6949 research outputs found
Sort by
Oxygen dynamics and organic carbon remineralization in the sands of high-energy beaches
No full textCoastal regions are hotspots of biological productivity and biogeochemical substance transformations, and play a key role in global element cycles. Within them, beach aquifers beneath sandy coasts, which make up about a third of the world's ice-free shorelines, function as prominent biogeochemical reactors that regulate the transport and fate of carbon, nutrients, and trace elements at the land-ocean interface. At high-energy coasts, tides and waves drive the infiltration of large volumes of seawater into the upper saline plume (USP) of the beach aquifer, introducing fresh organic carbon (OC) into the beach sands and inducing deep porewater advection. This input stimulates the activity of the sedimentary microbial communities, remineralizing OC and recycling nutrients. However the regulation of microbially mediated biogeochemical processes under variable boundary conditions, such as tides, waves, and seasonal inputs, in these systems remains poorly understood and substance transformations are not sufficiently quantified. This doctoral thesis aims to improve our mechanistic understanding of how OC remineralization and redox zonation in high-energy beach aquifers respond to variable physico-chemical boundary conditions, such as the seasonally variable inputs of oxygen (O2) and labile organic matter (OM). To achieve this, an integrated approach combining a year-long field campaign, direct rate measurements and reactive transport modelling captures both large-scale patterns and small-scale heterogeneity of OC mineralization rates and redox dynamics.
The first study examined the seasonal and spatial patterns of O2 consumption as a proxy for OC turnover in the upper sediment layers of the USP’s infiltration zone. This investigation employed novel batch incubations for a high sample throughput that spanned an entire year and provided a comprehensive dataset of aerobic respiration rates. The interpretation of the data was informed by the seasonal inputs of OM and O2, filtration efficiency of the sediment, and beach morphodynamics. The results revealed a pronounced seasonal regulation of OC remineralization, with a highly reactive surface layer that closely correlated with labile OC inputs and particulate organic matter (POM) retention. These patterns demonstrate that aerobic respiration in the USP is limited by the availability of labile OC and O2 is advected beyond the investigated upper sediment layers. The study characterized the USP of the beach aquifer as a high-throughput system for OC with rapid remineralization in a reactive retention layer, particularly in summer, but little OM storage.
Building on these findings, the second study integrated field data into a reactive transport model to simulate O2 dynamics and OC remineralization across the full depth of the USP under winter and summer conditions. The tide-resolving model accounted for periodic tidal desaturation of the surface layer and indicated that atmospheric O2 inputs during desaturation contributes significantly to total O2 supply. In contrast, O2 from infiltrating seawater largely bypassed the upper reactive layer, establishing an oxycline at depths over ten meters, depending on the season. The aeration mechanism dampened seasonal variability in deeper sediment layers while maintaining high aerobic OC remineralization rates in shallow sediments. The study provided a mechanistic understanding how variable boundary conditions and OM inputs as well as the desaturated layer within the infiltration zone determine the aerobic OC turnover and drive significant changes in redox conditions in the USP of beach aquifers.
The third study shifted focus from OC mineralization in the bulk sediment to the microscale variability of aerobic respiration. Using steady-state and transient flow-through reactor experiments with sandy sediments from the upper POM retention layer, it showed that small-scale heterogeneity in reactivity fosters localized anoxic zones within otherwise oxic sediments. This enables the spatial coexistence of aerobic and anaerobic processes and highlights the role microscale biogeochemical heterogeneity in creating microbial niches for anaerobic metabolism under bulk oxic conditions.
The fourth study expanded the scope of the thesis to include trace metal cycling in the USP, focusing on the redox-sensitive mobilization of elements such as cobalt. Experimental flow-through reactor incubations complemented field observations and geochemical analyses, contributing to a mechanistic understanding of trace metal behavior in sandy beach sediments. The fifth study presents experimental rate measurements as part of the DynaDeep Observatory, which provides the conceptual and infrastructural framework for this thesis.
