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71.
The linear dunes of the southern Kalahari dunefield constitute one of the major palaeoenvironmental proxies in the region. The application of optically stimulated luminescence (OSL) dating since the1990s and advancements in the depth of sampling using augering equipment over the past few years have permitted the reconstruction of linear dune accumulation chronostratigraphies for entire dune profiles from base to crest. These methods are applied to four dunes in the Mariental–Stampriet region of the southern Kalahari dunefield, sampled at predominantly 0.5 m intervals. Individual dunes record multiple phases of dune construction, but with only a few phases recorded consistently between two or more of the dunes. Results from the 48 OSL ages produced here extend the aeolian accumulation record for the southern Kalahari dunefield through the last three glacial–interglacial cycles with two ages from the early part of MIS6. A synthesis of all existing luminescence ages for the southern Kalahari reveals that the dunefield has been partially active throughout much of the past 120 ka. There are no clear clusters of ages within OSL age errors. This is in contrast to previous syntheses of ages for this region. In addition, these new data from Mariental–Stampriet dunes show that clusters in grouped dune OSL ages can be spuriously produced as a function of reducing the sampling frequency with depth within the dunes, from 0.5 to 1 mintervals. This has significant implications for previous conclusions regarding discrete phases of aeolian accumulation based on sampling at 1 m intervals and less vertically intensive sampling techniques. The total luminescence data set of 136 ages for the southern Kalahari implies that this dunefield has been close to the threshold of reactivation throughout much of the late Quaternary. 相似文献
72.
73.
Citrate released by plants, bacteria, and fungi into soils is subject to abiotic oxidation by MnO2(birnessite), yielding 3-ketoglutarate, acetoacetate, and MnII. Citrate loss and generation of products as a function of time all yield S-shaped curves, indicating autocatalysis. Increasing the citrate concentration decreases the induction period. The maximum rate (rmax) along the reaction coordinate follows a Langmuir-Hinshelwood dependence on citrate concentration. Increases in pH decrease rmax and increase the induction time. Adding MnII, ZnII, orthophosphate, or pyrophosphate at the onset of reaction decreases rmax. MnII addition eliminates the induction period, while orthophosphate and pyrophosphate addition increase the induction period. These findings indicate that two parallel processes are responsible. The first, relatively slow process involves the oxidation of free citrate by surface-bound MnIII,IV, yielding MnII and citrate oxidation products. The second process, which is subject to strong positive feedback, involves electron transfer from MnII-citrate complexes to surface-bound MnIII,IV, generating MnIII-citrate and MnII. Subsequent intramolecular electron transfer converts MnIII-citrate into MnII and citrate oxidation products. 相似文献
74.
V. Petoukhov M. Claussen A. Berger M. Crucifix M. Eby A. V. Eliseev T. Fichefet A. Ganopolski H. Goosse I. Kamenkovich I. I. Mokhov M. Montoya L. A. Mysak A. Sokolov P. Stone Z. Wang A. J. Weaver 《Climate Dynamics》2005,25(4):363-385
An intercomparison of eight EMICs (Earth system Models of Intermediate Complexity) is carried out to investigate the variation
and scatter in the results of simulating (1) the climate characteristics at the prescribed 280 ppm atmosphere CO2 concentration, and (2) the equilibrium and transient responses to CO2 doubling in the atmosphere. The results of the first part of this intercomparison suggest that EMICs are in reasonable agreement
with the present-day observational data. The dispersion of the EMIC results by and large falls within the range of results
of General Circulation Models (GCMs), which took part in the Atmospheric Model Intercomparison Project (AMIP) and Coupled
Model Intercomparison Project, phase 1 (CMIP1). Probable reasons for the observed discrepancies among the EMIC simulations
of climate characteristics are analysed. A scenario with gradual increase in CO2 concentration in the atmosphere (1% per year compounded) during the first 70 years followed by a stabilisation at the 560 ppm
level during a period longer than 1,500 years is chosen for the second part of this intercomparison. It appears that the EMIC
results for the equilibrium and transient responses to CO2 doubling are within the range of the corresponding results of GCMs, which participated in the atmosphere-slab ocean model
intercomparison project and Coupled Model Intercomparison Project, phase 2 (CMIP2). In particular EMICs show similar temperature
and precipitation changes with comparable magnitudes and scatter across the models as found in the GCMs. The largest scatter
in the simulated response of precipitation to CO2 change occurs in the subtropics. Significant differences also appear in the magnitude of sea ice cover reduction. Each of
the EMICs participating in the intercomparison exhibits a reduction of the strength of the thermohaline circulation in the
North Atlantic under CO2 doubling, with the maximum decrease occurring between 100 and 300 years after the beginning of the transient experiment.
After this transient reduction, whose minimum notably varies from model to model, the strength of the thermohaline circulation
increases again in each model, slowly rising back to a new equilibrium. 相似文献
75.
Increases in extreme precipitation greater than in the mean under increased greenhouse gases have been reported in many climate
models both on global and regional scales. It has been proposed in a previous study that whereas global-mean precipitation
change is primarily constrained by the global energy budget, the heaviest events can be expected when effectively all the
moisture in a volume of air is precipitated out, suggesting the intensity of these events increases with availability of moisture,
and significantly faster than the global mean. Thus under conditions of constant relative humidity one might expect the Clausius–Clapeyron
relation to give a constraint on changes in the uppermost quantiles of precipitation distributions. This study examines if
the phenomenon manifests on regional and seasonal scales also. Zonal analysis of daily precipitation in the HadCM3 model under
a transient CO2 forcing scenario shows increased extreme precipitation in the tropics accompanied by increased drying at lower percentiles.
