Bulletin of the Geological Survey of Japan, vol. 70 (1/2), p. 5–16, 2019

ArticlePaleoenvironmental analysis from fossil ostracod assemblages of the Middle
Pleistocene Naganuma Formation in the Sagami Group, central Japan
Hirokazu Ozawa1,* and Gengo Tanaka2
Hirokazu Ozawa and Gengo Tanaka (2019) Paleoenvironmental analysis from fossil ostracod assemblagesof the Middle Pleistocene Naganuma Formation in the Sagami Group, central Japan. Bull. Geol. Surv.
Japan, vol. 70 (1/2), p. 5–16, 5 figs, 4 tables.
Abstract: This study estimates paleoenvironmental changes in water depths and temperatures fromapproximately 0.6 Ma during the marine oxygen isotope stage (MIS) 15 of the Middle Pleistocene along
coastal Japan. We used the modern analog technique (MAT), which is based on ostracod assemblages
from the Naganuma Formation in the Sagami Group of Kanagawa Prefecture in central Japan. The
analysis revealed that the Naganuma Formation was deposited in the shallow-marine environment at
paleo-water depths of 25–41 m, similar to current depths of southwestern Japan coastal areas such as
the Setonaikai Sea. The estimated summer paleo-water temperature (ca. 28℃) was approximately 2℃
higher than current Sagami Bay water temperatures at depths of shallower than 30 m. The estimatedtemperatures were also similar to or slightly lower (~2℃) than current summer temperatures in Sagami
Bay at depths of 40 m. The estimated winter paleo-water temperatures (6–11℃) were slightly lower(~2℃) than current Sagami Bay temperatures at depths of shallower than 40 m. This study reports for the
first time fluctuations in paleo-water temperature estimates quantitatively from 0.6 Ma during the MIS 15
along coastal Japan.
Keywords: osracods, Middle Pleistocene, Naganuma Formation, MIS 15, modern analog technique,paleo-water temperature, Sagami Bay, Kanagawa Prefecture, central Japan

Osaka Bay, Lake Hamana, and Tokyo Bay (Yajima, 1978,
1982; Ishizaki, 1990a, b; Ozawa et al., 1995; Yasuhara
and Kumai, 2003; Irizuki and Seto, 2004; Irizuki et al.,2002, 2005, 2011) from 0.4 Ma in the MIS 11 (MIS age
from e.g. Lisiecki and Raymo, 2005). However, details of
the Middle Pleistocene paleoenvironment prior to 0.4 Ma
along the Japanese coast remain unknown.
The Middle Pleistocene Naganuma Formation (ca. 0.6
Ma, MIS 15; Machida, 2010) of the Sagami Group is
exposed on the northern side of Sagami Bay (Fig. 1; Ujiie
and Kagawa, 1963) and is known for its shallow marine
calcareous fossils, including molluscs (Yabe, 1906;
Oyama, 1973). There are no reports of fluctuations inpaleo-water temperatures from 0.6 Ma in the MIS 15 forcoasts of Japan and adjacent regions. This study reports,
for the first time, fluctuations in paleo-water temperaturesof coastal Japan from 0.6 Ma (MIS 15) using modern
analog technique (MAT) methods.

1. Introduction
The paleoenvironment and invertebrate faunas fluctuateddrastically during the Pleistocene in response to marine
climatic changes and glacioeustacy in shallow seas of thenorthwest Pacific Ocean margin near the Japanese Islands(Chinzei, 1991; Tada, 1994; Amano, 2004; Ozawa andKamiya, 2005; Ozawa and Ishii, 2014). Because of themid-Pleistocene transition (MPT; e.g. Berger and Jansen,
1994) and a shift in the glacioeustatic climatic cycle
periodicity from 41 kyr to 100 kyr, the Middle Pleistocene
represents an important paleoclimate period.
Benthic ostracods provide clues for understanding
paleoenvironments, because they lack a planktic growth stage
and are endemic, making them sensitive to environmental
changes (e.g. Boomer et al., 2003). Previous Japanesestudies of ostracods have documented environmental
changes since the Middle Pleistocene, evidenced byfossils in shallow seas along the Pacific coasts of the1

Earth Sciences Laboratory, College of Bioresource Sciences, Nihon University, Kameino 1866, Fujisawa, Kanagawa 252-0880, Japan
General Education Courses, Institute of Liberal Arts and Science, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan*
Corresponding author: H. Ozawa, Email: ozawa.
@nihon-u.ac.jp
2−5−Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

