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Cambridge NERC Doctoral Landscape Awards (Training Partnerships)

Graduate Research Opportunities

Lead supervisor: Sanne Cottaar, Earth Sciences

Co-supervisor: John Rudge, Earth SciencesJuliane Dannberg, University of Florida

Brief summary: 
Exploration of the role of strong heterogeneity that sits at the core-mantle boundary in mantle and outer core dynamiccs
Importance of the area of research concerned: 
Seismic imaging of the core-mantle boundary region shows it is highly heterogeneous. The strongest anomalies are so-called Ultra-Low Velocity Zones (ULVZs), for which the extremely slow velocities indicate the presence of anomalous dense heterogeneity, which is likely iron enriched. Our research team has imaged large ULVZs using Sdiff waveforms (e.g. Cottaar et al. 2022). From Sdiff data, which covers about 50% of the CMB, we estimate the total coverage of ULVZs to be 2-4% of the CMB (Carl Martin’s thesis). However, the debate is open if we can only observe local thickening of an otherwise global thinner layer (Russell et al. 2023). The question remains as to what role the ULVZs play in the mantle dynamics. Are the ULVZs swept up at the base of mantle plumes? Does this mean they are just passive tracers of the surrounding convection, or do they play an active role in rooting mantle plumes in place? Does entrainment of ULVZ material occur?
Project summary : 
The distribution of significant ULVZs across the CMB is slowly emerging. The nature and role in dynamics of these features is still unclear. In this project geodynamical models with various starting conditions will be developed to recreate seismic observations. This should provide understanding of the volume and density of material needed. We will assess the role of the ULVZs in the mantle plume dynamics. Potentially we will run specific models to test the degree of entrainment from our ULVZs. And we can test the effect of ULVZs on CMB topography.
What will the student do?: 
The student will be working closely with Juliane Dannberg on geodynamical models to create distributed mega-ULVZs or global layers with thickening. These models will use the open-source community code ASPECT (Bangerth et al., 2023) and run on the local High Performance Cluster. The student will analyze and visualize the outcomes of the runs to understand the key factors in ULVZ morphology and plume behaviour. As described above, the project could take various directions based on interest of the student. The project can take several multi-disciplinary directions depending on the interests of the student. We can also collaborate with geochemists to further understand if ULVZs are sampled at hotspot locations., or with mineral physicists to test certain compositional models. Other options are to work with geodynamo modellers, to test if heat flux variations caused by ULVZs could influence outer core convection and the associated magnetic field. Further seismic observations can also be made to fill in missing puzzle pieces.
References - references should provide further reading about the project: 
Bangerth, W., Dannberg, J., Fraters, M., Gassmoeller, R., Glerum, A., Heister, T., Myhill., R. and Naliboff, J., 2023. ASPECT v2.5.0. Zenodo. Doi:10.5281/zenodo.8200213
Cottaar, S., Martin, C., Li, Z. and Parai, R., 2022. The root to the Galápagos mantle plume on the core-mantle boundary. Seismica, 1(1).
Russell, S., Jagt, L., Irving, J., Cottaar, S., 2023 Evidence for a kilometre-scale seismically slow layer atop the core-mantle boundary from normal modes, Geophysical Research Letters, in revision.
You can find out about applying for this project on the Department of Earth Sciences page.