Geophysics: The buoyancy of Earth's deep mantle.

Abstract

The physical nature of two regions called large low-shear-velocity provinces at the base of Earth's mantle is uncertain. A measurement of their density has implications for our understanding of mantle dynamics. See Article p.321 The interior composition of Earth can be estimated by imaging seismic waves changing speed as they travel through different materials, but some anomalies in the deep mantle remain challenging to explain. Fast wave speed anomalies appear in areas with a history of subduction, indicating relatively cold and dense mantle material driving downward flow. However, slow wave speed anomalies in the form of large domes above the core–mantle boundary remain contentious—in particular their net buoyancy. Harriet Lau and co-authors estimate Earth's deep mantle buoyancy using GPS-based measurements of the daily deformation of Earth in response to the gravitational pull of the Sun and the Moon. They show that the mean excess density across the bottom two-thirds of these lower-mantle domes is about 0.5 per cent. The authors conclude that these structures are enriched with high-density chemical components, probably originating from subducted oceanic plates or primordial material associated with Earth's formation.