Early Cambrian renewal of the geodynamo and the origin of inner core structure

Abstract

Paleomagnetism can elucidate the origin of inner core structure by establishing when crystallization started. The salient signal is an ultralow field strength, associated with waning thermal energy to power the geodynamo from core-mantle heat flux, followed by a sharp intensity increase as new thermal and compositional sources of buoyancy become available once inner core nucleation (ICN) commences. Ultralow fields have been reported from Ediacaran (~565 Ma) rocks, but the transition to stronger strengths has been unclear. Herein, we present single crystal paleointensity results from early Cambrian (~532 Ma) anorthosites of Oklahoma. These yield a time-averaged dipole moment 5 times greater than that of the Ediacaran Period. This rapid renewal of the field, together with data defining ultralow strengths, constrains ICN to ~550 Ma. Thermal modeling using this onset age suggests the inner core had grown to 50% of its current radius, where seismic anisotropy changes, by ~450 Ma. We propose the seismic anisotropy of the outermost inner core reflects development of a global spherical harmonic degree-2 deep mantle structure at this time that has persisted to the present day. The imprint of an older degree-1 pattern is preserved in the innermost inner core.

MCADAM: A Continuous Paleomagnetic Dipole Moment Model for at Least 3.7 Billion Years

“…In total, 295 new sites from 34 studies have been added to the PINT v8.1.1 database, increasing the total number of site mean records ( N Sites ) to 4,648. These data include contributions constraining the field during the Cambrian/Ediacaran (e.g., Thallner, Biggin, & Halls, 2021; Thallner, Biggin, McCausland, & Fu, 2021; Thallner et al., 2022; Zhou et al., 2022) and Neoproterozoic (e.g., Lloyd, Biggin, et al., 2021), which remain under‐sampled relative to other geologic intervals.…”

Can homogeneous nucleation resolve the inner core nucleation paradox?

“…Prior to inner core growth, the geodynamo must have been powered by other means such as secular cooling, radiogenic heating or precipitation of light elements (O'Rourke and Stevenson, 2016;Hirose et al, 2017;Badro et al, 2018;Wilson et al, 2022). Because it presents such a fundamental change in regimes, the nucleation of the inner core is perhaps the most important event in the thermal history of the core and might present an observable signature in the paleaomagnetic record (Biggin et al, 2015;Bono et al, 2019;Zhou et al, 2022;Davies et al, 2022). Despite this the age of the inner core is unknown and controversy over the thermal conductivity of the core has led to a wide range of inner core age estimates (e.g.…”