Simulations of the thermo-chemical evolution of the Earth’s core with stable stratification

The Earth’s magnetic field is generated inside the liquid outer core via a dynamo process, converting the kinetic energy of fluid motions into magnetic energy. Stratified fluid at the top of the outer core inhibits fluid motions however, there is a lack of consensus on the volume of stratified fluid and its thermal and chemical structure. In this study I consider different scenarios of the very long term evolution of the core that result in present day stable stratification. I first construct a numerical model for a parameterised representation of the core, including a time dependent stratified layer beneath the Core-Mantle Boundary (CMB), which is evolved over the age of the Earth. This model, unlike previous models, has a general framework for both thermal and chemical stratification, allowing a range of scenarios to be tested. Successful models produce a layer compatible with seismic observations, 100-400 km thick, match the present inner core radius, and generate sufficient entropy to power the geodynamo for billions of years. My model is applied to investigate thermally stratified layers resulting from a sub- adiabatic CMB heat flow using recent high thermal conductivity estimates. I find that viable models require a rapid decrease in Qc, > 3 TW Gyr−1, over the inner core age, which is required to be even larger if entrainment is not negligible. This rate is difficult to reconcile with coupled core and mantle evolution models. When investigating a chemical layer resulting from a downward flux of oxygen from the mantle, very large fluxes inhibit the ability of the core to generate a dynamo for much of its history, providing upper bounds on the flux. The stratified layer is relatively insensitive to the thermal evolution of the core and so layers up to 150 km thick are consistently found for a range of parameters. Finally, I produce the first thermal history calculations of a chemical layer formed during core formation, demonstrating that a relatively thick chemical layer can persist until the present day, whilst always permitting a dynamo. Brunt-Väisälä frequencies resulting from thermal stratification gives periods of 8-28 hours, from mass transfer with the mantle ∼30 minutes, and finally from a primordial layer 1.5-4 hours. Additional future constraints upon the Brunt-Väisälä frequency may therefore allow distinction between the mechanisms.