Reconstruction of Sedimentary Environments for Late

第四紀研究(The Quaternary Research) 37 (2) p. 77-94 May 1998

Reconstruction of Sedimentary Environments for Late Pleistocene

to Holocene Coastal Deposits of Lake Kamo, Sado Island,

Central Japan

Van Lap Nguyen*1,Masaaki Tateishi*2 and Iwao Kobayashi*2

Multiproxy analyses including grain size, total organic carbon (TOC), total sulfur (TS), total nitrogen (TN), calcium carbonate (CaCO3) contents, diatom as well as sedimentary properties of the boring core KM-11 (54 m in length) collected from a brackish lake, Lake Kamo, are used to reconstruct paleoenvironment and coastal evolution of northeast coast of Sado Island in central Japan. Anoxic and normal marine sediments can be detected by TOC-TS relationship, and the origin of organic matter is inferred from the TOC/TN ratio. Geochemical characteristics of the sediments respond striking to sea-level change. In the Late Pleistocene, fluvial sediments are characterized by high TOC, very low TS and terrigenous organic matter. The overlying coastal marsh sediments are characterized by high TOC and abruptly increasing trend of TS suggesting sea water spilled into the site, and are overlain by estuarine sediments which obviously indicates the beginning of the Holocene transgression. Embayment sediments are characterized by low TOC, marine organic matter and low productivity condition indicating the rapid relative sea-level rise. The maximum Holocene transgression could be inferred between 6,500 and 5,000 yrs BP. After that fluctuations of relative sea-level are characterized by high TOC, high TS, a mixed type of marine/terrigenous organic matter and high productivity condition to be inferred marine/brackish lagoonal sediments. Diatom floral changes of the core KM-11 also support the geochemical data to interpret the paleoenvironment and evolution of the coastal deposits.

Key Words: Lake Kamo, total organic carbon, total sulfur, total nitrogen,

diatom, sedimentary environment, relative sea-level

I. Introduction

Lake Kamo is situated in Ryotsu City, Sado

Island, Niigata Prefecture (Fig. 1). Actually Lake Kamo is a lagoon and separated from the

Sea of Japan by a sand barrier. The area of this

lake is about 4.83km2 with a maximum water

depth of about 9.0m. The Lake Kamo boring core (KM-11) is located at the center of Lake

Kamo with 8m water depth offering a good

opportunity to investigate paleoenvironment,

coastal evolution of this area response to rela-

tive sea-level change. Because Lake Kamo is in

the boundary zone between land and sea, then

its sediments should vary significantly in chemi-

cal and physical properties. Lake Kamo could

be an efficient trap collecting marine sediment

transported onshore, as well as alluvium moved

seaward by rivers in the Late Pleistocene and

Holocene. There has been an increasing re-

search interest in recent years concerning the

paleoenvironment change in this area based on

Received June 3, 1997. Accepted January 10, 1998. *1 Environmental Science Section

, Graduate School of Science and Technology, Niigata University. 8050

Ninocho, Ikarashi, Niigata, 950-2181. Present actress: Vietnam National Research Center for Science and

Technology, Sub-Institute of Geography. Ho Chi Minn City, Vietnam. *2 Department of Geology

, Faculty of Science, Niigata University. 8050 Ninocho, Ikarashi, Niigata, 950-2181.

78 Van Lap Nguyen, Masaaki Tateishi and Iwao Kobayashi May 1998

Fig. 1 Location of Lake Kamo

black dot: boring core KM-11, open dots indicating previous boring cores :

diatom studies (Sato and Kumano, 1985, 1986;

Matsuki et al., 1987; Kobayashi et al., 1993;

Nguyen and Kobayashi, 1997). These investi-

gations are important for understanding the paleoenvironments in this studied area and Holocene sea-level changes. Moreover, organic

carbon-sulfur relationship has been increasing-ly used to interpret paleoenvironments of both

modern and ancient sedimentary sequences

(Raiswell and Berner, 1985, 1986; Lin and Morse, 1991). A linear relationship exists

between organic carbon and sulfide sulfur in

normal marine sediments shows the regression line passing through the origin (Berner, 1982,

