Evidence for an Exceptionally Weak Cambrian Geomagnetic Field: New Paleointensity Estimates From the Florida Mountains, New Mexico

The timing of the formation of the Earth's inner core remains a major mystery in the history of Earth's deep interior, with estimates ranging from >1 billion years to <500 million years. Inner core nucleation is expected to be characterized in the paleomagnetic record by a drop in magnetic field strength prior to inner core nucleation, followed by a sharp increase in strength after nucleation. Paleointensity data from the 1 Ga–500 Ma age range are, however, exceptionally sparse (with fewer than a dozen studies), which largely prevents any definitive conclusions. This study presents new paleointensity results from whole rocks aged 510 ± 5 Ma from the Florida Mountains in southwestern New Mexico. Exceptionally low paleointensity estimates (<2 μT) were measured from three sites in granite and syenite outcrops of the southwestern portion of the Florida Mountains. Detailed rock magnetic and imaging investigations, including Quantum Diamond Microscopy, suggested that the two were unreliable because their remanence was unlikely a pure thermoremanent magnetization. The third site is more trustworthy and gave an estimate of 1.2 ± 0.2 μT (N = 5) corresponding to a virtual dipole moment of $0.4\times {10}^{22}{\text{Am}}^2$, which is of a similar magnitude to the lower bound of estimates from the Ediacaran (635–541 Ma). Such a low estimate at 510 Ma appears inconsistent with recent claims that the field strengthened rapidly following inner core nucleation in the late Ediacaran. Nevertheless, the risk of this single estimate being unrepresentative of the long-term field should be recognized alongside the urgent need for more paleointensity data spanning the interval 540–440 Ma.