We established a mastercurve “Baikal 200” of relative paleointensity, which represents a new synthetic paleomagnetic archive for Central Eurasia. The synthetic record is composed of mean values of the six records with respect to a sliding time window of 2 ka. This compilation has been restricted to the last 200 ka in order to maintain a representative population of points (between 2 and 68). However, we present this synthetic record together with individual records and the reference paleointensity curve (Fig. 11) for the following reasons: – Each relative paleointensity record has a different resolution (e.g., sedimentation rates in CON 01-605-3 are five times higher than in VER 98-1-14). During stack procedure, smoothing of the data had the effect of lowering the resolution of the paleomagnetic information. – This stack does not provide more information on timing of the geodynamo changes since the records are tuned to ODP Site 984.
The concentration in ferromagnetic particles is much lower in the interglacial sediments than in glacial sediments. Therefore, the NRM needs to be normalised by a concentration-related parameter, such as κLF, ARM or SIRM, in order to generate relative paleointensity records free of the effects of concentration (Fig. 5). The three intensity records look quite similar in the topmost part of the sedimentary column, but intensity values are lower in the bottom of the column when using κLF or SIRM instead of ARM as a concentration parameter. Low values of ARM/SIRM are also observed in the bottom of the sedimentary column, while SIRM values remain constant. The discrepancies between the relative paleointensity records result from a relative high amount of coarse magnetic grains (high values of SIRM), which lower the intensity carried by small magnetic particles preferentially contributing to the ARM.
No significant HIRM change is observed at the transition between oxidising and reducing conditions in the sediment (Fig. 9A). This implies that HIRM is not affected by redox conditions and further confirms that the “hard” magnetic mineral content is the best tracer of detrital input (Peck et al., 1994). On the other hand, the S-ratio seems to be related to the redox conditions in the sediment (see Section 7.2). The ARM has also to be considered with caution as it is mainly influenced by the ferrimagnetic contribution, which is itself influenced by post depositional processes. This is seen in Fig. 9 where ARM variations are partly influenced by S-ratio variations.
Three representative vector endpoint diagrams: (A) sample from a clay-rich layer, exhibiting normal polarity; (B) sample from a clay-rich layer, exhibiting reverse polarity, with the separation of the stable remanence after removal of the normal viscous overprint; (C) instable remanence of a sample from a diatomaceous layer, characterised by a low S-ratio, indicative of reductive magnetite dissolution: see text for details. Axis labelling is in mA m−1.
Selected intervals of down-core variations of normalised relative paleointensity, ChRM inclination and declination, and the reversal angle. (A) Core CON 01-603-2: numbers in the simplified lithological column indicate marine isotope stages (MIS), after Fig. 3. The paleomagnetic data show a geomagnetic excursion with a short, but full, reversal of the local field vector at the beginning of MIS 6. (B) Core VER 98-1-1: in this case, the excursion represented by a strong deviation of the reversal angle during a period of low intensity occurs in MIS 3 and corresponds to the Laschamp event. (C) Core VER 98-1-1: in this case, the excursion is also represented by a strong deviation of the reversal angle during a period of low intensity. Again this occurs in MIS 3 and corresponds to the Laschamp event.
Selected intervals of down-core variations of normalised relative paleointensity, ChRM inclination and declination, and the reversal angle. (A) Core CON 01-603-2: numbers in the simplified lithological column indicate marine isotope stages (MIS), after Fig. 3. The paleomagnetic data show a geomagnetic excursion with a short, but full, reversal of the local field vector at the beginning of MIS 6. (B) Core VER 98-1-1: in this case, the excursion represented by a strong deviation of the reversal angle during a period of low intensity occurs in MIS 3 and corresponds to the Laschamp event. (C) Core VER 98-1-1: in this case, the excursion is also represented by a strong deviation of the reversal angle during a period of low intensity. Again this occurs in MIS 3 and corresponds to the Laschamp event.
Dashed lines mark some correlation levels of the cores from Lake Baikal with the dated δ18O record from ODP 677 (Shackleton et al., 1990), see also Fig. 3. Diagenetic features such as dissolution of magnetite and mineralization of greigite are marked according to Fig. 4. Laschamp and Iceland Basin excursions are indicated as La. and Ic., respectively. Inclination and declination records could provide information on paleosecular variations (periodicity ≤105 years and directional variability <20°, according to Butler, 1992). In the present study, we did not interpret inclination and declination records in terms of paleosecular variations since slight sediment disturbances could produce slight deviations of ChRM declinations and inclinations. (see Fig.7)
The concentration in ferromagnetic particles is much lower in the interglacial sediments than in glacial sediments. Therefore, the NRM needs to be normalised by a concentration-related parameter, such as κLF, ARM or SIRM, in order to generate relative paleointensity records free of the effects of concentration (Fig. 5). The three intensity records look quite similar in the topmost part of the sedimentary column, but intensity values are lower in the bottom of the column when using κLF or SIRM instead of ARM as a concentration parameter. Low values of ARM/SIRM are also observed in the bottom of the sedimentary column, while SIRM values remain constant. The discrepancies between the relative paleointensity records result from a relative high amount of coarse magnetic grains (high values of SIRM), which lower the intensity carried by small magnetic particles preferentially contributing to the ARM.
Selected intervals of down-core variations of normalised relative paleointensity, ChRM inclination and declination, and the reversal angle. (A) Core CON 01-603-2: numbers in the simplified lithological column indicate marine isotope stages (MIS), after Fig. 3. The paleomagnetic data show a geomagnetic excursion with a short, but full, reversal of the local field vector at the beginning of MIS 6. (B) Core VER 98-1-1: in this case, the excursion represented by a strong deviation of the reversal angle during a period of low intensity occurs in MIS 3 and corresponds to the Laschamp event. (C) Core VER 98-1-1: in this case, the excursion is also represented by a strong deviation of the reversal angle during a period of low intensity. Again this occurs in MIS 3 and corresponds to the Laschamp event.
Down-core variations of normalised relative paleointensity after removal of intervals affected by diagenesis (magnetite dissolution and/or greigite formation) and correlation to the relative paleointensity record from ODP Site 984 (Channell, 1999). In addition to AMS 14C dating and the geomagnetic excursions, the age model was completed and refined by tuning the relative paleointensity records to the equivalent record from ODP Site 984 (Channell, 1999; Fig. 8). The relative paleointensity variations in Lake Baikal and ODP Site 984 are well correlated. This confirms the global geomagnetic field origin of the relative paleointensity variations documented in the present study. In addition, it shows that local sedimentary variations have no effect on the paleomagnetic records.
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