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91.
Giant ground figures are widespread in the lower Colorado River area of southwestern North America, yet their chronology has remained unconstrained by numerical ages. Thirteen AMS 14C measurements reported here indicate that geoglyphs were made from before ˜A. D. 1200 to before ˜900 B. C. We account for potential contamination from prior organics in weathering rinds. All other potential errors point to 14C dates being minimum-limiting ages for the manufacturing of geoglyphs. Although these ages indicate considerable chronological complexity among geoglyphs, our data are consistent with the linguistic hypothesis that the Yuman people in the desert of southeastern California migrated from Baja California—rather than from the north. These results must, however, be placed under the cloud of uncertainty that hangs over the entire field of AMS dating of rock art: the untested assumption surrounding contemporeneity of organics in a surface context. © 1995 John Wiley & Sons, Inc. 相似文献
92.
Ejecta at North Ray crater (Apollo 16) sampled a unique section of the lunar highlands not accessible at most other landing sites and provide important constraints on the composition of late accreted materials. New data on multiple aliquots of four fragmental matrix breccias and a fragment‐laden melt breccia from this site display a variety of highly siderophile element patterns which may represent the signatures of volatile element‐depleted carbonaceous chondrite‐like material, primitive achondrite, differentiated metal, and an impactor component that cannot be related to known meteoritic material. The latter component is prevalent in these rocks besides characterized by depletions in Re and Os compared to Ir, Ru and Pt, chondritic Re/Os, and a gradual depletion of Pd and Au. The observed characteristics are more consistent with fractionations by nebular processes, like incomplete condensation or evaporation, than with lunar crustal processes, like partial melting or volatilization. The impactor signature preserved in these breccias may stem from primitive meteorites with a refractory element composition moderately different from known chondrites. The presence of distinct impactor components within the North Ray crater breccias together with observed correlations of characteristic element ratios (e.g., Re/Os, Ru/Pt, Pd/Ir) in different impact lithologies of four Apollo landing sites constrains physical mixing processes ranging from the scale of gram‐sized samples to the area covered by the Apollo missions. 相似文献
93.
94.
Karen Rene Stockstill Harry Y. McSween Robert J. Bodnar 《Meteoritics & planetary science》2005,40(3):377-396
Abstract— Nakhla contains crystallized melt inclusions that were trapped in augite and olivine when these phases originally formed on Mars. Our study involved rehomogenization (slow‐heating and fast‐heating) experiments on multiphase melt inclusions in Nakhla augite. We studied melt inclusions trapped in augite because this phase re‐equilibrated with the external melt to a lesser extent than olivine and results could be directly compared with previous Nakhla melt inclusion studies. Following heating and homogenization of encapsulated melt inclusions, single mineral grains were mounted and polished to expose inclusions. Major element chemistry was determined by electron microprobe. The most primitive melt inclusion analyzed in Nakhla NA03 is basaltic and closely matches previously reported nakhlite parent melt compositions. MELTS equilibrium and fractional crystallization models calculated for NA03 and previous Nakhla parent melt estimates at QFM and QFM‐1 produced phase assemblages and compositions that can be compared to Nakhla. Of these models, equilibrium crystallization of NA03 at QFM‐1 produced the best match to mineral phases and compositions in Nakhla. In all models, olivine and augite co‐crystallize, consistent with the hypothesis that olivine is not xenocrystic but has undergone subsolidus re‐equilibration. In addition, measured melt inclusion compositions plot along the MELTS‐calculated liquid line of descent and may represent pockets of melt trapped at various stages during crystallization. We attempt to resolve discrepancies between previous estimates of the Nakhla parental melt composition and to reinterpret the results of a previous study of rehomogenized melt inclusions in Nakhla. Melt inclusions demonstrate that Nakhla is an igneous rock whose parent melt composition and crystallization history reflect planetary igneous processes. 相似文献
95.
