Origin of stable remanent magnetization of siliceous sediments in the central equatorial Pacific

Earth and Planetary Science Letters, 105 (1991) 81-93

81

Elsevier Science Publishers B.V., A m s t e r d a m

[MK]

Origin of stable remanent magnetization of siliceous sediments in the central equatorial Pacific Toshitsugu Y a m a z a k i a, I k u o K a t s u r a b a n d K a t s u m i M a r u m o a,1 a Geological Survey of Japan, Tsukuba, lbaraki 305, Japan b Department of Geology and Mineralogy, Kyoto Unioersity, Kyoto 606, Japan Received November 13, 1990; revision accepted April 10, 1991

ABSTRACT Siliceous sediments distributed in the central equatorial Pacific have a stable remanent magnetization regardless of age or depth. This is in contrast with the unstable remanence of unfossiliferous red clay (defined as "pelagic clay" here) which accumulates in the middle latitudes of the Pacific. We have conducted a rock-magnetic study of the siliceous sediments to clarify the magnetic carriers of the stable remanence. For this purpose, we combined magnetic granulometry by the suspension method and observation of magnetic grains with a transmission electron microprobe (TEM). The advantage of the former is that the magnetic extraction procedure is not required for obtaining size distribution of the magnetic grains in sediments. The T E M observations revealed that most of the magnetic extracts are identical in size and shape to bacterial magnetosomes. The suspension method proved that the magnetic assemblages of the siliceous sediments of both Quaternary and early Miocene age have mean diameters of about 0.05 /~m, which is within the single-domain range of magnetite, and have narrow size distributions compared with pelagic clay. These characteristics can be explained if the magnetofossils observed by T E M are the major constituent of the magnetic grains in the sediments, and have been preserved for a long period of time. The difference in the stability of the remanent magnetization between the siliceous sediments and the pelagic clay can be explained by the difference in the size distribution of magnetic grains, which would reflect differences in their sources, i.e., biogenic vs. detrital (eolian).

1. Introduction

In the 1960's, deep-sea sediments were found to preserve reliable records of the earth's magnetic field [1,2]. Since then, studies of paleomagnetism in deep-sea sediments have yielded many fruitful results in establishing the history of magnetic polarity reversals, understanding the behavior of the geomagnetic field at the polarity change, and studying the time-averaged geomagnetic field. Siliceous sediments distributed in the biogenically highly productive province along the equator in the Pacific (Fig. 1) played a particularly important role in sediment paleomagnetic studies [3-7] not only because precise ages can be determined through microfossils independently of

1 Present address: Department of Geology, University of Toronto, Ont. M5S 3B1, Canada. 0012-821X/91/$03.50

© 1991 - Elsevier Science Publishers B.V.

the magnetostratigraphy, but also because the sediments have stable remanent magnetization regardless of their age. The sediments only suffer slight viscous overprinting, which can be easily cleaned by the alternating-field (AF) demagnetization of 5 to 15 mT [4,5,8]. Rock-magnetic studies of these sediments are still scarce, and the carriers of the stable remanence or their grain size distributions have not been well characterized. The stability of the remanent magnetization of the siliceous sediments contrasts with the unstable remanence of unfossiliferous red clay (defined here as "pelagic clay") [9-12], which accumulates very slowly in the middle latitudes of the Pacific (Fig. 1). Pelagic clay older than late Pliocene (deeper than tens of centimeters to several meters below the surface) usually has unstable remanence of viscous origin, which is caused by hyperfine superparamagnetic (SP) grains [12,13]. This suggests that the carriers of the remanence of the siliceous

82

T. Y A M A Z A K 1 E T A L

120°E

180 °

120°W

60°N

oO

60°S

7-3

Pelagic clay (red clay)

D

Siliceous sediments Calcareous sediments

Fig. 1. Distribution of deep-sea sediments in the Pacific (after [5Ol) and the location of the study area (GH8I-4 area) from which siliceous-sediment cores were collected (solid square).

