Tidal tomography constrains Earth’s deep-mantle buoyancy

Earth’s body tide—also known as the solid Earth tide, the displacement of the solid Earth’s surface caused by gravitational forces from the Moon and the Sun—is sensitive to the density of the two Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific. These massive regions extend approximately 1,000 kilometres upward from the base of the mantle and their buoyancy remains actively debated within the geophysical community. Here we use tidal tomography to constrain Earth’s deep-mantle buoyancy derived from Global Positioning System (GPS)-based measurements of semi-diurnal body tide deformation. Using a probabilistic approach, we show that across the bottom two-thirds of the two LLSVPs the mean density is about 0.5 per cent higher than the average mantle density across this depth range (that is, its mean buoyancy is minus 0.5 per cent), although this anomaly may be concentrated towards the very base of the mantle. We conclude that the buoyancy of these structures is dominated by the enrichment of high-density chemical components, probably related to subducted oceanic plates or primordial material associated with Earth’s formation. Because the dynamics of the mantle is driven by density variations, our result has important dynamical implications for the stability of the LLSVPs and the long-term evolution of the Earth system. An estimate of Earth’s deep-mantle buoyancy is derived from GPS-based measurements of body tide deformation and shown to be dominated by dense material possibly related to subducted oceanic plates or primordial rock. Deep mantle mystery 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