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Multiple seismic reflectors in Earth’s lowermost mantle

Significance Deep in the Earth’s interior, the region just above the core–mantle boundary exerts control on mantle convection and heat loss from the core. It has long been thought that the so-called D″ region is separated from a more uniform mantle above by a single interface, often attributed to a phase transition in Mg perovskite. Systematic deep-mantle exploration with massive seismic waveforms now yields evidence for multiple reflectors up to at least 600 km above the core–mantle boundary. Some of the newly discovered interfaces can be explained by postperovskite transitions in differentiated oceanic slab materials, transported from Earth’s surface through deep subduction and convection. The lowermost mantle appears more complex than hitherto thought, and this complexity is not confined to the canonical D″ region. The modern view of Earth’s lowermost mantle considers a D″ region of enhanced (seismologically inferred) heterogeneity bounded by the core–mantle boundary and an interface some 150–300 km above it, with the latter often attributed to the postperovskite phase transition (in MgSiO3). Seismic exploration of Earth’s deep interior suggests, however, that this view needs modification. So-called ScS and SKKS waves, which probe the lowermost mantle from above and below, respectively, reveal multiple reflectors beneath Central America and East Asia, two areas known for subduction of oceanic plates deep into Earth’s mantle. This observation is inconsistent with expectations from a thermal response of a single isochemical postperovskite transition, but some of the newly observed structures can be explained with postperovskite transitions in differentiated slab materials. Our results imply that the lowermost mantle is more complex than hitherto thought and that interfaces and compositional heterogeneity occur beyond the D″ region sensu stricto.