Palacomagnetic and rock magnetic investigation of Late Pleistocene loess deposits in Belgium

Physics of the Earth and Planetaiy Interiors, 44 (1986) 21—40

Elsevier Science Publishers B.V., Amsterdam

—

21

Printed in The Netherlands

Palaeomagnetic and rock magnetic investigation of Late Pleistocene bess deposits in Belgium J.J. Hus and R. Geeraerts Institut Royal Mêtèorologique, 3, Avenue Circulaire, 1180 Bruxelles (Belgium)

Hus, J.J. and Geeraerts, R., 1986. Palaeomagnetic and rock magnetic investigation of Late Pleistocene bess deposits in Belgium. Phys. Earth Planet. Inter., 44: 21—40. The results of a palaeomagnetic and rock magnetic investigation of two bess profiles of Late-Pleistocene age in Tongrinne (TG) and Rocourt (RC) in Belgium are given. We examined the possibility of using this type of sediment, which is, strictly speaking, eolian in origin, to gain information about the dynamic behaviour of the ancient geomagnetic field. The influence of geological processes, especially those known to occur in periglacial environments, were examined. A field test was applied to the upturned strata near a fossil frost wedge of Weichselian age in Tongrinne to demonstrate that at least some large directional changes found in periglacial sediments, and which could be mistaken for ‘excursions’, cannot be attributed to the Earth’s magnetic field, but are due to mechanical disturbances. The DRM/ARM method and the requirements for uniformity by Levi et al. to obtain relative geomagnetic palaeointensities were tested on those bess deposits which contain a large silt fraction.

1. Introduction 1.1. Aim

When we started a detailed palaeomagnetic and rock magnetic investigation of the bess deposits in Belgium, our main concerns were firstly to assess the fidelity of the geomagnetic record to ensure if valid information about the fine structure of the ancient geomagnetic field could be obtained in this type of sediment and, secondly, to improve our knowledge of the remanence acquisition, not only for the study of the geomagnetic field during the Quaternary, but also for magnetostratigraphical purposes. We used the fact that the bess extent in Belgium was deposited in a periglacial environment to examine the influence of periglacial activities on the remanent magnetization, and also to demonstrate that at least some of the large directional magnetization changes found are not induced by the geomagnetic field, but may be explained by mechanical disturbances. The existence of large 0031-9201/86/803.50

© 1986 Elsevier Science Publishers B.V.

directional deviations of the field, called ‘excursions’, being still a matter of dispute due to the lack of their spatial and temporal consistency (Verosub, 1982), is of great importance in finding constraints of processes taking place in the Earth’s core and/or core mantle interface. In the event that they are a real manifestation of the field, they would provide us also with new magnetostratigraphical markers for at least regional stratigraphical correlations, when they are dated with time-stratigraphic datums. As absolute dates for the bess sections studied are very scarce at the moment, it was very useful to look for the presence of bess older than Late-Pleistocene on the basis of magnetic polarity. Next, we wanted to find out to what extent we could make use of the palaeomagnetic and rock magnetic properties for between-site correlations, and to what degree the magnetic signature of the bess deposits could give us insight into weathering, erosion, pedogenesis, periglaciab and sedimentological changes during the Quaternary (see also Thompson et al., 1980). We report here on the pabaeomagnetic and rock

22

magnetic results obtained after the study of two exposures of the bess area of Belgium. 1.2. Definition of bess and remanence processes in bess

Although the exact mechanism of the acquisition of the natural remanent magnetization (NRM) in bess deposits is still unknown, the discovery of reversed magnetozones in outcrops in Europe and Asia prove that they can retain a stable remanence over a relatively barge period of time (Bucha et ab., 1975; Ko~iet al., 1973; Hebber and Tungsheng, 1984). To ascertain the fidelity of the geomagnetic signal carried by the bess particles, it is fundamental to understand exactly the recording process and to know to which degree the original record has been modified afterwards, especially if the intention is to reconstruct the secular variation of the field. Even problems rebated to the formation of bess, a silty deposit, and its origin and spatial distribution, have not yet been completely solved, Loess deposits in Europe are considered to be the consequence of Quaternary glaciations (Smabbey, 1980), and the most commonly accepted explanation for their formation is that the silt was produced by sub-glacial grinding followed by eolian transportation and deposition. A bess deposit is a elastic deposit, commonly non-stratified, occurring as wind-laid sheets, which consists predominantly of silt-size quartz particles in the range 20—50 ~tm (Smalley, 1980). Although it is commonly agreed that airborne dust is unable to acquire a detrital remanent magnetization (DRM) during deposition, experiments conducted by Kravchinskiy (1970) proved that magnetic particles smaller than 200 j.tm align along the field lines when settled in air. It is therefore necessary to consider all the events during the genesis of bess from the moment of the formation of its constituents (provenance), their transportation and deposition until subsequent diagenesis. Glacial grinding or basin outwash may supply the original particles, which are transported by wind and finally deposited, but rivers may have been the major agencies controlling the formation of bess found in basins today. Subsequent to deposi-

tion, the bess may have been taken up by running water and redeposited elsewhere, building up in this way well-sorted layers known as ‘schwemmbess’. It follows from this that we must distinguish between upland bess, which is, strictly speaking, eolian in origin, slope bess, where creep and solifluction processes may be important, and valley bess which was formed by redeposition in running water (Smalley, 1980). In the bess sections we studied, much evidence is present that water played an important robe either during or after deposition, so that we may assume that the preliminary remanence resulted from the statistical alignment of previously magnetized detrital grains in the geomagnetic field. The remanent magnetization is only ‘blocked in’ when the grains become immobilized by the physical constraints imposed by the new particles arriving. When exactly free rotation of the grains is inhibited, this is difficult to forecast, as realignment within the sediment may still occur as long as the water content is sufficiently high, or also when the magnitude of the constraining forces is temporarily reduced due, for instance, to mechanical disturbances such as bioturbation, tremors, etc. (see also Tucker, 1983). The remanence resulting from such a realignment will hereafter be called a post-depositionab remanence (PDRM). From our study, it became clear that after. deposition, soil formation, erosion, weathering and periglaciab activity had a great influence on the original DRM or PDRM, which may be severely altered through misalignment effects. Owing to the above-mentioned processes, bess deposits will be discontinuous in time and space, and there is field evidence that they formed in environments where periods of erosion and stability were probably more frequent than sedimentation. It is therefore crucial to have an idea of the intensity and duration of these processes, it we want to estimate the fidelity and completeness of the magnetic signab present. In the end, the success of the recoilstruction of the time variations of the palaeofield will largely depend on them, as will the success of bess dates based on magnetostratigraphicab methods.

