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排序方式: 共有354条查询结果,搜索用时 15 毫秒
191.
Manfred R. Brix Bernhard Stckhert Eberhard Seidel Thomas Theye Stuart N. Thomson Martina Küster 《Tectonophysics》2002,349(1-4)
A fossil partial annealing zone of fission tracks in zircon is described from high pressure–low temperature (HP–LT) rocks of the Phyllite–Quartzite Unit (PQ) on the island of Crete, Greece. Correlation of regional trends in fission track age populations with independent thermobarometric and microstructural data, and with new experimental annealing results, allows a calibration of this low temperature thermochronological method to a degree hitherto not available from other field examples.The zircon fission track (FT) ages of samples from the PQ across Crete range from original detrital signature through reduced to completely reset. The annealing is the result of a single heating period related to the HP–LT metamorphism with near-peak temperatures lasting for only a few million years some time between 24±1 and 20±1 Ma. In eastern Crete, where rocks have experienced temperatures of 300±50 °C and pressures of 0.8±0.3 GPa, zircon FT ages range from 414±24 to 145±10 Ma. Ages above 300 Ma occur mostly near the east coast of the island in rocks which have not been heated to above ca. 280 °C and probably represent a pre-Variscan source. Track lengths are already indicative of a substantial annealing at this temperature. Most of the zircon FT ages from eastern Crete scatter within error around the stratigraphic age. Samples with apparent zircon FT ages significantly younger than the depositional age are only observed in areas where temperatures exceeded ca. 320 °C. Towards the west, a sudden decrease to very young ages ranging from 17±2 to 18±1 Ma reflects a complete resetting at ca. 350 °C. Short tracks, however, are still observed. Throughout the central and western part of the island, ages are consistently below 22 Ma. Thermobarometric data for this area indicate maximum temperatures of 400±50 °C and pressures of 1±0.3 GPa. Only samples from western Crete, which have been exposed to 400±50 °C, show exclusively long tracks. Consequently, the high temperature limit of the zircon partial annealing zone (ZPAZ) appears to be between 350 and 400 °C.A significant influence of elevated confining pressure on the stability of fission tracks in zircon is ruled out by the results of annealing experiments at 0.5 GPa and at different temperatures, which fit the curves previously obtained by other authors at ambient pressure. 相似文献
192.
In sediments, diffusive transport of ions and molecules is basically influenced by two sediment characteristics: tortuosity and porosity. For the first time, the formation factor F, which combines the effect of tortuosity and porosity on diffusion, was quantified in freshwater sediments at submillimeter resolution. Sediment cores were treated with KCl and F was determined using a resistivity sensor and K+ selective electrodes.F was determined in sediments from different water depths of the eutrophic Lake Zug (Switzerland): In sandy sediments from a shallow site (12 m depth), F increased by approximately 50% within a few millimeters below the sediment surface. In clayey and silty sediments from the oxic (<80 m depth) and seasonally anoxic (80-120 m depth) zones of the lake, the initial increase in F was only 20%. In the permanent anoxic zone (>160 m depth), F increased by only 10% just below the sediment surface. Values of F were correlated with the porosity at each depth. We found close correlations of F = 1.02 · φ−1.81 for clay-silt sediments, and F = 1.04 · φ−1.21 for sandy sediments. The exponents are considerably smaller in Lake Zug than found for marine sediments, thus, diffusive transport seems to be less affected by tortuosity in this freshwater system. 相似文献
193.
Bernhard M. Krooss Lothar Friberg Yves Gensterblum Jan Hollenstein Dirk Prinz Ralf Littke 《International Journal of Earth Sciences》2005,94(5-6):1023-1038
Open-system non-isothermal pyrolysis up to 1,200°C in combination with elemental analysis was used to study the thermal liberation
of molecular nitrogen (N2) from sedimentary rocks and kerogen concentrates of Palaeozoic age from the Central European Basin system and an Eocene shale
(Liaohe Basin, China) with a high content (36%) of ammonium feldspar (buddingtonite). The N/Corg (atomic) ratios of the kerogen concentrates ranged from 0.005 to 0.014, which represents the range commonly observed for
coals. Bulk N/Corg ratios of the Palaeozoic shales extended from 0.035 to 0.108, indicating the presence of significant amounts of inorganic
nitrogen. Namurian A and A-B (CnA; CnA-B) samples typically exhibited the earliest onset of N2 generation with intense, characteristic peaks around 600°C. N2 liberation from the buddingtonite-rich sample occurred at higher temperatures, with a broad peak around 700°C. Pyrograms
of the kerogen concentrates showed no or strongly reduced N2 generation in the 500–700°C range. On-line isotope-specific analysis of the pyrolytically liberated N2 on one sample revealed a variability of ∼10‰ in the δ15N values and a steady increase in δ15N with temperature during the main phase of N2 generation. 相似文献
194.
Tectonic map and overall architecture of the Alpine orogen 总被引:9,自引:0,他引:9
195.
The present article describes the use of plant bioassays for the detection of genotoxins in the aquatic environment and gives an overview of test methods, their detection spectrum for environmental mutagens and carcinogens and their limitations and pitfalls. The most widely used test systems are micronucleus assays with meiotic cells of Tradescantia and with meristematic root tip cells of Allium and Vicia. In the last years, protocols for single cell gel electrophoresis assays have been developed, which can be conducted with a variety of species. Also various gene mutation test procedures have been developed with plants but they have hardly ever been used in environmental studies. Plants detect a broad variety of environmentally relevant genotoxins, in particular directly acting compounds. Many pesticides and industrial chemicals caused positive results; plant assays are also a unique tool for the detection of DNA‐reactive carcinogenic heavy metals in the environment. In many studies with complex mixtures, positive results were obtained which indicates that plants are sufficiently sensitive to detect effects without concentration of water samples. One of the shortcomings of the use of plants as indicators is their lack of sensitivity towards certain classes of promutagens such as nitrosamines, heterocyclic amines, and polycyclic aromatic hydrocarbons (PAHs). However, the former compounds are hardly encountered in the environment and PAHs can easily be detected chemically and in other mutagenicity tests. Taken together, the currently available data show that plant bioassays are a useful component of test batteries for environmental monitoring. 相似文献
196.
