Exploring geomagnetic field behaviour during polarity reversals and excursions
Creators: name: Ingham, Elizabeth Mary
Publisher: name: The Australian National University
Year: 2015

description: The Earth's magnetic field, which is driven by dynamo action within the molten outer core, is constantly changing. Variations occur over a wide range of both spatial and temporal scales, and contribute to a complex overall field morphology. Despite significant advances over the last 25 years, much remains unknown regarding the behaviour and driving mechanisms of the geomagnetic field, with increased understanding requiring collection of further high-quality paleomagnetic records and development of new analytical tools. Two significant areas of study are investigated in this thesis: geomagnetic field behaviour during polarity excursions, and whether there is evidence for a link between geomagnetic field intensity and the frequency of polarity reversals. Geomagnetic excursions are short-lived deviations of the geomagnetic field from the normal range of secular variation. Records of the Mono Lake (32 ka) and Laschamp (41 ka) excursions from three marine sediment cores from Bounty Trough, New Zealand, are presented, alongside a complementary record of the Laschamp excursion from fourteen lavas flows from Mt Ruapehu, New Zealand. Recovery of records of both excursions from New Zealand fills a significant gap in the global database of excursion records, and helps to confirm that these geomagnetic excursions were global in scale. Records of the Laschamp excursion obtained in this study suggest that the excursion consisted of two phases of transitional behaviour, centred at around 43 and 41 ka. The age of the later phase agrees with the currently accepted age of the Laschamp excursion of 41.0 ka. It is proposed that the initial phase of the Laschamp excursion was non-dipole dominated, which might explain why it is not as well recorded at previously studied locations as the main, later, phase. Overall, this study provides rare excursion records from the southern hemisphere, which provide an improved view of geomagnetic field morphology during excursions. In this research, a new tool for objectively analysing paleomagnetic time series is employed to investigate the possibility of a link between reversal frequency and paleointensity. Transdimensional Markov chain Monte Carlo techniques are applied to a quality-filtered version of the global paleointensity (PINT) database for the last 202 My to model long-term paleointensity behaviour. The resulting paleointensity model confirms published conclusions that the single silicate crystal method gives significantly different results from more conventional whole rock paleointensity methods. When the much larger whole rock dataset is considered, the resultant paleointensity model is consistent with a stable paleointensity for the last 202 My within the 95% confidence interval about the mean model. Statistical tests indicate no significant correlation between reversal frequency and field intensity at the 0.05 level. However, this result is likely due to the characteristics of the PINT database rather than being a genuine, physically representative conclusion. Given the paucity of data and the general state of the PINT database, concerted efforts to increase the number of high-quality, well-dated paleointensity data are required before conclusions about any link between geomagnetic field intensity and reversal frequency can be confidently drawn.