RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES1004, doi:10.2205/2006ES000191, 2006
Geomagnetic field in the vicinity of the Paleozoic-Mesozoic boundary and the Siberian superplumeD. M. PecherskyInstitute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia Contents
Abstract[1] The aim of this study was to generalize the data available for the paleointensity, polarity, and frequency of reversals, and variations in the direction of the geomagnetic field in the vicinity of the Paleozoic and Mesozoic (P/T) boundary which had been marked by a peak in the magmatic activity of series superplumes, including that of the Siberian traps (251 Ma). However, no specific features have been found for the behavior of the field at that time. Some notable changes in its paleointensity had occurred 30 million years before the P/T boundary: the reversals frequency and polarity of the field changed during a period of 15 million years before the P/T boundary. The global changes of the average magnitude of the field direction variations from the unstable state with variations of 6o-10o to 6o-7o marked the lower and upper boundaries of the Kiama hyperchron (the stable state of the reversed-polarity field). The transition of the Kiama hyperchron to the Illawara hyperchron of frequent polarity changes was marked by the growth of the field variation magnitude from 6o (265 Ma) to 8o-9o (240 Ma). The regular growth of the field variation magnitude is marked with approaching to the center of Sibirian traps from the normal state, averaging 7o-8o, to 11o-12o, which demonstrated a connection between the local disturbance in the Earth core at its boundary with the mantle and the formation of the Siberian superplume. The growth of the field variation magnitude continued during a time period of 20-50 million years befor the P/T boundary and maximum activity of the Siberian trap formation, reflecting the time of the superplume rise from the base of the mantle to the Earth surface. This pattern is similar to the World magnetic anomalies, modern plumes, and the plumes in the vicinity of the Mesozoic-Cenozoic (Mz-Kz) boundary, this proving the same origin of the lower-mantle plumes and world magnetic anomalies. Introduction[2] The boundary between the Paleozoic and Mesozoic eras was marked by the intensive plume-type magmatism, associated with the activity of the Siberian superplume and other plumes, the origin of which is believed to have been associated with the Earth core and mantle boundary [Ernst and Buchan, 2003; Grachev, 2000; and others]. In this case, this must have been reflected in the behavior of the geomagnetic field. As follows from the analysis of the geomagnetic field in the Cenozoic and in the vicinity of the Mz-Kz boundary [Pechersky, 2001; Pechersky and Garbuzenko, 2005], a change in the core conditions, leading to the geomagnetic field reversals and to changes in the paleointensity, is not directly related to this boundary, or to the generation of lower-mantle plumes, or to the generation of the magnetic field direction variations. The magnitude of the field direction variations grows closer to the epicenters of lower-mantle plumes, the vigorous magmatic activity of which being close to the modern one (Afar, the volcanoes of the Khamar-Daban Ridge and of the Bolshoi Anyui R. basin, and the Buve, Hawaii, Iceland, Reunion, and Samoa volcanic islands), or to the Mz-Kz boundary (North-Atlantic volcanic province and Deccan traps). However, the origin of these plumes, which is usually correlated with the growth of the magnetic field variation magnitude, took place 25-50 million years befor present-day or before the Mz-Kz boundary. This "retardation" is usually associated with the time of the plume rising from the core-mantle boundary to the Earth surface. A change in the core condition, which caused the geomagnetic field reversals, also began some 20 million years earlier than the Mz-Kz boundary.
[3] The retardations of the onsets of the geological
eras from the minima of the reversal frequencies
were reported for the Phanerozoic
[Khramov et al., 1982;
Molostovskii et al., 1976;
Pechersky and Didenko, 1995]
and for the whole of the Neogaean
[Pechersky, 1997, 1998].
This retardation from the reversal frequency
peaks is typical of the average velocity peaks of
the continent motions
[Pechersky, 1998].
