Abstract
Magnetic iron oxides and iron sulfides are present as trace constituents in sediments. These magnetic particles can carry a natural remanent magnetisation (NRM). When the sediment consolidates and lithifies after deposition, the direction of the then ambient field is preserved. The amount of magnetic material is often related to climate.
... read more
Moreover, the original magnetic material can be altered by diagenetic processes which are often linked to climate or the sedimentary environment. The NRM therefore contains information on the Earth’s magnetic field and on climate or sedimentary environment in the geologic past. The geomagnetic and the climatic signal, however, are often entangled. These signals need to be separated to be able to deduce information on either the geomagnetic field, or the climate and environment. For a correct interpretation of the NRM, its origin has to be known in detail, especially for variations on short geologic time scales of several hundreds to thousands of years. The aim of this thesis is twofold: first, we try to understand the acquisition of the NRM in various sedimentary environments, including the processes which might affect the magnetic signal (long) after deposition. Second, when the primary origin of the magnetic signal has been confirmed, the variations of the Earth’s magnetic field during the geologic past are studied.
A new rock-magnetic method has been developed which analyses the magnetic mineral content of sediments and rocks. The magnetic minerals carry (part of) the NRM and therefore might shed a light on the origin of the NRM. This method is applied to a number of marine and continental sections to identify the magnetic remanence carriers and their contributions to the NRM. First, an Eastern Mediterranean organic rich layer, a so-called sapropel, is studied. The magnetic content is shown to be a combination of dust magnetite and hematite. Exclusively in the oxidised part of the sapropel, the presence of an additional magnetite fraction was demonstrated with very specific rock-magnetic characteristics - presumably magnetosomes. For a late Miocene section, consisting of red beds from a distal alluvial fan, the new method indicated that a recorded subchron (which was not identified before) is real and not a sedimentary artefact. In the Messinian alluvial/palustrine section of Librilla (SE Spain) the method demonstrated the presence of goethite in the alluvial lithology. The regular alternation of alluvial and palustrine lithologies could be linked to precession and therefore to climate.
In the second part of the thesis, the variations of the geomagnetic field are investigated. The secular variation during the Permo-Carboniferous Reversed Superchron, as recorded by Permian red beds, appeared to be non-suppressed. This observation supports geodynamo models which regard a Superchron as a high-energy state of the geodynamo, rather than a low-energy state. The paleointensity of the last 276 kyr is studied in a marine core from near the Azores. The conventional normalisation paleointensity method is compared to the recently developed pseudo-Thellier method. Moreover, the climate influence on this core is investigated using fuzzy c-means analysis on rock-magnetic and geochemical parameters.
show less