1984; Berner and Raiswell, 1983, 1984). Sulfur is

available in excess as sulfate in sea water, thus

the limiting factor for pyrite formation under

normal oxic sea water condition is the amount of organic matter which controls the formation

of reducing conditions in the near surface sedi-

ments. Berner (1984) defined normal marine

condition in characteristic of the sediments that are deposited in bottom waters containing dis-

solved oxygen-breathing bottom fauna. The

term euxinic is applied to those environments where sediments are deposited in anoxic, H2

S-containing bottom waters (Leventhal, 1983;

Raiswell and Berner, 1985). In addition, TOC-TS relationship has been used to interpret the

paleoenvironments in the siliciclastic marine (Bruchert et al., 1995; Leventhal, 1995; Jansen et al., 1996) and brackish coastal lacustrine

(Sampei et al., 1996, 1997) sediments where TS occurs mainly as pyrite sulfur. The types of the

organic matter in the sediments have been characterized using TOC/TN ratio (Bordov-

skiy, 1965; Emerson and Hedges, 1988).

The present study aims to characterize lith-ologic and geochemical features of the Late

Pleistocene and Holocene sediments in the

Lake Kamo and to compare their features to

the diatom analysis. Both geochemical and diatom data could be applied to reconstruct

paleoenvironment and Holocene sea-level change in the coastal deposits.

II. Material and method

The 54.19 m thick sediment sequence of bor-ing core KM-11 was routinely sampled at the

5 cm intervals excepting for coarse sand

and gravel layers. 196 samples taken for particle size analysis

were air dried and organic material was di-

gested in H2O2; after that sediments were sieved to divide gravel, sand and mud owing to Wentworth's size classes.

133 samples taken for total organic carbon

(TOC), total sulfur (TS) and total nitrogen

1 and 2. Sato and Kumano (1985, 1986), 3. Matsuki et al. (1987).

1998年5月 Sedimentary Environments for Late Pleistocene to Holocene Deposits of Lake Kamo 79

(TN) analyses were dried in oven at 80℃ for

about 24 hours. The dried samples were crushed

to powder, about 15 mg homogenized sediments

of each sample were put in the silver capsule.

After treatment with 1M-HCl (or 10%) on hot

plate at 110℃ for about 1 hour to remove

carbonate content, the residuals were wrapped

in the tin capsule for combustion. For total

carbon analysis, samples were not treated with

HCl. Analyses were measured by combustion

method at about 1,800℃ in a Carlo Erba Ele-

mentary Analyser EA 1108 CHNS. The errors

(coefficient of variation) to this analysis were

within ±3% for TOC and TN, and ±4% for

TS. The calcium carbonate (CaCO3) content

was calculated as:

CaCO3=(TC-TOC)×8.333

where TC=total carbon, TOC=total organic

carbon (both in wt % of the bulk sample) and 8.333=ratio of CaCO3 mole weight and carbon

mole weight.

4 samples for radiocarbon dating analyses that are woods (-3.98 and-45.80m) and

shells (-25.00 and -31.14m) were examined

by the BETA ANALYTIC INC., USA.

III.Result

1.Lithology

The boring core KM-11 is divided into sixlithologic units from Unit I to Unit VI in ascend-

ing order (Fig. 2) (Nguyen and Kobayashi

1997). Unit I (-54.19 to -37.10m) is subdivid

ed into three subunits (Ia-Ic), each consisting

of gray fine sand to sandy silt bed with top peat.