sediments would be different from those of the pelagic clay. Recently, the importance of biogenic magnetites has become recognized as magnetic carriers of remanence in deep-sea sediments. It has been known for sometime that some living organisms such as mud bacteria [14], chitons [15] and honey bees [16] precipitate magnetites. Kirschvink and Lowenstam [15] first suggested that the biogenic magnetites would be a potential source of stable remanent magnetization in sediments because their grain sizes are within the single-domain (SD) range of magnetite. Since then, fossil bacterial magnetites (magnetofossils) have been discovered from deep-sea sediments at several localities [17-19] using transmission electron microscopy (TEM) on magnetic extracts. T E M is a powerful tool for

rock magnetism, but one cannot know whether the magnetic grains observed with a T E M faithfully represent their original magnetic assemblage because the magnetic extraction procedure may miss finer fractions of the magnetic assemblage and distort the original grain size distributions. We conducted a rock-magnetic study of siliceous sediments from the central equatorial Pacific to characterize the carriers of the stable remanent magnetization. We combine magnetic granulometry using the suspension method of Yoshida and Katsura [20] with T E M observation. The advantage of the suspension method is that the absolute grain size of a magnetic assemblage can be obtained without magnetic extraction procedure. These results are compared with those of pelagic clay.

ORIGIN

OF STABLE REMANENT

MAGNETIZATION

83

OF SILICEOUS SEDIMENTS

ness, which are underlain by early Miocene sediments (20.7 to 23.2 Ma) [21].

2. Samples The Geological Survey of Japan collected thirteen siliceous-sediment cores 8 m long from a narrow area (70 × 50 kin) in the central equatorial Pacific, centered at 3 ° N , 1 6 9 ° 4 0 ' W (GH81-4 area) (Fig. 1). The cores were composed of siliceous clay or ooze (here called "siliceous sediments" regardless of clay content). No calcareous sediments were found [21] because the water depth of the area (5200 to 5600 m) is deeper than the Carbonate Compensation Depth (CCD) (about 5000 m at present [22]). The color of the cores was generally dark brown (Munsell soil color chart, 10YR3/3), indicating oxidizing conditions. Many light yellowish brown burrows and Zoophycos (deposit-feeding burrows) occurred in the sediments, suggesting relatively high activity of benthic organisms. The biostratigraphy [21] and magnetostratigraphy [8] of the cores indicate that the average sedimentation rate of the area through the Quaternary was 3 to 6 m / m . y . Hiatuses of various durations were observed in some of these cores, and a few cores penetrated the sediments of as old as early Miocene age within several meters below the surface. Core P225 (position: 3 o 13.32' N, 169 °41.65' W; water depth: 5427 m) was chosen for detailed rock-magnetic analyses. Observations of siliceous microfossils revealed that this core has Quaternary sediments (younger than 1.3 Ma) 2.7 m in thick-

2 ~Q.4

magnetization

All cores taken from the GH81-4 area had stable remanent magnetizations regardless of age or depth [8]. Figure 2 shows the direction and intensity of the remanent magnetization of Core P225 after the AF demagnetization of 7.5 m T in a peak field. The measurements were done using an ScT cryogenic magnetometer. A soft secondary component of likely V R M (viscous remanent magnetization) origin could be removed by the AF demagnetization of 5 m T or less. The M D F (Median Destructive Field) was 10 to 15 mT. The coercivity of the remanence of the deeper, older (early Miocene) sediments did not differ significantly from that of the shallower, younger (Quaternary) sediments. A drop of the magnetization intensity below the hiatus at 2.7 m (Fig. 2) is probably caused by a lithological change, below which the abundance of siliceous organism tests increases. The remanence polarity was judged assuming that the soft secondary component is the V R M of the present geomagnetic field direction. The B r u n h e s / M a t u y a m a boundary exists just above the hiatus at 2.7 m in depth. The polarity reversal sequence below the hiatus would correspond to the Polarity Chrons C5EN to C6A.1N of the

Declination

Intensity

Inclination

(relative)

(10-5 A m 2 k g -1)

0.1

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10

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°

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-90

i1

14,........... ~-- ~

0

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-90

0

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4

6

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Fig. 2. Remanent magnetization of Core P225 after the alternating field demagnetization of 7.5 mT in a peak field (after [8]). Declination is relative because the core was not oriented azimuthally. The intensity is normalized by the weight of solids. The right column is the magnetostratigraphicinterpretation.

