Strange fields: non-uniformitarian paleomagnetic records imply that the geodynamo process has been substantially perturbed on multiple occasions

Abstract:

Paleomagnetic records have long indicated variations in field behaviour occurring on timescales of tens of millions of years and longer. Nevertheless, it is often assumed that, away from geomagnetic reversals and excursions, the geodynamo has always operated to maintain a field with stable, dominantly dipolar morphology and a dipole moment of the same order as the modern field. Here we will present a synthesis of new and extant paleomagnetic datasets that demonstrate this assumption unlikely to be valid for certain time periods in Earth history. At this time, there are two primary candidates for intervals where the paleomagnetic field was highly non-uniformitarian: the mid-Paleozoic (~ 415-335 Myr ago) and the Ediacaran (~635-540 Myr ago). Both are well-known as “problem times” for classical paleomagnetism applied to tectonic reconstructions and reversal chronology; many explanations have been put forward for the anomalous and poor quality directional data obtained from rocks of these ages. Only recently has it become apparent that rocks originating in both time periods yield paleomagnetic intensity estimates that are an order of magnitude weaker than in the present-day. These new paleointensity measurements very likely explain the unusual directional measurements both directly (e.g. weaker magnetisations are more easily corrupted) and indirectly (e.g. dynamo simulations with lower dipole moments generally produce more complex and erratic fields). The current distribution of paleomagnetic intensity data leaves open the question of how many other such intervals have occurred in Earth’s history. The follow-up question of what could be perturbing the geodynamo process to cause such shifts in behaviour will also be explored.