23

2. Area description and stratigraphy 2.]. Area description and lithostratigraphy

In the bess area in Belgium, which extends mainly north of the rivers Sambre and Meuse (Fig. 1), eobian sedimentation was the dominant process of the last glacial period, and some authors think that the material was blown up out of the North Sea basin during the ice ages (Tavernier, 1947). Oriented samples (222 in total) were taken along a vertical profile (TG) in the brickyard ‘Point du Jour’ at Tongrinne (Fig. 1) at a small topographic plateau (altitude about 163 m) situated between the basins of the rivers Meuse and Schelde. The thickness of the Pleistocene deposits, resting on Bruxellian sands, attains roughly 15 m, from which the visible top 6 m in the pit belong to the Late-Pleistocene. The bithostratigraphical description of this section is due to Paepe and Vanhoorne (1967). From bottom to top, we recognize a Saabian bess deposit (Fig. 2), in the upper part of which a truncated reddish coloured, textural-B-horizon occurs as a relic of a podzolic soil, which developed most probably during the Eemian interglacial (Paepe, 1968), and which corresponds with the

(274 samples in total) at a distance of about 70 km from the first site in the sandpit of the ‘S.A. Sables et Graviers’ at Rocourt (altitude about 185 m) near Liege. The lithostratigraphicab description

SEDIMENTATION AREAS OF THE LATE PLEISTOCENE 300 400 5~OO 6~oo NE 0

~ 5, 00

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~ I

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of this section is mainly due to Gulbentops (1954) and Haesaerts and Van Vliet (1981) From bottom to top, we could recognize a Saabian bess deposit resting on Oligocene sands (Fig. 4b). In the upper part of this, the brown-red Rocourt soil occurs, followed immediately by a thin layer (about 0.1

-‘

L

COVERSANO AREA TRANSITIONAL

~

ilbuvial Bt horizon of the Rocourt soil of the last interglacial age, introduced by Gullentops as early as 1954 in its type locality, Rocourt. It is followed by a greyish solifluction deposit containing a humic steppe-bike soil (chernozem), which was connotated as the Warneton soil complex of interstadial age by Paepe and Vanhoorne (1967) and which he correlated with either the Amersfoort (about 68000 bp), the Brörup (about 65000 bp) or the Odderade (about 58000 bp), all of Early Weichselian age. Immediately above lies a series of stratified loam layers of Middle Weichselian age with numerous small frost wedges, on top of which an interstadiab brown forest soil, called the Kesselt soil, occurs. This soil, which was defined by Gullentops (1954) in Kesselt, interfingers with a strongly cryoturbated horizon. The whole is overlain with a homogeneous eolian coverboam, on top of which we find the actual soil. Frost wedges are found at various bevels: in the loam formations, immediately above the Kesselt soil and in the coverboam. Oriented samples were also taken across a large fossil frost wedge, discovered at a distance of about 3.35 m from the vertical profile TG. In total, 109 samples were recovered at the level of the Warneton soil (between A and B in Fig. 5) and 101 samples at the level of the Rocourt soil (between C and D in Fig. 5). A second vertical profile (RC) was sampled

~

Fig. 1. Map of the Sedimentation areas of the Late-Pleistocene in Belgium, afterR. Paepe, 1967(1 Grand-Manil, 2 Kesselt, 3 = Rocourt, 4 = Tongrinne).

m) of whitish loam and a complex humic soil horizon of interstadial age. It is in this soil horizon and the bess immediately above it that Bastin (1969) found three successive extensions of pollen which he considered as the witnesses of the Amersfoort, Brörup and Odderade interstadials. Next follows non-calcerous and calcerous stratified bess with frost wedge rows, on top of which we find the cryoturbated horizon, and finally the

24 W

0

1

2

3

4

5m

E

I I

~ 4

‘‘~iiL

—~

iI’i~IIIIIII

III

I!It

~ liii

-

~

RSUDnISOIL

TONGRINNE. BRICKYARD “POINT DU JOUR”

5 m

Fig. 2. Description, brickyard ‘Point du Jour’, Tongrinne; after R. Paepe, 1967 (CRYOT brown, B = brown, G = grey, BH = brown homogeneous, Hum = humic, Cl = clayish).

coverloam with a Hobocene soil on top. Since the time that Paepe (1968) correlated the Al horizon of the Rocourt soil defined by Gublentops with the Warneton soil of Weichselian age, the concept of the Rocourt soil is limited to the truncated textural B horizon, also called ‘limon fendilbé’ (Langohr in Cahen and Haesaerts, 1984). Haesaerts et al. (1981) considered the Rocourt soil as a pedocomplex consisting of three superposed truncated ibluvial soils. It is important to mention that Van Vliet-Lanoë (1975) found two distinct deposits in the humic soil separated by a hiatus. 2.2. Chronostratigraphy At Rocourt, the humic horizon overlaying the 14 Rocourt times byet Cal.,~ (Haesaertssoil andwas Van dated Vbiet, several 1981; Haesaerts 1981) giving the following dates: horizon EA1 in contact with the Rocourt soil 47800 bp (+ / 2100) on extract (GrN-9080); horizon EA2 above the hiatus in horizon EA 35900 bp (+ / 1000) on extract (GrN-9081) and 38550 bp (+ / 700) on residue (GrN-9186) and 27900 bp (+ / 830) on residue (Lv-540), (Gibot in Cahen and Haesaerts, 1984). As these age determinations have been carried out on humic fractions, they only give an age indication of the horizon, as the migration of humic material cannot be excluded, and should therefore be considered as minimum ages; —

—

—

—

=

cryoturbated horizon, YB

=

yellowish

the cryoturbated horizon, cabled Kesselt B by Gullentops, gives a mean age of 22000 bp (Gublentops in Haesaerts et al., 1981). TL dates of both sections will be available in the near future (A. Wintle, personal communication,1984).

