. The mean plume width becomes independent of source size much more rapidly than the instantaneous plume width. Since it is the meandering of the instantaneous plume which generates most of the concentration fluctuations near the source, these are also dependent on source size. The flux of variance in the plume reaches a maximum, whose value is greatest for the smallest source size, close to the source and thereafter is monotonically decreasing. The intermittency factor reaches a minimum, whose value is lowest for the smallest source, and increases back towards one. Concentration fluctuations for the ground-level source are much less dependent on source size due to the effects of the surface. 相似文献
83.
Ground temperature, pressure and wind speed monthly averages in the area of the Italian Station at Terra Nova Bay, Antarctica, were analyzed for the period 1987–1991 by means of a network of nine AWS (automatic weather stations). Spatial configurations of temperature show a well-defined, relatively warm island in the area of Terra Nova Bay, between Drygalsky and Campbell ice tongues, throughout the year. A second warm island is present to the north along the coast, between Aviator and Mariner ice tongues, for most of the year. From February to March a rapid drop in temperature is observed at all stations. A strong thermal gradient develops during February, March, April and October, November, December, between the coastal region and inner highlands. The baric configuration follows the elevation of the area. Annual average pressure and temperature as functions of stations altitude show linear trends. Severe katabatic wind episodes are recorded at all stations, with wind speed exceeding 25 ms–1 and direction following the orographic features of the inner areas. Co-occurrences of these episodes were observed for stations located along stream lines of cold air drainage. The autocorrelation function of maximum wind speed time series shows wind persistence of 2–3 days and wind periodicity of about one week. 相似文献
84.
Thomas K. Nielsen Hans Christian Larsen John R. Hopper 《Earth and Planetary Science Letters》2002,200(3-4):271-286
We present new and reprocessed seismic reflection data from the area where the southeast and southwest Greenland margins intersected to form a triple junction south of Greenland in the early Tertiary. During breakup at 56 Ma, thick igneous crust was accreted along the entire 1300-km-long southeast Greenland margin from the Greenland Iceland Ridge to, and possibly 100 km beyond, the triple junction into the Labrador Sea. However, highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest Greenland margin at the same time. Thus, a transition from a volcanic to a non-volcanic margin over only 100–200 km is observed. Magmatism related to the impact of the Iceland plume below the North Atlantic around 61 Ma is known from central-west and southeast Greenland. The new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction. The extent of initial plume-related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere. Plume mantle is channelled to great distances provided that significant melting does not take place. Melting causes cooling and dehydration of the plume mantle. The associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift. We further suggest that thick Archaean lithosphere blocked direct flow of plume material into the magma-starved southwest Greenland margin while the plume was free to flow into the central west and east Greenland margins. The model is consistent with a plume layer that is only moderately hotter, 100–200°C, than ambient mantle temperature, and has a thickness comparable to lithospheric thickness variations, 50–100 km. Lithospheric architecture, the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism. 相似文献
85.
86.
Rare earth element distribution in some hydrothermal minerals: evidence for crystallographic control 总被引:1,自引:0,他引:1
Rare earth element (REE) abundances were measured by neutron activation analysis in anhydrite (CaSO4), barite (BaSO4), siderite (FeCO3) and galena (PbS). A simple crystal-chemical model qualitatively describes the relative affinities for REE substitution in anhydrite, barite, and siderite. When normalized to ‘crustal’ abundances (as an approximation to the hydrothermal fluid REE pattern), log REE abundance is a surprisingly linear function of (ionic radius of major cation—ionic radius of REE)2 for the three hydrothermal minerals, individually and collectively. An important exception, however, is Eu, which is anomalously enriched in barite and depleted in siderite relative to REE of neighboring atomic number and trivalent ionic radius. In principle, REE analyses of suitable pairs of co-existing hydrothermal minerals, combined with appropriate experimental data, could yield both the REE content and the temperature of the parental hydrothermal fluid.The REE have only very weak chalcophilic tendencies, and this is reflected by the very low abundances in galena—La, 0.6 ppb; Sm, 0.06 ppb; the remainder are below detection limits. 相似文献
87.
