D. K. Nourgaliev1, F. Heller2, A. C. Borisov1, P. G. Yasonov1, I. Yu. Chernova1, and I. Hajdas3
1Geological Faculty, Kazan State University, Kazan
2Institut für Geophysik, ETH Hönggerberg, CH-8093 Zürich, Switzerland
3Institut für Teilchenphysik, ETH Hönggerberg, CH-8093 Zürich, Switzerland
The acquisition of information for geomagnetic field variations is among the fundamental problems of geophysics, which allows the geologists to solve some very important problems associated with the state and evolution of the Earth's core and core-mantle boundary [Bloxham et al., 1989]. Of great interest are the geomagnetic field variations with periods of 102 -103 years, which are caused by processes that operate in the deep geospheres [Korte and Constable, 2003]. Paleosecular geomagnetic variations (PSV) of such periods were discovered using historical [Jackson et al., 2000], archeomagnetic [Daly and Le Goff, 1996], and limnomagnetic [Mackereth, 1971] data. The longest PSV records were obtained using the deposits of modern lakes, which had usually accumulated throughout the Holocene (the last 10-12 thousand years). By the present time about a hundred records of this kind have been obtained. Some of them reflect fairly reliably the geomagnetic field behavior during the Holocene and have been used to derive geomagnetic field models [Constable et al., 2000; Hongre et al., 1998; Korte and Constable, 2003]. Some of the models derived for the geomagnetic field and its secular variations, obtained from archeomagnetic and limnomagnetic data are far from being perfect ones [Korte and Constable, 2003]. This stems from a number of reasons, the most important of them being the small number of sites with high-quality records of declination (D) and inclination (I); the substantial distortion of PSV magnitudes and their morphologies at the expense of various processes operating in the sediments during their deposition and diagenesis, as well as in the course of drilling and collecting samples from the cores; the not sufficiently exact dating of the sediments because of the inability to account for carbon of different genesis; differences in the ages of ChRM and the sediments, caused by the effect of postsedimentation magnetization (PDRM).
Presented in this paper are the new curves of the paleomagnetic declination and inclination of the geomagnetic field in Belorussia, derived as a result of generalizing the PSV records in the sediments of the Naroch Lake [Nourgaliev et al., 2003] and in the sediments of the Svir Lake [Borisov et al., 2003]. Also discussed in this paper are the problems of the isotopic dating of sediments in modern lakes and of the PSV magnitude smoothing in the case of the PDRM effects.
The Naroch Lake (54o51 N, 26o51 E) is situated in the northwestern part of Belorussia, in the zone of the typical topography produced by the Valdai glaciation. The Naroch Lake [Yakushko, 1971] is the largest lake of Belorussia, measuring 79.6 km 2 in area with an average water depth of 9 m, the maximum depth being 24 m. The lake has an isometric form and consists of two parts. Its southern part, as deep as 24 m, has a floor of complex topography. The bottom current operating there is responsible for the washout and redeposition of the sediments. The lake has a flat bottom topography in its northwestern part. In low-water periods these parts of the lake might have been almost separated, being connected by a narrow channel situated in the middle of the modern lake. The topography of the surrounding territory and the types of the water streams flowing into the lake suggest the poor addition of the terrigenous material into this basin. The paleobiological and lithological data suggest the following history of the water level oscillations in the lake [Tarasov et al., 1996]: the water level was very low (1) from 13600 to 13150 years ago, varied from low to intermediate one (4) from 13150 to 10900 years ago, had a low level (2) from 10900 to 9500 years ago, had a high level (6) from 9500 to 8500 years ago, had an intermediate level (5) from 8500 to 5120 years ago, had a very high level (7) from 5120 to 3850 years ago, a relatively low level (3) from 3850 to 2360 years ago, and a high level (6) from 2360 years ago to the present time. The figures in the parentheses denote the numeric values of the Naroch Lake relative level after [Tarasov et al., 1996]. In terms of lithology this lake is of interest as an intracontinental basin of intensive carbonate accumulation. All sedimentary material is highly calcareous, being represented mainly by calcareous gyttia, except for the lower part which includes a layer of calcareous sand overlain by highly calcareous clay (almost marl).
