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Weathering and clay mineral formation in two Holocene soils and in buried paleosols in Tadjikistan: towards a Quaternary paleoclimatic record in Central Asia
Catena 34 Ž1998. 19–34
Weathering and clay mineral formation in two Holocene soils and in buried paleosols in Tadjikistan: towards a Quaternary paleoclimatic record in Central Asia A. Bronger
a,)
, R. Winter a , S. Sedov
b
a
b
Institute of Geography, UniÕersity of Kiel, D-24098 Kiel, Germany Department of Soil Science, Lomonosow UniÕersity, 119899 Moscow, Russian Federation
Abstract The upper part of the Karamaydan section, Tadjikistan, shows the most detailed loess–paleosol sequence yet known for the Brunhes chron, and the central and lower parts of the Chashmanigar section provide similar detail for most of the Matuyama chron. To enable paleoclimates to be deduced, the primary and secondary minerals in the silt and clay fractions must be determined separately to evaluate the type and intensity of mineral weathering and clay mineral formation. To distinguish between inherited and pedogenetically formed clay minerals, the original petrographic homogeneity of the parent material from which a soil developed must be established. The main sources of pedogenic clay minerals are phyllosilicates in the silt fractions. Illites and vermiculites are the dominant pedogenetically formed clay minerals in the B or Bt horizons of the Holocene climaphytomorphic soils and in all paleosols ŽS. and pedocomplexes ŽPK. in Karamaydan and Chashmanigar, except S XVII in which large amounts of smectites were formed. There is little difference in the type and amount of pedogenic clay mineral formation between the Holocene soils and the paleosols in the Brunhes epoch at Karamaydan as well as during most of the Matuyama epoch at Chashmanigar. These results indicate that the climates of the interglacials represented by the B or Bt horizons of the buried paleosols of young, mid and old Pleistocene age were similar to that of the Holocene. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Loess; Mineral weathering; Clay mineral formation; Micromorphology; Clay illuviation; Paleoclimate
)
Corresponding author. Fax: q49-431-880-4658; E-mail: [email protected]
0341-8162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 9 8 . 0 0 0 7 9 - 4
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1. Introduction Loess profiles in Tadjikistan, especially those of Karamaydan and Chashmanigar, contain loess–paleosol sequences representing about the last 2 million years, which are probably as detailed as any in Central and East Asia. They have been described several times from a geological–stratigraphical perspective, mainly by Dodonov Ž1979, 1984, 1991., Dodonov et al. Ž1977, 1982., Lazarenko Ž1977, 1984. and Lazarenko et al. Ž1981., and the first paleopedological work was presented by Bronger et al. Ž1995.. Penkov and Gamov Ž1977, 1980. and Forster and Heller Ž1994. identified the BrunhesrMatuyama ŽBrM. boundary in several loess profiles in Tadjikistan between the Pedocomplexes ŽPK. IX and X. An angular unconformity occurs below PK X at both Karamaydan and Chashmanigar, supporting the stratigraphic correlation between these profiles, which enables a reliable chronostratigraphical framework to be developed. Following earlier investigations of the micromorphology and particle size distribution of the loesses and paleosols in both sections ŽBronger et al., 1995., we present here mineralogical analyses of the Karamaydan and Chashmanigar profiles. We selected B or Bt horizons of eight PKs or single paleosols ŽSs. in the upper Karamaydan section and nine PKs or Ss in the central and lower part of the Chashmanigar section, and compared them with two Holocene soils of a polypedon near Karatau ŽFig. 1.. The upper part of the Karamaydan section seems to show the most detailed loess–paleosol sequence for the Brunhes chron, and the central and lower parts of the Chashmanigar section have the most detailed sequence for most of the Matuyama chron ŽBronger et al., 1995, 1998.. However, this does not mean that these profiles are complete. The aim of this paper is to give a quantitative paleoclimatic interpretation based on the type and amount of pedogenic clay material formation of the paleosols in comparison with two holocene soils and thus improve the knowledge of the history of Quaternary climatic changes.
2. Methods For paleoclimatic deductions from loess–paleosol sequences it is necessary to understand the processes involved in the genesis of the paleosols. However, this is often difficult because most of the buried paleosols are truncated and more or less recalcified from the overlying loess. In contrast to modern ŽHolocene. soils, buried paleosols have consequently lost some important diagnostic properties. However, thin sections allow primary and secondary carbonates to be distinguished and provide unequivocal evidence of the process of clay illuviation. This allows, for example, the distinction between B and Bt horizons and also between Bw and CB horizons, as noted in more detail in Bronger et al. Ž1998.. For further paleoclimatic interpretation mineralogical investigations are necessary to indicate the nature and intensity of weathering. Many of the soil-chemical methods of investigation applied to unburied Ž‘modern’. soils cannot be used because of post-pedogenetic changes in the paleosols. To enable paleoclimates the primary and secondary minerals of each fraction, especially that of the clay, must be determined separately to
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evaluate the type and intensity of mineral weathering. For clay mineralogical investigations, it is necessary to separate the clay fraction Ž- 2 mm. into coarse Ž2–0.2 mm. and fine clay Ž- 0.2 mm., enabling semiquantitative estimations to be made. The characteristics of clay minerals in Pleistocene soils and in modern ŽHolocene. soils and their parent materials must be compared to indicate how much clay has been formed by pedochemical weathering processes. To evaluate the type and intensity of weathering, it is first necessary to establish the original petrographic homogeneity of the parent material from which a soil developed. One way of determining this is to use resistant index minerals ŽBarshad, 1967.. These should be present in equal amounts in all horizons and are neither decomposed nor displaced by soil development. Preferably they should occur in quantities sufficiently large for precise quantitative determinations by counting a few hundred grains. In the loess soils of Central Europe and North America ŽBronger, 1966, 1969r1970, 1976, 1991; Bronger et al., 1976. and the Loess Plateau of China ŽBronger and Heinkele, 1989a, 1990; Heinkele, 1989. stable heavy minerals, such as zircon, anatase and rutile, are unsuitable because they occur only in very small quantities. However, quartz is suitable because it is resistant to pedochemical weathering, at least in temperate latitudes, and is always abundant in loess. If the percentage of quartz by weight in the different soil horizons is approximately constant, there is good evidence that the soil parent material was originally homogeneous ŽBronger, 1976, pp. 25–30; Bronger et al., 1976.. To determine the mineral composition of the 63–20 mm fraction, several slides were examined in polarized light. In this fraction the phyllosilicates could be separated into muscovites and biotites. At least 300 particles were identified per slide and the percentage of each mineral was calculated. The proportions of quartz, feldspar, phyllosilicates and other heavy minerals in the fractions 6–20 mm and 2–6 mm fractions were determined by phase-contrast microscopy. At least 500 grains in the 6–20 mm fraction and at least 700 grains in the 2–6 mm fraction were examined per slide. The percentages of the minerals in each fraction were then multiplied by the weight percentages of each fraction to give the percentage by weight of each mineral group in the whole sample, as shown in Figs. 2–10. The percentages by weight are little different from the percentages by volume ŽBronger, 1976, pp. 24–25; Bronger et al., 1976., although the minerals have different densities. The composition of the clay subfractions was determined by semiquantitative estimation based on the areas under selected XRD peaks. The 0.2–2 mm and - 0.2 mm fractions were analysed as oriented samples with a Philips PW 1710 X-ray diffractometer using CoK a radiation after Mg–ethylene–glycol treatment as well as after K saturation and heating to 25, 125, 400 and 5508C. We used the weighting factors recommended by Laves and Jahn ¨ Ž1972.: illites and vermiculites were given a weighting of 1, kaolinites and smectites 0.25. The results were compared with the cation exchange capacities of both fractions of selected paleosols and one Holocene soil to correct gross deviations. Each mineral name refers to a group of closely related clay minerals with slightly varying compositions. The weight percentages obtained by multiplying the estimated percentages of the clay minerals in each fraction by the weight percentage of each clay subfraction are consequently only estimates. However, the results allow
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Fig. 1. Genesis of the soil horizons of pedocomplexes ŽPK. of the loess profile of Karamaydan, Tadjikistan ŽA: upper part, right section; B: upper part, left section; C,D: lower part of the left section. and of the loess profile of Chashmanigar, Tadjikistan ŽE: central part; F: lower part..
