Earth and Planetary Science Letters, 87 (1988) 463-472 463 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

[41

The Matuyama-Brunhes transition recorded from Lake Tecopa sediments (California)

Jean-Pier re Valet *, Lisa T a u x e and D av id R. Cla rk

Scripps Institution of Oceanography, La Jolla, CA 92093 (U.S.A.)

Received September 1, 1987; revised version accepted December 17, 1987

The paleomagnetic record of the Matuyama-Brunhes reversal in the sediment of ancient lake Tecopa provided the first evidence that the geomagnetic field during transition is not dipolar.

We resampled the section to increase the time resolution of the record. Our new data show that the characteristic direction could not be completely isolated by AF demagnetization. The transitional directions obtained from thermal demagnetization are thus quite different from those of the previous study but the primary conclusion that the field was non-dipolar remains. The VGP paths lying close to the site longitude is consistent with an axisymmetric transition field.

1. Introduction

The work of Hillhouse and Cox [1] on the paleomagnetic record of the Matuyama-Brunhes reversal in sediments from the ancient lake Tecopa constituted a major step in our knowledge of the behaviour of the geomagnetic field during a rever- sal. These authors showed that the trajectories of virtual geomagnetic poles (VGPs) obtained for this transition in California and Japan [2] traced completely different paths, indicating that the transitional field is not dipolar. Hillhouse and Cox also observed that the dominant direction of the non-dipolar transitional component was parallel to the present non-dipolar field. Furthermore, it had the same direction during an excursion which occurred prior to the reversal. They interpreted these results in terms of the standing field model which invokes the predominance of a standing portion of the non-dipole field during the main part of the transition while the axial dipole field decays to zero and then grows back in the oppo- site sense.

* Permanent address: Centre des Faibles Radioactivitrs, Laboratoire mixte CNRS-CEA, Avenue de la Terrasse, Parc du CNRS, 91190 Gif-sur-Yvette, France.

0012-821X/88/$03.50 © 1988 Elsevier Science Publishers B.V.

Since this classic study, a large number of records of the Matuyama-Brunhes transition have been obtained from sediments cored from the deep sea or sampled on land [3-5]. Many of these are poorly defined by very few intermediate direc- tions. In fact Tecopa may still be regarded as one of the most detailed studies of the last reversal. However, improved instrumentation and advances in techniques have made possible a considerable increase in the time resolution of the record. It was with this in mind that we resampled the Tecopa section. In addition, we hoped to obtain a detailed record of field variation before the transi- tion in order to assess the relationship of the transitional field to secular variation.

2. Geologic setting and sampling

Lake Tecopa is located just to the east of Death Valley about 100 km west of Las Vegas. The geologic setting of the lake deposits has been extensively described by Sheppard and Gude [6] and also summarized by HiUhouse and Cox [1]. Lake Tecopa existed in the Middle Pleistocene during which time mudstone and sandstone inter- bedded with tuff layers accumulated until 500,000 years ago. Three diagenetic facies controlled by

464

the chemistry of the pore water are characterized by increasing degrees of zeolitic alteration (prin- cipally in the tufts) from the outer margin to the inner part of the basin. Paleomagnetic sampling was thus concentrated in the outer part of the basin.

Three successive ash layers are visible throughout the basin, the second being identified [6] as the Bishop Tuff on the basis of its petro- graphic and chemical properties. The Bishop Tuff is a rhyolite ash flow exposed along the east side of the Sierra Nevada between Bishop and Mono Lake over approximately 350 square miles. It was erupted from an area which then collapsed to form the Long Valley Caldera at the eastern base of the Sierra Nevada. The K-At age for the Bishop Tuff obtained by Dalrymple et al. [7] is 0.7 Ma. The results of a paleomagnetic study performed on 103 samples from 5 localities indicated that the Bishop Tuff has a normal polarity. The Bishop ash is thus a very useful stratigraphic marker of the Matuyama-Brunhes transition where it occurs.

