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321.
Kathleen R. Stoof-Leichsenring Annett Junginger Lydia A. Olaka Ralph Tiedemann Martin H. Trauth 《Journal of Paleolimnology》2011,45(3):353-367
Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth
century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea,
flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years
was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment
core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon
concentrations in the sediments. Core chronology was established by 210Pb dating and covered a ~186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic
zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later
a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm−2 year−1 correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century.
The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified
two major periods in the lake’s history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by
natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted
those of natural climate variation. 相似文献
322.
Jason W. Barnes Lawrence Lemke Rick Foch Christopher P. McKay Ross A. Beyer Jani Radebaugh David H. Atkinson Ralph D. Lorenz Stéphane Le Mouélic Sebastien Rodriguez Jay Gundlach Francesco Giannini Sean Bain F. Michael Flasar Terry Hurford Carrie M. Anderson Jon Merrison Máté ádámkovics Simon A. Kattenhorn Jonathan Mitchell Devon M. Burr Anthony Colaprete Emily Schaller A. James Friedson Kenneth S. Edgett Angioletta Coradini Alberto Adriani Kunio M. Sayanagi Michael J. Malaska David Morabito Kim Reh 《Experimental Astronomy》2012,33(1):55-127
We describe a mission concept for a stand-alone Titan airplane mission: Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR). With independent delivery and direct-to-Earth communications, AVIATR could contribute to Titan science either alone or as part of a sustained Titan Exploration Program. As a focused mission, AVIATR as we have envisioned it would concentrate on the science that an airplane can do best: exploration of Titan??s global diversity. We focus on surface geology/hydrology and lower-atmospheric structure and dynamics. With a carefully chosen set of seven instruments??2 near-IR cameras, 1 near-IR spectrometer, a RADAR altimeter, an atmospheric structure suite, a haze sensor, and a raindrop detector??AVIATR could accomplish a significant subset of the scientific objectives of the aerial element of flagship studies. The AVIATR spacecraft stack is composed of a Space Vehicle (SV) for cruise, an Entry Vehicle (EV) for entry and descent, and the Air Vehicle (AV) to fly in Titan??s atmosphere. Using an Earth-Jupiter gravity assist trajectory delivers the spacecraft to Titan in 7.5 years, after which the AVIATR AV would operate for a 1-Earth-year nominal mission. We propose a novel ??gravity battery?? climb-then-glide strategy to store energy for optimal use during telecommunications sessions. We would optimize our science by using the flexibility of the airplane platform, generating context data and stereo pairs by flying and banking the AV instead of using gimbaled cameras. AVIATR would climb up to 14?km altitude and descend down to 3.5?km altitude once per Earth day, allowing for repeated atmospheric structure and wind measurements all over the globe. An initial Team-X run at JPL priced the AVIATR mission at FY10 $715M based on the rules stipulated in the recent Discovery announcement of opportunity. Hence we find that a standalone Titan airplane mission can achieve important science building on Cassini??s discoveries and can likely do so within a New Frontiers budget. 相似文献
323.
324.
Impact of tributary DOM and nutrient inputs on the nearshore ecology of a large, oligotrophic lake (Georgian Bay, Lake Huron, Canada) 总被引:1,自引:0,他引:1
Serghei A. Bocaniov David R. Barton Sherry L. Schiff Ralph E. H. Smith 《Aquatic Sciences - Research Across Boundaries》2013,75(2):321-332
Two adjacent bays in a large oligotrophic lake (Georgian Bay, Lake Huron) were compared to determine how the inputs from relatively pristine, but moderately humic, tributaries may influence phytoplankton, nutrients and system metabolism. Dissolved organic carbon (DOC) concentrations decreased from 4 to 5 gC m?3 at inner sites to 2 gC m?3 or less at outer sites. The concentration gradient from inner to outer was greater in the bay with a major tributary, and optical properties (intensity and slope of light absorption spectrum) showed there was a loss of material with allochthonous characteristics along the gradient. Chlorophyll a (Chl a) and total phosphorus (TP) were also higher at inner (2–4 mg Chl a m?3 and 8–12 mgP m?3, respectively) than outer sites (≤1 mg Chl a m?3 and 4–5 mgP m?3). Chl a and TP, as well as particulate nutrient ratios (C:P, C:N, N:P), indicated significant eutrophication at inner sites, especially in the bay with the tributary, and there was a strong positive Chl a-phosphorus relationship. The stable oxygen isotope ratio (18O:16O) of dissolved oxygen indicated greater influence of biological oxygen fluxes at inner sites (where ratios were 2–3 ppt below atmospheric equilibrium) than at outer sites (where ratios were within 0.5 ppt of equilibrium). Community photosynthesis:respiration ratios inferred from 18O:16O varied positively with Chl a and inorganic nutrients, but negatively with DOC. Altered loading of allochthonous organic matter can be expected under changing climate and development scenarios and will have significant influence on optical properties and system metabolism through changes in DOC in this coastal system. The effects will nonetheless be strongly modulated by any accompanying change in inorganic nutrients. 相似文献
325.
