Relative paleointensity of the Earth's magnetic field from marine sedimentary records: a global perspective

Physics of the Earth and Planetary Interiors, 56 (1989) 59—68 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands

59

Relative paleointensity of the Earth’s magnetic field from marine sedimentary records: a global perspective Lisa Tauxe Scripps Institution of Oceanography, A-020, La Jolla, CA 92093 (U.S.A.)

Jean-Pierre Valet Centre des Faibles Radioactivités, Laboratoire mixte CNRS—CEA, Avenue de Ia Terrassé, Parcdu CNRS, 91190 Cif/Yvette (France) (Received November 23, 1987; revision accepted April 15, 1988)

Tauxe, L. and Valet , J.-P., 1989. Relative paleointensity of the Earth’s magnetic field from marine sedimentary records: a global perspective. Phys. Earth Planet. Inter., 56: 59—68. To assess the potential of marine sediments as a source of relative paleointensity information, we have undertaken an investigation of selected box cores stored in the Scripps core repository. We present here new results from two North Atlantic box cores. Magnetic remanence in the sediments is carried by magnetite and is umvectorial during alternating field and thermal demagnetization. The median destructive field is 25—30 mT. Normalization by anhysteretic remanent magnetization yields relative paleointensity data that are highly concordant within and between cores. Age control provided by oxygen isotope data from both cores allows comparison of these relative paleointensity data with contemporaneous archeomagnetic data from Czechoslavakia. Both data sets indicate a peak in paleofield intensity at around 8—10 ka. This peak is less prominent in box core data from the western Pacific and global averages of archeomagnetic and lava flow data. However, the peak is evident in data estimating changes in production of radiocarbon, which is controlled in part by changes in the dipole moment of the geomagnetic field. The excellent agreement between production variations predicted by the changes in dipole moment and those observed suggests that long-term variations in atmospheric radiocarbon are dominated by geomagnetic field variations.

1. Introduction The intensity of the Earth’s magnetic field in the past was among the first subjects of paleomagnetic studies (e.g., Thellier and Thellier, 1959). However, with the dawning of the plate tectonic revolution, attention turned to the directional information necessary to create apparent polar wander paths or polarity stratigraphies. In addition to a shift in interest, a change in emphasis partly resulted from the fact that intensity studies are extremely time consuming and often yield results of poor quality. Complexities in remanence caused by chemical alteration or viscous behavior, as well as the need for precise dates,

severely limit the availability of suitable material for paleointensity analysis. Thus, studies of paleosecular variation of the intensity of the magnetic field have been pursued with some hesitation. The ideal records for paleointensity studies from a rock magnetic point of view are lavas and baked archeological materials that carry a stable total thermal remanent magnetization (TRM) as these can provide absolute paleointensity values. However, such material, by nature, gives only spot readings of the geomagnetic field; the resulting record is discontinuous at best and very sparse before about 12000 yr (BP) (McElhinny and Senanayake, 1982).

60

In contrast to volcanic or archeological records, sedimentary sequences have the advantage of providing more continuous and longer records. Under the best of conditions, sediments can provide a record of relative paleointensity; marine sediment cores also have the potential of providing a global view. Moreover, marine carbonates may be precisely dated using variations in oxygen isotope ratios which provide a high-resolution global chronometer for at least the Brunhes Chron (e.g., Prell et al., 1986). Sedimentary records are not without problems, however. The process by which remanence is locked in is only poorly understood and even under the best of circumstances, only relative intensity is credible. In this paper, we discuss criteria by which the paleointensity records from marine sediments may be assessed. We then present results from our investigation of box cores from the North Atlantic. We compare these with other paleointensity information obtained from different recording media.

