Influence of thermochemical piles on topography at Earth's core–mantle boundary

Numerous seismic studies reveal the presence of two large, low velocity anomalies beneath Africa and the central Pacific. Efforts to characterize these anomalies have yielded a variety of interpretations over the years, both isochemical and thermochemical. Previous interpretations have included large, isochemical superplumes, clusters of smaller thermal plumes, and doming thermochemical superplumes. A conceptual mantle model that is presently growing favor involves long-lived thermochemical piles. In anticipation that nutation studies will provide better topographic constraints on the core–mantle boundary (CMB) in the future, we examine the effects of thermochemical piles at this boundary. In this study, we perform numerical modeling of thermochemical and isochemical convection as a function of convective vigor and temperature-dependent viscosity to predict the topography at Earth's CMB. Our results show that in isochemical convection, downwellings always lead to negative (depressed) CMB topography, while upwellings cause positive (elevated) topography, consistent with previous studies. However, in thermochemical convection, CMB topography and its relationship to downwellings and thermochemical piles are significantly affected by temperature-dependent viscosity. For isoviscous or weakly temperature-dependent viscosity, the piles cause negative CMB topography, while positive topography is often below cold downwellings. However, for realistic, more strongly temperature-dependent viscosity, the most negative CMB topography is below downwellings, while the topography below the piles is often slightly more positive and/or flat, similar to upwellings in isochemical models. We also show that although thermochemical piles are intrinsically more dense, the large thermal buoyancy associated with them leads to overall relative buoyancy that is on par with slabs. Consequently, thermochemical models lead to an overall reduction in magnitude of CMB topography with respect to isochemical models.