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71.
Mangrove forests and saltmarshes are important habitats for carbon (C) sequestration in the coastal zone but variation in rates of C sequestration and the factors controlling sequestration are poorly understood. We assessed C sequestration in Moreton Bay, South East Queensland in mangrove forests and tidal marshes that span a range of environmental settings and plant communities, including mangrove forests and tidal marshes on the oligotrophic sand islands of the eastern side of Moreton Bay and on the nutrient enriched, western side of the bay adjacent to the city of Brisbane. We found that rates of C sequestration in sediments were similar among mangrove forests over the bay, despite large differences in the C density of sediments, because of different rates of vertical accretion of sediments. The C sequestration on the oligotrophic sand island tidal marshes, dominated by Juncus kraussii, had the highest rate of C sequestration in the bay while the western saltmarshes, which were dominated by Sarcocornia quinqueflora, had the lowest rate of C sequestration. Our data indicate C sequestration varies among different tidal wetland plant community types, due to variation in sediment characteristics and rates of sediment accretion over time.  相似文献   
72.
The development of the new seismic hazard map of metropolitan Tehran is based on probabilistic seismic hazard computation using the non-Poisson recurrence time model. For this model, two maps have been prepared to indicate the earthquake hazard of the region in the form of iso-acceleration contour lines. They display the non-Poisson probabilistic estimates of peak ground accelerations over bedrock for 10 and 63 % probability of exceedance in 50 years. To carry out the non-Poisson seismic hazard analysis, appropriate distributions of interoccurrence times of earthquakes were used for the seismotectonic provinces which the study region is located and then the renewal process was applied. In order to calculate the seismic hazard for different return periods in the probabilistic procedure, the study area encompassed by the 49.5–54.5°E longitudes and 34–37°N latitudes was divided into 0.1° intervals generating 1,350 grid points. PGA values for this region are estimated to be 0.30–0.32 and 0.16–0.17 g for 10 and 63 % probability of exceedance, respectively, in 50 years for bedrock condition.  相似文献   
73.
The Saouaf syncline is located in Tunisia Centre north-east, south-east of the Tunisian dorsal. The sedimentary series, starting with limestone and marlstone, is essentially an alternation of three terms, sandstone, clay and lignite. It is in this basin that the Saouaf Formation was defined as a siliciclastic and lignite series dating back to the Serravallian-Tortonian age. The pollen associations identified in this series allowed for the organization of the vegetation in the era in altitudinal levels (coastal environment, rivers edges, plains, high zones). The evolution of vertical micro-floristic associations through the detailed analysis of the palynological diagram reflects the fluctuation of climatic and ecological parameters during this period. Moreover, the facies associations reflect an evolutionary curve in the depositional environment, showing its fluctuation over time. Accordingly, the synthesis of palynological data, correlated with lithosedimentological ones, suggests the type of environment prevailing at that time. This is a coast-lagoon fluvio-deltaic environment with a significant detrital influence in a subsiding context controlled by a fluctuating but generally warm climate.  相似文献   
74.
A sediment core from Chuna Lake (Kola Peninsula, northwest Russia) was studied for pollen, diatoms and sediment chemistry in order to infer post‐glacial environmental changes and to investigate responses of the lake ecosystem to these changes. The past pH and dissolved organic carbon (DOC) of the lake were inferred using diatom‐based transfer functions. Between 9000 and 4200 cal. yr BP, slow natural acidification and major changes in the diatom flora occurred in Chuna Lake. These correlated with changes in regional pollen, the arrival of trees in the catchment, changes in erosion, sediment organic content and DOC. During the past 4200 yr diatom‐based proxies showed no clear response to changes in vegetation and erosion, as autochthonous ecological processes became more important than external climate influences during the late Holocene. The pollen stratigraphy reflects the major climate patterns of the central Kola Peninsula during the Holocene, i.e. a climate optimum between 9000 and 5400/5000 cal. yr BP when climate was warm and dry, and gradual climate cooling and an increase in moisture during the past 5400/5000 yr. This agrees with the occurrence of the north–south humidity gradient in Fennoscandia during the Holocene. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
75.
Abstract— Calcium‐aluminum‐rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka. The 20Ne/22Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne‐HL, Ne‐Q, or solar wind Ne. The 21Ne/22Ne ratios range from 0.86 to 0.72, with fine‐grained, more altered CAIs usually showing lower values than coarse‐grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na‐rich alteration phases rather than to the presence of Ne‐G or Ne‐R (essentially pure 22Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The 36Ar/38Ar ratios are between 0.7 and 4.8, with fine‐grained CAIs within one meteorite showing higher ratios than the coarse‐grained ones. This agrees with higher concentrations of cosmogenic 36Ar produced by neutron capture on 35Cl with subsequent β?‐decay in finer‐grained, more altered, and thus, more Cl‐rich CAIs than in coarser‐grained, less altered ones. Although our data do not strictly contradict the presence of small amounts of Ne‐G, Ne‐R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na‐, Ca‐, and Cl‐rich minerals.  相似文献   
76.
