Rock magnetism of pelagic sediments from the Equatorial Pacific

Rock magnetism of pelagic sediments from the Equatorial Pacific

Earth and Planetary Science Letters, 89 (1988) 184-192 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 184 [51 Rock magnet...

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Earth and Planetary Science Letters, 89 (1988) 184-192 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

184

[51

Rock magnetism of pelagic sediments from the Equatorial Pacific H o r s t - U l r i c h W o r m i and N o r b e r t W e i n r e i c h 2,. I University of Minnesota, Department of Geology and Geophysics, 310 Pillsbury Drive SE, Minneapolis, MN 55455 (U.S.A.) 2 Ruhr-Universitiit Bochum, Institutfiir Geophysik, 4630 Bochum (F.R.G.) Received February 12, 1987; revised version received February 29, 1988 A detailed rock magnetic investigation has been carried out on Deep Sea Drilling Project (DSDP) pelagic sediments from the Central Equatorial Pacific. This comprises hysteresis and thermomagnetic measurements, Lowrie-Fuller test and, for the first time, ferromagnetic resonance (FMR). Nearly stochiometric magnetite in two grain size fractions, single domain (SD) and multi domain (MD), has been deduced to be the carrier of magnetic remanence. Comparatively strong paramagnetic contributions are carried by pyrite, being identified by X-ray analysis. The statistical analysis of paleomagnetic parameters (NRM, MDF, initial susceptibility, Kgnigsberger ratio Q) from a large number ( > 1000) of samples, supported by hysteresis measurements, indicates a latitude and sedimentation rate dependent ratio of S D / M D grains. Possible sources for the magnetic constituents are discussed in terms of bacterial, volcanic, meteoritic and authigenic origin.

1. Introduction

Since the early paleomagnetic works on marine sediments by Harrison and Funnel [1] and Opdyke [2] revealed their capability to carry reliable records of the earth's magnetic field and since the advent of SQUID magnetometers, numerous paleomagnetic studies on marine sediments have been conducted. Few studies, however, deal with the magnetic carriers themselves. This is intimately related to the problem of extracting significant proportions of the fine magnetic grains, which may constitute only a few ppm of the bulk material. Red clays from the North Pacific have been thoroughly studied rock magnetically [3,4]. Low temperature oxidized magnetite or low titanium content titanomaghemite were identified as magnetic carriers of these sediments with low magnetic stability, often giving unreliable paleomagnetic data. Less oxygenated conditions are met in carbonate-rich sediments, at least below the topmost few meters where pore waters are essentially cut off from oxygen-rich bottom waters [5]. Grain * Present address: Steinrtickenstr. 67, 8750 Aschaffenburg, F.R.G.

0012-821X/88/$03.50

© 1988 Elsevier Science Publishers B.V.

size counts of magnetite-like magnetic separates [6] gave an average size of a few microns, but submicron grains were also identified after an overall separation efficiency of around 60%. Depending on titanium content, magnetic separates from marine sediments have been classified into three groups of origin [7]: (a) continental, (b) andesitic volcanic ash, and (c) local mid-ocean ridge or seamount volcanics. Large grain size fractions ( d > 63 /~m) from Arctic Ocean sediments were recovered by sieving [8]. Upon electron microscopic characterization and microprobe analysis in addition to volcanic sources extraterrestrial input has been identified. Hysteresis loops were measured on numerous Leg 73 samples from the South Atlantic [9]. A positive correlation between saturation magnetization and non-carbonate content (and colour) has been demonstrated. Furthermore, a slight downhole decrease of coercivity is indicated, being more pronounced in the upper few meters. Only recently, with the introduction of more sophisticated extraction techniques, ultrafine magnetite crystals have been recovered [10,11], interestingly also from Leg 73 sediments. These grains have sizes in the stable single domain field [12] and are most likely of bacterial origin.

185

..... .......

20°s 160°w

I

I

..... .......

--

140°w

i

......... i

i

/i

.i

" 'l

120°w

I

lO0°w

Fig. 1. Location of DSDP Leg 85 sites in the Central Equatorial Pacific. Dashed lines are isocons of sediment thickness. Indicated are also the East Pacific Rise (solid lines), the Mathematician Ridge (hatched lines) and fracture zones (dotted lines).

