全文获取类型
收费全文 | 838篇 |
免费 | 26篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 73篇 |
地球物理 | 120篇 |
地质学 | 254篇 |
海洋学 | 80篇 |
天文学 | 233篇 |
综合类 | 6篇 |
自然地理 | 93篇 |
出版年
2021年 | 8篇 |
2020年 | 7篇 |
2019年 | 15篇 |
2018年 | 12篇 |
2017年 | 18篇 |
2016年 | 18篇 |
2015年 | 24篇 |
2014年 | 14篇 |
2013年 | 30篇 |
2012年 | 14篇 |
2011年 | 25篇 |
2010年 | 31篇 |
2009年 | 46篇 |
2008年 | 36篇 |
2007年 | 26篇 |
2006年 | 30篇 |
2005年 | 38篇 |
2004年 | 20篇 |
2003年 | 23篇 |
2002年 | 21篇 |
2001年 | 19篇 |
2000年 | 23篇 |
1999年 | 10篇 |
1998年 | 16篇 |
1997年 | 9篇 |
1995年 | 11篇 |
1994年 | 11篇 |
1993年 | 8篇 |
1992年 | 13篇 |
1990年 | 14篇 |
1989年 | 15篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 6篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1980年 | 15篇 |
1979年 | 23篇 |
1978年 | 15篇 |
1977年 | 11篇 |
1976年 | 10篇 |
1975年 | 10篇 |
1974年 | 8篇 |
1973年 | 14篇 |
1972年 | 6篇 |
1971年 | 6篇 |
1969年 | 7篇 |
排序方式: 共有871条查询结果,搜索用时 328 毫秒
61.
62.
Vertical distributions of particulate silica, and of production and dissolution rates of biogenic silica, were determined on two N-S transects across the Pacific sector of the Antarctic Circumpolar Current during the austral spring of 1978. Particulate silica profiles showed elevated levels in surface water and near the bottom, with low (35–110 nmol Si · 1?1) and vertically uniform values through the intervening water column. Both the particulate silica content of the upper 200 m and the production rate of biogenic silica in the photic zone increased from north to south, reaching their highest values near the edge of the receding pack ice. A significant, but variable, fraction (18–58%) of the biogenic silica produced in the surface layer was redissolving in the upper 90–98 m. Net production of biogenic silica in the surface layer (production minus dissolution) was proceeding at a mean rate of ca. 2 mmol Si · m?2 · day?1. This is ca. 4 times greater than the most recent estimate of the mean accumulation rate of siliceous sediments beneath the ACC. We estimate, based on mass balance, that the mean dissolution rate of biogenic silica in subsurface water column in the Southern Ocean is 1.2–2.9 mmol Si · m?2 · day?1. 相似文献
63.
Tectonic systems and the deposition of iron-formation 总被引:8,自引:0,他引:8
Gordon A. Gross 《Precambrian Research》1983,20(2-4)
The iron-formation group of chemically precipitated cherty iron-rich sediments ranges in age from Early Precambrian to Recent and is composed of a variety of lithological facies. Iron-formations are classified as Lake Superior and Algoma types to emphasize the contrasting conditions in the spectrum of sedimentary-tectonic environments in which they formed. Thick stratigraphic units of Lake Superior type iron-formation, which form the most extensive iron ranges of the world, are part of sedimentary-tectonic systems that developed along the margins of cratons or continental platforms and were deposited over extended periods of time under relatively stable tectonic conditions. The Algoma type iron-formations were deposited with volcanic rocks and greywacke in various tectonic-volcanic systems, formed under more dynamic tectonic conditions, but are otherwise comparable to present day spreading ridges on the ocean floor. The thick sequences of Lake Superior type iron-formation deposited between 2.7 and 2.0 Ga form part of major sedimentary-tectonic systems that extended along the margins of continents. Direct biogenic factors and the composition of the atmosphere probably had only limited influence on the precipitation of these chemical sediments. 相似文献
64.
In the context of challenging targets for renewable energy generation, this paper draws out social implications of moves towards low carbon energy systems. As renewable energy develops as a heterogeneous category, many potential forms of social relation between 'publics' and technologies are emerging. Utilising perspectives from science and technology studies, we outline five modes in which renewable energy has been implemented in the UK and how these involve different configurations of technology and social organisation. We argue that a multiplicity of roles for 'the public' are implicated across this increasingly complex landscape, cutting across established categories and raising questions of meaning, differentiation, interrelation and access. Policy assumptions and conceptions are questioned, highlighting that dominant characterisations of public roles have been part of a concentration on particular socio-technical pathways to the exclusion of others. 相似文献
65.
Hypothesized large-scale climatic extremes require verification from distantregions in order toconfirm the magnitude and timing of such events. Three of the most massivehypothesized volcanic events of the past two millennia, occurring in or aboutAD 536, 934 and1258, had profound climatic and demographic repercussions over much of Europe,the MiddleEast, and other areas, according to historical accounts recently described inStothers (1998, 1999,2000) as well as other research. Here we report on frost ring and otherdendrochronologicalevidence derived from a 1738-year tree-ring chronology from Mongolia andmillennial-scaletree-ring data from northern Siberia which demonstrate that these three eventsmay have alsoimpacted conditions in these distant regions. 相似文献
66.
