Zabel, Matthias
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Zabel, Matthias
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Zabel, Matthias
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Item type:Publication, Holocene climate variability and its feedbacks to hydrology and vegetation dynamics in South Africa: Insights from compound-specific stable carbon (d13C) and hydrogen (dD) isotopes(2017-05-05); ; ; This thesis aims to advance the understanding of Holocene climate changes and their environmental consequences in South Africa using plant wax derived n-alkanes and their compound-specific stable carbon and hydrogen isotopes (d13Cwax, dDwax). The first part deals with a source-to-sink approach of terrestrial organic material (TOM) deposited in the mudbelt offshore western South Africa. It is shown that TOM in the northern mudbelt is mainly delivered by the Orange River and contains an integrated homogenous catchment signal. The second part deals with the applicability of dDwax as a hydrological recorder using soil samples from across South Africa. It is shown that dDwax suits well as qualitative hydrological proxy in the summer rainfall zone (SRZ) but not in the winter rainfall zone due to more complex environmental conditions. The third part deals with the Holocene hydrological and vegetation reconstruction in South Africa using a marine sediment core. The investigation reveals that towards the middle Holocene increasingly C4 plants abundance and wetter conditions occurred in the SRZ. During the last 4,000 years the SRZ experienced a gradual drying.doctoral thesis331 160 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Terrigenous climate signals in deep-sea sediments: Insights from the inorganic and organic geochemistry(2011-12-20); ; ; Terrigenous material is transported to the marine environment in large quantities, mainly by fluvial and eolian transport. Its deposition within the marine sediments provides an archive of Earth s history, past climate conditions, source regions, transport processes, and their variations in time and space. The main objective of this thesis was the documentation of the transport of terrigenous material into the marine environment and the identification of terrigenous material (especially soil organic carbon) in sediments of continental margin systems. These investigations were based on the analysis of the chemical composition as well as specific organic compounds (biomarkers). This thesis contributes to the understanding of the chemical composition of Saharan/Sahelian dust which is deposited in the Atlantic Ocean off NW Africa. It also provides insights into the deposition of terrigenous material on two large deep sea fans in the equatorial Atlantic Ocean during the last ~35 ka.doctoral thesis449 160 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Geochemical conditions in continental margin sediments: implications for distribution and cycling of phosphorus(2009-12-22); ; ; This thesis investigated the sedimentary phosphorus cycle in different upper continental slope and shelf surface sediments. In this thesis a combined multi-parameter and geochemical approach has been used to improve the speciation of the phosphorus reservoir in selected continental margin surface sediments. In particular the determination of pore water constituents has the potential to examine sediment redox processes associated to organic matter degradation and their impact on phosphorus species distribution in different marine environments. Linked to that, the early diagenetic regeneration of organically bound phosphorus and reductive dissolution of iron-phosphate phases have been identified as important pathways for the release of phosphate in the investigated sediments. Evidence that reducible iron-phosphate minerals are the major reactive pool for phosphorus transformation has been provided by sequential extraction results. Moreover, the variety of phosphorus-containing species that contributed to the total sum of solid-phase phosphorus reservoir has been identified. A model has been applied, which successfully describe and quantify the processes that are responsible for sedimentary phosphorus cycling in Namibian sediments. As well as deduced from conservative mass balance calculations, the presented results revealed that the content in reactive iron phases is not sufficient to produce the quantified release rates of phosphate. Most important in terms of coupling of benthic phosphorus cycling with redox-sensitive iron phases is the influence of non-local particle transport. Such additional transport process has to be considered as determining factor in balancing the sedimentary phosphorus budget.doctoral thesis565 232 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Isotope insights into the phosphorus cycle of marinesediments(2009-10-26); ; ; The aim of the present dissertation was to investigate the microbial contribution to biogeochemical phosphorus (P) cycling in marine sediments. This essential nutrient controls marine primary production on geologic and recent time scales. Regeneration of phosphate (Pi) from organic matter and sequestration in phosphorites are major processes in the ocean's P cycle, in which the benthic microbial community has decisive influence on the source-sink balance. Two isotopic methods elucidated these