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81.
Saturn’s satellite Dione is becoming an increasingly important object in the outer Solar System, as evidence for its current activity accumulates. Infrared observations of the surface can provide clues to the history of the body and currently active processes. Using data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS), we perform three sets of analyses that are sensitive to the ice state, temperature, thermal history, grain size and composition of surface ice. These are calculation of a “crystallinity factor”, spectral ratios and water ice band depths. In our analysis, we focus on the dichotomy between the wispy and dark terrain on Dione’s trailing hemisphere, to better understand the source of the different materials and their current properties. Our results suggest two different scenarios: (1) the ice from the wispy region has a higher crystallinity and water ice content than the dark region or (2) the wispy region contains larger grains. Both of these models imply recent geologic activity on Dione. 相似文献
82.
83.
A. Georgakakis A. L. Coil E. S. Laird R. L. Griffith K. Nandra J. M. Lotz C. M. Pierce M. C. Cooper J. A. Newman A. M. Koekemoer 《Monthly notices of the Royal Astronomical Society》2009,397(2):623-633
We use morphological information of X-ray selected active galactic nuclei (AGN) hosts to set limits on the fraction of the accretion density of the Universe at z ≈ 1 that is not likely to be associated with major mergers. Deep X-ray observations are combined with high-resolution optical data from the Hubble Space Telescope in the All-wavelength Extended Groth strip International Survey, Great Observatories Origins Deep Survey (GOODS) North and GOODS South fields to explore the morphological breakdown of X-ray sources in the redshift interval 0.5 < z < 1.3 . The sample is split into discs, early-type bulge-dominated galaxies, peculiar systems and point sources in which the nuclear source outshines the host galaxy. The X-ray luminosity function and luminosity density of AGN at z ≈ 1 are then calculated as a function of morphological type. We find that disc-dominated hosts contribute 30 ± 9 per cent to the total AGN space density and 23 ± 6 per cent to the luminosity density at z ≈ 1 . We argue that AGN in disc galaxies are most likely fuelled not by major merger events but by minor interactions or internal instabilities. We find evidence that these mechanisms may be more efficient in producing luminous AGN compared to predictions for the stochastic fuelling of massive black holes in disc galaxies. 相似文献
84.
Kappler A Johnson CM Crosby HA Beard BL Newman DK 《Geochimica et cosmochimica acta》2010,74(10):2826-1852
Iron isotope fractionations produced during chemical and biological Fe(II) oxidation are sensitive to the proportions and nature of dissolved and solid-phase Fe species present, as well as the extent of isotopic exchange between precipitates and aqueous Fe. Iron isotopes therefore potentially constrain the mechanisms and pathways of Fe redox transformations in modern and ancient environments. In the present study, we followed in batch experiments Fe isotope fractionations between Fe(II)aq and Fe(III) oxide/hydroxide precipitates produced by the Fe(III) mineral encrusting, nitrate-reducing, Fe(II)-oxidizing Acidovorax sp. strain BoFeN1. Isotopic fractionation in 56Fe/54Fe approached that expected for equilibrium conditions, assuming an equilibrium Δ56FeFe(OH)3-Fe(II)aq fractionation factor of +3.0‰. Previous studies have shown that Fe(II) oxidation by this Acidovorax strain occurs in the periplasm, and we propose that Fe isotope equilibrium is maintained through redox cycling via coupled electron and atom exchange between Fe(II)aq and Fe(III) precipitates in the contained environment of the periplasm. In addition to the apparent equilibrium isotopic fractionation, these experiments also record the kinetic effects of initial rapid oxidation, and possible phase transformations of the Fe(III) precipitates. Attainment of Fe isotope equilibrium between Fe(III) oxide/hydroxide precipitates and Fe(II)aq by neutrophilic, Fe(II)-oxidizing bacteria or through abiologic Fe(II)aq oxidation is generally not expected or observed, because the poor solubility of their metabolic product, i.e. Fe(III), usually leads to rapid precipitation of Fe(III) minerals, and hence expression of a kinetic fractionation upon precipitation; in the absence of redox cycling between Fe(II)aq and precipitate, kinetic isotope fractionations are likely to be retained. These results highlight the distinct Fe isotope fractionations that are produced by different pathways of biological and abiological Fe(II) oxidation. 相似文献
85.
Elie Verleyen Dominic A. Hodgson Koen Sabbe Holger Cremer Steven D. Emslie John Gibson Brenda Hall Satoshi Imura Sakae Kudoh Gareth J. Marshall Andrew McMinn Martin Melles Louise Newman Donna Roberts Steve J. Roberts Shiv M. Singh Mieke Sterken Ines Tavernier Sergey Verkulich Evelien Van de Vyver Wim Van Nieuwenhuyze Bernd Wagner Wim Vyverman 《Earth》2011,104(4):199-212
We review the post-glacial climate variability along the East Antarctic coastline using terrestrial and shallow marine geological records and compare these reconstructions with data from elsewhere. Nearly all East Antarctic records show a near-synchronous Early Holocene climate optimum (11.5–9 ka BP), coinciding with the deglaciation of currently ice-free regions and the optimum recorded in Antarctic ice and marine sediment cores. Shallow marine and coastal terrestrial climate anomalies appear to be out of phase after the Early Holocene warm period, and show complex regional patterns, but an overall trend of cooling in the terrestrial records. A Mid to Late Holocene warm period is present in many East Antarctic lake and shallow coastal marine records. Although there are some differences in the regional timing of this warm period, it typically occurs somewhere between 4.7 and 1 ka BP, which overlaps with a similar optimum found in Antarctic Peninsula terrestrial records. The differences in the timing of these sometimes abrupt warm events in different records and regions points to a number of mechanisms that we have yet to identify. Nearly all records show a neoglacial cooling from 2 ka BP onwards. There is no evidence along the East Antarctic coastline for an equivalent to the Northern Hemisphere Medieval Warm Period and there is only weak circumstantial evidence in a few places for a cool event crudely equivalent in time to the Northern Hemisphere's Little Ice Age. There is a need for well-dated, high resolution climate records in coastal East Antarctica and particularly in Terre Adélie, Dronning Maud Land and Enderby Land to fully understand the regional climate anomalies, the disparity between marine and terrestrial records, and to determine the significance of the heterogeneous temperature trends being measured in the Antarctic today. 相似文献
86.
