Earth and Planetary Science Letters, 94 (1989) 371-384 371 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

[51

Equatorial and mid-latitude records of the last geomagnetic reversal from the Atlantic Ocean

Jean-Pier re Valet 1, Lisa Tau x e 2 and Brad fo rd C lemen t 3

i Centre des Faibles Radioactivit~s, Laboratoire mixte CNRS-CEA, Avenue de la Terrasse, 91198 Gifsur Yvette Cedex (France) 2 Scripps Institution of Oceanography, GRD, A-020, La Jolla, CA 92093 (U.S.A.)

3 Department of Geology, Florida International University, Miami, FL 33199 (U.S.A.)

Received January 25, 1989; revised version accepted June 8, 1989

Three records of the Matuyama-Brunhes reversal have been obtained from ODP deep-sea cores distributed along the West African margin in the Atlantic Ocean. These studies in addition to the record from DSDP Site 609B provide a latitudinal transect extending from the equator to 50 ° N. Simulations of natural smoothing by post-depositional processes show that characteristic features of a transitional field geometry dominated by axisymmetry should be preserved in the records, especially at the equator. The results do not seem to favour the hypothesis that axisymmetrical terms would dominate during this transition.

1. Introduction

Many models have been proposed in attempts to account for the non dipolar structure of the geomagnetic field during reversals. The first com- pilations of paleomagnetic records across reversal boundaries appeared to support models in which the transitional field was dominated by low-order zonal terms (e.g. [1]). Subsequent results [2-5], however, suggested that in most cases the transi- tional field cannot be purely axisymmetric and non-zonal components were thus incorporated in the models [6-8]. The most recent reversal is certainly the most appropriate for studies of the geometry of the reversing field because it is easily identified in sedimentary sequences which have undergone the least chemical alteration. The first attempts to analyze the harmonic content of the field from the existing records of Matuyama- Brunhes were mostly consistent with the presence of zonal axisymmetrical components [7,9,10]. In particular, the approach followed by Williams and Fuller [11] focussed on the relationship between the site latitude the duration and the signature of the inclination during the transition. However, most of the records used were poorly documented and the geographical distribution of the sites was not very adequate. Studies of Matuyama-Brunhes involving latitudinal transects as well as records from the equator and the southern hemisphere are

crucial for testing whether the transitional field is indeed dominated by axisymmetrical terms.

In this paper we present the results of three records of Matuyama-Brunhes obtained from Holes 659C (18 o 04' N, 21° 01' W), 664D (0 o 06' N, 23°16 'W °) and 665B drilled during ODP Leg 108. The complete results of the magnetostratigra- phy [12] as well as detailed paleomagnetic studies of the first two records are reported elsewhere [13] and we include here data from Hole 665B located also at the equator (23°16'W). Thus, together with Hole 609B (50°N, 25°W) drilled during DSDP Leg 94 [14] these records provide a latitudi- nal transect extending from the equator to 50 ° N along a mean longitude of 20 ° W.

2. Sampling and laboratory techniques

The sediments from Leg 108 are composed of light grey to dark grey (659C, 664D) or brown (665B) nannofossil ooze. No apparent change in the lithology was observed through the transition zones which were identified from the results of shipboard measurements at 23.2 meters below sea floor (mbsf) (659C), 28 mbsf (664D) and 13.70 mbsf (665B). Sampling was accomplished with " U " channels [15] 50-100 cm long. The " U " channels were then sliced into 0.5-1 cm thick wafers giving two or three specimen at the same level for Holes 659C and 664D; only one speci-

0012-821X/89/$03.50 © 1989 Elsevier Science Publishers B.V.

372

664D-4-S-100A UP

, I5, ' "T , ,3qnq./m-~'5~. , s ' ~ : ~ - I : : - : 1 ~ "

Ow

-W 664D-4-5- 50A

UP

DOWN • Dec • Inc

W

664-4-5-20A UP

~ I - - 50mT ' ' -"-"~N

w

s

659C-3-3-130

uP

N

DOWN

oDec • Inc

659C-3-4-25

5

'~OmT

35

63mT

. t

W UP

l

NRN 37,S3 :0E-B A/M

~ , , 665-4

NRM [ 1 3 7 1 0 E - 6 A /M

u~/~ 665B-19 up/w665B-23

2o

_ _ N / N

NRM 508 ' tOE-6 A /M

665B-20

7

N/N ~/ 'N

Fig. 1. Demagnetization diagrams relative to samples (most of them with intermediate directions) from Holes 659C, 664D and 665B. The samples 665B-19 and 20 are stratigraphically consecutive and adjacent to the transition.

men per level was taken in 665B as the magnetiza- tion intensity of the sediment was lower.