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Fig. 2. Mineralogical and clay mineralogical composition of two Holocene soils ŽTypic Haploxerolls or Haplic Phaeozems. in the Tadjik Depression Ž1700 m above sea level..
conclusions to be drawn regarding the original petrographic homogeneity of the parent material and the type and intensity of pedogenic weathering and clay mineral formation in the soils of the investigated sequences.
Fig. 3. Mineralogical and mineralogical composition of pedocomplexes ŽPK. I and II in the loess profile of Karamaydan ŽTadjik Depression.. For legend see Fig. 2.
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3. Results The designation of horizons of the pedocomplexes ŽPK. or single paleosols ŽS. in the Karamaydan and Chashmanigar sections based on field and micromorphological studies Žfor details see Bronger et al., 1995, 1998. are summarised in Fig. 1. Parts A and B show the left and right sections, respectively of the upper part of the Karamaydan sequence. Field studies in 1991, before a landslide covered much of the right section in spring 1992, clearly indicate that PK II was missing in the left section. Fig. 1 shows the position of the samples taken for mineralogical investigations, the results of which are summarised in Figs. 2–9. The main points worth emphasizing are the following. Ž1. The percentages of quartz by weight in the silt fractions increase only slightly between the C or Ck and the overlying B or Bt horizons in almost all PKs and Ss, thus indicating original petrographic homogeneity. However, in the quartz content from 19.5 to 23.5% in S XIV and especially from 21 to 26% in S XII at Chashmanigar ŽFig. 7., show a greater petrographic inhomogeneity in these paleosols. Ž2. Only some Bt horizons in the PKs I–VI and S XV show about 1% of illuviation argillans in the soil matrix, which is regarded as the minimum necessary for an argillic horizon ŽSoil Survey Staff, 1975, 1994.. Even the lower Bt horizon of PK I, the two Bt horizons of PK II and the lower Bt of PK IV have - 2% illuviation argillans ŽBronger et al., 1998.; in S XV it just meets the 1% criterion. Thus, the process of clay illuviation is mostly not responsible for the increase in clay content from the C or Ck to the Bt horizons. Most of the increase in clay content therefore results from pedogenic clay formation by weathering in situ. Ž3. The main sources of pedogenic clay minerals are phyllosilicates in the silt fractions: in most of the paleosols and the Holocene soils there is a considerable decrease in silt-size micas in the B or Bt horizons compared with the Ck horizons. Feldspars are a minor source of the pedogenically formed clay minerals ŽFigs. 2–9.. In the coarse silt fraction Ž20–63 mm., where a separate identification of different phyllosilicates is possible, 60–80% of the micas are muscovites and 20–40% are
Fig. 4. Mineralogical and mineralogical composition of pedocomplexes ŽPK. III and IV in the loess profile of Karamaydan ŽTadjik Depression..
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Fig. 5. Mineralogical and mineralogical composition of pedocomplexes ŽPK. V and VI in the loess profile of Karamaydan ŽTadjik Depression.. For legend see Fig. 2.
biotites. In the Holocene soils and most of the paleosols, a large decrease in biotite content in the B or Bt horizons compared with the C or Ck horizons was found, especially in the Karamaydan section. This agrees with micromorphological observations, which suggest that in the Bt and B horizons, the biotites show signs of weathering, as indicated by a decrease of pleochroism and precipitation of iron oxides Žgoethite. on their surfaces. In comparison with biotites, muscovites are regarded as rather stable, although they probably contribute to the increase of illite in the clay fractions through physical breakdown. Ž4. Illites and vermiculites are the dominant pedogenetically formed clay minerals in the B or Bt horizons of the Holocene soils. They are also dominant in all paleosols at Karamaydan and Chashmanigar, except in S XVII. Whereas muscovites are a major source of the pedogenic illite formation, transformation of biotites produces vermi-
Fig. 6. Mineralogical and mineralogical composition of pedocomplexes ŽPK. IX and XII in the loess profile of Karamaydan ŽTadjik Depression.. For legend see Fig. 2.
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Fig. 7. Mineralogical and mineralogical composition of pedocomplexes ŽPK. X, XII and XIV in the loess profile of Chashmanigar ŽTadjik Depression.. For legend see Fig. 2.
culites, a process which can occur quickly ŽFanning et al., 1989.. In some paleosols, especially in the Bt horizons of PK X and S XIX at Chashmanigar, regular interstratified ˚ were found in the fraction 0.2–2 mm. mica–vermiculites with a basal spacing of 24 A
Fig. 8. Mineralogical and mineralogical composition of pedocomplexes ŽPK. XV ŽqVI. and XVII in the loess profile of Chashmanigar ŽTadjik Depression.. For legend see Fig. 2.
