Numerical simulations of thermal-chemical instabilities at the core-mantle boundary

Dynamical effects at the core-mantle boundary have so far been modelled mainly with thermal convection^1,2, yet accumulating evidence^3,4 supports the idea of a combined thermal and chemical boundary layer as the likely explanation of the D″ zone. Here we present numerical simulations of thermal-chemical instabilities in the D″ layer which show that strong lateral heterogeneities in the composition and density fields can be initiated and maintained dynamically if there is continuous replenishment of material from subduced slabs coming from the upper mantle. These chemical instabilities have a tendency to migrate laterally and may help to support core-mantle boundary topography with short and long wavelengths. The thermal-chemical flows produce a relatively stagnant D″ layer with strong lateral and temporal variations in basal heat flux, which gives rise to thermal core-mantle interactions⁵, influencing the geodynamo.