After investigation of the outer margin of the basin, the freshest outcrops were found at the same location of the southwestern corner where Hillhouse and Cox collected their transitional samples. In this part of the section fluvial erosion has cut fresh vertical walls about 15 m high com- posed of well-consolidated massive tan siltstone. Because the sediment was not appropriate for core drilling we followed a similar procedure to the one described by Hillhouse and Cox [1]. Several successive columns of sediment were sculpted from the base of the Bishop Tuff and at least two or three flat oriented faces shaved on each block for orientation. A total of 19 oriented blocks were collected in this manner covering a total thickness of 2.4 m. In the laboratory vertical columns were cut from each block using a diamond saw and then indurated with an ammine derivative epoxy glue. Finally, 420 sIices of sediment 0.5-1 cm thick were cut from the columns using a Buehler Isonet low-speed saw equipped with a very thin blade and using oil as a coolant. The final orienta- tions of the samples obtained by this method are probably less accurate than samples obtained from drilled cores.

However, we can easily estimate the degree of accuracy by comparing the directions of strati- graphically twin samples obtained from adjacent

columns. We can also check whether a significant error has been introduced during the preparation of the columns since it would be reflected by systematic jumps in the directions. Such scrutiny reveals reproducibility of directions to within a few degrees.

3. The magnetic mineralogy

Magnetic separates prepared by Hillhouse and Cox [1] revealed that the magnetic mineralogy of Tecopa siltstones is composed of magnetite, mag- hemite and a lesser amount of hematite. Further analyses were conducted by us on a series of specimens regularly spaced over the sampled in- terval. Stepwise acquisition of the isothermal re- manent magnetization (IRM) (Fig. 1) indicates that 95% saturation is attained after 0.3 T as expected from magnetite. The median destructive

M / Mmax 1 - - , -e

.8 SAMPLE TIB3

.6

.4

.2

TESLAS

.I .2 .3 .4 .5 .6 .7 .8 .9 I

Fig. 1. Typical diagram of IRM acquisition showing that 95% of the saturation is attained after 0.3 T as expected for a mineralogy dominated by magnetite.

2 0 0

"801 ~ . 160 !

E 14-0; ' o

._~ 120

100

8o 8

4 0

2 0

O-

2urn

, , ' " ' " .~, °1"- , ,-~lOum

J L J b

2 4 6 2 3 8 10 SUSCEPTIBILITY in 10- m / k g

Fig. 2. Plots of the anhysteretic susceptibility as a function of the low-field susceptibility relative to samples distributed across the section. The grain sizes are within the pseudo-single do- main range.

fields of the SIRM between 20 and 25 mT as well as its maximum blocking temperatures of 580-590°C are also consistent with a magnetic mineralogy dominated by magnetite. Total satura- tion not completely attained after 0.3 T but usu- ally at 0.5 T may reflect the presence of a fraction with a higher coercivity which could be associated with a second mineral (as e.g. hematite) present in very small amounts.

Following the technique developed by King et al. [8] the approximate size range of the magnetic grains was estimated from the plots of the low-field susceptibility versus the susceptibility of the anhysteretic remanent magnetization (ARM). These plots reported in Fig. 2 are consistent with grain sizes between 2 and 10 #m.

4. The paleomagnetie directions

The paleomagnetic measurements were made with a CTF three-axis magnetometer in the shielded room of the Scripps Paleomagnetic Laboratory. The natural remanent magnetization of the samples (NRM) is of the order of 10 -1 A / m . AF and thermal demagnetization were car- ried out on two series of twin samples. AF de- magnetization was performed by steps of 4 -5 mT and beyond 20 mT a procedure of double de- magnetization was used routinely. For the major- ity of the samples an apparently coherent direc- tion was isolated beyond 20-25 mT. However, the demagnetization performed beyond 40-50 mT led to quite scattered directions and large intensity jumps so that no reliable direction could be iso- lated.

Thermal demagnetization was performed by steps of 3 0 - 5 0 ° C from room temperature to the limit of reproducible results, usually about 500 ° C. The low-field susceptibility measured at each step of demagnetization remains constant between room temperature and 360 °C and then increases slowly up to a factor of two culminating at 540 ° C. Because the magnetic susceptibility is proportional to the total induced magnetic moment of the samples and not strictly related to the remanent magnetization, it is difficult to determine to what extent changes of this parameter are connected to the fraction of grains carrying the remanence.

