Quantifying the Influence of Mantle Convection on Extreme Anomalies in long-term Geomagnetic Field Behavior

The geodynamo, driven by convective processes in Earth's outer core, is thought to have been active for at least 3.5 billion years. Records of the geomagnetic field and its variability over geological timescales can be used as a proxy to study the geodynamo as well as deep Earth processes, whereby major anomalies in long-term behavior can be correlated to deep interior evolution. One such anomaly is seen in the paleomagnetic record at the Precambrian-Cambrian transition. Here the geomagnetic field was an order of magnitude weaker than the long-term average, which has been interpreted as the transition from a thermally to a compositionally driven geodynamo at the onset of inner core nucleation, the timing of which is highly debated as uncertainties in core thermal conductivity lead to appreciable errors for the inner core age. However, recent estimates of weak geomagnetic field strengths in the Devonian period hint at a similar field behavior at this time, which could be related to a reconfiguration of flow patterns in the outer core at the transition between small and large inner core regimes. Both weak-field periods would also fit into cyclic decreases and increases of field strengths in the long-term paleointensity record that occur on similar timescales as the convective overturning of Earth's mantle. Even though the cooling history of the core is governed by the heat transport through the mantle, the influence of the mantle on the geodynamo is not yet fully explored.

New simulations of 3D mantle convection using the geodynamic modeling software ASPECT, predict a range of plausible heat maps at the core-mantle boundary (CMB) using Earth-like materials and a plate reconstruction of the past billion years. We use these maps as boundary conditions for thermally driven numerical geodynamo simulations to systematically explore the impact of the most extreme variations of CMB heat flux patterns and amplitudes on the geodynamo to see if extreme anomalies in geomagnetic field behavior, like the weak-field periods in the Ediacaran and Devonian, could be caused by the mantle alone.