御所浦白亜紀資料館報，第24号，9-20ページ，2023年 3 月．
Bull. Goshoura Cret. Mus., No.24, 9-20, March, 2023.

Seasonal deposition of non-glacial varved clay in the Asono Formationof the middle Pleistocene in central Kyushu, Japan
Yoshitaka Hase 1 and Yuko Ando 2( 1 Goshoura Cretaceous Museum, 4310-5 Goshoura, Goshoura Town, Amakusa City, Kumamoto 866-0313,
Japan )
( 2 Yuko Nishinaka (Ando), MSc. )Abstract
Abstract
The Asono Formation is composed of a lacustrine sediment which includes non-glacial varved clay. The non-glacial
The Asono Formation is composed of a lacustrine sediment which includes non-glacial varved clay. The non-glacial
varved clay is composed of light-colored and dark-colored laminae in alternating layers. The lamina includes fossil
varved clay is composed of light-colored and dark-colored laminae in alternating layers. The lamina includes fossil
diatom and pollen. Based on the idea that strata formed in lakes record the changes in the surrounding terrestrial
diatom and pollen. Based on the idea that strata formed in lakes record the changes in the surrounding terrestrial
environment, we have so far grasped those changes mainly based on pollen analysis. We understood the changes in theenvironment, we have so far grasped those changes mainly based on pollen analysis. We understood the changes in thelake environment from the analysis results of diatom fossils and pollen fossils for each non-glacial varve clay lamina,
lake environment from the analysis results of diatom fossils and pollen fossils for each non-glacial varve clay lamina,
and demonstrated that there was a part of the non-glacial varved sediments indicating seasonal changes of the lakeand demonstrated that there was a part of the non-glacial varved sediments indicating seasonal changes of the lake
environment.
environment.
Keywords: non-glacial varved clay, seasonal deposition, middle Pleistocene, Asono Formation
Keywords: non-glacial varved clay, seasonal deposition, middle Pleistocene, Asono Formation
Introduction(1975) on the lacustrine banded diatomaceous sedi-

Lake sediments including fossils of pollen andments of the Middle Pleistocene Hiruzenbara Forma-diatom are sensitive indicators of past environmen-

tion, Okayama Prefecture. Therefore, it was thought

tal changes such as lake conditions, surroundingthat a dark-colored and light-colored pair represent-

land vegetation and climate. Diatom assemblages

ed a year. It is possible that analysis of non-glacial

contained in fine sediments in lake beds show pastvarved clay reveals details of environmental changes

aquatic conditions, while pollen from sedimentsof surrounding land. It is also mentioned that oscilla-

indicate surrounding vegetation and climate in thetion of light- and dark-colored laminae demonstrates

area. These lake sediments are often composed ofa cyclic orbital activity of intervals of 11 or 22 years

alternating light- and dark-colored laminae; thin clay

(Sonett and Williams, 1985).

layers of 1-2 mm in thickness. These alternating lam-

Diatomite, mainly composed of frustules of fossil

inae are so-called non-glacial varved clay. Non-gla-

diatoms, is part of the composition of non-glacial

cial varved clay is generally known to be composed

varved clay. Diatoms live under aquatic conditionsof very fine grain, thin lamina formed by sedimentsthat have pH, temperature, nutrition availability andwhich were deposited as light-colored clay in thesalinity. Therefore, diatoms in sediment indicatessummer and dark-colored clay in the winter, showing

its life condition at the time of its deposition. Pol-an annual sediment by a set of two laminae. Ishihara

len included in sediment indicates its parent plantand Miyata (1999) reported that the breeding season

and shows how luxuriant it was under existing cli-of diatom (Stephanodiscus sp.), which constituted

matic conditions of temperature, precipitation andthe light-colored lamina, was winter, based on the

moisture. As a result, analysis of diatom and pollen

research of the Research Group for the Hiruzenbara

within laminae leads to the understanding of envi-

Yoahi ta k a H a s e a n d Yu k o A n d o

Seasonal deposition of non-glacial varved clay in the Asono Formation of the middle Pleistocene in central Kyushu, Japan(1985), Iwauchi and Hase (1986) and Ando (2004)

We studied laminated samples from the Asono For-

Kumamureyama Rhyolite (Fig.2). According to radio-

The Asono Formation in the middle Pleistocene

described the Asono Formation with regard to its

mation and clarified that the lamina deposited under

metric aging by the fission-track method, the Kumamu-

is formed by siltstone with intercalated sandstone

stratigraphy and environmental changes during theseasonal changes based on the diatom and pollen in-reyama Rhyolite is dated at 0.73±0.14 Ma and the Uchi-

layers at the Asono Basin of Syonai-Town, Oita Pre-

period of deposition based on pollen and diatom

clusion of each lamina.

yama Andesite is dated a 0.34±0.17 Ma (Hase andfecture in central Kyushu (Fig. 1). Hase and Iwauchi

analysis.

ronmental changes during deposition of sediments.

Iwauchi, 1985). Therefore, the Asono Formation was
Geologic setting of the Asono area

formed in the middle Pleistocene. The upper part of the

The Asono area is mainly composed of Kakura An-

Asono Formation includes very laminated clay, known

desite, Imaichi Pyroclastic Flow Deposit, Tokiyama

as non-glacial varved clay, at a thickness of about 5 m.

Andesite, Kumamureyama Rhyolite, Asono Forma-

The Hanamureyama Volcanic Rocks lay on the Asono

tion, Hanamure Volcanic Rocks, detritus and fan

Formation unconformably. The Hanamureyama Vol-deposits on mountain slopes and Serikawa Pyroclas-

canic Rocks are composed of a hypersthene hornblende

tic Flow Deposit, in an ascending order (Hase andIwauchi, 1985) (Table 1).
The Kakura Andesite and Tokiyama Andesite consist of lava and tuffbreccia of an augite hypersthene
andesite, and the Imaichi Pyroclastic Flow Deposit
is intercalated with these andesites and mainly composed of welded tuff of augite hypersthene andesite.
The Kumamureyama Rhyolite is composed of lava
and tuff breccia of biotite rhyolite.
The Asono Formation as identified by Ono (1963)
is mainly composed of diatomaceous siltstone with
tuffaceous sandstone and is intercalated with the
Uchiyama Andesite lava at the upper part. The formation is 50-80 m in thickness and abutted against theTable 1. Stratigraphy of the Asono area (After Haseand Iwauchi, 1985).

