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201.
Kevin R. Chamberlain Axel K. Schmitt Susan M. Swapp T. Mark Harrison Norbert Swoboda-Colberg Wouter Bleeker Tony D. Peterson Charles W. Jefferson Andrei K. Khudoley 《Precambrian Research》2010
An in situ U–Pb SIMS (IN-SIMS) method to date micro-baddeleyite crystals as small as 3 μm is presented with results from three samples that span a variety of ages and geologic settings. The method complements ID-TIMS geochronology by extending the range of dateable crystals to sizes smaller than can be recovered by physical separation. X-ray mapping and BSE imaging are used to locate target grains in thin section, followed by SIMS analysis on a CAMECA ims 1270, using the field aperture in the transfer column to screen out ions from host phases. Internal age precisions for the method are anticipated to range from 0.1% for Precambrian rocks to 3–7% for Phanerozoic rocks. Results establish a 2689 ± 5 Ma age for mafic dikes in the Wyoming craton, USA, a 1540 ± 30 Ma age for a subaerial lava flow from the Thelon Basin of northern Canada, and a 457 ± 34 Ma age for mafic dikes in the platform sequence of southeastern Siberia. The method is ideal for relatively non-destructive dating of small samples such as extraterrestrial rocks and precious terrestrial samples. 相似文献
202.
Evidence for a Long-Term Strength Threshold in Crystalline Rock 总被引:3,自引:1,他引:2
The mechanical response of brittle rock to long-duration compression loading is of particular concern in underground disposal of nuclear waste, where radionuclides must be isolated from the biosphere for periods of the order of a million years. Does the strength decrease without limit over such time, or is there, for some rock types, a lower “threshold” strength below which the rock will cease to deform? This paper examines the possibility of such a threshold in silicate crystalline rocks from several perspectives, including: (1) interpretation of the results of short-term creep tests on rock; (2) numerical analysis of the effect of decrease in fracture toughness due to stress corrosion on the strength of a crystalline rock; and (3) evidence from plate tectonics, and observations of in situ rock stress in granite quarries. The study concludes that there is clear evidence of threshold strength. The threshold is of the order of 40% of the unconfined compressive strength or higher for laboratory specimens under unconfined compressive loading, and increases rapidly in absolute value with confinement. Field evidence also leads to the conclusion that the long-term strength of crystalline rock in situ is of comparable magnitude to the laboratory value. 相似文献
203.
Human arrival in Sahul – Pleistocene Australia and New Guinea – has long been argued as the catalyst in the decline and disappearance of a suite of extinct animals referred to as megafauna. The debate concerning causality in Sahul is highly polarised, with climate change often cited as the alternative explanatory model. On continental Australia, there are few datasets available with which to explore the likely processes leading to the extinction events. At the present time, there is one site in New Guinea (Nombe Rockshelter) and one on continental Australia (Cuddie Springs) where the coexistence and temporal overlap of humans and megafauna has been identified. The Cuddie Springs Pleistocene archaeological site in southeastern Australia contains an association of fossil extinct and extant fauna with an archaeological record through two sequential stratigraphic units dating from c. 36 to c. 30 ka ago. A taphonomic study of the fossil fauna has revealed an accumulation of bone in a primary depositional context, consistent with a waterhole death assemblage. Overall the faunal assemblage studied here (n: 8146; NISP: 1355) has yielded little direct evidence of carnivore damage or human activities. Post depositional factors such as physical destruction incurred by trampling, compaction of sediments, and/or the hydrological status of the lake at that time have played important roles. As the only known site on continental Australia where megafauna and humans co-occur, the Cuddie Springs faunal assemblage yields equivocal evidence for a significant human role in the accumulation of the fauna here. At the present time there is no evidential basis to the argument that humans had a primary role in the extinction of the Australian megafauna. The first colonisers are likely to have preyed upon those few species known to have persisted to this time, but their impact may have been restricted to the tail end of a process that had been underway for millennia prior to human arrival. 相似文献
204.
