Magnetic properties of light and dark sediment layers from the japan sea: Diagenetic and...

Pergamon Qurrrmcrn Scirm r KP\iew.v. Vol. 16. pp. 1093-l I 14, 1997.

C 1998 Elsevier Science I-id.

PII: SO277-3791(96)00118-7 All rights reserved. Printed in Great Britain.

0277-3791197 1332.00

MAGNETIC PROPERTIES OF LIGHT AND DARK SEDIMENT LAYERS FROM THE JAPAN SEA: DIAGENETIC AND PALEOCLIMATIC

IMPLICATIONS

L. VIGLIOTTI Istituto di Geologia Marina, CNR, Via P. Gohetti 101, 40129 Bologna, Italy

{E-mail: M~f#@hoigm2.igm.ho.cnr.it)

Abstract - Rock magnetic/paleoclimatic/diagenetic relationships of sediments spanning the last 0.78 Ma have been investigated using samples collected from light and dark layers recovered at ODP Sites 794 (Yamato Basin) and 795 (Japan Basin). Rock-magnetic parameters (K. Kfd, ARM. SIRM, S-ratio) are shown to reflect diagenetic processes and climate-related variations in the concentration, mineralogy and grain-size of the magnetic minerals contained within the sediments. The magnetic mineralogy is dominated by ferrimagnetic (magnetite-type) minerals with a small contribution made by hematite and iron sulphides such as pyrrhotite and/or greigite. Magnetic mineral concentration and grain size vary between light and dark layers with the former characterized by a higher magnetic content and a finer magnetic grain size. Magnetite dissolution, related to sulfate reduction due to bacterial degradation of organic matter, is the process responsible for the magnetic characteristics observed in the dark layers. testifying to the reducing conditions in the basin. Variations in the rock magnetic properties of the sediments are strongly correlated with global oxygen isotope fluctuations. with glacial stager, characterized by a lower magnetic tnineral content and a coarser magnetic grain size relative to interglacial stages. Major downcore changes in the magnetic properties observed at Site 794 can be related to changes in the oceanographic conditions of the basin associated with the flow of the warm Tsushima Current into the Japan Sea at about 0.35-0.40 Ma ago. Q 1998 Elsevier Science Ltd. All rightx reserved

INTRODUCTION

Magnetic parameters provide a rapid and effective tool for monitoring the supply of terrigenous sediment to the oceans in response to climatic change, and for recon- structing the paleoenvironmental records of deep-sea sediments (Kent, 1982; Oldfield and Robinson, 1985; Doh et al., 1988; Bloemendal rf al., 1992; Sahota et ml., 1995). Rock-magnetic properties often vary with changes in the lithology of the sediments and these variations are in many cases controlled by climate. Thus changes in the concentration, mineralogy and grain size of magnetic mineral assemblages often reflect climatically-induced variations within the lithogenic fraction of the sediment (Robinson, 1986; Bloemendal el al., 1988; Hall and King, 1989; Sager and Hall, 1990; Robinson rf ul., 1995). Gross downcore changes in the magnetic properties reflect physical changes in the sedimentary environment that may indicate paleoceanographic variations and/or diagenetic processes.

The aim of this paper is to investigate the rock magnetic/paleoclimatic/diagenetic relationships of the light-dark sedimentary cycles deposited during the Brunhes Chron (Upper Quaternary, 0.78 Ma) in the Japan Sea at ODP Sites 794 and 795.

QSR

Tada et al. (1992) found that variations in sediment composition are closely related to glacial-interglacial cycles expressed in the standard oxygen isotope curve (Imbrie et crl., 1984). The detrital fraction of the sediment tends to be enriched and the maximum grain size tends to be larger during the glacial stages. Considering that often it is the terrigenous fraction of the sediment which contains magnetic minerals, the magnetic properties of the bulk sediment should exhibit distinct differences between glacial and interglacial horizons of the sequence.

GEOLOGIC SETTING

The Japan Sea is a semi-enclosed back-arc basin with a distinctive physiographic configuration which is crucial to its oceanographic regime. Most of the basin exceeds a water depth of 2000-3000 m, but it is connected to outer seas only by shallow water sills: the Mamiya (I2 m), Soya (55 m). Tsugaru (130 m) and Tsushima (130 m) straits (Fig. I ). The oceanographic conditions of the basin are regulated by the precarious balance of waters flowing through these shallow sills. Therefore, glacio-eustatic sea- level change should play an important role in the oceanographic conditions of the basin, and it is

1093

1094

I

FIG. I. Location map of the Japan Sea with sites drilled during ODP legs 127 (794-797) and 17-8 (798 and 799). This study focused on sediments recovered from Sites 794 and 795.

I I Sea of Japan

0 300 km 1.---l

predictable that, as a response to past climatic change, drastic paleoceanographic changes have occurred in the Japan Sea during the Quaternary particularly with respect to bottom water oxygenation and detrital organic matter preservation level.

Four sites were drilled in the Japan Sea during ODP leg 127: Sites 794 and 797 in the Yamato Basin and Sites 795 and 796 in the Japan Basin (Fig. 1). Fine-grained siliciclastic sediments with distinct dark- light sedimentary cycles were recovered from the uppermost part of the sedimentary sequences at all the sites. Volcaniclastic materials produced by explosive erup- tions from the nearby volcanic islands were also recovered throughout the drilled intervals. Light-dark rhythms occur in the Upper Miocene to Pleistocene sediments. but they are most prominent and persistent in the Late Quaternary (Follmi et d.. 1992). The rhythmical alternation of these sediments suggests cyclic changes between oxic and anoxic conditions related to basin-wide paleoceanographic changes which are likely to be linked to glacio-eustatic sea-level changes (Tada et al., 1992). On the basis of gray value (darkness) profiles, Tada et al. (1992) correlated the light and dark layers of Sites 794, 795 and 797 suggesting that the deposition of these cycles resulted from synchronous events. These cycles reflect high- frequency changes in the depositional environments of the basin with periodicities ranging between 100 ka and markedly less than 1000 years. which is shorter than those in the Milankovitch band (Follmi et (II.. 1992).

SAMPLES AND METHODS

For this study, 132 samples were selected from light and dark layers deposited during the Brunhes Chron (last 0.78 Ma) at Site 794 (Yamato Basin) and 795 (Japan Basin). The sediment consists of a rhythmical alternation of color-banded siliciclastic clay and silty-clay. Magne- tostratigraphic data indicate that the time-interval exam- ined in this study spans 24.6 m at Site 794 and 35 m at Site 795 (Hamano et crl.. 1992). The number of samples studied retlects the difference in sedimentation rates between the two sites. Fifty-three samples were collected from Site 794 (26 dark and 27 light layers) and 79 samples from Site 795 (38 dark and 31 light layers). The samples were collected by using paleomagnetic cubes of constant volume (7 cc). Not all the horizons are clearly light or dark and the latter have been subdivided into dark and semidark sediments based on their relative darkness.

In order to characterize the magnetic minerals in the sediment, detailed magnetic measurements were carried out which determine the response of the sediments to a variety of applied magnetic fields. This response is mainly determined by the mineralogy. concentration and grain size distribution of the magnetic phases.

The procedures used for the magnetic measurements were as follows. ( 1) Measurement of low-field volume magnetic suscept-

ibility (K) at two different frequencies (0.47 and 4.7 kHz) using an MS2 Bartington susceptibility meter. The difference between the two MS values was used to calculate the frequency dependence of

L. Vigliotti: Magnetic Properties of- Sediment Layers from the Japan Sea 109.5

susceptibility (Kcd). This parameter reflects the presence within the sediment of very fine (CO.04 pm for magnetite) ferrimagnetic grains in the superparamagnetic state (SP).

(2) Measurement of the natural remanent magnetization (NRM) after alternating-field (AF) demagnetization with peak fields of 20 millitesla (mT). The remanence was measured with a Jelinek Jr-4 spinner magnetometer (noise level: 4x 10 ’ A/m).

(3) Acquisition of anhysteretic remanent magnetization (ARM) by subjecting the samples to an AF field of 100 mT biased by a 0.05 mT direct field, followed by progressive AF demagnetization in three steps (20, 30 and 40 mT). The ARM is expressed as anhysteretic susceptibility (KARM). obtained by dividing the ARM by the strength of the DC field.