The ARM can be considered as the most ap- propriate parameter involving similar grain popu- lations as the detrital remanent magnetization

465

(DRM) (provided that magnetite has been identi- fied as the main cartier [9]. A complementary investigation was thus carried out on a series of 6 samples. ARMs (imparted in a 0.05 mT D.C. field and 50 mT AF field) were first measured, then demagnetized partially by heating at 100°C and finally completely demagnetized by AF. This en- tire procedure was repeated at successively higher temperatures. Mineralogical changes should thus be reflected by variations of the coercivity spectra of the grains carrying the ARM measured after each heating. No change of the ARM intensities was detected during the experiment. It is therefore reasonable to conclude that the mineralogical frac- tion carrying the remanence remains unaffected by heating.

The typical thermal demagnetization diagrams in Fig. 3a show the presence of two components of magnetization with overlapping unblocking temperatures.The first one directed along the pres- ent geomagnetic field is mostly removed by heat- ing at 300-350 ° C but is sometimes resistant up to 400 ° C. Beyond these steps the characteristic com- ponent is well defined by a trajectory decreasing linearly to the origin and having a maximum blocking temperature of 580-600 ° C. A significant comparison of the results obtained by both meth- ods of demagnetization is illustrated by the di- agrams of Fig. 3b obtained from two adjacent samples. The sample demagnetized by AF dis- plays a single linear component with a normal polarity. In the second case a reversed direction is isolated after heating to 400 ° C. This comparison repeated for many twin samples led to the conclu- sion that the very strong overprint is incompletely and sometimes not removed at all by AF. Conse- quently, at this stage of the study we found it preferable to rely on thermal demagnetization.

A first set of 15 samples, most of them located at the intermediate and reversed levels defined by Hillhouse and Cox was thus stepwise AF demag- netized up to 15 mT and then stepwise thermally demagnetized. The rest of the samples (188 speci- mens) was only stepwise thermally demagnetized. One hundred and twenty specimens, including all the transitional samples were demagnetized by steps of 20-50 °C and the remainder (mostly of reverse polarity) was demagnetized at three or four successive steps. The final paleomagnetic di- rections were determined by fitting a line through

466

~T, UP

£*Sl (a)

SOUTH

WEST, UP

.¢.

F.~T, DOWN

T H E R M A L =~'r. up

SOUT~ I

~sr. uP A. I-.

~0 20roT ...._~m_.~

E~ST 0o~ 1

T13 IOA ~ - (13) T13 11A F.J~. Dot~

Fig. 3. (a) Typical thermal demagnetization diagrams for samples with normal and reversed polarities (1 = 100°C, 2 = 200°C , 3 = 300 ° C etc.). (b) Demagnetization diagrams corresponding to two distinct samples located at the same stratigraphic level demagnetized by AF and by heating respectively. The strong overprint is not removed by AF and no coherent direction is isolated beyond 45-50 mT. Symbols: * = horizontal component, O = vertical component

467

the last points of the demagnetization diagrams. No reliable direction could be obtained from 15 of the demagnetization diagrams.

5. Characteristics of the transition

5.1. Directional variations Declination and inclination records obtained

from these different sets of samples extending to 120 cm below the Bishop Tuff are shown in Fig. 4. For comparison we have plotted the paleomagnetic results obtained by Hillhouse and Cox [1] (trian- gles) after AF demagnetization in a peak field of 15 mT together with the directions obtained in the present study from the 45 samples treated at the same AF level.

8 0

60

4 0

o

~ - 2 0 z

- 4 0

- 6 0

LAKE TECOPA

--8O I

o ='o ,'o 6'o 6'o 1 oo' 1 ,o S T R A T I G R A P H I C POSITION

40

3 5 0

3 0 0

2 5 0

2 0 0 1 80

15,0

~ 100 Z

J 5o

o

Zk HILLHOUSE &COX (1976) A F = 15mT

a This paper . . . . . . . . . . . . . AF = 15mT

• This paper . . . . . . . . . . . . . THERMAL

~ . = .

3100 2~0 40 ; 0 ; 0 ' J 1 O0 120

S T R A T I G R A P H I C POSITION

Fig. 4. Variations of the declination and inclination obtained from two distinct sets of samples treated by distinct methods of demagnetization and compared with the record obtained by HiUhouse and Cox [1]. The "intermediate" AF directions are clearly associated with reversed polarities isolated by thermal demagnetization. The stratigraphic positions of the samples are measured from the base of the Bishop Tuff.

468

There is rather good agreement between these two sets of samples suggesting that the transition occurs between 55 and 75 cm below the Bishop Tuff. However, the paleomagnetic directions ob- tained from thermal demagnetization in this inter- val are unambiguously reversed; clearly, the previ- ous "intermediate" directions were caused by the superposition of a normal overprint present in different amounts. Each intermediate direction published by Hillhouse and Cox represented the averaged measurements of 5 separate specimens. As the authors noted themselves there was a large scatter between the individual vectors which, in light of these new results, truly reflected different degrees in the resistance of the overprint.

The paleomagnetic record of the Matuyama- Brunhes transition, based on thermally demag- netized specimen directions, occurs 10 cm higher

80 .~. . . ~'-'.

-'-I- " " " :'i~ " " 60

4o

2o

0 • . ¢.) z ° • - - - - 20 ' . " , . . i - °

.: :...'.%-o ..~ .

--40 2 " "~- s."

- or i . . . . .

150 200 250 300 3~0 400 DECLINATION

Fig. 5. Plot of the inclinations versus declinations. The dashed lines correspond to the values expected at the site latitude for a reverse or normal dipole field. The intermediate inclinations are associated with reverse declinations. Such directions are difficult to explain by incomplete removal of the overprint because this situation would probably have produced inter- mediate values for both components.

6 O

40 A ~ A

- -20 1 & % - -40 - -

- -60

- - 8 0 i = i

0 510 100 150 200 C m

i

2 5 0

4 0 0

3 5 0

~) 3 O O

~-~ 250

A A

A

2 0 o

Cm 150 I J

0 50 1 O0 150 200 250

STRATIGRAPHIC POSIT ION

Fig. 6. Complete record of the declination and inclination obtained with stepwise thermal demagnetization of more than 200 specimens. For comparison, the triangles represent the results which would have been obtained from AF stepwise demagnetization. The stratigraphic positions are measured from the base of the Bishop Tuff.

10

8

~e

m 4.

2

•'o ' ' '

O0 1 O0 1 5 0 2 0 0

C m i

250

469

1 2 0

1 O0

E 8 0

~ ~o

~ 4 0

2 0

"10-2~Kg-,

~ 0 i i i 1 0 0 1 5 0 200

Cm, 250

1 8

1 6

1 4

12

8

6

4

2

0 0

lO-'m3KgJ

510 I I I

1 O0 1 5 0 2 0 0 S T R A T I G R A P H I C P O S I T I O N

C m i

2 5 O

Fig. 7a. Variations of the low-field susceptibility, ARM and SIRM as a function of the stratigraphic depth from the Bishop Tuff. The constancy and overall similarity of these parameters establish that the grain size distribution is uniform.

3 . 0

~ 2 . 5

: z 2 . 0

o~ 1.5

~1.0

i. t I " . I i. "t i ' . l i "1 r , i i " . l ,. ,,

I I I .0 i i I Cm i 0 50 1 O0 150 200 250

S T R A T I G R A P H I C P O S I T I O N

Fig. 7b. Variations of the ratio DRM (300 o C)/ARM(300 o C). This plot represents our best approximation of the field variations related to the reversal. Intervals of low intensity precede and follow the transitional directions (indicated by hatchured zones).

470

than the previous results, at only 45 cm below the base of the Bishop Tuff. At these intermediate levels the variations in declination and inclination cannot be compared to the former record; in particular the declination rotates westward (not eastward) and the inclination is characterized by high positive values. Another characteristic is the sharpness of the transitional zone which extends over 10-12 cm.