Leg 85 cores were recovered from the Central Equatorial Pacific by the hydraulic piston core technique (HPC) and comprise moderate to high carbonate content pelagic sediments. Sites 573 to 575 are located on a south-north profile from 0 o to 6 ° N at 134°W extending from the maximum of the equatorial high biogenic sediment accumulation zone (Site 573) to its northern margin (Site 575) (Fig. 1). A paleomagnetic investigation on a large number of samples ( N = 1384) with detailed demagnetization studies on each sample [13] allowed to investigate for systematic variations of magnetic properties with sedimentation rate.

2. Statistics from paleomagnetic data A statistical evaluation of standard paleomagnetic parameters, natural remanent magnetization NRM, median destructive field MDF, initial susceptibility Xi, Krnigsberger ratio Q (Table 1 and Fig. 2) deafly shows that significant increases in N R M intensities and susceptibilities are correlated

with decreases in M D F s and Q-factors as a function of increasing latitude or towards lower sedimentation rates (see sedimentation rate data listed in Table 2). The latitude of the sampling sites, of course, have not remained constant throughout the time interval of deposition. Paleolatitudes for the time span of interest, however, can be reconstructed from a plate tectonic model assuming a northward drift of the Pacific plate of 0.3°/m.y. [14-16]. These calculations result in latitudes between 0.4°S (3.3 m.y.) and 0 . 5 ° N for Hole 573, 0 . 5 ° N (12 m.y.) and 4 . 2 ° N for Hole 574 and 1 . 8 ° N (13.5 m.y.) and 4 . 2 ° N for Hole 575, respectively. The geological ages (in brackets) are based on magneto and biostratigraphical analyses.

3. Rock magnetic investigations For the identification of the magnetic carriers and elucidation of their magnetic domain state, a rock magnetic investigation has been performed. Strong field magnetization vs. temperature mea-

186 TABLE 1 Statistics of paleomagnetic parameters for three DSDP sites (573-575). Natural remanent magnetization (NRM) in 10-5 A/m, NRM of samples with normal polarity (NRM( + )), NRM(+) of samples less than 3.4 m.y. old, maximum sample depth (dmax), median destructive field of NRM(+) (MDF(+)) in roT, susceptibility (X) in SI units and K/Snigsberger ratio (Q= NRM/x.H) with H = 29.4 A/m. N denotes number of samples

NRM dmax (m) NRM(+) NRM(+) ( < 3.4 m.y.)

N

Site 573

N

Site 574

N

Site 575

181

1.41 (0.60-3.32) 57 2.07 (1.03-4.16) 2.07 (1.03-4.16) 46 25.2 (15.6-40.7) 25.2 (15.6-40.7) 9.2 (3.6-22.5) 5.2 (5.0-5.4)

365

1.80 (0.61-4.28) 89 2.92 (1.33-6.42) 3.43 (1.60-7.33) 18 14.6 (8.8-24.2) 16.1 (10.7-26.1) 32.2 (20.9-50.0) 1.9 (0.9-2.9)

497

2.12 (0.92-4.87) 131 3.14 (1.34-7.36) 4.24 (1.89-9.48) 7.6 9.2 (4.4-19.3) 12.9 (6.7-24.9) 50.9 (37.7-68.6) 1.4 (0.7-2.4)

89 89

dmax (m)

MDF(+) MDF(+) ( < 3.4 m.y.) x xl06 Q

81 86 86

170 42 68 42 107 107

10 2

NRM 10 3

]

10 4

Xi 10"5

O

~2

MDF 101



l

573





~o

~oI 574

~o

222 111 43 43

s u r e m e n t ( J J T ) a n d ferromagnetic resonance ( F M R ) as m e a n s for the identification of the m a g n e t i c minerals, Lowrie-Fuller test and, primarily, hysteresis m e a s u r e m e n t s for d o m a i n state detection have b e e n applied. All experiments were c o n d u c t e d o n b u l k samples, i.e. n o possible artifacts b y m a g n e t i c extraction techniques are involved. A d d i t i o n a l l y , X-ray diffraction analysis of b u l k samples helped i d e n t i f y i n g the m a j o r constituent minerals.

