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941.
942.
Defining the floodplain in hydrologically‐variable settings: implications for flood risk management 下载免费PDF全文
Flood risk management is an essential responsibility of state governments and local councils to ensure the protection of people residing on floodplains. Globally, floodplains are under increasing pressure from growing populations. Typically, the engineering‐type solutions that are used to predict local flood magnitude and frequency based on limited gauging data are inadequate, especially in settings which experience high hydrological variability. This study highlights the importance of incorporating geomorphological understanding into flood risk management in southeast Queensland (SEQ), an area badly affected by extreme flood events in 2011 and 2013. The major aim of this study is to outline the hydrological and sedimentological characteristics of various ‘inundation surfaces’ that are typical of catchments in the sub‐tropics. It identifies four major inundation surfaces; within‐channel bench [Q ~ 2.33 yr average recurrence interval (ARI)]; genetic floodplain (Q = 20 yr ARI); hydraulic floodplain (20 yr < Q ≤ 200 yr ARI) and terrace (Q > 1000 yr ARI). These surfaces are considered typical of inundation areas within, and adjacent to, the large macrochannels common to this region and others of similar hydrological variability. An additional area within genetic floodplains was identified where flood surfaces coalesce and produce an abrupt reduction in channel capacity. This is referred to here as a Spill‐out Zone (SOZ). The associated vulnerability and risk of these surfaces is reviewed and recommendations made based on incorporating this geomorphological understanding into flood risk assessments. These recommendations recognize the importance to manage for risks associated with flow inundation and sediment erosion, delivery and deposition. The increasing availability of high resolution topographic data opens up the possibility of more rapid and spatially extensive assessments of key geomorphic processes which can readily be used to predict flood risk. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
943.
Chris Koen 《Monthly notices of the Royal Astronomical Society》1999,309(3):769-802
The estimation of the frequency, amplitude and phase of a sinusoid from observations contaminated by correlated noise is considered. It is assumed that the observations are regularly spaced, but may suffer missing values or long time stretches with no data. The typical astronomical source of such data is high-speed photoelectric photometry of pulsating stars. The study of the observational noise properties of nearly 200 real data sets is reported: noise can almost always be characterized as a random walk with superposed white noise. A scheme for obtaining weighted non-linear least-squares estimates of the parameters of interest, as well as standard errors of these estimates, is described. Simulation results are presented for both complete and incomplete data. It is shown that, in finite data sets, results are sensitive to the initial phase of the sinusoid. 相似文献
944.
Critical loads of acidity have been estimated for Irish lakes (277) using survey data from the Republic of Ireland and Northern Ireland, and compatible inputs and methods across both regions. This is the first trans-region application of the Steady-State Water Chemistry model for the island of Ireland. Results indicate that a small percentage of lakes (10.8%, 30 lakes) have low critical loads of acidity (<50.0 mmolc m-2 yr-1). Based on bulk sulphate deposition, exceedance of critical load was observed in 6.9% of the sampled lakes (19 lakes). Inferences relating to the whole lake population cannot be made, as statistical lake surveys were not carried out. However, as the surveys were weighted towards acid-sensitive lakes, the current study suggests that Irish lakes are not under a significant threat from anthropogenic acidification. The acidifying impact of nitrogen was incorporated into the exceedance calcula tion by superimposing the present leaching of nitrogen onto sulphate deposition. This increased exceedance by ഌ% (i.e., from 11 to 19 lakes). Despite the potential uncertainties associated with using default values and relationships in the Steady-State Water Chemistry model, uncertainty analysis indicated that the critical loads were relatively stable. The critical acid neutralising capacity limit (specified water quality criterion) had the greatest effect on the critical load. In the current study, a catchment dependent limit was employed (ranged: 0-50 7molc L-1; mean: 35 7molc L-1) rather than a single fixed value. 相似文献
945.
Poor air quality has long been one of the dominant transboundary issues facing protected areas worldwide. In 1977, the United States Congress amended the Clean Air Act to address federally-managed lands containing valued air resources (i.e. scenic views). Forty years later, we interviewed professionals from the U.S. National Park Service, Forest Service, and Bureau of Land Management (n=38) regarding their perceptions of the legacy of the Clean Air Act and other federal policies relevant to air resources, as well as their perceptions of the future of air resource management on U.S federal lands. Results were analyzed using a Strengths, Weaknesses, Opportunities, and Threats (SWOT) analysis supported by in vivo, thematic, and salience coding. Our findings illustrate national progress in air resource management, potential policy directions, and highlight the influence of emerging technologies and governance in air resource management within protected areas. 相似文献
946.
