Compositional convection and stratification of Earth's core

Abstract

Traditionally, studies1,2 of the possible stratification of the outer core of the Earth have been based on thermal buoyancy, with the effects of composition being implicitly ignored. However, recent work3,4 has demonstrated the importance of compositional buoyancy in providing the power necessary to sustain the geodynamo. It seems likely, therefore, that the effects of composition are also important in determining the stratification of the core. We present here a study of the relative importance of thermal and compositional buoyancy in the core. We briefly review the idea that the growth of an iron-rich inner core leads to the release of material which is depleted in iron and therefore buoyant. We then calculate the density profile which results if both this light material and the latent heat released as the inner core grows are redistributed by diffusion. It is found that not only are compositional effects important, they dominate thermal effects in determining the stratification of the core. Finally, using parameter estimates relevant to the core, we show that the diffusive density profile is unstable throughout the outer core except for a layer some 70 km deep adjacent to the mantle–core boundary. We therefore expect that convection fills the entire outer core with the possible exception of a thin layer below the mantle–core boundary.