3.1. Thermomagnetic measurements

10-6

10"1

225 111

T~o

10 0

575

Fig. 2. Latitudinal variation of paleomagnetic parameters: natural remanent magnetization NRM (in A/m), initial susceptibility Xi ( S I units), K~Snigsbergerratio Q and medium destructive field MDF (in roT) with standard deviation in logarithmic display versus latitude ( o N) of the three sites 573, 574 and 575.

Fig. 3 shows a t h e r m o m a g n e t i c curve m e a s u r e d with a v i b r a t i n g sample m a g n e t o m e t e r o n sample 575/2-2/110 i n a v a c u u m of 1.2 Pa. Higher v a c u u m s resulted in more irreversible Js/T curves with higher Js values after heating due to alterations of the samples easily recognizable b y a d a r k e n i n g effect o n the samples surface. A relatively low applied field of H = 80 k A / m has been chosen for the m e a s u r e m e n t because higher fields e n h a n c e p a r a m a g n e t i c c o n t r i b u t i o n s to the b u l k m a g n e t i z a t i o n a n d m a k e Curie t e m p e r a t u r e det e r m i n a t i o n s more difficult. T h e heating curve has a clear m a g n e t i t e Curie t e m p e r a t u r e ( Tc = 580 o C). A l t h o u g h the cooling curve does n o t m a t c h the heating curve with a lower Tc a n d a higher r o o m t e m p e r a t u r e m a g n e t i z a t i o n we consider the J J T m e a s u r e m e n t to be solely indicative for m a g n e t i t e as the only " m a g n e t i c " mineral. N e i t h e r is there a

187

1.

J

16o

260

TI*C

36o

z,6o

56o

66o

Fig. 3. Thermomagnetic curve ( J s / T ) of sample 575/2-2/110 measured with a vibrating sample magnetometer in a field of 0.1 T at a heating rate of 20 ° / m i n in a vacuum of 1.2 Pa.

goethite Curie temperature (T~= 120°C) nor a significant magnetization remaining above 580 ° C that could be attributed to the presence of hematite ( Tc = 680°C). The irreversibility of the curve is most likely caused by chemical alterations during heating. 3.2. Ferromagnetic resonance

Ferromagnetic resonance (FMR) so far has found few applications in rock magnetic investigations [17,18]. Its advantage over other characterization techniques for magnetic minerals, e.g. M~Sssbauer spectroscopy, is its superior sensitivity, so that again no magnetic extraction treatment need to be applied. Although we are dealing with ferromagnetic material the same basic principles apply as for ferromagnetics. As in electron paramagnetic resonance (EPR) experiments the resonant absorption of the microwave radiation by the uncompensated electron magnetic moments of samples is measured. The resonance condition is given by Kittel [19]: p = Heffgl.tB/h

where 1, = the resonance frequency, g = spectroscopic splitting factor, #B = the Bohr magneton, h = Planck's constant, and Heft = effective magnetic field at the site of the electron.

For practical reasons in experiment the frequency is kept constant whereas the applied magnetic field, Ha, is swept. In contrast to paramagnetic materials the effective field Heff is not approximately equal to the applied fields Ha, but the (strong) demagnetization and anisotropy fields have to be accounted for. Demagnetizing fields, Hd, arise for each individual grain, depending on shape and orientation of the magnetization vector. Demagnetization effects owing to the internal field of bulk samples can be neglected here as the overall magnetization is orders of magnitude lower than that of an individual grain. If the magnetic components are saturated by the magnetic field where resonance occurs (magnetite grains are saturated at fields H > 160 k A / m ) , no domain walls are present and the resonance maximum and its line width are entirely determined by the grains saturation magnetization, their shape and crystal anisotropy. Strain induced anisotropies would have to be considered for materials with high magnetostriction constants and mechanically treated grains. Fig. 4 displays the absorption spectrum (a) and its (measured) first derivative (b) of sample 575/2-2/110. For comparison in Fig. 5 we see the F M R signal measured on dispersed magnetite grains prepared by the glass-ceramic method [20]. Indicated in both curves is the field where resonance occurs for the free electron (fie). From the

