Magnetic study of Late Pleistocene loess/palaeosol sections from Siberia: palaeoenvironmental...

Magnetic study of Late Pleistocene loess /palaeosol sections fromSiberia: palaeoenvironmental implications

G. Matasova,1 E. Petrovsky,2,3,* N. Jordanova,2,4 V. Zykina1 and A. Kapicka2

1 Institute of Geology UIGGM Siberian Division, Russian Academy of Sci., Ac. Koptyug ave. 3, 630090, Novosibirsk, Russia2 Geophysical Institute ASCR, Bocnı II /1401, 141 31 Prague 4, Czech Republic. E-mail: [email protected] CEREGE, University of Aix-Marseille III, Europole de l’Arbois, 13545 Aix en Provence, France4 Geophysical Institute, Bulgarian Academy of Sciences, Acad. Bonchev Str., bl.3, 11113 Sofia, Bulgaria

Accepted 2001 June 8. Received 2001 June 6; in original form 2001 January 10

SUMMARY

Rock-magnetic properties, including anisotropy of magnetic susceptibility, were investi-gated in detail for two loess /palaeosol sections in south-western Siberia (Bachat andKurtak). The results obtained agree with the ‘Alaskan’ type of susceptibility variationsfor the Kurtak section, showing maximum susceptibility values in loess horizons andminima in palaeosols. For the Bachat section the palaeoenvironmental record, expressedthrough susceptibility, is not clear, in spite of the geographical proximity of the twosections. Instead, frequency-dependent magnetic susceptibility (FD%) discriminates wellbetween loess and palaeosol units, showing maxima in pedocomplexes and minimain loess units. This suggests certain pedogenic formation of fine ferrimagnetic grainsin soils. Studies of anisotropy of magnetic susceptibility (AMS) reveal a well-definedsedimentary magnetic fabric for both sections. Different origins of loess deposition, pureaeolian at Bachat and aeolian affected by secondary processes at Kurtak are reflected inthe AMS patterns. The two sections also show different high-temperature behaviour ofmagnetic susceptibility. While for the Bachat section only one type of thermomagneticcurve was found, the Kurtak section shows different behaviour for loess and palaeosolunits. Magnetic properties of samples from the two sections are discussed in terms ofpalaeoclimatic and depositional conditions.

Key words: anisotropy of magnetic susceptibility, frequency dependence of magneticsusceptibility, magnetic susceptibility, palaeoclimate, palaeoenvironment, Siberia.

1 I N T R O D U C T I O N

Magnetic susceptibility of continental sediments, especially of

loess /soil sequences, has been widely used for palaeoclimatic

reconstructions (e.g. Heller & Liu 1984; Beget et al. 1990;

Maher & Thompson 1991; Liu et al. 1988, 1993; Forster et al.

1996; Chlachula et al. 1997, 1998; Maher 1998). Most of the

studies deal with Chinese loess deposits (e.g. Hovan et al. 1989;

Kukla et al. 1990; Heller et al. 1991; Evans et al. 1996), and

less with European ones (e.g. Reinders & Hambach 1995;

Oches & Banerjee 1996; Jordanova & Petersen 1999). Unlike the

‘pedogenic enhancement’ model, applied to explain the observed

relationship between high susceptibility values of palaeosols

and minima found in loesses, loess /soil sequences in Siberia

(Chlachula et al. 1997, 1998; Virina et al. 2000) and Alaska

(Beget et al. 1990; Rosenbaum et al. 1997; Vlag et al. 1999) in

general exhibit the opposite behaviour, i.e. relative magnetic

enhancement in loess and signal depletion in soils. Some authors

(Hayward & Lowell 1993) suppose that varying loess accumu-

lation rates from the source area or wind intensity (Beget et al.

1990; Chlachula et al. 1998) determine the susceptibility pattern

in these areas, although much more work is needed for better

understanding of this phenomenon. In a recent study, Evans

(2001) proposed qualitative analysis, explaining the relation-

ship between the two models and the conditions, determining

which one dominates in determining the magnetic susceptibility

behaviour.

Anisotropy of magnetic susceptibility (AMS) is known to

be a very powerful tool in magnetic fabric analysis, related

strongly to texture, tectonic deformations, strain and transport

direction (e.g. Hrouda 1982; Rochette et al. 1992; Tarling &

Hrouda 1993; Borradaile & Henry 1997). Difficulties in revealing

palaeoenvironmental significance of AMS in terms of transport

direction for non-consolidated sediments are mainly caused

by post-depositional tectonic deformations (e.g. Borradaile

1988) or artificially imposed magnetic fabric during sampling

(Gravenor et al. 1984; Jordanova et al. 1996; Copons et al.

1997).

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We have studied magnetic susceptibility and its anisotropy for

two loess /palaeosol sections in Siberia, Bachat (54.5uN, 87.1uE)

and Kurtak (55.1uN, 91.4uE). Siberian loesses have been studied

by other authors (i.e. Pospelova 1971, 1973), in particular the

Kurtak profile, due to its significance as a major palaeolithic

site and the fact that it is the thickest loess /palaeosol section in

southern Siberia (Chlachula et al. 1997, 1998). The aim of this

study is to obtain new magnetic data on the two profiles in

order to increase our knowledge about the lithological, palaeo-

climatological and depositional mechanisms controlling their

magnetic signatures. The two sampled profiles are suitable for

this purpose, because the Bachat profile is believed to be of

purely aeolian origin, while the deposits in the Kurtak profile

seem to have been influenced by secondary seasonal flooding

and solifluction processes (Chekha 1990). Therefore comparison

of various magnetic parameters may help us to better inter-

pret magnetic susceptibility data in terms of the mechanisms

controlling the formation of the two sequences.

2 G E O L O G I C A L S E T T I N G

Quaternary loess /soil deposits are widespread in the southern

part of Siberia (between latitudes of 50u–59uN and longitudes

of 66u–97uE), fragmentary covering an area of more than

700 000 km2 (Fig. 1). Thickness is variable, but can reach

100 m. According to the geological information available

(e.g. Volkov & Zykina 1982; Zudin et al. 1982; Drozdov et al.

1990), two principal types of loess deposits are present in

Siberia: pure aeolian sediments and sediments affected by

secondary processes. The thickest loess /soil profiles are related

to development of the fluvial system of major Siberian rivers

(Fig. 1). The altitude of both sections is between 240 and 270 m

above mean sea level.

Section Bachat is situated in the western part of Kuznetsk

Basin (54.5uN, 87.1uE, Fig. 1). The total thickness of Late

Pleistocene loess /soil sediments in this region is up to 12 m.

Sampling was carried out in the upper part of a coal quarry

wall, where the thickness of Late Pleistocene sediments is about

7 m. The section includes modern soil, two pedocomplexes Is

(PC1) and Br (PC2), corresponding in age to L1SS1 and S1 in

China, respectively (An et al. 1991) and two loesses LE1 and LE2,

corresponding to L1LL1 and L1LL2 in China. The palaeosols

in the Is and Br pedocomplexes are widespread in Siberia as

regional units and can be traced on the basis of their specific

profile constitution. The PC1 is subdivided into two palaeo-

pedological units with four horizons: A1 upper, BCk upper,

A1 lower and BCk lower, whereas PC2 in Bachat consists of

only A1 lower and BCk lower horizons. Therefore, the studied

Figure 1. Location map showing the sampling sites (Bachat and Kurtak) (redrawn from Volkov & Zykina 1984; modified by the authors).

368 G. Matasova et al.

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section can be considered as a sequence of loesses and two inter-

bedded chernozem-like palaeosols (PC1) and one chernozem-

leached soil (PC2) with humic (A) and illuvial (BCk) horizons.

According to geological information, LE1 and LE2 are typical

loesses with carbonate concretions and pseudomicelii. A thin

sandy layer is found near the middle part of LE1 and inter-

layers of coarse reworked material occur at the bottom of LE2

(see Fig. 2). Absolute age determinations (14C and thermo-

luminiscence) date the PC1 at 20–35 ka, and PC2 at 110–130 ka

(Volkov & Zykina 1982; Zykina et al. 1981; Zykina 1999). The

PC1 was formed during the Karginsky interstadial period,

corresponding to stage 3 of the oxygen isotope curve in deep-

sea sediments (Shackleton & Opdyke 1973). The PC2 was

formed during the Kazancevo interglacial period, corresponding

to stage 5e of the oxygen isotope curve (Shackleton & Opdyke

1973).

Section Kurtak is located in the upper Yenisey River

valley in the steppe zone of Northern Minusinsk depression

(55.1uN, 91.4uE, Fig. 1). The western side of the Krasnoyarsk

reservoir is covered by partly aeolian, dilluviated, alluviated

and colluviated loess deposits with buried palaeosols in between.

The loess /soil profile studied consists of loess units LE1–LE5

and four pedocomplexes (PC1, PC2, PC3, PC4) (Fig. 2). Most

humic horizons of palaeosols are usually pale, weakly developed

and contain secondary carbonate concretions. Therefore, they

were probably formed under steppe vegetation during periods of

continental arid climate. Clear traces of solifluction are visible

in all palaeosols. The Kr (PC1) unit is a chernozem-like soil,

with weakly developed humic horizon. PC1 is attributed to one

of the mid-last glacial interstadials. The Sh (PC2) kashtanozem

type, Km (PC3) and PC4 chernozem-leached type pedo-

complexes formed during the last interglacial stage and exhibit

well developed profiles with thick humic accumulation horizons.

This indicates that they were formed under relatively warmer

and more humid climate. Palaeosol PC3 is separated from PC2

by a thin (0.4 m) loam layer. Taking into account the tenuous

thickness of the horizon and its considerable secondary

alteration by pedogenic processes, it is not considered as an

individual loess horizon LE3 in the stratigraphic sequence at

Kurtak.

The PC4 is distinguished from the others by its dense

structure, high sand content and secondary Fe- and Mn-

concretions. Such concretions are present in smaller amounts in

all palaeosols. Loess horizons in the Kurtak section could be con-

sidered as an alternation of differently coloured aleurites (in the

middle part of LE1) and loams, with abundant Fe-concretions,

Mn spots and gleying.

3 S A M P L I N G A N D M E T H O D S

Samples were collected from clean, fresh outcrops as

block samples of c. 10r10r10 cm. Standard cubic samples

(2r2r2 cm) were cut in the laboratory, so that 4–6 specimens

were obtained from each sampling level.

Figure 2. Lithology, density, LF magnetic susceptibility and per cent frequency dependence (FD%) of susceptibility of Bachat (a) and Kurtak (b).

Loess /palaeosol sections from Siberia 369

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Altogether more than 630 orientated and non-orientated

specimens were obtained from the Kurtak section, while the

Bachat collection consists of about 320 specimens.

Laboratory measurements of magnetic susceptibility (x) were

carried out in a Bartington MS-2 Susceptibility Meter at two

frequencies (0.47 kHz and 4.7 kHz). Magnetic anisotropy of

samples was measured using Kappabrigde KLY-3S (Agico Ltd,

Czech Republic).

Thermomagnetic behaviour of magnetic susceptibility was

examined on selected samples, representing each lithologic unit,

in a CS-3 oven attached to a Kappabrigde KLY-3S (Agico Ltd,

Czech Republic).

4 R E S U L T S

All the measured magnetic parameters of loess-soil sediments

were mass-normalized. For this purpose density was calculated

and plotted as a function of depth (Fig. 2). The density data

reflect compositional differences in Bachat and Kurtak sediments

(Table 1).

4.1 Magnetic susceptibility

4.1.1 Bachat section

Variations of mass-normalized low-field magnetic susceptibility

of Bachat samples do not discriminate well between loesses

and soils, but on the other hand reflect changes in lithology

within the LE1 unit only (Fig. 2). Indeed, the x-values are

highly variable in the youngest loess LE1, with the highest

values (8–9.6r10x7 m3 kgx1) measured in the top of the gley

layer and the lowest value of 2.8r10x7 m3 kgx1 observed in the

thin sandy layer. The average value of low-field susceptibility

of the LE1 loess is higher than that of the LE2 loess, where

susceptibility is practically invariable.

Per centage frequency dependent magnetic susceptibility

[FD%=(xLFxxHF)*100 /xLF], as well as mass-specific frequency

dependent susceptibility Dx=(xLFxxHF), better discriminates

loess from soils, with high values (up to 7.5 per cent) in palaeosols

and low values in loesses (up to 2.5 per cent, in average) (Fig. 2a).

4.1.2 Kurtak section

The magnetic susceptibility pattern reflects changes in the

lithology with maxima corresponding to the periods of loess

deposition and minima related to pedocomplexes (Fig. 2b).

Humic horizons of all soils are characterized by the lowest

susceptibility values (5–12.6r10x7 m3 kgx1), with an average

value of 10r10x7 m3 kgx1. In general, higher values were

measured in loess horizons (up to 31r10x7 m3 kgx1) with the

average value of 18r10x7 m3 kgx1. The difference between

loess and soil susceptibilities is much more pronounced than at

Bachat.

Magnetic susceptibility of Kurtak sediments is 3–5 times

higher than at Bachat (Table 1). Most likely it is connected

with differences in concentration of ferromagnetic minerals in

the sediments. On the other hand, FD susceptibility of Kurtak

samples is nearly zero and almost uniform (Fig. 2b), indicating

minor content or almost absence of SP magnetic grains. This

result contradicts conclusions reached by Chlachula et al. (1998)

and will be discussed below.

4.2 Thermal behaviour of magnetic susceptibility

Representative samples of each lithological unit were selected

for measurements of temperature dependence of magnetic

susceptibility in order to identify the magnetic phases present.

Heating /cooling cycles from room temperature to 700 uC were

performed in air and typical results are shown in Fig. 3(a).


Thermomagnetic behaviour exhibited by Bachat samples can

be divided into two groups (Fig. 3a). Most samples show a

distinct decrease in susceptibility at 570u–580 uC, suggesting

magnetite as a major magnetic phase. The cooling branch is

characterized by a strong increase in susceptibility in the same

temperature range. In some palaeosol samples from humic

horizons, a small peak is observed at 450u–500 uC during heat-

ing and the maximum of the cooling curve occurs less or more

Table 1. Summarized data for density and magnetic susceptibility measurements for different stratigraphic units of the

two sections.

Section Strat.

unit

Number

of samples

Density (103 mx3 kg) LF susceptibility (10x7 m3 /kg)

min max average min max average

BACHAT LE1 62 1.19 1.44 1.32 2.79 9.62 6.14

LE2 58 1.48 1.68 1.56 2.47 4.18 3.51

PC1 59 1.18 1.41 1.29 3.34 5.46 3.89

PC2 38 1.25 1.52 1.45 1.83 3.90 2.80

KURTAK LE1 44 1.28 1.48 1.37 11.97 23.91 18.25

LE2 72 1.39 1.58 1.46 11.71 31.11 20.61

LE4 5 1.58 1.66 1.62 9.23 17.18 11.83

LE5 6 1.52 1.59 1.56 15.48 27.00 20.32

PC1 32 1.17 1.57 1.29 6.65 12.36 10.09

PC2 14 1.26 1.43 1.35 5.03 11.01 9.44

PC3 5 1.38 1.43 1.41 6.53 10.36 9.36

PC4 14 1.48 1.67 1.63 10.32 12.63 11.67


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in the same temperature range. Samples from loesses and

illuvial horizons of palaeosols do not show this additional

peak, and the maximum on the cooling curve is shifted to lower

temperatures to about 200u–300 uC.

In the case of palaeosols, significant increase in susceptibility

induced by heating can be attributed to neo-formation of

magnetite from phyllosilicate minerals present in soils. The

increase observed in loess units may reflect creation of a strongly

magnetic phase as a result of thermal destruction of primary

minerals in loess material (mica, feldspar, olivine, etc.) or clay

minerals formed during chemical weathering of the aeolian

dust. Different behaviour in magnetic susceptibility was only

observed for samples from interlayers (for example, sample

Bc8e in Fig. 3a), containing sandy and coarser material. In case

of sample Bc8e high-temperature susceptibility behaviour can

be explained by significant contribution of paramagnetic grains.

Figure 3. High-temperature (a) and low-temperature (b) susceptibility behaviour of loess and soil samples from Bachat (above) and Kurtak (below).


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Low-temperature dependence of magnetic susceptibility

(Fig. 3b) showed a more pronounced Verwey transition in

the loess sample compared to the palaeosol, where the para-

magnetic contribution prevails. This suggests the presence of

more multidomain particles in loess compared to palaeosol,

where pedogenesis has taken place.


Two distinct types of thermomagnetic curves were observed

in Kurtak samples (Fig. 3a) characterized by either increase

or decrease in magnetic susceptibility upon heating. All the

Kurtak samples show the Curie temperature of magnetite. In

the palaeosol samples magnetic susceptibility increases after heat-

ing, very similar to the behaviour of Bachat samples. However,

two additional peaks at 200u–300 uC and at 450u–500 uC were

observed on the heating curve of most of the soil samples. The

cooling pattern is similar for all soil samples: sharp increase

occurs after cooling from 580u to 450 uC, followed by gradual

decrease. The room-temperature value is always 3–4 times

higher than the original one.

On the other hand, loess samples exhibit a decrease in

magnetic susceptibility after heating (Fig. 3a). In most of

them a gradual increase of susceptibility during heating up to

250–300 uC is observed. Cooling curves are characterized by

sharp increase at 580u–550 uC and then the signal remains

constant. The final susceptibility value is some 60–80 per cent

lower than the initial one. Hematite is detected neither as a

starting mineral nor as a product of heating to 700 uC (Fig. 3a).

The local maximum at c. 250 uC on the heating branch, which

is absent on the cooling branch, together with a decrease

of susceptibility probably can be explained by the presence of

maghemite. The presence of a clear magnetite Curie temper-

ature on the cooling curves of the loess samples and absence

of magnetite neo-formation suggests that the stable magnetite

fraction is a significant component in the original material.

The presence of coarse multidomain magnetite was con-

firmed by a well pronounced Verwey transition on the low-

temperature curve of magnetic susceptibility (Fig. 3b). From

this point of view, no difference is observed between the loess

and palaeosol samples.

One can note that Bachat soils and loesses and Kurtak

soils show similar thermomagnetic behaviour, while the Kurtak

loess samples exhibit a different pattern. The sharp increase of

susceptibility during cooling observed for soil samples, probably

is the result of magnetite formation from initially non-magnetic

minerals during heating.

Figure 3. (Continued.)


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4.3 Anisotropy of magnetic susceptibility (AMS)


The degree of anisotropy Pk (Jelinek 1981) for samples from

section Bachat is low. For palaeosols Pk falls within the range

of 1.001–1.018 (Fig. 4a), most values (>90 per cent) are

lower than 1.012. Loess samples show Pk values up to 1.042,

but for most of them (>80 per cent) it varies in the range

1.004<Pk<1.02. The anisotropy degree of loess and soil is

similar (Figs 4a and 5a), although loess samples exhibit slightly

higher values. In most cases, these enhanced values correspond

to lithological heterogeneities in loesses (LE1, LE2). To reveal

possible variations in the shape of the susceptibility ellipsoid

with the degree of anisotropy, the magnetic foliations (F) and

lineations (L) are plotted as a function of Pk and fitted by a line

using least squares (Fig. 5). The correlation coefficient (R2)

characterizes goodness of the fit (Table 2). A strong correlation

between F and Pk, but no correlation between L and Pk, was

found for both LE1 and LE2 loesses. The shape of the

anisotropy ellipsoid of Bachat loess is oblate for some 50 per

cent of samples (especially in samples with high Pk) and neutral

and prolate for the others.

Magnetic fabric of loess LE1 is characterized by minimum

susceptibility axes grouped in an almost vertical direction,

while maximum and intermediate axes are scattered in the

horizontal plane with no preferred direction (Fig. 6a). This

pattern is considered as normal sedimentary fabric (Hrouda

1982). Distribution of the principal susceptibility axes in LE2 is

similar, except for the interlayers of reworked material in the

lower part of loess horizon.

The shape of the anisotropy ellipsoid for Bachat palaeosols

varies from prolate through neutral to oblate. There is no pre-

ference for any shape and the correlation coefficients between

degree of anisotropy and foliation or lineation are similar, but

not very high (Table 2).

Principal susceptibility axes for palaeosols are randomly

distributed (Fig. 6a), suggesting that pedogenesis played an

important role for modification of the initial magnetic fabric of

the parent loess material through bioturbation, which was

evident from field observations and also through formation of

magnetic minerals in situ, supported by the FD% values.


The anisotropy parameters for Kurtak loesses and palaeosols

are listed in Tables 2 and 3 and depicted in Figs 4(b) and 5(b).

A higher degree of anisotropy (in comparison with Bachat

section) along the whole profile was observed, with 1.01<Pk<1.115. Most of the samples (90 per cent) are characterized by

1.02<Pk<1.08, and for the lowest pedocomplex PC4 Pk is

Figure 4. Lithology and AMS parameters of loess and soils samples from two sections: Bachat (a) and Kurtak (b).


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between 1.018 and 1.045. Magnetic fabric in both loess and soil

is clearly oblate and shows strong foliation. All correlation

coefficients between the degree of AMS and foliation are greater

than 0.9 (Fig. 5).

The distribution of principal susceptibility axes shows typical

sedimentary magnetic fabric, with minimum susceptibility axes

close to the vertical. The maximum axes are well grouped along

north-west-south-east for LE1 and south-west-north-east for

LE2 direction (Fig. 6b). The exception is palaeosol PC4, which

shows no clear orientation of its principal susceptibility axes.

Mean directions of the AMS ellipsoid and the average AMS

characteristics for Kurtak stratigraphic units and of different

palaeosol horizons (humic A1, transitional Bk, Bm, BCk) are

shown in Table 3.

5 D I S C U S S I O N

5.1 LF magnetic susceptibility

Based on the data available up to now, it is generally accepted

that magnetic susceptibility variations of loess /soil deposits in

Alaska and Siberia mainly reflect wind strength during glacial

and interglacial periods. According to the model of Beget et al.

(1990), periods with intense loess deposition are accompanied

by higher concentration of magnetic material, in contrast to the

warm stadials.

The applicability of the wind-intensity model to our two

Siberian sections, which are relatively close to each other will be

discussed. The magnetic susceptibility data (Fig. 2) suggest that

while for the Kurtak section the expected ‘Alaskan’ mode of

variations is present (e.g. higher x in loesses and low x in soils),

for the other section (Bachat) it is difficult to observe significant

lithology-related changes. As mentioned above, loess deposits

at section Bachat are considered to be of pure aeolian origin,

while for Kurtak’s loess deposition controversial opinions

exist. Chlachula et al. (1997) claim that the upper part of the

section, which is examined in our study, consists of aeolian

loess deposits. On the other hand, Drozdov et al. (1990) do not

exclude combined aeolian–alluvial origin of certain horizons

in the section. Colluvial and solifluction processes are also

considered by the author.

The very different geomorphological conditions and sedi-

mentation processes, acting in the two sections, are reflected also

Table 3. The AMS data for Kurtak loess /soil section. N denotes the number of samples from the corresponding unit.

Stratigraphic

unit

N Kmax Kint Kmin Pk L F T

D (u) I (u) D (u) I (u) D (u) I (u)

LE1 27 290 2 199 3 50 86 1.03 1.01 1.03 0.60

PC1A1 6 281 2 191 3 47 86 1.06 1.00 1.05 0.89

PC1B 5 198 5 289 6 64 82 1.04 1.00 1.03 0.82

LE2 31 235 9 325 2 68 81 1.04 1.00 1.03 0.77

PC2A1 8 57 5 327 5 193 83 1.03 1.00 1.02 0.74

PC2B 12 167 5 76 6 293 82 1.03 1.00 1.02 0.71

PC3A1 4 160 0 250 5 70 85 1.05 1.01 1.04 0.71

PC3B 7 116 0 26 4 209 86 1.03 1.00 1.02 0.91

LE4 5 120 2 35 5 232 85 1.05 1.00 1.04 0.87

PC4A1 5 176 5 272 51 82 38 1.03 1.02 1.01 x0.52

PC4B 6 52 1 142 4 303 86 1.05 1.01 1.04 0.78

LE5 5 61 9 152 6 275 79 1.04 1.01 1.03 0.68

Table 2. Correlation between degree of anisotropy (Pk) and foliation (F) or lineation (L). N denotes the number of samples from the corresponding

unit.

Section Stratigr.

unit

N P–F P–L

Slope

(b)

Intercept

(a)

Fit goodness

(R2)

Slope

(b)

Intercept

(a)

Fit goodness

(R2)

BACHAT LE1 17 0.876 0.121 0.968 x0.002 1.005 x0.007

LE2 12 0.854 0.143 0.969 0.074 0.930 0.309

PC1 31 0.552 0.448 0.710 0.366 0.635 0.538

PC2 19 0.497 0.502 0.671 0.457 0.544 0.609

KURTAK LE1 27 0.840 0.016 0.996 0.036 0.967 0.292

LE2 31 0.830 0.170 0.981 0.071 0.935 0.348

LE4 5 0.663 0.346 0.944 0.083 0.170 0.644

LE5 5 1.300 x0.330 0.974 x0.70 1.737 x0.912

PC1 11 0.822 0.175 0.988 0.072 0.931 0.357

PC2 20 0.924 0.068 0.917 x0.053 1.064 x0.119

PC3 11 0.825 0.171 0.952 0.110 0.894 0.363

PC4 11 0.978 0.009 0.946 0.204 0.803 0.514


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by the thickness of the profiles—the Kurtak section is about

3 times thicker than the Bachat one, indicating a significantly

higher sedimentation rate.

