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71.
Sidney R. Hemming Gerard C. Bond Wallace S. Broecker Warren D. Sharp Mieczyslawa Klas-Mendelson 《Quaternary Research》2000,54(3):372
The abundance and lithic content of ice rafted detritus in glacial North Atlantic sediment cores vary abruptly on millennial time scales that have been correlated to Dansgaard-Oeschger cycles in the Greenland ice cores. There is growing evidence that various ice sheet outlets contributed increased iceberg fluxes at multiple discrete intervals, and the relative timing of iceberg discharges from different sources is important for understanding interactions between oceans and ice sheets. We present a provenance study based on 40Ar/39Ar dates of individual hornblende grains from 20 samples taken at 600 to 700 yr spacing between 10,500 and 22,000 yr B.P., from Orphan Knoll core EW9303-GGC31. Heinrich layers are characterized by a dominant Paleoproterozoic hornblende provenance consistent with published studies. A change in provenance between Heinrich events H2 and H1 indicates contributions of iceberg calving from the Newfoundland and southern Labrador margins. Between H1 and the Younger Dryas interval, Paleoproterozoic ice rafted grains remained dominant. The dominance of Baffin Island (or Greenland?) sources to the ice rafted detritus is ascribed to the retreat of the southern Laurentide ice sheet at about the time of H1—a retreat that isolated Newfoundland and southern Labrador ice from the shelf-slope boundary. 相似文献
72.
X. Hernandez Gerard Gilmore David Valls-Gabaud 《Monthly notices of the Royal Astronomical Society》2000,317(4):831-842
We use recent Hubble Space Telescope colour–magnitude diagrams of the resolved stellar populations of a sample of local dSph galaxies (Carina, Leo I, Leo II and Ursa Minor) to infer the star formation histories of these systems, SFR ( t ). Applying a new variational calculus maximum likelihood method, which includes a full Bayesian analysis and allows a non-parametric estimate of the function one is solving for, we infer the star formation histories of the systems studied. This method has the advantage of yielding an objective answer, as one need not assume a priori the form of the function one is trying to recover. The results are checked independently using Saha's W statistic. The total luminosities of the systems are used to normalize the results into physical units and derive SN type II rates. We derive the luminosity-weighted mean star formation history of this sample of galaxies. 相似文献
73.
The outer slope of the eastern reefs of Mauritius shows: from 0 to 15 m depth, a spurs-and-grooves zone of coral morphogenesis, sometimes replaced by a flagstone of coral morphogenesis (down to 20 m) or a spurs-and-grooves zone of volcanic morphogenesis; from 20 to 50 m, a volcanic flagstone with a thin coral-built layer and a buttresses-and-valleys zone of volcanic morphogenesis. The living cover of the upper part (0–15 m) is also quite similar to the cover of the leeward side. On the contrary, the volcanic basement in the lower part is poorly covered by reef-building organisms, while soft or slightly calcified algae are of widespread occurrence. 相似文献
74.
Coastal peatlands represent an interface between marine and terrestrial ecosystems; their hydrology is affected by salt and fresh water inflow alike. Previous studies on bog peat have shown that pore water salinity can have an impact on the saturated hydraulic conductivity (Ks) of peat because of chemical pore dilation effects. In this study, we aimed at quantifying the impact of higher salinities (up to 3.5% NaCl) on Ks of fen peat. Two experiments employing a constant‐head upward‐flow permeameter and differing in measurement and salinity change duration were conducted. Additionally, a third experiment to determine the impact of water salinity on the release of dissolved organic carbon (DOC) of the studied peat type was carried out. The results show a decrease of Ks with time, which does not depend on the water salinity but is differently shaped for different peat types. We assume pore clogging due to a conglomerate of physical, chemical, and biological processes, which rather depend on water movement rate and time than on water salinity. However, an increased water salinity did increase the DOC release. We conclude that salinity‐dependent behaviour of Ks is a function of peat chemistry and that for some peat types, salinity may only affect the DOC release without having a pronounced impact on water flow. 相似文献
75.
76.
D. Kesten M. Weber Ch. Haberland Ch. Janssen A. Agnon Y. Bartov I. Rabba The DESERT Group 《International Journal of Earth Sciences》2008,97(1):153-169
The left-lateral Dead Sea Transform (DST) in the Middle East is one of the largest continental strike-slip faults of the world.
