Extremely low geomagnetic field strength recorded in the Ediacaran Grenville Dykes.

Abstract:

Long-term variations of the geomagnetic field, observed in the paleomagnetic record, have the potential to shed much light on the evolution of Earth’s deep interior. In the Ediacaran period (635-541 Ma) 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 have been highly nondipolar and subject to rapid reversals. Paleointensity determinations of units that record the anomalous directions could potentially help to discriminate between morphologies but the spatial and temporal distribution of paleomagnetic data require improvement. Here we present new paleointensities from 11 sites from the western end of the Grenville Dyke swarm that recorded directionally anomalous geomagnetic fields around ~585 Ma. Paleointensities, obtained through microwave Thellier, Shaw and pseudo-Thellier methods, show field strength values of 3.2±2.3 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 site-mean paleointensity 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 surface field intensity. VGPs grouping in two distinct clusters with almost identical angular dispersions of VGPs may argue for the presence of an equatorial dipole. In contrast, the paleointensities 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.