Coercivities of the natural remanent magnetiza- tion (NRM) lower than 60 roT, blocking tempera- tures of the NRM of under 580 o C and saturation of isothermal remanent magnetization (IRM) at- tained below 0.2 T suggest that the magnetic mineralogy is dominated by magnetite in every section. Attempts to determine the range of grain size using the method of the low-field susceptibil- ity versus anhysteretic susceptibility (ARM) [16] are consistent with fine magnetite grains. The downcore variations in susceptibility and ARM do not show changes by more than a factor of 2 (see Fig. 3 and Valet et al. [13]). These results establish the uniformity of the magnetic properties along the transitional intervals.

The measurements were done in the shielded rooms of the CFR and Scripps paleomagnetic laboratories using LETI and CTF three-axis cryo- genic magnetometers. A total of 273 samples in- cluding all those located near or within the transi- tions were stepwise demagnetized by alternating fields (AF) and a procedure of double demagneti- zation used as a routine when anomalous direc- tions were observed between two successive steps; thermal demagnetization was also performed on selected samples. The remaining 231 specimens were demagnetized at three successive steps.

Typical demagnetization diagrams relative to the three sites are shown in Fig. 1. Samples from 659C and 664D have a soft component of magne- tization removed by fields between 5 mT (664D) and 15 mT (659C) and then a characteristic com- ponent decreasing linearly to the origin. Samples from 665B show also a secondary viscous compo- nent with a direction more or less close to the present field entirely removed by AF demagneti- zation at 15 roT. A total of 46 samples which yielded ambiguous directions (three of which were within polarity reversal) were rejected; no sample from 665B was rejected.

3. Paleomagnetic results

The directional changes obtained from 256 in- dividual samples at site 659C are plotted as vector means for each stratigraphic level in Fig. 2. Be-

373

cause the core was not oriented the declinations have been corrected by adjusting the mean normal value to 0 °. Using the same correction for the entire section, the mean reversed declination is 180 o. The declination changes associated with the reversal identified at 23.15 mbsf are characterized by an abrupt variation from 180 ° to 0 ° and a relatively gradual change in inclination. The mean normal (33 +_ 9.8 °) and reverse (29.7 + 7 °) in- clinations are in good agreement with the geo- centered dipole field inclination of 33 ° at the site latitude. The absence of data points within 23.37-23.44 mbsf is due to the lack of stable pseudo-single domain grains.

In Fig. 1 we show the demagnetization di- agrams associated with the uppermost reversed (665B-19) and the lowermost normal (665B-20) consecutive samples from Hole 665B. It is clear from these diagrams that no intermediate direc- tion was obtained. The complete record of the section shown in Fig. 2 is characterized by a sudden change in declination at 13.7 mbsf associ- ated with a large decrease of the magnetization intensity while the inclinations remain close to 0 o. The declinations associated with the reverse and normal polarities are perfectly antipodal.

The most detailed record was obtained from Hole 664D and is also plotted in Fig. 2 as vector means for each stratigraphic level. The inclina- tions are on average close to 0 o in agreement with the equatorial position of the site. The transition identified at 27.8 mbsf is characterized by easterly transitional declinations while the inclination val- ues remain close to 0°. The upper part of the U channel (80 cm above the transition record) show anomalously high inclinations with a general in- crease in scatter of the directions. These levels are also characterized by a change in sediment colour and they appear to be physically disturbed.

