@phdthesis{Liu2019,
  author    = {Liu, Jiabo},
  title     = {Dynamics of the geomagnetic field during the last glacial},
  doi       = {10.25932/publishup-42946},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-429461},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xv, 158},
  year      = {2019},
  abstract  = {Geomagnetic paleosecular variations (PSVs) are an expression of geodynamo processes inside the Earth's liquid outer core. These paleomagnetic time series provide insights into the properties of the Earth's magnetic field, from normal behavior with a dominating dipolar geometry, over field crises, such as pronounced intensity lows and geomagnetic excursions with a distorted field geometry, to the complete reversal of the dominating dipole contribution. Particularly, long-term high-resolution and high-quality PSV time series are needed for properly reconstructing the higher frequency components in the spectrum of geomagnetic field variations and for a better understanding of the effects of smoothing during the recording of such paleomagnetic records by sedimentary archives. In this doctorate study, full vector paleomagnetic records were derived from 16 sediment cores recovered from the southeastern Black Sea. Age models are based on radiocarbon dating and correlations of warming/cooling cycles monitored by high-resolution X-ray fluorescence (XRF) elementary ratios as well as ice-rafted debris (IRD) in Black Sea sediments to the sequence of 'Dansgaard-Oeschger' (DO) events defined from Greenland ice core oxygen isotope stratigraphy. In order to identify the carriers of magnetization in Black Sea sediments, core MSM33-55-1 recovered from the southeast Black Sea was subjected to detailed rock magnetic and electron microscopy investigations. The younger part of core MSM33-55-1 was continuously deposited since 41 ka. Before 17.5 ka, the magnetic minerals were dominated by a mixture of greigite (Fe3S4) and titanomagnetite (Fe3-xTixO4) in samples with SIRM/κLF >10 kAm-1, or exclusively by titanomagnetite in samples with SIRM/κLF ≤10 kAm-1. It was found that greigite is generally present as crustal aggregates in locally reducing micro-environments. From 17.5 ka to 8.3 ka, the dominant magnetic mineral in this transition phase was changing from greigite (17.5 - ~10.0 ka) to probably silicate-hosted titanomagnetite (~10.0 - 8.3 ka). After 8.3 ka, the anoxic Black Sea was a favorable environment for the formation of non-magnetic pyrite (FeS2) framboids. Aiming to avoid compromising of paleomagnetic data by erroneous directions carried by greigite, paleomagnetic data from samples with SIRM/κLF >10 kAm-1, shown to contain greigite by various methods, were removed from obtained records. Consequently, full vector paleomagnetic records, comprising directional data and relative paleointensity (rPI), were derived only from samples with SIRM/κLF ≤10 kAm-1 from 16 Black Sea sediment cores. The obtained data sets were used to create a stack covering the time window between 68.9 and 14.5 ka with temporal resolution between 40 and 100 years, depending on sedimentation rates. At 64.5 ka, according to obtained results from Black Sea sediments, the second deepest minimum in relative paleointensity during the past 69 ka occurred. The field minimum during MIS 4 is associated with large declination swings beginning about 3 ka before the minimum. While a swing to 50°E is associated with steep inclinations (50-60°) according to the coring site at 42°N, the subsequent declination swing to 30°W is associated with shallow inclinations of down to 40°. Nevertheless, these large deviations from the direction of a geocentric axial dipole field (I=61°, D=0°) still can not yet be termed as 'excursional', since latitudes of corresponding VGPs only reach down to 51.5°N (120°E) and 61.5°N (75°W), respectively. However, these VGP positions at opposite sides of the globe are linked with VGP drift rates of up to 0.2° per year in between. These extreme secular variations might be the mid-latitude expression of the Norwegian-Greenland Sea excursion found at several sites much further North in Arctic marine sediments between 69°N and 81°N. At about 34.5 ka, the Mono Lake excursion is evidenced in the stacked Black Sea PSV record by both a rPI minimum and directional shifts. Associated VGPs from stacked Black Sea data migrated from Alaska, via central Asia and the Tibetan Plateau, to Greenland, performing a clockwise loop. This agrees with data recorded in the Wilson Creek Formation, USA., and Arctic sediment core PS2644-5 from the Iceland Sea, suggesting a dominant dipole field. On the other hand, the Auckland lava flows, New Zealand, the Summer Lake, USA., and Arctic sediment core from ODP Site-919 yield distinct VGPs located in the central Pacific Ocean due to a presumably non-dipole (multi-pole) field configuration. A directional anomaly at 18.5 ka, associated with pronounced swings in inclination and declination, as well as a low in rPI, is probably contemporaneous with the Hilina Pali excursion, originally reported from Hawaiian lava flows. However, virtual geomagnetic poles (VGPs) calculated from Black Sea sediments are not located at latitudes lower than 60° N, which denotes normal, though pronounced secular variations. During the postulated Hilina Pali excursion, the VGPs calculated from Black Sea data migrated clockwise only along the coasts of the Arctic Ocean from NE Canada (20.0 ka), via Alaska (18.6 ka) and NE Siberia (18.0 ka) to Svalbard (17.0 ka), then looping clockwise through the Eastern Arctic Ocean. In addition to the Mono Lake and the Norwegian-Greenland Sea excursions, the Laschamp excursion was evidenced in the Black Sea PSV record with the lowest paleointensities at about 41.6 ka and a short-term (~500 years) full reversal centered at 41 ka. These excursions are further evidenced by an abnormal PSV index, though only the Laschamp and the Mono Lake excursions