Magnetomineralogy and revised excursions for the last interglacial-glacial cycle in the Grande Pile lacustrine sequence, France

QUATERNARY RtSEARCH 37, if+28 (ic)Y?I

Magnetomineralogy and Revised Excursions for the Last Interglacial-Glacial Cycle in the Grande Pile Lacustrine

Sequence, France

KARI LEE ELLINGSEN AND REIDAR LENLIE

Institure of Solid Earth Ph?‘sic,s-~;aomuyrlPti.sm. Allegt. 70. 5007 Bergen. Nowtr~

AND

GUY SEREI

Received May 23. 1990

A paleomagnetic record for the Grdnde Pile lacustrine sequence deposited during the last inter-

glacial-glacial cycle has been constructed based on continuous sampling (n = 792) of a I-m-long oriented core (GPXX). The NRM intensity. magnetic susceptibility. and saturation isothermal

remanent magnetization show pronounced variations related to the climatic zonations in the s- quence. The stratigraphic consistency of the paleomagnetic directions vary considerably due to

variations in signal/noise ratios. Paleosecular variation patterns occur throughout most of the core. as well as several zones of inferred geomagnetic excursions. The application of magnetic fabric (AMS) shows that some apparent Lanterne (Weichselian) excursions reside in deformed \edi-

ments. while anomalous paleomagnetic directions in the 70.000 to 170.000 yr B.P. time interval appear in sediments with low signal/noise ratios and a high degree of compaction. The signature is

hence not likely to reflect genuine records of geomagnetic field variations in these levels. Discrep- ancies between the magnetostratigraphy of GPXX and a previously investigated core implie\ that

the earlier claimed regional correlations based on the paleomagnetic signature of low intensity levels should be critically reassessed. The Grande Pile sequence is not regarded as a magneto-

stratigraphic standard for the last interglacial-glacial cycle. 1 IVV? I:nwer,~ty of Wa\hmgton

INTRODUCTION

Paleomagnetic excursions are short duration (usually ~5000 yr), anomalously high amplitude directional variations of the geomagnetic field, and may be readily recognized in paleomagnetic time records. Ex- cursions may thus serve as an important chronostratigraphic tool in Quaternary research for time periods beyond the range of the radiocarbon dating method (Lovlie, 1989a). Although there is a mounting number of reported excursions for the last interglacial-glacial cycle (e.g., Bleil, 1987; Barbetti and McElhinny, 1976; Gillot rr al.. 1979; Levi and Karlin, 1989: Liddicoat and Coe. 1979; Lovlie et al., 1986; Lovlie and Sandnes, 1987; Negrini et ul., 1984; Smith and Foster, 1969), ambiguity still remains

concerning the reality of most of them. The controversy is partly due to poor age con- trol, the absence of proper procedures for assessing the fidelity of paleomagnetic records, and the lack of consistency of anomalous paleomagnetic signatures in co- eval sediments (Verosub. 1982).

The lacustrine sediment sequence of the Grande Pile (GP) peat bog, northeastern France (Fig. I). was continuously deposited during the last 140.000 yr, and has a relatively well established chronostratigraphy (Woillard and Mook. 1982). A previous paleomagnetic investigation of the GP sequence reports numerous (9-l 1) excursions (Morner. 1977. 1979. 1981, 1986) that, however, have not been recognized in a recent paleomagnetic investigation of a correlative lacustrine sequence from Lac du Bouchet.

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GRANDE PILE MAGNETOSTRATIGRAPHY 17

southeastern IFrance (O-120,000 yr B.P.) (Creer et al., 1986; Smith and Creer, 1986; Thouveny et al., 1990) (Fig. 1). The reality of a number of the reported paleomagnetic excursions from GP may be questioned, justifying a parallel investigation of this unique sequence. The present work is in- tended to assless the paleomagnetic relia- bility of the GP sequence by adding magnetomineralogic analysis and subsequent determination of the magnetic fabric properties.