The magnetization intensities from Holes 664D and 665B have been normalized with respect to variations in the content of magnetic minerals by the anhysteretic remanent magnetization (0.1 mT direct field, 100 mT AF field) demagnetized at 20 roT. The N R M / A R M variations (Fig. 3) obtained at Hole 664D start decreasing before the direc- tional changes and reach a minimum of 20% dur- ing the reversal. The N RM intensity (after 20 mT) and the N R M / A R M variations plotted in Fig. 3 show identical variations. The N R M / A R M de-

374

9d

o o

i INCLINATION

6 6 5 B

- ; = H "~mJ~mlHr m

13.60

8 0

(50

4 0

2 0

0

- - 2 0

- - 4 0

INCLINATION

664D ~, ~J .I

DEPTH in METERS --80 13..70 13.80 273 27.7 28.1 28.5

DEPT t

I DEC LI NATION

180 I

ol 13,60 13.65 13.70

DECLINATION

METERS (~ ., , METERS

13.75 1380 27.3 27,7 28.1 28.5

8 0

6 0

,40

N 20 ~ o

- - 4 0

- - 6 0

- - 8 0

2 5 O

2 0 0

1 5 0

100

5 0

0

~ 5 o 2 3 •

659C

23`2

i

2 3 . 2 2 3 . 4

DE PTH t , a j

23.6 24

i i M E T E R S j

2 3 , 6 2 3 . 8 24 D E P T H

Fig. 2. Records of the declination and inclination variations across the Ma tuyama-Brunhes reversal obtained from ODP Holes 659C, 664D and 665B after stepwise demagnetization.

creases symmetrically around the polarity change represented by the latitude of the virtual geomag- netic pole (VGP) and exhibits a minimum syn- chronously with the directional jump.

375

4. Resolution and reliability of the records

Before discussing these results in terms of tran- sitional field behavior we must first investigate

NRM/ARM

6 6 4 D

i i J I

I

I

I

27.3 27.6 28 28.4 METERS'

665B

. . . . . V," , , , D E P T H

13.60 13.70 13.80

Fig. 3. Variations of the N R M (20 m T ) / A R M ( 2 0 mT) relative to the records from Holes 664D and 665B. The directional changes (represented as the V(3P latitudes) and the N R M changes relative to Hole 665B are shown also for comparison.

376

carefully the quality of these sediments as paleomagnetic recorders. The possible influence of post-depositional remanent magnetization (PDRM) on recordings of reversals have been discussed by several authors [17-19] and there is a common agreement among the studies that PDRM may introduce a smoothing of the transitional directions which could explain in particular the overall longitudinal confinement of the trajecto- ries. It is thus important to attempt to distinguish specific features which may be compatible with post-depositional processes from those inherent to the field behaviour.

A first approach to these questions is given by the record from Hole 665B characterized by an absence of transitional directions while the de- crease and the recovery of the magnetization in- tensity are symmetric around the polarity change. Such characteristics might be the consequence of a sedimentary hiatus. However, assuming that the N R M / A R M variations would then be representa- tive of the relative variations of the paleofield intensity, the relationship between the directional changes and the N R M / A R M plot shown in Fig. 3 would imply that the hiatus occurred exactly when the field direction was reversing. Although such an hypothesis cannot definitely be ruled out, the ab- sence of any visible discontinuity in the sedimen- tary column and the constancy in the mineralogy and chemical composition of the sediment suggest that no major break occurred.

Another hypothesis is that this record is the result of mixing of grains with opposite polarities within the individual samples due to post-deposi- tional reorientations. When the number of grains oriented along the normal polarity becomes pre- dominant the declination jumps abruptly from reverse to normal polarity. The downcore changes in the magnetization intensity reflect the varia- tions in the amount of grains with antipodal polar- ities within the samples. It must be emphasized that this interpretation implies a very rapid transi- tion at the scale of the deposition rate quite com- patible with the low value (1.5 cm/kyr ) obtained at Hole 665B. It is also required that both polari- ties be perfectly antipodal as seen effectively in Fig. 2. Therefore, the changes in the intensity of magnetization are symmetric with respect to the directional changes (Fig. 3) and the depth over which intensity variations are recorded corre-

sponds to the depth over which PDRM was active, i.e. 22 cm at Hole 665B. Interestingly, this result agrees quite well with values reported in the litera- ture based on laboratory experiments [20,21].

