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The Barberton Greenstone Belt (BGB) in the northwestern part of South Africa belongs to the few well-preserved remnants of Archean crust. Over the last centuries, the BGB has been intensively studied at surface with detailed mapping of its surfacial geological units and tectonic features. Nevertheless, the deeper structure of the BGB remains poorly understood. Various tectonic evolution models have been developed based on geo-chronological and structural data. These theories are highly controversial and centre on the question whether plate tectonics - as geoscientists understand them today - was already evolving on the Early Earth or whether vertical mass movements driven by the higher temperature of the Earth in Archean times governed continent development.
To get a step closer to answering the questions regarding the internal structure and formation of the BGB, magnetotelluric (MT) field experiments were conducted as part of the German-South African research initiative Inkaba yeAfrica. Five-component MT data (three magnetic and two electric channels) were collected at ~200 sites aligned along six profiles crossing the southern part of the BGB. Tectonic features like (fossil) faults and shear zones are often mineralized and therefore can have high electrical conductivities. Hence, by obtaining an image of the conductivity distribution of the subsurface from MT measurements can provide useful information on tectonic processes.
Unfortunately, the BGB MT data set is heavily affected by man-made electromagnetic noise caused, e.g. by powerlines and electric fences. Aperiodic spikes in the magnetic and corresponding offsets in the electric field components impair the data quality particularly at periods >1 s which are required to image deep electrical structures. Application of common methods for noise reduction like delay filtering and remote reference processing, only worked well for periods <1 s. Within the framework of this thesis two new filtering approaches were developed to handle the severe noise in long period data and obtain reliable processing results. The first algorithm is based on the Wiener filter in combination with a spike detection algorithm. Comparison of data variances of a local site with those of a reference site allows the identification of disturbed time series windows for each recorded channel at the local site. Using the data of the reference site, a Wiener filter algorithm is applied to predict physically meaningful data to replace the disturbed windows. While spikes in the magnetic channels are easily recognized and replaced, steps in the electric channels are more difficult to detect depending on their offset. Therefore, I have implemented a novel approach based on time series differentiation, noise removal and subsequent integration to overcome this obstacle. A second filtering approach where spikes and steps in the time series are identified using a comparison of the short and long time average of the data was also implemented as part of my thesis. For this filtering approach the noise in the form of spikes and offsets in the data is treated by an interpolation of the affected data samples. The new developments resulted in a substantial data improvement and allowed to gain one to two decades of data (up to 10 or 100 s).
The re-processed MT data were used to image the electrical conductivity distribution of the BGB by 2D and 3D inversion. Inversion models are in good agreement with the surface geology delineating the highly resistive rocks of the BGB from surrounding more conductive geological units. Fault zones appear as conductive structures and can be traced to depths of 5 to 10 km. 2D models suggest a continuation of the faults further south across the boundary of the BGB. Based on the shallow tectonic structures (fault system) within the BGB compared to deeply rooted resistive batholiths in the area, tectonic models including both vertical mass transport and in parts present-day style plate tectonics seem to be most likely for the evolution of the BGB.
Spectral fingerprinting
(2015)
Current research on runoff and erosion processes, as well as an increasing demand for sustainable watershed management emphasize the need for an improved understanding of sediment dynamics. This involves the accurate assessment of erosion rates and sediment transfer, yield and origin. A variety of methods exist to capture these processes at the catchment scale. Among these, sediment fingerprinting, a technique to trace back the origin of sediment, has attracted increasing attention by the scientific community in recent years. It is a two-step procedure, based on the fundamental assumptions that potential sources of sediment can be reliably discriminated based on a set of characteristic ‘fingerprint’ properties, and that a comparison of source and sediment fingerprints allows to quantify the relative contribution of each source.
This thesis aims at further assessing the potential of spectroscopy to assist and improve the sediment fingerprinting technique. Specifically, this work focuses on (1) whether potential sediment sources can be reliably identified based on spectral features (‘fingerprints’), whether (2) these spectral fingerprints permit the quantification of relative source contribution, and whether (3) in situ derived source information is sufficient for this purpose. Furthermore, sediment fingerprinting using spectral information is applied in a study catchment to (4) identify major sources and observe how relative source contributions change between and within individual flood events. And finally, (5) spectral fingerprinting results are compared and combined with simultaneous sediment flux measurements to study sediment origin, transport and storage behaviour.
