Rapid Recovery of Earths Magnetic Field Strength after Nucleation of the Inner Core

Ultra-low paleointensity values and geodynamo modeling results independently indicate an Ediacaran age for the nucleation of Earths solid inner core. At ca. 565 Ma, the field was close to a state of collapse, with time-averaged intensities ten times less than the present day. However, because of the scarcity of paleointensity data, it is unclear when the period of ultra-low field strength ended and how the recovery of the geomagnetic field proceeded. Herein, we report new paleointensity results from the early Cambrian (~532 Ma) Glen Mountains Layered Complex anorthosites to investigate these questions. We apply the single crystal paleointensity method on plagioclase feldspar to isolate ideal magnetic carriers able to record and preserve paleofield records. Comprehensive rock magnetic and scanning electron analyses confirm the dominance of single-domain magnetic needle-shaped grains (~200 nanometer width, several microns length) in the plagioclase with a titanomagnetite composition. Paleointensities were measured with the Thellier-Coe method on crystals 1 to 4 mm in size lacking opaque inclusions visible under 10 magnification. For thermal treatments, we used a CO2 laser which affords short heating times which can limit laboratory alteration. Data collected to date from sixteen crystals pass our selection criteria, and from these we derive a time-averaged dipole moment of 4.9 1.4 1022 A m2. We apply cooling rate and anisotropy corrections, and these yield a value of 3.5 0.7 1022 A m2. These newly acquired early Cambrian paleointensity data indicate that the ultra-low field state had ended before ~532 Ma. Thus, the intensity of Earths magnetic field had recovered to about 70% of its present value, 30 Myr or less after the end of the ultra-low paleointensity period linked to nucleation of the inner core.