Heterogeneity in core-mantle heat flow influenced the ancient geodynamo

Studies of the behaviour of the geodynamo through time, preserved in palaeomagnetic records, have a unique potential to inform us about the time-varying thermal structure of Earth’s core-mantle boundary region. In practise, however, such insights have proved difficult to obtain because of the limited spatiotemporal resolution of palaeomagnetic records and uncertainties in how to interpret them. Here we show that certain key characteristics of the palaeomagnetic field observed over the last 23 Myr can only be reproduced by numerical simulations of the geodynamo that have heterogeneous heat flux conditions imposed on their outer boundaries. We further demonstrate that a signal of azimuthal asymmetry in field behaviour, characteristic of such heterogeneity, is evident in the variance of records of palaeosecular variation recovered from low latitudes, that are up to 265 Myr old. Finally, we show that imposing strong heterogeneity in the heat flow boundary conditions significantly increases the parameter range over which simulations can reproduce aspects of palaeosecular variation that have been observed to be stable through much of geological time. In conjunction, these findings provide compelling new evidence that significant thermal heterogeneity at the core-mantle boundary has influenced the geodynamo for at least the last few hundred million years. The identified palaeomagnetic signatures of thermal heterogeneity at the core-mantle boundary provide a new means to constrain the properties and time-evolution of this region of Earth’s interior. Furthermore, our insights into how thermal heterogeneity at the base of the mantle can break the axial symmetry of the time-averaged magnetic field may provide an important step towards resolving longstanding palaeogeographic controversies.