Local patches of negative core-mantle boundary heat flux : insights from numerical models of thermo-chemical convection

Heat flux at the Earth’s core-mantle boundary (CMB) partially controls the outer core dynamics and its associated geodynamo. On the mantle side, spatial and temporal variations in this flux are, in turn, controlled by details of mantle convection. Previous simulations of mantle dynamics showed that CMB heat flux may be locally negative, i.e., in these regions heat flows from the mantle to the core. Here, we investigate the conditions needed to generate such patches of negative CMB heat flux. For this, we perform a series of high-resolution numerical simulations of thermo-chemical convection in spherical annulus geometry using the code StagYY. The compositional initial condition consists in a thin basal layer of chemically denser material (alos referred to as primordial material), which subsequently evolves into piles of hot, primordial material, modelling the large low shear-wave velocity provinces (LLSVPs) observed on global seismic tomography maps. We more specifically explore the influence of two key parameters that promote temperature increase within the piles of primordial material: the excess internal heating within these piles ; and the temperature-dependence of thermal conductivity. We quantify the size and amplitude of negative heat flux patches depending on these parameters. As one would expect, a larger internal heating excess and a stronger temperature dependence of thermal conductivity both favor the development of negative heat flux patches within piles of dense material. However, these parameters also alter the piles stability, such that there is no straightforward relationship between them and the size and amplitude of the negative heat flux patches. Finally, we discuss possible consequences of our findings for core dynamics and geodynamo.