Constraints from seismic anisotropy on the nature of the lowermost mantle

THE D'' layer lies at the bottom of the Earth's rocky mantle, and separates it from the liquid metal-alloy core. This region, extending from the core-mantle boundary to a few hundred kilometres above (Fig. 1), is geodynamically analogous to the more easily studied lithosphere, at the top of the mantle. The structure of D'' may reflect the style of lower-mantle convection, the nature of core-mantle interaction and perhaps even the fate of subducting lithosphere¹. Observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper-mantle boundary layer, but discussion of lower-mantle seismic anisotropy has been somewhat contentious^2-5. Here we present evidence, from seismic waves that have traversed the lowermost mantle beneath the Caribbean region, for a zone of seismic anisotropy below the D'' discontinuity, which in this region lies 250 km above the core-mantle boundary. The anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear-wave velocity. If D'' is a graveyard for subducted lithosphere, a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust, which may have lower shear-wave velocity owing to the presence of melt.