Core–mantle coupling: New insights into the magnetic and thermal evolution of Earth

The Earth has possessed a magnetic field for at least ~4.3 Ga, as indicated by paleomagnetic data. To constrain Earth’s thermal and magnetic evolution, parameterized core models have traditionally relied on a parameterized mantle assumed to be vigorously convecting due to plate tectonics. By neglecting spatial variations in mantle temperature and viscosity, these models typically predict an inner core nucleation (ICN) age of 0.5–0.8 Ga, which requires a thermally driven dynamo prior to that time. Recent experimental constraints indicating higher core thermal conductivities have therefore led to the “new core paradox,” in which sub-adiabatic conditions can result in gigayear-long interruptions of the modeled geodynamo.

Alternatively, studies that couple higher-dimensional mantle convection models with parameterized core evolution have found that hot initial core temperatures and an insulating primordial lid above the core–mantle boundary (CMB) are required to reproduce the present-day inner core size, with minimal influence from the surface tectonic regime. However, these studies did not predict magnetic field strengths and showed that the available magnetic dissipation overestimates Earth’s magnetic field in the early evolution and underestimates it at later times.

Here, we present a new two-dimensional mantle convection model coupled to a core evolution model that incorporates state-of-the-art mineral physics data and magnetic field strength scaling laws. Our results require a ~200 km thick primordial dense layer and the presence of the post-perovskite phase at the base of the mantle, forming a CMB thermal lid that inhibits strong early core-cooling. By varying surface plasticity and the maximum density contrast of the lower mantle relative to the ambient mantle, we identify best-fit models that reproduce both inner core growth and the secular variation of the magnetic field.

Assuming a bulk silicate Earth (BSE) composition, 12 wt.% light elements in the core, a core thermal conductivity of 125 W m⁻¹ K⁻¹, an initial CMB temperature of 4564 K, and a CMB lid that is 7% denser than peridotite, ICN occurs at ~1.3 Ga, while the thermal dynamo ceases after ~3 Ga. Future constraints on the presence and evolution of a thermally stable layer in the core will further refine models of Earth’s magnetic field evolution.