Late Weichselian palaeomagnetic secular variation from the Torreberga basin, south Sweden

160

Physics of the Earth and Planetary Interiors, 43 (1986) 160—172 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands

Late Weichselian palaeomagnetic secular variation from the Torreberga basin, south Sweden Per Sandgren Department of Quaternary Geology, Tornavagen 13, S-223 63 Lund (Sweden) (Received September 17, 1985; revision accepted January 24, 1986)

Sandgren, P., 1986. Late Weichselian palaeomagnetic secular variation from the Torreberga basin, south Sweden. Phys. Earth Planet, Inter., 43: 160—172. Three 6 m long sediment profiles from the Torrebergabasin (55°37’N,13°14’E)in south Sweden have been analysed palaeomagnetically. The sediment successions cover the period 12800—10000 years B.P. which almost corresponds to the entire Late Weichseian (13000—10000 years B.P. sensu Mangerud et al.). The sediments were recovered with a Russian peat corer, a method which allows the declination to be determined in absolute degrees. Around the mean declination two maxima with an intervening minima is recorded, with a peak to peak amplitude of 60°. In the inclination records more variation can be seen, however, the amplitudes are much smaller. The mean inclination is 5° lower than expected with respect to the site latitude. On a general trend of gradually steeper inclination upcore 5 maxima and 5 minima are found. The peak to peak amplitudes range between 5°and 14°.The amplitudes of both parameters are comparable in magnitude to those documented in other records of Late Quarternary age from, e.g., North America or of Holocene age from Europe. The records show that there is no evidence of any geomagnetic excursion between 12800 and 10000 years B.P., as previously discussed in the literature, but the field has been of normal polarity. The palaeomagnetic records from the Torreberga basin, which give consistent and repeatable values, must so far be considered to be the best and most detailed European palaeomagnetic record from this period.

1. introduction Many high quality and good records of the geomagnetic field secular variations have been published in recent years. Geomagnetic master curves of Holocene age from different regions of the world have been put together and analysed by Thompson (1984). The European records are based on studies of lake sediments in Great Britain (Thompson and Turner, 1979) and Finland (Huttunen and Stober, 1980). Geomagnetic master curves from Sweden, situated approximately between these two regions are still lacking. South Sweden is for several reasons suitable for palaeomagnetic studies, (1) the bedrock is generally rich in magnetite, (2) accessibility of a large number of both 14C dated and pollen analysed lacustrine sediment successions extending back to 0031-9201/86/$03.50

© 1986 Elsevier Science Publishers B.V.

the deglaciation (c. 13 000 years ago), (3) accessibility of long sequences of varved clays linked to the Swedish time-scale in calendar years. The reasons for putting efforts into palaeomagnetic studies in this area are many, (1) well dated geomagnetic records, from this stifi unknown region, would improve our knowledge in the studies of geophysical models concerning the Earth’s magnetic field, (2) the possibility of establishing two independent sets of master curves based on (a) sediment dated with radiocarbon or by pollenstratigraphy, and on (b) varved clays linked to the Swedish time-scale in calendar years in order to establish a relationship between these two dating methods, (3) study the occurrence of geomagnetic excursions previously discussed in the literature. A well dated record further could be used in dating sediments that otherwise cannot be dated, e.g., non varved

161

clays or sediment contaminated by ancient radiocarbon. The present results from the Torreberga basin are the first steps in creating a geomagnetic master curve for south Sweden back to the deglaciation.

there is no evidence of dead ice in the northern part (Bergiund and Digerfeldt, 1970). The basin is located within the ‘clay till’ area but the drainage area also includes the sandy till area. Thus two different types of material may have contributed to the sediment in the basin.

2. General description 3. Collection of cores The Torreberga basin is situated in Skàne in the southernmost part of Sweden (55°37’N, 13°14’E),10 km south of the city of Lund (Fig. 1). During the glaciation two ice streams influenced this area (for a review see Lagerlund, 1977). At the deglaciation the main ice receded towards the northeast depositing a sandy till, containing shales and igneous rocks. In the lower flat parts of the landscape of southwest Skâne a very clayey material exists, stratigraphically above the sandy till. This clayey material contains Cretaceous rocks and according to the traditional view it is a clayey lodgment till, deposited by the Low Baltic ice stream, advancing from the southeast (e.g., Ringberg, 1980, 1983). Recent sedimentological investigations, however, suggest that it is a marine sediment, partly deformed by rafted icebergs (Lagerlund, 1980, 1983; Berglund and Lagerlund, 1981; Adrielsson, 1984; Malmberg, 1984). The basin is 2.5 km long and 1 km wide. It is divided into a northern and a southern part by an island composed of till. Although dead ice seems to have existed in the southern part of the basin

) HAL’i

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raphy, samples for palaeomagnetic analyses were immediately taken out in the field by pushing

L EK I N G E

S KNE

______

•Lund

Four profiles were cored in the northern part of the basin (Fig. 2), where there is no evidence for stagnant ice. Profile 2 lies 50 m north of profile 1 and profile 3 lies 25 m east of 2. The fourth profile was collected 400 m northwest of the other three profiles, closer to ancient shore of the basin. Because the directional data of profile 1 was partly very scattered it is not discussed further. The total length of profile 3 is 5.5 m, with an overlapping sequence of 0.5 m between 6.5—7.0 m. Profile 4 is 6 m. In both these profiles the underlying till was reached. Profile 2 is also 6 m, but in this case the till was not reached. The vertical cores were recovered with a Russian peat sampler, modified for palaeomagnetic sampling. With the Russian sampler a 1 m long and undisturbed core segment of the profile was obtained. By using two separate coring holes at each site a ‘continuous’ profile of the sediment succession was recovered. The rods of the corer are constructed with markers being parallel to the blade of the corer. By applying a stick into these markers the orientation of each core segment can be detennined with an ordinary compass to within ±3o~ The orientation procedure, in a few cases, as discussed below.however, failed After inspection and description of the stratig-

~

“:‘

small plastic boxes (2x2x2 cm) into the sediment. Samples were taken at intervals between 3 and 4cm.

OTorreberga

________

1. Fig. 1 Map showing the location of the investigated site, situated in south Sweden, 10 km south of Lund.

4. Stratigraphy The stratigraphy of the Torreberga basin is well documented and described in Berglund and Di-

162

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Fig. 2. Topographical map of the area around the Torreberga basin (shaded). The northern part. in which the profiles (1—4) were cored is delimited from the southern part by islands of till. The basin is situated within a low lying relatively flat area immediately north of the border to a hilly landscape. Contour levels are in metres and distance scale in kilometres. Depth Cm)

o

gerfeldt (1970). The Late Weichseian sequence

Profile Tor3 Tor4

Tor2 —

BD—70

—

years BP

—

P

1 2 3

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organic sediment clay gyttja clay ‘clay till’

Fig. 3. Stratigraphy of profiles 2, 3 and 4, related to the stratigraphy described in Berglund and Digerfeldt (1970). The chronology is based on their biostratigraphy.

.

.

consists of grey clay, occasionally with millimetre thicknesses of horizontal laminae of silt. Some horizons with water mosses also occur. The clay is divided into an upper and a lower part by an easily recognizable layer of dark brown gyttja clay, a few centimetres thick. In the northern part of the basin, where the profiles were collected, the total thickness of the clay ranges between 5 and 7 m. This mainly inorganic sequence is overlain by an approximately 3 m thick sequence of predominantly organic deposits (calcareous gyttja or detritus gyttja, overlain by peat). The stratigraphy of the three profiles (Fig. 3), discussed in this paper, can easily be correlated to the stratigraphy presented in Berglund and Digerfeldt (1970). All the corings started 3 m below the surface, i.e., approximately at the organic/minerogenic transition.

5. Chronology The chronology is based on Bergiund and Digerfeldt’s (1970) biostratigraphic (pollen and mi-

163

crofossil) studies of two near shore profiles. According to Berglund and Digerfeldt (1970) the transition between the upper clay and overlying gyttja represents the onset of the Younger Dryas—Preboreal transition period (Berglund, 1966) and is dated to c. 10200 14C years B.P. in southern Sweden by Bjorck (1979). The upper clay was thus mainly deposited during the Younger Dryas Chronozone (10000—11 000 years B.P. sensu Mangerud et al., 1974). The well delimited layer of gyttja clay within the clay represents deposition during the latest part of the Allerod Chronozone. The upper boundary of this layer has been estimated to be 11200 years B.P. and the lower boundary to be 11400 years B.P. These dates are based on the incomplete pollen analytical record by Berglund and Digerfeldt (1970). The lower clay is partly considered to represent the earlier part of the Allerød Chronozone (starting 11 800 years ago sensu Mangerud et al., 1974) and possibly reaches down into the Bølling Chronozone (starting 13 000 years B.P. sensu Mangerud et al., 1974). The onset of clay sedimentation has been estimated to be 12800 years B.P., based on approximately the same sedimentation rate in the lower as in the upper clay. As southwestern Skàne became ice free around 13 000 years ago this estimation seems to be fairly correct, The average clay sedimentation rate can be calculated to c. 2 mm a 1, with an apparently lower sedimentation rate around the Allerød gyttja clay layer.