Fig. 1. Locality and geologic map of the Asono area (Hase and Iwauchi, 1985).
1: alluvium, 2: terrace deposit, 3: Serikawa Pyroclastic Flow Deposit, 4: volcanic fan deposit, piedmont
deposit and ash bed, 5: Hanamureyama Volcanic Rocks, 6: Uchiyama Hornblende Andesite, 7: Asono
Formation, 8: Kumamureyama Rhyolite, 9: Tokiyama Andesite, 10: Imaichi Pyroclastic Flow Deposit,
11: Kakura Andesite, 12: strike and dip, 13: fault, 14: localities of macrofossils, U: Uchiyama, K: Kozuno, M: Makinoharu, I:Iwashita, A: Ageju, H: Harunaka, N: Nagano.

Pleistocene

Holocene

Geologic
Age?
Pliocene

10

Stratigraphic Sequence
Alluvium
Loam layer

Terrace deposits

Fig. 2. Columnar sections of the Asono Formation

Serikawa Pyroclastic Flow Deposit(Hase and Iwauchi, 1985).

volcanic fan deposits/piedmont deposits

A: Serikawa Pyroclastic Flow Deposit, B: volcanic fan

Hanamure Volcanic Rocks

deposit, piedmont and ash bed, C: Hanamure Vol-

Uchiyama Holnblende Andesite
Asono Formation

canic Rocks, D: Uchiyama Hornblende Andesite, E-L:

Kumamureyama Ryolite

silt, F; sand bed, G; sandy gravel bed, H; diatomite,

Tokiyama Andesite

I; pumice bed, J; ash bed, K; “Shiromaru band”, L;

Imaichi Pyroclastic Flow Deposit
Kakura Andesite

地質時代

層序

Asono Formation (E; alternation of tuffaceous sand andslump bed), M: Kakura Andesite, N: Macrofossils ofplants, O: sampling horizon for the pollen analysys.
← : Showing the sampling point of non-varved clay.

11

Yoahi t a k a H a s e a n d Yu k o A n d oronmental changes during deposition of sediments.

Seasonal deposition of non-glacial varved clay in the Asono Formation of the middle Pleistocene in central Kyushu, Japan(1985), Iwauchi and Hase (1986) and Ando (2004)

We studied laminated samples from the Asono For-

Kumamureyama Rhyolite (Fig.2). According to radio-

The Asono Formation in the middle Pleistocene

described the Asono Formation with regard to its

mation and clarified that the lamina deposited under

metric aging by the fission-track method, the Kumamu-

is formed by siltstone with intercalated sandstone

stratigraphy and environmental changes during theseasonal changes based on the diatom and pollen in-reyama Rhyolite is dated at 0.73±0.14 Ma and the Uchi-

layers at the Asono Basin of Syonai-Town, Oita Pre-

period of deposition based on pollen and diatom

clusion of each lamina.

yama Andesite is dated a 0.34±0.17 Ma (Hase andfecture in central Kyushu (Fig. 1). Hase and Iwauchi

analysis.

Iwauchi, 1985). Therefore, the Asono Formation was
Geologic setting of the Asono area

formed in the middle Pleistocene. The upper part of the

The Asono area is mainly composed of Kakura An-

Asono Formation includes very laminated clay, known

desite, Imaichi Pyroclastic Flow Deposit, Tokiyama

as non-glacial varved clay, at a thickness of about 5 m.

Andesite, Kumamureyama Rhyolite, Asono Forma-

The Hanamureyama Volcanic Rocks lay on the Asono

tion, Hanamure Volcanic Rocks, detritus and fan

Formation unconformably. The Hanamureyama Vol-deposits on mountain slopes and Serikawa Pyroclas-

canic Rocks are composed of a hypersthene hornblende

tic Flow Deposit, in an ascending order (Hase andIwauchi, 1985) (Table 1).
The Kakura Andesite and Tokiyama Andesite consist of lava and tuffbreccia of an augite hypersthene
andesite, and the Imaichi Pyroclastic Flow Deposit
is intercalated with these andesites and mainly composed of welded tuff of augite hypersthene andesite.
The Kumamureyama Rhyolite is composed of lava
and tuff breccia of biotite rhyolite.
The Asono Formation as identified by Ono (1963)
is mainly composed of diatomaceous siltstone with
tuffaceous sandstone and is intercalated with the
Uchiyama Andesite lava at the upper part. The formation is 50-80 m in thickness and abutted against theTable 1. Stratigraphy of the Asono area (After Haseand Iwauchi, 1985).

Fig. 1. Locality and geologic map of the Asono area (Hase and Iwauchi, 1985).
1: alluvium, 2: terrace deposit, 3: Serikawa Pyroclastic Flow Deposit, 4: volcanic fan deposit, piedmont
deposit and ash bed, 5: Hanamureyama Volcanic Rocks, 6: Uchiyama Hornblende Andesite, 7: Asono
Formation, 8: Kumamureyama Rhyolite, 9: Tokiyama Andesite, 10: Imaichi Pyroclastic Flow Deposit,
11: Kakura Andesite, 12: strike and dip, 13: fault, 14: localities of macrofossils, U: Uchiyama, K: Kozuno, M: Makinoharu, I:Iwashita, A: Ageju, H: Harunaka, N: Nagano.

Pleistocene

Holocene

Geologic
Age?
Pliocene

10

Stratigraphic Sequence
Alluvium
Loam layer

Terrace deposits

Fig. 2. Columnar sections of the Asono Formation

Serikawa Pyroclastic Flow Deposit(Hase and Iwauchi, 1985).

volcanic fan deposits/piedmont deposits

A: Serikawa Pyroclastic Flow Deposit, B: volcanic fan

Hanamure Volcanic Rocks

deposit, piedmont and ash bed, C: Hanamure Vol-

Uchiyama Holnblende Andesite
Asono Formation

canic Rocks, D: Uchiyama Hornblende Andesite, E-L:

Kumamureyama Ryolite

silt, F; sand bed, G; sandy gravel bed, H; diatomite,

Tokiyama Andesite

I; pumice bed, J; ash bed, K; “Shiromaru band”, L;

Imaichi Pyroclastic Flow Deposit
Kakura Andesite

地質時代

層序

Asono Formation (E; alternation of tuffaceous sand andslump bed), M: Kakura Andesite, N: Macrofossils ofplants, O: sampling horizon for the pollen analysys.
← : Showing the sampling point of non-varved clay.

