Earth’s magnetic field originates in fluid motion of the liquid outer core. This geodynamo is probably more than 4.2 billion years old. The protection that the magnetic field provides from solar particles that might otherwise erode our atmosphere plays an important role in the formation of Earth as a habitable planet and for the evolution of life.
Over long-time scales, Earth’s magnetic field can be viewed as if it is generated by a bar magnet placed at its center and along its spin axis. This is known as a geocentric axial dipole (GAD). Billions of years ago Earth had only a liquid iron core. Only later did a solid inner core start to nucleate and it has been growing ever since. The emergence of a solid inner core in the previous liquid core changed the flow of iron, perturbed the geodynamo, and Earth’s magnetic field.
The timing of inner core nucleation has been debated, but mounting evidence points to start ~570 million year ago in the Ediacaran Period. At this time the magnetic field exhibits highly frequent polarity reversals and the weakest strength known over Earth history. This unusual field behavior is thought to be the surface expression of the onset of inner core nucleation. However, little had been known as to when and how the magnetic field transitioned to a relatively stable mode that characterize most the Phanerozoic.
A new study by scientists from China and the University of Rochester (USA) (Li, Tarduno et al., 2023) reported in Nature Communications reports data from a Cambrian global stratotype section and point (GSSP) section in South China that document Earth’s magnetic field behavior ~495 million years ago (Fig. 1). The field at this time is characterized by an interval of frequent reversals (I), followed by a brief interval of stable reversed polarity (II) and an ~80-thousand-year long interval of unprecedented polar shifts (III) before it entered into an interval of stable normal polarity (IV). During interval III, the field exhibits shifts of up to 90° from the GAD direction (Fig. 2). This type of anomaly mimics polar movement associated with inertial interchange true polar wander (TPW), the hypothetical rotation of the entire solid Earth relative to the spin axis due to a redistribution of mass (e.g., change in the distribution of subducted slabs of oceanic crust). However, significant TPW can occur only on timescale on many millions-of-years due to limits imposed by the viscosity of Earth’s mantle. The 90° polar shift observed we have discovered took place in less than 80 thousand years. Thus, it reflects unprecedent extreme unstable geomagnetic field behavior rather than a TPW.
Fig. 1 Late Cambrian geomagnetic field anomalies
Fig. 2 Non-GAD field during Interval III
A recent study shows the geomagnetic field gained strength rapidly in early Cambrian about 530 million years ago consistent with new energy sources becoming available after an onset of inner core nucleation in late Ediacaran Period. The highly unstable geomagnetic field behavior we have documented in our study (Fig. 3a) suggests that the inner core had not grown large enough to stabilize the geomagnetic field approximately 495 million years ago. Calculations of Earth’s thermal evolution suggests that the radius of the inner core was only about 35-40% of its modern value at this time.
Fig. 3 (a) Inner core is too small to stabilize the field; (b) Inner core growth stabilizes the field
The Cambrian is an important interval in the evolution of life. The well documented appearance of new animals is often called the Cambrian explosion. One hypothesis suggests that rapid inertial interchange TPW occurred in a million-year time scale during the earliest Cambrian and this stimulated evolution through environmental change, contributing to the Cambrian explosion. One important implication of our findings is that previously interpreted inertial interchange TPW could be an artifact because some data used to define it– namely from relatively quickly cooled lavas and dikes, could have recorded an unstable geomagnetic field in early Cambrian when Earth’s inner core was still small. We believe the balance of data are more compatible with stability of the solid Earth relative to the spin axis during the Cambrian explosion. This stability may actually have been a key factor fostering this profound event in evolution.
The details are reported in the paper below:
Li, Y.-X., Tarduno, J. A., Jiao, W.J., Liu, X.Y., Peng, S.C., Xu, S.H., Yang, A.H., Yang, Z.Y., 2023. Late Cambrian geomagnetic instability after the onset of inner core nucleation. Nature Communications. https://doi.org/10.1038/s41467-023-40309-7