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241.
242.
When measuring samples from southern Peru, we discovered an age offset between feldspar infrared-stimulated luminescence (IRSL) and quartz optically stimulated luminescence (OSL) ages. This age offset was not only found in these particular samples but was also observed in samples from a range of geographical localities. Extensive analysis revealed that it is actually the quartz age that underestimates the true depositional age, and this is due to a very weak fast component combined with other thermally unstable OSL signal components. We discuss properties of these poorly behaving samples and how to detect such samples prior to the OSL single-aliquot regenerative-dose (SAR) measurement. Finally, existing approaches to improve equivalent dose (De) determination are discussed for samples from southern Peru. 相似文献
243.
244.
Bryan Shuman Anna K. Henderson Steven M. Colman Jeffery R. Stone Sherilyn C. Fritz Lora R. Stevens Mitchell J. Power Cathy Whitlock 《Quaternary Science Reviews》2009,28(19-20):1861-1879
The availability of water shapes life in the western United States, and much of the water in the region originates in the Rocky Mountains. Few studies, however, have explicitly examined the history of water levels in the Rocky Mountains during the Holocene. Here, we examine the past levels of three lakes near the Continental Divide in Montana and Colorado to reconstruct Holocene moisture trends. Using transects of sediment cores and sub-surface geophysical profiles from each lake, we find that mid-Holocene shorelines in the small lakes (4–110 ha) were as much as 10 m below the modern lake surfaces. Our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000–2000 years ago. We also discuss evidence for millennial-scale moisture variation, including an abruptly-initiated and -terminated wet period in Colorado from 4400 to 3700 cal yr BP, and find only limited evidence for low-lake stands during the past millennium. The extent of low-water levels during the mid-Holocene, which were most severe and widespread ca 7000–4500 cal yr BP, is consistent with the extent of insolation-induced aridity in previously published regional climate model simulations. Like the simulations, the lake data provide no evidence for enhanced zonal flow during the mid-Holocene, which has been invoked to explain enhanced mid-continent aridity at the time. The data, including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium, confirm the ability of large boundary-condition changes to push western water supplies beyond the range of recent natural variability. 相似文献
245.
Detlef Rost Thomas Stephan Ansgar Greshake Jörg Fritz Iris Weber Elmar K. Jessberger Dieter Stöffler 《Meteoritics & planetary science》2009,44(8):1225-1237
Abstract— A rare three‐phase symplectite consisting of Ca‐rich pyroxene, Fe‐rich olivine, and a silica phase is frequently found rimming pyroxene in the Martian meteorite Los Angeles. This assemblage is usually interpreted as the breakdown product of metastable pyroxferroite, a very rare pyroxenoid mineral itself. However, its origin is not entirely understood, mainly because the extremely small average size of the constituent phases represents a challenge for precise high‐resolution analysis. In addition to electron microbeam methods, the present study uses time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) to overcome the limits of spatial resolution and to comprehensively study this mineral assemblage. The prevailing breakdown hypothesis is supported by the following results: (1) The three symplectite phases are very homogenous in composition from 100 μm down to the micrometer scale. (2) The silica phase could be shown to be almost pure SiO2. (3) The symplectite bulk composition is consistent with pyroxferroite. Sub‐micrometer sized Ti‐oxide grains are found within the symplectite (but not within the Ca‐rich pyroxene) and probably represent a minor breakdown phase in addition to the three main phases. 相似文献
246.
Looking at kriging problems with huge numbers of estimation points and measurements, computational power and storage capacities
often pose heavy limitations to the maximum manageable problem size. In the past, a list of FFT-based algorithms for matrix
operations have been developed. They allow extremely fast convolution, superposition and inversion of covariance matrices
under certain conditions. If adequately used in kriging problems, these algorithms lead to drastic speedup and reductions
in storage requirements without changing the kriging estimator. However, they require second-order stationary covariance functions,
estimation on regular grids, and the measurements must also form a regular grid. In this study, we show how to alleviate these
rather heavy and many times unrealistic restrictions. Stationarity can be generalized to intrinsicity and beyond, if decomposing
kriging problems into the sum of a stationary problem and a formally decoupled regression task. We use universal kriging,
because it covers arbitrary forms of unknown drift and all cases of generalized covariance functions. Even more general, we
use an extension to uncertain rather than unknown drift coefficients. The sampling locations may now be irregular, but must
form a subset of the estimation grid. Finally, we present asymptotically exact but fast approximations to the estimation variance
and point out application to conditional simulation, cokriging and sequential kriging. The drastic gain in computational and
storage efficiency is demonstrated in test cases. Especially high-resolution and data-rich fields such as rainfall interpolation
from radar measurements or seismic or other geophysical inversion can benefit from these improvements. 相似文献
247.
In this work, the interaction of natural organic matter (NOM) with metal(loid)s (Cu, Pb, Zn, Pt, As) and the role of NOM on
the metal(loid) transport in a water-saturated quartz sand column were investigated. For detailed information, size exclusion
chromatographic (SEC) measurements and “short pulse” laboratory transport experiments with online metal(loid) and NOM detection
were used. The SEC measurements showed the formation of metal–NOM complexes. Cu, Pb, Zn and Pt were predominantly bound to
the high molecular mass NOM molecules. The binding capacity of the NOM for metals increased with increasing pH value and in
the following order: Zn < Pb < Cu < Pt. No evidence for the formation of As–NOM complexes was found. The transport experiments
showed no significant influence of NOM on the mobility of Cu, Pb and Zn. The metal–NOM complexes detected in the SEC experiments
were obviously sorbed completely onto the grain surfaces in case of the quartz sand system, or they were dissociated partially
during passage through the column. No influence of NOM was observed on the transport of As as well. Inorganic Zn and As species
were transported through the column with increasing retardation as the pH value increased. Pt showed a high mobility at a
pH of 5, and it decreased at a pH of 7 especially in the presence of NOM. The results support the known fact that a decrease
in the pH value results in enhanced transport of inorganic metal(loid) species in water-saturated porous media. On the other
hand, the presence of NOM can immobilise the metals through metal–NOM complex formation and the deposition of the complexes
onto the stationary phase. 相似文献
248.
