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Vegetation changes after the late period of the Last Glacial Age based on pollen analysis of the northern area of Aso Caldera in central Kyushu, Southwest Japan
Quaternary International 254 (2012) 107e117
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Vegetation changes after the late period of the Last Glacial Age based on pollen analysis of the northern area of Aso Caldera in central Kyushu, Southwest Japan Yoshitaka Hase a, *, Akiko Iwauchi b, Utako Uchikoshiyama c, Eri Noguchi d, Naoko Sasaki e a
Goshoura Cretaceous Museum, Goshoura 4310-5, Goshoura Town, Amakusa City, Kumamoto Prefecture 866-0313, Japan Avance Co. Ltd., Ezu 1-3-48, Kumamoto City, Kumamoto Prefecture 862-0942, Japan c Kumamoto Prefectural Government, Culture Promotion Division, Suizenji 6-18-1, Kumamoto City, Kumamoto Prefecture 862-8570, Japan d Echo Electronic Industry Co. Ltd., Higashihie 3-1-2, Hakata-ku, Fukuoka City, Fukuoka 812-0007, Japan e Research Institute for Humanity and Nature, Kamogamo-motoyama 457-4, Kita-ku, Kyoto City, Kyoto-fu 603-8047, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 21 January 2011
Within the Aso Caldera in central Kyushu, Japan, the Asodani valley is located in the northern part of the inner area, encircled by a wide caldera rim plain consisting largely of grassland. Pollen analyses and radiocarbon dating of samples from the Senchomuta core of the upper part of the basal sediments in the inner area revealed a change in vegetation after around 14,000 cal BP. According to the analytic results from the Hosenbashi core, the forest was mainly formed under cool-temperate to sub-arctic conditions at about 24,000e17,000 cal BP, followed by a change to temperate conditions with predominance of deciduous trees, and subsequently followed by warm-temperate conditions to the present consisting largely of evergreen trees. Herb pollen abundance indicated that the grassland on the broad caldera rim plain 300 to 500 m above Asodani developed at the Last Glacial Age. During the climatic warming of the Post Glacial Age, the grassland basically continued with trees until just after the K-Ah ash layer (7280 cal BP) was deposited. At about 6500 to 5000 cal BP, the herb pollen rate decreased, and the grassland changed to some scattered low-density forest vegetation composed of pine, deciduous and/or evergreen Quercus, and willow. After that period, it is suspected that the grassland was restored by human activity. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Japan is a long, narrow archipelago lying in a northeast to southwest orientation, flanked by the Pacific Ocean to the east and the Japan Sea to the west (Fig. 1, A). The Aso area is located in the central part of Kyushu in Southwest Japan (Fig. 1, B). The Aso area has a caldera (Matumoto, 1933) formed by a depression after several large eruptions. The caldera rim has a diameter of 18 km from east to west and 25 km from north to south, and is mainly composed of three parts: the depression floor, the caldera rim plain and the central cones of Aso volcano. The study area is situated in the northern part of Aso Caldera (Fig. 1, C). The northern part of the caldera is divided into a wide valley named Asodani (Aso valley), which was depressed after the fourth huge eruption (Aso-4 pyroclastic flow) of the Aso Caldera activity about
* Corresponding author. Fax: þ81 96 383 5421. E-mail addresses: [email protected] (Y. Hase), [email protected] (A. Iwauchi), [email protected] (E. Noguchi), [email protected] (N. Sasaki). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.01.018
