全文获取类型
收费全文 | 2656篇 |
免费 | 157篇 |
国内免费 | 33篇 |
专业分类
测绘学 | 110篇 |
大气科学 | 229篇 |
地球物理 | 609篇 |
地质学 | 993篇 |
海洋学 | 227篇 |
天文学 | 403篇 |
综合类 | 14篇 |
自然地理 | 261篇 |
出版年
2023年 | 15篇 |
2022年 | 23篇 |
2021年 | 65篇 |
2020年 | 89篇 |
2019年 | 79篇 |
2018年 | 90篇 |
2017年 | 108篇 |
2016年 | 130篇 |
2015年 | 94篇 |
2014年 | 116篇 |
2013年 | 165篇 |
2012年 | 127篇 |
2011年 | 173篇 |
2010年 | 148篇 |
2009年 | 153篇 |
2008年 | 136篇 |
2007年 | 99篇 |
2006年 | 95篇 |
2005年 | 97篇 |
2004年 | 82篇 |
2003年 | 80篇 |
2002年 | 66篇 |
2001年 | 51篇 |
2000年 | 45篇 |
1999年 | 33篇 |
1998年 | 27篇 |
1997年 | 32篇 |
1996年 | 30篇 |
1995年 | 28篇 |
1994年 | 14篇 |
1993年 | 16篇 |
1992年 | 22篇 |
1991年 | 18篇 |
1990年 | 22篇 |
1989年 | 12篇 |
1988年 | 15篇 |
1987年 | 19篇 |
1986年 | 8篇 |
1985年 | 21篇 |
1984年 | 26篇 |
1983年 | 18篇 |
1982年 | 19篇 |
1981年 | 22篇 |
1980年 | 14篇 |
1979年 | 8篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1976年 | 10篇 |
1975年 | 14篇 |
1974年 | 16篇 |
排序方式: 共有2846条查询结果,搜索用时 15 毫秒
231.
Veronika S. HEBER Roger C. WIENS Amy J. G. JUREWICZ Nadia VOGEL Daniel B. REISENFELD Heinrich BAUR Kevin D. McKEEGAN Rainer WIELER Donald S. BURNETT 《Meteoritics & planetary science》2011,46(4):493-512
Abstract– We report concentrations and isotopic compositions of He, Ne, and Ar measured with high spatial resolution along a radial traverse of a silicon carbide (SiC) quadrant of the Genesis mission concentrator target. The Ne isotopic composition maps instrumental fractionation as a function of radial position in the target: the maximum observed isotopic fractionation is approximately 33‰ per mass unit between the center and periphery. The Ne fluence is enhanced by a factor of 43 at the target center and decreases to 5.5 times at the periphery relative to the bulk solar wind fluence. Neon isotopic profiles measured along all four arms of the “gold cross” mount which held the quadrants in the concentrator target demonstrate that the concentrator target was symmetrically irradiated during operation as designed. We used implantation experiments of Ne into SiC and gold to quantify backscatter loss and isotopic fractionation and compared measurements with numerical simulations from the code “stopping and range of ions in matter.” The 20Ne fluence curve as a function of radial distance on the target may be used to construct concentration factors relative to bulk solar wind for accurate corrections for solar wind fluences of other light elements to be measured in the concentrator target. The Ne isotopic composition as a function of the radial distance in the SiC quadrant provides a correction for the instrumental mass‐dependent isotopic fractionation by the concentrator and can be used to correct measured solar wind oxygen and nitrogen isotopic compositions to obtain bulk solar wind isotopic compositions. 相似文献
232.
The use of invariant manifolds for transfers between unstable periodic orbits of different energies 总被引:1,自引:0,他引:1
Kathryn E. Davis Rodney L. Anderson Daniel J. Scheeres George H. Born 《Celestial Mechanics and Dynamical Astronomy》2010,107(4):471-485
Techniques from dynamical systems theory have been applied to the construction of transfers between unstable periodic orbits
that have different energies. Invariant manifolds, trajectories that asymptotically depart or approach unstable periodic orbits,
are used to connect the initial and final orbits. The transfer asymptotically departs the initial orbit on a trajectory contained
within the initial orbit’s unstable manifold and later asymptotically approaches the final orbit on a trajectory contained
within the stable manifold of the final orbit. The manifold trajectories are connected by the execution of impulsive maneuvers.