188

/ 575/2-2/110

rfe b)

o :a 0

irfe

//' /

0

/

1§5 24'22;/8 317

H kA/m

860

Fig. 4. Ferromagnetic resonance (FMR) of sample 575/2-2/110: (a) microwave absorption curve, (b) its measured first derivative. u = 9.40 GHz, modulation frequency Umoa= 100 kHz, modulating field Hmod = 0.16 kA/m. derivative spectrum it becomes evident that two different resonance curves overlap. The stronger component ( I ) is centered around the field where resonance occurs for the free electron (rfe). The second one ( H ) is displaced to lower fields in the region of the magnetite field. Line widths are also

comparable (83 and 110 k A / m , respectively). We therefore interpret the spectrum to be composed of, first, a magnetite like and, second, a paramagnetic component, although for the latter the line width is too broad to be constituted of free magnetic m o m e n t s only. T o o few studies on

Fe304

iSo zoo zso F i g . 5. F M R

kA/m.

H kA/m

curve of dispersed magnetite grains prepared by the glass-ceramic method,

860 v = 9.77 G H z , Vmod = 100 k H z , Hmo d =

0.10

189

JI01~

lapping curves are neither indicative for pure single domain (SD) nor multi domain (MD) grains but rather for pseudo-single domain (PSD) grains or a mixture of SD and M D components. Only the resistance of SIRM to demagnetization at higher fields suggests the presence of a hard magnetic component.

57512-1111.1

ARM: SIRM : •

3.4. Hysteresis measurements

o

o

i0

2'o

3'o

4'o

s'o

H_ kA/m

Further and less ambiguous information on the domain state can be obtained from hysteresis measurements. Despite the low magnetization of pelagic sediments we were able to obtain high resolution hysteresis loops (Fig. 7) by using the vibrating sample magnetometer in the rock magnetism laboratory at the University of Minnesota. Hysteresis properties have been measured on 10 selected samples (4 of hole 573 and 575, respectively, and 2 of hole 574) in order to determine the magnetic state of the components and to elucidate the causes for the latitudinal variations of paleomagnetic parameters (Table 2). At first inspection all curves appear to be very similar, varying strongly only in magnitude of magnetization. A striking feature is the high field linear magnetization/field behavior. Often from such non-saturating loops, the presence of hematite is inferred. However, we have to bear in mind that the low content of ferrimagnetic minerals increases the significance of paramagnetic contributions giving rise to a linear field dependence of

8o

Fig. 6. Normalized AF-demagnetization curves of saturation remanence (SIRM) and anhysteretic remanent magnetization (ARM), H= = 40 A/m (Lowrie-Fuller test), sample 575/22/110. materials of relevance exist for comparison. Hence the interpretation is by no means considered to be unambiguous. However, future studies on well characterized samples (ferri- and paramagnetic) may help to establish such F M R studies as a sensitive diagnostic tool. 3.3. Lowrie-Fuller test

A widely applied examination on the domain state is the so called Lowrie-Fuller test [21] comparing normalized saturation remanence (SIRM) and anhysteretic remanent magnetization (ARM) demagnetization curves (Fig. 6). The nearly over-

J Aim 57512-2/60 Hc=12.2 k A / m

60 2.3

-soo

-ioo

100

500

H kA/m

Fig. 7. Hysteresisloop of sample 575/2-2/60. The maximum applied field is H = 800 kA/m.

