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71.
Tobias Dürig Fabio Dioguardi Ralf Büttner Pierfrancesco Dellino Daniela Mele Bernd Zimanowski 《Bulletin of Volcanology》2012,74(4):895-902
Brittle magmatic fragmentation plays a crucial role in explosive eruptions. It represents the starting point of hazardous
explosive events that can affect large areas surrounding erupting volcanoes. Knowing the initial energy released during this
fragmentation process is fundamental for the understanding of the subsequent dynamics of the eruptive gas-particle mixture
and consequently for the forecasting of the erupting column’s behavior. The specific kinetic energy (SKE) of the particles
quantifies the initial velocity shortly after the fragmentation and is therefore a necessary variable to model the gas-particle
conduit flow and eruptive column regime. In this paper, we present a new method for its determination based on fragmentation
experiments and identification of the timings of energy release. The results obtained on compositions representative for basaltic
and phonolitic melts show a direct dependence on magma material properties: poorly vesiculated basaltic melts from Stromboli
show the highest SKE values ranging from 7.3 to 11.8 kJ/kg, while experiments with highly vesiculated samples from Stromboli
and Vesuvius result in lower SKE values (3.1 to 3.8 kJ/kg). The described methodology presents a useful tool for quantitative
estimation of the kinetic energy release of magmatic fragmentation processes, which can contribute to the improvement of hazard
assessment. 相似文献
72.
Dirk Enters Hermann Behling Christoph Mayr Lydie Dupont Bernd Zolitschka 《Journal of Paleolimnology》2010,44(1):265-277
Environmental changes of the last 9,300 years were reconstructed by geochemical and pollen analyses of a 14-m-long, laminated
sediment core from Lago Aleixo, south-eastern Brazil. Fossil pollen assemblages indicate open savannah vegetation (campo cerrado)
and gallery forests until approximately 6,900 cal. BP. During that time, siderite laminae were deposited under anoxic conditions
at the lake bottom. Then, increased rainfall and a shorter annual dry period allowed gallery forests and semi-deciduous forests
to expand, leading to more closed cerrado vegetation. High-intensity rainfall events during this period are recorded as peaks
in K and Ti concentrations. The sediment facies during this period consists of alternating layers of diatoms and minerogenic
matter. C/N ratios imply that algae and perhaps soils, too, were the main contributors to sediment organic matter. Biogenic
silica and δ13Corg variations indicate increasing primary productivity, which was related to higher nutrient flux from intensified leaching
of soils, as shown by rising K/Al ratios. Around 800 cal. BP, a closed, semi-deciduous forest developed under present-day
climate conditions. Slope stabilization diminished erosion processes in the catchment and caused reduced input of minerogenic
matter into the basin. Human impact is evident in the topmost homogeneous sediments, as removal of the stabilizing forest
cover amplified soil erosion. The continuous trend to more humid conditions during the Holocene probably reflects increased
influence of the Amazon Basin as a moisture source. We conclude that the Lago Aleixo sediment archive was a sensitive recorder
of environmental dynamics in tropical South America, which were mainly controlled by changes in precipitation patterns. 相似文献
73.
Elena Dunca Harry Mutvei Peter Göransson Carl-Magnus Mörth Bernd R. Schöne Martin J. Whitehouse Mikael Elfman Susanne P. Baden 《International Journal of Earth Sciences》2009,98(1):3-17
Shells of Arctica islandica collected between 1884 and 2004 from Öresund, Kattegat and Skagerrak (Swedish West Coast) were used to monitor local climate variations and the influence of human activities on the local environment. For this purpose, we analysed the growth, structure and chemical composition of these shells and compared them with shells collected from Kiel Bay, Norway and Iceland. The growth rate of the studied shells registers an NAO periodicity of ca 8 years. However, the observed signal is weak because of other environmental interferences that are either of natural or anthropogenic origin. For example, the oxygen isotope ratios show temperature fluctuation, but also the influx of low salinity water. Higher contents of S, N, Cu, Zn, As, Cd and P in shell portions formed during the last century are related to human activities such as mining and industrial development. Our study indicates that in order to use Arctica shells as archives of climate change it is necessary to study the full range of environmental data that is recorded in the shells by using a multi element and isotope approach in combination with different analytical techniques including investigation of growth rates and shell structure. 相似文献
74.
