全文获取类型
收费全文 | 1611篇 |
免费 | 46篇 |
国内免费 | 23篇 |
专业分类
测绘学 | 53篇 |
大气科学 | 159篇 |
地球物理 | 284篇 |
地质学 | 626篇 |
海洋学 | 124篇 |
天文学 | 290篇 |
综合类 | 11篇 |
自然地理 | 133篇 |
出版年
2023年 | 9篇 |
2021年 | 16篇 |
2020年 | 17篇 |
2019年 | 17篇 |
2018年 | 34篇 |
2017年 | 29篇 |
2016年 | 40篇 |
2015年 | 25篇 |
2014年 | 44篇 |
2013年 | 71篇 |
2012年 | 54篇 |
2011年 | 74篇 |
2010年 | 74篇 |
2009年 | 117篇 |
2008年 | 80篇 |
2007年 | 96篇 |
2006年 | 77篇 |
2005年 | 86篇 |
2004年 | 55篇 |
2003年 | 49篇 |
2002年 | 54篇 |
2001年 | 41篇 |
2000年 | 40篇 |
1999年 | 32篇 |
1998年 | 26篇 |
1997年 | 22篇 |
1996年 | 20篇 |
1995年 | 19篇 |
1994年 | 21篇 |
1993年 | 16篇 |
1992年 | 17篇 |
1991年 | 14篇 |
1990年 | 14篇 |
1988年 | 12篇 |
1986年 | 12篇 |
1985年 | 16篇 |
1984年 | 13篇 |
1983年 | 22篇 |
1982年 | 16篇 |
1981年 | 16篇 |
1980年 | 12篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1976年 | 15篇 |
1975年 | 8篇 |
1973年 | 15篇 |
1972年 | 15篇 |
1971年 | 11篇 |
1970年 | 9篇 |
排序方式: 共有1680条查询结果,搜索用时 93 毫秒
771.
S. Frank M. C. Bentz K. Z. Stanek S. Mathur M. Dietrich B. M. Peterson D. W. Atlee 《Astrophysics and Space Science》2007,312(3-4):325-330
We examine the recent report by Prochter et al. that gamma-ray burst (GRB) sight lines have a much higher incidence of strong
Mg II absorption than quasar sight lines. We propose that the discrepancy is due to the different beam sizes of GRBs and quasars,
and that the intervening Mg II systems are clumpy with the dense part of each cloudlet of a similar size as the quasars, i.e.
≲1016 cm, but bigger than GRBs. We also discuss observational predictions of our proposed model. Most notably, in some cases the
intervening Mg II absorbers in GRB spectra should be seen varying, and quasars with smaller sizes should show an increased
rate of strong Mg II absorbers. In fact, our prediction of variable Mg II lines in the GRB spectra has been now confirmed
by Hao et al., who observed intervening Fe II and Mg II lines at z=1.48 to be strongly variable in the multi-epoch spectra of z=4.05 GRB 060206. 相似文献
772.
Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects 总被引:1,自引:0,他引:1
The detection of induced magnetic fields in the vicinity of the jovian satellites Europa, Ganymede, and Callisto is one of the most surprising findings of the Galileo mission to Jupiter. The observed magnetic signature cannot be generated in solid ice or in silicate rock. It rather suggests the existence of electrically conducting reservoirs of liquid water beneath the satellites' outermost icy shells that may contain even more water than all terrestrial oceans combined. The maintenance of liquid water layers is closely related to the internal structure, composition, and thermal state of the corresponding satellite interior. In this study we investigate the possibility of subsurface oceans in the medium-sized icy satellites and the largest trans-neptunian objects (TNO's). Controlling parameters for subsurface ocean formation are the radiogenic heating rate of the silicate component and the effectiveness of the heat transfer to the surface. Furthermore, the melting temperature of ice will be significantly reduced by small amounts of salts and/or incorporated volatiles such as methane and ammonia that are highly abundant in the outer Solar System. Based on the assumption that the satellites are differentiated and using an equilibrium condition between the heat production rate in the rocky cores and the heat loss through the ice shell, we find that subsurface oceans are possible on Rhea, Titania, Oberon, Triton, and Pluto and on the largest TNO's 2003 UB313, Sedna, and 2004 DW. Subsurface oceans can even exist if only small amounts of ammonia are available. The liquid subsurface reservoirs are located deeply underneath an ice-I shell of more than 100 km thickness. However, they may be indirectly detectable by their interaction with the surrounding magnetic fields and charged particles and by the magnitude of a satellite's response to tides exerted by the primary. The latter is strongly dependent on the occurrence of a subsurface ocean which provides greater flexibility to a satellite's rigid outer ice shell. 相似文献
773.
