ge ecs aos2007

Acta Oceanolo#ca Sinica 2007，Vo1．26，No．5，P．54～65

http：／／www．oceanpress．tom．on

E—mail：hyxbe@263．net

Rock gnetic property of gravity core CSH1 frq the northernRock magnetic prope gravity H1[rom the northern

Okinawa Trough and the effect of early diagenesis

GE Shulan 一，SHI Xuefa ，W U Yonghlla ，LEE Tehquei ，

XIONG Yingqian ，SAITO Yoshiki

Marine Sedimentation and Environmental Geology Key Laboratory of State Oceanic Administration，First Institute of Oceanogra—

phy，State Oceanic Administration，Qingdao 26606 1，China

Institute of Geology and Geophysics，Chinese Academy of Sciences，Bering 100029，China

Institute of Earth Sciences，Taipei，China

Geology Survey of Japan，Tsukuba 306—8567，Japan

Received 20 November 2006；accepted 22 May 2007

Abstract

Detailed rock magnetic investigations were undertaken at 2～4 cm interval for the gravity core CSHI(with a length of 17．36 m)

from the northern Okinawa Trough．Time—scale of the core was constructed by two characteristic tephras and foraminferal assembla—

ges，indicating an age of 50 ka for the bottom of the core．Except for three tephras and abrupt decrease in surface，there are little

changes in all kinds of rock magnetic parameters that can be correlated to the climate change for the last 50 ka．Different from the

common sediments，most S—ratios(S equals the negative ratio of IRM一0 3T to SIRM，which is an indicator of low coercivity con-

tent)of the sediments are smaller than 0．9，which implies a substantial amount of magnetic minerals with high coercivity．The ex—

istence of iron sulphide(greigite or pyrrhotite)is revealed by representative susceptibility—temperature curves showing 200～

350 oC Curie temperature in addition to 580℃ of magnetite．and also by awful smell during heating and dark heating products．

Both the occurrence of authigenic iron sulphide and quick decrease of magnetic parameters near the surface clearly show that sedi—

ments from Core CSHI have undergone early diagenesis．The featureless magn etic changes of the whole core except for three teph—

ras mean that the post—depositonal alteration is so strong that most original sign als have been destroyed．For the same reason，the

organic matter in sediment and sulphate in pore water must have been consumed along with dissolution，precipitation of iron and

manganese happening sequencially during the redox reaction series．Great caution must be taken when using these altered chemi—

cal parameters for the interpretation of climatic changes．

Key words：northern Okinawa Trough，gravity core of CSHI，rock magnetic property，characteristic tephra，early diagenesis

1 Introduction

Most areas of the continental sea east of China

have once been exposed to the air with the exception

} Corresponding author。E—mail：xfshi@rio．org．ca

of the Okinawa Trough where sediments continuously

deposited and uninterrupted geological records since

last glacial periods would be expected．Furthermore，

the flowing across of Kuroshio Current also makes

this area a key place of interest for the study of his—

torical evolution of the Kuroshio，and the above two

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GE Shulan et a1．Acta Oceanologica Sinica 2007，Vo1．26，No．5， P．54～65

