Quaternary International 246 (2011) 233e238

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Late Pleistocene buried humic soils within a tephra-soil sequence near Unzen
volcano, Kyushu, Japan
Yudzuru Inoue a, *, Shinji Nagaoka b, Shinji Sugiyama c
a
Center for Advanced Instrumental Analysis, Kyushu University, 6-1 Kasuga-kouen, Kasuga, Fukuoka 816-8580, Japan
b
Department of Geography, Faculty of Education, Nagasaki University, 14-1 Bunkyou-machi, Nagasaki 852-8521, Japan
c
Paleoenvironment Research Institute Co. Ltd, 1417 Akae, Miyazaki 880-0912, Japan

a r t i c l e i n f o a b s t r a c t

Article history: Relatively few late Pleistocene buried humic soils in volcanic areas have been studied to date, because
Available online 28 June 2011 they are not common worldwide. The present study examined the origin of soil organic matter (SOM) in
mainly late Pleistocene buried humic soils within a tephra-soil sequence near Unzen volcano, north-
eastern Shimabara Peninsula, Kyushu, SW Japan. The buried humic soil horizons were divided into seven
horizons in a tephra-soil sequence. The total carbon contents were very high for late Pleistocene soils,
with a value of >60 g kg
1 obtained for the oldest buried humic soil (8Ab horizon). The main phytolith
types in the buried humic soils are Pleioblastus sect. Nezasa in A and 2Ab, and Sasa sect. Crassinodi in 3Ab,
4Ab, 5Ab, 6Ab, 7Ab/C, and 8Ab. SOM was derived mainly from Gramineae. In the oldest buried humic soil
(8Ab horizon), the d13C values indicate that C3 plants contributed more than 50% of the plant-derived
carbon in the SOM, and the phytoliths indicate the dominance of C3 grasses (e.g., Sasa sect. Crassi-
nodi) in the NE and SE of the peninsula. The late Pleistocene humic soils formed due to an abundant
supply of organic matter from mainly grass vegetation such as Sasa sect. Crassinodi, regardless of climate
change at w30 ka or earlier in the Unzen volcano area. The present study contributes to the paleo-
environmental reconstruction for the late Pleistocene.
Ó 2011 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction 1989). Sase et al. (2006) chronologically studied the tephra-soil

sequence at the foot of Ashitaka Volcano, Central Japan, to discuss
When a volcano produces tephra that covers the surface soil the formative history of Late Pleistocene buried humic soils by
with eruption, the soil is a so-called ‘buried soil’. If the buried soil examining soil chemical properties and paleo-phytolith assem-
contains abundant humus (soil organic matter; SOM), it is called blages. In the Kyushu region, SW Japan, Yamada and Kubotera
a ‘buried humic soil’. Few previous studies have examined buried (1996) examined the tephra-soil sequence containing buried
humic soils in late Pleistocene (MIS 3-2) sequences within volcanic humic soils of late Pleistocene age in the western hills and terraces
areas, because such soils are rare in the world. However, these soils of Aso volcano. Sugiyama et al. (2002) studied the history of humic
in late Pleistocene age have been described from the Nevado de soil formation by using phytolith analysis in a tephra-soil sequence
Toluca volcano in Mexico (Sedov et al., 2001, 2003; Yakimenko containing late Pleistocene buried humic soil in southern Kyushu.
et al., 2007), the Kamchatka volcano in Russia (Braitseva et al., The Unzen volcano area on Shimabara Peninsula (western
1993), and Cordillera de Talamanca in Costa Rica (Driese et al., Kyushu, SW Japan) also contains buried humic soils of late Pleis-
2007). In Japan, some areas near volcanoes located to the west of tocene (MIS 3-2) age that are rich in carbon, within a tephra-soil
the Kanto region in Central Japan have buried humic soils of late sequence (Inoue et al., 2006). The tephras of Unzen volcano have
Pleistocene age. Previous studies have examined the humic been described by Watanabe and Hoshizumi (1995), Hoshizumi
composition within the tephra-soil sequence containing late et al. (1999), and Xu et al. (2004), among others. Xu et al. (2004)
Pleistocene buried humic soils in the Kanto-Tokai regions of Japan reported that humic soil beneath a pyroclastic flow deposit yiel-
(Yoshida et al., 1978; Sakai et al., 1982; Tsutsuki and Kuwatsuka, ded a 14C age of 29,300  250 BP (2s; 14C calibrated ages of
33,300e34,600 cal BP), obtained from a borehole within the central
part of Unzen graben, on the east side of Unzen volcano. However,
* Corresponding author. Fax: þ81 92 583 8421. there seems to have been no examination of the origin of SOM in
E-mail address: yuinoue@mm.kyushu-u.ac.jp (Y. Inoue). the late Pleistocene buried humic soils of this area.

