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

Tinghong Zhou, John A. Tarduno (), Francis Nimmo, Rory D. Cottrell, Richard K. Bono, Mauricio Ibanez-Mejia, Wentao Huang, Matt Hamilton, Kenneth Kodama, Aleksey V. Smirnov, Ben Crummins and Frank Padgett
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Tinghong Zhou: University of Rochester
John A. Tarduno: University of Rochester
Francis Nimmo: University of California
Rory D. Cottrell: University of Rochester
Richard K. Bono: Florida State University
Mauricio Ibanez-Mejia: University of Arizona
Wentao Huang: Chinese Academy of Sciences
Matt Hamilton: University of Oklahoma
Kenneth Kodama: Lehigh University
Aleksey V. Smirnov: Michigan Technological Univ.
Ben Crummins: University of Rochester
Frank Padgett: University of Rochester

Nature Communications, 2022, vol. 13, issue 1, 1-7

Abstract: 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.

Date: 2022
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DOI: 10.1038/s41467-022-31677-7

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