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This research was conducted in the Rwenzori Region of the Western Branch, East African Rift System (EARS). The EARS is a tectonic structure extending over a length of more than 3000 km from the Afar Triple Junction, in Ethiopia, to Lake Malawi in the south. The Western Rift System is a roughly NE to ENE trending sector of the EARS, which runs along the western boundary of Uganda and the neighboring Democratic Republic of Congo (D.R.C). It stretches 2100 km from Nimule, NW on Uganda-Sudan border, extending to Lake Malawi in the SE of Africa. The unusual uplift of the Rwenzori Mountains within an extensional regime and the mechanisms associated with the high frequency of seismic activity in the region was hardly understood and therefore, had remained a subject of contention that needed to be critically addressed in detail. To my knowledge, this was probably the first study to be performed and documented in great depth within the domains of seismic noise variation, seismic anisotropy and b value analyses beneath the Rwenzori Region. After about six years of operation (2006-2012), the seismology group of the RIFTLINK Research Project (www.riftlink.org) acquired a vast amount of high-quality, digital data that were collected using a seismic network of well calibrated seismic equipment. The project was divided into two phases. Phase I, that operated between February 2006 - September 2007, consisted of thirty-two temporary seismic stations, which were selectively spread out in the Rwenzori Region on the Ugandan side, to detect and record extremely weak as well as strong naturally occurring earthquakes. The seismic equipment used included EDL and REFTEK digitizers, which were coupled with Güralp and MARK sensors respectively (REFTEKS: only short-period MARK sensors, EDLs: short-period MARK plus few broadband Güralp Sensors). Exactly 22375 earthquakes were recorded. The data were processed using the SEISAN software package. About 14413 earthquakes were carefully localized using the velocity model of Bram (1975) that implements a Vp=Vs ratio fixed at 1.74. Phase II, that extended between 2009-2012 consisted of thirty-two seismic stations, which were spread out around the Rwenzori Mountains, both on the Ugandan side and the neighboring D.R.C. Only Taurus digitizers that were coupled with Trillium sensors were used in the D.R.C. On the Ugandan side however, both EDL and Taurus digitizers, which were coupled with Trillium and Güralp sensors were used. ...
The LPJ-GUESS dynamic vegetation model uniquely combines an individual- and patch-based representation of vegetation dynamics with ecosystem biogeochemical cycling from regional to global scales. We present an updated version that includes plant and soil N dynamics, analysing the implications of accounting for C-N interactions on predictions and performance of the model. Stand structural dynamics and allometric scaling of tree growth suggested by global databases of forest stand structure and development were well-reproduced by the model in comparison to an earlier multi-model study. Accounting for N cycle dynamics improved the goodness-of-fit for broadleaved forests. N limitation associated with low N mineralisation rates reduces productivity of cold-climate and dry-climate ecosystems relative to mesic temperate and tropical ecosystems. In a model experiment emulating free-air CO2 enrichment (FACE) treatment for forests globally, N-limitation associated with low N mineralisation rates of colder soils reduces CO2-enhancement of NPP for boreal forests, while some temperate and tropical forests exhibit increased NPP enhancement. Under a business-as-usual future climate and emissions scenario, ecosystem C storage globally was projected to increase by c. 10 %; additional N requirements to match this increasing ecosystem C were within the high N supply limit estimated on stoichiometric grounds in an earlier study. Our results highlight the importance of accounting for C-N interactions not only in studies of global terrestrial C cycling, but to understand underlying mechanisms on local scales and in different regional contexts.
Mantle convection is the process by which heat from the Earth’s core is transferred upwards to the surface and it is accepted to explain the dynamics of the Earth’s interior. On geological time-scales, mantle material flows like a viscous fluid as a consequence of the buoyancy forces arising from thermal expansion. Indeed, mantel convection provides a framework which links together the major disciplines, such as seismology, mineral physics, geochemistry tectonic and geology. The numerical model has been applied to understand the dynamic, structure and evaluation of the Earth, and other terrestrial planets and the investigations continue to explore, different aspects of the mantle convection.
