全文获取类型
收费全文 | 1486篇 |
免费 | 51篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 25篇 |
大气科学 | 130篇 |
地球物理 | 353篇 |
地质学 | 477篇 |
海洋学 | 153篇 |
天文学 | 194篇 |
综合类 | 4篇 |
自然地理 | 203篇 |
出版年
2021年 | 8篇 |
2020年 | 13篇 |
2019年 | 22篇 |
2018年 | 42篇 |
2017年 | 29篇 |
2016年 | 36篇 |
2015年 | 34篇 |
2014年 | 51篇 |
2013年 | 83篇 |
2012年 | 69篇 |
2011年 | 79篇 |
2010年 | 71篇 |
2009年 | 54篇 |
2008年 | 72篇 |
2007年 | 61篇 |
2006年 | 59篇 |
2005年 | 61篇 |
2004年 | 41篇 |
2003年 | 40篇 |
2002年 | 51篇 |
2001年 | 28篇 |
2000年 | 28篇 |
1999年 | 23篇 |
1998年 | 23篇 |
1997年 | 24篇 |
1996年 | 13篇 |
1995年 | 22篇 |
1994年 | 15篇 |
1993年 | 25篇 |
1992年 | 15篇 |
1991年 | 8篇 |
1990年 | 17篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 12篇 |
1986年 | 19篇 |
1985年 | 28篇 |
1984年 | 20篇 |
1983年 | 24篇 |
1982年 | 21篇 |
1981年 | 12篇 |
1980年 | 19篇 |
1979年 | 18篇 |
1978年 | 21篇 |
1977年 | 13篇 |
1976年 | 16篇 |
1975年 | 8篇 |
1974年 | 12篇 |
1973年 | 15篇 |
1971年 | 6篇 |
排序方式: 共有1539条查询结果,搜索用时 15 毫秒
111.
Robert F. Anderson 《Geochimica et cosmochimica acta》1982,46(7):1293-1299
Concentrations and fluxes of particulate U were measured throughout the water column at several locations in the Atlantic and Pacific Oceans with in situ filtration systems and sediment traps. The results indicate that dissolved U is fixed to particles in surface seawater. Organic matter appears to be the carrier phase. Formation of particulate authigenic U below the surface waters could not be detected. Authigenic U is remineralized within the bathypelagic layers at the open ocean sites studied. In the Panama Basin, an upwelling area with high biological productivity, remineralization of authigenic U in the deep water column was not observed. The rate of remineralization of authigenic U in the deep sea is insufficient to produce a measurable concentration gradient between surface and deep waters within the mixing time of the oceans. Formation of authigenic U in the water column in areas such as the Panama Basin is not a significant sink for U on an ocean wide basis. 相似文献
112.
James R. Anderson 《Contributions to Mineralogy and Petrology》1983,84(2-3):253-271
Peraluminous and metaluminous plutonic rocks of the Peninsular Ranges batholith near Borrego Springs in southern California were mylonitized in the large shear zone known as the eastern Peninsular Ranges mylonite zone (EPRMZ). Accompanying mylonitization in this portion of the EPRMZ was metamorphism at intermediate-low-pressure amphibolite-facies conditions. Deformation in the zone overlapped in time with Cretaceous intrusion of the batholith. In the San Ysidro Mountain — Pinyon Ridge area, four north-south trending zones of differing intensity of deformation have been defined; from east to west the degree and style of deformation gradually change from undeformed or weakly deformed rocks to strongly mylonitized rocks. Electron microprobe analysis shows that recrystallized hornblende, biotite, and plagioclase are variable in composition, probably reflecting a range of metamorphic conditions accompanying deformation. Comparison of mineral compositions with those from mafic schists of Vermont suggests conditions ranged from andalusite-staurolite through sillimanite-muscovite grades as defined for pelitic rocks. Stability of muscovite+quartz in mylonite assemblages and lack of remelting of granitic rocks indicate that temperature did not exceed about 650° C during mylonitization and lithostatic pressure did not exceed about 5 kbar. Over time, any given rock volume experienced a range of temperature, lithostatic pressure, and perhaps fluid pressure and differential stress. Mineral reactions in the zone involved hydration, requiring introduction of water. The possibility of large-scale migration of K and Fe is suggested by whole-rock chemical data. Brittle and ductile deformation features are closely associated in one part of the EPRMZ. The combined evidence suggests the presence of a pore fluid with fluid pressure close to lithostatic pressure. Short periods of low fluid pressure and possible high differential stress cannot be ruled out. 相似文献
113.
114.
