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251.
Automated digital photogrammetry was used to produce digital elevation models of experimental model landscapes under controlled laboratory conditions as part of a series of rainfall erosion experiments looking at the evolution of landforms in response to erosion. The method allowed the elevations of the experimental landscapes to be studied in great detail on a regular grid digital terrain map with relatively very little effort. Digital photogrammetry produced elevation data at a resolution of 6 mm with a standard deviation of 2·0 mm over an experimental catchment relief of approximately 200 mm; this resolution is considerably better than that achievable by conventional manual photogrammetry. The density of grid points was sufficiently high that small‐scale details such as knickpoints developing in channels were represented. The method can facilitate the study of both experimental and natural landscapes in great detail. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
252.
253.
A. Downey J. D. Jasper J. J. Gras S. Whittlestone 《Journal of Atmospheric Chemistry》1990,11(1-2):43-68
Radon and particle concentrations obtained at Macquarie Island, halfway between Australia and Antarctica, and on Tasmania's north-west coast at Cape Grim during 1987 are examined. Four-day trajectories based on 12 hourly analyses over the Australian region are used to explore the transport of continental material across the Southern Ocean. This study suggests that at least 25% of the variance of radon concentration at Macquarie Island can be accounted for by transport from the Australian continent. Trajectories at intervals of one hour are used to demonstrate the agreement between episodes of trajectories passing over land and episodes of elevated radon and particle concentration. Wind roses of percentage probability of various levels of radon and particle concentration are compared with back trajectories for their accuracy in depicting episodes of continental air incursion. 相似文献
254.
At two locations in the Atlantic Ocean (DSDP Sites 367 and 530) early to middle Cretaceous organic-carbon-rich beds (“black shales”) were found to have significantly lower δ15N values (lower15N/14N ratios) than adjacent organic-carbon-poor beds (white limestones or green claystones). While these lithologies are of marine origin, the black strata in particular have °15N values that are significantly lower than those previously found in the marine sediment record and most contemporary marine nitrogen pools. In contrast, black, organic-carbon-rich beds at a third site (DSDP Site 603) contain predominantly terrestrial organic matter and have C- and N-isotopic compositions similar to organic matter of modern terrestrial origin.The recurring15N depletion in the marine-derived Cretaceous sequences prove that the nitrogen they contain is the end result of an episodic and atypical biogeochemistry. Existing isotopic and other data indicate that the low15N relative abundance is the consequence of pelagic rather than post-depositional processes. Reduced ocean circulation, increased denitrification, and, hence, reduced euphotic zone nitrate availability may have led to Cretaceous phytoplankton assemblages that were periodically dominated by N2-fixing blue-green algae, a possible source of this sediment15N-depletion. Lack of parallel isotopic shifts in Cretaceous terrestrially-derived nitrogen (Site 603) argues that the above change in nitrogen cycling during this period did not extend beyond the marine environment. 相似文献
255.
Closure of the gap at the abutment joints of bridges has been the source of extensive damage during the 1971 San Fernando and more recent earthquakes. In this paper a model for the investigation of the effects of this gap closure is presented and analysed. The focus of the model is the representation of the non-linear response of the bridge abutments, the foundation and the columns. The model is used for the investigation of the response of a short bridge located in California. 相似文献
256.
The U/Pb dating of ophiolite and arc complexes in the Caledonides of SW Norway has demonstrated that these spatially associated
rocks are also closely related in time. A sequence of tholeiitic island arc volcanics, and an unconformably overlying sequence
of calc-alkaline volcanics have been dated as 494 ± 2 Ma (2σ) and 473 ± 2 Ma respectively. Ophiolitic crust formed both prior
to, and during the first 10 Ma after the tholeiitic island arc volcanism. Boninitic and island arc tholeiitic dyke swarms
intruded the ophiolites soon after they formed and represent a second phase of spreading-related magmatism in the ca 20 Ma
period that separated the tholeiitic and the calc-alkaline island arc volcanism. The magmatism ended with the formation of
alkaline, ocean island basalt (OIB)-like magmas. Quartz dioritic and S-type granitic plutons, dated to 479 ± 5 Ma and 474 +3/−2 Ma
respectively, intruded into the base of the arc crust during and subsequent to the boninitic magmatism, and at the time when
calc-alkaline volcanic centres developed. The quartz dioritic and the granitic rocks contain inherited zircons of Precambrian
age which prove the involvement of a continental source. This together with the geology of the terrain and the geochemistry
of these plutons suggests that the granitic magmas were partly derived from subducted clastic sediments. The Sr and Nd isotope
systematics indicate that the same continental source was a component in the boninitic and the calc-alkaline magmas. While
the calc-alkaline magmas may have gained this continental component at a crustal level by assimilation, both geology and isotope
systematics suggest that the continental component in the boninitic rocks was introduced by source contamination – possibly
by a direct interaction between the mantle source and the S-type granitic magmas. A modified mid ocean ridge basalt-like mantle
source was the principal source during the earliest and the main crust forming stage. This source became replaced by an OIB-like
source during the later stages in the evolution of this ancient arc.
