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311.
312.
Miriam Peña Manuel Peimbert Silvia Torres-Peimbert Maria Teresa Ruiz Jose Maza 《Astrophysics and Space Science》1996,238(1):55-58
The recent evolution of the central star of the planetary nebula LMC-N66 is presented. Before 1987, it showed a weak continuum with aT
eff120 000 K andL
bol25 000L
and in a few years it developed strong WR features (P Cygni line profiles in N v at 124.0 nm and C IV at 155.0 nm, wide Heii emission, etc.) typical of a WN 4.5. Additionally the stellar continuum increased by a large factor and the absolute visual magnitude of the star changed from + 1.24 in 1987 to–2.57 in January, 1995. The WR features and enhanced continuum, evidencing a powerful mass-loss event remained with small variations for more than 5 years. Recent ultraviolet and optical data shows that the mass-loss seems to have diminished abruptly in the last three months. 相似文献
313.
A model is proposed which describes the boundary zone between two transcurrent plates as a viscoelastic body, with rheological properties changing with depth. In this model, the brittle-ductile transition is defined as the depth at which the time derivative of shear stress changes from positive to negative values. Variations of this depth are studied as functions of geothermal gradient, rheological parameters and strain rate, using a power law rheology with exponent ranging from 1 to 4. Stress relaxation in the ductile zone is controlled by a local characteristic time, which depends on petrology, temperature and, in the case of non-Newtonian rheology, on strain rate. The composition and the hydration degree of crustal rocks may also sensibly influence the depth of the brittle-ductile transition. The model predictions are compared with observations regarding the San Andreas, Imperial Valley and North Anatolian Faults: it is found that values ofn from 1 to 3 are more appropriate to reproduce the transition depth inferred by the seismicity distribution. 相似文献
314.
Andrea Borsato Silvia Frisia Andrea Somogyi Jean Susini 《Geochimica et cosmochimica acta》2007,71(6):1494-1512
Stalagmite ER78, from Grotta di Ernesto cave in NE Italy displays clear annual lamination consistent with its shallow depth below a forest ecosystem subject to autumnal peaking of water infiltration. Synchrotron radiation scanning micro-X-ray fluorescence analyses of heavy elements at European Synchrotron Radiation Facility beamline ID22, and light elements at ID21, with 1-3 μm resolution has been combined with data from ion microprobe analyses to reveal chemical variability across the visible layers of these annual laminae. A series of elements display a symmetrical peak, centered around the thin, dark layer at the top of each lamina. The peak concentration is ordered Y > Zn, Cu and Pb > P and Br. This hierarchy is thought to reflect the selectivity of transport of these elements, possibly by organic colloids flushed from the soil zone during autumn infiltration. Ion microprobe analysis indicates Na and F also increase, as does H, the latter reflecting increased microporosity. Sr displays a trough around the dark and thin autumn layer implying that its incorporation may be limited by competition with other elements. Mg and S show a different pattern of annual variation and Fe displays none. The trace metals, Br and Y display peak abundance in the early 20th century, which appears to reflect a period of tree-felling rather than a climatic anomaly. The results demonstrate the power of the high spatial resolution and low detection limits of the synchrotron technique, and its ability to produce quantitative maps that allow distinction of layered structure from that of isolated particles, or irregular inhomogeneities. 相似文献
315.
Silvia N. Csari 《Gondwana Research》2007,11(4):529-536
Absolute radiometric data obtained recently in Upper Paleozoic basins of South America and Southern Africa are available to constrain the age of some palynological biozones defined in the Western Gondwana. Some dates obtained from Argentina coincide with southern Africa's and are useful to constrain the palynological associations. In this way, the Permian Argentinian Lueckisporites–Weylandites (LW) Biozone is compared with the Lueckisporites virkkiae Biozone of Brazil and Zone 3 of South Africa. On the other hand, the palynofloras from Dwyka (South Africa) are comparable with Argentinian and Brazilian associations recovered near the Carboniferous–Permian boundary. Tuffaceous intervals of the Dwyka Group in South Africa suggest the beginning of that sequence at 307 Ma reaching the 290 Ma at the top. The available palynological, paleoenvironmental and radiometric data from these three western Gondwanan areas are analyzed and compared. 相似文献
316.
Documentary Evidence on Climate in Sixteenth-Century Europe 总被引:2,自引:0,他引:2
317.