(4) Acquisition of isothermal remanent magnetization (IRM) in steps up to a maximum field of 1 tesla (T). The acquired IRM (referred to as saturation isothermal remanence; SIRM) was subsequently demag- netized in three steps (IS, 25 and 35 mT) and subjected to a reversed DC field up to 0.4 T (five steps). The latter measurements were used to calculate the coercivity of the isothermal remanence Bo(,.,, and the S-ratio (-IRMmo &SIRM) (Bloemen- dal, 1983). Low field ‘soft’ IRM (IRM20mT) was used to approximate the total concentration of remanence carrying ferrimagnets.

(5) A composite IRM (1 .I.5 T along the Z-axis, 0.4 T in the Y-axis and 0.12 T in the X-axis) was given to a small collection of 16 samples (10 from Site 795 and six from Site 794) which were then subjected to progressive thermal demagnetization following the method proposed by Lowrie ( 1990).

The light and dark layers were also characterized on the basis of sulfur content (S). total carbon (C,,,,) and organic carbon (C,,,) and the total nitrogen (N) content. These values, expressed as percentages, are given in Tables 1 and 2 together with the various magnetic parameters.

Time Control and Isotopic Stage Assignment

The sediments considered in this study belong to the Late Quaternary. Unfortunately, within the Brunhes Chron (0.78 Ma) we have very poor time control for these sediments. A couple of biostratigraphic markers such as the last occurrence (LO) of the Pseudoemilima lacunosa (0.46 Ma; Thierstein ef al., 1977) and the LO of the Rhiznsolenia curvirostris (0.30 Ma; Koizumi. 1992) were identified at both the sites (Tada et al., 1992), but in intervals too large to be really significant. The only useful marker is the ash layer termed Aso- (0.86-0.90 Ma; Oba er al., 1991) identified at 3.86 mbsf at Site 794 and at 5.51 mbsf at Site 795 (Tada et al., 1992).

Oxygen isotope (6’sO) data are not available for the sites studied. However, Tada et ul. (1992) used diatom abundance as a substitute for the 6’“O curve at Site 797. As stated above, these authors correlated the light-dark cycles of Site 797 with the cycles of Sites 794 and 795.

On this basis, it has been possible to ‘tune’ the age of the sediments studied to the oxygen isotopic stages of Imbrie et [I/. (1984). According to this correlation, 20 samples from each site appear to belong to stratigraphic intervals deposited at the same time in the Japan and Yamato basins. In Fig. 2, the magnetic susceptibility (2a) and the sulfur content (2b) are plotted as an age function for these samples. The curves exhibit a good agreement between the two sites suggesting that the correlation is correct and that paleoceanographic processes in the Japan Sea are synchronous basin-wide events. Although the correlation can be considered correct, the assignment to the oxygen isotope stages is ambiguous with respect to glacial stage 8. The values observed in this stage, as discussed below, appear anomalous with respect to other glacial stages. Considering also that, at Site 797, this stage exhibited contrasting data (Tada et (I/., 1992 1. it is possible that the interval representing this stage has not been identified accurately. Evidence for this is given by the LO datum of the Rhi:osolmia curvirmtris. In the oxygen isotope assignment proposed by Tada et (11. ( 1992) for Site 797, this datum falls in an interval belonging to stage 8. In fact, in the North Pacific, it has been tied to the oxygen isotope record at an age of 0.276 Ma (Morley et al., 1982). and it is only assumed to be time-correlative in the Japan Sea (Burckle et rll., 1992). However, at Site 798, it appears to fall in oxygen isotope stage 9 (Dunbar rt crl., I992), suggesting a diachronism for this datum. The intervals tied to the oxygen isotope stages X-9 should therefore be considered ambiguous. Furthermore the interval related to the oxygen isotope stage 8 at Site 794 corresponds to a small gap in the sedimentary record of the core (Tuda ct rd., 1992).

RESULTS

Magnetic Mineralogy

IRM acquisition experiments are helpful for a rough identification of the magnetic minerals present in a sample. Fig. 3 shows typical IRM acquisition curves for sediments from sites 794 and 795. Most of the IRM is acquired below 0.2 T suggesting that magnetite dom- inates the magnetic properties of these sediments. The highest IRM values correspond to light coloured samples, implying a higher ferrimagnetic mineral content for these layers. Nevertheless, S-ratio values (Table 1). which retlect the proportion of ferrimagnetic minerals to high- coercivity minerals such as hematite and/or goethite. indicate that the latter minerals are also present. For this reason a small subsample of 16 specimens (6 from Site 794 and IO from Site 795) were sub.jected to thermal demagnetization of orthogonal composite IRMs. This procedure reveals the unblocking temperatures of three different coercitivity fractions which correspond to different magnetic minerals (Lowrie, 1990). Four typical examples are shown in Fig. 4. Most of the IRM resides in the soft fraction (<O. 12 T) in all the samples. This fraction sometimes shows a single unblocking temperature above 500°C (Fig. 4, samples 794 2Hl 114-l 16 and 795 2H2

TABL

E 1.

Org

anic

carb

on

cont

ent

(C,,,

,). t

otal

car

bon

cont

ent

(C,,,

,), s

ulfu

r co

nten

t (s

), ni

troge

n co

nten

t (N

) an

d m

agne

tic

para

mct

crs

for

sam

ples

fro

m

Site

794

Sam

ple

Dept

h (m

) Co

lor

C,,,

, (%

) C

,,,, (

%‘I

S,,,,

(%:)

N (%

) C

/N

K Kf

d (%

) Q

NRM

AR

M

IRM

,,,,,

SIRM

SI

RM/K

B,

., (m

T)

S-ra

tio

KARM

KA

RMlK

AL

F

IH1-

78

0.78

lH

1-85

0.

85

I H2-

54

2.04

1H

2-12

0 2.

70

lH3-

I I

3.1

I lH

3-71

3.

71

I H3-

90

3.90

lH

4-IO

4.

60

I H4-

68

5.18

38

4-13

2 5.

82

IHS-

I I

6.1

I 1 H

5-29

6.

29

I HS-

47

6.47

2H

l-IX

6.98

2H

l-55

7.35

2H

I-Y

I 7.

71

2HI-I

I4

7.

94

2H2-

39

8.69

2H

2-85

Y.

15

2H2-

IO

9 9.

39

2H2-

I33

9.63

2H

3-I2

9.

92

2H3-

39

IO.1

9 2H

3-69

10

.4Y

2H3-

I33

I I.1

3 2H

4- I

O

I I .4

0 2H

4-9

I 12

.21

2H4-

I IX

12

.48

2H5-

28

13.0

8 2H

5-7

1 13

.51

2H5-

I20

14

.00

2H6-

32

14.6

2 2H

6-68

14

.98

2H6-

78

15.0

8 2H

h-I

I4

15.4

4 2H

7-I5

15

.95

2H7-

55

16.3

5

D 2.

86

L 0.

54

L 0.

34

L 0.

31

L 0.

35

D I.5

2 L

0.29

L

0.37

L

0.49

SD

0.

28

L 0.

35

D I.1

9 L

0.43

SD

0.

3 I

SD

I .os

L 0.

4x

D I .7

3 L

0.3’

) SD

0.

64

L 0.

48

D I .

03

SD

0.44

SD

0.

9x

1.

0.48

D

I .53

L

0.63

D

4.46

L

0.34

L

0.35

L

0.37

D

I .90

SD

I .7

2 D

3.60

L

0.24

L

0.62

SD

0.

42

D 1.

23

2.86

0.

40

3.86

0.

18

0.49

0.

14

0.44

0.

09

0.35

0.

34

I.52

0.27

0.

55

0.15

0.

50

0.15

0.

5 I

0.22

0.

28

0.22

0.

43

0.17

l.l

Y 2.

92

0.43

0.

98

0.44

0.

18

I .os

0.27

0.

x3

0.20

I .

90

I .2s

0.

39

0.1

I 0.

66

0.28

0.

49

0.30

I.1

I 0.

60

0.44

7.

63

O.Y

X 0.

83

0.48

0.

23

I .64

4.

14

0.78

0.

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4.47

3.

04

0.35

0.

40

0.35

0.

13

0.37

0.

22

I .90

5.13

7.

