Changing Patterns in Core-Mantle Boundary Heat Flux Throughout the Past Billion Years of Earth's History

The Earth's magnetic field is generated by the dynamo in the outer core. The dynamo is driven by the heat flux out of the core into the overlying mantle, and the freezing of iron at the solid inner core-liquid outer core boundary. Recent studies have ranged the age of the inner core from a few billion years to as young as 250 million years. The age of nucleation remains unclear because the precise effects of inner core nucleation on the magnetic field are unknown and the magnitude of magnetic field variations caused by mantle convection are not well constrained. We here test if changes in heat flux at the core-mantle boundary (CMB), which is controlled by mantle convection, are capable of generating some of the observed magnetic field changes that are currently attributed to inner core nucleation.

To constrain both the absolute amplitude and spatial pattern of heat flux at the CMB we use a plate reconstruction that describes supercontinent formation and dissolution patterns of the last 1 Gyr, and apply it as boundary condition to 3D global mantle convection models using the community code ASPECT. The models use a compressible material description based on mineral physics data, and feature a dense layer at the CMB that we track using a particle method and that forms piles during the model evolution. We computed heat flux maps at the CMB from 900 Myr to the present, and analyzed their spatial patterns and average heat flux in relation to mantle convection.

The model displays changes in pile shape and number and corresponding changes in the CMB heat flux. The highest average heat flux occurs 580 Myr ago, at a time when the large amount of subducted material at the CMB shaped the dense layer into many medium-sized piles. The lowest average heat flux (750 Myr ago) is caused by larger, more consolidated piles with only little subducted material between them. Overall, the model cools down over time. Our model shows that the subduction history controls the distribution of thermochemical structures in the lowermost mantle. We will use this computed CMB heat flux as boundary condition for geodynamo simulations that will improve our understanding of the connection between paleomagnetic data and changes in the Earth's deep interior by illuminating the role of plate tectonics and inner core size in regulating surface magnetic field behavior over Earth's history.