Nephelines from the Somma-Vesuvius volcanic complex (Southern Italy): crystal-chemical,structural and genetic investigations     

Giuseppina Balassone  Volker Kahlenberg  Angela Altomare  Angela Mormone  Rosanna Rizzi  Michele Saviano  Nicola Mondillo 《Mineralogy and Petrology》2014,108(1):71-90

Sixteen nephelines from different geological occurrences were sampled at the type-locality, the Somma-Vesuvius volcanic complex (southern Italy), and investigated for their chemistry and crystal structure obtained by both single-crystal and powder X-ray diffraction. Nepheline-bearing samples are metamorphic or from magmatic ejecta and pumice deposits. The lower K contents characterize the pumice- and some metamorphic-derived nephelines, whereas the higher ones are found in some samples from magmatic nodules. The amount of the anorthite molecule, quite low on average, can be more variable in the metamorphic nephelines. The crystal-structure investigations on Somma-Vesuvius samples compare well with previous studies of natural nephelines. All 16 nepheline samples adopt space group P6₃. The observed lattice parameters vary between 9.9768–9.9946 Å (for a) and 8.3614–8.3777 Å (for c), respectively. Furthermore, chemical analysis revealed that all specimens exhibit an excess of Si relative the ideal Si:Al ratio of 1:1. The analysis of the T-O distances in our samples clearly indicates a distinct ordering process of aluminium and silicon on the tetrahedral sites which is an agreement with Loewenstein’s rule. A linear correlation between the distance of symmetry equivalent split atoms O(1)-O(1)’ and the T(1)-O(1)-T(2) tilt angle was observed. The average <B-O> (B = Na) distances of all crystals are very similar which is consistent with the outcome of the site population refinement indicating full occupancy with sodium. Oriented precession-type sections of reciprocal space indicated the presence of at least the most intense family of satellite peaks, demonstrating that this group of satellite reflections can occur not only in nephelines from pegmatites and ijolites but also in rocks from completely different petrological settings.  相似文献   

Stable Sulfur Isotopic Evidence for Historical Changes of Sulfur Cycling in Estuarine Sediments from Northern Florida     

Volker Brüchert  Lisa M. Pratt 《Aquatic Geochemistry》1999,5(3):249-268

Data on abundance and isotopic composition of porewater and sedimentary sulfur species are reported for relatively uncontaminated and highly contaminated fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong contrast in amount and composition of sedimentary organic matter at the two sites allows a comparative study of the historical effects of increased organic loading on sulfur cycling and sulfur isotopic fractionation. In the contaminated sediments, an increase in organic loading caused increased sedimentary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences are well reflected in the isotopic composition of dissolved sulfate, sulfide, and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomposition of organic matter is most active in the upper 8cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of pyrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changing diagenetic conditions. Sediment profiles of the sulfur isotopic composition of pyrite reflect present-day higher rates of bacterial sulfate reduction and lower rates of sulfide oxidation, and record a profound change in the diagenetic cycling of sulfur in the contaminated sediments coincident with urban and industrial development of the St. Andrew Bay area.  相似文献   

Long‐range terrestrial laser scanning for geomorphological change detection in alpine terrain – handling uncertainties          下载免费PDF全文

Christine Fey  Volker Wichmann 《地球表面变化过程与地形》2017,42(5):789-802

Long‐range terrestrial laser scanning (TLS) is an emerging method for the monitoring of alpine slopes in the vicinity of infrastructure. Nevertheless, deformation monitoring of alpine natural terrain is difficult and becomes even more challenging with larger scan distances. In this study we present approaches for the handling of spatially variable measurement uncertainties in the context of geomorphological change detection using multi‐temporal data sets. A robust distance measurement is developed, which deals with surface roughness and areas of lower point densities. The level of detection (LOD), i.e. the threshold distinguishing between real surface change and data noise, is based on a confidence interval considering the spatial variability of TLS errors caused by large laser footprints, low incidence angles and surface roughness. Spatially variable positional uncertainties are modelled for each point according to its range and the object geometry hit. The local point cloud roughness is estimated in the distance calculation process from the variance of least‐squares fitted planes. Distance calculation and LOD assessment are applied in two study areas in the Eastern Alps (Austria) using multi‐temporal laser scanning data sets of slopes surrounding reservoir lakes. At Finstertal, two TLS point clouds of high alpine terrain and scanned from ranges between 300 and 1800 m are compared. At Gepatsch, the comparison is done between an airborne laser scanning (ALS) and a TLS point cloud of a vegetated mountain slope scanned from ranges between 600 and 3600 m. Although these data sets feature different conditions regarding the scan setup and the surface conditions, the presented approach makes it possible to reliably analyse the geomorphological activity. This includes the automatic detection of rock glacier movement, rockfall and debris slides, even in areas where a difference in vegetation cover could be observed. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Erosion pattern of artificial gravel deposits   总被引：1，自引：1，他引：0  