reasons together lead to a hot sediment study in the

trough(Jian et a1．，2000；Liu et a1．，2000；Li，

Shi et a1．，1999；Li，Wang et a1．，1999；Xu and

Oda，1999；Wang et a1． 1996；Marine Geology

Bureau of China Geological Survey，1985)．Since

Okinawa Trough is an active back·-arc spreading cen．-

ter，faults，volcanoes and submarine thermal fluid

occur extensively at these areas(Wu，2000)．Di—

verse deposition environments and complex sediment

sources result in a big discrepancy of sedimentary

rates in different locations and different periods in

the same place(Meng et a1．，2001；Li，Wang et

a1．，1999)．The recent study shows that the sedi—

mentary rate in the trough is much higher than that

previously thought(Liu et a1．，2000；Li，Shi et a1．，

1999；Li，W ang et a1．，1999；Marine Geology Bu—

reau of China Geological Survey，1985)．High accu—

mulation rate and large amount of organic matters

brought by the Kuroshio may contribute to the possi—

ble reducing environment in many areas of the trough

(Meng et a1．，2001)，which provides an objective

condition for the early diagenetic development．The

preliminary study of early diagenesis began early

abroad and the detailed rock magnetic and geochemi—

cal behaviors of early diagenesis from sediments of

continental shelf and slope have been reported fre—

quently in recent years，and the effects on paleomag-

netism and environmental magnetism were also dis—

cussed(Robinson and Sahota，2000；Roberts and

Pillans，1993；Karlin，1990；Leslie，Hammond et

a1．，1990；Leslie， Lund et a1． 1990；Karlin et

a1．，1987；Karlin and Levi，1983)．Researchers at

home also recognize gradually the importance of early

diagenesis and its effect on the southern Huanghai

Sea(SHS)sediments(Liu，Zhu et a1．，2003；Liu

et a1．，2004)，but the early diagenesis in the Oki—

nawa Trough and its effect on record have not attrac—

ted enough attention．

Early diagenesis can change the magnetic prop—

erty through dissolution and re—precipitation of the

55

original magnetic carriers． Secondary remanence of

newly form ed minerals superposes natural remanence

with unknown delay time，

pretation of paleomagnetic

complex(Karlin，1990)．

investigated the deglacial

which can make the inter—

and rock magnetic results

Liu，Zhu et a1．(2003)

muddy sediments in Core

YSDP103 from SHS and postglacial muddy sediments

in Cores SSDP102 and SSDP103 in the Korean Strait

(Liu et a1．，2004)。The rock magnetic and geo—

chemical property revealed high—resolution processes

of early diagenesis and proved that the rock magnetic

behavior did not respond to climatic changes．Except

for the influence on the magnetic parameters，the

dissolution and re—precipitation of minerals with

iron，manganese and sulphur in the early diagenetic

process will affect geochemical results to a certain

extent．The geochemical researches in the Okinawa

Trough are very active for the moment(Xiong and

Liu，2004；Liu，Meng et a1．，2003；Meng et a1．，

200 1)and provide powerful proof for the climatic

and sediment source changes．Possible existence of

early diagenesis will hamper the recording of geo—

chemical signature of climate and environment，and

should not be neglected in the geochemical studies．

This paper discussed the rock magnetic behav—

ior of gravity core CSH1 and revealed the effect of

early diagenesis on magnetic parameters．Core CSH1

(31。13．7 N，128。43．4'E)has a length of 17．36 m

(the longest gravity core at home)and a water depth

of 703 m．It lies in the north of west slope adjacent

to the outer slope of East China Sea and is actually

the foot of the slope(see Fig．1)．According to the

upper two tephras and foraminiferal assemblage，the

age of the core may be 50 ka．

2 Sample and method

Cubic nonmagnetic plastic box with a 2 am side

length was used to get 844 samples．Magnetic meas—

urements were undertaken in the paleomagnetic labo—



34。

N

32。

30。

28。

26

GE Shulan et a1．Acta Oceanologica Sinica 2007， Vo1．26，No．5，P．54～65

120。 122 124。 126。 128。 130。 132。E

Fig．1． Locations of CSH1(star)and other related cores

(dot)．KH82—4 —14 from Xu and Oda(1999)；

DGKS9603 from Liu et a1．(2000)and Xiong and Liu

(2004)；EA03—180 from Liu，Meng et a1．(2003)；Z1

— 4 from Yan(1990)．

ratory of the Institute of Earth Sciences in Taipei．

Low frequency and high frequency susceptibilities

(Klf and Kl1f)were acquired by Bartington MS2 mag—

netometer，the difference between them was used to

calculate frequency dependent susceptibility[Kfd

(Klf—Kl1f)／Klf x 100％]．Anhysteresis remanent

magnetization(ARM)was imparted by a decreasing

alternate field of maximum field 80 mT and a direct

field of 0．05 mT． Saturation isothermal remanent

magnetization(SIRM)was done in a maximum field

of 1 T and a back field of 0．3 T was used to get low

coercivity remanence(IRM-0．3T)and the above two

were used to get high coercivity remanence

(HIRM)and S ratio．We used the formula of

HIRM equaling SIRM plus IRM一 0．3T

divided by 2

(Thompson and Oldfield，1986；Robinson，1986)

and S equaling the negative ratio of IRM一

0．3T to

SIRM (King and Channell，1 99 1)to calculate high

coercivity remanence and S ratio． Susceptibility—

temperature(k—T)curve was measured in the pal—

eomagnetic labortary of the Institute of the Geology

and Geophysics， Chinese Academy of Sciences，

temperature range is from room temperature to

700~C．Samples from the three tephras in core were

collected for tephra measurement and the results are

provided by the National Institute of Advanced In—

dustrial Science and Technology of Japan．

The interval for grain size analysis is 4 cm and

t0tal samples are 434．Before measurement．the sam—

ples were deleted organ ic matter and carbonate by H2

O．and diluted HC1． A1l the above measurements

were done in the granulometry laboratory in the First

Institute of Oceanography，State Oceanic Administra—

tion with a Mastersizer 2000 granulometer．

3 Results

3．1 Sedimentary and rock magnetic properties of

the depositional sequence

3．1．1 Sedimentaryproperty

Dark—grey and grey silty mud and mud dominate

in the sequence，which implies a redox depositional

condition． Dark—grey ooze is at 2．04 ～ 4．42．

4．42～7．08 and 15．42～17．3 m where strong smell

0f H．S occurs． Shell debris and big forams can be

seen in the whole cnre．

3．1．2 Three tephras and the establishment of age

framework ofthe C0，．e

Dark—gray silty mud and mud alternate in the

whole core(see Fig．2)．There occur three obvious

susceptibility peaks in very low (<5 x 10 )and

stable background values，peak depths are 0．73～

1．11，7．91 and 15．63～l6．04 m (see Fig．2b)．

Distribution of mean grain size also has three abnor—

mal coarse peaks(see Fig．2a)．Previous studies

show that volcanic glasses commonly present uncom—

mOB coarse component and high susceptibility， so

the grain size and susceptibility peaks in this core

imply the existence of volcanic materials(Rea et

a1．，1998；Thompson and Oldfield，1986)．



58 GE Shulan et a1．Acta Oceanologica Sinica 2007，Vo1．26，No．5，P．54～65

Recently，the tephra—chronolgy in the sediment

of Japan and nearby area has gained rapid progress，

characteristic tephras preserved in marine sediments

have become an effective tool for Quaternary strati—

graphic correlation (Machida，1999a，b；Machida

and Ami，1983)．According to the research of

Machida et a1．，there are two tephras which can

reach the core location in the last 50 ka．one is K—

Ah tephra originating from the subaquatic Kikai vent

south of Kyfishca：the other is AT tephra of Arai vent

in the land of south Kvashfi．The HC dating of K—

Ah is 6．3 ka，deposited in the tran sgressive maximum

of Holocene：calendar year of AT tephra is 25～24 ka

and in the transition period of MIS3 and MIS2 of ma—

rine cores(Machida，1999b)．If the three tephras in

this core can be correlated with certain tephras，then

they can provide indubitable age indication．

Th e three coarse grains from latent tephras are

examined under microscope and volcanic dass and

debris of all kinds of shapes are discovered．Th e first

tephra is mainly tran sparent ass slices，the ratio in

the coarse grains is high；the second tephra is also

the transparent glass．the shapes include angular，

bunchy and slices；the third tephra is transparent and

mainly the brown color glass with big size，low con

centration and less slices(Wu，2003)．

Refractive index is the foundation for the identi—

ficati0n 0f certain tephras (Macihda，1999a，b；

Machida and Arai． 1 983)． After pretreatment，

62．5～125 Ixm components from the three tephras

were made into rock slices for analysis of mineral

components，heavy minerals and volcanic shapes，

als0 for measurement of refractive index．Th e follow—

ing result is the statistics from 200 grains of above

size range(see Table 1)．The first tephra is com—

posed of 90％ volcanic glass．5．5％ light minerals，

4．5％ planktonic foram and little heavy minerals．

Th e heavy minerals are orthopyroxene，clinopyrox—

ene，opaque mineral and apatite．The glass is of the

bubble—walled type，more than pumiceous type and

the refractive index is 1．5 1 1．which is coherent with

the petrographic property of K —Ah from Machida

(1999b)．The second tephra includes 80．5％ vol—

canic glass，7．5％ light mineral，5．5％ rock debris

and trace heavy minera1． Heavy minerals are 0卜

thopyroxene，clinopyroxene，amphibole and apatite．

The glass type is bubble—walled and horizontal and

the refractive index is 1．499．in agreement with AT

tephra(Machida，1 999b)．The third tephra is com—

posed of 22％ volcanic glass， 1 1％ light mineral ，

1．5％ heavy mineral and 50％ micropaleontology．

Heavy minerals are orthopyroxene，clinopyroxene，

brown amphibole，zircon，biotite and apatite． The

bubble—walled and pumiceous glass types have simi—

lar number and the refractive index has two peaks of

1．499 and 1．511．which are identified as K —Tz

and Ata tephra respectively(see Table 1)．

The refractive identification further confirills the

upper two tephras ，but conflict still exists for the

third one．The TL(thermoluminescence)age for K—

Tz is 9O～95 ka and that for Ata tephra is 95～110 ka

(Machida，1999a)，which are both different with the

extrapolation according to the upper two tephra ages．

We already know that the sedimentation rate in the

Okinawa Trough changes a lot，so it still needs de—

tailed age dating to prove the validity of extrapola—

tion．Maybe the tephra is K —Tz or Ata，so the bot—

tom sediment may belong to MISS． An inadequate

proof is that a small number of G．ruber(pink)is

found discontinuously under depth 16．21 m which

species is extinct in MIS5e(Cang and Yan，1992)．

According to the element sequ ence correlation be—

tween cores，Yan(1990)deduced that the bottom of

Core Z1—4(see Fig．1)(from 4．5 to 6．93 m)be一

10ngs t0 MIS3 and seems back the conclusion that the

c0re bott0m of the paper is MIS5．In general，the ol—

dest age of this core is still open to disoussion．

The ab0ve tephras also occur in other cores of

Okinawa Trough．Core KH82—4—14(simplified as

KH)lying at the northeast，60 km away from CSH1



GE Shulan et a1．Acta Oceanologica Sinica 2007，Vo1．26， No．5，P．54 65

Table 1． Three tehras’type，refractive index and identification results in the long core CSH1

59

calendar vear／a

KH82—4—14 CSH

0

K—A

2o0

400

600

嘻

0 AI’

800

1 0oO

Calendar vear／a

DCK$9603

一 U一目ii§i

‘一10 340 一一 2O

<--15 602

一 40

《 18 844 一 I [ n

《一 22 579 w

25 630

8O

tept ·∈一30 996

‘一35138 一 1 C

一 1：

s1 Jtv c Jav—c Jav

l

一

grav 1(

tep| ·__-__- [tephra

1 S

E

b

Fig．3． Tephra correlation and age from Cores CSH1，KH

and DGKS9603 in the Okinawa Trough．There is no detailed

description of tephra in Core DGKS9603，the depth is ven

according to the paper of Liu et a1．(2000)．

(see Fig．1)，is located at the flat foot of continental

shelf．Core KH presents detailed H C datings(Fig．

3 a)．which serve as the base for the identification of

tephras K —Ah and AT．The correlation between the

two cores makes it clear that the occurrence depth of

tephra 1 and tephra 2 in this core are almost identi—

cal with K —Ah and AT in Core KH f Figs 3a and

b)．The close location，flatness and little change in

water depth between the two cores all contribute to a

similar sedimentary environment and sedimentation

rate．So tephra 1 and tephra 2 should be the two

tephras in Core KH，that is to say，K —Ah with an

age of 6．3 ka and AT with an age of 24 ka．

Furthermore， tephras also occur in Core

DGKS9603 faraway from the above two cores(Fig．

3 c)．Detailed H C datings prove that the calendar

year for the first tephra is 5 300～8 950 a， agreeable

with the age of K —Ah．There is no record of AT

tephra，which may be ignored．The calendar year of

second recorded tephra is 41 400 a．It can be seen

that the tephras in Core CSH1 are not locally but ex—

tensively distributed in the Okinawa Trough，and can

serve as time markers for correlation in this area．

Foraminifera assemblage indicates that the Hol—

OCene began at 2．04 m，the transition from intersta—

dial to LGM of last

6．84 m ．6．84～l7．36

sediment(MIS3)(Wu，

glacial period occurred at

m belongs to th．

e interstadial

2003)．

3．1．3 Rock magnetic properties of the sequence

Figure 2 shows all kinds of rock magnetic pa—

rameters and mean size changes in the core． Th e

mean size results show that the sediment below l 6 m



GE Shulan et a1．Acta Oceanologica Sinica 2007，Vo1．26，No．5，P．54～65

has finer grain with a mean value of 74(7．8 p．m)，

and the finest is about 7．5 at 12～15 m and gradu—

ally coarsens upwards(see Fig．2a)．Apart from the

three tephras(shown as shadows in Fig．3)，there

are no obvious changes for magnetic susceptibility，

ARM ，SIRM and S ratio，even in MIS1，MIS3 and

MIS2 when drastic environmental and climatic chan—

ges happened．W hat causes the failure of rock mag—

netic recording of climate?The answer is the early

diagenesis in the sediment．

To clearly show the rock magnetic behavior of

early diagenesis， the upper 2．2 m was enlarged

(Fig．4)．It can be seen that all the parameters de—

cline abruptly in scores of centimeters near the sur—

face of sediment．ARM，SIRM and magnetic suscep—

tibility are the highest of the core at 0～0．11 m，all

the three parameters decrease from 0．1 1 m down to

0．33 m and stabilized at 0．33 m． Values below

0．33 m for ARM，SIRM and magnetic susceptibility

declined 86％，80％ and 58％ respectively． Be—

tween 0．1 1 ～0．33 m coarsening of magnetic size

downwards can be seen(the ratio of ARM to SIRM，

indicator of magnetic grain size)，the S ratio also de—

creases from the upper 0．92～0．99 down to 0．81～

0．92(except 0．998 at 0．69 m)．The abrupt decline

and coarsening of magnetic minerals is the same as

deglacial sediments in south Huanghai Sea cores，

which can all be interpreted by prior dissolution of

fine magnetite．Th e decrease of S ratio indicates the

formation of magnetic minerals of high coercivity

(pyrrhotite and greigite)(Liu et a1．，2004；Liu，

Zhu et a1．，2003)．The gradual fining below 0．33

m (increasing ratio of ARM to SIRM)and first

slight decline and following stabilization may be the

dissolution of high coercivity magnetic minerals．

This fining includes further dissolution of magnetic

minerals and sequencial form ation of selfborn sul—

phide at certain redox conditions：first is mackinaw—

ite(FeS0．9)，then high coercivity pyrrhotite or

greigite(Fe3 S4)and ultimately paramagnetic pyrite

(FeS2)(Thompson and Oldfield，1986；Berner，

1984)．

HIRM SIRM／x10 A·nl! ARM／x10一 A·nl! mag．SUS．f10一 1 ARM／SIRM S

0．00 0．o02 0．o0 0．02 0．04

0

Il

33

E

专 100

0

coarse．．．．．． fine

0．000 2 0．00040 5 l0 l5 20 250．000 0．008 0．016 0．4 0．6 0．8 1．0

l⋯ ‘1 I⋯ 。l

一 ’ l__ 一一： ⋯ 一一i一一一一

一

_ 。一

：一一一一一一一一一一一一一{ 一一一一 ■ 一＼

i -

一

。 ● ‘

—

；臻 0譬苍0≮1 誊囊鬟善茅毒每霉霉 0 蠢每冬恐毫蕊怒芬撼薯

尊蠹翼羹誊攀爱

0

嚣：0 0 ：：‘ ：：

枣鬻誊；奢奢殳囊：客奢囊念．警母k Ah t品；瓣鬣毒搿冬套 ≥

譬：：：：

萼戮婺露菇尊曩港

蓉 I、

＼

’

i i

t ～

气 _{ 一、

Fig．4． The abrupt decline of rock magnetic parameters in the upper 2．20 in of Core CSH1

0～0 11 m is oxidizing zone，below 0．33 ill is reducing zone．



GE Shulan et a1．Acta Oceanologica Sinica 2007， Vo1．26，No．5，P．54～65

3．2 Evidences of early diagenesis from rock mag—

netism and other items

To further make sure the existence of iron sul—

phide，we measured magnetic susceptibility — ten—

perature curves for two samples in the course of re—

ducing and after reducing to reveal the magn etic

phase and transformation in the process of heating

and cooling．The whole experiment is done in argon

to avoid possible oxidation．

Sample from the process of dissolution(Sample

CSH1 — 9) has a low magnetic susceptibility

(≈30×10 m ／kg)，but increases a lot during

heating and cooling，showing the form ation of mag—

netite during heating(see Figs 5a and b)．Small de—

cline from room temperature to 1 20 22 indicates a

small amount of goethite．Small peak form s from 120

to 19022 ，revealing new form ation of magnetic min—

erals．Magnetic susceptibility declines steadily in a

wide range of 190～37022 ，which may be the Curie

temperature of greigite rather than pyrrhotite(pyr—

rhotite has a narrow Curie temperature of 320 —-

32522)(Hu et a1．，2002；Roberts，1995)．Slow

decline at 380—-55022 and quick decline at 550—-

58022 both show the Curie point of magnetite．S ra—

tio slightly above 0．86 means that the original mag—

netite still has its proportion．Magnetite includes the

original and new form ing magn etite．

Sample from the stable section of magnetic SUS—

ceptibility in the lower part of the core(CSH1—

54)(see Figs 5c and d)has even low original

magnetic susceptibility，which may be the result of

formation of weak magnetic greigite or pyrite through

magnetite reduction．Magnetic susceptibility gradu—

ally decreases from room temperature to 300 22 and

shows an obvious fall between 90 ～l10℃ which

may be goethite． Magnetic susceptibility increases

from 300～43022 ，which may be the dissolution of

greigite or pyrite to magnetite(Dekkers et a1．，

61

2000； Geiss and Baneriee，1997； Roberts and

Turner，1993)．From 430 to 600℃ ．magnetic SUS—

ceptibility first drastically (430～470 22) then

gradually falls to null，showing the Curie point of

magnetite．The content of greigite or pyrite of Core

CSH1—54 is higher than Core CSH1—9 because

more new magnetite is form ed after 300℃ and cool—

ing magnetic susceptibility is four times and two

times the original value respectively． There is an

awful smell at the high temperature process，char—

acteristic of iron sulphide．

In addition，X．ray diffractive results show an

extensive occurrence of pyrite in the whole core．

Core CSH1 lies in the far north of Okinawa Trough

where no volcanism and hydrothermalism exist(Wu，

2000)，so these pyrites are not the product of vol—

canic eruption but mainly authigenic，which indi—

cates early diagenesis．

It has been discussed above that there are no

obvious variation in the petrographic property of the

whole core，so the drastic changes of rock magnetic

parameters above are the characteristic behavior of

early diagenesis． These characteristics have been

found in many cores located in the continental shelf

and slope and have become powerful indicators for

early diagenesis(Liu et a1．，2004；Liu，Zhu et

a1．，2003； Robinson and Sahota， 2000； Roberts

and Turner．， 1993； Leslie，Lund et a1．， 1990；

Le slie，Hammond et a1．，1990；Karlin，1990；Kar—

lin et a1．，1987；Karlin and Levi，1983)．Owing to

the similarity of sediment sources and dynamics，the

magnetic property of Core CSH1 is much more alike

with sediments from the muddy area of the adjacent

south Huanghai Sea and Korean Strait． Those sedi—

ments have increasing total iron and sulphur content

near surface and remanence loss between 200 and

320℃ which testifies the occurrence of early diagen—

esis and formation of iron sulphide(Liu et a1．，

2004；Liu，Zhu et a1．，2003)．



62 GE Shulan et a1． Acta Oceanologica Sinica 2007

，Vo1．26，No．5，P．54～65

l20

80

0

l20

80

0

0 200 400 600

Temperature／。C

0 200 400 600

Temperature／。C

J0

0

5

0

90～l20

0 20o

370-386

40o

Temperature／。C

580～60o

6o0

430-470

0 200 400 600

temperature／。C

Fig．5． Representative susceptibility—temperature curve in core CSH1．Bold lines represent heating and thin lines repre．

sent cooling；the right curve is enlarged heating curve of left sample，the number at abscissa is the Curie point range． Sample