2. Geological setting
The Naganuma Formation in the Sagami Group is exposedat the base of the Miura Peninsula in the southernmost
region of Yokohama City and in the northeastern regionof Fujisawa City, Kanagawa Prefecture, central Japan(Fig. 1). The geology and fossils of this formation havebeen studied (e.g. Otuka, 1937; Ujiie and Kagawa, 1963;
Oyama, 1973; Mitsukoka et al., 1977; Mitsunashi and
Kikuchi, 1982) since 1906 (Yabe, 1906). This stratum
unconformably overlies the Lower Pleistocene Ofuna
Formation in the Kazusa Group (age from Fujioka et al.,2003), and the unconformable surface is known as the“Naganuma Unconformity”(Machida, 2010; Fig. 2).
The Middle Pleistocene Byobugaura Formation in the
Sagami Group unconformably overlies the Naganuma
Formation (Osada et al., 1982; age from Machida, 2010).
The Naganuma Formation primarily consists of silt andfine- to medium-grained sand, and contains calcareous
fossils accompanied by layers of tuff (Otsuka, 1937; Ujiie
and Kagawa, 1963; Fig. 2). Fossil ostracods have also
been found in the Naganuma Formation (Ozawa, 2009).
The chronology of tuff layers suggests that the Naganuma
and Byoubugaura formations in the Sagami Group of
the Miura Peninsula are correlated with the lower andupper parts, respectively, of the Kasamori Formation in
the Kazusa Group of the Boso Peninsula in central Japan(Machida, 2010; Fig. 2). The two unconformities of the
Naganuma Formation in the study area (Fig. 2) are also
seen in the Kazusa Group of the Boso Peninsula. Basedon the chronology of the Ks18 tuff layer of the lower partof Kasamori Formation (Machida, 2010), the Naganuma
Unconformity is estimated to have been formed while
the sea-level was low during a glacial period (MIS 16).
Similarly, the Byobugaura Formation is correlated withthe upper part of the Kasamori Formation (Fig. 2). Basedon estimations from the Ks11 tuff layer chronology in
the Kasamori Formation, previous researches (Machida
et al., 1980; Machida, 2010) determined that anotherunconformity between the Naganuma and Byobugaura
formations was also formed while sea-level was low during
a glacial period (MIS 14). The Naganuma Formation isthought to have formed from deposits during the highsea-level period of the interglacial period between MIS
16 and 14, and therefore to be correlated with MIS 15,
approximately 0.6 Ma (Machida, 2010; MIS age from
Lisiecki and Raymo, 2005), although Ozawa (2009)referred previous studies of nannofossils and tuff layers
pre-2009 to estimate that the formation was deposited in
ca. 0.5 Ma (MIS 13).
Fig. 1(A) A map of locations described in the text. (B) A map of
the study area. (C) A map of the distribution of the Naganuma
Formation of central Japan, with the study site, modified from
Ujiie and Kagawa (1963). (A)–(C) are modified from Ozawa(2009).

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Paleoenvironment from fossil ostracods of the Middle Pleistocene Naganuma Formation (Ozawa and Tanaka)

3. Materials and methods
We used the modern analog technique (MAT) to analyze
the Naganuma Formation fossil ostracod assemblage
dataset found in Table 1 of Ozawa (2009). Ozawa (2009)
described fossil ostracod assemblages from eight sediment
samples (Ty1–Ty8) collected from the Naganuma
Formation outcrop, which is exposed in the Taya area of
Sakae-ku, Yokohama City, Kanagawa Prefecture, central
Japan (latitude: 35º 21.77’ N, longitude: 139º 31.42’ E;
Figs. 1 and 2). Then a total of 58 ostracod species wereidentified.
The MAT is a method for estimating paleoenvironmental
conditions, such as the water depth and temperature, bycomparing similar recent environmental data to fossil
assemblages (Overpeck et al., 1985). The degree of
nonsimilarity between objective fossil assemblages andrecent assemblages is determined by the formula asfollows;(SCD: square chord distance; Fpi: proportion of thei-th fossil species; Mpi: proportion of the i-th modern
species). Ikeya and Cronin (1993) first employed MAT tofossil ostracod assemblages in order to estimate Pliocene
and Pleistocene paleoenvironments of Japan, based on
273 recent samples from Japan and adjacent areas. In
the current study, we summarized data from 419 recent
samples (including 367 species) from Japan and adjacentareas (Zhou, 1993; Tsukawaki et al., 1997, 1998, 1999,
2000; Yamane, 1998, Ozawa et al., 1999; Yasuhara andIrizuki, 2001; Nakao and Tsukagoshi, 2002; Irizuki etal., 2006; Tanaka, 2008) using the modern analog data
matrix of the United States Geological Survey (Ikeya
and Cronin, 1993). Squared-chord distances (SCDs) werecomputed using Microsoft Excel 2007 in the Visual Basics
for Applications (VBA) environment, following methods
described by Tanaka and Nomura (2009).
The SCD is a value between 0 and 2. When the SCD is 0,
the objective fossil assemblage is completely concordant
with recent assemblages. To obtain the minimum andmaximum bottom water temperatures for each recent
ostracod sample, temperatures for the sites used in ourstudy were obtained from the sectioned squares of theJapan Oceanographic Data Center (JODC) database within
one degree of latitude and longitude (JODC, 2017; http://
www.jodc.go.jp/jodcweb/index_j.html). We estimatedpaleo-water temperatures in both summer and winterat paleo-water depths for eight Naganuma Formation
samples described by Ozawa (2009).

Fig. 2 (A) A columnar section of the Naganuma Formation withsample horizons, modified from Osada et al. (1982) andOzawa (2009). FB: dominant occurrences of fragmentsof barnacles. (B) Schematic stratigraphic correlation for
the Naganuma, Byoubugaura and Kasamori formations
between Yokohama and Boso areas. Data for tuff layersin (A) and (B) referred from Mitsuoka et al. (1977),
Mitsunashi and Kikuchi (1982), Osada et al. (1982),
Machida and Arai (2003), and Machida (2010). MIS(Marine oxygen Isotope Stage) nos. are referred from
Machida (2010).

4. Results
The MAT results for changes in the paleo-water
depth and temperature in summer and winter based on
the ostracod assemblage dateset from the Naganuma−7−Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Table 1 Results of modern analog area based on the MAT (modern analog technique) for each fossil ostracod sample from the Naganuma
Formation. From left to right: sample number from Ozawa (2009); estimated water depth (WD); summer bottom water temperature(SBT); winter bottom water temperature (WBT); square chord distance (SCD); longitude (Long.) and latitude (Lat.) of the mostsimilar modern site in terms of the ostracod assemblage; reference for the recent ostracod assemblage of a modern analog area.
N: northern; C.: central; SW: southwestern.
Sample no. WD (m) SBT (℃) WBT (℃) SCD Long. (ºN) Lat. (ºE)
Ty1
41
16.0
11.0
1.06
127.09
34.16
Ty2
25
28.4
6.3
0.87
133.25
34.13
Ty3
25
28.4
6.3
0.71
133.25
34.13
Ty4
35
30.6
10.3
1.22
131.37
34.43
Ty5
25
28.4
6.3
0.81
133.25
34.13
Ty6
31
28.4
6.3
0.89
133.20
34.08
Ty7
31
28.4
6.3
0.72
133.20
34.08
Ty8
25
28.4
6.3
0.58
133.25
34.13

Modern analog area
N East China Sea
C. Setonaikai Sea
C. Setonaikai Sea
SW Sea of Japan
C. Setonaikai Sea
C. Setonaikai Sea
C. Setonaikai Sea
C. Setonaikai Sea

Reference
Choe (1985)
Yamane (1998)
Yamane (1998)
Tsukawaki et al. (2000)
Yamane (1998)
Yamane (1998)
Yamane (1998)
Yamane (1998)

Table 2 MAT (modern analog technique) method results (SCD analysis; SCD means square chord distance) for the three ostracod
assemblages (A–C) from the Naganuma Formation (Ozawa, 2009). From left to right: assemblage names; five dominant species
from Ozawa (2009); assemblage (Ass.); estimated water depth (WD) range; summer bottom water temperature (SBT) range;
winter bottom temperature (WBT) range; square chord distance (SCD) range; modern analog area based on the modern analog
technique (MAT). C.: central; SW: southwestern; N: northern.
Ass.