Together, these studies identify the USP as a high-throughput, yet carbon-limited system, where biogeochemical functioning is strongly influenced by seasonal variability, POC retention and tide-induced desaturation at the upper infiltration boundary. By combining field observations, laboratory experiments, and numerical modelling, this thesis offers an integrative framework for understanding O2 dynamics and OC remineralization in intertidal permeable beach sediments. It emphasizes their role as effective biogeochemical filters at the land–ocean interface and highlights the importance of continued research on these systems
AI Pioneers Final Conference: AI and the Future of Education
No full textAI Pioneers Final Conference: Abstracts of keynote speech, presentations and workshops
The Health Care System in Mexico
No full textThis country report provides a description of the emergence of a health care system under public responsibility in Mexico. The inception of the health care system refers to the first legislation stipulating entitlements to medical care. The report also includes a brief description of major health care reforms, and the current organization of the health care system in Mexico. This report is part of the CRC 1342 Social Policy Country Briefs Series.Deutsche Forschungsgemeinschaft (DFG)5
AI ethics unwrapped: understanding and shaping ethical AI through value sensitive design
No full textThis dissertation examines the real-world effects of artificial intelligence (AI) system design and explores how AI systems can be designed to achieve ethical outcomes. It focuses on the social, technical, and institutional dimensions of design, grounded in the understanding that AI is not merely a computational artifact but a sociotechnical system. As AI systems increasingly shape decision-making in high-stakes domains such as healthcare and hiring, concerns around fairness, transparency, and inclusion move to the forefront. Instead of retrofitting alignment with these values, this dissertation explores how Value Sensitive Design can support the proactive integration of ethical principles throughout the AI lifecycle, while also identifying where current approaches fall short.
The work is structured around five research questions (RQs), divided into two parts. Part 1 is based on two publications and focuses on understanding the effects of AI design, specifically on decision-making and inclusion. For addressing RQ1: How do design choices in algorithmic decision-making systems influence transparency and explainability of AI systems?, formal modeling is used to simulate ethical trade-offs in healthcare. The findings demonstrate that by making the decision architecture of AI systems explicit, we can generate measurable ethical impacts that can support us in designing systems that align with our values. RQ2: How does the design of AI-systems affect persons with disabilities, and what ethical, legal, and technical shortcomings contribute to these outcomes?, is explored through an interdisciplinary analysis combining ethics, legal evaluation, and technical assessments. The study reveals that current AI-based hiring tools often neglect the needs of persons with disabilities due to their design, legislation, and technical functionality. The findings point to the necessity of inclusive design practices that extend beyond narrow bias mitigation.
Part 2 is also based on two publications and one additional paper currently under review for publication, and turns to the conditions that shape ethical AI design, structured in three RQs. In response to RQ3: How do emergency contexts influence the development, ethical evaluation, and perceived trustworthiness of AI systems?, the dissertation presents a focus group-based case study on an AI-assisted diagnostic system. The study shows how urgency, uncertainty, and resource scarcity shaped both the technical design and its ethical framing, revealing how perceived trustworthiness of an AI system is co-constructed by contextual and organizational factors. RQ4: How do AI developers discuss and apply ethical principles in the process of designing prototypes of AI-based systems? is examined using the qualitative methods of design thinking workshops and interviews. The results demonstrate that developers engage with ethical values, but in context-specific ways that are influenced by individual motivations, and the best interests of users. Finally, RQ5: What is the current state of applying VSD to AI as reported in research studies, and which approaches have been proposed in the literature to address challenges for this application? is addressed through a systematic literature review of VSD applications to AI. The analysis reveals a lack of consistency in stakeholder identification, ambiguity in value translation processes, and limited documentation of iterative practices. While VSD provides a flexible framework for integrating ethics into AI development, its application can and must be refined. The dissertation proposes such refinements, including structured stakeholder identification, context-sensitive value translation, and iterative design.
These contributions advance our understanding of how design decisions of AI systems impact individuals and social groups, and how design decisions of AI systems are influenced by context, humans, and methodologies. This dissertation demonstrates that ethical impact is never incidental but embedded in AI architectures, organizational routines, and developer practices. By demonstrating this, it also shows that ethical AI development requires not only methodological rigor but also institutional support and awareness for the importance of design. Future research can build on these findings by defining ethical maintenance of AI systems post-deployment, refining ethical design methodologies such as VSD, and establishing social practices for AI developers
Light and strain responsive polymeric materials
No full textStimuli-responsive polymers have garnered significant attention for their ability to undergo reversible changes in response to external cues. This thesis contributes to the development of smart materials by the covalent incorporation of molecular switches into two categories of polymers: linear elastomers and crosslinked thermoresponsive polymers, known as hydrogels.
