Fakultät für Biologie, Chemie und Geowissenschaften
Measurement of elastic properties of silicates at realistic mantle pressures
- Measurements of seismic wave travel times through the Earth’s interior provide one of the few sources of information on the properties of deep mantle rocks. These travel times can be interpreted in terms of their implications for the mineralogy, chemistry and temperature of the mantle if they can be compared with models determined from laboratory measurements for the elastic properties of mantle minerals at high pressures and temperatures. In this study MHz ultrasonic measurements have been used to determine the velocities of P and S acoustic waves in mantle minerals at mantle conditions. These results are used to constrain the properties of the mantle through comparison with seismic data.
Subduction zones exhibit faster seismic wave velocities compared to the surrounding mantle due to the recycling of relatively cold oceanic lithosphere. In certain subduction zones, however, a 5-10 km thick low velocity layer (LVL) has been inferred to exist along the top surface of the subducting slab at depths of up to 250 km. Shear-wave velocities, in particular, within these layers have been estimated as up to 10% slower than in the surrounding mantle. High-pressure ultrasonic interferometric measurements were performed to gain insight into the elastic properties of lawsonite [CaAl2(Si2O7)(OH)2.H2O], a hydrous mineral phase stabilized under cold subduction zone conditions. It was found that lawsonite has an unusually low shear modulus at high pressure and its formation in subducted oceanic crust can explain some seismic evidence for LVL at depths exceeding 100 km. To approach estimated LVL velocities requires lawsonite to form in the subducting crust as a result of a fluid influx due to the breakdown of other hydrous minerals such as serpentine. The formation of lawsonite additionally lowers seismic velocities because it forms at the expense of garnet, a mineral with relatively fast seismic velocities. LVL observations may therefore be used to place constraints on the amount of H2O subducted into the deep mantle.
Chemical heterogeneities in the transition zone are potentially developed through the subduction of basaltic oceanic crust and lithospheric ultramafic mantle. The mineralogy of these lithologies in the transition zone will mainly differ in terms of the proportion and chemistry of garnet. Garnets formed from originally basaltic and ultramafic compositions will differ in the proportions of Ca, Fe and majoritic component, with the former being richer in all components except majorite. Most models use partial derivatives of elastic properties with respect to composition to determine the elastic properties of complex garnet compositions in the mantle. In this study the effect of chemical variations on seismic wave velocities of garnets up to transition zone conditions have been intensively studied by examining the effects of Ca, Fe and majoritic substitutions on the elasticity of pyrope. In addition, complex multicomponent garnet compositions, expected to form from both subducted basaltic material and typical ultramafic mantle at transition zone conditions, have been studied. The results indicate that elastic properties of multicomponent garnets can be quite reliably interpolated from the end member properties, with an accuracy that is generally within the experimental uncertainties. No evidence was found that large excess properties exist that cause non-linear contributions to multicomponent elastic properties for the mineral garnet. Deviation between the two calculated mantle models, pyrolitic and MORB, is about 3% for both VP and VS, with the modelled slab composition being slower. Within the uncertainties of the seismic data therefore a pyrolite mantle composition is quite consistent with velocities at the base of the transition zone. Relatively low temperatures and olivine-rich nature of the stagnant slab support that there would be few grounds to argue that significant accumulation of MORB composition material may occur at the base of the transition zone.
Ultrasonic interferometry measurements in conjunction with in situ X-ray techniques have been used to measure compressional and shear wave velocities and densities of MgSiO3 perovskite (Mg-Pv) and perovskite ((Mg, Fe)-Pv) in the multianvil at pressures up to 25 GPa and temperatures to 1200 K. Data for Mg-Pv are consistent with previous studies and the (Mg, Fe)-Pv sample has almost identical shear properties to Mg-Pv. The adiabatic bulk modulus, Ks, for (Mg, Fe)-Pv, however, is found to be substantially lower than Mg-Pv, with a refined value of 236 GPa and a pressure derivative of 4.7. It is proposed that this low KS value results from a change in the elasticity of Fe-bearing perovskite at low pressures <30 GPa.
High temperatures measurements of VP and VS of and perovskite samples lead to the conclusion that the substitution of Mg by Fe seems to have a consistent effect on Ks, strongly lowering it in both Al and Al-free perovskites while it does not have any noticeable effect on the shear elastic properties. By substituting Al into the Si octahedral site, the rigidity of this framework is being reduced which clearly has an influence on the shear modulus G. G of the Al-bearing perovskites is obviously controlled by the Al content in the structure. Seismic observations indicate low visibility of underside PP reflections from the 660 km discontinuity. Seismologists have used this in the past to argue that the P velocity contrast at the discontinuity is much lower than would be expected from seismic reference models. The data collected on Fe-Al perovskite in this study provide excellent support for this argument as model calculations based on these data show very little contrast in Vp at 660 km depth. Instead, the model derived in this study indicates that much of the change in P velocity between the transition zone and lower mantle occurs over a much broader depth, 200km, interval and is mainly attributed to the transformation of garnet to perovskite. A major conclusion therefore is that a sharp 660 km discontinuity reflection should be visible in S but not P wave seismic data. The model calculation for VS is in a good agreement with the seismic reference models up to depths as great as 1071 km, while VP remains 3 to 4% lower at lower mantle conditions. The extrapolations of pressure and temperature dependences significantly out of the ranges over that they were measured may well explain this discrepancy however.