11

12

Yoahi ta k a H a s e a n d Yu k o A n d o

Seasonal deposition of non-glacial varved clay in the Asono Formation of the middle Pleistocene in central Kyushu, Japan

Results2) Diatom analysis

1) Measurement of lamina thickness(a) Light-colored lamina (Fig. 5)

Figure 4 shows the change of thickness of light- anddark-colored lamina in a 16 cm vertical thickness.

Fig. 3. Photograph of the non-glacial varved clay
layers of the Asono Formation.

Light-colored lamina was composed of more than
95% diatom frustule. There are Cyclotella comta that

Each colored lamina of light and dark is given a sequential number from 1 to 50 and 1’ to 50’ from thewere planktonic and oligotrophic species, and Steph-bottom to the top, respectively.

phic species. These two species combined were seen(a) Light-colored lamina: about 1 mm in thickness,some looseness, white to yellowish white in color,in more than 95% of frustules in each light-coloredanodiscus astraea that were planktonic and mesotro-

lamina, and basically these species mainly appearedboundary of the lower dark-colored lamina gradually changes, but the upper dark-colored lamina

in alternating slides.

change is sharper. Laminae numbered 1-6 have a
1mm thickness on average, 7-21 have from 1 to 1.5

are alternatively composed of Cyclotella comta and

By detailed observation, laminae of number 1 to 6
Synedra ulna + Stephanodiscus astraea at a high rate
(more than 90%). Cyclotella comta changes at 40-

sthene andesite in the southern slope of Mt. Hana-mm, 22-26 have about 2 mm thickness consistently, and 27-35 have changes from 0.9 to 1.7 mm inmureyama. These andesitic activities made tuffbrec-

thickness gradually.

in the laminae of number 7 to 24. Above number

andesite in the northern slope and an augite hyper-(b) Dark-colored lamina: 0.5-1.5 mm in thickness isdenser than light-colored lamina, gray to black in

cia at the lower part and lava flows at the upper part.
Detritus and fan deposits on mountain slopes are
made of less sorting conglomerate with pumice andFig. 4. Thickness of light- and dark-colored laminae.

tuff derived from surrounding mountains. The Seri-

Number 1～50 and 1’～50’ are shown the light-col-kawa Pyroclastic Flow Deposit was correlated withored and the dark-colored laminae from bottom to

the Aso-4 Pyroclasitic Flow Deposit by Ono (1963).

top, respectively. Number 9’, 12’, 26’, 39’, 42’ and 47’

color, some dark-colored lamina have.

50% and Stephanodiscus astraea changes at 30-40%
25 lamina, the samples have a high rate (more than90%) of Cyclotella comta and Stephanodiscus astraea. Synedra ulna is dominant in rate as aquaticin the dark-colored laminae are missing.
Material and method
A rectangular column of 10 cm wide and 50 cm long

1) Measurement of lamina thickness

with 5 cm in thickness was taken vertically at a bed-

The thickest part of each lamina is directly meas-

ding plane of the Asono Formation (Fig. 3) from anured by using a ruler. Each lamina colored light or

outcrop of a cliff in a branched stream of the Nabetani

dark has a common thickness from 0.5 to 1.5 mm (Fig. 4).

River in the eastern part of the Asono Basin. After a2) Diatom analysis

slow drying period, the column was shaved off along

0.05 g of material was prepared for diatom analy-

its vertical plane for cleaning, then it was photo-

sis by cutting down a plane of lamina and preparing

graphed and sketched to detail its sedimentary fea-a smear slide with a drop of liquid and the sample

tures.

and enclosing it in Mountmedia (Daiichi Yakuka

The samples were taken from a thin lamina of theCo.). Diatoms in the slides were observed under acolumn using a knife. The thin laminae were col-microscope at 600x. Diatom frustules in each slide

ored light-colored as white to yellowish white, andnumbered more than 500 counts under microscopic

dark-colored as black to dark gray, and they basicallyobservation.

alternated. The column had 50 sets of light-colored3) Pollen analysis

and dark-colored laminae with five laminae missing

All samples of light-colored lamina weighing 20-(Fig. 4). All of 95 laminae were treated to measure

40 mg were analyzed by the HF and washing method

their thicknesses and perform diatom analysis. Fur-because diatom frustules were mainly composed ofthermore, the light-colored laminae were used for

SiO 2. This was followed by viewing on preparedpollen analysis.

slides with glycerin jelly for observation under a microscope with a magnifying power of 600 x.

Fig. 5. Diatom diagram of the light-colored laminae.

13

Yoahi t a k a H a s e a n d Yu k o A n d o

12Fig. 3. Photograph of the non-glacial varved clay
layers of the Asono Formation.
andesite in the northern slope and an augite hypersthene andesite in the southern slope of Mt. Hanamureyama. These andesitic activities made tuffbreccia at the lower part and lava flows at the upper part.
Detritus and fan deposits on mountain slopes are
made of less sorting conglomerate with pumice andFig. 4. Thickness of light- and dark-colored laminae.

tuff derived from surrounding mountains. The Seri-

Number 1～50 and 1’～50’ are shown the light-col-kawa Pyroclastic Flow Deposit was correlated withored and the dark-colored laminae from bottom to

the Aso-4 Pyroclasitic Flow Deposit by Ono (1963).

top, respectively. Number 9’, 12’, 26’, 39’, 42’ and 47’

Seasonal deposition of non-glacial varved clay in the Asono Formation of the middle Pleistocene in central Kyushu, Japan

Results2) Diatom analysis

1) Measurement of lamina thickness(a) Light-colored lamina (Fig. 5)

Figure 4 shows the change of thickness of light- andLight-colored lamina was composed of more thandark-colored lamina in a 16 cm vertical thickness.