If we consider this hypothesis as valid in the case of Hole 665B we must also examine the influence of time averaging due to post deposi- tional reorientations on the other records. Based on the value of 22 cm obtained above for the depth of the lock-in zone we have thus examined possible effects of PDRM on a record similar to 664D characterized by a deposition rate between 3.8 and 4.4 cm/kyr . As the main question un- derlying this paper deals with the predominance of axisymmetrical components we used the field intensity and directional variations predicted by the model of Fuller et al. [1] for the Brunhes- Matuyama transition as seen from the equator by assuming that 20, 30 and 50% of the total energy of the dipole term are respectively redistributed to the gO, g03 and g4 ° Gauss coefficients. Fuller et al. point out that the form of the synthetic record depends on how much energy goes to each harmonic and their signs. Any other model in which only zonals are included could also have been choosen (as well as the most recent model developed by this group [7] combining low-order zonals with a drifting non-dipole field). A striking and common feature is that (with the exception of a pure octupole) they all are characterized by inclinations passing through + 90 ° or - 90 °.

We assumed that the process of remanence smoothing could be simulated by a weighted run- ning average with a Gaussian window (other shapes of window led to identical observations) performed on the vector data X, Y, Z individu- ally. The results and conclusions given below must thus be considered within the limits of this as- sumption. Variations of the inclination, declina- tion and intensity obtained by using successive windows with lengths of 7, 15 and 22 cm for a transitional interval 7 cm long are shown in Fig. 4a. The main feature is that the amplitude of the inclination peak of the original record remains totally unchanged while the intensity variations are spread over a larger zone and the transition is displaced downwards (wider windows lead to same characteristics). These results are the consequence of competing effects produced by averaging direc- tions restricted within the N - S plane. As the

377

(a)

l -56 -40 -24

.4

-56 -40 -24

cm - 10

DEPTH

-lO

360

(b)

27o DECLINATION

-40 -24 180

DEPTHincm ~10

Fig. 4. (a) Simulation of PDRM smoothing by a weighted running average with Gaussian windows of 7, 15 and 22 cm on a transition entirely dominated by axisymmetrical components. The inclination peak is persistent whatever the width of the window. (b) Same calculations as in figure a for a transitional field with zonal and non zonal components. The declination variations are shortened and the inclination changes still characterized by a significant peak.

declination can only be 180 ° or 0 ° the horizontal components are cancelled by averaging and conse- quently the vertical components are enhanced. The introduction of non-zonal components as (schematically) illustrated in Fig. 4b by a gradual westward rotation of the declination from 180 ° to 360 °, of course, limits these effects and smooths the inclination more effectively. The main feature, however, is that the smoothing does not become stronger for windows wider than 7 cm so that a significant peak of more than 40 ° in amplitude persist in the records. The declination variations appear shorter than on the original record.

It is also interesting that the duration of the transition is not significantly affected by PDRM especially if seen from the intermediate directions deviating by more than 30 ° from full polarity. On the contrary, the duration of the intensity changes is considerably enhanced and cannot be reconciled with the original variations if the window of smoothing is larger than the transition length. This result precludes any interpretation of the intensity variations from sediments when recorded with low deposition rates.

The main conclusion emanating from this simu- lation is that the pattern of directions characteris- tic for axisymmetric terms dominating during the transition could not be eliminated by the PDRM (if the smoothing in Nature occurs by a mecha- nism compatible with our calculations). In particu- lar, there is no way of observing significant devia- tions from the N - S axis (i.e. significant deviations of the declination from 0 ° or 180 °) if the transi- tional field remained confined close to this axis and the recording of vertical inclinations could not be smoothed out. Because the full polarity states are strongly dominated by zonal terms, these results suggest that zonal harmonics (for records of reversals obtained with low deposition rates) cannot be eliminated but rather advantaged by PDRM.

Since these calculations have been performed for a record similar to 664D we can now dis- tinguish whether zonal harmonics were dominat- ing the transition recorded at this site. We are well aware however that the absence of these compo- nents would not mean that the record was not affected by post-depositional processes.

378

5. Discussion

5.1. Inclination variations The equatorial position of Hole 664D is ideal

for testing the hypothesis of axisymmetry. Because the field must pass through the vertical, an equa- torial site would have the largest variation in inclination (from 0 o to either + 90 o or - 90 ° ) if quadrupolar terms are dominating. Octupolar terms or in a more general sense fields which are symmetric about the equator would have no change in inclination at the equator. Recently, Williams et al. [7] proposed a model combining low-order zonal harmonics with a drifting non-dipole field of intensity and drift similar to the present non-di- pole field. The authors show that the directional changes resulting from this configuration are con- trolled by the phase relationship of the variation of strength of the zonal harmonics and the drifting non-dipole field. However, high inclinations should still be observed at some time during the transi- tion in equatorial records, either related to the zonal terms or to the passage of features of the non-dipole field over the site. The absence of marked changes in inclination (which remained close to 0 °) during the transition in the record from 664D (Fig. 2) is thus inconsistent with the hypothesis that low-order zonal harmonics dominated. The alternative hypothesis, that an octupolar field family dominated the reversal may be difficult to account for since it requires the absence of any other component (in particular the declination should jump from 180 ° to 0°) . In- deed, the presence of other components even of weak intensity combined with an octupolar field

