全文获取类型
收费全文 | 2525篇 |
免费 | 146篇 |
国内免费 | 32篇 |
专业分类
测绘学 | 88篇 |
大气科学 | 289篇 |
地球物理 | 651篇 |
地质学 | 775篇 |
海洋学 | 215篇 |
天文学 | 353篇 |
综合类 | 6篇 |
自然地理 | 326篇 |
出版年
2022年 | 13篇 |
2021年 | 49篇 |
2020年 | 59篇 |
2019年 | 53篇 |
2018年 | 66篇 |
2017年 | 80篇 |
2016年 | 106篇 |
2015年 | 88篇 |
2014年 | 97篇 |
2013年 | 182篇 |
2012年 | 117篇 |
2011年 | 156篇 |
2010年 | 119篇 |
2009年 | 140篇 |
2008年 | 140篇 |
2007年 | 139篇 |
2006年 | 132篇 |
2005年 | 108篇 |
2004年 | 91篇 |
2003年 | 92篇 |
2002年 | 81篇 |
2001年 | 51篇 |
2000年 | 57篇 |
1999年 | 38篇 |
1998年 | 34篇 |
1997年 | 30篇 |
1996年 | 28篇 |
1995年 | 30篇 |
1994年 | 18篇 |
1993年 | 19篇 |
1992年 | 12篇 |
1991年 | 18篇 |
1990年 | 15篇 |
1989年 | 28篇 |
1988年 | 11篇 |
1987年 | 20篇 |
1986年 | 11篇 |
1985年 | 22篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 12篇 |
1981年 | 18篇 |
1980年 | 10篇 |
1979年 | 14篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1972年 | 5篇 |
排序方式: 共有2703条查询结果,搜索用时 609 毫秒
831.
Diatom assemblages in recent versus pre-industrial sediments were examined in 40 relatively undisturbed lakes from the Experimental
Lakes Area (ELA). The ELA region of northwestern Ontario receives low amounts of acidic deposition and the lakes have been
minimally disturbed by watershed development or other human activities. Consequently, this region represents an important
location to detect possible changes in lakes due to climate change. In over half of the lakes, planktonic taxa (especially
Discostella stelligera) increased between 10 and 40% since pre-industrial times. Changes in diatom assemblages are consistent with taxa that would
benefit from enhanced stratification and a longer ice-free season. We hypothesized that there should be a relationship between
stratification and measured chemical and physical characteristics of the study lakes. Multiple correlation analysis was undertaken
to see the relationship between planktonic taxa and D. stelligera since pre-industrial times and the physical and chemical characteristics of the study lakes. Lake depth was consistently
identified as an important variable. The timing of the increase in planktonic taxa within cores from these lakes will be needed
to rule out other possible regional changes that may also be occurring in the ELA region. 相似文献
832.
Andrew A. Skabar 《Natural Resources Research》2011,20(3):143-155
Assuming a study region in which each cell has associated with it an N-dimensional vector of values corresponding to N predictor variables, one means of predicting the potential of some cell to host mineralization is to estimate, on the basis
of historical data, a probability density function that describes the distribution of vectors for cells known to contain deposits.
This density estimate can then be employed to predict the mineralization likelihood of other cells in the study region. However,
owing to the curse of dimensionality, estimating densities in high-dimensional input spaces is exceedingly difficult, and
conventional statistical approaches often break down. This article describes an alternative approach to estimating densities.
Inspired by recent work in the area of similarity-based learning, in which input takes the form of a matrix of pairwise similarities
between training points, we show how the density of a set of mineralized training examples can be estimated from a graphical
representation of those examples using the notion of eigenvector graph centrality. We also show how the likelihood for a test
example can be estimated from these data without having to construct a new graph. Application of the technique to the prediction
of gold deposits based on 16 predictor variables shows that its predictive performance far exceeds that of conventional density
estimation methods, and is slightly better than the performance of a discriminative approach based on multilayer perceptron
neural networks. 相似文献
833.
