Ultralow velocity zone at the core-mantle boundary

Accumulating seismic evidence for a laterally varying thin ultralow velocity zone (ULVZ) at the base of the mantle is summarized, and shown to have far reaching implications. Anomalous SPdKS delays and amplitudes, and precursory energy to PcP, ScP, and PKP require seismic velocity reductions of at least 10% in some regions of the boundary layer. Estimates of ULVZ thickness depend on poorly constrained parameters such as the shear velocity and density, but it is likely less than 20-40 kilometers, where present. The most probable cause of the ULVZ is partial melting of the lowermost mantle, with chemical heterogeneity and dynamic effects playing an important role as well. Strong spatial correlations between regions with detected ULVZ zones and surface positions of buoyancy flux-weighted hot spots, as well as between regions lacking a detectable ULVZ and areas of accumulated subducted material, suggest a direct relationship between the top and bottom boundary layers of the mantle, strongly supporting the idea of circulation between the upper and lower mantle.