i.a Depth (m) 655

.~

4

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—

..•‘

----

0.5

~ ~ ~ £

.

~O

‘

40

‘

60

‘

80

‘

100

H(mT)

Fig. 4. Representative progressive demagnetizationcurves from four levels of profile 3. The median destructive field is in all the four curves between 25 and 30 mT.

6. Palaeomagnetic results

pilot samples were picked out from profile 3 and stepwise partially demagnetized in peak alternating fields of 10, 20, 30, 40, 50, 60, 80 and 100 mT to find a suitable field for cleaning the other samples. Representative demagnetization curves and orthogonal plots are shown in Figs. 4 and 5. The small angular changes during the partial demagetization imply stable directions. The small amount of scatter about the curves probably is a result of noise introduced by the demagnetization procedure. The median destructive field of the samples ranges between 20 and 30 mT. Based on these results all samples were cleaned using a 10 mT field. The samples are, based on the demagnetization results considered to be of good palaeomagnetic stability.

6.1. Laboratory methods

6.2. Magnetic susceptibility and remanence intensity

All measurements have been performed at the palaeomagnetic laboratory in Lund. For measurements of the remanence parameters a 7 Hz Digico fluxgate magnetometer (Molineux, 1971) was used. Demagnetization was carried out using AF demagnetization equipment. The magnetic susceptibility was measured on a Digico bulk susceptibility unit. Initially the normal remanent magnetism (NRM) of all samples was measured. Then 14

The magnetic susceptibility of recent sediments mainly depends on the concentration of fernmagnetic iron-oxides in the sediment (Thompson et al., 1975). Because variations of magnetic susceptibility with depth in a lake sediment are independent of geomagnetic field changes, susceptibility is a useful parameter in correlating cores. However, differences have also been noted, e.g., between marginal and central cores in the same basin (Thompson and Edwards, 1982). The con-

164 2)

(nAm

‘~

UP

) 10

\ \

5

west

east

Depth Irn) 7.60

—*

4.63

layer are denoted. Correlatable susceptibility features (labelled 1—4) are also noted. The most striking difference between the profiles is that following the lower values around the gyttja clay layer the susceptibility in profile 4 remains low. In profiles 2 and 3, where parts of the overlying organic sediment were also cored, the rapid decrease at the 10200 years B.P. level is clearly seen. The intensity logs show less clear similarities and are more difficult to correlate. Considering the variations in the magnetic susceptibility in terms of erosion in the watershed, there seems to be a close correlation between the stratigraphy and the susceptibility. The higher values recorded in the lower and upper clay units could be associated with a higher degree of erosion in the catchment, with little or no vegetation cover, in comparison to the lower values around the gyttja clay layer and in the organic sediments.

S0mT 6OmT 5OmT

6.3. Directional data

4OrnT

The NRM and 10 mT declination records of the

3OmT

1/ 2OmT

1 OmT

15

down

Fig. 5. Orthogonal plots showing the progressive AF demagnetization of the same four levels as in Fig. 4. The upper part of the diagram shows the horizontal component of the NRM vector at each demagnetization step, the lower part the vertical component.

centration of ferrimagnetic ironoxides has further been found to reflect patterns of erosion in the watershed as recorded by pollen percentages or chemical variations (Thompson et al., 1975). The susceptibility logs of the three Torreberga profiles are presented in Fig. 6. The three marker horizons, the 10200 years B.P. level and the upper and lower boundaries of the Allerød gyttja clay

three profiles, related to depth below the surface, are shown in Fig. 7. The average scatter of data is low. For the cleaned records it is about 20°, 16° and 12° for profiles 2, 3 and 4, respectively. The ‘pattern of th~uncleaned and cleaned data sets are practically identical. With respect to the circular correlation coefficient (Holmquist, 1985) there are in all the three profiles an almost perfect correlation (= 0.97, 0.99 and 0.99 for profiles 2, 3 and 4, respectively) between the NRM and 10 mT records, and the average angular difference is very low (less than 3 The individual core segments are noted in Fig. 7. With the exception of the three lowermost core segments of profile 2, the points of profile 3 and 4 make up a more or less continuous smooth curve with no abrupt changes at the core boundaries. It can also be noted that within the overlapping part (6.5—7.0 m) of profile 3 the values are similar. The clear offset at the core boundaries between the three lowermost core segments of profile 2 (C, D and E in Fig. 7) is considered to be due to coring difficulties. There are different techniques described in Lund and Banarjee (1985) for ‘reorientation’ of core segments by (a) matching end ).

165 NRM INTENSITY Tor2 Cm)

Tor4

~

~

~

3

SUSCEPTIBILITY

Tor3

Tor2

Tor3

Tor4

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I

I

I

—I—

I

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I

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60 I

•..,~

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\

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•

C

-

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5

6

__________

2~

6

0

0

Gauss/Os ,1o6

Fig.Gauss.10~ 6. NRM intensity and susceptibility logs of profiles 2, 3 and 4, related to depth. Arrows (labelled 1—4) show correlatable susceptibility features, ages in ‘4C years B.P. according to Fig. 3.

points, (b) matching trends, or (c) cross correlation in reconstructing the declination. As the sampling in this case in ‘continuous’ it is assumed that the first and simplest method, matching end points, accurately can be used. Based on this, core segment C and E have been moved to the east, 20 and 30°,respectively. The corrected values are noted in Fig. 7. This correction results in a smooth curve, The corrected values have been used in the following comparison between the profiles. The dated levels (Fig. 3) and the identified susceptibility peaks (Fig. 6) are noted in Figs. 7 and 8 to facilitate comparisons. Visually there seems to be a good correlation in the declination trend between the curves. In the uppermost part of profile 3 and 4, around the 10200 years B.P. level, easterly directions are prevalent. Combined with the declination record of profile 2, which has not been corrected in this part, and in which somewhat younger sediments are present, it is evident that there is an easterly peak around this level. In

all cores there is a gradual westerly trend. In profiles 3 and 4, it can be seen that there is a broad inflexion point somewhat above the Allerod gyttja clay layer. In the bottom of profile 2, corresponding to the Allerod level, the values also are at their lowest. Further downcore, as reflected in the two longest profiles, the declination gradually swings back again. Altogether at least half a period is documented in these records, with a peak to peak amplitude of about 60°,which is in agreement with other Late Weichselian and Holocene palaeomagnetic record (e.g., Thompson and Turner, 1979; Banarjee et al., 1979; Huttunen and Stober, 1980; Barton and McEthinny, 1982; Lund and Banarjee, 1985). The NRM and 10 mT inclination record, related to depth below the surface, are shown in Fig. 8. Again there is very little noise in data. The calculated average scatter is 6°, 8° and 4° for profiles 2, 3 and 4, respectively. Only minor differences between the uncleaned and cleaned data .

166 DECLINATION Tor2 Depth (m) 3

Tor3

NRM 300’

10 mT

I

330’ I~I

0’ I

30’300’ .+4

330’

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Fig. 7. Declination records (NRM and 10 ml) of profiles 2, 3 and 4, related to depth, A—F core segments, arrows (1—4) show the level of the identified susceptibility peaks in Fig. 6, ages in 14C years B.P. according to Fig. 3. The open circles (profile 2, 10 mT) represent the declination after the correction, described in the text. The NRM and 10 mT records are practically identical. With respect to the position of the susceptibility peaks and the dated levels there is a clear correlation in the trend of the three profiles.

sets can be noted. Superimposed on the general trend of gradually steeper inclination upcore a number of minor swings (labelled a—j) can be seen.

