Abstract
<title>Abstract</title> <p>Core-mantle boundary heat transfer plays a key role in Earth’s convection dynamics and magnetic field generation. Longstanding evidence of lowermost mantle thermochemical heterogeneity predicts large spatial variations in core-mantle boundary heat flux. However, observational evidence for coupled heterogeneity in the outermost core is scarce. Here, expanded spatial sampling of seismic phases sensitive to the outermost core and 3-D global wavefield modeling provide evidence for subtle regionalized stratification. Restriction of stratification to regions beneath Large Low Shear Velocity Provinces (LLSVPs), interpreted as long-lived thermochemical piles at the base of the mantle, fits the seismic measurements better than global, near-equatorial, or absent stratification. Regionally increased outermost core compressional velocity beneath LLSVPs may result from lower CMB heat flux that reduces convective vigor and promotes light element accumulation. The results support an important role for lowermost mantle heterogeneity in shaping the geomagnetic field and outermost core structure.</p>