A statistical boundary layer model for the mantle D" region

The D '' region above the core-mantle boundary (CMB) plays critical roles in the dynamics of both the mantle and the core; however, the complexity of this region observed over a broad range of spatial scales defies simple interpretations as either purely thermal or purely chemical heterogeneity. Here, we formulate a 1-D, time-dependent boundary layer model for the D '' region, which provides statistical properties of the dynamics and seismic heterogeneity, by coupling thermal, chemical, and phase (TCP) variability in the layer. We assume a Gaussian-like time-dependent mantle flow, compositional stratification due to variations in iron content, heat flow variations because of changes in the local temperature gradient, and a postperovskite (PPV) phase transformation. We compute a range of TCP boundary layer model cases that are consistent with the observed seismic shear wave velocity heterogeneity in the region 50-300 km above the CMB. These results imply an average CMB heat flow near 13 TW with +/- 3 TW variations, CMB temperature of 4000 K, a large positive PPV Clapeyron slope, and an average heat transport of about 3 TW associated with deep mantle plumes.