95% diatom frustule. There are Cyclotella comta that

Each colored lamina of light and dark is given a se-

were planktonic and oligotrophic species, and Steph-quential number from 1 to 50 and 1’ to 50’ from theanodiscus astraea that were planktonic and mesotro-

bottom to the top, respectively.

phic species. These two species combined were seen(a) Light-colored lamina: about 1 mm in thickness,in more than 95% of frustules in each light-coloredsome looseness, white to yellowish white in color,

lamina, and basically these species mainly appearedboundary of the lower dark-colored lamina grad-in alternating slides.

ually changes, but the upper dark-colored lamina

By detailed observation, laminae of number 1 to 6change is sharper. Laminae numbered 1-6 have a

are alternatively composed of Cyclotella comta and

1mm thickness on average, 7-21 have from 1 to 1.5

Synedra ulna + Stephanodiscus astraea at a high ratemm, 22-26 have about 2 mm thickness consistent-(more than 90%). Cyclotella comta changes at 40-

ly, and 27-35 have changes from 0.9 to 1.7 mm in

50% and Stephanodiscus astraea changes at 30-40%

thickness gradually.

in the laminae of number 7 to 24. Above number(b) Dark-colored lamina: 0.5-1.5 mm in thickness is

25 lamina, the samples have a high rate (more thandenser than light-colored lamina, gray to black in90%) of Cyclotella comta and Stephanodiscus as-

color, some dark-colored lamina have.

traea. Synedra ulna is dominant in rate as aquaticin the dark-colored laminae are missing.
Material and method
A rectangular column of 10 cm wide and 50 cm long

1) Measurement of lamina thickness

with 5 cm in thickness was taken vertically at a bed-

The thickest part of each lamina is directly meas-

ding plane of the Asono Formation (Fig. 3) from anured by using a ruler. Each lamina colored light or

outcrop of a cliff in a branched stream of the Nabetani

dark has a common thickness from 0.5 to 1.5 mm (Fig. 4).

River in the eastern part of the Asono Basin. After a2) Diatom analysis

slow drying period, the column was shaved off along

0.05 g of material was prepared for diatom analy-

its vertical plane for cleaning, then it was photo-

sis by cutting down a plane of lamina and preparing

graphed and sketched to detail its sedimentary fea-a smear slide with a drop of liquid and the sample

tures.

and enclosing it in Mountmedia (Daiichi Yakuka

The samples were taken from a thin lamina of theCo.). Diatoms in the slides were observed under acolumn using a knife. The thin laminae were col-microscope at 600x. Diatom frustules in each slide

ored light-colored as white to yellowish white, andnumbered more than 500 counts under microscopic

dark-colored as black to dark gray, and they basicallyobservation.

alternated. The column had 50 sets of light-colored3) Pollen analysis

and dark-colored laminae with five laminae missing

All samples of light-colored lamina weighing 20-(Fig. 4). All of 95 laminae were treated to measure

40 mg were analyzed by the HF and washing method

their thicknesses and perform diatom analysis. Fur-because diatom frustules were mainly composed ofthermore, the light-colored laminae were used for

SiO 2. This was followed by viewing on preparedpollen analysis.

slides with glycerin jelly for observation under a microscope with a magnifying power of 600 x.

Fig. 5. Diatom diagram of the light-colored laminae.

13

14

Seasonal deposition of non-glacial varved clay in the Asono Formation of the middle Pleistocene in central Kyushu, Japan

100%

to m
Dia
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lac
os
va eirar. a grng anuus latiss taimaea

Yoahi ta k a H a s e a n d Yu k o A n d o

Cyclotella comta
Au. granulata* 9’,12’,26’,39’,42’,47’:missing

Cymbella sp.
othersFig. 6. Diatom diagram of the dark-colored laminae.

Fig. 7. Pollen diagram of the light-colored laminae
occupied a large percentage alternating in the light-i) Occurrence rates of Cyclotella comta and Steph-flow. Each light-colored lamina was treated for pollen
analysis (Fig. 7). Pollen assemblages were mainly(b) Dark-colored lamina (Fig. 6)

anodiscus astraea + Synedra ulna + others alternate-composed of tree species with herb species. Broad-leafby blooming in spring, and the other by blooming in

The dark-colored lamina was composed of diatom

ly change in light-colored laminae. Occurrence rates oftree species consisted of Fagus, Ulmus/Zelkova, andautumn. Some research suggests that Cyclotella comta

frustules and very fine organic materials. There are

Cyclotella comta and other species change gradually inusually occurred in spring and autumn, but Stephan-Aulacoseira granulata var. angustissima that were

dark-colored laminae.

Al nus, which are accompanied by Quercus, Betula,
Castanea/Castanopsis and Elaeagnus, Conifers suchditions. In this study, it is assumed that the dominantconditions occurred prominently in the laminae of
number 4, 6 and 43.

1’-6’(i), 7’-24’(ii) and 25’-50’(iii) (Fig. 6).

colored lamina. It has been shown that one occurredodiscus astraea occurred under cool temperate con-planktonic and eutrophic species, Cymbella cistulaii) Cyclotella comta changed from 50-70% in light-that is epiphytic and eutrophic species, and Cyclotel-

colored laminae. Aulacoseira granurata var. angustis-as, Pinus were also included, accompanied by Larix/
Pseudotsuga and Tsuga.

la comta.

sima, Cyclotella comta and others changed 20-30% andFagus occurred with 30-50% stably, sometimes 50-

laminae of Stephanodiscus astrea were constructed

A characteristic of occurrence of diatom in the dark-

70-90% alternately in dark-colored laminae.

80%. Ulmus/Zelkova also occurred around 20% andin the autumn.

laminae of Cyclotella comta in the spring, and thecolored lamina is that Aulacoseira granulata var. an-iii) Cyclotella comta at 20-40% and 80-90%, and St-

sometimes 50%, excluding numbers 1-8 light-coloredDark-colored laminae were composed of diatom frus-

gustissima and Cymbella cistula were dominant alter-ephanodiscus astraea + others at 10-20% and 50-80%,

lamina which were less than 10%. Occurrence rate oftules of Cyclotella comta and other eutrophic species

nately in number 1’-6’ and 27’-50’. Aulacoseira gra-alternately changed in light-colored laminae, and Cy-

Alnus varied from less than 5% to 50%.

such as Aulacoseira granulata var. angustissima, Cym-

nulate var. angustissima occurred varyingly at 20-90%,

clotella comta at 60-70% and Aulacoseira granulata var.

but Cymbella cistula did not occur in number 7’-25’

angustissma + Cymbella cistula + others at 30-50%,

Consideration to sedimentary conditionareas. It is assumed that the dark-colored laminae were

samples. Synedra ulna did not occur at the same ratechanged alternately.

1) Light- and Dark-colored laminae
The composition of diatom frustule consisted ofmade by diatom reacting to eutrophic conditions madeAulacoseira granulata var. angustissima and Cymbella cistula are both eutrophic diatoms. In this study,

as in light-colored lamina except for number 6’, 35’andbella cistula, and organic matter from surrounding landby material from the land.

