Rock-magnetic investigation of Siberia loess and its implication

Transcription

1 Rock-magnetic investigation of Siberia loess and its implication ZHU Rixiang 1, Kazansky Alexey 2, Matasova Galina 2, GUO Bin 1, Zykina Valentina 2, Petrovsky Eduard 3 & Jordanova Neli 3 1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing , China; 2. Institute of Geology, UIGGM SD RAS, Novosibirsk, Russia; 3. Geophysical Institute, Prague, Czech Republic Abstract Multiple-rock magnetic investigations conducted on the loess-paleosol sequences at Kurtak in Southwestern Siberia reveal that the mass-normalized low-field magnetic susceptibility profiles reflect changes in lithology between relatively unweathered primary loess of glacial periods and the interglacial paleosols. Maxima in susceptibility values correspond with the least-weathered loess horizons, and minima with the humic horizons of soils. Frequency-dependent susceptibility of the loess-paleosol sequences at Kurtak is very low and practically uniform, indicating the dominance of non-sp ferrimagnetic minerals and negligible pedogenesis. The history of temperature-dependence of susceptibility (TDS) and stepwise acquisition of the isothermal remanent magnetization (SIRM) have confirmed that magnetite is predominant magnetic mineral, and only few maghemite and probably hematite are present within the studied section. Anisotropy of the magnetic susceptibility (AMS) can be used to monitor tilt and disturbance of the sedimentary layers, and also to provide information about the paleo-transport direction for Siberia loess. Keywords: Siberia, loess, rock-magnetism, paleoclimate. The wind-blown sediments in the Loess Plateau of north-central China have received significant attention in the last two decades for its correlated records of global and regional climatic and environmental changes. Low-magnetic susceptibility profiles in the Chinese Loess Plateau correlate well with marine sediment δ 18 O data, which have been interpreted as a global ice-volume signal and hence as a proxy for global climatic changes [1 4]. However, the loess deposits elsewhere in the world have not been given adequate attention. We present here the results of a rock-magnetic study of loesspaleosol sequences at Kurtak (55.1 N, 91.4 E), one of the most previously studied loess sequences in Southwestern Siberia wind-blown deposits [5 8]. This study shows the utility of applying what has been learned from Chinese/Alaskan loess and presents a detailed rock-magnetic information preserved in loess-paleosol sequences over the Southwestern Siberia. Of course, the depositional and postdepositional environment in Southwestern Siberia was different from that in the Chinese Loess Plateau or Alaskan. Thus, we have undertaken this detailed study of a representative record and its rock magnetic characteristics as the first stage in our regional investigation of loess-paleosol sequences in Southwestern Siberia. 1 Geologic setting and sampling Extremely thick loess sediments of both aeolian origin and reworked by secondary processes cover an area of more than km 2 in the southern Siberia, between latitudes of N and longitudes of E (fig. 1). The loess sediments have accumu- lated to thicknesses of about 100 m [9 11]. Due to the uplift in southern Siberia during the Late Pliocene and the Early Fig. 1. Schematic map of the southern Siberia loess and sampling site Chinese Science Bulletin Vol. 45 No. 23 December 2000

2 Pleistocene, the loess- paleosol sequences are connected with development of fluvial system along the Siberian major rivers. In this note we only focused on the record over the Upper Pleistocene time. This part of the section consists of two loess units (LE1, LE2) and two buried soils (SO1, SO2). The SO1 unit is characterized by several chernozem with weakly developed humic horizons and illuvial ones. The SO2 unit exhibits well developed humic profile interbedded by illuvial layers (fig. 2). Humic horizons within the SO1 unit are usually of primitive structure, slightly colored and clearly labeled by traces of soilifluction and contain secondary carbonate concretions (fig. 2), indicating that they are probably formed under the steppe vegetation arid climate condition. On the other hand, the SO2 unit contains relatively high humic materials, suggesting that it be formed under the relatively warm and humid climate condition. Alleviate horizons at Kurtak are of different colors (in the middle part of LE1), with abundant Fe-concretions, Mn punctuation and gleying. 14 C and thermoluminescence dating has shown that the SO1 and SO2 were formed at the time intervals of ka and ka, Fig. 2. Rock magnetic parameters as function of stratigrapgic depth for the top most 20 m at Kurtak in Southwestern Siberia. (a) Lithology; (b) low-field susceptibility attains high values in loess and low values in paleosol; (c) frequency-dependent susceptibility shows the dominance of non-sp ferrimagnetic minerals and negligible pedogenesis; (d) inclination of the K max ; (e) inclination of the K min. 1, Humus of S(Al); 2, sand; 3, illuvium of S (Bck, Bm, Bcca); 4, coarse sand; 5, loess; 6, gley; 7, secondany Fe- and Mg-concretions. Chinese Science Bulletin Vol. 45 No. 23 December