CSH1—9 is from the transitional zone(between oxidizing and reducing zone)；CSH1—54 is from the reducing zone．

4 Discussion

The ultimate cause of early diagenesis of marine

sediments iS the anoxia resulted from oxidation of or—

ganic matter through microorganism (Karlin et a1．，

1987；Karlin and Levi，1983)．Reduction by micro—

organism (bacteria)generally begins first from the

high—energy compound．SO the reducing process in

pore water is in sequence from high to low free ener—

gy．More than 20 a ago，Froelich et a1．(1979)

found a complete chemical series from the top down：

first the direct oxidation under oxic condition，then

the nutrient and manganese oxides reducing under

the anoxic condition，lastly the reducing of iron OX—

ides．If organic matter still exists，sulphate begins to

be reduced and the product of H，S will react with the

bivalent iron to form iron sulphide， which COl-re—

sponds to the drastic decline of magnetic parameters

(Froelich et a1．，1979)．The existence of iron sul—

phide and smell of H2 S in this core indicate the

process has reached the sulphate reducing period．

Selfborn magnetic minerals may produce de—

layed remanence(Roberts and Turner，1 993；van

Hoof and Langereis，1 99 1)and could be a disturb—

ance to norm al magnetostratigraphy．The greigite and

一．

)1．巨呷0一×，̂兰IqIl ∞譬= 苟＆日巨日至一I爿)1．巨叩0一×，̂兰IqIl 基=∞ 苟=Ⅱ日暑豢日至



GE Shulan et a1．Aeta Oceanologica Sinica 2007，Vo1．26，No．5，P．54～65

pyrrhotite in sediments can always be remagnetized

and complicate the polarity record． Roberts and

Turner(1 993)thought greigite could have a rather

long—time lag of remanence．Study of early diagene—

sis from postglacial sediments from south of Korea

Peninsula by Liu et a1．(2004)proves that the time

lag of greigite would be several thousands of years．

Because of the same reason，further work will still be

needed to prove the validity of magnetostratigraphy

from Core CSH1．The happening of early diagenesis

can be a reason for the unstable remanence in the

upper 1 m of the core(paleomagnetic results of this

core)．The co—settlement of tephra with sediment

avoids the lag and superposition of secondary rema—

nences． Furthermore， early diagenesis makes the

rock magnetic c

climatic change

should be taken

tal significance．

hanges be the combined response to

and early diagenesis， so caution

in the interpretation of environmen—

The occurrence of early diagenesis can point to

two specialities in this core：the large sedimentation

rate leaves no enough time for sediment oxidation；

decomposing of rich organic matter quickly consumes

the limited oxygen．These two aspects are also coher—

ent with the location of the foot of continental shelf，

near land and flowing across of Kuroshio with a large

amount of organic matter， so the early diagenesis

quickly revealed by rock magnetic property actually

pinpoints the particular conditions and sedimentary

process．A large amount of selfborn magnetic miner—

als produced by early diagenesis destroy original

magn etic sign als and the featureless change is no

longer adapted to the paleoclimatic discussion and

will also have effect on the record of geochemical pa—

rameters related with early diagenesis．

(1)Tephra K—Ah and AT are convincing time

markers for Core CSH1．

63

(2)Except for the three tephras，no feature

variations occur in the rock magnetic parameters for

the whole core．

(3)Rock magnetic

decline quickly within the

parameters of Core CSH1

surface scores of centime—

ters．which is the characteristic behavior o±early di—

agenesis．Original signals were intensively destroyed

by early diagenesis and show featureless variation

that could not be proxy of climate changes for the last

50 ka．

(4)Early diagenesis brings disadvantage to the

paleoceanographical and paleoclimatical studies by

means of magnetostratigraphy and rock magnetism ，

but its particular indication for certain paleo-·environ·-

ment(paleo redox boundary)will open a new win—

dow for the study of environmental magnetism．

Acknowledgements

This work is supported by the Key National Sci—

ence Foundation Program under contract No．

4043 1 002，the National Science Foundation Program

under contract No．40574029 and the State Oceanic

Administration Foundation Program for Youth under

contract No．2004303．

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