In fact, to model this phenomenon, two complementary approaches are possible. On the one hand, one can solve self-consistently the equations of thermal convection, including parameters and employing physical relationships derived from mineral physics. Our understanding of mantle convection depends ultimately upon the success of such fully self-consistent dynamic models in explaining observable features of the flow. Although, these models presently unable to predict the actual convection pattern of the Earth, they are extremely useful to investigate general characteristics of given physical systems. On the other hand, to permit comparison with specific observables associated with the flow, one can consider a more restricted problem. Instead of focusing on the time evolution of mantle flow, if we know a priori the temperature - and hence presumably the density - anomalies that drive the convection, we can try to build a snapshot of the present-day flow pattern, consistent with those anomalies, that can successfully predict the observables. As matter of fact, the aim of this study is to investigate both approaches in comparison with the main geophysical constraints on mantle structure. These constraints include the geoid anomalies, the dynamic surface and core-mantle boundary topography and tectonic plate motions.
The most appropriate mathematical basis functions for describing a bounded and continuous function on a spherical surface are spherical harmonics. We may therefore expand the geodynamic observables in terms of spherical harmonics. We have investigated two methods of the global spherical harmonic analysis by specific attention to the dynamic geoid computation of the geodynamic models. The first method is the quadrature method in which the loss of the orthogonality of the Legendre functions in transition from continues to discrete case is the major drawback to the method. Particularly, we showed that in the absence of the tesseral harmonics, quadrature formulation leads to obtain inaccurate results. The second method is the least-squares which can be considered as the best linear unbiased estimator that provides the exact results. We showed that even with a low resolution grid data it is possible to reconstruct the data and achieve an accurate result by using this method, which is extremely remarkable in three-dimensional global convection studies. However, special care has to be taken since there is some source of errors that might influence the efficiency of this method.
In general, to better understanding of the properties of the mantle, it is useful to assess observable characteristics of plumes in the mantle, including geoid, topography and heat flow anomalies. However, only few studies exist on geoid and topography for axi-symmetric convection and their models were restricted to isoviscous (or stratified) mantle and low Rayleigh numbers. We studied fully coupled depth and temperature dependent Arrhenius type of viscosity in axi-symmetric spherical shell geometry in order to investigate the shape of geoid anomalies and dynamic topography above a plume. Indeed, the topography and geoid anomalies produced from plumes are sensitive to rheology of the mantle and rheology of the plume; both have effects on shape and amplitude of the geoid anomalies. As results we are able to define different classes of plumes by their geoid signals.
Mainly depth-dependent viscosity models show a geoid with negative sign above the plume which can turn to the positive sign by decrease the viscosity contrast. This can be considered as a transition between the strongly depth dependent and the constant viscosity case. Our results basically support the idea by Morgan [1965] and McKenzie [1977]. They have shown the magnitude and even the sign of the total gravity anomaly depend on the spatial variation in effective viscosity. In addition, Hager [1984] has concluded that the total gravity field is depend on the radial distribution of effective viscosity, and a small change in viscosity contrast leads to varying sign of the response function.
In the case of temperature-dependent viscosity, the formation of an immobile lithosphere is a natural result, and the flow as well as the total geoid becomes strongly time dependent. When we increase the activation energy, all geoids associated with the first arriving plumes look like bell shaped whereas for typical plumes, after reaching a statistical steady state, bell-shaped geoids with decreasing amplitude as well as linear flank shaped geoids are observed. It is surprising that in spite of large differences in lateral and depth varying viscosities, the shapes of the geoid anomalies remained rather similar. We also identified different behaviors in the combined model with temperature-and pressure-dependent viscosity. In fact, in spite of the strongly different rheology, the geoid anomalies in all cases were surprisingly similar. Furthermore, we proposed a scaling law for the geoid which makes our results directly applicable to other planets. Moreover, we can apply the results of our calculation to find relations between different rheology and sub-lid temperature, since we know that the mantle temperature can change significantly with variation in pressure-temperature dependent viscosity. It is also possible to define a range of stagnant lid thickness related to the amplitude of the geoid which can be reasonable for study of the lid thickness in Venus or Mars.
Nevertheless, in these series of models, we simplified a number of complexities within the Earth. One of the most important of such simplification is the Boussinesq approximation. This approximation is valid if the temperature scale height (i.e. the depth over which temperature increases by a factor of “ ” due to adiabatic compression) is much greater than the convection depth. However, a temperature scale height in the Earth’s mantle is at best only slightly greater than the mantle depth. Hence, the Boussinesq approximation could mask some very important stratification and compressibility effects that influence both the spatial and temporal structure of the convection. Therefore, in more advance models we considered compressibility in our mantle convection models, assuming that density vary both radially and laterally, being determined as a function of pressure and temperature through an appropriate equation of the state. Moreover, thermodynamic properties assumed to be a function of depth.
