Mantle Control of the Geodynamo: Consequences of Top-Down Regulation

Abstract:

The mantle global circulation, including plate motions, 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 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 exceeds 90 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 1000 Ma, plus heterogeneous heat flux at the core-mantle boundary (CMB) with peak-to-peak lateral variations exceeding 100 mW.m^-2. Extreme lateral variations in CMB heat flux indicate that the liquid outer core is thermally unstable beneath the high seismic velocity regions in the lower mantle and thermally stable beneath the large low seismic velocity provinces. Numerical dynamo simulations show how this pattern of boundary heat flux affects the flow in the outer core below the CMB, tends to align the time average geomagnetic dipole axis with the rotation axis, while promoting polarity reversals. It also contributes to longitudinal variations in the solidification rate on the inner core boundary, a possible cause of the hemispherical pattern of seismic heterogeneity observed in the inner core.