Towards a Resolution of the New Core Paradox

Abstract:

Paleomagnetic observations that indicate a geodynamo as old as 4.2 Gyr are at odds with the “new core paradox”, which claims insufficient energy to drive the ancient geodynamo prior to inner core nucleation. Recent upward revisions to the thermal conductivity of iron in the core strain the energy budget by increasing the conductive heat flow that must be overcome in order to drive thermal convection and maintain an ancient geodynamo. We address this paradox by computing thermal-magnetic evolutions of the Earth using a 1-D model with parameterizations for heat sources and sinks in the mantle and core. This model includes a number of important new features that have not been previous coupled in a single model. New features in the mantle model include enhanced mantle heat loss to due extrusive volcanism, crust formation and insolation, and latent heating due to magma ocean solidification. New features in the core model include realistic iron conductivities, time-dependent partitioning of light elements (O, S, and Si) between solid and liquid and their effect on the liquidus depression and gravitational energy release. Core evolutions derived from an energy model are compared to an energy-entropy model, which accounts for ohmic dissipation. We identify the conditions necessary to avoid shut-down of the geodynamo prior to inner core nucleation and to maintain a strong magnetic field over the last 4.2 Gyr. Using a dipole field scaling law derived from numerical dynamo models we compare the predicted paleo-dipole intensity to the paleointensity record and speculate on possible observational evidence for the inner core nucleation event.