We examined the details of the structure of the spherical axi-symmetric Anelastic Liquid Approximation model (ALA) with special attention to the Arrhenius rheology, and compare it to the cases of compressible convection without depth dependent thermodynamical properties, and to cases of the extended Boussinesq approximation. At the same time, the effects of the interaction between temperature and pressure-dependent viscosity and thermodynamic parameters in the compressible mantle convection on the geoid and topography have been studied. We showed that assuming compressible convection with depth-dependent thermodynamic properties strongly influence the geoid undulations. Using compressible convection with constant thermodynamic properties is physically inconsistent and may lead to spurious results for the geoid and convection pattern. Indeed, by a systematic study of different approaches of compressibility in the spherical shell convection for different Arrhenius viscosity laws we proved that only in the unrealistic case of zero activation energy the different compressibility modes result in comparable convection and geoid patterns. In all other rheological cases, large differences have been obtained, that stressing the important role of consistent compressible thermodynamic properties for mantle convection.
In addition, we examine the impact of compressibility as well as different rheologies on the power law relation that connects the Nusselt number to the Rayleigh number. We have discovered that the power law index of the relationship is controlled by the rheology, independent of which approximation is used. Instead, the bound of this relation is controlled by a combination of different approximation and rheology.
Next, instead of focusing on the time evolution of mantle flow, we have carried out three-dimensional spherical shell models of mantle circulation to investigate the effects of joint radial and lateral viscosity variations on the Earth’s non-hydrostatic geoid, surface and core-mantle boundary topographies. These models include realistic lateral viscosity variations (LVV) in the lithosphere, upper mantle and lower mantle in combination with different stratified viscosity structures. We have demonstrated that the contradictory results concerning the effects of LVV can be clarified by the most straight-forward problem in geoid modeling; namely, rather poorly known stratified viscosity structure. We explored three classes of dynamic geoid models due to lateral viscosity variations. In the first class, the LVV strongly improved the fit to the observed geoid. Indeed, when the viscosity contrast between lower and upper mantles is not large enough to produce a good fit to geoid the LVVs are able to perform this action by adjusting amplitudes, so that it becomes comparable with observation. In the second class, inducing the LVV moderately improved the fit. Actually, when the geoid induced by a stratified viscosity structure already has a good correlation with observation, then the LVV causes its amplitude to further improve. In the last class, if the viscosity contrast between upper and lower mantle would be high enough, inducing LVV deteriorate the fit to the observed geoid.. Indeed, depending on the stratified viscosity, inducing the LVV may take place in one of these categories.
We also quantified the effects of LVV in the mantle and lithosphere individually. We found that the presence of LVV in the mantle (upper and lower) improves the fit to the observed geoid regardless of stratified viscosity. While LVV in the lithosphere is a crucial parameter, and dependent of the stratified viscosity, may increase or decrease the geoid fit. In fact, when the lower mantle considers being viscous enough, it would support the negative buoyancy of subducting slabs. Thus, it transmits some of the stress back to the top boundary and causes a weak coupling between slab and surface. Therefore, by including the low viscous plate boundaries in this model, the slabs and overriding plates decouples and the fit to the observed geoid degrades. In contrast, when the lower mantle viscosity is not sufficiently stiff, the presence of the low viscous plate boundaries assists to weaken the strong mechanical coupling between slab and surface. Hence, a better fit achieved.
Whereas fossil evidence indicates extensive treeless vegetation and diverse grazing megafauna in Europe and northern Asia during the last glacial, experiments combining vegetation models and climate models have to-date simulated widespread persistence of trees. Resolving this conflict is key to understanding both last glacial ecosystems and extinction of most of the mega-herbivores. Using a dynamic vegetation model (DVM) we explored the implications of the differing climatic conditions generated by a general circulation model (GCM) in “normal” and “hosing” experiments. Whilst the former approximate interstadial conditions, the latter, designed to mimic Heinrich Events, approximate stadial conditions. The “hosing” experiments gave simulated European vegetation much closer in composition to that inferred from fossil evidence than did the “normal” experiments. Given the short duration of interstadials, and the rate at which forest cover expanded during the late-glacial and early Holocene, our results demonstrate the importance of millennial variability in determining the character of last glacial ecosystems.
