全文获取类型
收费全文 | 75316篇 |
免费 | 1218篇 |
国内免费 | 705篇 |
专业分类
测绘学 | 1810篇 |
大气科学 | 5388篇 |
地球物理 | 15064篇 |
地质学 | 26564篇 |
海洋学 | 6531篇 |
天文学 | 17053篇 |
综合类 | 217篇 |
自然地理 | 4612篇 |
出版年
2022年 | 407篇 |
2021年 | 741篇 |
2020年 | 817篇 |
2019年 | 878篇 |
2018年 | 1958篇 |
2017年 | 1819篇 |
2016年 | 2316篇 |
2015年 | 1411篇 |
2014年 | 2270篇 |
2013年 | 3982篇 |
2012年 | 2400篇 |
2011年 | 3306篇 |
2010年 | 2763篇 |
2009年 | 3730篇 |
2008年 | 3420篇 |
2007年 | 3197篇 |
2006年 | 3019篇 |
2005年 | 2480篇 |
2004年 | 2385篇 |
2003年 | 2242篇 |
2002年 | 2063篇 |
2001年 | 1881篇 |
2000年 | 1813篇 |
1999年 | 1465篇 |
1998年 | 1558篇 |
1997年 | 1438篇 |
1996年 | 1170篇 |
1995年 | 1199篇 |
1994年 | 1017篇 |
1993年 | 924篇 |
1992年 | 903篇 |
1991年 | 821篇 |
1990年 | 905篇 |
1989年 | 763篇 |
1988年 | 691篇 |
1987年 | 877篇 |
1986年 | 703篇 |
1985年 | 911篇 |
1984年 | 991篇 |
1983年 | 918篇 |
1982年 | 880篇 |
1981年 | 773篇 |
1980年 | 718篇 |
1979年 | 650篇 |
1978年 | 661篇 |
1977年 | 596篇 |
1976年 | 584篇 |
1975年 | 532篇 |
1974年 | 541篇 |
1973年 | 501篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
281.
James M. Stroh 《Contributions to Mineralogy and Petrology》1976,54(3):173-188
The addition of Fe and Cr to the simple system MgO-SiO2-Al2O3 markedly affects the activities of phases involved in the equilibrium
\textMg\text2 \textSiO\text4 \text + MgAl\text2 \textSiO\text6 \text = MgAl\text2 \textO\text4 \text + Mg\text2 \textSi\text2 \textO\text6 \textOlivine + Opx\textsolid solution \text = Spinel + Opx\textsolid solution \begin{gathered} {\text{Mg}}_{\text{2}} {\text{SiO}}_{\text{4}} {\text{ + MgAl}}_{\text{2}} {\text{SiO}}_{\text{6}} {\text{ = MgAl}}_{\text{2}} {\text{O}}_{\text{4}} {\text{ + Mg}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{6}} \hfill \\ {\text{Olivine + Opx}}_{{\text{solid solution}}} {\text{ = Spinel + Opx}}_{{\text{solid solution}}} \hfill \\ \end{gathered} 相似文献
282.
G. S. Ripp A. G. Doroshkevich M. V. Badmatsyrenov N. S. Karmanov 《Geochemistry International》2007,45(6):538-545
The composition and nature of high-Cr minerals in lithic clasts from the carbonatites of the Veseloe occurrence, northern Transbaikalia, were considered. In order to determine their source, the Cr-bearing phases were compared with chromite, magnetite, and rutile from ultrabasic rocks, mantle xenoliths, and eclogites. It was suggested that the xenoclasts studied were formed at great depths, whereas the carbonatites were directly derived from the mantle rather than formed by the crustal differentiation of a silicate-carbonate melt. 相似文献
283.
284.
The paper presents the experimental investigation and analysis of the non-linear elasto-plastic stress–strain behaviour of
normally consolidated lacustrine clay. Drained triaxial stress path tests were performed on natural block samples of Swiss
lacustrine clay. Data were analysed using plasticity theory and the shape and extent of kinematic yield and bounding surfaces
were determined and found to be elliptical but not congruent. Cross-anisotropic elasticity was used to quantify elastic strains
to permit plastic strain increment vectors and hence a plastic potential surface to be defined. 相似文献
285.