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Fig. 9. Mineralogical and mineralogical composition of pedocomplexes ŽPK. XVIII, XIX, XXV and XVII in the loess profile of Chashmanigar ŽTadjik Depression.. For legend see Fig. 2.
These minerals are known to be products formed during the degradation of biotites to vermiculites. Large amounts of smectites were formed only in the Bt horizons of S XVII at Chashmanigar ŽFig. 8.. Very small amounts of pedogenic smectites were formed in the Bt horizons of PKs I, II, III and IV at Karamaydan ŽFigs. 3 and 4. and of PK X and S XII at Chashmanigar ŽFig. 7.. In other pedocomplexes, smectites, if present, seem to be inherited. Pedogenetically formed smectites can be regarded as an advanced stage of degradation of micas Že.g., Borchardt, 1989. or may be formed from feldspars ŽBronger et al., 1976; Bronger and Heinkele, 1989b, 1990.. In contrast, virtually no kaolinites were formed pedogenetically. In the Central Asian Kashmir Valley, India ŽPant and Dilli, 1986; Heinkele, 1986. and especially in the loess plateau of Central China ŽBronger and Heinkele, 1989a. pedogenetically formed illites are dominate the coarse and fine clay fractions of Pleistocene and Holocene soils. Pedogenic vermiculites are less frequent and smectites are only formed pedogenetically in small amounts, if at all. In contrast, in Central European Holocene and Pleistocene loess soils as well as in relict loess soils in the central and northern part of the Great Plains of the USA, smectite is the main pedogenetically formed mineral in the dominating fine clay fraction Žgreater than 0.2 mm. whereas illites are dominant in the coarse clay fraction ŽBronger, 1976, 1991; Bronger and Heinkele, 1989b, 1990.. The general predominance of pedogenic illites
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may result from the large amounts of muscovites in the silt fractions of loess in the Central Loess Plateau ŽHeinkele, 1989. and the Tadjik Depression. Ž5. There is little difference in the type and amount of pedogenic clay minerals between the Holocene soils and the buried paleosols ŽB or Bt horizons.. The same is true when the B or Bt horizons are compared with their parent materials ŽCk horizons. in the young, mid and old Pleistocene PKs or Ss ŽFigs. 2–9.. The amount of pedogenic clay mineral formation in the Quaternary soils is mostly in the range of 10–15%. Only in the lower Bt horizons of PKs I and IV at Karamaydan is it is large as 20% ŽFigs. 3 and 4.. In S XII at Chashmanigar ŽFig. 7. the increase in clay content is almost 20%, although this interpretation is complicated by a certain petrographic discontinuity in this soil Žsee above.. A limited amount of pedogenic clay mineral formation Ž10–15%. is typical also of the lower paleosols of the Matuyama epoch in the Chashmanigar section, although S XIX and XXV, which are more rubefied soils ŽFig. 1E,F., seem to be more strongly developed than the other paleosols. For instance their Munsell notations have 7.5-5YR or 5YR hues Žwhen dry., whereas most other paleosols have 10YR, some being close to 7.5YR. However, the Žclay. mineralogical composition of the Bt horizons of S XIX and XXV ŽFig. 9. were only compared with the ŽB.Ck horizons immediately beneath. The increase in clay content from the Bt horizon to the underlying loess is about 15% ŽBronger et al., 1995, Fig. 5.. The criterion of rubefication or the formation of hematite at the expense of geothite is often exaggerated as an indicator of a warm climate; it depends more on a distinctly seasonal climate with dry periods, on good drainage and on little or no humus. The Holocene climate in the Tadjik Depression south of the Tien-shan and west of the Pamir is characterized by cool and moist winters and very warm, very dry summers. This results in a distinct xeric soil moisture regime ŽSoil Survey Staff, 1975., though a considerable water deficit in the summer is mitigated by a large water utilization supplied by the high plant available water capacity of loess soils. In the late autumn and winter, water recharge occurs to give a large water surplus ŽBronger et al., 1995, Fig. 2.. In about the last 100 years the landscape has been strongly degraded and severe soil erosion has occurred because of a large increase in the population. However, Zapriagaeva Ž1964, 1968. and Staninkovitch Ž1968. concluded that the potential natural vegetation in the Tadjik Depression, which lies at 1000–2200 Ž2400. m above sea level, is broad-leaved forest consisting of maple Ž Acer turkestanicum., plane Ž Platanus orientalis., ash Ž Fraxinus potomolina., wild-growing fruit trees such as walnut Ž Juglans regia. and almond Ž Amygdalus communis, A. bucharica. and different species of Celtis, Pyrus, hawthorn Ž Crataegus., etc. In several areas juniper Ž Juniperus seraÕschanica. occurs at elevations of 1700–2300 m above sea level. The two Holocene soils were selected from a site near Karatau at about 1700 m above sea level on a plateau not far from the Chormazak Pass, as shown in Fig. 1 Žsee also Bronger et al., 1995, Figs. 1 and 2.. The two soils are free of CaCO 3 and the pH-values Ž0.1 M KCl. were as low as 4.9. Both have a dark grey Ah horizon, 27 cm in thick, which meets the requirement of a mollic epipedon ŽSoil Survey Staff, 1975. or a mollic horizon ŽSpaargaren, 1994.. Their Bt horizons, with G 7.5YR hues, are 105 cm and 80 cm thick, respectively. Both have a sharp boundary with a whitish Ck horizon, which is typical of forest rather than steppe soils. Micromorphologically several parts of
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Fig. 10. Mineralogical and clay mineralogical composition of the strongly rubefied mid-Pleistocene F6 complex in the loess profile of Stari Slankamen, Yugoslavia. For legend see Fig. 2.
the Ah horizons have a fine spongy fabric rich in aggregates and pores, indicating extensive biological activity. Most parts of the Bt horizons have a more dense fabric with far fewer aggregates and voids. Few illuviation argillans are visible: they comprise much less than 1% by area. Furthermore, the very small increase in clay content from the Ah to the B horizons ŽFig. 2; see also Bronger et al., 1995, Fig. 3. is not sufficient for these subsurface horizons to qualify as argillic horizons in Soil Taxonomy ŽSoil Survey Staff, 1975. or as argic horizons in the World Reference Base ŽSpaargaren, 1994.. The designation as a Bt horizon is a genetic and not a diagnostic one ŽSoil Survey Staff, 1994, p. 293.. These broad-leaved forest soils have to be classified as Typic
Fig. 11. Pedostratigraphic correlation of loess–paleosol sequences of Karamaydan, Central Asia, Luochuan, East Asia Žcf. Fig. 9B in Bronger et al., 1998. and Stari Slankamen, Yugoslavia with stages 15 to 13 of the deep-sea oxygen record of Bassinot et al. Ž1994..
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Haploxerolls or Haplic Phaeozems. It is surprising that the process of clay illuviation has scarcely occurred despite the considerable water surplus in March and April. More research on Holocene soils, especially in the moister Khovaling–Chashmanigar area, is needed.
4. Discussion and conclusions Our clay mineralogical results, detailed in point 5 above, indicate that the climates of the interglacials represented by the B or Bt horizons of the buried paleosols were comparable to that of the Holocene, because there is little difference in the type and amount of pedogenic clay mineral formation between the Holocene soils and the paleosols in the Brunhes as well as in most parts of the Matuyama epoch. The paleosols are mostly truncated and only in the upper soil parts of PKs III ŽFig. 1B. and V ŽFig. 1C. is the topsoil preserved. Nevertheless, they are genetically Žvery. similar to the Holocene soils. The presence of argillans as unequivocal signs of clay illuviation, especially in the lower Bt horizons of PKs I and IV Žwhere it probably coincident with greater pedogenic clay mineral formation. may indicate a somewhat moister climate during these interglacials than during the Holocene. The same is probably true for the two interglacials represented by the two Bt horizons of PK II. However, Ck horizons with distinct calcareous nodules are much better developed in most paleosols ŽFig. 1. than in the Holocene soils, though the reason for this remains uncertain. Recently published low-field magnetic susceptibility data from the same Karamaydan section show high values only in PKs II and III ŽShackleton et al., 1995. with somewhat lower values in PK I ŽForster and Heller, 1994.. Both publications found much lower susceptibility values in all other pedocomplexes or much smaller differences between the paleosols and the accompanying loesses. This is not in agreement with the clay mineralogical results and micromorphological observations, especially for the lower Bt horizons of PKs I and IV, which show more pedogenic clay mineral formation than other paleosols. It is important to determine which ferrimagnetic or superparamagnetic minerals are primary or lithogenic and which are pedogenic, Že.g., Liu et al., 1995. before low-field susceptibility variations of paleosols and accompanying loesses can be ‘taken as a reliable paleoclimate proxy’ ŽForster and Heller, 1994, p. 502.. Our conclusion that the interglacials in the Brunhes and most parts of the Matuyama epochs were climatically similar to the Holocene disagrees with earlier conclusions ŽBronger, 1976; Bronger and Heinkele, 1989b. that in the temperate climatic zone of Central Europe the paleosols derived from loess are much more strongly developed and rubefied in the mid and early Pleistocene than in the Holocene, and were probably formed in a subtropical climate. For example, the mid-Pleistocene F6 soil at Stari Slankamen, Yugoslavia, shows much more pedochemical weathering and clay mineral formation than the Holocene soils in the same area ŽBronger, 1976.. More than 40% of the feldspars and almost 80% of the micas are decomposed ŽFig. 10.; in thin sections only muscovites are found and no biotites remained. Later the F6 soil was correlated with the three soils of the S5 pedocomplex in the Luochuan section, China ŽBronger and Heinkele, 1989b.. As F6 corresponds chronostratigraphically Žvia the S5 pedocomplex at
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Luochuan. with at least two PKs ŽV and VI. at Karamaydan ŽFig. 11., we suggest that the much greater pedochemical weathering and clay mineral formation in the F6 results from a longer period of pedogenesis and does not indicate a warmer or wetter climate. Reasons for this correlation are given by Bronger et al. Ž1998.. Correlation with the oxygen isotope record of Bassinot et al. Ž1994. suggests that PKs V and VI were formed over a period of about 140,000 years, although pedogenesis was interrupted several times by loess formation. Thus the formation of the F6, which should now be regarded as F6 pedocomplex, at Stari Slankamen occurred over a period several times longer than the Holocene, and this explains its much greater pedochemical weathering and clay mineral formation.
Acknowledgements We thank the Deutsche Forschungsgemeinschaft for supporting field, micromorphological and laboratory work Žgrants Br 303r26-1,2..
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Dodonov, A.E., 1984. Stratigraphy and correlation of Upper Pliocene–Quaternary deposits of Central Asia. In: Pecsi, M. ŽEd.., Lithology and Stratigraphy of Loess and Paleosols. Geographical Research Institute, Hungarian Academy of Sciences, Budapest. Dodonov, A.E., 1991. Loess of Central Asia. Geoj. 24 Ž2., 185–194. Dodonov, A.E., Melemed, Y.R., Nikiforova, K.V. ŽEds.., 1977. International Symposium on the Neogene– Quaternary Boundary. IGCP 41 Excursion Guidebook, Moscow. Dodonov, A.E., Mavlyanov, G.A., Tetyukhin, G.F., 1982. Guidebook for Excursions A-II and C-Uzbek SSR, Tajik SSR. INQUA XI Congress, Moscow. Fanning, D.S., Keramidas, V.Z., El-Desoky, M.A., 1989. Micas. In: Dixon, J.B., Weed, S.B. ŽEds.., Minerals in Soil Environments. Soil Science Society of America, Madison, pp. 551–634. Forster, T., Heller, F., 1994. Loess deposits from the Tajik depression ŽCentral Asia.: magnetic properties and paleoclimate. Earth Planet. Sci. Lett. 128, 501–512. Heinkele, T., 1986. Zur holozanen landschaftsentwicklung der loss-plateaus im Kashmir Valley auf boden¨ ¨ geographischer Grundlage. Diplomarbeit, Kiel Žunpublished.. Heinkele, T., 1989. Bodengeographische und palaopedologische Untersuchungen im zentralen lossplateau von ¨ ¨ China—ein beitrag zur quartaren ¨ klima- und landschaftsgeschichte. Dissertation der Math.-Nat. Fakultat ¨ der Christian-Albrechts-Universitat, ¨ Kiel, 120 pp. Laves, D., Jahn, G., 1972. Zur quantitativen rontgenographischen bodenton-mineralanalyse. Arch. Acker¨ ¨ Pflanzenb. Bodenk. 16, 735–739. Lazarenko, A.A. 1977. Loess cover of the Tajik depression Žstratigraphy, lithology, problems of genesis.. Abstracts of the IGCP International Symposium on the Neogene–Quaternary Boundary, Moscow, pp. 35–36. Lazarenko, A.A. 1984. The Loess of Central Asia. In: Velichko, A.A. ŽEd.., Late Quaternary Environments of the Soviet Union. University of Minnesota Press, Minneapolis, pp. 125–131. Lazarenko, A.A., Bolikhovskaya, N.S., Semenov, V.V., 1981. An attempt at a detailed stratigraphic subdivision of the loess association of the Tashkent region. Int. Geol. Rev. 23 Ž11., 1335–1346. Liu, X.M., Rolph, T., Bloemendal, J., 1995. The citrate–bicarbonate–dithionite ŽCBD. removable magnetic component of Chinese loess. Quat. Proc. 4, 53–58. Pant, R.K., Dilli, K., 1986. Loess deposits of Kashmir, Northwest Himalaya, India. J. Geol. Soc. India 28, 289–297. Penkov, A.V., Gamov, L.N., 1977. Paleomagnetic datums in Pliocene–Quaternary strata of southern Tajikistan. Abstracts of the IGCP International Symposium on the Neogene–Quaternary Boundary, Moscow, pp. 46–47. Penkov, A.V., Gamov, L.N., 1980. Paleomagnetic datums in the Pliocene to Quaternary strata of southern Tajikistan. In: The Neogene–Quaternary Boundary ŽIGCP Project Nr. 41., Nauka, Moscow, pp. 189–194. Shackleton, N.J., An, Z., Dodonov, A.E., Gavin, J., Kukla, G.J., Ranov, V.A., Zhou, L.P., 1995. Accumulation rate of loess in Tadjikistan and China. Relationship with global ice volume cycles. Quat. Proc. 4, 1–6. Soil Survey Staff, 1975. Soil Taxonomy. A basic system of soil classification for making and interpreting soil surveys. USDA Agriculture Handbook No. 436, US Government Printing Office, WA. Soil Survey Staff, 1994. Keys to Soil Taxonomy, 6th edn. USDA, Soil Conservation Service, US Government Printing Office, WA. Spaargaren, O. ŽEd.., 1994. World reference base for soil resources Ždraft.. ISSS-ISRIC-FAO, Rome, 161 pp. Staninkovitch, K., 1968. Geobotanic zonation of Tadjikistan. Atlas of Tajik SSR, Academy of Sciences of Tajik SSR. Zapriagaeva, V.I., 1964. Wildgrowing Fruits of Tajikistan. Nauka, Moscow, 695 pp. Žin Russian.. Zapriagaeva, V.I., 1968. Map of Wildgrowing Fruits, 1:1.5 Mill. Atlas of Tajik SSR, Academy of Sciences of Tajik SSR, Moscow Žin Russian..
Weathering and clay mineral formation in two Holocene soils and in buried paleosols in Tadjikistan: towards a Quaternary paleoclimatic record in Central Asia A. Bronger
a,)
, R. Winter a , S. Sedov
b
a
b
Institute of Geography, UniÕersity of Kiel, D-24098 Kiel, Germany Department of Soil Science, Lomonosow UniÕersity, 119899 Moscow, Russian Federation
Abstract The upper part of the Karamaydan section, Tadjikistan, shows the most detailed loess–paleosol sequence yet known for the Brunhes chron, and the central and lower parts of the Chashmanigar section provide similar detail for most of the Matuyama chron. To enable paleoclimates to be deduced, the primary and secondary minerals in the silt and clay fractions must be determined separately to evaluate the type and intensity of mineral weathering and clay mineral formation. To distinguish between inherited and pedogenetically formed clay minerals, the original petrographic homogeneity of the parent material from which a soil developed must be established. The main sources of pedogenic clay minerals are phyllosilicates in the silt fractions. Illites and vermiculites are the dominant pedogenetically formed clay minerals in the B or Bt horizons of the Holocene climaphytomorphic soils and in all paleosols ŽS. and pedocomplexes ŽPK. in Karamaydan and Chashmanigar, except S XVII in which large amounts of smectites were formed. There is little difference in the type and amount of pedogenic clay mineral formation between the Holocene soils and the paleosols in the Brunhes epoch at Karamaydan as well as during most of the Matuyama epoch at Chashmanigar. These results indicate that the climates of the interglacials represented by the B or Bt horizons of the buried paleosols of young, mid and old Pleistocene age were similar to that of the Holocene. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Loess; Mineral weathering; Clay mineral formation; Micromorphology; Clay illuviation; Paleoclimate
)
Corresponding author. Fax: q49-431-880-4658; E-mail: [email protected]
0341-8162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 9 8 . 0 0 0 7 9 - 4
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1. Introduction Loess profiles in Tadjikistan, especially those of Karamaydan and Chashmanigar, contain loess–paleosol sequences representing about the last 2 million years, which are probably as detailed as any in Central and East Asia. They have been described several times from a geological–stratigraphical perspective, mainly by Dodonov Ž1979, 1984, 1991., Dodonov et al. Ž1977, 1982., Lazarenko Ž1977, 1984. and Lazarenko et al. Ž1981., and the first paleopedological work was presented by Bronger et al. Ž1995.. Penkov and Gamov Ž1977, 1980. and Forster and Heller Ž1994. identified the BrunhesrMatuyama ŽBrM. boundary in several loess profiles in Tadjikistan between the Pedocomplexes ŽPK. IX and X. An angular unconformity occurs below PK X at both Karamaydan and Chashmanigar, supporting the stratigraphic correlation between these profiles, which enables a reliable chronostratigraphical framework to be developed. Following earlier investigations of the micromorphology and particle size distribution of the loesses and paleosols in both sections ŽBronger et al., 1995., we present here mineralogical analyses of the Karamaydan and Chashmanigar profiles. We selected B or Bt horizons of eight PKs or single paleosols ŽSs. in the upper Karamaydan section and nine PKs or Ss in the central and lower part of the Chashmanigar section, and compared them with two Holocene soils of a polypedon near Karatau ŽFig. 1.. The upper part of the Karamaydan section seems to show the most detailed loess–paleosol sequence for the Brunhes chron, and the central and lower parts of the Chashmanigar section have the most detailed sequence for most of the Matuyama chron ŽBronger et al., 1995, 1998.. However, this does not mean that these profiles are complete. The aim of this paper is to give a quantitative paleoclimatic interpretation based on the type and amount of pedogenic clay material formation of the paleosols in comparison with two holocene soils and thus improve the knowledge of the history of Quaternary climatic changes.
