THE ULTRA-WEAK EDIACARAN GEOMAGNETIC FIELD AND THE RENEWAL OF FIELD STRENGTH IN THE CAMBRIAN: IMPLICATIONS FOR THE STABILITY OF EARTH AND THE RISE OF ANIMALS

The discovery of an ultra-weak geomagnetic field, some 10x weaker than the present-day at ca. 565 Ma during the Ediacaran Period by Bono et al. (Nature Geoscience, 2019), together with the recent documentation of a renewal of high field intensity during the early Cambrian (Zhou et al., Nature Communications, in press, 2022), shows the two predicted signatures for the onset of inner core growth. The weak field is predicted because over billions of years thermal power for the geodynamo wanes; in the Ediacaran Period the field appears to be close to collapse. The weak field is accompanied by highly unusual magnetic directions and a hyper-reversal frequency. Some of these phenomena have been interpreted as instability of the entire solid Earth relative to the spin axis (true polar wander). Instead, these are best interpreted as manifestations of an unstable dynamo in a rotationally stable Earth, implying that true polar wander was not the main driver of late Proterozoic glaciations. The inherent protection of early animal life by the atmosphere and water implies that any direct connection between increased energetic particle flux during the ultra-weak field period and radiation or extinction events is unlikely. However, indirect effects including atmospheric change are possible. Moreover, the rapid recovery of field strength in the Cambrian, expected as new energy sources to power the geodynamo become available from inner core growth, provided critical magnetic shielding of the atmosphere just as it was becoming more vulnerable to loss from erosion by the solar wind.