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401.
Edward A. Cloutis Stephen E. Grasby Barbara L. Sherriff 《Planetary and Space Science》2010,58(4):522-537
Spectroscopic analysis of carbonate-bearing samples from a variety of terrestrial environments provides important insights into spectroscopy-based investigations of Mars designed to detect the presence of carbonate minerals. In order to better address the spectral detectability of carbonates on Mars, we examined the spectral reflectance properties of carbonates and carbonate-bearing lithologies from a variety of terrestrial environments, including impact structures (Haughton, St. Martin, Eagle Butte), landslides (Frank), quarrying operations (Hecla), carbonates affected by weathering (Haughton, East German Creek), and sulfide-sulfate-carbonate assemblages (Central Manitoba). The goal is to identify processes and environments that can affect spectroscopy-based carbonate detection, for more detailed follow-on studies. Common carbonates appear to be stable, from a spectroscopic perspective, to various tectonic processes. Iron oxides/hydroxides do not appear to significantly affect spectral detectability of carbonates, as the spectrum-altering effects of these phases are largely restricted to the region below ∼1 μm, while useful carbonate absorption bands occur longward of ∼1.8 μm. Carbonate detection and characterization in the 0.35-2.5-μm region is largely restricted to a single absorption feature in the 2.3-μm region, which can be problematic for robust carbonate identification. While tectonic processes and iron oxide/hydroxide staining do not appear to significantly impair carbonate detection based on the 2.3-μm region absorption band, a number of other factors can affect carbonate detection. These include the fact that this absorption band is weak compared to many other minerals, a number of other minerals also exhibit absorption bands in this wavelength region (leading to possible misidentifications), and that even small abundances of minerals that absorb strongly in this region will reduce the strength of the carbonate absorption band. Identifying the nature of accessory minerals associated with carbonates can be used to constrain possible formation environments. Ongoing research at carbonate-bearing terrestrial analogue sites will continue to provide new insights into the occurrence and detection of carbonates on Mars. 相似文献
402.
Nate Bastian Mark Gieles Barbara Ercolano Rob Gutermuth 《Monthly notices of the Royal Astronomical Society》2009,392(2):868-878
We present an analysis of the spatial distribution of various stellar populations within the Large Magellanic Cloud (LMC). We combine mid-infrared selected young stellar objects, optically selected samples with mean ages between ∼9 and ∼1000 Myr and existing stellar cluster catalogues to investigate how stellar structures form and evolve within the LMC. For the analysis we use Fractured Minimum Spanning Trees, the statistical Q parameter and the two-point correlation function. Restricting our analysis to young massive (OB) stars, we confirm our results obtained for M33, namely that the luminosity function of the groups is well described by a power law with index −2, and that there is no characteristic length-scale of star-forming regions. We find that stars in the LMC are born with a large amount of substructure, consistent with a two-dimensional fractal distribution with dimension and evolve towards a uniform distribution on a time-scale of ∼175 Myr. This is comparable to the crossing time of the galaxy, and we suggest that stellar structure, regardless of spatial scale, will be eliminated in a crossing time. This may explain the smooth distribution of stars in massive/dense young clusters in the Galaxy, while other, less massive, clusters still display large amounts of structure at similar ages. By comparing the stellar and star cluster distributions and evolving time-scales, we show that infant mortality of clusters (or 'popping clusters') has a negligible influence on the galactic structure. Finally, we quantify the influence of the elongation, differential extinction and contamination of a population on the measured Q value. 相似文献
403.
Barbara Lo Faro Pierluigi Monaco Eros Vanzella Fabio Fontanot Laura Silva Stefano Cristiani 《Monthly notices of the Royal Astronomical Society》2009,399(2):827-841
It has recently been shown that galaxy formation models within the Λ cold dark matter cosmology predict that, compared to the observed population, small galaxies (with stellar masses <1011 M⊙ ) form too early, are too passive since z ∼ 3 and host too old stellar populations at z = 0 . We then expect an overproduction of small galaxies at z ≳ 4 that should be visible as an excess of faint Lyman-break galaxies. To check whether this excess is present, we use the morgana galaxy formation model and grasil spectrophotometric + radiative transfer code to generate mock catalogues of deep fields observed with Hubble Space Telescope Advanced Camera for Surveys. We add observational noise and the effect of Lyman α emission, and perform colour–colour selections to identify Lyman-break galaxies. The resulting mock candidates have plausible properties that closely resemble those of observed galaxies. We are able to reproduce the evolution of the bright tail of the luminosity function of Lyman-break galaxies (with a possible underestimate of the number of the brightest i -dropouts), but uncertainties and degeneracies in dust absorption parameters do not allow to give strong constraints to the model. Besides, our model shows a clear excess with respect to observations of faint Lyman-break galaxies, especially of z 850 ∼ 27 V -dropouts at z ∼ 5 . We quantify the properties of these 'excess' galaxies and discuss the implications: these galaxies are hosted in dark matter haloes with circular velocities in excess of 100 km s−1 , and their suppression may require a deep rethinking of stellar feedback processes taking place in galaxy formation. 相似文献
404.