2. Reliability of sedimentary paleointensity records It is well known that the intensity of magnetic remanence in laboratory-deposited marine sediments is linearly related to the applied magnetic field (Kent, 1973). If there are no changes in mineralogy, then, the remanent intensity record reflects relative paleointensity changes. If there are changes in the concentration of the remanencecarrying grains, these must be taken into account by using a normalization parameter. The appropriate method for normalizing sedimentary remanent intensity is the subject of some concern and has been eloquently discussed by Levi and Baneijee (1976). Low field bulk susceptibility and saturation isothermal remanent magnetization (sIRM) tend to reflect the concentration of coarser grains rather than the concentration of those responsible for the stable remanence (Johnson et al., 1975). For this reason, it is customary to use anhysteretic remanent magnetization (ARM) as a normalization parameter (Opdyke et al., 1973; Johnson et al., 1975; Levi and Banerjee, 1976) although ARM cannot be considered an analogue

(x)

to the actual process of magnetization in the case of sediments. Further insight into the rock magnetic aspects of paleointensity records in sediments was provided by King et al. (1983). They described several criteria that sediments should meet for the ARM normalization procedure to yield reliable relative intensity information. They pointed out that ARM is only appropriate for magnetite (or a similar mineral, such as titanomagnetite), that the remanence should be stable, carried by pseudo-single domain grains and that the concentration should not vary by more than a factor of 20 or 30. Thse criteria are necessary, but not sufficient to ensure data reliability. Many subtle factors (e.g., sediment flux, clay content, rate of dewatering, bottom current or bioturbation activity, to name a few) may affect the remanence of sediments; the effect of these may not be apparent from rock magnetic tests alone. Indeed, it is likely that each sedimentary package has a unique lock-in depth and degree of smoothing. For these reasons, we wish to stress the need to obtain multiple records, preferably from different recording media. In general, agreement among multiple specimens per level and multiple records per area lends credence to the reliability of paleointensity information. Such internally consistent local records may then be compared with contemporaneous records around the globe.

3. North Atlantic box cores To assess the potential of marine sediment cores as a source of global relative intensity data, we have undertaken an investigation of selected box cores in the Scripps core repository. These have been kept refrigerated and have not been allowed to dry out. Also, several target areas have associated piston cores, providing the possibility of extending the records back hundreds of thousands of years. Constable and Tauxe (1987) presented results from box cores taken on the Ontong—Java plateau in the western Pacific during the Eurydice expedition. They found substantial intra- and inter-core agreement. Their stacked record agreed reasona-

61 Table I Core locations and water depths Core

Water depth

Latitude

Longitude

280 34.4’N 29 °48.8’N

~

(m) INMD 42Bx INMD 48Bx

3774 2838

°21.4’W

43013.3~W

bly well with the relative paleointensity estimates from Australian lakes (Constable, 1985). There was overall agreement with the broad trends in the archeomagnetic paleointensity results compiled by McElhinny and Senanayake (1982), but significant differences were noted upon detailed comparison, The ‘global’ average is heavily influenced by data from Europe; it was thought, therefore, that cores taken from the North Atlantic would be more similar to the archeomagnetic data set. For this reason, we chose two box cores taken during the Indomed Expedition (Leg 2) for which oxygen isotope data were available (Berger et al., in preparation). Locations and water depths of our box cores are listed in Table I. Pains were taken by Wolf Berger on board the ship to assess the damage caused by the coring process, which is at times disturbed by rough sea-surface conditions. Each core was described and photographed immediately after recovery and subcores were taken in the least-disturbed portions of the boxes. INMD 42Bx was judged to be complete, but the top few centimeters were somewhat distorted. It is likely that the top of INMD 48Bx was lost during coring because photographs show that the pteropods characteristically abundant at the surface were washed into the lowest corner of the box core. The box corer does not always penetrate vertically; slight deviations ( a few degrees) are common. Therefore, deviations in inclination from the expected dipole inclination can be interpreted in terms of non-vertical coring as well as non-dipole contributions,

3. Methods Paleomagnetic specimens were prepared and analyzed using the techniques described by Constable and Tauxe (1987). A 2 cm slice was cut

from the center of a subcore from each box core. The center slice was then cut into horizontal strips 1 cm thick using an Exacto knife. These in turn were cut into three specimens, spanning 1 cm of stratigraphic thickness with a 4 cm3 volume. Each stratigraphic level from each core, then, was represented by three specimens. Duplicate specimens from every 5 cm were subjected to stepwise alternating (AF) and thermal demagnetization. The natural remanent magnetization (NRM) and the NRM after demagnetization to 10 mT were then measured on all remaining specimens. An ARM was imparted in a 0.1 mT steady field with a 100 mT peak alternating field. These were also measured before and after demagnetization at 10 mT. All remanence measurements were made on the CTF Inc. three-axis cryogenic magnetometer housed in the magnetically shielded room at Scripps. Bulk magnetic susceptibility was measured on a Sapphire Instruments SI-i susceptibility bridge. In general, our data are the means of from three to five separate measurements to assure reproducibility of such low susceptibilities.