For the sediment fingerprinting approach, soil samples were collected from potential sediment sources within the Isábena catchment, a meso-scale basin in the central Spanish Pyrenees. Undisturbed samples of the upper soil layer were measured in situ using an ASD spectroradiometer and subsequently sampled for measurements in the laboratory. Suspended sediment was sampled automatically by means of ISCO samplers at the catchment as well as at the five major subcatchment outlets during flood events, and stored fine sediment from the channel bed was collected from 14 cross-sections along the main river. Artificial mixtures of known contributions were produced from source soil samples. Then, all source, sediment and mixture samples were dried and spectrally measured in the laboratory. Subsequently, colour coefficients and physically based features with relation to organic carbon, iron oxide, clay content and carbonate, were calculated from all in situ and laboratory spectra. Spectral parameters passing a number of prerequisite tests were submitted to principal component analyses to study natural clustering of samples, discriminant function analyses to observe source differentiation accuracy, and a mixing model for source contribution assessment. In addition, annual as well as flood event based suspended sediment fluxes from the catchment and its subcatchments were calculated from rainfall, water discharge and suspended sediment concentration measurements using rating curves and Quantile Regression Forests. Results of sediment flux monitoring were interpreted individually with respect to storage behaviour, compared to fingerprinting source ascriptions and combined with fingerprinting to assess their joint explanatory potential.
In response to the key questions of this work, (1) three source types (land use) and five spatial sources (subcatchments) could be reliably discriminated based on spectral fingerprints. The artificial mixture experiment revealed that while (2) laboratory parameters permitted source contribution assessment, (3) the use of in situ derived information was insufficient. Apparently, high discrimination accuracy does not necessarily imply good quantification results. When applied to suspended sediment samples of the catchment outlet, the spectral fingerprinting approach was able to (4) quantify the major sediment sources: badlands and the Villacarli subcatchment, respectively, were identified as main contributors, which is consistent with field observations and previous studies. Thereby, source contribution was found to vary both, within and between individual flood events. Also sediment flux was found to vary considerably, annually as well as seasonally and on flood event base. Storage was confirmed to play an important role in the sediment dynamics of the studied catchment, whereas floods with lower total sediment yield tend to deposit and floods with higher yield rather remove material from the channel bed. Finally, a comparison of flux measurements with fingerprinting results highlighted the fact that (5) immediate transport from sources to the catchment outlet cannot be assumed. A combination of the two methods revealed different aspects of sediment dynamics that none of the techniques could have uncovered individually.
In summary, spectral properties provide a fast, non-destructive, and cost-efficient means to discriminate and quantify sediment sources, whereas, unfortunately, straight-forward in situ collected source information is insufficient for the approach. Mixture modelling using artificial mixtures permits valuable insights into the capabilities and limitations of the method and similar experiments are strongly recommended to be performed in the future. Furthermore, a combination of techniques such as e.g. (spectral) sediment fingerprinting and sediment flux monitoring can provide comprehensive understanding of sediment dynamics.
Climate change is likely to impact the seasonality and generation processes of floods in the Nordic countries, which has direct implications for flood risk assessment, design flood estimation, and hydropower production management. Using a multi-model/multi-parameter approach to simulate daily discharge for a reference (1961–1990) and a future (2071–2099) period, we analysed the projected changes in flood seasonality and generation processes in six catchments with mixed snowmelt/rainfall regimes under the current climate in Norway. The multi-model/multi-parameter ensemble consists of (i) eight combinations of global and regional climate models, (ii) two methods for adjusting the climate model output to the catchment scale, and (iii) one conceptual hydrological model with 25 calibrated parameter sets. Results indicate that autumn/winter events become more frequent in all catchments considered, which leads to an intensification of the current autumn/winter flood regime for the coastal catchments, a reduction of the dominance of spring/summer flood regimes in a high-mountain catchment, and a possible systematic shift in the current flood regimes from spring/summer to autumn/winter in the two catchments located in northern and south-eastern Norway. The changes in flood regimes result from increasing event magnitudes or frequencies, or a combination of both during autumn and winter. Changes towards more dominant autumn/winter events correspond to an increasing relevance of rainfall as a flood generating process (FGP) which is most pronounced in those catchments with the largest shifts in flood seasonality. Here, rainfall replaces snowmelt as the dominant FGP primarily due to increasing temperature.We further analysed the ensemble components in contributing to overall uncertainty in the projected changes and found that the climate
projections and the methods for downscaling or bias correction tend to be the largest contributors. The relative role of hydrological parameter uncertainty, however, is highest for those catchments showing the largest changes in flood seasonality, which confirms the lack of robustness in hydrological model parameterization for simulations under transient hydrometeorological conditions.