41’ laminae.

3) Pollen analysis(c) Occurrence change of diatom assemblage in the

Almost all light-colored laminae are composed of

one or two species, while almost all in the light-colored lamina were assumed to have formed by diatom

sequence of laminae

diatom frustules. Therefore, pollen in the light-col-blooming in the mixture zone of a lake in spring andthey occurred dominant and alternating in the dark-

The sequence is divided into three parts: numbers

ored laminae settled originally, and not from lake

autumn. Cyclotera comta and Stephanodiscus astraea

colored lamina. What caused this? It is assumed that

15

to m
Dia
os
n th

ira
lia
sp
.

Be

Au

Seasonal deposition of non-glacial varved clay in the Asono Formation of the middle Pleistocene in central Kyushu, Japan

Fra
g

a
uln
ra
ed
Sy
n

no
ha
Ste
p

Cy
mb
e

as
tra

lac
os
va eirar. a grng anuus latiss ta
im
a

ea
100%

dis
cu
s

om
ta
tel
la
c
clo
Cy

50’
49’
48’
47’
46’
45’
44’
43’
42’
41’
40’
39’
38’
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36’
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9’
8’
7’
6’
5’
4’
3’
2’
1’

lla
cis
tu l
a

Yoahi t a k a H a s e a n d Yu k o A n d o

14

Cyclotella comta
Au. granulata* 9’,12’,26’,39’,42’,47’:missing

Cymbella sp.
othersFig. 6. Diatom diagram of the dark-colored laminae.
conditions occurred prominently in the laminae of

1’-6’(i), 7’-24’(ii) and 25’-50’(iii) (Fig. 6).

Fig. 7. Pollen diagram of the light-colored laminae
flow. Each light-colored lamina was treated for pollen

occupied a large percentage alternating in the light-i) Occurrence rates of Cyclotella comta and Steph-analysis (Fig. 7). Pollen assemblages were mainlycolored lamina. It has been shown that one occurred(b) Dark-colored lamina (Fig. 6)

anodiscus astraea + Synedra ulna + others alternate-composed of tree species with herb species. Broad-leafby blooming in spring, and the other by blooming in

The dark-colored lamina was composed of diatom

ly change in light-colored laminae. Occurrence rates oftree species consisted of Fagus, Ulmus/Zelkova, andautumn. Some research suggests that Cyclotella comta

frustules and very fine organic materials. There are

Cyclotella comta and other species change gradually in

Alnus, which are accompanied by Quercus, Betula,usually occurred in spring and autumn, but Stephan-Aulacoseira granulata var. angustissima that were

dark-colored laminae.

Castanea/Castanopsis and Elaeagnus, Conifers suchodiscus astraea occurred under cool temperate con-number 4, 6 and 43.

planktonic and eutrophic species, Cymbella cistulaii) Cyclotella comta changed from 50-70% in light-

as, Pinus were also included, accompanied by Larix/

ditions. In this study, it is assumed that the dominantthat is epiphytic and eutrophic species, and Cyclotel-

colored laminae. Aulacoseira granurata var. angustis-

Pseudotsuga and Tsuga.

laminae of Cyclotella comta in the spring, and thela comta.

sima, Cyclotella comta and others changed 20-30% andFagus occurred with 30-50% stably, sometimes 50-

laminae of Stephanodiscus astrea were constructed

A characteristic of occurrence of diatom in the dark-

70-90% alternately in dark-colored laminae.

80%. Ulmus/Zelkova also occurred around 20% andin the autumn.

colored lamina is that Aulacoseira granulata var. an-iii) Cyclotella comta at 20-40% and 80-90%, and St-

sometimes 50%, excluding numbers 1-8 light-coloredDark-colored laminae were composed of diatom frus-

gustissima and Cymbella cistula were dominant alter-ephanodiscus astraea + others at 10-20% and 50-80%,

lamina which were less than 10%. Occurrence rate oftules of Cyclotella comta and other eutrophic species

nately in number 1’-6’ and 27’-50’. Aulacoseira gra-alternately changed in light-colored laminae, and Cy-

Alnus varied from less than 5% to 50%.

such as Aulacoseira granulata var. angustissima, Cym-

nulate var. angustissima occurred varyingly at 20-90%,

clotella comta at 60-70% and Aulacoseira granulata var.

but Cymbella cistula did not occur in number 7’-25’

angustissma + Cymbella cistula + others at 30-50%,

samples. Synedra ulna did not occur at the same ratechanged alternately.

as in light-colored lamina except for number 6’, 35’andbella cistula, and organic matter from surrounding land
Consideration to sedimentary conditionareas. It is assumed that the dark-colored laminae were

1) Light- and Dark-colored laminae

made by diatom reacting to eutrophic conditions madeThe composition of diatom frustule consisted ofby material from the land.

41’ laminae.

3) Pollen analysis

one or two species, while almost all in the light-col-Aulacoseira granulata var. angustissima and Cym-(c) Occurrence change of diatom assemblage in the

Almost all light-colored laminae are composed ofored lamina were assumed to have formed by diatom

bella cistula are both eutrophic diatoms. In this study,

sequence of laminae

diatom frustules. Therefore, pollen in the light-col-blooming in the mixture zone of a lake in spring andthey occurred dominant and alternating in the dark-

The sequence is divided into three parts: numbers

ored laminae settled originally, and not from lake

autumn. Cyclotera comta and Stephanodiscus astraea

colored lamina. What caused this? It is assumed that15

Yoahi ta k a H a s e a n d Yu k o A n d o

Seasonal deposition of non-glacial varved clay in the Asono Formation of the middle Pleistocene in central Kyushu, Japan

Fig. 8. Seasonal changes of the environmental conditions based on non-glacial varved clay of the Asono
Formation.

Fig. 9. Connection between pollen and dominant diatom assemblages.

16

Aulacoseira granulata var. angustissima was plank-

The light-colored laminae mainly included pollen of

According to the above consideration, it is assumed

tonic and bred under a highly eutrophic condition,

Fagus, Ulmus/Zelkova, Alnus and Pinus. Fagus blos-that the dark-colored lamina laid on the light-col-while Cymbella cistula was epiphytic, and could notsomed in May, and Ulmus/Zelkova from April to June.

ored lamina of spring was made in summer, and thebreed under oligotrophic conditions. Conversely, if

Pinus also blossomed in April and May. Therefore, theydark-colored lamina laid on the light-colored lamina

the conditions were nutrient deficient, Cyclotella comta

were blossoming in spring. Conversely, Alnus blossomsof autumn was made in winter.

could not breed, and Cymbella cistula would breed un-

from autumn to winter. Figure 7 shows the changing

der the eutrophic conditions of the benthic area in apattern of pollen rate blooming in spring and in autumn.