Michel Faure Alain Cocherie Eugne B Mzme Nicolas Charles Philippe Rossi 《Gondwana Research》2010,18(4):369
In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pbtot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale. 相似文献
205.
Medium- and Long-term Recovery of Estuarine and Coastal Ecosystems: Patterns,Rates and Restoration Effectiveness 总被引:1,自引:0,他引:1
Ángel Borja Daniel M. Dauer Michael Elliott Charles A. Simenstad 《Estuaries and Coasts》2010,33(6):1249-1260
Many estuarine and coastal marine ecosystems have increasingly experienced degradation caused by multiple stressors. Anthropogenic
pressures alter natural ecosystems and the ecosystems are not considered to have recovered unless secondary succession has
returned the ecosystem to the pre-existing condition or state. However, depending upon the scales of time, space and intensity
of anthropogenic disturbance, return along the historic trajectory of the ecosystem may: (1) follow natural restoration though
secondary succession; (2) be re-directed through ecological restoration, or (3) be unattainable. In order to address the gaps
in knowledge about restoration and recovery of estuarine and coastal ecosystems, this special feature includes the present
overview and other contributions to provide a synthesis of our knowledge about recovery patterns, rates and restoration effectiveness.
From the 51 examples collated in this contribution, we refine the recovery from the list of stressors into six recovery mechanisms:
(1) recovery from sediment modification, which includes all aspects of dredging and disposal; (2) recovery by complete removal
of stressors limiting natural ecosystem processes, which includes tidal marsh and inundation restoration; (3) recovery by
speed of organic degradation, which includes oil discharge, fish farm wastes, sewage disposal, and paper mill waste; (4) recovery
from persistent pollutants, which includes chemical discharges, such as TBT; (5) recovery from excessive biological removal,
related to fisheries and (6) recovery from hydrological and morphological modification. Drawing upon experience both from
these many examples and from an example of one comprehensive study, we show that although in some cases recovery can take
<5 years, especially for the short-lived and high-turnover biological components, full recovery of coastal marine and estuarine
ecosystems from over a century of degradation can take a minimum of 15–25 years for attainment of the original biotic composition
and diversity may lag far beyond that period. 相似文献
206.
Adnand Bitri Jean-Pierre Brun Denis Gapais Florence Cagnard Charles Gumiaux Jean Chantraine Guillaume Martelet Catherine Truffert 《Comptes Rendus Geoscience》2010,342(6):448-452
We present results and interpretation of a 72 km long deep seismic reflection profile acquired across the internal zone of the Hercynian belt of South Brittany. The profile is of excellent quality, most of the crust being highly reflective. The “ARMOR 2 South” profile, is correlated with the “ARMOR 2 North” profile that was published in 2003. Correlation of the main subsurface reflections with surface geological and structural data provides important information about the crustal structure that resulted from thickening during Late Devonian and regional-scale extension during Late Carboniferous. In particular, seismics image shows a very high reflectivity zone, lying flat over more than 40 km at about 10–12 km depth. This zone is interpreted as a major zone of ductile crustal thinning. 相似文献
207.
Paul Augustinus Charles E. Barton Atun Zawadzki Kate Harle 《Environmental Earth Sciences》2010,61(3):625-639
Macquarie Harbour in southwest Tasmania, Australia, has been affected severely by the establishment of mines in nearby Queenstown
in the 1890s. As well as heavy metal-laden acid rock drainage from the Mount Lyell mine area, over 100 Mt of mine tailings
and slag were discharged into the Queen and Ring Rivers, with an estimated 10 Mt of mine tailings building a delta of ca.
