Untersuchungen an spilitischen Gesteinen der variskischen Geosynkline in Nordwestdeutschland

21 samples of spilitic rocks of Devonian to Carboniferous age from NW-Germany have been analyzed for major elements, a large group of minor elements and the modal mineral composition. Their major element and mineral composition is comparable to Recent spilites from oceanic ridges. Major emphasis has been given to the absolute and relative abundance of the lanthanides and yttrium. Their pattern, which is most probably not altered during processes of spilitization is that of tholeiitic basalts occuring on the continents and in oceanic islands.Spilitization of the rocks under investigation is explained as a metamorphic process in a partially open system (loss of Si, Ca, and gain of H₂O, CO₂, Na) in a temperature range from 200° to about 400° C. Most abundant new minerals are: albite and chlorite; basaltic relict minerals are: plagioclase (high in Ca) and diopsidic pyroxene. Minerals typical for the temperature range mentioned are: prehnite, pumpellyite, actinolithe, epidote biotite. Microprobe analyses of several typical minerals are listed.

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Unsere Untersuchungen wurden im Rahmen des Sonderforschungsbereichs 48 durchgeführt und die Probenpräparation sowie die Spektrenauswertung aus dessen Mitteln finanziert.

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Wir danken den Herren Dr. Alfred Schneider und Dr. Malte Gramse für die Durchführung der quantitativen Mikrosondenanalysen von zahlreichen Spilitmineralen (Tabelle 6).

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Die Herren Professor D. Meischner und Dr. J. Schneider, Göttingen, unterstützten uns bei der Probenauswahl im Kellerwald. Herr Dozent Dr. S. Meisl (Wiesbaden) überließ uns Proben Pumpellyit-führender Spilite für die Mikrosondenanalyse und das Zentrale Geologische Institut, Berlin, stellte die Probe Mellenbach (Gesteinsstandard BM) zur Verfügung.     

Measurements and numerical simulations of the degree of activity and vegetation cover on parabolic dunes in north-eastern Brazil

In this work we present measurements of vegetation cover over parabolic dunes with different degrees of activation along the north-eastern Brazilian coast. We extend the local values of the vegetation cover density to the whole dune by correlating measurements with the relative brightness index C of high resolution QuickBird panchromatic satellite images of the dune field. We then introduce the vegetation data into a continuous model for vegetated dunes, coupling sand erosion and vegetation growth, and perform simulations of the evolution of the morphology and vegetation cover of parabolic dunes. Finally, from the comparison of both, the measurements and the simulation results, we show that the model is able to predict the dune shape and the vegetation distribution of real parabolic dunes as result of the evolution of a blow-out.     

The nature and origin of pigments in black opal from Lightning Ridge,New South Wales,Australia

Abstract

Black opal (opal-AG) owes its dark coloration to a fine-grained pigment commonly inferred to be mainly carbon, yet chemical compositions for black opals suggest there may be additional components. Here we search for such components in pigment concentrates prepared by dissolving black opal nodules (nobbies) from Lightning Ridge (NSW) in hydrofluoric acid, using electron microscopy (scanning electron microscopy, transmission electron microscopy), X-ray diffraction and laser-ablation ICP-MS. The results demonstrate the presence of sulfides—predominantly pyrite and chalcopyrite, with minor galena and Ti-oxide phases, as additional components of the pigment. ATR-FTIR analysis indicates the presence of C=O and C–H groups, consistent with an organic origin. Transmission electron microscopy images of pigment show variously deformed, originally spherical ～100?nm particles rich in sulfide and carbon, which are interpreted as thin coatings of pigment on now dissolved opaline silica spheres. Laser-ablation ICP-MS analysis identifies remnant silica in pigment concentrates, which may be interpreted as opaline silica surviving HF treatment protected as inclusions in sulfides. When examined within the context of petrographic observations from more than 1000 opal nodules (nobbies) at Lightning Ridge, these new results suggest that pigment carbon and sulfides in the nodules formed microbially under initially anoxic groundwater conditions, within pre-existing cavities concurrently being filled with silica sol ultimately derived from chemical weathering of feldspar-rich volcaniclastic sediment. Intensely black pigment layers observed at the floor of many nodules indicate settling of dark, high-density (sulfide–Ti-oxide-rich) pigment within cavities, with the implication that sulfate-reducing bacterial (SRB) activity commences early during the silica sol-gel ripening process. Microbial activity may persist until after the cavity has completely filled with the silica sol, as illustrated by abundant black opals with uniformly distributed pigment. Pigment formed at this stage may no longer be able to settle out within the ripening and increasingly viscous silica gel, thus forming pigmentation throughout the opal cavity. The existence of ‘amber’, pigment-poor opal with intensely black basal pigment layers is interpreted as signalling a lack of sulfate to sustain further SRB activity, or a change to more oxidising conditions, possibly related to interaction with surface waters within a downward-penetrating weathering front. A change in redox conditions would shut off activity of SRB and thus sulfide pigment production and allow development of aerobic microbial activity as described by others.     