4) Unexpected sedimentation in lakelake.

The diagram shows an alternating high rate of spring

During the blooming of Stephanodiscus astraea inblooming pollen and autumn blooming pollen. It is

autumn, it sometimes happens that Cymbella cistula

2) Change of environmental condition in lakeshown that the light-colored laminae having spring

was dominant. It is assumed that while Stephanodis-

According to the result of diatom analysis of bothblooming species deposited in spring, and the light-

cus astraea is breeding and light-colored lamina is

numbers 1-6 and 25-50 of light-and dark-colored lam-colored laminae having autumn blooming species de-being created, an unexpected sedimentation occurs

inae, it is assumed that the lake was under oligotro-

posited in autumn.

such as those triggered by typhoons. Dark-coloredphic conditions. During the sedimentary interval from

Figure 9 shows diatom species in the light-coloredlamina is made with much material including epi-

number 7 to 24 laminae, the lake was assumed to belamina deposited in spring and autumn respectively.

phytic and flowable diatom such as Cymbella cistula.

eutrophic because a high percentage of Aulacoseira

It also shows that the laminae of spring blooming

These dark-colored laminae were intercalated with

granurata var. angustissima and Cymbella cistula,included Cyclotella comta dominantly, while the lam-light-colored laminae including Stephanodiscus as-

occurring from much precipitation existed (Fig. 8).

inae of autumn blooming included Stephanodiscus

traea as a dominant feature.

astraea dominantly. Therefore, Cyclotella comta bloom3) Determination of season based on pollen

ed in spring, and Stephanodiscus astraea bloomed inanalysisautumn except for laminae number 7-24.

Process of annual sedimentation of laminae
It is clarified according to this study that there

Fig. 10. Diagram showing annual interval indicatedby the light-colored lamina diatom and pollen
composition.

17

Yoahi t a k a H a s e a n d Yu k o A n d o

Seasonal deposition of non-glacial varved clay in the Asono Formation of the middle Pleistocene in central Kyushu, Japan

Fig. 8. Seasonal changes of the environmental conditions based on non-glacial varved clay of the Asono
Formation.

Fig. 9. Connection between pollen and dominant diatom assemblages.

16

Aulacoseira granulata var. angustissima was plank-

The light-colored laminae mainly included pollen of

According to the above consideration, it is assumed

tonic and bred under a highly eutrophic condition,

Fagus, Ulmus/Zelkova, Alnus and Pinus. Fagus blos-that the dark-colored lamina laid on the light-col-while Cymbella cistula was epiphytic, and could notsomed in May, and Ulmus/Zelkova from April to June.

ored lamina of spring was made in summer, and thebreed under oligotrophic conditions. Conversely, if

Pinus also blossomed in April and May. Therefore, theydark-colored lamina laid on the light-colored lamina

the conditions were nutrient deficient, Cyclotella comta

were blossoming in spring. Conversely, Alnus blossomsof autumn was made in winter.

could not breed, and Cymbella cistula would breed un-

from autumn to winter. Figure 7 shows the changing

der the eutrophic conditions of the benthic area in apattern of pollen rate blooming in spring and in autumn.

lake.

The diagram shows an alternating high rate of spring4) Unexpected sedimentation in lake
During the blooming of Stephanodiscus astraea inblooming pollen and autumn blooming pollen. It is

autumn, it sometimes happens that Cymbella cistula

2) Change of environmental condition in lakeshown that the light-colored laminae having spring

was dominant. It is assumed that while Stephanodis-

According to the result of diatom analysis of bothblooming species deposited in spring, and the light-

cus astraea is breeding and light-colored lamina is

numbers 1-6 and 25-50 of light-and dark-colored lam-colored laminae having autumn blooming species de-being created, an unexpected sedimentation occurs

inae, it is assumed that the lake was under oligotro-

posited in autumn.

such as those triggered by typhoons. Dark-coloredphic conditions. During the sedimentary interval from

Figure 9 shows diatom species in the light-coloredlamina is made with much material including epi-

number 7 to 24 laminae, the lake was assumed to belamina deposited in spring and autumn respectively.

phytic and flowable diatom such as Cymbella cistula.

eutrophic because a high percentage of Aulacoseira

It also shows that the laminae of spring blooming

These dark-colored laminae were intercalated with

granurata var. angustissima and Cymbella cistula,included Cyclotella comta dominantly, while the lam-light-colored laminae including Stephanodiscus as-

occurring from much precipitation existed (Fig. 8).

inae of autumn blooming included Stephanodiscus

traea as a dominant feature.

astraea dominantly. Therefore, Cyclotella comta bloom3) Determination of season based on pollen

ed in spring, and Stephanodiscus astraea bloomed inanalysisautumn except for laminae number 7-24.

Process of annual sedimentation of laminae
It is clarified according to this study that there

Fig. 10. Diagram showing annual interval indicatedby the light-colored lamina diatom and pollen
composition.

17

18

Yoahi ta k a H a s e a n d Yu k o A n d o

In autumn, sun energy decreased and the surface

the light-colored lamina made by Stephanodiscus sp.

water (epilimnion) gradually moved deeper creat-

bloomed mainly deposited in winter while the dark-

The environmental changes in lakes during lamina

ing circular contamination making the differencescolored lamina with Cyctella comta, resulted in lam-formation are described as below (Fig. 11).

between the deep water and shallow water less obvi-

ina made by an annual deposition. In Sasaki et al.

are five original patterns for depositional environ-

Environmental changes of lamina deposition inment of non-glacial varved lamina in the Asono For-lake

mation.
Light-colored lamina originated during blooming

Seasonal deposition of non-glacial varved clay in the Asono Formation of the middle Pleistocene in central Kyushu, Japanof diatom in the spring and autumn. Laminae of Cy-

In spring, the temperature of the surface waterous. The planktonic species Stephanodiscus astraea(2019), there was a normal dark-colored layer with aclotella comta and Fagus, Ulmus/Zelkova and other(epilimnion) gradually rises causing a circulation

bloomed and made a light-colored lamina. Whenlight-colored layer that was about half the thickness

dominant forms, were created in the spring and lam-with the hypolimnion in the lake. The eutrophic

storms came, debris entered the lake and Cymbellaof the normal layer in the Miyajima Formation of theinae of Stephanodiscus astraea and Alnus dominantmaterial rose from the bottom to the top of the wa-

cistula became dominant forming a dark lamina.

