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71.
72.
High performance liquid chromatography-mass spectrometry has been used to identify the cyanophyta-derived ultraviolet screening pigment scytonemin and its reduced counterpart in an Antarctic lake sediment. The formation of an artefact during the analysis has been demonstrated, enabling the recovery of improved estimates of the scytonemin and reduced scytonemin signals. The overall pigment composition records a primary producer community comprising both oxygenic and anoxygenic photoautotrophs, with variations in the abundance of scytonemin relative to the chlorophylls and carotenoids that are consistent with changes in response to variations in the light regime over time.  相似文献   
73.
During the 1999 Athens Earthquake the town of Adàmes, located on the eastern cliff of the Kifissos river canyon, experienced unexpectedly heavy damage. Despite the significant amplification potential of the slope geometry, topography effects cannot alone explain the uneven damage distribution within a 300 m zone behind the crest, characterized by a rather uniform structural quality. This paper illustrates the important role of soil stratigraphy, material heterogeneity, and soil–structure interaction on the characteristics of ground surface motion. For this purpose, we first perform elastic two-dimensional wave propagation analyses utilizing available geotechnical and seismological data, and validate our results by comparison with aftershock recordings. We then conduct non-linear time-domain simulations that include spatial variability of soil properties and soil–structure interaction effects, to reveal their additive contribution in the topographic motion aggravation.  相似文献   
74.
Accurate identification of fossil pigments is essential if they are to be used as biomarker compounds in palaeolimnological studies. In recent years High Performance Liquid Chromatography (HPLC) has greatly enhanced the efficiency with which fossil pigments can be characterised and quantified. Using HPLC, undegraded pigments are typically identified through retention times, absorbance spectra and co-chromatography with authentic reference standards. However, lake sediments may also contain degraded pigments for which there are often no standards, and which may be difficult to identify using HPLC alone. In this study, we submitted HPLC fractions of fossil pigments and pigment derivatives collected from a meromictic lake in south west Tasmania, to a combination of Mass Spectrometry (MS) techniques including Electron Impact (EI) and static Liquid Secondary Ion MS (LSIMS) to identify their molecular ion characteristics and organic chemical composition. Mass Spectrometry permitted the detection of specific mass ions which were used to verify the identity of pigments and their derivatives. These included five carotenoids, chlorophyll a and derivatives, three previously described bacteriochlorophyll c derivatives with molecular weights of 770, 784, and 802, and two undescribed derivatives of bacteriochlorophyll c with molecular weights of 766 and 788. With these improved identifications we speculate on the pathways and modes of pigment degradation in the lake and asses the value of the degraded pigments as biomarkers. The use of MS permitted the identification of a greater number of signature pigments of algal and bacterial communities thus increasing the palaeolimnological value of the sediments. These methods are best applied in fossil pigment studies where there are a large number of unknown pigments and pigment degradation products, and where there are no authentic standards for co-chromatography. Practical suggestions for pigment MS are included in the discussion.  相似文献   
75.
Beach evolution models are normally applied in a prognostic fashion, with parameters and boundary conditions estimated from previous experience or other forecasts. Here, we use observations of beach profiles to solve a beach profile evolution equation in an inverse manner to determine model parameters and source function. The data used to demonstrate the method are from Christchurch Bay in Dorset, UK. It was found that there is a significant contribution from diffusive processes to the morphodynamic evolution of the beach profiles and that the development and disappearance of near-shore coastal features such as upper beach berms and inter- and sub-tidal bars are well captured by the source function in the governing equation.  相似文献   
76.
Miocene strata in the southern Taranaki Basin (STB), up to 3 km thick, provide a distal record of erosion associated with plate boundary deformation in New Zealand. 2D and 3D seismic reflection data tied to drillhole stratigraphy have been used to constrain four main phases of basin development. These are: (a) Early Miocene (22–19 Ma) subsidence, dominantly bathyal water depths and deposition of minor submarine fans along the eastern basin margin. (b) Middle Miocene (19–14 Ma) widespread submarine fan deposition on a bathyal basin floor in the central STB. (c) Rapid Middle–Late Miocene (14–7 Ma) progradation of the shelf break northwards across the STB. (d) Widespread uplift and erosion of the STB during the latest Miocene–Pliocene (7–4.5 Ma). Bathyal water depths and fan deposition in the Early Miocene were influenced by vertical motions on major reverse faults and regional subsidence produced by subduction of the Pacific plate beneath northern New Zealand. Subsequent submarine fan deposition and northward shelf‐break progradation reflect increasing input of terrigenous material, primarily eroded from an uplifting region to the south of the STB. Sedimentation patterns in the STB are consistent with the age and locations of conglomerates deposited in onshore West Coast basins, related to this uplift and erosion. Sediment transport in the West Coast region was mainly parallel to NNE trending active reverse faults, and in the STB was perpendicular to the NE‐SW orientated shelf break, especially from ca. 14–7 Ma, when sedimentation rates exceeded fault‐displacement rates. Increases in sedimentation rates in the STB coincide with regional increases in the rates of shortening that appear to reflect plate boundary‐wide events and have been attributed to, or correlated with, increases in the plate convergence rate. Miocene sedimentation patterns in the STB thus reflect both intra‐basinal deformation and tectonic signals from the wider developing New Zealand plate boundary.  相似文献   
77.
