Mantle control of the geodynamo: Consequences of top‐down regulation

Abstract

The mantle global circulation, including deep subduction and lower mantle superplumes, exerts first-order controls on the evolution of the core, the history of the geodynamo, and the structure of the geomagnetic field. Mantle global circulation models that include realistic plate motions, deep subduction, and compositional heterogeneity similar to the observed large low seismic velocity provinces in the lower mantle predict that the present-day global average heat flux at the core-mantle boundary (CMB) exceeds 85 mW m−2. This is sufficient to drive the present-day geodynamo by thermochemical convection and implies a very young inner core, with inner core nucleation between 400 and 1100 Ma. The mantle global circulation also generates spatially heterogeneous heat flux at the CMB, with peak-to-peak lateral variations exceeding 100 mW m−2. Such extreme lateral variability in CMB heat flux, in conjunction with the high thermal conductivity of the core, implies that the liquid outer core is thermally unstable beneath the high seismic velocity regions in the lower mantle but thermally stable beneath the large low seismic velocity provinces. Numerical dynamo simulations show how this pattern of heterogeneous boundary heat flux affects flow in the outer core, producing localized circulation patterns beneath the CMB tied to the mantle heterogeneity and long-lived deviations from axial symmetry in the geomagnetic field.