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991.
Stefan Br?nnimann Andrea N. Grant Gilbert P. Compo Tracy Ewen Thomas Griesser Andreas M. Fischer Martin Schraner Alexander Stickler 《Climate Dynamics》2012,39(7-8):1577-1598
We compare the daily, interannual, and decadal variability and trends in the thermal structure of the Arctic troposphere using eight observation-based, vertically resolved data sets, four of which have data prior to 1948. Comparisons on the daily scale between historical reanalysis data and historical upper-air observations were performed for Svalbard for the cold winters 1911/1912 and 1988/1989, the warm winters 1944/1945 and 2005/2006, and the International Geophysical Year 1957/1958. Excellent agreement is found at mid-tropospheric levels. Near the ground and at the tropopause level, however, systematic differences are identified. On the interannual time scale, the correlations between all data sets are high, but there are systematic biases in terms of absolute values as well as discrepancies in the magnitude of the variability. The causes of these differences are discussed. While none of the data sets individually may be suitable for trend analysis, consistent features can be identified from analyzing all data sets together. To illustrate this, we examine trends and 20-year averages for those regions and seasons that exhibit large sea-ice changes and have enough data for comparison. In the summertime Pacific Arctic and the autumn eastern Canadian Arctic, the lower tropospheric temperature anomalies for the recent two decades are higher than in any previous 20-year period. In contrast, mid-tropospheric temperatures of the European Arctic in the wintertime of the 1920s and 1930s may have reached values as high as those of the late 20th and early 21st centuries. 相似文献
992.
Active and inactive carbonate chimneys from the Lost City Hydrothermal Field contain up to 0.6% organic carbon with diverse lipid assemblages. The δ13C values of total organic carbon range from −21.5‰ vs. VPDB at an extinct carbonate chimney to −2.8‰ at a 70 °C, actively venting carbonate chimney. Samples collected at locations with total organic carbon with δ13C > −15‰ also contained high abundances of isoprenoidal and nonisoprenoidal diether lipids. Samples with TOC more depleted in 13C lacked or contained lower amounts of these diethers.Isoprenoidal diethers, including sn-2 hydroxyarchaeol, sn-3 hydroxyarchaeol, and putative dihydroxyarchaeol, are likely to derive from methanogenic archaea. These compounds have δ13C values ranging from −2.9 to +6.7‰ vs. VPDB. Nonisoprenoidal diethers and monoethers are presumably derived from bacteria, and have structures similar to those produced by sulfate-reducing bacteria in culture and at cold seeps. In samples that also contained abundant hydroxyarchaeols, these diethers have δ13C values between −11.8 and +3.6‰. In samples without abundant hydroxyarchaeols, the nonisoprenoidal diethers were typically more depleted in 13C, with δ13C as low as −28.7‰ in chimneys and −45‰ in fissures.The diethers at Lost City are probably derived from hydrogen-consuming methanogens and bacteria. High hydrogen concentrations favor methanogenesis over methanotrophy and allow the concurrent growth of methanogens and sulfate-reducing bacteria. The unusual enrichment of 13C in lipids can be attributed to nearly complete consumption of bioavailable carbon in vent fluids. Under carbon-limited conditions, the isotope effects that usually lead to 13C-depletion in organic material cannot be expressed. Consequently, metabolic products such as lipids and methane have δ13C values typical of abiotic carbon. 相似文献
993.
Highly siderophile element evidence for early solar system processes in components from ordinary chondrites 总被引:1,自引:0,他引:1
Separated magnetic and nonmagnetic components from the ordinary chondrites Dhajala (H3.8) and Ochansk (H4) were analyzed for their Re-Os isotopic compositions, as well as for the abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt and Pd. The Re-Os isotopic systematics of these components are used to constrain the timing of HSE fractionations, and assess the level of open-system behavior of these elements in each of the different components. The high precision, isotope dilution mass spectrometric analyses of the HSE are used to constrain the origins of, and possible relations between some of the diverse components present in these chondrites. The relative and absolute abundances of the HSE differ considerably among the components. Metal fractions have Re/Os that are factors of ∼2 (Dhajala) to ∼3 (Ochansk) higher than those of their nonmagnetic fractions. The isotopic data for both meteorites are consistent with the largest Re-Os fractionations occurring between metal and nonmagnetic components early in solar system history, although minor to moderate late stage, open-system behavior, and limited variations in Re/Os preclude a precise determination of the age for that fractionation. Open-system behavior is generally absent to minor in the metal fractions, and highly variable in nonmagnetic fractions. Re/Os ratios of nonmagnetic fractions deviate as much as 40% from a primordial isochron. Although some deviations are large for isochron applications, nearly all are negligible with respect to consideration of fractionation processes controlling the HSE.Metal from both meteorites contains about 90% of the total budget of HSE. Metal in Ochansk has ∼2 to 10 times the abundances of the bulk meteorite, while metal from the matrix of Dhajala has ∼2 to 4 times the abundances of the bulk. Fine metal in both meteorites has higher abundances than coarse metal, as has been previously observed. Nonmagnetic components, consisting of chondrules and matrix from which metal was removed in the laboratory, have highly fractionated HSE, characterized by much lower Re/Os than the bulk meteorites, as well as large relative depletions in Pd. The abundances of Re, Os, Ir, Ru and Pt in the nonmagnetic fractions are 14-120 ng/g, much higher than would be expected if they had equilibrated with the metal phases present (150-16,000 ng/g). Collectively, the data are consistent with the HSE budget in ordinary chondrites being dominated by two HSE-bearing carrier phases with distinct compositions. These phases formed separately, and never subsequently equilibrated. Metal components incorporated a HSE carrier that formed at high through moderate temperatures and relatively high pressures, such that the relatively volatile Pd behaved coherently with the more refractory HSE. Nonmagnetic fractions from both chondrules and matrix have HSE compositions that likely require at least two processes that fractionated the HSE. Depletions in Pd are consistent with the presence of HSE carriers that formed as either highly refractory condensates, or residues of high degrees of metal melting. Depletions in Re may implicate a period of relatively high fO2 during which a volatile form of Re was separated from the other HSE. 相似文献
994.
