4. Discussion

[19]  Reconstructing the paleointensity from the sediment rocks, the arguments are usually presented showing that the obtained data manifest, namely, the geomagnetic field dynamics. In the practice of paleomagnetic studies such arguments used to be considered: (1) petromagnetic homogeneity of the sediment strata used to recover the paleointensity dynamics; (2) independence of the dynamics of the obtained paleointensity on the behavior of petromagnetic parameters; and (3) identity of the behavior of the paleointensity obtained from sediments of the same age. In application to our study the corresponding arguments are presented below.

[20]  1. While building the paleointensity, we used the sediments having an orientation nature of the natural residual magnetization. The Ird/Irs parameter is the parameter characterizing the ability of particles to be oriented in the magnetic field [Kurazhkovskii and Kurazhkovskaya, 2001]. In the sediment strata used by us it almost did not varied along their thickness. In this sense the sediments may be considered homogeneous. The petromagnetic homogeneity of the sediment strata is also manifested in small changes in Irs and  K.

Figure 2

[21]  2. The most variable petromagnetic parameter in the Albian-Cenomanian and upper Barremian (M1) sediments was found the increase of the magnetic receptivity of the samples (TK) after heating up to a temperature above 500^o. The variations in this parameter are due to the physical and chemical situation in the process of sediment accumulation; that is, they demonstrate the changes in the sediment accumulation conditions. The comparison of the behavior of TK and paleointensity presented in Figure 2 shows that the dynamics of the geomagnetic field intensity does not coincide with variations in the sediment accumulation conditions.

Figure 3

[22]  Significant changes in Irs were observed in the Aptian sediments. The comparison of Irs with the behavior of the paleointensity (Figure 3) shows that even very significant changes in the amount of the bearer of the residual magnetization most probably do not influence the obtained picture of the paleointensity. Thus it is evident that the results of the performed paleoreconstruction are not a consequence of the changes in the sediment accumulation.

Figure 4

[23]  3. We studied the paleointensity of some polar intervals over different sediment strata. For example, the M2 zone of the lower Barremian was studied on the basis of the North Caucasus sediment objects located at a distance of 400 km from each other. Therein the reconstructed dynamics of the paleointensity was found the same [Guzhikov et al., 2002]. The comparison of the paleointensity dynamics of the upper Barremian (M1) obtained on the basis of the sediments in the Penza region and Crimea (which are distanced by about 2000 km) showed that they coincide (Figure 4). The magnetization bearers of the Crimea sediments had mainly authigenic (chemical) origin [Pimenov et al., 2003]. These sediments are usable for creation of the paleointensity dynamics but do not allow obtaining the absolute values of the magnetic field intensity using reprecipitation. The nature of the residual magnetization of the Crimea and Saratov sediments is different; however, the paleointensity dynamics derived from them coincide. The only known factor capable to provide such a coincidence is the geomagnetic field.

[24]  The obtained material demonstrate that in the main features the studied magnetopolar intervals had similar structure of the paleointensity dynamics. Each magnetopolar epoch started with a quiet regime which was changed to a burst-like regime and then was transformed into a quiet regime of the geomagnetic field generation. This picture of the paleointensity structure of the epoch of direct geomagnetic polarity in Cenomanian-Barremian should be considered as a preliminary one, because it seems impossible to determine how completely geomagnetic events have been fixed in magnetic properties of sediment thickness. Nevertheless, the very existence of different regimes of geomagnetic field generation and their changes during a magnetopolar interval seems to us fairly convincing.

[25]  The comparison of the mean value of the paleointensity determined by us to the mean paleointensity value of the current magnetopolar epoch (0.75  H₀ ) performed by Guyodo and Valet [1999] and Petrova et al. [2002] showed that they coincide. Thus the paleointensity of the current magnetopolar epoch and the Cretaceous magnetopolar epochs of direct polarity differ not by its mean value, but by the character of its variations. According to Valet and Meynadier [1993] no such high (3  H₀ ) values of the paleointensity has been observed during the current magnetopolar epoch. The bursts of the paleointensity detected by us in Barremian-Cenomanian are not unique events in the history of the terrestrial magnetic field. According to the results of paleointensity determinations compiled in the Tanaka and Kono [1994] database, the geomagnetic field intensity values in the Tertiary reached 4  H₀. Evidently in those times the geomagnetic field generation also was of a burst-like character. The detailed picture of the paleointensity behavior obtained in this paper made it possible to formulate problems which arise while interpreting results of its determination. The paleointensity fragments of Cenomanian and Albian can be referred to the same magnetopolar interval separated be a series of short events of geomagnetic polarity changes. Such an approach shows that within one magnetopolar interval there is observed more than one burst of the paleointensity. That does not change the evaluation of the duration of paleointensity variation cycles related to its bursts, but is important for formation of the picture of the geomagnetic field structure of magnetopolar intervals.

[26]  The differences in the behavior of the mean paleointensity of separate parts of magnetopolar intervals may be due to two causes: the paleointensity behavior and the completeness of cuts studied and detailedness of the studies. Moreover, it should be noted that the geomagnetic field intensity has a complicated and still weakly studied structure. Therefore the problem (absolutely unsolved in the paleomagnetism practice) on how much data are enough for grounded interpretation of the paleointensity behavior gets a special actuality.

[27]  The rate of sedimentation accumulation similar to that used in this paper usually is evaluated as tenths of millimeter per year. This estimate means that in one sample there is averaged information on the geomagnetic field for at least 200 years. Probably postsedimentation processes and thickening of the sediments in the process of their burial increase considerably the duration of this estimate. Therefore the above obtained picture of the paleointensity behavior should have a smoothed (averaged) character, and its true amplitude values should exceed the values obtained in this paper.

[28]  The problem of the lower Cretaceous paleointensity is rather complicated and evidently can not be solved in the scope of the currently available paleomagnetic materials. It has been noted above that the mean paleointensity of the analyzed time interval is 0.8  H₀. This value exceeds the paleointensity value of the lower Cretaceous (0.5  H₀ ) obtained by Solodovnikov [1995]. At the same time, a fragmentary character of our knowledge on the lower Cretaceous paleointensity does not provide great hops that the above estimates are final.