GEOLOGICAL SETTING AND STRATIGRAPHY

The peat bog of Grande Pile is situated in the Vosges mountains, outside the maximum extent of the post-Lure (-Eemian) gla- ciers (Fig. 1). Continuous sedimentation

could occur in the former lake since the area was deglaciated some 140,000 yr B.P. The minerogenic constituents of these highly organic lake sediments consist ex- clusively of eolian-derived material and lo- cal hillwash because the site is situated on a watershed plateau (330 m altitude). The re- sulting lithostratigraphy comprises alternat- ing layers of silty clay and organic mud (gyttja to gyttja-clay) deposited during re- spective cold stades and warmer interstades to interglaciations (Seret, 1985; Woil- lard, 1978).

The chronostratigraphy of the GP sequence has been established by its re- nowned pollen stratigraphy (Woillard, 1975, 1978), several 14C dates and correlation to deep-sea oxygen isotope stages 1 to 6 (Woillard and Mook, 1982). Correlation of the present GP core (GPXX) to the estab-

0 GRANDE PILE

0 maximum glaciated area:

I 1 - - - - POST-LURE (-EEMIAN)

- LINEXERT

FIG. 1. Map showing the situation of the Grande Pile peat bog (modified after Woillard. 1978). Lac du Bouchet is indicated on the small-scale inset map.

lished GP stratigraphy has been obtained CORING AND SUBSAMPLING by organic carbon stratigraphy. by which the top of the core is indirectly dated at (:ore GPXX was drilled in 1Y78 using ;I about 12,000 yr B.P. (Fig. 2). The correla- Grond Mechanika (Dutch) piston drillsys- tion of the Lure interglaciation to the Ee- tern which introduces minimal disturbances mian interglaciation of northwestern Eu- to the sediments. A total of I5 geographi- rope is well founded (Woillard, 1978: Zag- tally oriented cores ( I m long) were subse- wijn, 1961), and the latter term is used here. quently recovered below about 1 m of Ho-

FIG. 2. Organic carbon curve (weight %) and chronostratigraphy of Grande Pile core XX. All “C ages (Woillard and Mook, 1982) are given approximately to emphasize that the datings are indirect, achieved by organic carbon correlations ( * = 14C dating obtained by isotopic enrichment). Correlation to deep sea ‘so stages is achieved as suggested by Woillard and Mook (1982) and Seret et u/. (1990). while stage boundaries and stage optimum ages follow Martinson et al. (1987).


locene peat. The core sections have been PALEOMAGNETIC MEASUREMENTS

carefully stored at about 4”C, and showed no serious drying effects when opened for The natural remanent magnetization paleomagnetic subsampling in 1988. Sam- (NRM) was measured within 1 month after pling was performed by cutting 50-cm-long subsampling, using a single axis cryogenic bars from the center of the split surface of magnetometer with a 45” inclined sensor. the 6-cm-diameter cores. The bars were NRM intensities are in general very low, continuously subsampled with cubic plastic varying from 7.40 mAm -’ to the noise boxes (5.6 and 6.9 cm3), sealed with tight level of the instrument (0.02 mA m- ‘>. Low lids, and stored at 4°C to avoid drying of the NRM intensities are associated with signif- sediment. The applied sampling procedure icant scatter of the paleomagnetic direc- reduces sediment deformation (Gravenor ef tions (Fig. 3). The reproducibility of the pa- al., 1984; Lovlie et al., 1986) and enabled leomagnetic measurements were deter- continuous sampling (n = 792). mined by repeated measurements of single

NRM Kmin AXIS CHRONO-

DECLINATION INCLINATION inclination STRATIGRAPHY

1 mAm-l -90” 0” 90" -45" 0" 45" 30" 60" go"

I .I. . .

.,:.:..::. : . . .

r; . . . :p

5- ..:.w . . .*. . tq .

. ‘,:.g . 6

. ‘. :: . .

. .

-. : + .‘.‘. I. ‘. i. . . . . *.

