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971.
We use a viscous slide model of Jiang and LeBlond (1994) coupled with nonlinear shallow water equations to study tsunami waves in Resurrection Bay, in south-central Alaska. The town of Seward, located at the head of Resurrection Bay, was hit hard by both tectonic and local landslide-generated tsunami waves during the M W 9.2 1964 earthquake with an epicenter located about 150 km northeast of Seward. Recent studies have estimated the total volume of underwater slide material that moved in Resurrection Bay during the earthquake to be about 211 million m3. Resurrection Bay is a glacial fjord with large tidal ranges and sediments accumulating on steep underwater slopes at a high rate. Also, it is located in a seismically active region above the Aleutian megathrust. All these factors make the town vulnerable to locally generated waves produced by underwater slope failures. Therefore it is crucial to assess the tsunami hazard related to local landslide-generated tsunamis in Resurrection Bay in order to conduct comprehensive tsunami inundation mapping at Seward. We use numerical modeling to recreate the landslides and tsunami waves of the 1964 earthquake to test the hypothesis that the local tsunami in Resurrection Bay has been produced by a number of different slope failures. We find that numerical results are in good agreement with the observational data, and the model could be employed to evaluate landslide tsunami hazard in Alaska fjords for the purposes of tsunami hazard mitigation. 相似文献
972.
Methylhopane biomarker hydrocarbons in Hamersley Province sediments provide evidence for Neoarchean aerobiosis 总被引:1,自引:0,他引:1
Jennifer L. Eigenbrode Katherine H. Freeman Roger E. Summons 《Earth and Planetary Science Letters》2008,273(3-4):323-331
Bacteriohopanepolyols with A-ring methyl substituents are bacterial lipid biomarkers that are presently known to originate from just a few specific taxa. When preserved in ancient rocks, the fossilized hydrocarbon skeletons of these molecules have the potential to provide biogeochemical records of those taxa and their respective physiologies. Considering both their occurrences in modern organisms, and their Proterozoic and Phanerozoic sedimentary distributions, hopanes carrying 2-methyl or 3-methyl substituents are proposed to be derived from cyanobacteria and oxygen-respiring methanotrophs, respectively. Here we report the distribution of methylhopanes in 2.72–2.56 billion-year-old, Neoarchean rocks from the Hamersley Province on the Pilbara Craton. The relative abundance of C31-3β-methylhopane, but not that of C31-2α-methylhopane, shows a strong correlation to the carbon isotopic composition of co-occurring kerogen (insoluble particulate organic matter). The unanticipated nature of this relationship provides evidence for a shallow-water locus of carbon cycling through aerobic oxidation of CH4 and, coincidentally, a means to demonstrate biomarker syngenicity. 2α-Methylhopanes are most abundant in both shale and carbonate from shallow-water sediments, and a positive correlation to carbonate abundance, contrasts to variable, but generally lower, 2α-methylhopane abundances of deeper water facies. These observations are consistent with their origin from cyanobacteria which were likely the cornerstone of microbial communities in shallow-water ecosystems providing molecular oxygen, fixed carbon, and possibly fixed nitrogen. 相似文献
973.
Scott A. Whattam Roger H. Hewins Bosmat A. Cohen Nicholas C. Seaton David J. Prior 《Earth and Planetary Science Letters》2008,269(1-2):200-211
Granoblastic olivine aggregates (GOA) have been discovered in some Type I magnesian chondrules within carbonaceous chondrites by Libourel and Krot [Libourel, G., Krot, A.N., 2007. Evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin. Earth Planet. Sci. Lett. 254, 1–8], who proposed an origin from pre-existing planetesimals. Amoeboid olivine aggregates (AOA), generally considered as aggregates of solar nebula condensates and found within similar carbonaceous chondrites, display similar equilibrium texture, though on a finer scale. For these reasons, we conducted experiments to determine if annealing of olivine required time scales appropriate to planetesimal or nebular heating. Pressed < 43 µm and < 63 µm San Carlos olivine powder (Fo88.4) was isothermally heated at temperatures ranging from 1350–1550 °C for 1–100 h. The 100 h runs yield olivine aggregates with well-developed granoblastic texture at all temperatures, manifest as a network of randomly-oriented and sutured olivine grains with 120° triple junctions. Individual olivine grains are 4–6 sided and polygonal by 1450 °C and equigranular texture is developed at high temperature (1500–1550 °C). Melting of olivine commences at 1450–1500 °C and aids in ‘ripening’ and suturing (grain coarsening and grain boundary migration). Textural equilibrium is clearly met at 1550 °C. A planetesimal origin cannot be ruled out; however, the experimental evidence reveals that granoblastic texture can be reproduced in an interval not inconsistent with heating times for nebular objects. GOA may have experienced higher degrees of thermal processing than the finer-grained AOA. If the precursors were the same, grain coarsening would have to be accompanied by modification to bulk and isotopic compositions. However, the precursors could have been olivine condensates formed later than AOA. Annealing may have been a widespread process operating in the primordial solar nebula responsible for thermal processing and formation of GOA prior to their incorporation into chondrules. 相似文献
974.
