sedimentary environments based on textures of surface

67

堆積学研究,48号,67-84,1998

J.Sed.Soc.Japan.No.48,67-84,1998

Sedimentary environments based on textures of surface sediments and sedimentation rates in the South Yatsushiro Kai (Sea),

southwest Kyushu, Japan

Rifardi*, Kimihiko Oki** and Takashi Tomiyasu**

The main purpose of this study is to clarify the relation between characteristics of sedi-

ments and environmental conditions, based on the analysis of core samples from the South

Yatsushiro Kai (Sea) which is located off the west coast of central Kyushu Island, Japan. Bottom sediment samples were collected at 74 stations in the southern part of the

Yatsushiro Kai using gravity corers in March 1996. 62 core samples were used for the

mechanical analysis by the settling-tube method, and Q-mode cluster analysis was carried

out based on the proportion of grain size distribution (Wentworth size classes) . 61 core sam-

ples were available to estimate the sedimentation rates (during c. 50 years) which were rec-ognized from the first appearance of high mercury contents of cores. On the other hand, the topmost one centimeter of 73 core samples, which is approximately 10 cc of wet sediments,

was utilized for foraminifera) study.

The general trend of the bottom current system in the area was discussed, based on the

mechanical analysis data of bottom sediments, the distribution of sedimentation rates and

the distributions of the number of individuals of planktic and living benthic foraminifera. Based on the results mentioned above, the South Yatsushiro Kai is subdivided into the follow-

ing five areas:1) the area near the straits characterized by coarse sediments under the influ-

ence of strong tidal and bottom currents; 2) the southern part and the area off the mouth of

the Minamata River, both of which are characterized by relatively coarse sediments under

the influence of longshore currents and poorly sorted sediments supplied by the Komenotsu and Minamata Rivers; 3) the northwestern and eastern parts characterized by fine-grained

sediments accumulated under conditions of rather stagnant water masses; 4) the central

area, characterized by relatively fine sediments and a large number of planktic empty tests

deposited on the sea bottom under the condition of rather weak tidal current systems; 5) the

northern part characterized by gravels or gravelly sands which were derived from shallow coastal areas.

Judging from the distribution pattern of the maximum mercury content at some horizons

of each core, the fine-grained sediments polluted by mercury were transported northeast-

ward and southward by weak longshore currents and spread toward north and west across

the northern and southern parts of the South Yatsushiro Kai.

Received: June 29, 1998, Accepted : July 21, 1998 * Science of Marine Resources

, United Graduate School

of Agricultural Sciences, Doctoral Course, Kagoshima

University, Korimoto 1-21-24, Kagoshima City 890-0065,

Japan. **Department of Earth and Environmental Sciences

,

Faculty of Science, Kagoshima University, Korimoto 1-

21-35, Kagoshima City 890-0065, Japan.

Key words: Yatsushiro Kai, bottom sedim-

ents, sedimentation rates, foraminifera, dis-

charged mercury

68 Rifardi, Kimihiko Oki and Takashi Tomiyasu 1998

INTRODUCTION

Since the outbreak of Minamata disease, the Yatsushiro Kai (Sea) has become one of the are-

as most intensively studied in relation to the pol-

lution of water, sediments and marine fauna. Sig-

nificant studies in ecological and oceanographical

aspects of the area have been carried out by Hy-drographic Department, Maritime Safety Agency

(1974), Hirota and Hara (1975), Hydrographic Department, Maritime Safety Agency (1978),

Kikuchi and Tanaka (1978), Geographical Survey Institute (1982, 1984), Kikuchi (1983), Tsuruta

et al. (1986, 1987) and Tanaka et al. (1987).

However, only a few papers among them are

concerned with bottom sediments from the sedi-

mentological point of view.

The main purpose of this study is to clarify the

relation between characteristics of sediments and

environmental conditions, based on analysis of core samples from the South Yatsushiro Kai.

PHYSIOG1IAPHIC SETTING

The Yatsushiro Kai, a semi-closed inland sea, is located off the west coast of central Kyushu

Island, Japan (Fig. 1). It is elongate in outline

with a length of about 75 km from south to north

and a width of about 10 to 20km. The northern

part of the Sea has a rather flat bottom topogra-

phy less than 30m deep and is occupied by low-salinity water masses due to fresh water influ-

ence from the Kuma River with a large drainage

Fig.1 Index map of the study area (Kj: Karajiro Seto; Gn: Gannoshiri Seto; Mf: Mefuki Seto; Ir: Ikara Seto; Kr: Kurono Seto; Ec: Euchi River; Nd: Noda River; Kn: Komenotsu River; Mi: Minamata River; Ts: Tsunagi River; Ss: Sashiki River; Km: Kuma River; ----: watershed).

69J. Sed. Soc. Japan, No. 48 Sedimentary emvironments in the South Yatsushiro Kai

area (Fig. 1).

Climatologically, the Yatsushiro Kai is referable

to the region of the humid mesothermal (sub-

tropical) climate (Finch and Trewartha, 1949).

According to temperature data for 30 years

(1961-1990) in Kumamoto City near the Yatsu-

shiro Kai, annual mean temperature is 16.2•Ž,

mean temperature in winter is 6.0•Ž and that in

summer is 25.8•Ž. The highest annual mean pre-

cipitation during the past 30 years (1961-1990) is

1967.7 mm, the monthly mean precipitation being

lowest in December (49.1mm) and highest in

June (392.8 mm). The rainy season is from early

June to middle July in the Kumamoto area.

The area studied is restricted to the southern

part of the Yatsushiro Kai, being about 39 km

long and about 14 km wide. It is located between

Fig.2 Bathymetric map showing the sampling stations (contour interval 10m; Ak: Amakusa-kamishima; As: Amakusa-shimoshima; flu: Hino-shima; Sb: Shiba-shima; Go: Goshonoura-

jima; Sh: Shishi-jima; Ik: Ikara-jima; Na: Naga-shima; Iz: Izumi City; Mc: Minamata City; tc: Tsunagi-cho; at: Ashikita-cho; tn: Tanoura-cho; Mw: Minamata Wan).

the lines of 32•‹08'00" N and 32•‹ 23' 00" N Lat.

and between 130•‹12'00" E and 130•‹29'00" E

Long. This inland sea has five outlets or straits

(Karajiro Seto, Gannoshiri Seto, Mefuki Seto,

Ikara Seto and Kurono Seto) leading to the East

China Sea (Fig. 2) . The studied area is rather

flat in bottom topography and gets gradually

shallower eastwards (Fig. 2) . The Sashiki,

Tsunagi, Minamata and Komenotsu Rivers flow

into the eastern part, all of which have rather

large drainage areas as shown in Figure 1. On

the other hand, rivers with small drainage areas

flow into the southern part of the study area,

which consist of the Euchi and Noda Rivers, and

a few small rivers on the islands to the west,

which separate the Yatsushiro Kai from the East

China Sea (Fig. 1).

Strong tidal currents flow through the straits of

the western and southern parts of the area.

According to the Hydrographic Department,

Maritime Safety Agency (1978), the velocity of

the flood current coming into the Yatsushiro Kai

reaches 1.1 knots in the Gannoshiri Seto, 0.8

knots in the Mefuki Seto, and 4.9 knots in the

Kurono Seto.

On the contrary, the velocity of the ebb current

reaches 1.5 knots, 2.0 knots, and 4.8 knots at each

strait, respectively. The velocity and direction of

tidal currents in the Yatsushiro Kai were report-

ed by the Geographical Survey Institute (1984).

The areas with more than 0.5 knots are shown in

Figure 3 based on the map of Geographical Sur-

vey Institute (1984).

Except for the Izumi plain, the coastal area

shows rocky beaches around the southern part of

the Yatsushiro Kai. The northeastern (Tanoura-

cho and Ashikita-cho) , northwestern (Goshono-

ura-jima and Shishi jima) and southern (Izumi

City) parts of the studied area have a hinterland

of Carboniferous to Cretaceous sedimentary

rocks mainly composed of slate, chert, limestone,

sandstone and shale. Eastern (mainly Minamata

City) and southwestern (Naga-shima) parts of

the area are occupied by Pliocene to Pleistocene

volcanic rocks mainly of andesite. The Izumi

70 Rifardi,Kimihiko Oki and Takashi Tomiyasu 1998

Fig.3 Velocity and direction of tidal currents (after the Geographical Survey Institute, 1984); left: the flood current; right: the ebb current.

plain consists of Pliocene to Pleistocene terrace and pyroclastic deposits.

MATERIAL AND METHODS

The bottom samples used for this study were collected at 74 stations in the southern part of the

Yatsushiro Kai with depths from 14 to 54 m in

March 1996 (Fig. 2) . Positioning for the sampling

sites was determined with the Furuno GPS

Plotter GP-1500 (FCV-663 Model) installed on the research vessel of the Fisheries Reseach Labora-

tory, Kagoshima University. All the bottom sam-

ples were collected by using the gravity corer made by Hisanaga Co., Ltd, Kagoshima. The 1 to

2 cm (about 10cc in volume) of 62 core samples

was used for mechanical analysis and 61 core samples were used for estimation of sedimenta-

tion rates. The top one centimeter of 73 core sam-

ples was analysed for foraminiferal study.

1. Bottom sediment analyses

a) Mechanical analysis

Gravels were collected by sieving and were

weighed in dry condition. The Emery settling-

tube method was utilized for determining the rel-

ative proportion of sand. The mud proportion

was calculated by using Hiranuma Particle Size

Analyser PSA-2. Median diameter (MdƒÓ) , sort-

ing coefficient (ƒÐi) , and skewness (Ski) were

obtained with the graphical method of Folk and

Ward (1957). The textural proportions of sand,

silt and clay were plotted on the Shepard's trian-

gle (Shepard, 1954) for all the samples.

b) Cluster analysis

Based on the proportion (%) of grain size dis-

tribution excluding gravels (Wentworth size

classes), a dendrogram was obtained by the Q-

mode cluster analysis that is made up by average

linkage method (Norusis, 1993). The samples

were grouped according to similarity indicated by

the cosine similarity coefficient *.

The cosine similarity coefficient is computed by

* Similarity(X,Y)=(ΣiXiYi)/√(ΣXi2)(ΣYi2)

71J.Sed.Soc.Japan,No.48 Sedimentary emvironments in the South Yatsushiro Kai

2. Analysis of mercury contents and sedimenta-

tion rates Mercury contents at 12 horizons of 2-3, 5-6, 8-9,

13-14,18-19, 23-24, 28-29, 33-34, 38-39, 43-44, 48-49

and 53-54 cm* of core samples were measured in

laboratory. The Sansou Automatic mercury ana-

lyser Model Hg-3500 was used. The procedure for determination of total mercury (Akagi and

Nishimura, 1991) was as follows: a known a-

mount of sample was placed in a 50 ml volumet-

ric flask, to which 2 ml of 1:1 nitric acid-perchlo-

ric acid solution and 5 ml of concentrated sulfuric acid were added, and heated on a hot plate at

230°C for 30 min. After cooling, the digested sam-

* Below the surface of a core in cm.

ple was made up to 50 ml with water and the

mercury in a suitable aliquot of the resulting

solution (<20ml) was analysed by CVAAS.