3. Methods and techniques 3.1. Sampling technique

Non-disturbed oriented samples were obtained by gently hammering about 15 cm long thin-walled plastic tubes (2.54 cm diameter) for about 8 cm perpendicularly the cleaned walls of the pits with the aid of ainto hollow aluminium cylinder and brass piston. A graduated table with a clinometer that can be bevelled, and which is equipped with a slotted tube, was used to obtain the strike and dip of the protruding plastic tubes. An orientation mark was inscribed on the plastic tubes along the slot and a known geographical azimuth carried over with a theodolite. This resulted in well onented samples with a maximum dip and bearing error of less than 10. The filled plastic tubes were sliced into slightly less than 2.5 cm long samples immediately after arrival in the laboratory, and the top and bottom sealed with a cold setting epoxy. This sampling technique proved to be very suitable in the bess sections of Belgium, especially

25

cab disturbances induced during sampling were observed in the laminated sediments. Some of the plastic tubes had a non-negligible, hard remanence, even after acid cleaning, and had to be discarded. Only tubes that gave a magnetic signal not higher than the noise bevel of i0—~Am~ of

Mo /

0.8

the remanence measuring equipment were used.

~

0.6

3.2. Measuring equipment 04

0.2

0~ 0

_________________________________ ~~1 I

20

40

60

80

700 (mT)

a

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remanences were measured with a spinner magnetometer JR-4 (Jelinek, 1966). Progressive stepwise demagnetization in increasing alternating fields

0.8~

0.6

successively axes (AF) up to aalong peakthree valueperpendicular of 0.1 T wassample performed

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-

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in triple mumetal shielded field Schonstedta single GSD-1axis, demagnetizer. The residual inside the demagnetizer was regularly checked with

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a 0.2

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Before starting the NRM measurement, the specimens were stored in the laboratory magnetic field (about 47 ~tT) in the same direction they had in the field for a period longer than the time that elapsed between sampling and preparation. In this manner, viscous magnetization components can recover. After the NRM measurement, the specimens were kept in an upturned position after rotation around the magnetic E—W axis for about 1 month, to study the spontaneous build-up of viscous remanent magnetization (VRM). The NRM ‘in situ’ and all other laboratory induced

20

I

40

60

80

100 fmT)

a Fig. 3. Alternating field demagnetization curves of natural remanent magnetizations at: (a) Tongrinne, (b) Rocourt (* = actual soil, + = bess, X = Warneton soil, 0 = Rocourt soil).

in early spring, when the exposures are still damp (the method cannot be used after a long period of dryness on in compacted sediments). No mechani-

single axis fluxgate probe to be less than 5 nT. Coaxial with the AF coil, a pair of Helmholtz coils were added to apply small constant fields of the order of 100 1tT to give samples an anhystenetic remanent magnetization (ARM). The ARM was imparted to the samples by a combined action of a constant field of 40 itT and a peak alternating field of 0.1 T, followed by a slow and smooth decrease of the AF towards zero and removal of the constant field. Isothermal remanent magnetizations (IRM) were given to the samples in a steady field of about 0.8 T in the air-gap of a 15 cm field controlled Varian electromagnet. Steady low field bulk susceptibility was obtained in a Kappabridge KLY-1 by averaging three measurements along three orthogonal sample directions.

26

I

o

0 ~

5(W,) 3Q0

I 75(E)

30°

m

9~ 0

00

900(W) 0

~

750

0

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3

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m

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6

TG I

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RC 8 m

b

Fig. 4. Inclination and declination records in bess sequences at Tongrinne (a) and Rocourt (b). (1 = present soil, 2 = coverloam, 3 = cryoturbated horizon (Kesselt soil), 4 = bess, 5 = Warneton soil, 6 = whitish loam, 7 = Rocourt soil, 8 = bess (Saale), 9 = sand (Tertiary)).

27

ing fields up tocurves a peakofvalue of 0.1into T. The alternating AF demagnetization TG split 0~

I

two bands (Fig. 3a), depending on the lithology; one band contains the bess samples, the other the soil samples. Within the two groups, the coverboam differentiates further from the old boesses by its

—

•

•

•

U

••

~

.

higher resistance to alternating fields, as does the Warneton soil from the Rocourt soil. The median destructive fieldto(MDF), which is the alternating field necessary reduce the remanence to half its

•••• —

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0

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.

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initial value, is high and varies between 20 and 80 mT, except in the horizon the top where values as low as Ap 10 mT are of found. Thesoilresidual

I

~ 101

I

~

S .•

•

______________________

remanence, left after a treatment in the maximum AF field of 0.1 T, varies between 24 and 44% of the initial remanence, except again in the topsoil

I

~

A/rn .02

I

I

•

• •

I

______________________

A

B

WNW

ESE

--

CRYOTURBA TED HORIZON

WARNET0N Sc!

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Fig. 5. Inclination, declination and intensity changes across a fossil frost wedge at the bevel A B in the brickyard ‘Point du Jour’, Tongrinne.

4. Results and discussion 4.]. Alternating field stability tests and NRM intensity

To get an idea of the nature and stability of the magnetization components present in the samples and to find an AF value necessary to remove unwanted components, in order to isolate the stable characteristic remanence, 26 pilot samples of TG were progressively demagnetized in increas-

where only about 10% remains. The samples from the topsoil were intentionally taken to demonstrate the effect of disturbing factors of a biobogical and anthropogenic kind, and to prove that they cannot yield valid information of the ancient field. Their low stability in AF fields points to a viscous origin of their remanence, acquired each time after ceasing disturbances such as bioturbation and plowing. The AF demagnetization curves of 22 pilot samples of RC (Fig. 3b) are quite similar to the ones of TG. In both sections, the coverloams and older boesses display the highest stability in AF demagnetization, while the pabaeosoils have a significantly lower one. In RC, the MDF ranges from 15 to 85 mT, and the residual remanence after treatment in an AF of 0.1 T drops to 9—47% of the initial remanence. This is comparable to that found in TG, except for the lower values in the Warneton soil. In contrast to the TG section, where the Warneton soil has a higher MDF and higher residual remanence than the Rocourt soil, the opposite is true in the RC section. The lower stability of the palaeosoils can be explained by pedogenetic and palaeoclimatic processes when partial degradation of the magnetic fraction may have occurred and/or new minerals formed. As we will see later in this chapter, the VRM attains the highest values in the upper part of the soils (column 6, Fig. 6a) and seems to reside in a small fraction of superparamagnetic particles. This is, for instance, the case for the Rocourt soil,

28 NRMC,

X

0666d

NRM~,,0003 -3

X -6

IS A!,,

ARM

10 5.1.

IRM

ARM

1PM

10 Al,,,

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~

c

‘I

~

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_______ Is

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a

~..

S

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(NRM - NRMcI)

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..

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~0.

