Search

—The previous paleomagnetic studies of the Upper Vendian Zigan Formation sediments on the western slope of Southern Ural revealed uncommonly numerous magnetic polarity zones (Bazhenov et al., 2016). The dating of magmatic zircons from the tuff interlayer constrained the age of this formation to 547.6 ± 3.8 Ma. The rate of reversals estimated from most general considerations is about 20–30 per Myr which is approximately two to three times higher than the highest reversal frequency in Phanerozoic. In this work, a more accurate estimate of the rate of reversals is obtained from cyclostratigraphic study of the lower red-rock part (74 m) of the Zigan Formation in its continuous section along the Sterlitamak–Magnitogorsk road—the longest one among those previously studied by the paleomagnetic method. The correlation of the detailed cyclostratigraphic studies with the magnetostratigraphic column allowed a more reliable estimation of the reversal rate. The studied section in which we identified 20 reversals was accumulated during 1.6 Myr, i.e., the reversal frequency in this interval was 12–13 per Myr. Our quantitative assessment shows that the previous studies overestimated the geomagnetic reversal frequency by a factor of two. However, even the reversal rate estimated in this study can be regarded anomalously high, and the previous conclusions about the existence of a period with anomalously frequent reversals in terminal Ediacaran 547.6 ± 3.8 Myr ago can be considered validated.

—The analysis of paleomagnetic data from Late Neoproterozoic complexes of Siberia and Australia is carried out. We show that the existing paleomagnetic datasets are in a disagreement with the concept of the axial-dipole configuration of the Late Neoproterozoic geomagnetic field: proposed non-actualistic models of the field do not reasonably explain the distribution of the paleomagnetic poles. We carried out analysis of paleomagnetic and virtual geomagnetic poles distribution based on simple geometric calculations. The analysis suggests that the configuration of the Late Neoproterozoic geomagnetic field was determined by the coexistence of a weak, long-lived source that was stably fixed in space, with a main dipole source that experienced sporadic multidirectional jumps within a certain preferred region of the Earth. Predominantly equatorial orientation of the main dipole source is substantiated by paleoclimate proxies. We propose a descriptive non-actualistic model of the Late Neoproterozoic geomagnetic field—the Devious Equatorial Dipole hypothesis, which brings paleomagnetic and paleoclimate data into accordance.

Various geodynamo models proposed recently in the different ways explain the processes of geomagnetic field generation and its evolution over the course of the geological history. The testing capabilities of these models are strongly limited, inter alia, by the scarcity of the reliable magnetostratigraphic data for the Paleozoic and more ancient eras of geologic time, especially for the time periods before the establishment of the Kiama and Moyero superchrons. In this work, we present the results of the magnetostratigraphic and geochemical studies of the top part of the Upper Cambrian section in the Chopko river valley, Norilsk region, which is one of the most complete Upper Cambrian reference sections in the Siberian Platform. Our studies have shown that in the Late Cambrian there has been an interval of the reversed magnetic polarity which lasted at least 1.5 million years. Together with the previous data obtained on the Upper Cambrian of the Kulumbe river section and on the Early Ordovician of the Kotuy river (Pavlov and Gallet, 1998; Pavlov et al., 2017), our result presented in this study means that between the mid-Cambrian epoch of the extremely frequent reversals (Gallet et al., 2019) and the Ordovician Moyero superchron of the reversed polarity (Pavlov et al., 2005), there intervened at least two magnetic polarity intervals (reversed and normal) whose duration was at least one million years. This conclusion quite definitely points to a low frequency of the reversals on the eve of the Ordovician superchron and supports the concept according to which a process preparing the onset of the superchron takes place at the core/mantle boundary. The data obtained in our study support the hypothesis of the existence of three geodynamo regimes with sharp, on the order of a few million years, transitions between them (Gallet and Pavlov, 2016). At the same time, overall, the quality and amount of the currently existing magnetostratigraphic data for the pre-Mesozoic and, in particular, for the Paleozoic are still insufficient for confident testing this hypothesis.

—Paleo- and rock magnetic, sedimentological, and isotope geochemical study is carried out for the carbonate member of Late Neoproterozoic Nichatka Formation (Siberian Platform, western slope of the Aldan Shield) enclosed within glacial deposits corresponding to the hypothetical event of “Snowball Earth” global glaciation. Based on the sedimentological, rock magnetic, and geochemical indications it is established that sediments composing this member have varve-type seasonal stratification and, according to our estimates, have been accumulated for at most 13 thousand years. Obtaining the detailed paleomagnetic data for the Precambrian varves allowed us to reveal a linear trend in the distribution of the virtual geomagnetic poles and to link it with the peculiarity of secular variation of the geomagnetic field during the time span of the Nichatka Formation. The paleomagnetic record in the periglacial sediments of the Nichatka Formation testifies to their deposition close to the equator which might be considered as supporting the Snowball Earth hypothesis. However, the absence of annual temperature fluctuations within the equatorial belt makes the formation of seasonal deposits at low latitudes barely possible and completely excludes such a possibility in the conditions close to total glaciation. The contradiction between paleoclimatic and paleomagnetic data is not explained in the context of the actualistic model of the geomagnetic field. The peculiarities of the paleomagnetic record in the Nichatka Formation, similar to the record of the field during the reversal, suggest that the geomagnetic field in the Neoproterozoic could be determined by substantial contribution of the low-latitude non-axial-dipole component. This peculiarity of the Neoproterozoic geomagnetic field can explain the entire set of the worldwide paleomagnetic data implying low latitude glaciations in the Neoproterozoic.