2. Methods For paleoclimatic deductions from loess–paleosol sequences it is necessary to understand the processes involved in the genesis of the paleosols. However, this is often difficult because most of the buried paleosols are truncated and more or less recalcified from the overlying loess. In contrast to modern ŽHolocene. soils, buried paleosols have consequently lost some important diagnostic properties. However, thin sections allow primary and secondary carbonates to be distinguished and provide unequivocal evidence of the process of clay illuviation. This allows, for example, the distinction between B and Bt horizons and also between Bw and CB horizons, as noted in more detail in Bronger et al. Ž1998.. For further paleoclimatic interpretation mineralogical investigations are necessary to indicate the nature and intensity of weathering. Many of the soil-chemical methods of investigation applied to unburied Ž‘modern’. soils cannot be used because of post-pedogenetic changes in the paleosols. To enable paleoclimates the primary and secondary minerals of each fraction, especially that of the clay, must be determined separately to
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evaluate the type and intensity of mineral weathering. For clay mineralogical investigations, it is necessary to separate the clay fraction Ž- 2 mm. into coarse Ž2–0.2 mm. and fine clay Ž- 0.2 mm., enabling semiquantitative estimations to be made. The characteristics of clay minerals in Pleistocene soils and in modern ŽHolocene. soils and their parent materials must be compared to indicate how much clay has been formed by pedochemical weathering processes. To evaluate the type and intensity of weathering, it is first necessary to establish the original petrographic homogeneity of the parent material from which a soil developed. One way of determining this is to use resistant index minerals ŽBarshad, 1967.. These should be present in equal amounts in all horizons and are neither decomposed nor displaced by soil development. Preferably they should occur in quantities sufficiently large for precise quantitative determinations by counting a few hundred grains. In the loess soils of Central Europe and North America ŽBronger, 1966, 1969r1970, 1976, 1991; Bronger et al., 1976. and the Loess Plateau of China ŽBronger and Heinkele, 1989a, 1990; Heinkele, 1989. stable heavy minerals, such as zircon, anatase and rutile, are unsuitable because they occur only in very small quantities. However, quartz is suitable because it is resistant to pedochemical weathering, at least in temperate latitudes, and is always abundant in loess. If the percentage of quartz by weight in the different soil horizons is approximately constant, there is good evidence that the soil parent material was originally homogeneous ŽBronger, 1976, pp. 25–30; Bronger et al., 1976.. To determine the mineral composition of the 63–20 mm fraction, several slides were examined in polarized light. In this fraction the phyllosilicates could be separated into muscovites and biotites. At least 300 particles were identified per slide and the percentage of each mineral was calculated. The proportions of quartz, feldspar, phyllosilicates and other heavy minerals in the fractions 6–20 mm and 2–6 mm fractions were determined by phase-contrast microscopy. At least 500 grains in the 6–20 mm fraction and at least 700 grains in the 2–6 mm fraction were examined per slide. The percentages of the minerals in each fraction were then multiplied by the weight percentages of each fraction to give the percentage by weight of each mineral group in the whole sample, as shown in Figs. 2–10. The percentages by weight are little different from the percentages by volume ŽBronger, 1976, pp. 24–25; Bronger et al., 1976., although the minerals have different densities. The composition of the clay subfractions was determined by semiquantitative estimation based on the areas under selected XRD peaks. The 0.2–2 mm and - 0.2 mm fractions were analysed as oriented samples with a Philips PW 1710 X-ray diffractometer using CoK a radiation after Mg–ethylene–glycol treatment as well as after K saturation and heating to 25, 125, 400 and 5508C. We used the weighting factors recommended by Laves and Jahn ¨ Ž1972.: illites and vermiculites were given a weighting of 1, kaolinites and smectites 0.25. The results were compared with the cation exchange capacities of both fractions of selected paleosols and one Holocene soil to correct gross deviations. Each mineral name refers to a group of closely related clay minerals with slightly varying compositions. The weight percentages obtained by multiplying the estimated percentages of the clay minerals in each fraction by the weight percentage of each clay subfraction are consequently only estimates. However, the results allow
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Fig. 1. Genesis of the soil horizons of pedocomplexes ŽPK. of the loess profile of Karamaydan, Tadjikistan ŽA: upper part, right section; B: upper part, left section; C,D: lower part of the left section. and of the loess profile of Chashmanigar, Tadjikistan ŽE: central part; F: lower part..
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Fig. 2. Mineralogical and clay mineralogical composition of two Holocene soils ŽTypic Haploxerolls or Haplic Phaeozems. in the Tadjik Depression Ž1700 m above sea level..
conclusions to be drawn regarding the original petrographic homogeneity of the parent material and the type and intensity of pedogenic weathering and clay mineral formation in the soils of the investigated sequences.
Fig. 3. Mineralogical and mineralogical composition of pedocomplexes ŽPK. I and II in the loess profile of Karamaydan ŽTadjik Depression.. For legend see Fig. 2.
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3. Results The designation of horizons of the pedocomplexes ŽPK. or single paleosols ŽS. in the Karamaydan and Chashmanigar sections based on field and micromorphological studies Žfor details see Bronger et al., 1995, 1998. are summarised in Fig. 1. Parts A and B show the left and right sections, respectively of the upper part of the Karamaydan sequence. Field studies in 1991, before a landslide covered much of the right section in spring 1992, clearly indicate that PK II was missing in the left section. Fig. 1 shows the position of the samples taken for mineralogical investigations, the results of which are summarised in Figs. 2–9. The main points worth emphasizing are the following. Ž1. The percentages of quartz by weight in the silt fractions increase only slightly between the C or Ck and the overlying B or Bt horizons in almost all PKs and Ss, thus indicating original petrographic homogeneity. However, in the quartz content from 19.5 to 23.5% in S XIV and especially from 21 to 26% in S XII at Chashmanigar ŽFig. 7., show a greater petrographic inhomogeneity in these paleosols. Ž2. Only some Bt horizons in the PKs I–VI and S XV show about 1% of illuviation argillans in the soil matrix, which is regarded as the minimum necessary for an argillic horizon ŽSoil Survey Staff, 1975, 1994.. Even the lower Bt horizon of PK I, the two Bt horizons of PK II and the lower Bt of PK IV have - 2% illuviation argillans ŽBronger et al., 1998.; in S XV it just meets the 1% criterion. Thus, the process of clay illuviation is mostly not responsible for the increase in clay content from the C or Ck to the Bt horizons. Most of the increase in clay content therefore results from pedogenic clay formation by weathering in situ. Ž3. The main sources of pedogenic clay minerals are phyllosilicates in the silt fractions: in most of the paleosols and the Holocene soils there is a considerable decrease in silt-size micas in the B or Bt horizons compared with the Ck horizons. Feldspars are a minor source of the pedogenically formed clay minerals ŽFigs. 2–9.. In the coarse silt fraction Ž20–63 mm., where a separate identification of different phyllosilicates is possible, 60–80% of the micas are muscovites and 20–40% are
Fig. 4. Mineralogical and mineralogical composition of pedocomplexes ŽPK. III and IV in the loess profile of Karamaydan ŽTadjik Depression..
