Mixing and chemical interdiffusion of trachytic and latitic magma in a subvolcanic complex of the Tertiary Westerwald (Germany)     

Ulrich Schreiber  Dirk Anders  Joachim Koppen 《Lithos》1999,46(4):263-714

A special kind of magma mixing is extraordinarily well exposed in the Bittersberg subvolcanic complex in the Tertiary volcanic field of the German Westerwald: A trachytic melt has been penetrated by a latitic dyke which has been dispersed within the host magma as small spherical enclaves (globules). Whole rock analyses of the globules show a change in composition that cannot be explained by a simple mechanical mixing between the endmembers. The most evolved globules have a phonolitic composition. Microprobe measurements in the microlithic matrix of the host rock and the guest indicate a diffusive motion of the alkalis from the host into the globules. On the other hand, an opposite trend can be observed for Ca, Mg, Fe and Ti, which are impoverished in the globules. The trace elements and the middle rare earth elements (MREE) has also been involved in the diffusive exchange. The REE-pattern of the most evolved (phonolitic) globules shows a characteristic trough in the area of the MREE which is almost identical to the REE-pattern of many phonolites. The phonolites and the alkali-rich trachytes of the Westerwald show similar globular textures as the Bittersberg volcanics. Therefore, generation of these rocks involving diffusive element exchange during mixing processes in a magma reservoir situated on a deeper crustal level may be possible.  相似文献   

History of scoria-cone eruptions on the eastern shoulder of the Kenya–Tanzania Rift revealed in the 250-ka sediment record of Lake Chala near Mount Kilimanjaro     

Catherine Martin-Jones  Christine Lane  Maarten Van Daele  Thijs Van Der Meeren  Christian Wolff  Heather Moorhouse  Emma Tomlinson  Dirk Verschuren 《第四纪科学杂志》2020,35(1-2):245-255

Reconstructions of the timing and frequency of past eruptions are important to assess the propensity for future volcanic activity, yet in volcanic areas such as the East African Rift only piecemeal eruption histories exist. Understanding the volcanic history of scoria-cone fields, where eruptions are often infrequent and deposits strongly weathered, is particularly challenging. Here we reconstruct a history of volcanism from scoria cones situated along the eastern shoulders of the Kenya–Tanzania Rift, using a sequence of tephra (volcanic ash) layers preserved in the ~250-ka sediment record of Lake Chala near Mount Kilimanjaro. Seven visible and two non-visible (crypto-) tephra layers in the Lake Chala sequence are attributed to activity from the Mt Kilimanjaro (northern Tanzania) and the Chyulu Hills (southern Kenya) volcanic fields, on the basis of their glass chemistry, textural characteristics and known eruption chronology. The Lake Chala record of eruptions from scoria cones in the Chyulu Hills volcanic field confirms geological and historical evidence of its recent activity, and provides first-order age estimates for seven previously unknown eruptions. Long and well-resolved sedimentary records such as that of Lake Chala have significant potential for resolving regional eruption chronologies spanning hundreds of thousands of years.  相似文献   

Enhancement of prestack diffraction data and attributes using a traveltime decomposition approach     

Alexander Bauer  Benjamin Schwarz  Dirk Gajewski 《Studia Geophysica et Geodaetica》2016,60(3):471-486

Diffractions not only carry important information about small-scale subsurface structures, they also possess unique properties, which make them a powerful tool for seismic processing and imaging. Since a point diffractor scatters an incoming wave to all directions, a diffraction event implies better illumination than a reflection, because the rays travel through larger parts of the subsurface. Furthermore, unlike the reflection case, in which the emergence location of the reflected wave depends on the source position, in the case of non-Snell scattering, up-going and down-going raypaths are decoupled. Based on this decoupling, we introduce a diffraction traveltime decomposition principle, which establishes a direct connection between zero-offset and finite-offset diffraction wavefield attributes. By making use of this approach, we are able to enhance diffractions and obtain high-quality diffraction wavefield attributes at arbitrary offsets in the prestack domain solely based on zero-offset processing without any further optimization of attributes. We show the accuracy of the method by fitting diffraction traveltimes, and on simple waveform data. Application to complex synthetic data shows the ability of the proposed approach to enhance diffractions and provide high-quality wavefield attributes even in sparsely illuminated regions such as subsalt areas. The promising results reveal a high potential for improved prestack data enhancement and further applications such as efficient diffraction-based finite-offset tomography.  相似文献   

The Deformational Response of a Viscoelastic Solid Earth Model Coupled to a Thermomechanical Ice Sheet Model     

Hannes Konrad  Malte Thoma  Ingo Sasgen  Volker Klemann  Klaus Grosfeld  Dirk Barbi  Zdeněk Martinec 《Surveys in Geophysics》2014,35(6):1441-1458

We apply a coupled thermomechanical ice sheet—self-gravitating viscoelastic solid Earth model (SGVEM), allowing for the dynamic exchange of ice thickness and bedrock deformation, in order to investigate the effect of viscoelastic deformation on ice dynamics and vice versa. In a synthetic glaciation scenario, we investigate the interaction between the ice sheet and the solid Earth deformation, the glacial-isostatic adjustment (GIA), accounting for an atmospheric forcing depending on the ice sheet surface altitude. We compare the results from the coupled model to runs with the common elastic lithosphere/relaxing asthenosphere (ELRA) model, where the lithosphere is represented by a thin plate and the mantle relaxes with one characteristic relaxation time, as well as to a rigid Earth without any deformation. We find that the deformational behaviour of the SGVEM on ice dynamics (i.e. stored ice volume, ice thickness and velocity field) is comparable to the ELRA for an optimal choice of the parameters in steady state, but exhibits differences in the transient behaviour. Beyond the ice sheet, in the region of peripheral forebulge, the differences in the transient surface deformation between ELRA and SGVEM are substantial, demonstrating the inadequacy of the ELRA model for interpreting constraints on GIA in the periphery of the ice sheet, such as sea-level indicators and GPS uplift rates.  相似文献   

