Author's Personal Copy Evolving Core Conditions Ca. 2 Billion Years Ago Detected by Paleosecular Variation

Paleomagnetic data provide one of the few probes available to interrogate early evolution of the core. Here we apply this probe by examining the latitudinal dependence of paleosecular variation (PSV) data derived from high-quality paleomagnetic data collected from Proterozoic and Neoarchean rocks. These data define a Neoarchean geomagnetic field that was more dipolar than that during Proterozoic times, indicating a change in core conditions. The signals observed may reflect a change in forcing of the dynamo and an early onset of inner core growth. We propose a model that links evolution of the core, mantle and crust in three principal phases: (i) Before approximately 3.5 Ga, an entirely liquid core may not have hosted a geodynamo. If heat transport was sufficient across the core–mantle boundary, however, a geodynamo could have been generated. If so, sources in the shallow outer core could have been more important for generating the dynamo relative to deeper convection, resulting in a field that was less dipolar than that generated in later times. (ii) Cooling of the lower mantle between ca. 2 and 3.5 billion years ago was promoted by deep subduction and possibly coincided with inner core growth. The geodynamo during this episode was deeply-seated producing a highly dipolar surface magnetic field. (iii) After ca. 2 billion years ago, continued subduction led to large-scale core–mantle boundary compositional and heat flux heterogeneity. With these changes, shallow core contributions to the geomagnetic field grew in importance, resulting in a less dipolar field. 2011 Elsevier B.V. All rights reserved.