84

T. Y A M A Z A K I E T A L .

s t a n d a r d time scale [23] ( a b o u t 20 to 22 Ma), b u t o t h e r possibilities c a n n o t be e x c l u d e d (for e x a m ple, C 6 A . 1 N to 6 A A N , a b o u t 22 to 23 Ma).

4. Magnetic mineralogy

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W e e v a l u a t e d the m a g n e t i c m i n e r a l o g y of C o r e P225 from strong field t h e r m o m a g n e t i c b e h a v i o r and isothermal remanent magnetization (IRM) acquisition. Such i n f o r m a t i o n is necessary to c a l i b r a t e the m a g n e t i c g r a n u l o m e t r y b y the suspension method. T h e r m o m a g n e t i c analyses were p e r f o r m e d on m a g n e t i c extracts using a m a g n e t i c balance. Samples were h e a t e d f r o m r o o m t e m p e r a t u r e to 700 ° C in a v a c u u m of - 10 -3 Pa. T h e t h e r m o m a g n e t i c ( J s - T ) curves of the s a m p l e s a b o v e a n d b e l o w the h i a t u s were quite similar each other. A s shown in Fig. 3, these curves suggest that m a g n e t i t e is the m a j o r carrier of the m a g n e t i z a t i o n . T h e Curie t e m p e r a t u r e s were a b o u t 580 ° C, which is close to that of p u r e magnetite. N o m a g n e t i z a t i o n rem a i n e d a b o v e 580 ° C, i n d i c a t i n g that h e m a t i t e is insignificant. The curves were slightly irreversible, p o s s i b l y d u e to c h e m i c a l a l t e r a t i o n s d u r i n g heating. This suggests that the m a g n e t i t e s suffered some low-temperature oxidation.

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I R M was m e a s u r e d after p l a c i n g a s a m p l e in a s t e a d y m a g n e t i c field whose s t r e n g t h was increased stepwise up to 0.9 T. T h e I R M a c q u i s i t i o n curves of all s a m p l e s were c h a r a c t e r i z e d b y a steep initial increase a n d s a t u r a t i o n at fields of 0.3 T or less (Fig. 4). This b e h a v i o r is c o n s i s t e n t with the results of the t h e r m o m a g n e t i c analysis a n d suggests that m a g n e t i t e is the d o m i n a n t m a g n e t i c m i n e r a l [24]. I s o t h e r m a l L o w r i e - F u l l e r tests [25,26] were cond u c t e d to evaluate the d o m a i n state of the m a g netic grains in the s e d i m e n t s (Fig. 4). T h e M D F of the S I R M (here d e f i n e d as the I R M i n d u c e d in a field of 0.9 T) is lower t h a n that o f the A R M ( a n h y s t e r e t i c r e m a n e n t m a g n e t i z a t i o n ) which was p r o d u c e d in a 0.1 m T s t e a d y field s u p e r i m p o s e d on a parallel A F having a p e a k field of 95 mT. These results suggests that the r e m a n e n t m a g n e t i zation is d o m i n a n t l y c a r r i e d b y s i n g l e - d o m a i n magnetites.

600

(°C)

Fig. 3. Thermomagnetic curves of magnetic extracts from 1.1 to 1.3 m of Core P225. Samples were heated in a vacuum of 10 -3 Pa. The heating and cooling rate was about 8°C/min. The applied field intensity was 0.4 T.