At mid- to high-latitudes there is increased precipitation over all percentiles. The greatest agreement with Clausius–Clapeyron
predicted change occurs at mid-latitudes. This pattern is consistent with other climate model projections, and suggests that
regions in which the nature of the ambient flows change little give the greatest agreement with Clausius–Clapeyron prediction.
This is borne out by repeating the analyses at gridbox level and over season. Furthermore, it is found that Clausius–Clapeyron
predicted change in extreme precipitation is a better predictor than directly using the change in mean precipitation, particularly
between 60°N and 60°S. This could explain why extreme precipitation changes may be more detectable then mean changes. 相似文献
76.
Peter H. Stone 《地球物理与天体物理流体动力学》2013,107(1):147-164
Abstract The stability of a baroclinic zonal current to symmetric perturbations on an equatorial β-plane is considered. The fluid is assumed to be Boussinesq, inviseid, adiabatic, hydrostatic, and stably stratified. The solutions exhibit the same stability properties as those on an f-plane: instability occurs whenever Ri < 1/(1 + d), where Ri is the Richardson number and d is a measure of the horizontal shear of the current; the most unstable motions tend to parallel the isotherms of potential temperature; and they have infinitely small scales of variation perpendicular to the isotherms. The variation of Coriolis parameter leads to one important difference in the structure of the eigenfunctions: the rapidly growing modes are concentrated in high latitudes, and the slowly growing ones in low latitudes. The suggestion that the symmetric cloud bands observed at low latitudes in Jupiter's atmosphere are caused by symmetric instabilities is re-examined in the light of these results. These cloud bands would have to be associated with the slowly-growing, low-latitude modes. These modes consist of small scale motions parallel to the isotherms, with the magnitude of the motions having a large scale modulation as a function of latitude. The time scales of these modes and the latitude scales of their modulation agree qualitatively with the observations of Jupiter's cloud bands, so long as Ri is not very close to zero or to its critical value. 相似文献
77.
介绍了如何将互联网技术应用于石油行业的局域网或广域网中,在服务器端提供图形信息数据库的服务,并在客户端利用浏览器进行图形信息的查询,实现勘探开发图形信息的共享。 相似文献
78.
Ages of fractures in the Eye-Dashwa granite pluton north of Atikokan, Ontario, are estimated from analyses of their filling materials (pegmatite, aplite, hornblendeporphyry, epidote, chlorite, gypsum, diabase, iron oxides, carbonate and clays). The analyses include radiometric age determinations of filling materials, comparison of crystallization temperatures of filling materials with the expected temperature of the pluton through time, and paragenetic relations derived from textures. The major fractures (primarily epidote and chlorite-filled) were produced by penetrative deformation during cooling immediately after intrusion of the pluton. From 2.6 Ga to 2.4 Ga they transected the pluton and largely defined the present overall fracture network. Less severe penetrative deformation occurred about 1.1 Ga, when a few diabase dikes and small gypsum fractures developed. A second source of fracturing appears to have existed through most of geological time, and it reopened segments of old fractures within about 400 m of the present erosional surface. It is unknown whether the near-surface, or supergene, fractures formed more or less continuously through time or in distinct episodes, such as during glacial periods. Open fractures, which are most common in the near-surface layer, tend to transmit groundwater, but fracture sealing due to the accumulation of filling materials seems to occur with time. Underground excavations such as nuclear fuel waste disposal vaults, would be less susceptible to the incursion of groundwater, if they were located below the near-surface layer. 相似文献
79.
吉林南部辉南-靖宇地区岩石圈地幔氧化-还原状态及研究意义 总被引:1,自引:2,他引:1
吉林省南部辉南-靖宇地区第四纪碱性玄武岩中的地幔包体主要为尖晶石相二辉橄榄岩和方辉橄榄岩。二辉橄榄岩和方辉橄榄岩的平衡温度分别为770~1000℃和850~1025℃,对应的氧逸度 (fO2)值分别为FMQ -0.70至+0.34 (均值为FMQ -0.06) 和FMQ -0.46至+0.05 (均值为FMQ -0.15),它们与深海橄榄岩(abyssal peridotites)以及软流圈地幔的fO2相似。橄榄岩的fO2值,连同其全岩化学成分(如Mg#、Al2O3、CaO、Ni、Co和Cr)和矿物化学成分(如橄榄石的Fo、尖晶石的Cr#和Mg#,以及辉石的Mg#)特征,表明辉南-靖宇地区龙岗火山群下面的岩石圈地幔很可能是在晚中生代以来,伴随着华北克拉通和扬子板块的碰撞以及来自东侧太平洋板块和北侧蒙古-额霍次克(Mongolo-Okhotsk)板块分别向西和向南的俯冲叠加,原来的古老岩石圈失衡、塌陷(拆沉?),取而代之的深部软流圈底辟、上涌,又经历了低度部分熔融的产物。 相似文献
80.
Summary ?The PGE contents of chromite separated from peridotite layers of Archaean mafic–ultramafic flows, Abitibi belt (Canada),
indicate enrichment in Os–Ir–Ru (600 ppb) relative to Pd–Au (<5 ppb). Evidently, chromite was a sink for Ir–Os–Ru during melt-chromite
fractionation in each of the flows. However, an additional phase, probably olivine, is required to explain the bulk Ir content
of the sulphide-poor peridotites. In contrast, the chromite Pt contents range from <10 ppb to 400 ppb, with large variation
in Pt/Ru (0.02–2.76) and Pt/Pd (5–400) ratios. The Pt enrichment may be related to the presence of Pt spinel structure compounds
in oxidised melt, reflecting Fe–Ti spinel-related mineralisation in higher pyroxenite-gabbro layers.
Received December 5, 2002; revised version accepted January 7, 2003 相似文献