Dan Thallner , Andrew John Biggin, Henry Halls

Long-term variations of the geomagnetic field, observed in the palaeomagnetic record, have the potential to shed much light on the evolution of Earth’s deep interior. With a geomagnetic field characterised by anomalous directions and ultra-low intensities, the Ediacaran period (635-541 Ma) is a time of special interest. Steep and shallow directions, leading to virtual geomagnetic poles (VGPs), separated by angles of up to 90° and very close in age could have recorded a geomagnetic field switching between axial and equatorial dipole-dominated states. Alternatively, the field may simply have been highly nondipolar and subject to rapid reversals. Palaeointensity determinations of units that record the anomalous directions could potentially help to discriminate between morphologies but the spatial and temporal distribution of palaeomagnetic data require improvement. Here we present new palaeointensities from 11 sites from the western end of the Grenville Dyke swarm that recorded directionally anomalous geomagnetic fields around ~585 Ma. Palaeointensities, obtained through microwave Thellier, Shaw and pseudo-Thellier methods, show field strength values of 2.9±2.2 µT and corresponding virtual dipole moments of 0.3-1.7 x1022 Am2. These field strengths are an order of magnitude weaker than the present-day field. The most extreme palaeointensity values of 1.4-2.1 µT are half as strong as seen in previous studies of the Ediacaran field and as low as Mars’ recently measured crustal field intensity, giving a new lower bound for the Earth. VGPs grouping in two distinct clusters with almost identical angular dispersions of VGPs (SB = 18.5° and 18.9°) may argue for the presence of an equatorial dipole. In contrast, the palaeointensities associated with the steep and shallow components are indistinguishable. This observation, together with the overall very large VGP dispersion may rather support that the Grenville Dykes have recorded enhanced secular variation linked to a highly unstable, multipolar and reversing field.