Outer core compositional layering and constraints on core liquid transport properties

- Outermost outer core wavespeeds can be fit to diffusion-based profiles.
- The best-fitting diffusion profile is for a layer, 186±20 km thick.
- Core diffusion coefficients (D) may be seismically estimated.
- Hard-sphere liquid theory adequately explains the estimated D values.
- The layer may be a remnant of an early impact that formed the Earth.

A variety of studies of the Earth's outer core report wave speeds near the top of the core slightly lower than reference models for core properties. One interpretation of the slower wavespeed profile is that it could represent a change in the core's light element concentration with depth in the core. I explore the consequences of this idea by interpreting the velocity profile as arising from diffusion gradients imposed in the outer core by various mechanisms. In order to estimate relative diffusion rates for light elements in liquid iron I also examine theories for transport properties of high pressure metallic liquids that are based on hard-sphere models. From the seismic wavespeed profile, an effective diffusivity may be obtained, which ranges from 0.1 to 10×10−7 m2s−1 depending on the particular boundary condition or initial condition chosen. The upper bound of the range is higher than expected from high pressure experiments and models of diffusivity in liquid metals for all elements except H. The lower bound is within the uncertainty of theoretical predictions and experimental determinations given the range of expected outer core temperatures if diffusion involves low Z elements. Plausible agreement arises from a class of models that represent diffusion out of a compositionally different layer existing from the time of the formation of the Earth. If the wavespeed profile in the core is diffusive in nature, the data suggest that it is an original feature of the core.