49

2.24

3.

60

3.47

0.

27

0.61

0.

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0.3

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0.25

1.

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0.57

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5 1.

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0.07

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5.88

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170

X.93

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2.8

2.52

0.

073

3.94

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078

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188.

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.86

90.3

0.

81

0.23

4.

29

210.

6 6.

44

75.1

0.

76

0.12

3.

76

150.

1 3.

36

67.0

0.

80

0.09

1.

96

0 W

o\

5.3

6.5

11.7

12

.2

II.1

10.4

9.

9 12

.3

12.2

13

.7

8.8

0.9

9.1 7.2

6.6

m-l.

1 14

.6

5.8

9.6 6.2

6.6

6.8 3.7

4.6

-0.7

6.

1 -9

.6

-2.6

I.7

-2.5

-5

.3

-6.0

-4

.2

-2.4

-1

7.7

-7.8

-6

.7

TABL

E 1.

Org

anic

carb

on

cont

ent

(C,,,

,). t

otal

ca

rbon

co

nten

t (C

,,,).

sulfu

r co

nten

t (S

). ni

troge

n co

nten

t (N

) an

d m

agne

tic

para

met

ers

for

sam

ules

fro

m S

ite 7

94

Sam

ple

Dept

h (m

) Co

lor

Corg

(‘%

) C

,,,, (

%)

S,,,

(%)

N (%

) C

/N

K Kf

d (%

) Q

NRM

AR

M

IRM

,,rt

SIRM

SI

RM/K

B,

, (m

T)

S-ra

tio

KARM

KA

RM/K

AL

F

381-

46

16.7

6 L

0.39

0.

57

0.23

3H

1-10

5 17

.35

D 3.

49

3.65

4.

98

3Hl-1

33

17.6

3 L

0.46

0.

52

0.23

3H

2-47

18

.27

D 2.

40

6.03

2.

23

3H2-

77

18.5

7 1,

0.

43

3.03

0.

43

3H2-

104

18

.84

L 0.

44

0.44

0.

26

383-

24

19.5

4 L

0.32

0.

32

0.27

3H

3-75

20

.05

D 0.

96

0.96

3.

77

3H3-

IO6

20.3

6 D

3.02

3.

02

6.33

3H

3-I

33

20.6

3 D

3.01

5.

06

3.05

3H

4-28

2

I .08

L

0.21

0.

53

0.39

3H

4-47

21

.27

L 0.

89

1.04

0.

29

3H4-

X6

21.6

6 L

0.26

0.

38

0.49

3H

5-17

22

.47

SD

0.66

0.

70

0.43

38

5-55

22

.85

SD

0.86

0.

95

1.19

38

5-12

0 23

.50

SD

I.59

I .59

0.

42

0.06

1 6.

459

56.5

1.

47

0.06

0.

3 6.

0 16

.7

222.

9 3.

94

0.3

16

I 1.0

46

30.9

0.

81

0.06

0.

2 3.

x 14

.6

204.

6 6.

63

0.09

7 4.

742

38.5

0.

49

0.13

0.

5 5.

4 15

.3

239.

8 6.

23

0.19

4 12

.345

9.

6 1.

57

0.17

0.

2 2.

9 13

.8

76.7

8.

03

0.06

3 6.

X73

39.1

1.

53

0.08

0.

4 5.

5 17

.4

243.

9 4.

97

0.07

5 5.

827

42.1

1.

78

0.25

I.1

7.

0 17

.6

299.

5 7.

12

0.06

I 5.

279

38.4

2.

77

0.14

0.

5 5.

2 17

.0

249.

2 6.

48

0.10

9 8.

747

55.3

0.

67

0.08

0.

4 5.

2 17

.1

156.

0 2.

82

0.36

9 8.

195

29.7

1.

78

0.08

0.

2 3.

3 x.

‘) 11

3.4

3.82

0.

187

16.1

18

14.6

7.

04

0.12

0.

2 2.

2 8.

1 61

.8

4.23

0.

055

3.91

6 64

.4

1.47

0.

09

0.6

6.6

22.7

23

3.2

3.62

0.

106

X.41

5 58

.3

1.72

0.

08

0.5

6.6

18.3

25

2.0

4.33

0.

060

4.38

3 93

.3

0.54

0.

20

1.8

13.9

25

.4

2459

.0

26.3

7 0.

102

6.43

1 43

.8

1.28

0.

08

0.3

6.1

18.2

22

2.0

5.07

0.

121

7.09

1 65

.0

0.96

0.

04

0.3

7.0

16.2

19

5.1

3 0.

I so

IO.5

87

60.1

2.

08

0.07

0.

4 5.

3 13

.6

164.

3 2.

73

Magn

etic

param

etera

(NRM

. AR

M.

IRM

,,,,,.

SI

RM)

are

expr

c\\c

d in

mA/

m.

Mag

nrtic

su

scep

titxl

ily (K

) va

lues

are

in 10

’

SI u

nits

(di

men

sionl

ess)

.

71.0

0.

79

0.15

2.

69

-6.6

66

.0

0.89

0.

10

3.08

2.

8 73

.0

0.86

0.

14

3.55

-5

.4

41.0

0.

95

0.07

7.

74

-1.3

73

.0

0.87

0.

14

2.79

-7

.8

72.0

0.

86

0.18

4.

21

-2.5

78

.0

0.86

0.

13

3.40

3.

5 70

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0.91

0.

I3

2.38

1.

4 68

.0

0.88

0.

08

2.82

-4

.3

53.0

O

.Yl

0.05

3.

72

-2.1

50

.9

0.95

0.

17

2.58

0.

4 70

.0

0.87

0.

17

2.84

-6

.1

74.0

0.

98

0.35

3.

75

1.0

66.0

0.

86

0.15

3.

48

-4.3

56

.0

0.92

0.

18

2.71

0.

X 61

.0

0.88

0.

13

2.21

-3

.0

TABL

E 2.

Org

anic

carb

on

cont

ent

(C,,,

,), t

otal

ca

rbon

co

nten

t (C

,,,,),

sul

fur

cont

ent

(S),

nitro

gen

cont

ent

(N)

and

mag

netic

pa

ram

eter

s fo

r sa

mpl

es

from

Site

795

Sam

ple

Dept

h (m

) Co

lor

Cc,

,u (%

,) C

,,,, (

%)

S,,,,

(o/c

) N

(%)

C/N

K

Kfd(

%)

Q NR

M

ARM

IR

M,,,

,, SI

RM

SIRM

/K

B,,

(mT)

S-

ratio

KA

RM

KARM

/K

ALF

iHl-7

0 0.

70

IHI-X

0 0.

80

lHl-9

3 0.

93

I H2-

22

I .72

182-

82

2.32

I H

3-20

3.

20

I H3-

50

3.50

I H

3-78

3.

78

I H3-

I20

4.

20

lH4-

IS

4.68

I H

4-4

I 4.

91

I H4-

66

5.16

1 H

4-02

5.

42

185-

29

6.29

I H

S-48

6.

48

I HS-

54

6.54

I H

5-92

6.

92

I H6-

4X

7.98

1 H

6-68

X.

18

IHh-

I04

8.54

18

6-14

3 8.

93

2HI-2

1 9.

51

2Hl-7

6 10

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ZHI-I

IS

IO

.45

2Hl-I

42

IO.7

2 2H

2-5

IO.8

5 2H

2-66

1 I

.46

2H2-

79

I I .5

9 2H

2-97

I I

.77

2H3-

22

12.5

2 2H

3-54

12

.84

2H3-

90

13.2

0 2H

3-I

16

13.4

6 2H

4-24

14

.04

2H4-

65

14.4

5 2H

4-Y

I 14

.71

2H4-

I30

15

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2H5-

20

ISSO

28

5-50

15

.80

2HS-

121

l6Sl

SD

I .93

D 2.

26

D I.9

1 SD

I.1

5 1~

0.

48

SD

I .85

SD

I .o

o SD

0.

90

L 0.

38

SD

2.15

L

0.78

SD

2.

88

L 0.

41

SD

I.93

L 0.

52

SD

2.77

L

0.52

D

I .os

L 0.

37

D 0.

91

L 0.

32

SD

I.51

SD

I.01

SD

0.38

D

2.94

L

0.33

D

2.21

L

0.43

SD

I .