Sediments of Lake Fidler, a meromictic lake in south-west Tasmania, contain distinctive laminae. In order to determine their composition and formation, these laminae were studied using a combination of X- ray analysis, scanning electron microscopy, X-ray diffraction and vibrational spectrometry. Results indicated that the laminae were composed of clay originating from the adjacent Gordon River estuary. The clay was also found as part of the general sediment matrix of the core. The evidence indicates that the laminae are formed during incursions of brackish water from the adjacent Gordon River estuary into Lake Fidler which cause the flocculation of clay minerals in the water column. These clay minerals then sink rapidly to the sediments to form laminae. Although the formation of these laminae pre-dates meromixis, their mode of formation corroborates the theory that meromixis is maintained ectogenically by periodic inflows of brackish water replenishing monimolimnetic salts.  相似文献   
78.
This paper describes the application of Canonical Correlation Analysis (CCA) to derive forcing–response relations between the wave climate and shoreline position on a macrotidal gravel barrier located in the southwest of the U.K., and to develop a tool to determine shoreline positions from wave records. The data sequences comprise wave climate recorded by a nearshore directional wave buoy and video-derived shorelines over a time span of one year and a half. The hydrodynamic conditions are used to determine the probability density function of wave heights and alongshore energy fluxes. These are then related to shoreline change through a CCA analysis. The CCA analysis identifies patterns of behaviour of the wave conditions and the shoreline position, and the relation between both patterns is found to provide useful information about the beach response to wave action. The analysis shows that the movement of sediment is greater at the southern end of the study area and that there is an immediate shoreline response to the wave action. In the case of coastal management it is more often the case that wave forecasts are available on a routine basis. The ability of the CCA to provide useful estimates of shorelines from wave conditions was tested by using measured waves to calculate the corresponding shoreline position from additional data at the end of the sequences. Shoreline positions determined with the CCA agreed well with the measured ones. Thus, the CCA is found to be a useful tool to determine unknown shoreline positions and support effective coastal management if good quality hydrodynamic and morphological data are available to input into the initial set-up of the technique.  相似文献   
79.
Near-bed oscillatory flows with acceleration skewness are characteristic of steep and breaking waves in shallow water. In order to isolate the effects of acceleration skewness on sheet flow sand transport, new experiments are carried out in the Aberdeen Oscillatory Flow Tunnel. The experiments have produced a dataset of net transport rates for full-scale oscillatory flows with varying degrees of acceleration skewness and three sand sizes. The new data confirm previous research that net transport in acceleration-skewed flow is non-zero, is always in the direction of the largest acceleration and increases with increasing acceleration skewness. Large transport rates for the fine sand conditions suggest that phase lag effects play an important role in augmenting positive net transport. A comparison of the new experimental data with a number of practical sand transport formulations that incorporate acceleration skewness shows that none of the formulations performs well in predicting the measured net transport rates for both the fine and the coarser sands. The new experimental data can be used to further develop practical sand transport formulations to better account for acceleration skewness.  相似文献   
80.
This paper presents an integrated investigation of physical processes generating impulsive pressures under the action of plunging breakers impinging on gravel beaches. This work is an extension of a recent investigation which suggested that wave impacts from plunging breakers acting on gravel beaches may be a key mechanism to enhance sediment mobilisation. In particular, comparisons of full scale laboratory measurements against model results from a well-validated phase/depth-resolving numerical model based on the Reynolds–Averaged Navier–Stokes (RANS) equations are presented. This represents the first attempt at comparison with such a tool against observed hydrodynamics on steep (slope~1/8) gravel beaches at prototype scale. In order to understand how impulsive pressures are generated under plunging waves, the numerical model is used to carry out a detailed investigation to examine the role of each of the acceleration terms in the momentum balance. Consistent with prior studies, numerical results show that under plunging breakers the local acceleration (∂u/∂t) alone cannot be used as a proxy for pressure gradients. However, the contribution of the third term (wu/∂z) of total acceleration is recognized for the first time and indicates that this term has an important role in both the induced pressure gradient and sediment mobilisation as induced by this particular type of breaking. Furthermore, results suggest that a parameterisation of the pressure gradient in terms of ∂u/∂t+uu/∂x, may not suffice when dealing with plunging breakers and hence there is a lack of a suitable parameterisation of this process in the present literature. Thus, for different types of breaking it may be necessary to consider a different characterisation of the pressure gradient toward the parameterisation of sediment transport inside the surf zone.  相似文献   
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