Convective Erosion of a Primordial Stratification Atop Earth's Core

M. Bouffard,A. Goument,M. Landeau

doi:10.1029/2020gl087109

Abstract

AbstractSeismic and geomagnetic observations suggest the presence of a stratified layer atop Earth's core. Previous laboratory experiments showed that this layer could be primordial, produced by a collision between the primitive Earth and a giant impactor. However, paleomagnetic data require turbulent flow motions in the core for the last 3.4 Ga. Such flows can erode an existing stratification. It is therefore unclear whether a primordial stratification still exists nowadays. Here, we use numerical simulations to investigate the erosion by thermal convection of a chemical layer atop Earth's core. Our scaling law predicts that a primordial layer thicker than 1 km with a density anomaly above 0.01% can survive 4.5 Ga of convective erosion. We conclude that the observed present‐day stratification could be a vestige of core formation. We also observe strong double‐diffusive flows in the layer. These might reconcile the existence of a stratification with the present‐day structure of the geomagnetic field.

Highlights

Seismic and geomagnetic observations have long suggested the presence of a stratified layer at the top of Earth's core (Buffett, 2014; Buffett et al, 2016; Braginsky, 1994; Lay & Young, 1990; Souriau & Poupinet, 1991; Tanaka, 2007; Whaler, 1980)
We use numerical simulations to investigate the erosion by thermal convection of a chemical layer atop Earth's core
Some found no evidence for stratification (Alexandrakis & Eaton, 2010; Irving et al, 2018), recent seismic investigations indicate low P-waves velocities within a 100 to 500 km thick layer below the core-mantle boundary (CMB) (Helffrich & Kaneshima, 2010; Kaneshima, 2018; Kaneshima & Helffrich, 2013; Kaneshima & Matsuzawa, 2015; Tang et al, 2015)

Summary

Introduction

Seismic and geomagnetic observations have long suggested the presence of a stratified layer at the top of Earth's core (Buffett, 2014; Buffett et al, 2016; Braginsky, 1994; Lay & Young, 1990; Souriau & Poupinet, 1991; Tanaka, 2007; Whaler, 1980). Landeau et al (2016) proposed that a chemically stratified layer could be produced early in the Earth's history, as the result of the incomplete turbulent mixing between the terrestrial core and that of a giant impactor. Other authors have modeled the effect of stable layers on convection in rotating spherical shells (Christensen, 2006, 2018; Manglik et al, 2010; Masada et al, 2013; Mound et al, 2019; Nakagawa, 2015; Olson et al, 2018; Stanley & Mohammadi, 2008) These investigations focus on a quasi-steady stratified layer, with a constant thickness. Our scaling predicts that core convection likely eroded less than 1 km of primordial stratification over the age of the Earth

Numerical Model

Flow Regimes

Erosion Scaling

Findings

Implications for Core Stratification