TABLE I Comparison of the average inclination of the core segments between the three profiles

Core segment

Profile 2

A B C D E

71 2°

X

66.6°

70:30

64.8° 62.3° 64.3°

3

4

700° 65.8° 62.6°

68 3° 63.3°

63.10

62.30

58.3°

59.3°

63.7°

_________________________________________

The average inclinations are 66.6°,62.4°and 61.7° for profiles 2, 3 and 4, respectively. These are between 5° and 10° lower than expected on the basis of a general axial dipole field with respect to the site latitude of 70,9°.The average inclination of each core segment is presented in Table I. If it is taken into account that the oldest sediments are not present in profile 2, then it is obvious that there is close agreement in the average inclinations between the core segments. Down to the gyttja clay layer the inclination is only about 5° lower than the axial dipole value direction and comparable between the core segments. The low inclination values can either be due to some kind of inclination error (Ising, 1942; Griffiths et al., 1960), or alternatively be a real reflection of low geomagnetic inclination.

167 INCLINATION Tor2

Tor3

NRM

Depth

Cm) 3

10 mT

50’ I

70’ I

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Fig. 8. Inclination records (NRM and 10 ml) of profiles 2, 3 and 4, related to depth. Core segments, susceptibility peaks and ages as in Fig. 7. The vertical line represents the axial dipole inclination ( = 70.9°).The NRM and 10 mT records are similar. Comparable inclination peaks are labelled a—j.

The inclination swings are most prominent in profiles 2 and 3. The inclination peaks (a—j) seem to correlate well between all the profiles. Inclination above 70°(peak a) in all the three profiles correlates with the 10200 years B.P. level. The f, g and h peaks correlate with the dated levels around the gyttja clay layer. Differences in the absolute distance between the peaks between the profiles probably reflect differences in the sedimentation rate between the coring points. 6.4. Preliminary transformation to a time scale It is generally difficult to compare profiles that exhibit differences in the sedimentation rate. Therefore the 10 mT declination and inclination records of the three profiles have been transformed to a time scale. The greatest uncertainty probably is the age given to the onset of sedimen-

tation. Between the dated levels the sedimentation rate is considered constant. Because there is a random noise in the directional data, assumed to be independent of the secular variations, it is difficult to undertake detailed comparisons of curve trends to decide which features are ‘real’. Therefore the transformed data sets havebeen smoothed with a linear filter (Björck et al., 1986) having a band width varying between 100 and 200 hundred years. The smoothed curves represent a preferred model of the (true) geomagnetic field variations at the site. With respect to the smoothed declination records (Fig. 9) there is an almost perfect correlation between profiles 3 and 4 (circular correlation coefficient = 0.9952, with a lag of 200 years). There is, however, a minor average angular difference between the curves of about 6°.The reason for this is not clear. The westerly broad declination swing

168 DECLINATION Tor2

“C

300’

~

DP 10,000

I

Tor3

300’

0

3300 I

3300 I

I

Tor4 00 30’ —~-~°

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—4—4

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-

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.

___________

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(f’

10.500

330’

I

00

I

I

____________

_~-_

/

//J

Profile 2

3

j

a

49’

b c d e f g h

10200 10600 10750 10900 11100 11300 11500

10200 10550 10625 10850 11000 11200 11500

11.000 ____________

(i2~?) ~

11,500

-

____________

___________

—

___________

Ii 12,000

“)“)

I 12,500

)

(

—

11625

i

—

11900

10200 10550 10650 10850 11100 11300 11450 11600 11700

j

—

12425

12525

0 50 125 50 100 100 50 25 200 100

TABLE III

Comparison of the inclination peaks (in degrees) between the three profiles.

13,000

I

300’

I

I

330°

I

—4————I

0°

30~ I 3t0° 330°

I

1

~

0°

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330°

I

0’

30°

30°

INCLINATION

wear,

50°

BP

I

Tor2 70° I

Tor3 50° I

‘

50°

I

—~ I

Profile _________________________________________

2

3

4

a b c

73

73

>

62 64

62

62

d

59

68 59

64 59

e g

65 58 70

66 57 65

66 57 64

h

—

65

64

i

—

61

62

—

47

49

f

j

Tor4 70°

—4 1

Inclination

peak

Fig. 9. Comparison of the smoothed and age transformed declination records (10 ml) of profiles 2, 3 and 4 (smoothing degree 200 years). The age transformation is based on the dated synchronous lithological levels presented in Fig. 3, and an assumed bottom age of 12800 years B.P. The corrected values have been used for profile 2. There is a clear correlation in the trend of the curves.