Das Thema dieser Arbeit war die Untersuchung der natürlichen Variationen von den zwei primordialen Uranisotopen (238U und 235U) mit einem Schwerpunkt auf Proben, die (1) die kontinentale Kruste und ihre Verwitterungsprodukte (d.h. Granite, Shales und Flusswasser) repräsentieren, (2) Produkte der hydrothermalen Alteration vom mittelozeanischen Rücken widerspiegeln (d.h. alterierte Basalte, Karbonatgänge und hydrothermales Wasser) und (3) aus abgegrenzten euxinischen Becken (d.h. Proben aus der Wassersäule und den dazugehörigen Sedimenten) stammen. Das allgemeine Ziel war das Verständnis, unter welchen Bedingungen und Mechanismen eine Fraktionierung der zwei häufigsten Uranisotope (238U und 235U) in der Natur erfolgt, zu verbessern.
Die untersuchten Haupt- und Nebenflüsse unterscheiden sich sowohl in Ihrer Urankonzentration (c(U)) als auch in Ihrer Uranisotopenzusammensetzung (δ238U), wobei die Nebenflüsse eine geringere Urankonzentration (0.87 nmol/kg bis 3.08 nmol/kg) und eine schwerere Uranisotopenzusammensetzung aufweisen (-0.29 ‰ bis +0.01 ‰ im δ238U) im Vergleich zu den Hauptflüssen (c(U) = 5.19 nmol/kg bis 11.69 nmol/kg und d238U = -0.31 ‰ bis +0.13 ‰) aufweisen. Die untersuchten Gesteinsproben fallen alle in einen recht schmalen Bereich von δ238U, zwischen -0.45 ‰ und -0.21 ‰, mit einem Durchschnittswert von -0.30 ‰ ± 0.04 ‰ (doppelte Standardabweichung). Deren Uranisotopenvariationen sind unabhängig von der Urankonzentration (11.8 µg/g bis 1.3 µg/g), dem Alter (3.80 Ga bis 328 Ma), der Probenlokalität und Grad der Differenzierung. Basierend auf den Ergebnissen der Hauptflüsse, die die Uranhauptquelle für den Ozean darstellen, schlagen wir für zukünftige Berechnungen in der Massenbilanz des Urans einen neuen Wert als beste Abschätzung für die Quelle des Urans im Ozean vor, δ238U = -0.23 ‰.
Die Produkte der hydrothermalen Alteration, alterierte Basalte und Kalziumkarbonatgänge, zeigten etwas stärkere Isotopenvariationen (δ238U zwischen -0.63 ‰ und +0.27 ‰) als erwartet und die hydrothermalen Fluide wiesen eine etwas leichtere Uranisotopenzusammensetzung als Meerwasser ((-0.43 ± 0.25) ‰ vs. (-0.37 ± 0.03) ‰) auf. Diese Ergebnisse sind in Übereinstimmung mit einem Modell, dass annimmt, dass die beobachtete Isotopenfraktionierung hauptsächlich ein Ergebnis von Redoxprozessen ist, z.B. die partielle Reduktion von löslichem UVI aus dem Meerwasser während der hydrothermalen Alteration, was zu einer Anreicherung der schweren Uranisotope in der reduzierten Uranspezies (UIV) führt und 2) das bevorzugte Entfernen von UIV aus den hydrothermalen Fluid und der Einbau in die alterierte ozeanische Kruste. Durch diesen Prozess wird das hydrothermale Fluid an schweren Uranisotopen verarmt und somit würden auch die alterierten Basalte und Karbonate ein niedriges δ238U aufweisen, wenn sie mit dem isotopisch leichten hydrothermalen Fluid in Kontakt gekommen sind.
Die Untersuchung von Wasser- und Sedimentproben aus der Ostsee und dem anoxischen Kyllaren Fjord (Norwegen) auf deren Uran- und Mo-Isotopenzusammensetzung zeigte, dass die Uranisotopenzusammensetzung der Sedimente abhängt von (1) dem Ausmaß des Uranaustrags aus der Wassersäule (in einer ähnlichen Art und Weise wie bei den Molybdänisotopen) und (2) der Sedimentationsrate, d.h. der Fraktion von authigenem- relativ zum dedritischen Uran in den Sedimenten. Aufgrund der hohen Sedimentationsrate zeigen die Sedimente aus dem Kyllaren Fjord nur eine moderate authigene Urananreicherung und eine leichtere Uranisotopenzusammensetzung als Sedimente aus dem Schwarzen Meer. In den anoxischen Becken der Ostsee erfolgt dagegen eine starke Mo- und schwache U-Isotopenfraktionierung zwischen Wasser und Sediment. Durch die regelmäßigen auftretenden Spülereignisse mit sauerstoffreichem Wasser wurden vermutlich die ursprünglichen anoxischen Mo- und U-Isotopensignaturen der Sedimente verändert. Demzufolge müssen die Sedimente durchgehend anoxischen Bedingungen ausgesetzt sein, um eine Mo- und U-Isotopensignatur von den Redoxbedingungen während der Ablagerungen zu speichern.