High-Resolution Modelling of Surface-Atmosphere Interactions and Convection Development at Nam Co Lake, Tibetan Plateau
- The Tibetan Plateau has recently become an area of increased interest for the atmospheric and environmental sciences. Surface-atmosphere interactions and specifically the exchange of momentum, turbulent energy and water vapour as well as the development of convection are not only important for the surface energy balance and local water resources, but also have influence on the evolution of the monsoon system and climate. High-resolution, numerical atmospheric models with a fully coupled surface model are a valuable tool for the systematic investigation of surface-atmosphere interactions. Nam Co Lake, located at the northern extent of the monsoon's influence, was selected as a complex system in order to study the interaction of the land and the lake in the generation of mesoscale circulations and the development from boundary-layer clouds to moist convection.
Turbulent fluxes estimated by eddy-covariance and atmospheric profiles measured by radiosondes are used in this work in conjunction with the ATHAM (Active Tracer High-resolution Atmospheric Model) and Hybrid models. Substantial model development is undertaken for both ATHAM and Hybrid. This means a more consistent formulation of tracer and heat transport in ATHAM and improved model stability. Hybrid has been modified with an extrapolated surface temperature, to be used for the calculation of turbulent fluxes. A quadratic temperature profile based on the layer mean and surface model base temperature is assumed in each layer and extended to the surface. Compared to eddy-covariance measurements and a Surface-Vegetation-Atmosphere Transport (SVAT) Model there is an overall reasonable model performance, when tested on four days for two sites with variable environmental conditions during the 2009 summer monsoon season. At the same time, errors are reduced by 40-60% compared to the unmodified Hybrid.
Subsequently, the coupled modelling system is used for 2-dimensional cross-sections through the Nam Co Lake basin with horizontal resolutions of
200 m and at least 150 vertical layers between the surface and the model top located in the lower stratosphere. The 2-dimensional modelling approach has a
tendency to overestimate convective strength due to the underestimation of dry air entrainment and cannot reproduce fully realistic flow fields. Nevertheless,
it provides a valuable tool for systematic investigations of environmental factors, where 3D simulations are prohibitively expensive.
In simulations with several background wind speeds it is found that the model adequately simulates the mesoscale circulation system between the lake and the surrounding mountain chains. Dependent on the geostrophic wind direction there are two different mechanisms for the triggering of convection: Convective triggering, when overflowing topography, and triggering due to convergence between the lake-breeze front and the background wind. It is concluded that coupled modelling setup is capable of reproducing the system's most important dynamics, such as realistic turbulent surface fluxes, mesoscale circulations and cloud evolution.
Thereafter, the influence of the atmospheric profiles of temperature and relative humidity and the uncertainty that arises from them is discussed.
Simulations are initialised with profiles based on direct measurements (radiosondes), NCEP-I and ERA-INT reanalysis and GFS-FNL analysis data on two
days during the summer of 2012. The simulated convection from radiosondes compares reasonably well with weather observations for the first day, but
less well for the second day, when large-scale synoptic effects, which are not included in the model, gain importance. The choice of vertical profile information leads to strongly differing convection development, causing modifications of the surface energy balance and thus of the energy and water
cycle for Nam Co Lake.
With respect to precipitation it is found that a large fraction of the precipitation that is generated in the simulations is deposited within the basin
and on the slopes of the surrounding mountain chains and thus locally recycled. This also means that a weather station in the centre of the basin is not representative of the system. Furthermore, Nam Co Lake may be of importance as a water supply for the region. Additionally, the choice of profile and the initial water vapour contents determine the amount of precipitation so that there are strong differences spanning one order of magnitude in the generated precipitation between the model simulations driven by different vertical profiles.
The findings from the thesis provide an example of the impacts of surface-atmosphere interactions, mesoscale circulations and convective evolution on
the Tibetan Plateau. Scaled to the entire plateau these processes are highly relevant to ecosystems, climate and the water cycle.