Two-body parameters dictate the selection of the individual manifold trajectories used to construct efficient transfers. A
bounding sphere centered on the secondary, with a radius less than the sphere of influence of the secondary, is used to study
the manifold trajectories. A two-body parameter, κ, is computed within the bounding sphere, where the gravitational effects of the secondary dominate. The parameter κ is defined as the sum of two quantities: the difference in the normalized angular momentum vectors and eccentricity vectors
between a point on the unstable manifold and a point on the stable manifold. It is numerically demonstrated that as the κ parameter decreases, the total cost to complete the transfer decreases. Preliminary results indicate that this method of
constructing transfers produces a significant cost savings over methods that do not employ the use of invariant manifolds. 相似文献
233.
Igor I. Alexeev Elena S. Belenkaya Haje Korth Daniel N. Baker Catherine L. Johnson Menelaos Sarantos 《Icarus》2010,209(1):23-1862
The “paraboloid” model of Mercury’s magnetospheric magnetic field is used to determine the best-fit magnetospheric current system and internal dipole parameters from magnetic field measurements taken during the first and second MESSENGER flybys of Mercury on 14 January and 6 October 2008. Together with magnetic field measurements taken during the Mariner 10 flybys on 29 March 1974 and 16 March 1975, there exist three low-latitude traversals separated in longitude and one high-latitude encounter. From our model formulation and fitting procedure a Mercury dipole moment of 196 nT · (where RM is Mercury’s radius) was determined. The dipole is offset from Mercury’s center by 405 km in the northward direction. The dipole inclination to Mercury’s rotation axis is relatively small, ∼4°, with an eastern longitude of 193° for the dipole northern pole. Our model is based on the a priori assumption that the dipole position and the moment orientation and strength do not change in time. The root mean square (rms) deviation between the Mariner 10 and MESSENGER magnetic field measurements and the predictions of our model for all four flybys is 10.7 nT. For each magnetic field component the rms residual is ∼6 nT or about 1.5% of the maximum measured magnetic field, ∼400 nT. This level of agreement is possible only because the magnetospheric current system parameters have been determined separately for each flyby. The magnetospheric stand-off distance, the distance from the planet’s center to the inner edge of the tail current sheet, the tail lobe magnetic flux, and the displacement of the tail current sheet relative to the Mercury solar-magnetospheric equatorial plane have been determined independently for each flyby. The magnetic flux in the tail lobes varied from 3.8 to 5.9 MWb; the subsolar magnetopause stand-off distance from 1.28 to 1.43 RM; and the distance to the inner edge of the current sheet from 1.23 to 1.32 RM. The differences in the current systems between the first and second MESSENGER flybys are attributed to the effects of strong magnetic reconnection driven by southward interplanetary magnetic field during the latter flyby. 相似文献
234.
Alexandra V. Turchyn Volker Brüchert Gregory S. Engel Daniel P. Schrag 《Geochimica et cosmochimica acta》2010,74(7):2011-6698
Kinetic isotope effects related to the breaking of chemical bonds drive sulfur isotope fractionation during dissimilatory sulfate reduction (DSR), whereas oxygen isotope fractionation during DSR is dominated by exchange between intercellular sulfur intermediates and water. We use a simplified biochemical model for DSR to explore how a kinetic oxygen isotope effect may be expressed. We then explore these relationships in light of evolving sulfur and oxygen isotope compositions (δ34SSO4 and δ18OSO4) during batch culture growth of twelve strains of sulfate-reducing bacteria. Cultured under conditions to optimize growth and with identical δ18OH2O and initial δ18OSO4, all strains show 34S enrichment, whereas only six strains show significant 18O enrichment. The remaining six show no (or minimal) change in δ18OSO4 over the growth of the bacteria. We use these experimental and theoretical results to address three questions: (i) which sulfur intermediates exchange oxygen isotopes with water, (ii) what is the kinetic oxygen isotope effect related to the reduction of adenosine phosphosulfate (APS) to sulfite (SO32−), (iii) does a kinetic oxygen isotope effect impact the apparent oxygen isotope equilibrium values? We conclude that oxygen isotope exchange between water and a sulfur intermediate likely occurs downstream of APS and that our data constrain the kinetic oxygen isotope fractionation for the reduction of APS to sulfite to be smaller than 4‰. This small oxygen isotope effect impacts the apparent oxygen isotope equilibrium as controlled by the extent to which APS reduction is rate-limiting. 相似文献
235.