190 TABLE 2 Magnetic hysteresis properties from three different sites: saturation magnetization, Js; high field (paramagnetic) susceptibility, Xp; saturation magnetization corrected for paramagnetic contributions, Jsc ( = J ~ - XpH); saturation remanence, Jr~; coercivity, H~; coercivity of remanence, H~r; low field susceptibility, Xi. Approximate ages and sedimentation rates are derived from the magnetostratigraphy Hole:

573

Section/cm:

5-2/26

5-2/142

5-4/128

5-6/86

1-1/105

1-3/105

2-1/141

2-2/50

2-2/60

2-2/110

Js ( A / m ) Xp × 106 J~ ( A / m ) Jrs ( A / m )

7.26 6.41 2.13 0.68 0.32 14.4 26.2 1.82 12.2 1.9 2.27

1.94 1.74 0.55 0.21 0.39 15.0 24.8 1.66 5.2 3.0 2.36

5.53 4.36 2.04 0.78 0.38 16.8 30.2 1.80 10.9 2.5 2.56

2.68 2.62 0.58 0.26 0.44 16.0 28.0 1.75 6.8 2.6 2.75

19.4 17.3 5.55 2.22 0.40 14.4 24.6 1.71 0.47

10.1 9.25 2.70 1.11 0.41 13.8 24.0 1.73 17.2 1.9 0.65

24.2 21.0 7.40 2.59 0.35 12.2 19.2 1.57 60.9 2.9 3.8

19.9 17.4 5.98 2.21 0.37 12.8 22.4 1.75 66.1 3.8 5.17

22.1 20.1 6.01 2.28 0.38 12.2 23.4 1.91 68.3 3.4 5.19

20.8 18.9 5.68 2.16 0.38 12.2 22.4 1.83 58.2 2.8 5.28

13.9

13.9

13.9

13.9

5.4

5.4

2.4

5.4

5.4

5.4

Jrs/Jsc Hc (kA/m) Her ( k A / m )

H~r/H c Xi × 106

Xi/Xp Age (m.y.) Sed. rate (m/m.y.)

574

magnetization. In addition, for hematite, some bending of the magnetization curve towards saturation at high fields, non-closing hysteresis branches and a higher coercivity would also be expected. For the determination of the saturation magnetization, and in principle also for the coercivity of the ferrimagnetics, paramagnetic contributions have to be substracted. Table 2 lists hysteresis parameters together with age and sedimentation rate as deduced from paleomagnetic stratigraphy data [13]. The corrected ratios Jrs/Jsc around 0.38 and a (uncorrected) Hc/Hcr = 1.75 are indicative of PSD grains, or, alternatively, of an ensemble of SD grains together with approximately 25% M D grains, as SD magnetite grains have a Jrs/Js = 0.5 (when shape anisotropy is prevailing) and for MD grains Jrs/Js < 0.05 [22] (or a larger fraction of PSD grains instead of MD grains). Differences in hysteresis parameters between the two sites (573 and 575) are significant for H c a n d - - m o r e p r o n o u n c e d - - f o r Her. The latter is a measure of solely irreversible magnetization processes and thus not influenced by the paramagnetic contributions, hence being a more sensitive parameter of the ferrimagnetic constituents. Hole 573 samples, though lower in saturation magnetization by an order of magnitude than 575 samples have clearly higher coercivities of rema-

575

nence Her, they are magnetically "harder', or, with other words, their SD fraction is larger.

3.5. X-ray diffraction analysis Strong paramagnetic contributions to the bulk magnetization require a relatively high concentration of their carriers as paramagnetic susceptibilities are orders of magnitudes lower than those of (ferri-) magnetics. Therefore, it appeared feasible to detect paramagnetic minerals by X-ray diffraction analysis of the bulk material, and indeed besides diamagnetic calcite and quartz, (antiferromagnetic) pyrite has been identified, whereas magnetite escaped under the resolution limit. Although the atomic moments of an antiferromagnet are ordered its bulk magnetization appears like that of a paramagnetic material. 4. Discussion and conclusion

All the measurements taken together provide convincing evidence that nearly stochiometric magnetite is the sole carrier of magnetic remanence. Bearing in mind what has been learned from earlier investigations on magnetic sediment components we interprete the magnetite minerals to be composed of two fractions: a SD bacterial derived one and a M D fraction, their ratio being