4. Results 4.1. Magnetic mineralogy and stability of remanence The behavior of the Indomed specimens was uninformly excellent. Typical vector end-point diagrams for AF and thermal demagnetization are shown in Fig. la, b. The highest unblocking temperature of just over 550°C and the median destructive fields of between 25 and 30 mT suggest that pure or nearly pure magnetite is the carrier of magnetic remanence. The remanence is quite stable, decaying linearly to the origin. In Fig. ic, we plot ARM versus magnetic susceptibility. Based on this diagram, there is no significant change in grain size, and the ratio of the parameters is controlled primarily by concentration, which varies by less than a factor of three. The results shown in Fig. 1 therefore suggest that the Indomed sediments pass the minimum criteria butlined by King et al. (1983) and may retain reliable relative paleointensity information.

62 NORTH, UP

7~2

io

NORTH. IJ’

~iiIiiu

I

(a)

EAST

WEST

EAST

~i~iiIIIuI

(b) SOUTH, DOWN

SOUTH. DOWN 10

a. N

E 0 x

4.

2

0

I

0 (C)

2 4 6 8 SUSCEPTIBILIfl’ x 106 (SI)

I

10

Fig. 1. Summary of rock magnetic characteristics of the Indomed sediments. Upper diagrams are representative vector end-point diagrams for (a) alternating field and (b) thermal demagnetization. Stars are vector projections onto the horizontal plane and ovals onto the vertical plane. The cores were unoriented with respect to North. (c) ARM intensity versus bulk susceptibility.

Two further tests are: agreement among specimens from a given level and agreement between the two cores for a given age.

Results from magnetic remanence and isotopic analyses are shown in Figs. 2 (INMD 42Bx) and 3 (INMD 48Bx) as a function of stratigraphic thick-

63 INMD 428x —.8

INMD 48Bx —1.0

‘

—.4

.4

•

.8

.

.

lb 0

5

10

15

20

5

10

15

20

25

30

1.

0

~3.0

I

10

20

‘.

25

30

‘:;~Is::,

s;.,•I ‘

~200

I

I

I

I

I

5

10

15

20

25

z.C.

30

5

10

152025

•‘:‘

25

.30

30

•~I••I

0

5

10

15

20

25

30

25

30

•,•

~

0 z300

I

I

I

p

5

10

15

20

I

I

p

~

~200

‘

~

20

0 520

,:Is••ss2~:s~..~.~8.s;.

~300

15

:j40

520 C 0

10

.

z80 260

•

~40

5

,

I

15

~‘

lb

22.0

5

.

1.0

•

80 z ~60

•

.5

•

•

:‘~•

0

,

~.5

•

~100

~100. o

0 I

0

5

I

I

10 15 20 STRATIGRAPHIC POSmON (cm)

25

30

Fig. 2. Paleomagnetic and oxygen isotope data for INMD 42Bx. Isotopes were measured on G. ruber (pink) at SlO

0 0

5

10 15 20 SiRATIGRAPHIC POSmON (cm)

I

25

30

Fig. 3. Same as Fig. 2 for INMD 48Bx.

(Berger et al., in preparation).

4.2. Age calibration ness. The NRM vector results (after demagnetization to 10 mT) are shown as well as ARM intensity (also demagnetized to 10 mT) and oxygen isotopic data. It should be noted that the shapes of the magnetization curves before demagnetization are identical to those shown in the figures (after demagnetization to 10 mT). The remanence measurements for a given level are highly consistent in both cores, indicating good stability of remanence. The core-top disturbance of INMD 42Bx is reflected by a swing in declination over the top 4 cm. Measurements on these specimens have been removed from the relative intensity data set presented in the next section.

