Superchron at the Mesoproterozoic-Neoproterozoic transition

Abstract

The search for old superchrons represents an important direction in the study of geomagnetic field evolution. According to recent terminology, the superchron is a time interval (and corresponding state of the geomagnetic field), during which the process of reversal of the geomagnetic field polarity that is characteristic of any other period of geological history seems to become frozen and the field remains in one of two possible stationary states for many millions to tens of millions of years. It may confidently be postulated that the superchrons are the most remarkable and, probably, the most enigmatic events in the history of the geomagnetic field. Until recently, only two such superchrons had been reliably established: Cretaceous normal polarity and Carboniferous‐Permian (Kiama Superchron) reversed polarity. Recent studies revealed a third Phanerozoic superchron in the Early‐Middle Ordovician [1‐3]. In this communication, we present evidence for a new geomagnetic superchron at the Mesoproterozoic‐Neoproterozoic transition with outlining its boundaries and duration. The statistical analysis of the duration of magnetic polarity intervals [4] demonstrates that superchrons are most likely characterized by their own distribution mode, which is different from that typical of all other magnetic polarity intervals. This inference may be considered as pointing to the relation of superchrons to some basic changes in the Earth’s dynamo work. These changes might be induced by different factors: (1) layer D' evolution and formation of plumes; (2) mantle avalanches; (3) redistribution of “cryptocontinents” at the core surface; (4) transformation of the outer core shape; or (5) lateral variations in the heat flux across the core‐ mantle boundary regardless of the above-mentioned factors [5‐10 and others]. Some researchers are pessimistic believing that superchrons reflect the complex nonlinear nature of the Earth’s dynamo, which is completely independent from any external impact and, consequently, bears no information on changeable environments in the core and at the core‐mantle boundary [11]. At the same time, the discovery of the third Phanerozoic superchron supports the earlier assumption [12] that there is a certain characteristic period existing in the frequency variation of geomagnetic field reversals and that this period is 150‐200 Ma long. Our data [3] indicate that this inference is true, at least, of the Phanerozoic, i.e., the last 550 Ma. It is understandable that the characteristic period of superchron development favors the assumption that this process is controlled by some mechanism located in the Earth’s interior. It is clear, however, that the existence of a characteristic period favorable for the superchron appearance itself needs additional evidence. Such data may be obtained only for the Precambrian by studying the frequency of Late Precambrian geomagnetic reversals. In order to understand the nature of superchrons and the mechanisms responsible for them as well as to prove the reality of these mechanisms, we should find out whether superchrons existed in the Late Precambrian and what their age is if there were any of them. When studying the Late Mesoproterozoic Malgina Formation in the Uchur‐Maya Riphean hypostratototype section of the southeastern Siberian Platform, we established that frequent variations in the magnetic field polarity at the beginning of the Malgina time (1043 ± 14 Ma) were followed by a relatively long period when it remained unchanged [13]. The existence of the interval with stable magnetic polarity was confirmed by a limited number of samples taken at several stratigraphic levels from four remote sections of the Malgina Formation. In total, not more than one-fourth of the Malgina Formation thickness was previously studied with respect to its magnetostratigraphy. In the course of this work, we carried out detailed magnetostratigraphic study of the entire Malgina Formation and