The chemistry of glycolaldehyde (hydroxyacetaldehyde) relevant to the troposphere has been investigated using UV absorption spectrometry and FTIR absorption spectrometry in an environmental chamber. Quantitative UV absorption spectra have been obtained for the first time. The UV spectrum peaks at 277 nm with a maximum cross section of (5.5± 0.7)×10–20 cm2 molecule–1. Studies of the ultraviolet photolysis of glycolaldehyde ( = 285 ± 25 nm) indicated that the overall quantum yield is > 0.5 in one bar of air, with the major products being CH2OH and HCO radicals. Rate coefficients for the reactions of Cl atoms and OH radicals with glycolaldehyde have been determined to be (7.6± 1.5)×10–11 and (1.1± 0.3)×10–11 cm3 molecule–1 s–1, respectively, in good agreement with the only previous study. The lifetime of glycolaldehyde in the atmosphere is about 1.0 day for reaction with OH, and > 2.5 days for photolysis, although both wet and dry deposition should also be considered in future modeling studies. 相似文献
115.
116.
North End Lake is a polluted and eutrophic freshwater system located in Port Elizabeth, South Africa. Since the lake is expected
to be used for recreational/tourist purposes by 2010, a rehabilitation program will have to be designed. For this reason,
we retrieved a sediment core from the central region of the lake to decipher the effect of historical human impacts on the
water body. Pre-disturbance paleolimnological inferences indicate that the lake was likely mesotrophic. After ∼1831, when
sheep farming activities were undertaken in the catchment, increases in trophic state and changes in sediment composition
were observed. After ∼1937, increases in trace metal levels, organic matter, spheroidal carbonaceous particles (SCP) and changes
in sediment composition were recorded. The system became eutrophic as indicated by the dominance of the diatom Actinocyclus normanii, a cosmopolitan species often observed in systems where water quality has been dramatically degraded. The conditions worsened
after 1986 because of the construction of a storm-water retention system, which intentionally channeled storm-water runoff
into the lake. Because of this, extremely high values of fecal coliforms (i.e. 2 × 106 every 100 ml) have been measured in the water column. The paleolimnological information identified the sharp increase in
organic content in the uppermost section of the core, and this could be correlated to the operation of the storm-water retention
system. Therefore, as an immediate management measure, we suggest that the storm-water retention system should either no longer
be utilized, or the storm-water runoff should be treated before disposal into the lake. In addition, an effective sewage system
has to be constructed. 相似文献
117.
118.
Controls on Spatial Variability of Uranium in Sandstone Aquifers 总被引:1,自引:0,他引:1
119.
Karen A. Hudson-Edwards Mark G. Macklin Jerry R. Miller Paul J. Lechler 《Journal of Geochemical Exploration》2001,72(3)
The Río Pilcomayo rises on the Cerro Rico de Potosí precious metal-polymetallic tin deposits of Bolivia, and flows in a southeasterly direction for ca. 600 km to Bolivia's southern border with Argentina. Mining of the Potosí deposits has occurred continuously since 1545, generating large quantities of waste materials in the headwater of the basin. In addition, a tailings dam breach at the Porco mine in 1996 released an estimated 235 000 m3 of tailings and fluid into the upper reaches of the Río Pilaya, the largest tributary to the Pilcomayo.Concentrations of As, Sb, Cd, Cu, Pb, Hg, Ag, Tl and Zn in contemporary channel sediments upstream of the Pilaya confluence are significantly elevated above background values. Elevated levels appear to be associated with pyrite- and other sulphide mineral-bearing tailings materials transported more than 200 km downstream of the Potosí mines. Significant downstream declines in elemental concentrations occur within 15 km, and again between 150 and 200 km, from the mines. The initial decrease in concentrations is due to the rapid dilution of nearly pure tailings effluent released to the river from milling facilities near Potosí. The latter decrease results from a combination of geomorphic processes including the storage of sediment-borne metals within the channel bed and the influx of ‘clean’ sediment from several large tributaries. Downstream of the Pilaya confluence, concentrations of Cu, Pb, Hg and Zn are only slightly elevated above background values, and Ag, Cd, Sb and Tl cannot be distinguished from background levels. These data suggest that while the Porco tailings spill may have had a significant short-term impact on sediment and water quality along the lower reaches of the Río Pilcomayo, its longer-term impacts were limited. Metals stored and eroded from alluvial deposits of historical age in upstream reaches appear to be an important source of metals to the river today. An additional, and perhaps more significant source, is the release of tailings effluent to the river from modern milling operations. The transport of these contaminants downstream of Icla (203 km from Potosí) appears to be restricted by aggradational processes occurring in the vicinity of Puente Sucre. In addition, downstream of the confluence of the Río Pilaya, inputs of large amounts of ‘clean’ sediment have caused dilution of the metal contaminants. Data from other studies where similar geomorphic processes have occurred suggest that the metals in the upper Pilcomayo may eventually be moved downvalley as the aggradational processes are reversed and channel stabilisation occurs. Thus, the most significant impacts of metal contamination may not be realised in downstream areas for decades. 相似文献
120.