Assessment of a Stable Region of Earth's Core Requiring Magnetic Field Generation over Four Billion Years

We discuss whether the emergence of a sta ble region at the top of Earth's core is consistent with the continuous generation of a magnetic field over 4 billion years using a one-dimensional (1-D) thermal and compositional evolution model. The key points of this study are to revise the recent assessment scheme of a stable region for a 1-D radial convective structure, apply for a realistic reference state of the Earth's core, and incorporate the core-mantle chemical coupling modeled as the downward chemical flux across the core-mantle boundary (CMB). First, for a given present-day thermal and compositional structure of Earth's core, a stable region can be found when the present-day CMB heat flow is lower than the isentropic heat flow, which is below 12 TW without core-mantle chemical coupling, whereas this threshold becomes as large as 14 TW when the chemical coupling operates at the CMB. Second, we perform a backward time integration of the thermal and magnetic evolution model from the present-day state to t = 4.6 Ga. With current constraints on the present-day CMB heat flow (15–17.5 TW), a stable region at the top of Earth's core would not be consistent with the continuous generation of the magnetic field. However, there are exceptional cases that may satisfy both the existence of a stable region and continuous generation of the magnetic field over 4 billion years when CMB heat flow allows lower values between 11 TW and 13.75 TW, resulting in the maximum thickness of the stable region of 50 km and 75 km with and without CMB chemical coupling, respectively. Therefore, a stable region at the top of Earth's core cannot be confirmed at the present point because of uncertainties in the present-day CMB heat flow and thermal conductivity of Earth's core.