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5

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Fig. 6. (a) ‘Cleaned’ NRM (AF = 30 mT), magnetic susceptibil-

}

ity, anhysteretic remanence ARM, isothermal remanence IRM, ratio residual remanence after ‘cleaning’ to initial remanence, viscous remanence and remanent coercive force HCR in the bess sequence at Tongrinne. (b) ‘Cleaned’ NRM (AF = 22.5 mT), magnetic susceptibility x, ARM, IRM and ratio NRM — NRM,,1 /NRM in the bess sequence at Rocourt.

29

which is strongly ‘marmorized’, showing several vertical, bight-coloured patches in the field with Fe-concentrations at the rims, indicating the migration of Fe which had precipitated at the border. As mentioned earlier, field and laboratory observations yield evidence of the polycyclic character of this soil and further indicates that climatic changes to a colder era at the beginning of the Weichsel are responsible for many features in it, such as ‘limon fendilbé’. A first indication that the characteristic remanence could be a DRM or PDRM comes from the high AF stability and shape of the AF demagnetization curves, although we cannot exclude a chemical remanent magnetization (CRM) or hard-VRM from the very beginning. Field evidence favouring a DRM origin is hidden in the stratified bess. Its distinct layering indicates that some kind of sorting occurred in the presence of water. Several authors agree that the stratified bess is of niveoeolian origin, the bess being present as dust in the snow during settling (Gulbentops et al., 1981). Hence, field alignment of the grains could occur in the snow mebtwater. Also, during periods when a permafrost was present, grains in the active layer could orient in the ambient field during seasonal melting or when the permafrost completely melted away during climatic improvement. Also, if the characteristic remanence was a hard-VRM, we could expect it to increase in depth (time), if the magnetic mineralogy is uniform, while in fact the opposite is true (Fig. 6a). A large strong magnetic fraction was easily collected in all the horizons of the bess sections after dispersing the material in water and pumping it through the air gap of a permanent magnetron magnet. This fraction was identified by X-ray diffraction as being mainly composed of magnetite. Traces of goethite and glauconite which were revealed should only play a minor role as contributors to the remanence properties. From the aforementioned field evidence and uniform magnetic mineralogy, we conclude that the remanence carried by the bess in the sections studied is mainly a DRM or PDRM. This contrasts strikingly with the behaviour in other bess areas, such as the Chinese bess near Lochuan, where Helber and Tungsheng (1984) observed a

large overprinting of the NRM with a strong secondary component residing in oxidized magnetite. They could not isolate a stable end-direction even after treatment in AF fields higher than 100 mT. Instead, a heat treatment to temperatures of 525 to 575 K was necessary and successful. The authors conclude that the NRM of the Lochuan bess was mainly controlled by chemical alteration processes reflecting climatic changes during the Late-Pliocene and Pleistocene. In both TG and RC profiles, the coverboam carries a higher NRM intensity compared to the older boesses (column 1, Fig. 6a). The lowest values in TG are attained in and immediately below the cryoturbated horizon and in the Rocourt soil. A remarkably barge intensity increase is noticed in both profiles at the bevel of the Warneton soil, characterized by a sudden increase in RC and a broad local maximum in TG. All the samples of TG and RC underwent ‘cleaning’ in a relatively high AF of 30.0 and 22.5 mT, respectively, to remove the VRM and eventually bow-coercivity contributions of large grains which were submitted to the largest gravitational misalignment torques during deposition. AF cleaning did not modify the trend of the remanence intensity changes with depth (column 5, Fig. 6a, b), but diminished slightly the contrast between young and old boesses and the maximum in the Warneton soil becomes less pronounced. We could differentiate between three bess horizons on the basis of the ‘cleaned’ intensity. In both sections, the highest values were found in the youngest bess, followed by the oldest Saalian bess, and finally the bess occurring in between the two of the former. Rock magnetic measurements in the next chapters show that the intensity changes, with a factor as barge as 3 in TG and 7 in RC, cannot be completely explained by variations in the magnetic content or granubometry, and that part of it is probably due to field changes. There is no evidence that the intensity peak in the greyish humic Warneton soil is the consequence of an enhancement of the soil magnetic properties caused by reduction or some Le Borgne effect (Le Borgne, 1960; Mullins, 1974). The large increase is merely due to an enhancement caused by the contribution of volcanic minerals. Indeed,

30

it is in this humic horizon in RC that Gublentops (1954) found heavy minerals of volcanic origin for the first time, and Juvigne (1977) the largest concentration in the whole profile. The latter author, who examined only transparent grains larger than 105 itm, counted 700 dense volcanic minerals per 50 g of soil. Even grains larger than 300 itm were revealed and the spectrum consists, in decreasing order, of 58.7% monoclinic pyroxenes (amongst which acicular augite was the most common), 31.8% basaltic hornblende and 9.4% enstatite. Juvigne (1977) assumed that the level of maximum occurrence in RC in the humic soil corresponds with the bevel where the volcanic grains were ‘in situ’ and called this horizon the ‘tuff of Rocourt’. He also found about the same mineralogical association of transparent volcanic minerals in Tongrinne (monoclinic pyroxenes: 49.7%, basaltic hornbbende: 24.6% and enstatite: 25.6%) at about the same stratigraphicab bevel in the ebuviab horizon between the top of the humic soil and the Rocourt soil. The level where the maximum concentration of volcanic minerals of the Rocourt tuff occurs has an important tephra-stratigraphicab value, especially as enstatite has not yet been found in other fallouts of volcanic origin during the Quaternary in Belgium. The magnetic signature of the ‘cleaned’ NRM allows us to trace the bevel of maximum concentration, as well as the vertical distribution, in a rapid and accurate way. The shape of the distribulion may inform us about the migration of volcanic particles in depth, due to biological and physical agencies, and also inform us about the eventual presence of sediments which were removed elsewhere and which contain altered volcanic dust particles, lithe volcanic dust happens to be spread out on a soil surface, then it is very likely that it had been eroded afterwards or submitted to climatic degradation, so that the actual level of maximum occurrence does not coincide with the initial bevel of maximum concentration. The actual bevel of maximum occurrence based on the magnetic remanence falls webb within the humic steppe soil in both sections, also in TG, not in complete agreement with the findings of Juvigne. A granubometric examination by Juvigne (1977) of the basaltic hornbbende of the tuff of Rocourt -

.

.

in several pits showed a decrease in grain sizes from east to west, indicating an Eifeb (Germany) origin. This explains the pronounced peak found in the NRM in RC, which is closer to the source region. It is astonishing that the volcanic minerals contribute to the NRM intensity, so that the environmentab conditions prevailing at the time must have been suitable for field alignment to occur. The humic soil dates from the early Weichselian, when climate degraded and forest vegetation was replaced by steppe-bike vegetation. A micromorphobogical analysis by Van Vliet-Lanoë (1975) indicates that the bower third part of the humic soil underwent creep movement and that the upper two-thirds is a colbuviab deposit. This explains why field alignment could occur as well as the asymmetric distribution found in RC. .

4.2. Directional behaviour In Fig. 4, the declination (D) and the inclination (I) of the ‘cleaned’ NRM is plotted against the stratigraphic variation with depth. No great change in direction was observed after AF treatment, except for a slight increase of the scatter, as can be seen in Table I, where the overall mean direction is given before and after cleaning, based on a Fisher distribution. In both sections, the largest directional deviations occur in the topsoil and in the Rocourt soil. In the batter, the magnetization direction fluctuates from one sample to the next. In RC, barge fluctuations also occur immediately above the humic soil. Instead of a shabbowing of I in the Rocourt soil in TG, an increase of I is noticed in the same soil horizon in RC. Although we can advance several plausible cxplanations for the large directional deviations found in the Rocourt soil, such as pedogenesis, bioturbation, or the manifestation of a field ‘excursion’, it is clear that it is the result of periglacial activity and in particular of disturbances caused by the segregation of ice and solifluction. There was a fair chance to find the Blake event in the bess on top of which we find the relics of the Eemian Rocourt soil, or further downwards in the profile. Owing to the lack of internal and temporal consistency, many ‘excursions’ reported in the

31 TABLE I Average directions and intensities of the NRM before and after alternating field cleaning (AF for TG: 30 mT, for RC: 22.5 mT) 3 TG N I D K a 95 Mill NRM 221 65°20’ 0°00’ 85 1.0° 21.4 NRMCI NRMCJfld

221 186

64°40’ 66°20’

3°30’ 2°40’

65 108

1.2° 1.0°

13.8 14.7

RC NRM NRM,,

274

63°20’

5°20’

80

1.0°

21.1

272 232

63°50’ 62°10’

3°20’ 1°30’

65 83

1.1° 1.0°

15.0 15.6

1 NRMCIfld

N = number of samples, 1= inclination of the average magnetization direction; D = mean declination in °E; K = Fisher’s estimate of the precision parameter: a95 = radius of the 95% confidence circle; M= average magnetization in Am 1; cI = cleaned;

n.d

=

non-disturbed samples.

literature can no longer be attributed to a departure of the field from its average near axial configuration. A serious candidate remains the Blake event, which could even be a true reversal of the field. It was first discovered in seven cores from the Caribbean and Indian Oceans by Smith and Foster (1969), who dated it between 105 000 and 114 000 years bp and estimated its duration as 5000—7000 years. Several authors claimed to have found the Blake event: Denham (1976) and Denham et ab. (1977) in two cores from the Greater Antilles Outer Ridge; Manabe (1977) in a marine terrace in Japan; Creer et al. (1980) in a core from Gioia Tauro in Italy; Yokoyama and Hehanussa (1982) in the Si-Guragura younger Toba tuff around Lake Toba (Sumatra, Indonesia) with a fission track age of 0.10 Ma (+ / — 0.02) and Liu Chun et ab. (1982) in basalts in the Heishan area in Datong district in North China, where they obtained a TL dating of 0.16 Ma in underlying baked bess (see also Jacobs, 1984). Its apparent long duration is estimated to be between 5000 and 50000 years. In the Italian and Japanese records, it appears as two reversed intervals separated by a normal interval. It was also reported in bess seetions by Tucholka (1977) in Poland. Not less than 13 excursions of the geomagnetic field during the Brunhes period are distinguished by Pospebova (1982) from which the most recent ones are dated about 12000; 24000; 42000 and 110000 years bp. The author mentioned that in continental sediments, the ‘excursions’ seem to occur either in soil horizons or in glacial drifts! The inclination de-

crease found in the Rocourt soil in TG corresponds to an increase in RC, which shows a lack of internal consistency and therefore excludes any possibility of deriving geomagnetic field behaviour from soil horizons. In RC, the whole soil complex underwent cryoreptation along a small slope (Van Vliet-Lanoë, 1975), as is also witnessed by the inflected degradation patches visible in the field. No other large field deviations were found in both exposures. Except for the large directional deviations resubting from mechanical disturbances, inclination changes are smoother than declination changes, both with amplitudes of the same order as secular variation changes recorded in Hobocene bake sediments (Creer, 1981; Tucker, 1983) and known from archaeomagnetic measurements (Aitken, 1974; Thellier, 1981; Kovacheva, 1982). From Table I, we can see that the overall mean declination is slightly eastwards after AF treatment, and the average inclination only slightly less than that corresponding to a geocentric axial dipole (67047~ for TG, 67 o43~ for RC). Although one might conclude from this that no inclination error (King, 1955; Griffiths et al., 1962) did occur, this is not certain, as the average should be taken over a time interval large enough so that the time-averaged field corresponds with an axial dipole field. We did not notice any marked changes in the position of the geomagnetic pole, such as a shifting from the Pacific to the North American continent, in the interval 12000 to 10000 years ago, as found by Bucha (1976).

32

4.3. Periglacial activity As the bess examined accumulated in a penglacial environment, it is important to study its influence on the remanence properties. Frost action gives rise to different kinds of physical mechanisms such as thermal contraction, appearance of segregation ice, and an increase of the water volume which transforms into ice (Pissart, 1970; Washburn, 1973). These mechanisms may result in different structures, such as frost cracks, involutions, etc. When a humid soil starts freezing, its volume may increase by as much as 9%, but once transformed into ice, the volume will decrease with a decrease in temperature. A sudden decrease in temperature to very low values will contract the frozen ground and internal tensions appear due to the difference in the thermal contraction coefficient between pure ice and the silicates. Finally, the soil will become fissured when the horizontal tensions exceed the tensile strength near the surface. We only consider vertical V-shaped cracks, assumed to be the result of horizontal tensions. In early spring when defrosting starts, the melting water enters the cracks and freezes when it comes into contact with the frozen soil, producing a vertical vein of ice that penetrates the permafrost. Horizontal compression caused by the re-expansion of the permafrost during the following summer results in the upturning of the permafrost strata by plastic deformation. During the cooling period of the next winter, the thermal tensions may create new fissures and will generally reopen the already-existing ice-cemented cracks which are zones of weakness (Lachenbruch, 1962). Another increment of ice will be added when the spring mebtwater enters the reopened crack and freezes, resulting in the growth of a wedge-shaped vein of ice. The frost wedge we sampled is composite in nature;~it first opened on top of the Warneton soil, pointing to an age younger than Early Weichsebian, and reopened at least once above the Kesselt soil, probably during the intensive cooling-off during the polar desert phase in the middle of the Upper Weichsebian. Figure 5 gives I, D and the magnetization intensity of a series of 109 samples taken across the fossil wedge at the bevel