C

Greatest difference

,~

if

____________

inFig.10

Inclination peak

/77

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30°

—+——‘4

TABLE II Comparison of the age (in years B.P.) of the inclination peaks based on the smoothed and age transformed inclination curves

70

70° -

—1

10,000

1~ ~ 10.500-

b

b(~

-

11,000

d(&~ I

I’E~~

‘-~

—

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9

11.600

I

12,000

The transformed and smoothed inclination curves also exhibit a high degree of similarity (Fig. 10). The absolute inclination values and corresponding ages are listed in Tables II and III. As can be seen from Table III, there are only small

-

12,600

Fig. 10. Comparison of the smoothed and age transformed inclination records (10 ml’) of profiles 2, 3 and 4 (smoothing degree 100 years). Age transformation and bottom age as in Fig. 9. The pattern of the identified peaks show a close agreement between the profiles, both with respect to age and de-

I

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~

13,000

—

)

~

can be dated to 11500—11 000 years B.P. while the more accentuated eastern declination peak can be dated to c. 10100 years B.P.

F 50°

I

,

—4 I

70°

50°

I

I

70°

50°

I 70°

grees.

169

angular differences between the peaks, in most cases 3° or less. With respect to age correlation (Table II) there is also a very close agreement between the profiles. For 9 of the 11 peaks the age difference is less than 75 years. This difference could easily be explained by slight differences in the sedimentation rate between the dated levels at the coring points. A sediment thickness of 20 cm corresponds to about 100 years, and a much longer period around the Allerød layer. With respect to the few dated levels the correlation is considered good.

7. Discussion and conclusion Clayey sediments from the Torreberga basin have been investigated with respect to the geomagnetic field secular variations. The palaeomagnetic data from the three profiles indicate that the records are high in resolution and without systematic error, Based on dated levels by means of pollen stratigraphy the records have been transformed to an age relationship and smoothed by a filtering technique resulting in a preliminary model of Late Weichselian field secular variations. The declination records of two of the three profiles (profile 3 and 4) are almost identical. The same pattern is found in the third profile (2), but as two core segments in the lower part of this profile had to be corrected some caution must be taken when interpreting these results. The smoothed and age transformed data sets of the declination reveal an easterly peak around 10100 years B.P. and a broad westerly peak between 11 500 and 11000 years B.P. More detailed features can be seen in the inclination records. Also in this case the records are most similar. Ten inclination maxima and minima are recorded on a long scale trend of gradually steeper inclination upeore. The average inclination is lower than expected with respect to the site latitude, but similar between correlatable core segments. This small difference can probably be attributed to a long-term non-dipole component in the palaeomagnetic field rather than a systematic error in the DRM/PDRM mechanism. Based on

the smoothed and age transformed inclination curves, and taking into account that there are not many dated levels, the correlation between the identified peaks is considered very good. As the sediment of the two longest profiles (3 and 4) extend back to 12800 years B.P. these two sections cover approximately one-third of the time that magnetostratigraphically has been referred to as the Gothenburgh Geomagnetic Excursion. Evidence of this short Late Glacial period of reversed polarity was first reported from a core in Sweden (Mörner et al., 1971). The Gothenburgh Geomagnetic Excursion is ‘now considered to consist of (1) a period of irregular magnetism from 13700 years B.P. and (2) a fully reversed “Flip” at 12 400—12 350 years ago’, i.e., during the Fj’áràs stadial (Mörner, 1977). Evidence of the Gothenburgh Geomagnetic Flip are scattered all over the world (Morner and Lanser, 1974). The Gothenburgh Magnetic Excursion and Flip are proposed as a standard magnetostratigraphic unit in global correlations (Mörner, 1979). In Sweden the Gothenburgh Flip is documented in some 10 cores (Mörner, 1977). Five of these, from the Swedish west coast, are closely correlated and dated on other grounds (stratigraphy and climate) than the magnetic results. Four of the cores are from marine environments and the fifth core from a lacustrine environment. A short and rapid, completely reversed inclination is recorded in these cores in the layer representing the Fjäräs stadial. The Fjllrãs stadial lasted for about 50 years (Mörner, 1969, 1971) or possibly 85 years (Mörner, 1975a,b). In the marine deposits this Fjärãs layer consists of ‘clayey sandy silt with abundant molluscs and ice rafted material including pebbles with attached barnacles and pieces of Cretaceous chalk and chert ... This layer serves as a regional marker bed in the marine deposits ... along the Swedish west coast’ (Mörner, 1977, p. 416). Mörner further states ‘that extensive exposures in clay pits indicate that this layer was formed by heavy ice rafting (and not turbidity currents) in combination with normal setting of particles’. His conclusion is that this layer is suitable for palaeomagnetic studies, indicated by the consistent declination and inclination records in all the cores, with exception of core B873, In that

