RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES3003, doi:10.2205/2005ES000175, 2005

Ancient Reversals

[42]  The study of the Earth core evolution requires the data available for Precambrian and Paleozoic reversals. It is a change in the characteristic time and magnitude values of the geomagnetic field fine-structure elements that can provide information for changes in the liquid core state for a long period of time. It is believed that the most important parameter in this respect is the MAC wave spectrum. However, the above-mentioned doubts concerning the real existence and persistence of some "basic spectrum'' even for the case of the field during the young epochs do not allow us to believe that we can correlate the data available for different periods of time with an accuracy sufficient for judging whether some characteristic variation time periods did actually change during the period of time concerned. Unfortunately, we can just set hopes only on the correlation between the time periods of the operation and character of the geomagnetic field variations during reversals

[43]  The first results for polarity reversals in Paliozoic time were obtained during the early studies of transitional periods [Gurarii, 1968; Khramov, 1987; Khramov and Rodionov, 1980; Khramov et al., 1974; Kravchinskii, 1968; Rodionov, 1969; Rodionov and Osipova, 1985]. However, it was only recently that the repeated studies of some previously investigated reversals and the use of the data obtained for new rock sequences yielded the results which could be used to compare the processes of the reversals that had occurred about 0.6-0.5 billion years ago with those of the Late Cenozoic reversals.

[44]  The studies of the reversals recorded in the Middle Ordovician rocks (the south of the Siberian Craton), in the Middle Cambrian rocks (same region), and in the Late Riphean rocks (Southern Ural) [Komissarova et al., 1997; Rodionov et al., 1998; Surkis et al., 1999] allowed the authors to interpret the stationary field of these epochs as the sum of the fields of the basic and equatorial dipoles with the M e/M a=0.2. During reversals the axial field diminishes and passes across the zero, while the equatorial dipole (and the sectorial harmonics) remains almost unaltered. A similar model was proposed in the description of the Early Jaramillo reversal, and the potential existence of the axial and equatorial dipoles with the M value of the latter being about 8-10% of the M value of the former was confirmed by the results of studying the stationary field both at a distance from and in the vicinity of the Matuyama-Jaramillo reversal [Gurarii et al., 2000a] which, as mentioned above, agrees with the data reported in [Constable, 1992; Gubbins, 1994]. The decline of the magnetic moment during the above mentioned ancient reversals was estimated, using the Koenigsberger ratio, to be ~5, 3-5, and 2.5-5, respectively. The duration of the Late Riphean reversal was estimated to be about 20 thousand years, that of the Late Cambrian reversal, to be less than 30 thousand years.

[45]  Of particular interest was the attempt to estimate the characteristics of the behavior of the geomagnetic reversals during the periods of their frequent repetitions (Early Ordovician). The study of the four reversals following one another during this time period in the south of Siberia yielded the results that were somewhat different from the previous ones. With the duration of the subchrons between the inversions measuring 13-32 thousand years, the duration of the transitional period was merely 3-4 thousand years, the magnetic moment being 5-10 times lower [Surkis et al., 1999].

[46]  To sum up, no significant differences were found in the characteristics of the magnetic field during its ancient and Late Cenozoic reversals.


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Citation: Gurarii, G. Z. (2005), Geomagnetic field reversals: Main results and basic problems, Russ. J. Earth Sci., 7, ES3003, doi:10.2205/2005ES000175.

Copyright 2005 by the Russian Journal of Earth Sciences

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