1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved.
doi:10.1016/j.quaint.2011.06.039
234 Y. Inoue et al. / Quaternary International 246 (2011) 233e238

The origin of SOM in the buried humic soils can be roughly presumed that the buried humic soil is similar to the surface humic
estimated from analyses of stable carbon isotopes (d13C). Such an soil in distribution. The sampling site (32 470 3500 N, 130190 1300 E;
approach is effective for identifying vegetation in terms of the Tateno-cho, Shimabara City, Nagasaki Prefecture) lies on the flat
proportion of C3 and C4 plants. In Japan, typical C3 plants are grass landform on the piedmont at an altitude of 280 m and is located
and forest plants such as Bambusoideae, Oryza sativa and trees, about 4 km NE of Unzen volcano (Fig. 1). The basement of the
while typical C4 plants are grass plants such as Miscanthus, Imper- sampling profile is the Ipponmatsu pyroclastic flow deposit from
ata, and Echinochloa. In addition, phytolith analysis enables the the Nodake, Unzen volcano (Watanabe and Hoshizumi, 1995). Data
identification of some plant genera or species for investigating the from the nearby Shimabara weather station, at Shimabara City,
origin of SOM. Previous studies have attempted paleoenvir- Nagasaki Prefecture (32 44.50 N, 130 22.40 E), indicate a temper-
onmental reconstructions based on combined analyses of stable ateehumid climate with an udic moisture regime (Soil Survey Staff,
carbon isotopes and phytoliths within soils (e.g., Fredlund and 2010). The mean annual temperature is w17.2  C and the mean
Tieszen, 1997; Kelly et al., 1998; Alexandre et al., 1999; Kerns annual rainfall is 2092 mm (based on monthly averages for the 29-
et al., 2001; Sedov et al., 2003; Smith and White, 2004; Inoue year period between 1977 and 2005; Japan Meteorological Agency,
et al., 2010). 2010). The site sampled by Inoue et al. (2006) (32 420 4000 N,
Very few attempts have focused on elucidation of the origin of 130170 5800 E; Arie-cho, Minami-Shimabara City, Nagasaki Prefec-
SOM in tephra-soil sequences containing buried humic soils of late ture), which is compared with the present site to the NE, lies on the
Pleistocene age. Therefore, the origin of SOM was examined in flat landform on the piedmont an altitude of 350 m and is located
buried humic soils around Unzen volcano, NE Shimabara Peninsula, about 5 km SE of Unzen volcano. The basement of the sampling
SW Japan. The natural abundance of d13C within SOM in each layer profile is Tawaraishi debris avalanche deposits from the Nodake,
was examined, and using the same samples, the nature of the Unzen volcano (Watanabe and Hoshizumi, 1995).
vegetation was analyzed based on the phytolith composition. The
results were compared with those published previously for the SE 2.2. Sampling profile
Shimabara Peninsula (Inoue et al., 2006). The results of this study
will contribute to paleoenvironmental reconstruction. The tephra-soil sequence is about 5 m thick; its stratigraphy and
age data for the interlayered tephra are shown in Fig. 2. The
2. Materials and methods sequence is dominated by interlayered tephras derived from Unzen
Volcano. The names of the tephras are based on Watanabe and
2.1. Sampling site Hoshizumi (1995) and Hoshizumi (2005), and the ages are based
on Okuno (2002), Kobayashi and Kato (1986), and Kobayashi and
Fig. 1 shows the location of the sampling site and the distribu- Nakada (1991). Soil descriptions followed the Guidelines for Soil
tion of the surface humic soil on the Shimabara Peninsula. It is Description, 4th edition (FAO, 2006). Forty samples were collected

