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

Abstract

S U M M A R YInner 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.

Partitioning of sulfur between solid and liquid iron under Earth’s core conditions: Constraints from atomistic simulations with machine learning potentials

“…This would support a stable stratification of sulfur in the Earth's inner core, since more sulfur precipitates in the inner core as the outer core sulfur concentration increases over time (Cottaar and Buffett, 2012;Deguen and Cardin, 2011). By contrast, without counting the T-P dependence of the relevant parameters in the equations of chemical potentials (as listed in Table 4 and analyzed in the previous section), recent numerical simulations propose continuously decreasing partition coefficient with time and find its destabilizing buoyancy effects on the stratification of the Earth's inner core (Gubbins et al, 2013;Labrosse, 2014;Lythgoe et al, 2015). Our results in this study show that these simulations may need to be re-evaluated at least for sulfur based on our results.…”

Constraints from material properties on the dynamics and evolution of Earth’s core

“…Compositionally unstable conditions may have arisen once the inner core grew beyond O(10) km, but probably have not persisted to the present day 59,70 . The case of thermochemical buoyancy is complicated by possible double-diffusive effects; initial studies indicate that the net buoyancy force is stabilising 90 . Overall it seems that inner core convection, either in the plume 87 or translation 39, 91 regimes, is unlikely at present.…”

A Boussinesq slurry model of the F-layer at the base of Earth’s outer core

“…Compositional effects have been proposed as an alternative pathway to inner core convection, since freshly created solid at the ICB over the lifetime of the inner core gives rise to unstable stratification as the concentration of iron is progressively refined (Deguen et al 2013). Gubbins et al (2013) find a weak chemical stratification caused by temperature dependent partitioning of light elements, though Labrosse (2014) and Lythgoe et al (2015) find that unstable compositional effects are dominated by thermal stratification and so inner core convection is unlikely to occur. These models of convective translation are mainly concerned with explaining the hemispherical asymmetry of the inner core, and it is unclear whether the magnitude of this convection can explain an F-layer hundreds of kilometres thick.…”