Abstract
Stable stratification at the top of the Earth's outer core has been suggested based upon seismic and geomagnetic observations, however, the origin of the layer is still unknown. In this paper we focus on a thermal origin for the layer and conduct a systematic study on the thermal evolution of the core. We develop a new numerical code to model the growth of thermally stable layers beneath the CMB, integrated into a thermodynamic model for the long term evolution of the core. We conduct a systematic study on plausible thermal histories using a range of core properties and, combining thickness and stratification strength constraints, investigate the limits upon the present day structure of the thermal layer. We find that whilst there are a number of scenarios for the history of the CMB heat flow, Qc, that give rise to thermal stratification, many of them are inconsistent with previously published exponential trends in Qc from mantle evolution models. Layers formed due to an exponentially decaying Qc are limited to 250–400 km thick and have maximum present-day Brunt-Väisälä periods, TBV = 8 − 24 hrs. When entrainment of the lowermost region of the layer is included in our model, the upper limit of the layer size is reduced and can fully inhibit the growth of any layer if our non-dimensional measure of entrainment, E > 0.2. The period TBV is insensitive to the evolution and so our estimates remain distinct from estimates arising from a chemical origin. Therefore, TBV should be able to discern between thermal and chemical mechanisms as improved seismic constraints are obtained.
Highlights
The Earth’s large scale magnetic field is generated within the liquid iron outer core by the geodynamo process, which converts the me chanical energy of fluid motion into magnetic energy
We explore a wide range of input parameters including different core chemical and thermal properties and core-mantle boundary (CMB) heat flows and focus on high values of the thermal conductivity, since this favours thicker layers
The inner core boundary (ICB) is located at radius r = ri(t), the base of the stable layer is at r = rs(t), which varies with time t, and the CMB is at r = rc
Summary
The Earth’s large scale magnetic field is generated within the liquid iron outer core by the geodynamo process, which converts the me chanical energy of fluid motion into magnetic energy. A range of seismic studies (Lay and Young, 1990; Garnero et al, 1993; Helffrich and Kaneshima, 2010; Kaneshima, 2017), but not all (Alexandrakis and Eaton, 2010), find significant P-wave ve locity reductions relative to the Preliminary Reference Earth Model (PREM, Dziewonski and Anderson, 1981) ranging up to 400 km deep into the core This has been interpreted as a layer of anomalously light fluid (Helffrich and Kaneshima, 2013) that is resistant to the convective motion beneath it, this interpretation has been recently chal lenged (Irving et al, 2018). The existence of a stratified layer has important implications for interpreting geomagnetic observations because stable regions filter the signal from the deeper core (Christensen, 2006) and support unique classes of wave motions such as MAC waves, which have been invoked to explain certain periodic vari ations in the observed magnetic field and length of day (Buffett et al, 2016)
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