Five dominant species

WD
SBT
range (m) range (°C)

WBT
range (°C)

SCD
range

Modern analog
areas

25–41

16.0–30.6

6.3–11.0

C. Setonaikai Sea
0.71–1.22 SW Sea of Japan,
N East China Sea

25–31

28.4

6.3

0.72–0.81 C. Setonaikai Sea

25

28.4

6.3

Krithe japonica
Amphileberis nipponica
A Buntonia hanaii
Bicornucythere bisanensis
Krithe surugensis
Amphileberis nipponica
Bicornucythere bisanensis
B Nipponocythere bicarinata
Krithe japonica
Loxoconcha tosaensis
Bicornucythere bisanensis
Amphileberis nipponica
C Trachyleberis scabrocuneata
Loxoconcha tosaensis
Nipponocythere bicarinata

0.58

C. Setonaikai Sea

Ozawa & Tanaka Table 2

Formation (Ozawa, 2009) are shown in Table 1 and Fig.
3. Species compositions of fossil ostracod assemblages
from eight Naganuma Formation samples are similar tomodern assemblages found in present-day shallow seassurrounding southwestern Japanese Islands, such as thecentral Setonaikai Sea (Hiuchinada Sea), the southwestern
Sea of Japan, and the northern East China Sea (Tables 1
and 2).
Our results suggest that paleo-water depths fluctuatedbetween 25 and 41 m (Fig. 3; Table 1). These water depths
are slightly shallower for Ty1 through Ty8 (41–25 m). The
paleo-water temperatures in summer are estimated to beapproximately 16ºC for the one lower horizons (Ty1),

and 28–31ºC for the seven upper horizons (Ty2–Ty8).
Winter paleo-water temperatures are estimated to beapproximately 10–11ºC for the two lower horizons (Ty1and Ty4) and 6ºC for the six upper horizons (Ty2, Ty3
and Ty5–Ty8).
Our results showing a realtively wide SCD range (0.71–
1.22; Fig. 3 and Table 2) suggest that the modern analog
areas of the assemblage A of Ozawa (2009) are the central
Setonaikai Sea (Hiuchinada Sea), the southwestern Sea ofJapan, and the northern East China Sea, from longitudes
127–134ºE and latitudes 34–36ºN and at water depths
ranging from 25–41 m. The modern analog area for theassemblage B is the central Setonaikai Sea (Hiuchinada−8−Paleoenvironment from fossil ostracods of the Middle Pleistocene Naganuma Formation (Ozawa and Tanaka)

Fig. 3 Vertical changes in results from the MAT (modern analog technique) method (SCD analysis; SCD means square chord
distance) for each sample in the Naganuma Formation within a columnar section, with sample horizons. Ostracod
assemblages A–C and Continental shelf species are the same as in Ozawa (2009). Data for appearance horizons of theassemblages A–C and percentages for Continental shelf species are referred from Ozawa (2009). Continental shelf
species: Abrocythereis guangdongensis, Actinocythereis kisarazuensis, Bradleya japonica, Bradleya sp., Hirsutocythere?
hanaii, Krithe surugensis and Loxoconcha tamakazura. Columnar section is modified from Ozawa (2009).

Ty6 of Ozawa (2009)), was slightly shallower than thatof the lower horizon.
The three steps of the water-depth decrease during thedepositional periods of the horizons Ty1–Ty8 were definedfrom the ostracod assemblages A–C (Fig. 3), which include
dominantly Amphileberis nipponica, Bicornucythere
bisanenesis and Krithe japonica (Fig. 4; Table2) by Ozawa(2009). That study stated that the water depth decreased
upwards in the studied sequence from around 40 m water
depth to 10–15 m water depths, and that the influenceof the open-sea water was weaker during depositional
periods of the upper six horizons of Ty3–Ty8, comparedwith the lower two horizons Ty1–Ty2, based on the ratioof continental species of Ozawa (2009); i.e., Abrocythereis
guangdongensis, Actinocythereis kisarazuensis, Bradleya
japonica, Bradleya sp., Hirsutocythere? hanaii, Krithe
surugensis and Loxoconcha tamakazura (Figs. 3 and 4).
These species are reported from modern continental shelfarea around Japan (e.g. Ikeya and Suzuki, 1992; Zhou andIkeya, 1992; Zhou, 1995; Tsukawaki et al., 1998, 2000;
Tanaka, 2008, 2012).
Therefore paleo-water depths reconstructed from MATSea) from longitudes 133–134ºE and latitudes 34–35ºN
at water depths of 25–31 m, as evidenced by a realtively
low SCD range (0.72–0.81). The modern analog area of
the assemblage C has the lowest SCD score (0.58), and
it is also the central Setonaikai Sea (Hiuchinada Sea) at
longitude 133ºE and latitude 34ºN at a water depth of 25m.

