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- Bazargan, Mohsen
(författare)
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Numerical and laboratory studies of seismic properties and elements of rock fabric from the microscale to the macroscale
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Physical properties of rocks studied in the laboratory are useful to provide constraints on the dynamics of Earth’s interior. This may include direct constraints on in-situ seismic properties, such as elastic wave velocity measurements that can be compared to seismological data, or petrofabric indicators such as anisotropy of magnetic susceptibility (AMS). Another approach that provides predictive insight into the physical properties of Earth’s interior are computer models. Numerical modelling, in particular, can be used to investigate the dynamic propagation of elastic waves or the flow of a material to generate a fabric or texture (i.e., petrofabric in rocks). This thesis focuses on an integrative approach, utilizing both laboratory measurements and numerical modelling, to understand physical properties and petrofabric development in rocks originating in Earth’s crust. The physical properties of rocks are affected by both intrinsic sources (e.g., inherent to crystals) and extrinsic sources (e.g., layering, microcracks, shape preferred orientation of crystals, grain size, presence of geological fluids). A versatile numerical elastic wave propagation model is constructed with COMSOL Multiphysicsâ and benchmarked against a stainless-steel standard used for laboratory elastic wave measurements. The numerical model is flexible and enables setup of composite materials with different sample geometries, which is of importance when modeling the physical properties of rocks in realistic geological scenarios. Using the elastic wave propagation model, this thesis explores different scenarios and their influence on seismic properties, including the effect of grain size on bulk elastic wave speed and compositional layering on seismic anisotropy. The first application focuses on a joint laboratory and numerical study of similar composition gabbro samples, with distinctly different grain size. The numerical model is used to evaluate the relationship between wavelength and grain size. The second application utilizes laboratory measurements as input data for the models, to determine seismic properties of compositionally layered materials. It is shown that the seismic properties, and in particular anisotropy, of a layered material depends on 1) the combination of the inherent rock properties and layering and 2) the wavelength (l) to layer thickness ratio (d). Importantly, independent of scale, the physical properties are wavelength dependent, showing a decrease (apparent dispersion) in velocity, when transitioning from a ray medium (l/d < 1) to an effective medium (l/d > 10). In the second part of this thesis, another COMSOL Multiphysics modelling approach is used to investigate how crystals rotate in a magmatic flow, and how a petrofabric in different magmatic rocks may develop. A set of different magmatic flow scenarios are explored, with direct application to natural examples of dykes and magma chambers. These numerical models may serve as useful predictors of petrofabric in igneous rocks where determination of flow direction of magma is of interest.
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| 2. |
- Költringer, Chiara Amalia
(författare)
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Past environment and sediment dynamics in the Black Sea-Caspian Sea region from Southern Russian loess sequences
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Loess deposits are excellent past climate and environment archives and contain records of aeolian mineral dust accumulation. Loess deposits stretch across Eurasia and sequences along the Lower Volga in the Northern Caspian lowland in the South East European Plain represent a key link between European and Asian loess. They were deposited during phases of Caspian Sea sea-level low stands in the Late Quaternary and are confined by deposits of marine sediments from transgressive phases. These sea-level fluctuations of the Caspian Sea and their driving factors are not well resolved, and one obstacle is the lack of palaeoclimate information during phases of sea-level low stands. In contrast to the marine sediments, the continental deposits are understudied and the origin and formation of the material is debated. As a consequence, loess along the Lower Volga is often left out from palaeoclimate reconstructions in the region. This thesis aims to resolve the origin, formation and post-depositional modification of this loess and determines its aeolian origin. Provenance analyses show that Lower Volga loess particles were formed due to glacial grinding and were transported over long distances via rivers prior to near-source aeolian deposition, where the loess experienced phases of pedogenic and cryogenic reworking. While no temporal changes in material supply can be identified for Lower Volga loess, a remarkable spatial variability is seen in dust sources over the wider East European Plain and in the South Caspian Sea region. This variability is linked to material input from multiple local sources, while the larger material supply to the region has broadly similar provenance and derives mainly from areas of continental and mountain glaciation, transported via rivers prior to aeolian deflation. This thesis explores the potential of Lower Volga loess for palaeoclimate reconstructions in Eurasia, and shows a generally cold and arid climate in the Northern Caspian lowland during the Late Quaternary Caspian Sea regression, with strong dusty winds, punctuated by slightly more humid and/or warmer periods. The thesis shows that enviromagnetic methods are suitable for palaeoclimate reconstructions from such cold climate loess, as long as several methods are applied in combination and interpreted appropriately. It also provides information about the magnetic fabric of Lower Volga loess. The findings of this thesis in broader implication show that the Caspian Sea level is only secondarily influenced by local-regional climate variation and that Quaternary Northern Hemisphere glaciation is the primary controller via river discharge to the Caspian Sea.
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| 3. |
- Wiers, Steffen, 1985-
(författare)
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The Arctic Ocean Palaeomagnetic Record : A Stratigraphic Approach
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Arctic Ocean is an essential component of the global climate system, yet understanding of its geological archives is hampered by difficulties in age modelling. In addition to the scarcity of dateable microfossils, the palaeomagnetic record of Arctic sediments is unusual. Palaeomagnetic inclination sequences from Arctic sediments display numerous changes from steep positive to steep negative values that do not match the established geomagnetic polarity timescale. Independent age constraints suggest that most changes in the upper few meters below the sea-floor took place within the Brunhes normal chron. It has been suggested that zones of reversed inclination contain reversed titanomaghemite, formed by sea-floor oxidation of titanomagnetite. Until now, self-reversed components of Arctic records have not been studied in the context of regional stratigraphic frameworks, which could elucidate the relative timing of significant diagenetic changes, or their synchronicity between different records.This thesis examines marine sediments from three different areas of the Arctic. All records were evaluated within their regional stratigraphic frameworks and cross-correlated with existing records. A combined approach that used magnetic and sedimentological data was employed to identify the effect of depositional and diagenetic processes on the palaeo- and rock magnetic properties. Chemical, palaeo- and rock magnetic investigations on sediments from the Arlis Plateau and the Lomonosov Ridge revealed a complex magnetic mineralogy and constrained reversed inclinations to a medium to high coercivity magnetic phase. The important role of manganese in the Arctic Ocean, its involvement in iron (oxyhydr)oxide enrichment during interglacial periods and its role in diagenesis led to the hypotheses that an unidentified magnetic ferromanganese phase is involved in the anomalous palaeomagnetic record. Elevated pore water manganese concentration a few meters below the sea-floor in central Arctic sediments is evidence of ongoing diagenesis that involves manganese-oxides. A relationship with iron-oxides is likely and can lead to alteration of existing magnetic minerals and the precipitation of new magnetic phases. This approach revealed 1) an inconsistent alignment of zones of negative inclination when different records were correlated using lithological parameters, 2) no correlation of inclination changes with stratigraphic boundaries and 3) a link between diagenesis and the palaeomagnetic record.
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