Magnetic susceptibility data of the Bachat section (Fig. 2)

indicate that in this case susceptibility variations are not such a

good proxy of palaeoclimatic changes, related to alternation of

loesses and buried soils. Variations of magnetic susceptibility

within the LE1-unit probably reflect changes in sedimentary

environment and superimposed effects of the initial sedi-

mentation and secondary alterations. There is no plausible

explanation at the moment for the maximum susceptibility

values in the top of the gley layer and minimum ones in

the sandy layer. These variations probably resulted from

abrupt changes in sedimentary environment, caused by local

events. Likely, the changes in magnetic content are the

main reason, however, their true nature requires additional

investigations. Further down, magnetic susceptibility is nearly

constant and does not discriminate between loesses and

palaeosols (Fig. 2, Table 1). On the other hand, variations

of FD% clearly indicate relative enhancement in fine super-

paramagnetic grains in pedocomplexes. FD% is a sensitive

parameter to the presence of ultra-fine ferrimagnetic (magnetite)

Figure 5. Lineation (L), foliation (F) and shape-factor (T) versus degree of anisotropy (P’) for loesses and palaeosols of two sampled profiles:

Bachat (a) and Kurtak (b).


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grains of c. 0.016–0.023 mm in size for the frequencies used

(Maher 1988, 1998). High values reflect enhanced concentration

of superparamagnetic (SP) grains. The observed pattern could

be explained by ‘in situ’ pedogenic susceptibility enhancement

of primary loess material, due to production of authigenic

(magnetite?) SP grains. Thus, minor SP contribution (maxi-

mum FD% y7.5 per cent and low values of Dx) in palaeosols

causes only a slight increase of low-field magnetic susceptibility

in these units. As a consequence, x-values along PC1, LE2

and PC2 are very similar (Fig. 2, Table 1). One can note that

the values of FD% in Bachat palaeosols are in general low; the

average for PC1 is c. 3.5 per cent and 3 per cent for PC2. These

values are significantly lower than those for well studied

palaeosols from China (e.g. Maher & Thompson 1991; Liu

et al. 1993). This suggests that the Bachat soils contain a lower

quantity of SP grains in comparison with Chinese palaeosols.

This distinction probably indicates that pedogenic processes in

Bachat are significantly weaker than in the temperate belt of

China or Central /South Europe, suggesting a colder and dryer

climate in interglacial periods in Siberia.

Section Kurtak exhibits rather different behaviour of

susceptibility and FD% (Fig. 2b). In agreement with the results

obtained by Chlachula et al. (1997, 1998) for the same

section, our data show almost three-fold decrease of x in

Figure 6. Equal-area projection of AMS principal axes of measured samples for all stratigraphic units from section Bachat (a) and section

Kurtak (b).


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pedocomplexes, compared to parent loess units. At the same

time, variations in FD% could not be considered as significant

since all values oscillate near zero, hardly ever exceeding

2 per cent. Considering both the measured values of magnetic

susceptibility of palaeosols and their FD% values, we assume

that pedogenic production of SP grains in Kurtak soils is either

absent, or minimal (e.g. Maher 1988; Eyre 1997). Another

possible reason for the low values of FD% may be advanced

dissolution of magnetic minerals as a result of specific soil-

forming conditions (Maher 1998). However, as will be shown

later on, according to the results of anisotropy of magnetic

susceptibility, such a hypothesis is not very probable. In

contrast, Chlachula et al. (1998) argue that in spite of the

generally low FD% values along the profile, slightly higher

FD%, measured in most developed soils, reflect pedogenic

enhancement.

5.2 AMS behaviour

With our study of anisotropy of magnetic susceptibility (AMS)

we tried to test the conflicting viewpoints concerning the origin

of the loess deposits at Kurtak (Chlachula et al. 1997, 1998;

Drozdov et al. 1990). As shown by Liu et al. (1988) this method

is a powerful tool for discriminating wind-blown sediments

from water-lain deposits.

There are only few published studies on AMS of loess /soil

deposits (e.g. Liu et al. 1988, 1993; Noel & Bull 1982; Noel

1983; Thistlewood & Sun 1991) and most of them deal with

Chinese loess. The main difficulty in studying this important

magnetic property in sediments is probably the sampling tech-

nique. Since loess and palaeosol material is not sufficiently con-

solidated, it is very susceptible to stress influence. Therefore,

application of any kind of sampling method based on pressure

can disturb the magnetic fabric and the results are thus biased

and not reliable for environmental implications (Gravenor et al.

1984; Jordanova et al. 1996; Copons et al. 1997). In the case of

orientated hand-cut blocks, used in our study, this disturbance

is greatly reduced but possible errors could arise from non-

exact orientation, which could cause increased scatter of

measured mean directions.

