The Paradox of Low Field Values and the Long-Term History of the Geodynamo

Abstract

The long-term strength of the geomagnetic field has been defined on the basis of Thellier-Thellier paleointensity experiments using igneous whole-rock samples and submarine basaltic glass. Yet many of the virtual dipole moments derived from these analyses 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 contrast to what is known from the history of paleosecular variation and the frequency of geomagnetic reversals. Taken at face value, the paleointensity data imply the field has been extraordinarily energetic during the last 10 million years. But factors that could change the magnetic field energy are characterized by time scales orders of magnitude greater than those needed to account for this apparent signal. We suggest instead that processes that lead to field underestimates (natural and experimental alteration) are far more common than is usually supposed. In particular, we focus here on the roles of low-temperature oxidation (maghemitization) and hydrothermal alteration. 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 available data and, together with Thellier paleointensity estimates based on analyses of single plagioclase crystals, suggest that reversal rate and field intensity are inversely related.