02

D I .

49

SD

0.84

D

I.91

L 0.

32

SD

I s4

L 0.

57

D I .

63

SD

I .XY

L 0.

47

L 0.

49

D I.7

5

I .Y3

2.

26

I .99

I .

20

0.43

2.

02

1.00

I .

Oh

0.38

2.

30

0.81

I .

54

0.47

2.

06

0.5

I 2.

77

3.15

I.1

2 0.

43

0.90

0.

89

0.68

I.1

9 0.

50

2.94

0.

36

331

I.I

0.4

I 0.

85

I .49

0.

93

2.49

0.

32

I .57

0.

57

1.77

1.

89

0.57

0.

85

I .76

0.29

0.

210

9.18

95

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0.61

2.

85

0.22

3 IO

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206.

X 2.

06

0.42

0.

196

9.77

15

6.5

2.45

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51

0. I3

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88

58.8

2.

77

0.46

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7.50

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5.8

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19

0.19

6 9.

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373.

2 1.

36

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117

X.56

30

4.5

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78

202.

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10

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588

0.65

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23

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5 0.

95

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3.13

26

X.3

2.12

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166.

0 2.

91

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076

5.37

32

8.8

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53

0.20

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66

182.

X 1.

46

0.64

0.

08s

6.16

18

9.3

1.06

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73

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19

0.07

7 6.

73

210.

5 2.

49

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X.67

62

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0.70

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5.52

29

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x 0.

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55.0

0.

97

6.16

IO

.47

7.9

0.76

45

.1

435.

3 72

5.0

2973

I .o

50

.09

71.5

0.

99

IO.9

4 I x

.44

2.5

TABL

E 2.

Org

anic

carb

on

cont

ent

(C,,,

,), t

otal

car

bon

cont

ent

(C,,,

,), s

ulfu

r co

nten

t (S

), ni

troge

n co

nten

t (N

) an

d m

agne

tic

para

met

ers

for

sam

ples

fro

m

Site

795

Sam

ple

Dept

h (m

l Co

lor

Cor

f (%

) C

,,, (

‘ir)

S,,,

(%)

N (%

) C

/N

K Kf

d(%

) Q

NRM

AR

M

IRM

,,,,,

SIRM

SI

RM/K

B,

, (m

T)

S-ra

tio

KARM

KA

RM/K

AL

F

2H6-

20

17.0

0 28

6-64

17

.44

2H6-

IO1

17.8

1 2H

7-17

18

.47

287-

33

18.6

3 3H

l-24

lY.0

4 3H

l-97

19.7

7 3H

2-I9

20

.4’)

3H2-

64

20.9

4 3H

2-I

I6

21.4

6 38

2-13

2 21

.62

3H3-

1X

21.9

8 3H

3-3

I 22

. I I

3H3-

40

22.2

0 38

3-68

22

.48

3H4-

19

23.4

9 3H

4-66

23

.Yh

3HS-

6 24

.86

3HS-

4 I

25.2

1 3H

5-77

25

.51

3H5-

I 14

2S

.Y4

3H6-

SO

26.8

0 3H

h-IO

0 27

.30

3H6-

I32

27

.62

3H7-

36

28.1

6 3H

7-55

28

.35

4H l-

20

2X.S

O

4H I-

90

29.2

0 4H

l-I

17

29.4

7 4H

2-20

30

.00

4H2-

44

30.2

4 4H

2-I

IS

30.9

5 4H

3m20

3 1

.50

4H3-

68

3 I .9

8 4H

3-95

32

.25

4H3-

I I7

32

.47

4H4-

I5

32.9

s 4H

4-I

I7

33.9

7 4H

4-13

0 34

.10

SD

L SD

L D L L SD

D L L SD

L D L SD

SD

SD

I<

D I>

L SD

SD

D L SD

D D I. D L L D L D D D L

0.92

0.

70

0.46

0.

57

0.84

0.

55

0.40

I .

04

0.91

I .

42

0.57

2.

72

0.39

2.

94

0.27

0.

17

0.74

I .

07

0.85

2.

76

0.45

0.

38

0.72

0.

8X

2.63

0.

41

0.37

0.

65

1.72

0.

27

? ?6

-.-

0.

25

0.3x

2.

30

0.32

0.

97

I .49

3.82

0.

38

0.92

0.

58

0.57

0.

14

0.63

0.

11

0.84

0.

24

0.84

3.

76

0.55

0.

25

0.49

0.

12

1.04

0.

16

0.89

3.

54

5.03

I .7

9 0.

76

I .25

I.4

1 0.

93

0.36

I .

29

2.80

5.

25

0.27

0.

20

0.77

0.

26

0.60

0.

16

I .07

0.

34

0.59

0.

I7

3.75

3.

7Y

0.59

0.

22

0.38

0.

IX

0.89

0.

33

O.Y

.7

0.53

2.

60

7.45

0.

42

0.18

0.

36

0.26

0.

62

4.99

1.

72

5.37

0.

92

0.40

3.

64

3.35

0.

25

0.3

I 3.

00

0.14

2.

30

0.93

0.

39

0.29

0.

99

3.95

I .3

8 I .

49

3.82

3.

19

0.38

0.

63

0.10

7 8.

63

186.

8 0.

98

0.08

8 7.

97

171.

5 2.

19

0.07

2 6.

39

103.

2 0.

87

0.07

3 7.

79

134.

7 0.

62

0.09

4 8.

91

75.3

0.

66

0.07

4 7.

43

212.

8 1.

73

0.08

2 4.

84

3 17.

0 2.

05

0.06

4 16

.27

247.

8 1.

61

0.10

3 X.

79

57.9

1.

47

0.12

5 Il.

39

63.0

0.

92

0.07

7 7.

3X

118.

3 1.

27

0.12

9 21

.10

65.3

2.

54

0.05

3 7.

26

56.4

0.

20

0.24

9 11

.79

29.6

1.

31

0.05

I 5.

37

‘13.

5 0.

39

0.08

9 X.

63

145.

8 1.

03

0.06

7 1 I

.OO

285

.2

1.60

0.

1 I.7

0.

50

176.

3 0.

57

0.06

2 13

.71

666.

5 0.

90

0.23

3 11

.x.3

61

.0

I.64

0.06

9 6.

45

706.

3 0.

73

0.09

6 3.

YX

178.

5 0.

98

0.08

6 8.

36

94.3

0.

84

0.10

9 X.

10

97.3

1.

29

0.24

4 10

.77

80.8

1.

02

0.07

3 5.

67

102.

5 I.7

1 0.

061

6.02

10

7.3

I .S6

0.

074

X.82

61

.0

2.1s

0.

I40

12.2

6 47

.X

2.25

0.

057

4.72

65

.8

2.01

0.

223

IO.1

38

.5

3.02

0.

057

4.32

7X

.5

1.48

0.

064

5.94

52

.2

2.86

0.

217

10.6

0 56

.0

1.04

0.

072

4.50

10

3.8

1.93

0.

11 I

8.

74

57.X

1.

73

0.13

9 IO

.71

139,

s 1.

07

0.32

2 I I

.85

47.6

0.

64

0.07

7 4.

88

113.

0 1.

33

0.77

14

.4

47.4

27

9.8

2819

.7

IS.1

0 37

.0

0.40

6.

9 42

.7

217.

8 22

37.7

13

.05

37.0

0.

37

3.8

15.4

21

6.6

1019

.9

9.89

39

.0

1.84

24

.X

17.5

14

7.7

1324

.5

9.83

37

.0

0.28

2.

1 x.

4 41

.1

416.

3 5.

53

59.0

1.

26

26.7

18

0.9

456.

2 37

93.6

17

.83

37.0

0.

75

23.6

22

9.0

612.

8 62

82.0

19

.82

37.0

0.

69

17.1

12

9.7

294.

3 39

77.3

16

.05

40.0

0.

08

0.4

6.7

25.3

30

4.9

5.27

60

.0

0.06

0.

4 5.

X 33

.7

281.

7 4.

47

55.0

0.

39

4.7

20.5

79

.7

3506

.0

29.6

5 70

.0

0.06

0.

4 7.

9 44

.1

399.

5 6.

12

45.0

0.

07

0.4

6.1

24.1

32

0.1

5.68

68

.0

0.08

0.