5. TEM observations A n a d v a n t a g e of T E M o b s e r v a t i o n s of m a g netic extracts is that the m o r p h o l o g i c a l s h a p e s of

Fig. 5. (a)-(b) TEM images of magnetic extracts from the siliceous sediments of early Miocene age (Core P225, 7.0 to 7.5 m in depth). (a) (Octahedra (rectangular in projection); (b) Bullet-shaped grains. The shape and size of these grains are identical to magnetosomes in magnetotactic bacteria. (c) A TEM image of magnetic extracts of the pelagic clay (Core P411 [12], 1.5 m). Particles of the characteristic morphology suggesting biogenic origin were rare. Scale bars represent 0.1 ~m.

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86

T. YAMAZAKI

magnetic grains provide information regarding the origin of the magnetic minerals. Magnetic grains of Core P225 were examined on a Philips CM12 T E M operating at 100 keV with an energy-dispersive X-ray analysis (EDXA) system. Magnetic grains were extracted from the sediments, and d i s p e r s e d c o m p l e t e l y with s o d i u m h e x a metaphosphate in a ultrasonic bath. A drop of the suspension was dried on a copper electron-microscope grid. A striking feature of the magnetic grains in the siliceous sediments is that most of them are identical in shape and size to magnetosomes (intracellular magnetite grains) in living magnetotactic bacteria [19,27,28]. The photomicrographs showed octahedra (rectangular in projection; Fig. 5a), slightly rounded parallelepipeds, and bullet-shaped grains (Fig. 5b). The bullet-shaped magnetites are, in particular, morphologically quite unique, and magnetites of such a shape cannot be produced by detrital or authigenical process [28]. The existence of the bullet-shaped grains unequivocally establishes the presence of bacterial magnetite (magnetofossils) in the siliceous sediments. The length and width of the magnetic grains were measured on T E M micrographs (Fig. 6). Most of the magnetic grains were within the narrow SD size range of magnetite calculated by Butler and Banerjee [29]. This fact also suggests the bacterial origin of these grains [17,18]. Compositional analyses with the E D X A on individual

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6. Magnetic granulometry method

by the suspension

6.1. Principle of the suspension method Yoshida and Katsura [20] developed a new method for determining the amount and magnetic moment distribution of fine magnetic grains in sediments. This method is applied to a dilute suspension made of the sediments. An assemblage of fine magnetic grains in a noninteracting dilute suspension attains an equilibrium magnetization state in a magnetic field under the combined effects of magnetic torque and thermal agitation (Brownian motion). The average effective moment, meff, of each magnetic grain in the direction of the applied magnetic field (here the intensity of external field is represented by flux density B) is expressed by the Langevin function ( L ) : (1)

t" ~ +"

sP

0.01 0.0

magnetic grain showed that only Fe was present. This agrees well with the result of the thermomagnetic analysis. We compare the T E M micrographs of the magnetic extracts of the siliceous sediments with those of pelagic clay Core P411 [12] from the south Pacific. The grains which have characteristic biogenic morphology were scarce in the pelagic clay (Fig. 5c) in contrast with the siliceous sediments. The shapes of the grains were rather rounded compared with those of the siliceous sediments. This suggests that the magnetic grains of the pelagic clay could be of detrital origin, which is consistent with the general view that the major source of pelagic clay is atmospherically transported dust [30-32].

meff=mL(~T)=m[coth(~T)-~B ]

m ~r

ET AL.

0.8

1.0

Axial ratio

Fig. 6. Size and shape distribution of magnetic grains in the siliceous sediments measured on TEM micrographs. Boundaries between the multi-domain (MD), single-domain (SD) and superparamagnetic (SP) fields were drawn after [29]. The SP threshold is shown for relaxation times of 4 × 10 9 years (upper curve) and 100 seconds (lower).

where m is the magnetic moment of the grain (we use the system of units in E-B analogy, then the unit of the moment is A m2), k is the Boltzmann's constant, and T is the absolute temperature. The equilibrium magnetization, M(B), and the complete alignment magnetization (CAM) represent the sum of metf and the sum of m, respectively, for the number of grains in the assemblage. When we adopt an appropriate probability density function of the grain moment distribution, f(m), the

