A moment of weakness: The anomalous geomagnetic field in the Ediacaran period

Long-term variations in geomagnetic field behaviour can contain essential information on deep Earth processes. A major topic of debate has been the age of the onset of Earth’s inner core nucleation, as uncertainty in the value of thermal conductivity of the core allows for a wide range in age estimates. Recently, an inner core age in the late Ediacaran period (635-538 Ma) has been inferred from an ultra-weak palaeointensity estimate at 565 Ma. Palaeomagnetic investigations using rocks from this time are known to give anomalous results and show ambiguous apparent polar wander paths in a hyper-reversing field. To date, a full characterisation of the geomagnetic field at that time has been prevented by a lack of field strength estimates. The studies presented in this thesis add 27 new and reliable field strength estimates between 0.31 ± 0.11 and 2.25 ± 0.39 ×10^22Am^2 to the record. These exceptionally weak estimates were obtained from rocks of the Grenville dykes (Canada), the Skinner Cove Formation (Newfoundland) and the Volyn traps (Ukraine). These cover the time period between 550 - 600 Ma and suggest that the geomagnetic field has been remarkably weak over a much longer time period than previously seen from palaeointensity data and starts to increase in strength at the Ediacaran-Cambrian transition, as suggested by less chaotic palaeomagnetic directions. These results could correspond to considerably lower field strengths predicted by geodynamo simulations for fields with low dipolarity before the onset of inner core nucleation. However, the context of these extremely weak palaeointensities is still unclear, as field strengths are still mostly unexplored for the time periods that surround the Ediacaran, highlighting the ubiquitous need for additional high quality palaeointensity data. New estimates of VGP dispersion and dipole moment variance for the Ediacaran were calculated using filtered data from the literature combined with the new data from this thesis. These estimates, although based on a low number of data, suggest enhanced palaeosecular variation in the Ediacaran compared to other time periods. With the high reversal frequencies, the field might have been in a transitional state throughout the Ediacaran and none of the currently available geodynamo model solutions was able to capture the full range of the unique Ediacaran field behaviour.