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.

Detecting dipolarity of the geomagnetic field in the paleomagnetic record

“…The obtained pole intensity ( Table 1 ) indicates that the dipole strength may have been on average 25% weaker but not significantly different from the present-day value [around 60 µT according to IGRF13 model ( 40 )], with the two following caveats. First, the estimate of should be seen as an average value and does not preclude the existence of variations in dipole strength, reflecting potential distinct energy regimes of the geodynamo—dipole low during the Devonian ( 41 – 43 ), dipole low during the Cambrian-Ediacaran ( 44 , 45 ) or intensity spike during the Stenian ( 46 , 47 ). Second, based on the present version of the ( B , I ) record, with only 29 high-quality determinations for 14 independent ages ( N ≥ 5, RSE ≤ 10%), deriving statistically robust conclusions about the existence of distinct geodynamo regimes in terms of dipolarity and/or dipole strength prior to 130 Ma deserves caution.…”

Seismic insights into Earth’s core

“…Dynamically, the IMIC may mark a gradual transition in the growth processes of the inner core, observed as a change in the crystalline structure of iron 3,10 ; the relic of two-stage formation, also detected in paleomagnetic data 18 ; or the onset or cessation of external mechanisms creating texturing 13 . We anticipate ongoing improvements to the current IMIC models will provide the underpinning measurements to orient the next generation of core evolution models.…”

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.…”