ИСТИНА

Recently geomagnetic field hyperactivity has been suggested for the Terminal Ediacaran and its impact on the Cambrian explosion of Life has been supposed [Meert et al., 2016]. The suggestion of hyperactivity is based on the unusually high amount of magnetic polarity zones recorded in some late Ediacaran rocks of Baltica and Siberia. However, quantitative estimates of geomagnetic field reversal frequency have not yet been presented. In Siberia, an anomalous magnetostratigraphic record was detected in the Lopata Fm. redbeds (Yenisei Ridge). Here 58 zones of magnetic polarity were recorded in a 60 m sedimentary sequence. To determine the frequency of magnetic field reversal, the accumulation time of the Lopata Fm. deposits were studied using sedimentological and cyclostratigraphic approaches. The depositional settings of the Lopata Fm. were interpreted as analogues of the wadi channels in modern deserts, which can provide sedimentation rates as low as 0.2 m/1000 years. To estimate the duration of the formation of the sequence examined by the cyclostratigraphic method, magnetic susceptibility (MS) was measured along a section with a step of 0.2 m in the field. According to sedimentological estimates, such measurement frequency should be enough to recognize precession, obliquity and eccentricity astronomic cycles possibly affecting the magnetic characteristic of the rock. The multi-taper spectral analysis of measured MS variations shows a number of peaks on the periodogram which correspond to significant periods of 29.3, 7.1, 1.8, 1.3, 1.0, 0.8 m. Their ratios are very similar, and compared with those for orbital cycles of long and short eccentricity, obliquity and precession (405.6, 94.9, 27.6-36.2, 15.5-22.0 ky) [Berger et al., 1994; Waltham, 2015]. Based on this assumption the 57 inversions of geomagnetic field inversions recorded at Lopata Fm., happened during 1.2 My, suggesting an ultra-high frequency of magnetic field reversal in the Late Ediacaran, which has no analogues in the Phanerozoic eon.