U. Morgenstern R. G. Ditchburn E. G. Vologina M. Sturm 《Journal of Paleolimnology》2013,50(3):345-352
We applied the 32Si dating technique to a sediment core from Lake Baikal to obtain the sediment chronology for the last millennium. The core was recovered about 4 km offshore from the north slope of the South Basin in 1,366 m water depth. The sediment material consisted of continuously accumulated diatom-rich geogenic-terrigenous mud, intercalated with a number of dark olive-grey turbidite layers. The sediment layers containing the turbidites were excluded from 32Si sampling to obtain the chronology that is representative of the continuous sedimentation. The initial 32Si activity of 31.3 dpm kg?1 SiO2, measured in sediment trap samples, confirms the trend of 32Si specific activities of biogenic silica found in other Northern Hemisphere lakes. The four sediment core samples from depth 0–48 cm have 32Si specific activities between 23.5 and 0.5 dpm kg?1 SiO2, with corresponding ages between 60 and 860 years and constant sedimentation rate of 0.036 ± 0.004 cm year?1 over the most recent 800 years. 32Si allowed us for the first time to date the uppermost turbidites in the South Basin of Lake Baikal, to 1030, 1310 and 1670 ad. Given these dates, the last long-distance turbidity current triggered by slope instabilities had occurred 330 years before 2000 ad, and the intervals between the 1310 and 1670 ad event and between the 1030 and 1310 ad event were 360 and 280 years, respectively. The 32Si ages allow unprecedented time resolution for reconstruction of the former environmental and climatic conditions during the past millennium. 相似文献
88.
éric J.M.?DelhezEmail author Geneviève?Lacroix éric?Deleersnijder 《Ocean Dynamics》2004,54(2):221-231
The constituent-oriented age theory (CAT) worked out by Delhez et al. (1999) is a flexible tool that can be applied to diagnose complex models. It is shown here how this can be used to quantify the pace at which an ecosystem model works. At the cost of the introduction of one additional evolution equation for each compartment of the ecosystem model, the mean age of the biological material forming these compartments can be computed. The information obtained in this way complements the information provided by the concentration data; while the latter measures the standing stocks, the former provides an integrated assessment of the interaction rates and matter fluxes. The benefits of the method are demonstrated with a simple Lotka–Volterra system and a one-dimensional vertical model of the nitrogen cycle in the Ligurian Sea. The theory can be used to study the biological compartments individually or the ecosystem as a whole. In particular, the age is a valuable tool to quantify the overall cycling rate of nitrogen in the food web.Responsible Editor: Phil Dyke 相似文献
89.
The attenuation of chemical elements in acidic leachates from coal mineral wastes by soils 总被引:1,自引:0,他引:1
The chemical attenuation of acidity and selected elements (aluminum, arsenic, cadmium, cobalt, chromium, copper, fluorine,
iron, manganese, nickel, and zinc) in acidic leachates from coal mineral wastes by four natural subsurface soils has been
investigated using laboratory column methods Leachate solutions were allowed to percolate through the soils under simulated
natural flow conditions, and the elemental concentrations in the influents and effluents were measured periodically Elemental
retentions were substantial for all species except managanese, which was eluted in excess from all soils except the most calcareous
Two processes appeared to operate in decreasing influent concentrations: (1) precipitation of solid phases caused by increased
pH of the leachate as it percolated through the soil, and (2) adsorption of elements onto exchange and sorption sites naturally
present in the soil and on iron and aluminum oxide precipitates formed in situ from leachate components because of the increased
pH The soil property most important in retention was its alkalinity Thus, carbonaceous soils provide the best control material
for acidic leachates from coal mineral wastes. Results show that natural soils can substantially reduce pollutant fluxes to
the environment from acidic coal waste dumps and should be considered when selecting waste disposal sites
Performed under the auspices of the U.S. Department of Energy 相似文献
90.