There is a light brownish gray sandy gravel bed,

4m in thickness, at the base of this unit, 15 to 30

% gravel in 3-30mm diameter. A 14C age of

the obtained wood at-45.80m indicates 10,050

±120 yrs BP. Unit II (-37.10 to -31.55m) is

composed of intercalated beds of coarse-

medium sands and sandy silts, fining upward. A

sandy gravel bed consisting of 35% gravel, 50%

sand and 15% mud is at the base of this unit

(Fig. 3). Unit III (-31.55 to -27.60 m) is com-

posed of two subunits, each consisting of fine

sand to sandy silt bed with humus matters,

fining upward. A 14C age of the obtained shell

at -31.14m indicates 7,710±70 yrs BP. Unit IV

(-27.60 to -21.50m) is composed of 87 to 98%

homogenous dark gray clays commonly bearing

shell fragments at the top. A 14C age of the

obtained shell at -25.00m indicates 7,570±60

yrs BP., and Akahoya tephra bed (K-Ah) was

found at -21.65m indicating 6,300 yrs BP. Unit

V (-21.5 to -8.5m) is composed of coarse-

fine sands and pebbly sands bearing shell frag-

ments.65 to 90% fine sands and sandy silts

occupy at the lower part, 12 to 25% pebbly

sands at -14.5 to -16.8m. Unit VI (-8.5 to 0

m) is composed of 73 to 96% dark gray clays

bearing shell fragments. Finely laminated

clayey silt with humus matters occurred in the

intervals of -5.90 to -7.16m and weakly lam-

inated clayey silt from -2.49 to -3.0m. Bio-

turbations are clear at -0.5 to -1.0m and

-7 .75 to -8.0m. A 14C age of the obtained

wood at -3.98m indicates 2,700±50 yrs

BP.

2. Total organic carbon, sulfur and nitrogen concentrations

The stratigraphical change of TOC, TS and

TN concentrations can be divided into four

groups, Group 1 to Group 4 and the list of

geochemical data is shown in Table 1. Group 1 can be divided into subgroup la and

lb (Fig. 3). The subgroup la (-50.93 to -38.70 m) corresponds to the lithologic subunit Ia, Ib

and the lower part of the subunit Ic. The sub-

group la is characterized by high amplitude variations of TOC between 0.43 and 3.14%, very low TS (0-0.21%), TN (0.04-0.16%)

and relatively high TOC/TN ratios (6.9-2.1 %) . The subgroup lb (-38.70 to -37.34m)

corresponds to the upper part of the lithologic

subunit Ic. It is characterized by slighly high

TOC (1.11-3.98 %) , TS (0.32-0.92 %) and TN (0.13-0.29%).

Group 2 can be divided into three subgroups

2a, 2b and 2c (Fig. 3). The subgroup 2a H37.34

to -30.40m) corresponds to the lithologic Unit II, although there is only one datum between


Fig. 2 Geological column and sampling points of boring core KM-11

After Nguyen and Kobayashi (1997).

37.34 and 32.00m where coarse grained (pebble

to sand) materials are predominant. It is char-acterized by slightly high value of TOC between

0.44 and 1.83 %. TS content shows increasing

trend ranging from 0.56 to 1.46 % between 32.00 and 38.00m. The subgroup 2b (-30.40

to -27.00m) corresponds to the lithologic Unit

III. It is characterized by relatively constant

TOC from 0.50 to 0.95% . TS content shows a

decreasing trend ranging from 1.94 to 0.42%. The subgroup 2c (-27.00 to -22.75m) corre-

sponds to the lithologic Unit IV. It is character-

ized by relatively constant TOC from 0.33 % to

0.85%. TS content shows increasing trend rang-



Table 1 List of geochemical data of the boring core KM-11 corresponding to diatom divisions


ing from 0.46 to 1.72•}. Concentration of TN in

Group 2 is low from 0.05 to 0.18% and shows a

decreasing trend upward.

Group 3 (-22.75 to -8.65m) corresponds to

the lithologic Unit V and is divided into three

subgroups 3a, 3b and 3c (Fig. 3). An obviously

increasing trend of TS is observed in the lowest

part between 22.75 and 21.28m. Then the TS

trend suddenly increases from 0.20% at 21.28 m

to 1.79% at 19.60m, although the analitical

data points are restricted only in the lowermost

part of the section, because of too coarse mate-

rials. Samples at the uppermost parts of sub-

groups 3b and 3c contain high TS (1.79 and 1.84

%) and very low concentrations of TOC and

TN (Fig. 3) .

Group 4 (-8.37 to 0m) corresponds to the

lithologic Unit VI. It is characterized by high

amplitude variations of TOC between 1.90

and 5.50% (Fig. 3). Very high concentrations

of TOC of 5.50%, 5.26% and 5.16% occur at

-6 .85, -6.00 and -2.55m respectively and

correlate to the laminated f acies. High ampli-

tude variations of TS range between 0.86 and

2.04%. High concentrations of TS, reaching

1.93%, 1.96% and 2.04% correlate to the

laminated f acies. TS content shows a decreas-

ing trend ranging from 2.04 to 0.86% in the

interval of -2.55m to -0.40m and a strong

increase to 1.77% from -0.40 to 0m. TN

varies in high amplitude between 0.14% and

0.46%. The high concentrations of TN of 0.45

% and 0.46% also correlate to the laminated

facies.

