The feasibility of thermal and compositional convection in Earth’s inner core

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Lythgoe, Karen H ; Rudge, John Frederick ; Neufeld, Jerome Anthony ; Deuss, Arwen (2015)
  • Publisher: OUP
  • Subject: sub-02 | Composition of the core | Heat flow | Numerical approximations and analysis

Inner core convection, and the corresponding variations in grain size and alignment, has been proposed to explain the complex seismic structure of the inner core, including its anisotropy, lateral variations and the F-layer at the base of the outer core. We develop a parametrized convection model to investigate the possibility of convection in the inner core, focusing on the dominance of the plume mode of convection versus the translation mode. We investigate thermal and compositional convection separately so as to study the end-members of the system. In the thermal case the dominant mode of convection is strongly dependent on the viscosity of the inner core, the magnitude of which is poorly constrained. Furthermore recent estimates of a large core thermal conductivity result in stable thermal stratification, hindering convection. However, an unstable density stratification may arise due to the pressure dependant partition coefficient of certain light elements. We show that this unstable stratification leads to compositionally driven convection, and that inner core translation is likely to be the dominant convective mode due to the low compositional diffusivity. The style of convection resulting from a combination of both thermal and compositional effects is not easy to understand. For reasonable parameter estimates, the stabilizing thermal buoyancy is greater than the destabilizing compositional buoyancy. However we anticipate complex double diffusive processes to occur given the very different thermal and compositional diffusivities. We would like to thank Chris Davies for help with comparison to his results, plus Deputy Editor Stephane Labrosse, Renaud Deguen and ´ an anonymous reviewer for constructive comments that improved the manuscript. KHL and AD are funded by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement number 204995. JAN is partially funded by a Royal Society University Research Fellowship. This is the final published version. This article has been accepted for publication in Geophysical Journal International ©: 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
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