Climate change is likely to impact the seasonality and generation processes of floods in the Nordic countries, which has direct implications for flood risk assessment, design flood estimation, and hydropower production management. Using a multi-model/multi-parameter approach to simulate daily discharge for a reference (1961–1990) and a future (2071–2099) period, we analysed the projected changes in flood seasonality and generation processes in six catchments with mixed snowmelt/rainfall regimes under the current climate in Norway. The multi-model/multi-parameter ensemble consists of (i) eight combinations of global and regional climate models, (ii) two methods for adjusting the climate model output to the catchment scale, and (iii) one conceptual hydrological model with 25 calibrated parameter sets. Results indicate that autumn/winter events become more frequent in all catchments considered, which leads to an intensification of the current autumn/winter flood regime for the coastal catchments, a reduction of the dominance of spring/summer flood regimes in a high-mountain catchment, and a possible systematic shift in the current flood regimes from spring/summer to autumn/winter in the two catchments located in northern and south-eastern Norway. The changes in flood regimes result from increasing event magnitudes or frequencies, or a combination of both during autumn and winter. Changes towards more dominant autumn/winter events correspond to an increasing relevance of rainfall as a flood generating process (FGP) which is most pronounced in those catchments with the largest shifts in flood seasonality. Here, rainfall replaces snowmelt as the dominant FGP primarily due to increasing temperature.We further analysed the ensemble components in contributing to overall uncertainty in the projected changes and found that the climate projections and the methods for downscaling or bias correction tend to be the largest contributors. The relative role of hydrological parameter uncertainty, however, is highest for those catchments showing the largest changes in flood seasonality, which confirms the lack of robustness in hydrological model parameterization for simulations under transient hydrometeorological conditions.
Stream water and groundwater are important fresh water resources but their water quality is deteriorated by harmful solutes introduced by human activities. The interface between stream water and the subsurface water is an important zone for retention, transformation and attenuation of these solutes. Streambed structures enhance these processes by increased water and solute exchange across this interface, denoted as hyporheic exchange.
This thesis investigates the influence of hydrological and morphological factors on hyporheic water and solute exchange as well as redox-reactions in fluvial streambed structures on the intermediate scale (10–30m). For this purpose, a three-dimensional numerical modeling approach for coupling stream water flow with porous media flow is used. Multiple steady state stream water flow scenarios over different generic pool-riffle morphologies and a natural in-stream gravel bar are simulated by a computational fluid dynamics code that provides the hydraulic head distribution at the streambed. These heads are subsequently used as the top boundary condition of a reactive transport groundwater model of the subsurface beneath the streambed. Ambient groundwater that naturally interacts with the stream water is considered in scenarios of different magnitudes of downwelling stream water (losing case) and upwelling groundwater (gaining case). Also, the neutral case, where stream stage and groundwater levels are balanced is considered. Transport of oxygen, nitrate and dissolved organic carbon and their reaction by aerobic respiration and denitrification are modeled.
The results show that stream stage and discharge primarily induce hyporheic exchange flux and solute transport with implications for specific residence times and reactions at both the fully and partially submerged structures. Gaining and losing conditions significantly diminish the extent of the hyporheic zone, the water exchange flux, and shorten residence times for both the fully and partially submerged structures. With increasing magnitude of gaining or losing conditions, these metrics exponentially decrease.
Stream water solutes are transported mainly advectively into the hyporheic zone and hence their influx corresponds directly to the infiltrating water flux. Aerobic respiration takes place in the shallow streambed sediments, coinciding to large parts with the extent of the hyporheic exchange flow. Denitrification occurs mainly as a “reactive fringe” surrounding the aerobic zone, where oxygen concentration is low and still a sufficient amount of stream water carbon source is available. The solute consumption rates and the efficiency of the aerobic and anaerobic reactions depend primarily on the available reactive areas and the residence times, which are both controlled by the interplay between hydraulic head distribution at the streambed and the gradients between stream stage and ambient groundwater. Highest solute consumption rates can be expected under neutral conditions, where highest solute flux, longest residence times and largest extent of the hyporheic exchange occur. The results of this thesis show that streambed structures on the intermediate scale have a significant potential to contribute to a net solute turnover that can support a healthy status of the aquatic ecosystem.
We study segregation of the subducted oceanic crust (OC) at the core mantle boundary and its ability to accumulate and form large thermochemical piles (such as the seismically observed Large Low Shear Velocity Provinces - LLSVPs). Our high-resolution numerical simulations suggest that the longevity of LLSVPs for up to three billion years, and possibly longer, can be ensured by a balance in the rate of segregation of high-density OC-material to the CMB, and the rate of its entrainment away from the CMB by mantle upwellings.
