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861.
In Snowdonia there is a marked contrast between weathered summits, often with well‐developed blockfields or tors, and lower ice‐moulded terrain. The boundary is interpreted as a trimline marking the upper surface of the last ice sheet. This interpretation is supported by the presence of gibbsite, an end‐product of prolonged weathering, at the base of soils above but not below the trimline. The reconstructed ice surface reaches about 850 m above present sea‐level along an ice divide running NE–SW through the massif. There is no evidence to support the popular view that ice centred further south extended over Snowdonia, and breaching to form the major glacial troughs can be explained by the action of local ice. The field evidence presented here demonstrates that most models of the southern British and Irish Sea ice sheets are significantly flawed, the earliest being far too thick and the most recent far too thin. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
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863.
C. Vance Haynes T. A. Maxwell A. El Hawary K. A. Nicoll S. Stokes 《Geoarchaeology》1997,12(8):819-832
A small concentration of Acheulian cleavers and handaxes within the driest region on Earth adds to the increasing evidence that the eastern Sahara was considerably more verdant during the Middle Pleistocene than it is today. The similarities to stone artifact assemblages of Acheulian sites in sub-Saharan Africa and in the Levant support the evidence for the movement of hominids, utilizing the Kombewa lithic technology, between Africa and the Middle East during the Middle Pleistocene. © 1997 John Wiley & Sons, Inc. 相似文献
864.
Colin Snodgrass Jessica Agarwal Michael Combi Alan Fitzsimmons Aurelie Guilbert-Lepoutre Henry H. Hsieh Man-To Hui Emmanuel Jehin Michael S. P. Kelley Matthew M. Knight Cyrielle Opitom Roberto Orosei Miguel de Val-Borro Bin Yang 《Astronomy and Astrophysics Review》2017,25(1):5
We review the evidence for buried ice in the asteroid belt; specifically the questions around the so-called Main Belt Comets (MBCs). We summarise the evidence for water throughout the Solar System, and describe the various methods for detecting it, including remote sensing from ultraviolet to radio wavelengths. We review progress in the first decade of study of MBCs, including observations, modelling of ice survival, and discussion on their origins. We then look at which methods will likely be most effective for further progress, including the key challenge of direct detection of (escaping) water in these bodies. 相似文献
865.
Annama H chondrite—Mineralogy,physical properties,cosmic ray exposure,and parent body history 下载免费PDF全文
Tomáš Kohout Jakub Haloda Patricie Halodová Matthias M. M. Meier Colin Maden Henner Busemann Matthias Laubenstein Marc. W. Caffee Kees C. Welten Jens Hopp Mario Trieloff Ramakant R. Mahajan Sekhar Naik Josep M. Trigo‐Rodriguez Carles E. Moyano‐Cambero Michael I. Oshtrakh Alevtina A. Maksimova Andrey V. Chukin Vladimir A. Semionkin Maksim S. Karabanalov Israel Felner Evgeniya V. Petrova Evgeniia V. Brusnitsyna Victor I. Grokhovsky Grigoriy A. Yakovlev Maria Gritsevich Esko Lyytinen Jarmo Moilanen Nikolai A. Kruglikov Aleksey V. Ishchenko 《Meteoritics & planetary science》2017,52(8):1525-1541
The fall of the Annama meteorite occurred early morning (local time) on April 19, 2014 on the Kola Peninsula (Russia). Based on mineralogy and physical properties, Annama is a typical H chondrite. It has a high Ar‐Ar age of 4.4 Ga. Its cosmic ray exposure history is atypical as it is not part of the large group of H chondrites with a prominent 7–8 Ma peak in the exposure age histograms. Instead, its exposure age is within uncertainty of a smaller peak at 30 ± 4 Ma. The results from short‐lived radionuclides are compatible with an atmospheric pre‐entry radius of 30–40 cm. However, based on noble gas and cosmogenic radionuclide data, Annama must have been part of a larger body (radius >65 cm) for a large part of its cosmic ray exposure history. The 10Be concentration indicates a recent (3–5 Ma) breakup which may be responsible for the Annama parent body size reduction to 30–35 cm pre‐entry radius. 相似文献
866.
