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Title: The Matuyama-Brunhes geomagnetic field reversal: a global and temporal perspective
Author: Brown, Maxwell Christopher
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2008
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Understanding the global and temporal evolution of the geomagnetic field during reversals can provide important constraints on conditions at the core-mantle boundary and on geodynamo processes. However, the behaviour of the geomagnetic field during reversals is unclear. Many palaeomagnetic· studies from both volcanic and sedimentary sequences have aimed to uncover details of the reversal processes; however, a paucity of data has made robust conclusions about reversal behaviour difficult. The model CALS7K.2 has been used to explore the possible influence of the timevarying non-dipole components of the geomagnetic field during field reversals. Results generated by the model suggest that non-dipole components could add significant structure to the field during the reversal processes. Globally, the main polarity reversal is variable in duration and rapid reversals on sub-decadal time-scales are seen for a small number of locations. The model generates variable reversal paths; however, there is a longitudina} preference for certain longitudinal paths, both spatial~y and, more weakly, temporally. Directional reversal features are not globally synchronous: some polarity changes finish before they start elsewhere; however, variations in intensity appear more coherent. The Matuyama-Brunhes reversal is the most recent geomagnetic field reversal ('''780 ka) and is recorded in numerous sedimentary and volcanic sequences. It provides our best chance to characterise a reversal globally. Volcanic sequences from La Palma and Guadeloupe that record the Matuyama-Brunhes boundary, possible precursors, and fluctuations in the magnetic field prior to the main polarity reversal, have been investigated. Absolute palaeointensity was determined using the microwave technique and palaeodirectional results were obtained from thermal demagnetisation. Ten flows from the twenty-nine flows studied gave reliable mean pafaeointensities; however, only one of the ten flows records a transitional direction. The VDM determined from this flow is 1.9 ± 0.6 (±IT) x 1022 Am2 . Analysis of all Matuyama-Brunhes data suggests that the main directional changes occurred once the VDM was reduced below 3 x 1022 Am2 ('''40% of the present field value). Further comparisons with palaeointensity data compiled from all reversals younger than 20 Ma supports this conclusion; however, the lack of transitional data does not allow us to make any truly robust statements about the transitional field behaviour.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available