185Journal of Paleolimnology 23: 185–199, 2000.© 2000 Kluwer Academic Publishers. Printed in the Netherlands.

Stratigraphy of multiple piston-core sediments for the last 30,000 years fromLake Biwa, Japan

Keiji Takemura1, Akira Hayashida2, Makoto Okamura3, Hiromi Matsuoka3, Mohammad Ali1*, Yoshiki Kuniko4

& Masayuki Torii1**

1Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan2Science and Engineering Research Institute, Doshisha University, Kyo-Tanabe, Kyoto 610-0321, Japan3Department of Geology, Kochi University, Kochi 780-8520, Japan4Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan*Present address: Geoscience Laboratory, Geological Survey of Pakistan, Islamabad, Pakistan**Present address: Department of Biosphere-Geosphere System Science, Okayama University of Science,Okayama 700-0005, Japan

Received 28 July 1998; accepted 18 February 1999

Key words: Lake Biwa, piston core, volcanic ash, magnetic susceptibility, tephrostratigraphy, sedimentationrate, paleoenvironment

Abstract

Analysis of seven piston cores obtained from three sites in Lake Biwa provide a detailed stratigraphy of the lakesediments for the last 30 kyrs. Description of lithology and measurement of magnetic susceptibility reveal occurrenceof volcanic ashes at ten horizons, that were visually observed or microscopically found from clayey sediments. Mostash layers are observed in multiple cores from the three sites and also in earlier cores, establishing correlation and ageassignment of the core sediments. This result demonstrates usefulness of magnetic susceptibility data in findingdispersed volcanic ashes. The age-depth curves deduced from published tephra ages show that significant differencein sedimentation rates occurs in Lake Biwa. The central part of the depression is characterized by homogeneoussediments deposited with a high sedimentation rate. The 250-m-thick lacustrine clay in this sub-basin has great potentialfor high-resolution study of the past environment and paleomagnetism for the last 0.5 Myr.

Introduction

High resolution study of past environmental changerequires continuous and homogeneous recordingmedia. Continuous lacustrine clay with a high sedimen-tation rate is one such material. Lake Biwa, the largestlake in Japan and situated in the central part of the mainisland of Japan, is a sedimentary basin that can providecontinuous record of past environments since theMiddle Pleistocene. Several attempts to recover coresediments from Lake Biwa have been made, mainly inthe 65–70 m-deep depression situated south of theNorth Basin. Horie and his colleagues first recovereda 6 m-long sediment core in 1965 and then a 11.5 m-

long piston core in 1967 (Horie et al., 1971). In 1971,they made an epoch-making effort to drill the sedimentsin the same depression (Figure 1), and successfullyobtained core samples totaling about 200 m long(Horie, 1984). From 1982–1983, they succeeded inrecovering the entire lake sediment record in the samearea. This record confirmed that the basin is filled withlacustrine and fluvial sediments about 800 m thick(Takemura & Yokoyama, 1989; Horie, 1991). Theuppermost unit consists of lacustrine clay and silt about250 m thick, which is estimated to have depositedcontinuously during the last 430 kyrs (Takemura, 1990;Meyers et al., 1993). About 5 km northeast of thesedrilling sites, samples of 141 m thick sediment were

186

recovered in 1986 (Figure 1; Yoshikawa & Inouchi,1991, 1993). Several shorter cores have been collectedfrom near-shore areas of the northern lake and used forsedimentological study of recent sediments (Yamamuraet al., 1990; Inouchi et al., 1993).

The core samples from Lake Biwa have providedimportant information on geomagnetic fluctuationduring Pleistocene time. Kawai et al. (1972) measurednatural remanent magnetization of the 200-m coresample obtained by Horie’s team and found severalshort polarity reversals. The reversal record at 50.0–55.0 m deep was correlated to the Blake excursion at

around 100 kyrs BP, whereas the lower two (at 80.5–85.0 m and 130.0–132.5 m) were identified as newexcursions, called the Biwa I and Biwa II ‘events’(Kawai et al., 1972). Torii et al. (1986), however,reported no reversed polarity data in the Brunhessediments recovered by the drilling in 1982-83. Mean-while a re-examination of tephra deposits in the 200-mcore sample was made by Machida et al. (1991), whoassigned the volcanic ash around the horizon of theBiwa I event of Kawai et al. (1972) to the tephradeposits erupted in the last interglacial period. The newchronology of the Lake Biwa sediment is supported byexamination of the paleoclimatic indices from the LakeBiwa cores by Meyers et al. (1993).

Although the Lake Biwa sediments have beenanalyzed by various methods, high-resolution studieson both paleoenvironment and paleomagnetism havenot yet been attained. In most studies a single core wasobtained at single site. It was difficult to evaluatecompleteness of core recovery and disturbance of coresamples in the previous studies. In the deep drilling in1982–1983, rotary coring caused disturbance of theupper sediment samples to various extent. For detailedstudy of sedimentary records, it is necessary to recoverundisturbed samples representing complete sections. Itis also important to compare the data between differentsites having different sedimentation rates in order toidentify regional or global environmental change. Inorder to attain these purposes, we are planning torecover complete sediments for the last 430 kyrs frommultiple holes at several localities in Lake Biwa,hopefully by using a long hydraulic piston corer suchused in the Ocean Drilling Program.

In preparation for the future project, the presentstudy was created to penetrate the uppermost sedimentwith a piston corer to study sedimentary records sincethe last glacial time. In the summer of 1995, werecovered seven piston cores (10-15 m long) at threelocalities in the northern part of Lake Biwa (Figure 1).We designed the coring plan (1) to take at least twocores from each site; (2) to take cores at three locationshaving different sedimentation rates; and (3) to recoverundisturbed sediments as far as possible. Analysis ofthe core samples are now ongoing in various disciplinesincluding paleomagnetism, environmental magnetism,physical properties, organic and inorganic chemistry,pollen analysis and 14C dating. Results of these studieswill be published soon elsewhere. In this report, wepresent initial results obtained from lithological descrip-tion, measurements of magnetic susceptibility andtephrostratigraphic correlation of the core sediments.