A statistical test has revealed that abrupt regional climate changes are produced in a coupled atmosphere-ocean general circulation
model. Abrupt changes are detected over much of the globe although the occurrence frequency is small over the continents.
Over the tropical Pacific Ocean and northern Pacific Ocean, surface air temperature (SAT) and sea level pressure (SLP) shift
rapidly on decadal time scales. The regional climate changes presented here have been classified into three types. The first
type consists of statistically significant shifts in SLP and statistically significant shifts in SAT which are of opposite
sign, and which are reinforced through a positive feedback between the atmosphere and the ocean. The second type is for those
occurrences where changes are of the same sign. The third type includes those with a significant shift in only one meteorological
element. The second and third types are generally generated by changes in air pressure and wind fields induced by changes
of the first type. For example, when SLP increases and sea surface temperature (SST) decreases abruptly in the tropical Pacific
Ocean, it triggers abrupt regional changes in middle and high latitudes. The abrupt changes in the model climate have characteristics
which are very similar to those of observed rapid shifts. Thus, it is concluded that abrupt changes are a predominant part
of regional climate change on decadal time scales.
Received: 11 February 1999 / Accepted: 18 May 2000 相似文献
67.
Large woody debris (LWD) is an integral component of forested streams of the Pacific Northwest and elsewhere, yet little is known about how far wood is transported and where it is deposited in streams. In this paper, we report the results of flume experiments that examine interactions among hydraulics, channel geometry, transport distance and deposition of floating wood. These experiments were carried out in a 1.22-m-wide×9.14-m-long gravel bed flume using wooden dowels of various sizes as surrogate logs. Channel planforms were either self-formed or created by hand, and ranged from meanders to alternate bars. Floating pieces tended to orient with long axes parallel to flow in the center of the channel. Pieces were deposited where channel depth was less than buoyant depth, typically at the head of mid-channel bars, in shallow zones where flow expanded, and on the outside of bends. We hypothesize that the distance logs travel may be a function of the channel's debris roughness, a dimensionless index incorporating ratios of piece length and diameter to channel width, depth and sinuosity. Travel distance decreased as the ratio of piece length to both channel width and radius of curvature increased, but the relative importance of these variables changed with channel planform. Large pieces can move further than our debris roughness models predict if greater than 50% of the active channel area is deeper than the buoyant depth of the piece, or if momentum is high enough to carry pieces across shallows. Our debris roughness model allows first-order prediction of the amount of wood transport under various channel geometries. 相似文献
68.
Gordon B. Bonan 《Climate Dynamics》1996,12(4):279-285
A subgrid parameterization of infiltration and surface runoff was evaluated using a land surface model coupled to an atmospheric general circulation model. Averaged over 5 year simulations, the subgrid parameterization resulted in significantly less infiltration of water into the soil compared to a simulation without subgrid hydrologic processes. As a result, the soils were drier, latent heat flux decreased, and surface air temperature increased. These results are consistent with other studies of subgrid hydrologic parameterizations, which also resulted in drier soils, decreased latent heat flux, and warmer surface temperatures. Several river basins were studied in detail. In the Amazon and Lena basins, the subgrid parameterization resulted in better annual runoff compared to observed annual river flow; surface air temperature was unchanged in the Amazon and better, compared to observations, in the Lena. In the Ob, Yenisey, and Amur basins, the subgrid parameterization resulted in too much annual runoff; July surface air temperature was unchanged or worse (Amur). Annual runoff for the Mississippi basin was better with the subgrid parameterization, but July surface air temperature was worse. These results suggest the utility of subgrid hydrologic parameterizations vary among river basins depending on the relative importance of Horton and Dunne runoff and the geologic factors affecting runoff generation. 相似文献
69.
A general circulation model is used to simulate the atmospheric response to a prescribed, idealized time varying warm sea surface temperature anomaly (SSTA) in the equatorial Pacific characteristic of ENSO events. The model incorporates the full seasonal cycle and an ensemble of five anomaly simulations, each of 14 months duration, is performed and the results compared with those from a ten year control simulation involving climatological SSTs. In a test of the model, simulated perturbations to precipitation patterns are compared with the analysis by Ropelewski and Halpert which identifies regions where precipitation perturbations are associated with the ENSO cycle. Strong and statistically significant perturbations are simulated throughout much of the equatorial Pacific and the Australian region which agree with the analysis. Perturbations over the Americas and the Indian sub-continent show part agreement with observations, while elsewhere, at more remote and high latitude locations, there is less agreement, providing some indication of model limitations in simulating the hydrological cycle. Over the equatorial Pacific most of the anomalous precipitation is accounted for by moisture flux convergence. Only in the vicinity of the maximum anomaly, located in the eastern Pacific, do warmer SSTs contribute to the perturbed circulation. Elsewhere, anomalous wind speeds mainly determine anomalous heat fluxes. As a result, the large scale perturbations to wind, moisture and precipitation appear to be forced indirectly, rather than directly as assumed in simple ocean-atmosphere models. 相似文献
70.
Gordon A. Macdonald 《Bulletin of Volcanology》1958,19(1):221-229