The Arctic Ocean Palaeomagnetic Record : A Stratigraphic Approach

Sammanfattning: The Arctic Ocean is an essential component of the global climate system, yet understanding of its geological archives is hampered by difficulties in age modelling. In addition to the scarcity of dateable microfossils, the palaeomagnetic record of Arctic sediments is unusual. Palaeomagnetic inclination sequences from Arctic sediments display numerous changes from steep positive to steep negative values that do not match the established geomagnetic polarity timescale. Independent age constraints suggest that most changes in the upper few meters below the sea-floor took place within the Brunhes normal chron. It has been suggested that zones of reversed inclination contain reversed titanomaghemite, formed by sea-floor oxidation of titanomagnetite. Until now, self-reversed components of Arctic records have not been studied in the context of regional stratigraphic frameworks, which could elucidate the relative timing of significant diagenetic changes, or their synchronicity between different records.This thesis examines marine sediments from three different areas of the Arctic. All records were evaluated within their regional stratigraphic frameworks and cross-correlated with existing records. A combined approach that used magnetic and sedimentological data was employed to identify the effect of depositional and diagenetic processes on the palaeo- and rock magnetic properties. Chemical, palaeo- and rock magnetic investigations on sediments from the Arlis Plateau and the Lomonosov Ridge revealed a complex magnetic mineralogy and constrained reversed inclinations to a medium to high coercivity magnetic phase. The important role of manganese in the Arctic Ocean, its involvement in iron (oxyhydr)oxide enrichment during interglacial periods and its role in diagenesis led to the hypotheses that an unidentified magnetic ferromanganese phase is involved in the anomalous palaeomagnetic record. Elevated pore water manganese concentration a few meters below the sea-floor in central Arctic sediments is evidence of ongoing diagenesis that involves manganese-oxides. A relationship with iron-oxides is likely and can lead to alteration of existing magnetic minerals and the precipitation of new magnetic phases. This approach revealed 1) an inconsistent alignment of zones of negative inclination when different records were correlated using lithological parameters, 2) no correlation of inclination changes with stratigraphic boundaries and 3) a link between diagenesis and the palaeomagnetic record.