:-I-- _ - - - - - . - _ _ _ _ _ .- ST. GERM. II

.: -- MELISEY II

. . . .-

. . .* . . ; ‘ - 4 . ..’ ST. GERM. I

.;i a.*. . : . . . ;*; .;::.

2 ,

. .::. *. ..‘. --MELISEY I

*-. . 4 . .;.

. .:. -... . . . . . .: ‘I . . LURE . . . . .

: -*..;A * .-. . :. (EEMIAN)

. i-y’ *-I f . . Q

. . . y . . . .*: . :/.:.

. ‘,. . . LINEXERT

FIG. 3. Intensity, declination and inclination of the natural remanent magnetization (NRM), compared to the inclination of the Kmin (minimum susceptibility axis), Grande Pile core XX.

30 L:l 1 INGSlzN. Lk9VI.IE~.. AND 5tKE 1

samples, and are within 5” of arc for intensities above 0.2 mAm ‘. deteriorating se- riously for intensities below 0.1 mAm ‘.

Due to the low NRM intensities, only 70 samples were subjected to progressive al- ternating field (an demagnetization (maximum field of 50 mT). Several samples showed large loss of NRM intensity during storage, complicating the obtainment of MDF values which was based on these lowered NRM intensities as 100%~. The NRM typically shows median destructive fields (MDF) of 12-20 mT. seldom exceeding 30 mT. Characteristic remanent magnetiza- tions (ChRM). defined as the high coerciv- ity components, do not generally deviate significantly from the NRM directions, suggesting minor if any overprinting due to the acquisition of viscous or chemical magneti- zations.

MAGNETIC CONSTITUENTS

Thermomagnetic Analysis

The sediments have extremely low concentrations of magnetic minerals, thus. magnetic extracts had to be obtained for thermomagnetic analysis. The applied procedure (Lgvlie et al., 1971) resulted in vis- ible amounts of black, highly magnetic grains, in addition to a larger proportion of less magnetic material, obtained from glacial (Lanterne II, within 9.75 and 10.75 m) and interglacial sediments (Eemian, within 17.75 and 18.30 m).

Thermomagnetic curves were obtained by heating the air (20Wmin) to 700°C. The highly magnetic extracts reveal highly irre- versible curves; the heating curves define single Curie temperatures around 58O”C, indicative of pure magnetite (Nagata, 1961). The cooling curves do not exhibit any in- flexions, suggesting complete oxidation of magnetite to hematite during the analysis.

The less magnetic fractions are dominated by paramagnetic minerals, reflected by monotonously decreasing thermomagnetic curves with weakly developed Curie

points around 580°C. attributrtt to \mall concentrations of magnetite. Thr Lanterne I1 thermomagnetic curves. in addition. exhibit a weak inflexion above 650°C. indicative of initial hematite. Both paramagnetic samples define Curie points during cooling around 22O”C, probably representing the magnetic “cristobalite” phase reported to form by the breakdown of clay minerals (Moskowitz and Hargraves. IY84).

IRM-H Anulysis

Progressive acquisition of isothermal remanent magnetization (IRM) to a maximum field (H) of 860 mT was performed on 47 samples representing the whole range of NRM intensities. Saturation was obtained in fields above 700 mT, associated with remanent coercive forces (H,,.) ranging between 5 1.3 and 102.1 mT.

The ratio (F3 between the median acquisition coercive force (H’,,.) and H,,. F = H’JH,,. has been proposed as a diagnostic parameter for natural magnetite. hematite, and titanomagnetite (grain sire <S-250 km) (Dankers, 1981). The F-ratios for GPXX range between I .05 and 1.35, which is not within the empirically derived range for either pure magnetite (F = I .6 t 0.2) or hematite (F = 1 .O) (Dankers. I981 I (Fig. 4). Subsequent IRM-H analysis of the most magnetic extract fraction however, revealed an F-ratio of 1.75, indicative of pure magnetite. Thus a tentative interpretation is that the observed F-ratios of bulk GPXX samples can be attributed to different pro- portions of both pure magnetite and hematite (ironhydroxides) with depth (Fig. 4). Other as yet unidentified parameters may however influence the F-ratios. which hence should not be interpreted as a direct quantitative measure of the proportion between the two components.