90,000 years ago (Matsumoto et al., 1991), and a broad caldera rim plain, separated by the steep cliffs of the caldera rim between 350 and 500 m above Asodani. The caldera rim plain radiates from the Aso Caldera rim, and consists of wide, flat grassland with many recently formed ravines. The study sites in Asodani are located at Senchomuta on the north side of the Kurokawa river in Mikubo, and at Ryozanbashi bridge on an alluvial fan in Ichinomiya town, Aso city, as well as three sites on the caldera rim plain: Sanno on the northeast, Zogahana on the north and Daikanbo at the entrance road to the Daikanbo viewpoint (Fig. 2). The strata in Asodani mainly consists of silt, sand and gravel deposited in lake, river, swamp and alluvial fan settings, and the intercalated vitric volcanic Kikai-Akahoya ash layer, commonly known as K-Ah (Machida and Arai, 1978). The stratified sediments on the caldera rim plain consist of yellowish to reddish brown ashy soil and black ashy Kuroboku humus soil in ascending order, with the K-Ah ash layer intercalated. There are four climatic and vegetation zones from north to south in Japan: sub-arctic, cool-temperate, warm-temperate and subtropic. Kira (1949) described the ‘Warmth-Index’ which correlates vegetation to temperature. The lowland area of Kyushu mainly
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too coarse for pollen analysis. This study describes pollen assemblages based on samples from the Senchomuta and Ryozanbashi drill cores in Asodani at an altitude of 450e500 m, as well as samples from the Sanno and Zogahana hand-drill cores, and an outcrop of the Daikanbo entrance road on the caldera rim plain at an altitude of 750e890 m (Fig. 2). The aim is to clarify and discuss the vegetation changes in the inner area of the caldera and on the caldera rim plain after the late period of the Last Glacial Age, combining data from this study with that of Iwauchi and Hase (1992). This study will verify the vegetation changes in the Aso Caldera area, and give evidence to suggest that human activity is implicated in the extended existence of grassland on the caldera rim plain. 2. Material and methods
Fig. 1. Map showing Japan (A), the Aso area in Central Kyushu (B) and the study area in the Aso Caldera (C).
belongs to the warm-temperate zone, having a Warmth-Index of 85e180. At the Aso-Otohime observation point (Automated Meteorological Data Acquisition System) at an altitude of 497 m in Asodani, the mean annual temperature observed is about 13 C, and the Warmth-Index is estimated at about 97 which suggests the warm-temperate zone. The caldera rim plain with its upper height at about 800 m in altitude has an estimated mean annual temperature of about 10 C with a Warmth-Index estimated at 80, placing the area in the cool-temperate zone. According to Ito (1981), the potential natural vegetation in the Aso Caldera area mainly belongs to the warm-temperate zone and is characterized by the forest on Kitamuki Mountain, located at the west part of the caldera rim at an altitude of 300e800 m, mainly covered by evergreen forest composed of oak and horse-chestnut with some deciduous species such as Japanese zelkova, Alangium and Helwingia. On the walls of the caldera around Asodani, artificial forests of Japanese cedar (Cryptomeria japonica) and cypress (Chamaecyparis obtuse) exist among natural evergreen forest, including pine. The broad grassland of the caldera rim plain mainly consists of eulalis (Miscanthus sinensis) and low bamboo (Pleioblastus chino). Iwauchi and Hase (1992) previously mentioned the vegetation change in Asodani based on pollen analysis of samples from the Hosenbashi core at Uchinomaki, but detailed change after ca. 9300 cal BP was not sufficiently explained, because sediments were
The samples were taken from machine drill and hand-drill cores at the sites shown in Fig. 2. The cores were mainly composed of silt, fine sand and tuffaceous fine material, including medium to coarse sand and gravel deposited after the late period of the Last Glacial Age (Hase et al., 2010) in Asodani, and ashy soil and Kuroboku on the caldera rim plain (Fig. 3). Columnar sections of the cores were correlated using the horizon of the K-Ah ash layer. Some samples were dated by radiocarbon age determination. In this study, pollen analyses were performed on 32 samples from the 25 m long Senchomuta core and 16 samples from the 34 m long Ryozanbashi core taken from Asodani, and 32 samples from the 7.3 m long Sanno core, and 24 samples from the 6.3 m long Zogahana core. Eight samples from a 1.1 m thick outcrop on the Daikanbo entrance road were also studied. Samples weighing 0.5e1.5 g in a dry condition were analyzed by the HFeKOHeAcetolysis method (Nakamura, 1976) with