Figure 1. Map showing location of the coring sites: Site 1 offTakashima Harbor (BIW95-2 and BIW95-3), Site 2 off ShirahigeCape (BIW95-4 and BIW95-5) and Site 3 off Hira (BIW95-6,BIW95-7 and BIW95-1). Also shown are locations of the previousdeep drilling of 200 m (BB) and 141 m long (BT).

187

We also demonstrate that magnetic susceptibilitydata are very useful to find microscopic tephra horizons,and establish correlation and age assignment of the coresediments from the different locations.

Sites, coring method and samples

Coring operation was done from July 18 to July 21 in1995. We first carried out acoustic survey by using aKAIJO SP-3 seismic profiler (Okamura et al., 1992) toidentify coring sites with undisturbed sediments. Thelocations of sites were determined using a GPS receiver.A rectangular flat boat (lighter) of about 20 × 10 m anda crane (10 tons) were employed for the coring on thelake. We used a piston corer constructed from severalaluminum tubes of 90 mm in outer diameter and 80 mmin inner diameter. We connected as many as five of5-m-long tubes, expecting penetration as deep as 25 m,and used a weight of 850 kg. Details of the corer andits operation were published elsewhere (Okamura andMatsuoka, 1995).

Locations of the three sites are shown in Figure 1.The first site (BIW95 Site 1) is situated off TakashimaHarbor where the water depth is 63 m. We used two20-m-long corers at Site 1 to penetrate the west(BIW95-2: 35°17.49´N, 136°04.28´E) and east(BIW95-3: 35°17.47´N, 136°04.29´E) sides of anunder-water fault at locations separated by about 40 m.We had no previous data about the sedimentation ratehere, but a relatively higher rate was expected becausethe site is located near the mouth of the Ado River. Thesecond site (BIW95 Site 2) is located off ShirahigeCape (35°15´N, 136°03´E) at a water depth of 67 m.This site is close to the location where the 141-m-deepdrilling was made in 1986 (Yoshikawa & Inouchi,1991). We used two 20-m-long corers (BIW95-4 andBIW95-5), expecting a relatively slow sedimentation ofabout 30 cm/kyr based on the previous data (Yoshikawa& Inouchi, 1991). The third site (BIW95 Site 3) waschosen near the previous drilling sites of Horie and hiscolleagues, where a sedimentation rate of about100 cm/kyr was expected. It is located betweenOmatsugasaki and the Okinoshima Island (35°13´N,136°00´E) and the water depth is 76 m. We employedtwo 20-m-long corers (BIW95-6 and BIW95-7) andone 25-m-long corer (BIW95-1).

After recovery from the lake bottom, the corer wasdisassembled into its original 5-m-long pieces andthen cut into half lengths onboard. Two cores fromSite 1 (BIW95-2 and BIW95-3) and one from Site 2

(BIW95-5) were divided into 1-m-long sections, fromwhich the core sediment was slowly pressed out andsplit into two vertical halves. We obtained a series ofcubic samples of 7 cm3 by pressing a plastic box into asplit face. One half of Core BIW95-5 was immediatelyfrozen and dedicated to analysis of organic chemistry.Another core from Site 2 (BIW95-4) and all three coresfrom Site 3 were chosen for targets of paleomagneticstudies. In order to avoid mechanical distortion of thesesamples, we did not press out the sediment from thecorer, but we cut aluminum pipes lengthwise using anelectric saw. Each section of 2.5-m-long was split intotwo vertical halves using monofilament fishing line.One half of the vertical splits was extensively sampledby inserting plastic boxes (7 cm3) into the split face.Thus, we obtained two or three cubic samples from onehorizon at intervals of about 2.3 cm. From another half,samples for 14C dating were collected. We also obtainedu-channel samples of 2 × 2 cm in cross section and aslong as 1.5 m. This method allows continuous archivingof undisturbed sediment near the center of a core forhigh-resolution paleomagnetic measurement (e.g.,Weeks et al., 1993).

In collecting discrete and u-channel samples and alsofor description of lithology, we first measured thehorizon as a position in each section. Then we cal-culated its depth from the core top, subtracting anyvoids in the core sediments. The lithological descrip-tions (Figures 2 to 4, Table 1) are presented using thisdepth scale for each core. When we compared horizonsof a key volcanic ash in multiple cores from the samesite, their depths show small but significant differenceas great as 1 m. Such discrepancy arises from missingsurface sediments in the core top and from expansionor shrinkage of sediment after core recovery. Weadjusted the depth of two cores from Site 2 and threecores from Site 3 to develop composite depths for eachsite. Comparison of magnetic susceptibility data betweentwo cores from Site 2 suggests that at least 1 m ofsediment is missing from the top of Core BIW95-4. Wetherefore adjusted the susceptibility peaks at 0.89 m inBIW95-4 to the peak occurring at 2.12 m in BIW95-5,by adding 1.23 m to the original depth of BIW95-4.Then, assuming core deformation would be minor in the2.5-m-long sections compared to the sediments pressedout, we matched the depth of susceptibility peaks in CoreBIW95-5 to those in the corrected depth of BIW95-4.The composite depth for Site 3 was developed by Aliet al. (1999), who adjusted the depth of Cores BIW95-6 and BIW95-7 to that of Core BIW95-1, utilizingmagnetic susceptibility and paleomagnetic secular

188

Figure 2. Lithology of cores BIW95-2 and BIW95-3 from site 1.

variation data. The two cores of Site 1 were obtainedfrom locations separated by a fault, as mentioned before.The discrepancy of tephra horizons between BIW95-2and BIW95-3 should therefore represent the differentaccumulation histories of the two holes.

Lithology and volcanic ashes

We described sedimentary structure, coring-relateddisturbance and colors and classified sedimentaryfacies on each section of the split cores as shown in

189

Figure 3. Lithology of cores BIW95-4 and BIW95-5 from site 2.

190

Figures 2 to 4. In case a layer of volcaniclastic grainswas found, we observed the sample under a microscopeand determined its mineral composition and the

refractive index of its glass shards. Thicknesses of thevisually observed ash layers range from 1 mm to about15 cm. These volcanic ash layers were correlated to

Figure 4. Lithology of cores BIW95-6, BIW95-7 and BIW95-1 from site 3.

191

Tabl

e 1.

Cha

ract

er o

f vo

lcan

ic a

shes

fou

nd in

the

pist

on c

ore

sedi

men

ts f

rom

Lak

e B

iwa

Site

Cor

eD

epth

(m

)V

isua

l te

xtur

eM

icro

scop

ic f

eatu

reH

eavy

min

eral

sR

efra

ctiv

e in

dex

of g

lass

Cor

rela

tion

|Sit

e 1

BIW

95-2

2.13

disp

erse

d te

phra

not

pure

tep

hra,

pum

iceo

us g

lass

,bu

bble

wal

l gl

ass

GH

o, O

px, B

i, O

pq1.

501–

1.50

2K

g?B

IW95

-22.

67–2

.70

pale

yel

low

ish

brow

n, f

ine

pum

iceo

us g

lass

yB

i, G

Ho,

Cpx

, Opq

, Ap,

Opx

1.52

1–1.

523

U-O

kiB

IW95

-28.

47ve

ry f

ine

sand

siz

e as

hcr

ysta

l do

min

ant,

pum

iceo

us g

lass

GH

o, O

pq, B

Ho,

Bi,

Cu

m1.

498–

1.50

1Sa

kate

Site

1B

IW95

-31.

53di

sper

sed

teph

rano

t pu

re t

ephr

a, p

umic

eous

gla

ssH

Go,

Opx

, Bi,

Opq

1.50

1–1.

503

Kg

BIW

95-3

2.25

disp

erse

d te

phra

not

pure

tep

hra

(gla

sses

der

ived

from

old

er te

phra

)B

IW95

-32.

54–2

.56

fine

gla

ssy

ash

glas

sy, b

ubbl

e w

all,

brow

n gl

ass

Opx

, Cpx

, Opq

, Ap

1.51

1–1.

513

K-A

hB

IW95

-33.

54–3

.57

silt

siz

e as

h, p

ale

yell

owis

h br

own

pum

iceo

us g

lass

yB

i, G

Ho,

Cpx

, Opq

, Ap,

Opx

1.52

1–1.

523

U-O

kiB

IW95

-38.

52ve

ry f

ine

sand

- s

ilt s

ize

ash

crys

tal

dom

inan

t, pu

mic

eous

gla

ssG

Ho,

Opq

, Cum

, Bi,

BH

o1.

498–

1.50

0Sa

kate

Site

2B

IW95

-40.

63di

sper

sed

teph

rapu

mic

eous

gla

ssy,

obs

idia

nG

Ho,

Opx

, Cpx

, Opq

, Ap

1.50

0–1.

503

Kg

BIW

95-4

0.89

disp

erse

d te

phra

crys

tal

dom

inan

t, gl

assy

fra

gmen

t 30

%G

Ho,

Opx

, BH

o, O

pq, Z

r, A

p, B

iB

IW95

-41.

46di

sper

sed

teph

racr

ysta

l do

min

ant,

glas

sy f

ragm

ent

30%

GH

o, B

Ho,

Opq

, Ap,

Bi

1.49

7–1.

500,

1.5

11–1

.514

BIW

95-4

1.95

–1.9

8gl

assy

, gr

adin

ggl

assy

, bro

wn

glas

sO

px, C

px, O

pq, A

p1.

510–

1.51

5K

-Ah

BIW

95-4

2.87

–2.8

9fi

ne-m

ediu

m a

sh (

1.5

cm b

ase)

,cl

ayey

ligh

t gre

y as

hpu

mic

eous

gla

ss, a

lkal

i fe

ldsp

arB

i, G

Ho,

Cpx

, Opq

, Ap,

Opx

1.52

1–1.

523

U-O

kiB

IW95

-45.

6li

ght

grey

fin

e as

hpu

mic

eous

gla

ssG

Ho,

Opq

, BH

o, C

um, B

i, A

p1.

498–

1.50

0Sa

kate

BIW

95-4

9.64

pum

iceo

us c

oars

e (9

mm

bas

e),

pum

iceo

us, c

ryst

al a

shG

Ho,

Opx

, Opq

, Bi,

Cum

1.50

2–1.

504

DH

gbr

owni

sh g

rey

fine

(8m

m)

BH

o, Z

rB

IW95

-410

.21

fine

ash

pum

iceo

us, c

ryst

al a

shG

Ho,

Opx

, Opq

, BH

o, A

p, B

i1.

498–

1.50

4D

SsB

IW95

-410

.22

fine

ash

(pu

mic

eous

)pu

mic

eous

, cr

ysta

lG

Ho,

Opx

, Opq

, Bi,

BH

o, A

p, Z

r1.

501–

1.50

4D

SsB

IW95

-410

.38–

10.5

2fi

ne (

0.5c

m b

ase)

, gra

ding

(coa

rse

to f

ine)

glas

syO

px, C

px, O

pq, G

Ho,

Ap,

Bi

1.49

8–1.

501

AT

Site

2B

IW95

-52.

12di

sper

sed

teph

racr

ysta

l do

min

ant,

pum

iceo

us g

lass

GH

o, O

pq, O

px, B

i, A

p1.

498–

1.50

0, 1

.512

–1.5

13B

IW95

-53.

08–3

.10

glas

sy,

grad

ing

glas

sy, b

row

n gl

ass

Opx

, Cpx

, Opq

, Ap

1.51

0–1.

515

K-A

hB

IW95

-53.

90–3

.92

fine

ash

(0.

5 cm

bas

e),

clay

ey d

ark

grey

pum

iceo

us g

lass

, alk

ali

feld

spar

Bi,

GH

o, C

px, O

pq, A

p, O

px1.

521–

1.52

4U

-Oki

BIW

95-5

6.56

ligh

t gr

ey a

shpu

mic

eous

gla

ssG

Ho,

Opq

, BH

o, C

um, B

i, A

p1.

499–

1.50

4Sa

kate

BIW

95-5

10.3

5pu

mic

eous

cou

rse

(0.5

cm

bas

e),

fine

yel

low

pum

iceo

us, c

ryst

al a

shG

Ho,

Opx

, Opq

, Bi,

Cum

, BH

o, Z

r1.5

01–1

.504

DH

gB

IW95

-510

.75

fine

ash

pum

iceo

us, c

ryst

al a

shG

Ho,

Opx

, Opq

, BH

o, A

p, B

i1.

499–

1.50

4D

SsB

IW95

-510

.76

fine

ash

pum

iceo

us, c

ryst

alG

Ho,

Opx

, Opq

, Bi,

BH

o, A

p, Z

r1.

502–

1.50

5D

SsB

IW95

-510

.95–

11.0

9fi

ne (

0.5

cm b

ase)

, gra

ding

(coa

rse

to f

ine)

glas

syO

px, C

px, O

pq, G

Ho,

Ap,

Bi

1.49

9–1.

501

AT

BIW

95-5

13.4

6di

sper

sed

teph

racr

ysta

l do

min

ant,

scor

ia g

lass

,pl

agio

clas

e (h

igh

RF)

Ol,

Opx

, CPx

, GH

o, O

pq, A

p

Site

3B

IW95

-64.

28fi

ne a

shpu

mic

eous

gla

ssy,

obs

idia

nG

Ho,

Opx

, Cpx

, Opq

, Ap

1.49

9–1.

503

Kg

BIW

95-6

5.18

disp

erse

d te

phra

crys

tal d

omin

ant,

glas

sy fr

agm

ent 3

5-40

%G

Ho,

BH

o, O

px, O

pq, Z

r, B

i, A

p1.

499–

1.50

2B

IW95

-66.

77di

sper

sed

teph

racr

ysta

l dom

inan

t, gl

assy

frag

men

t 35-

40%

GH

o, B

Ho,

Opq

, Bi,

Ap

1.49

7–1.

499,

1.5

11–1

.514

BIW

95-6

8.64

–8.6

9gl

assy

glas

sy, b

row

n gl

ass

Opx

, Cpx

, Opq

, Ap

1.51

0–1.

515

K-A

h

Site

3B

IW95

-79.

12–9

.17

glas

sygl

assy

, bro

wn

glas

sO

px, C

px, O

pq, A

p1.

510–

1.51

5K

-Ah

Site

3B

IW95

-18.

97–9

.07

pale

bro

wn

to b

row

nish

gra

y, g

lass

ygl

assy

, bro

wn

glas

sO

px, C

px, O

pq, A

p1.

510–

1.51

5K

-Ah

Bi:

bio

tite

, GH

o: g

reen

hor

nble

nde,

BH

o: b

row

n ho

rnbl

ende

, Cum

: cum

min

gton

ite,

Opx

: ort

hopy

roxe

ne, C

px: c

lino

pyro

xene

, Ap:

apa

tite,

Zr:

zir

con,

Ol:

oliv

ine,

Opq

: opa

que

min

eral

s, K

g:A

mag

i-K

awag

odai

ra, K

-Ah:

Kik

ai-A

kaho

ya, U

-Oki

: Ulr

eung

-Oki

, DH

g: D

aise

n-H

igas

hi D

aise

n, D

Ss: D

aise

n-Sa

saga

naru

, AT:

Air

a-T

n.

192

known widespread tephra deposits based on the mineralcomposition and refractive index data and their strati-graphic positions (Table 1).

Site 1 (Cores BIW95-2 and BIW95-3)

The two core samples from Site 1 are mainly composedof massive clayey silt with plant fragments, vivianite,laminated fine to very fine sand and small amounts ofmedium to coarse sand with granule gravels (Figure 2).The core length of BIW95-2 was 10.05 m and that ofBIW95-3 was 11.30 m.

Three volcanic ash layers were found in CoreBIW95-3. The upper ash layer, intercalated at 2.54–2.56 m deep, is a fine vitric tephra that is composed ofbubble-wall type glasses of a brown color. This ash iscorrelated to the Kikai-Akahoya (K-Ah) ash for whichthe age is estimated at 6300 yrs BP (Machida & Arai,1978) by the heavy mineral composition and refractiveindex values summarized in Table 1. The middle ashlayer at 3.54–3.57 m deep is composed of fine silt-sizegrains of a pale yellowish brown color. This ash layeris correlated to the Ulreung-Oki (U-Oki) ash layererupted at around 9300 yrs BP (Arai et al., 1981) . Avery fine sand layer at 8.52 m in BIW95-3 wasidentified as the Sakate volcanic ash (Yoshikawa et al.,1986) based on its refractive index value and heavymineral composition.

The U-Oki and Sakate volcanic ashes were observedat depths of 2.67–2.70 m and 8.47 m, respectively, inCore BIW95-2.

Site 2 (Cores BIW95-4 and BIW95-5)

The two cores from Site 2 are mainly composed ofhomogeneous gray clay, partly including black to darkgray seams, coaly layers and vivianite crystals (Figure3). The core length of BIW95-4 was 15.03 m and thatof BIW95-5 was 14.45 m. Below about 4 m deep inboth cores, a zone of abundant black to dark gray seamswas observed. These black seams consist of plantfragments and siderite (H. Fukusawa, personal com-munication). Moreover, densely laminated light grayor gray seams are observed in an interval about 20 cmthick above the basal volcanic ash horizon (Figure 3).

Seven volcanic ash layers were found in both cores(Table 1), and the two cores were correlated to eachother by these tephra layers.

The uppermost volcanic ash is at 1.95–1.98 m inBIW95-4 and at 3.08–3.10 m in BIW95-5. This ash isvitric, mainly composed of bubble-wall type glasses of

brown color. This ash is correlated with the K-Ahvolcanic ash on the basis of the heavy mineral composi-tion (mainly orthopyroxene and clinopyroxene) andrefractive index value.

The second ash layer was observed at 2.87–2.89 min BIW95-4 and at 3.90–3.92 m in BIW95-5. This ashis fine-grained and is composed of pumiceous glassshards. This ash is correlated with the U-Oki ash layer.

The third one was found at 5.60 m in BIW95-4 andat 6.56 m in BIW95-5. This ash has light gray color andis composed of fine grains of pumiceous glasses. Thisash is correlated with the Sakate volcanic ash layer.

The fourth is a pumiceous crystal ash found at 9.64 min BIW95-4 and at 10.35 m in BIW95-5. This ash iscorrelated with Daisen-Higashi Daisen (DHg) tephralayer (Tsukui, 1984; Machida & Arai, 1992).

The fifth and sixth ash layers were found at veryclose horizons, barely separated by a clay bed of 0.6-1.0 cm thick. The upper ash layer is a pumiceous crystalash of 1–2 mm thickness occurring at 10.22 m inBIW95-4 and at 10.75 m in BIW95-5. The lower ashlayer has a thickness of 4–5 mm with a similar compo-sition of heavy minerals and a slightly higher refractiveindex of glass shards. These two ash layers can beassigned to the plinian and co-ignimbrite units of awidespread tephra that are known as the Daisen-Sasaganaru (DSs) tephra (Tsukui, 1984; Machida &Arai, 1992).

The lowermost tephra is a vitric ash with gradingstructure intercalated at 10.38–10.52 m in BIW95-4and at 10.95–11.09 m in BIW95-5. This ash is mainlycomposed of bubble-wall type glass shards. This iscorrelated with the Aira-Tn (AT) volcanic ash (Machida& Arai, 1976) erupted at about 25 kyrs BP.

Site 3 (Cores BIW95-6, BIW95-7 and BIW95-1)

The main lithology of the cores from Site 3 is bluish-gray, homogenous clay (Figure 4). Length of the threecores were 11.61 m (BIW95-6), 11.55 m (BIW95-7)and 11.29 m (BIW95-1).

A vitric ash layer of light gray color was found at8.69 m in BIW95-6 with at least 5 cm thickness. Thesame vitric ash is observed at 9.17 m in BIW95-7 witha thickness of 5 cm, and also at 9.07 m in BIW95-1with a thickness of 10 cm. These ashes are mainlycomposed of brown-colored glass shards of bubble-wall type with the refractive index ranging from 1.510–1.515. This ash layer is correlated with the K-Ah tephra.A crystal ash of gray color is located at 4.28m inBIW95-6 with 1 cm thickness. The heavy minerals are

193

composed of green hornblende, orthopyroxene andclinopyroxene. The refractive index value of volcanicglasses ranges from 1.499–1.503. This is correlatedwith the Amagi-Kawagodaira (Kg) tephra, which waserupted from the Izu Peninsula and transported to theLake Biwa region by east winds (Machida & Arai,1992; Nishida et al., 1993). Its age is best estimated at2.8–2.9 kyrs BP (Machida & Arai, 1992).

Magnetic susceptibility

We measured low-field (0.1 mT) magnetic susceptibilityof the cubic samples for paleomagnetic and rock mag-netic analysis. The measurement was made immediately

after the sampling, using a Bartington Instrumentssusceptibility meter with MS1B sensor operating at afrequency of 0.47 kHz. The diamagnetic susceptibilityof the plastic sample holder (about –0.3 µSI) was notcorrected, because it constitutes less than 1% of thebulk readings. The results primarily represent ferri-magnetic mineral concentration in sediments andprovide a clue for stratigraphic correlation and paleo-environmental analysis.

Pass-through measurements of whole-core suscepti-bility were also made on Cores BIW95-2 and BIW95-3 by using a loop sensor (Bartington InstrumentsMS2C) at Kochi University. Figure 5 shows the resultsobtained from the whole-core and discrete samples ofthe two cores from Site 1. While the two methods

Figure 5. Down-core plots of volume magnetic susceptibility of Cores BIW95-2 and BIW95-3 from Site 1. The susceptibility peaks correspondto the tephra horizons 1 (Kg), 4 (K-Ah), 5 (U-Oki) and 6 (Sakate). Dashed lines show the result from the whole-core samples, and solid linesindicate the result from discrete samples.

194

yielded consistent results, the discrete measurementrevealed more detailed features including short-termfluctuations and outstanding peaks. The three peaksat 2.56, 3.57 and 8.52 m deep in Core BIW95-3correspond to the K-Ah, U-Oki and Sakate volcanicashes, respectively. The U-Oki and Sakate ashes werealso found in BIW95-2, associated with the suscep-tibility peaks at 2.70 and 8.47 m deep. Although noother tephra deposit was visually observed at Site 1, thesusceptibility data suggested possible concentrationof ferrimagnetic material at several horizons. Bydispersing 2–3 cube samples from each magneticsusceptibility peak and checking mineralogical charac-

teristics (abundance, type, refractive index of glass andminerals) by the procedure of Takemura and Danhara(1994), we detected volcanic glass shards and heavyminerals from the depth of 2.13 m in BIW95-2 andfrom 1.53 and 2.26 m in BIW95-3. The sample from1.53 m deep in BIW95-3 was recognized as the Kg ashfrom the abundance of volcanic originated material andrefractive index value of glass, while the other two werepossibly derived from older tephra deposits.

Figure 6 shows susceptibility data obtained fromCores BIW95-4 and BIW95-5 at Site 2. These profilesindicate occurrence of prominent susceptibility peakscorrelative between the two cores. Six of these peaks

Figure 6. Down-core plots of volume magnetic susceptibility of Cores BIW95-4 and BIW95-5 from Site 2. The susceptibility peaks correspondto the tephra horizons of 1 (Kg), 2, 3, 4 (K-Ah), 5 (U-Oki), 6 (Sakate), 7 (DHg), 8 (DSs), 9 (AT) and 10.

195

correspond to widespread tephra layers, K-Ah, U-Oki,Sakate, DHg, DSs and AT, which were visually foundin both cores. Although no visible volcanic ash wasobserved above the K-Ah ash, susceptibility peaks occurat the composite depth of 2.12, 2.69 and 3.2 m. Wedetected volcanic glass shards and heavy minerals fromthese three horizons, among which the topmost one wasidentified as the Kg ash. We also recognized a small peakat 13.44 m in BIW95-5 (at 14.87 m in the compositedepth scale). This horizon yielded crystal ash grainsmainly composed of plagioclase and olivine. It is thusapparent that the spikes of the susceptibility valuesoriginated from abundant ferrimagnetic minerals involcanic ashes.

In Figure 7, susceptibility profiles of Cores BIW95-6,BIW95-7 and BIW95-1 from Site 3 show quite similarfeatures; they comprise low amplitude fluctuations

between 90 and 160 µSI and outstanding peaks exceeding250 µSI. In BIW95-6, two peaks at 4.49 and 8.90 mcorrespond to the Kg and K-Ah ashes, respectively,which were found intercalating in the clayey sediments.Although no visible volcanic ash was found around theother two susceptibility maxima at 5.43 and 7.03 m, weagain detected volcanic glasses and heavy mineralsfrom these horizons. In BIW95-1 and BIW95-7, we didnot collect cubic samples from the K-Ah ash layer,which had been disturbed in coring process, so that highsusceptibility data are absent at around 9 m deep. Theother peaks and background fluctuations in the suscep-tibility records show consistent patterns between thethree cores and thereby establish the core-to-corecorrelation within Site 3 in detail . We also found asimilarity of the susceptibility profiles between Site 3and Site 2, while the former is about three times thicker

Figure 7. Down-core plots of volume magnetic susceptibility of Cores BIW95-6, BIW95-7 and BIW95-1 from Site 3. The susceptibilitypeaks correspond to the tephra horizons of 1 (Kg), 2, 3 and 4 (K-Ah).

Com

posi

te D

epth

(m

)

196

than the later. The similarity in the susceptibility recordis concordant with occurrence of the four volcanic ashhorizons including the Kg and K-Ah ashes. Thus,supported by tephrostratigraphic data, the susceptibilityprofiles are correlative between the two site, while theyare separated by about 5 km in distance and depositedwith different accumulation rates.

The magnetic susceptibility variations in the clayeysediments afford some accordance with paleoclima-tological information from previous studies on the LakeBiwa cores. At Site 3 (Figure 7), the susceptibility valuesare noticeably decreased at the depth between 5.5 and10 m and then enhanced in the upper part of the core.The interval of low susceptibility is correlative to a levelof increases in the number of total diatom cells pervolume obtained from the 200-m-long core near our site(Mori, 1975; Mori & Horie, 1984). The susceptibilityrecord from Site 2 (Figure 6) has increased values from9–12 m deep, which corresponds to the last glacialperiod. This record also shows in general a negativecorrelation with the total number of diatom cells in thenearby 140-m-long core, which was recently reportedby Kuwae et al. (1997). Meyers et al. (1993) suggestedthat the high concentration of diatom cells indicatesenhanced aquatic productivity, which might result fromwash-in nutrient flux into the lake increased by greaterprecipitation in a warm period. Therefore, the decreasedsusceptibility may be attributed to dilution of ferri-magnetic minerals by increased biogenic productivity ordiagenetic dissolution of fine magnetic particles underthe nutrient rich environment.

More detailed rock magnetic properties of the coresediments are now investigated and will be reportedelsewhere.

Tephrostratigraphy and accumulation history

From the visual description and the susceptibilitymeasurement, we recognize the occurrence of volcanicashes at ten horizons in our cores (Table 2). Correlationbetween the three sites is shown in Figure 8. Seven tephrahorizons are correlated to widespread tephra layerspreviously reported from Lake Biwa and other areas ofSouthwest Japan: the Kg, K-Ah, U-Oki, Sakate, DHg,DSs and AT tephra layers. In order to delineate theaccumulation history of the three sites and also for theprevious cores (Figure 9), we used published 14C datesfor the Kg (2.8–2.9 kyrs BP; Machida & Arai, 1992), K-Ah (6.3 kyrs BP; Machida & Arai, 1978), U-Oki (9.3kyrs BP; Arai et al., 1981) and AT (24.3–24.7 kyrs BP;Matsumoto et al., 1987; Murayama et at., 1993). Addi-tionally, the Sakate volcanic ash, originally described byYoshikawa et al. (1986), was recently found in a peatlayer on the shore of Lake Biwa and its age was estimatedbetween 15800 ± 200 and 15900 ± 200 yrs BP by 14Cdating (Togo et al., 1997). Fukusawa (1995) assumedthat the Sakate volcanic ash in core samples from LakeSuigetsu is correlative to the Sambe-Ukinuno (SUP)tephra. The refractive index of the glass shards clearlydistinguishes the Sakate and SUP ashes, however,while these two ash layers are both dated at around16 kyrs BP.

The age-depth curves obtained from Site 2 offShirahige Cape show significant variations in sedi-mentation rate; between the K-Ah and U-Oki ashes itdecreased to nearly half (0.30 m/kyrs) that of the lowerpart of the core (0.57 m/kyrs). Similar change insedimentation rate is observed in the core samplesrecovered in 1986 near Site 2 (Yoshikawa & Inouchi,

Table 2. Tephra horizons in the core sediments from Lake Biwa for the last 30 kyrs

Site 1 Site 2 Site 3 141-m core 200-m coreTephra Correlation to Age BIW95-2 BIW95-3 Composite Depth Composite Depth (Yoshikawa and Inouchi) (Machida et al.)Horizon Widespread Tephra (kyrs BP) (m) (m) (m) (m) (m) (m)

1 Kg 2.8–2.9 1.53 1.86 4.49 1.52 (BT1) 6.0 (BB7)2 2.12 5.433 2.69 7.034 K-Ah 6.3 2.56 3.21 8.90 2.23 (BT3) 10.0 (BB15)5 U-Oki 9.3 2.70 3.57 4.12 2.65 (BT4) 13.3 (BB23)6 Sakate 15.9 8.47 8.52 6.83 4.76 (BT6) 19.0 (BB37)7 DHg 10.878 DSs 11.459 AT 24.3–24.7 11.75 8.62 (BT10) 26.0 (BB55)10 14.87

Kg: Kawagodaira, K-Ah: Kikai-Akahoya, U-Oki: Ulreung-Oki, DHg: Daisen-Higashi Daisen, DSs: Daisen-Sasaganaru, AT: Aira-Tn.