Clumped isotope thermometry of carbonatites as an indicator of diagenetic alteration 总被引:2,自引:0,他引:2
We measure the clumped isotopic signature of carbonatites to assess the integrity of the clumped isotope paleothermometer over long timescales (107-109 years) and the susceptibility of the proxy to closed system re-equilibration of isotopes during burial diagenesis. We find pristine carbonatites that have primary oxygen isotope signatures, along with a Carrara marble standard, do not record clumped isotope signatures lighter than 0.31‰ suggesting atoms of carbon and oxygen freely exchange within the carbonate lattice at temperatures above 250-300 °C. There is no systematic trend in the clumped isotope signature of pristine carbonatites with age, although partial re-equilibration to lower temperatures can occur if a carbonatite has been exposed to burial temperatures for long periods of time. We conclude that the solid-state re-ordering of carbon and oxygen atoms is sufficiently slow to enable the use of clumped isotope paleothermometry on timescales of 108 years, but that diagenetic resetting can still occur, even without bulk recrystallization. In addition to the carbonatite data, an inorganic calibration of the clumped isotope paleothermometer for low temperature carbonates (7.5-77 °C) is presented and highlights the need for further inter-lab standardization. 相似文献
236.
Lars-Eric Heimbürger Daniel Cossa Christophe Migon Bernard Averty Josephine Ras 《Geochimica et cosmochimica acta》2010,74(19):5549-4052
Recent findings on the distribution of methylated mercury (MeHgT) in waters have highlighted the importance of organic carbon remineralization on the production of these compounds in the open ocean. Here, we present the first time-series (20 monthly samplings between July 2007 and May 2009) of high-resolution vertical profiles (10-12 depths in a 2350 m water column) of MeHgT distributions in an open ocean environment, the Ligurian Sea (North-western Mediterranean Sea). Concentrations varied within the sub-picomolar range (general mean: 0.30 ± 0.17 pmol L−1, n = 214) with the lowest values at the surface, increasing with depth up to the oxygen minimum zone, and decreasing slowly at greater depth. Concentrations in the surface waters never exceeded 0.15 pmol L−1, while the highest concentrations (up to 0.82 pmol L−1) were associated to the hypoxycline during the autumn bloom. A detailed vertical MeHgT profile reveals a “double-peak” pattern, coincidental with the two microbial layers described by Tanaka and Rassoulzadegan (2002), the so-called “microbial food web” in the euphotic zone (<100 m) and the “microbial loop” in the aphotic zone (>100 m). Temporal variations in the MeHgT abundance and distribution in the water column were linked to seasonality. The highest MeHgT concentrations were found in the oxygen minimum zone during the period of stratification, and coincide with the greatest abundance of nano- and picophytoplankton (cyanobacteria, nanoflagellates, etc.) in the euphotic layer. None of our deep MeHgT measurements (∼100 m above the sea bottom) revealed a significant sedimentary source of MeHgT. We explored the correlation between MeHgT concentrations and the apparent oxygen utilization, a proxy of organic matter remineralization, over the study period. Results of this study strengthen the hypothesis that net mercury methylation in the open ocean occurs in the water column, is linked to organic matter regeneration, and is promoted by the presence of small-sized nano- and picophytoplankton, that dominate under oligotrophic conditions. 相似文献
237.
Daniel Novik Warshawsky 《Geoforum》2010,41(5):763-775
While some non-profits have suffered under the political and economic pressures of neoliberal urban governance reform, others have emerged as important institutional players in local governance regimes. This article highlights how non-profits food banks in Chicago have commercialized in order to respond to increased demand, enhance their institutional independence from government, and reassert their local dominance in emergency food service delivery. From a food bank member agency perspective, the consolidation of food bank institutional power has produced new bureaucratic limitations, user fees, and increased competition. These shifts reveal the rise of metropolitan food banks as important players in neoliberal urban governance regimes, as they control the conceptualization of hunger, management of poverty, and organization of food distribution systems. 相似文献
238.