The Svir Lake (54o47N; 26o30E), located 15 km southwest of the Naroch Lake, has an elongated shape which suggests that this lake might have been a well-drained basin at some periods of time. At the present time the lake is 18 km long, its width varying from 1.5 km to 3 km with the maximum water depth of 8 m. The lake floor topography has a simple U-shaped form. The territory surrounding the lake is fairly flat with minor elevations, less than a few tens of meters high, yet the river and springs flowing to the lake carry significant amounts of terrigenous material. In contrast to the sediments in the Naroch Lake, the sediments in the Svir Lake are poor in carbonate and consist of sapropel with the high content of organic matter. The maximum thickness of the sediments is 7-8 m. The rock sequence begins with dark green compact clay which is overlain by sapropel (organic mud) with the high content of argillaceous material at its base. The sedimentary material is not stratified, this precluding the visual correlation of the core samples in lithology. The top of the sedimentary material is distinguished by its patchy pattern. The sapropel color varies downward from dark green to almost black.
Samples of lake floor sediments were collected using a piston corer designed and manufactured at the Kazan University [Borisov, 2000]. Used as a prototype was a piston corer which had been designed and used by F. J. H. Mackereth [Mackereth, 1958, 1971]. The Borisov corer is equipped with a bell-shaped sucking disk used to fix the tool at the lake floor. After the lowering of the tool to the lake floor, the bell is hermetized and calls for a significant effort to tear it off from the lake floor. Attached to its upper hole is an outer pipe with an internal movable pipe inside. The top of the outer pipe is hermetically closed by a lid to which a steel line is connected to fix the piston inside the movable pipe at the level of the upper end of the bell. Fixed to the top of the movable pipe is a piston with collars. After this equipment is fixed at the lake floor, the compass located in the upper part of the bell is arrested. The compressed gas is fed into the top of the outer tube, and the piston is lowered, together with the mobile tube, until the piston reaches the lower part of the outer pipe where it joins the bell, and the whole of the mobile tube enters the lake floor sediments. Thanks to the presence of a fixed piston the sedimentary material enters freely into the mobile pipe and does not fall out of it during its lifting. This sampling device differs from the known analogs [Mackereth, 1958] by the fact that after the inner pipe is driven into the rock material it can be drawn into the outer pipe again by delivering compressed air into the lower segment of the outer pipe equipped with a special coupling. This is an important advantage of our sampling device compared to the Mackereth sampler which can collect core samples 6 m long only in the lakes with the water depth not less than 6 m, whereas our sampling device does not require any limitations of this kind. It can be used in shallow lakes with a water depth of 1.5-2 m. The pressure, not higher than 15-20 atmospheres, is usually created in our system using cylinders filled with some compressed gas (air, CO2, and the like). However, recently, like in the case of the project described in this paper, we used a hydraulic system for intruding the inner pipe into the rock material and its subsequent removal. This system prevents the sediment from deformation and is more safe. To prevent the rotation of the mobile tube in the course of its sinking into the ground, this tube has a figure profile, conformable with the form of the piston cup. This simplifies the orientation of the core in terms of its inclination because of the fixed positions of the compass and the profile on the tube. After this equipment is raised to the ground surface the internal pipe with the core is removed and placed into a horizontal position and the cores are squeezed out of the pipe through its upper part using a special device. The upper segment of the pipe is equipped with cutters which cut cubes with a side of 20 mm from the central part of the core samples, each cube passing immediately into a nonmagnetic (polystyrene) container. This container is marked, closed, and hermetized. All cubes are placed into hermetic plastic bags and are transported carefully to the laboratory. As a rule several core columns are collected in each lake for laboratory and paleobotanic studies and 1-3 columns for paleomagnetic analysis. The Naroch and Svir lakes were investigated in 1997. Twelve columns of their bottom material cores were collected, nine of which were used for paleomagnetic studies (Table 1). The positions of the core samples in the lake were determined using a GPS receiver, the water depth was measured using an echo sounder. The rock material that remained after collecting cubes for isotopic analysis was used to carry out paleomagnetic analyses. The surface of the core remains was removed using a clean knife and placed first into aluminum foil and then into a hermetic plastic packet. The core lumps were usually 4-6 cm long.