197

1991) and also in Core BIW95-3 from Site 1 offTakashima Harbor. At Site 2, the interval with thehigher sedimentation rate is characterized by abundantoccurrence of black to dark gray seams of plantfragments (Figure 3). The lower part of Core BIW95-3from Site 1 includes several layers of coarse sands orgranules and dark bands of plant fragments (Figure 2).It is thus evident that in the last glacial time terrigenous

materials were abundantly supplied to the two siteslocated on the margin of the sub-basin. On the otherhand, the core sediments from Site 3, located within thesub-basin, consist of homogeneous clay except the K-Ah tephra deposit (Figure 2). The ages of the Kg andK-Ah ashes provide the higher sedimentation rate of1.1–1.3 m/kyrs for this site. While our piston cores ofabout 12 m long did not penetrate the U-Oki ash here,

Figure 8. Correlation between Site 1 to 3 based on the tephra horizons 1 to 10 (Table 1). Dashed lines show horizons where volcanic asheswere found dispersed in clayey sediments under microscope, while solid lines indicate tephra deposits which were visually observed in thecore.

198

the age-depth curves for the previous cores suggest thatthe sedimentation rate was rather small between 10 and25 kyrs BP compared to the upper 15-m interval. Thisvariation may be explained by the increased biogenicproductivity and greater sedimentation rate during thewarm period (Meyers et al., 1993; Meyers & Takemura,1997). Though above discussion is made on the basisof conventional 14C age data during about 30 kyrs, theconclusion is not affected even in the case of calendaryear time scale.

Our work demonstrated the great utility of continuoussusceptibility data in locating the dispersed volcanicashes in lake sediments.Besides the tephra horizonsassigned to the known widespread tephra units, volcanicashes were found dispersed in clayey sediments at threehorizons. The tephra horizons 2 and 3 were identifiedboth at Sites 2 and 3, and dated at about 3.5 and 4.8 kyrsBP from the age-depth curves in Figure 8. The tephrahorizon 10 only occurs in the longest core from Site 2

and is dated at about 29.6 kyrs BP. Although theircorrelation has not been clarified yet, these ashes maybe used as new datum levels in tephrostratigraphy oflacustrine sediments in Southwest Japan.

Conclusions

We described lithology and measured magnetic suscep-tibility of seven piston cores from three sites in andaround the southern sub-basin in Lake Biwa. Signi-ficant peaks of the susceptibility profiles correspond tohorizons of the volcanic ashes that were visuallyobserved or microscopically found from clayey sedi-ments. We thus identified 10 tephra horizons, whichwere used for correlating the three sites and the previouscores from Lake Biwa. The age-depth curves delineatedfrom the tephrostratigraphic data suggest that on themargin of the depression the sedimentation rate washigher in the last glacial time. In the central part of thedepression, we observed sedimentation rate about fourtimes higher than the margin site and it conversely wasless in glacial time. The homogeneous clay in this sub-basin appears to be continuous and therefore has greatpotential for high-resolution study of upper Pleistocenesedimentary records.

Acknowledgments

We thank S. Ohmichi of the Katata Fishery Union fortheir enthusiastic help with the coring work in LakeBiwa. We would also like to thank M. Miyamoto, M.Tamai, K. Negishi, H. Oda, H. Zaman, M. Okuda, K.Yamaguchi, K-L. Du, and T. Tachibana for their helpin coring and sampling. Thanks are also due to R.Ishiwatari, H. Fukusawa, K. Toyoda, H. Kitagawa, N.Ishikawa, K. L. Verosub and P. Meyers for their supportin this project. Analyses of volcanic ashes were madeby T. Danhara and T. Yamashita of Kyoto Fission-TrackCo., Ltd. Part of this study was financially supportedby the grant-in-aid from Monbusho (the “Decoding theEarth Evolution” Program; 1995-1997), the SumitomoFoundation (1995–1997), the Nissan Science Founda-tion (1996–1998) and Doshisha University (1997).

References

Ali, M., H. Oda, A. Hayashida, K. Takemura & M. Torii, 1999.Holocene paleomagnetic secular variation from Lake Biwa.Geophys. J. Int. 136: 218–228.

Figure 9. Age-depth relations of the core sediments from Site 1 to3 based on reported ages of the Kg, K-Ah, U-Oki and AT tephralayers (Table 1). The depth of these ashes found in the previous200-m (BB) and 140-m (BT) deep drillings are also shown.

199

Arai, F., T. Oba, H. Kitazato, S. Horibe & H. Machida, 1981. LateQuaternary tephrochronology and paleo-oceanography of thesediment of the Japan Sea. Quat. Res. (Japan) 20: 209–230.

Fukusawa, H., 1995. Non-glacial varved lake sediment as a naturaltimekeeper and detector on environmental changes. Quat. Res.(Japan) 34: 135–149.

Horie, S. (ed.), 1984. Lake Biwa. Dr. W. Junk Publishers, Dordrecht,654 pages.

Horie, S. (ed.), 1991. Die Geschichte des Biwa-See in Japan: seineEntwicklung, dargestellt anhand eines 1400 m langen Tief-bohrkerns. Wagner, Insbruck, 349 pages.

Horie, S., O. Mitamura, S. Kanari, H. Miyake, A. Yamamoto & N.Fuji, 1971. Paleolimnological study on lacustrine sedimentsof Lake Biwa-ko. Contribution from the Geological Institute,Kanazawa University 18: 745–762.

Inouchi, Y., Y. Kinugasa, F. Kumon, S. Yasumatsu, S. Nakano & T.Shiki, 1993. Turbidites in Lake Biwa as indicators of intensityof paleoearthquakes. Mem. Geol. Soc. Japan 39: 61–70.

Kawai, N., K. Yaskawa, T. Nakajima, M. Torii & S. Horie, 1972.Oscillating geomagnetic field with a recurring reversaldiscovered from Lake Biwa. Proc. Japan Acad. 48: 186–190.

Kuwae, M., S. Yoshikawa & Y. Inouchi, 1997. Diatom records fromlacustrine sediments of Lake Biwa during the past 400,000years. Quat. Res. (Japan) 36: 113–122.

Machida, H. & F. Arai, 1976. The very widespread tephra - TheAira-Tn ash -. Kagaku (Science) 46: 339–347.

Machida, H. & F. Arai, 1978. Akahoya Ash - a Holocene widespreadtephra erupted from the Kikai Caldera, south Kyushu, Japan.Quat. Res. (Japan) 17: 143–163.

Machida, H. & F. Arai, 1992. Atlas of Tephra in and around Japan.Tokyo University Press, Tokyo, 276 pages.

Machida, H., F. Arai & T. Yokoyama, 1991. Re-examination ofmaker-tephra layers in the 200 m Biwa-lake core. QuaternaryRes. (Japan) 30: 439–442.

Matsumoto, E., Y. Maeda, K. Takemura & S. Nishida, 1987. Newradiocarbon age of Aira-Tn ash (AT). Quat. Res. (Japan) 26:79–83.

Meyers, P. A. & K. Takemura, 1997. Quaternary changes in deliveryand accumulation of organic matter in sediments of Lake Biwa,Japan. J. Paleolim. 18: 211–218.

Meyers, P. A., K. Takemura & S. Horie, 1993. Reinterpretation ofLate Quaternary sediment chronology of Lake Biwa, Japan,from correlation with marine glacial-interglacial cycles. Quat.Res. 39: 154–162.

Mori, S., 1975. Diatom succession in a core from Lake Biwa. InHorie, S. (ed.), Paleolimnology of Lake Biwa and the JapanesePleistocene. Kyoto University, Otsu 2: 247–254.

Mori, S. & S. Horie, 1984. Diatom Analysis. In Horie, S. (ed.),Lake Biwa. Junk, Dordrecht, 531–543.

Murayama, M., E. Matsumoto, T. Nakamura, M. Okamura, H.Yasuda & A. Taira, 1993. Re-examination of the eruption age

of Aira-Tn Ash (AT) obtained from a piston core off Shikoku -determined by AMS 14C dating of planktonic foraminifera -. J.Geol. Soc. Japan 99: 787–798.

Nishida, S., Y. Takahashi, K. Takemura, S. Ishida & Y. Maeda, 1993.A Late-Jomon volcanic ash transported by an east wind. Quat.Res. (Japan) 32: 129–138.

Okamura, M. & H. Matsuoka, 1995. Sediment sampling method. InKoizumi, I. & Y. Yasuda (ed.), Cyclicities in Earth andCivilization History. Asakura-Shoten, Tokyo, 105–114.

Okamura, M., K. Shimazaki, T. Nakata, N. Chida, T. Miyatake, H.Maemoku, H. Tsutsumi, T. Nakamura, C. Yamaguchi & M.Ogawa, 1992. Submarine active faults in the northwestern partof Beppu Bay, Japan - On a new technique for submarine activefault survey -. Mem. Geol. Soc. Japan 40: 65–74.

Takemura, K., 1990. Tectonic and climatic record of the Lake Biwa,Japan, region provided by the sediments deposited sincePliocene times. Paleogeogr. Paleoclimatol. Paleoecol. 78: 185–193.

Takemura, K. & T. Yokoyama, 1989. Sedimentary environmentsinferred from lithofacies of the Lake Biwa 1400 m core sample,Japan. Japan. J. Limnol. 50: 247–254.

Takemura, K. & T. Danhara, 1994. A method for determination ofvolcanic glass concentrations in sedimentary sequences andapplication to Quaternary studies. Geoarchaeology 9: 301–316.

Togo, M., H. Sato, A. Okada & N. Matsuyama, 1997. Trenchingstudy of the Katata fault at Hieitsuji, Otsu, Shiga Prefecture in1994. Active Fault Research 16: 53–58.

Torii, M., H. Shibuya, A. Hayashida, I. Katsura, S. Yoshida, T.Tagami, Y.-i. Otofuji, Y. Maeda, S. Sasajima & S. Horie, 1986.Magnetostratigraphy of sub-bottom sediments from Lake Biwa.Proc. Japan Acad. 62: 333–336.

Tsukui, M., 1984. Geology of Daisen Volcano. J. Geol. Soc. Japan90: 643–658.

Weeks, R. J., C. Laj, L. Endignoux, M. D. Fuller, A. P. Roverts, R.Manganne, E. Blanchard & W. Goree, 1993. Improvements inlong core measurement techniques: Application in palaeomagne-tism and palaeoceanography. Geophys. J. Int. 114: 651–662.

Yamamura, T., Y. Miyata, T. Kodato, A. Nabetani, T. Iwata, M.Obata, T. Yuuki & S. Tokuhashi, 1990. Deposits and sedi-mentary process of lacustrine delta - an example from Echi-River delta, Lake Biwa -. Mem. Geol. Soc. Japan 36: 219–231.

Yoshikawa, S. & Y. Inouchi, 1991. Tephrostratigraphy of theTakashima-oki boring core samples from Lake Biwa, centralJapan. Chikyu Kagaku (Earth Sci.) 45: 81–100.

Yoshikawa, S. & Y. Inouchi, 1993. Middle Pleistocene to Holoceneexplosive volcanism revealed by ashes of the Takashima-okicore samples from Lake Biwa, central Japan. Chikyu Kagaku(Earth Sci.) 47: 97–109.

Yoshikawa, S., T. Nasu, H. Taruno & M. Furutani, 1986. LatePleistocene to Holocene volcanic ash layers in central Kinkidistrict, Japan. Chikyu Kagaku (Earth Sci.) 40: 18–38.

200