Fig. 1. Map showing the location of the sampling site and the distribution of surface humic soils on the Shimabara Peninsula (modified from the following soil maps, Scale 1:50,000,
Fundamental Land Classification Survey: Shimabara/Arao (Matsuo, 1977), Kuchinotsu/Misumi (Matsuo and Ono, 1977), Hizen-Obama (Ono, 1973), and Isahaya (Waki et al., 1971).
The distribution of volcanoes (triangles) is based on Hoshizumi et al. (1999). Shading shows the distribution of humic soils. Circles indicate sampling points in the northeastern area
(the present study) and the southeastern area (Inoue et al., 2006). Fu ¼ Fugendake, Ma ¼ Mayuyama, No ¼ Nodake.
 Y. Inoue et al. / Quaternary International 246 (2011) 233e238 235

Crassinodi
Sasa etc.

Fig. 2. Vertical distribution of pH (H2O), total carbon content, d13C values of soil organic matter, and phytolith composition in the tephra-soil sequence of the NE Shimabara
Peninsula. Shading indicates buried humic soil horizons. Tephras: K-Ah ¼ Kikai-Akahoya tephra (erupted from Kikai caldera); AT ¼ Aira-Tn tephra (erupted from Aira caldera).
afa ¼ ash fall deposit, pfa ¼ pumice fall deposit, pfl ¼ pyroclastic flow deposit. *Okuno (2002), **Kobayashi and Nakada (1991), ***Kobayashi and Kato (1986).

at 10-cm intervals, sampling from each horizon within the 5-m- most recent volcanic eruption. The 2nd horizon (2Ab-2C)
thick sequence (Fig. 2). comprises the youngest buried humic soil (2Ab) within the
sequence and the Kikai-Akahoya (K-Ah; Machida and Arai, 1983)
2.3. Analytical methods afa (2C) of a widespread tephra erupted at 7.3 cal ka BP (Okuno,
2002) from Kikai caldera, located about 220 km south of the
The total carbon content, soil pH (H2O), d13C values of SOM, and study site (Fig. 1). The 3rd horizon (3Ab-3C) consists of a buried
phytolith composition of the 40 samples were examined. The pH humic soil (3Ab) and the Yuegawa afa (3C) of hornblende dacite,
(H2O) was measured using a glass electrode in a suspension with which is a block-and-ash flow (Hoshizumi et al., 1999) erupted at
a soil:deionized water ratio of 1:2.5. The isolation of phytoliths 16.5 cal ka BP (Kobayashi and Nakada, 1991) from Fugendake/
followed Sugiyama (2000), and Miyabuchi and Sugiyama (2006, Mayuyama (Fig. 1). The 4th horizon (4Ab-4C-4Bwb-4C0 ) consists of
2008, 2010). The total carbon content and d13C values were a buried humic soil (4Ab) and the Kureishibaru pyroclastic flow
measured using a Finnigan MAT 252 mass spectrometer housed at deposit (pfl) (4C) of hornblende dacite, which is a block-and-ash
the Faculty and Graduate School of Social and Cultural Studies, flow (Hoshizumi et al., 1999) erupted at 22.5 cal ka BP (Kobayashi
Kyushu University, Japan. The analytical method of the d13C fol- and Kato, 1986) from Fugendake/Mayuyama. The 4Bwb horizon
lowed Inoue et al. (2001). The percentages of C4 and C3 plant- and 4C0 horizon consist of a pyroclastic surge accompanied by the
derived carbon calculated from d13C values could be estimated Kureishibaru pfl. The pyroclastic surge pre-dates the Kureishibaru
using the formula by Inoue et al. (2010). A d13C value of
20& is pfl. The 5th horizon (5Ab-5Ab/C) consists of a buried humic soil
indicative of a 1:1 ratio between C4 and C3 plants. (5Ab) and the Kitasenbongi pfl (5Ab/C), which includes humic soils.
The 6th horizon (6Ab-6C) consists of a buried humic soil (6Ab) and
3. Results the Haraguchimachi afa (6C), which is a block-and-ash flow
(Hoshizumi et al., 1999) erupted from one of the Unzen volcanoes.
3.1. Morphological characteristics The 7th horizon (7Ab/C) consists of a buried humic soil the Aira-Tn
(AT) afa of a widespread tephra erupted at 29 cal ka BP (Okuno,
Table 1 lists the morphological characteristics of the tephra-soil 2002) from Aira caldera, located about 130 km SSE of the study
sequence. Nine horizons are recognized in the sequence. The site (Fig. 1). The 8th horizon (8Ab-8ABwb-8Bwb) consists of the
uppermost horizon (A-C) consists of Heisei ash fall deposits (afa) oldest buried humic soil (8Ab) within the sequence. The humic soil
erupted from Fugendake volcano (within the Unzen field) in AD developed before 30 cal ka BP (MIS 3). The 9th horizon (9C) consists
1991 (Watanabe and Hoshizumi, 1995; Hoshizumi et al., 1999). The of the Ipponmatsu pfl, which contains a high density of hornblende
degree of soil development is weak and the horizon shows no andesite blocks, and subordinate vesicular hornblende dacite and
structure or consistency, because little time has passed since the mixed andesiteedacite blocks (Hoshizumi et al., 1999) erupted at
236 Y. Inoue et al. / Quaternary International 246 (2011) 233e238