would induce variations of the inclination at the equator. It must be noted that no evidence of a perfect axisymmetrical field during a reversal has been reported so far.

Recently Merrill and McFadden [22] have pro- posed that the condition of occurrence of reversals is that the coupling between the dipole family (characterized by the sum of degree n and order m of the spherical harmonic being odd) and the quadrupole families (n + m = even) be critically enhanced at a time when the relative magnitude of the quadrupole terms is high with respect to the dipole family contribution. The present results do not seem to support the presence of zonal harmon- ics issued from the quadrupole family.

5.2. Duration of the transition Another interesting aspect of our data set is

based on a comparison of the duration of the reversal at various latitudes. Assuming for the moment that the transition of field is dominated by octupolar terms, durations of the reversal at equatorial latitudes should be significantly shorter than at mid-northern latitudes. Such a latitudinal dependency has been reported for the Matuya- ma-Brunhes reversal by Clement and Kent [10] from deep-sea cores of the Pacific Ocean. Their study dealt, however, with records with low sedi- mentation rates (1 c m / k y r and less) which were thus very sensitive to the criteria used in the determinations of the length of the transitional intervals as well as to variations in the deposition rate. Before calculating durations, therefore, it is useful first to examine the consistency between the lengths of the transitional intervals and the sedi-

TABLE 1

Durations of the t ransi t ion--calcula t ions of the transition duration from the lengths of the transitional intervals and estimates of the sedimentation rate associated with each record

Site Location Transition length (cm) Deposition rate ( cm/kyr ) Duration

min. max. min. max. (kyr)

Hole 659C 1 8 ° 0 ' N , 2 1 ° W 2 6 2.7 3.1 0.6-2.2 kyr d = 1.4 kyr

Hole 664D 0 ° 06 'N, 23 ° W 9 17 3.8 4.4 2.0-4.4 kyr

d = 3.2 kyr Hole 609B 49 ° 8 'N, 25 ° W 15 17 5.8 8.2 1.8-2.9 kyr

d = 2.3 kyr Lake Tecopa 35 ° 5 ' N, 116 o W 10 12 4.5 5.0 2.0-2.6 kyr

d = 2.3 kyr

UJ ~ E

t -

mentation rate. The boundaries of the transition zones may be defined by using such criteria as the level at which the directions exceed the maximal amplitude of the pre- and post-reversal variations or the occurrence of VGPs latitudes lower than 45 o or 60 o. In order to account for the uncertain- ties related to the use of different criteria we considered the minimum and maximum thickness obtained for each record using the various meth- ods (Table 1). We assume that differences between the values are representative of the error. Values of the deposition rates using the magnetostrati- graphic results obtained for the Brunhes chron and the Matuyama-Upper Jaramillo interval were both taken into account although they are similar.

We calculated durations for the data from Holes 659C, 664D, DSDP Hole 609B (all located along the same longitude between 0 ° and 50 ° N) and Lake Tecopa [13]. It can be seen in Fig. 5 that there is a good correlation between the lengths of the transitional zones and the deposition rates. Because no transitional direction was recorded at Site 665B, no reliable determination of the transi- tion length can be obtained although the results are consistent with a very short duration of the reversal. Within the limits of the errors described above, the best-guess durations obtained for the ODP Holes and DSDP Hole 609B (Fig. 6) do not show any dependency with site latitude and thus does not support the concentration that octupolar terms be dominant. The mean duration of the reversal is about 2.3 kyr and this value agrees also with the record of Matuyama-Brunhes from Lake Tecopa in California [13] obtained from a very different kind of sediment. This result indicates that the duration of Brunhes-Matuyama was very likely short. Our value differs by almost a factor of two with values usually reported in the litera- ture for other reversals [3,4,9,11].