Stratigruphic Vuriutions

Variations in the relative concentration and composition of magnetic minerals


6

10

I1

12

13

14

15

I6

ORGANIC CARBON

F H’cr I Her

1;l 1;2 1;3

. I

.

.

.

. .

. -

: .

. .

. * .

. --i-------

.

. **

CHRONO-

STRATIGRAPH)

III

L A N

_T E R N E

II

I

ST. GERM. II - - MELISEY II

ST. GERM. I

- - MELISEY I

LURE (EEMIAN)

LINEXERT

FIG. 4. Inversed organic carbon curve (weight 761, saturation isothermal remanent magnetization (SIRM), bulk magnetic susceptibility (K) (SI units * 10m6) and F-ratio (H’,, = median acquisition coercive force, H,, = remanent coercive force) for Grande Pile core XX. Susceptibility values are added by IO * lO-6 SI units to obtain positive values for the logaritmic scale.

throughout a ssediment sequence may be de- tected by determinations of bulk magnetic susceptibility (ZC) and saturation isothermal remanent magnetization (SIRM).

The susceptibility was determined for each sample on an induction bridge KLY-2 with a sensitivity of 2 x lop8 (SI units), and ranges between -5.55 and 607.09 x lop6 (Fig. 4). Sediments show in general positive K values due to the presence of paramagnetic, antiferro- and ferrimagnetic minerals. Negative values of K in parts of the Eemian and St. Germain warm stages reflect a dom-

inance of diamagnetic constituents (e.g., water, organic matter), implying extremely low concentrations of remanence-carrying minerals. This is also evident from the low NRM intensities and high organic carbon content in these zones (Figs. 3 and 4).

The saturation isothermal remanent magnetization (SIRM) reflects the concentration of remanence-carrying (antiferro- and ferrimagnetic) minerals only. SIRM has been determined for every second to third sample in a field of 860 mT, and ranges between 8 and 1562 mAme (Fig. 4).

22 EI.LINGSEN. LOVLIL. AND SbRE2

The SIRM and K values generally show a distinct negative correlation with the organic carbon content along the sequence (Fig. 4).

MAGNETIC FABRIC

Magnetic fabric is derived from the anisotropy of magnetic susceptibility (AMS), and is described by the lengths and directions of the principal axes of the susceptibility ellipsoid (k,,,, ki”t, and km,“). All magnetic minerals possess a weak, intrinsic magnetocrystalline anisotropy. The AMS in magnetite is however completely dominated by a shape-dependent anisotropy, and the magnetic fabric in magnetite bear- ing sediments effectively reflects the statis- tical distribution of nonequant grain axes (Collinson, 1983).

Magnetic fabric may be modified by postdepositional deformations such as slump- ing, folding, water-escape, and compaction, which are likely also to influence any remanent magnetization carried by physi- cally aligned mineral grains (Verosub. 1975). Thus, magnetic fabric has been adopted as a useful property for assessing the fidelity of paleomagnetic signals (Marino and Ellwood, 1978).

In order to obtain a reasonable precision. AMS was only determined in samples with K > 2 x 10e6 SI. This limit effectively ex- cludes several samples from the St. Ger- main and Eemian warm stages. Approxi- mately every fifth sample along the core was measured (using the KLY-2 induction bridge), in addition to all samples exhibiting highly scattered paleomagnetic directions. The magnetic particles in undisturbed, sub- parallel, horizontal sediment layers deposited from suspension are expected to ex- press a foliation dominated fabric (ablate susceptibility ellipsoid) with kmin axes per- pendicular to the bedding plane (Hamilton and Rees, 1970; Rees and Woodall, 1975). Consequently, samples with significant an- isotropies (k,,,lk,i, > 0.5%) and inclinations of the kmin axis of less than 70” were

considered to carry deformed magnetic fabric and were rejected from further analysis due to the inferred distortion of the paleomagnetic signal. These rather <onservativc criteria mainly reject samples in two zones in core GPXX which are characterized by highly scattered paleomagnetic NRM directions (4.0 to 4.5 m and 8.5 to I I .O m depth) (Fig. 3).

INTERPRETATION OF THE PALEOMAGNETIC SIGNATURE

The natural remanent magnetization is probably mainly carried by eolian-derived magnetite grains (dominating the weak net magnetic moment of any hematite) which aligned parallel to the geomagnetic field by syn- and/or postdepositional processes. Af- demagnetization seldom revealed any indi- cation of secondary magnetic components, suggesting the absence of significant viscous or chemical magnetization components.

The high scatter of the paleomagnetic signal in some zones is attributed to low signal/ noise ratios. Hence. a five-point weighted running mean procedure (Fisherian) was applied to the NRM data after excluding all samples with deformed magnetic fabric properties. An exponential weight function was constructed, which gives unit weight to samples with intensities above 0.2 mA m ‘, decreasing exponentially with decreasing intensities (63% for an intensity of 0.1 mAm -I). Samples with intensities twice the adjacent samples have also been excluded, since spurious high intensity samples would completely dominate the av- eraging procedure. With the applied restric- tions, 94 samples were excluded. and the five-point running weighted mean of NRM directions is thus based on 88% of the original NRM data-set (N = 698) (Fig. 5). The virtual geomagnetic pole (VGP) has been calculated from the running mean directions, representing a time-integrated record of the geomagnetic tield during the last climatic cycle (Fig. 5).


E 1

NRM VGP CHRONO- I: i

(smoothed) latitude STRATIGRAPHY ” DECLINATION INCLINATION

11

1

1:

1:

11

l!

II

1:

II

-90”

\

0” 90”

. . . G .) - ’

. j. . .

I

$ - .

t* . \; . -.-a

.? *. . .

0" 30" 60" T

0"30"60"

.:

v- - .,.

l . : ” 97 - - - --MELISEY II

.,. * 4 . . . .* ST. GERM. I

--III----MELISEY I

--,3(J _____ ---.---__--- LINEXERT

FIG. 5. Smoothed (see text) natural remanent magnetization (NRM) directions and virtual geomagnetic pole (VGP) latitude for Grande Pile core XX. Question marks and vertical dashed line indicate levels of unreliable paleomagnetic signal (apparent geomagnetic excursions). Ages are approximate (Fig. 2), retrieved by ‘T datings and oxygen isotope correlations (italic numbers).

Lanterne 111-U (4-l 1.8 m)

The axial dipole field at the Grande Pile locality (47”44’N and 6”30’E), is defined by an inclination of 65” with a declination due north. The smoothed NRM record for the Lanterne III-l.1 sequence typically shows declinations between - 30” and 60”, and inclinations steeper than 35” (Fig. 5). Thus, the amplitude of the oscillations are com-

parable to the paleomagnetic pattern observed in sediments from Lac du Bouchet, southeastern France (Fig. 1). A merged directional record (demagnetized) from 12 Lac du Bouchet cores exhibits paleosecular variation amplitudes for the last 94,000 yr of the order of -+45” in declination and between 25” to 85” in inclination (Thouveny et al., 1990).

The anomalous VGP-latitudes observed

in the zones around 4.3 and 10.0 to 10.6 m in GPXX resemble paleomagnetic excursions (Fig. 5). However, several samples were removed from the original NRM data- set at these levels due to deformed magnetic fabric characteristics (Fig. 3). The lower zone also shows a highly fragmented sediment character. The fidelity of these zones as recorders of the geomagnetic field is therefore questioned, and the anomalous directions within the Lanterne III-II sequence are concluded to reflect a disturbed paleomagnetic signal. The remaining sections of this sequence probably record paleosecular variations of varying amplitudes.