Since 1946, methyl mercury had been dis-

charged into the Yatsushiro Kai through the

drainpipes from the Minamata factory of Shin-

nihon Chisso Hiryo Co., Ltd. The mercury con-

tents of less than 0.1ppm (0.036•`0.094 ppm) in

the deepest part of the cores have been regarded

as the chemical situation of the Yatsu-shiro Kai.

On the contrary, samples of more than 0.1 ppm

(maximum value: 3.457ppm at Station 19) are

discovered in the relatively upper part of the

cores (Fig. 4). The first appearance of high mer-

cury contents at each core sample indicates the

arrival time of pollutants at the points. Each sam-

Fig.4 Vertical distribution of mercury contents in core samples.


pling point has a time-lag depending on the dis-

tance from the drainage of the factory and cur-

rent systems in the area. The high mercury con-

tents (0.31•`0.55 ppm) in the bottom surface

sediments in the northern part of the Sea, off the

east coast of Goshonoura-jima, were known

(Kitamura et al., 1960) . These facts suggest that

the time-lag between the eastern and western

parts of the area, may have been a few years,

and a rough estimate of sedimentation rates is

inferable from the core length above the first

appearance of high mercury contents (>0.1

ppm), and this length is roughly equivalent to the

period for about 50 years from 1946 to 1996.

3. Foraminiferal analysis

The topmost one centimeter of core samples,

which is approximately 10cc of wet sediments,

was preserved in alcohol and stained with Rose

Bengal for discriminating the living foraminifera.

Each sample was washed through a 200-mesh

(0.0074mm opening) sieve, and ovendried after

Fig.5 Distribution of the median diameter

(MdƒÓ) of bottom surface sediments.

removal of dye stuff (Rose Bengal) . All the speci-

mens were then picked out from the aliquot.

Aliquots containing less than 200 individuals of

benthic foraminifera were supplemented by addi-

tional splits. Specimens were spread on a tray,

and all the specimens of benthic species were

identified and counted. Specimens of planktic

foraminifera within each aliquot were also count-

ed.

RESULTS

1. Bottom sediments analyses

a) mechanical analysis

The results of mechanical analysis of the 62

samples are shown in Table 1. The distributions

of median diameter (MdƒÓ) and mud content

(%) are shown in Figures 5 and 6, respectively.

The bottom sediments in the South Yatsushiro

Kai are characterized by fine to very fine sand

(MdƒÓ:2•`4ƒÓ).•

Core samples could not be collected at stations

1, 6, 15, 29, 35, 42, 47, 48, 53, 70 and 73, which are

Fig.6 Distribution of the mud content (%) of

bottom surface sediments.


1able 1 Results of mechanical analysis of the bottom sediments.

74 Rifardi,Kimihiko Okiand Takashi Tomiyasu 1998

all located near the straits (Gannoshiri Seto,

Mefuki Seto, Ikara Seto and Kurono Seto) and in

the northern part of the studied area. The surface

sediments collected for foraminiferal analysis at

these stations contain shell fragments, granule

and coarse sand grains. These samples indicate

that the bottom character in the areas must be

gravel and/or sand. The Geographical Survey

Institute (1982, 1984) reported a bottom surface

character showing medium sand to gravel near

the straits of Gannoshiri Seto> Mefuki Seto, Ikara

Seto and Kurono Seto. Stations 65 (gravel con-

tent: 5.8%), 68 (gravel content: 12.4%) and 74

(gravel content: 41.9%) with relatively low medi-

an diameters (MdƒÓ: about 1ƒÓ) are located in

the northern part of the studied area.

The areas with high median diameters (Md ƒÓ

:>4ƒÓ) can be seen in the northeastern (Stn.

40, 46, 51, 52, 56, 57, 63, 67 and 72) and north-

western (Stn. 64 and 69) parts, where all the

stations have high mud contents more than 50%

(Figs. 5 and 6).

Fig.7 Distribution of the sorting coefficient (ƒÐi)

of bottom surface sediments.

The distributions of sorting coefficient (ƒÐi)

and skewness (Ski) are shown in Figures 7 and

8, respectively. A general trend of sorting coeffi-

cients in the studied area is that sediments are

poorly sorted (1.0•`2.0ƒÓ). Moderately sorted

sediments (0.71•`1.00ƒÓ) are recognized in the

areas influenced by strong tidal currents as in the

Gannoshiri Seto area (Stn. 27, 31, 32, 36, 37, 41

and 43) and the Kurono Seto area (Stn. 2, 12, 16

and 20) . Very poorly sorted sediments (2.00•`

4.00ƒÓ) are distributed off the estuaries of the

Minamata and Komenotsu Rivers and at Stn. 74

near Shiba-shima.

Strongly fine skewed sediments are recognized

in nearshore areas of the northern part of the

Yatsushiro Kai, and the areas off Minamata Wan

(Bay) and extended from Gannoshiri Seto to the

southeast. Coarse skewed and near symmetrical

skewed sediments are distributed in the belt-like

area drawing an arc in the central part of Yatsu-

shiro Kai, and in the belt-like area having east-

west direction in the southern part of the sea.

Fig.8 Distribution of the skewness (Ski) of bottom surface sediments.


b) cluster analysis

The sediment samples can largely be classified

by the cluster analysis into five clusters (I to V)

at the level of 0.83 similarity coefficient in den-

drogram (Fig. 9) . Among the five clusters,

Cluster II is subdivided into four subclusters;

Cluster III into two subclusters; and Cluster IV

into three subclusters at 0.90 level. Geographical

distribution of these sediment clusters is shown

in Figure 10. The character of each cluster is

shown in Table 2, and the relationship between

clusters and three parameters; median diameter

(MdƒÓ) , sorting coefficient (ƒÐi) and skewness

(Ski), is shown in Figure 11.

Cluster I (23 stations) is characterized by pre-

dominantly fine sand to coarse silt sediments (56

-81%), and median diameter (MdƒÓ) ranging

from 3.0 to 3.75ƒÓ except for Station 21 (MdƒÓ:

2.4) which have high gravel content (21.2%). In

the central and southern parts of the sea, Cluster

I is distributed in the elongate area extending

from northeast to southwest in the middle part

between the east and west coastlines, and in the

Fig.9 Dendrogram for the bottom surface sedi-

ments.

elongate areas extending from northwest to

southeast in the southwestern part of the sea

(Fig. 10).

Subclusters IIa (6 stations) and IIb (14 sta-

tions) are characterized by fine-grained sedi-

ments (MdƒÓ: 3.4•`4.3; mud contents: 33.0•`

65.2%) and are mainly distributed in the north-

ern and eastern parts in the studied area.

Table 2 Results of cluster analysis of the

grain-size composition of sediment samples.

Fig.10 Distribution of the sediment clusters

recognized by cluster analysis.


Subcluster IIb differs from the ha in showing a

very fine skewed nature (0.33•`0.6) .

Subcluster IIc is only recognized at Station 74

located in the northernmost part of the area, and

is characterized by very coarse median diameter

(0.7ƒÓ) , high gravel content, and a trimodal dis-

tribution of grain size; granule (41.9%), coarse

sand (10.1%) and very fine sand (18.2%).

Fig.11 The relationship between

clusters and three parameters;

median diameter (MdƒÓ), sorting

coefficient (ƒÐz) and skewness

(Ski), Stn. 21; gravel content

21.2%.

Subcluster lid is mostly of fine-grained sedi-

ments in the area (MdƒÓ: 4.5ƒÓ;mud content:

73.4%). This subcluster is shown only at Station

64.

Cluster III (8 stations) is characterized by pre-

dominantly medium to very fine sand sediments

(77•`88%) , median diameter (MdƒÓ) ranging

from 2.2 to 2.85ƒÓ, and fine to very fine skewed.

Subcluster IIIb (Stn. 54) shows a bimodal distri-

bution of grain size; one is fine sand (47.2%)

and the other is coarse silt (9.7%) . Cluster III is

distributed in the nearshore areas off Goshono-

ura-jima and in the southern part of the area

(Stn. 2, 9 and 25).

Subclusters IVa (5 stations) and IVb (2 sta-

tions) are characterized by fine sand-grained

median diameter (MdƒÓ) ranging from 1.9 to 2.9

ƒÓ,relatively symmetrical skewed (-0.11•`0.16),

and poorly and very poorly sorted (1.23•`3.04).

All of the stations include gravel (1.1•`25.6%).

These subclusters are distributed in the shore

area off the estuaries of the Komenotsu (Stn. 5,

10 and 14) and the Minamata (Stn. 33) Rivers,

and in the southernmost part of Yatsushiro Kai

(Stn. 3 and 7) near the Kurono Seto except for

Stn. 17.

Subcluster IVc (Stn. 65) shows a trimodal dis-

tribution of grain size; granule (5.8%) , medium

sand (44.8%) and coarse silt (7.8%) , with a

medium sand-grained median diameter (1.4ƒÓ).

Station 65 is located in the northern part of the

area off Hino-shima.

Cluster V (Stn. 68) shows a trimodal distribu-

tion of grain size; granule (12.4%), coarse sand

(46.9%) and very fine sand (11.7%), with a

coarse sand-grained median diameter (0.8ƒÓ) .

Gravel is approximately composed of shell frag-

ments. Station 68 is located in the Karajiro Seto.

2. Sedimentation rates based on mercury con-

tents

The mercury contents of less than 0.1 ppm

(0.036•`0.094 ppm) are regarded as a chemical

condition of the Yatsushiro Kai as mentioned

above. The horizon of core showing the first


appearance of high mercury contents are shown

in Table 3.

Table 3 Horizon of core length showing the

first appearance of high mercury contents.

* Below the surface of a core in centimeter .

> Low mercury contents (less than 0.1ppm) cannot be detected in the core.

The distribution of sedimentation rates have

been roughly estimated by the first appearance of high mercury contents, as shown in Figure 12.

Generally, the low sedimentation rates (less than

11cm / 50 years) are shown in the western part

of the area except for Stations 25, 43, 58 and 69.

Very low sedimentation rates of less than 4 cm are seen at Stations 64 and 65 located in the strait

between Hino-shima and Goshonoura-jima

lslands. In contrast, high sedimentation rates

(more than 21cm) can be observed in the north-eastern and southeastern areas in addition to Station 40 off the estuary of the Tsunagi River.

3. Foraminifera

a) Planktic foraminifera

At each station, the number of individuals* of

planktic foraminifera was roughly estimated con-sidering the rate of sedimentation and their dis-

Fig.12 Distribution of the sedimentation rates.

tribution as shown in Figure 13.

In general, a large number of individuals can

be seen in the southwestern part of the South

Yatsushiro Kai except for Stations 17, 21 and 25,

and the central part of the area. The stations

with large numbers of more than 300 are distrib-

uted in the belt-like areas with northwest-south-

east direction (Stn.12, 16 and 20; Stn. 27, 31, 36

and 41) and in the center part (Stn. 50) . In con-

trast, the stations where the number is less than

50 individuals are found in the northwestern part

of the area and in the shallow area along the

coastline from Izumi City to Tanoura-cho (Fig.

13).

b) Living benthic foraminifera

The individual numbers of living benthic

foraminifera range from 17 to 838 in 10cc of the

* Number of individuals =Number of individuals in

10cc•~core length above the horizon of the first appear-

ance of high mercury / 50 years (since 1946 to 1996).


Fig.13 Distribution of the number of individu-als of planktic foraminifera.

surface sediments in the South Yatsushiro Kai,

and their distribution is shown in Figure 14. In general, a large number of individuals, more

than 200, is found at the stations in the central

part of the South Yatsushiro Kai (Stn. 27, 28, 31, 32, 33, 36, 37, 38, 39, 41, 43, 44) , and Station 11 in

the southern part of the area. On the other hand, a small number of individuals, less than 100, is

seen at the stations in the northern and south-

eastern parts.

DISCUSSION

The South Yatsushiro Kai is divided into the

following five areas according to the character of

the bottom sediments: 1) areas near the straits

characterized by coarse-grained sediments

(Cluster III) ; 2) the southern part of the Sea and off the mouth of the Minamata River (Clusters IVa and b) ; 3) the northern and eastern parts of

the Sea characterized by fine-grained sediments

(Clusters ha, b and d); 4) the area mainly in the central part of the Sea (Cluster I); 5) the north-

Fig.14 Distribution of the number of individu-

als of living benthic foraminifera.

emn part of the Sea characterized by gravel or

gravelly sand sediments (Clusters IIc, IVc and

V) . The relationship between the characters of

bottom sediments and the depositional environ-

ments in these five areas is discussed below.

1. Areas near the straits characterized by coarse-

grained sediments (Cluster III)

Strong main currents are seen in the

Gannoshiri Seto and Kurono Seto (Hydrographic

Department, Maritime Safety Agency, 1978; Geo-

graphical Survey Institute, 1984; Fig. 3) . Core

samples at Stations 1, 6, 42, 47, 48 and 53 near

both the straits could not be collected, but a small

amount of surface sediments mainly composed of

shell fragments and gravels was recovered at the

same stations. These facts indicate that strong

tidal currents as rapid as more than 1 knot influ-

ence the bottom sediments in spite of the com-

paratively deep water depths (32.5•`54.3 m) at

the stations. Tidal currents flowing through the

Mefuki Seto and Ikara Seto are also strong at


Stations 15, 29 and 35 near the straits.

Cluster III is characterized by fine sand-

grained median diameter (2.2 to 2.85ƒÓ) and

relatively well sorted sediments (less than 1.35

ƒÓ) . The seven stations which belong to Cluster

III except for Station 9 are distributed in the

areas that extend from the straits to the South

Yatsushiro Kai. Stations 37, 43, 49, 54 and 58 are

distributed particularly in the elongate area off

the southeast coast of Goshonoura-jima. The fea-

tures described above suggest that strong bottom

currents constantly occur in and near the straits.

The grain-size composition of sediments at

Station 9 is rather similar to that at Stations 5, 10

and 14 (Cluster IVa) in the shore area off the

estuary of the Komenotsu River, but differs

slightly from the latter by its in well sorted and

fine skewed nature (0.23) .This situation may be

explained by the location of Station 9 in the vicin-

ity of sea mounds.

2. The southern part of the Sea and off the mouth

of the Minamata River (Clusters IVa and b)

All the stations included in Clusters III and IV

show the character of bottom sediments (MdƒÓ:

1.9•`2.9) which are influenced by rather strong

bottom current (Figs. 3, 5 and 6) . Five stations

which belong to Cluster IVa and b except for two

stations (Stn. 17 and 33) are located in the

coastal area off Izumi City (Fig. 10) . Judging

from the distribution pattern of sorting coeffi-

cients (ƒÐi; Fig. 7) , the bottom sediments sup-

plied by the Komenotsu River are assumed to

have spread toward the north. The feature sug-

gests that the counterclockwise longshore cur-

rents occur along the coast of Izumi City. The

Geographical Survey Institute (1984) reported

that the velocities of flood and ebb currents are

relatively strong in this area and the former is

stronger than the latter (Fig. 3) .These facts sup-

port the existence of longshore currents men-

tioned above.

Station 33 is located off the mouth of the

Minamata River. The poorly sorted sediments

supplied by the Minamata River were deposited

in the area (Fig. 7) , but the deposition of mostly

mud sediments was prevented by the flood cur-

rents which flow across the South Yatsushiro Kai

from the Gannoshiri Seto (Fig. 3) . Judging from

the distributions of median diameter (MdƒÓ; Fig.

5) and mud content (Fig. 6) , the mud part of the

sediments must have been transported from the

mouth of the Minamata River northeastward and

southwestward by very weak bottom currents,

and was accumulated in the areas off Minamata

Wan and off Tsunagi-cho where the high sedi-

mentation rates are seen (Fig. 12).

Station 17 is isolated from the stations which

are grouped into Clusters I and IIb. This may be

explained by the fact that Station 17 is located in

the vicinity of sea mounds with irregular subma-

rine topography.

3. The northern and eastern parts of the area

characterized by fine-grained sediments

(Clusters ha, b and d)

Fine-grained sediments, which belong to

Clusters ha, b and d, are distributed in the north-

ern and eastern parts of the South Yatsushiro Kai

(Figs. 5 and 6) . These sediments show the medi-

an diameter ranging from 3.4 to 4.5ƒÓ and the

near symmetrical to strongly fine skewed nature

(Ski: 0.0•`0.63) . All the sediments belonging to

Cluster IIb show the strongly fine skewed nature

of Ski 0.33 to 0.63 (Fig. 11) . Judging from the

high mud contents (33.0•`73.4%) at every sta-

tion, suspended sediments accumulate on the sea

bottom in the areas, and rather high sedimenta-

tion rates in the areas support the accumulation

of mud (Fig.12).

According to the Hydrographic Department,

Maritime Safety Agency (1978) and the Geo-

graphical Survey Institute (1984) , velocities of

the flood and ebb currents are relatively strong,

more than 0.5 knots in the northeastern part of

the South Yatsushiro Kai. However, bottom cur-

rents in the area must be very weak in spite of

the strong surface tidal currents, and this area is

occupied by rather stagnant water mass through-

out the year. Suspended sediments are assumed


to be supplied by the ebb current flows south-

ward from the North Yatsushiro Kai where a

great amount of sediment derived from the hin-terland through the Kuma River, has been deposited.

The Sashiki and Tsunagi Rivers flow down into

the northern part of the area, but coarse-grained

sediments could not be seen in the area. The rea-

sons for deficiency of such sediments may be inferred as follows: 1) the Sashiki River has a

comparatively large drainage area, but this area

is mainly occupied by Paleozoic and Mesozoic

hard sedimentary rocks which are very resistant

against weathering; 2) the Tsunagi River has a small drainage area; 3) both rivers flow down

into small bays, respectively Nosakano-ura and

Tsunagi Wan, and the coarse sediments were

deposited in those bays (Geographical Survey

Institute, 1984). On the other hand, very fine sediments sup-

plied mainly by the Minamata River, seem to spread southwestward by very weak bottom cur-

rents and have accumulated in the area off

Minamata Wan (Figs. 5, 6 and 8) . The feature mentioned above is supported by high sedimen-

tation rates of more than 24 cm at Stations 18, 19

and 24 as well as by the presence of stagnant

water masses off Minamata through the spring

and neap tides (Hydrographic Department, Maritime Safety Agency, 1978).

Figure 15 shows the distribution of the maxi-

mum mercury content which have been analysed

at some horizons of each core. Judging from their

distribution patterns, the sediments polluted by mercury were transported both northeastward

and southward by longshore currents and spread

north and west across the sea in the northern

and southern parts of the South Yatsushiro Kai.

These facts harmonize quite well with the current

system based on the character of sediments in the South Yatsushiro Kai.

At Stations 64, bottom sediments show the

highest mud content (73.4%) in the studied area.

This station is located in the vicinity of

Goshonoura-jima. This point must be situated out

Fig.15 Distribution of the maximum mercury content (ppm) which have been analysed at some horizons of each core.

of the main tidal current system.

4. Area recognized mainly in the central part of

the Sea (Cluster I)

In the studied area, bottom sediments with an

intermediate grain size (Cluster I; Fig. 10) are distributed between two areas. One of these is

characterized by coarse-grained sediments which

are influenced by rather strong bottom current

(Clusters III, IVa and IVb) and the other is char-acterized by fine-grained sediments which are

deposited in stagnant water masses (Clusters ha and b) .

The area where the bottom sediments show an

intermediate grain size, is divided into three

areas. One of these areas (Stn. 28, 32, 38, 39, 44,

45, 50 and 55) could be interpreted as the transi-tional zone from the coarse-grained sediments to

the fine-grained sediments. However, considering

rather high contents of planktic foraminiferal

tests in the bottom sediments, this area may cor-


respond to the boundary between the water

masses which enter into the South Yatsushiro Kai

through the Gannoshiri Seto and which flow

northward as the flood current along the east coast of the northern part of the sea, and the

rather stable surface water masses off the south-

east coast of Goshonoura-jima. Planktic forami-

niferal tests may be accumulated by a down

stream along the frontal interface which occurs in a current rip area (Sakamoto, 1982; Oki,

1989).

The other two areas extend across the South

Yatsushiro Kai; one extends from off the south-

east coast of Shishi-jima to the southeast (Stn. 22, 26, 27, 30, 31, 36 and 41) and the other

extends from off the east coast of Ikara-jima to

the southeast (Stn. 8, 11, and 20) . Both

the areas are sandwiched between areas where

strong tidal currents flow into and out through

the straits; Gannoshiri, Mefuki and Kurono Seto

(Fig. 3) . These areas must be out of the strong tidal current system, and suspended matters

accumulate on the sea bottom. This may best be

explained by the high contents of planktic

foraminiferal tests in the bottom sediments (Fig. 13).

Large number of individuals of living benthic

foraminifera (Fig. 14) in the bottom sediments

can be seen in the central part of the South

Yatsushiro Kai (Stn. 31, 32, 36, 37, 38, 41 and 44). The abundance of living benthic foraminifera

may be related to the supply of detritus by tidal

current systems, but further studies are needed

on the ecology of living benthic foraminifera.

5. The northern part of the sea characterized by

gravel or gravelly sand sediments (Clusters IIc, IVc and V)

The bottom sediment (Cluster IIc) at Station

74 located in the vicinity of a small island, Shiba-

shima, is characterized by a trimodal distribution

of grain size; granule (41.9%), coarse sand

(10.1%) and very fine sand (18.2%) . The high

gravel content may have resulted from the direct supply from the coastal area of Shiba-shima dur-

ing storms.

The area around Hino-shima shows rather

complex submarine topography with steep slopes and many small caldrons. Off the east coast of

Hino-shima, there is a depression more than 40

m deep (Fig. 2) . Core samples at the two stations

(Stn. 70 and 73) situated in this depression, could not be collected. According to the Geographical Survey Institute (1974), bottom

sediments in this area are characterized by

almost medium-grained sand. At Station 65 locat-

ed in the southernmost part of this depression,

bottom sediments show a trimodal distribution of

grain size; granule (5.8%), medium sand (44.8%) and coarse silt (7.8%) . These facts suggest that

the bottom currents are not so weak in the

depression area and gravels are derived from the

coastal area of Hino-shima.

Station 68 is located in the caldron near a nar-row channel. Bottom sediments at the station

show a trimodal distribution of grain size; gran-

ule (12.4%) , coarse sand (46.9%) and very fine

sand (11.7%) , and this character harmonizes

with the strength of current which flows through the narrow channel (Geographical Survey In-

stitute, 1984; Fig. 3).

The Karajiro Seto can be described as a small

inner bay which is surrounded on three sides by the islands of Hino-shima, Amakusa-kamishima

and Goshonoura-jima. Bottom sediments at two

stations (Stn. 64 and 69) , located in the Karajiro

Seto, are characterized by high mud contents of

more than 55%. The Geographical Survey Insti-

tute (1974) also reported that bottom sediments in the Karajiro Seto are characterized by silt at

almost all the points. These data indicate that the

area is occupied by rather stagnant water mass

except for the areas near the small channels, and

fine-grained sediments were accumulated in the Karajiro Seto area. The abnormal mercury con-

tents were found in the muddy sediments at

Station 69 (Fig. 15) . It is of particular interest

that the sediments polluted by mercury migrated

across the South Yatsushiro Kai off Minamata to


this area, but this is not enough to explain the

process of sediment migration only by bottom

current system, based on the mechanical analysis

of bottom sediments. Owing to a lack of biological

evidence, a definite conclusion must be reserved.

CONCLUSIONS

The results of the present study are summa-

rized as follows:

1. General feature of the bottom sediments in the

South Yatsushiro Kai is characterized by one to

very fine sand (Figs. 5 and 11) . High mud con-

tents of more than 50% are found in the north-

eastern and northwestern parts of the Sea.

2. The South Yatsushiro Kai is divided into the

following five areas according to bottom sedi-

ment characters:

1) Areas near the straits characterized by

coarse-grained sediments (Cluster III).

2) The southern part of the Sea and off the

mouth of the Minamata River character-

ized by rather poorly sorted coarse-

grained sediments (Clusters IVa and b).

3) The northern and eastern parts of the Sea

characterized by fine-grained sediments

(Clusters ha, b and d).

4) Areas mainly in the central part of the

Sea characterized by the bottom sedi-

ments with an intermediate grain size

and the high contents of planktic forami-

niferal empty tests (Cluster I).

5) The northern part of the Sea character-

ized by gravel or gravelly sand sediments

(Clusters IIc, IVc and V).

3. The strong tidal currents flowing through the

straits (Gannoshiri, Mefuki and Kurono Seto)

influence the bottom sediments in spite of deep

water depth (32.5•`54.3 m) near the straits

(Area 1 mentioned above).

4. The poorly sorted sediments supplied by the

Komenotsu River spread northwards by the

counterclockwise longshore current which

occurs along the coast of Izumi City (Area 2).

5. Judging from the high mud contents and

rather high sedimentation rates, the northeast-

ern and northwestern (the Karajiro Seto area)

parts of the Sea and the area off Minamata Wan are occupied by rather stagnant water

masses throughout the year (Area 3).

6. Judging from the distribution pattern of the maximum mercury content at some horizons of

each core, the fine-grained sediments polluted

by mercury were transported northeastward

and southward by weak longshore currents and

spread toward north and west across the north-

ern and southern parts of the South Yatsushiro Kai.

7. Bottom sediments with an intermediate grain

size (Cluster I) and high contents of planktic

foraminiferal empty tests are distributed in the areas which are situated at the boundary

between different water masses.

ACKNOWLEDGEMENTS

We wish to express our deep gratitude to Dr.

Hakuyu Okada, former Professor of Kyushu

University, for critical reading of the manuscript and valuable suggestions concerning sedimen-

tology. Particular thanks are due to Professor

Hiroshi Ujiie of Takushoku University who was

an advisor to Rifardi during his M. Sc. study at Department of Marine Sciences, College of Sci-

ence, University of the Ryukyus, Okinawa, for

encouragements and reading of the manuscript;

Professor Johann Hohenegger of the Institute of

Paleontology, University of Vienna, for his

encouragement and valuable suggestions con-cerning the relationship between bottom sedi-

ments and benthic foraminifera; Mr. Teruo

Kasedo, the captain of R.V. Azuma and the crew,

the Fisheries Research Laboratory, Faculty of

Fisheries, Kagoshima University, for their kind help in on-board work, and Ms Hiroko Ijiri, tech-

nical official of the Kagoshima University Infor-

mation Processing Center, for her technical assis-

tance with the computer work. We also thank

Professor Hiroyuki Otsuka of the Department of Earth and Environmental Sciences, Faculty of

Science, and Professor Gunzo Kawamura of the

Department of Fisheries Science and Technology,


Faculty of Fisheries, Kagoshima University, for

their encouragement.

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Akagi, H. and Nishimura, H., 1991: Speciation of

mercury in the environment. In Suzuki, T.,

Imura, N. and Clarkson, T. W., eds., Ad-

vances in Mercury Toxicology, Plenum Press,

New York, 53-76. Finch, V.C. and Trewartha, G.T., 1949: Elements

of geography, physical and culture. McGraw-

Hill Book Co. Inc., 711p.

Folk, R. L, and Ward, W. C., 1957: Brazos River

bar: a study in the significance of grain size

parameter. Jour. Sed. Pet., 27, 3-26. Geographical Survey Institute, 1974: The 1:25, 000

topographic and land condition maps of the

coastal sea and land areas "Kyoragi" (in

Japanese). Geographical Survey Institute, Tsukuba, Japan.

Geographical Survey Institute, 1982: Reports of

fundamental survey of the coastal sea areas "Mi

namata" (in Japanese). Geographical

Survey Institute, Tsukuba, Japan, 119p.

Geographical Survey Institute, 1984: Ibid. "Izumi"

(in Japanese). Geographical Survey Institute, Tsukuba, Japan, 97p.

Hirota, R. and Hara, M., 1975: Zooplankton inves-

tigation in Yatsushiro Kai, Western Kyushu,

I. Regional and seasonal occurrences of the important zooplankton. J. Oceanogr. Soc.

Japan, 31,115-123.

Hydrographic Department, Maritime Safety

Agency, 1974: Report on oceanographic sur-

vey in Ariake-Kai and Yatsushiro Kai (in

Japanese). Maritime Safety Agency, Tokyo, Japan, 39p.

Hydrographic Department, Maritime Safety

Agency, 1978: Charts of tidal currents in

Simabara Wan and Yatsushiro Kai (in

Japanese). No.6217, Maritime Safety Agency, Tokyo, Japan.

Kikuchi, T., 1983: The composition and distribu-

tion of macrobenthos communities in

Shiranuhi Kai (in Japanese). In Yamagata,

N., Doi, R., Saishu, S. and Taguchi, T., eds., "Endeavour against environme

ntal pollution-biological effect of heavy metals", Koseisha-

ku Ltd., 71-84, Tokyo.

Kikuchi, T. and Tanaka, M., 1978: Studies on ben-

thos of polluted water: I. Benthic communi-ties of Ariake Sea and Yatsushiro Sea. In

Irie, H., ed., Fundamental study on the cy-

cling of pollutants and long term variation of

fauna in embayment waters, Reseach Rep.,

1975-1977, 75-80. Kitamura, S., Ueda, K., Niino, J., Ujioka, T.,

Misumi, H. and Kakita, T., 1960: Minamata-

byo ni kansuru Kagaku-dokubutsu Kensaku

Seiseki (in Japanese). Jour. Kumamoto Medi-

cal Society, 34, 593-601. Norusis, M.J., 1993: SPSS for UNIX, professional

statistics release 5.0. SPSS Inc., 345p.

Oki, K., 1989: Ecological analysis of benthonic

foraminifera in Kagoshima Bay, South

Kyushu, Japan. South Pacific Study., 10 (1), 1-191.

Sakamoto, W., 1982: Detritus-roles of suspended

and bottom sediment on coastal marine

ecosystem (in Japanese). Marine Sci. Month-

ly, 14 (8), 482-489.

Shepard, F.P., 1954: Nomenclature based on sand-silt-clay ratio. Jour. Sed. Pet., 24, 151-

158.

Tanaka, M., Kikuchi, T., Tsutsumi, H., Nojima, S.,

Mori, K., Tamaki, A., Asakura, A., Yun, S. G.,

Nishihama, S., Soliman, F. E., Suzuki, H., Rodrigues, C. L., Aoki, M., Fischer, S. and

Aryuthaka, C., 1987: Ecological studies on the

benthic ecosystem in Yatsushiro Kai, West

Kyushu: I. Physico-chemical measurement of

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Tsuruta, A., Ueno, S., Ohgai, M. and Yamada, M.,

1986: Study on the seasonal and horizontal

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Yatsushiro Sea, using a cluster analysis (in

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Tsuruta, A., Ueno, S., Ohgai, M. and Yamada, M.,

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表層堆積物の組成と堆積速度からみた南八代海の堆積環境

Rifardi・大木公彦・冨安卓滋,1998,堆積学研究,No,48,67-84

Rifardi, Oki, K, and Tomiyasu, T., 1998: Sedimentary environments based on textures of

surface sediments and sedimentation rates in the South Yatsushiro Kai (Sea), southwest

Kyushu, Japan. Jour. Sed. Soc. Japan, No. 48, 67-84.

九州西部の,天草の島々によって外洋と分けられている南八代海の表層堆積物と海洋環境と

の関係を明らかにすることを目的として,1996年3月に74点のコア試料を採取した.62のコア

の試料は,settling-tube methodによって粒度分析に使われ,61の試料は,コア中の水銀含有

量の異常値の初出現層準から堆積速度(cm/約50年)を見積もるために使われた.一方、73の

コア試料の最上部1cm(10ccに相当)は,底生・浮遊性有孔虫殻の個体数の地理的分布,底生

有孔虫生体殻の生息分布を知るために使われた.

それらの研究結果から,南八代海は次の5つの海域にまとめることができる:1)西側の海

峡近くの海域は,強い潮流・底層流を反映して粗い堆積物で特徴づけられる;2)南部と水俣

川河口沖の浅海域は,沿岸流の影響と米ノ津川・水俣川から供給される堆積物から,比較的粗

粒な,淘汰の悪い堆積物で特徴づけられる;3)北西部と東部海域は,比較的停滞した水塊の

存在から細粒堆積物で特徴づけられ,堆積速度も一般に早い;4)中央部では,弱い潮流と異

なる水塊の潮目付近に位置することを反映して,比較的細粒な,浮遊性有孔虫遺骸殻を多く含

んだ堆積物によって特徴づけられる;5)北部の島に近い海域では,沿岸浅海域からもたらさ

れた礫あるいは礫質砂によって特徴づけられる.

各コアで測定された水銀含有量最大値の地理的分布から,水銀に汚染された細粒堆積物は水

俣湾沖から沿岸流によって北東と南へ移動し、南八代海の北部では北へ,南部では西へ,海域

を横切って拡散していることが明らかになった.

sedimentary environments based on textures of surface

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