Alexandre Raphael Cabral Bernd Lehmann Miguel Tupinambá Michael Wiedenbeck Michael Brauns 《Journal of Geochemical Exploration》2011
The Palaeo-Mesoproterozoic metasiliciclastic rocks of the southern Serra do Espinhaço, Minas Gerais, Brazil, are host to historically important alluvial deposits of diamonds and gold. Detrital gold grains often comprise Au–Pd–Pt intermetallic compounds, with low Ag contents, which contain inclusions of tourmaline and titaniferous hematite (up to ~ 6 wt.% TiO2). The latter minerals connect the alluvial mineralisation to the rutile–hematite–quartz veins and tourmalinisation observed in the quartzitic country rocks of the alluvial gravel. The quartzite (Sopa-Brumadinho Formation of lacustrine to fan-deltaic origin) is affected by pervasive B metasomatism with F-bearing tourmaline replacing the recrystallised quartz fabric. The tourmaline belongs to the alkali group, with Mg/(Mg + Fe) and X/(X + Na) ratios in the ranges from 0.5 to 0.7 and 0.18 to 0.29, respectively, where X represents vacancies in the X site. Boron-isotopic values of tourmaline vary from ~ 1 to − 10.4‰ δ11B. The B-isotope range, in conjunction with the Na–Mg-rich tourmaline composition, and the widespread occurrence of tourmalinite in the Sopa-Brumadinho Formation suggest a derivation from non-marine evaporitic brines. Brines are capable of transporting otherwise immobile Ti and explain, under oxidising conditions, the fractionation of Ag from Pd to precipitate palladiferous gold with extremely high Pd/Ag ratios. Zirconium-in-rutile and Ti-in-quartz temperatures for a variety of hematite-rich veins suggest episodic vein emplacement over a temperature range from around 500 °C to ~ 350 °C. Cross-cutting relationships and episodic vein emplacement indicate a late-Brasiliano age. 相似文献
75.
Axel Müller Bernd Leiss Klaus Ullemeyer Karel Breiter 《International Journal of Earth Sciences》2011,100(7):1515-1532
The lattice-preferred orientation (LPOs) of two late-Variscan granitoids, the Meissen monzonite and the Podlesí dyke granite,
were determined from high-resolution time-of-flight neutron diffraction patterns gained at the diffractometer SKAT in Dubna,
Russia. The results demonstrate that the method is suitable for the LPO analysis of polyphase, relatively coarse-grained (0.1–6 mm)
rocks. The Meissen monzonite has a prominent shape-preferred orientation (SPO) of the non-equidimensional minerals feldspar,
mica and amphibole, whereas SPO of the Podlesí granite is unapparent at the hand-specimen scale. The neutron diffraction data
revealed distinct LPOs in both granitoids. The LPO of the non-equidimensional minerals feldspar, mica and amphibole developed
mainly during magmatic flow. In the case of the Meissen monzonite, the magmatic flow was superimposed by regional shear tectonics,
which, however, had no significant effect on the LPOs. In both samples, quartz shows a weak but distinct LPO, which is atypical
for plastic deformation and different in the syn-kinematic Meissen monzonite and the post-kinematic Podlesí granite. We suggest
that, first of all, the quartz LPO of the Meissen monzonite is the result of oriented growth in an anisotropic stress field.
The quartz LPO of the Podlesí granite, which more or less resembles a deformational LPO in the flattening field of the local
strain field, developed during magmatic flow, whereby the rhombohedral faces of the quartz crystals adhered to the (010) faces
of aligned albite and to the (001) faces of zinnwaldite. Due to shape anisotropy of their attachments, the quartz crystals
were passively aligned by magmatic flow. Thus, magmatic flow and oriented crystal growth are the major LPO-forming processes
in both granitoids. For the Meissen monzonite, the solid-state flow was too weak to cause significant crystallographic re-orientation
of the minerals aligned by magmatic flow. Finally, the significance of our results for the evaluation of the regional tectonic
environment during magma emplacement is discussed. The discussion on the regional implications of the more methodologically
oriented results provides the basis for future, more regionally aimed studies in view of the fabric characteristics of such
plutons and their developing mechanisms. 相似文献
76.