O R I G I N OF STABLE R E M A N E N T M A G N E T I Z A T I O N OF SILICEOUS SE D I M E N T S

equilibrium magnetization, M ( B ) , is given by a rearrangement of eq. (1):

M ( B ) = N fo~f ( m ) m efrdm =Nfo°°f(m)mL(~T)dm

(2)

where N is the number of the magnetic grains. If the value of M(B) is measured as a function of magnetic field intensity and temperature, both C A M and m can be determined by fitting a theoretical M ( B ) curve to the observed data. The mean volume (v) of magnetic grains can be derived from the mean magnetic moment ( m 6 ) if the magnetic minerals constituting magnetic grains in a suspension and their domain state are determined. When all magnetic grains are in an SD state and have a certain saturation magnetization, m,, the relation is expressed as m 6 = m s • v.

6.2. Experiments and analyses Fifteen samples from Core P225 were subjected to the experiments: six from the Quaternary sedi-

~q

ments (0.5 to 1.3 m in depth) and nine from the lower Miocene (6.9 to 7.5 m). The apparatus and procedure used in this study were almost the same as reported previously [12,20]. The sediments were thoroughly dispersed and diluted until their magnetic grains attained a noninteracting state as determined by observing exponentially decreasing magnetic relaxation in a field-free space. The concentration of the suspensions (the weight of solids in a unit volume) in a noninteracting state ranged from 2 to 3 kg m 3 for the sediments of Quaternary age, and from 7 to 13 kg m -3 for those of early Miocene age. The equilibrium magnetization of a noninteracting suspension was measured at eight levels of magneticfield intensity (less than 1.1 × 10 5 T) (Fig. 7) using an ScT cryogenic magnetometer equipped with a flux-compensated coil. Five runs of the measurement were performed at each level of the field. For the probability density function of magnetic moment, f ( m ) in eq. (2), we adopted a lognormal distribution, which has a geometric-

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88

T. Y A M A Z A K I

mean (median) moment, mG, with a log standard deviation, log a (the base of logarithms is e), 1

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(3) Three parameters, CAM, log m G and log a, were evaluated using the nonlinear least squares fitting program, SALS [33] (Table 1). Assuming the lognormal distribution, the theoretical M ( B ) curves fitted observed data well (Fig. 7), as demonstrated previously in other sediments [12,20]. We also tried the fitting based on a log uniform distribution of grain moments between 0 and m ma× [34,35] and a unit distribution having an identical moment m c, but the theoretical curves showed systematic departure from the observed data. On calculating the magnetic grain sizes from the magnetic moments, we assumed that all magnetic grains in the suspensions are SD magnetites, which is supported by the thermomagnetic analysis, the I R M acquisition experiment, and the Lowrie-Fuller test. There is a possibility that the

E T AL.

magnetites may have suffered low-temperature oxidation, and the goethites may exist. Maghemitization, however, does not seriously affect the magnetic grain size determinations because the saturation magnetization of maghemite, 85 A m 2 kg 1, is close to that of magnetite, 90 A m 2 k g [36]. The presence of goethites also would not significantly affect the magnetic grain size determinations unless the amount of goethites is much larger than that of magnetites, since the saturation magnetization of the former is at most two orders of magnitude smaller than that of the latter [37]. The domain state of the magnetic grains can be estimated also from the ratio of C A M to SIRM. The ratio will be 2 for an assemblage of SD grains with uniaxial anisotropy which does not have preferred orientation of the axis of easy magnetization [38]. On the other hand, the ratio falls to the order of 0.001 for multi-domain (MD) grains [39]. The mean of the CAM of the suspensions from the upper part of the core is 4.2 (_+0.6) × 10 3 A m 2 kg t (per unit weight of solids), and that of the lower part is 5.1 ( + 1 . 3 ) × 1 0 -3 A m 2 kg -~

TABLE1 S u m m a ~ of m a g n e t i c p r o p e r t i e s o f C o r e P225 No.