Table 1
Main morphological characteristics of the tephra-soil sequence in the NE Shimabara Peninsula.

Horizon Depth (cm) Moist color Field Structureb Consistencec Compactnessd Tephras name Eruption agee Samples
(Munsell) texturea (mm) number
A 0e10 7.5YR4/2 SL e e 6 1
C 10e20 7.5YR5/2 S e e 7 Heisei afa AD 1991 2
2Ab 20e50 7.5YR1.7/1 CL WE FI SB SSt VFR 14 3e5
2C 50e80 7.5YR4/4 CL WE FI SB SSt SP VFR 22 Kikai-Akahoya afa 7.3 cal ka BP* 6e8
3Ab 80e120 7.5YR1.7/1 LiC WE FI SB St P FR 21 9e12
3C 120e140 7.5YR2/3 LiC ST ME SB St P FI 23 Yuegawa afa 16.5 cal ka BP** 13e14
4Ab 140e160 7.5YR1.7/1 LiC WE ME SB SSt SP FR 23 15e16
4C 160e260 2.5Y5/1, 2.5Y8/6 gravel e e e Kureishibaru pfl 22.5 cal ka BP*** 17e18
4Bwb 260e270 7.5YR3/3 L WE ME SB SSt SP FR 28 19
4C0 270e280 7.5YR5/4 SiL WE FI SB SSt P VFR 28 Pyroclastic surge deposit accompanied with 20
Kureishibaru pfl
5Ab 280e300 10YR1.7/1 CL MO FI SB SSt SP FR 26 21e22
5A/C 300e320 10YR1.7/1 SiL MO FI SB SSt SP VFR 24 Kitasenbongi pfl e 23e24
6Ab 320e330 10YR1.7/1 LiC WE FI SB SSt SP FR 27 25
6C 330e360 10YR4/2 LiC WE FI SB SSt P FR 27 Haraguchimachi afa e 26e28
7Ab/C 360e390 7.5YR2/2 LiC MO ME SB St P FR 27 included AT afa 29 cal ka BP* 29e31
8Ab 390e410 7.5YR1.7/1 LiC MO ME SB St P FI 23 32e33
8ABwb 410e430 7.5YR2/3 LiC MO ME SB St P FI 24 34
8Bwb 430e480 7.5YR4/4 LiC ST ME SB VSt VP FI 23 35e39
9C 480e530þ 10YR4/6 S e e 21 Ipponmatsu pfl e 40
a
CL ¼ clay loam, L ¼ loam, LiC ¼ light clay, SiL ¼ silty loam, SL ¼ sandy loam, S ¼ sand.
b
WE ¼ weak, MO ¼ moderate, ST ¼ strong, FI ¼ fine, ME ¼ medium, SB ¼ subangular blocky.
c
SSt ¼ slightly sticky, St ¼ sticky, VSt ¼ very sticky, SP ¼ slightly plastic, P ¼ plastic, VP ¼ very plastic, VFR ¼ very friable, FR ¼ friable, FI ¼ firm.
d
Yamanaka hardness meter.
e
Okuno, (2002)*, Kobayashi and Kato (1986)** and Kobayashi and Nakada (1991)***.

