Regional stratification at the top of Earth's core due to core–mantle boundary heat flux variations

Abstract

Earth’s magnetic field is generated by turbulent motion in its fluid outer core. Although the bulk of the outer core is vigorously convecting and well mixed, some seismic, geomagnetic and geodynamic evidence suggests that a global stably stratified layer exists at the top of Earth’s core. Such a layer would strongly influence thermal, chemical and momentum exchange across the core–mantle boundary and thus have important implications for the dynamics and evolution of the core. Here we argue that the relevant scenario is not global stratification, but rather regional stratification arising solely from the lateral variations in heat flux at the core–mantle boundary. Using our extensive suite of numerical simulations of the dynamics of the fluid core with heterogeneous core–mantle boundary heat flux, we predict that thermal regional inversion layers extend hundreds of kilometres into the core under anomalously hot regions of the lowermost mantle. Although the majority of the outermost core remains actively convecting, sufficiently large and strong regional inversion layers produce a one-dimensional temperature profile that mimics a globally stratified layer below the core–mantle boundary—an apparent thermal stratification despite the average heat flux across the core–mantle boundary being strongly superadiabatic.

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Fig. 1: Thermal structure in a simulation with a tomographic pattern of CMB heat flux.
Fig. 2: Profiles of the time-averaged temperature gradient in the top half of the core.
Fig. 3: The thermal signature of stratification.
Fig. 4: Flow ~100 km below the CMB.

Data availability

The data that support the findings of this study are available from the corresponding author on request.

Code availability

The code used to model the core dynamics is described in Willis et al.55 and is available on request from the corresponding author.

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Acknowledgements

C.D. is supported by a Natural Environment Research Council Independent Research Fellowship (NE/L011328/1). J.A. is supported by the NSF Geophysics programme via award no. EAR-1853196. This work used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk) and ARC2, part of the High Performance Computing facilities at the University of Leeds, UK.

Author information

All authors discussed and developed the central ideas and contributed to the writing of the manuscript. J.M. and C.D. carried out the numerical modelling and analysis.

Correspondence to Jon Mound.

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Supplementary Figures

Supplementary Video 1

Equatorial slices (viewed from above; Pacific to the left, Africa to the right) of the thermal structure in the simulation with a tomographic pattern of CMB heat flux presented in Fig. 1. Left, temperature field. Right, radial gradient of temperature.

Supplementary Video 2

Equatorial slices (viewed from above; Pacific to the left, Africa to the right) of the thermal structure in the simulation with a hemispheric pattern of CMB heat flux presented in Supplementary Fig. 1. Left, temperature field. Right, radial gradient of temperature.

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Mound, J., Davies, C., Rost, S. et al. Regional stratification at the top of Earth's core due to core–mantle boundary heat flux variations. Nat. Geosci. 12, 575–580 (2019). https://doi.org/10.1038/s41561-019-0381-z

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