5. Discussion
5. 1 Paleo-water depth
The paleo-water depth of the Naganuma Formation wasreconstructed by several previous studies of molluscan
fossils. Osada et al. (1982) reported the detailed species
composition of the molluscan fossils from the two upperhorizons above the Tayasuko tuff layer at the same location
as the current study. They recognized one molluscan fossil
assemblage, which was dominated by Acila divaricata(Hinds) in the two horizons, and reported that the
paleoenvironment in their lower horizon (near sample Ty1of Ozawa (2009)) was the inner bay area influenced by
the open sea with 20–60 m water depths. They estimatedthat the water depth of their upper horizon (near sample−9−Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Fig. 4 SEM images of fossil ostracods from the Naganuma Formation, modified from Ozawa (2009). LV: left
valve, RV: right valve, CA: carapace. 1: Amphileberis nipponica (Yajima), male LV, sample no. Ty6, 2–3:
Bicornucythere bisanensis (Okubo), 2: female LV, Ty7, 3: A-1 juvenile LV, Ty1, 4: Bradleya japonica
Benson, female RV, Ty1, 5: Buntonia hanaii Yajima, male LV, Ty3, 6: Hirsutocythere? hanaii Ishizaki, A-1
juvenile RV, Ty5, 7: Krithe japonica Ishizaki, female RV, Ty6, 8: Krithe surugensis Zhou and Ikeya, male
RV, Ty7, 9: Loxoconcha tosaensis Ishizaki, male LV, Ty7, 10: Nipponocythere bicarinata (Brady), male
RV, Ty6, 11: Sinoleberis tosaensis (Ishizaki), female CA left side, Ty5., 12: Trachyleberis scabrocuneata(Brady), A-1 juvenile RV, Ty5. All scale bars are equal to 0.1 mm.

in our study (Fig. 3; Tables 1 and 2) are mostly consistent
with results from previous molluscan and ostracod studies.
5. 2 Paleo-water temperature
The SCD scores (0.58–1.22) for paleotempeartures
based on ostracods and the MAT were fairly high (Fig. 3;
Tables 1 and 2). Ikeya and Cronin (1993) demonstrated
that a SCD value below 0.5 indicates analogs at a faunal
provincial level, although we could obtain only these SCD
scores herein. However we had tried to attempt to compare
paleotemperatures to average modern data, including
the minimum and maximum water temperature in andaround Sagami Bay (Table 3), in an effort to reconstruct
the Japanese paleoenvironment from 0.6 Ma, because thisstudy reported for the first time fluctuations in paleo-watertemperature estimates quantitatively during the MIS 15
along coastal Japan.

We compared the estimated Naganuma Formation
paleotemperatures (Figs. 3 and 5; Tables 1 and 2) tomodern water temperatures in and around Sagami Bay,located within latitude 35–36˚N and longitude 139–140˚E(Table 3). We utilized data for this region from Japan
Oceanographic Data Center (2017)’
s website (http://
www.jodc.go.jp/jodcweb/JDOSS/index_j.html), as thisis the area closest to the Naganuma Formation samplingsite. Modern water temperatures are presented as averages
with maximum and minimum values for 97 years between
1906 and 2003. Data are given for latitude 35–36˚N andlongitude 139–140˚E (within one degree of latitude andlongitude for a given site) at water depths of 20, 30, and
50 m in summer (August) and winter (February) (Table3). The water depths are not the same as the estimatedpaleo-water depths, so comparisons were made between
similar water depths. For example, a paleo-water depth of− 10 −

Paleoenvironment from fossil ostracods of the Middle Pleistocene Naganuma Formation (Ozawa and Tanaka)

Table 3 Modern average, maximum, and minimum water temperatures from 1906 to 2003 for latitude 35–36˚N and longitude
139–140˚E (within one degree of latitude and longitude for a given site) at three water depths (20, 30, and 50 m) in summer(August) and winter (February). From the J-DOSS database of Japan Oceanographic Data Center (2017).
Summer temperature (°C)
Water
depth (m) average maximum minimum
20
21.6
26.6
15.5
30
20.0
26.1
14.0
50
17.5
23.2
12.7

Fig. 5Winter temperature (°C)
average maximum minimum
13.9
17.5
8.6
14.4
17.4
10.9
14.5
17.3
11.5

 esults for the paleo-water depth and water tempearature in summer and winter from the MAT (modernRanalog technique) method (SCD analysis; SCD means square chord distance) for each sample in the
Naganuma Formation, and modern average, maximum, and minimum water temperatures from 1906 to 2003
for latitude 35–36˚N and longitude 139–140˚E (within one degree of latitude and longitude for a given site)at three water depths (20, 30, and 50 m) in summer (August) and winter (February). Data of the modern
water temperature are referred from the J-DOSS database of Japan Oceanographic Data Center (2017).