The oxygen isotopic records of the two cores may be compared with each other and with other, calibrated, records of the North Atlantic. Duplessy et al. (1981) termed the second, steeper portion of the Glacial—Holocene transition (Termination 1 of Broecker and Van Donk, 1970) step lb. We have identified lb in Figs. 2 and 3. The age of this deglaciation event is the subject of some debate (summarized by Berger et al., in press). Duplessy et al. (1981) and Berger et al. (1985) argued that the lb termination immediately follows the Younger Dryas climatic event in Europe. Stuiver et al. (1986a) compared varve counting, ice stratigraphy and 14~ dating to bracket the age of the

64

Younger Dryas event between 11 and 10.7 ka. Our discussion is not sensitive to small differences in age estimates and we have chosen an age of 10 ka for the lb termination, 4.3. Paleointensity Changes in intensity of the Earth’s magnetic field observed at a single location can be caused by changes in dipole tilt (Smith, 1967; Barton et aL, 1979). Such changes would be accompanied by changes in inclination and may therefore be identified in records with complete vector information. We have calculated relative virtual dipole moments (‘vdm’) by the formula ‘vdm’ = (NRM/ARM)(l + 3 cos2(lat)) where, lat = arctan(0.5 tan(Inc)).

—1 2

/

(1) We enclose the vdm in quotation marks to remind us that this is not an absolute value and may be scaled by a linear multiplication factor when compared with other records. Estimates of paleointensity based on NRM/ARM ratios and ‘vdm’ as calculated above are plotted in Fig. 4. These have been drawn such that the lb termina-

:~ 16

INMD 42Bx

.16 .12

~:.

~ ;;,

.02 .000

‘!:~~.

‘

.08

.04

10

.04

I

~:.IS,..; ;‘

I

o

20

~

I

10

5. Comparison with other paleointensity data To assess the reliability of our paleointensity record, we require a contemporaneous record in another recording medium from nearby. The oldest nearby record was obtained from Neolithic hearths in Czechoslovakia (Bucha, 1967; Bucha and Mellart, 1967). In Fig. 5 we plot the Czechoslovakian data. We also show our North Atlantic data (INMD 48Bx) for comparison. We have assumed that the top 2 cm were lost during coring and that the lb termination is 10 ka. These relative virtual dipole moment data have been multiplied by a

INII4D 428,

.

.12 .10

~:

20

tions are aligned on the page. There is not much variation in inclination down-core; hence there is little difference between the NRM/ARM data and the ‘vdm’ data. Nevertheless, we will use the ‘vdm’ data in further discussions. The data shown in Fig. 4 suggest that when plotted against a common time-scale, there is a great deal of similarity in the two records, indicating a peak in paleofield intensity immediately following the lb termination, probably around 8 or 9 ka. The upper portion of the INMD 42Bx record was eliminated owing to core disturbance, but the INMD 48Bx record suggests another upswing in intensity at the core top. It is unlikely that the core top represents the present, but how much time is missing is impossible to judge from core descriptions and photographs.

I

20

constant scaling factor for direct comparison with the absoluteto measurements. theuse conespondence be encouragingWe andfind may the scaling factor thus derived to convert our measurements to units of absolute intensity. We also

.20

INMD 488,

.20

P .16 • .

.12 .08

.16 ~‘:.;,:!.,. ,, .12

~

i~

will hereafter assume that the top 2 cm are mis-

INMD 48Bx

Ii,

.04

-

“:,.~

.04

‘ II’

08

I

0 10 20 30 STRATIGRAPHIC POSmON (cm)

‘~

III

0 10 20 30 STRATIGRAPHIC POSITION (cm)

Fig. 4. Plots of NRM/ARM and ‘vdm’ as calculated in the text for the INMD box cores. Arrows indicate the position of the lb termination shown in Figs. 2 and 3.

from the INMD 48our boxdata core.with We those are now in asing position to compare from around the globe. In Fig. 6 we plot the data from both INMD box cores against time. Plotted for comparison are the data from western Pacific box cores and Australian lakes discussed by Constable and Tauxe (1987). Also plotted are the, absolute paleointensity data compiled by McEllunny and Senanayake (1982). The data from the different sources over.