In thermoresponsive hydrogels, two types of azobenzene derivatives were co-polymerized to add light as a remote stimulus for controlling the phase transition behaviour. The first type is a hydrophobic azobenzene which can modulate the phase transition temperature by 6 °C and induce light-driven hydrophilicity in the network. This hydrogel formulation was further used to develop precise photo-actuating micropillars for cell sorting applications. The second type, an amphiphilic azobenzene resulted in broadening of the phase transition temperature enabling super swelling and super deswelling in the hydrogels. Through a combination of synthesis, characterization, and functional analyses, the work compiled in this thesis advances the understanding of photo-, mechano- and thermos-sensitive polymeric systems (linear and crosslinked) and paves the way for their future applications in responsive and adaptive materials.
This thesis focuses on processing spiropyran functionalized elastomers into mechanchromic microfibers. The strategy of polymer blending is used to stabilize the micro-fibers, improving their durability, functionality, and reliability for advanced applications. Mechanochromic polymers exhibit color changes in response to mechanical stimuli, offering potential applications in stress sensing and smart materials. Additionally, the covalent integration of diazocine units into polymers via an atom transfer radical polymerization (ATRP) led to photochromic elastomers. The significant changes in the π-systems, resulting in color and size changes in the respective molecular switches, were utilized for developing mechano- and photo-sensitive smart materials
Freundschaftsdynamiken junger Menschen im Kontext ethnisch kultureller Diversität
No full textDie Dissertation untersucht, wie junge Menschen in einer ethnisch-kulturell diversen Gesellschaft Freundschaften bilden, gestalten und aufrechterhalten. Im Zentrum steht die Frage, warum Intergruppenfreundschaften – trotz zunehmender Vielfalt – seltener entstehen und weniger stabil sind als gleichethnische Beziehungen. Theoretisch verbindet die Arbeit Opportunitätsstrukturen, individuelle Präferenzen und sozialen Einfluss mit dem Konzept ethnischer Grenzziehungen. Diese Grenzen wirken als zusätzliche „Kosten“, die die Erfolgserwartung interethnischer Beziehungen senken und deren Entstehung erschweren. Empirisch zeigt die Dissertation anhand von drei Studien, dass gruppenbezogene Identifikation die Freundschaftsbildung junger Menschen stark prägt, während multikulturalistische Einstellungen sich zwar über sozialen Einfluss verbreiten, aber weniger relevant für die gezielte Auswahl von Freundschaften sind. Zudem weisen Intergruppenfreundschaften eine geringere Stabilität auf, was maßgeblich durch weniger außerschulischen Kontakt und geringere elterliche Unterstützung vermittelt wird. Auch bei jungen Erwachsenen zeigt sich, dass interethnische Freundschaften sowohl von individuellen Merkmalen – etwa Offenheit oder Mehrsprachigkeit – als auch von Kontexten wie Wohnvierteln, Freizeitorten oder urbanen Räumen beeinflusst werden. Insgesamt macht die Dissertation deutlich, dass ethnische Grenzen trotz wachsender Diversität fortbestehen, jedoch unter günstigen Bedingungen – etwa in heterogenen Sozialräumen oder durch offene Einstellungsmuster – an Relevanz verlieren können. Die Befunde betonen die Bedeutung intergenerationeller und multikontextueller Perspektiven, um Intergruppenfreundschaften nachhaltig zu fördern
Projektbericht Verbundvorhaben „WärmewendeNordwest“: Potenziale für eine stadtweite und quartiersbezogene Transformation der Wärmeversorgung sowie Bildungsformate für eine nachhaltige Entwicklung