Joint investigations of the Middle Pliocene climate I: PRISM paleoenvironmental reconstructions 总被引:1,自引:0,他引:1
Harry Dowsett Robert Thompson John Barron Thomas Cronin Farley Fleming Scott Ishman Richard Poore Debra Willard Thomas Holtz Jr. 《Global and Planetary Change》1994,9(3-4)
The Pliocene epoch represents an important transition from a climate regime with high-frequency, low-amplitude oscillations when the Northern Hemisphere lacked substantial ice sheets, to the typical high-frequency, high-amplitude Middle to Late Pleistocene regime characterized by glacial—interglacial cycles that involve waxing and waning of major Northern Hemisphere ice sheets. Analysis of middle Pliocene (3 Ma) marine and terrestrial records throughout the Northern Hemisphere forms the basis of an integrated synoptic Pliocene paleoclimate reconstruction of the last significantly warmer than present interval in Earth history. This reconstruction, developed primarily from paleontological data, includes middle Pliocene sea level, vegetation, land—ice distribution, sea—ice distribution, and sea-surface temperature (SST), all of which contribute to our conceptual understanding of this climate system. These data indicate middle Pliocene sea level was at least 25 m higher than present, presumably due in large part to a reduction in the size of the East Antarctic Ice Sheet. Sea surface temperatures were essentially equivalent to modern temperatures in tropical regions but were significantly warmer at higher latitudes. Due to increased heat flux to high latitudes, both the Arctic and Antarctic appear to have been seasonally ice free during the middle Pliocene with greatly reduced sea ice extent relative to today during winter. Vegetation changes, while more complex, are generally consistent with marine SST changes and show increased warmth and moisture at higher latitudes during the middle Pliocene. 相似文献
96.
97.
Abstract— Recent measurements of ordinary chondrite physical and thermal properties along with new geothermometry studies have provided the necessary parameters for updating a previously proposed model (Miyamoto et al., 1981) for the thermal evolution and internal structure of ordinary chondrite parent bodies. Model calculations assumed a heat source term derived from the decay of 26Al (justification is provided). Differences from the previous model include: varying the thermal diffusivity parameter with increasing temperature (and decreasing porosity), using variable physical and thermal parameters to provide end member models, and incorporating a shortened thermal history of 60 Ma (obtained from new Pb-Pb chronology of phosphates) rather than 100 Ma. Times of isotopic closure in chondrite phosphates overlap the thermal model estimates, and postmetamorphic cooling rates from the model approximately coincide, in both trend and magnitude, with metallographic and fission track cooling rate data. Model calculations attempt to match peak metamorphic conditions in the central portions of these bodies and yield accretion ages between 1.4 to 3.1 Ma after calcium-aluminum inclusion (CAI) formation. Model calculations also predict that both the H and the L chondrite parent asteroids consisted of ~80% equilibrated and 20% unequilibrated chondritic material and that their original radii ranged from 80 to 95 km. 相似文献
98.
Aloys Borgers Frank Hofman Maarten Ponjé Harry Timmermans 《Journal of Geographical Systems》2001,3(4):347-367
None of the currently developed activity-based models of transport demand explicitly models task allocation among household
members. To fill this gap, the present paper suggests to complement activity-based models of activity scheduling with a context-dependent
model of task allocation. That is, it is assumed that the allocation of tasks within households is partly based on such contextual
variables as the amount of time a member has to spend on mandatory activities and car availability. In particular, the paper
advocates a conjoint-based approach, based on an assignment task as opposed to the traditional ranking, rating or choice response
formats. By definition, an assignment task involves a combinatorial explosion of choice alternatives, implying that additional
operational decisions to estimate the context-dependent model are required. This study presents the results of various numerical
experiments, conducted to better understand the impacts of those decisions on the degree of bias in the parameter estimates
of the choice model. The results of these simulations indicate that under particular assumptions, the development and estimation
of a conjoint-based, context-dependent model of task allocation within households is feasible.
Received: March 1999 / Accepted: June 2001 相似文献
99.
100.
Possible mechanisms for the production of the extended dust coma observed in comet Bowell (1980b) at the large heliocentric distance of 7.17 AU are considered. It is concluded that a plausible mechanism is electrostatic blow-off of fine, loose dust from an electrically charged H2O dominated nucleus, as recently proposed by Mendiset al. (1981). Of all the other processes considered, dust entrainment by a chemical species more volatile than H2O is considered the most plausible. However, the non-observation of prominent CO+ lines of theA
2-X
2 band, seems to mitigate against a CO dominated nucleus, particularly if its radius is 1 km. A CO2 dominated nucleus, however, cannot be ruled out by this non-observation unless its radius is considerably larger ( 5 km). The electrostatic blow-off process predicts that the dust grains in the coma of comet Bowell at 7.17 AU must be very small (r
g0.4 ), with a total mass 4×108 g, and a spatial extension 105 km, which is consistent with the observations (observed coma diameter 6×104 km). Both the size and spatial distribution of the dust given by electrostatic blow-off and the gas entrainment processes are shown to be different, and this would be a way of discriminating between the two processes. However, the present observations do not have the resolution to do so.Department of Physics.Department of Electrical Engineering and Computer Sciences. 相似文献