Abstract
AbstractSeismic and geomagnetic observations have been used to argue both for and against a global stratified layer at the top of Earth's outer core. Recently, we used numerical models of turbulent thermal convection to show that imposed lateral variations in core‐mantle boundary (CMB) heat flow can give rise to regional lenses of stratified fluid at the top of the core while the bulk of the core remains actively convecting. Here, we develop theoretical scaling laws to extrapolate the properties of regional stratified lenses measured in simulations to the conditions of Earth's core. We estimate that regional stratified lenses in Earth's core have thicknesses of up to a few hundred kilometres and Brunt‐Väisälä frequencies of hours, consistent with independent observational constraints. The location, thickness, and strength of the stratified regions would change over geological time scales in response to the slowly evolving CMB heat flux heterogeneity imposed by mantle convection.
Highlights
Independent inferences from seismology (Helffrich & Kaneshima, 2010; Kaneshima, 2018; Tanaka, 2007), geomagnetism (Buffett, 2014; Buffett et al, 2016; Olson et al, 2018; Yan & Stanley, 2018), and geodynamics (Davies et al, 2015; Nimmo, 2015) have been used to suggest that a stably stratified layer exists at the top of Earth's liquid core
Compared to the dynamics of the relatively low‐viscosity core, solid‐state convection in the overlying mantle is associated with long time scales and large temperature variations, such that the core is subjected to large lateral variations in core‐mantle boundary (CMB) heat flux (Nakagawa & Tackley, 2008; Olson et al, 2015; Stackhouse et al, 2015; Zhang & Zhong, 2011)
We first establish that our simulations obey the expected scaling by restricting ourselves to the subset of simulations with a hemispheric pattern of CMB heat flux heterogeneity
Summary
Independent inferences from seismology (Helffrich & Kaneshima, 2010; Kaneshima, 2018; Tanaka, 2007), geomagnetism (Buffett, 2014; Buffett et al, 2016; Olson et al, 2018; Yan & Stanley, 2018), and geodynamics (Davies et al, 2015; Nimmo, 2015) have been used to suggest that a stably stratified layer exists at the top of Earth's liquid core. Compared to the dynamics of the relatively low‐viscosity core, solid‐state convection in the overlying mantle is associated with long time scales and large temperature variations, such that the core is subjected to large lateral variations in CMB heat flux (Nakagawa & Tackley, 2008; Olson et al, 2015; Stackhouse et al, 2015; Zhang & Zhong, 2011). We show that our simulations match this theoretical expectation, enabling us to extrapolate to parameter values plausibly representative of the Earth's core
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