2.5 km2 and ca. 10 Mt of fine tailings in the harbour beyond the delta. Coring of sediments throughout Macquarie Harbour indicated
that mine tailings accreted most rapidly close to the King River delta source with a significant reduction in thickness of
tailings and heavy metal contamination with increasing distance from the King River source. Close to the King River delta
the mine tailings are readily discriminated from the background estuarine sediments on the basis of visual logging of the
core (laminations, colour), sediment grain size, sediment magnetic susceptibility and elemental geochemistry, especially concentrations
of the heavy metals Cu, Zn and Pb. The high heavy metal concentrations are demonstrated by the very high contamination factors
(CF > 6) for Cu and Zn, with CF values mostly >50 for Cu for the mine-impacted sediments. Although the addition of mine waste
into the King River catchment has ceased, the catchment continues to be a source of these heavy metals due to acid rock drainage
and remobilisation of mine waste in storage in the river banks, river bed and delta. The addition of heavy metals to the harbour
sourced from the Mount Lyell mines preceded the advent of direct tailings disposal into the Queen River in 1915 with the metals
probably provided by acid rock drainage from the Mount Lyell mining area. 相似文献
208.
209.
Jean‐Charles Leclerc Pascal Riera Laure M.‐L. J. Noël Cédric Leroux Ann C. Andersen 《Marine Ecology》2014,35(2):261-270
The diet of Pomatoschistus microps has been studied using both gut content and stable isotope analyses. In the Roscoff Aber Bay (Brittany, France), this fish is commonly found on sandy muddy intertidal flats. Gut content analyses were also interpreted using trophic indices. Owing to the large diversity of prey consumed, these indices emphasised the opportunistic feeding behaviour of P. microps. Here, this species fed mainly on endofauna with meiofauna being of high relative importance. The main biotic components of its trophic habitat, characterized by δ13C and δ15N, provided evidence of a major trophic pathway based on drift Enteromorpha sp. Trophic positions estimated by both diet analyses and isotopic analyses led to similar results. In this bay, P. microps is a first‐order predator with a low degree of omnivory. Despite a preferential consumption of the amphipod Corophium arenarium, we assumed that this goby behaves as a generalist feeding on a uniform variety of endofauna taxa. 相似文献
210.
Xiquan Dong Behnjamin J. Zib Baike Xi Ryan Stanfield Yi Deng Xiangdong Zhang Bing Lin Charles N. Long 《Climate Dynamics》2014,43(1-2):53-70
Along with significant changes in the Arctic climate system, the largest year-to-year variation in sea-ice extent (SIE) has occurred in the Laptev, East Siberian, and Chukchi seas (defined here as the area of focus, AOF), among which the two highly contrasting extreme events were observed in the summers of 2007 and 1996 during the period 1979–2012. Although most efforts have been devoted to understanding the 2007 low, a contrasting high September SIE in 1996 might share some related but opposing forcing mechanisms. In this study, we investigate the mechanisms for the formation of these two extremes and quantitatively estimate the cloud-radiation-water vapor feedback to the sea-ice-concentration (SIC) variation utilizing satellite-observed sea-ice products and the NASA MERRA reanalysis. The low SIE in 2007 was associated with a persistent anticyclone over the Beaufort Sea coupled with low pressure over Eurasia, which induced anomalous southerly winds. Ample warm and moist air from the North Pacific was transported to the AOF and resulted in positive anomalies of cloud fraction (CF), precipitable water vapor (PWV), surface LWnet (down-up), total surface energy and temperature. In contrast, the high SIE event in 1996 was associated with a persistent low pressure over the central Arctic coupled with high pressure along the Eastern Arctic coasts, which generated anomalous northerly winds and resulted in negative anomalies of above mentioned atmospheric parameters. In addition to their immediate impacts on sea ice reduction, CF, PWV and radiation can interplay to lead to a positive feedback loop among them, which plays a critical role in reinforcing sea ice to a great low value in 2007. During the summer of 2007, the minimum SIC is 31 % below the climatic mean, while the maximum CF, LWnet and PWV can be up to 15 %, 20 Wm?2, and 4 kg m?3 above. The high anti-correlations (?0.79, ?0.61, ?0.61) between the SIC and CF, PWV, and LWnet indicate that CF, PWV and LW radiation are indeed having significant impacts on the SIC variation. A new record low occurred in the summer of 2012 was mainly triggered by a super storm over the central Arctic Ocean in early August that caused substantial mechanical ice deformation on top of the long-term thinning of an Arctic ice pack that had become more dominated by seasonal ice. 相似文献