RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES1004, doi:10.2205/2006ES000191, 2006

Conclusion

[34]  The aim of this study was to generalize the data available for the paleointensity, reversal frequency, and the variation of the direction of the geomagnetic field in the vicinity of the Paleozoic-Mesozoic boundary, which was marked by the peak of the igneous activity of the Siberian traps (251 Ma). For this purpose I used the data, available in the Data Base and the geomagnetic polarity time-scale. However, I did not find any specific features in the behavior of the geomagnetic field for that period of time.

[35]  1. The paleointensity of the field was found to be elevated in the time interval of 330-280 Ma and then declined abruptly (averagely twice as much), remaining the same up to 200 Ma. The time interval of the igneous activity of the Siberian traps was situated within the time interval of low paleointensity values and "lagged" 30 million years behind the abrupt changes in the paleointensity.

[36]  2. The Paleozoic-Mesozoic boundary and the time of the maximum trap activity coincided with the period of frequent reversals and, hence, with the frequent changes of the geomagnetic polarity, without being recorded in the specific features of the geomagnetic field. The peak of the biota change, coinciding with the peak of the highest Siberian trap activity, lagged 15 million years behind the boundary between the Kiama hyperchron of the stable reversed polarity field and the Illawara hyperchron of frequent polarity changes.

[37]  3. The global changes of the average magnitude variations of the field direction from its unstable state with oscillations of 6o-10o to 6o-7o fell on the lower and upper boundaries of the Kiama hyperchron. As the Kiama hyperchron was superseded by the Illawara hyperchron, the variation magnitude began to grow from 6o (265 Ma) to 8o-9o (240 Ma). The P/T boundary proper did not show any specific features in the field variation magnitude. Therefore, the Paleozoic-Mesozoic boundary is not recorded in the paleomagnetic data.

[38]  4. With approaching to the center of the Siberian traps, the field variation magnitude showed a regular growth of the field direction variation magnitude from its normal state (7o-8o) to the average value of 11o-12o. This can be explained by a connection between the local excitation in the outer core of the Earth and the formation of the Siberian superplume. This growth of the variation magnitude occurred during the period of 20-50 million years before the Paleozoic-Mesozoic boundary and the maximum activity of the Siberian traps. This "retardation" seems to have been the time of the Siberian superplume rise from the core-mantle boundary to the Earth surface. This long time lagging can be explained by the inexact dating of the objects of the paleomagnetic studies and/or by the NRM age, yet, the most probable explanation is the formation of a series of plumes at that time, in the same region of the core and mantle boundary. This interpretation is validated by the existence of the compact concentrations of the high-magnitude magnetic field directions, as the potential regions of the formation of world magnetic anomalies and plumes in the time interval between 300 Ma and 200 Ma. Main part of such groups concentrated relatively close to one another, between the longitudes of 0o E and 80o E and between the latitudes of 10o N and 60o N. It is possible that the region of the exited state of the upper part of the Earth core (270-300 Ma), which was situated south of the region underlain by the Siberian traps, was the region of the Siberian superplume generation.

2006ES000191-fig17
Figure 17
[39]  5. The comparison of the results obtained for the behavior of the geomagnetic field in the vicinity of the Paleozoic-Mesozoic boundary with the data reported earlier [Pechersky, 2001; Pechersky and Garbuzenko, 2005], suggested the following regularity: the boundaries of the geological eras are not recorded in any specific features of the paleointensity, polarity, reversal frequency, and in the variations of the geomagnetic field direction. The boundaries between the eras reside in the region of frequent geomagnetic field polarity changes which occurred 15-20 million years later than the preceding hyperchrons of the steady state (single-polarity) of the magnetic field. Against the background of the "normal" field it is seen an almost similar trend toward the growth of the magnetic field variation magnitude closer to the epicenters of the modern, Greenland, Deccan, and Siberian lower mantle plumes, as well as of the world magnetic anomalies (Figure 17), this suggesting the same origin of the lower mantle plumes, differing in the time of their formation, of the world magnetic anomalies, and of the growing magnitudes of the geomagnetic field direction variations, i.e. all of them being the results of the local disturbances at the top of the liquid core. This unity is proved by the brief existence of both, the world magnetic anomalies and the activity of the Siberian and Deccan traps. At the same time the Siberian and Deccan superplumes originated during the time intervals devoid of geomagnetic field reversals. This fact suggests the different sources of the global magnetic anomalies, local geomagnetic field variations and plumes, on the one hand, and of the field reversals, on the other. The "retardation" of the magmatic activity of the plumes at the Earth surface from the time of their origin at the core-mantle boundary, is the time of the plume rise. In all cases it fits in the time periods of 20-50 million years. Differences in their rise time seem to be associated with the differences in plume rising routes, various obstacles delaying their rising, and the like.


RJES

Citation: Pechersky, D. M. (2006), Geomagnetic field in the vicinity of the Paleozoic-Mesozoic boundary and the Siberian superplume, Russ. J. Earth Sci., 8, ES1004, doi:10.2205/2006ES000191.

Copyright 2006 by the Russian Journal of Earth Sciences

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