65

Pacific data (and may be the peak observed at the top of INMD 48Bx). The prominent peak in the

14 INMD 48Bx 12

•

10

European average at 10—8 ka (entirely controlled by the Czechoslovakian data) is also evident in the North Atlantic data as discussed previously. The 10—8 ka peak is more subdued in the

Czechoslovakia

• •

~

‘•

—

.•

~

.4

%

8

o

6

~

4 •

8

~

£ £

A A A

A

oa. 2

2

I 4 6 8 10 12 ThOUSAND YEARS BEFORE PRESENT Fig. 5. Comparison of archeomagnetic data from Czechoslovakia (Bucha, 1967; Bucha and Mellart, 1967) and INMD 48Bx.

lap in age and exhibit broad similarities. The well-documented peak in field intensity occurring between 3 and 2 ka is also seen in the western 14 12 ° io p~0~ P 8 ~ ~ 6

v GloXI averag, 0 European average 00

0

~

V

‘

~‘

V

02

a O _____________________________________ I I I I I 5 10 15 20 25 30

12

a•

~

Australian ERDC 830xlake, and lO2Bx

.u.

~

12 8

5

Oat

0

4 -

10

15

20

o

25

30

INMD ~aex

I

I

C O

5

the efficiency of shielding by the magnetosphere. Shielding of cosmic rays occurs at the edge of the

magnetosphere and thus is a function of the geomagnetic dipole moment. Changes in non-dipole fields are attenuated at the outer bounds of the magnetosphere (a result of upward continuing any potential field) and therefore cannot affect the shielding of cosmic rays, contrary to the suggestion of Bucha (1970). Elsasser et al. (1956) developed a model for the control of dipole moment on radiocarbon production

Q

=

I M

~

(2)

where Q is the production of radiocarbon, Q

• 0

globally averaged data and is barely noticeable in the western Pacific data. The question arises: is the 10—8 ka peak purely a non-dipole feature which has not been averaged out, or is it indeed a dipole feature as implied by the globally averaged data? In light of the sparse nature of the available absolute paleointensity data of this age, we must turn to another record of changes in dipole moment: variations in radiocarbon production. 14C in the atmoChanges the production sphere resultinfrom changes in of the flux of cosmic rays, either owing to fluctuations in cosmic rays reaching the Earth’s magnetosphere or changes in

10 15 20 ThOUSAND YLARS BEFORE PRESENT

25

30

Fig. 6. Summary of paleointensity data. The top curve is of the dipole moments compiled from archeomagnetic and lava flows (Mclilhinny and Senanayake, 1982). Relative intensity from marine sediment cores: western Pacific and Australian lake data are from Constable and Tauxe (1987) and the INMD data are from this paper.

0 is the present radiocarbon production, M is the 4C mo(redipole moment and M0 is the present dipole ment. variation atmospheric ferred The to astrue i~4C)can be in determined by ‘‘dating’ tree rings or lake sediments of known age; Stuiver et al. (1986b) compiled all such data for the past 10 ka. We may now compare the observed ~4C variations with those predicted if the variation is solely a function of geomagnetic field variations

(i.e., solar flux is constant and reservoir changes

are unimportant). Using the globally averaged (dipole?) moment data of McElhinny and Senanayake

66 15

curve (Fig. 7) using a decrease in NADW formation at 10 ka superimposed on sinusoidal variations m prouuction rate ~presumaoiy contronea oy —

observed’-~, 10

mo~1eI 0

I

I

2

4

6

8

10

the magnetic field). Andree et al. (1986) found no evidence for changes in ‘ventilation rate’ of the deep ocean in their high sedimentation rate cores. However, Berger (in press) re-interpreted the data of Andree et al. (1986) to support the hypothesis that ventilation rates have changed. The data shown here suggest that there is no need to call on changes in ventilation rates to explain the longterm trends in ~14C.

6. Implications for periodicity in paleointensity

THOUSAND YEARS BEFORE PRESENT

Fig. 7. Comparison of observed variations in 14C production (Stuiver et al., 1986b) and those predicted from the globally averaged data set of McEthinny and Senanayake (1982).

If we accept marine sediments as reliable recorders of relative paleomtensity, we may extend our understanding of the behavior of the Earth’s magnetic field intensity back to 25 ka in a more continuous fashion. Companion piston cores could provide a record of at least the past 500 ka. The North Atlantic and western Pacific box core data support the record pieced together by McElhinny and Senanayake (1982), as do the i~4Cdata of Stuiver et al. (1986b). There is no basis for the proposed periodicity in dipole moment of 8—9 ka proposed by Cox (1968). Longer periodicities of 40 ka suggested by Kent and Opdyke (1977) are not excluded, however. —

