Other
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmgp33a..03v&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #GP33A-03
Other
1521 Paleointensity, 1522 Paleomagnetic Secular Variation, 1535 Reversals: Process, Timescale, Magnetostratigraphy
Scientific paper
Volcanic and sedimentary records of reversals favour a model which involves a control of the lower mantle on the dynamo processes prevailing during the transition. This has been constrained either by the existence of preferred longitudinal bands of VGPs and/or by clusters of VGPs linked to flux patches. Recent excursion records show VGP paths which are relatively confined within the same large loop and could thus be similarly related to some persisitent flux patches at the surface of the core. Reversals and excursions have been simulated by decreasing the axial dipole field to zero while leaving up a time-varying non-dipolar field. As a first step, we investigated the relationship between the non-axial dipole field structure and the configuration of the virtual geomagnetic pole (VGP) paths using snapshots of the historical field. We found that clusters of poles do not coincide with flux concentrations and therefore do not indicate a control of the the lower mantle on the dynamo processes during reversals. The individual VGPs resulting from complete simulations of reversals show all kinds of trajectories, some being very simple with rather strong longitudinal confinment, while others are quite complex with alternances of slow and fast changes as well as loopings of VGPs. Preference for a specific longitude frequently happens in individual records, but this is not systematic. Because reversals and excursions occur during periods of very weak dipole field intensity and cannot really be distinguished in terms of duration, they could be associated with similar magnetic processes within the Earth's core leading to either successful or incomplete reversals depending on conditions that could be related to the influence of the inner core. Alternately, excursions can be regarded as secular variation events of large amplitude. We simulated field excursions using the same model but assuming either the disappearance of the dipole or a short episode of reversed dipolar field before returning to its initial polarity. The model accounts for the different characteristics inherent to the volcanic and sedimentary records of the recent excursions. The simple structure of the sedimentary VGP paths dominated by a large loop between northern and southern latitudes partly results from smearing of the signal due to the magnetization process. We have compiled the most detailed records of excursions that occurred during the Brunhes and Matuyama chrons. It is striking that almost all of them reach the opposite polarity. We show that this is impossible if these events were not associated with a short period of reversed dipole field that can be seen as a nanochron. Therefore we consider that both excursions can be seen as reversals. It remains to document and to understand what factors control the success of a new polarity state. Field intensity variations may provide some clues.
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