Der Vergleich zwischen Molybdän- und Uranisotopen in der Ostsee und dem anoxischen Kyllaren Fjord zeigte, dass sich Uran- und Molybdänisotope in stark euxinischen Wassersäulen (c(H2S) > 11 µmol/L) entgegengesetzt verhalten. Dementsprechend ergänzen sich die beiden Isotopensysteme und können genutzt werden, um die Ablagerungsbedingungen in abgeschlossenen Becken und die Redoxentwicklung des Paläoozeans zu untersuchen.
Reduction of greenhouse gas (GHG) emissions to minimize climate change requires very significant societal effort. To motivate this effort, it is important to clarify the benefits of avoided emissions. To this end, we analysed the impact of four emissions scenarios on future renewable groundwater resources, which range from 1600 GtCO2 during the 21st century (RCP2.6) to 7300 GtCO2 (RCP8.5). Climate modelling uncertainty was taken into account by applying the bias-corrected output of a small ensemble of five CMIP5 global climate models (GCM) as provided by the ISI-MIP effort to the global hydrological model WaterGAP. Despite significant climate model uncertainty, the benefits of avoided emissions with respect to renewable groundwater resources (i.e. groundwater recharge (GWR)) are obvious. The percentage of projected global population (SSP2 population scenario) suffering from a significant decrease of GWR of more than 10% by the 2080s as compared to 1971–2000 decreases from 38% (GCM range 27–50%) for RCP8.5 to 24% (11–39%) for RCP2.6. The population fraction that is spared from any significant GWR change would increase from 29% to 47% if emissions were restricted to RCP2.6. Increases of GWR are more likely to occur in areas with below average population density, while GWR decreases of more than 30% affect especially (semi)arid regions, across all GCMs. Considering change of renewable groundwater resources as a function of mean global temperature (GMT) rise, the land area that is affected by GWR decreases of more than 30% and 70% increases linearly with global warming from 0 to 3 ° C. For each degree of GMT rise, an additional 4% of the global land area (except Greenland and Antarctica) is affected by a GWR decrease of more than 30%, and an additional 1% is affected by a decrease of more than 70%.
Water footprints have been proposed as sustainability indicators, relating the consumption of goods like food to the amount of water necessary for their production and the impacts of that water use in the source regions. We have further developed the existing water footprint methodology by globally resolving virtual water flows and import and source regions at 5 arc minutes spatial resolution, and by assessing local impacts of export production. Applying this method to three exemplary cities, Berlin, Delhi and Lagos, we find major differences in amounts, composition, and origin of green and blue virtual water imports, due to differences in diets, trade integration and crop water productivities in the source regions. While almost all of Delhi's and Lagos' virtual water imports are of domestic origin, Berlin on average imports from more than 4000 km distance, in particular soy (livestock feed), coffee and cocoa. While 42% of Delhi's virtual water imports are blue water based, the fractions for Berlin and Lagos are 2% and 0.5%, respectively, roughly equal to local drinking water abstractions of these cities. Some of the external source regions of Berlin's virtual water imports appear to be critically water scarce and/or food insecure. However for deriving recommendations on sustainable consumption and trade, further analysis of context-specific costs and benefits associated with export production will be required.
The SAWNUC microphysical aerosol nucleation model is used to study the effect of reactor walls on the interpretation of nucleation experiments with respect to nucleation theory. This work shows that loss processes, such as wall losses, influence the interpretation of nucleation experiments, especially at low growth rates and short lifetime of freshly nucleated particles. In these cases the power dependency of the formation rates, determined at a certain particle size, with respect to H2SO4 does not correspond to the approximate number of H2SO4 molecules in the critical cluster as expected by the first nucleation theorem. Observed ∂log(J)/∂log([H2SO4]) therefore can vary widely for identical nucleation conditions but different sink terms.