Oxygenic primary production below the oxycline and its importance for redox dynamics     

Andreas Brand  Hannah Bruderer  Kirsten Oswald  Carole Guggenheim  Carsten J. Schubert  Bernhard Wehrli 《Aquatic Sciences - Research Across Boundaries》2016,78(4):727-741

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The Rock–Water–Ice Topographic Gravity Field Model RWI_TOPO_2015 and Its Comparison to a Conventional Rock-Equivalent Version     

Thomas Grombein  Kurt Seitz  Bernhard Heck 《Surveys in Geophysics》2016,37(5):937-976

RWI_TOPO_2015 is a new high-resolution spherical harmonic representation of the Earth’s topographic gravitational potential that is based on a refined Rock–Water–Ice (RWI) approach. This method is characterized by a three-layer decomposition of the Earth’s topography with respect to its rock, water, and ice masses. To allow a rigorous separate modeling of these masses with variable density values, gravity forward modeling is performed in the space domain using tesseroid mass bodies arranged on an ellipsoidal reference surface. While the predecessor model RWI_TOPO_2012 was based on the \(5'\times 5'\) global topographic database DTM2006.0 (Digital Topographic Model 2006.0), the new RWI model uses updated height information of the \(1'\times 1'\) Earth2014 topography suite. Moreover, in the case of RWI_TOPO_2015, the representation in spherical harmonics is extended to degree and order 2190 (formerly 1800). Beside a presentation of the used formalism, the processing for RWI_TOPO_2015 is described in detail, and the characteristics of the resulting spherical harmonic coefficients are analyzed in the space and frequency domain. Furthermore, this paper focuses on a comparison of the RWI approach to the conventionally used rock-equivalent method. For this purpose, a consistent rock-equivalent version REQ_TOPO_2015 is generated, in which the heights of water and ice masses are condensed to the constant rock density. When evaluated on the surface of the GRS80 ellipsoid (Geodetic Reference System 1980), the differences of RWI_TOPO_2015 and REQ_TOPO_2015 reach maximum amplitudes of about 1 m, 50 mGal, and 20 mE in terms of height anomaly, gravity disturbance, and the radial–radial gravity gradient, respectively. Although these differences are attenuated with increasing height above the ellipsoid, significant magnitudes can even be detected in the case of the satellite altitudes of current gravity field missions. In order to assess their performance, RWI_TOPO_2015, REQ_TOPO_2015, and RWI_TOPO_2012 are validated against independent gravity information of current global geopotential models, clearly demonstrating the attained improvements in the case of the new RWI model.  相似文献   

A Wavelet-Based Assessment of Topographic-Isostatic Reductions for GOCE Gravity Gradients   总被引：3，自引：2，他引：1  

Thomas Grombein  Xiaoguang Luo  Kurt Seitz  Bernhard Heck 《Surveys in Geophysics》2014,35(4):959-982

Gravity gradient measurements from ESA’s satellite mission Gravity field and steady-state Ocean Circulation Explorer (GOCE) contain significant high- and mid-frequency signal components, which are primarily caused by the attraction of the Earth’s topographic and isostatic masses. In order to mitigate the resulting numerical instability of a harmonic downward continuation, the observed gradients can be smoothed with respect to topographic-isostatic effects using a remove–compute–restore technique. For this reason, topographic-isostatic reductions are calculated by forward modeling that employs the advanced Rock–Water–Ice methodology. The basis of this approach is a three-layer decomposition of the topography with variable density values and a modified Airy–Heiskanen isostatic concept incorporating a depth model of the Mohorovi?i? discontinuity. Moreover, tesseroid bodies are utilized for mass discretization and arranged on an ellipsoidal reference surface. To evaluate the degree of smoothing via topographic-isostatic reduction of GOCE gravity gradients, a wavelet-based assessment is presented in this paper and compared with statistical inferences in the space domain. Using the Morlet wavelet, continuous wavelet transforms are applied to measured GOCE gravity gradients before and after reducing topographic-isostatic signals. By analyzing a representative data set in the Himalayan region, an employment of the reductions leads to significantly smoothed gradients. In addition, smoothing effects that are invisible in the space domain can be detected in wavelet scalograms, making a wavelet-based spectral analysis a powerful tool.  相似文献   

Karst morphology and groundwater vulnerability of high alpine karst plateaus   总被引：3，自引：0，他引：3  

Lukas Plan  Kurt Decker  Robert Faber  Michael Wagreich  Bernhard Grasemann 《Environmental Geology》2009,58(2):285-297

