HR: 08:45h
AN: GP41B-04 INVITED     [Abstracts]
TI: The Anatomy of a Geomagnetic Excursion: The Iceland Basin Excursion (188 ka)
AU: * Roberts, A P
EM: arob@soc.soton.ac.uk
AF: Southampton Oceanography Centre, European Way, Southampton, SO143ZH United Kingdom
AU: Kissel, C
EM: kissel@lsce.cnrs-gif.fr
AF: Laboratoire des Sciences du Climat et de l'Environnement, Campus du CNRS, Avenue de la Terrasse,, Gif-sur-Yvette, 91198 France
AU: Winklhofer, M
AF: University of Munich, Department of Earth and Environmental Sciences Theresienstrasse 41/IV, Munich, D-80333 Germany
AU: Laj, C
AF: Laboratoire des Sciences du Climat et de l'Environnement, Campus du CNRS, Avenue de la Terrasse,, Gif-sur-Yvette, 91198 France
AB: The origin of geomagnetic excursions has been debated ever since their discovery. Several testable mechanisms for excursions have been suggested: (i) if an excursion represents a significant departure of the dipole field from the rotation axis, it would be expected to have a global manifestation with similar VGP paths; (ii) if excursions occur when the non-dipole to dipole field ratio becomes large, a global manifestation would also be expected, with different VGP paths (and possibly different apparent polarity) at varying locations on the globe, and (iii) if excursions represent relatively localized perturbations in the Earth's outer core, excursions would have restricted rather than global geographic extent. Few geomagnetic excursions are represented by high-quality data from a broad geographic coverage of well-dated sites, which has prevented adequate testing of these hypotheses. The Iceland Basin excursion (IBE; 188 ka) is now an exception. It occurred at the boundary between marine oxygen isotope stages 7 and 6 and is therefore easily dated in marine sediments. It also coincides with a clear paleointensity minimum, so it is also easily identifiable in continuous relative paleointensity time series. We have compiled more than 20 records of the IBE from a broad distribution of northern hemisphere sites, including several of our own new records. The level of detail of these records is variable, but many are from relatively rapidly deposited sediments (>10 cm/kyr) that provide detailed representation of field behaviour during the excursion. The VGP paths are highly consistent and can be split into two groups. One group has VGP paths that loop counter-clockwise from high northern latitudes over Africa to high southern latitudes and that then return to high northern latitudes through Australia/east Asia. The other group has VGP paths that loop clockwise from high northern latitudes over the North Atlantic/eastern North America to South America and then to high southern latitudes and that return to high northern latitudes through the Pacific and over Kamchatka. The widespread distribution of sites that record the IBE, and the fact that many of the records have high southern latitude VGPs, indicate that the IBE is a global feature and that it does not represent a localized perturbation of fluid flow in the outer core. The fact that a single VGP path is not observed for all excursions indicates that the excursional field is not a simple dipolar field. However, the dominance of only two VGP pathways indicates that the excursional field had a relatively simple geometry. We therefore prefer the hypothesis that geomagnetic excursions occur globally when the non-dipole to dipole field ratio becomes large at times when the dipole field is weak.
DE: 1507 Core processes (8115)
DE: 1513 Geomagnetic excursions
DE: 1520 Magnetostratigraphy
DE: 1560 Time variations--secular and long term
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting