On Destruction of a Thermally Stable Layer by Compositional Convection in the Earth's Outer Core

Shin-Ichi Takehiro,Youhei Sasaki

doi:10.3389/feart.2018.00192

Abstract

We discuss destruction of a thermally stable layer in the upper part of the Earth's outer core by compositional convection excited at the inner core boundary. We propose to use the radial distribution of power induced by thermal and compositional buoyancy (rate of kinetic energy production) as a measure of occurrence of thermal and compositional convection. The power consists of the terms proportional to convective entropy flux and convective compositional flux. In the region with positive power, convection is active because kinetic energy can be produced by buoyancy force, and a stably stratified layer could not be formed there. On the other hand, in the region with negative power, convection is suppressed and a stably stratified layer may be produced. Considering penetration effect of convection, we discuss possible maximum and minimum thicknesses of the stable layer based on the radial distribution of power and its radial integral, respectively. We construct a 1-dimensional thermal and compositional balance model of the Earth's core with a larger value of thermal conductivity recently suggested by high-pressure experiments and first principle calculations, and estimate radial distributions of power for various values of core mantle boundary (CMB) heat flux Q_CMB. When Q_CMB > Q_sCMB no thermally stable layer can exist, where Q_sCMB is conductive heat flux along the adiabat at CMB. On the other hand, when Q_CMB < Q_sCMB, formation of an upper thermally stable layer becomes possible, depending on the extent of penetration of compositional convection excited below. When Q_CMB is sufficiently lower than Q_sCMB, a thermally stable layer survives the maximum penetration of compositional convection. The results show that a thermally stable layer becomes effectively thinner when the effect of compositional convection is considered compared with the results of previous studies where the existence of a stable layer is evaluated based on the convective flux only.

Highlights

It is widely thought that fluid motion driven in the Earth’s outer core by cooling due to mantle convection generates and maintains the geomagnetic field through dynamo action
Alternative models considering formation of a stably stratified layer below the core mantle boundary (CMB) were proposed (Labrosse et al, 1997; Lister and Buffett, 1998). They assumed that, when heat flow at the CMB QCMB determined by strength of mantle convection becomes smaller than upward conductive heat flow along the adiabatic temperature profile Qs, heat accumulates below the CMB, and a thermally stable layer starts to develop by conduction
A stable layer survives below the CMB when penetration is weak, whereas a stable layer is completely destroyed by compositional convection when its penetration is strong

Summary

INTRODUCTION

It is widely thought that fluid motion driven in the Earth’s outer core by cooling due to mantle convection generates and maintains the geomagnetic field through dynamo action. Labrosse et al (1997) assumed that erosion of the stable layer by mixing due to compositional convection generated from the deeper region does not occur, whereas Lister and Buffett (1998) considered accumulation of light elements due to inner core growth explicitly. By using 1-dimensional thermal balance models with the updated values of thermal conductivity, generation and existence of a stably stratified layer in the Earth’s outer core was discussed (Gomi et al, 2013; Labrosse, 2015) Their results showed that a stable layer with a thickness of O(1,000 km) could be produced when the heat flux across the CMB is small.

Estimation of Stratified Layer

Analytical Expressions of the Fluxes in a Simple Model

RESULTS

CONCLUSIONS AND DISCUSSION