Fast directional changes during geomagnetic transitions: Global reversals or local fluctuations?

Abstract

Paleomagnetic investigations from sediments in Central and Southern Italy found directional changes of the order of 10◦ per year during the last geomagnetic field reversal (which took place about 780,000 years ago). These values are orders of magnitudes larger than what is expected from the estimated millennial timescales for geomagnetic field reversals. It is yet unclear whether these extreme changes define the timescale of global dipolar change or whether they indicate a rapid, but spatially localised feature that is not indicative of global variations. Here, we address this issue by calculating the minimum amount of kinetic energy that flows at the top of the core required to instantaneously reproduce these two scenarios. We found that optimised flow structures compatible with the global-scale interpretation of directional change require about one order of magnitude more energy than those that reproduce local change. In particular, we found that the most recently reported directional variations from the Sulmona Basin, in Central Italy, can be reproduced by a core-surface flow with rms values comparable to, or significantly lower than, present-day estimates of about 8 to 22 km/y. Conversely, interpreting the observations as global changes requires rms flow values in excess of 77 km/y, with pointwise maximal velocities of 127 km/y, which we deem improbable. We therefore concluded that the extreme variations reported for the Sulmona Basin were likely caused by a local, transient feature during a longer transition.