Depth (m)

NRM intensity (10 S A m e kg l)

CAM ( 1 0 - 3 A m2 kg-1)

Magnetic moment distribution (lognormal)

G ra i n diameter (ttm)

Mean(mG)(lO-l~Am 2) s.d.(a) (+)

(-)

(+)

(-)

(+)

(-)

Upper part (Quaternary) 17221 17224 17230 17236 17239 17251

0.56 0.63 0.77 0.91 0.97 1.25

5.39 6.55 5.48 5.41 6.83 6.58

3,91 +0.26 4.97 + 2.30 3.69+0.36 3.78_+0.42 3.71 _+0.95 5.19+0.50

4.9 0.70 2.1 1.4 4.3 2.0

8.2 31 4.3 3.3 31 3.6

2.9 0.016 1.0 0.58 0.61 1.1

5.0 7.2 5.9 6.4 5.0 5.9

5.6 14 6.8 7.5 7.6 6.6

4.5 3.7 5.1 5.5 3.3 5.3

0.058 0.030 0.044 0.038 0.056 0.043

0.069 0.10 0.056 0,051 0.11 0.052

0.049 0.0086 0.034 0.028 0.029 0.035

5.92+0.55 5.97_+0.46 4.84 _+0.32 4.50_+0.25 2.81 -+ 0.14 4.74-+0.31 4.22-+0.22 7.92 + 1.05 5.20_+0.38

3.2 2.0 7.5 12 7.1 4.0 6.8 0.39 3.8

4.9 2.9 10 16 9.0 5.4 8.7 0.70 5.1

2.2 1.4 5.5 9.5 5.6 2.9 5.3 0.21 2.8

3.8 4.4 3.3 2.9 3.5 3.8 3.5 5.8 3.7

4.2 4.7 3.5 3.1 3.7 4.1 3.7 6.4 4.0

3.5 4.1 3.0 2.7 3.3 3.6 3.3 5.2 3.5

0.051 0.043 0.067 0.079 0.066 0.054 0.065 0.025 0.053

0.058 0.048 0.074 0.086 0.071 0.060 0.070 0.030 0.059

0.044 0.038 0.060 0.072 0.061 0.049 0.060 0.020 0.048

Lower part (early Miocene) 17497 17500 17503 17506 17509 17512 17515 17518 17524

6.89 6.96 7.03 7.10 7.17 7.24 7.31 7.37 7.51

2.09 1.95 2.23 1.78 1.82 1.81 2.05 1.72 1.39

N R M is natural remanent magnetization without partial demagnetization. C A M (complete alignment magnetization) and N R M intensity are normalized by the weight of solids. ( + ), ( - ) are the upper and lower limits of parameters within the standard deviation of the uncertainties of leas t-squares fitting.

89

ORIGIN OF STABLE REMANENT MAGNETIZATION OF SILICEOUS SEDIMENTS

(Table 1). The S I R M was about 1.1 and 0.7 × 10 2 A m 2 kg -~, respectively. Thus, the ratio of the C A M to the SIRM of the upper part is about 0.4, and the lower part about 0.7. This evidence suggests that most of the magnetic grains in the suspensions are in an SD or PSD (pseudo-singledomain) state, which agrees with the results of the Lowrie-Fuller test.

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6. 3. Magnetic grain size The results of the magnetic granulometry are summarized in Table 1. Table 1 contains the upper and lower limits of the parameters within the standard deviation of fitting uncertainties, which reflect the error of the equilibrium-magnetization measurements. The mean diameters of the magnetic grains were calculated assuming that all grains are of spherical shape. This is a rather crude approximation, but would be sufficient for the granulometry by this method considering other simplifications made on magnetic mineralogy and domain states. The mean diameters of the magnetic grains in the suspensions ranged around 0.05 /~m (Fig. 8a), which is within the SD grain size range of magnetite [29,40]. The mean diameters are fairly close

