A two-phase two-component slurry model of the F-layer at the base of Earth's core

Abstract

We present a new two-phase two-component slurry model of the F-layer at the base of Earth's liquid outer core. Seismic observations indicate that the F-layer is stably stratified, which challenges conventional models of core dynamics that assume outer core convection and dynamo action are powered by heat and light element release at the inner core boundary (ICB). Previous work (Wong et al., 2021) has shown that an F-layer comprising a “snow” of solid iron particles falling through an iron-oxygen liquid can account for the inferred thickness, density and velocity anomaly of the F-layer; however, the model prescribed simplified fluid dynamical descriptions of the solid and liquid phases. Here we build on the work of Wong et al. (2021) by incorporating a self-consistent description of two-phase fluid dynamics. Analysing a suite of 1D time-independent solutions reveals that the solid fraction ${\varphi }^s$ and liquid velocity $u^l$ decrease with increasing bulk oxygen concentration and buoyancy number B, while ${\varphi }^s$ and $u^l$ increase with increasing ICB heat flux and solid/liquid viscosity ratio ${\lambda }_{\eta }$. Extrapolating to core conditions suggests that ${\varphi }^s,u^l\ll 1$ while the solid velocity $u^s$ is comparable to velocities at the top of the liquid core inferred from geomagnetic secular variation. Our results suggest that stable stratification in the F-layer arises from compositional variations maintained by outward barodiffusion and flux of light element that balance the inward flux of solid.