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particular core the declination (but not the incination) is assumed to have been displaced by turbidity currents. According to my opinion it can be discussed to what extent this layer actually is suitable for palaeomagnetic studies and in any case it must be difficult to determine the nature of this layer in corings. Based on the figures (Mörner, 1977, Figs. 5, 6, 7, 8 and 10) there seem to be no correlation of the declination and inclination between the cores, with the exception of the reversed inclination confined to the Fjärãs stadial layer. The magnetic records have not proved to be reproducible in a duplicate core from any of the sites. From the Torreberga profiles (3 and 4) there is no evidence of reversed inclination between 12400—12 350 years B.P. (Fig. 10) or irregular magnetism prior to this age (Figs. 9 and 10). The magnetic results of Mörner’s (1977) five cores can not be correlated to the magnetic records from the Torreberga basin discussed in this paper. Even if the duration of the Gothenburgh Geomagnetic Flip is short (50 years) it should have been recorded in these sediment successions. With a sedimentation rate of 2 mm a1 each sample represents 10 years and having one sample at every 4 cm, as in this case, at least one sample must fall within this critical period. Based on the above discussion my conclusion is that the existence of the Gothenburgh Geomagnetic Excursion and Flip is contradicted by the Torreberga records. The reversed inclination recorded in the Fjäràs stadial layer is not a reflection of the geomagnetic field but most probably a reflection of unsuitable palaeomagnetic conditions. With respect to the lacustrine core (BjOrkerods mosse), Thompson and Berglund (1976) concluded that the Flip recorded in that particular profile was caused by disturbances in the sediment and severely questioned the existence of the excursion. The existence of the Gothenburgh Excursion as a standard magnetostratigraphic unit in global correlations was also criticized by Banarjee et al. (1979). Varved clays from two localities in Blekinge, southeast Sweden were analysed palaeomagnetically by Noel (1975). At each locality a single core was recovered with a 36 mm foil piston corer. The two sediment successions (the Stärno core and the Stilleryd core) partly overlap in time and cover a

total period of 393 years. The varved clay in Blekinge was deposited between c. 12500—12100 14C years B.P. (BjOrck, 1984). According to the data of Noel (1975) there is an almost linear eastern change in the declination record (average 0.12°a”), starting at 250° in the bottom of the Stärnö core and ending at 20° in the top of the Stilleryd core. Within the overlapping time period there is a discrepancy of 30°.Noel (1975, p. 357) concluded that the largely westerly direction cornbined with low inclinations ‘provide an accurate, annual record of part of the Laschamp geomagnetic event and indicate that the magnetic pole was reversed between 10153 and 10127 years B.C.’ (years B.C. according to Nilsson’s (1968) varve chronology). As previously discussed this is not in agreement with the results from the Torreberga basin. My conclusion is that the sediments in the two different Blekinge cores most probably have been distorted by mechanical sedimentation processes or slumping, and support the three minimum requirements proposed by Thompson and Berglund (1976) in the recognition of geomagnetic excursions in recent sediment. Owing to the rapid sedimentation rate in cornbination with suitable grain size and obviously calm depositional conditions my conclusion is that the Torreberga profiles provide the most detailed and well dated palaeomagnetic record from the Late Weichselian in Sweden. More intense work from other suitable and well dated basins in the study of the Late Weichselian secular variations is, however, required to confirm the pattern documented in the Torreberga basin.

Acknowledgements I thank Dr. S. BjOrck, the leader of the project on Late Glacial Magnetostratigraphy in south Sweden, for fruitful discussions; J. Ising and L. Barnekow for assistance in the field; B. Callahan for correcting my English. The work has been funded by the Bank of Sweden Tercentenary Foundation (Riksbankens Jubileumsfond). The mathematical statistical analyses were carried out by Dr. B. Holmquist, Inst. of Mathematics and Statistics at Lund University, and supported by a

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grant from the Swedish Natural Research Council (Naturvetenskapliga Forskningsradet).

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