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91.
Susan TAYLOR Keith W. JONES Gregory F. HERZOG Claire E. HORNIG 《Meteoritics & planetary science》2011,46(10):1498-1509
Abstract– To determine the role played by sulfides in the formation of vesicles and FeNi metal beads, we mapped the locations and tabulated the numbers of sulfides, metal beads, and vesicles in 1583 sectioned micrometeorites (MMs) using conventional microscopy and in 190 whole MMs using synchrotron computed microtomography (SCMT). Both the section and the SCMT images show that sulfides melt, coalesce, and migrate to the MMs surface. The decomposition of sulfides may occur during all these stages. Given the sulfide morphologies and compositions that we see in section, we think the breakdown of Ni sulfides produces the FeNi beads. The SCMT images show that metal beads are common in melted MMs, >50% have them. Vesicles in porphyritic and scoriaceous MMs are also probably formed as sulfides decompose. Not only do sulfides abut the vesicles but also the temperatures at which sulfides decompose overlap those at which MM surfaces first melt and temporarily seal, suggesting that S gases could produce most of these vesicles. As the vesicle shapes and patterns of distribution differ among MM classes, tomography can be used to nondestructively screen for specific types of MMs. Tomography is a powerful tool for visualizing the three‐dimensional distribution of metal beads, sulfides, mean densities, and vesicles in MMs. 相似文献
92.
Keith D. Hamill 《新西兰海洋与淡水研究杂志》2013,47(5):1057-1059
Toxic cyanobacteria have been linked to dog deaths in the Waikanae River (1998) and Mataura River (1999 and 2000), New Zealand. These were Oscillatoria‐like species which formed benthic mats. This is the first time in New Zealand that animal deaths have been linked to toxins from benthic cyanobacteria. 相似文献
93.
An inflection in the rate of early mid-Holocene eustatic sea-level rise: A new sea-level curve from Singapore 总被引:3,自引:0,他引:3
M.I. Bird L.K. Fifield T.S. Teh C.H. Chang N. Shirlaw K. Lambeck 《Estuarine, Coastal and Shelf Science》2007,71(3-4):523-536
This study presents a sea-level curve from 9500 to 6500 cal BP for the farfield location of Singapore, on the Sunda Shelf in southeast Asia. The curve is based on more than 50 radiocarbon dates from elevations of +1.43 m to −15.09 m representing sea-level index points in intertidal mangrove and shallow marine sediments deposited by sea-level rise accompanying deglaciation. The results indicate that mean sea level rose rapidly from around −17 m at 9500 cal BP to around −3 m by 8000 cal BP. After this time, the data suggest (but do not unequivocally prove) that the rate of sea-rise slowed for a period of 300–500 years centred on 7700 cal BP, shortly after the cessation of meltwater input to the oceans from the northern hemisphere. Renewed sea-level rise amounting to 3–5 m began around 7400 cal BP and was complete by 7000 cal BP. The existence of an inflection in the rate of sea-level rise, with a slow-down centred on 7700 cal BP, is broadly consistent with other available sea-level curves over this interval and is supported by evidence of stable shorelines and delta initiation elsewhere at this time, as well as evidence of comparatively rapid retreat of the West Antarctic ice sheet beginning around 7500 cal BP. ‘Stepped’ sea-level rise occurring shortly after 7500 cal BP and also earlier during deglaciation may have served to focus significant post-glacial episodes of human maritime/coastal dispersal, into comparatively narrow time intervals. 相似文献
94.
