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Laser Diffraction Size Analysis of Loess-Paleosol Sequences– Pretreatment, Calculation, Interpretation

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Laser Diffraction Size Analysis of Loess-Paleosol Sequences– Pretreatment, Calculation, Interpretation Laser diffraction size analysis of loess-paleosol sequences– pretreatment, calculation, interpretation Von der Fakultät für Georessourcen und Materialtechnik der Rheinisch -Westfälischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation vorgelegt von M.Sc. Philipp Schulte aus Lennestadt Berichter: Univ.-Prof. Dr. rer. nat. Frank Lehmkuhl Prof. Dr. rer. nat. Bernhard Diekmann Tag der mündlichen Prüfung: 18.04.2017 Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar II Zusammenfassung Die Korngrößenverteilung ist eine der wesentlichsten Eigenschaften von Korngemischen und wird daher seit Jahrhunderten in vielen technischen und wissenschaftlichen Bereichen mit unterschiedlichen Methoden analysiert. In der Industrie werden Korngrößenverteilungen von zum Beispiel Baustoffen, Farbpigmenten, Lebensmitteln oder Medikamenten bestimmt, um die Bestandteile und Eigenschaften der Produkte besser zu verstehen und die Produktionsprozesse zu optimieren (Produkt und Prozessoptimierung) sowie eine gleichbleibende Qualität zu sichern (Compliance). In den Geowissenschaften werden Korngrößenanalysen angewendet um Sedimente und Böden zu klassifizieren (Granulometrie). Die granulometrischen Eigenschaften (Form und Größe) erlauben Rückschlüsse auf die Entstehungsgeschichte der Boden- und Sedimentpakete. Die gewonnenen Informationen können als Proxy (indirekter Indikator) zur Rekonstruktion der Klima und Umweltbedingungen genutzt werden, welche zur Zeit der Entstehung geherrscht haben. Das Wissen über die känozoische und vor allem über die quartäre Umweltentwicklung hilft uns, aktuelle Prozesse und zukünftige Veränderungen besser einschätzen zu können. Korngrößenanalysen können mit diversen Methoden (Mikroskopie, Siebanalyse, Sedimentationsverfahren, Laser-Streulicht-Analyse) an unterschiedlichen Archiven (lakustrine, marine, (peri-)glaziale, äolische und kolluviale Sedimente) durchgeführt werden. Aktuell werden häufig Löss-Paläoboden-Sequenzen untersucht, da diese große Bereiche der Erdoberfläche bedecken und teilweise weit in der Zeit zurückreichen. Eine hochaufgelöste Bestimmung der Korngrößenverteilung solcher Sequenzen wurde erst durch die Entwicklung der Laser-Streulicht-Analyse ermöglicht. Obwohl dieses Verfahren deutlich zeit- und kosteneffektiver ist als alle verfügbaren Alternativen, ist es insbesondere in der Bodenkunde bis heute umstritten und wird daher nicht standartmäßig angewendet. Die vorliegende Dissertation liefert einen Beitrag zum Verständnis der Einschränkungen der Laser- Streulicht-Analyse bezüglich der Erfassung und Interpretation von Korngrößendaten und liefert Ansätze, diesen zu begegnen. Dabei liegt ein besonderer Fokus auf der Bestimmung der vertikalen Korngrößenvariabilität in Löss-Paläoboden-Sequenzen sowie die Etablierung derselben als Proxy zur Rekonstruktion vergangener Klima und Umweltbedingungen. Im Rahmen einer Literaturübersicht wurden die wichtigsten etablierten Methoden, welche zur Korngrößenbestimmung von Lösssedimenten eingesetzt werden können, zusammengefasst um im Hinblick auf ihre Eignung bewertet. Eine detaillierte Validierung der Laser-Streulicht-Analyse wurde im Rahmen der einzelnen bereits publizierten wissenschaftlichen Artikel durchgeführt. Bei der Untersuchung eines Schlüsselprofils der Gebirgsregion auf dem nordöstlichen Tibet Plateau wurde eine klassische granulometrische Analyse durchgeführt, um die Korngrößenvariabilität innerhalb dieser Löss-Paläoboden-Sequenz zu bestimmen. Basierend auf einer kombinierten Betrachtung der horizontalen (innerhalb der gemessenen Klassen einer Probe) und vertikalen (über alle gemessenen Proben) Variabilität I mittels verschiedener etablierter Parameter, konnten Aussagen über holozäne Klima und Umweltveränderungen getroffen werden. Durch die Generalisierung der horizontalen Variabilität in Form eines einzelnen Wertes (z.B. eines Korngrößenverhältnisses) werden geringe aktive Korngrößenänderungen verschleiert. Solche können als Folge leichter klimatischer Veränderungen in schwach frequentierten Korngrößenklassen auftreten und werden durch die relative Veränderungen der hoch frequentierten Klassen maskiert. Dies wurde unter anderem anhand des Vergleichs zweier Fallstudien aus Rumänien und der Ukraine nachgewiesen. Im Rahmen dieser Arbeit wurde die Heatmap als Darstellungsform für hochaufgelöste (große Anzahl an Größenklassen) Korngrößendatensätze von langen und dicht beprobten Sediment Sequenzen vorgeschlagen. Dies ist die einzige Möglichkeit um die Variabilität innerhalb von Löss- Paläoboden-Sequenzen ohne horizontalen oder vertikalen Datenverlust darzustellen. Mittels