Martin Wipf Ulrich Anton Glasmacher Daniel Fritz Stockli Axel Emmerich Thilo Bechstädt Heinrich Baur 《International Journal of Earth Sciences》2010,99(3):675-686
Miocene Intrusives and Lower Cretaceous siliciclastic sedimentary rocks from the Basal Complex in western-Fuerteventura were
analyzed with low-temperature thermochronometric methods such as fission-track, and (U–Th–Sm)/He dating, in order to reveal
the evolution of the island’s exhumation history. The obtained thermochronometric data yields a very slow rate of cooling
in the order of 1.5–3°C/Myr from ~50 to 20 Ma for the Early Cretaceous siliciclastic rocks. These sedimentary units have never
been heated significantly above 240°C after deposition and still record the submarine onset of the island’s formation in the
Eocene. Intrusive bodies associated with the early Miocene magmatic activity of the central volcanic complex of the island
show rapid initial cooling rates of 50–70°C/Myr from ~20 to 14 Ma. Contemporaneous with the intrusions the cooling rate of
the Cretaceous sedimentary units increased to 25–35°C/Myr and it is inferred that this increase is associated with enhanced
uplift and erosion of the Central Volcanic Complex. After ~14 Ma rates slowed down to 3–6°C/Myr. Palaeosols overlying the
sedimentary units are themselves covered by Pliocene basalt flows and reveal that the sedimentary rocks reached the surface
before ~5 Ma. The thermochronometric data obtained in this study for central Fuerteventura is difficult to reconcile with
the cooling history derived from previously obtained fission-track and K–Ar data from the north-western part of the island.
This inconsistency is likely to indicate that the exhumation history of Fuerteventura is more complex and regionally subdivided
than previously believed. 相似文献
249.
Biljana Starijaš Axel Gerdes Dražen Balen Darko Tibljaš Fritz Finger 《Swiss Journal of Geoscience》2010,103(1):61-82
For a long time the Moslavačka Gora Massif in Croatia has been regarded as a major outcrop of Variscan crystalline basement
of the South Tisia block. However, new geochronological data indicate that this massif consists of a Cretaceous S-type granite
pluton intruding a Cretaceous low-pressure/high-temperature (LP/HT) metamorphic envelope. The age of the LP/HT metamorphism
is estimated at ~90–100 Ma using the method of electron microprobe based monazite dating. The Central Granite was dated at
82 ± 1 Ma (LA-SF-ICP-MS zircon age). The metamorphic complex comprises mainly felsic anatexites and orthogneisses of granitic
composition, some metapelites (paragneisses and mica schists) and amphibolites. Zircons from three different samples of metagranite
were dated at 486 ± 6, 483 ± 6, and 491 ± 1 Ma, suggesting that most of the metamorphic complex represents an Early Ordovician
granitic series. The Cretaceous regional metamorphism culminated in granulite facies conditions of ~750°C and 3–4 kbar. A
retrograde metamorphic event at lower amphibolite facies conditions overprinted the metamorphic complex. This event is probably
related to the intrusion of the Central Granite. The southeastern sector of the massif was additionally affected by post-granitic,
predominantly NE oriented shearing at greenschist facies conditions. As yet there is no clear evidence for Variscan events
in the Moslavačka Gora Massif. Mineral relics of a medium-pressure amphibolite facies metamorphism are preserved in amphibolites.
They are older than the Cretaceous LP/HT regional metamorphism, but their age is presently unknown. Some indications for a
Permian regional metamorphic event are provided by inherited zircons in the Central Granite that have been dated with a Permian
age, and by Permian monazite relics in metapelites. The Cretaceous high heat flow regime recorded in the Moslavačka Gora Massif
is unique in the subcrop of the Pannonian Basin and may be a local feature triggered by a mafic intrusion in the lower crust. 相似文献
250.
Bertrand Fritz Emmanuel Jacquot Benoit Jacquemont Armelle Baldeyrou-Bailly Michel Rosener Olivier Vidal 《Comptes Rendus Geoscience》2010,342(7-8):653-667
The development of the Enhanced Geothermal System (EGS) at Soultz-sous-Forêts (France) has given to scientists an interesting opportunity for the application of geochemical modelling of water–rock interactions, combining theoretical studies with field and experimental data. The main results of four successive and complementary studies are summarized: geochemical modelling of fluid–rock interactions with prediction of dissolution/precipitation of minerals, feed-back effects on the mineralogy and petrography of the rock (major role of silicates in the geological past and of carbonates in the near future of the exploitation), experimental control of the dynamics of silicates under thermal gradient and relation between the evolution of the petrophysics of the rocks and the heat and mass transfers. The thermal cycle of the fluid, between 200 °C and 65 °C in the geothermal loop, may be responsible for dissolution/precipitation of minerals which modify the porosity and permeability of the granite, as it happened in the geological past, in relation with hydrothermal circulations in the Rhine Graben. 相似文献