3 respectively [9,12]. In comparison with the Chinese loess-paleosol sequence the SO1 and SO2 units correspond to L1SS1 and S1, respectively, whereas LE1 and LE2 correspond to L1LL1 and L1LL2, respectively [13]. We collected samples from the interval between 4 20 m depth in Kurtak section with wellexposed LE1, LE2, SO1, SO2 layers. All the samples were taken after recessing at least 30 cm inward from the surface in order to eliminate possible weathering effect. The sampling procedure consisted of cutting overlapping block samples about 10 cm 10 cm on the base and up to 10 cm in vertical height, in such a way the section was sampled continuously. The samples were oriented using a magnetic compass. Two parallel sets of samples were obtained for each stratigraphic horizon, so that different experiments could be performed on samples from the same stratigraphic height. In the laboratory, these long samples were cut into slices of about 2 cm vertical thickness. 2 Experimental results Laboratory measurements were carried out at the Institute of Geophysics, the Chinese Academy of Sciences and Geophysical Institute in Prague. Low field susceptibilities were measured using Bartington MS-2 Susceptibility Meter. Mass-normalized low-field magnetic susceptibility profiles reflect the changes in lithology between relatively unweathered primary loess of glacial periods and the interglacial paleosols, and correlated with oxygen isotope stages. Humic horizons of soils are characterized by the lowest susceptibility values of m 3 kg -1, with an average value of m 3 kg -1. In general, higher values were measured in loess horizons (up to m 3 kg -1 ) with the average value of m 3 kg -1. The susceptibility variations themselves are in the opposite sense to that found in the classic sections of the Chinese Loess Plateau, but are similar to the pattern observed in loess sequence in Alaska [7,8,14]. The remarkable feature of this study is very high susceptibility values within loess. On the other hand, frequency-dependent susceptibility of both loess and paleosol at Kurtak is very low and practically uniform (fig. 2), indicating the dominance of non-sp ferrimagnetic minerals and negligible pedogenesis [15,16]. Anisotropy of the magnetic susceptibility (AMS) can be used to monitor tilt and disturbance of the sedimentary layers, and also to provide information about the paleo-transport direction. AMS measurements were performed using a KLY-3s Kappabridge and AMS tensor was calculated according to Jelinek [17]. We can describe this tensor using an ellipsoid having three principal axes: maximum, intermediate and minimum susceptibility (K max, K int and K min ). K max /K int and K int /K min are defined as the magnetic lineation (L) and the magnetic foliation (F), respectively. AMS measure- ments performed every 10 cm along the section show that L is smaller than F, indicating that the AMS ellipsoid is stave. Inclination of K min is generally higher than 75, otherwise the inclina- tions of K max are very shallow (fig. 2(d), (e)). The maximum axes determined from loess units LE1 and LE2 are well grouped along NW-SE and SW-NE, respectively, indicating the paleodirection of the dust transportation, while the phenomenon was very blurred for paleosol units (fig. 3). Fig. 3. The directions of the AMS ellipsoid axis, squares/triangles/circles stand for the directions of the K max /K int /K min. Stepwise acquisition of the isothermal remanent magnetization (SIRM) in the field up to 2.7 T showed that more than 90% of the SIRM were acquired in the field of 400 mt for both loess and paleosol (fig 4(a)). Progressive removal of the SIRM by applying reversed fields showed that the remanent coercivities (H cr ) of the SIRM are approximately 45 and 60 mt for loess and paleosol, respectively (fig. 4(b)), indicating that the low coercive magnetic minerals, such as magnetite/ 2194 Chinese Science Bulletin Vol. 45 No. 23 December 2000

4 Fig. 4. (a) Typical curves of normalized isothermal remanence magnetization (IRM) acquisition curves; (b) DC demagnitization curves of IRM. Circles (Triangles) stand for loess (paleosol) samples. maghemite, might be predominant within the loess-paleosol sequences at Kurtak. In order to determine the magnetic properties of the loess-paleosol sequences at Kurtak, the temperature-dependence of susceptibility (TDS) was measured continuously with a furnace-equipped KLY-3 Kappa-bridge. The powdered whole-rock specimens were heated and cooled in air condition. In addition to the distinct decrease of TDS near 580, the TDS for both loess and paleosol showed a small peak at about 300 on the heating curves (fig. 5), perhaps indicating that magnetite is dominant and some maghemite is being converted into cation-deficient magnetite during the heating. The susceptibility values are decreased for loess after heating up to 600 or 700 (fig. 5(a)), but Fig. 5. Typical TDS curves of (a) loess and (b) paleosol, thick (thin) curves stand for heating (cooling). Chinese Science Bulletin Vol. 45 No. 23 December