Angela Dikou Colin Ackerman Carly Banks Alex Dempsey Michael Fox Meagan Gins Patricia Hester Adam Parnes Stephanie Roach Jessica Rohde Cliff Spital Mackai Tapleshay Luke Thomas 《Marine Ecology》2009,30(4):425-436
Coral reefs of the Turks and Caicos Islands (TCIs) (Caribbean Sea) constitute some of the few pristine coral reef systems in the world and play a crucial role in the islands’ economy because they support rich fisheries catches and tourism development. Ambitious development plans involving increase in fishing and tourism pressures are about to bring changes in coastal zone resources of the TCIs associated with increased sediments and nutrients and reduced predation by herbivorous fish on coral reefs. Understanding change is critical when attempting to protect the resources that these coral reefs support and to adopt proper management strategies. Yet, an environmental assessment program to detect imminent human‐induced changes on the surrounding reefs of the TCIs is lacking. Thus, (i) we obtained baseline data on benthic composition and coral community structure at seven reef sites of representative reefs of the TCIs within the Admiral Cockburn Land and Sea National Park (ACLSNP) of South Caicos Island and (ii) performed a priori statistical power analysis to calculate replication requirements for safely and confidently detecting small (δ = 0.1), medium (δ = 0.3), and large (δ = 0.5) effect sizes for a number of relevant to anticipated changes, univariate, benthic indices and for power β = 0.95. The platforms of the margin reefs studied (9–12 m depth) appeared rather variable regarding benthic composition but quite homogeneous regarding hard coral community structure. Mean percent cover of algal functional groups was 0.1 ± 0.3 (mean ± sd) percent for coralline algae and Halimeda, 0.1 ± 0.6 (mean ± sd) percent for macroalgae, 21.7 ± 33 (mean ± sd) percent for turf algae and 4.8 ± 4.0 (mean ± sd) percent for hard coral cover. The dominant benthic component, however, was carbonate substrate (mean ± sd = 30.4 ± 34.3), thus indicating an accreting reef framework. Mean hard coral density, colony size and recruit density were 5.5 ± 1.8 (mean ± sd) corals per 20‐m line transect, 13.0 ± 2.3 (mean ± sd) cm maximum colony diameter, and 1.3 ± 1.4 (mean ± sd) recruits per square foot, respectively. Due to high natural variance, hard coral colony size and density were practically the most sensitive indices in detecting even small size changes on benthos. Also, the geometric mean of log‐transformed colony size‐frequency distributions of the most abundant hard coral taxa, i.e. Montastrea annularis, Agaricia spp., Siderastrea spp. and Porites asteroides were practically sensitive for the same purpose. We hope that the study will optimize the spatial component of a necessary environmental impact assessment program on coral reefs of the TCIs once the natural spatial variability of the system has been assessed and sensitive, benthic, univariate indices have been identified for representative reference coral reef sites of the TCIs. 相似文献
867.
1INTRODUCTIONBARRETT(1992)demonstratedaphenomenoninwhichtheinter-governmentalbehaviorsareeffectivelydescribedbyanumberofslipperyfishermen.Thefisherydilemmaillustratesaphenomenonthatiscommontomanysocialandeconomicproblemsinwhichtheprivateincentivesofindependentagents(liketheslipperyfishermenasabove)preventtheagentsfromreachinganoutcomewhichmakesalltheagentsbetteroff.Iftheresourceisunderthejurisdictionofasinglegovernment,theexploitationofitcanbeeasilycoordinatedbythegovernmentitself.Butif… 相似文献
868.
Short JW Springman KR Lindeberg MR Holland LG Larsen ML Sloan CA Khan C Hodson PV Rice SD 《Marine environmental research》2008,66(5):487-498
We deployed semipermeable membrane devices (SPMDs) on beaches for 28 days at 53 sites in Prince William Sound (PWS), Alaska, to evaluate the induction potential from suspected sources of cytochrome P450 1A (CYP1A)-inducing contaminants. Sites were selected to assess known point sources, or were chosen randomly to evaluate the region-wide sources. After deployment, SPMD extracts were analyzed chemically for persistent organic pollutants (POPs) and polycyclic aromatic hydrocarbons (PAH). These results were compared with hepatic CYP1A enzyme activity of juvenile rainbow trout injected with the same extracts prior to clean-up for the chemical analyses. Increased CYP1A activity was strongly associated with PAH concentrations in extracts, especially chrysene homologues but was not associated with POPs. The only apparent sources of chrysene homologues were lingering oil from Exxon Valdez, asphalt and bunker fuels released from storage tanks during the 1964 Alaska earthquake, creosote leaching from numerous pilings at one site, and PAH-contaminated sediments at Cordova Harbor. Our results indicate that PWS is remarkably free of pollution from PAH when nearby sources are absent as well as from pesticides and PCBs generally. 相似文献
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870.