We model the particle velocity distribution functions around the entrance window of the Suprathermal Ion Imager (SII). The SII sensor was mounted on a 1 m boom carried by the scientific payload of NASA rocket 36.234 as part of Joule II mission to investigate Joule heating in the E-region ionosphere. The rocket flew above Northern Alaska on 19 January 2007. The payload was spin-stabilized with a period of 1.6 s, giving an apparent rotation of the ion flow velocity in the frame of reference of the payload. The SII sensor is an electrostatic analyzer that measures two dimensional slices of the distribution of the kinetic energies and arrival-angles of low energy ions. The study is concerned with the interpretation of data obtained from the SII sensor. For this purpose, we numerically investigate ram velocity effects on ions velocity distributions in the vicinity of the SII sensor aperture at an altitudes of approximately 150 km. The electrostatic sheath profiles surrounding the SII sensor, boom and payload are calculated numerically with the PIC code PTetra. It is observed that the direction of the ion flow velocity modifies the plasma sheath potential profile. This in turn impacts the velocity distributions of NO+ and \(\mathrm{O}_{2}^{+}\) ions at the aperture of the particle sensor. The velocity distribution functions at the sensor aperture are calculated by using test-particle modeling. These particle distribution functions are then used to inject particles in the sensor, and calculate the fluxes on the sensor microchannel plate (MCP), from which comparisons with the measurements can be made.  相似文献   
77.
78.
RETRACTED ARTICLE: Numerical analysis of multiphase flow in porous material   总被引:7,自引:7,他引:0  
Recent developments in the application of x-ray micro-tomography in laboratory geomechanics have allowed all the individual grains of sand in a test sample to be seen and identified uniquely in 3D. Combining such imaging capabilities with experiments carried out “in situ” within an imaging set-up has led to the possibility of directly observing the mechanisms of deformation as they happen. The challenge has thus become extracting pertinent, quantified information from these rich time-lapse 3D images to elucidate the mechanics at play. This paper presents a new approach (ID-Track) for the quantification of individual grain kinematics (displacements and rotations) of large quantities of sand grains (tens of thousands) in a test sample undergoing loading. With ID-Track, grains are tracked between images based on some geometrical feature(s) that allow their unique identification and matching between images. This differs from Digital Image Correlation (DIC), which makes measurements by recognising patterns between images. Since ID-Track does not use the image of a grain for tracking, it is significantly faster than DIC. The technique is detailed in the paper, and is shown to be fast and simple, giving good measurements of displacements, but suffering in the measurement of rotations when compared with Discrete DIC. Subsequently, results are presented from successful applications of ID-track to triaxial tests on two quite different sands: the angular Hostun sand and the rounded Caicos Ooids. This reveals details on the performance of the technique for different grain shapes and insight into the differences in the grain-scale mechanisms occurring in these two sands as they exhibit strain localisation under triaxial loading.  相似文献   
79.
The recent sediments from two deep arctic lakes, Mitrofanovskoe and Vanuk-ty, situated in the permafrost belt within the Bolshezemelskaya Tundra in the northern Ural region, were studied for diatoms, chironomids, spheroidal carbonaceous particles and stable lead isotopes. The magnitudes and rates-of-change in diatom and chironomid assemblages were numerically estimated. Instrumental climate records were used to assess statistically the amount of variance in diatom and chironomid data explained by temperature. August and September air temperatures have a statistically significant effect on diatom composition at both lakes. At Mitrofanovskoe Lake, major compositional changes in diatom and chironomid assemblages occurred at the turn of the 20th century and might be related to the regional increase in temperature. Chironomid-inferred air temperature also increased by approximately 1 °C since the early 1900s. At both lakes diatom compositional changes, coincident with the increase in June and September temperatures, also occurred in the late 1960s. These compositional changes are correlated with the increase in diatom production, sediment organic content and diatom species richness, and are likely to be a diatom response to the lengthening of the growing season. These changes are also correlated with the circum-Arctic temperature increase from the 1960s. A chironomid response to the late 1960s temperature increase was less pronounced at both lakes. Pollution levels are relatively low and pollution history is unrelated to ecological changes. Both lead isotopes and spheroidal carbonaceous particles show a clear atmospheric pollution signal, peaking in the 1980s.  相似文献   
80.
Mid-Holocene changes in vegetation, palaeohydrology and climate were investigated from the sediments of Lake Vankavad in the northern taiga of the Usa Basin, NE European Russia, through the analysis of pollen, plant macrofossils, Cladocera and diatoms. Lake Vankavad was probably formed at ca. 5000 BP (ca. 5600 cal. BP) and initially it was shallow with a littoral cladoceran fauna. Macrofossil and pollen results suggest that dense Betula-Picea forests grew in the vicinity and the shore was close to the sampling point. At ca. 4600 BP (ca. 5400 cal. BP) the water level rose coincident with the decrease in the density and area of forests, probably caused by cooling climate and accelerated spread of mires. There was also a further rise in the water level at ca. 3500 BP (ca. 3800 cal. BP). The initiation of the lake, followed by two periods of rising water-level, as well as the increase in mire formation, was a consequence of a rise in groundwater level. This probably reflects lower evapotranspiration in a cooling mid-Holocene climate and/or higher precipitation in the lowland area. Also the decreased forest density and area may have contributed to the lower evapotranspiration. It is also possible that permafrost aggradation or changes in peat ecosystems might have affected the hydrological conditions in the area.  相似文献   
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