In order to constrain the duration of the transi- tion, we took an additional 11 sites spanning from 524 to 1326 cm below the Bishop Tuff from a nearby section. Two normal zones were identified below the Matuyama-Brunhes transition interval which we correlate to the Jaramillo and Cobb Mountain subchrons. The upper boundary of the JaramiUo is constrained to fall between 864 and 959 cm and the lower between 1159 and 1242 cm. The Cobb Mountain subchron was found at 1346 cm below the Bishop Tuff. Given the wide sam- piing interval of about a meter, we estimate the sedimentation rate to be between 3 and 5 cm per thousand years implying a duration of between 2000 and 4000 years for the transition interval studied here.

Despite the sharpness of the transition zone large fluctuations occur on a very short time scale. Several fluctuations are defined by successive di- rections and therefore are certainly not related to any kind of random noise. Some of these varia- tions could result from an incomplete cleaning of the very strong overprint. However, as it can be seen in Fig. 5, most of the positive inclinations are associated with reverse declinations and it is dif- ficult to envisage a mechanism by which the over- print would be selectively demagnetized along the horizontal component. I t is much more likely that in many cases strong vertical components resulted in poor definition of the horizontal components. However, despite these arguments we are well aware that in some cases it is difficult to dis- tinguish to what extent these fluctuations are truly related to large and sudden directional jumps of the geomagnetic field.

The complete declination and inclination rec- ords extending to 2.40 m below the Bishop Tuff are shown in Fig. 6. No significant secular varia- tion is apparent preceding the transition. The mean value of the inclination (55 ° ) agrees with the value of the geocentered dipole at this latitude,

although the zone between 130 and 180 cm is associated with lower values. No evidence of the excursion reported by Hillhouse and Cox 2.15 m below the Bishop Tuff from a site 2.5 km of Soshone has been found. However, in this part of the section, no reliable samples could be obtained from the unconsolidated sediment and there is a small gap of 20 cm in our record.

5.2. The relative variations of the field intensity Estimates of the relative paleointensity changes

can be obtained by normalizing the variations in the concentration of magnetic minerals carrying the remanence.

King et al. [10] have shown that the ARM can be used as a suitable parameter only if the mag- netic mineralogy is dominated by magnetite pres- ent in pseudo-single domain grain size range and if its concentration does not vary by more than a factor of 20-30. As discussed above, magnetite has been identified as the main magnetic carrier of Tecopa sediment and there is no large variation of the grain sizes which are between 2 and 10 ~m. Moreover, the amount of magnetic material does not change by more than a factor of two as illustrated by the variations in susceptibility, ARM and SIRM (Fig. 7a) over the sampled section. Therefore, we tentatively conclude that this sedi- ment is appropriate for paleointensity estimates.

Assuming that characteristic and reproducible intensities of the D R M have been obtained after thermal demagnetization at 300 o C, the variations of D R M (300 o C ) / A R M (300 ° C) represent our best approximation of the field intensity changes related to the reversal (identical results were ob- tained by using D R M (400 o C ) / A R M (400 o C)). This ratio plotted in Fig. 7b as a function of the stratigraphic height suggests that the field inten- sity was reduced to about 10-20% of its original value. Within the limits of this normalisation, there is also evidence that this period of low intensity started earlier and lasted longer than the directional changes. There is an additional period of low intensity which corresponds to the interval associated with low inclinations between 130 and 180 cm.

6. Discussion and conclusion

The VGP positions shown in Fig. 8 have been calculated from the paleomagnetic directions ob-

NORTH

471

Fig. 8. VGP path plotted along Tecopa longitude. The overall confinement of the VGPs along the site meridian is expected from an axisymmetric transitional field.

tained after thermal demagnetization. The transi- tion is well described by more than 20 VGPs at intermediate latitudes and many rotated directions lying more than 30 o away from the axial dipole direction.