191 dependent of the sedimentation ratb. The origin of the MD magnetite is not clear at hand. Magnetite grains exhibiting M D behavior must have grain sizes of at least a few microns [22]. Continental detritus or mid-ocean ridge basalts as sources are highly unlikely due to the remoteness of the sampling sites. Hence, atmospheric fallout and authigenic formation remain as possible sources. It is well known that volcanic ash entering the stratosphere can be spread all over the globe. The ashes, however, are pyroclastic, often glassy materials [23,24] and unlikely to contain micron-sized magnetite grains as the upper grain size of the ashes is around a few microns themselves [25]. They may, however, contribute to the paramagnetic constituents, either directly or indirectly as an iron source for the authigenic pyrite. According to Lisitzin [26], two thirds of all cosmic material recovered from Antarctic ice are magnetic spherules larger than 10 # m in size. Although most of these entered the atmosphere as metal meteoroids, they may have suffered extensive oxidation (to magnetite) during their fragmentation and heating in the atmosphere and in the course of deposition in the ocean [27]. Extraterrestrial materials thus provide the sediments with M D magnetite grains at an approximately constant input rate with time, whereas bacterial SD input rates may vary with food supply for the bacteria and bulk sediment accumulation. Alternatively, chemical alterations by either dissolving preferentially the small SD magnetite crystals or authigenic growth of MD grains would have similar effects on the S D / M D ratio. Then one would suspect this process to be time or depth dependent correlating age or depth with magnetic hardness. Indeed, N R M s and MDFs within a site decrease with increasing age and depth and age (Table 1), for Site 575 the mean N R M ( + ) value is 35% larger for sediments younger than the total N R M ( + ) mean for which the maximum age is 18.1 m.y., while the M D F ( + ) is 40% higher for the younger samples. Similarly, for Site 574 the N R M ( + ) mean for the samples younger than 3.4 m.y. is 17% higher than the total (maximum age: 12.1 m.y.) and the M D F ( + ) mean is 10% higher. Nevertheless, the age dependence within sites cannot account for the site to site variation of paleomagnetic parameters because here the decrease in N R M is accompanied by an increase in

M D F while the trend within sites is just the opposite. Hence, the finding that slowly accumulating sediments are magnetically soft albeit high in magnetic remanence can only be explained by (latitudinal) variations in the geochemical conditions concerning the formation/dissolution of iron oxides or a significant input from atmospheric (meteoritic) fallout.

Acknowledgements We are grateful to Subir Banerjee and three reviewers for helpful discussions. Waldemar St~Scklein (Universit~it Bayreuth, F.R.G.) kindly performed the F M R measurements. Special thanks go to Jim Marvin for putting the vibrating sample magnetometer in a state of high sensitivity. This work was supported in part by the Deutsche Forschungsgemeinschaft ( D F G ) and N S F grant E A R 87-8610838-01.

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20 H.-U. Worm and H. Markert, The preparation of dispersed titanomagnetic particles by the glass-ceramic method, Phys. Earth Planet. Inter. 46, 263-269, 1987. 21 H.P. Johnson, W. Lowrie and D.V. Kent, Stability of anhysteretic remanent magnetization in fine and coarse magnetite and maghemite particles, Geophys. J. 41, 1-10, 1975. 22 H.-U. Worm and H. Markert, Magnetic hysteresis properties of fine particle titanomagnetites precipitated in a silicate matrix, Phys. Earth Planet. Inter. 46, 84-92, 1987. 23 M. de Angelis, L. Fehrenbach, C. Jehanno and M. Maurette, Micrometer sized volcanic glasses in polar ices and snows, Nature 317, 52-54, 1985. 24 D. Ninkovich and J.H. Robertson, Volcanogenic effects on the rates of deposition of sediments in the northwest Pacific Ocean, Earth Planet. Sci. Lett. 27, 127-136, 1975. 25 J.P. Kennett, Marine Geology, 441 pp., Prentice-Hall, New York, N.Y., 1982. 26 A.P. Lisitzin, Sedimentation in the world oceans, SEPM Spec. Publ. 17, 195 pp., 1972. 27 D.E. Brownlee, Extraterrestrial components, in: The Sea, Vol. 7. The oceanic lithosphere, C. Emiliani, ed., pp. 733-762, 1981; D.E. Brownlee, personal communication, University of Washington, 1986.