Compositional instability of Earth's solid inner core

TitleCompositional instability of Earth's solid inner core
Publication TypeJournal Article
Year of Publication2013
AuthorsGubbins D, Alfe D, Davies CJ
JournalGeophysical Research Letters
Volume40
Pagination1084-1088
Date Published2013/03
Type of ArticleArticle
ISBN Number0094-8276
Accession NumberWOS:000319215700012
Keywords1st-principles; base; convection; viscosity
Abstract

All models that invoke convection to explain the observed seismic variations in Earth's inner core require unstable inner core stratification. Previous work has assumed that chemical effects are stabilizing and focused on thermal convection, but recent calculations indicate that the thermal conductivity at core temperatures and pressures is so large that the inner core must cool entirely by conduction. We examine partitioning of oxygen, sulfur, and silicon in binary iron alloys and show that inner core growth results in a variable light element concentration with time: oxygen concentration decreases, sulfur concentration decreases initially and increases later, and silicon produces a negligible effect to within the model errors. The result is a net destabilizing concentration gradient. Convective stability is measured by a Rayleigh number, which exceeds the critical value for reasonable estimates of the viscosity and diffusivity. Our results suggest that inner core convection models, including the recently proposed translational mode, can be viable candidates for explaining seismic results if the driving force is compositional.

DOI10.1002/grl.50186
Student Publication: 
No