全文获取类型
收费全文 | 3419篇 |
免费 | 93篇 |
国内免费 | 39篇 |
专业分类
测绘学 | 51篇 |
大气科学 | 301篇 |
地球物理 | 778篇 |
地质学 | 1208篇 |
海洋学 | 330篇 |
天文学 | 485篇 |
综合类 | 11篇 |
自然地理 | 387篇 |
出版年
2021年 | 45篇 |
2020年 | 40篇 |
2019年 | 47篇 |
2018年 | 57篇 |
2017年 | 62篇 |
2016年 | 68篇 |
2015年 | 74篇 |
2014年 | 69篇 |
2013年 | 180篇 |
2012年 | 93篇 |
2011年 | 140篇 |
2010年 | 147篇 |
2009年 | 163篇 |
2008年 | 129篇 |
2007年 | 115篇 |
2006年 | 119篇 |
2005年 | 108篇 |
2004年 | 90篇 |
2003年 | 82篇 |
2002年 | 82篇 |
2001年 | 72篇 |
2000年 | 43篇 |
1999年 | 52篇 |
1998年 | 53篇 |
1997年 | 43篇 |
1996年 | 66篇 |
1995年 | 43篇 |
1994年 | 48篇 |
1993年 | 44篇 |
1992年 | 52篇 |
1991年 | 55篇 |
1990年 | 49篇 |
1989年 | 46篇 |
1988年 | 50篇 |
1987年 | 50篇 |
1986年 | 37篇 |
1985年 | 65篇 |
1984年 | 83篇 |
1983年 | 81篇 |
1982年 | 66篇 |
1981年 | 46篇 |
1980年 | 52篇 |
1979年 | 50篇 |
1978年 | 49篇 |
1977年 | 44篇 |
1976年 | 38篇 |
1975年 | 30篇 |
1974年 | 28篇 |
1973年 | 33篇 |
1972年 | 24篇 |
排序方式: 共有3551条查询结果,搜索用时 0 毫秒
181.
182.
Paul E. Fell Nancy C. Olmstead Elaine Carlson William Jacob Diane Hitchcock Gregory Silber 《Estuaries and Coasts》1982,5(3):234-239
The distribution of macroinvertebrates on Connecticut tidal marshes corresponds well with that reported for other marshes along the Atlantic and Gulf coasts of the United States. The greatest densities and biomass of the ribbed mussel,Geukensia demissa, were found on marshes in the central and western part of the state where both the annual production ofSpartina alterniflora and tidal range are large. *** DIRECT SUPPORT *** A01BY019 00011 相似文献
183.
184.
185.
Some approaches for location of centroids of quartz grain outlines to increase homology between Fourier amplitude spectra 总被引:2,自引:0,他引:2
The ability to test for similarities and differences among families of shapes by closed-form Fourier expansion is greatly enhanced by the concept of homology. Underlying this concept is the assumption that each term of a Fourier series, when compared to the same term in another series, represents the same thing. A method that ensures homology is one which minimizes the centering error, as reflected in the first harmonic term of the Fourier expansion. The problem is to chose a set of edge points derived from a much larger, but variable, number of edge points such that a valid homologous Fourier series can be calculated. Methods are reviewed and criteria given to define a proper solution. An algorithm is presented which takes advantage of the fact that minimization of the error term can be accomplished by minimizing the distance between the origin of the polar coordinate system in the calculation of the Fourier series and the shape centroid. The use of this algorithm has produced higher quality solutions for quartz grain provenance studies. 相似文献
186.
187.
William M. Last 《Environmental Geology》1983,5(4):177-190
Lake Manitoba, North America's thirteenth largest lake, occupies a glacier-scoured basin in south-central Manitoba. Despite its large size, the lake is extremely shallow with a mean depth of 4.5 m. The lake can be subdivided into two connected but distinctly different basins: a small, irregular-shaped North Basin and a much larger and smoother South Basin. Most of the water inflow is from the Waterhen River (42% of the inflow) and from precipitation directly on the lake's surface (40%), while nearly 60% of the outflow is by evaporation. Lake Manitoba water is alkaline and brackish with the salinity dominated by sodium and chloride ions. The surficial offshore deposits of the main South Basin of the lake consist mainly of silt and clay-sized sediments composed of detrital components (clay minerals, quartz, carbonates, and feldspars) and endogenic/authigenic components (carbonates, sulfides, and organic matter). In addition to these modern sediments, several areas of relict fluvial-shoreline sand and till deposits occur in the South Basin. The lacustrine processes presently operating in Lake Manitoba reflect the influence of (1) the extreme shallow depth of the lake, (2) the basin morphology, and (3) the water chemistry. In addition, land clearing and increased watershed drainage have resulted in substantially increased sedimentation rates in the South Basin during the past century. 相似文献
188.
189.
Luigi Beccaluva Gianluca Bianchini Massimo Coltorti William Perkins Franca Siena Carmela Vaccaro Marjorie Wilson 《Contributions to Mineralogy and Petrology》2001,142(3):284-297
Peridotite xenoliths entrained in Plio-Pleistocene alkali basalts from Sardinia represent fragments of the uppermost lithospheric mantle, and are characterised by an anhydrous four-phase mineral assemblage. They range in bulk rock composition from fertile spinel-lherzolites to residual spinel-harzburgites. The Sr-Nd isotope and trace element composition of clinopyroxene mineral separates varies between LREE-depleted samples with 87Sr/86Sr as low as 0.70262 and 143Nd/144Nd up to 0.51323 and LREE-enriched samples with 87Sr/86Sr up to 0.70461 and 143Nd/144Nd down to 0.51252. The available data suggest that all the studied peridotite samples suffered variable degrees of partial melting during Pre-Mesozoic times (based on Nd model ages relative to CHUR and DMM). The overprinted enrichment is related to a subsequent metasomatism, induced by fluids rising through the lithosphere that preferentially percolated the originally most depleted domains. Despite the occurrence of orogenic volcanism in the area, preferential enrichment in elements typically associated with slab derived fluids/melts (K, Rb, Sr, Th) relative to LREE has not been detected, and metasomatism seems to be more likely related to the infiltration of highly alkaline basic melts characterised by an EM-like Sr-Nd isotopic composition. Similar 87Sr/86Sr-143Nd/144Nd compositions, characterised by an EM signature, are observed in anorogenic mafic lavas and peridotite xenoliths from widespread localities within the "European" plate, whereas they have not previously been recorded in peridotite xenoliths and associated alkaline mafic lavas from the stable "African" lithospheric domain. 相似文献
190.
William F. Ruddiman 《Climatic change》2005,69(2-3):427-434