Plot showing a compilation the virtual dipole moment of the geomagnetic field during the Ediacaran and Cambrian periods.
A compilation of the virtual dipole moment (VDM) of the geomagnetic field during the Ediacaran and Cambrian periods (487-635 Ma). Most of the Ediacaran features very low VDMs (about 10 percent of today’s field), values that prevail during polarity transitions of the modern field, likely indicating an instable geomagnetic field at Ediacaran times. This could be an explanation for the marked inconsistencies in Ediacaran paleogeographic reconstructions. As shown by this study (green diamonds), just before the Ediacaran/Cambrian boundary at about 539 Ma, VDMs double, a value seemingly typical of most of the Cambrian. Note the low number of paleointensity determinations that pass modern selection criteria as applied in the present study. Error bars show one standard deviation for VDM strength and published uncertainties of age estimates. Credit: Thallner et al. [2021], Figure 6
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Solid Earth

Paleogeographic reconstructions during the Ediacaran period (685-539 million years ago) are plagued with ambiguity, often related to a postulated instable configuration of the geomagnetic field at those times. Support for this is found in recent ultra-low paleointensity determinations from the Ediacaran that yield intensities typical of the field strength during polarity transitions of the modern field.

Ediacaran paleointensities have a corresponding dipole moment (with the samples’ inclination the determined intensity can be recalculated to the moment of an imaginary dipole field) that ranges between 0.5*1022 and 1.0*1022 Am2, which is about 10 percent of the moment of today’s stable field (~8.2*1022 Am2). By using a combination of methods to determine the paleointensity, Thallner et al. [2021] report a notably higher but a still low dipole moment of ~2.0*1022 Am2 from the Skinner Cove Formation (Newfoundland, eastern Canada) with an age of 550 Ma, i.e. latest Ediacaran, that heralds the change-over to a stable dipolar field in the Cambrian and later.

The authors discuss a perceived inconsistency with magnetostratigraphic information that indicates a continued ‘hyper-active’ field (i.e., a frequently reversing field) from the Ediacaran into the mid-Cambrian. However, the reversal frequency in the Cambrian is poorly constrained hampering robust conclusions. Thus, the question whether or not there would be distinct phases in the inner-core-growth regime can only be answered with an improved magnetostratigraphic record and an increased number of paleointensity data, as underscored in this study.

Citation: Thallner, D., Biggin, A. J., McCausland, P. J. A., & Fu, R. R. [2021]. New paleointensities from the Skinner Cove Formation, Newfoundland, suggest a changing state of the geomagnetic field at the Ediacaran-Cambrian transition. Journal of Geophysical Research: Solid Earth, 126, e2021JB022292.

—Mark J. Dekkers, Associate Editor, JGR: Solid Earth

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