For a range of parameters tested in this study, a large-scale compositional anomaly forms at the CMB, similar in shape and size to the LLSVPs. Neutrally buoyant thermochemical piles formed by mechanical stirring - where thermally induced negative density anomaly is balanced by the presence of a fraction of dense anomalous material - best resemble the geometry of LLSVPs. Such neutrally buoyant piles tend to emerge and survive for at least 3Gyr in simulations with quite different parameters. We conclude that for a plausible range of values of density anomaly of OC material in the lower mantle - it is likely that it segregates to the CMB, gets mechanically mixed with the ambient material, and forms neutrally buoyant large scale compositional anomalies similar in shape to the LLSVPs.
We have developed an efficient FEM code with dynamically adaptive time and space resolution, and marker-in-cell methodology. This enabled us to model thermochemical mantle convection at realistically high convective vigor, strong thermally induced viscosity variations, and long term evolution of compositional fields.
Two of the most controversial issues concerning the late Cenozoic evolution of the Andean orogen are the timing of uplift of the intraorogenic Puna plateau and its eastern border, the Eastern Cordillera, and ensuing changes in climatic and surface-process conditions in the intermontane basins of the NW-Argentine Andes. The Eastern Cordillera separates the internally drained, arid Puna from semi-arid intermontane basins and the humid sectors of the Andean broken foreland and the Subandean fold-and-thrust belt to the east. With elevations between 4,000 and 6,000 m the eastern flanks of the Andes form an efficient orographic barrier with westward-increasing elevation and asymmetric rainfall distribution and amount with respect to easterly moisture-bearing winds. This is mirrored by pronounced gradients in the efficiency of surface processes that erode and re-distribute sediment from the uplifting ranges. Although the overall pattern of deformation and uplift in this sector of the southern central Andes shows an eastward migration of deformation, a well-developed deformation front does not exist and uplift and associated erosion and sedimentary processes are highly disparate in space and time. In addition, periodic deformation within intermontane basins, and continued diachronous foreland uplifts associated with the reactivation of inherited basement structures furthermore make a rigorous assessment of the spatiotemporal uplift patterns difficult.
This thesis focuses on the tectonic evolution of the Eastern Cordillera of NW Argentina, the depositional history of its intermontane sedimentary basins, and the regional topographic evolution of the eastern flank of the Puna Plateau. The intermontane basins of the Eastern Cordillera and the adjacent morphotectonic provinces of the Sierras Pampeanas and the Santa Bárbara System are akin to reverse fault bounded, filled, and partly coalesced sedimentary basins of the Puna Plateau. In contrast to the Puna basins, however, which still form intact morphologic entities, repeated deformation, erosion, and re-filling have impacted the basins in the Eastern Cordillera. This has resulted in a rich stratigraphy of repeated basin fills, but many of these basins have retained vestiges of their early depositional history that may reach back in time when these areas were still part of a contiguous and undeformed foreland basin. Fortunately, these strata also contain abundant volcanic ashes that are not only important horizons to decipher tectono-sedimentary events through U-Pb geochronology and geochemical correlation, but they also represent terrestrial recorders of the hydrogen-isotope composition of ancient meteoric waters that can be compared to the isotopic composition of modern meteoric water. The ash horizons are thus unique recorders of past environmental conditions and lend themselves to tracking the development of rainfall barriers and tectonically forced climate and environmental change through time.
U-Pb zircon geochronology and paleocurrent reconstructions of conglomerate sequences in the Humahuaca Basin of the Eastern Cordillera at 23.5° S suggest that the basin was an integral part of a largely unrestricted depositional system until 4.2 Ma, which subsequently became progressively decoupled from the foreland by range uplifts to the east that forced easterly moisture-bearing winds to precipitate in increasingly eastward locations. Multiple cycles of severed hydrological conditions and drainage re-capture are identified together with these processes that were associated with basin filling and sediment evacuation, respectively. Moreover, systematic relationships among faults, regional unconformities and deformed landforms reveal a general pattern of intra-basin deformation that appears to be linked with basin-internal deformation during or subsequent to episodes of large-scale sediment removal. Some of these observations are supported by variations in the hydrogen stable isotope composition of volcanic glass from the Neogene to Quaternary sedimentary record, which can be related to spatiotemporal changes in topography and associated orographic effects. δDg values in the basin strata reveal two main trends associated with surface uplift in the catchment area between 6.0 and 3.5 Ma and the onset of semiarid conditions in the basin following the attainment of threshold elevations for effective orographic barriers to the east after 3.5 Ma. The disruption of sediment supply from western sources after 4.2 Ma and subsequent hinterland aridification, moreover, emphasize the possibility that these processes were related to lateral orogenic growth of the adjacent Puna Plateau. As a result of the hinterland aridification the regions in the orogen interior have been characterized by an inefficient fluvial system, which in turn has helped maintaining internal drainage conditions, sediment storage, and relief reduction within high-elevation basins.
