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541.
Sean A. Hartnoll Eric G. Blackman 《Monthly notices of the Royal Astronomical Society》2001,324(1):257-266
The central engines of active galactic nuclei (AGN) contain cold, dense material as well as hot X-ray-emitting gas. The standard paradigm for the engine geometry is a cold thin disc sandwiched between hot X-ray coronae. Strong support for this geometry in Seyferts comes from the study of fluorescent iron line profiles, although the evidence is not ubiquitously airtight. The thin disc model of line profiles in AGN and in X-ray binaries should still be benchmarked against other plausible possibilities. One proposed alternative is an engine consisting of dense clouds embedded in an optically thin, geometrically thick X-ray-emitting engine. This model is also motivated by studies of geometrically thick engines such as advection-dominated accretion flows (ADAFs). Here we compute the reprocessed iron line profiles from dense clouds embedded in geometrically thick, optically thin X-ray-emitting discs near a Schwarzschild black hole. We consider a range of cloud distributions and disc solutions, including ADAFs, pure radial infall and bipolar outflows. We find that such models can reproduce line profiles similar to those from geometrically thin, optically thick discs and might help alleviate some of the problems encountered from the latter. Thus, independent of thin discs, thick disc engines can also exhibit iron line profiles if embedded dense clouds can survive long enough to reprocess radiation. 相似文献
542.
Matthew R. Patrick John L. Smellie Andrew J. L. Harris Robert Wright Ken Dean Pavel Izbekov Harold Garbeil Eric Pilger 《Bulletin of Volcanology》2005,67(5):415-422
The MODVOLC satellite monitoring system has revealed the first recorded eruption of Mount Belinda volcano, on Montagu Island in the remote South Sandwich Islands. Here we present some initial qualitative observations gleaned from a collection of satellite imagery covering the eruption, including MODIS, Landsat 7 ETM+, ASTER, and RADARSAT-1 data. MODVOLC thermal alerts indicate that the eruption started sometime between 12 September and 20 October 2001, with low-intensity subaerial explosive activity from the islands summit peak, Mount Belinda. By January 2002 a small lava flow had been emplaced near the summit, and activity subsequently increased to some of the highest observed levels in August 2002. Observations from passing ships in February and March 2003 provided the first visual confirmation of the eruption. ASTER images obtained in August 2003 show that the eruption at Mount Belinda entered a new phase around this time, with fresh lava effusion into the surrounding icefield. MODIS radiance trends also suggest that the overall activity level increased significantly after July 2003. Thermal anomalies continued to be observed in MODIS imagery in early 2004, indicating a prolonged low-intensity eruption and the likely establishment of a persistent summit lava lake, similar to that observed on neighboring Saunders Island in 2001. Our new observations also indicate that lava lake activity continues on Saunders Island.Editorial responsibility: J. Gilbert 相似文献
543.
This paper reports the first reliable quantitative determination of the thermal metamorphism grade of a series of nine CV3 chondrites: Allende, Axtell, Bali, Mokoia, Grosnaja, Efremovka, Vigarano, Leoville, and Kaba. The maturity of the organic matter in matrix, determined by Raman spectroscopy, has been used as a powerful metamorphic tracer, independent of the mineralogical context and extent of aqueous alteration. This tracer has been used along with other metamorphic tracers such as Fe zoning in type-I chondrules of olivine phenocrysts, presolar grain abundance and noble gas abundance (bulk and P3 component). The study shows that the petrologic types determined earlier by Induced ThermoLuminescence were underestimated and suggests the following values: PT (Allende-Axtell) >3.6; PT (Bali-Mokoia-Grosnaja) ∼3.6; PT (Efremovka-Leoville-Vigarano) = 3.1-3.4; PT (Kaba) ∼3.1. The most commonly studied CV3, Allende, is also the most metamorphosed. Bali is a breccia containing clasts of different petrologic types. The attribution suggested by this study is that of clasts of the highest petrologic types, as pointed out by IOM maturity and noble gas bulk abundance. CV3 chondrites have complex asteroidal backgrounds, with various degrees of aqueous alteration and/or thermal metamorphism leading to complex mineralogical and petrologic patterns. (Fe,Mg) chemical zoning in olivine phenocrysts, on the borders of type I chondrules of porphyritic olivine- and pyroxene-rich textural types, has been found to correlate with the metamorphism grade. This suggests that chemical zoning in some chondrules, often interpreted as exchanges between chondrules and nebular gas, may well have an asteroidal origin. Furthermore, the compositional range of olivine matrix is controlled both by thermal metamorphism and aqueous alteration. This does not support evidence of a nebular origin and does not necessarily mirror the metamorphism grade through (Fe,Mg) equilibration. On the other hand, it may provide clues on the degree of aqueous alteration vs. thermal metamorphism and on the timing of both processes. In particular, Mokoia experienced significant aqueous alteration after the metamorphism peak, whereas Grosnaja, which has similar metamorphism grade, did not. 相似文献
544.
