The Composition of Low-Velocity Layers at the Top and Bottom of the Earth's Core

Abstract:

We use results from ab initio simulations on the properties of Fe-Ni-S-C-O-Si liquids to put constraints on the composition of reported low-velocity layers at the top of the Earth's outer core.

At the top of the core, we find that increasing the concentration of any light-element in liquid iron under core conditions always increases velocity. It is, therefore, impossible to make a low velocity and low density layer – required for buoyant stability – at the top of the core by changing the concentration of a single light element; replacement of one light element with another is required. Assuming a bulk core with a given composition in oxygen and silicon, a layer containing more oxygen and less silicon can satisfy the observation of a low velocity layer along with the requirement that it has a lower density.

The F-layer at the bottom of the core is easier to account for, since decreasing the concentration of any light-element leads to lower velocity and simultaneously a higher density required for dynamic stability. An F-layer with low oxygen compared to the bulk core of would provide the observed velocity drop and be denser. However, the density increase is probably too large in view of the observed density jump between the inner and outer core. An alternative is to remove oxygen while increasing the silicon concentration.