The diachronous nature of basin formation and impacts on the fluvial system in the adjacent broken foreland is underscored by the results of detailed sediment provenance and paleocurrent analyses, as well as U-Pb zircon geochronology in the Lerma and Metán basins at ca. 25° S. This is particularly demonstrated by the isolated uplift of the Metán range at ~10 Ma, which is more than 50 km away from the presently active orogenic front along the eastern Puna margin and the Eastern Cordillera to the west. At about 5 Ma, Puna-sourced sediments disappear from the foreland record, documenting further range uplifts in the Eastern Cordillera and hydrological isolation of the neighboring Angastaco Basin from the foreland. Finally, during the late Pliocene and Quaternary, deformation has been accommodated across the entire foreland and is still active. To elucidate the interactions between tectonically controlled changes in elevation and their impact on atmospheric circulation processes in this region, this thesis provides additional, temporally well-constrained hydrogen stable isotope results of volcanic glass samples from the broken foreland, including the Angastaco Basin, and other intermontane basins farther south. The results suggest similar elevations of intermontane basins and the foreland sectors prior to ca. 7 Ma. In case of the Angastaco Basin the region was affected by km-scale surface uplift of the basin. A comparison with coeval isotope data collected from sedimentary sequences in the Puna plateau explains rapid shifts in the intermontane δDg record and supports the notion of recurring phases of enhanced deep convection during the Pliocene, and thus climatic conditions during the middle to late Pliocene similar to the present day.
Combined, field-based and isotope geochemical methods used in this study of the NW-Argentine Andes have thus helped to gain insight into the systematics, rate changes, interactions, and temporal characteristics among tectonically controlled deformation patterns, the build-up of topography impacting atmospheric processes, the distribution of rainfall, and resulting surface processes in a tectonically active mountain belt. Ultimately, this information is essential for a better understanding of the style and the rates at which non-collisional mountain belts evolve, including the development orogenic plateaus and their bordering flanks. The results presented in this study emphasize the importance of stable isotope records for paleoaltimetric and paleoenvironmental studies in mountain belts and furnishes important data for a rigorous interpretation of such records.
Methanhydrate sind besonders in Verbindung mit den steigenden Weltmarktpreisen für Öl und Gas in den vergangenen Jahren mehr und mehr in den Fokus der Energiewirtschaft geraten, was zu einer starken Zunahme der angewandten Forschungsprojekte auf diesem Gebiet führte. Da Methanhydrat nur unter hohem Druck und niedrigen Temperaturen stabil ist, ist die Gewinnung natürlicher Proben für Laboruntersuchungen technisch sehr aufwendig und vor allem teuer. Zur Charakterisierung der Eigenschaften hydratführender Reservoire ist man häufig auf die Herstellung synthetischer Proben angewiesen. Die Eigenschaften der synthetisierten Proben sind dabei abhängig von der Herstellungsmethode und man ist noch immer auf der Suche nach Verfahren, mit denen sich möglichst „naturnahe“ Proben mit vertretbarem Aufwand erzeugen lassen.
In der vorliegenden Arbeit wurde eine neue, relativ schnell durchführbare Methode getestet, die im Porenraum von Sedimenten schwimmende bzw. gefügestützende Hydrate bildet, wie sie in der Natur vorkommen. Gleichzeitig erzeugt sie eine gleichmäßige Verteilung des Hydrats über die Probe und bietet gute Kontrolle über den Hydratgehalt. Sie funktioniert wie folgt: Eine mit einer KCl-Lösung gesättigte Sedimentprobe wird zu einem bestimmten Teil ausgefroren und das übrige Wasser mit Methan verdrängt. Durch Anlegen eines Methandrucks im Stabilitätsbereich wird das Eis zu Methanhydrat umgesetzt. Im Anschluss wird die Probe erneut mit einer KCl-Lösung gesättigt. Anhand seismischer Messungen konnte bestätigt werden, dass Hydrat mit dem gewünschten Hydrathabitus erzeugt wurde. Des Weiteren wurde gezeigt, dass die eishaltigen Proben aufgrund ähnlicher physikalischer Eigenschaften bereits vor der Umsetzung des Eises zu Methanhydrat als Näherung für Proben mit Porenraumhydrat verwendet werden können.