biogeochemical transformations of Pi in detail and allowed new insights that extended the scope of investigations of Pi concentrations and pools alone.Oxygen isotopes of dissolved Pi (delta-18OP) have proven a suitable biosignature for microbial Pi regeneration and cycling. We refined and validated a micro extraction protocol for the separation and mass spectrometry of ultra low Pi samples (< 1 micromoles) of marine pore waters. We obtained a novel dataset on delta-18OP from two sediment cores off Morocco. With increasing depth, isotopic equilibration of Pi with water indicated thorough microbial Pi turnover, while disequilibria at the top pointed to Pi release from organic matter.In a comprehensive study on benthic Pi cycling in the Benguela upwelling system, we combined pore water inventories and modeling with the investigation of delta-18OP, in order to detect the microbial control on Pi regeneration and the influence of different sedimentary settings of mineralization and Pi availability. Pattern of delta-18OP in the sediments were more complex than expected from geochemical evidence. Isotope disequilibria were preserved at low Pi deep-sea sites, and thorough equilibria at sites with high mineralization and Pi concentration. Bottom water data pointed to a discontinuity between water column and pore water Pi. An isotope mass balance model, considering enzyme and substrate systems, allowed us to estimate the Pi recycling efficiency of the benthic community. We believe that different microbial Pi uptake strategies, controlled by ambient Pi availability, strongly influence the preservation of pore water delta-18OP.We finally sought to elucidate the role of microorganisms in authigenic phosphorite formation, and applied a radioisotope tracer (33P) to track sedimentary Pi transfers in incubation experiments. Our results constitute the first direct evidence that the presence of live bacteria is pivotal to phosphorite precipitation. Sequestration of Pi outbalanced regeneration under anoxic conditions, and Pi uptake by Thiomargarita namibiensis strongly suggested that these large sulfur bacteria provide a transient Pi reservoir. The unique physiology of Pi uptake and release in these bacteria may antagonize the positive feedback between enhanced Pi regeneration, stimulated primary production, and anoxia in bottom waters of upwelling regions.doctoral thesis679 279 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Lateral particle transport in nepheloid layers - a key factor for organic matter distribution and quality in the Benguela high-productivity area.(2005-10-14); ; ; This thesis aims to identify and estimate the significance of lateral transport of particulate organic matter in the deep water-column of the Benguela upwelling system (BUS) offshore southwestern Africa. Although the BUS is regarded as the most productive of the four major eastern boundary systems of the ocean, and therefore of high importance for the global marine carbon cycle, little information is available on processes and fluxes involved in particulate matter transport in this area. Above the continental slope, water layers strongly enriched in particle content are identified in intermediate water depths and in the lowermost water column. These layers are termed intermediate and bottom nepheloid layers (INLs and BNLs, respectively). In such layers, the major part of the advective particle transport is supposed to take place. A pronounced BNL covers the entire study area with maximum intensity above the outer shelf and at the shelf break. The detachment of this BNL at the shelf break feeds a major INL in 250 to 400 m water depth at 25.5Ã °S, positioned at the lower boundary of the oxygen minimum zone. Together, these strong subsurface nepheloid layers are indicators of intensive lateral particle transport from the outer shelf towards a depocenter of organic matter (OM) on the upper continental slope. The results on distribution, transport and quality of organic matter presented in this thesis confirm the major significance of lateral transport of pre-aged, but well-preserved organic particles in the highly-productive Benguela upwelling area. Many indications suggest that the results from this study are also applicable to other continental margin settings. For example, high 14C-ages of SPM and the deposition of OC-rich sediments with high hydrocarbon potential on the upper continental slope is also known from other high-productive areas of the modern ocean. Thus, the results may improve our understanding of the genesis of black shales and petroleum source rocks in the geological record, to which these depocenters may represent a modern analogue. Additionally, it is shown that advective processes have the potential to displace areas of enhanced OC burial effectively from maximum production cells along the coast towards the slope and possibly the deep-sea. These results limit the validity of often used, merely vertically oriented particle flux models. The findings from the BUS identify lateral transport as an important, yet hardly recognized, secondary mechanism, effectively transferring carbon from the atmosphere to long-term sequestration in the sediments. Therewith, the results contribute to the improvement of global models to predict the impact of environmental changes on the oceanic system.doctoral thesis855 466