Future development and climate change pose potentially serious threats to estuarine fish populations around the world. We
examined how habitat suitability for delta smelt (Hypomesus transpacificus), a state and federally protected species, might be affected by changes in outflow in the San Francisco Estuary due to future
development and climate change. Forty years of sampling data collected during fall from 1967 to 2008 were examined to define
abiotic habitat suitability for delta smelt as a function of salinity and water transparency, and to describe long-term trends
in habitat conditions. The annual habitat index we developed, which incorporated both quantity and quality of habitat, decreased
by 78% over the study period. Future habitat index values under seven different development and climate change scenarios,
representing a range of drier and wetter possibilities, were predicted using a model which related estuarine outflow to the
habitat index. The results suggested that each of the scenarios would generally lead to further declines in delta smelt habitat
across all water year types. Recovery targets for delta smelt will be difficult to attain if the modeled habitat conditions
are realized. 相似文献
87.
A general circulation model is used to evaluate changes to the circulation and dust transport in the martian atmosphere for a range of past orbital conditions. A dust transport scheme, including parameterized dust lifting, is incorporated within the model to enable passive or radiatively active dust transport. The focus is on changes which relate to surface features, as these may potentially be verified by observations. Obliquity variations have the largest impact, as they affect the latitudinal distribution of solar heating. At low obliquities permanent CO2 ice caps form at both poles, lowering mean surface pressures. At higher obliquities, solar insolation peaks at higher summer latitudes near solstice, producing a stronger, broader meridional circulation and a larger seasonal CO2 ice cap in winter. Near-surface winds associated with the main meridional circulation intensify and extend polewards, with changes in cap edge position also affecting the flow. Hence the model predicts significant changes in surface wind directions as well as magnitudes. Dust lifting by wind stress increases with obliquity as the meridional circulation and associated near-surface winds strengthen. If active dust transport is used, then lifting rates increase further in response to the larger atmospheric dust opacities (hence circulation) produced. Dust lifting by dust devils increases more gradually with obliquity, having a weaker link to the meridional circulation. The primary effect of varying eccentricity is to change the impact of varying the areocentric longitude of perihelion, l, which determines when the solar forcing is strongest. The atmospheric circulation is stronger when l aligns with solstice rather than equinox, and there is also a bias from the martian topography, resulting in the strongest circulations when perihelion is at northern winter solstice. Net dust accumulation depends on both lifting and deposition. Dust which has been well mixed within the atmosphere is deposited preferentially over high topography. For wind stress lifting, the combination produces peak net removal within western boundary currents and southern midlatitude bands, and net accumulation concentrated in Arabia and Tharsis. In active dust transport experiments, dust is also scoured from northern midlatitudes during winter, further confining peak accumulation to equatorial regions. As obliquity increases, polar accumulation rates increase for wind stress lifting and are largest for high eccentricities when perihelion occurs during northern winter. For dust devil lifting, polar accumulation rates increase (though less rapidly) with obliquity above o=25°, but increase with decreasing obliquity below this, thus polar dust accumulation at low obliquities may be increasingly due to dust lifted by dust devils. For all cases discussed, the pole receiving most dust shifts from north to south as obliquity is increased. 相似文献
88.
The DEEP2 Galaxy Redshift Survey: the relationship between galaxy properties and environment at z∼ 1
Michael C. Cooper Jeffrey A. Newman † Darren J. Croton Benjamin J. Weiner Christopher N. A. Willmer Brian F. Gerke Darren S. Madgwick † S. M. Faber Marc Davis Alison L. Coil † Douglas P. Finkbeiner Puragra Guhathakurta David C. Koo 《Monthly notices of the Royal Astronomical Society》2006,370(1):198-212
89.
Leo Polansky Ken B. Newman Matthew L. Nobriga Lara Mitchell 《Estuaries and Coasts》2018,41(2):572-581
Understanding the distribution and abundance of organisms can be exceedingly difficult for pelagic fish species that live in estuarine environments. This is particularly so for fish that cannot be readily marked and released or otherwise tracked, such as the diminutive delta smelt, Hypomesus transpacificus, endemic to the San Francisco Estuary. The environmental factors that influence their distribution operate at multiple scales, from daily tidal cycles and local perceptual fields to seasonal and annual changes in dominant environmental gradients spanning the entire San Francisco Estuary. To quantify scale-specific patterns and factors shaping the spatiotemporal abundance dynamics of adult delta smelt, we fit a suite of models to an extensive, spatially resolved, catch survey time series from 13 annual cohorts. The best model included cohort-specific abundance indicators and daily mortality rates, a regional spatial adjustment, and haul-specific environmental conditions. The regional adjustment identified several density hot spots that were persistent across cohorts. While this model did include local environmental conditions, the gain in explained variation was relatively slight compared to that explained by the regional adjustment. Total abundance estimates were derived by multiplying habitat volume by catch density (design-based) and modeled density (model-based), with both showing severe declines in the population over the time period studied. The design-based approaches had lower uncertainty but potentially higher bias. We discuss the implications of our results for advancing the science and improving management of delta smelt, and future data collection needs. 相似文献