This retardation varies from 20
to 60 million years with the average value being
35
[4] The aim of this paper is to analyze the behavior
of the geomagnetic field in the time interval of
340 Ma to 200 Ma and the potential association of
the Siberian superplume with it. This time
interval includes the P/T boundary and the
potential time interval prior to, during, and
after the formation of the Siberian plume and its
manifestations on the Earth surface. According to
many data, the time of the igneous activity of
the Siberian traps was dated 251
Paleointensity[5] The results of determining paleointensity using the Thellier, Wilson-Burakov, van-Zijl, and Shaw methods [Khramov et al., 1982; Merrill and McElhinny, 1983] and the dipole magnetic moments calculated on its basis are collected in the database and generalized. The authors of the latest generalization [Shcherbakov et al., 2002] used only the most reliable determinations, performed using the Thellier method and its modifications. These data suggest that in the time interval of 330-280 Ma the paleointensity of the magnetic field was high and then declined abruptly (averagely two times) and remained as such up to the time of 200 Ma. It follows that the P/T boundary, and hence the peak of the igneous activity of the Siberian traps (251 Ma) fell into the interval of low paleointensity values [Shcherbakov et al., 2002], and were not fixed in the paleointensity. Yet, the data available are not sufficient to judge about the fine details of the global paleointensity behavior and, especially, about its local specific features. It should be emphasized that the paleointensity decline and the high growth of paleointensity variations was dated 280 Ma, that is, earlier than the boundary between the Kiama reversed polarity hyperchron (steady-state field) and the hyperchron Illawara of the frequent changes of the polarity (unstable state of the field), which has been dated 265 Ma. The Geomagnetic Polarity and Frequency of Geomagnetic Reversals
Variations in the Geomagnetic Field Direction
[7] The total magnitude of the geomagnetic field
direction variations was determined using the
standard angular deviation
S=81/ K1/2,
where
[8] In contrast to the other geomagnetic field characteristics, briefly reported above, the abundant data available for the magnitude of the field direction variations, both in time and space, are sufficient to analyze not only the global behavior of this field characteristic, but also some local features, relative, in particular, to the center of the Siberian traps. [9] In order to describe the S behavior in the vicinity of the Paleozoic-Mesozoic (P/T) boundary, I used the Paleomagnetic Database (GPMDB-2005), and chose the paleomagnetic data, ranging roughly from 200 Ma to 340 Ma in age. This choice was based on the following reasons: (a) the basic impulse of the Siberian trap activity can be dated 251 Ma, some less intensive magmatic activity was recorded later, during the Triassic [Gurevich et al., 2004; Ivanov et al., 2005]. In order to reconstruct a more complete pattern of the geomagnetic field variation, I used the time interval after the end of the Siberian trap magmatism, namely, 250 Ma to 200 Ma; (b) earlier, using the examples of the modern igneous activity of the plumes [Pechersky, 2001], and of the plumes in the vicinity of the Mz-Kz boundary [Pechersky and Garbuzenko, 2005], we found that closer to the plume epicenter the S value and its scatter increase notably, this growth being especially significant during the period of ~20-50 million years before the beginning of the high magmatic activity of the plume at the Earth surface. In this connection I used the time interval overlapping the potential time of the plume rise, namely, 270-300 Ma. For comparison, I added the time interval of 310-340 Ma, preceding the beginning of the Siberian Plume formation. [10] The next step was to sort out the data chosen from the paleomagnetic data base into four categories: [11] (1) the unreliable paleomagnetic data obtained for not more than ten samples, the thermal demagnetization was not higher than 200o C, the AF demagnetization was not higher than 15 mT, the precision parameter K<7, the confidence angle a95>25, the coordinates of the paleopole were different greatly from the average pole for the given time for the continent in question, the paleomagnetic determinations being considered as the results of the remagnetization of the older rocks. The determinations of this kind were discarded; [12] (2) the low reliability of the paleomagnetic determinations with the number of the samples not more than 20, the cleaning temperature lower than 400o C, the AF cleaning being lower than 30 mT. The index of the paleomagnetic reliability was 0.1 for this group of samples. This index was used as a weight value in calculating the average S values; [13] (3) the intermediate reliability of the paleomagnetic determinations with the number of the samples higher than 20, the thermal cleaning was performed at temperatures not lower than 500o C, the AF cleaning was not less than 50 mT. There were examples of positive geophysical tests (fold, pebble, reversal or baking tests). The index of paleomagnetic reliability was 0.5. This index was used as the weight in calculating the average S values. [14] (4) the high reliability of paleomagnetic determinations with the number of samples larger than 20, the obligatory complete thermal demagnetization and AF demagnetization with the component analysis, the identification of the characteristic NRM component, and the positive fold, pebble, baking, and reversal tests. The index of paleomagnetic reliability, as high as 1.0, was used as the weight in calculating the average S values. [15] It should be emphasized that the geophysical tests of paleomagnetic reliability, such as the fold, pebble, reversal, and baking tests, are important to prove the identification of the primary NRM component, yet, they cannot be used to prove that the S value corresponds to the variation magnitude of the geomagnetic field direction. As will be demonstrated below, the scatter of the S values is very large, primarily, because of some technical and methodological reasons, such as not complete cleaning, measurement errors, magnetic biasing in the course cleaning, the uncertainty of the age interval during the calculation of the paleomagnetic direction, and the like. For this reason we can be sure only of the average and modal S values.
[18] In determining the coordinates of the paleomagnetic observation site, we used two factors: the paleolatitude, calculated from the paleomagnetic inclination of the given site, and the position of the site in the map of the "suitable" age reconstruction. The fact is that in many cases the Database offers a fairly broad range of rock and paleomagnetic determinations ages. For this reason, I selected a map where the position of the point in question corresponded best of all to its paleolatitude determined using the paleomagnetic inclination. Also, we took into account the agreement of the polarity of the given paleomagnetic determination with the time-scale of geomagnetic polarity [Gradstein et al., 2004], within a five-million year averaging.