Guy R. Holdgate Brian McGowran Tom Fromhold Barbara E. Wagstaff Stephen J. Gallagher Malcolm W. Wallace Ian R.K. Sluiter Michael Whitelaw 《Global and Planetary Change》2009,65(1-2):89-103
The carbon-isotope and palynological record through 580 m thick almost continuous brown coal in southeast Australia's Gippsland Basin is a relatively comprehensive southern hemisphere Middle Eocene to Middle Miocene record for terrestrial change. The carbon isotope δ13Ccoal values of these coals range from ? 27.7‰ to ? 23.2. This isotopic variability follows gymnosperm/angiosperm fluctuations, where higher ratios coincide with heavier δ13C values. There is also long-term variability in carbon isotopes through time. From the Eocene greenhouse world of high gymnosperm-heavier δ13Ccoal values, there is a progressive shift to lighter δ13Ccoal values that follows the earliest (Oi1?) glacial events around 33 Ma (Early Oligocene). The overlying Oligocene–Early Miocene brown coals have lower gymnosperm abundance, associated with increased %Nothofagus (angiosperm), and lightening of isotopes during Oligocene cooler conditions.The Miocene palynological and carbon-isotope record supports a continuation to the Oligocene trends until around the late Early Miocene (circa 19 Ma) when a warming commenced, followed by an even stronger isotope shift around 16 Ma that peaked in the Middle Miocene when higher gymnosperm abundance and heavier isotopes prevailed. The cycle between the two major warm peaks of Middle Eocene and Middle Miocene was circa 30 Ma long. This change corresponds to a fall in inferred pCO2 levels for the same period. The Gippsland data suggest a link between gymnosperm abundance, long-term plant δ13C composition, climatic change, and atmospheric pCO2. Climatic deterioration in the Late Miocene terminated peat accumulation in the Gippsland Basin and no further significant coals formed in southeast Australia.The poor correspondence between this terrestrial isotope data and the marine isotope record is explained by the dominant control on δ13C by the gymnosperm/angiosperm abundance, although in turn this poor correspondence may reflect palaeoclimate control. From the brown coal seam dating, the coal appears to have accumulated during a considerable part of the allocated 30 Ma Cenozoic time period. These brown coal carbon isotope and palynological data appear to record a more gradual atmospheric carbon isotope change compared to the marine record. 相似文献
405.
Thorsten Bauersachs Barbara Kremer Stefan Schouten Jaap S. Sinninghe Damsté 《Organic Geochemistry》2009,40(2):149-157
Early Silurian cherts from the Holy Cross and Bardzkie Mountains (Poland) contain abundant microfossils morphologically resembling contemporary cyanobacteria. Most of the organic matter preserved in the cherts is highly mature and extensively degraded because of biological decomposition and progressive thermal alteration. These processes may have changed the original morphology of the deposited microbial remains, so the microfossil origin could be easily misinterpreted. The cherts were therefore examined using organic geochemical and stable isotope techniques to provide support for the presence of cyanobacterial remains. The nitrogen isotopic composition of bulk sediments and extractable organic matter ranged from +0.1‰ to ?2.2‰ and from +1.8‰ to ?1.7‰, respectively. The δ15N values are thus in good agreement with a contribution of diazotrophic cyanobacteria for both locations. Biomarkers in the Holy Cross Mts. cherts included mid-chain branched monomethyl alkanes, indicative of a cyanobacterial contribution. However, molecular fossils of a cyanobacterial origin were not detected in the Bardzkie Mts. cherts, most likely because of the greater maturity than those from the Holy Cross Mts. 相似文献
406.
Fifty soil samples collected from agricultural land in four regions of Poland with different anthropopressure were analysed for their content of 16PAHs by GC/MS. The regions correspond to Polish administrative units (voievodeships): Podlaskie and Lubelskie are situated in the rural East part of the country and more industrialised Slaskie and Dolnoslaskie voievodeships – in the South-West part. Basic physicochemical properties as well as the content of selected potentially harmful metals (Pb and Zn) were included in the soil analysis. Overall accumulation of Σ16PAHs in the upper soil layer was within the range 73–1800 μg kg−1 with a geometric mean (GM) of 252 μg kg−1, while the mean benzo(a)pyrene (BaP) load was 20 μg kg−1. This corresponds with data for other European countries. Carcinogenic compounds contributed nearly in 50% to the total PAHs loads. In uncontaminated rural regions the mean Σ16PAHs and BaP contents were 113–159 μg kg−1 and 11–13 μg kg−1, respectively. Regional conditions strongly influenced the accumulation of PAHs ?4-rings, which were highly dependent (over 95%) on local anthropopressure expressed as dust and 4PAHs emission indexes. Soil acidity was the main soil parameter related to the accumulation of higher molecular weight PAHs in soils. In more contaminated regions a significant link between soil OM and PAH loads was noted. The same regions were characterised by associations between PAHs and potentially harmful metals implying common sources of pollution. Those relationships were not observed in the uncontaminated part of the country. The lower molecular weight PAHs contributed to a smaller extent (about 20%) to the total PAHs content in soils, and were less affected by anthropogenic factors. 相似文献
407.