Changqun Cao Wei Wang Lujun Liu Shuzhong Shen Roger E. Summons 《Earth and Planetary Science Letters》2008,270(3-4):251-257
New analyses reveal two intervals of distinctly lower δ13C values in the terrestrial organic matter of Permian–Triassic sequences in northern Xinjiang, China. The younger negative δ13Corg spike can be correlated to the conspicuous and sharp δ13C drops both in carbonate carbon and organic carbon near the Permian–Triassic event boundary (PTEB) in the marine section at Meishan. The geochemical correlation criteria are accompanied by a magnetic susceptibility pulse and higher abundances of distinctive, chain-like organic fossil remains of Reduviasporonites.The older negative δ13Corg spike originates within a latest Permian regression. Significant changes in organic geochemical proxies are recorded in the equivalent interval of the marine section at Meishan. These include relatively higher concentrations of total organic carbon, isorenieratane, C14–C30 aryl isoprenoids and lower ratios of pristane/phytane that, together, indicate the onset of anoxic, euxinic and restricted environments within the photic zone. The massive and widespread oxidation of buried organic matter that induced these euxinic conditions in the ocean would also result in increased concentrations of 13C-depleted atmospheric CO2. The latest Permian environmental stress marked by the older negative δ13Corg episode can be correlated with the distinct changeover of ostracod assemblages and the occurrences of morphological abnormalities of pollen grains. These observations imply that biogeochemical disturbance was manifested on the land at the end of the Permian and that terrestrial organisms responded to it before the main extinction of the marine fauna. 相似文献
975.
Measurements of coupled fluid and sediment motion over mobile sand dunes in a laboratory flume 总被引:1,自引:1,他引:0
The relationship between turbulent fluid motions and sediment particle motions over mobile sand dunes was investigated by using a laser Doppler velocimeter and an acoustic backscatter system in laboratory experiments performed at the USDA-ARS-National Sedimentation Laboratory. Profiles of acoustic backscatter from particles and at-a-point turbulence data were collected while translating both measurement devices downstream at the speed of mobile dune bedforms. The resulting data set was used to examine the frequency (recurrence frequency) at which the fluctuating backscatter and fluid velocity signals exceeded magnitude thresholds based on the standard deviation (σ) of the local velocity and the magnitude the acoustic signal resulting from backscatter from suspended particles. The slope of the downstream and vertical velocity recurrence frequencies generally indicated a gradually increasing recurrence time with increasing elevation. The recurrence frequency for acoustic backscatter data was not strongly variable with elevation. The closest correspondence between the recurrence frequencies of sediment backscatter and vertical velocities at the 1σ magnitude threshold was in a region defined by X/L〈0.4 and y〈6 cm. The downstream velocity was most closely related to backscatter in a small region at 0.4〈X/L〈0.8 and less than 3-4 cm from the bed. 相似文献
976.
Streams and rivers, particularly smaller ones, often do not maintain steady flow rates for long enough to reach equilibrium conditions for sediment transport and bed topography. In particular, streams in small watersheds may be subject to rapidly changing hydrographs, and relict bedforms from previous high flows can cause further disequilibrium that complicates the prediction of sediment transport rates. In order to advance the understanding of how bedforms respond to rapid changes in flow rate,... 相似文献
977.
978.
979.
980.
Jonathon Dale Tim Holland Roger Powell 《Contributions to Mineralogy and Petrology》2000,140(3):353-362
Calibrations are presented for an independent set of four equilibria between end-members of garnet, hornblende, plagioclase
and quartz. Thermodynamic data from a large internally-consistent thermodynamic dataset are used to determine the ΔG° of the equilibria. Then, with the known mixing properties of garnet and plagioclase, the non-ideal mixing in amphibole is
derived from a set of 74 natural garnet–amphibole–plagioclase–quartz assemblages crystallised in the range 4–13 kbar and 500–800 °C.
The advantage of using known thermodynamic data to calculate ΔG° is that correlated variations of composition with temperature and pressure are not manifested in fictive derived entropies
and volumes, but are accounted for with non-ideal mixing terms. The amphibole is modelled using a set of ten independent end-members
whose mixing parameters are in good agreement with the small amount of data available in the literature. The equilibria used
to calibrate the amphibole non-ideal mixing reproduce pressures and temperatures with average absolute deviations of 1.1 kbar
and 35 °C using an average pressure–temperature approach, and 0.8 kbar with an average pressure approach. The mixing data
provide not only a basis for thermobarometry involving additional phases, but also for calculation of phase diagrams in complex
amphibole-bearing systems.
Received: 8 November 1999 / Accepted: 7 July 2000 相似文献