31 m was compared to average values at the 30 m depth
in modern seas.
Results from these comparisons suggest that estimatedpaleo-water temperatures at depths of 25–35 m (16–31℃;
Fig. 5, Tables 1 and 2) were slightly higher than those in
and around the present-day Sagami Bay (ca. 18–22℃ of
20–50 m; Fig. 5; Table 2) in summer for seven of eightsamples. For the assemblage A samples (Ozawa, 2009), thesummer estimated temperatures at 25 m and 35 m for threesamples (28.4℃ for samples Ty2 and Ty4, and 30.6℃ for
Ty3, respectively; Fig. 5; Table 2) were 7–10℃ higher thanpresent-day average temperatures at depths of 20 m and 30
m (21.6℃ and 20.0℃; Fig. 5; Table 3). These estimatedtemperatures are approximately 2–4℃ higher than themodern maximum temperatures in this region (26.6℃ and
26.1℃). Estimated summer temperatures at depths of 41
m for another sample in the assemblage A (16.0℃ for Ty1)
were 2–4℃ lower than the modern average temperatures at
30 and 50 m in Sagami Bay (17.5℃ and 20.0℃). This value
is also 2–3℃ higher than modern minimum temperaturesin this area (12.7℃ and 14.0℃). Estimated summer
temperatures of the assemblages B and C at 25 m and 31
m (28.4℃ for Ty5–Ty8) were 7–8℃ higher than averagetemperatures at 20 m and 30 m (20.0℃ and 21.6℃; Table3), and are 2℃ higher than recent maximum Sagami Bay
temperatures (26.1℃ and 26.6℃).

− 11 −

Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Estimated winter water temperatures (6–11℃) werelower than recent Sagami Bay water temperatures at
depths of 20–50 m (14–15℃; Fig. 5; Table 2) for all
samples. Estimated winter temperatures of the assemblage
A at 25 m, 35 m, and 41 m (6.3℃ for Ty2 and Ty3, 10.3℃
for Ty4, 11.0℃ for Ty1) were 4–8℃ lower than recent
average temperatures at 20 m, 30 m, and 50 m (13.9℃,
14.4℃, and 14.5℃, respectively; Table 3), and 1–2℃
lower than modern minimum temperatures (8.6℃, 10.9℃,
and 11.5℃, respectively). Estimated temperatures of theassemblages B and C at 25 m and 31 m (6.3℃ for Ty5–8)
were 8℃ lower than average temperatures at depths of 20
m and 30 m (13.9℃ and 14.4℃, respectively), and were
2–5℃ lower than modern minimum temperatures in andaround Sagami Bay (8.6℃ and 10.9℃).
The estimated summer water temperature for depths of
25–35 m was slightly higher (~2℃) than that in present-day
Sagami Bay at depths of 20–30 m, although the estimatedsummer temperature at 41 m depths was similar to orslightly lower (~3℃) than current temperatures at 30–50
m depths (Fig. 5). The estimated winter temperatures
were slightly lower (~2℃) than those recorded recently
in Sagami Bay at depths of 30–50 m.
The ostracod species Sinoleberis tosaensis (Fig. 4) was
found in assmblage B of the Naganuma Formation (Ozawa,2009). This shallow-marine species has been mainly foundalong the Pacific coast of southwestern Japan (south of35ºN) from Lake Hamana of Honshu Island to Uranouchi
Bay of Shikoku Island (water depths of less than 15 m,
and in the Taiwan Strait of the southern East China Sea(Ishizaki, 1968; Malz and Ikeya, 1982). This species is notfound around modern Tokyo Bay (e.g. Frydl, 1982; Zhou,
1995). S. tosaensis fossils are found in Pliocene deposits insouthwestern Taiwan along the coast of the northern South
China Sea and in Plio-Pleistocene deposits in Taiwan andJapan along the Pacific Ocean coasts (Malz and Ikeya,1982). Modern-day and fossil distributions of this species
suggest that it favors warm envinronments influenced bywarm Kuroshio Current along the Pacific Ocean coast(Ozawa, 2009). Therefore, the presence of this species in
the Naganuma Formation might suggest that paleo-watertemperatures in shallow marine areas were slightly higher
0.6 Ma compared to those in the present-day Sagami Bay,especially in summer.
5. 3 Comparison with results of Ozawa (2009)
Based on the species compositions of three ostracod
fossil assemblages from the Naganuma Formation, as
determined through Q-mode cluster analysis, Ozawa(2009) described three types of paleoenvironments withthree stepwise changes during the depositional period of
the horizons Ty1–Ty8; i.e., the outer bay (assemblage A),
central bay (assemblage B), and inner bay (assemblageC) (Fig. 3; Table 2). In our MAT analysis, the species
composition of the fossil assemblages B and C, describedby Ozawa (2009), is most similar to the assemblages fromthe modern area of the Setonaikai Sea (Hiuchinada Sea)

