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81.
Ricardo Conceição Hugo Gonçalves Silva Alec Bennett Rui Salgado Daniele Bortoli Maria João Costa Manuel Collares Pereira 《Boundary-Layer Meteorology》2018,166(1):69-81
The spectral response of atmospheric electric potential gradient gives important information about phenomena affecting this gradient at characteristic time scales ranging from years (e.g., solar modulation) to fractions of a second (e.g., turbulence). While long-term time scales have been exhaustively explored, short-term scales have received less attention. At such frequencies, space-charge transport inside the planetary boundary layer becomes a sizeable contribution to the potential gradient variability. For the first time, co-located (Évora, Portugal) measurements of boundary-layer backscatter profiles and the 100-Hz potential gradient are reported. Five campaign days are analyzed, providing evidence for a relation between high-frequency response of the potential gradient and strong dry convection. 相似文献
82.
C. R. L. Friend A. P. Nutman V. C. Bennett M. D. Norman 《Contributions to Mineralogy and Petrology》2008,155(2):229-246
Modern chemical sediments display a distinctive rare earth element + yttrium (REE + Y) pattern involving depleted LREE, positive
La/La*SN, Eu/Eu*SN, and YSN anomalies (SN = shale normalised) that is related to precipitation from circumneutral to high pH waters with solution complexation
of the REEs dominated by carbonate ions. This is often interpreted as reflecting precipitation from surface waters (usually
marine). The oldest broadly accepted chemical sediments are c. 3,700 Ma amphibolite facies banded iron-formation (BIF) units
in the Isua supracrustal belt, Greenland. Isua BIFs, including the BIF international reference material IF-G are generally
considered to be seawater precipitates, and display these REE + Y patterns (Bolhar et al. in Earth Planet Sci Lett 222:43–60,
2004). Greenland Eoarchaean BIF metamorphosed up to granulite facies from several localities in the vicinity of Akilia (island),
display REE + Y patterns identical to Isua BIF, consistent with an origin by chemical sedimentation from seawater and a paucity
of clastic input. Furthermore, the much-debated magnetite-bearing siliceous unit of “earliest life” rocks (sample G91/26)
from Akilia has the same REE + Y pattern. This suggests that sample G91/26 is also a chemical sediment, contrary to previous
assertions (Bolhar et al. in Earth Planet Sci Lett 222:43–60, 2004), and including suggestions that the Akilia unit containing G91/26 consists entirely of silica-penetrated, metasomatised,
mafic rock (Fedo and Whitehouse 2002a). Integration of our trace element data with those of Bolhar et al. (Earth Planet Sci Lett 222:43–60, 2004) demonstrates that Eoarchaean siliceous rocks in Greenland, with ages from 3.6 to 3.85 Ga, have diverse trace element signatures.
There are now geographically-dispersed, widespread examples with Isua BIF-like REE + Y signatures, that are interpreted as
chemically unaltered, albeit metamorphosed, chemical sediments. Other samples retain remnants of LREE depletion but are beginning
to lose the distinct La, Eu and Y positive anomalies and are interpreted as metasomatised chemical sediments. Finally there
are some siliceous samples with completely different trace element patterns that are interpreted as rocks of non-sedimentary
origin, and include metasomatised mafic rocks. The positive La/La*SN, Eu/Eu*SN and YSN anomalies found in Isua BIFs and other Eoarchaean Greenland samples, such as G91/26 from Akilia, suggests that the processes
of carbonate ion complexation controlling the REE − Y patterns were already established in the hydrosphere at the start of
the sedimentary record 3,600–3,850 Ma ago. This is in accord with the presence of Eoarchaean siderite-bearing marbles of sedimentary
origin, and suggests that CO2 may have been a significant greenhouse gas at that time. 相似文献
83.
Kevin G. Taylor Karen A. Hudson-Edwards Andrew J. Bennett Vladimir Vishnyakov 《Applied Geochemistry》2008
The sediments in the Salford Quays, a heavily-modified urban water body, contain high levels of organic matter, Fe, Zn and nutrients as a result of past contaminant inputs. Vivianite [Fe3(PO4)2 · 8H2O] has been observed to have precipitated within these sediments during early diagenesis as a result of the release of Fe and P to porewaters. These mineral grains are small (<100 μm) and micron-scale analysis techniques (SEM, electron microprobe, μ-EXAFS, μ-XANES and Raman) have been applied in this study to obtain information upon the structure of this vivianite and the nature of Zn uptake in the mineral. Petrographic observations, and elemental, X-ray diffraction and Raman spectroscopic analysis confirms the presence of vivianite. EXAFS model fitting of the FeK-edge spectra for individual vivianite grains produces Fe–O and Fe–P co-ordination numbers and bond lengths consistent with previous structural studies of vivianite (4O atoms at 1.99–2.05 Å; 2P atoms at 3.17–3.25 Å). One analysed grain displays evidence of a significant Fe3+ component, which is interpreted to have resulted from oxidation during sample handling and/or analysis. EXAFS modelling of the Zn K-edge data, together with linear combination XANES fitting of model compounds, indicates that Zn may be incorporated into the crystal structure of vivianite (4O atoms at 1.97 Å; 2P atoms at 3.17 Å). Low levels of Zn sulphate or Zn-sorbed goethite are also indicated from linear combination XANES fitting and to a limited extent, the EXAFS fitting, the origin of which may either be an oxidation artifact or the inclusion of Zn sulphate into the vivianite grains during precipitation. This study confirms that early diagenetic vivianite may act as a sink for Zn, and potentially other contaminants (e.g. As) during its formation and, therefore, forms an important component of metal cycling in contaminated sediments and waters. Furthermore, for the case of Zn, the EXAFS fits for Zn phosphate suggest this uptake is structural and not via surface adsorption. 相似文献
84.