einer Versuchsreihe wurde der Einfluss der Probenvorbehandlung, insbesondere der Zugabe von Salzsäure, auf das Ergebnis der Laser-Streulicht-Analyse evaluiert. Es ist festzuhalten, dass mit zunehmender post-depositioneller Veränderung der analysierten Proben die Wirkung der einzelnen Vorbehandlungsschritte undurchsichtiger wird. Besonders die Vorbehandlung mit Salzsäure kann zu missverständlichen Interpretationen führen. Generell ist jede Vorbehandlung eine zum Teil selektive und unergründliche Modifikation der natürlichen Sedimentprobe. Daher sollte die Probenvorbehandlung nur die essentiellsten Schritte umfassen (Oxidation der Organik und Dispergierung). Nach der eigentlichen Messung wird das detektierte Laser-Streulicht-Muster mithilfe verschiedener optischer Modelle in eine Korngrößenverteilung umgerechnet. Die gängigsten Modelle sind die Fraunhofer Annäherung und das Lorenz-Mie Modell. Obwohl mit letzterem, vor allem für sehr feine Partikel (< 7,8 µm), deutlich validere Ergebnisse berechnet werden können, wird es bis heute nicht standardmäßig angewandt. Der Nachteil dieses Modells besteht darin, dass zur Berechnung der komplexe Brechungsindex, welcher die optischen Eigenschaften der in der Probe enthaltenden Minerale beschreibt, benötigt wird. Da dieser bei polymineralischen Gemischen nicht eindeutig zu bestimmen ist und eine Bestimmung extrem zeit- und kostenintensiv ist, wird in der Regel ein Näherungswert angenommen. Letztendlich liefert keines der beiden Modelle die tatsächliche Größe sehr feiner Partikel. Allerdings konnte nachgewiesen werden, dass die Differenz zweier Korngrößenverteilungen jeweils berechnet mit einem der beiden Modelle von der mineralischen Zusammensetzung der Feinkomponente Probe abhängig ist. Hiervon ausgehend, wurde im Rahmen dieser Arbeit ein neuer Korngrößenparamter, welcher vom Autor als delta grain size distribution (ΔGSD) bezeichnet wird, entwickelt, evaluiert und bereits vielfach zur Charakterisierung von Löss- Paläoboden-Sequenzen angewandt. Der Parameter reagiert sensitiv auf die post-depositionelle Bildung von Tonmineralen als Folge chemischer Verwitterung und kann als Proxy für fossile Bodenbildung und die damit assoziierten Paläoklima- und Paläoumweltbedingungen genutzt werden. II Summary Grain size distribution is one of the most important properties of particle mixtures, and, for centuries, it has been analyzed in various technical and scientific fields using different techniques. In industry, grain size distributions of, for example, building materials, color pigments, foodstuffs, or medicines are identified in order to better understand the ingredients and the properties of products, to optimize the production processes (product and process optimization), and to ensure a consistent quality (compliance). In the geosciences, grain size analysis are used to classify sediments and soils (granulometry). The granulometric properties (degree of roundness, shape, and size) allow conclusions about the genesis of the sediment and soil packages. The information obtained can be used as a proxy (indirect indicator) for the reconstruction of climate and environmental conditions which prevailed at the time of its origin. Knowledge about the Cenozoic Era, especially Quaternary period environmental development, helps us to better assess current processes and possible future changes. Grain size analysis can be conducted using various methods (microscopy, sieve analysis, sedimentation methods, and laser diffraction size analysis) at different archives (lacustrine, marine, (peri-glacial, and aeolian and colluvial sediments). Currently, loess-paleosol sequences are regularly investigated because they cover large areas of the earth’s surface and some date back far in time. Highly resolved determinations of the grain size distribution of such sequences was enabled by the development of laser diffraction size analysis. Although this technique is significantly more time- and cost-effective than all available alternatives, especially in soil sciences, it remains controversial, and was, therefore, not applied by default until recently. This study intends to disclose and defuse the constraints of the laser diffraction technique concerning grain size date acquisition, subsequent analysis, and interpretation. Especial focus is on the methodological progress of the investigation of vertical grain size variability within loess- paleosol sequences and its validation as a proxy to reconstruct past climate and environmental conditions. In the literature review, the most important and well-established methods used for grain size analysis of loess sediments were summarized and assessed with regard to their susceptibility. A detailed validation of laser diffraction size analysis was carried out in the framework of the individual previously published chapters. For the investigation
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