(1982) and eqn. (2), we calculate the L~4Cfor the past 10.5 ka. In Fig. 7 we show our resulting predicted variations as well as those observed by Stuiver et al. (1986b). Unless the correspondence is coincidence, we may take it to support both the contention that the iS.~4Cvariations are controlled primarily by geomagnetic field variations and that the McElhinny and Senanayake compilation is a reasonably accurate estimate of the dipole intensity variations. The 10—8 ka dip in production (presumably resulting from the higher dipole moment), although clearly evident, is somewhat less than predicted, suggesting that the global average could be slightly biased toward the European field values. Changes in concentration of 14~in the atmosphere are not only a function of changes in production, but are also influenced by changes in partitioning among the various reservoirs (see discussion by Suess (1970)). The ocean is by far the largest reservoir and atmospheric 14C is pumped down by the process of formation of Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) at high latitudes (see Broecker and Peng, 1982). Thus, changes in the rate of

deep-water formation could influence the concentration of radiocarbon in the atmosphere. Indeed, Keir (1983) was able to model the carbon

7. Conclusions (1) The sediments of the North Atlantic collected with a box corer on the Indomed expedition pass the rock magnetic criteria suggested by King et al. (1983) and may therefore retain a reliable paleointensity record. As a further test, the sethments display low within-level dispersion and, when placed on a common time-scale, excellent between-core agreement. These characteristics encourage us to suggest that the normalized remanent intensity record reflects relative paleointensity of the Earth’s magnetic field. (2) By assuming that the top several centimeters were lost during coring (an assumption con-

sistent with core photographs taken on board the

67

ship), the relative paleointensity record from our box cores agrees remarkably well with the archeomagnetic record from Czechoslovakia compiled by Bucha (1967). Furthermore, the peak in field intensity centered at 9 ka is supported by changes in l4~ concentration revealed by study of tree rings and lake sediment varves (Stuiver et al., 1986b). (3) If we consider the marine sedimentary record to preserve reliable paleointensity information, there is no support for periodicity in the Earth’s dipole moment with cycles of 8—9 ka as proposed by Cox (1968). Longer periods are not excluded, however. —

Acknowledgments We are pleased to acknowledge many helpful discussions with Wolf Berger, Catherine Constable, Juan Carlos Herguera, Robin Keir, and Devendra Lal, all of whom contributed in significant ways. This work was partially supported by NSF grant 0CE85-01578 to Wolf Berger and EAR8515743 to the author. Annette Mennell made most of the paleomagnetic measurements. Many thanks are also due to Wolf Berger for allowing us access to unpublished data and core photographs. Funds for paleomagnetic equipment were kindly provided by the W.M. Keck Foundation.

References Andree, M., Oeschger, H., Broecker, W., Beaven, N., Kias, M., Mix, A., Bonani, G., Hofman, H.J., Suter, M., Woéfli, W. and Peng, T.-H., 1986. Limits on the ventilation rate for the deep ocean over the last 12,000 years. Climate Dynamics, 1: 53—62. Barton, C.E., Merrill, R.T. and Barbetti, M., 1979. Intensity of the Earth’s magnetic field over the last 10,000 years. Phys. Earth Planet. Inter., 20: 96—110. Berger, W.H., 1987. Ocean ventilation during the last 12,000 years: hypothesis of counterpoint deep water production. Mar. Geol., 78: 1—10. Berger, W.H., Killingley, J.S. and Vincent, E., 1985. Timing of deglaciation from an oxygen isotope curve for Atlantic deep-sea sediments. Nature, 314: 156—158. Berger, W.H., Kilhingley, J.S. and Vincent, E., 1987. Time scale of the Wisconsin/Holocene transition: oxygen isotope re-