85.
Electrical conductivity measurements provide useful geotechnical information, particularly porosity data, and are well suited
for use in conjunction with acoustic profiling techniques. A proposed conductivity measurement system for surveying the seabed
is characterized by a towed inductive multifrequency probe operating underwater. It is integrated with a shipboard data acquisition
and processing system. 相似文献
86.
Douglas N. Lambert Peter A. Rona Richard H. Bennett John W. Kofoed 《Geo-Marine Letters》1981,1(1):69-72
Two pendulum-type inclinometers, providing accurate direct measurements of seafloor gradient from a submersible, were developed
and field tested. A manipulator-held inclinometer was used in Great Abaco Canyon to measure the angle of repose of taluslike
carbonate deposits on the canyon wall. A submersiblemounted inclinometer continuously measured, and a camera recorded, variations
in seafloor gradient on the lower continental slope off Cape Hatteras. This area has been previously identified seismically
as containing rotational slump blocks. Slump deposits were found to have slopes of up to 10° while their associated slump
scarps had gradients between 30 and 50°. 相似文献
87.
88.
89.
Andrew F. Bennett 《地球物理与天体物理流体动力学》2013,107(1):113-142
AbstractThe flow in a mechanically driven thin barotropic rotating fluid system is analysed. The linear theory of Baker and Robinson (1969) is modified and extended into the non-linear regime.An internal parameter, the “local Rossby number”, is indicative of the onset of nonlinear effects. If this parameter is 0(1) then inertial effects are as important as Coriolis accelerations in the interior of the transport-turning western boundary layer and both of its Ekman layers. The inertial effects in the Ekman layers, ignored in previous explorations of non-linear wind driven oceanic circulation, are retained here and calculated using an approximation of the Oseen type. The circulation problem is reduced to a system of scalar equations in only two independent variables; the system is valid for non-small local Rossby number provided only that the approximate total vorticity is positive.To complete the solution for small Rossby number a boundary condition for the inertially induced transport is needed. It is found by examining the dynamics controlling this additional transport from the western boundary layer as the transport recirculates through the rest of the ocean basin. The strong constraint of total recirculation within the western boundary layer (zero net inertial transport) is derived.The calculated primary inertial effects are in agreement with the observations of the laboratory model of Baker and Robinson (1969).The analysis indicates the extent to which three-dimensional non-linear circulation can be reduced to a two dimensional problem. 相似文献
90.
Climate moderates potential shifts in streamflow from changes in pinyon‐juniper woodland cover across the western U.S. 下载免费PDF全文
Pinyon‐juniper (PJ) cover has increased up to 10‐fold in many parts of the western U.S. in the last 140+ years. The impacts of these changes on streamflows are unclear and may vary depending on the intra‐annual distribution and amount of precipitation. Given the importance of streamflow in the western U.S., it is important to understand how shifts in PJ woodland cover may produce changes in streamflow across the region's diverse hydroclimates. To this end, we simulated the land surface water balance with contrasting woodland and grassland cover with the Hydrologiska Byråns Vattenbalansavdelning (HBV) model at a 4‐km resolution across the distribution of PJ woodlands in the western U.S. We used shifts in evapotranspiration (ET) between woodland and grassland cover as a proxy for potential changes in streamflows. Comparison of HBV model results with paired catchment studies indicated the model reasonably simulated annual decreases in ET with changes from woodland to grassland cover. For the northern and western ecoregions of the PJ distribution in the western U.S. where precipitation predominantly occurs in the winter, HBV simulated a 25 mm (37%) annual decrease in ET with conversion to grassland from woodland. Conversely, in southern ecoregions of PJ distribution with prominent summer monsoons, annual differences in ET were only 6 mm (19%). Our results suggest that only 29% of the PJ distribution, compared to an estimated 45% based on precipitation amount alone, has the potential for meaningful increases in streamflow with land cover change from woodland to grassland. 相似文献