We have shown above that the interpretation of the intensity changes is difficult. According to the results in Fig. 3 the dipole intensity (Hole 664D) would have been reduced for about 4-5 kyr (about twice as long as the transition defined by direc- tional changes) and this duration may be exag- gerated due to post depositional processes. Larger secular variation can be expected during periods of low intensity before and after the transition. Both declination and inclination records are effec- tively characterized by distinctive oscillations but

i i i • z

200 400 600 800 AGE in Kyr

0

-10"

- 2 0

- 3 0 "

- 4 0

-50"

-60

379

a

1000

D E P O S I T I O N

R A T E in cmlkyr

659C I

665B

l .1

T R A N S I T I O N L E N G T H

5 10 15 in cm

Fig. 5. (a) Age versus depth of the transition boundaries for every core. The sedimentation rates do not show major changes. (b) Relationship between the lengths of the transitional zones and the deposition rates. There is a good correlation between the data sets. The deposition rate corresponding with Lake Tecopa was deduced from the thickness of sediment between the Matuyama-Brunhes transition and the lower and upper limits obtained on the position of the upper Jaramillo from preliminary magnetostratigraphy (no estimate of the thickness relative to the Brunhes period can be obtained from the section).

the time resolution of the record as well as the number of directions associated with full polarity states are too short to draw a firm conclusion.

5.3. Rotated directions D', I ' and VGP paths We calculated the rotated directions D' , I ' [9]

and the VGP paths associated with each record from the declination, inclination data presented in

380

5i DURAT ION in Kyr

3 664D

2

l i

659C[

TECOPA

: 609B ±

,LATITUDE ~ 6 ° 2~ 5o~

Fig, 6. Plot of the durations calculated for each record of Matuyama-Brunhes with error bars versus site latitude. The duration associated with the record from Lake Tecopa is shown in dashed lines.

Fig. 2. The few intermediate directions at Hole 659C result in a poorly defined trajectory with three intermediate VGPs (latitudes > 30 ° from the rotational axis) in the northern hemisphere and no VGPs in the southern hemisphere (Fig. 7a). The equal area plot of the intermediate " ro ta ted" directions (Fig. 7b) show three inter- mediate directions restricted to the lower hemi- sphere with declinations between 125 ° and 180 °.

The VGP path recorded at Hole 664D (Fig. 8a) is described by 17 intermediate points as defined above distributed along a mean longitude 70 °E, i.e. 90 ° to the east of the site longitude. The D ' , I ' plots shown in Fig. 8b are characterized by 22 " intermediate" rotated directions with declina- tions close to the east axis.

5. 4. Geometry of the transitional field Any attempt to test possible field geometries

during the Matuyama-Brunhes reversal requires a large number of detailed paleomagnetic records. The last reversal can easily be identified (in princi- ple) in sedimentary sequences and consequently many distinct records have been published. In a review paper on reversals, a classification of rec- ords in terms of quafity A, B and C was published by Fuller et al. [1]. Owing to the subsequent developments in Paleomagnetism additional re- quirements (including stepwise demagnetization of all the transitional samples and analysis of the magnetic mineralogy) have been proposed [13]. We are aware that such criteria may be considered subjective and their use is a matter of debate. It is frustrating, however, that after selecting records consistent with these requirements,only the two

records of Matuyama-Brunhes from Lake Tecopa in California and that from ODP Hole 664D are retained. In fact most of the other studies are excluded either because the samples have not been stepwise demagnetized, the VGP paths are too poorly defined or the presence of a pr imary mag- netization is not clearly established.

The results obtained from Lake Tecopa and Hole 664D, considered separately, lead to differ- ent interpretations. The lack of inclination varia- tions as well as the pattern of the directional changes at Hole 664D indicate that axisymmetri- cal components were not recorded at the equator. In contrast the results from Lake Tecopa taken alone could be interpreted in terms of axisymme- try. If we deal strictly with these two records we are thus somewhat tantalized by ambiguity.