High alpine karst plateaus are recharge areas for major drinking water resources in the Alps and many other regions. Well-established methods for the vulnerability mapping of groundwater to contamination have not been applied to such areas yet. The paper characterises this karst type and shows that two common vulnerability assessment methods (COP and PI) classify most of the areas with high vulnerability classes. In the test site on the Hochschwab plateau (Northern Calcareous Alps, Austria), overlying layers are mostly absent, not protective or even enhance point recharge, where they have aquiclude character. The COP method classifies 82% of the area as highly or extremely vulnerable. The resulting maps are reasonable, but do not differentiate vulnerabilities to the extent that the results can be used for protective measures. An extension for the upper end of the vulnerability scale is presented that allows identifying ultra vulnerable areas. The proposed enhancement of the conventional approach points out that infiltration conditions are of key importance for vulnerability. The method accounts for karst genetical and hydrologic processes using qualitative and quantitative properties of karst depressions and sinking streams including parameters calculated from digital elevations models. The method is tested on the Hochschwab plateau where 1.7% of the area is delineated as ultra vulnerable. This differentiation could not be reached by the COP and PI methods. The resulting vulnerability map highlights spots of maximum vulnerability and the combination with a hazard map enables protective measures for a manageable area and number of sites.  相似文献   

Amphibole megacrysts as a probe into the deep plumbing system of Merapi volcano,Central Java,Indonesia   总被引：1，自引：0，他引：1  

Stefan?T.?M.?PetersEmail author  Valentin?R.?Troll  Franz?A.?Weis  Luigi?Dallai  Jane?P.?Chadwick  Bernhard?Schulz 《Contributions to Mineralogy and Petrology》2017,172(4):16

Amphibole has been discussed to potentially represent an important phase during early chemical evolution of arc magmas, but is not commonly observed in eruptive arc rocks. Here, we present an in-depth study of metastable calcic amphibole megacrysts in basaltic andesites of Merapi volcano, Indonesia. Radiogenic Sr and Nd isotope compositions of the amphibole megacrysts overlap with the host rock range, indicating that they represent antecrysts to the host magmas rather than xenocrysts. Amphibole-based barometry suggests that the megacrysts crystallised at pressures of >500 MPa, i.e., in the mid- to lower crust beneath Merapi. Rare-earth element concentrations, in turn, require the absence of magmatic garnet in the Merapi feeding system and, therefore, place an uppermost limit for the pressure of amphibole crystallisation at ca. 800 MPa. The host magmas of the megacrysts seem to have fractionated significant amounts of amphibole and/or clinopyroxene, because of their low Dy/Yb ratios relative to the estimated compositions of the parent magmas to the megacrysts. The megacrysts’ parent magmas at depth may thus have evolved by amphibole fractionation, in line with apparently coupled variations of trace element ratios in the megacrysts, such as e.g., decreasing Zr/Hf with Dy/Yb. Moreover, the Th/U ratios of the amphibole megacrysts decrease with increasing Dy/Yb and are lower than Th/U ratios in the basaltic andesite host rocks. Uranium in the megacrysts’ parent magmas, therefore, may have occurred predominantly in the tetravalent state, suggesting that magmatic fO₂ in the Merapi plumbing system increased from below the FMQ buffer in the mid-to-lower crust to 0.6–2.2 log units above it in the near surface environment. In addition, some of the amphibole megacrysts experienced dehydrogenation (H₂ loss) and/or dehydration (H₂O loss), as recorded by their variable H₂O contents and D/H and Fe³⁺/Fe²⁺ ratios, and the release of these volatile species into the shallow plumbing system may facilitate Merapi’s often erratic eruptive behaviour.  相似文献   

On a nonlinear hybrid method for multiscale analysis of a bearing-capacity test of a real-scale segmental tunnel ring     

Jiao-Long Zhang  Herbert A. Mang  Xian Liu  Yong Yuan  Bernhard Pichler 《国际地质力学数值与分析法杂志》2019,43(7):1343-1372

A nonlinear hybrid method is developed for multiscale analysis of a bearing-capacity test of a real-scale segmental tunnel ring subjected to point loads. The structural analysis consists of two parts. Part I refers to modeling of bending-induced tensile cracking of the segments, resulting from the external loading. The segments are subdivided into elements, according to the crack spacing. Each element is either intact or contains one central crack band, flanked by lateral undamaged domains. A multiscale model for tensile softening of concrete is used to describe the progressive deterioration of the crack bands. After iterative determination of their state of damage, the effective bending and extensional stiffnesses of the corresponding elements are quantified by means of Voigt-Reuss-Hill estimates. The effective stiffnesses are used for linear-elastic simulations of the segmental tunnel ring. Part II refers to the relative rotation angles at the joints, which are estimated from monitoring data, using the Bernoulli-Euler hypothesis. Since the validity of this hypothesis is questionable for neck-like joints, the relative rotation angles are post-processed such that they refer to rigid body displacements of the segments. The following conclusions are drawn: The presented approach yields good estimates of crack widths. Relative rotation angles at the joints mainly result in rigid body displacements of the segments, governing the convergences. Because realistic interface models are lacking, hybrid analysis based on displacement-monitoring data allows for performing ultimate-load analysis of segmental tunnel rings.  相似文献   

