A multidisciplinary assessment of heat flux at the core mantle boundary

Earth's core-mantle boundary (CMB) comprises the largest density jump within the planet; its thermal structure determines the energetics of both the core and mantle. We use thermal boundary layer (TBL) theory based on a global thermochemical convection model with temperature-dependent viscosity to estimate temperature profiles of the lowermost mantle, TBL thickness, and stability timescale. We make predictions of the observable geophysical signals such a TBL would produce.

Based on temperature tie points in the upper mantle and inner core boundary and assuming adiabatic temperature gradients, a TBL in the lowermost mantle could reach a maximum thickness between 170-210 km and could persist over geologic timescales (10² Myr). It could be seismically observable using tomography, and potentially seen using single phase amplitudes and travel times. Compositionally-dependent thermal conductivity of lowermost mantle minerals help constrain these temperature gradients. Geodynamic simulations with temperature-dependent viscosity support the use of TBL theory. A TBL originating from a CMB temperature of 4000 K is consistent with estimates of the mantle heat budget, while temperatures much higher or much lower are inconsistent with the Urey ratio and the estimated secular cooling rate of the mantle.

The heat flux at the CMB also defines the energy budget for the core. Paleomagnetic observations constrain the longevity and morphology of the geomagnetic field. For a CMB temperature of 4000 K, there is sufficient energy to drive the magnetic field currently. If the thermal conductivity of iron alloys is high, the core heat flux will be slightly subadiabatic and there will be a small ( 100 km) thermally stratified layer at the top of the outer core. This region may not be seismically observable based on small temperature differences, but can be constrained geomagnetically. However, prior to the growth of the inner core the CMB heat flux must have been 30% higher than present day to sustain the geomagnetic field.