cord in the Western Equatorial Pacific. Quaternary Res., 28: 295-306. Berger, W.H., Killingley, J.S., Vincent, E and Whitman, J., in preparation. The INMD box core data, SlO Reference Becker, W.S. and Peng, T-H., 1982. Tracers in the Sea. Eldigio Press, Palisades, NY, 690 pp. Broecker, W.S. and Van Donk, J., 1970. Insolation changes, ice volumes and the 180 record in deep sea cores. Rev. Geophys. Space Phys., 8: 169—191. Bucha, V., 1967. Intensity of the Earth’s magnetic field during archeological times in Czechoslovakia. Archeometry, 10: 12—22. Bucha, V., 1970. Influence of the Earth’s magnetic field on radiocarbon dating. In: I.U. Olsson (Ed.), Radiocarbon Variations and Absolute Chronology. Almqvist & Wikssell, Stockholm, pp. 501—511. Bucha, V. and Mellart, J., 1967. Archeomagnetic intensity measurements on some Neolithic samples from çatal Huyuk (Anatolia). Archeometry, 10: 23—25. Constable, C.G., 1985. Eastern Australian geomagnetic field intensity over the past 14,000 years. Geophys. J. R. Astron. Soc., 81: 121—130. Constable, C.G. and Tauxe, L, 1987. Paleointensity in the pelagic realm: marine sediment data compared with archeomagnetic and lake sediment records. Geophys. J. R. Astron. Soc., 90: 43-59. Cox, A., 1968. Lengths of geomagnetic polarity intervals. J. Geophys. Res., 73: 3247-3260. Duplessy, J.C., Deigrias,warming G., Turon, Pujol, C. andAtlantic Duprat, J., 1981. Deglacial of J.L, the northeastern Ocean, correlation with the paleoclimatic evolution of the European continent. Paleogeogr., Paleodlimat., Paleoecol., 35: 121-144. Elsasser, W.M., Ney, E.P. and Wenkker, J.R., 1956. Cosmic ray intensity and geomagnetism. Nature, Johnson, H.P., Lowrie, W. and Kent, D.V., 178: 1975.1226. Stability of anhysteretic remanent magnetization in fme and coarse magnetite and maghemite particles. Geophys. J. R. Astron Soc., 41: 1—10. Keir, R.S., 1983. Reduction in thermohaline circulations during glaciation: the effect on atmospheric radiocarbon and CO 2. Earth Planet. Sci. Lett., 64: 445—456. Kent, D.V., 1973. Post-depositional remanent magnetization in deep-sea sediment. Nature, 246: 32—34. Kent, D.V. and Opdyke, N.D., 1977. Paleomagnetic field intensity variation recorded in a Brunhes epoch, deep-sea sediment core. Nature, 266: 156—159. King, J.W., Banei~ee,S.K. and Marvin, J., 1983. A new rock magnetic approach to selecting sediments for geomagnetic paleointensity studies: application to paleointensity for the last 4000 years. J. Geophys. Res., 88: 5911—5921. Levi, S. and Baneijee, S.K., 1976. On the possibility of obtaintag relative paleointensities from lake sediments. Earth Planet. Sci. Lett., 29: 219—226. McElhinny, MW. and Senanayake, W.E., 1982. Variations in the geomagnetic dipole 1: the past 50,000 years. J. Geomagn. Geoelectr., 34: 39—51.

68 Opdyke, N.D., Kent, transitions D.V. and Lowrie, Details of magnetic polarity recordedW., in a 1973. high deposition rate deep-sea core. Earth Planet. Sci. Lett., 20: 315—324. Prell, W.L., Imbrie, J., Martinson, D.G., Morley, J.J., Pisias, N.G., Shackleton, N.J. and Streeter, H.F., 1986. Graphic correlation of oxygen isotope stratigraphy application to the Late Quaternary. Paleoceanography, 1: 137—162. Smith, P.1., 1967. The intensity of the ancient geomagnetic field: a review and analysis. Geophys. J. R. Astron. Soc., 12: 321—362. Stuiver, M., Kromer, B., Becker, B. and Ferguson, C.W., 1986a. Radiocarbon age calibration back to 13,300 years B.P. and

14C age matching of the German oak and U.S. bristlethe cone pine chronologies. Radiocarbon, 28: 969—979. Stuiver, M., Pearson, G.W. and Braziunas, T., 1986b. Radiocarbon age calibration of marine samples back to 9000 cal yr B.P.. Radiocarbon, 28: 980—1021. Suess, H.E., 1970. The three causes of the secular ‘4C fluctuations, their amplitudes and time constants. In: I.U. Olsson (Ed.), Radiocarbon Variations and Absolute Chronology. Ahnqvist & Wikssell, Stockholm, pp. 594—605. Thellier, E. and Thellier, 0., 1959. Sur l’intensité du champ magnétique terrestre dans Ic passé historique et géologique. Ann. Geophys., 15: 285—376.