2 4.

6 24

.3

243.

1 x.

21

53.0

0.

30

6.3

29.S

22

5.4

3X79

.0

IX. I

7 61

.O

0.x0

II.

7 12

8.5

23x.

9 27

30.0

I x

.73

41 .o

I.1

2 31

.Y

251.

5 52

8.6

5999

. I

21.0

4 4

I .o

0.73

12

.9

243.

1 28

9. I

60

0 I .

6 34

.05

63.0

I.1

1 13

.7

194.

9 38

4.0

362

18.0

54

.34

61.8

0.

20

I.2

I I.2

46

.7

631.

1 IO

.35

71.4

0.

08

I.6

30.7

25

3.9

3347

-.- 1

I I .3

6 39

.0

0.26

4.

6 36

.1

237.

5 ‘1

02.3

1 I

.78

37.0

0.

04

0.4

I I.2

64

.8

557.

7 5.

91

40.0

0.

10

I .o

17.4

Y9

.S

1028

. I

IO.5

7 41

.0

0.08

0.

6 IO

.6

70.8

6.

50.6

X.

06

44.0

0.

I I

I.1

19.3

11

8.3

ll3S.

S I I

.0x

43.0

0.

IO

I.1

II.8

x2.9

70

7.5

6.60

36

.0

0.07

0.

4 6.

6 31

.7

249.

5 4.

09

63.0

0.

14

0.7

6.X

71.2

30

7.2

6.42

73

.0

0.26

I.7

8.

4 26

.7

631.

1 9.

60

75.4

0.

I2

0.5

5.8

19.0

22

I .s

5.

75

64.0

0.

23

1.x

13.2

34

. I

1744

.2

22.2

2 75

.8

0.27

1.

4 12

.7

73.7

62

7.5

12.0

3 64

.0

0.13

0.

7 7.

1 29

.6

402.

2 7.

18

62.0

0.

17

I.7

14.3

XI

.5

1042

.8

10.0

5 62

.0

0.08

0.

5 6.

X 23

.6

274.

7 4.

76

64.0

0.

26

3.7

23.1

95

.5

3 10

9.8

22.2

9 61

.0

0.05

0.

3 6.

X 32

. I

329.

4 6.

92

56.0

0.

26

3.0

14.4

10

7.2

1011

.0

8.95

42

.0

0.94

I.1

9 6.

38

8.3

0.88

I .

07

6.25

6.

0 0.

83

0.39

3.

74

4.9

0.88

0.

44

3.27

7.

6 0.

79

0.21

2.

79

2.5

0.93

4.

55

21.3

8 12

.2

0.96

5.

76

18.1

6 13

.4

0.93

3.

26

13.1

6 16

.6

0.86

0.

I 7

2.93

0.

5 0.

82

0.15

2.

31

0.4

0.98

0.

51

4.35

2.

7 0.

87

0.20

3.

06

2.8

0.79

0.

15

2.72

-4

.5

0.86

0.

12

3.92

-

1.4

0.92

0.

74

3.48

-2

.6

O.Y

S 3.

23

22.1

7 10

.9

0.96

6.

32

22.1

7 17

.0

0.97

6.

1 I

34.6

7 2.

5 0.

84

4.90

7.

35

I I.4

0.

75

0.28

4.

63

m-3

.6

0.91

0.

77

3.74

9.

5 0.

89

0.91

5.

08

1.9

0.86

0.

28

2.99

X.

9 0.

88

0.44

4.

49

5.8

0.8

I 0.

27

3.30

6.

0 0.

82

0.48

4.

73

4.3

0.89

0.

30

2.76

4.

0 0.

X7

0.17

2.

71

-2.1

0.

86

0. I7

3.

57

6.5

0.93

0.

21

3.20

-6

.X

0.87

0.

15

3.80

-3

.1

0.98

0.

33

4.24

-0

.2

0.82

0.

32

6.10

-6

.Y

0.x0

0.

IX

3.20

-3

.6

0.83

0.

36

3.46

-1

.2

0.77

0.

17

2.96

-1

.3

0.97

0.

58

4.16

0.

1 0.

83

0. I7

3.

60

3.4

0.79

0.

36

3.21

5.

4

Mag

netic

par

amet

ers

(NRM

. AR

M.

IRM

,,,,,.

SIR

M)

are

expr

erse

d in

mA/

m.

May

neG

c su

scep

tibilq

(K

) va

lues

are

in I

O?’

SI u

nits

(di

men

sionl

ess)

.

1100

Time

-0:

10 ;;3;’ 1 ~ 5 7 ‘X?~ 9; i 11 : 12 I 13 15 17 18:

Time

FIG. 2. Sulphur content (%J) and magnetic susceptibility (K) (dimensionless, IO ’ SI Units) ‘tuned’ to the Oxygen Isotope Stages for samples deposited during the same time interval in the Yamato Basin (Site 794; open symbols) and the Japan Basin (Site 795; full symbols) according to the correlation with Site 797 made by Tada PI al. (1992). The isotopic stage 8

boundaries are not well defined. and are thus drawn with a different (dashed) line.

66-68), which in some cases is coupled with a lower unblocking temperature just below 300°C (Fig. 4, samples 794 3H2 77-79 and 795 4H4 15-17). The medium coercivity fraction (0.12-0.4 T) has essentially the same unblocking temperatures which suggests a mixture of (titano)magnetite with higher coercivity minerals. Iron sulfides are present in these sediments (pyrite is common throughout the cores) and an iron-rich form of pyrrhotite (Fe9S ,,,; Tc=260”C, O’Reilly, 1984; or Tc=29O”C. Thompson and Oldfield, 1986) may be the explanation for the low unblocking temperature ferrimagnetic phase present in the samples shown in Fig. 4. A similar interpretation has been given by Torii et al. ( 1992) for the magnetic mineralogy of the sediments recovered at Site 797. However, it is possible even that greigite (Fe&) is represented in these sediments. This mineral is not common in deep sea sediments, but its occurrence is certainly underestimated (Verosub and Roberts, 1995).

The hard IRM fraction (0.4-l. 1.5 T) is a very small portion of the total IRM and it is difficult to identify unblocking temperatures from the overall plots of Fig. 3. For this reason it is plotted separately in Fig. 5. This figure shows that many samples in this coercivity fraction have unblocking temperatures in excess of 58O”C,

indicating that hematite is also present in these sediments. The magnetic mineralogy of the sediments is dominated by magnetite. However, we should take into account that canted antiferromagnetic minerals such as hematite are two orders of magnitude less magnetic than ferrimagnetic minerals. Therefore, even if the concentration of hematite is much greater than that of magnetite. it will not influence significantly the bulk magnetic properties of the sediment, which will be dominantly ferrimagnetic. The hematite observed in the samples cannot be a heating artifact because, if this were the case, it would have been oriented randomly rather than along the z-axis with highest coercivity.

All these data suggest that the magnetic mineralogy of the sediments at Sites 794 and 795 is characterized by a mixture of magnetic minerals including (titano)magnetite, pyrrhotite and/or greigite, and hematite.

Carbon, Sulfur and Nitrogen Analyses

The Cclrg content of the Japan Sea sediments is variable, ranging between 0.26 and 7.63% at Site 794 (Fig. 6), and between 0.35 and 3.82% at Site 795 (Fig. 7). The sulphur

L. ViEliotti: Magnetic Properties of Sediment Layers from the Japan Sea I IO1

s 100

2

2 10

SITE 794

10000

1000

g

3 - 100

E

10

1

‘I I ’ I ’ I ’ I I ’ I

0 200 400 600 800 1000 1200 mT

SITE 795

0 200 400 600 800 1000 1200 mT

FIG. 3. Typical IRM acquisition plots for light (open symbols) and dark (full symbols) sediment layers from the Japan Sea.

IRM intensity is expressed in mA/m.

content is also variable ranging between 0.09 and 5.13% at Site 794 (Fig. 6), and between 0.11 and 5.36% at Site 795 (Fig. 7). The observed ranges of Corg and sulfur are similar to those found by Tada et al. (1992) at Site 797 in the Yamato Basin. Fig. 8 shows the magnetic susceptibility plotted against the Corg and sulfur content at the two sites. Dark sediments, with very low susceptibility values exhibit the highest C,,., and sulfur content. Except in a very few cases, both Core and S content yield values above 1% only for the dark layers, suggesting that the presence of organic carbon contributes to the dark color (Tada, 1991). Corg and S content make the dark layers similar to the sapropelitic layers found in the Mediterra- nean Sea and the Black Sea (Rohling and Hilgen, 1991).

The ratio between the Corf and the St<,, (C/S) can be used to distinguish euxinic from non-euxinic conditions (Berner and Raiswell, 1983). According to these authors the C/S ratio in modern non-euxinic marine environments is about 3. This means that most of the dark cycles were

deposited under anoxic conditions. Marine organic carbon enrichment may have been caused by increased surface- water productivity or by increased preservation rate of organic matter under anoxic conditions. In general, these anoxic conditions occur during glacial stages (Figures 6 and 7). The glacial sea of Japan has been described as a poorly ventilated basin with low surface productivity (Ujiie and Ichikura. 1973), so anoxic deep-water conditions are the dominant mechanism controlling the preservation of the organic carbon. In spite of the low carbonate content of the sediments, a surprising inverse correlation exists between variations in magnetic susceptibility and C,,,, (Figures 9 and IO). Maximum values of both C<,rg and S were measured below a depth of I I m (Six0 Stage IO) at Site 794.

The carbon/nitrogen (C/N) ratio, (Tables 1 and 2), ranges between 4 and 16 at Site 794, and although, in general, the same range is spanned at Site 795, some spikes reach values above 20 at this site. The observed C/ N ratios suggest a mixed marine/terrigenous type of organic matter with the marine proportion dominant. The lower values of this ratio (generally below 8) belong to the light layers at both the sites. This parallels the minimum organic carbon content of these layers.

Differences in Magnetic Properties between Light and Dark Layers

Individually, magnetic parameters like K, NRM, ARM and SIRM reflect the concentration of the ferrimagnetic minerals (i.e. magnetite-type) in the sediments, but also respond in different ways to variations in the average domain state (grain-size) of these minerals. In a mixed domain state assemblage. K is more strongly influenced by coarser-grained ferrimagnets while SIRM, ARM and NRM are biased toward finer-grained ferrimagnets (Dankers, 1978; Harstra, 1982; King et al., 1982; Ozdemir and Banerjee, 1982). Thus relative changes in the ratio of coarser to finer grained ferrimagnets. as well as in the total ferrimagnetic concentration. will influence these parameters. The presence of very fine-grained (<0.04 pm) ferrimagnets can be detected by the occurrence of significant frequency dependent susceptibility (e.g. Mul- lins and Tite, 1973; Thompson and Oldfield, 1986).

The magnetic properties of the sediments at Sites 794 and 795 appear to discriminate between dark, semidark and light samples on the basis of their ferrimagnetic concentration. At both sites the dark sediments are usually characterized by lower values for concentration dependent magnetic parameters, suggesting a lower magnetic content for these layers. At Site 795, however, the minimum and maximum values of SIRM, ARM and K are found in the dark layers suggesting a wider variation in ferrimagnetic concentration within the dark sediments at this site. In most cases the maximum values are observed in the dark layers deposited during the interglacial stages.

The modified Koenigsberger ratio (Q=NRM/K) provides a measure of magnetic stability as well as an indicator of changes in the magnetic properties. The

794 2HI 114-l 16

I “.m

‘I 795 2H2 66-68

0.12 - O.?T i

0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700

“C “C

200-a 794 3H2 77-79 ‘. ‘.

250 , I

3000 ,

0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700

“C “C

2500 - ! - .1~ 795 4H4 15-17

- - 2000 E

i

;2 0.12 T .g 1500-

“.,.’

FIG. 4. Types of thermal demagnetization curves of composite orthogonal IRMs. IRM values are expressed in mA/m

coercivity of the isothermal remanence Boc,,-, is indica- In order to check the domain state of the magnetic tive of the magnetic hardness of the sediment which minerals we can compare the spectrum of coercivity of is a function of both mineralogy and grain-size. the ARM and IRM by applying a modified Lowrie-Fuller Thompson and Oldfield (1986) gave a Boc,,-, of’ 33 mT test (Johnson cv crl., 1975). To make a quantitative for single domain (SD) magnetite, IS mT for multi evaluation of this test Petersen et crl. (1986) introduced a domain (MD) magnetite and 700 mT for hematite. parameter called ALF. that is the difference between the Histograms of Q and B,,(,,-, values (Fig. 1 1) show that median destructive field (MDF) of the ARM and MDF of the dark layers are characterized by lower Koenigsber- the IRM. Positive values of _1LF indicate a dominance of ger ratio values (generally below 0.2) and higher single domain magnetic grains, while negative values coercivity of remanence. Low Q values can be attrib- imply the presence of multi-domain grains. This para- uted to the presence of MD or SP magnetite grains, meter provides a numeric value for the Lowrie-Fuller test both of which contribute more strongly to susceptibility and is similar to the interparametric quotient AMDF than to remanence. However, frequency dependent (I -MDFSIKM/MDFARM) introduced by Dunlop (1953). susceptibility is below 2% in most of the samplej Increasing values of AMDF suggest a change in the (Table 1) which indicates that superparamagnetic grains domain structure from multidomain (MD, negative

are almost absent from these samples. so rhe results values) to stable single domain (SSD. positive values)

suggest that the dark layers are characterized by a larger to a mixed assemblage of SSD+MD (higher values); the grain size. This contradicts the observation that B,, same changes in the domain structure can be predicted for values are higher in these layers. but this can be ALF. This latter parameter is plotted against anhysteretic explained by a relative increase in the contribution of remanence. MDFIRbl and S-ratio for sites 794 and 795 in higher coercivity minerals. Fig. I? and Fig. 13. respectively. A positive correlation is

L. Vigliotti: Magnetic Properties of Sediment Layers from the Japan Sea 1103

SITE 794

0 100 200 300 400 500 600 700 “C

SITE 795 , \ I

F 600

3

400

200

0 0 100 200 300 400 500 600 700

“C

FIG. 5. Thermal demagnetization curves for the hard IRM fraction (0.4-1.15 T). Open (closed) symbols for light (dark)

sediments. IRM intensity in mA/m.

T

observed between ALF and ARM. but surprisingly a negative correlation is observed between ALF and MDFIKM (Figure 12 and 13). This contradicts theory by suggesting that coercivity (expressed by the MDFIRM) increases with increasing grain size (expressed by ALF). An interpretation of this apparent contradiction is that the samples with larger magnetic grain size are also characterized by an increasing contribution from minerals with higher coercivity. The positive correlation between ALF and S-ratio (Figure 12 and I3), as well as the higher Bo,r,-, values observed in the dark layers, support this interpretation.

For magnetic mineral assemblages dominated by ferrimagnetic minerals. the ratios KARM/K and SlRM/K respond to changes in magnetic grain size within the magnetically stable fraction. K increases through the SD- PSD-MD range, while KAKM and SIRM show an opposite trend. so that higher (lower) values of these ratios represent an increasing contribution from finer (coarser) magnetite grains (King et rrl., 1982). The results which best illustrate the changes in the magnetic properties within light and dark layers are represented by the samples collected from Section 5 of Core 794- 1 H and from Section 6 of Core 795 IH. These samples were collected in adjacent. well defined, light and dark horizons as can be seen in Fig. 14. In the same figure are plotted the magnetic properties, as well as the Corg and the S,clt measured in the samples. The data show clearly that the change from light to dark layers is accompanied by:

(a) increasing sulfur and organic carbon content; (b) lower magnetic mineral content expressed by a decrease in the values of K. ARM and SIRM; (c) higher coercivity defined by higher Boc,,, and lower S-ratio values; (d) coarsening of the magnetic grain size expressed by a decrease in the ilLF values. as well as in the interparametric quotients K,,,,/K and SIRM/K.

SITE 974

IO IS Depth (mbsf)

l-

FIG. 6. Downcore profiles for Cr,rf (solid symbol) and S,,,, (open symbol) content for Site 794. Ilashed lines represent boundaries of the Oxygen Isotope Stages.

1104 Qurrtrrnar~~ Science Revievcs: Volume 16

SITE 79.5

6 :7; 8?; 9 j II 13 ~ iI. i

P

17: : : : :

: : / ~

I5 20 Depth (mbsf)

FIG. 7. Downcore profiles for C,,,., (solid symbol) and S,,,, (open symbol) content for Site 7%. Dashed lines represent boundaries of the Oxygen Isotope Stages.

q Q*

&& q * @SO&

OcBBo 0 I

4 10 100 400 K (IO-6 SI units)

10 100 1000 K (lo-6 SI units)

FIG. 8. Plots of magnetic susceptibility K (dimensionless) versus sulfur content. Open (solid) symbols for light (dark)

sediments, half-solid symbol for semidark sediments.

Downcore Variations in Magnetic Properties

Downcore variations in rock magnetic parameters exhibit a significant difference between the two sites. Two major features characterize the magnetic profiles of Site 794. A significant drop in the values of all concentration-dependent bulk magnetic parameters (K, ARM, SIRM, NRM) occurs below 11 mbsf and at about 6 mbsf (Fig. 15). Downcore profiles of IRM,,,r, (indicative of ferrimagnetic mineral concentration) and of KARM/K (indicative of magnetic grain size) (Fig. 16) suggest that, at these depths, the decrease in magnetic concentration is coupled with an increase in the grain size. Values for ALF also suggest a coarsening of magnetic grain size and a shift from SD to MD domain structures. Furthermore, the sediments exhibit an increase in magnetic hardness, testified to by lower S-ratio values and higher coercivities of IRM (Fig. IS). Higher values for Corg and sulfur measured in these two intervals indicate a more anoxic environment at these times. Covariations with shifts in the ‘“0 isotopic stages suggest a correlation with the glacial stages 6 (6 mbsf) and 10 ( 1 1 mbsf).

At Site 795 there are not such well defined features in the profiles of magnetic parameters. Only KARM and IRM,,,r, (IRMZO,,,r) profiles (Fig. 17) exhibit a clear trend reflecting downcore variations in the concentration of SD magnetite. A significant decrease in the values of these parameters is observed below about 25 mbsf. Above this depth, high and low values of ARM and IRM,,,r, correspond to interglacial and glacial stages, respectively. The same glacial-interglacial signature is observed in grain size parameters ALF and KARM/K (Fig. Ig), as well as in magnetic hardnecs parameters S-ratio and Boc,,-, (Fig. 19). Maximum magnetic grain sizes and higher coercivities of IRM are typical of glacial stages, A general increase in magnetic hardness (lower S-ratio) seems to occur below about I8 mbsf.

0,l

g

g 1,

c

10

L. Vigliotti: Magnetic Properties of Sediment Layers from the Japan Sea

SITE 794

10 1.5 Depth (mbsf!

032

6,

1105

FIG. 9. Downcore profiles of C,,, (open symbol) and K (solid symbol) for Site 794.

DISCUSSION significant difference exists between each layer in the

Effects of Diagenesis on the Magnetic Properties

The magnetic properties of the Japan Sea sediments show that their magnetic mineralogy is dominated by ferrimagnetic (magnetite-type) components with a minor contribution made by iron sulfides (pyrrhotite and/or greigite) and canted-antiferromagnetic minerals (hematite). Iron sulfides such as greigite or pyrrhotite may be produced by magnetotactic bacteria in sediments rich in sulfide (Mann et al., 1990; Farina ef ul., 1990); however, their presence in these sediments is attributed to the reductive diagenesis of magnetite. Although a similar mineralogy is exhibited by both light and dark layers, a

SITE 795

content and grain size of the ferrimagnetic mineral present in the sediment. A lower concentration of magnetic minerals and a larger magnetic grain size are typical of the latter (Fig. 16).

Sulfate reduction due to bacterial degradation of organic matter has been recognized in several suboxic/ anoxic environments (Karlin and Levi, 1983, 1985; Canfield and Berner, 1987; Channel1 and Hawthorne, 1990; Leslie et al., 1990; Karlin, 1990a, b). This process leads to the dissolution of the magnetic minerals and subsequent transformation into iron sulfides. Pyrite is the most stable iron sulfide phase under reducing conditions (Berner, 1984), and this mineral has been observed

0 5 10 15 20 Depth (mbsf)

25 30 35

FIG. 10. Downcore profiles of C,,,, (open symbol) and K (solid symbol) for Site 795.

1106

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SITE 794

1107

FIG

1 10 100 1000 ARM (mA/m)

““/ I ‘, ! I /

10 20 30 40 50 60

MDF-IRM (mT)

12. Lowrie-Fuller test parameter ALF plotted versus ARM, MDF of IRM and S-ratio (-IRMP,,,,,/SIRM) for Site 794. Open (solid) symbols for light (dark) sediments, half-solid symbol for semidark sediments.

0.7 0.x 0.9 I .(I S-ratlo (-0.3T)

clearly in the sediments collected from the Japan Sea (Tamaki et al., 1990). The presence of pyrite implies the formation of intermediate ferrimagnetic iron sulfides such as pyrrhotite and greigite. In reducing environments, the reductive process proceeds through an initial dissolution of the finest magnetic fraction followed by the destruction of the remaining coarse grains (Karlin and Levi, 1983, 1985). This process can explain the magnetic properties observed in the sediments of the Japan Sea, in particular the differences between the dark and light layers as shown in Fig. 11. All the samples showed very little frequency dependent susceptibility (generally ~2%) indicating that superparamagnetic particles (SP) constitute an insignif- icant fraction. Diagenetic dissolution of magnetite explains the lower magnetic concentration and the coarser magnetic grain-size observed in the dark layers.

Paleoclimatic Implications and Origin of the Tem’genous Minerals

Major changes in the magnetic parameters correlate with glacial-interglacial cycles suggesting that climate change is the principal mechanism driving variations in magnetic properties of the Japan Sea sediments. Warm interglacial stages are generally associated with a higher magnetic mineral content and a higher proportion of fine grained ferrimagnetic minerals. Cold glacial stages have a lower magnetic mineral content and coarser assemblages of ferrimagnetic grains.

Downcore variations in the magnetic properties of the sediments can be also related to changes in the paleoclimatic/paleoceanographic conditions of the basin. At site 794. a significant change in both magnetic

-lO1.. loo0 ARM (mA/m)

SITE 795

101 i ! /

10 20 30 40 50

MDF-IRM (mT)

0.7 0.8 I).“, IO s (-0.3Tl

FIG. 13. Lowrie-Fuller test parameter ALF plotted versus ARM. MDF of IRM and S-ratio (-IRM ,, IT/SIRM) for Site 795 Open (solid) symbols for light (dark) sediments, half-solid symbol for semidark sediments.

1108

794-lH5

7951H6

0

20

E30

2

240

50

60

0

30

60

-2 3 c E. 0”

90

120

150

Quaternaq Science Reviews: Volume 16

\\

- \\\

\\ - & CEI

- ARM

I

SNM/K

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SIRMIK

/ \ I,

I \ \ I ', \

I \ I

'KAk$K'

Arbitrary Units

BCK

*;\ ,?

u;

/’

“P F

/ /

FIG. 14. Variations of physical and magnetic properties within light and dark sediments layers from two typical sections of the studied sites: 794-l H5 and 79%1H6. For units refer to Table I.


500

g t

2 100

c

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10

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-0

5 l 0

5

3

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i

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0

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SITE 974

Depth (mbsf)

10 15 20 25 Depth (mbsf)

FIG. IS. Downcore profiles of B OCcr) (solid symbol) and S-ratio (-IRM_ (, 3T/SIRM) (open symbol) for sediments from Site 794. Dashed lines represent boundaries of the oxygen isotope stages.

SITE 794

I / ,’ ,’

, / I I , $ I I I ,

10 1s Depth (mbsf)

20 25

FIG. 16. Downcore profiles of IRMlof, and K A&K for sediments from Site 794. Dashed lines represent boundaries of the oxygen isotope stages.

1110 Quatema? Science Reviews: Volume 16

SITE 795 12

1 ;3 / ,5 6 15

5 10 15 20 25 Depth (mbsf)

15 20 2s Depth (mbsf)

FIG. 17. Downcore profiles of KARM and IRM,,r, for sediments from Site 795. Dashed lines represent boundaries of the oxygen isotope stages. Open (solid) symbols for light (dark) sediments, half-solid symbol for semidark sediments.

concentration and grain size occurs below a depth of 11 m, close to the boundary between glacial/interglacial stages IO/l 1. Possible explanations for this change may take into account a different source of magnetic material, or a possible change in oceanographic conditions. The lithological description of the sediments (Tamaki et al., 1990) does not support a possible change in source for the magnetic material such as may be suggested by the occurrence of ash layers, or by differences in the biogenic content of the sediments. A possible explanation of this sudden change in the magnetic properties may be related to oceanographic factors that caused changes in the paleoenvironmental conditions. Organic carbon and sulfur data suggest that anoxic conditions were more highly developed before 6”O stage IO. A significant paleoceanographic change, such as the inflow of the Tsushima Current into the Japan Sea, may be responsible for the observed variations. Site 794 is situated exactly within the flow of this warm current, while Site 795 is offset (Fig. I ). This may explain the differences in downcore magnetic properties between the two sites. Nevertheless, at Site 795, a decrease in coercivity can be seen between 17-20 m depth which largely corresponds to the same isotope stage IO (Fig. 19). This may reinforce the evidence for an intensification of oceanic circulation in the Japan Sea since about 0.35-0.40 Ma. If this interpretation is correct. the sudden change in the

magnetic properties observed at a depth of about 6 m (glacial stage 6), may represent a major interval of lowered sea level and restricted circulation, with the Tsushima Current no longer flowing into the basin. At ODP Site 798 (Yamato Basin), the benthic foraminiferal ‘so values are lighter during this glacial stage than they are during the adjacent interglacials, suggesting an unusual warming and/or freshening of deep water (Dunbar et LZ~., 1992). At the same site. a planktonic foraminiferal assemblage rich in Globigerinu bulloides suggests upwelling conditions, and increased productivity between 0.4 and 0. IX Ma (Kheradyar, 1992). Minimum abundance values for this species, coupled with a maximum in the occurrence of Neoglohoquarlrin~l pc~chylerma (sinistral) occur exactly in the interval corresponding to the 61h0 stage 6 providing evidence for variations in sea-surface temperature, surface-water masses, and circulation during this stage.

The rock-magnetic/paleoclimatic relationships imply that climate is the main driving mechanism responsible for changes in the magnetic content of the sediment. The data suggest that both ferrimagnetic and canted-antiferromagnetic minerals contribute to the magnetic mineralogy of the sediments. Ferrimagnetic minerals may be of terrigenic, biogenic. volcanogenic. diagenetic or authi- genie origin, while canted-antiferromagnetic minerals are most probably of terrigenic origin. Among the potential


SITE 195

20 ! /

15 20 2s 30 Depth (mbsf)

: 1,

1 3j:5 6 7 ‘X?’ 9 : II 13 i IS 17 15

I”“l”’ I”’ I”“1 / 0 5 10 IS 20 25 30 3.5

Depth (mbsf)

FIG. 18. Downcore profiles of rock-magnetic parameters ALF and KARM/K for sediments from Site 795. Dashed lines represent boundaries of the Oxygen Isotope Stages. Open (solid) symbols for light (dark) sediments. half-solid symbol for

semidark sediments.

SITE 795

60

I I9 0,75

> ~

il

0,80

0,85 E 6

~ 0,90 *

0,9s

20 L-J- I,00 0 5 10 15 20 2.5 30 35

Depth (mbsf)

FIG. 19. Downcore profiles of Bo,,,-, (solid symbol) and S-ratio (-IRM (1 +SIRM) (open symbol) for sediments from Site 195.

1111

sources, a terrigenous origin is the one most likely to be influenced by climate as suggested by Robinson ( 1986). Accepting a terrigenous origin for the magnetic minerals of the Japan Sea implies an eolian or fluvial source. Even if the Yellow River is a potential source for the detrital minerals of the Japan Sea, several factors indicate that an eolian source is more likely, as has been suggested by a number of authors (Uematsu et trl., 1983: Mizota and Matsuhisa, 1984: Tada er al.. 1992). The presence of hematite supports an eolian origin. Hematite, which is not common in deep-sea sediments, is a typical component in the soils of arid regions, and thus an eolian origin may explain its presence in the Japan Sea sediments. Furthermore, the magnetic grain-size parameters (KARM/ K. ALF) exhibit a coarsening during the glacial stages (Fig. IS) similar to the maximum grain siLe observed by Tada et trl. (1992) in the detrital (probably of eolian origin) minerals at Site 797. The Japan Sea is situated along the dust trajectory of the prevailing westerly winds that s~~pply a high quantity of dust from the Mongolian and Chinese deserts. Wind-blown Asian dust is deposited as far away as the northwest Pacific Ocean (Hovan et trl., 1989) and it is one of the world’s most important eolian sources (Uematsu et ~1.. 1983). Atmospherically trans- ported dust is considered to be the source of the paleoclimatic signal observed in the pelagic clay sequences of the North Pacific (Yamazaki and Katsura. 1990). The hematite observed within the Japan Sea sediments is strongly suggestive of an aeolian origin for at least one of the terrigenous components in the Japan Sea sediments, and this may be the reason for their paleoclimatological record.

CONCLUSIONS

The magnetic properties of the light and dark layers from the Japan Sea are controlled by climatically-induced changes in oceanographic conditions in the Sea of Japan, leading to systematic changes in sediment (and magnetic mineral) diagenesis. The magnetic mineralogy of the sediments is dominated by ferrimagnetic (magnetite-type) minerals with a small contribution made by canted- antiferromagnetic minerals (hematite) and iron sulphides (pyrrhotite and/or greigite). Whilst mineralogically similar. differences exist between lighl and dark layers in terms of magnetic mineral concentration and grain size. Reductive diagenesis involving magnetite dissolution controls the magnetic properties of the dark layers. Sulfur and organic carbon content show that the euxinic conditions responsible for this process occur during glacial stages and are probably related to sea level changes. This strongly suggests that climate exerts ;I major control on the magnetic properties. Notwithstand- ing the difficulties involved in decoupling the climatic signal from the diagenetic overprint, the former is clearly dominating the magnetic properties as indicated by relationships between magnetic parameters and oxygen isotope stages. The oceanic circulation influenced by the arctic cold fronts system and cold sea current dominated

the climate of Japan through the Brunhes as recognked by palynological studies carried out by Fuji and Horowitz (1989) on lacustrine sediments from Lake Biwa.

Wind-blown dust containing hematite, probably in the form of red coated desert quartz grains, and ferrimagnetic minerals probably of volcanogenic origin are the main contributors to the terrigenous input of the Japan Sea sediments. Grain-size dependent rock magnetic parameters (K,\&K: ALF) provide evidence for coarser ferrimagnetic grain size assemblages during glacial stages.

During the Brunhes Chron more deeply developed anoxic conditions occurred in the Japan Sea before about 0.35-0.40 Ma when the warm Tsushima Current started to flow into the basin with the resulting development of more oxidizing conditions. Since that time the data imply that an interruption to this flow may also have occurred during Isotope Stage 6.

The magnetic record of the Japan sea sediments parallel the magnetic signature of the Chinese loess and both match the SPECMAC oxygen isotope stack (Imbrie et (11.. 1984). Since very different factors such as the source of the sediments and the global storage of ice control these records. this implies that they respond to a global model of the atmospheric and oceanic circulation. Rock magnetic variations in Late Quaternary sediments from the Japan Sea can be used for paleoclimatic and paleoceanographic reconstructions, as tracers of a terrigenous component source and as indicators of reductive diagenetic processes related to anoxic conditions of the basin. Magnetic measurements provide a high-resolution constraint on the paleoclimatic/paleoceanographic conditions of the basin and confirm that at least during the Upper Quaternary. the Japan Sea behaved as a single sedimentary system responding to climatic changes.

ACKNOWLEDGEMENTS

The author thank\ Proi’eswr M.B. Cita for i‘inanclal support of this work, A. Boschrtti wd A. Ccwri for help during the experimentc He is xlw grateful to T. Rolph for critxal reading of the manuscript and to S. Robinson for hi\ helpful suggcstion~ to improve the paper. This is IGM pubhcntion N 1055.

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Magnetic properties of light and dark sediment layers from the japan sea: Diagenetic and...

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