Élisa Morichon Thierry Allard Daniel Beaufort David Quirt 《Physics and Chemistry of Minerals》2010,37(3):145-152
Natural radiation-induced defects were identified in specimens of sudoite (Al–Mg di-trioctahedral chlorite) related to unconformity-type
uranium deposits at the base of the Athabasca Group (Saskatchewan, Canada), using electron paramagnetic resonance (EPR) spectroscopy
at X- and Q-band frequencies. X-band spectra indicate the presence of a main native defect, named the As-center, whose EPR signal is dominated by an axially distorted spectrum with apparent principal components as follows: g
// = 2,051 and g
⊥ = 2,005, and a secondary defect with apparent component g = 2,025. The study of oriented specimens shows that the main defect has its g
// component perpendicular to the (ab) plane of sudoite. The As-center corresponds to an electron hole located on oxygen atoms of the structure and is likely associated with Si, according
to the lack of hyperfine structure. The As-center in sudoite has EPR parameters similar to the A-center in kaolinite and dickite, and the Ai-center in illite. The saturation behavior of EPR spectra as a function of power demonstrates that native defects of sudoite
are different from those known in other clays, such as kaolinite, dickite or smectite, but are similar to those of illite.
The isochronal annealing data suggest that the main defect in sudoite is stable to more than 300°C. The corresponding defects
characterized in sudoite may have the potential for tracing past radionuclide migration around unconformity-type uranium deposits. 相似文献
239.
Volker Kahlenberg Daniela Girtler Erik Arroyabe Reinhard Kaindl Daniel M. Többens 《Mineralogy and Petrology》2010,100(1-2):1-9
Single crystals of devitrite (Na2Ca3Si6O16) were synthesized as pale-yellow transparent needle shaped crystals using a Na2MoO4-flux. Experiments aiming to prepare the K-equivalent of devitrite from the corresponding K2MoO4-flux were unsuccessful. The crystal structure of devitrite was determined from single-crystal X-ray diffraction data (Mo-Kα radiation, 2θmax.?=?25.34°, Rint?=?2.66%) and refined in space group P $ \bar{1} $ (no. 2) to R(|F|)?=?3.08% using 2,513 observed reflections with I?>?2σ(I). Unit-cell parameters are: a?=?7.2291(8), b?=?10.1728(12), c?=?10.6727(12) Å, α?=?95.669(9), β?=?109.792(10), γ?=?99.156(9)°, V?=?719.19(14) Å3, Z?=?2. The structure belongs to the group of multiple chain silicates consisting of dreier quadruple chains, i.e. the crystallochemical formula can be written as $ {\hbox{N}}{{\hbox{a}}_2}{\hbox{C}}{{\hbox{a}}_3}\left\{ {{\mathbf{uB}}{,4}_\infty^1} \right\}\left[ {^3{\hbox{S}}{{\hbox{i}}_6}} \right.\left. {{{\hbox{O}}_{16}}} \right\} $ . Linkage between the bands running along [100] is provided by double chains of edge sharing CaO6-octahedra as well as additional more irregularly coordinated Na- and Ca-cations located in the tunnel-like cavities of the mixed tetrahedral-octahedral framework. Structural investigations were completed by Raman and infrared spectroscopical studies. The allocation of the bands to certain vibrational species was aided by density functional theory (DFT) calculations. 相似文献
240.
Geochemical and mineralogical evidence for Sahara and Sahel dust additions to Quaternary soils on Lanzarote,eastern Canary Islands,Spain 总被引:1,自引:0,他引:1
Daniel R. Muhs James Budahn Gary Skipp Joseph M. Prospero DeAnna Patterson E. Arthur Bettis III 《地学学报》2010,22(6):399-410
Africa is the most important source of dust in the world today, and dust storms are frequent on the nearby Canary Islands. Previous workers have inferred that the Sahara is the most important source of dust to Canary Islands soils, with little contribution from the Sahel region. Soils overlying a late Quaternary basalt flow on Lanzarote, Canary Islands, contain, in addition to volcanic minerals, quartz and mica, exotic to the island’s bedrock. Kaolinite in the soils also likely has an exotic origin. Trace‐element geochemistry shows that the soils are derived from varying proportions of locally derived basalt and African dust. Major‐element geochemistry, clay mineralogy and interpretation of satellite imagery suggest that dust additions to the Canary Islands come not only from the Sahara Desert, but also from the Sahel region. 相似文献