全文获取类型
收费全文 | 458篇 |
免费 | 23篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 11篇 |
大气科学 | 39篇 |
地球物理 | 136篇 |
地质学 | 195篇 |
海洋学 | 43篇 |
天文学 | 45篇 |
综合类 | 5篇 |
自然地理 | 19篇 |
出版年
2023年 | 4篇 |
2022年 | 10篇 |
2021年 | 9篇 |
2020年 | 15篇 |
2019年 | 18篇 |
2018年 | 18篇 |
2017年 | 16篇 |
2016年 | 27篇 |
2015年 | 20篇 |
2014年 | 26篇 |
2013年 | 32篇 |
2012年 | 23篇 |
2011年 | 25篇 |
2010年 | 21篇 |
2009年 | 29篇 |
2008年 | 26篇 |
2007年 | 13篇 |
2006年 | 24篇 |
2005年 | 16篇 |
2004年 | 15篇 |
2003年 | 11篇 |
2002年 | 10篇 |
2001年 | 11篇 |
2000年 | 6篇 |
1999年 | 7篇 |
1998年 | 8篇 |
1997年 | 1篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1979年 | 3篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有493条查询结果,搜索用时 0 毫秒
1.
2.
Xochicalco is a Mesoamerican locality that developed after the fall of Teotihuacan, and became one of the most important urban, ceremonial and military communities of its time. Chronologically, it is situated in the Epiclassic period (700–900 A.D.) and its buildings show a marked hierarchical distribution. The site is located in the current State of Morelos (Mexico), embedded in the Mexican Transition Zone between the two major American zoogeographic regions: Nearctic and Neotropic. The animal remains identified show a mixed assemblage from both regions. With the aim of exploring the relationship between the zoogeographical affinities of identified fauna and differential uses due to the hierarchical distribution of the site, the data were processed using two multivariate analyses in order to find the animals that characterize each social level. This comparison shows that some animals are common to all the social levels, suggesting easy availability, while a few others, mainly from the Neotropic area, were associated only with the elite places suggesting their quality as social markers. This approach offers an initial hypothesis about the different cultural attitudes toward the animals in a Mesoamerican locality, little studied, showing a positive relationship between the zoogeographic affinity and the social hierarchy. 相似文献
3.
Periodic orbits in the Stormer problem are studied using the symmetry lines of the Poincaré map introduced by De Vogelaere. Many known facts are explained by mean of these lines. The dynamics of four special symmetry lines when the Stormer parameter 1 changes is presented, and we obtain a clear global view of the structure of the simple periodic orbits and their bifurcations, including the asymmetrical ones. New asymmetrical multiple periodic orbits are obtained. 相似文献
4.
Effects of trace metal and exposure to air on serotonin and dopamine levels in tissues of the mussel Perna perna 总被引:1,自引:0,他引:1
We evaluated levels of serotonin (5HT) and dopamine (DOPA) in muscle and digestive glands of the mussel, Perna perna, collected at different times of day; exposed to air for 24 h, followed by re-submersion; and after exposure to different metals. Mussels collected at different periods of day showed little oscillation in 5HT and DOPA levels. Mussels exposed to metals showed significant changes in 5HT and DOPA levels in digestive gland and muscle, as did mussels exposed to air. Our data suggest that analyses of 5HT and DOPA in tissues of mussels could serve as a tool to evaluate the presence and effects of heavy metal contamination in mussels. Care in data interpretation is required, however, since other environmental factors such as exposure of mussels to air (i.e. at low tides) can also cause changes in DOPA and 5HT levels. Additional research is necessary to separate such natural environmental effects from effects of contaminants. 相似文献
5.