Nitrate-nitrogen (nitrate-N) in groundwater is a potential problem in many parts of Kansas.From 1990 to 1998, 747 water samples were collected from domestic, irrigation, monitoring,and public water supply wells primarily from the western two-thirds of the state, and analyzedfor nitrate-N by the Kansas Geological Survey. Nitrate-N concentrations of the 747 samplesanalyzed range from 29% with less than or equal to 3 mg/L, 51% between 3 and 10 mg/L,and 20% greater than or equal to 10 mg/L. Factors that show a statistically significantrelationship with the occurrence of nitrate-N in Kansas groundwater in this assessment includegeographic area of the state, depth of well, and age of well. Nitrate-N levels of wells screenedin the High Plains aquifer in south-central Kansas showed a statistically higher concentrationthan samples collected from the High Plains aquifer in the western portion of the state.Comparison of nitrate-N with depth of well indicated that shallower wells throughout the stategenerally have higher nitrate-N values than deeper wells. Irrigation wells older than 1975showed statistically higher nitrate-N concentration than wells installed during and since 1975,possibly related to changes in well-construction practices and regulations that occurred in 1975. 相似文献
286.
287.
288.
Thermal characteristics of anisotropic media with inclusions 总被引:1,自引:0,他引:1
289.
K.D. Wake-Dyster M.J. Sexton D.W. Johnstone C. Wright D.M. Finlayson 《Geophysical Journal International》1987,89(1):423-430
Summary. In 1984, the Australian Bureau of Mineral Resources and the Geological Survey of Queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin. A relatively transparent upper crustal basement with an underlying, more reflective lower crust is characteristic of much of the region. Prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the Clarence Moreton Basin and the adjacent Beenleigh Block, and provide some of the most interesting features of the entire survey. A wide angle reflection/refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the Nebine Ridge. The only clear deep reflectors are interpreted as P-to-SV or SV-to-P converted reflections from a mid-crustal boundary at a depth of about 17 km. The combined Nebine Ridge data provide well-constrained P and S wave velocity models of the upper crust, and suggest a crustal structure quite different from that beneath the adjacent Mesozoic basins. 相似文献
290.
Potential implications of differential preservation of testate amoeba shells for paleoenvironmental reconstruction in peatlands 总被引:1,自引:0,他引:1
Edward A. D. Mitchell Richard J. Payne Mariusz Lamentowicz 《Journal of Paleolimnology》2008,40(2):603-618
Testate amoebae are now commonly used in paleoenvironmental studies but little is known of their taphonomy. There is some
experimental evidence for differential preservation of some testate amoeba shell types over others, but it is unclear what,
if any impact this has on palaeoenvironmental reconstruction. To investigate this issue we looked at palaeoecological evidence
for the preservation of different shell types. We then investigated the possible impact of selective preservation on quantitative
palaeoenvironmental inference. We first used existing palaeoecological data sets to assess the vertical patterns of relative
abundance in four testate amoeba shell types: (1) shells made of secreted biosilica plates (idiosomes, e.g. Euglypha), (2) idiosomes with thick organic coating (Assulina), (3) proteinaceous shells (e.g. Hyalosphenia), (4) shells built from recycled organic or mineral particles (xenosomes) (e.g. Difflugia, Centropyxis). In three diagrams a clear pattern of decay was only observed for the idiosome type. In order to assess the implications
of differential preservation of testate amoeba taxa for paleoenvironmental reconstruction we then carried out simulations
using three existing transfer functions and a wide range of scenarios, downweighting different test categories to represent
the impact of selective test decomposition. Simulation results showed that downweighting generally reduced overall model performance.
However downweighting a shell type only produced a consistent directional bias in inferred water table depth where that shell
type is both dominant and shows a clear preference along the ecological gradient. Applying a scenario derived from previous
experimental work did not lead to significant difference in inferred water table. Our results show that differential shell
preservation has little impact on paleohydrological reconstruction from Sphagnum-dominated peatlands. By contrast, for the minerotrophic peatlands data-set loss of idiosome tests leads to consistent underestimation
of water table depth. However there are few studies from fens and it is possible that idiosome tests are not always dominant,
and/or that differential decomposition is less marked than in Sphagnum peatlands. Further work is clearly needed to assess the potential of testate amoebae for paleoecological studies of minerotrophic
peatlands. 相似文献
|