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51.
The Fosdick Mountains migmatite–granite complex in West Antarctica records episodes of crustal melting and plutonism in Devonian–Carboniferous time that acted to transform transitional crust, dominated by immature oceanic turbidites of the accretionary margin of East Gondwana, into stable continental crust. West Antarctica, New Zealand and Australia originated as contiguous parts of this margin, according to plate reconstructions, however, detailed correlations are uncertain due to a lack of isotopic and geochronological data. Our study of the mid-crustal exposures of the Fosdick range uses U–Pb SHRIMP zircon geochronology to examine the tectonic environment and timing for Paleozoic magmatism in West Antarctica, and to assess a correlation with the better known Lachlan Orogen of eastern Australia and Western Province of New Zealand.NNE–SSW to NE–SW contraction occurred in West Antarctica in early Paleozoic time, and is expressed by km-scale folds developed both in lower crustal metasedimentary migmatite gneisses of the Fosdick Mountains and in low greenschist-grade turbidite successions of the upper crust, present in neighboring ranges. The metasedimentary rocks and structures were intruded by calc-alkaline, I-type plutons attributed to arc magmatism along the convergent East Gondwana margin. Within the Fosdick Mountains, the intrusions form a layered plutonic complex at lower structural levels and discrete plutons at upper levels. Dilational structures that host anatectic granite overprint plutonic layering and migmatitic foliation. They exhibit systematic geometries indicative of NNE–SSW stretching, parallel to a first-generation mineral lineation. New U–Pb SHRIMP zircon ages for granodiorite and porphyritic monzogranite plutons, and for leucogranites that occupy shear bands and other mesoscopic-scale structural sites, define an interval of 370 to 355 Ma for plutonism and migmatization.Paleozoic plutonism in West Antarctica postdates magmatism in the western Lachlan Orogen of Australia, but it coincides with that in the central part of the Lachlan Orogen and with the rapid main phase of emplacement of the Karamea Batholith of the Western Province, New Zealand. Emplaced within a 15 to 20 million year interval, the Paleozoic granitoids of the Fosdick Mountains are a product of subduction-related plutonism associated with high temperature metamorphism and crustal melting. The presence of anatectic granites within extensional structures is a possible indication of alternating strain states (‘tectonic switching’) in a supra-subduction zone setting characterized by thin crust and high heat flow along the Devonian–Carboniferous accretionary margin of East Gondwana. 相似文献
52.
David Hodgkinson Hakim Benabderrahmane Mark Elert Aimo Hautojärvi Jan-Olof Selroos Yasuharu Tanaka Masahiro Uchida 《Hydrogeology Journal》2009,17(5):1035-1049
An overview is presented of a 4-year study by the Äspö Task Force on Modelling of Groundwater Flow and Transport of Solutes, whose primary aim was to build a bridge between the approaches used for site characterisation (SC) and performance assessment (PA) associated with nuclear waste repositories. Eleven modelling teams representing six national radioactive waste organisations participated in eight modelling exercises whose objectives were: to assess simplifications used in PA models; to determine how, and to what extent, experimental tracer and flow experiments can constrain the range of parameters used in PA models; to support the design of SC programmes to assure that the results have optimal value for PA calculations; and to improve the understanding of site-specific flow and transport behaviour at different scales using SC models. The modelling tasks were concerned with flow and transport through single and multiple near-planar features on SC and PA timescales, including the diffusion of solutes into multiple immobile zones adjacent to fracture surfaces. In general, tracer tests provide only limited quantitative constraints on retention parameter values relevant to PA but nevertheless provide insight about the flow and transport processes, which is a key element of the bridge between SC and PA. 相似文献
53.