Fabian Friedl  Volker Weitbrecht  Robert M. Boes 《国际泥沙研究》2018,33(1):57-67

Sediment replenishment with artificial gravel deposits is an option to compensate for sediment deficits in rivers and to improve their ecological conditions. Predicting and quantifying the erosion rate of an artificial gravel deposit is important to successfully perform river restoration projects. Laboratory experiments have been done to investigate the influence of various parameters on the erosion pattern of artificial gravel deposits. In the present paper the effects of deposit geometry, bulk density, grain size distribution, and hydraulic load on the erosion process are described. The temporal evolution of the deposit geometry and the corresponding mean erosion rates were studied. The mean erosion rate increases with deposit height, deposit width, and decreasing grain size. Furthermore, no significant impact of the bulk density was observed. Equations to predict the mean erosion rate are proposed. This investigation helps to determine the design frequency of gravel dumping and deposit volumes for restoration projects.  相似文献   

Modification of the deep salinity-maximum in the Southern Ocean by circulation in the Antarctic Circumpolar Current and the Weddell Gyre     

Matthew Donnelly  Harry Leach  Volker Strass 《Ocean Dynamics》2017,67(7):813-838

The evolution of the deep salinity-maximum associated with the Lower Circumpolar Deep Water (LCDW) is assessed using a set of 37 hydrographic sections collected over a 20-year period in the Southern Ocean as part of the WOCE/CLIVAR programme. A circumpolar decrease in the value of the salinity-maximum is observed eastwards from the North Atlantic Deep Water (NADW) in the Atlantic sector of the Southern Ocean through the Indian and Pacific sectors to Drake Passage. Isopycnal mixing processes are limited by circumpolar fronts, and in the Atlantic sector, this acts to limit the direct poleward propagation of the salinity signal. Limited entrainment occurs into the Weddell Gyre, with LCDW entering primarily through the eddy-dominated eastern limb. A vertical mixing coefficient, κ_V of (2.86 ± 1.06) × 10^?4 m² s^?1 and an isopycnal mixing coefficient, κ_I of (8.97 ± 1.67) × 10² m² s^?1 are calculated for the eastern Indian and Pacific sectors of the Antarctic Circumpolar Current (ACC). A κ_V of (2.39 ± 2.83) × 10^?5 m² s^?1, an order of magnitude smaller, and a κ_I of (2.47 ± 0.63) × 10² m² s^?1, three times smaller, are calculated for the southern and eastern Weddell Gyre reflecting a more turbulent regime in the ACC and a less turbulent regime in the Weddell Gyre. In agreement with other studies, we conclude that the ACC acts as a barrier to direct meridional transport and mixing in the Atlantic sector evidenced by the eastward propagation of the deep salinity-maximum signal, insulating the Weddell Gyre from short-term changes in NADW characteristics.  相似文献   

Robert John Russell,Nancey Murphy,C. J. Isham (eds.): Quantum Cosmology and the Laws of Nature. Scientific Perspectives on Divine Action; Vatican Observatory Foundation 1993, Preis: 19.95 US$ ISBN 0-268-03975-5, paperback: 0-268-03976-3     

Volker Müller 《Astronomische Nachrichten》1996,317(1):42-42

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Maurice M. Shapiro,Rein Silberberg,John P. Wefel (Eds.): Particle Astrophysics and Cosmology; Nato AS1 Series C: Mathematical and Physical Sciences - Vol. 394, Kluwer Academic Publishers 1993, VII+168 Seiten (Preis: 79.00 $) ISBN 0-7923-2235-5     

Volker Müller 《Astronomische Nachrichten》1996,317(5):367-368

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Book reviews: Current Aspects of Neutrino Physics. By David O. Caldwell     

Volker Müller 《Astronomische Nachrichten》2003,324(5):507-507

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