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546.
Eric?HellebrandEmail author Jonathan?E.?Snow Smail?Mostefaoui Peter?Hoppe 《Contributions to Mineralogy and Petrology》2005,150(5):486-504
Clinopyroxenes (cpx) in abyssal and ophiolitic peridotites are commonly analyzed for lithophile trace element abundances in
order to estimate degrees of melting and porosity conditions during melt extraction, assuming that these data reflect near-solidus
conditions. During cooling, however, cpxs always exsolve into parallel lamellae of low-Ca enstatite and high-Ca diopside.
This may potentially lead to redistribution of the initial trace element budget. Since orthopyroxene (opx) cannot significantly
host most incompatible trace elements, exsolution will lead to an enrichment in the cpx lamellae. In order to address a possibly
exsolution-controlled partitioning between cpx and opx, we have obtained major and trace element mineral compositions on 14
plagioclase-free ocean floor mantle rocks. They cover the entire abyssal peridotite compositional spectrum from very fertile
to highly depleted compositions. The mean volume proportion of opx lamellae in cpx porphyroclasts lies around 15% of the original
cpx. For the light to middle rare earth elements, the enrichment in the measured cpx exsolution is exclusively controlled
by these phase proportions. Relative to these highly incompatible trace elements, solely Ti and Yb partition significantly
into opx. Lamellar interpyroxene partition coefficients, estimated from NanoSIMS analyses, are around three times as high
as the ones for near-solidus bulk pyroxene. The equilibration temperatures for the exsolution lamella are slightly higher
than 800°C. The bulk cpx can be reconstructed using the lamellar proportions and their relative partitioning. The implication
of such a reconstruction is that the cpx rare earth element patterns shift almost in parallel to lower values. These shifts,
however, do not affect mantle melting models proposed thus far for mid-ocean ridges.
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
547.
548.
549.
Valrie Masson Franoise Vimeux Jean Jouzel Vin Morgan Marc Delmotte Philippe Ciais Claus Hammer Sigfus Johnsen Vladimir Ya. Lipenkov E. Mosley-Thompson Jean-Robert Petit Eric J. Steig Michel Stievenard Rein Vaikmae 《Quaternary Research》2000,54(3):348
A comparison is made of the Holocene records obtained from water isotope measurements along 11 ice cores from coastal and central sites in east Antarctica (Vostok, Dome B, Plateau Remote, Komsomolskaia, Dome C, Taylor Dome, Dominion Range, D47, KM105, and Law Dome) and west Antarctica (Byrd), with temporal resolution from 20 to 50 yr. The long-term trends possibly reflect local ice sheet elevation fluctuations superimposed on common climatic fluctuations. All the records confirm the widespread Antarctic early Holocene optimum between 11,500 and 9000 yr; in the Ross Sea sector, a secondary optimum is identified between 7000 and 5000 yr, whereas all eastern Antarctic sites show a late optimum between 6000 and 3000 yr. Superimposed on the long time trend, all the records exhibit 9 aperiodic millennial-scale oscillations. Climatic optima show a reduced pacing between warm events (typically 800 yr), whereas cooler periods are associated with less-frequent warm events (pacing >1200 yr). 相似文献
550.
Michelle Denfle Anne-Marie Lzine Eric Fouache Jean-Jacques Dufaure 《Quaternary Research》2000,54(3):423
Pollen data from Lake Maliq, the first from Albania, contribute new information to the discussion of the vegetational, hydrological, and climatological history of the Balkans since 12,000 yr B.P. During late-glacial time, a perennial lake expanded at Maliq. It was surrounded by a complex vegetation association composed of steppe and mixed forest elements. The highly diverse forest flora suggest that late-glacial forest refugia were more developed here at middle altitude, rather than at higher altitude as previously suggested. The forest developed after 9800 yr B.P., while the water level remained high in the Korçë basin until 5000 yr B.P. Different environmental conditions, characterized by lower available moisture and warmer winters, progressively took place after this date. Human activity in the Korçë basin ca. 4500 yr B.P. was coeval with conditions characterized by an increase in winter temperatures and a decrease in summer moisture. 相似文献