Extremely Weak Early Cambrian Dipole Moment Similar to Ediacaran: Evidence for Long-Term Trends in Geomagnetic Field Behaviour?

Paleointensity data can yield insight on the state of the geodynamo, providing constraints on deep Earth events and enabling analysis of long-term trends in the paleomagnetic field. The Ediacaran (635 Ma- 539 Ma) is a period of discrepant paleomagnetic behaviour that was recently characterised by sustained, extremely weak, paleointensity. The interval also coincides with some of the most recent estimates for Earth's inner core nucleation (ICN) age, determined from numerical geodynamo models and analysis of long-term paleointensity data. However, the field strength during the subsequent Cambrian period (540 Ma-485 Ma) is largely unknown with almost no data. Here, we provide high-quality paleointensity results for the Cambrian. A Grenville dyke (& SIM;590 Ma) that was baked by the Chatham-Grenville stock (532 Ma), slowly cooled at a rate controlled by the stock and recorded the paleointensity averaged over this interval (up to several tens of thousands of years). The characteristic paleomagnetic directions of the dyke are well-defined and consistent with those previously obtained from the Chatham-Grenville and Mont Riguad stocks. Paleointensity data were obtained using multiple methods and indicate an extremely weak field during a period coincidental with evidence for hyper-reversing activity extending into the late Cambrian. The dipole strength is similar to that of the 'ultra-weak' Ediacaran and may suggest that this paleomagnetic behaviour persisted into the Cambrian. The cause of this weak-field interval remains enigmatic but an ap-proximate 200-million-year quasi-periodicity in dipole strength extending across the entire Phanerozoic is not ruled out. (C) 2022 The Author(s). Published by Elsevier B.V.