a microsphere count of 40,000 grains/cm3, and prepared on two slides per sample with glycerin jelly for observation under a microscope with a magnification of 600. Pollen grains and spores were observed and counted in all areas of each slide. If counted pollen on the first slide was less than 100 grains, the duplicate slide was used. The percentage of pollen grains of each taxon to the sum count of arboreal pollen grains was calculated. Samples with a count of less than 100 grains are shown by open lines in the diagrams (Figs. 5, 6 and 7). Table 1 shows the age determination of each core by the radiocarbon method and the horizon of the K-Ah ash layer. The age of a horizon was estimated by linear interpolation from dated positions in the core. 3. Results 3.1. Vegetation change in Asodani based on pollen analyses of samples from the Senchomuta and Ryozanbashi drill cores 3.1.1. Senchomuta core Miyabuchi et al. (2010) and Hase et al. (2010) observed the sedimentary facies of core samples at the Senchomuta drill site. Samples for pollen analysis were taken at intervals of 40 and/or 80 cm and consisted of silt and fine sand, often including organic matter. The resulting pollen analysis of the samples is shown in a pollen diagram with four local pollen assemblage zones in ascending order (Fig. 4). Pollen zone Se-A, LepidobalanuseUlmus/ZelkovaeCeltis zone, ca.14,000e8500 cal BP, was mainly composed of deciduous oak accompanied by Ulmus/Zelkova, Celtis/Aphanante and Castanea/ Castanopsis. Cyclobalanopsis gradually increased in the upper part of the zone. Poaceae and Artemisia pollen were evident. This type of forest is categorized as belonging to the lower part of the cooltemperate zone.
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Fig. 2. Locality map of the study area with the sites of drill cores. The Hosenbashi site indicates the Hosenbashi core in Iwauchi and Hase (1992).
Pollen zone Se-B, PodocarpuseCyclobalanopsiseUlmus/ZelkovaeCeltis zone, 8500e5500 BP was composed of evergreen oak, increasing in the upper part. Lepidobalanus existed, while Ulmus/ Zelkova and Celtis/Aphanante were gradually reduced in number. This was categorized as a warm-temperate forest. Pollen zone Se-C, CyclobalanopsiseCastanea/Castanopsis zone, 5500e1800 cal BP, included abundant Cyclobalanposis and Castanea/Castanopsis. Therefore, this forest was also categorized as warm-temperate. Poaceae and Cyperaceae increased in this zone. In Pollen zone Se-D, PinuseCryptomeriaeCyclobalanopsiseCastanea/ Castanopsis zone, 1800 cal BP e present, coniferous Pinus and Cryptomeria characteristically increased in the upper part. It is assumed that human activity affected the forests at this time. Poaceae and Cyperaceae, along with Artemisia pollen grains markedly increased in this zone. 3.1.2. Ryozanbashi core The Ryozanbashi core is composed of coarse-grained materials (coarse sand to gravel) that were intercalated with fine-grained layers. Samples composed of fine sand, silt and clay with organic matter were taken from the core at irregular intervals.
The pollen diagram is displayed in Fig. 5. More than half of slides have counts of less than 100 grains of pollen, but it is assumed that the vegetation on the site was mainly composed of Pinus, Lepidobalanus, Cyclobalanopsis, Castanea/Castanopsis, Alnus of the arboreal variety, and Poaceae, Artemisia and other Compositae representing nonarboreal pollen with fern spores. The pollen composition shows vegetation of a warm-temperate forest. However, this represents a low density of trees, accompanied by some grass areas on the alluvial fan.
3.2. Vegetation change on the rim plain based on pollen analyses Pollen analyses of samples from two hand-drill sites at Sanno and Zogahana, and an outcrop of the entrance road to the Daikanbo viewpoint on the caldera rim plain, are described below. 3.2.1. Sanno core Based on pollen analysis of the samples from the Sanno handdrill core at intervals of 20 cm, four local pollen assemblage zones were recognized, in ascending order (Fig. 6).
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Fig. 3. Columnar sections of the drill cores correlated by the horizon of the K-Ah ash layer.