Ground-motion modelling at regional distances for earthquakes in a continental interior,II. Effect of focal depth,azimuth and attenuation

Multiple-mode surface-wave signals are used to model ground motion at distances of 50 to 500 km for an earthquake source in a continental interior. Motion on a thrust fault is used as the earthquake model. Theoretical ground-motion time histories are generated for this source for various focal depths, receiver azimuths and medium-attenuation models. A shallow source will generate greater values for the ground motion than the same source at a greater depth. Two anelastic attenuation models are considered, one appropriate to the central and eastern United States and the other to southern California. The effects of the difference in the attenuation models are seen at distances greater than 100 km for periods greater than 1.5 sec.     

Vegetation and Climate Reconstruction of the Planalto in Rio Grande do Sul, South Brazil (Late Glacial and Holocene)

1IntroductionThe ecosystem of Araucaria-Forest is one of the most important plant-associations in South Brazil. Today,however,the spreading area is a fraction of its natural size,originally spread over 200 km. It spanned from 18°to 30°of southern lati-t…     

Time-domain sparsity promoting least-squares reverse time migration with source estimation

Least-squares reverse-time migration is well known for its capability to generate artefact-free true-amplitude subsurface images through fitting observed data in the least-squares sense. However, when applied to realistic imaging problems, this approach is faced with issues related to overfitting and excessive computational costs induced by many wave-equation solves. The fact that the source function is unknown complicates this situation even further. Motivated by recent results in stochastic optimization and transform-domain sparsity promotion, we demonstrate that the computational costs of inversion can be reduced significantly while avoiding imaging artefacts and restoring amplitudes. While powerful, these new approaches do require accurate information on the source-time function, which is often lacking. Without this information, the imaging quality deteriorates rapidly. We address this issue by presenting an approach where the source-time function is estimated on the fly through a technique known as variable projection. Aside from introducing negligible computational overhead, the proposed method is shown to perform well on imaging problems with noisy data and problems that involve complex settings such as salt. In either case, the presented method produces high-resolution high-amplitude fidelity images including an estimate for the source-time function. In addition, due to its use of stochastic optimization, we arrive at these images at roughly one to two times the cost of conventional reverse-time migration involving all data.     

Seasonal characteristics of tropical marine boundary layer air measured at the Cape Verde Atmospheric Observatory

Observations of the tropical atmosphere are fundamental to the understanding of global changes in air quality, atmospheric oxidation capacity and climate, yet the tropics are under-populated with long-term measurements. The first three years (October 2006–September 2009) of meteorological, trace gas and particulate data from the global WMO/Global Atmospheric Watch (GAW) Cape Verde Atmospheric Observatory Humberto Duarte Fonseca (CVAO; 16° 51′ N, 24° 52′ W) are presented, along with a characterisation of the origin and pathways of air masses arriving at the station using the NAME dispersion model and simulations of dust deposition using the COSMO-MUSCAT dust model. The observations show a strong influence from Saharan dust in winter with a maximum in super-micron aerosol and particulate iron and aluminium. The dust model results match the magnitude and daily variations of dust events, but in the region of the CVAO underestimate the measured aerosol optical thickness (AOT) because of contributions from other aerosol. The NAME model also captured the dust events, giving confidence in its ability to correctly identify air mass origins and pathways in this region. Dissolution experiments on collected dust samples showed a strong correlation between soluble Fe and Al and measured solubilities were lower at high atmospheric dust concentrations. Fine mode aerosol at the CVAO contains a significant fraction of non-sea salt components including dicarboxylic acids, methanesulfonic acid and aliphatic amines, all believed to be of oceanic origin. A marine influence is also apparent in the year-round presence of iodine and bromine monoxide (IO and BrO), with IO suggested to be confined mainly to the surface few hundred metres but BrO well mixed in the boundary layer. Enhanced CO₂ and CH₄ and depleted oxygen concentrations are markers for air-sea exchange over the nearby northwest African coastal upwelling area. Long-range transport results in generally higher levels of O₃ and anthropogenic non-methane hydrocarbons (NMHC) in air originating from North America. Ozone/CO ratios were highest (up to 0.42) in relatively fresh European air masses. In air heavily influenced by Saharan dust the O₃/CO ratio was as low as 0.13, possibly indicating O₃ uptake to dust. Nitrogen oxides (NO_x and NO_y) show generally higher concentrations in winter when air mass origins are predominantly from Africa. High photochemical activity at the site is shown by maximum spring/summer concentrations of OH and HO₂ of 9?×?10⁶ molecule cm^?3 and 6?×?10⁸ molecule cm^?3, respectively. After the primary photolysis source, the most important controls on the HO_x budget in this region are IO and BrO chemistry, the abundance of HCHO, and uptake of HO_x to aerosol.