The record obtained from Hole 609B by Cle- ment and Kent [14] stands out from the rest of the studies because the transitional samples have been demagnetized at at least three steps. Another rea- son why we consider this record is that its location is at the same longitude as Hole 664D but much further north. Indeed, similar VGP paths are pre- dicted from sites located on the same longitude, if the transitional field is dominated by zonal terms. We observe in Fig. 9 that the two trajectories not only do not follow a common path but are (surprisingly) constrained along almost antipodal longitudes 90 ° west and 90 ° east from the sites meridian respectively. Moreover, a normal- t rans i - t ional -normal rebound is apparent on the 609B record which is not observed at the equator. Simi- lar observations are drawn from the equal area plots of the rotated directions D ' , I ' (Fig. 8) which clearly indicate that the discrepancy results from the declination variations rotating eastwards in the first case (664D) and westwards in the other case (609B).

We are thus faced with puzzling observations which could leave us somewhat suspicious as to the reliability of the present records. However, no artefact due to incomplete cleaning of secondary components of magnetization or problems linked to the magnetic mineralogy can be strongly sus- pected. We have also discussed above possible effects of PDRM on these records. The global appearance of the VGP paths (as, e.g., their longi- tudinal confinement) may be the consequence of smoothing effects and the influence of this factor

(cl) N

S

381

(b) 6 5 9 C

Fig. 7. VGP paths and rotated directions D', 1' obtained from ODP Hole 659C.

certainly cannot be underestimated but it is dif- ficult to imagine that antipodal transitional direc- tions would have resulted from smoothing if the field variations were identical at both Sites 664D and 609B. It is also difficult to explain why the structure of the directional changes would be so different. Obviously, high-resolution records are required to observe in detail the transitional field variations. Unfortunately, none of these records could be duplicated and this condition seems to be necessary in order to firmly establish the reliabil- ity of paleomagnetic studies of reversals. Such a requirement is however extremely difficult to be

met especially when dealing with marine cores from ODP protected by a severe (but maybe justified) sampling policy.

6. Conclusion

The present results give insights on field behav- ior during the last reversal by suggesting that zonal terms did not dominate during the transi- tional period. It seems that the characteristics of the records would be consistent with a com- plicated structure of the transitional field involv- ing harmonics of at least degree three but the

382

S

(b) 664D

(c) 6 0 9 B

Fig. 8. VGP paths and rotated directions D ' , I ' from ODP Hole 664D. For comparison we show also the D ' , f record obtained from DSDP Hole 609B.

383

120 180 60 •

664D

o ,

240 300 0

TECOPA 609B

6 0 ° N

o

' ' 6 o 6 0 ° S

Fig. 9. VGP paths relative to the records of Matuyama-Brunhes from ODP Hole 664D, Lake Tecopa and DSDP Hole 609B. The location of the sites is shown on the planisphere.

resolution and the number of records are too poor to draw firm conclusions.

In a paper related to a statistical model for geomagnetic reversals, Constable [23] suggests that during historical and archeological times the geo- dynamo exhibited complex field variations and there is no apparent reason for the field to be simpler during transitions. The model proposed is based on a statistical description of the geomag- netic secular variation over the last 5 Myr that has been extended to include reversals by introducing a time dependence so that any Gauss coefficient at any instant in time depends on its value at present time plus a random component. Reversals are simulated by allowing the axial dipole to decay and grow back with opposite sign while letting the rest of the secular variation continue as usual. Many of the features of reversal records are pre- dicted, such as looping and loitering in the VGP paths and a fairly rapid transition between north- ern and southern latitudes. There is no require- ment that the VGP path be related to the site longitude but appears to be located within a re- gion determined by the field configuration at the onset of the reversal and for any given reversal may vary from site to site. A model of interacting dipoles undergoing spontaneous reversals pro- posed recently by Mazaud and Laj [24] predict VGP trajectories not related to the site but mostly constrained in longitude. Such features could be compatible with the present results. Further tests of these models require a large number of detailed

records with a special attention to periods im- mediately preceeding the transition.

Acknowledgements

The paper benefited greatly from the reviewers comments (R. Merrill and two anonymous re- viewers). We are pleased to thank C. Laj for very valuable suggestions. We enjoyed discussions with R. Weeks and C. Constable. A. Mennel and D. Clark are acknowledged for help in measurements and technical assistance. The authors enjoyed dis- cussions with P. Tucholka, R. Coe, and K. Hoff- man. Support was provided for this study by NATO, the CNRS and the CEA for J.-P.V. and by NSF grants EAR 8515743 and Texas A & M 777348 to L.T.

This is CFR Contribution No. 1024.

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