71  11 or 82  9 ka (K-Ar ages) from Nodake (Fig. 1) within the the buried humic soil horizons of the present study. Only the 6Ab
Unzen volcanoes (Hoshizumi, 2005). horizon has a notable component of C4 plants. The top horizon,
which contains the Heisei afa (C horizon), contains the highest
3.2. pH (H2O) percentage of C3 plant-derived carbon (97.6%), indicating the effect
of C3 plants growing within thicket vegetation. The upper part of
The pH (H2O) values tend to be lower in the upper horizons than the Haraguchimachi afa (6C horizon) contains the highest
in the lower horizons, ranging from 4.0 to 5.4 (Fig. 2). In particular, percentage of C4 plant-derived carbon (60.8%). The percentage of
the top horizon, which includes the Heisei afa, is strongly acidic, C3 plant-derived carbon of the oldest buried humic soil horizon
with pH (H2O) ¼ 4.0. The upper and lower horizons in the Kur- (8Ab) indicated 63.3e69.0%; C3 plant-dominated vegetation is the
eishibaru pfl (4C horizon) are divided by a threshold pH value of 5. majority.
The pH (H2O) values are lower than the threshold of pH 9 which
makes a phytolith dissolve drastically (Kondo, 2005). 3.5. Phytolith composition

3.3. Total carbon content Fig. 2 shows the vertical distribution of phytolith composition.
Many phytoliths are found in soil horizons 2Ab, 3Ab, 5Ab, 7Ab/C,
The buried humic soil horizons (Ab horizons) have a high total and 8Ab. The main phytolith types in the buried humic horizons are
carbon content: 116 g kg
1 (maximum value) in the 2Ab horizon, Pleioblastus sect. Nezasa in A and 2Ab, and Sasa sect. Crassinodi in
85 g kg
1 in the 3Ab horizon, 34 g kg
1 (minimum value) in the 4Ab 3Ab, 4Ab, 5Ab, 6Ab, 7Ab/C, and 8Ab. Most of the phytoliths are from
horizon, 62 g kg
1 in the 5Ab horizon, 38 g kg
1 in the 6Ab horizon, Gramineae.
and 68 g kg
1 in the 8Ab horizon. The 8Ab horizon below AT tephra A few phytoliths of arboreal type were counted in the samples
(ca.29 cal ka BP eruption) is the oldest buried humic soil in the from the 3Ab horizon, with Lauraceae type>Quercus subgen.
studied sequence, older than 30 ka (MIS 3). Cyclobalanopsis type<Distylium type (not including ‘others’). Only
a few phytoliths of arboreal type (’others’) were observed in the
3.4. Natural abundance of stable carbon isotopes (d13C) 5Ab, 5A/C, 6Ab, 7Ab/C, 8Ab, 8ABwb, and the upper part of the 8Bwb
horizons.
The mean d13C values for the buried humic soil horizons (Ab
horizons) are as follows:
22.3& for the 2Ab horizon,
21.1& for 4. Discussion
the 3Ab horizon,
21.0& for the 4Ab and 5Ab horizons,
18.9& for
the 6Ab horizon, and
20.5& for the 8Ab horizon. The lowest d13C 4.1. Origin of soil organic matter in the buried humic soil
value (
26.7&) was obtained for the top horizon, which contains
the Heisei afa (C horizon). The highest d13C value (
18.5&) was In a borehole within the central part of Unzen graben, on the
found for the upper part of the Haraguchimachi afa (6C horizon). east side of Unzen volcano, Xu et al. (2004) reported that humic soil
The d13C values of C3 and C4 plants are generally distinct, beneath a pyroclastic flow deposit yielded a 14C age of
reflecting their photosynthetic pathways (Smith and Epstein, 1971). 29,300  250 BP (2s; 14C calibrated ages of 33,300e34,600 cal BP).
Accordingly, the mean d13C values for C3 and C4 plants are
27& This finding is consistent with the age of the base of the buried
and
13&, respectively (Boutton, 1996). Based on these values, humic soil horizon below Aira-Tn tephra (ca.29 ka eruption)
C3 plants are the dominant plant type in the source of SOM within examined in the present study. The 8Ab horizon is the oldest buried
 Y. Inoue et al. / Quaternary International 246 (2011) 233e238 237