Shiobara Group, and two sets of stripes were formed

were formed in autumn (Figs. 9, 10).

ter body, and planktonic species such as Cyclotella

After that, Stephanodiscus astraea bred and madein one year. The “double couplet'' indicates that dia-

Dark-colored lamina were formed with organic

comta occurred and bloomed during the formation oflight-colored lamina. Around the lake, large amounts

tom bloom peaks occurred twice a year. Also Sasaki

matter in summer and winter. Laminae dominated bylight-colored lamina. Broad-leaf trees grew and pro-of Alnus blossomed causing that byproduct to flow

et al. (2019) discussed event sediments in lacustrine

the eutrophic species Aulacoceira granulata var. an-duced much pollen around the lake.

into the lake.

laminated sediments.

gustissima and Cymbella cistula show that the lake

In summer, the temperature of the surface waterIn winter, the surface temperature fell to about 0

By understanding the pollen fossil content of each

was in a eutrophic condition. Lamina on light-col-(epilimnion) in the lake rose and the thermocline

Celsius while bottom temperatures gradually rose up

light-colored lamina, we were able to incorporate in-ored lamina made in spring was formed in summer

stratified on the cool and heavy water (hypolimnion)

to 4 Celsius. The lake condition was stable (stagna-

formation on seasonal changes into lamina formation.

and other lamina on light-colored lamina made in au-

in the lake. The water of the lake was stable (stag-tion period in winter). Eutrophic diatom species such

For this reason, the formation process of sediments

tumn was formed in winter. Intercalated lamina into

nation period in summer) except for the epilimnion.

as planktonic Aulacoseira granurata var. angustissi-

showing non-glacial lacustrine varved clay cannot belight-colored lamina made in autumn was formed un-

Planktonic species such as Aulacoseira granulata

ma or epiphytic Cymbella cistula bred and dark-col-generally regarded as a set of light- and dark-colored

der special conditions such as during typhoons. Fig.

var. angustissima were dominant under the condition

ored lamina was made.

lamina representing the details of the sedimentary

10 shows one year as indicated by the diatom andwith eutrophic and organic matter in the lake. When

pollen compositions of the white laminae, although

there was much precipitation, epiphytic species suchin samples 9-24 the diatom-pollen compositions do

as Cymbella cistula were dominant and a dark-col-not show a clear seasonal change.

ored lamina was formed.

environment.
Conclusion and discussion

Based on the idea that strata formed in lakes record

Based on the laminae that clearly formed sea-

the changes in the surrounding terrestrial environment,

sonally, the annual sequence had two patterns of

we have so far understood the changes mainly based

sedimentation. One is basically made of the lighton pollen analysis. We grasped the changes in the lake

spring lamina dark summer lamina, light-coloredenvironment from the analysis results of diatom fossils

autumn lamina and dark-colored winter lamina inand pollen fossils for each non-glacial varve clay lami-

an ascending order. And the other having a sequencena, and demonstrated that there is a part of the non-in ascending order with unexpected sedimentation. Thatglacial varved sediments indicating the seasonal changes

is, an annual sequence made of light-colored springof lake environment. In addition to the part that clearlylamina, dark-colored summer lamina, light-coloredshows seasonal changes, it was also revealed that there

autumn lamina within the unexpected dark-coloredwere parts that were formed by events such as typhoons.

lamina and the dark-colored winter lamina.

Considering previous studies, the formation of varved

Seasonal change was not clearly recognized for thelamina does not occur under the same conditions

annual sequence of laminae. Though the laminae

throughout the lake. It seems that what the set of light-

must have been made under the environment of sea-

and dark-colored lamina have different meanings.

sonal change, the area had much precipitation and aThis suggests that seasonal changes can be read in the

eutrophic condition of the lake, resulting in no changes

varved sediments taken up this time in the Asonoof diatom association and pollen assemblage.

Formation.

Fukusawa (1995) discussed the origin of the non-Fig. 11. Seasonal environment during deposition of the Asono Formation.

glacia l varved clay based on diatom assemblage andAcknowledgments

mineral conposition.

Dr. Akiko Iwauchi of Avance Co., Ltd. provided var-

Ishihara and Miyata (1999) clarified sedimentary

ious discussions on the stratigraphy and sedimentary

process of the non-glacial varved clay of the Hiruzen-

environment of the Asono Formation, and Dr. Utako

baru Formation of the Middle Pleistocene as follows:

Uchikoshiyama of the Uozu Buried Forest Museum

19

Yoahi t a k a H a s e a n d Yu k o A n d o

18

Seasonal deposition of non-glacial varved clay in the Asono Formation of the middle Pleistocene in central Kyushu, Japanare five original patterns for depositional environ-

Environmental changes of lamina deposition in

In autumn, sun energy decreased and the surface

the light-colored lamina made by Stephanodiscus sp.

ment of non-glacial varved lamina in the Asono For-lakewater (epilimnion) gradually moved deeper creat-

bloomed mainly deposited in winter while the dark-mation.

The environmental changes in lakes during lamina

ing circular contamination making the differencescolored lamina with Cyctella comta, resulted in lam-formation are described as below (Fig. 11).

between the deep water and shallow water less obvi-

ina made by an annual deposition. In Sasaki et al.

Light-colored lamina originated during bloomingof diatom in the spring and autumn. Laminae of Cy-

In spring, the temperature of the surface waterous. The planktonic species Stephanodiscus astraea(2019), there was a normal dark-colored layer with aclotella comta and Fagus, Ulmus/Zelkova and other(epilimnion) gradually rises causing a circulation

bloomed and made a light-colored lamina. Whenlight-colored layer that was about half the thickness

dominant forms, were created in the spring and lam-with the hypolimnion in the lake. The eutrophic

storms came, debris entered the lake and Cymbellaof the normal layer in the Miyajima Formation of theinae of Stephanodiscus astraea and Alnus dominantmaterial rose from the bottom to the top of the wa-

cistula became dominant forming a dark lamina.