Lunterne I-Linexert (I I .8-18.5 nz)

The high-amplitude, high-frequency variations of paleomagnetic directions in this sequence reside in well-preserved sediments with primary depositional magnetic fabrics (where it could be measured). The precision of the paleomagnetic record is. however, marginal due to the low signal/ noise ratios. The sequence has a high organic carbon content, and has probably ex- perienced a postsedimentary compaction of >50% (Seret, 1982). Compaction may mod- ify a detrital remanent magnetization caus- ing significant flattening of the inclination (Anson and Kodama, 1987; Blow and Ham- ilton, 1978). This process may partly ex- plain the shallow inclinations in the St. Ger- main II to Eemian interval (Fig. 5). How- ever, there is no evidence that compaction can produce changes in the sign of the inclination (e.g., from positive to negative). Hence. the high-frequency variations in the 70,000 to 120,000 yr B.P. time interval are mainly attributed to low signal/noise ratios. Any real geomagnetic field instability (excursion(s)) during this time period can hardly be extracted from the low-precision data set, which should not be interpreted as a genuine record of the geomagnetic field.

DISCUSSION AND CONCLUSIONS

The stratigraphic variations in organic carbon content constitute a climatic indica-

tor depending on the biomass o1 the Grande Pile area. During periods of intcr\tadial to interglacial climates, the production and in- put of organic matter to the lake was high, decreasing significantly during glacial climates (stades). The supply of minerogenic particles (loess) fluctuated in an opposite sense, being high during cold periods and low during warmer periods when winnow- ing was inhibited by vegetation (Dricot er ~1.. in press: Seret, 1983: Seret rt trl.. 1990). These relationships are reflected by the negative correlations of K and SIRM with organic carbon content (Fig. 41. Thus. these magnetic parameters represent climatic in- dicators for the Grande Pile area, as earlier reported on the susceptibility characteristics of certain loess and deep-sea sediments (e.g., BegCt and Hawkins. 1989: Kent. 1982; Kukla et (11.. 1988; Miniert and Bloe- mendal. 1989: Robinson, 1986). .4 homogeneous magnetomineralogy is indicated by the relative uniform F-ratios throughout the Grande Pile sequence, and makes possible the climatic implications of the K and SIRM parameters since quantitative changes seem to dominate any qualitative ones. Smaller magnetomineralogic changes may, however. be indicated at about X.5-m depth where uniform F-ratios close to I. I appear (Fig. 31, corresponding to a change from dominately metamorphic to granitic heavy mineral source during the Grand Bois inter- stade (Seret et ul., 1990). The lowered F-ratios above 8.5-m depth may be caused by an increased proportion of hematite to magnetite across this 8.5-m boundarq

The susceptibility peaks between Il.5 and 12.5-m depth are not accompanied by similar peaks in the SIRM values. and may indicate a high content of paramagnetic or superparamagnetic ferrimagnetic grains (Thompson and Oldfield. 1986). These characteristic K and SIRM signatures appear in early Lanterne sediments which cover the much disputed Ognon interstades. Bulk magnetic properties may therefore ease future correlations between different cores from the GP basin. thereby


clarifying the latteral consistency of the pa- lynological signal at this level.

The upper part of core GPXX covers the period from about 70,000 to 12,000 yr B .P., during which a number of paleomagnetic excursions has previously been reported (Lovlie, 1989b). The absence of distinct records of paleomagnetic excursions throughout this interval in GPXX are in ac- cord with the paleomagnetic results from Lac du Bouchet (Thouveny et al., 1990), implying that a significant smoothing due to postdepositional acquisition of magnetization (Irving and Major, 1964; Kent, 1973; Lovlie, 1974; Payne and Verosub, 1982) may be a primary process in certain lacustrine environments.