Received: 27 June 1994 / Accepted: 16 September 1996 相似文献
257.
258.
Scott D. Samson James P. Hibbard Greg L. Wortman 《Contributions to Mineralogy and Petrology》1995,121(2):171-184
Nd isotopic analyses of whole-rock samples from the older portion of the Carolina terrane, one of the largest terranes in
the Appalachian orogen, demonstrate that part of this terrane is composed of juvenile, mantle-derived crust. These data suggest
that the terrane may not have originally been built upon old, evolved basement material but rather may have been built upon
oceanic crust. A recent study by other workers demonstrates a more crustally evolved Nd isotopic signature for younger components
of the Carolina terrane. These data may indicate that the terrane interacted with evolved crust at a later time, possibly
by amalgamation with a more evolved crustal fragment before final accretion to Laurentia, rather than indicating a primary
old basement. A juvenile nature for the older portion of the terrane contrasts with models that suggest it is an evolved crustal
fragment that formed in a continental margin setting — a scenario proposed to explain the high proportion of felsic volcanic
rocks within the terrane. It is herein suggested that Carolina is a chemically evolved but isotopically juvenile crustal fragment,
because it remained in an oceanic setting for an unusually long time. In this regard the Carolina terrane is similar to some
of the large accreted terranes in the Canadian Cordillera, such as Wrangellia and Alexander. The presence of juvenile crust
in the Carolina terrane documents that at least part of the southern Appalachian orogen is not composed solely of reactivated
pre-existing continental crust. The importance of this part of the orogen in terms of the volume of juvenile Phanerozoic crustal
material in North America may be larger than previously thought. However, until additional major Appalachian terranes have
been isotopically characterized the volume of juvenile crust in the whole orogen remains unknown. The isotopic make-up of
a terrane can be an important aspect of terrane analysis as different terranes may have significantly different isotopic compositions,
while even widespread pieces of a single terrane should have very similar isotopic characteristics. The Nd isotopic data for
the Carolina terrane form the beginning of an isotope database for terranes in the southern Appalachians.
Received: 15 June 1994/Accepted: 31 January 1995 相似文献
259.
The rate of sandstone weathering in the semi-arid climate of the Hunter Valley, New South Wales, Australia has been estimated
from observations of gravestone weathering in the area. The gravestone data points to two distinct stages in the weathering
process. The first stage covering the first century of exposure is characterised by a relatively low recession rate of 0.5 mm/100 years.
This is followed by a second stage in which the rate of weathering increases sharply to ca 2.5 mm/100 years. The non-linear
nature of the weathering trends over time suggests that during the first century of exposure, structural changes took place
within the sandstone material, which lay the foundation for accelerated weathering after further exposure. Laboratory trials
were also conducted to identify the effectiveness of different weathering processes in the decay of sandstone in this region.
Of the four processes examined, only the freeze–thaw cycle produced a significant degree of mass loss and is therefore most
likely a strong contributor to the weathering of sandstone in this region. 相似文献
260.
Understanding the nature and relative importance of various drivers of change is crucial for sustainable management of natural resources and in prioritizing management efforts, allocating limited resources, and understanding cumulative effects. For this article, we employed structured an expert judgments approach to identify, characterize, and assess the relative importance of the key biophysical and socioeconomic drivers of change within the Volta River Basin, West Africa. Precipitation variability, water availability, land use change, drought events, and population growth were perceived as most important, while biodiversity loss, social conflicts, pest and disease occurrence, urbanization, and pollution were viewed as less critical. A majority of these drivers were characterized as “slow” acting processes as compared to rapidly changing drivers. Intra- and interexpert groups agreement were found to be significant and convergent, indicating the reliability of the results. The implications of these results for sustainable water resources management and agricultural production are discussed. 相似文献