3. Calcium carbonate concentration

CaCO3 content is very low ranging from 0.05

to 1.85% throughout the core. Especially low

values are found at the subgroups lb and from

upper part of 2a to 2b (Fig. 3). CaCO3 contents

above 2c remain between 0.5 and 1.9% in the

sediments. Low CaCO3 indicates that total

organic carbon is the main part of total carbon

in the sediments. CaCO3 content in sediment

is primarily controlled by the flux of biogenic

carbonate and supply of terrigenous material.

From -26. 50 to -19.50m , CaCO3 increases

abruptly from 0.90 to 1.85%. Group 4 is char-

acterized by high amplitude variations of

CaCO3 between 0.50 and 1.60% (Fig. 3). 4. TOC/TN ratio and grain size texture

In the subgroup la, high amplitude variations

of TOC/TN ratio range between 7.5 to 22.1 at

the lower part, but relatively stable values between 14.5 to 18.8 at the upper part. TOC/

TN ratio ranges between 7.8 to 15.8 in sub-

group lb; 3.3 to 9.0 in group 2; 4.2 to 11.0 in

group 3 and 7.7 to 12.5 in group 4 (Table 1). Grain size texture in the lithologic Unit I

indicates coarsening upward and ranges from

85% sand in the lower part, especially 44

pebble in the base, to 90% mud in the upper

part. Lithologic Unit II also shows coarsening upward with 49% pebble in the base, following

by 80% sand in the middle and 86% mud in the uppermost part. Lithologic Units III, IV and

VI are dominated by 75 to 95% mud, but Unit V

is composed of 55 to 94% sand, especially 16 to 26% pebble in the middle part (Fig. 3).

IV. Discussion

1. Paleoenvironmental change The following discussion is based on the

concentrations of TOC, TS, relationship of

TOC-TS and types of the organic matter covar-

ying throughout the core. The result of diatom investigation (Nguyen and Kobayashi, 1997) is

also integrated in interpreting the changes of

sedimentary environments and relative sea-level.

In subgroup la, almost of the data plotted fall

within the range of non-marine fresh water on the TOC-TS diagram (Berner, 1984) showing

TOC enrichment in relation to TS (Fig. 4-A).

Low TS and relatively high contents of TOC/ TN ratios and TOC at the upper part of sub-

group la (Fig. 3) indicate terrigenous input as rivers discharging directly on the site. Low

TOC contents at the lower part of this subgroup could be due to predominance of sand contents.

In the terrigenous organic matter, nutrients are

low, restricting microbial decomposition and H2S generation, thus inhibiting the pyrite for-


Fig. 4 Plots of weight percent of TOG versus TS of Group 1, 2 and 4

mation. Sediment has, therefore, relatively high TOC and low TS concentrations and reflects an

area of terrigenous organic matter deposition

(Rao et al.,1994) . Diatoms flora also supports

the fluvial environments as shown in Al diatom subdivision (Fig. 5). Fresh water epiphytic,

epipelic and brackish/fresh aerophilous diatom

groups are dominant with representative taxa of Synedra ulna, Pinnularia spp., Gomphonema acuminatum and Epithemia turgida in this sub-

group la section (Figs. 5 - 7). In the subgroup 1b, TS increases abruptly in

comparison with the underlying fluvial sedi-

ments (Fig. 3). Almost all the data plotted on

the TOC-TS diagram fall within the range of normal marine sediments (Fig. 4-A) . TOC/TN

ratio ranges from 8 to 15, suggesting a mixed

type of marine/terrigenous organic matters as a consequence of sea water spilled into the site.

A2 diatom subdivision is representative of


Fig. 5 Diagram showing the relative abundance of the individual diatom species of boring core KM-11 See Fig. 2 for legends. After Nguyen and Kobayashi (1997).