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In Pollen zone Sn-1, LipidobalanuseArtemisiaePoaceae zone, ca. 14,000e12,000 cal BP, Lipidobalanus represented 10e15% of the total arboreal pollen grains. Artemisia and Poaceae concentrations were more than 100% of the total arboreal pollen grains. In Pollen zone Sn-2, PinuseLepidobalanuseSalix zone, 12,000e 7500 cal BP, each prepared slide included a small amount of pollen less than 50 grains. It is assumed that the arboreal vegetation roughly consisted of pine, deciduous and/or evergreen oak and willow on the grassland. In Pollen zone Sn-3, LepidobalanuseCyclobalanopsiseCastanea/ Castanopsis-Salix zone, ca. 7500e5000 cal BP, Lipidobalanus was dominant and accompanied by Cyclobalanopsis. Castanea/Castanopsis and Salix were also dominant in the upper half of the zone. Pinus and Tsuga appeared in smaller amounts. Artemisia concentrations were more than 60% of the total arboreal pollen, with other Compositae and Poaceae in lesser amounts. In Pollen zone Sn-4, PinuseCryptomeriaeLepidobalanus zone, 5000 e present, Pinus mainly occurred as arboreal pollen with Lepidobalanus, Cyclobalanopsis, Castanea/Castanopsis and Salix. Cryptomeria slightly increased toward the top. In the lowermost part of the zone, arboreal pollen grains represented 90%, decreasing gradually to 10% at the uppermost horizon of the samples. Artemisia, Poaceae and other Compositae were dominant in occurrence. 3.2.2. Zogahana core The samples were irregularly taken at intervals of 10 cm from the top to 70 cm depth, and at intervals of 20 cm from 70 cm to 4 m depth. The K-Ah ash layer was present at 2.5 m in depth. The formation was divided into lower and upper parts. The lower part
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was tuff and tuffaceous material, and the upper half was Kuroboku soil above 3.3 m depth. The pollen diagram of the Zogahana hand-drilling core samples was characterized by an excellent count of herbaceous grains such as Poaceae, Artemisia and other Compositae with a high percent occurrence of Pinus (Fig. 7). Three local pollen assemblage zones are recognized. Pollen zone Zh-1, PinuseCryptomeriaeLepidobalanus-Cyclobalanopsis zone, ca. 10,000e7500 cal BP, is mainly composed of Conifer pollen such as Pinus, Cryptomeria and Abies, and deciduous and evergreen Quercus pollen. Herbaceous pollen such as Poaceae and Compositae including Artemisia are abundant. Pollen zone Zh-2, PinuseCyclobalanopsiseSalix zone, 7500e5000 cal BP, is mainly composed of Pinus, Cyclobalanopsis and Salix arboreal pollen. This zone is characterized by a decrease of herbaceous pollen, especially Poaceae and Artemisia. Pollen zone Zh-3, PinuseLepidobalanuseCyclobalanopsis zone, 5000 cal BP e present, shows an increase of coniferous pollen, mainly composed of Pinus, and Quercus. Lepidobalanus and Cyclobalanopsis existed with relatively equal counts. The Zh-3 zone is characterized by a large amount of herbaceous pollen, Poaceae and Compositae including Artemisia. 3.2.3. Daikanbo outcrop The Kuroboku soil crops out at the west side of the entrance road to the Daikanbo viewpoint. The K-Ah layer was included in the lower part at 80 cm depth in the outcrop. A pollen diagram was characterized by many counts of Lepidobalanus and Cyclobalanopsis with Castanea/Castanopsis. Importantly, herbaceous pollen of
Fig. 4. Pollen diagram of samples from the Senchomuta core.
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Fig. 5. Pollen diagram of samples from the Ryozanbashi core.