Shiobara Group, and two sets of stripes were formed

were formed in autumn (Figs. 9, 10).

ter body, and planktonic species such as Cyclotella

After that, Stephanodiscus astraea bred and madein one year. The “double couplet'' indicates that dia-

Dark-colored lamina were formed with organic

comta occurred and bloomed during the formation oflight-colored lamina. Around the lake, large amounts

tom bloom peaks occurred twice a year. Also Sasaki

matter in summer and winter. Laminae dominated bylight-colored lamina. Broad-leaf trees grew and pro-of Alnus blossomed causing that byproduct to flow

et al. (2019) discussed event sediments in lacustrine

the eutrophic species Aulacoceira granulata var. an-duced much pollen around the lake.

into the lake.

laminated sediments.

gustissima and Cymbella cistula show that the lake

In summer, the temperature of the surface waterIn winter, the surface temperature fell to about 0

By understanding the pollen fossil content of each

was in a eutrophic condition. Lamina on light-col-(epilimnion) in the lake rose and the thermocline

Celsius while bottom temperatures gradually rose up

light-colored lamina, we were able to incorporate in-ored lamina made in spring was formed in summer

stratified on the cool and heavy water (hypolimnion)

to 4 Celsius. The lake condition was stable (stagna-

formation on seasonal changes into lamina formation.

and other lamina on light-colored lamina made in au-

in the lake. The water of the lake was stable (stag-tion period in winter). Eutrophic diatom species such

For this reason, the formation process of sediments

tumn was formed in winter. Intercalated lamina into

nation period in summer) except for the epilimnion.

as planktonic Aulacoseira granurata var. angustissi-

showing non-glacial lacustrine varved clay cannot belight-colored lamina made in autumn was formed un-

Planktonic species such as Aulacoseira granulata

ma or epiphytic Cymbella cistula bred and dark-col-generally regarded as a set of light- and dark-colored

der special conditions such as during typhoons. Fig.

var. angustissima were dominant under the condition

ored lamina was made.

lamina representing the details of the sedimentary

10 shows one year as indicated by the diatom andwith eutrophic and organic matter in the lake. When

pollen compositions of the white laminae, although

there was much precipitation, epiphytic species suchin samples 9-24 the diatom-pollen compositions do

as Cymbella cistula were dominant and a dark-col-not show a clear seasonal change.

ored lamina was formed.

environment.
Conclusion and discussion

Based on the idea that strata formed in lakes record

Based on the laminae that clearly formed sea-

the changes in the surrounding terrestrial environment,

sonally, the annual sequence had two patterns of

we have so far understood the changes mainly based

sedimentation. One is basically made of the lighton pollen analysis. We grasped the changes in the lake

spring lamina dark summer lamina, light-coloredenvironment from the analysis results of diatom fossils

autumn lamina and dark-colored winter lamina inand pollen fossils for each non-glacial varve clay lami-

an ascending order. And the other having a sequencena, and demonstrated that there is a part of the non-in ascending order with unexpected sedimentation. Thatglacial varved sediments indicating the seasonal changes

is, an annual sequence made of light-colored springof lake environment. In addition to the part that clearlylamina, dark-colored summer lamina, light-coloredshows seasonal changes, it was also revealed that there

autumn lamina within the unexpected dark-coloredwere parts that were formed by events such as typhoons.

lamina and the dark-colored winter lamina.

Considering previous studies, the formation of varved

Seasonal change was not clearly recognized for thelamina does not occur under the same conditions

annual sequence of laminae. Though the laminae

throughout the lake. It seems that what the set of light-

must have been made under the environment of sea-

and dark-colored lamina have different meanings.

sonal change, the area had much precipitation and aThis suggests that seasonal changes can be read in the

eutrophic condition of the lake, resulting in no changes

varved sediments taken up this time in the Asonoof diatom association and pollen assemblage.

Formation.

Fukusawa (1995) discussed the origin of the non-Fig. 11. Seasonal environment during deposition of the Asono Formation.

glacia l varved clay based on diatom assemblage andAcknowledgments

mineral conposition.

Dr. Akiko Iwauchi of Avance Co., Ltd. provided var-

Ishihara and Miyata (1999) clarified sedimentary

ious discussions on the stratigraphy and sedimentary

process of the non-glacial varved clay of the Hiruzen-

environment of the Asono Formation, and Dr. Utako

baru Formation of the Middle Pleistocene as follows:

Uchikoshiyama of the Uozu Buried Forest Museum

19

20

Yoahi ta k a H a s e a n d Yu k o A n d oadvised us on identification and habitat environment

Okayama Prefecture. Jour. Geol. Japan, 105(7),of diatom fossils. We would like to express our grati-

461-472. (In Japanese with English abstract)

tude to both of them.

Iwauchi, A. and Hase, Y. (1986): Fossil Diatom
Assemblages of the Asono Formation (Middle

References

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Ando Y. (2004): Accurate paleoenvironmental anal-

Research, 24(4), 283-293. (In Japanese with Eng-

ysis based on fossil diatom assemblages andlish abstract)pollen composition in lake varved sediments.

Sasaki, H., Sasaki, Y., Saito-Kato, M., Naruse, H.,

Kumamoto Graduate School of Science andYumi, M. and Ishihara, Y. (2016): Stratigraphic

Technology, master’s thesis. (In Japanese)veriations in lacustrine sediment gravity-flow

Fukusawa, H. (1995): Non-Glacial Varved Lake

deposits intercalated in varved diatomite: An

Sediment as a Natural Timekeeper and Detector

example from the Hiruzenbara Formation,

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Hase, Y. and Iwauchi, A. (1985): Late Cenozoic veg-

mentary facies and the recognition of event de-etation and paleoenvironment of Northern andposits intercalated in lacustrine varved deposits

Central Kyushu, Japan. Jour. Geol. Soc. Japan,

in the Middle Pleistocene Miyajima Formation:

91(11), 753-770. (In Japanese with English ab-

Sedimentary processes which are taken accountstract)of lateral changes. The Quaternary Research,

Research Group for the Hiruzenbara (1975): Qua-58, 237-249. (In Japanese with English abstract)ternary System of the Hiruzenbara, Okayama

Sonett, C.P. and Williams, G.E. (1985): Solar perio-

Prefecture (2). Earth Science, (Chikyu Kagaku),

dicities expressed in varves from glacial Skilak

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Ishihara, Y. and Miyata, Y. (1999): Periodic variation

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extracted from lacustrine laminated diatomite of
the middle Pleistocene Hiruzen-bara Formation,（2022 年 12 月 20 日受理）