Eduardo O. Zappettini Carlos J. Chernicoff Joao O. S. Santos Marcelo Dalponte Elena Belousova Neal McNaughton 《International Journal of Earth Sciences》2012,101(8):2083-2104
The studied Carboniferous units comprise metasedimentary (Guaraco Norte Formation), pyroclastic (Arroyo del Torreón Formation), and sedimentary (Huaraco Formation) rocks that crop out in the northwestern Neuquén province, Argentina. They form part of the basement of the Neuquén Basin and are mostly coeval with the Late Paleozoic accretionary prism complex of the Coastal Cordillera, south-central Chile. U–Pb SHRIMP dating of detrital zircon yielded a maximum depositional age of 374?Ma (Upper Devonian) for the Guaraco Norte Formation and 389?Ma for the Arroyo del Torreón Formation. Detrital magmatic zircon from the Guaraco Norte Formation are grouped into two main populations of Devonian and Ordovician (Famatinian) ages. In the Arroyo del Torreón Formation, zircon populations are also of Devonian and Ordovician (Famatinian), as well as of Late Neoproterozoic and Mesoproterozoic ages. In both units, there is a conspicuous population of Devonian magmatic zircon grains (from 406?±?4?Ma to 369?±?5?Ma), indicative of active magmatism at that time range. The εHf values of this population range between ?2.84 and ?0.7, and the TDM-(Hf) are mostly Mesoproterozoic, suggesting that the primary sources of the Devonian magmatism contained small amounts of Mesoproterozoic recycled crustal components. The chemical composition of the Guaraco Norte Formation corresponds to recycled, mature polycyclic sediment of mature continental provenance, pointing to a passive margin with minor inputs from continental margin magmatic rocks. The chemical signature of the Huaraco Formation indicates that a magmatic arc was the main provenance for sediments of this unit, which is consistent with the occurrence of tuff—mostly in the Arroyo del Torreón Formation and very scarcely in the Huaraco Formation—with a volcanic-arc signature, jointly indicating the occurrence of a Carboniferous active arc magmatism during the deposition of the two units. The Guaraco Norte Formation is interpreted to represent passive margin deposits of mostly Lower Carboniferous age (younger than 374?Ma and older than 326?Ma) that precede the onset of the accretionary prism in Chile and extend into the earliest stage of the accretion, in a retrowedge position. The Arroyo del Torreón and Huaraco formations are considered to be retrowedge basin deposits to the early frontal accretionary prism (Eastern Series) of Chile. The presence of volcanism with arc signature in the units provides evidence of a Mississippian magmatic arc that can be correlated with limited exposures of the same age in the Frontal Cordillera (Argentina). The arc would have migrated to the West (Coastal Batholith) during Pennsylvanian–Permian times (coevally with the later basal accretionary prism/Western Series). The source of a conspicuous population of Devonian detrital zircon interpreted to be of magmatic origin in the studied units is discussed in various possible geotectonic scenarios, the preferred model being a magmatic arc developed in the Chilenia block, related to a west-dipping subduction beneath Chilenia before and shortly after its collision against Cuyania/Gondwana, at around 390?Ma and not linked to the independent, Devonian–Mississippian arc, developed to the south, in Patagonia. 相似文献
6.
Low calcification rates and calcium carbonate production in Porites panamensis at its northernmost geographic distribution 下载免费PDF全文
Carlos Orión Norzagaray‐López Luis Eduardo Calderon‐Aguilera José Martín Hernández‐Ayón Héctor Reyes‐Bonilla Juan P. Carricart‐Ganivet Rafael Andrés Cabral‐Tena Eduardo Francisco Balart 《Marine Ecology》2015,36(4):1244-1255
Porites panamensis is a hermatypic coral present in the eastern Pacific Ocean. Skeletal growth parameters have been reported, but studies of the relationship between annual calcification rates and environmental controls are scarce. In this study, we investigated three aspects of the annual calcification rates of P. panamensis: growth parameters among three P. panamensis populations; the sea surface temperature as a calcification rate control spanning a latitudinal gradient; and calcium carbonate production among three sites. Growth parameters varied among the sites due to the colony growth form. Massive colonies in the north showed a higher calcification rate than encrusting colonies in the south (mean: 1.22–0.49 g CaCO3 · cm?2 · yr?1), where variations in calcification rates were related to growth rate (0.91–0.38 cm · yr?1) rather than to skeletal density differences (overall mean ± SD, 1.31 ± 0.04 g CaCO3 · cm?3). Our results showed a positive linear relationship between annual calcification rates and sea surface temperatures within these P. panamensis populations. Differences were related to distinct oceanographic environments (within and at the entrance of the Gulf of California) with different sea surface temperature regimes and other chemical properties. Different populations calcified under different environmental conditions. Calcium carbonate production was dependent upon the calcification rate and coral cover and so carbonate production was higher in the north (coral cover 12%) than in the south (coral cover 3.5). Thus, the studied sites showed low calcium carbonate production (0.25–0.43 kg CaCO3 · m?2 · yr?1). Our results showed reduced calcification rates, regional temperature regime control over calcification rates, different growth forms, low coral cover and low calcium carbonate production rates in P. panamensis. 相似文献
7.
