HR: 14:40h
AN: GP32A-05    [PDF]
TI: Was the Ancient Geomagnetic Field Dipolar?
AU: * Tauxe, L
EM: ltauxe@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0220 United States
AU: Kent, D V
EM: dvk@ldeo.columbia.edu
AF: Lamon-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Kent, D V
EM: dvk@ldeo.columbia.edu
AF: Department of Geological Sciences, Rutgers University, Piscataway, NJ 08854 United States
AB: One of the most useful assumptions in paleomagnetism is that the time-averaged geomagnetic field is closely approximated by a geocentric axial dipole (GAD). This has been found to be true for at least the last 5 million years with the largest non-GAD contribution to the spherical harmonic expansion generally being of the order of 5%. For the more ancient past, it is difficult to test the GAD (or any other field) hypothesis owing to plate motions, rock deformation and accumulating problems of overprinting. Although most paleomagnetic studies make the implicit assumption of a GAD field, several recent studies have called the essential GAD nature of the ancient field into question and have suggested large (up to 20%) contributions of the axial octupolar term to the geocentric axial dipole in the spherical harmonic expansion even in the Cenozoic. In this paper, we develop a new statistical model for the geomagnetic field to diagnose directional dispersion resulting from sedimentary inclination error, a widespread process that plausibly explains many of the observed discrepancies from the GAD field hypothesis. We also present a methodology to correct the resulting persistent shallow bias. Application of this technique to one of the few published studies from the Cenozoic of Asia with adequate data shows that the reported discrepancies from a GAD field in this region are most probably due to sedimentary inclination error rather than a non-GAD field geometry or undetected crustal shortening. Although non-GAD fields cannot in general be strongly rejected (actually, only GAD is a well posed and testable, i.e., refutable, hypothesis), the principle of least astonishment requires us to consider plausible geological mechanisms such as sedimentary inclination error as the cause of persistent shallow bias prior to the very ``expensive" option of throwing out the GAD hypothesis.
DE: 1522 Paleomagnetic secular variation
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1532 Reference fields (regional, global)
DE: 1599 General or miscellaneous
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting