Inner core heterogeneity induced by a large variation in lower mantle heat flux

TL;DR

This paper demonstrates how large variations in heat flux at the core-mantle boundary can induce significant heterogeneity in Earth's inner core, affecting seismic and magnetic observations.

Contribution

It introduces a two-component dynamo model showing that large CMB heat flux variations produce inner core heterogeneity consistent with seismic data.

Findings

Inner core heterogeneity correlates with CMB heat flux variations.

East-west hemispherical differences match seismic velocity observations.

Magnetic flux variability aligns with the proposed convection regime.

Abstract

Seismic mapping of the top of the inner core indicates two distinct areas of high P-wave velocity, the stronger one located beneath Asia, and the other located beneath the Atlantic. This two-fold pattern supports the idea that a lower-mantle heterogeneity can be transmitted to the inner core through outer core convection. In this study, a two-component convective dynamo model, where thermal convection is near critical and compositional convection is strongly supercritical, produces a substantial inner core heterogeneity in the rapidly rotating strongly driven regime of Earth's core. While the temperature profile that models secular cooling ensures that the mantle heterogeneity propagates as far as the inner core boundary (ICB), the distribution of heat flux at the ICB is determined by the strength of compositional buoyancy. A large heat flux variation $q^*$ of $O(10)$ at the core-mantle boundary (CMB), where $q^*$ is the ratio of the maximum heat flux difference to the mean heat flux at the CMB, produces a core flow regime of long-lived convection in the east and time-varying convection in the west. Here, the P-wave velocity estimated from the ICB heat flux in the dynamo is higher in the east than in the west, with the hemispherical difference of the same order as the observed lower bound, 0.5%. Additional observational constraints are satisfied in this regime -- the variability of high-latitude magnetic flux in the east is lower than that in the west; and the stratified F-layer at the base of the outer core, which is fed by the mass flux from regional melting of the inner core and magnetically damped, attains a steady-state height of $\sim$ 200 km.