Seismological evidence for a non-monotonic velocity gradient in the topmost outer core

The Earth's core is mostly an Fe-Ni alloy with a fraction of light elements (~10 wt%, mainly O, S and Si). Accumulation of these light elements under the core-mantle boundary (CMB) may lead to chemical stratification. Seismic observations have been presented both for and against the stratification in the topmost region of the outer core. Here we investigate the structure under the CMB using differential travel times between SKKS and S3KS waves. We obtain 606 high-quality S3KS-SKKS differential travel times with global path coverage. Result from a Bayesian inversion of these differential times indicates that the seismic velocity in the top 800 km of the outer core is ~0.07% on average lower than that in model PREM. The depth-dependent velocity profile, in particular a low-velocity zone of up to ~0.25% lower than PREM at ~80 km below the CMB, strongly favors the existence of stratification at the top of the outer core.