Crassinodi
Sasa etc.
Fig. 3. Vertical distribution of pH (H2O), organic carbon content, and phytolith composition in the tephra-soil sequence within the SE Shimabara Peninsula (from Inoue et al., 2006).
Tw ¼ Tawaraishi debris avalanche deposit.

humic soil in the studied sequence, older than 30 ka (MIS 3). Among (except the 4Bwb horizon) above the oldest buried humic soil
tephra-soil sequences worldwide, it is rare that the oldest buried horizon (8Ab). Inoue et al. (2006) proposed that the main parent
humic soil in MIS 3 has a high carbon content (>60 g kg
1), materials of the humic soil were re-deposited sediments on bare
although there are some precedents in Ashitaka volcano, Central land around the volcanic body, or fine-grained tephra deposits that
Japan. For buried humic soils of MIS3 near the Ashitaka volcano, erupted from Unzen volcano. Sase et al. (1993) estimated that
Sakai et al. (1982), Tsutsuki and Kuwatsuka (1989), and Sase et al. humic soils were formed when aeolian dust composed mainly from
(2006) reported total carbon contents of up to 94 and 55 g kg
1, tephras had been accumulating slowly under grasses and forest
respectively. vegetation in the Towada volcano area, Tohoku region, NE Japan.
C3 plants are the dominant plant type in the source of SOM Moreover, in Iwate volcano area, Tohoku region, NE Japan, the long-
within the oldest buried humic soil horizon (8Ab) of the present term deposition of aeolian dust transported from the Asian conti-
study (Fig. 2). Moreover, the dominant phytolith in the oldest nent has contributed to pedogenesis, especially during the cold
buried humic soil horizon (8Ab) is Sasa sect. Crassinodi, which stages of the Last Glacial period (Inoue and Sase, 1996). Thus, the
implies Sasa gracillima, although these Sasa cannot be distinguished main parent materials during pedogenesis were re-deposited
visually. Sasa gracillima is endemic to Japan, locally known as tephra and aeolian dust, with the tephra deposits in the humic
“Unzen-zasa”, and is currently widely distributed in the area of soils. The humic soils formed as a mixture from an abundant supply
Unzen volcano. Rare arboreal-type phytoliths are found in the 8Ab of organic matter from mainly grass vegetation such as Sasa sect.
horizon. In general, arboreal plants produce few phytoliths; e.g., Crassinodi and concomitant ongoing deposition of small amounts
coniferous leaves generally contain <0.5% phytoliths, compared of parent materials derived from tephras, re-deposited tephra and
with w3% for grass (Klein and Geis, 1978). The present findings aeolian dust (i.e. the soils formed via progressive upbuilding
raise the possibility that the vibrant growth of S. gracillima pedogenesis), regardless of climate change at w30 ka or earlier in
community with sparse trees was the main source of SOM in the the Unzen area.
oldest buried humic soil horizon, and possibly in other horizons.
At the southeastern site on Unzen volcano (Fig. 1), humic soil
5. Conclusions
developed continuously, because the humic soil horizons are not
buried by tephras produced during the period from ca.
In the late Pleistocene, buried humic soils are uncommon in the
30e7.3 cal ka BP (Inoue et al., 2006). Phytoliths within horizons at the
world. The total carbon contents were very high for late Pleistocene
southeastern site (Fig. 3) are dominated by Sasa sect. Crassinodi (Inoue
soils, with a value of >60 g kg
1 obtained for the oldest buried
et al., 2006), as at the northeastern site (Fig. 2). This result indicates
humic soil (8Ab horizon) within a tephra-soil sequence near Unzen
that the main source of SOM at the southeastern site was the same as
volcano. The main phytolith types in the buried humic soils are
that at the northeastern site. Therefore, the source of SOM around
Pleioblastus sect. Nezasa in A and 2Ab horizons, and Sasa sect.
Unzen volcano was dominated by Sasa sect. Crassinodi (probably
Crassinodi in 3Ab, 4Ab, 5Ab, 6Ab, 7Ab/C, and 8Ab horizons. The soil
S. gracillima), regardless of climate change at w30 ka or earlier.
organic matter (SOM) was derived mainly from Gramineae. In the
oldest late Pleistocene buried humic soil (8Ab horizon), the d13C
4.2. Estimation of pedogenesis in the buried humic soil values indicate that C3 plants contributed more than 50% of the
plant-derived carbon in SOM, and the phytoliths indicate the
Pedogenesis was successive at the southeastern site because the dominance of Sasa sect. Crassinodi (C3 grass) type at sampling
period of soil formation did not coincide with a large-scale eruption points on the NE side of the volcano (the present study) and on the
(Inoue et al., 2006). The humic soil horizon developed successively SE side (a previous study). The source of the SOM within late
at the northeastern site, as thick tephras were not deposited. Pleistocene buried humic soils is likely to be mainly Sasa sect.
Supporting the interpretation of the successive humic soil forma- Crassinodi. The late Pleistocene humic soils formed due to an
tion in the northeastern site is the general absence of B horizons abundant supply of organic matter from mainly grass vegetation,
238 Y. Inoue et al. / Quaternary International 246 (2011) 233e238

