The new core paradox, the stable layer, and double diffusive convection in Earth’s core

• Applicants: Dr. Johannes Wicht; Dr. Xiaojue Zhu, Ph.D.
• Subject Area: Geophysics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 521319293

Both thermal and compositional convection contributed to driving the geodynamo over its evolution. However, most numerical dynamo models to date neglect the differences between both convection types, describing the evolution of density variation with only one variable. Therefore, a number of peculiar instabilities that rely of the significant differences in diffusivities in double-diffusive dynamics can never arise in these simulations. This proves particularly problematic in the buildup and the dynamics of a stable stratified layer where one component is driving why the other suppresses motions. Seismic observations indeed suggest that a stable layer with a thickness of perhaps 100 km may exist underneath Earth’s core mantle boundary. It may actually have been there since Earth’s early history. Once Earth’s inner core started to grow, the light elements released in the crystallization process significantly contributed to powering the geodynamo. Before that time, however, the exsolution of, for example, MgO may have played a similar role. A significant contribution of compositional driving could solve the New Core Paradox, which states that the large heat flux required by a purely thermally driven dynamo would yield an unrealistically hot core and lower mantle earlier in Earth’s history. We propose to explore double-diffusive dynamos in the full-sphere (without inner core) and the spherical shell (with inner core) geometry with the numerical code MagIC. In order to better understand the peculiar doubly-diffusive small-scale instabilities arising in a stable layer, we will also conduct high-resolution box simulations with the code AFiD. The results will then be parametrized for a more realistic implementation of the outer stable layer in global MagIC simulations. Understanding the role of double-diffusive dynamics in general and the stable layer in particular for the core evolution, for the dynamo process, and the impact of the lower mantle on the dynamo is a first order problem for the SPP DeepDyn.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2404: Reconstructing the deep dynamics of planet Earth over geologic time (DeepDyn)
• International Connection: China
• Co-Investigator: Dr. Stephan Stellmach
• Cooperation Partner: Professor Dr. Yantao Yang