Localized negative core-mantle boundary heat flux

The Earth’s core-mantle boundary (CMB) hosts structures and processes that strongly affect our planet’s thermal and chemical evolution. Spatial and temporal changes in CMB heat flux, Φ _CMB , influence core dynamics and its associated geodynamo. These variations, in turn, are controlled by details of mantle dynamics and may be affected by lowermost mantle structures, in particular the large low shear-wave velocity provinces (LLSVPs), which commonly interpreted as reservoirs of hot, chemically differentiated material. Here, we perform simulations of thermo-chemical convection that account for changes in thermal conductivity with temperature and for excess heating in piles of dense material, modelling LLSVPs, and show that beneath these piles, Φ _CMB can be locally negative, i.e., heat flows to the core. These patches do not appear if excess heating is too large. We further show that throughout the piles lateral extension, Φ _CMB remains lower than the adiabatic core heat flux, while slab arrival at the CMB triggers temporary high Φ _CMB spikes, increasing lateral heterogeneity in Φ _CMB . Our findings support the hypothesis that partial stratification occurs at the top of the core, reconciling present-day geomagnetic and seismic observations in this region. At timescales of 100s Myrs they may explain the onset and termination of superchrons.