77.
Louise H. Kellogg Gerald W. Bawden Tony Bernardin Magali Billen Eric Cowgill Bernd Hamann Margarete Jadamec Oliver Kreylos Oliver Staadt Dawn Sumner 《Pure and Applied Geophysics》2008,165(3-4):621-633
The geological sciences are challenged to manage and interpret increasing volumes of data as observations and simulations increase in size and complexity. For example, simulations of earthquake-related processes typically generate complex, time-varying data sets in two or more dimensions. To facilitate interpretation and analysis of these data sets, evaluate the underlying models, and to drive future calculations, we have developed methods of interactive visualization with a special focus on using immersive virtual reality (VR) environments to interact with models of Earth’s surface and interior. Virtual mapping tools allow virtual “field studies” in inaccessible regions. Interactive tools allow us to manipulate shapes in order to construct models of geological features for geodynamic models, while feature extraction tools support quantitative measurement of structures that emerge from numerical simulation or field observations, thereby enabling us to improve our interpretation of the dynamical processes that drive earthquakes. VR has traditionally been used primarily as a presentation tool, albeit with active navigation through data. Reaping the full intellectual benefits of immersive VR as a tool for scientific analysis requires building on the method’s strengths, that is, using both 3D perception and interaction with observed or simulated data. This approach also takes advantage of the specialized skills of geological scientists who are trained to interpret, the often limited, geological and geophysical data available from field observations. 相似文献
78.
79.
Deep sea drilling in the Central Gulf of California, a young and active spreading centre, shows that the high heat flow typical of these regions causes extensive alteration of sediment organic matter, especially near sills and above magma chambers where hydrothermal activity is concentrated. Even on the nearby passive margin, where there are no sills, heat flow is moderately high and hydrocarbon generation has begun in immature sequences. Migrating light hydrocarbons are detected especially where hydrothermal activity is concentrated. Thermogenic methane is more widespread, though not in the passive margin bordering the spreading centre. Despite the incidence of hydrocarbon generation and migration, the amounts of hydrocarbons involved are relatively small and apparently do not lead to commercially significant accumulations of petroleum.The organic matter in these sediments is mostly marine because the Gulf of California generally has low runoff from land and highly productive surface waters. Turbidites rich in terrigenous organic material are locally abundant in the mainly pelagic section in the Guaymas Basin. The highest concentrations of organic matter are found in laminated diatomites deposited on the Guaymas passive margin within the oxygen minimum zone. 相似文献
80.
Organic matter in eolian dusts over the Atlantic Ocean 总被引:1,自引:0,他引:1
Bernd R.T Simoneit 《Marine Chemistry》1977,5(4-6)
The elemental and mineralogical composition and the microfossil and detritus content of particulate fallout from the lower troposphere over the Atlantic Ocean have been extensively documented in earlier work, and it was possible to ascribe terrigenous source areas to such fallout. A brief review of the organic geochemistry of eolian dust is also presented here. The lipids of eolian dusts sampled from the air mass over the eastern Atlantic from about 35°N to 30°S were analyzed here.These lipids consisted mainly of normal alkanes, carboxylic acids and alcohols. The n-alkanes were found to range from n-C23 to n-C35, with high CPI values and maximizing at n-C27 in the North Atlantic, at n-C29 in the equatorial Atlantic and at n-C31 in the South Atlantic. The n-fatty acids had mostly bimodal distributions, ranging from n-C12 to n-C30 (high CPI), with maxima at n-C16 and in the northern samples at n-C24 and in the southern samples at n-C26. The n-alcohols ranged from n-C12 to n-C32, with high CPI values and maxima mainly at n-C28. The compositions of these lipids indicated that their terrigenous sources were comprised mainly of higher plant vegetation and desiccated lacustrine mud flats on the African continent. 相似文献