![]() [22] Discussed below in more detail is the behavior of the variation of the geomagnetic field direction during the main time intervals relative to the P/T boundary and to the center of the Siberian trap, which is believed to have been the epicenter of the plume. It should be emphasized that the distance from the plume epicenter to the region of its origin at the core-mantle boundary was about 3000 km, and its rising path being often inclined [Ernst and Buchan, 2003]. Combined with the insufficient accuracy of the paleomagnetic direction determinations and reconstructions, this fact might have caused the significant scatter of the S p values. It is pertinent to remind that each of these time intervals embraces 15 to 40 million years. Consequently, with the above mentioned uncertainty of the distance to the source of the plume and the long period of time compared to the time of the unstable state at the core-mantle boundary and, hence, the unstable state of the geomagnetic field, the conditions closer to the plume epicenter must be reflected, first, in the growth of the variation magnitude, that is, in the S p value and, secondly, in the growth of the S p value scatter. Moreover, the S p distribution relative to the Siberian plume epicenter could be disturbed notably by the other plumes of different ages, which might have been active during the time period discussed. Apart from the reasons mentioned above, one should not forget technical errors, the inaccuracies of the measurements, the "purity" of identifying the primary NRM components (the presence of stable secondary components), and the like. All of them augmenting the scatter of the S p unit values, the mean and modal values being more objective quantitative characteristics.
Discussion of the Results[28] The bimodal distribution of the S p values was caused by two factors: (1) the global first S p mode exists throughout the time interval of 340-200 Ma, irrespective of the distance to the Siberian traps (Figures 8 and 9); (2) the local second, higher S p mode appears only in the time interval of 300-270 Ma (Figure 9) and at relatively small distances from the center of the Siberian traps and disappears away from it (Figures 11 and 12). Consequently, during the time period discussed, the variation magnitude of the geomagnetic field direction was averagely 7o-8o (Figure 6b), this being a global effect characterizing the normal state of the geomagnetic field predominantly of reversed polarity. Moreover the transition from the Kiama reversed polarity hyperchron to the Illawara hyperchron of frequent polarity changes had a poor effect on the average variation magnitude (Figure 6b): it grew 1o-2o larger. This background was overlapped by the anomalous state of the geomagnetic field, which was of local character. Obviously, this was caused by the high disturbance of the "normal" state of the geomagnetic field in the area where the Siberian plume was being generated. [29] The large scatter of the S p values in the vicinity of the Siberian trap center suggests the relatively short existence of any large-magnitude variations. It is known that the S p values get into each of the time intervals concerned during the time period of 15-40 million years. The short-time existence of the anomalous magnitudes of the field variations and plume existence is proved by the following fact. Approaching to the centers of modern world magnetic anomalies, we observe the similar pattern of the growing magnitudes of the geomagnetic field direction variations, the lifetime of the world anomalies being shorter than 20 thousand years [Pechersky, 2001]. We can suggests a close relationship between the sources of the world magnetic anomalies and plume formation, this being emphasized by the short existence of the world magnetic anomalies and the flows of most of the Siberian traps. However, apart from the short-term intensive trap formation, there are some long-lived hot spots, such as, the Hawaii, Galapagos, Iceland, and others, the sources of which had been lower-mantle plumes [Courtillot et al., 2003; Ernst and Buchan, 2003]. There is no contradiction here. The impulses causing the generation of the geomagnetic field variations and plumes are very short, yet, the plume chambers produced at the base of the mantle can exist longer than a hundred million years and they are not associated with the core events. [30] The S p dependence on the distance to the center of the Siberian traps is obvious in the time interval of 300-270 Ma (Figures 11 and 12). Accordingly, the "retardation" of the magmatism from the exited state of the core, which caused the higher variation of the geomagnetic field was 20-50 million years. These estimates agree with those obtained earlier for the magmatic activities of the modern plumes and for the plumes refer to the Mz-Kz boundary [Pechersky, 2001; Pechersky and Garbuzenko, 2005]. This long interval can be explained by the following two alternatives: [31] The first trivial cause is associated with the uncertainty of dating the rocks and paleomagnetic determinations. The highest S p values are referred, as mentioned above, to the time interval of 300-270 Ma, the average time of "retardation" being 35 million years. [32] The second nontrivial cause allows us to suggest that this significant event was caused by the repeated "bursts" of the core reactivation which caused the formation of the series of plumes, all of them representing the Siberian superplume. It appears that not all of the rising plumes, which had originated in the time interval of 300-270 Ma in the vicinity of the core-mantle boundary and reached the surface of the Earth, except for the largest and most powerful "burst" which reached the Earth surface in the form of the main stage of the Siberian trap magmatism.
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.
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Index Terms: 1521 Geomagnetism and Paleomagnetism: Paleointensity; 1535 Geomagnetism and Paleomagnetism: Reversals: process, timescale, magnetostratigraphy; 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism. ![]() Citation: 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 SciencesPowered by TeXWeb (Win32, v.2.0). |