Our data on AMS of two loess /soil sections in Siberia indicate

that different prevailing mechanisms of loess deposition and

soil formation play role in the determination of the magnetic

fabric. The process of loess deposition can depend on the path-

ways of eroded rock material, the type of rocks being eroded, the

nature of erosion processes, palaeogeomorphology (leeward or

windward deposition) and on various post-depositional changes

(Tarling & Hrouda 1993). The most significant physical

processes affecting the formation of magnetic fabrics are

gravitational and hydrodynamic forces. The Earth’s magnetic

field represents another orientation factor but it is weak,

affecting only small grains (<1 mm). When deposition occurs

in quiet conditions (in absence of any current or strong wind),

gravitational settling is the main controlling force, which causes

all grains to lie with their long axes in the plane of the

depositional surface or bedding plane. Most oblate and prolate

grains also align randomly within the bedding plane (although

small grains will show some preferential alignment correspond-

ing to the local geomagnetic field) and with the minimum axes

perpendicular to the bedding plane (e.g. Reinders & Hambach

1995). When the deposition is influenced by a current, the

longer axes of the grains will be grouped (depending on

velocity) along the flow direction or perpendicular to it in

case of a sloping bed (e.g. Rees & Woodall 1975; Noel & Bull

1982; Noel 1983). The deviation of the xmin direction from

the vertical can be also due to imbrication of the grains in a

moderate current, but its effect will decrease due to compaction.

Another key question in AMS studies is the identification of

the main mineral determining the anisotropy behaviour through

its crystallographic anisotropy or shape anisotropy (Hrouda

1982; Jackson 1991; Rochette et al. 1992; Tarling & Hrouda

1993). The results of thermomagnetic analysis show that the

main ferromagnetic mineral is magnetite. In case of section

Kurtak, high values of susceptibility both in loess and palaeosol

horizons (Fig. 2) allow us to suppose that the AMS is governed

by the shape anisotropy of magnetite grains. For Bachat section,

however, low susceptibilities especially along Is, LE2 and Br

horizons suggest that phillosilicate minerals also play a role in

the determination of magnetic fabric (Hrouda 1982).

The AMS pattern obtained for the Kurtak section raises the

question about the origin of the well defined magnetic texture.

Several possible ‘orienting’ factors could be considered: (1) wind

(2) solifluction and colluvial sedimentation (3) water flow. In

case of aeolian loess deposits wind is the main factor con-

trolling the sedimentation process. However, usually it cannot

give strong alignment of dust material because of the signi-

ficantly lower viscosity of air, compared to water. Taking into

account the observed high degree of anisotropy for Kurtak

profile and the well-defined mean xmax direction in loesses,

wind is probably not the main orienting factor. On the other

hand, according to the geological information (Volkov 1971),

the prevailing palaeowind direction in the area was SW–NE,

coinciding with xmax in LE2, but nearly perpendicular to xmax

in LE1 (Fig. 6b).

Colluvial sedimentation and solifluction involve flow-like

movement of deposited material. Consequently, the observed

AMS pattern could reflect such processes. However, in a case

when the material is re-deposited from a slope usually the

depositional surface is inclined. In our case, almost no slope in

LE1 can be revealed by AMS, and only a slight one (to the NE)

in LE2 is observed. On the other hand, seasonal flooding can

considerably re-oriented grains in the sediment. The obtained

xmax directions in the loess units coincide well with the

orientation of the Yenisey river valley at this site (Chekha 1990)

and may indicate the prevailing seasonal flooding processes

during loess deposition.

The third suggested mechanism involves the influence of

water flow (alluvial sedimentation), which has the strongest

effect on the orientation of silt loess particles. As pointed out

by Tarling & Hrouda (1993), weak to moderate currents cause

mechanical ordering of the deposited material, with xmax axes

parallel to the flow direction. The alluvial type of sedimentation

could also explain most of the observed AMS characteristics of

Kurtak data – high degree of anisotropy (Fig. 4), well defined

xmax direction, and systematically higher susceptibility values,

compared to Bachat section.

Simple comparison between the anisotropy parameters for

the two sections (Bachat and Kurtak) with those obtained by

Liu et al. (1988) for wind-blown and water-lain loess also shows

that AMS of Kurtak section is likely controlled by flow.

Whatever the origin of the orienting mechanism, progressive

change of the xmax direction probably indicates differences in

prevailing transport direction from c. N–S to NW–SW (Fig. 6b)

during different interglacial /glacial periods.


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Comparison between the two sections also clearly shows

(Fig. 4,5) that the degree of anisotropy for the two sites is

different, being much higher in Kurtak than in Bachat. The

main reason for the observed pattern could be different

deposition mechanism, as discussed also by Liu et al. (1988) in

their study of anisotropy of wind-blown and water-lain loess /

soil sequences in China. Pure aeolian origin, characteristic for

section Bachat, would yield low degree of anisotropy, but well

expressed ‘sedimentary’ magnetic fabric of loess beds (Figs 4–6),

with no defined lineation. This feature is also confirmed by the

correlation coefficients between F or L and Pk (Table 2). The

strong correlation between F and Pk in loess units shows that

the anisotropy is almost entirely controlled by the distribution

of magnetic grains within the bedding plane. In contrast to

loess units, Bachat palaeosols are characterized by random

magnetic fabric (Fig. 6), suggesting that disturbing factors like

biological activity, weathering and formation of new magnetic

minerals played significant role. AMS studies of Chinese loess /

palaesols (Liu et al. 1988), as well as a number of weathered

soils developed on different parent materials (Mathe et al.