The outer vents in the Auckland Volcanic Field lie within 19 to 559 m (mean 224.75 m) of a ‘best-fit' ellipse with a 28.9-km-long major axis trending almost north–south, and a minor axis 16.5 km long. The ellipse has formed the outer boundary of the field since the inception of volcanism 140,000 years ago. We present the following testable hypotheses as an explanation of this pattern: The boundary is the expression of a corresponding elliptical source area at depth in the lithospheric mantle (possibly asthenosphere material trapped at this level). The ellipse may represent a depth contour on a very small upper mantle dome or of a lens intruding into a neck of an extensional structure. Alternatively it could be the boundary of a flat elliptical area where tensional stresses allow decompressional melting. The elliptical tensional region may either have developed in a releasing bend during strike-slip faulting along a fundamental lithosphere structure inherited from Mesozoic tectonics, associated with the NNW-trending Dun Mountain ophiolite belt, or may represent the tip of a fracture along which the Auckland Volcanic Province is propagating northward. 相似文献
197.
Bernhard Schulz Claude Triboulet Claude Audren Hans-Rudolf Pfeifer Albert Gilg 《International Journal of Earth Sciences》2001,90(4):871-889
On Ile de Groix, Variscan metamorphic former tholeiitic and alkaline basalts occur as glaucophane-eclogites, blueschists and greenschists in isolated lenses and layers within metapelites. Whole-rock '18OSMOW values of the metabasites show limited variations (10.4-12.0) and no systematic differences among rock types and metamorphic grades. This provides no argument for large-scale blueschist-to-greenschist transformation driven by infiltration of externally derived fluids. Metamorphic mineralogical changes should have been triggered by internal fluids. Element variations in interlayered blue- and greenschists can be attributed to magmatic fractionation. Assemblages with garnet, clinopyroxene and glaucophane of a high-pressure/low-temperature (HP-LT) metamorphism M1, and NaCa-amphiboles (barroisite, magnesiohornblende, actinolite) of a medium-pressure/medium-temperature metamorphism M2 crystallized during deformation D1. Detailed core-rim zonation profiles display increasing and then decreasing AlIV in glaucophane of M1. NaCa-amphiboles of M2, mantling glaucophane and crystallized in porphyroblasts, show first increasing, then decreasing, AlIV and AlVI. Empirically calibrated thermobarometers allowed P-T path reconstructions. In glaucophane-eclogites of a metamorphic zone I, a prograde evolution to M1 peak conditions at 400-500°C/10-12 kbar was followed by a retrograde P-T path within the glaucophane stability field. The subsequent M2 evolution was again prograde up to >600°C at 8 kbar and then retrograde. Similarly, in metamorphic zones II and III, prograde and retrograde paths of M1 and M2 at lower maximal temperatures and pressures exist. The almost complete metamorphic cycle during M2 signalizes that the HP-LT rocks escaped from an early erosion by a moderate second burial event and explains the long-lasting slow uplift with low average cooling rates. 相似文献
198.
199.
The paper deals with hydraulic problems, arising with the development of geothermal deep ground-water flow. It begins with a brief comment upon the most important principles of flow, regarding the influence of temperature of water. Further it is explained what kind of hydraulic investigations on deep boreholes are necessary and how they have to be done.Finally, it will be reported about hydraulic investigations on particular geothermal water resources in Austria. Results of a pumping test, made at a deep well and lasting 15 months, show that by thermohydraulic investigations, particularly in connexion with geophysical measurements, fundamental statements can be made about available geothermal resources of the earth's crust for the technical planning and economic working of a geothermal energy production plant. The paper ends with a brief report about experiences made with the first geothermal heat supply plant that has been installed in Austria (Waltersdorf/East Styria). 相似文献
200.
Estimating the Impact of Global Change on Flood and Drought Risks in Europe: A Continental, Integrated Analysis 总被引:11,自引:0,他引:11
Bernhard Lehner Petra Döll Joseph Alcamo Thomas Henrichs Frank Kaspar 《Climatic change》2006,75(3):273-299
Most studies on the impact of climate change on regional water resources focus on long-term average flows or mean water availability,
and they rarely take the effects of altered human water use into account. When analyzing extreme events such as floods and
droughts, the assessments are typically confined to smaller areas and case studies. At the same time it is acknowledged that
climate change may severely alter the risk of hydrological extremes over large regional scales, and that human water use will
put additional pressure on future water resources. In an attempt to bridge these various aspects, this paper presents a first-time
continental, integrated analysis of possible impacts of global change (here defined as climate and water use change) on future
flood and drought frequencies for the selected study area of Europe. The global integrated water model WaterGAP is evaluated
regarding its capability to simulate high and low-flow regimes and is then applied to calculate relative changes in flood
and drought frequencies. The results indicate large ‘critical regions’ for which significant changes in flood or drought risks
are expected under the proposed global change scenarios. The regions most prone to a rise in flood frequencies are northern
to northeastern Europe, while southern and southeastern Europe show significant increases in drought frequencies. In the critical
regions, events with an intensity of today's 100-year floods and droughts may recur every 10–50 years by the 2070s. Though
interim and preliminary, and despite the inherent uncertainties in the presented approach, the results underpin the importance
of developing mitigation and adaptation strategies for global change impacts on a continental scale. 相似文献