We reconstructed changes in inclination and declination in time using the data available (Table 2). We had six isotopic datings: one for each core column collected in the Naroch Lake area (except for the column N-1) and one dating for the S-1 core column collected in the Svir Lake.
The isotopic dating of the sediments was carried out at the Institute of Elementary Particle Physics of the Federal Technological University of Switzerland using an acceleration-type mass-spectrometer. The results of this work are presented in Table 2. The isotopic ages of the samples were calculated using the relative concentrations of 14C in the rock samples. However, this dating is not correct, because the initial 14C contents in the rock samples varied in time as a function of various factors. The correct dating could be performed using a calibration curve obtained as a result of using a large number of various methods, such as, the other types of dating, dendrochronology, varvometry, historical dating, and the like. In our case we calculated the isotopic ages using a calibration curve and the OxCal program [Bronk Ramsey, 1995]. This calibration curve has a fairly complex shape and can be used to find different versions for the most probable time intervals. Listed in Table 2 are the dates falling in the 2 s interval (with 95% falling into this time interval). We chose the time intervals of the highest probability.
To represent declination and inclination variations in a time scale we used the isotopic age datings available (Table 2). We had six values of dating: one for each rock column in the Naroch Lake, except for the N-1 rock column, and one isotopic age value for the S-1 column of the Svir Lake. Proceeding from the results of correlating the magnetic susceptibility in the rock columns, all isotopic age datings were transferred to column N-2. In all cases this operation was correct because very distinct specific features were discovered in the magnetic susceptibility variation curve. This was also proved by the good correlation of the other parameters obtained from the core columns (Figures 1 and 3).
The good agreement of the D and I variations among all of the study rock columns, as well as with the records obtained for the other objects, including the archeomagnetic ones, suggests that the observed D and I variations had been most probably caused by the perennial secular variations (PSV) and represent their records for the last ~12 thousand years. We also compared the geomagnetic field variations recorded in our study with the records reported in the literature for the sediments in the lakes of northern Sweden [Snowball and Sandgren, 2002] and with the sediments of three maar lakes from the West Eifel region (Germany). The inclination variation trend (the inclination growth toward the present time) discovered in the British record, is not as distinct in the other records. All declination records (Figure 10) show good correlation, yet, have some fairly significant differences. For instance, in the time interval of 6000 to 4000 years ago, the record for the Belorussian lakes differs from the records of the nearest objects, such the Nautajarvi Lake and the North Sweden lakes. This difference seems to have been caused by a real PSV difference even at a distance as small as this. At higher latitudes, this inclination behavior is possible because of the relatively small value of the horizontal component of the geomagnetic field intensity. The declination recorded for the West Eifel lakes correlates fairly well with all of the other records in spite of the high noise level. This is the longest record of all records available, its lower part correlating well, both in inclination and declination, with the records obtained for the Belorussian lakes. It can thus be inferred that the ChRM variations, identified in this study in the sediments of the Naroch and Svir lakes represent the PSV records for the last 12 thousand years. Proceeding from the general similarity and some minor differences of the Belorussian lakes compared to the other European records available, it can be concluded that the PSV master curve obtained in this study can be used to model Holocene geomagnetic variations.
With a very good similarity between the PSV records obtained for the sediments of the Belorussian lakes and the data obtained for the other objects, our results show a substantial number of problems associated with the reconstruction of the PSV records obtained for modern lake deposits.
