Signature of inner core nucleation on the geodynamo

Abstract:

Energy considerations indicate that the power delivered to the present-day geodynamo comes mainly from the growth of the solid inner core, through light element and latent heat release. However, estimates of present-day core heat loss predict that inner core nucleation (ICN) occurred by 1 Ga, implying that the geodynamo operated over most of its history without the help of inner core growth. We use numerical dynamo simulations linked by thermochemical evolution of the core to determine signatures of ICN in the paleomagnetic field and possible ICN footprints in paleomagnetic data. First, we select a set of dynamo simulation parameters that produces a magnetic field structure compliant with the present-day geomagnetic field. Then, we use thermochemical evolution models to take this dynamo model backward in time, varying the inner core size and the forcing parameters accordingly. We find that the paleointensity is greater prior to ICN than at present-day, which contrasts with predictions from classical dynamo scalings. Our results suggest that the surface magnetic field is well-approximated by an axial dipole prior to ICN, although it contains an axial octupole component larger than at present-day. This stronger octupole is caused by the emergence of flow downwellings near the poles of the outer core prior to ICN, and may be resolvable using paleomagnetic data.