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Fig. 9. Examples of the shapes of the probability density function of the magnetic moment distribution (a lognormal distribution, eq. (3), was assumed). The samples having approximately equal mean moment are compared; a bold curve with large solid circles represents sample No. 17512 (7.24 m) of the siliceous-sediment Core P225 [m G = 4.0 × 10-lVm m2 (d = 0.054 /~m), a = 3.8 (Table 1)], dashed curve with small open circles represents sample No. 888 of the pelagic-clay Core P411 (m G = 4.1 × 10-17A m2 (d = 0.055/tm), a = 5.5 [12]).

to the grain sizes determined from the T E M micrographs. The nominal magnetic grain diameters from the Quaternary sediments did not differ significantly from those of early Miocene age. Another interesting result of the granulometry is the narrowness of the distribution of magnetic grain diameters of the siliceous sediments, particularly of early Miocene age. The broadness of the magnetic moment (grain size) distribution is expressed in terms of the standard deviation a of the lognormal distribution (3). The standard deviation a of the siliceous sediments (Table 1) is smaller than that of the pelagic clay Core P411 [12] if the samples having similar mean magnetic moment (order of 10-17A m 2) are compared. This result suggests that magnetic grain size distribution of the siliceous sediments is narrower than that of the pelagic clay. The shapes of the probability density function (3) of these samples are exemplified in Fig. 9. The mean grain diameters of the two examples are almost the same, but the pelagic clay sample has a larger amount of SP grains than the siliceous sediments.

7. Frequency dependence of magnetic susceptibility The relative grain size variations of fine magnetic grains in deep-sea sediments can be esti-

90

T. YAMAZAKI ET AL.

mated from the frequency dependence of magnetic susceptibility [12,13]. When an assemblage of magnetite grains contains a larger amount of SP grains, the ratio of high-frequency susceptibility to low-frequency is expected to be more reduced because the hyperfine SP grains contribute less to the high-frequency susceptibility [41,42]. Lowfrequency (0.47 kHz) and high-frequency (4.7 kHz) susceptibility (X L and X u, respectively) were measured using a Bartington M.S. 2 susceptibility meter. The frequency dependence of susceptibility is defined as (XL-XH)/XL × 100. The frequency dependence of the susceptibility of Core P225 did not change remarkably with depth or age (Fig. 8b). This agrees with the uniformity of the grain diameters determined by the suspension method. Furthermore, the frequency dependence above the hiatus of Core P225 and other three cores from the GH81-4 area [43] whose ages range from 0 to 4.5 Ma were similar (Figs. 8b and 10). This implies that the amount of SP grains in the siliceous sediments has not varied since 4.5 Ma. Together with the result of the suspension method, the mean magnetic grain size of the siliceous sediments appears to have been in the SD

Frequency dependence o f susceptibility 100 x (ZL-XH)/XL 0 0

2

4

6

8

10

'

~'2

o <

3

5 p218, P219, P225, p228 Fig. 10. The frequency dependence of magnetic susceptibility plotted versus age. The data above the hiatus of Core P225 and other three cores from the GH81-4 area (Cores P218, 219 and P228 [43]) were superimposed. The ages of the cores were based on the magnetostratigraphy and microfossils [8,21]. The definition of the frequency dependence is the same as that of Fig. 8b.

range of magnetite since at least 4.5 Ma, and may extend to the early Miocene. 8. Discussion

