Variations in geomagnetic reversal frequency during the Earth's middle age

We have obtained new magnetostratigraphic results from two Precambrian sedimentary sections from eastern Siberia and the southern Urals dated between similar to 1100 Ma and similar to 800 Ma. Sample magnetizations from the uppermost Mesoproterozoic Talakh-Khaya section in the Siberian Uchur-Maya region appear to be mostly carried by a mixture of magnetite and hematite. A sequence of 33 magnetic polarity intervals is recorded within the section. All reversals occur in the first similar to 24 m, while the upper similar to 160 m are characterized by a single magnetic interval of normal polarity assuming a Northern Hemisphere position of Siberia around 1000 Ma. The Lower Neoproterozoic Uralian Minyar section also possesses an ancient magnetization carried by magnetite and hematite. The high-temperature magnetization component obtained from the Minyar section establishes a sequence of 43 magnetic polarity intervals. Positive reversal tests obtained from the data sets presented here and from other previously analyzed Proterozoic sections provide no convincing evidence for a long-standing asymmetric geomagnetic field during the Proterozoic that would make the Precambrian field markedly less dipolar than during the Phanerozoic. The new data further reveal the occurrence of sharp transitions and alternations between long periods without any reversal (one superchron is observed from the Talakh-Khaya section) and periods with high reversal frequencies, possibly larger than 5-10 reversals per Myr. This characteristic may well be an important property of the Precambrian field, although rather sudden transitions between reversing and nonreversing states of the geodynamo may not be unique to this period.