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Fig. 5. Mineralogical and mineralogical composition of pedocomplexes ŽPK. V and VI in the loess profile of Karamaydan ŽTadjik Depression.. For legend see Fig. 2.
biotites. In the Holocene soils and most of the paleosols, a large decrease in biotite content in the B or Bt horizons compared with the C or Ck horizons was found, especially in the Karamaydan section. This agrees with micromorphological observations, which suggest that in the Bt and B horizons, the biotites show signs of weathering, as indicated by a decrease of pleochroism and precipitation of iron oxides Žgoethite. on their surfaces. In comparison with biotites, muscovites are regarded as rather stable, although they probably contribute to the increase of illite in the clay fractions through physical breakdown. Ž4. Illites and vermiculites are the dominant pedogenetically formed clay minerals in the B or Bt horizons of the Holocene soils. They are also dominant in all paleosols at Karamaydan and Chashmanigar, except in S XVII. Whereas muscovites are a major source of the pedogenic illite formation, transformation of biotites produces vermi-
Fig. 6. Mineralogical and mineralogical composition of pedocomplexes ŽPK. IX and XII in the loess profile of Karamaydan ŽTadjik Depression.. For legend see Fig. 2.
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Fig. 7. Mineralogical and mineralogical composition of pedocomplexes ŽPK. X, XII and XIV in the loess profile of Chashmanigar ŽTadjik Depression.. For legend see Fig. 2.
culites, a process which can occur quickly ŽFanning et al., 1989.. In some paleosols, especially in the Bt horizons of PK X and S XIX at Chashmanigar, regular interstratified ˚ were found in the fraction 0.2–2 mm. mica–vermiculites with a basal spacing of 24 A
Fig. 8. Mineralogical and mineralogical composition of pedocomplexes ŽPK. XV ŽqVI. and XVII in the loess profile of Chashmanigar ŽTadjik Depression.. For legend see Fig. 2.
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Fig. 9. Mineralogical and mineralogical composition of pedocomplexes ŽPK. XVIII, XIX, XXV and XVII in the loess profile of Chashmanigar ŽTadjik Depression.. For legend see Fig. 2.
These minerals are known to be products formed during the degradation of biotites to vermiculites. Large amounts of smectites were formed only in the Bt horizons of S XVII at Chashmanigar ŽFig. 8.. Very small amounts of pedogenic smectites were formed in the Bt horizons of PKs I, II, III and IV at Karamaydan ŽFigs. 3 and 4. and of PK X and S XII at Chashmanigar ŽFig. 7.. In other pedocomplexes, smectites, if present, seem to be inherited. Pedogenetically formed smectites can be regarded as an advanced stage of degradation of micas Že.g., Borchardt, 1989. or may be formed from feldspars ŽBronger et al., 1976; Bronger and Heinkele, 1989b, 1990.. In contrast, virtually no kaolinites were formed pedogenetically. In the Central Asian Kashmir Valley, India ŽPant and Dilli, 1986; Heinkele, 1986. and especially in the loess plateau of Central China ŽBronger and Heinkele, 1989a. pedogenetically formed illites are dominate the coarse and fine clay fractions of Pleistocene and Holocene soils. Pedogenic vermiculites are less frequent and smectites are only formed pedogenetically in small amounts, if at all. In contrast, in Central European Holocene and Pleistocene loess soils as well as in relict loess soils in the central and northern part of the Great Plains of the USA, smectite is the main pedogenetically formed mineral in the dominating fine clay fraction Žgreater than 0.2 mm. whereas illites are dominant in the coarse clay fraction ŽBronger, 1976, 1991; Bronger and Heinkele, 1989b, 1990.. The general predominance of pedogenic illites
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may result from the large amounts of muscovites in the silt fractions of loess in the Central Loess Plateau ŽHeinkele, 1989. and the Tadjik Depression. Ž5. There is little difference in the type and amount of pedogenic clay minerals between the Holocene soils and the buried paleosols ŽB or Bt horizons.. The same is true when the B or Bt horizons are compared with their parent materials ŽCk horizons. in the young, mid and old Pleistocene PKs or Ss ŽFigs. 2–9.. The amount of pedogenic clay mineral formation in the Quaternary soils is mostly in the range of 10–15%. Only in the lower Bt horizons of PKs I and IV at Karamaydan is it is large as 20% ŽFigs. 3 and 4.. In S XII at Chashmanigar ŽFig. 7. the increase in clay content is almost 20%, although this interpretation is complicated by a certain petrographic discontinuity in this soil Žsee above.. A limited amount of pedogenic clay mineral formation Ž10–15%. is typical also of the lower paleosols of the Matuyama epoch in the Chashmanigar section, although S XIX and XXV, which are more rubefied soils ŽFig. 1E,F., seem to be more strongly developed than the other paleosols. For instance their Munsell notations have 7.5-5YR or 5YR hues Žwhen dry., whereas most other paleosols have 10YR, some being close to 7.5YR. However, the Žclay. mineralogical composition of the Bt horizons of S XIX and XXV ŽFig. 9. were only compared with the ŽB.Ck horizons immediately beneath. The increase in clay content from the Bt horizon to the underlying loess is about 15% ŽBronger et al., 1995, Fig. 5.. The criterion of rubefication or the formation of hematite at the expense of geothite is often exaggerated as an indicator of a warm climate; it depends more on a distinctly seasonal climate with dry periods, on good drainage and on little or no humus. The Holocene climate in the Tadjik Depression south of the Tien-shan and west of the Pamir is characterized by cool and moist winters and very warm, very dry summers. This results in a distinct xeric soil moisture regime ŽSoil Survey Staff, 1975., though a considerable water deficit in the summer is mitigated by a large water utilization supplied by the high plant available water capacity of loess soils. In the late autumn and winter, water recharge occurs to give a large water surplus ŽBronger et al., 1995, Fig. 2.. In about the last 100 years the landscape has been strongly degraded and severe soil erosion has occurred because of a large increase in the population. However, Zapriagaeva Ž1964, 1968. and Staninkovitch Ž1968. concluded that the potential natural vegetation in the Tadjik Depression, which lies at 1000–2200 Ž2400. m above sea level, is broad-leaved forest consisting of maple Ž Acer turkestanicum., plane Ž Platanus orientalis., ash Ž Fraxinus potomolina., wild-growing fruit trees such as walnut Ž Juglans regia. and almond Ž Amygdalus communis, A. bucharica. and different species of Celtis, Pyrus, hawthorn Ž Crataegus., etc. In several areas juniper Ž Juniperus seraÕschanica. occurs at elevations of 1700–2300 m above sea level. The two Holocene soils were selected from a site near Karatau at about 1700 m above sea level on a plateau not far from the Chormazak Pass, as shown in Fig. 1 Žsee also Bronger et al., 1995, Figs. 1 and 2.. The two soils are free of CaCO 3 and the pH-values Ž0.1 M KCl. were as low as 4.9. Both have a dark grey Ah horizon, 27 cm in thick, which meets the requirement of a mollic epipedon ŽSoil Survey Staff, 1975. or a mollic horizon ŽSpaargaren, 1994.. Their Bt horizons, with G 7.5YR hues, are 105 cm and 80 cm thick, respectively. Both have a sharp boundary with a whitish Ck horizon, which is typical of forest rather than steppe soils. Micromorphologically several parts of
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Fig. 10. Mineralogical and clay mineralogical composition of the strongly rubefied mid-Pleistocene F6 complex in the loess profile of Stari Slankamen, Yugoslavia. For legend see Fig. 2.