In the first place, this is the problem of ChRM dating. It has been demonstrated above that the ChRM age of the Belorussian lake sediments differs substantially from the radiocarbon ages of the sediments. The errors of dating the sediments of the Naroch Lake arise first because this lake water contains some "aged'' carbon which is transported by ground and surface waters from the surrounding carbonate rocks. The lake plants the remains of which were used in this study for radiocarbon dating had used not only atmospheric carbon (CO2 ), but also carbon from the water. As a result, the 14C content in the organic remains turned out to be lower compared to the remains of terrestrial plants which absorb CO2 only from the atmospheric air. As a result these plant remains show a higher radiocarbon age [Deevey et al., 1954]. It follows that the radiocarbon dating of sediments should take into account the "effect of the water reservoir'' (or the "effect of hard water'') [Deevey et al., 1954], especially in the lakes surrounded by carbonate rocks, and also in the lakes using subsurface water. The introduction of a correction for water hardness is a fairly complicated procedure [Stiller et al., 2001] which can be used in very rare cases, where the evolution history of a basin is well known. Naturally, we could not introduce this correction in the course of our radiocarbon dating of the Naroch Lake sediments. Nevertheless, we can estimate the order of this value. The sediments of the Naroch Lake differ substantially in terms of their composition from the sediments of the Svir Lake because of its present-day carbonate accumulation. We can, therefore, assume that the "effect of hard water'' in the Naroch Lake is greater than in the Svir Lake. Unfortunately, we have only one isotopic age dating for the Svir Lake sediments (Figure 8), yet, this date is actually ~350 years younger than that available for the Naroch Lake sediments and ~500 years older than the ChRM data obtained from correlating the Belorussian PSV curve with the records obtained for the other objects. This proves that at least ~40% of the difference between the radiocarbon age of the sediments and the ChRM age of the Naroch Lake sediments was produced by the "effect of hard water''. The remaining part of the shift seems to have been caused by the fact that the bulk of the NRM, both in the Naroch and Svir lakes, has a postdeposition origin (PDRM).
The difference between the age of the sediments and the age of magnetization, similar to the difference observed in the sediments of the Belorussian lakes, complicates any PSV correlation using the sediments of different lakes. In particular, where the difference between the age of magnetization and the age of the sediments varies over the rock sequence, the accuracy of correlating the characteristic peaks of the geomagnetic field elements becomes very low and does not allow one to estimate the spatial evolution of the field morphology. It is not inconceivable that these processes of postsedimentation magnetization can be observed in all types of sediments [Bleil and von Dobeneck, 1999; Franke et al., 2004] causing a similar systematic error. For this reason, the datings reported in the literature may call for revision. For instance, the sediments of the Trichonis Lake in Greece [Creer et al., 1981] showed a difference between the age of the sediments and the age of their magnetization to be more than 1000 years. This was clearly demonstrated by the comparison of the inclination and declination records obtained for this lake sediments with the records obtained for the British lakes [Frank et al., 2002]. Yet, the latter cannot be used as a reference because their magnitudes and ages were not properly corrected [Turner and Thompson, 1982]. Therefore, at the present time we cannot estimate quantitatively the drifting of the geomagnetic field features using merely the data available only for the lake deposits of Europe. The shifting time of the morphological features of the declination and inclination variations, caused by the drifting of the specific features of the geomagnetic field, might have been substantially lower than the variations caused by the inexact dating of the magnetization.
1. The comparison and summation of the data available for the nine columns of the sedimentary rock samples collected in the Naroch and Svir lakes (Belorussia) resulted in plotting a PSV master curve for this region, which is similar in morphology to the archeomagnetic and limnomagnetic records obtained for some nearest regions and can be used to derive models of the geomagnetic field for the last ~12000 thousand years.
2. The comparison of the resulting record of the paleosecular geomagnetic filed variation with the archeomagnetic data revealed a difference between the radiocarbon age of the sediments and their magnetization age, amounting to ~2300 years, which was caused by the "ageing'' of the carbon in the sedimentation basin, on the one hand, and by the postsedimentation magnetization, on the other.
3. The PSV magnitudes recorded in the sediments of the Belorussian lakes are 5 times lower compared to the archeomagnetic and historical data, this possibly having been caused by the large depth (up to 0.6-0.8 m) of the PDRM recording in the sedimentary rocks.
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