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Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008) THE LAST GEOMAGNETIC REVERSAL MATUYAMA-BRUNHES IN LOESS-PALEOSOL SEQUENCES <strong>OF</strong> PRIOBSKOE PLATEAU Z.N. Gnibidenko Petroleum Institute Geology and Geophysics of Siberian Branch of Russian Academy of Sciences, Novosibirsk, 630090, Russia, e-mail: magnit@uiggm.nsc.ru Abstract. Paleomagnetic record of the Matuyama-Brunhes polarity reversal has been studied. For our study, two closely spaced sections of loess-paleosol sequences (one section is 14 m thick and another is 1.8 m thick) on the left bank of Ob’ River near village Volodarka (52°41'N, 83°38'E) were sampled continuously. Stepwise thermal demagnetization up to 685ºC yielded two natural remanent magnetization (NRM, Jn) components: secondary and primary. The secondary NRM component is broken in the 250-500°С intervals. The primary NRM component, which is isolated at the 250-500°C, is characteristic component (ChRM). Magnetite, maghemite, and haematite are as the main magnetic mineral - carriers of natural remanent magnetization. The characteristic component forms the complex succession of normal, reversal, and intermediate directions. This succession reflects the Matuyama-Brunhes polarity reversal. Introduction One of the actual problems of paleomagnetology and geophysics, as a whole, is study reversals of the Earth’s magnetic field, caused by processes in the core and on the core-mantle boundary. The reversals are significant for development of the theory of geomagnetic dynamo. Therefore, interest in study of reversal transitions increased in the last time. The reversal transitions are studied in the age interval of the whole geological time, but at present, investigators focus their efforts mostly on young reversals. Thus, characteristics of a magnetic field are studied during reversal transition, as well as, during stationary periods of this field before and after reversals. This work presents the results of study of geomagnetic field during the Matuyama-Brunhes reversal that took place 780 ka ago and recorded while magnetostratigraphic investigations were carried out on the Priobskoe Plateau. Geology and rocks sampling Studied section is located on the high left bank of Ob’ River 70 km upstream of Barnaul city. Geographical coordinates of the section are as follows: 52°41' N, 83°38' E. Here in the steep of the bank about 50 m in height, deposits represented by loess-paleosol sequences are exposed. The subsurface geology of the loess-paleosol sequence on the Priobskoe Plateau as a whole, and, particular, that of this section being studied is represented by three thicknesses. The lower thickness, which is revealed above the water level, is consists of gray and blue colored clays and loams. This thickness is the upper part of Kochkovka suite [Martynov, 1966]. Loams are heavy, limous, and unstratified. Paleontological remains (small rodents, ostracoda, and seed flora) revealed in Kochkovka suite deposits allow these rocks to be dated by Upper Pliocene [Martynov and Nikitin, 1968]. These deposits are of subaqual and subaerial genesis. The middle thickness represented by loams and white farinaceous sands with horizons of paleosols is the lower part of the Krasnodubrovka suite. The deposits are of alluvial genesis. The upper part of the Krasnodubrovka suite represented by loess-like loams, sandy loams, and sands with paleosols is the upper thickness of the loesspaleosol sequences. Based on paleontological remains (mammal fauna, ostracoda, spores, pollen, and seed assemblages) the age range of Kochkovka suite is defined as Late Pliocene-Early Pleistocene, and that of Krasnodubrovka suite is dated as Early Pleistocene-Late Pleistocene. Studied section Volodarka is represented by Pliocene-Pleistocene deposits composed of alternating horizons of loess-like loams, paleosols, and compact loams lying horizontally. From two closely-spaced (at the distance of 1 km) sections of the Volodarka, 490 oriented specimens representing 200 temporal stratigraphic levels were taken. The most of these specimens were taken continuously; other specimens were taken 10, 20, and 40 cm apart. At first, the samples were taken in the shape of a lump 5-20 cm thick, the upper part of such is horizontal. At a later time, the specimens were cut into slices (stratigraphic levels) 2 cm thick, from which cubic specimens of 2×2×2 cm were made. 375
Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008) Methods Both magnetic and paleomagnetic characteristics of deposits such as – susceptibility (χ), intensity and direction of NRM were studied. These experiments were done in the IPGG paleomagnetic group (Novosibirsk) and in the VNIGRI paleomagnetic laboratory (St. Petersburg). The NRM intensity and direction 376
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vertical fields. The vertical field
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a Proceedings of the 7th Internatio
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Introduction ON THE NATURE OF PLASM
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Vy Here index j =1,2 characterizes,
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2. Burlaga & Klein method. The main
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