(Tables 1, 2 and 4). The fossil assemblage A is correlatedwith the three modern assemblages from three different
areas; i.e., the Setonaikai Sea, Sea of Japan, and East
China Sea (Tables 1, 2 and 4). Fossil assemblages fromsix samples (Ty2, Ty3, and Ty5–Ty8) out of a total ofeight samples collected from the Naganuma Formation
are most similar to those from the modern Setonaikai
Sea (Hiuchinada Sea) (Tables 2 and 4). Ozawa (2009)
identified the paleoenvironment of the assemblage A as
the outer bay area. The SCD scores for two (Ty1 andTy4) out of four samples of the assemblage A have thehighest range in our study, ca. 1.1–1.2 (Fig. 3; Tables 1 and
4). Thus, our results reflect the utility of relatively small
datasets for MAT analysis by including assemblages fromjust four localities in outer bay areas of the Setonaikai Sea(Hiuchinada Sea).
Comparing the results of Ozawa (2009) with those from
our MAT study (Table 4), their paleoenvironments alignwell with bay areas, with the assemblage A as outer bay(samples Ty1 and Ty4) and B as central bay (Ty5–Ty7),
and differ slightly for the assemblages A (Ty2 and Ty3) andC (Ty8). The SCD scores of these three samples (Ty2, Ty3
and Ty8) were greater than 0.5; i.e., 0.6–0.9 (Fig. 3; Tables
1 and 4). Therefore, our data may not yet support a detailed
comparison between paleoenvironments and modern sea
environments around Japan, especially for these threesamples. On the other hand, Ozawa (2009) might haveregarded the percentages of Amphileberis nipponica and
Loxoconcha tosaensis, which are often reported from outerand central bay areas (e.g. Yamane, 1998; Tsukawaki etal., 2000), in assemblages A (Ty2, Ty3) and C (Ty8; Table4) as more important in paleoenvironments. However,
our MAT results show a similar tendency with regard
to paleoenvironmental fluctuations to those reported by
Ozawa (2009), with the water depth decreasing upwards
in this sequence from around 40 m to 20 m, and a weakerinfluence from open-sea water in the upper horizons, ascompared to the lower horizons.
5. 4 Paleoenvironment of 0.6 Ma in and around Japan
Several recent studies of other microfossils were used
to generate new details of the Japanese paleoenvironment
around 0.6 Ma. Radiolarian fossils from 0.74 Ma were
present in a drill core that was obtained from the bottomof the sea at a depth of 1,200 m from east of the Shimokita
Peninsula in northeastern Japan (Matsuzaki et al., 2014).
This study determined that the influenece of the warm
Tsugaru Current (a branch of the Tsushima and Kuroshio
Currents) significantly increased from 0.62 Ma to 0.48 Ma(from MIS 15 to MIS 13). Based on the fossil radiolarianassemblages, researchers estimated that sea surface watertemperatures 0.6 Ma were slightly higher than modernday temperatures off the Shimokita Peninsula. They
hypothesized that warm Kuroshio Current, which flows
along the northwest margin of the Pacific Ocean near theJapanese Islands, became more influential for central andsouthwestern Japan during this time.