No full textDieser Bericht fasst die abgeschlossenen Meilensteine des Forschungsfelds 5.2 „Potenziale für eine stadtweite und quartiersbezogene Transformation der Wärmeversorgung“ im Rahmen des Projekts Wärmewende Nordwest zusammen. Aufbauend auf einer strukturierten Methodik zur Ermittlung von Eignungsbereichen für erneuerbare Wärmequellen (Solarthermie, Abwasser, Geothermie) wurden die Bremer Neustadt und das Quartier Neu-Schwachhausen als Fallbeispiele analysiert. Die Integration von GIS-basierten Werkzeugen, Wärmebedarfsdaten und Sinus-Milieu-Informationen ermöglicht eine differenzierte, zielgruppenorientierte Planung. Die Ergebnisse wurden in die Querschnittsaktivität 2 (QA2) transferiert und bereits in Lehre, Forschung und Weiterbildung genutzt. Die Arbeit liefert eine nachvollziehbare, übertragbare Grundlage für die klimaneutrale Transformation städtischer Wärmeversorgung sowie Ausblicke auf weiteren Forschungsbedarf
Stadtgefühle: Ausstellungen studentischer Kunstprojekte zum Thema „Emotionen und Raum“
No full text-2-202
Mass spectrometric characterisation and chemometric evaluation of dissolved organic matter
No full textThe oceans contain the largest active carbon reservoir on earth. Dissolved organic matter (DOM) is one of the most complex organic mixtures on earth and thus challenges analytical and statistical tools. Changes in the molecular composition reflect biogeochemical gradients and transformation processes. The aim of this thesis is to improve current chemometric approaches to characterise the DOM composition to enable a deeper understanding of organic matter flux. The thesis investigated mixing of DOM constituents in two Arctic fjords along a gradient of glacier-derived meltwater to sea water by mass spectrometry and chemometrics. Chemometric methods identify differences in composition, therefore the choice of the multivariate method is not trivial. A variety of chemometric approaches have been used in the DOM community. Here, they were systematic evaluated to improve the statistical analysis of chemodiversity. Lastly the thesis asks if the relation of chromatography to physico-chemical properties of known structures can constrain the functional feature traits of DOM
Härtungsverhalten von Klebstoffen im Offshore-Bereich für die Anwendung auf maritimen Beschichtungen
No full textDie Offshore-Infrastruktur bietet Potenzial für den Ausbau von Offshore-Windenergie und trägt damit maßgeblich zur Energiewende bei. Nachrüstungen oder Montagen erfordern kontinuierlich die Entwicklung und Validierung neuer Technologien. Aufgrund dieser hohen Varianz im Anlagendesign, immer größer werdenden Bauwerke und dem fortschreitenden Stand der Technik, werden häufig Sekundärstrukturen im Feld nachgerüstet. Durch das Kleben von Halterungen werden nachteilige Effekte wie Bauteilverzug, Gefügeänderungen und Schäden am Stahl oder an der Korrosionsschutzbeschichtung der Primärstruktur vermieden. Die Klebtechnik ermöglicht einen Verbund mit dem Baustahl und der Beschichtung. Für den Nachweis wurde zunächst das thermische und mechanische Verhalten von Klebstoffen, sowohl unter atmosphärischen Umgebungsbedingungen als auch in künstlichem Meerwasser, detailliert untersucht. Der Vorteil des Klebstoff-Benchmarks bestand darin, dass eine Vielzahl an Klebstoffen systematisch auf eine geringe Anzahl reduziert wurde. Dabei wurden die leistungsfähigsten und geeignetsten Optionen für die spezifischen Anforderungen identifiziert. Ein weiterer Schwerpunkt lag auf dem Nachweis der Haftfestigkeit der Klebung nach dem Injektionsprozess der Halterungen unter Alterungsbedingungen. Hierbei wurden sowohl Freibewitterungen als auch entwickelte Laboralterungsszenarien im Nordseelabor erfolgreich durchgeführt. Die Beständigkeit der geklebten Halterungen wurde nachgewiesen. Teils niedrige Temperaturen erschweren das Härtungsverhalten von Klebstoffen im Offshore-Bereich. Eine Etablierung der induktiven Erwärmung der Klebung ermöglichte eine gezielte und reproduzierbare Härtung der Klebstoffe. Dafür wurden spezialisierte Induktoren und Induktionsprogramme entwickelt, angewendet und validiert. Die Induktoren sind speziell für den Einsatz im Offshore-Bereich konzipiert. Die Qualität der Klebungen wurde signifikant gesteigert