RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES3004, doi:10.2205/2005ES000173, 2005
[13] - the group I sediments contain predominantly SD ferrimagnetic grains composed of magnetite and
magnetic iron sulfide, the coercivity of the ferromagnetic component ( Bcr ) being 61.9
5.8 mT;
[14] - the group II sediments contain predominantly MD ferrimagnetic grains composed of magnetite,
the magnetization ( Jrs ) of these sediments is 4-5 times lower than that of the group I sediments, the
coercitivity of the ferrimagnetic component ( Bcr ) being 38.1
7.7 mT which is substantially lower
than the of the group I sediments.
[15] The data obtained in this study suggest some assumptions concerning the origin of these magnetic rock fractions and use their concentration variations for reconstructing changes in the environment. We believe that the magnetic fraction of the group I rock samples is represented by single-domain grains of biogenic origin. In the first place, this is suggested by the very narrow range of the Bcr values and by the peaks of the coercive spectra of the samples (Figure 3), in the second place, the allothigenic ferrimagnetic fraction cannot be so well "sorted'': it must contain large (multidomain) grains, and in the third place, the same samples may contain single-domain grains of magnetite and magnetic iron sulfide (Figure 4) only in the case of their authigenic (biogenic) origin. It is known that the same "magnetotactic'' bacteria can produce magnetite or grayite (magnetic iron sulfide) [Bazylinski and Moskowitz, 1997]. Besides, it is necessary to mention, that the maximum of a coercitive spectrum in the first group of samples, equals the number, fixed earlier for bacterial magnetite and greigite [Egli, 2004; Moskowitz et al., 1993; Snowball, 1994; Snowball et al., 2002]. The magnetic material, dominating in the rock samples of Type II seems to be represented by allothigenic magnetite grains, often oxidized in the course of their transportation, as indicated by a low magnetization decline in the temperature range of 350-450o C in the first-heating curves (Figure 4a), usually associated with the transition of magnetite to hematite [Burov et al., 1986].
[16] This rock sequence can be subdivided into two segments in terms of changes in the magnetic properties of the sediments. The upper sedimentary rocks extending to a depth of 19.8 m differ from the underlying rocks in terms of their total magnetization, coercitivity, and magnetic property variation, as well as in the values and behavior of their paramagnetic magnetization ( Jp ). The variation of the Jp trends and average values of the sediments could be caused by changes in the relative contents of clay minerals in the sediment. This fact can also be used for estimating the content of the terrigenous component in the sediments, this component reflecting the intensity of the removal of the rock material to the sediment accumulation basin under the effects of tectonic and climatic factors. Below the depth of 19.8 m the Jp value is averagely higher than in the overlying rocks. The rocks residing above this depth level show a stable growth (trend) of the Jp value. This variation trend in of the paramagnetic component content in the sediments might have been associated with tectonic factors. The Jp peaks at the depth of about 20 m seem to have been caused by tectonic events. For example, the variation trends of the Bcr/Bc and Jrs/Js parameters, as well as the decline of the Jp value from the depth of ~36 m to ~25 m, suggest a decline in the addition of the allothigenic component to the sedimentation basin.
[17] The most important result of this study is the discovery of an abrupt change in the ratio of the allogenic and authigenic (biogenic) material in the Khubsugul Lake sediments, this proving the complex sedimentation pattern with numerous breaks and abrupt changes in the rate of sedimentation. This is most distantly recorded in the spasmodic Bda parameter variations (Figure 1 and Figure 2d), associated with an abrupt change in the ferrimagnetic fraction of the sediments (a ratio between the biogenic and terrigenous components). The relative concentration of biogenic materials in the sediments seems to be controlled by the following main factors:
[18] (1) The rate of allothigenic material supply to the sedimentation basin. The higher the content of the allothigenic material in the sediment, the naturally lower the content of its biogenic material. Variations of this factor may be caused both by tectonic and climatic factors.
[19] (2) The biologic productivity of the water and sediments, which is controlled, in its turn, by the lake's trophic state index [Wetzel, 2001]. This controls the position of the boundary between the oxidation and reduction zones in the water and in the sediments. For instance, magnetotactic bacteria are most productive where this boundary resides in water rather than in the sediments [Bazylinski and Moskowitz, 1997]. This is possible only where the lake's trophic state index is sufficiently high. Changes in this factor can be caused mainly by climatic factors.
[20] It is natural that these factors produce a combined effect on the formation of the sediment properties. The type of their interaction is controlled by the type of the lake water supply and by the morphology and profile of the basin.
[21] Therefore, it is most likely that the rate of sediment accumulation in this basin with the large amount of a biogenic magnetic material is substantially lower than that of the sediments predominated by the terrigenous material. The paleoclimatic interpretation of the data available can be performed using the results of the paleobiological (paleontological) data available. At the present time it can only be mentioned that a decline in the addition of a terrigenous material into the Khubsugul Lake basin might have taken place during a decline in the general humidity and during the lake level decline, but under the conserved conditions of sufficiently high bioproductivity. On the other hand, the diminished addition of a terrigenous material might have been caused by the long periods of time when the lake was covered by ice, which were followed by the periods of ice thawing and the abrupt growth of the allogenic material added to the lake.
Citation: 2005), The origin of magnetic minerals in the Lake Khubsugul sediments (Mongolia), Russ. J. Earth Sci., 7, ES3004, doi:10.2205/2005ES000173.
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