Chris?HarrisEmail authorView authors OrcID profile Tanya?Dreyer Petrus?le?Roux 《Contributions to Mineralogy and Petrology》2018,173(1):8
Peralkaline syenite and granite dykes cut the Straumsvola nepheline syenite pluton in Western Dronning Maud Land, Antarctica. The average peralkalinity index (PI?=?molecular Al/[Na?+?K]) of the dykes is 1.20 (n?=?29) and manifests itself in the presence of the Zr silicates eudialyte, dalyite and vlasovite, and the Na–Ti silicate, narsarsukite. The dykes appear to have intruded during slow cooling of the nepheline syenite pluton, and the petrogenetic relationship of the dykes and the pluton cannot be related to closed-system processes at low pressure, given the thermal divide that exists between silica-undersaturated and oversaturated magmas. Major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition, and internal mineral segregation prior to final solidification. The distribution of accessory minerals is consistent with late-stage crystallization of isolated melt pockets. The dykes give an Rb–Sr isochron age of 171?±?4.4 Ma, with variable initial 87Sr/86Sr ratio (0.7075?±?0.0032), and have an average ε Nd of ? 12.0. Quartz phenocrysts have δ18O values of 8.4–9.2‰, which are generally in O-isotope equilibrium with bulk rock. Differences in the δ18O values of quartz and aegirine (average Δquartz?aegirine = 3.5‰) suggest aegirine formation temperatures around 500 °C, lower than expected for a felsic magma, but consistent with poikilitic aegirine that indicates subsolidus growth. The negative ε Nd (< ? 10) and magma δ18O values averaging 8.6‰ (assuming Δquartz?magma = 0.6‰) are inconsistent with a magma produced by closed-system fractional crystallization of a mantle-derived magma. By contrast, the nepheline syenite magma had mantle-like δ18O values and much less negative ε Nd (average ??3.1, n?=?3). The country rock has similar δ18O values to the granite dykes (average 8.0‰, n?=?108); this means that models for the petrogenesis of the granites by assimilation are unfeasible, unless an unexposed high-δ18O contaminant is invoked. Instead, it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali-metasomatised gneiss that surrounds the nepheline syenite, followed by fractional crystallization. 相似文献
947.
Alexander Knebe Bastian Arnold Chris Power Brad K. Gibson 《Monthly notices of the Royal Astronomical Society》2008,386(2):1029-1037
We present a comparison of the properties of substructure haloes ( subhaloes ) orbiting within host haloes that form in cold dark matter (CDM) and warm dark matter (WDM) cosmologies. Our study focuses on selected properties of these subhaloes, namely their anisotropic spatial distribution within the hosts; the existence of a 'backsplash' population; the age–distance relation; the degree to which they suffer mass loss; and the distribution of relative (infall) velocities with respect to the hosts. We find that the number density of subhaloes in our WDM model is suppressed relative to that in the CDM model, as we would expect. Interestingly, our analysis reveals that backsplash subhaloes exist in both the WDM and CDM models. Indeed, there are no statistically significant differences between the spatial distributions of subhaloes in the CDM and WDM models. There is evidence that subhaloes in the WDM model suffer enhanced mass loss relative to their counterparts in the CDM model, reflecting their lower central densities. We note also a tendency for the (infall) velocities of subhaloes in the WDM model to be higher than in the CDM model. Nevertheless, we conclude that observational tests based on either the spatial distribution or the kinematics of the subhalo population are unlikely to help us to differentiate between the CDM model and our adopted WDM model. 相似文献
948.
The most important geomorphic responses to storms are qualitative changes in system state. Minor storms produce no state change or very rapid recovery to pre‐storm state, and extinction events wipe out the system. In other cases disturbance results in a state change, which may be transitional (change to a previously existing state), state space expansion (change to a new state), and clock‐resetting events that return the system to its initial state. Recovery pathways are much more varied than the monotonic progressions represented in classic vegetation succession and linear channel evolution models. Those linear sequential pathways are only one of several archetypal recovery pathways, which also include binary, convergent, divergent, and more complex networks. Filter‐dominated systems are more likely to follow linear sequential or convergent patterns, whereas amplifier‐dominance is characteristic of divergent and more complex mesh or fully‐connected patterns. Amplifier domination is also more likely to lead to evolutionary or state space expansion responses. Amplification and filtering in geomorphic response and recovery can be assessed using the 'Four R's' framework of response, resistance, relaxation, and recursion. High resistance and resilience, rapid relaxation times, and stable recursive feedback networks reduce or offset effects of disturbances, thus filtering their impacts. Conversely, low resistance and resilience, slow relaxation, and dynamically unstable feedbacks can exaggerate disturbances, creating disproportionately large and long‐lived impacts, thereby amplifying disturbances. Unless new filter mechanisms evolve (either autogenically or anthropically), or the number of extinction or clock‐resetting events increases, intensified storminess will result in more geomorphic variability. These ideas are applied to a case study of a flood on the Clark Fork River, Montana, USA. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
949.