exhibit excursional VGP positions. The stacked Black Sea paleomagnetic record was also converted into one component parallel to the direction expected from a geocentric axial dipole (GAD) and two components perpendicular to it, representing only non-GAD components of the geomagnetic field. The Laschamp and the Norwegian-Greenland Sea excursions are characterized by extremely low GAD components, while the Mono Lake excursion is marked by large non-GAD contributions. Notably, negative values of the GAD component, indicating a fully reversed geomagnetic field, are observed only during the Laschamp excursion. In summary, this doctoral thesis reconstructed high-resolution and high-fidelity PSV records from SE Black Sea sediments. The obtained record comprises three geomagnetic excursions, the Norwegian-Greenland Sea excursion, the Laschamp excursion, and the Mono Lake excursion. They are characterized by abnormal secular variations of different amplitudes centered at about 64.5 ka, 41.0 ka and 34.5 ka, respectively. In addition, the obtained PSV record from the Black Sea do not provide evidence for the postulated 'Hilina Pali excursion' at about 18.5 ka. Anyway, the obtained Black Sea paleomagnetic record, covering field fluctuations from normal secular variations, over excursions, to a short but full reversal, points to a geomagnetic field characterized by a large dynamic range in intensity and a highly variable superposition of dipole and non-dipole contributions from the geodynamo during the past 68.9 to 14.5 ka.},
  language  = {en}
}
@phdthesis{Kwiecien2008,
  author    = {Kwiecien, Olga},
  title     = {Paleoenvironmental changes in the Black Sea region during the last 26,000 years : a multi-proxy study of lacustrine sediments from the western Black Sea},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-19180},
  school      = {Universit{\"a}t Potsdam},
  year      = {2008},
  abstract  = {Paleoenvironmental records provide ample information on the Late Quaternary climatic evolution. Due to the great diversity of continental mid-latitude environments the synthetic picture of the past mid-latitudinal climate changes is, however, far from being complete. Owing to its significant size and landlocked setting the Black Sea constitutes a perfect location to study patterns and mechanisms of climate change along the continental interior of Central and Eastern Europe and Asia Minor. Presently, the southern drainage area of the Black Sea is characterized by a Mediterranean-type climate while the northern drainage is under the influence of Central and Northern European climate. During the Last Glacial a decrease in the global sea level disconnected the Black Sea from the Mediterranean Sea transforming it into a giant closed lake. At that time atmospheric precipitation and related with it river run-off were the most important factors driving sediment supply and water chemistry of the Black 'Lake'. Therefore studying properties of the Black Sea sediments provides important information on the interactions and development of the Mediterranean and Central and North European climate in the past. One significant outcome of my thesis is an improved chronostraphigraphical framework for the glacial lacustrine unit of the Black Sea sediment cores, which allowed to refine the environmental history of the Black Sea region and enabled a reliable correlation with data from other marine and terrestrial archives. Data gathered along a N-S transect presented on a common time scale revealed coherent changes in the basin and its surrounding. During the glacial, the southward-shifted Polar Front reduced moisture transport to the northern drainage of the Black Sea and let the southern drainage become dominant in freshwater and sediment supply into the basin. Changes in NW Anatolian precipitation reconstructed from the variability of the terrigenous input imply that during the glacial the regional rainfall variability was strongly influenced by Mediterranean sea surface temperatures and decreased in response to the cooling associated with the North Atlantic Heinrich Events H1 and H2. In contrast to regional precipitation changes, the hydrological properties of the Black Sea remained relatively stable under full glacial conditions. First significant modification in the freshwater/sediment sources reconstructed from changes in the sediment composition, lithology, and 18O of ostracods took place at around 16.4 cal ka BP, simultaneous to the early deglacial northward retreat of the oceanic and atmospheric polar fronts. Meltwater pulses, most probably derived from the disintegrating European ice sheets, changed the isotopic composition of the Black Sea and increased the supply from northern sediment sources. While these changes signalized a mitigation of the Northern European and Mediterranean climate, a decisive increase in local temperature was indicated only later at the transition from the Oldest Dryas to the B{\o}lling around 14.6 cal ka BP. At that time the warming of the Black Sea surface initiated massive phytoplankton blooms, which in turn, induced the precipitation of inorganic carbonates. This biologically triggered process significantly changed the water chemistry and was recorded by simultaneous shifts in the elemental composition of ostracod shells and in the isotopic composition of the inorganically-precipitated carbonates. Starting with the B/A warming and continuing through the YD cold interval and the Early Holocene warming, the Black Sea temperature signal corresponds to the precipitation and temperature changes recorded in the wider Mediterranean region. Early Holocene conditions, similar to those of the B{\o}lling/Aller{\o}d, were punctured by the marine inflow from the Mediterranean at ~ 9.3 cal ka BP, which terminated the lacustrine phase of the Black Sea and had a substantial impact on the chemical and physical properties of its water.},
  language  = {en}
}