Understanding chemical, biogenic, and detrital magnetic signatures in sediments

Roud, Sophie (2022): Understanding chemical, biogenic, and detrital magnetic signatures in sediments. Dissertation, LMU München: Faculty of Geosciences

Abstract

Much of Earth’s history is recorded by sediments and sedimentary rocks. Paleo- and environmental magnetism help to explore these geologic records by providing paleoenvironmental proxies, stratigraphic tie-points, information about continental plate motion and about geomagnetic field variations through time. A major challenge in paleo- and environmental magnetism is to distinguish depositional from post-depositional signatures. This thesis explores both types of magnetic signatures in natural sediment archives and provides fundamental research aiming to elicit bio- and geochemical processes that can modify the magnetic mineral inventory and overprint paleomagnetic records. A magnetostratigraphic study of fluvio-lacustrine sediments from Central Asia provides a chronology for the Mio-Pleistocene syn-tectonic deposits of the Issyk-Kul Basin. From these age constraints we determined the onset of mountain building and the timing of tectonic events, which transformed the area into a closed basin that now hosts one of the deepest mountain lakes on Earth. In a study of modern surface sediments, we investigated magnetotactic bacteria (MTB), which are ubiquitous in benthic environments worldwide and biosynthesize intracellular magnetic minerals. When preserved in the rock-record these magnetofossils represent ideal recorders of the Earth’s magnetic field and their concentration has been linked to paleoclimatic variations. Understanding what controls MTB abundance remains, however, limited. By monitoring spatiotemporal changes in the population of MTB in a freshwater pond, we inferred coeval changes of the population size among nearby sites that were largely independent of season, temperature, and bottom water oxygen concentration. Variations of the magnetofossil concentration are treated in a separate study on marine sediments from the western Tropical Atlantic. Systematic glacial-interglacial variations were linked to changes in palaeoceanographic conditions at the site, which supports that magnetofossils represent sensitive paleoclimatic biomarkers. Finally, post-depositional chemical remagnetization is addressed through novel laboratory experiments that recreate this process under controlled conditions. Greigite (Fe3S4), a common authigenic magnetic mineral, was synthesized in artificial sediments under controlled magnetic field conditions. The formation pathway and magnetic properties of greigite are characterized, and the evolution of chemical remanent magnetization (CRM) monitored via real-time, in-situ magnetic measurements. Redeposition of the greigite-bearing sediments lead to depositional magnetizations that were 5-6 times weaker than the CRMs recorded by the same grains under the same magnetic field conditions. The results are consistent with theoretical models of the different magnetic recording mechanism and show for the first time how magnetic mineral growth in sediments can bias paleomagnetic reconstructions of Earth’s magnetic field intensity.

• Item Type: Theses (Dissertation, LMU Munich)
• Subjects: 500 Natural sciences and mathematics; 500 Natural sciences and mathematics > 550 Earth sciences
• Faculties: Faculty of Geosciences
• Language: English
• Date of oral examination: 28. January 2022
• 1. Referee: Gilder, Stuart
• Signature of the printed copy: 0001/UMC 29342