Ryan E. Sherrill Andrew J. Sinclair S. C. Sinha T. Alan Lovell 《Celestial Mechanics and Dynamical Astronomy》2014,119(1):55-73
The relative motion of chief and deputy satellites in close proximity with orbits of arbitrary eccentricity can be approximated by linearized time-periodic equations of motion. The linear time-invariant Hill–Clohessy–Wiltshire equations are typically derived from these equations by assuming the chief satellite is in a circular orbit. Two Lyapunov–Floquet transformations and an integral-preserving transformation are here presented which relate the linearized time-varying equations of relative motion to the Hill–Clohessy–Wiltshire equations in a one-to-one manner through time-varying coordinate transformations. These transformations allow the Hill–Clohessy–Wiltshire equations to describe the linearized relative motion for elliptic chief satellites. 相似文献
834.
Philipp R. Heck Frank J. Stadermann Dieter Isheim Orlando Auciello Tyrone L. Daulton Andrew M. Davis Jeffrey W. Elam Christine Floss Jon Hiller David J. Larson Josiah B. Lewis Anil Mane Michael J. Pellin Michael R. Savina David N. Seidman Thomas Stephan 《Meteoritics & planetary science》2014,49(3):453-467
Atom‐probe tomography (APT) is currently the only analytical technique that, due to its spatial resolution and detection efficiency, has the potential to measure the carbon isotope ratios of individual nanodiamonds. We describe three different sample preparation protocols that we developed for the APT analysis of meteoritic nanodiamonds at sub‐nm resolution and present carbon isotope peak ratios of meteoritic and synthetic nanodiamonds. The results demonstrate an instrumental bias associated with APT that needs to be quantified and corrected to obtain accurate isotope ratios. After this correction is applied, this technique should allow determination of the distribution of 12C/13C ratios in individual diamond grains, solving the decades‐old question of the origin of meteoritic nanodiamonds: what fraction, if any, formed in the solar system and in presolar environments? Furthermore, APT could help us identify the stellar sources of any presolar nanodiamonds that are detected. 相似文献
835.
Eleanor R. Mare Andrew G. Tomkins Belinda M. Godel 《Meteoritics & planetary science》2014,49(4):636-651
Ordinary chondrite meteorites contain silicates, Fe,Ni‐metal grains, and troilite (FeS). Conjoined metal‐troilite grains would be the first phase to melt during radiogenic heating in the parent body, if temperatures reached over approximately 910–960 °C (the Fe,Ni‐FeS eutectic). On the basis of two‐pyroxene thermometry of 13 ordinary chondrites, we argue that peak temperatures in some type 6 chondrites exceeded the Fe,Ni‐FeS eutectic and thus conjoined metal‐troilite grains would have begun to melt. Melting reactions consume energy, so thermal models were constructed to investigate the effect of melting on the thermal history of the H, L, and LL parent asteroids. We constrained the models by finding the proportions of conjoined metal‐troilite grains in ordinary chondrites using high‐resolution X‐ray computed tomography. The models show that metal‐troilite melting causes thermal buffering and inhibits the onset of silicate melting. Compared with models that ignore the effect of melting, our models predict longer cooling histories for the asteroids and accretion times that are earlier by 61, 124, or 113 kyr for the H, L, and LL asteroids, respectively. Because the Ni/Fe ratio of the metal and the bulk troilite/metal ratio is higher in L and LL chondrites than H chondrites, thermal buffering has the greatest effect in models for the L and LL chondrite parent bodies, and least effect for the H chondrite parent. Metal‐troilite melting is also relevant to models of primitive achondrite parent bodies, particularly those that underwent only low degrees of silicate partial melting. Thermal models can predict proportions of petrologic types formed within an asteroid, but are systematically different from the statistics of meteorite collections. A sampling bias is interpreted to explain these differences. 相似文献
836.
Andrew R. Bodman 《The Professional geographer》1985,37(3):288-295
Analyses of the geography of British elections have typically stressed the continuity in patterns of party support. This paper challenges that conclusion. Employing an alternative method of analysis, it identifies significant regional trends in the Conservative and Labour vote between 1950 and 1983. Pro-Labour trends in Scotland and Northwest England are contrasted with secular changes favoring the Conservatives in non-metropolitan Southern England. These long-term trends have not been associated with growing regional polarization in party support. While the findings are specific to Britain, the method of assessing electoral change might be usefully applied to patterns in other countries. 相似文献
837.
838.
839.
840.