Diffusion of proteins inside crowded structures generated using microemulsions
- Microemulsions are thermodynamically stable mixtures of water, oil and surfactant. In the case of microemulsions based on sugar surfactants a cosurfactant as an additional component is necessary, most often a short-chain alcohol. In this thesis mainly pure surfactants are used. The focus is on n-nonyl-ß-d-maltosid and n-dodecyl-ß-d-maltosid as model systems for microemulsions based on technical grade surfactants. Depending on the ratio of the components microemulsion form different structures. In the bicontinuous phase continuous oil and water domains are present separated by a surfactant/co-surfactant film. At higher surfactant amounts the bicontinuous structure passes over to a droplet phase. The size of these structures is in the scale of up to 100nm, therefore microemulsions look transparent for the human eye. Bicontinuous microemulsions are a promising carrier medium for decontamination applications. Chemical warfare agents are mainly lipophilic, in contrast degradation agents are hydrophilic. A microemulsion is able to solubilize lipophilic and hydrophilic molecules. At the interface the warfare agent is close to the degradation agent and can be eliminated. The protein diisopropyl-fluorophosphatase (DFPase) is a promising candidate for decontamination applications and is able to degrade different warfare agents. Therefore, the knowledge of the dynamics and properties of enzymes inside a bicontinuous structure is of big importance. In this thesis different microemulsion systems based on C9G2 or C12G2, water, cyclohexane and 1-pentanol are systematically characterized by X-ray and neutron scattering experiments. By using an improved Green Fluorescent Protein (GFP+) as a counterpart of DFPase with a similar size, the diffusion inside a microemulsion can be studied with the fluorescence correlation spectroscopy (FCS) method. Here, fluorescent impurities in the used components are a problem. By the choice of a suitable concentration of GFP+ and regarding the dynamics of the microemulsion structure, it could be shown that the protein is able to move inside the microemulsion structure. Hence, microemulsions are interesting model systems to produce crowding effects in a controlled way. This is probably the most important result of this thesis. In the C9G2 system a bicontinuous phase is present. At small oil/water ratios o/w with a high water amount big water domains exist with length scales of approximately 10nm, which allows protein diffusion. With increasing oil/water ratio the protein diffusion is more and more hindered. This results in an increase of the diffusion time and a decrease of the anomalous diffusion exponent. At even higher oil/water ratios the diffusion of the protein is nearly identical with the microemulsion dynamics. The length scales are too small to allow protein diffusion, therefore it is stuck in the structure. The C12G2 system shows an oil-in-water droplet phase which also hinders the GFP+ diffusion due to crowding effects. For small oil/water ratios nearly free diffusion is present, at o/w = 0.3 - 0.5 the diffusion is hindered, the anomalous diffusion exponent decreases. At o/w = 0.6 the protein diffusion reflects the microemulsion dynamics. Again at higher values of o/w, GFP+ is stuck in the structure because of the high oil fraction. Two microemulsion systems with different structures were studied, which allow protein diffusion due to their length scales. These results are related to applications: For the formulation of decontamination media the knowledge of the dynamics of the decontaminating protein is of importance. Moreover, pure surfactants can be easily replaced by technical grade surfactants. This enables cheap systems for a production-scale. Furthermore, the situation in living cells can be simulated by a microemulsion structure, where a confined diffusion is present as well. Additional neutron spin echo experiments emphasize the complexity when studying a microemulsion system based on four components. The contrast matching procedure, which is necessary when protein dynamics inside the microemulsion are investigated, is very difficult and requires a high amount of surfactant.
Zur Halogenaktivierung im Aerosol und in Salzpfannen
- Diese Arbeit zeigt die weltweit ersten Messungen der Halogenaktivierung mit direkter Beobachtung der Halogenoxide BrO, ClO und OClO unter kontrollierten Bedingungen eines Laboratoriums. Dazu wurde die an Forschungsstelle für atmosphärische Chemie vorhandene Teflon-Umweltkammer mit einem Sonnensimulator für troposphärisches Licht und diverser Sensorik ausgestattet. Zusätzlich wurde in Zusammenarbeit mit der Universität Heidelberg ein Langpfad-DOAS (Differential Optical Absorption Spectroscopy) installiert. In diesem Aufbau wurden zwei Arten von Experimenten durchgeführt: mit künstlichem Meerwasseraerosol und mit Salzpfannen. Nachgewiesen wurde ein starker Einfluss der Stickoxide auf die Aktivierung der Halogene, so dass in Extremfällen das Bromid und das Chlorid komplett freigesetzt wurde, was Mischungsverhältnisses von bis zu 17 ppb OClO, 6 ppm ClO und 1.5 ppb BrO führte. Zur eingehenden Untersuchung der Resultate wurden Modelle eingesetzt: das eigens für die Salzpfannenexperimente erstellte HaloBox-Modell und das CAABA/MECCA-Modell für Aerosolexperimente. Durch die Anpassung des CAABA/MECCA-Modells an die Rahmenbedingungen der Umweltkammer, kann dieses nun die Beobachtungen hinreichend wiedergeben und über unbekannte Größen wie pH-Wert oder die Konzentration von Halogenionen Aufschluss geben. In den Experimenten mit Salzpfannen wurden Mischungsverhältnisse von bis zu 6 ppb BrO beobachtet. Dort wurde eine Korrelation der Bromaktivierung zur relativen Luftfeuchte nachgewiesen. Untersucht wurden auch die einleitende Mechanismen der Aktivierung, die bereits bei Dunkelheit ablaufen. Hier wirkt Ozon als auslösende Spezies.