Gain and loss of uranium by meteorites and rocks,and implications for the redistribution of uranium on Mars     

Gerlind DREIBUS  Reinhard HAUBOLD  Wolfgang HUISL  Bernhard SPETTEL 《Meteoritics & planetary science》2007,42(6):951-962

Abstract Terrestrial alteration of meteorites results in the redistribution, gain, or loss of uranium and other elements. We have measured the maximum U adsorption capacity of a meteorite and two geochemical reference materials under conditions resembling terrestrial ones (pH 5.8). The basaltic eucrite Sioux County adsorbs 7 ppm of U. The result for the terrestrial granite AC‐E is similar (5 ppm), while the basalt BE‐N adsorbs 34 ppm of U. We have also investigated U adsorption in the presence of phosphate (0.01 M or less) in imitation of conditions that probably occurred in the earlier history of Mars. Such a process would have alterated Martian surface material and would be noticeable in Martian meteorites from the affected surface. The experiments demonstrated the counteracting effects of phosphate, which increases U adsorption, but decreases the quantity of dissolved U that is available for adsorption. U adsorption by AC‐E increases to 7 ppm. The lowered value for BE‐N of 8 ppm results from the low quantity of dissolved U in the volume of solution used. The results from the adsorption experiments and from leaching the Martian meteorite Zagami and a terrestrial basalt imply that the aqueous redistribution of U on Mars was moderate. Acidic liquids mobilized uranium and other metals, but present phosphate impeded the dissolution of U compounds. Some mobilized U may have reached the global sinks, while most of it probably was transported in the form of suspended particles over a limited distance and then settled.  相似文献   

Geochemical-focusing of manganese in lake sediments — An indicator of deep-water oxygen conditions   总被引：1，自引：0，他引：1  

Tobias Schaller  Bernhard Wehrli 《Aquatic Geochemistry》1996,2(4):359-378

The lateral distributions of Mn concentrations in the sediments of two Swiss lakes under varying oxygen conditions have been determined. The comparison of Mn distribution patterns with oxygen in the deep-water provides strong evidence for a geochemical-focusing effect, which is driven by the redox cycle of manganese. Conditions essential for this process to occur are anoxic sediments in contact with oxic deep-water. Average sedimentary manganese concentrations determined for different water-depth ranges are directly proportional to the area of shallower sediments. This result indicates that geochemical-focusing of manganese in lake sediments is a promising proxy indicator for the reconstruction of oxygen conditions during deposition.  相似文献   

The late Palaeozoic Idusi Formation of southwest Tanzania: a record of change from glacial to postglacial conditions     

H. Wopfner  B. Diekmann 《Journal of African Earth Sciences》1996,22(4):575-595

The Idusi Formation forms the basal depositional unit of Karoo basins of southwestern Tanzania. It is defined as the rock unit limited by the unconformable contact with underlying Precambrian metamorphics and the base of the overlying, fluviatile Mpera Sandstone Member of the Mchuchuma Formation. The Idusi Formation is subdivided into the Lisimba Member, comprising diamictities, lutites with dropstones, slump masses and laminites, and the overlying Lilangu Member, consisting of black, pyritic and kerogenous lutites. The type section is at Idusi Gorge, situated 6 km east of Lake Nyasa on 10°17′50″S. The thickness of the formation at the type section is 240 m, with the maximum observed thickness being 715 m. Plant fossils and palynological assemblages indicate an Asselian age, probably extending down into the Late Carboniferous.The basal diamictites contain striated and facetted clasts, which identify them as glacial deposits. They are mainly wash-out and slurried tillites and also, less frequently, lodgment tillites. These are overlain by proximal and distal deposits of proglacial lakes, which were laid down during the retreat of the glaciers. Laminates of the upper Lisimba Member demonstrate progressively stronger seasonal control. Further amelioration of the climate, with substantially increased biological production both on land and in the water, is demonstrated by the black lutites of the Lilangu Member. They represent euxinic lake sediments formed during the final deglaciation phase. Deposition of the over-lying, coal-bearing Mchuchuma Formation occurred under a temperate climate. The duration of the Late Palaeozoic glaciation is estimated at about 20 to 25 Ma. The last quarter of this time span was characterized by climatic amelioration. Global warming was accompanied by an eustatic rise in sea level and a marked decrease in atmospheric CO₂.  相似文献   

Adaptive wavelet‐based finite‐difference modelling of SH‐wave propagation     

Stéphane Operto  Jean Virieux  Bernhard Hustedt  Fabrizio Malfanti 《Geophysical Journal International》2002,148(3):476-498

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