The VGP path, confined inside a narrow band of longitude centered on 220°E, differs signifi- cantly (by 120 o) from the path obtained by Hill- house and Cox [1] but it does not overlap with the path obtained by Nii tsuma [2] from the sediments of Boso Peninsula in Japan. Therefore, the main conclusion that the transitional field is not dipolar is not affected by these new results. On the basis of this observation, the very close proximity of the VGPs to the site longitude a n d / o r the rotated directions [11] lying in the vertical plane contain- ing the N-S direction and the axial dipole field direction indicate that the transitional field was strongly axisymmetrical.

Any further investigation of the field geometry requires a comparison of the Tecopa record with other detailed records of the Matuyama-Brunhes reversal obtained from a broad geographic distri- bution of sites.

Before attempting such a test it is necessary to set up criteria so that only very reliable records can be used. In a review paper, Fuller et al. [3]

characterized the paleomagnetic studies of rever- sals by the number of intermediate directions of the records. In light of the results obtained in the present paper we feel that there is a crucial need to define new criteria which would deal mainly with experimental precautions.

First of all, complete stepwise demagnetization should be performed on all the transitional sam- pies (not just pilot specimens). Any study should be accompanied by an investigation of the mag- netic mineralogy through usual standard methods such as the I R M acquisition test. Furthermore, a minimum number of four intermediate directions should be required to obtain a reliable description of the field characteristics.

Unfortunately, to our knowledge, none of the records published so far meets these three criteria, mainly because pilot samples were used in most of the studies. Faced with this problem, we feel that it is hazardous to at tempt any analysis of the data from the existing records of the Matuyama- Brunhes reversal.

Acknowledgements

The authors are pleased to acknowledge A. Mennel who helped with measurements and R. Myers who provided technical assistance for cut-

472

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m a n u s c r i p t . S u p p o r t fo r th i s s t u d y w as p r o v i d e d

to J .P.V. b y N A T O , t he C N R S a n d t h e C E A a n d

b y N S F g r a n t E A R 8515743 to L.T.

References

1 J.W. Hillhouse and A. Cox, Brunhes-Matuyama polarity transition, Earth Planet. Sci. Lett. 29, 51-64, 1976.

2 N.D. Niitsuma, Paleomagnetic and paleoenvironment study of sediments recording Matuyama-Brunhes geomagnetic reversal, N. Tohuku Univ. Sci. Rep. 2nd, 43(1), 1-30 (1971).

3 M. Fuller, I. Williams and K.A. Hoffman, Paleomagnetic records of geomagnetic reversals and the morphology of the transitional field, Rev. Geophys. Space Phys. 17, 172-203, 1979.

4 B.M. Clement, D.V. Kent and N.D. Opdyke, Brunhes- Matuyama polarity transition in three deep-sea cores, Philos. Trans. R. Soc. London, Ser. A 306, 113-119, 1982.

5 F. Theyer, E. Herrero-Bervera and V. Hsu, The zonal harmonic model of polarity transitions: a test using succes- sive reversals, J. Geophys. Res. 90(B2), 1963-1982, 1985.

6 R.A. Sheppard and A.J. Gude, Distribution and genesis of authigenic minerals in tufts of Pleistocene Lake Tecopa, Inyo Co., California, U.S. Geol. Surv. Prof. Pap. 597, 1968.

7 G.B. Dalrymple, A. Cox and P.R. Doell, Potassium-argon age and Paleomagnetism of the Bishop tuff, California, Geol. Soc. Am. Bull. 76, 665, 1965.

8 J.W. King, S.K. Banerjee, J. Marvin and O.A. Izdemir, comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials. Some results for lake sediments, Earth Planet. Sci. Lett. 59, 404-419, 1982.

9 S. Levi, S.K. Banerjee and J. Marvin, On the possibility of obtaining relative paieointensities from lake sediments, Earth Planet. Sci. Lett. 29, 219-226, 1976.

10 J.W. King, S.K. Banerjee and J. Marvin, A new rock magnetic approach for selecting sediments for geomagnetic paleointensity studies: application to paleointensity for the last 4000 years, J. Geophys. Res. 88, 5911-5921, 1983.

11 K.A. Hoffman, A method for the display and analysis of transitional paleomagnetic data, J. Geophys. Res. 89, 6285-6292, 1984.