HR: 16:30h
AN: U42B-03    [PDF]
TI: The Long-term Strength of the Geomagnetic Field
AU: * Tarduno, J A
EM: john@earth.rochester.edu
AF: University of Rochester, Department of Earth & Environmental Sciences 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Smirnov, A V
AF: University of Rochester, Department of Earth & Environmental Sciences 227 Hutchison Hall, Rochester, NY 14627 United States
AB: The long-term strength of the geomagnetic field, an essential parameter constraining the nature and history of the geodynamo, has been defined on the basis of Thellier-Thellier paleointensity experiments using igneous whole-rock samples. However, many of the virtual dipole moments derived from the Thellier data are comparable to values that characterize geomagnetic excursions and reversal transitions. The low field stability implied by these values represents a paradox: it is in stark constrast to what is known from the history of paleosecular variation and frequency of geomagnetic reversals. Taken at face value, the paleointensity data imply the field has been extraordinarily energetic during the last 10 million years. Because factors that could change the magnetic field energy are characterized by time scales orders of magnitude longer than this apparent signal, we suggest natural processes that lead to underestimates of field values (natural and experimental alteration) are far more common than is usually supposed (resulting in a bias toward low values in paleointensity databases). Using secular variation as a guide, we conclude that the mean strength of the field since Mesozoic times was probably similar to that of the last 10 million years (7 to 8 x 10$^{22}$ Am$^{2}$), except during periods of very low (superchron) and very high (e.g. the Late Jurassic) reversal frequency. Paleointensity estimates for the latter interval are anomalously low relative to all data and, together with Thellier estimates based on analyses of single plagioclase crystals from the Cretaceous Normal Polarity Superchron, suggest that reversal rate and field intensity are inversely related. These interpretations suggest that the modern decay of the dipole should be viewed relative to the ``threshold" value of paleointensity, estimated from analyses of sediments. At this threshold value (approximately 1/2 the modern field strength), excursional field behavior can be expected.
DE: 1510 Dynamo theories
DE: 1513 Geomagnetic excursions
DE: 1521 Paleointensity
SC: U
MN: 2003 Fall Meeting