Niveo-aeolian deposits and denivation forms,with special reference to the great Kobuk Sand Dunes,Northwestern Alaska     

Eduard A. Koster  Jos W. A. Dijkmans 《地球表面变化过程与地形》1988,13(2):153-170

The geomorphology, lithology and chronostratigraphy of extensive, late Pleistocene inland and river dune sands, aeolian sand sheets (‘cover sands’) and loess deposits of periglacial origin in northwestern Europe are well known. However, the idea that some of these aeolian sediments result from niveo-aeolian processes is still an open question, as no diagnostic sedimentary features have yet been reported. Moreover, actual niveo-aeolian sediments and related denivation forms, reported from various cold-climate regions, are not suitable analogues. Recent observations in active dune fields in northwestern Alaska indicate that interstratification of wind-driven snow and sand preferentially occurs on slip faces of transverse, barchanoid or parabolic dune ridges. Annual denivation forms develop: e.g. snow ramparts, sinkholes, snow hummocks, snow meltwater fans and tensional cracks. The surface consists of a cracked wet sand layer with a dimpled surface and spongy structure. Although the preservation potential of these features is low in this specific case, similar features may be observed in ancient sediments elsewhere and provide useful palaeoclimatic indicators. The niveo-aeolian concept should therefore not specifically be related to late Pleistocene cover sand deposition in northwestern Europe, as previously assumed.  相似文献   

Fractal properties of simulated bed profiles in coarse-grained channels   总被引：2，自引：0，他引：2  

André Robert 《Mathematical Geology》1991,23(3):367-382

Bed roughness characteristics in coarse-grained channels are fairly complex. A hierarchy of roughness elements can be observed, ranging from variable particle sizes and shapes and small-scale sedimentary structures, to large-scale bedforms such as riffle-pool sequences. The effects of these scales of roughness on the flow geometry still remain to be thoroughly investigated. The semivariogram has been suggested in the past as a means of quantifying bed roughness effects on streamflow, as well as for distinguishing between scales of roughness. However, field measurements are rather time-consuming. The low number of bed profiles measured in the field precludes the identification of generally applicable relationships between the statistical properties derived from the semivariograms (such as the Hausdorff dimensions and the scale of autocorrelation corresponding to each fractal band) and the bed configuration itself (geometrical and sedimentological properties). Simulation results of gravel-bed profiles are, therefore, presented in order to complement the original investigation of Robert (1988a). The simulation experiments, based on grain characteristics of sizes and shapes and on morphological properties of small-scale bedforms, yield very significant information on boundary roughness at the microscale and give insight into the interpretation of empirical semivariograms (derived from field measurements). Bed-material sorting, variable grain shapes, and height and spacing of cluster bedforms control the fractal dimensions obtained from the semivariograms, as well as the location of the break of slope and the range of the process.  相似文献   

Mixture of populations     

Hua Zhu  André G. Journel 《Mathematical Geosciences》1991,23(4):647-671

A simple example simulating a mixture of two normal populations results in some important observations, nonnormality and nonsymmetry of the mixture conditional pdf, nonlinearity of the conditional mean as a function of the conditioning data, heteroscedasticity of the conditional variance and its nonmonotonicity as a function of distance of the unknown to the conditioning data. A comparison of the mixture statistics with those predicted by traditional models ignoring the mixture reveals the inadequacy and inappropriateness of these traditional approaches. A mixture of two multivariate normal populations is illustrated through the analytical expressions of its conditional distribution and moments.  相似文献   

Size of earthquakes in southern Mexico from indirect methods     

José A. Canas 《Pure and Applied Geophysics》1991,136(4):479-498

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Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano,Panama   总被引：3，自引：0，他引：3  

Mare J. Defant  Lee F. Clark  Robert H. Stewart  Mark S. Drummond  Jelle Z. de Boer  René C. Maury  Hervé Bellon  Thomas E. Jackson  Juan F. Restrepo 《Contributions to Mineralogy and Petrology》1991,106(3):309-324

The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites. The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics. We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano is related to the subduction of relatively hot lithosphere.  相似文献   

Diabatic wind and temperature profiles obtained from a 100 m tower: A statistical comparison of model performances     

R. San José  J. Regueroa  J. Casanova 《Boundary-Layer Meteorology》1986,37(1-2):1-15

More than 8000 10-min profiles of wind and temperature obtained during an extensive field experiment have been analyzed. The study was carried out in the environs of Valladolid (Spain) where a 100-m and a 6-m meteorological tower are located. Less than 20% of data were discarded due to equipment failure. Because of its exceptional flatness, the site is almost ideal for model performance comparisons.Predicted profiles of wind speed and potential temperature at 12, 26, 51, and 100 m heights were obtained using the methods proposed by Berkowicz and Prahm (1982) and San José (1983) based on values observed on a 6 m tower. Statistical t, F, and R tests were used to determine bias, scatter and correlation. The data were classified according to atmospheric stratification and height above ground. Finally, a determination was made of the predicted wind speeds and potential temperatures that fell within the 1 and 20% confidence ranges centered at the measured value. San José's method performed better than did that of Berkowicz and Prahm for the temperature profiles in both unstable and stable conditions. However, no significant differences were found for the wind speed profiles.  相似文献   

A modified parameterization of flux-profile relationships in the surface layer using different roughness length values for heat and momentum   总被引：1，自引：0，他引：1  

Patrick Mascart  Joel Noilhan  Hervé Giordani 《Boundary-Layer Meteorology》1995,72(4):331-344

The surface-layer flux-profile formulae of Louis (1979), used in many atmospheric models, are modified in a simple way to allow for different values of the roughness lengths for heat and momentum. The modified set of formulae simplifies the calculation of surface-layer fluxes over most natural land surfaces, where the roughness length for momentum can be almost an order of magnitude greater than that for heat.  相似文献   

Observed dependence of the water vapor and clear-sky greenhouse effect on sea surface temperature: comparison with climate warming experiments     

Sandrine Bony  Jean-Philippe Duvel  Hervé Le Trent 《Climate Dynamics》1995,11(5):307-320

This study presents a comparison of the water vapor and clear-sky greenhouse effect dependence on sea surface temperature for climate variations of different types. Firstly, coincident satellite observations and meteorological analyses are used to examine seasonal and interannual variations and to evaluate the performance of a general circulation model. Then, this model is used to compare the results inferred from the analysis of observed climate variability with those derived from global climate warming experiments. One part of the coupling between the surface temperature, the water vapor and the clear-sky greenhouse effect is explained by the dependence of the saturation water vapor pressure on the atmospheric temperature. However, the analysis of observed and simulated fields shows that the coupling is very different according to the type of region under consideration and the type of climate forcing that is applied to the Earth-atmosphere system. This difference, due to the variability of the vertical structure of the atmosphere, is analyzed in detail by considering the temperature lapse rate and the vertical profile of relative humidity. Our results suggest that extrapolating the feedbacks inferred from seasonal and short-term interannual climate variability to longer-term climate changes requires great caution. It is argued that our confidence in climate models' predictions would be increased significantly if the basic physical processes that govern the variability of the vertical structure of the atmosphere, and its relation to the large-scale circulation, were better understood and simulated. For this purpose, combined observational and numerical studies focusing on physical processes are needed.  相似文献   

Evolution estivale de la turbidité dans le lac de Neuchâtel     

Par André Bapst  Bernard Kübler 《Aquatic Sciences - Research Across Boundaries》1987,49(1):1-15

Some turbiditic measures carried out by a limnological diffusiometer along with the physico-chemical determinations of the Lake of Neuchatel have permitted to subdivide the water column in 4 distinct layers: the Epilimnium, the Clear Water Layer, the Pelagic Nepheloid Layer and the Benthic Nepheloid Layer.   相似文献   

Least-squares prediction of horizontal coordinate distortions in Canada     

Gérard Lachapelle  André Mainville 《Journal of Geodesy》1982,56(3):242-257

A least-squares prediction method is described to estimate horizontal coordinate distortions at lower order points of a network using known coordinate differences (NAD27 coordinate distortions Δϕ′s and Δλ′s) at higher order points between NAD27 coordinates and coordinates derived from a recent (MAY 76), relatively distortion free, adjustment of these points. Empirical autocovariance functions of Δϕ and Δλ and crosscovariance function between Δϕ and Δλ are derived from some 5,250 data points and modelled using series of exponential functions. Empirical mean square values of Δϕ and Δλ, which are a measure of the distortions in NAD27 ϕ and λ, are 0.051 and 0.645 arcsecs² respectively. The corresponding mean value of the product ΔϕΔλ, which is a measure of the correlation between Δϕ and Δλ, is 0.056 arcsecs². The accuracy obtainable for predicted Δϕ and Δλ at an arbitrary point (e.g., lower order station) is a function of the accuracy and configuration of known Δϕ′s and Δλ′s in the surrounding area. Accuracies obtainable for various types of data configuration are given. Under favorable conditions taking place in about 60% of cases, accuracies in terms of ms agreement with known values of 0″.02 (0.6 m) and 0″.01 (0.2 m along parallel at latitude 50°) for the predicted latitude and longitude distortions are obtainable. Finally, a comparison with a method based on the use of complex polynomials is made. Presented at International Symposium on Geodetic Networks and Computations, Munich, August–September 1981.  相似文献