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 cure-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 lithosphere(1). 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.