Poaceae and Artemisia decreased in the middle part of the column, and the upper two horizons of the diagram included many counts of Pinus and Cryptomeria (Fig. 8). 4. Discussion 4.1. Stratigraphic correlation among the core samples 4.1.1. Hosenbashi, Senchomuta and Ryozanbashi cores in Asodani The Hosenbashi core was not dated in the work of Iwauchi and Hase (1992). In this study, the five samples from the core were dated by the radiocarbon dating method (Table 1). Samples at 69.1 m and 16.3 m in depth respectively indicated 23,770e22,930
and 9540e9450 cal BP. This indicates a deposition of the former sample during the Last Glacial Age, while the latter sample was deposited before the setting of the K-Ah ash layer. The K-Ah ash layer was not observed in the Hosenbashi core, but this horizon is radiocarbon dated in the correlative Senchomuta core (Fig. 3 in Hase et al., 2010). The Senchomuta core has more fine material than the Hosenbashi core after about 9000 cal BP. The Ryozanbashi core intercalates the K-Ah ash layer at 13.7 m in depth and also correlates to the Senchomuta core (Fig. 3). 4.1.2. Sanno and Zogahana cores and Daikanbo outcrop The base of the Sanno core originated from the Aso-4 pyroclastic flow deposit, about 90,000 a. The basal part of dark gray silt layer was
Fig. 6. Pollen diagram of samples from the Sanno core.
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Fig. 7. Pollen diagram of samples from the Zogahana core.
dated at 14,250e13,860 cal BP by the radiocarbon method (Table 1). The K-Ah ash layer was not recognized, but it is assumed that a horizon of the layer existed at nearly 2.7 m in depth in the lower part of the Kuroboku, because it was usually intercalated in the lower horizon of Kuroboku on the Aso Caldera rim plain. A sample of 3.5 m in depth of the Sanno core was dated at 2260 40 BP (Table 1), but it drastically differed from other data collected and is suspect and therefore omitted. The Zogahana core and Daikanbo outcrop can be correlated using the K-Ah ash layer (Fig. 3). 4.1.3. Asodani and the caldera rim plain There are different sedimentary environments between the inner floor area and the outer rim plain of the caldera. In the inner floor area, sedimentary environments included lake, river and swamp on the west side, and an alluvial fan with river and swamp conditions on the east side. On the outer rim plain, sedimentary conditions were usually aeolian, with occasional short rivers and small swamps. The
inner floor area and the outer rim plain were correlated using the KAh ash layer and some radiocarbon data (Fig. 3). 4.2. Vegetation changes in the Aso Caldera area after the late period of the Last Glacial Age 4.2.1. Asodani and surrounding area The upper part of the Hosenbashi drill core indicated river and/ or swamp conditions, due to its composition of coarse sand and granules, and sparse pollen grains. The samples from the Senchomuta core, correlated to the upper part of the Hosenbashi core (Hase et al., 2010), were mainly composed of fine materials deposited in a small lake existing on the outer side of a riverbank of the Kurokawa River, including pollen grains that were in abundance. The age of a sample of 69.1 m in depth of the Hosenbashi core was 23,770e22,930 cal BP (Table 1). Forest transformation in the
Table 1 14 C Age of samples from the Hosenbashi and Senchomuta cores in Asodani and the Sanno and Zogahana cores on the caldera rim plain. Core site Hosenbashi
Depth of sample (m) d13C (permil)
16.3 48.0 56.4 62.6 69.1 Senchomuta 1.45 2.26 4.69 6.6(K-Ah) 12.25 22.97 Sanno 0.96e1.0 2.35e2.39 6.62e6.66 Zogahana 0.28e0.29 2.36e2.70(K-Ah) 3.27e3.28
14
C Age (BP) (conventional R.A.)a,b Calibrated (calendar) agec (cal BP) Lab code no.