Hydrologic and sedimentologic aspects of mining aggregate from the shipsk bank (Baltic sea)     

L. S. Gajewski  S. Uścinowicz 《Marine Georesources & Geotechnology》2013,31(3):229-244

Abstract

Investigations aimed at obtaining knowledge about the environmental effects of mining aggregate from the sea bottom were carried out on the Stupsk Bank. Special attention was given to the magnitude and reach of changes of hydrologic conditions, in that to changes in concentration of matter suspended in sea water, and to dynamics of the sea bottom—change of bottom relief, resulting from both natural causes and from mining of the aggregate.  相似文献   

Carry-over effects of the membrane interface probe     

Bumberger J  Radny D  Berndsen A  Goblirsch T  Flachowsky J  Dietrich P 《Ground water》2012,50(4):578-584

The membrane interface probe (MIP) is widely used to characterize the subsurface distribution of volatile organic compounds (VOCs). One problem that arises during MIP application is that disproportionately high MIP signals are obtained after passing source zones which contain mobile or residual phases. This serious problem occurs because of a carry-over effect, in particular caused by compound-specific retention times in the conventional unheated transfer line, commonly used during such an investigation. The objective of this study was to perform a qualitative methodical field evaluation of the carry-over effect of a conventional MIP system with a conventional unheated transfer line. This was achieved by coupling a mobile mass spectrometer to the MIP device. Results obtained were then further compared with those achieved using a laser induced fluorescence (LIF) system. Because of this coupling, time- and depth-dependent signals for different substances became known. Field evaluation data obtained showed complex superpositions of compounds with MIP system results. As a result of this superposition, MIP signals from the saturated zone beneath the source zone (zone with free and/or residual phase) are blurred and are therefore not representative of particular depths. However, utilizing multidirectional probing alongside conventional MIP probing (forwards and backwards), it was possible to detect the upper and lower phase boundary of the source zone. These MIP results correlated excellently with the LIF results. An important conclusion that can be drawn from the field investigation is that coupling a mobile mass spectrometer to the MIP system enables advanced MIP signal interpretation to be successfully achieved.  相似文献   

Does a one point sample adequately characterize the lake environment for paleoenvironmental calibration studies?     

J. Bunbury  K. Gajewski 《Journal of Paleolimnology》2008,39(4):511-531

A major goal of paleolimnological studies is the quantification of past environmental conditions. This is accomplished by computing transfer functions relating organism assemblages to environmental factors. The environmental data are typically comprised of a point sample of water chemistry and other environmental parameters that are collected at the same time as a surface sediment sample. We explore whether the year of sampling of the environmental variables affects the parameterization of organism-environment relations, in particular chironomids, ostracodes and diatoms. Canonical correspondence analyses revealed that the year of sampling is of secondary importance when relating the organism assemblages to environmental variables, but only with the major explanatory variables. A chironomid-inferred bottom water temperature partial least squares transfer function revealed similar performance statistics between the years. Taxon optima and tolerances were computed for both years using weighted averaging, and the results are comparable. A paired t-test computed on the proxy-inferred bottom water temperature values indicated that the results between the 2 years are not statistically different. The results of this study provide guarded optimism that the methodology of estimating transfer functions as currently applied is not entirely determined by the particular year when the data were collected, although more case studies are needed.  相似文献   

Responses of Fragilarioid-dominated diatom assemblages in a small Arctic lake to Holocene climatic changes, Russell Island, Nunavut, Canada     

S. A. Finkelstein  K. Gajewski 《Journal of Paleolimnology》2008,40(4):1079-1095

We present a paleolimnological record spanning the Holocene from a small lake on Russell Island (Lake PW02), in the central Canadian Arctic Archipelago (74.07° N, 97.77° W, 182 m asl). Fragilarioid diatom types in the genera Pseudostaurosira, Staurosira and Staurosirella constitute >90% of valves in fossil samples. Using modern biogeographic data which specify the temperature optima of the Fragilarioid diatom taxa, we present new inferences about the timing of paleoclimatic changes in the central Arctic islands. The early Holocene was characterized by maximum values for sediment organic matter, and lower ratios of Staurosirella pinnata to Staurosira construens v. venter, suggesting warm summer air temperatures between about 9500−6500 cal year BP. Influxes of biogenic silica and diatom valves decreased following 4000 cal year BP, the sediment accumulation rate slowed and diatom taxa of the littoral zone diversified, suggesting cooler summers and more persistent lake ice. Variations in the species composition of the assemblages indicate paleoclimatic changes that are in broad agreement with other paleoenvironmental records from the Arctic including melt records from the Agassiz Ice Cap. Although autecological data remain incomplete for Fragilarioid taxa, our results indicate differences in these taxa in responses to paleoenvironmental change and underline the potential for the increased use of these taxa in paleoenvironmental reconstructions. The record from Lake PW02, as in other records from Arctic lakes with low algal diversity throughout the Holocene, shows a pronounced increase in diatom diversity since the 1920s, and diatom production since the 1970s far exceeds any recorded during the Holocene.  相似文献   

Effect of algae on flocculation of suspended bed sediments in a large shallow lake. Consequences for ecology and sediment transport processes     

Miguel Angel de Lucas Pardo  Dirk Sarpe  Johan Christian Winterwerp 《Ocean Dynamics》2015,65(6):889-903

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Holocene environmental dynamics of south-eastern Brazil recorded in laminated sediments of Lago Aleixo     

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 δ¹³C_org 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