The difference in the stabilities of the remanent magnetization between siliceous sediments and pelagic clay can be explained by the difference in their grain size distributions. The magnetic granulometry revealed that the mean diameters of magnetic grains in the siliceous sediments are around 0.05 /zm, within the SD range of magnetite, and have not changed with age or depth. Further, the distribution of magnetic grain sizes is narrow. Majority of the magnetic grains in the siliceous sediments appear to be in a stable SD state. In contrast, the magnetic grain sizes of pelagic clay varies with age. Unstable-to-stable transitions of the remanence of pelagic clay are accompanied by an increase in magnetic grain size [12]. The pelagic clay has a larger amount of viscous SP grains than the siliceous sediments even when the nominal mean magnetic grain size of the former is the same as that of the latter. The difference in the size distribution of the magnetic grains may reflect difference in their sources. The characteristics of the magnetic grain size distribution of the siliceous sediments can be explained if the magnetofossils observed by a T E M are the major constituent of their magnetic grains. The grain size of bacterial magnetosomes lies within the narrow range of an SD state [17,18]. On the other hand, majority of magnetic grains in pelagic clay would be of eolian origin because the major source of pelagic clay is considered to be atmospherically transported dust [30-32], although other sources such as biogenic cannot be excluded as discussed later. Magnetic grains of eolian origin would have broader size distributions than those of biogenic origin. The magnetic minerals of eolian origin would not be the major constituent of magnetic assemblages in siliceous sediments in the equatorial Pacific. The reasons for this inference are as follows. Present-day atmospheric concentration of mineral aerosol in the equatorial Pacific is one order of magnitude less than that in the middle latitudes of the north Pacific [44]. Quaternary sedimentation rates in the central equatorial Pacific are several times faster than in the pelagic

ORIGIN OF STABLE REMANENT MAGNETIZATION

OF SILICEOUS SEDIMENTS

clay provinces in the middle latitudes [8,9,11]. The concentration of magnetic minerals of siliceous sediments should be one to two orders of magnitude less than that of pelagic clay on the assumption that all magnetic grains in both siliceous sediments and pelagic clay are of eolian origin and that the higher sedimentation rate of siliceous sediments is caused by a larger amount of siliceous organism tests. However, the actual difference of the concentration seems to be several times or less. A pelagic-clay core (GPC3) from the central north Pacific had sedimentation rate of about 1.8 m / m . y . [11] and magnetic susceptibility of about 4 x 10 - 4 SI [13] in the Quaternary. Siliceous sediments in the GH81-4 area, on the other hand, have sedimentation rate of 3 to 6 m / m . y . and susceptibility of 1 to 2 × 10 - 4 SI in this period [8,43]. It is hence suggested that magnetic grains in the siliceous sediments must have some source other than eolian dust. We consider that biogenic magnetites are a possible candidate. The bacterial magnetites found in the siliceous sediments would have been produced in subsurface sediments because magnetotactic bacteria occur in microaerobic condition [45]. It has been recognized nowadays that the phytodetrital aggregates which sink rapidly from surface waters to the sea floor are the major source of energy for deep-sea ecosystem and are decomposed by bacterial activity within a short period of time after their deposition [46-48]. High biogenic productivity in the surface water of the siliceous-sediment province along the equator would produce a larger amount of organic aggregates, which would support higher population of benthic organisms. The amount of the production of the bacterial magnetites is hence estimated to be larger in siliceous along the equator than in pelagic clay in the middle latitudes. Another possible explanation for the difference in the magnetic grain size distribution is that the magnetic grains in both siliceous sediments and pelagic clay are of biogenic origin but their preservations are different. Recently, Vali and Kirschvink [49] studied a Miocene calcareoussediment core from the south Atlantic and revealed that dissolution of magnetofossils occurs under oxidizing conditions, which is responsible for unstable remanent magnetization. They suggested that high clay content would cause the

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dissolution of magnetofossils. Siliceous sediments in the central equatorial Pacific contain abundant siliceous organism tests, which would be suitable for preserving magnetofossils for a long period of time. Higher clay content of pelagic clay, on the other hand, may cause corrosion of magnetites, resulting in a larger amount of viscous SP grains.

Acknowledgements We greatly appreciate M. Torii, A. Nishimura and H. Shibuya for valuable discussions throughout this work. We are also indebted to T. Furuta for use of his magnetic balance, H. Kanaya for the magnet of the IRM acquisition, and E. Kikawa for help in the experiments. We would also like to thank S. Nishimura and Y. Shimazaki for their encouragement. The cores were collected with the cooperation of all scientists, officers and crew of R / V Hakureimaru GH81-4 cruise. Constructive comments by D.V. Kent and two anonymous reviewers improved the manuscript, which is greatly acknowledged. A.G. McKay kindly corrected the English. This study was supported by research programs of the Geological Survey of Japan.

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