5 increased for paleosol after heating up to 600 or 700 (fig. 5(b)). This might be explained either by the transformation from iron-bearing silicate/clay minerals, or by reduction of maghemite to magnetite during the laboratory heating and cooling processes for paleosol. In order to control changes in magnetic phase during the heating two successive cycles of thermal demagnetization of SIRM were carried out (fig. 6), which indicates that thermal behavior of the SIRM is no sufficient difference between loess and paleosol, except initial SIRM value. In both cases there is a significant change in the slope of the heating curves between 200 and 350ºC, whereas it disappears at the second heating. This confirms the presence of maghemite. Also, 97% decay of SIRM is observed at 580ºC, suggesting that magnetite is dominant, and only few hemitite may be present. 3 Discussion and conclusion Fig. 6. Two successive cycles of thermal demagnetization of SIRM. Solid (empty) circles stand for first (second) heating cycle for loess sample, solid (empty) triangles represent first (second) heating cycle for paleosol sample. The most interesting question involving the susceptibility profile at Kurtak is what mechanism results in the completely opposite patterns observed in China. Moreover, the mean susceptibility value of Kurtak loess is about 3 times higher than that of Alaska ones, which partly confirms the windintensity model for the Alaska loess. Of course, the explanation of such phenomenon in terms of wind intensity changes does not consider pedogenic processes in paleosol. Frequency-dependent susceptibility (χ fd ) reflects variations in magnetic grain size from the single domain to superparamagnetic state [18]. Chinese loess-paleosol sequences are generally characterized by χ fd values varying between 2% 7%, while paleosols are characterized by χ fd values between 7% 11%. This suggests that the fined grain sizes have strong contribution to susceptibility due to magnetite formation during pedogenesis [19 22]. On the contrary, low χ fd values at Kurtak indicate the absence of ultrafine-grained superparamagnetic magnetite, so the pedogenic process here should be weak and different from that in the Chinese Loess Plateau. The AMS data show comparatively high values of P' for both loess-paleosol sequences, and have subvertical orientation of K int axis and distinct grouping of K max axis. In addition to clearly expressed sedimentary magnetic fabric, the AMS results for LE1 and LE2 (fig. 3) show well defined mean K max direction. Thus, we could expect the prevailing transport direction during the formation of these loess units NW-SE in LE1 and SW-NE in LE2. Although the K max direction in SO1 is not so well defined, it is basically similar to that in LE1, while in the illuvial horizon the AMS directions are more scattered, probably because of the secondary process. The first humic layer in the SO2 unit exhibits quite scattered principal susceptibility directions, but the mean K max direction still coincides with the welldetermined one in LE2. We expect well-defined mean K max directions in the studied section due to colluvial or deluvial sedimentation (e.g. more or less flow-influenced). This comes in conflict with the previous idea of wind-blowing origin of magnetite enhancement in Siberian loess [7,8], taking into account the fact that the wind usually cannot give very strong orientation of the fine loess particles. In addition, wind turbulence just above the ground, connected with the specific geomorphologic features, can also play a sensitive role. Typical sedimentary fabric is only developed following the deposition of loess particles under the influence of gravitational force. Recently, the AMS results determined from Malan loess to the west of Liupan Mts. in China show prefered orientation for K max axis, while the random directions of K max axis was found in the Malan loess at the east of the Liupan Mts. [23]. The susceptibility pattern at the Kurtak loess-soil section actually exhibits the behavior opposite to the aeolian sediments deposited in the Chinese Loess Plateau, Bulgaria and the Czech Republic, and similar to ones in Alaska and Argentina. Low χ fd values at Kurtak indicate the absence of SP magnetite, 2196 Chinese Science Bulletin Vol. 45 No. 23 December 2000

6 so the pedogenic process here was very weak and quite different from Chinese soils. There is no principle difference in magnetic composition of studied loess and paleosol. Magnetite is predominant within both loess and paleosol, only few maghemite and probably hematite are present. Thus, the variations of magnetic susceptibility are mainly controlled by concentration and grain size of magnetite. However, loess and soils demonstrate the difference in non-magnetic composition, which leads to different behavior of susceptibility when heating temperature up to 700. At the moment, we can explain it by the presence of organic material within paleosol at Kurtak. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No ) and RFBR-NFNS Grant (Grant No ). References 1. Heller, F., Liu, T. S., Magnetism of Chinese loess deposits, Geophys. J. R. Astr. Soc., 1984, 77: Hunt, C. P., Singer, M. 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