Long-wavelength variations in Earth’s geoid: physical models and dynamical implications

Abstract

The seismic velocity anomalies resolved by seismic tomography are associated with variations in density that lead to convective flow and to dynamically maintained topography at the Earth's surface, the core--mantle boundary (CMB), and any interior chemical boundaries that might exist. The dynamic topography resulting from a given density field is very sensitive to viscosity structure and to chemical stratification. The mass anomalies resulting from dynamic topography have a major effect on the geoid, which places strong constraints on mantle structure. Almost 90% of the observed geoid can be explained by density anomalies inferred from tomography and a model of subducted slabs, along with the resulting dynamic topography predicted for an Earth model with a low-viscosity asthenosphere (ca. 10$^{20}$ Pa s) overlying a moderate viscosity (ca. 10$^{22.5}$ Pa s) lower mantle. This viscosity stratification would lead to rapid mixing in the asthenosphere, with little mixing in the lower mantle. Chemically stratified models can also explain the geoid, but they predict hundreds of kilometres of dynamic topography at the 670 km discontinuity, a prediction currently unsupported by observation. A low-viscosity or chemically distinct D$^{\prime \prime}$ layer tends to decouple CMB topography from convective circulation in the overlying mantle. Dynamic topography at the surface should result in long-term changes in eustatic sea level.