全文获取类型
收费全文 | 278篇 |
免费 | 5篇 |
专业分类
测绘学 | 2篇 |
大气科学 | 42篇 |
地球物理 | 78篇 |
地质学 | 49篇 |
海洋学 | 30篇 |
天文学 | 45篇 |
自然地理 | 37篇 |
出版年
2021年 | 6篇 |
2020年 | 1篇 |
2019年 | 2篇 |
2017年 | 2篇 |
2016年 | 7篇 |
2015年 | 5篇 |
2014年 | 11篇 |
2013年 | 11篇 |
2012年 | 12篇 |
2011年 | 8篇 |
2010年 | 11篇 |
2009年 | 9篇 |
2008年 | 10篇 |
2007年 | 12篇 |
2006年 | 19篇 |
2005年 | 12篇 |
2004年 | 13篇 |
2003年 | 6篇 |
2002年 | 19篇 |
2001年 | 14篇 |
2000年 | 6篇 |
1999年 | 8篇 |
1998年 | 6篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1987年 | 3篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1972年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有283条查询结果,搜索用时 171 毫秒
1.
The gray crystalline hematite at Meridiani Planum first discovered by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) instrument occurs as spherules that have been interpreted as concretions. Analysis of the TES and mini-TES spectra shows that no 390 cm−1 feature is present in the characteristic martian hematite spectrum. Here, we incorporate the mid-IR optical constants of hematite into a simple Fresnel reflectance model to understand the effect of emission angle and crystal morphology on the presence or absence of the 390 cm−1 feature in an IR hematite spectrum. Based on the results we offer two models for the internal structure of the martian hematite spherules. 相似文献
2.
The Ocean Drilling Program Leg 175 recovered a unique series of stratigraphically continuous sedimentary sections along the SW African margin, an area which is presently affected by active coastal upwelling. The accumulation rates of organic and inorganic carbon are a major component of this record. Four Leg 175 sites (1082, 1084, 1085, 1087) are chosen as part of a latitudinal transect from the present northern to southern boundaries of the Benguela Current upwelling system, to decipher the Pliocene–Pleistocene history of biogenic production and its relationship with global and local changes in oceanic circulation and climate. The pattern of CaCO3 and Corg mass accumulation rates (MARs) over 0.25-Myr intervals indicates that the evolution of carbon burial is highly variable between the northern and the southern Benguela regions, as well as between sites that have similar hydrological conditions. This, as well as the presence over most locations of high-amplitude, rapid changes of carbon burial, reflect the partitioning of biogenic production and patterns of sedimentation into local compartments over the Benguela margin. The combined mapping of CaCO3 and Corg MARs at the study locations suggests four distinct evolutionary periods, which are essentially linked with major steps in global climate change: the early Pliocene, the mid-Pliocene warm event, a late Pliocene intensification of northern hemisphere glaciation and the Pleistocene. The early Pliocene spatially heterogeneous patterns of carbon burial are thought to reflect the occurrence of mass-gravitational movements over the Benguela slope which resulted in disruption of the recorded biogenic production. This was followed (3.5–3 Ma) by an episode of peak carbonate accumulation over the whole margin and, subsequently, by the onset of Benguela provincialism into a northern and a southern sedimentary regime near 2 Ma. This mid and late Pliocene evolution is interpreted as a direct response to changes in the ventilation of bottom and intermediate waters, as well as to dynamics of the subtropical gyral circulation and associated wind stress. 相似文献
3.
