Late Ediacaran magnetic field hyperactivity: Quantifying the reversal frequency in the Zigan Formation, Southern Urals, Russia

Abstract The behavior and strength of the Late Ediacaran-Early Cambrian magnetic field may provide a unique background for unravelling controversial plate reconstructions, the nature of the inner core and even biological evolution. Recent studies suggest that the Ediacaran dipole field strength was extremely low and that the magnetic field was in a hyperactive reversing state (>4–6 R/Ma) well into the Cambrian. Data supporting hyperactive reversals are derived from numerous studies of sedimentary successions in Baltica including the 547 Ma Zigan formation in the Urals. Although some of the younger, Cambrian-age, sedimentary sequences have robust age control, estimates of reversal frequency in the Zigan Formation was based on average sedimentation rates. In this study, we returned to a thick (~75 m) section of the Zigan and sampled for both magnetostratigraphic and magnetic susceptibility at a finer scale (~20 cm). The magnetic susceptibility data were used to evaluate orbitally-forced frequency signals that my provide a higher resolution estimate of reversal frequency. In particular, the stable long eccentricity signal of 405 Kyr was recognized in a frequency analysis of magnetic susceptibility. The identification of this periodic signal allowed us to refine the estimate for reversal frequency in the section to 20 R/Ma. Although slightly smaller than an original estimate of 25 R/Ma, this rate is well above the Phanerozoic background reversal rate and 3–5 times higher than the cut-off for hyperactivity. Our study thus confirms the hyperactive reversal interval from at least the latest Ediacaran until sometime in Epoch 3 of the Cambrian.