Mapping block-and-ash flow hazards based on Titan 2D simulations: a case study from Mt. Taranaki,NZ 总被引:1,自引:0,他引:1
Jonathan N. Procter Shane J. Cronin Thomas Platz Abani Patra Keith Dalbey Michael Sheridan Vince Neall 《Natural Hazards》2010,53(3):483-501
Numerical models for simulation of mass flows are typically focussed upon accurately predicting the paths, travel times and
inundation from a single flow or collapse event. When considering catchment-based hazards from a volcano, this is complicated
by often being faced with several possible scenarios. Over the last 800 years at Mt. Taranaki/Egmont, a number of dome growth
and collapse events have resulted in the genesis and emplacement of block-and-ash flows (BAFs). Each BAF was directed northwestward
by a breach in the crater rim. The latest dome collapse events in the AD 1880s and AD 1755 inundated the northwestern flank
and had run-out lengths 10 km from source. Future activity of this type could have a devastating effect on the Taranaki region’s
communities, infrastructure and economy. Hazard planning has involved constructing volcanic hazard maps based upon the areas
inundated by past volcanic flows, with little consideration of present-day topography. Here, a numerical geophysical mass
flow modelling approach is used to forecast the hazards of future comparable BAF events on NW Mt. Taranaki. The Titan2D programme
encompasses a “shallow water”, continuum solution-based, granular flow model. Flow mechanical properties needed for this approach
include estimates of internal and basal friction as well as the physical dimensions of the initial collapse. Before this model
can be applied to Taranaki BAFs, the input parameters must be calibrated by simulating a range of past collapse events. By
using AD 1860 and AD 1755 scenarios, initial collapse volumes can be well constrained and internal and basal friction angles
can be evaluated through an iterative approach from previous run-out lengths. A range of possible input parameters was, therefore,
determined to produce a suite of potentially inundated areas under present-day terrain. A suite of 10 forecasts from a uniformly
distributed range were combined to create a map of relative probabilities of inundation by future BAF events. These results
were combined in a GIS package to produce hazard zones related to user-specified hazard thresholds. Using these input parameter
constraints, future hazard forecasts for this scale and type of event can also take into account changing summit and topographic
configurations following future eruptive or collapse events. 相似文献
95.
Susan H. BeVille Benjamin B. Mirus Brian A. Ebel George G. Mader Keith Loague 《Environmental Earth Sciences》2010,61(6):1249-1257
Hydrologically driven mass wasting in the form of landslides on steep slopes is a worldwide occurrence. High-profile events
in, for example, Brazil, Chile, the Philippines, Puerto Rico, and Venezuela during the last three decades all clearly illustrate,
based upon significant losses of life and property, that hydrologically driven slope instability in developed (urban) areas
can be a major geologic/environmental hazard. The focus of this study is the 1973 hydrologically driven Lerida Court landslide
in Portola Valley, CA, USA. Physics-based hydrologic-response simulation, with the comprehensive Integrated Hydrology Model,
was employed to forensically estimate the spatiotemporal pore pressure distributions for the Lerida Court site. Slope stability,
driven by the simulated pore pressure dynamics, was estimated for the Lerida Court site with the infinite slope/Factor of
Safety approach. The pore pressure dynamics for the Lerida Court site were reasonably captured by the hydrologic-response
simulation. The estimated time of slope failure for the Lerida Court site compares well with field observations. A recommendation
is made that hydrologically driven slope stability estimates including variably saturated subsurface flow be standard protocol
for development sites in steep urban settings. 相似文献
96.
We report high-resolution macroscopic charcoal, pollen and sedimentological data for Agua Caliente, a freshwater lagoon located in southern Belize, and infer a late Holocene record of human land-use/climate interactions for the nearby prehistoric Maya center of Uxbenká. Land-use activities spanning the initial clearance of forests for agriculture through the drought-linked Maya collapse and continuing into the historic recolonization of the region are all reflected in the record. Human land alteration in association with swidden agriculture is evident early in the record during the Middle Preclassic starting ca. 2600 cal yr BP. Fire slowly tapered off during the Late and Terminal Classic, consistent with the gradual political demise and depopulation of the Uxbenká polity sometime between ca. 1150 and 950 cal yr BP, during a period of multiple droughts evident in a nearby speleothem record. Fire activity was at its lowest during the Maya Postclassic ca. 950–430 cal yr BP, but rose consistent with increasing recolonization of the region between ca. 430 cal yr BP and present. These data suggest that this environmental record provides both a proxy for 2800 years of cultural change, including colonization, growth, decline, and reorganization of regional populations, and an independent confirmation of recent paleoclimate reconstructions from the same region. 相似文献
97.