Correlation by Rb-Sr geochronology of garnet growth histories from different structural levels within the Tauern Window,Eastern Alps 总被引:1,自引:1,他引:0
John N. Christensen Jane Selverstone John L. Rosenfeld Donald J. DePaolo 《Contributions to Mineralogy and Petrology》1994,118(1):1-12
In order to evaluate rates of tectonometamorphic processes, growth rates of garnets from metamorphic rocks of the Tauern Window, Eastern Alps were measured using Rb-Sr isotopes. The garnet growth rates were determined from Rb-Sr isotopic zonation of single garnet crystals and the Rb-Sr isotopic compositions of their associated rock matrices. Garnets were analyzed from the Upper Schieferhülle (USH) and Lower Schieferhülle, (LSH) within the Tauern Window. Two garnets from the USH grew at rates of 0.67
–0.13
+0.19
mm/million years and 0.88
–0.19
+0.34
mm/million years, respectively, indicating an average growth duration of 5.4±1.7 million years. The duration of growth coupled with the amount of rotation recorded by inclusion trails in the USH garnets yields an average shear-strain rate during garnet growth of 2.7
–0.7
+1.2
×10-14 s-1. Garnet growth in the sample from the USH occurred between 35.4±0.6 and 30±0.8 Ma. The garnet from the LSH grew at a rate of 0.23±0.015 mm/million years between 62±1.5 Ma and 30.2±1.5 Ma. Contemporaneous cessation of garnet growth in both units at 30 Ma is in accord with previous dating of the thermal peak of metamorphism in the Tauern Window. Correlation with previously published pressure-temperature paths for garnets from the USH and LSH yields approximate rates of burial, exhumation and heating during garnet growth. Assuming that theseP — T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garnet in the USH and LSH were approximately 4
–2
+3
mm/year and 2±1 mm/year, respectively. 相似文献
4.
5.
6.
We are using a three-dimensional convection-driven numerical dynamo model without hyperdiffusivity to study the characteristic structure and time variability of the magnetic field in dependence of the Rayleigh number (Ra) for values up to 40 times supercritical. We also compare a variety of ways to drive the convection and basically find two dynamo regimes. At low Ra, the magnetic field at the surface of the model is dominated by the non-reversing axial dipole component. At high Ra, the dipole part becomes small in comparison to higher multipole components. At transitional values of Ra, the dynamo vacillates between the dipole-dominated and the multipolar regime, which includes excursions and reversals of the dipole axis. We discuss, in particular, one model of chemically driven convection, where for a suitable value of Ra, the mean dipole moment and the temporal evolution of the magnetic field resemble the known properties of the Earth’s field from paleomagnetic data. 相似文献
7.
A new 3-D model REGINA (REGIonal high resolutioN Air pollution model) is under development at the National Environmental Research Institute (NERI). The model is based on pieces from several models developed over the last decades at NERI. The aim of the work is to develop a nested model which can operate with very high resolution in both space and time. To fulfill this aim the choice and implementation of accurate numerical methods is crucial. The model will be applied for studying air pollution phenomena (both monitoring, forecasting and scenarios) over Denmark. The present paper is focussed on the modification, implementation and testing of a numerical method for treating the horizontal advection in the model as well as the implementation of two-way nesting techniques. The horizontal transport in the model is solved using an accurate space derivative algorithm. This method is traditionally implemented with periodic boundary conditions, however, this is not an option for nested modelling. A new method for calculating non-periodic boundary conditions has been developed in order to overcome this problem. Extensive testing of the numerical solution of the advection and the coupling of the solution of advection and chemistry in the model using Molenkamp–Crowley rotation tests have been carried out. The results show that the model with the current implementation of numerical methods is suitable for calculating air pollution levels with high resolution. 相似文献
8.
9.
Steen Christensen 《Ground water》1997,35(1):131-139
10.
We investigate the use of social capital in the form of social ties in the face of commercialization, urbanization and climate change. While discussions of social capital often focus on whether people possess certain social ties or not our study shows that it is also necessary to consider under what circumstances people can make use of their ties. The use of different kinds of ties varies with context and is not as clear cut as suggested in the literature. For example families closer to the city are in a better position to take advantage of new opportunities. Using a combination of ties people have engaged in high-input agriculture, business and paid employment. Diversification of livelihoods has made many people less sensitive to climate change, but this does not translate into decreased vulnerability for the community. Intensive agriculture and lower community cohesion seems unsustainable in the long run. Thus, decreased vulnerability at the household level may come at the price of increased vulnerability at higher levels and negative consequences for the wider social–ecological system. Evaluating vulnerability and the role of social ties depends on the unit and sector of analysis, and the temporal and spatial scale. 相似文献