This paper presents a full‐scale thermal remediation of a brownfields site near San Francisco, California. In Situ Thermal Desorption (ISTD) was used for treatment of chlorinated solvents in a tight clay below the water table. The site had contaminants in concentrations indicating that a tetrachloroethene (PCE)‐rich DNAPL was present. A target volume of 5097 m3 of subsurface material to a depth of 6.2 m was treated for a period of 110 d of heating. Energy was delivered through 126 thermal conduction heater borings, and vapors were extracted from a combination of vertical and horizontal vacuum wells. Approximately 2540 kg of contaminants were recovered in the extracted vapors by the end of treatment. The PCE concentration in the clay was reduced from as high as 2700 mg/kg to an average concentration of 0.012 mg/kg within 110 d of heating (a reduction of >99.999%). Similar effectiveness was documented for TCE, cis‐1,2‐DCE, and vinyl chloride. A total of 2.2 million kWh of electric power was used to heat the site. Approximately 45% of this energy was used to heat the subsurface to the target temperature. Another 53% was necessary to boil approximately 41% of the groundwater within the treatment zone, creating approximately 600 pore volumes of steam by the end of the 110‐d heating and treatment period. Steam generation thus occurred within the clay. Partitioning of the contaminants into the steam and its removal comprised the dominant remedial mechanism. The steam migrated laterally toward the ISTD heaters, where it encountered a small dry region adjacent to each of the heaters, which served as a preferential pathway allowing the steam to migrate upward along the heaters to the more permeable vadose zone. There the steam was captured by a system of vertical and horizontal vacuum extraction wells. This vapor removal strategy facilitated effective thermal treatment of the tight clays located below the water table. Features of a robust design are extension of the heaters at least 1.2 m deeper than the treatment depth, and the installation of shallow horizontal vapor collection wells which allow for establishment of pneumatic control. 相似文献
326.
Sean Matt † Ralph E. Pudritz † 《Monthly notices of the Royal Astronomical Society》2005,356(1):167-182
327.
Ralph E. Pudritz 《Astrophysics and Space Science》2004,292(1-4):471-478
This article summarizes recent theoretical, numerical and observational progress on the nature of jets and outflows in regions of star formation. The emphasis is placed on the role of hydromagnetic phenonmena in understanding these new results. The confluence of sophisticated 3D numerical MHD simulations and the recent observations of rotating jets makes it possible, for the first time, to rigorously test jet models. 相似文献
328.
Abstract— The carbonate mineralogy of several complex carbonate‐rich regions in Allan Hills (ALH) 84001 has been examined. These regions contain familiar forms of carbonate, as well as textural forms previously unreported including carbonate rosettes, planiform “slab” carbonates, distinct “post‐slab” magnesites, and carbonates interstitial to feldspathic glass and orthopyroxene. Slab carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were calcite‐rich. The kinetically controlled growth of rosettes and slab carbonates was followed by an alteration event that formed the magnesite‐siderite layers on the exterior surfaces of the carbonate. Post‐slab magnesite, intimately associated with silica glass, is compositionally similar to the magnesite in these exterior layers but represents a later generation of carbonate growth. Feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of thermal decomposition or any compositional changes associated with glass/carbonate contacts. 相似文献
329.
Peter McGregor Michael Dopita Ralph Sutherland Tracy Beck Thaisa Storchi-Bergmann 《Astrophysics and Space Science》2007,311(1-3):223-230
We present first results from the Near-infrared Integral Field Spectrograph (NIFS) located at Gemini North. For the active
galaxies Cygnus A and Perseus A we observe rotationally-supported accretion disks and adduce the existence of massive central
black holes and estimate their masses. In Cygnus A we also see remarkable high-excitation ionization cones dominated by photoionization
from the central engine. In the T-Tauri stars HV Tau C and DG Tau we see highly-collimated bipolar outflows in the [Fe II]
λ 1.644 micron line, surrounded by a slower molecular bipolar outflow seen in the H2 lines, in accordance with the model advocated by Pyo, T.-S., et al., Astrophys. J. 570, 724 (2002). 相似文献
330.
Laura Duncanson Wenli Huang Kristofer Johnson Anu Swatantran Ronald E. McRoberts Ralph Dubayah 《Carbon balance and management》2017,12(1):18