A U-series study of lavas from Kamchatka and the Aleutians: constraints on source composition and melting processes 总被引:1,自引:0,他引:1
Simon Turner Frank McDermott Chris Hawkesworth Pavel Kepezhinskas 《Contributions to Mineralogy and Petrology》1998,133(3):217-234
New data are presented for lavas from the Kamchatka Peninsula and the Aleutian arc. Radiogenic isotopes are strikingly homogeneous
in the Kamchatka lavas and although incompatible trace element ratios exhibit much greater variability, much of this appears
to result from shallow level, crystal fractionation. The data reveal little systematic across-arc change in radiogenic isotopes
or trace element ratios. The Nd and Pb isotope data overlap those for Pacific MORB and limit the amount of sediment that could
be incorporated in the mantle source region to <1% which is insufficient to account for the observed La/Ta ratios (50–68)
in the high-MgO lavas. The lack of a positive correlation between La/Ta and depth to the slab suggests that melt–wall rock
interaction was not important in controlling this ratio. Instead it is inferred that La/Ta increased during partial melting
and that DLa/DTa = 0.11–0.06, possibly due to residual amphibole. Ba, U, Sr and Pb were added to the source by an aqueous fluid from the subducting
slab and its inferred isotopic composition indicates that this fluid was derived from the altered oceanic crust. The addition
of U resulted in a large range of (238U/232Th) from 0.79–2.48 similar to that observed in the Mariana and Lesser Antilles island arcs. However, (230Th/232Th) = 0.79–2.34, and the majority of samples lie close to the equiline indicating that the time since U/Th fractionation is
generally ≥150 thousand years. The large width of the volcanic zone is assumed to reflect protracted fluid release from the
subducting slab over the depth interval 170–380 km possibly coupled with extension across the Central Kamchatka Depression.
The data from the Aleutians contrast strongly with those from Kamchatka. Radiogenic isotope data indicate that the Aleutian
lavas contain a significant recycled sedimentary component, consistent with elevated 10Be/9Be ratios. The Aleutian lavas have (230Th/232Th) = 0.79–2.34 and exhibit a significant range of U/Th disequilibria [(238U/230Th) = 0.75–1.01]. However, 10Be/9Be is positively correlated with (238U/230Th) suggesting that the 10Be signal was carried by the aqueous fluid from the slab. The U/Th disequilibria for the Aleutian lavas lie close to a 30
thousand year reference line suggesting that this fluid was released from the slab ∼30 thousand years ago similar to recent
estimates from the Lesser Antilles, Marianas, and Tonga-Kermadec island arcs from which it is inferred that fluid addition
was the trigger for partial melting. Given that the rate of convergence in Kamchatka is similar to that in the Aleutians,
Marianas and Tonga-Kermadec the inferred greater time since␣fluid release in Kamchatka requires further investigation.
Received: 24 September 1997 / Accepted: 7 July 1998 相似文献
950.
Physical, Chemical, and Mineralogical Properties of Comet 81P/Wild 2 Particles Collected by Stardust
George James Flynn 《Earth, Moon, and Planets》2008,102(1-4):447-459
NASA’s Stardust spacecraft collected dust from the coma of Comet 81P/Wild 2 by impact into aerogel capture cells or into Al-foils.
The first direct, laboratory measurement of the physical, chemical, and mineralogical properties of cometary dust grains ranging
from <10−15 to ∼10−4 g were made on this dust. Deposition of material along the entry tracks in aerogel and the presence of compound craters in
the Al-foils both indicate that many of the Wild 2 particles in the size range sampled by Stardust are weakly bound aggregates
of a diverse range of minerals. Mineralogical characterization of fragments extracted from tracks indicates that most tracks
were dominated by olivine, low-Ca pyroxene, or Fe-sulfides, although one track was dominated by refractory minerals similar
to Ca–Al inclusions in primitive meteorites. Minor mineral phases, including Cu–Fe-sulfide, Fe–Zn-sulfide, carbonate and metal
oxides, were found along some tracks. The high degree of variability of the element/Fe ratios for S, Ca, Ti, Cr, Mn, Ni, Cu,
Zn, and Ga among the 23 tracks from aerogel capture cells analyzed during Stardust Preliminary Examination is consistent with
the mineralogical variability. This indicates Wild 2 particles have widely varying compositions at the largest size analyzed
(>10 μm). Because Stardust collected particles from several jets, sampling material from different regions of the interior
of Wild 2, these particles are expected to be representative of the non-volatile component of the comet over the size range
sampled. Thus, the stream of particles associated with Comet Wild 2 contains individual grains of diverse elemental and mineralogical
compositions, some rich in Fe and S, some in Mg, and others in Ca and Al. The mean refractory element abundance pattern in
the Wild 2 particles that were examined is consistent with the CI meteorite pattern for Mg, Si, Cr, Fe, and Ni to 35%, and
for Ca, Ti and Mn to 60%, but S/Si and Fe/Si both show a statistically significant depletion from the CI values and the moderately
volatile elements Cu, Zn, Ga are enriched relative to CI. This elemental abundance pattern is similar to that in anhydrous,
porous interplanetary dust particles (IDPs), suggesting that, if Wild 2 dust preserves the original composition of the Solar
Nebula, the anhydrous, porous IDPs, not the CI meteorites, may best reflect the Solar Nebula abundances. This might be tested
by elemental composition measurements on cometary meteors. 相似文献