20.5 23.5 29.6 27.7 29.3 22.3 23.4 31.3
8480 12,500 13,140 14,190 19,560 1760 3520 5290
40 50 80 140 90 40 40 50
22.3 24.5 18.4 20.7 24.0 18.6
7970 10,010 2450 2260 12,220 910
60 40 30 40 50 40
19.7
7440 50
9540e9450 15,060e14,210 16,530e15,240 17,690e16,880 23,770e22,930 1810e1560 3900e3690 6200e5930 7280 9010e8610 11,710e11,300 2550e2360 2270e2160 14,250e13,860 920e730 7280 8370e8170
GX-29942-AMS GX-29943-AMS GX-29944-AMS GX-29945-AMS GX-29946-AMS Bata-274696 Bata-274697 Bata-248317
Material and analyzer Source
Silt # Silt # Silt # Silt # Silt # Pollen þ Pollen þ Charcoal þ Ash Bata-246468 Organic sediment þ Bata-247887 Organic sediment þ GX-28949-AMS Humus soil # GX-28950-AMS Humus soil # GX-28951-AMS Humus soil # Bata-269119 Organic sediment þ Ash Bata-269120 Organic sediment þ
Analyzer: þ BETA ANALYTIC INC., USA; # GEOCRON LABORATORIES, USA. Source: (1) Hase et al. (2010); (2) Miyabuchi et al. (2010); (3) This paper; (4) Fukusawa (1995). a 14 C ages were analyzed based on the Libby’s 14C half life of 5568 and 5570 years by the BETA and GEOCRON laboratories, respectively. b Conventional 14C ages were calculated using d13C values. c 2 Sigma calibrated result: 95% probability.
(1)
(3) (2) (4) (2) (3)
(3) (4) (3)
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Fig. 8. Pollen diagram of samples from an outcrop at the entrance road to the Daikanbo viewpoint.
Asodani area at the late period of the Last Glacial Age started with a cool-temperate coniferous forest that was accompanied by cooltemperate deciduous broad-leaved forest (Iwauchi and Hase, 1992). Cool-temperate deciduous forest including birch and beech was transitional into warm-temperate deciduous forest including hackberry, clearly followed by evergreen broad-leaved
forests such as evergreen oak in the Post Glacial Age in the Senchomuta core. After 1800 cal BP, an extreme increase of Poaceae pollen seems to suggest rice growing. In the youngest part of the zone, large contents of Pinus and Cryptomeria pollen show that vegetation change may have been artificial due to human involvement.
Fig. 9. Correlation of pollen counts in samples from the Senchomuta, Ryozanbashi in Asodani, and Sanno and Zogahana cores and the Daikanbo outcrop on the caldera rim plain.
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Fig. 10. Arboreal pollen/Non-arboreal pollen in the pollen samples of the Senchomuta, Ryozanbashi, Sanno and Zogahana cores and the Daikanbo outcrop.
The Ryozanbashi diagram shows that more than half of samples include less than 100 pollen grains. The numbers of grains per 1 g are less than 5000 (Fig. 9) where the percentage of arboreal pollen is highest (Fig. 10). This clarifies that the small amount of pollen grains in the Ryozanbashi site differ from the Senchomuta site which was mainly deposited under lake and swamp conditions, while its arboreal pollen rate was similar to that of the Senchomuta site (Fig. 10). The Ryozanbashi site lies on an alluvial fan plain composed of coarse material such as gravel and coarse sand with silt layers (Hase et al., 2010). The pollen diagram characteristically shows that the vegetation is composed of pine, shrubs and herbaceous plants such as Artemisia and other Compositae with fern spores, and existed on an alluvial fan plain deposited during the Post Glacial Age. 4.2.2. Formation process of the grasslands on the Aso Caldera rim plain The numbers of pollen grains per 1g of the Sanno samples are small compared with those of the Zogahana samples (Fig. 9), but their diagrams depict similar pollen compositions and rates (Fig. 10). Therefore, vegetation changes on the caldera rim plain can be represented by the Sanno pollen diagram. The result of pollen analysis of samples from the Sanno site showed that the grassland already existed at 14,000 cal BP. Vegetation of the late period of the Last Glacial Age suggests that some grassy species invaded the Aso Caldera rim plain from northern Japan in the Last Glacial Age. The vegetation, composing an open, low-density forest, was represented by deciduous oak with evergreen oak, pine, and hemlock ca. 7500 to 5000 cal BP. The forest