Keith C. Clarke 《GeoJournal》2014,79(2):129-136
This paper consists of three parts. After an introduction that stresses the historical progression of modeling methods, the motivation for urban modeling and simulation is explored, and the terms defined. Next, a meta-review of the literature is conducted, partially in an attempt to show that urban models resemble, and indeed share many overlapping issues with models in parallel fields such as economics and ecology. Lastly, the specific lessons learned from the author’s fifteen-year experience developing and supporting a cellular urban growth and land use change model (SLEUTH) are shared, in the interest of making these issues generic to current and future modeling and simulation efforts. The conclusion stresses that future models face new computing power, new theoretical paradigms, vastly improved ways of visualizing simulations, and a rapidly changing audience for modeling and simulation. 相似文献
98.
Manuela Magliocchetti Steve J. Maddox Ed Hawkins John A. Peacock Joss Bland-Hawthorn Terry Bridges Russell Cannon Shaun Cole Matthew Colless Chris Collins Warrick Couch Gavin Dalton Roberto de Propris Simon P. Driver George Efstathiou Richard S. Ellis Carlos S. Frenk Karl Glazebrook Carole A. Jackson Bryn Jones Ofer Lahav Ian Lewis Stuart Lumsden Peder Norberg Bruce A. Peterson Will Sutherland Keith Taylor 《Monthly notices of the Royal Astronomical Society》2004,350(4):1485-1494
99.
Multiscale asymptotics are used to derive three systems of equations connecting the planetary geostrophic (PG) equations for gyre-scale flow to a quasigeostrophic (QG) equation set for mesoscale eddies. Pedlosky (1984), following similar analysis, found eddy buoyancy fluxes to have only a small effect on the large-scale flow; however, numerical simulations disagree. While the impact of eddies is relatively small in most regions, in keeping with Pedlosky’s result, eddies have a significant effect on the mean flow in the vicinity of strong, narrow currents.First, the multiple-scales analysis of Pedlosky is reviewed and amplified. Novel results of this analysis include new multiple-scales models connecting large-scale PG equations to sets of QG eddy equations. However, only introducing anisotropic scaling of the large-scale coordinates allows us to derive a model with strong two-way coupling between the QG eddies and the PG mean flow. This finding reconciles the analysis with simulations, viz. that strong two-way coupling is observed in the vicinity of anisotropic features of the mean flow like boundary currents and jets. The relevant coupling terms are shown to be eddy buoyancy fluxes. Using the Gent-McWilliams parameterization to approximate these fluxes allows solution of the PG equations with closed tracer fluxes in a closed domain, which is not possible without mesoscale eddy (or other small-scale) effects. The boundary layer width is comparable to an eddy mixing length when the typical eddy velocity is taken to be the long Rossby wave phase speed, which is the same result found by Fox-Kemper and Ferrari (2009) in a reduced gravity layer. 相似文献
100.
The mid to late‐Holocene climates of most of Scotland have been reconstructed from seven peat bogs located across north–south and east–west geographical and climatological gradients. The main techniques used for palaeoclimatic reconstruction were plant macrofossil, colorimetric humification, and testate amoebae analyses, which were supported by a radiocarbon‐based chronology, aided by markers such as tephra isochrons and recent rises in pine pollen and in spheroidal carbonaceous particles (SCPs). Field stratigraphy was undertaken at each site in order to show that the changes detected within the peat profiles were replicable. Proxy climate records were reconstructed using detrended correspondence analysis (DCA) of the plant macrofossil data and a mean water table depth transfer function on the testate amoebae data. These reconstructions, coupled with the humification data, were standardised for each site and used to produce a composite record of bog surface wetness (BSW) from each site. The results show coherent wet and dry phases over the last 5000 years and suggest regional differences in climate across Scotland, specifically between northern and southern Scotland. Distinct climatic cycles are identified, all of which record a millennial‐scale periodicity which can be correlated with previously identified marine and ice core Holocene cycles. The key role of the macrofossil remains of Sphagnum imbricatum, a taxon now extinct on many sites, is discussed in relation to the identified climatic shifts. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献