Fig. 6 Correlation of diatom floral changes and paleoenvironments of previous works and boring core KM-11 See Fig. 2 for legends. After Nguyen and Kobayashi (1997).

brackish/fresh epiphytic, fresh water epiphytic and epipelic diatom groups (Fig. 5). Rhopalodia

gibba and Navicula radiosa are dominant. These data indicate that sediments were deposited

in the coastal marsh environment (Figs. 6 and 7).

The overlying subgroup 2a shows a strongly

increasing trend of TS. In order to decipher the

processes that produced from the moderately high concentrations of TOC and TS in the

subgroup 1b to low TOC and high TS in the

subgroup 2a, it is necessary to consider the


Fig. 7 Correlation data of geochemical characteristics, diatom divisions and

inferred sedimentary environments of boring core KM 11

See Fig. 2 for legends.

environmental changes that took place during

the filling of the Lake Kamo by sea water. The

distribution of TOC and TS in Figure 4-B indi-

cates the paleoenvironmental changes from

normal marine to anoxic bottom condition. The

process started with the spilling of saline water into the site; this influx probably sank to the

bottom. Surface water, however, remained


fresh, as indicated by A2 diatom subdivision.

Stratification of the water column increased

during the deposition of the section 2. As a consequence of the increasing inflow of sea

water relative to fresh water, the interface

between the saline and overlying fresh water rose throughout the water column and the sur-

face water gradually became more saline, as

recorded by B diatom division (Fig. 5). Marine/

brackish eipelic, epiphytic and fresh water epi-

phytic diatom groups are dominant. Diploneis smithii, Rhopalodia gibberula and Melosira jur-

gensii occur with high frequency. In spite of the

presence with low frequency of marine plank-tonic and epiphytic diatom groups, it demon-

strates the increasing inflow of sea water into the site. These data indicate that sediments

were deposited under anoxic bottom condition

in the estuarine environments (Figs. 6 and 7).

As a consequence of the estuarine environments in the upper part of this section, the sandy

pebbles in the lower one could be suggested as the base of estuarine valley fill environments

that indicates more sea influences under tidal and wave actions in comparison to the under-

lying coastal marsh sediments to be formed in

the supratidal zone. The subgroup 2b and 2c are characterized by

the decreasing and increasing trends of TS

respectively, and both with low constant TOC

(Fig. 3). All the data plotted on the TOC-TS diagram flanked above-left side of the regres-

sion line for normal marine sediments indicat-

ing TS enrichment in relation to TOC (Fig. 4-

B) . They indicate anoxic benthic conditions, sulfidic bottom waters and formation of sulfide

minerals in the water column (Raiswell and

Berner, 1985). Because of the presence of abun-

dant benthic fauna in some parts of the lith-ologic Unit IV indicates that the bottom waters

did not contain H2S during the deposition of

these sediments, then the semi-euxinic or poor-oxic conditions could be inferred in that

periods. Sediments are characterized by very low TOC to be mostly controlled by less intense

organic input, reflecting the low productivity

conditions in deep-water sediments of open sea condition (Berner, 1982, 1984; Lallier-Verges et

al., 1993) . TOC/TN ratio ranging from 3.5 to 9

suggests that major amounts of marine organic matter are preserved in the sediments (Fig. 4-

D) . It is probable sea-level had been rising

significant and the site being located in an open bay under conditions of low productivity and

slow sedimentation rate. Because environments

of slow sedimentation rates and low concentra-

tions of TOC should allow decomposition of all labile organic carbon (Berner, 1982; Gardner

and Dartnell, 1995; Stein and Rack, 1995), TOC

should reach minimum value that represents the

amount of the residual refractory organic car-bon. These data can also correlate with diatom

investigation. The subgroup 2b corresponds to

C diatom division which is representative of marine/brackish eipelic and epiphytic diatom

groups (Fig. 5). Rhopalodia gibberula and Di-ploneis smithii are dominant. The subgroup 2c corresponds to D1 diatom subdivision in which marine planktonic and marine/brackish eipelic

diatom groups increase significantly (Fig. 5). Thalassionema nitzschioides, Thalassiosira ec-

centrica and Nitzschia obtusa are dominant. TS

enrichments in both intervals of -30.8 to -30.4

m and -22.75 to -22.5m suggest paleoenviron-mental changes towards more influence of sea

water getting into the site. The first interval

corresponds with the change from B to C

diatom divisions or changing from brackish to marine environments (Nguyen and Kobayashi,

1997). The second one corresponds with Dl

diatom subdivision and an increase of CaCO3

content (Fig. 3) indicate a strong influence of sea water into the site. Mud contents in this

section increase from the lower part of 2a to the

upper part of 2c (Fig. 3). These data indicate that sediments were deposited in the shallow

and deep embayments correspond to the sub-

group 2b and 2c respectively. It is inferred that the sea-level rose during the deposition of the

group 2 with two times fluctuations (Fig. 7). Group 3 corresponds to the lithologic Unit V

consist of coarse-fine sands and pebbly sands


with shell fragments overlying the embayment

sediments. It is characterized by very low TOC

and an increasing trend of TS ranging from 0.33 to 1.80% at the lowermost part of this section

(Fig. 3). The low values of TS is coincided with fine sand layers and the moderately high values

is of sandy silt layers. TOC/TN ratio ranges

from 4.0 to 9.5. The data suggest that major amounts of marine organic matter are pre-

served in the sediments. Several diatom species

are found in sandy silts. Dimeregramma minor, Plagiogramma staurophorum and Nitzschia

obtusa occurred with low frequency. The sedi-

ments of this section intercalated the embay-ment sediments which are representative of

underlying and overlying D1 and D2 diatom

subdivisions respectively (Figs. 5 and 6). It is likely that these sediments were deposited dur-

ing a rising trend of sea-level to form transgres-

sive sand barrier (Fig. 7). The appearance of

pebbly sands in the middle part of this section (Fig. 3) could be due to either errosive proceses of tidal and wave actions or sediment supply

during the sea-level rise.

Group 4 corresponds to the lithologic VI and indicates high concentrations of TOC, TS and

TN (Fig. 3) comparing with Groups 1, 2 and 3.

Most of the data plotted on the TOC-TS dia-

gram fall within the range of normal marine sediments, except some data from the lower-

most part of this section fall above this range

(Fig. 4-C). These data can therefore be inter-

preted as paleoenvironmental changes from poor-oxic to normal oxic bottom water and high productive conditions. TOC/TN ratio rang-ing from 7.7 to 12.5 suggests a mixed type of

marine/terrigenous organic matters (Fig. 4-D).

Diatom subdivisions D2, D3, D4 and F are repre-

sentative of marine planktonic, brackish plank-

tonic and marine / brackish epipelic diatom

groups (Fig. 5). Thalassiosira eccentrica and Cyclotella caspia are dominant. The enrichment in marine organic carbon is probably result of

high plankton production brought about by

displacement of nutrients from the deep water

into the euphotic zone (Pedersen and Calvert,

1990; Middelburg et al., 1991) . Increased phyto-

plankton productivity and/or increased preser-vation of marine organic matter may have

caused enrichment of the marine organic car-

bon (Stein and Rack, 1995) . High amounts of diatoms also support a high surface water pro-

ductive environment and relationship between

high TOC and high productivity were suggested

(Pendersen and Calvert,1990; Lallier-Verges et at., 1993; Hinrichs et al., 1995; Stein and Rack,

1995). As a consequence of the high sedimenta-tion rate together with high value of TOC, all

metabolizable organic carbon could not be

decomposed before burial below the depth of

active diagenesis Very high concentrations of TOC, TS and

TN from the two intervals of -6.85 to -6.0m

and - 3.5 to -2.5m can correlate to the finely and/or weakly laminated facies. Enhanced con-

tribution of readily metabolizable organic mat-

ter from marine or bacterial sources may be responsible for enrichment in sulfur of the

laminated sediments, TS is consistently higher

in the laminated intervals compared to adjacent

bioturbated or homogenous intervals (Bruchert et al., 1995). The laminated sediments corre-

spond roughly to the marine/brackish and

brackish/fresh sediments obtainted by D3 and F diatom subdivisions. Moderately high value

of TS is origin, but the downward diffusion of

sulfide, sulfate and their quantitative fixation in

iron sulfide compounds from the overlying marine sediments may be important. TS enrich-

ment can be interpreted as follow: in the time

just after the deposition of laminated clayey silts, the enhanced production of sulfide in the bottom sediments and water column promoted

sulf illation of the underlying reactive Fe by

downward diffusion of sulfide and sulfate from

the overlying marine sediments. At this time the laminated sediments would have been largely

unconsolidated, uncompacted and still

contained more than 80% water. This sulfida-tion ultimately resulted in high TS. The similar

interpretations of depositional history in the

sediments of Kau Bay has been suggested by


Middelburg (1991), Leventhal (1995). Sedimen-

tation took place under embayment or marine/

brackish lagoonal environments. Between - 2.0m and -0.4m, TS concentra-

tion shows a regressive trend from 1.50 to 0.86

G1 diatom subdivision is representative of

fresh and brackish water planktonic diatom

groups (Fig. 5). Melosira granulata and Cos-cinodiscus lacustris are dominant. From -0. 4 to

0 m the obvious increase of TS to 1.77 % corre-

sponds to G2 diatom subdivision in which Cyclotella caspia , Actinocyclus ehrenbergii and

Nitzschia granulata are dominant. Sedimenta-

tion took place under the fresh/brackish lake in the former and marine/brackish lagoon in

the latter.

2. Relative sea-level change

The first Holocene transgression in Japan is called Jomon transgression after the prehistoric

Jomon culture of Japan, and inferred the rising rate of sea-level had been rapid during 8,000 to

6,000 yrs BP. Most the relative sea-level curves show that the maximum Holocene transgres-

sion occurred during the period of 6, 500 to 5, 000

yrs BP. (Matsumoto, 1981, 1985; Matsushima, 1984; Fujimoto, 1990; Ota et al., 1990; Umitsu,

1991) and reached about +4m above present

sea-level in the Kanto Plain and Sendai (Ma-

tsumoto, 1981, 1985; Saito, 1991, 1995). In the Lake Kamo, during the last glacial

period shoreline was further to the northeast in comparing to present Ryotsu coastline. The fluvial sediments of the lowermost part of the

KM-11 core were formed in the Late Pleis-

tocene due to a 14C age of 10, 050±120 yrs BP.

at -45.80m and could be compared with the fluvial sediments in the lowermost part of a

Fukushima-gata core, Echigo Plain of the

Niigata city, dated 30, 000 yrs BP. (Nguyen and Kobayashi, 1996a, b ). The sediment

sequence which consist of the coastal marsh,

estuarine valley fill and embayment or bay fill in ascending order overlying the fluvial sedi-

ments could be inferred the Holocene trans-

gression deposits. Because of two 14C data in the uppermost part of the estuarine valley and

embayment fill sediments dated 7,710±70 and

7,570±60 yrs BP. respectively, the coastal

marsh sediments could be, therefore, interpret-

ed as the beginning of the Holocene transgres-

sion, and a boundary of the Late Pleistocene

and Holocene deposits could be at - 38.50 m. Moreover, owing to the swamp deposits,

Matsumoto (1981) suggested the beginning of

the Holocene transgression and sea-level at

about -38m in the coastal plain along Sendai bay in 9,280 yrs BP. Around 9, 000 to 10, 000

yrs BP., sea-level at about -40 to -45m in Sendai and Kanto Plain was also suggested by Matsumoto (1981, 1985) and Saito (1991,1995) .

In the Lake Kamo, on the basic of the pa-

leoenvironmental changes and corresponding ages are shown in Figure 6, the first Holocene

transgression reached the maximum level

between 6,500 and 5,000 yrs BP, during which

time the 13m thick sand-gravel barrier was formed. Two minor cycles of relative sea-level

fall and rise could be inferred in this lake (Fig.

7). There are the second and third Holocene transgression in 3,000 and 2,000 yrs BP. respec-

tively. Embayment sediments occupied the

site during these transgressions. The Holocene regressions occurred intercalary the transgres-

sions in 4,500 yrs BP. and the Yayoi regression

(3,000 to 2,000 yrs BP.). Marine/brackish and brackish/fresh lagoonal sediments were

formed respectively. After 1,800 yrs BP.,

fresh/brackish lacustrine sediments were pro-bably formed due to the formation of the sand

barrier on which the Ryotsu city is located now.

Since 1902, Lake Kamo has been occupied by marine-brackish lagoonal sediments because an

artificial channel was constructed in connect-

ing Ryotsu Bay.

V. Conclusions

This study indicates that geochemical char-

acteristics obtained from 54m sediment se-

quence in a boring core KM-11 retrieved from Lake Kamo, Sado Island in central Japan corre-

spond to the diatom floral changes in the sedi-ments, and can be applied to reconstruct pa-


leoenvironment and relative sea-level change in

the coastal deposits. Boundary of Late Pleistocene and Holocene

sediments can be estimated at -38.50m depth.

In the Late Pleistocene, fluvial sediments are

characterized by high TOC, very low TS, rela-tively low CaCO3 and high TOC/TN implying

high input of terrigenous organic matter. The

beginning of Holocene transgression is inferred by the coastal marsh and estuarine sediments

that are characterized by the abruptly increas-ing trend of TS and high TOC. The overlying

sediments are characterized by low TOC, low

TOC/TN implying marine organic matter, low

productivity condition and slow sedimentation rate indicating the rapidly relative sea-level rise

and probably reached the maximum in between

6, 500 and 5,000 yrs BP. After that, fluctuations of relative sea-level are characterized by high

TOC and TS, high productivity condition and

a mixed type of marine/terrigenous organic matter. High concentrations of TOC and TS

can correlate to the weakly and/or finely

laminated sandy silts of the lithologic Unit VI, whereas the homogenous sediments in the lith-

ologic Unit III and Unit IV show low TOC

concentration. The enrichment of TS of the laminated sandy silts is probably due to the

enhanced contribution of readily metabolizable

organic matter from marine or bacterial sources, and the downward diffusion of sulfide

and sulfate from the overlying sea water and

marine sediments to the sulfate-moderate/poor

bearing sediments which have been largely in unconsolidated and uncompacted conditions.

Acknowledgements

The authors wish to express gratitude to

Niigata Prefecture Office and Kowa Co., Ltd.

for offering materials. We would like to express

sincere thanks Mr. Y. Kamoi, Kowa Co. Ltd.,

members of our research group for their sugges-

tions, Prof. K. Takayasu, Dr. Y. Sampei in

Shimane University for assistance in using

CHNS analyzer and discussion. We express our

thanks to Prof. Hisao Kumai, Faculty of Sci-

ence, Osaka City University, for providing

many suggestions for improvement of the man-

uscript. This study was supported in part by

Grant-in-Aid for Scientific Research from the

Ministry of Education, Science and Culture,

project no. (C) 07640598.

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*in Japanese with English abstract .


佐渡島加茂湖の後期更新世～完新世堆積物の堆積環境の復元

グエンラップバン*1・立石雅昭*2・小林巌雄*2

(要旨)

佐渡島加茂湖の湖心部から採取された約54mのボー

リングコア(KM-11)について,粒度,全有機炭素

(TOC),全イオウ(TS),全窒素(TN),炭酸カルシウム

(CaCO3)含有量を分析し,ヶイソウ化石群集組成とあわ

せて,後期更新世から完新世にかけての堆積環境の変遷

を検討した.その結果,ヶイソウ化石群集組成によるユ

ニット区分と対応した化学組成の系統的変化が確認され

た.後期更新世の河川性堆積物は高TOC,低TS,低

CaCO3であり,有機物は主として陸源である.コアの深

さ38.5mに更新世と完新世の境界が推定される.海進の

初期には高TOC,高TSに変化するとともに,海起源の

有機物が増加する.海進の後期には低TOC,高TSへと

変化し,海起源の有機物の割合が高い.およそ6,500

～5,000年前の最大海進時の砂州形成後,海水準の変動

に対応して,TOC,TS,TN,CaCO3は周期的に変動

する.高TOCと高TSは葉理構造の発達した砂質シル

ト堆積時に見ら礼低TOCは均質(塊状)な堆積時に見

られる.

*1新潟大学自然科学研究科〒950 -2181新潟市五十嵐二の町8050 .

*2新潟大学理学部地質科学教室〒950-2181新潟市五十嵐二の町8050 .

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Reconstruction of Sedimentary Environments for Late