regardless of climate change at w30 ka or earlier in the Unzen Matsuo, T., 1977. Scale 1:50,000 Fundamental Land Classification Survey. Soil Map
“Shimabara/Arao”. Nagasaki Prefecture. Fuji Microservice center Co Ltd,
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Kumamoto City, Japan.
Matsuo, T., Ono, S., 1977. Scale 1:50,000 Fundamental Land Classification Survey.
Soil Map “Kuchinotsu/Misumi”. Nagasaki Prefecture. Fuji Microservice center
Acknowledgements Co Ltd, Kumamoto City, Japan.
Miyabuchi, Y., Sugiyama, S., 2010. Phytolith and macroscopic charcoal analysis of
the Senchomuta drilling core in Asodani valley, northern part of Aso caldera,
We deeply thank Professor David J. Lowe of the University of
Japan. Journal of Geography 119, 17e32 (in Japanese with English abstract).
Waikato, New Zealand, for his valuable help in improving this Miyabuchi, Y., Sugiyama, S., 2006. A 30,000-year phytolith record of a tephra
paper. We thank Prof. Shin-ichiro Wada, Prof. Yuko Koike, Dr. Yuki sequence, east of Aso caldera, southwestern Japan. The Quaternary Research
(Daiyonki-Kenkyu) 45, 15e28 (in Japanese with English abstract).
Mori, and Dr. Ame Garong of Kyushu University for performing
Miyabuchi, Y., Sugiyama, S., 2008. A 30,000-year phytolith record of a tephra
analyses of pH and d13C. This study was supported in part by sequence at the southwestern foot of Aso volcano, Japan. Journal of Geography
a Grant-in-Aid for Scientific Research (C) (Project Leader, Y. Inoue; 117, 704e717 (in Japanese with English abstract).
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last 30,000 years. Quaternary Research (Daiyonki-Kenkyu) 41, 225e236 (in
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