Massimiliano Ghinassi Andrè C. Colonese Zelia Di Giuseppe Lisa Govoni Domenico Lo Vetro Giulia Malavasi Fabio Martini Silvia Ricciardi Benedetto Sala 《第四纪科学杂志》2009,24(4):383-398
Clastic sediments deposited in caves and rock shelters bear peculiar sedimentological characteristics and have seldom been considered as a high‐resolution proxy record of climatic or environmental changes. The Romito Cave has its entrance at 275 m above sea level, about 25 km from the Tyrrhenian coast of Calabria, southern Italy. New archaeological excavation performed since 2000 has revealed a sedimentary succession spanning the record of Gravettian to Late Epigravettian cultures (Late Pleistocene). The present study focuses on the lower part (2.5 m thick) of the succession, where three main unconformity‐bounded stratigraphic units have been recognised (labelled RM1–3). Each unit consists of water‐lain deposits indicating high‐ to low‐competence flow, capped with anthropogenic deposits. The gradual deactivation and reactivation of the water drainage between 23 475 ± 190 and 16 250 ± 500 cal. a BP is correlated with regional precipitation changes due to the onset of dry climatic conditions of the Last Glacial Maximum. However, the deactivation of cave drainage after the deposition of unit RM3, around 15 400 ± 500 cal. a BP, deviates from the regional hydrological trend of progressively increasing water discharges and is attributed to the drainage cut‐off by probable cave wall collapses. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
318.
The epi-benthic dinoflagellate Ostreopsis cf. ovata Fukuyo has an increasingly global distribution. In Brazil there are reports of O. cf. ovata along the coast from 8°S to 27°S latitude and blooms have been registered on the Rio de Janeiro coastline. In the current study, an O. cf. ovata bloom is reported at Saint Paul's Rocks (0°55'10″N; 29°20'33″W), between the southern and northern hemispheres. The Archipelago is not inhabited and not subjected to eutrophication, due to isolation, and sustains a number of endemic species. Therefore, blooms of O. cf. ovata may potentially cause demise to trophic chains by affecting marine invertebrates and vertebrates. 相似文献
319.
In the first decades of the 20th century, the Ebro River was the Iberian channel with the most active fluvial dynamics and the most remarkable spatial‐temporal evolution. Its meandering typology, the dimensions of its floodplain, and the singularities of its flow regime produced an especially interesting set of river functions. The largest dynamics of the Ebro River are concentrated along the meandering profile of the central sector. During the 20th century, this sector experienced a large alteration of its geomorphological structure. We present here an analysis of this evolution through the cartographic study of a long segment of the river (~250 km) in 1927, 1956 and 2003. The results show a large reduction in bank sinuosity, a progressive loss of fluvial territory, and a large decrease in channel width. These changes are especially clear in the areas previously most ecologically connected with the active channel. The fluvial territory of the river in 2003 was approximately half that found during the first decades of the 20th century. Forest plantations, which were non‐existent in 1927, occupied more than 1500 ha of the study area in the last decade. This intense geomorphological transformation becomes ecologically visible in (i) a 35% reduction of the area occupied by riparian vegetation; (ii) a loss of the heterogeneity of riparian forest spots, which were formerly structured in an irregular mosaic far from the river thalweg; and (iii) a modification of the riparian forest structure, which is currently linear, uniform, thin and very close to the river axis. The ecomorphological alteration was intensified by the remarkable reduction in bank length (13%) and the reduced dynamism of the present river system, indicated by an increase in the percentage of fluvial territory occupied by riparian forests and a reduction in the area occupied by the active channel. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
320.
Bérengère Parise Arnaud Belloche Silvia Leurini Peter Schilke 《Astrophysics and Space Science》2008,313(1-3):73-76
Highly-collimated outflows are believed to be the earliest stage in outflow evolution, so their study is essential for understanding
the processes driving outflows. The BHR71 Bok globule is known to harbour such a highly-collimated outflow, which is powered
by a protostar belonging to a protobinary system. Using the APEX telescope on Chajnantor, we mapped the BHR71 highly-collimated
outflow in CO(3-2), and observed several bright points of the outflow in the molecular transitions CO(4-3), CO(7-6), 13CO(3-2), C18O(3-2), CH3OH(7-6) and H2CO(4-3). We use an LVG code to characterise the temperature enhancements in these regions. These observations are particularly
interesting for investigating the interaction of collimated outflows with the ambient molecular cloud. In our CO(3-2) map,
the second outflow driven by IRS2, which is the second source of the binary system, is completely revealed and shown to be
bipolar. We also measure temperature enhancements in the lobes. The CO and methanol LVG modelling points to temperatures between
30 and 50 K in the two lobes. The methanol emission in the southern lobe bright knot is barely resolved with the APEX single-dish.
ALMA will thus be a central tool to study the shock chemistry in these regions. 相似文献