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Paleoenvironment from fossil ostracods of the Middle Pleistocene Naganuma Formation (Ozawa and Tanaka)

Table 4 Comparisons for MAT (modern analog technique) method results (SCD analysis; SCD means square chord
distance) for the three ostracod assemblages (A–C) from the Naganuma Formation (Ozawa, 2009). From left
to right: sample no., modern analog area (outer bay and central bay are referred from Choe, 1985; Yamane,
1998, Tsukawaki et al., 2000) and square chord distance (SCD) based on the modern analog technique (MAT),
paleoenvironment and assemblage names of Ozawa (2009). C.: central; SW: southwestern; N: northern.

Sample no.
Ty1
Ty2
Ty3
Ty4
Ty5
Ty6
Ty7
Ty8

This study
Modern analog area
N East China Sea (outer bay)
C. Setonaikai Sea (central bay)
C. Setonaikai Sea (central bay)
SW Sea of Japan (outer bay)
C. Setonaikai Sea (central bay)
C. Setonaikai Sea (central bay)
C. Setonaikai Sea (central bay)
C. Setonaikai Sea (central bay)

Compared to estimates from radioralian and ostracod
studies, Japanese Middle Pleistocene pollen studies(Hongo, 2009; Hongo and Mizuno, 2009; Nirei, 2017)
differed in paleoclimate estimations for present-day central
Japan. Based on the assemblages of the dominant genera
Cryptomeria and Fagus in boring core samples frominland areas near Tokyo and Osaka bays, results from thepollen studies determined that the paleo-air temperatures
in inland areas of the Kanto and Osaka plains during MIS
15 were slightly lower than present-day temperatures.
Results from the current study and othermicropaleontological studies of 0.6 Ma demonstrateevidence of both warmer and cooler paleotemperaturescompared to present temperatures among inland, coastal,and offshore regions of Japan. The SCD scores from ourostracod MAT analysis were relatively high (Fig. 3; Tables
1 and 2). So yet, our study could not address a detailed
comparison of paleotemperatures and modern-day sea
water temperatures.
The paleotemperatures of the Northwest Pacific margin of
the Japanese Islands during the MPT remain unknown.
Research on paleoclimates in marine and inland
regions during the MPT will clarify changes related
to glacioeustacy in both the paleoenvironment and ininvertebrate faunas in shallow marine regions along theNorthwest Pacific margin. Further paleoenvironmental
research on the Middle Pleistocene is needed in Japanand adjacent regions.
Acknowledgement
We extend our thank to Y. Tanimura and M. Saito(National Museum of Nature and Science, Tsukuba,
Japan) for their kind assistance in various aspects of
preparing the manuscript. For using by scanning electron
microscopes, we thank the National Museum of Natureand Science, Tsukuba, Japan. Thanks are also due toK. Yamada (Shinshu University, Japan), I. Motoyama(Yamagata Unibersity, Japan), T. Itaki and A. Suzuki(AIST, Geological Survey of Japan) for critically reading

SCD
1.06
0.87
0.71
1.22
0.81
0.89
0.72
0.58

Ozawa (2009)
paleoenvironment
outer bayouter bayouter bayouter bay
central bay
central bay
central bay
inner bayassemblages
A
A
A
A
B
B
B
C

the manuscript and providing many helpful advice. This
research was financially supported in part by a Grant-inAid from the Japan Society for the Promotion Science(No. 15K07199 to H. Ozawa).

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Recieved September 29, 2017
Accepted September 18, 2018
Published on-line January 18, 2019− 15 −

Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

相模層群長沼層（中部更新統）産の介形虫化石群に基づく古環境解析
小沢広和・田中源吾

要 旨
本研究は，神奈川県に分布する相模層群長沼層産の海生介形虫化石群に基づき，海洋酸素同位体ステージ15（MIS 15）の約60万年前の古環境（水深，水温）を現生アナログ法（MAT）を用いて推定した．その結果，長沼層の古環境は，現在の瀬戸内海などの西南日本沿岸の水深25～41 mの海域に，最も類似することが明らかになった．これらの西南日本沿岸域の水温値（夏：16～28℃，冬：6～11℃）を，現在の相模湾の水温データと比較したところ，夏の水温は水深30 m以浅では少なくとも2℃ほど高いが，40 m付近では現在とほぼ同じか2℃ほど低く，冬の水温は40 m以浅で少なくとも2℃ほど低いことが明らかになった．本研究は約60万年前（MIS 15）の日本列島沿岸浅海域における古水温の変動を，定量的に推定した初の研究例である．

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