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Stratified or not-stratified or somewhere in between: Multi-disciplinary study of partially stratified layers in Earth's outer core

Dr Sebastian Rost (SEE), Dr Jonathan Mount (SEE), Dr Andy Nowacki (SEE)

Contact email: s.rost@leeds.ac.uk

Summary

The Earth’s outer core is the source of Earth’s global magnetic field, with a combination of chaotic convection and Earth’s rotation producing the flow structures essential for the geodynamo process. The low viscosity of the liquid iron-nickel alloy and the fast convection (with velocities on the order of 10s km/yr) suggest that the bulk of the fluid core should be well mixed. However, evidence from seismological, geodynamic, and mineral-physics studies have indicated that the outer core close to the core-mantle boundary is not vigorously convecting and well mixed like the majority of the outer core but instead forms a stable stratified layer.

Stratified layers in the outer core have important implications for the long-term evolution of the Earth’s deep interior, physical and chemical interactions between the core and mantle, and the dynamics of the outer core and the global magnetic field. Failure to properly understand core stratification will affect our ability to predict the development of the magnetic field on different time-scales; e.g., the structure of the paleomagnetic field and the mechanism by which reversals happen, and the present pattern of secular variation and hence medium-term geomagnetic forecasting for space-weather predictions. 

Seismological studies of the seismic velocity structure of the outer core resolve a change in the seismic velocities compared to 1D reference models that indicate a change of either composition or temperature in this layer consistent with the existence of a stratified layer. Furthermore, structures of the Earth’s magnetic field such as periodic variations in secular variations can be matched by models of waves within in a stratified layer. Several mineral physical models can explain stratification through accumulation of light material below the CMB or material flux from mantle to core.

While several lines of evidence point towards stratification in the outer core the evidence is not conclusive due to the limitations of our sampling of the outer core structure using seismology, the non-uniqueness of the interpretation of the variations of Earth’s magnetic field, and our limited understanding of material properties at high pressure and temperature.

A recent study by Mound et al. (preprint doi: 10.31223/osf.io/dvfjp) proposes the possibility that the stratified layer is not global but a regional feature of the core controlled by the heat flow through the core-mantle boundary. A partially stratified layer will develop in regions with reduced core-mantle boundary heat-flow, e.g. beneath the lower mantle large low shear velocity provinces, while in other regions the full core from the inner core boundary to the core-mantle boundary is convecting. This scenario has so far not been considered in seismic studies.

This project will aim to better understand the possibility of partially stratified layers at the top of the outer core by using a combination of seismic data analysis, geodynamic modelling and synthetic seismic wave propagation. We will use new seismic data from globally distributed seismic networks and novel analysis techniques to resolve smaller scale seismic velocity variations in the outer core. We will test the influence of the seismic 3D mantle structure on the seismic measurements of core structure using synthetic seismic wave propagation through complex 3D velocity fields. Forward modelling of core convection models will allow us to test whether the seismically resolved structures are compatible with interpretations based on measurements of the magnetic field and Earth’s rotation. 

The PhD student will work with experts in this field of research and will be part of the Deep Earth research group of the Institute of Geophysics and Tectonics (School of Earth and Environment) of the University of Leeds. The Leeds Deep Earth research group is one of the largest groups of researchers studying deep Earth structure in the world. It is a multi-disciplinary research group including seismology, mineral physics, geodynamics, and geomagnetism. The PhD student will benefit from training from the Leeds-York-Hull Doctoral Training Partnership as well as from training at the university, and will be attending national and international conferences and workshops. Possibilities for attendance at fieldwork might arise in other projects at the Institute for Geophysics and Tectonics. 

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Related undergraduate subjects:

  • Applied mathematics
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  • Earth science
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