Abstract
Convection in the Earth's outer core is driven by buoyancy sources of both thermal and compositional origin. The thermal and compositional molecular diffusivities differ by several orders of magnitude, which can affect the dynamics in various ways. So far, the large majority of numerical simulations have been performed within the codensity framework that consists in combining temperature and composition, assuming artificially enhanced diffusivities for both variables. In this study, we use a particle-in-cell method implemented in a 3D dynamo code to conduct a first qualitative exploration of pure compositional convection in a rotating spherical shell. We focus on the end-member case of infinite Schmidt number by totally neglecting the compositional diffusivity. We show that compositional convection has a very rich physics that deserves several more focused and quantitative studies. We also report, for the first time in numerical simulations, the self-consistent formation of a chemically stratified layer at the top of the shell caused by the accumulation of chemical plumes and blobs emitted at the bottom boundary. When applied to likely numbers for the Earth's core, some (possibly simplistic) physical considerations suggest that a stratified layer formed in such a scenario would be probably weakly stratified and may be compatible with magnetic observations.
Highlights
Convection in the Earth’s outer core is presently driven by buoyancy sources of both thermal and compositional origin
A major difference between these buoyancy sources is tied to their molecular diffusivities which differ by several orders of magnitude, the chemical diffusivity being much weaker than its thermal counterpart
We performed a total of 11 numerical simulations summarized in Table 1 to conduct a first exploratory study of non-magnetic pure chemical convection in a rapidly-rotating spherical shell in the end-member case of infinite Schmidt number
Summary
Convection in the Earth’s outer core is presently driven by buoyancy sources of both thermal and compositional origin. A second interesting phenomenon envisioned by Moffatt and Loper (1994) is that the blobs may not completely mix with the bulk of the fluid core but accumulate below the core mantle boundary, thereby gradually forming a chemically stratified layer at the top of the core via a mechanism analogous to the “fillingbox” models (Baines and Turner, 1969) This prediction is noteworthy since the existence of a stably stratified layer in the outermost region of the core has been suggested by numerous seismic observations for several decades (Lay and Young, 1990; Tanaka, 2007; Helffrich and Kaneshima, 2010; Kaneshima, 2018). We propose to explore pure chemical convection at high Schmidt number by means of numerical simulations and test whether the accumulation of chemically buoyant parcels of fluid can lead to the formation of a stably stratified layer at the top of the core.
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