was formed in the upper part of a warm-temperate zone during the warming period in the Post Glacial Age. The grassland continued until deposition of the K-Ah ash layer, and afterwards Poaceae and other herb pollen rates decreased from 6500 to 5000 cal BP. From that point, some scattered forests with pine and deciduous and/or evergreen oak appeared. After 5000 cal BP, herbaceous pollen substantially increased, and therefore it is assumed that the grassland was restored. The research of Miyabuchi and Sugiyama (2006) which was based on phytolith analysis in the east of Aso Caldera mentioned that the intervals of ca. 32e30 cal ka of the east of Aso Caldera was dominated by a Sasa grassland (mainly Sasa sect. Crassinodi), suggesting a cool and dry climate. However during 30e13.5 cal ka, which corresponds to the Last Glacial Maximum, the Sasa grassland declined due to violent volcanic activity of Aso Volcano. During the Holocene, a Miscanthus grassland continued consistently for more than 10,000 years. The long continuation of the Miscanthus grassland might be related to artificial burning. Miyabuchi and Sugiyama (2008) noted vegetation at the southwest foot of Aso Caldera before 31 cal ka dominated by phytoliths of Sasa (mainly Sasa sect. Crassinodi), a cool-temperature dwarf bamboo. However, during 31e13.5 cal ka, which corresponds to the Last Glacial Maximum, vegetation composed mainly of Sasa dwarf bamboo had declined slightly. During the Holocene, the Sasa grassland dominated consistently, but phytoliths of Pleioblastus (a warm-temperature dwarf bamboo) and arboreal pollen such as Distylium increased from 7300 years ago. The reason for the restoration of the grassland is assumed to be the burning of forests mentioned by Miyabuchi and Sugiyama
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Table 2 Vegetation changes after the late period of the Last Glacial Age in the Aso Caldera area, central Kyushu, Japan.
Geologic Age (cal yBP) 1800
5,000
Asodani
Caldera rim plain
Iwauchi & Hase(1992) This study (Pollen)
Miyabuchi et al. (2010) (Phytolith)
Se-D: Pinus-Cryptomria -Cyclobalanopsis -Castanea/Castanopsis Z.
Miscanthus grassland
Sn-4: Pinus-Cryptomeria
existed continuously
-Lepidobalanus Z.
Se-C: Cyclobalanopsis -Castanea/Castanopsis Z. 7.3-6.6 cal ka:
5,500 8,500
10,000 12,300
Se-B: Podocarpus -Cyclobalanopsis -Ulmus/Zelkova-Celtis Z. U-V and Se-A: Lepidobalanus-Ulmus/Zeikova -Celtis Z.
U-IV: Lepidobalanus -Fagus-Carpinus Z. 15,000 16,500
Sasa increased 11-9 cal Ka: Sasa (cool-temperature dwarf bamboo) grassland and/or forests
This study (Pollen)
Sn-3: Lepidobalanus -Cyclobalanopsis -Castanea/Castanopsis -Salix Z.
Southwest foot of Caldera Miyabuchi and Sugiyama (2006) (Phytolith)
East of Caldera Miyabuchi and Sugiyama (2006) (Phytolith) Holocene: Sasa grassland dominated
Holocene:
consistently
a Miscanthus grassland
Pleioblastus and arboreal
continued
pollen (Distylium) increased
Sn-2: Pinus -Lepidobalanus-Salix Z.
from 7,300 years ago
Sn-1: Lepidobalanus -Artemisia-Poaceae Z.
U-III: Lepidobalanus -Fagus-Carpinus Z.
U-II: Pinus-Lepidobalanus -Carpinus-Betula Z. 20,000
31-13.5 cal ka: vegetation composed 30-13.5 cal ka:
mainly of Sasa dwarf
Sasa grassland declined
bamboo had declined
U-I: Pinus-Picea-Abies -Tsuga Z.