U-Pb isotopic thermochronometry of rutile, apatite and titanite from kimberlite-borne lower crustal granulite xenoliths has been used to constrain the thermal evolution of Archean cratonic and Proterozoic off-craton continental lithosphere beneath southern Africa. The relatively low closure temperature of the U-Pb rutile thermochronometer (~400-450 °C) allows its use as a particularly sensitive recorder of the establishment of "cratonic" lithospheric geotherms, as well as subsequent thermal perturbations to the lithosphere. Contrasting lower crustal thermal histories are revealed between intracratonic and craton margin regions. Discordant Proterozoic (1.8 to 1.0 Ga) rutile ages in Archean (2.9 to 2.7 Ga) granulites from within the craton are indicative of isotopic resetting by marginal orogenic thermal perturbations influencing the deep crust of the cratonic nucleus. In Proterozoic (1.1 to 1.0 Ga) granulite xenoliths from the craton-bounding orogenic belts, rutiles define discordia arrays with Neoproterozoic (0.8 to 0.6 Ga) upper intercepts and lower intercepts equivalent to Mesozoic exhumation upon kimberlite entrainment. In combination with coexisting titanite and apatite dates, these results are interpreted as a record of postorogenic cooling at an integrated rate of approximately 1 °C/Ma, and subsequent variable Pb loss in the apatite and rutile systems during a Mesozoic thermal perturbation to the deep lithosphere. Closure of the rutile thermochronometer signals temperatures of °C in the lower crust during attainment of cratonic lithospheric conductive geotherms, and such closure in the examined portions of the "off-craton" Proterozoic domains of southern Africa indicates that their lithospheric thermal profiles were essentially cratonic from the Neoproterozoic through to the Late Jurassic. These results suggest similar lithospheric thickness and potential for diamond stability beneath both Proterozoic and Archean domains of southern Africa. Subsequent partial resetting of U-Pb rutile and apatite systematics in the cratonic margin lower crust records a transient Mesozoic thermal modification of the lithosphere, and modeling of the diffusive Pb loss from lower crustal rutile constrains the temperature and duration of Mesozoic heating to °C for ka. This result indicates that the thermal perturbation is not simply a kimberlite-related magmatic phenomenon, but is rather a more protracted manifestation of lithospheric heating, likely related to mantle upwelling and rifting of Gondwana during the Late Jurassic to Cretaceous. The manifestation of this thermal pulse in the lower crust is spatially and temporally correlated with anomalously elevated and/or kinked Cretaceous mantle paleogeotherms, and evidence for metasomatic modification in cratonic mantle peridotite suites. It is argued that most of the geographic differences in lithospheric thermal structure inferred from mantle xenolith thermobarometry are likewise due to the heterogeneous propagation of this broad upper mantle thermal anomaly. The differential manifestation of heating between cratonic margin and cratonic interior indicates the importance of advective heat transport along pre-existing lithosphere-scale discontinuities. Within this model, kimberlite magmatism was a similarly complex, space- and time-dependent response to Late Mesozoic lithospheric thermal perturbation. 相似文献
54.
Government policies intended to reduce flood losses can increase the potential for catastrophe by stimulating development
inside the floodplain, a phenomenon referred to as the “safe development paradox.” New Urbanist design has the potential to
both exacerbate and alleviate flood risks. Because they are built at relatively high densities, New Urbanist developments
can exacerbate risk by placing more people and property in harm’s way. Conversely, New Urbanist design features theoretically
better enable designers of New Urbanist developments to avoid floodplain portions of project sites than designers of conventional
subdivisions. Using a sample of New Urbanist developments in the US that contain floodplain portions within their boundaries,
this paper focuses on whether and why these developments locate built structures inside the floodplain. The authors find that
roughly 30% of the developments locate structures inside the floodplain, and that the odds of locating structures inside the
floodplain increase with the proportion of the project site located inside the floodplain and decrease with the presence of
government policies that prohibit residential development in the floodplain. The authors also identify confusion among government
planners regarding the distinction between pre and postconstruction floodplain boundaries. A subset of New Urbanist developments
is found to have built structures located outside the postconstruction floodplain boundary, but inside the preconstruction
floodplain boundary. This finding is cited as an example of the “safe development paradox” in action. The authors recommend
changes in New Urbanist design codes and local government floodplain management to increasingly direct new development away
from the floodplain. 相似文献
55.
The objective of this study was to experimentally evaluate the effects of simulated herbivory on the ability of a freshwater marsh plant to recover from temporary saltwater intrusion such as can be caused by tropical storms. Sods containingSagittaria lancifolia, a dominant plant in interior coastal marshes, were manipulated in the field so as to subject plants to a pulse of 15‰ salt water for a duration of 1 wk. In addition to the exposure to salt water, some plants were also subjected to both short-term and long-term flooding treatments of 20 cm, and to simulated herbivory (clipping). Following exposure to salt water, plants were allowed to recover over the winter and were harvested the next June. Neither simulated herbivory, nor salinity, nor flooding caused any long-term effect either singly or in pairwise combinations. However, when plants were subjected to herbivory, salt water, and flooding simultaneously, reduced growth and plant death occurred. These results suggest that high levels of grazing by herbivores may increase the susceptibility of coastal marsh plants to damage from saltwater intrusion. *** DIRECT SUPPORT *** A01BY073 00002 相似文献
56.