Seasonal variations in diversity and biomass of diatoms, tintinnids, and dinoflagellates and the contribution of microplankton and faecal material to the vertical flux of particulates were investigated at one time series station T (station 18) between 2002 and 2005 and at a grid of stations during November 2004 in the coastal and oceanic area off Concepción (36°S), Chile. The variations were analysed in relation to water column temperature, dissolved oxygen, nutrient concentration, offshore Ekman transport, and chlorophyll-a concentration. Abundance was estimated as cell numbers per litre and biomass in terms of biovolume and carbon units.A sharp decrease with depth was observed in the abundance of both phytoplankton and microzooplankton during the whole annual cycle; over 70% of their abundance was concentrated in the upper 10 m of the water column. Also, a clear seasonality in microplankton distribution was observed at station T, with maxima for diatoms, tintinnids, and dinoflagellates every summer (centred on January) from 2002 to 2005.On the grid of stations, the maximum integrated (0-50 m) micro-phytoplankton abundances (>1 × 109 cells m−2) occurred at the coastal stations, an area directly influenced by upwelling. A similar spatial distribution was observed for the integrated (0-200 m) faecal carbon (with values up to 632 mg C m−2). Tintinnids were distributed in all the first 300 miles from the coast and dinoflagellates were more abundant in oceanic waters.At station T, the average POC export production (below 50 m depth) was 16.6% (SD = 17%; range 2-67%; n = 16). The biological-mediated fluxes of carbon between the upper productive layer and the sediments of the continental shelf off Concepción depend upon key groups of phytoplankton (Thalassiosira spp., Chaetoceros spp.) and zooplankton (euphausiids) through the export of either cells or faecal material, respectively. 相似文献
8.
Cione Alberto L. Tonni Eduardo P. San Cristóbal Jorge Hernández Pedro J. Benítez Adrián Bordignon Fernando Perí Jorge A. 《Earth, Moon, and Planets》2002,91(1):9-24
Ten oblong aligned depressions in the Río Cuarto area (provincia de Córdoba, Argentina) were supposed to be the result of very-low-angle Holocene meteoroid impacts. However, we consider that authors that studied the structures did not demonstrated their extraterrestrial origin. We suggest that an eolian origin for the structures of Río Cuarto is more likely. Actually, these landforms integrate large systems of similar deflation/accumulation geoforms aligned according to predominant winds during different periods. 相似文献
9.
10.
We have carried out an investigation of the environments of low redshift H ii galaxies by cross-correlating their positions on the sky with those of faint field galaxies in the Automatic Plate Measuring Machine (APM) catalogues. We address the question of whether violent star formation in H ii galaxies is induced by low-mass companions by statistically estimating the mean space density of galaxies around them. We argue that even if low-mass companions were mainly intergalactic H i clouds, their optical counterparts should be detectable at faint limits of the APM scans.
A significantly positive signal is detected for the H ii galaxy–APM galaxy angular cross-correlation function, but the amplitude is poorly determined. The projected cross-correlation function has a higher signal-to-noise ratio, and suggests that the amplitude is slightly lower than for normal field galaxies. This implies that these bursting dwarf galaxies inhabit slightly lower density environments than those of normal field galaxies, consistent with other studies of emission-line galaxies. This suggests that in these dwarf starburst galaxies, star formation is not always triggered by tidal interactions, and a significant fraction must have a different origin. 相似文献
A significantly positive signal is detected for the H ii galaxy–APM galaxy angular cross-correlation function, but the amplitude is poorly determined. The projected cross-correlation function has a higher signal-to-noise ratio, and suggests that the amplitude is slightly lower than for normal field galaxies. This implies that these bursting dwarf galaxies inhabit slightly lower density environments than those of normal field galaxies, consistent with other studies of emission-line galaxies. This suggests that in these dwarf starburst galaxies, star formation is not always triggered by tidal interactions, and a significant fraction must have a different origin. 相似文献