Garrit Voggesser Kathy Lynn John Daigle Frank K. Lake Darren Ranco 《Climatic change》2013,120(3):615-626
Climate change related impacts, such as increased frequency and intensity of wildfires, higher temperatures, extreme changes to ecosystem processes, forest conversion and habitat degradation are threatening tribal access to valued resources. Climate change is and will affect the quantity and quality of resources tribes depend upon to perpetuate their cultures and livelihoods. Climate impacts on forests are expected to directly affect culturally important fungi, plant and animal species, in turn affecting tribal sovereignty, culture, and economy. This article examines the climate impacts on forests and the resulting effects on tribal cultures and resources. To understand potential adaptive strategies to climate change, the article also explores traditional ecological knowledge and historical tribal adaptive approaches in resource management, and contemporary examples of research and tribal practices related to forestry, invasive species, traditional use of fire and tribal-federal coordination on resource management projects. The article concludes by summarizing tribal adaptive strategies to climate change and considerations for strengthening the federal-tribal relationship to address climate change impacts to forests and tribal valued resources. 相似文献
774.
We analyse the dependence of the cloud radiative effect (CRE) and cloud amount on mid-tropospheric pressure velocity (ω 500) and sea surface temperature (SST) and point out the shortcomings of using these two proxies separately as means to separate cloud regimes. A bivariate approach is proposed to overcome these shortcomings and it is used to systematically investigate marine cloud properties at different spatial and time scales in the present-day (1985–2001) tropical climate. During the 1997–1998 El Niño, the greatest regional change in CRE and cloud cover coincides with the greatest local change in circulation and SST. In addition, we find that the cooling effect of the stratiform low clouds reduces at the rate of approximately 1 W/m2 per percent of cloudiness reduction in the subsident cold pools of the Pacific ocean. During El Niño, the transition between different cloud regimes gives rise to opposing cloud feedbacks. The sign of the total feedback is controlled by the cloud optical thickness. More generally, we find that the largest part of the cloud response to El Niño, when averaged over the tropical Pacific, is not directly associated with ω 500 and SST changes, so other factors must play a role as well. 相似文献
775.
Abstract Economic studies suggest that market leakage rates of greenhouse gas abatement can reach the two-digit percentage range. Although the Marrakesh Accords require Clean Development Mechanism (CDM) projects to account for leakage, most projects neglect market leakage. Insufficient leakage accounting is facilitated by a lack of applicable methods regarding the quantification and attribution of project-related leakage effects. This article proposes a method for attributing CDM-related market leakage effects to individual projects. To this purpose, alternative attribution methods are analysed. We find that project-specific approaches fail to take account of market leakage effects. Consequently, we propose to estimate aggregate market leakage effects and attribute them proportionally to individual projects. We suggest that predetermined commodity-specific leakage factors are applied by project developers to any emission reductions that are associated with a project's leakage-relevant demand or supply changes. This approach is conservative, equitable, incentive-compatible and applicable at manageable costs. 相似文献
776.
777.
Alan J. Wainwright Richard M. Tosdal Joseph L. Wooden Frank K. Mazdab Richard M. Friedman 《Gondwana Research》2011,19(3):764-787
Uranium–Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu–Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (εNd(t) range from + 3.1 to + 7.5 and 206Pb/204Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have < 1.5% TiO2) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu–Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium–Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu–Au deposits are ~ 372 Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu–Au mineralization are ~ 366 Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu–Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu–Au deposits from younger magmatic suites in the district. 相似文献
778.
779.
The oxidation of pyrite results in sulphate concentrations from 1,300 to 2,000 mg/l in the dump water of the open cast mine
Inden, which is located in Germany. Under near-neutral pH and reduced redox conditions, precipitation of siderite (FeCO3) retains up to 85% of the pre-liberated ferrous iron, so that iron concentrations in the dump water are relatively low as
compared to the concentrations of sulphate. The highly mineralised dump water will enter the aquifer system northwards of
Inden during the next centuries. This area is used intensely for water extraction. Model calculations show that there will
be no problem of acidification in the adjacent aquifer system and, therefore, no problem of high heavy metal concentrations
during the next centuries. The most important problem will be high sulphate concentrations in the groundwater. Besides dispersion
and diffusion, there are no chemical mechanisms that lower the sulphate concentrations downstream. On the basis of the simulations,
sulphate plumes of about 1–2 km in width and up to 15 km in length are expected. Within the core of these plumes, sulphate
concentrations will be above the drinking water limits. 相似文献
780.
Veerle Beelaerts Maite Bauwens Emma Versteegh Frank Dehairs Rik Pintelon 《Mathematical Geosciences》2011,43(6):635-662
Identifying a periodic time-series model from environmental records, without imposing the positivity of the growth rate, does
not necessarily respect the time order of the data observations. Consequently, subsequent observations, sampled in the environmental
archive, can be inversed on the time axis, resulting in a non-physical signal model. In this paper an optimization technique
with linear constraints on the signal model parameters is proposed that prevents time inversions. The activation conditions
for this constrained optimization are based upon the physical constraint of the growth rate, namely, that it cannot take values
smaller than zero. The actual constraints are defined for polynomials and first-order splines as basis functions for the nonlinear
contribution in the distance-time relationship. The method is compared with an existing method that eliminates the time inversions,
and its noise sensitivity is tested by means of Monte Carlo simulations. Finally, the usefulness of the method is demonstrated
on the measurements of the vessel density, in a mangrove tree, Rhizophora mucronata, and the measurement of Mg/Ca ratios, in a bivalve, Mytilus trossulus. 相似文献