1997), show that weathering and pedogenesis cause lowering

of degree of anisotropy, although the initial foliated fabric

is preserved but principal susceptibility directions are more

scattered.

Similarly to the results, obtained for Chinese water-lain

loess /soil sediments, the Kurtak section shows notably higher

anisotropy (Figs 4–6, Table 2,3). The significant correlation

between F and Pk in both loess and palaeosol, in contrast to

Bachat, suggests that continuing deposition of finer material

even during warm stadials controls the magnetic fabric in

Kurtak. In addition to clearly expressed sedimentary magnetic

fabric, AMS results for LE1 and LE2 (Fig. 6b) show well-

grouped xmax directions. Thus, we could suppose that the

prevailing transport direction during formation of these loess

units is NW–SE in LE1 and LE4, and SW–NE in LE2 and LE5,

however, the statistical significance of those directions for LE4

and LE5 is relatively low. As also shown by Liu et al. (1988),

Chinese water-lain loess sediments are also characterized by

stronger foliation (F) than lineation (L). Well defined mean

xmax direction in loess horizons LE1 and LE2, discussed above

as an indicator of probable transport direction, could be con-

sidered also as a result of the alignment development during

seasonal flooding or /and colluvial sedimentation (e.g. more

or less flow-influenced). On the other hand, if we accept the

hypothesis of Chlachula et al. (1997) that the upper loess

horizons are of aeolian origin, it can also reflect prevailing

wind direction. There is no direct evidence allowing us to

exclude either of these possibilities, because Chlachula et al.

(1997) suggests more or less pure aeolian sedimentation, based

on palaeopedological and morphological data. The opposite

opinion, supporting flow effect as a result of solifluction and

colluvial processes (Drozdov et al. 1990), however, seems more

in agreement with our magnetic fabric data.

The distribution of principal susceptibility axes for palaeosol

horizons (PC1A, PC1B, PC2A, PC2B, PC3A, PC3B, PC4A,

PC4B—Fig. 6b) still suggests the presence of original sedi-

mentary magnetic fabric, as far as xmin are well grouped. xmax

mean directions, however, are determined with low accuracy,

probably as a result of pedogenic disturbances, although not

very strong so as to destroy completely the sedimentary mag-

netic fabric. Other reason is also the small number of samples

from different soil horizons. Well preserved sedimentary mag-

netic fabric in palaeosols could be regarded as an indirect

indication that strong destruction /dissolution processes did not

occur in them. Similar magnetic mineralogy of loesses and

palaeosols also supports such a conclusion. Therefore, we

suppose that magnetic susceptibility pattern at Kurtak reflects

wind strength or different deposition rates, rather than magnetic

depletion of palaeosols due to destructive processes.

6 C O N C L U S I O N S

Magnetic susceptibility and AMS study of two loess /palaeosol

sections from Siberia yield new data on the mechanism of

magnetic response of these sediments to changing climate

conditions during the Late Pleistocene. Based on the results

obtained, the following conclusions can be drawn.

(1) Variations of low-field magnetic susceptibility and

frequency-dependent magnetic susceptibility of the two profiles

differ in general. In case of pure aeolian origin like the Bachat

deposits, during warm periods susceptibility enhances due to

pedogenic processes (similar to the mechanism, suggested for

Chinese loess). Since the initial content of magnetite in wind-

blown loess decreases during interglacial periods (as suggested

for the wind-intensity model), the pedogenic enhancement

‘adds’ its signal to the total susceptibility of soils, compensating

the decrease and smoothing the overall variations along

the section. In Kurtak, the main mechanism controlling mag-

netic properties of loess /palaeosols, is most probably different

concentration of ferrimagnetic fraction, reflected by clear

susceptibility maxima in loess and minima in pedocomplexes,

while practically no significant contribution of fine SP grains,

presumably of pedogenic origin, is found.

(2) Although palaeoclimate conditions were probably the

same in both Bachat and Kurtak sections, the ‘wind-intensity’

model alone can not explain the magnetic susceptibility

variations. In the Bachat section authigenic formation of

magnetite due to certain but not intense pedogenesis must be

taken into account. If further evidences confirm that Kurtak

section contains alluvial and dilluvial horizons in its upper part,

the ‘wind-intensity’ model can not be applied to these horizons

as well.

(3) Study of anisotropy of magnetic susceptibility reveals

clear sedimentary magnetic fabric in both sections. One of

the sections, Kurtak, situated in the Yenisey River valley, is

characterized by well grouped xmax directions in loess units

LE1 and LE2. We suppose that they indicate prevailing tran-

sport direction during sediment deposition or re-deposition.

All stratigraphic units show clear sedimentary magnetic fabric

(except the humic horizon of the most developed palaeosol

PC4). Loess /soil sediments at Kurtak exhibit higher degree

of anisotropy than the corresponding deposits at Bachat.

All palaeosols in Bachat are magnetically isotropic, while

loess units exhibit weak magnetic anisotropy and also show

sedimentary magnetic fabric. Most probably it is also

connected with different depositional and post-depositional

environments.

A C K N O W L E D G M E N T S

The authors appreciate constructive comments by the reviewers

Prof. J. Hus and Dr A. van Velzen, which helped to improve

the manuscript.


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