of the Warneton soil between A and B in Fig. 5, after AF field ‘cleaning’ in 30 mT. At a barge distance from the wedge, the magnetization vectors point nearby in the same direction, while large directional changes occur near the wedge indicating that the strata have been deformed. In the deformed zones, the inclination is more shallow by about 20 to 25°, compared to the inclination in the non-disturbed samples, and deviations in declination as large as 500 are observed, especially in the upturned Rocourt soil. Inside the wedge, the magnetization vectors point again in nearby the same direction, and two fillings can be distinguished on the basis of their magnetization intensity. These wedge fillings have probably acquired their magnetization during the melting of the permafrost. The directional change in the deformed zones is rebated to the degree of movement which had occurred, if the forces acting during the deformation were insufficient to change the magnetization. Indeed, deflection of the magnetization vectors may also have occurred by shearing, associated with the deformation, and a shear remanent magnetization may have appeared, caused by shear strain and squeezing (Games, 1977). This field stability test demonstrates very clearly that periglacial features may have influenced greatly the initial remanence in sediments. Deformation due to periglacial activity may not be recognized when the sediment is sampled by coring and the anomalies direction changes found may be mistaken as ‘excursions’ of the geomagnetic field (see also Verosub, 1982). The retention of directional anomalous in the deformed strata further suggests that the sediment is capable of maintaining at beast part of the original magnetization for a relatively bong period of time, in this case estimated at beast 50000 years (see also Graham, 1949). 4.4. The determination of relative palaeointensities At present, there is no universal technique to relate the magnetization intensity of a sediment to the absolute intensity of the geomagnetic field responsible for it. The reason is that the NRM intensity is controlled by many factors other than

33

field intensity, e.g., shape, mineral type and concentration of the magnetic grains (see also Levi and Banerjee, 1976; and Tucker, 1983). Instead, relative palaeo-intensities can be obtained in a profile, if we normalize the variations of the NRM intensity with some kind of magnetic property, which compensates for the depth variation of the factors other than the field intensity, on the condition that the particular normalizing procedure chosen activates the same relative spectrum of magnetic particles which are responsible for the NRM. It is obvious that the only normalizing parameter fulfilling these conditions is the same magnetization type responsible for the NRM, so that for sediments carrying a DRM or PDRM, the best normalization procedure would be to redeposit the sediments in a known magnetic field. Besides constraints due to the sedimentation tank’s boundaries, it is impossible to simulate exactly the conditions prevailing in nature, neither are we assured that the magnetic content, texture and shape of the sample is maintained. It is also clear that the normalization procedure chosen can only lead to success if the sediment was not altered after deposition, and if fluctuations of variables such as compaction rate, temperature, water content, have a negligible effect on the remanence intensity during the NRM acquisition (Tucker, 1983). Because of these limitations, and the time-consuming nature of the redeposition experiments, several other normalizing parameters have been tried in the past: bow-field magnetic susceptibility (x), saturation magnetization (J5), isothermal remanent magnetization (IRM), saturation isothermal remanence (SIRM) and anhysteretic remanent magnetization (ARM) (see also Levi and Banerjee, 1976). The most difficult variable to allow for is grain size. It is a well known fact that x and IRM overemphasize the role of multidomain (MD) partides, and also x and J~(measured in the presence of the field) activate a disproportionately large fraction of the superparamagnetic (SP) and MD-particles which are relatively less important as stable remanence carriers, especially the former. Laboratory-induced remanences used to normalize NRM intensity changes at least have the advantage that the integrity of the sample is main-

tamed, compared to redeposition experiments. One serious drawback is that a DRM or PDRM occurs due to grain rotation, while the laboratory-induced remanences affect the magnetic states (the domain state, for instance) in the interiors of the grains. A criterion which is commonly used to see if the normalization remanence activates the same grains which contribute to the NRM process is that its AF demagnetization curve should closely approximate that of the NRM (Levi and Banerjee, 1976). This criterion ensures that only the normalization remanence may have the same stability or coercivity spectrum as the NRM. It is obvious that the NRM may only contain a DRM or PDRM, and that secondary components such as VRM must be removed beforehand, so that in practice the comparison with the induced remanence will be limited to certain alternating field intervals. The best normalization parameter for sediments is generally considered to be the ARM, which was used for the first time by Johnson et ab. (1975) and Levi and Banerjee (1976). Indeed, the ARM and DRM often have similar AF field demagnetization curves, while an IRM, which is much greater than the ARM, is less resistant to alternating fields. We first normalized the NRM changes with the volume susceptibility x. x depends on the nature, concentration and the effective grain sizes of the magnetic fraction, while the NRM intensity depends on the mineralogical composition of the grains, grain size and the effectiveness of the alignment in the case of a DRM and thus of the ancient field intensity. The change of x with depth, given in column 2 of Fig. 6a for TG and in column 2 of Fig. 6b for RC has the same trend as the NRM, except in the Warneton soil in TG. The correlation between x and the NRM intensity indicates that the batter is in the first instance determined by the concentration of magnetic minerals. The susceptibility is higher in the coverloams compared to the older loesses by about a factor of 1.7. When making the ratio NRM~3/x (Fig. 7), the contrast between the young and older boesses becomes smaller, while the peak observed in the Warneton soil becomes more pronounced. The different response of the Warneton soil in

34 2 0

NRM00

~200

PPM/APR I

2 0

IPM!X 0

QIPOLEMQMENT ‘0.I0~0

IO~Am

~,.

0

0

NRM~i/X NRM/ARM 120 0 2 0

IRM/X 18. 10~

0

~Z8RL2J ,,, 1±07jSI.SAWRNIN ,0000

I

.‘

•

2.~02~ I)

2

.!

.

‘ .

•

.‘

:

2

6 ‘0

‘I

.1 ••

0

Fig. 7. (a) ‘Cleaned’ NRM normabised with x, NRM normalised with ARM, IRM/x ratio in the bess sequence at Tongrinne. In the last column, dipole moment during the Late-Pleistocene based on archaeomagnetic intensity determinations. (b) Idem in the bess sequence at Rocourt.

•

a 4

0

• •

•

• •.

••

:?~• ‘~•

5

both sites confirms some of the observations of Juvigne (1977) on the volcanic minerals contained in this soil. As RC is closer to the source region, the highest concentration and the largest grains are found here. The x values found at RC are without doubt partly due to large multi-domain grains, while their absence in TG is the result of a shifting towards smaller grains, probably of single domain (SD) or pseudo single domain size (PSD). The latter are responsible for the local maximum observed in the NRM intensity in TG, but of lesser amplitude due to a smaller magnetic content, as the place is far more remote from the source area. From the NRMCI /x ratio, it is clear that the same spectrum of grains is not always activated in the NRM and x processes. Also, paramagnetic silicates and other minerals which contribute to the susceptibility, do not contribute to the remanence. To test the DRM/ ARM method, 22 specimens were given an ARM, as described in section 3.2. Comparison of the AF demagnetization curves of the NRM and ARM by plotting the (NRM/

:

•

0

••

•

6

••

•

•

••

•~

•

:

•

:

•

..•

flU

~

I,

ratio against HAF (Fig. 8) shows that this ratio is, in general, constant to better than 10% from 5 to 15 mT. This ratio can no longer be considered as constant outside this interval, for bower AF values due to the presence of a VRM in the NRM, and for higher AF values because the ARM was obtained in a limited AF field of 0.1 T, ARM)AF

35

thus excluding highly coercive parts. As all the specimens of TG and RC were already ‘cleaned’ in the AF fields of 30 and 22.5 mT, respectively, we partially demagnetized the ARM’s to the same levels; although a lower AF value would have been more adequate (see Fig. 8). For about half the total number of specimens (NRM/

SIRM 10

~,

ç~

•

•

~, ~

~

°~ 0.7-

ARM)AF

30 for TG and (NRM/ARM)AF225 for RC was plotted against depth in Fig. 7a,b. If we disregard the top metre, of which the upper part was severely disturbed, we notice a decrease of the (NRM/ARM) ratio with depth, with the lowest values attained in the upper part of the Rocourt soil, followed by a gradual increase further downwards in the profile. To further check that the NRM/ARM changes are not a by-product of variations in the nature of the magnetic minerals or grain-size fluctuations, we measured the SIRM obtained in a steady field of 0.8 T. Indeed, besides x, the SIRM is also a parameter which can be used to rapidly monitor changes in the magnetic mineralogy. For haematite, the ratio SIRM/x is about 4—130 times larger than in magnetite (Thompson et al., 1980). The SIRM, which is plotted against depth in Fig. 6, follows the same trend as x, and varies only by a factor of about 2.5 in the boesses. Isothermal remanent magnetization acquisition curves obtained

0.8-

+

0.5

o.~. 0.3~ 0 2-

o.i0

o..~ oJ~ o~ o.~ oJ’

o.~ o.~ I-id(T)

Fig. 9. IRM acquisition curves for whole sediment samples at Tongrinne.

~.8

for some whole sediment samples (Fig. 9) all approach saturation within 10% in a steady field exceeding 0.6 T, indicating that the dominant magnetic mineral is magnetite, as was already confirmed by the X-ray diffraction patterns of the magnetic separates. The ratio SIRM/x (Fig. 7) only changes by a factor slightly more than 2, so that the lower values observed between the cryoturbated horizon and the upper part of the Rocourt soil can hardly be attributed to a change in the

0

magnetic mineralogy. The SIRM, which is equal

?.0

particles and constant in the SD range, decreases gradually with increasing grain size in the MD range Assuperparamagnetic a magnetization) mixture of submicron magnetite to 1/2(Dunlop, J~(J5 with= 1981). ultrafine saturation formagSD

~

sass,,,,

netite could yield the same SIRM/x ratio as coarse we also measured remanent coercivemagnetite, force (HCR) which will be the higher in the

0.0

0.6

~i

(or)

Fig. 8. Ratio NRM/ARM at each AF demagnetization step against HAF for samples of the bess profile at Tongrinne.

first case, as SP grains will neither contribute to the SIRM nor to the HCR. The HCR parameter is also an indicator of the nature of the magnetic grains. In magnetite, depending on the grain size, it vanes between values less than 20 mT for grains larger than 100 itm and 50 mT for grains of about

36

1 !.Lm, while much higher values are attained in haematite (Thompson et ab., 1980). The HCR vabues found, which are only plotted for TG in Fig. 6, vary between 50 and 60 mT in the boesses, while lower values of about 35 mT are obtained in the Warneton soil. A peak value of about 80 mT is noticed immediately below the cryoturbated horizon in the Kesselt soil. The minimum observed in the Warneton soil is, of course, the result of the presence of coarse MD particles of volcanic origin. Although x, HCR and SIRM/x can be used to distinguish between different kinds of magnetic minerals, they are less sensitive to small grain-size variations. A better and also rapid method to book for fine-scale variations in the grain size is to plot ARM, which is sensitive to the finer grain-size fraction (SD and small PSD grains) against the low field susceptibility, which is in turn relatively more sensitive to the coarse grain-size fraction (barge PSDs and MD grains), (Banerjee et al., 1981; King et al., 1983). These graphs (Fig. 10) confirm our conclusions already reached that the grain sizes change within a narrow range, and that the highest concentration and largest particles of volcanic origin occur in the humic soil horizon at RC. The grain sizes of the volcanic minerals are smaller than the grain sizes of the boesses. In the boesses, the highest concentration of magnetic minerals occurs in the coverboams, while the largest grain size variations are noticed in the bess deposited between the coverboam and Saalian bess. Although the predominant magnetic mineral is magnetite, and the variation of the magnetic content along the profile less than 20—30 times the minimum concentration, the requirements for uniformity (King et a!., 1983) are not completely met, as we expect the grain sizes to change mainly between 20 and 50 itm, which is much larger than the PSD range of 1—15 itm. It is true that above 1 itm particle size variations have a negligible effect on the DRM/ARM ratio, but the DRM becomes too ‘soft’. The stable NRM and relatively high HCR values indicate that the effective grain sizes are much smaller than those assumed. This may be the result of a sub-division of the magnetic grains into SD and PSD regions and the contribution of magnetite inclusions that were revealed in tourmaline grains.

ARM A/rn 0.25

TG

~.*÷*

*

-

0 I

I

I

~I

I

I

I

I

0.5

0.1

1

SI

a

A RM A/rn a

a

0.25

. -

a

RC

.

0 o a a a .

0 I

I

0.1

I .

X

.

Fig. 10. Variation of the ARM intensity versus the low-field .

magnetic susceptibility in the bess sequences at Tongnnne (a) and Rocourt (b). (* = surface soil, + = coverboam, X = Kessebt soil, s~= bess, 0 = Warneton soil, <)= Rocourt soil, = bess (Saabe).)

37

An independent control of the DRM/ARM method would be the direct comparison with absolute palaeointensity data,time which arethat unfortunateby scarce within the period the sediments were formed. In Fig. 7, absolute intensity determinations based on the Thelbier—Thellier method on Late-Pleistocene archaeologic materiabs of Western Europe and compiled by Barbetti and Flude (1979) are plotted for comparison. Barbetti and Flude concluded, on the basis of worldwide absolute intensity determinations, that the Late-Pleistocene field was lower than the present field. If we eliminate the disturbed samples in the top metre, the same trend is noticed in both outcrops. This would lead us to the conclusion that, except for lower amplitude, shorter period palaeointensity fluctuations, which are observed in the sediment record, but not in the absolute intensity plot (which are only single determinations), that the average Late-Pleistocene field was bower than the present field, and that a minimum seems to have occurred during the Eemian interglacial. Nevertheless, it cannot be said with certainty that the NRM/ARM variation in Fig. 7a, b truly reflects the relative changes of the ancient Earth magnetic field. Indeed, even if allowance was made for the effect of grain-size changes, the method will only give valid results if other magnetization components such as VRMs or CRMs, which may have occurred at a different time from the acquisition of the DRM, are completely removed, and if the original DRM did not change under the influence of disturbing factors such as bioturbation or solifluction, etc. In some samples, the spontaneous acquisition of a VRM was followed in the Earth’s magnetic field (= 47 jiT) during a period of about 1 month (Fig. 11). Before storage of the samples in the field, they were all AF demagnetized in a field of 0.1 T to obtain a reproducible initial magnetic state. A linear increase of the VRM with the logarithm of time, with a single slope, was found for all the samples examined. The viscosity coefficient, determined from the slopes of the VRM acquisition curves, was low in the bess, but about three times higher in the Warneton soil. In the latter, the VRM resides in coarse MD particles, Column 6 in Fig. 6a and 6b gives the VRM as a

Y~tL V/tM

A

0

0.8-

2A/rn IVRM .1 I

0.9-

.2 I

.3

5 .710

.4I

L *

‘~

0.7

os.

-‘

-2

0.4.

log A

TIme (sec)

0.2 0 ~-

0-

I

30

I

. -.

I

HAP (ml)

I .

.

.

-

Fig. ii. The acquisition and alternating field demagnetization curves of VRM’s at Tongrinne. The VRM was acquired in a field of 47 pT after AF field demagnetization in a field of 100 mT.

percentage of the initial remanence acquired by the specimens after a storage test of about 1 month in an upturned position in the laboratory’s field (= 47 ~sT).The VRM, which is bow in the youngest and oldest bess, becomes high in the topsoil and the bevels between the cryoturbated horizon and the Rocourt soil. Comparison with the fraction of the NRM that was removed during the ‘cleaning’ process and plotted in column 5 of Fig. 6a and 6b proves that this fraction is of viscous origin. The steady increase of the VRM from the cryoturbated horizon, until the upper part of the Rocourt soil, points to the presence of SP particles or grains near the SP—SD boundary. This could explain why the NRM/ARM method gives the smallest values in these bevels. 5. Conclusions From the palaeomagnetic and rock-magnetic investigation of the two bess profiles of LatePleistocene age in Belgium, it follows that magnetite is the dominant remanence carrier, and that the environmental conditions prevailing at the time these sediments were formed were suitable for field alignment to occur.

38

Directional field changes did not prove to be very different from the secular variation changes known from the study of Hobocene bake sediments and from archaeomagnetic measurements. For the sites examined, directional changes very clearly manifest themselves in declination, while the inclination changes more smoothly. The average Late-Pleistocene field is not very different from an axial dipole field, and large directional deviations, called ‘excursions’, were not detected. A magnetic field test applied to the upturned strata near a fossil frost wedge of Weichsebian age suggests that at beast some of the large directional changes found in periglacial sediments, which were cabled ‘field excursions’, should be attributed to mechanical disturbances due to frost action. The high silt fraction in bess sediments render them generally suitable for determining relative intensity changes of the geomagnetic field based on the ARM method. However, the intensity minimum observed in the Rocourt soil of Eemian age cannot be entirely due to a decrease of the ancient field, as part of it can be explained by the presence of a VRM residing in SP or small SD particles, rendering the ARM normalization method uncertain in this case. No clear correlation is thus found between the NRM intensity and climate to support the suggestion of Wollin et al. (1971, 1978) that there is a fundamental direct cause and effect relationship between climate and geomagnetism. Burned soils and erosional features indicate that the sedimentation was not always uniform, and that the magnetic record present should be considered as fragmentary in nature. Nevertheless a long-term change in inclination can be noticed in both profiles. A spectrum analysis is not yet feasible, not only because of the discontinuous nature of the record, but also because a reliable time-frame to transform the depth scale into a time-scale is still backing. TL dates and the examination of other sediments to fill the gaps will cope with these problems in the future. Several magnetostratigraphical results were obtamed from which one of the most important is that we can correlate both profiles on the basis of their magnetic characteristics. From the magnetic signature, combined with the volcanic mineral

investigations by Juvigne (1977), we reached the conclusion that the humic soil complex overlaying the Rocourt soil in both sections in Tongrinne and Rocourt is contemporaneous. The remanence properties and magnetic susceptibility proved to be most useful to search for the maximum occurrence and spatial distribution of minerals of volcanic origin, the former being a stratigraphic marker.

Acknowledgements The authors are greatly indebted to all the members of the Centre for Quaternary Stratigraphy of Belgium for their encouragement and helpful discussions. We benefited greatly from the field experience of Professor Paepe, and thank him for providing a copy of Figs. 1 and 2. We extend our thanks to the personnel of the Centre de Physique du Globe of the Royal Meteorological Institute in Brussels, especially to R. Rosebbe for several measurements and G. Simon and A. Bourtembourg for the computer graphs and ink drawings. We thank the owners of the ‘Point du Jour’ and ‘S.A. Sables et Graviers’ pits for their kind cooperation. Last but not least, the authors also thank the reviewers of the manuscript for their helpful comments.

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