Tornado shelter-seeking behavior and tornado shelter options among mobile home residents in the United States 总被引:2,自引:2,他引:0
Residents of 401 mobile homes in Georgia, Mississippi, Illinois, and Oklahoma were surveyed after they heard a tornado warning.
Most residents (69%) did not seek shelter during the warning. Half of those who sought shelter went to the frame house of
a friend, neighbor, or relative, and 25% of those sought shelter in a basement or underground shelter. Some of the places
where residents sought shelter were of dubious quality, such as their own mobile home, another mobile home, or in an out-building.
Twenty-one percent of mobile home residents believed that they had a basement or underground shelter available as shelter
during a tornado warning, and about half of those said they would drive to the shelter. Residents said they would drive if
the shelter was more than 200 m away. Fifteen percent actually had a basement or underground shelter suitable as shelter within
200 m of their mobile home, but only 43% of the residents would use those shelters. The most common reason cited for not using
the shelters was that they did not know the people who lived there. Likewise, a frame house or other sturdy building was within
200 m of 58% of the mobile homes, but only 35% of the residents stated they would use those houses for shelter. Thirty-one
percent of mobile home residents had a ditch that was at least 0.5 m deep within 200 m of the mobile home. However, 44% of
these ditches had utility lines overhead, 23% had water in them, and 20% had trees overhead. The limited tornado shelter options
among mobile home residents in the United States needs to be incorporated into safety instructions so that residents without
nearby shelter are allowed to drive to safer shelter. 相似文献
408.
Mixed land use and obesity: an empirical comparison of alternative land use measures and geographic scales 总被引:1,自引:0,他引:1
Yamada I Brown BB Smith KR Zick CD Kowaleski-Jones L Fan JX 《The Professional geographer》2012,64(2):157-177
Obesity is a growing epidemic in the United States. Walkable neighborhoods, characterized as having the 3Ds of walkability (population Density, land use Diversity, and pedestrian-friendly Design), have been identified as a potentially promising factor to prevent obesity for their residents. Past studies examining the relationship between obesity and walkability vary in geographic scales of neighborhood definitions and methods of measuring the 3Ds. To better understand potential influences of these sometimes arbitrary choices, we test how four types of alternative measures of land use diversity measured at three geographic scales relate to body mass index for 4960 Salt Lake County adults. Generalized estimation equation models demonstrate that optimal diversity measures differed by gender and geographic scale and that integrating walkability measures at different scales improved the overall performance of models. 相似文献
409.
Sub-bottom profiling and coring were undertaken at eight sub-basins along the lower French River and at five small lakes near
North Bay, Ontario, to collect stratigraphical and chronological evidence to investigate whether lakes occupying the Huron–Georgian
basins during the early- to mid-Holocene became hydrologically closed. All of the coring sites are located within the route
of the North Bay outlet that carried outflow from the upper Great Lakes during this period. Sand beds containing organic detritus
are present within five cores from Muskrat, Crombie and Deep bays that otherwise are composed of glaciolacustrine rhythmites
or fine-grained lacustrine deposits. These sand beds are interpreted to represent intervals when water levels within the sub-basins
were lower than present, based on chronology, sediment texture, and macrofossil assemblages. It is inferred that the water
surface in the Huron–Georgian basins fell below the level of the Dalles Rapids sill isolating the lower French River sub-basins
from the large lake. A core from Depensier Lake, North Bay, contains an organic-rich sand interval within a thicker sand unit
barren of organic materials. Macrofossils within this organic-rich interval are interpreted to be evidence of substantially
diminished flow through the North Bay outlet channel. Radiocarbon dates of terrestrial macrofossils provide correlation of
the sand beds between the French River cores as well as with the organic-rich sand in the Depensier Lake core. The possibility
that the sand beds in the French River cores represent flood deposits rather than evidence of hydrologically closed conditions
is considered, but rejected, based on the occurrence of multiple peaty layers and the record of shallow water conditions inferred
from macrofossils within the upper sand bed of core MUS1, Muskrat Bay, in combination with the evidence of quiescent depositional
conditions from similarly aged macrofossils in the core from Depensier Lake. Eight radiocarbon dates from the French River
cores are incorporated into an elevation-age plot of paleo-indicators of water levels in the Huron–Georgian basins, using
additional data from the literature. This plot and stratigraphic evidence from the Muskrat Bay cores indicates that separate
closed-basin intervals occurred between 9.0 and 8.4, and 9.5 and 9.3 ka cal BP (~ 8.1 and 7.6, and ~ 8.5 and 8.3 ka BP).
The occurrence of these two closed-basin intervals between 9.6 and 8.4 ka cal BP (~ 8.7 and 7.6 ka BP) implies that run-off
derived exclusively from precipitation within the non-glaciated portions of the upper Great Lakes drainage basins was likely
insufficient at this time to support an open-basin lake hydrology during the contemporary climate, which was colder and drier
than present, without being supplemented from glacial Lake Agassiz overflow and/or Laurentide Ice Sheet meltwater. 相似文献
410.