Alexander S. Bradley Helen Fredricks Kai-Uwe Hinrichs Roger E. Summons 《Organic Geochemistry》2009,40(12):1169-1178
The Lost City Hydrothermal Field hosts a distinctive microbial ecosystem that is supported by the products of serpentinization reactions. The calcium carbonate chimneys here contain abundant isoprenoidal and non-isoprenoidal ether lipids, the structural diversity of which is similar to that found in carbonate crusts at cold seeps where anaerobic oxidation of methane (AOM) is the dominant biogeochemical process. The microbial community at Lost City includes abundant archaea, which largely belong to a single phylotype within the methanogenic Methanosarcinales. Isoprenoidal diethers derived from these archaea have polar head groups comprising phosphatidylglycerol or monoglycosyl moieties, although many isoprenoidal diethers detected in these carbonates lack head groups.The non-isoprenoidal diether lipids at Lost City are largely glycolipids. Glycosydic non-isoprenoidal diethers have not previously been reported from any environments or bacterial isolates. Glycolipids are common in archaea where they contain isoprenoidal hydrocarbon cores ether linked to glycerol. Glycolipids also occur in some bacteria and in these non-isoprenoidal fatty acid chains are ester linked to glycerol. However, the glycosylated non-isoprenoidal diether lipids at Lost City contain a previously undetected combination of archaeal and bacterial traits that might be an adaptive response to the vent environment. We hypothesize that utilization of glycosyl head groups instead of phosphatidyl head groups is a strategy for conservation of phosphate by organisms growing in fluids low in this essential nutrient. 相似文献
995.
Study of the upper loess strata within the profile of surface soils highlighted the role of pedogenesis in the formation of characteristic features of loess. Loess-paleosol sequences within the study area are influenced by their position in paleocryogenic microrelief. Clear evidence of sequential loess sedimentation, accompanied by slope processes and pedogenesis, is present in soil profiles within former thermokarst depressions. Different stages of loess sedimentation are marked by cryomorphic features, solifluction stripes and buried humus horizons. The balance between the rate of sedimentation, intensity of slope processes and pedogenesis changed within the upper 3 m of loess strata. Corresponding loess strata in inter-depression areas were also formed by sequential accumulation of aeolian dust, gradually altered by initial pedogenesis that left weakly developed soil profiles without clear horizonation. Pedogenesis resulted in diverse complexes of secondary carbonates, loose soil fabric and microfabric with abundant pores of biogenic and cryogenic nature, as well as other features, characteristic of soils of cold arid environments. The uniformity of these features throughout the upper loess strata confirms the synlithogenic nature of pedogenesis that accompanied loess accumulation. 相似文献
996.
Alexander?Alexandrovich?KonovalenkoEmail author Igor?Savelievich?Falkovich Nikolay?Nikolaevich?Kalinichenko Alexander?Alexandrovich?Gridin Igor?Nikolaevich?Bubnov Alain?Lecacheux Carlo?Rosolen Helmut?O.?Rucker 《Experimental Astronomy》2003,16(3):149-164
An effective wide-band (10 to 60 MHz) active antenna element has been developed. The cost of one short (3 m), thin dipole with built-in amplifier and metal construction is less than 45 euro. It was shown both theoretically and experimentally that the upper limiting frequency is at least 60 MHz, the dynamic range is 90 dB/V and the share of the amplifier noise to the background antenna temperature is about 10%. The developed active dipole was tested by building a 30-element antenna array and comparing its parameters with one of the subpart of the UTR-2 radio telescope having passive dipoles of 8.6 m in length and 1.8 m in diameter. The 3C461 ionospheric scintillation spectra observed in the experiments show that the sensitivities and noise-immunities of both antennas are close. This proves the availability using of a short cheap active dipole in new generation giant radio telescopes. 相似文献
997.
998.
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1000.
Robert Alexander Robert Ian Kagi Alfons Valentino Larcher 《Geochimica et cosmochimica acta》1982,46(2):219-222
The exchange of aromatic hydrogen in the presence of clay catalysts has been studied by measuring the rates of detritiation of some tritiated naphthalene derivatives on homoionic bentonites. The rate of reaction is influenced by the position of tritium on the ring, the presence of a methoxysubstituent on the ring and the acidity of the clay catalyst. It is inferred that the mechanism of exchange involves an adsorbed species similar to the arenium ion intermediate of electrophilic aromatic substitutions in homogeneous systems. In some cases exchange of hydrogen between acidic clay surfaces and naphthalene derivatives could be detected at temperatures as low as 23°C, and in aqueous slurries at 70°C. This observed reactivity suggests that these reactions could occur in sedimentary environments where organic matter, such as petroleum hydrocarbons, is in contact with clay surfaces. The hydrogen isotopic composition of aromatic hydrogen in petroleum may therefore be related to that of the acidic water adsorbed on clay surfaces with which the petroleum has been in contact. 相似文献