T. Mark Harrison Julien Célérier Joerg Hermann 《Geochimica et cosmochimica acta》2009,73(4):1039-1051
Hydrothermal treatment of closely sized muscovite aggregates in a piston-cylinder apparatus induced 40Ar∗ loss that is revealed in 40Ar/39Ar step heating spectra. Age spectra and Arrhenius data, however, differ from that expected from a single diffusion length scale. A numerical model of episodic loss assuming the presence of multiple diffusion domains yields excellent fits between synthetic and actual degassing spectra. We used this model to isolate 40Ar∗ loss from the grains that remained intact during hydrothermal treatment at 10 kbar permitting calculation of diffusion coefficients in the temperature range 730-600 °C. Diffusion data generated in this manner yield an activation energy (E) of 63 ± 7 kcal/mol and frequency factor (Do) of 2.3 cm2/s. Experiments at 20 kbar yield diffusivities lower by about an order of magnitude and correspond to an activation volume of ∼14 cm3/mol. Together, these parameters predict substantially greater retentivity of Ar in muscovite than previously assumed and correspond to a closure temperature (Tc) of 425 °C for a 100 μm radius grain cooling at 10 °C/Ma at 10 kbar (Tc = 405 °C at 5 kbar. Age and log (r/ro) spectra for the run products show strong correlations indicating that muscovites can retain Ar diffusion boundaries and mechanisms that define their natural retentivity during vacuum step heating. This may permit the application of high resolution, continuous 40Ar/39Ar thermochronology to low grade, regionally metamorphosed terranes. 相似文献
57.
58.
Fe and O isotope composition of meteorite fusion crusts: Possible natural analogues to chondrule formation? 下载免费PDF全文
Dominik C. Hezel Graeme M. Poole Jack Hoyes Barry J. Coles Catherine Unsworth Nina Albrecht Caroline Smith Mark Rehkämper Andreas Pack Matthew Genge Sara S. Russell 《Meteoritics & planetary science》2015,50(2):229-242
Meteorite fusion crust formation is a brief event in a high‐temperature (2000–12,000 K) and high‐pressure (2–5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with δ56Fe values up to +0.35‰ relative to the solar system mean. On average, the δ56Fe of fusion crusts from finds is +0.23‰, which is 0.08‰ higher than the average from falls (+0.15‰). Higher δ56Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The δ56Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher δ18O (0.9–1.5‰) than their host chondrites, and the same is true for Δ17O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high‐pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an 16O‐poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts. 相似文献
59.
Mark A. Vincent 《Celestial Mechanics and Dynamical Astronomy》1986,39(1):15-21
The relativistic equations of motion are derived for N self-gravitating, rotating finite bodies. These equations are then applied to the near-Earth satellite orbit determination problem. The apparent change of the shape of the Earth from the Earth centered frame to the Solar System barycentric frame changes the value of the Newtonian potential term in the metric. This in turn leads to a simplification of the equations of motion in the barycentric frame. 相似文献
60.
The Global Ozone Monitoring Experiment (GOME) is the first of a series of European satellite instruments monitoring global ozone and other relevant trace constituents in the UV/visible spectral range. On 20 April 1995, the European Space Agency (ESA) launched the GOME from Kourou, French Guyana, aboard the second European Remote Sensing satellite (ERS-2). In order to obtain the geometric albedo from the backscattered terrestrial radiance measurements, a solar irradiance measurement sequence in the spectral range between 240 nm and 790 nm is carried out once every day. The GOME solar irradiance is recorded at a moderate spectral resolution (0.2–0.4 nm), thus providing an excellent opportunity to contribute to the long-term investigation of solar flux variation associated with the 11-year solar activity cycle from space, which started in 1978 with SBUV (Solar Backscatter UV Experiment) observations on Nimbus-7 and covers solar cycles 21 and 22. This paper briefly describes the GOME spectrometer and measurement mode which are relevant to the solar viewing. Preliminary results from the solar irradiance measurements between 1995 and 1997 and comparisons to SSBUV-8 (Shuttle SBUV) in January 1996 are presented. Solar activity indices used as proxies for solar flux variation are often used to find a correlation with observed variation in atmospheric quantities, for instance, total ozone. Initial results from the GOME Mgii (280 nm) and Caii K (393 nm) solar activity index calculation are presented and discussed. The coupling of solar irradiance variability to global change is a current source of scientific and public concern. This study shows that GOME/ERS-2 (1995–2001) and the next generation of European remote sensing instruments, SCIAMACHY and GOME/METOP, have the potential to provide continuity in the measurements of solar irradiance from space well into the next century. 相似文献