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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, B11101, doi:10.1029/2005JB003970, 2005

Dipole strength and variation of the time-averaged reversing and nonreversing geodynamo based on Thellier analyses of single plagioclase crystals

J. A. Tarduno

Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York, USA


R. D. Cottrell

Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York, USA


Abstract

Single plagioclase crystals separated from lavas can contain minute magnetic inclusions that faithfully record the geomagnetic field, while the silicate host limits natural and experimentally induced alteration. Here we use paleointensity analyses of plagioclase crystals to examine three characteristic geodynamo regimes spanning the last 200 Myr: an interval of moderate reversal frequency during the Paleogene, the nonreversing field of the Cretaceous Normal Polarity Superchron, and a period of very high reversal occurrence during the Late Jurassic. An inverse relationship between reversal rate and intensity is supported. Furthermore, the strength of the time-averaged reversing field is more variable than that of the nonreversing field. These observations are consistent with an active lower mantle, shaping core-mantle boundary heat flux and efficiency of the geodynamo on timescales of tens of millions of years.

Received 29 July 2005; accepted 16 August 2005; published 5 November 2005.

Keywords: geodynamo; paleointensity; mantle.

Index Terms: 1510 Geomagnetism and Paleomagnetism: Dynamo: theories and simulations; 1521 Geomagnetism and Paleomagnetism: Paleointensity; 1535 Geomagnetism and Paleomagnetism: Reversals: process, timescale, magnetostratigraphy.

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Citation: Tarduno, J. A., and R. D. Cottrell (2005), Dipole strength and variation of the time-averaged reversing and nonreversing geodynamo based on Thellier analyses of single plagioclase crystals, J. Geophys. Res., 110, B11101, doi:10.1029/2005JB003970.