the Ah horizons have a fine spongy fabric rich in aggregates and pores, indicating extensive biological activity. Most parts of the Bt horizons have a more dense fabric with far fewer aggregates and voids. Few illuviation argillans are visible: they comprise much less than 1% by area. Furthermore, the very small increase in clay content from the Ah to the B horizons ŽFig. 2; see also Bronger et al., 1995, Fig. 3. is not sufficient for these subsurface horizons to qualify as argillic horizons in Soil Taxonomy ŽSoil Survey Staff, 1975. or as argic horizons in the World Reference Base ŽSpaargaren, 1994.. The designation as a Bt horizon is a genetic and not a diagnostic one ŽSoil Survey Staff, 1994, p. 293.. These broad-leaved forest soils have to be classified as Typic
Fig. 11. Pedostratigraphic correlation of loess–paleosol sequences of Karamaydan, Central Asia, Luochuan, East Asia Žcf. Fig. 9B in Bronger et al., 1998. and Stari Slankamen, Yugoslavia with stages 15 to 13 of the deep-sea oxygen record of Bassinot et al. Ž1994..
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Haploxerolls or Haplic Phaeozems. It is surprising that the process of clay illuviation has scarcely occurred despite the considerable water surplus in March and April. More research on Holocene soils, especially in the moister Khovaling–Chashmanigar area, is needed.
4. Discussion and conclusions Our clay mineralogical results, detailed in point 5 above, indicate that the climates of the interglacials represented by the B or Bt horizons of the buried paleosols were comparable to that of the Holocene, because there is little difference in the type and amount of pedogenic clay mineral formation between the Holocene soils and the paleosols in the Brunhes as well as in most parts of the Matuyama epoch. The paleosols are mostly truncated and only in the upper soil parts of PKs III ŽFig. 1B. and V ŽFig. 1C. is the topsoil preserved. Nevertheless, they are genetically Žvery. similar to the Holocene soils. The presence of argillans as unequivocal signs of clay illuviation, especially in the lower Bt horizons of PKs I and IV Žwhere it probably coincident with greater pedogenic clay mineral formation. may indicate a somewhat moister climate during these interglacials than during the Holocene. The same is probably true for the two interglacials represented by the two Bt horizons of PK II. However, Ck horizons with distinct calcareous nodules are much better developed in most paleosols ŽFig. 1. than in the Holocene soils, though the reason for this remains uncertain. Recently published low-field magnetic susceptibility data from the same Karamaydan section show high values only in PKs II and III ŽShackleton et al., 1995. with somewhat lower values in PK I ŽForster and Heller, 1994.. Both publications found much lower susceptibility values in all other pedocomplexes or much smaller differences between the paleosols and the accompanying loesses. This is not in agreement with the clay mineralogical results and micromorphological observations, especially for the lower Bt horizons of PKs I and IV, which show more pedogenic clay mineral formation than other paleosols. It is important to determine which ferrimagnetic or superparamagnetic minerals are primary or lithogenic and which are pedogenic, Že.g., Liu et al., 1995. before low-field susceptibility variations of paleosols and accompanying loesses can be ‘taken as a reliable paleoclimate proxy’ ŽForster and Heller, 1994, p. 502.. Our conclusion that the interglacials in the Brunhes and most parts of the Matuyama epochs were climatically similar to the Holocene disagrees with earlier conclusions ŽBronger, 1976; Bronger and Heinkele, 1989b. that in the temperate climatic zone of Central Europe the paleosols derived from loess are much more strongly developed and rubefied in the mid and early Pleistocene than in the Holocene, and were probably formed in a subtropical climate. For example, the mid-Pleistocene F6 soil at Stari Slankamen, Yugoslavia, shows much more pedochemical weathering and clay mineral formation than the Holocene soils in the same area ŽBronger, 1976.. More than 40% of the feldspars and almost 80% of the micas are decomposed ŽFig. 10.; in thin sections only muscovites are found and no biotites remained. Later the F6 soil was correlated with the three soils of the S5 pedocomplex in the Luochuan section, China ŽBronger and Heinkele, 1989b.. As F6 corresponds chronostratigraphically Žvia the S5 pedocomplex at
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Luochuan. with at least two PKs ŽV and VI. at Karamaydan ŽFig. 11., we suggest that the much greater pedochemical weathering and clay mineral formation in the F6 results from a longer period of pedogenesis and does not indicate a warmer or wetter climate. Reasons for this correlation are given by Bronger et al. Ž1998.. Correlation with the oxygen isotope record of Bassinot et al. Ž1994. suggests that PKs V and VI were formed over a period of about 140,000 years, although pedogenesis was interrupted several times by loess formation. Thus the formation of the F6, which should now be regarded as F6 pedocomplex, at Stari Slankamen occurred over a period several times longer than the Holocene, and this explains its much greater pedochemical weathering and clay mineral formation.
Acknowledgements We thank the Deutsche Forschungsgemeinschaft for supporting field, micromorphological and laboratory work Žgrants Br 303r26-1,2..
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