The southern segment of the DST in the Arava/Araba Valley between the Dead Sea and the Red Sea, called Arava/Araba Fault (AF),
has been studied in detail in the multidisciplinary DESERT (DEad SEa Rift Transect) project. Based on these results, here,
the interpretations of multi-spectral (ASTER) satellite images and seismic reflection studies have been combined to analyse
geologic structures. Whereas satellite images reveal neotectonic activity in shallow young sediments, reflection seismic image
deep faults that are possibly inactive at present. The combination of the two methods allows putting some age constraint on
the activity of individual fault strands. Although the AF is clearly the main active fault segment of the southern DST, we
propose that it has accommodated only a limited (up to 60 km) part of the overall 105 km of sinistral plate motion since Miocene
times. There is evidence for sinistral displacement along other faults, based on geological studies, including satellite image
interpretation. Furthermore, a subsurface fault is revealed ≈4 km west of the AF on two ≈E–W running seismic reflection profiles.
Whereas these seismic data show a flower structure typical for strike-slip faults, on the satellite image this fault is not
expressed in the post-Miocene sediments, implying that it has been inactive for the last few million years. About 1 km to
the east of the AF another, now buried fault, was detected in seismic, magnetotelluric and gravity studies of DESERT. Taking
together various evidences, we suggest that at the beginning of transform motion deformation occurred in a rather wide belt,
possibly with the reactivation of older ≈N–S striking structures. Later, deformation became concentrated in the region of
today’s Arava Valley. Till ≈5 Ma ago there might have been other, now inactive fault traces in the vicinity of the present
day AF that took up lateral motion. Together with a rearrangement of plates ≈5 Ma ago, the main fault trace shifted then to
the position of today’s AF. 相似文献
77.
An experiment is described showing that an optical scintillation instrument gives reliable values of heat and momentum fluxes in the surface layer, subject to the usual restrictions of homogeneity and steady state. This instrument measures the turbulence inner scale and refractive-index structure parameter, which are used to obtain the fluxes from Monin-Obukhov similarity relationships. The instrument gives space-averaged values over a propagation path that can range in length from tens to hundreds of meters. The history of the use of optical propagation to estimate fluxes is reviewed. 相似文献
78.
E. G. Beek A. Stein L. L. F. Janssen 《Stochastic Environmental Research and Risk Assessment (SERRA)》1992,6(3):209-221
Daily precipitation amounts show spatial variation over sub-continential regions. Point measurements, representative for regions of land, have to be interpolated towards unobserved locations. In this study four days in 1984 were selected to investigate the spatial variability of daily precipitation amount in North-western Europe in relation to the meteorological conditions. Data were interpolated using Kriging. Crossvalidation was used to compare interpolated values with measured values. Large differences in the spatial structure of daily precipitation amount are obsered as a result of different meterological conditions. Stratification of the study area into a coastal, a mountainous and an interior stratum proved to be successful, reducing the Mean Squared Error of Prediction with up to 55%. 相似文献
79.
80.
Eugene E. Epstein Michael A. Janssen Jeffrey N. Cuzzi William G. Fogarty John Mottmann 《Icarus》1980,41(1):103-118
We have used 3-mm Saturn observations, obtained from 1965 through 1977 and with Jupiter as a reference, to derive a ring brightness temperature of 18 ± 8°K. Thebrightness temperature of the disk of Saturn is 156 ± 9° K. Part of the ring brightness (≈62K) may be accounted for as disk emission which is scattered from the rings; the remainder (12 ± 8° K we attributed to ring particle thermal emission. Because this thermal component brightness temperatures is so much less than the particle physical temperature, limits are placed on the mean size and composition of the ring particles. In particular, as found by others, the particles cannot be rocky, but must be either metallic or composed of extremely low-loss dielectric material such as water ice. If the particles are pure water ice, for example, then a simple slab model and a multiple-scattering model both give upper limits to the particle sizes of ≈ 1 m, a value three times smaller than previously available. The multiple-scattering model gives a particle single-scattering albedo at 3 mm of 0.83±0.13. 相似文献