Close inspection of the paleomagnetic signature in GPXX between 5.0- and 8.5-m depth (about 15,000 to 32,000 yr B.P.), an interval with a homogeneous magnetomineralogy and low directional scatter, reveals relatively high-amplitude paleosecular variations with corresponding VGP latitudes of less than 60”. Depending on the degree of postdepositional grain alignment and the sedimentation rate, these intermediate VGP positions may represent time- integrated records of short-duration excursions. A VGP latitude of <?40”-45” as indicative of geomagnetic excursions (Bar- betti and McElhlinny, 1976; Watkins, 1976) is probably not an adequate criterion in all sediments. This fact, however, complicates the identification and discrimination of ex- cursional directions as compared to large- amplitude secular variation (Thouveny and Williamson, 198’8).

The secular variation record of the upper part of core GP:XX has not been correlated to Lac du Bouichet or other synchronous high-quality records since such an attempt should preferably be based on demagnetized data-records from several cores within a single lake.

The St. Germain II-Eemian sequence chronologically spans the Blake episode which is dated at about 115,000 yr B.P. (oxygen isotope stage 5d-5e boundary, esti-

mated duration about 6000 yr; Tucholka et al., 1987) and encountered on a global scale in different sedimentary environments (e.g., Bleil, 1987; Creer ef al., 1980; Den- ham et al., 1977; Herrero-Bervera et al., 1989; Kukla and Koci, 1972; Manabe, 1977; Sasajima et al., 1984; Smith and Foster, 1969; Tucholka et al., 1987; Yaskawa et al., 1973). It can not be excluded that one or more excursion(s) has influenced the highly fluctuating paleomagnetic record uncov- ered throughout this time interval, but the data quality in this part of the GPXX sequence does not justify a reliable definition or the chronostratigraphic limitation of the Blake. This conclusion is confirmed by dis- crepancies between the GP and Lac du Bouchet record where precisely recorded excursions are present prior to 94,000 yr B.P. only, the end of an excursion correlated to the Blake being recorded near the bottom of the core, just after 117,000 yr B.P. (Thouveny et al.. 1990).

The previous magnetostratigraphy of the Grande Pile sediments show some 9-l 1 excursions throughout the sequence (Morner, 1977, 1979, 1981, 1986). The paleomagnetic signature of the two records are significantly different, attributed to differences in directional precision, data-weighting procedure, and postsedimentary compaction/ modification of the remanent magnetization, which may also vary laterally across the basin. In addition, the number of apparent excursions in GPXX were reduced by rejecting samples with disturbed magnetic fabric and spurious high intensities. Hence, it is likely that some of the earlier-reported excursions may reflect unreliable magnetic signals.

The present results do not give support to the claim that the St. Germain I and II interstades may be distinguished and correlated to the Early Weichselian interstades of northern Europe based on their paleomagnetic signatures alone (Morner, 1979, 1981, 1986, 1987). In conclusion, the present magnetostratigraphy of the Grande Pile sediments is not applicable as a stan-

dard sequence for paleomagnetic cxcur- I)rrcot. b Petlllon. M.. and Set< 1 iin pre\\i

sions during the last interglacial-glacial cy- When and why did glacier\ grou 01 melt in the Vo\-

cle. ge\ Mountain\ (France)? In “lnternattonal Sympo-

\lurn on Paleoclimate. Mainr IYX”’ I R. Frenrrl.

ACKNOWLEDGMENTS Ed.1

Gillot. P. Y.. Labeyrie. J.. Laj? i balladas. G..

Many thanks to J. Mangerud who initiated the co-

operation between the authors in Norway and Bel-

gium, to N. Thouveny who allowed us to study the

Lac du Bouchet data. and to D. V. Kent and two

anonymous reviewers who critically read and im-

proved the manuscript. This project was financially

supported by the Norwegian Research Council for Sci-

ence and Humanities (NAVF).

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Documents

Magnetomineralogy and revised excursions for the last interglacial-glacial cycle in the Grande Pile lacustrine sequence, France