slightly
24,000
25,000
30,000 32,000
(2006, 2008) and Miyabuchi et al. (2010). Some northern species of Honshu district, such as Lily-of-the-valley (Convallaria majalis L.), Iris (Iris ensata var. spontanea (Makino) Nakai), and Lobelia (Lobelia sessilifolia Lamb.) are reduced as relic species of the cool environment today (Ito, 1981). At this point, it is not clear whether the grassland existed in a natural state, however it is clear that the grassland is maintained at present by human burning. The pollen diagram of the Zogahana core is characterized by more than 10%Cryptomeria pollen rate in the lower to middle parts of the core. It is assumed that Japanese cedar grew on the caldera rim plain with pine and Quercus (deciduous and evergreen) forming some scattered open forests in the grassland before the K-Ah ash layer in the Holocene. The Kuroboku outcrop at the entrance road to the Daikanbo viewpoint is assumed to have been formed by aeolian activity. However, the Daikanbo pollen diagram indicates that the arboreal pollen has a higher percentage than that of both the Sanno and Zogahana sites (Fig. 10). It is assumed that arboreal pollen accumulation on the Daikanbo site, situated on the edge of the caldera rim, mainly originated by aeolian transport from the forest of the inner caldera. 5. Conclusion Vegetation change from cool-temperate forest to warmtemperate forest in the Asodani area during the late period of the
ca. 32-30 cal ka: dominated by a Sasa grassland (mainly Sasa sect. Crassinodi)
before 31 cal ka : dominated by of Sasa (mainly Sasa sect. Crassinodi)
Last Glacial Age and Post Glacial Age was recognized based on pollen analysis of samples from drill cores of the Hosenbashi, Senchomuta and Ryozanbashi sites. Vegetation change in the Uchinomaki Formation based on the Hosenbashi core (Iwauchi and Hase, 1992) and the Senchomuta core is summarized as follows in ascending order (Table 2). Pollen zone U-1: PinusePiceaeAbieseTsuga zone. ca. 24,000e 20,000 cal BP Sub-arctic coniferous forests, consisting of Picea, Abies, Tsuga and Pinus, widely covered the mountain slopes, accompanied by some deciduous broad-leaved trees, such as Betula, Lepidobalanus and Carpinus. Pollen zone U-II: PinuseLepidobalanuseCarpinus-Betula zone. 20,000e16,500 cal BP Cool-temperate deciduous forests developed; however, the coniferous forests were gradually reduced in the area. Pollen zone U-III: LepidobalanuseFaguseCarpinus-Betula zone. 16,500e15,000 cal BP Cool-temperate deciduous forests predominated in this period, accompanied by a few sub-arctic conifers and Betula trees.
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Pollen zone U-IV: LepidobalanuseFaguseCarpinus zone. 15,000e 12,300 cal BP Cool-temperate forest continued to be widely distributed; however, sub-arctic elements did not appear any longer. In addition, in the later part of this period, the cool-temperate deciduous forests were slightly reduced in area. Pollen zone U-V and Se-A: LepidobalanuseUlmus/ZelkovaeCeltis zone. 12,300e8500 cal BP Warm-temperate deciduous broad-leaved forests, mainly composed of Lepidobalanus, Celtis and Ulmus/Zelkova, were widespread on the mountain slopes; in contrast, the cool-temperate deciduous forests were rapidly reduced in this period. Pollen zone Se-B: PodocarpuseCyclobalanopsiseUlmus/ZelkovaCeltis zone. 8500e5500 cal BP Warm-temperate deciduous forests, except for Ulmus/Zelkova and Celtis, were gradually reduced in this period; instead, the warm-temperate evergreen broad-leaved forests, mainly consisting of Podocarpus, Cyclobalanopsis and Castanopsis began to develop on the rim slopes. Pollen zone Se-C: Cyclobalanopsis-Castanea/Castanopsis zone. 5500e1800 cal BP Warm-temperate forest was composed of abundant Cyclobalanposis and Castanea/Castanopsis. Poaceae and Cyperaceae increased in this zone. Pollen zone Se-D: PinuseCryptomeriaeCyclobalanopsis-Castanea/Castanopsis zone. 1800 cal BP-present Coniferous trees of Pinus and Cryptomeria characteristically increased at the upper part of this zone. It is assumed that human activity affected the forests at this time. Poaceae and Cyperaceae, along with Artemisia pollen grains markedly increased in this zone, and it is suggested that this indicates rice growing through human activity. On the Aso Caldera rim plain, the grassland existed at least from 16,500 cal BP and continued until the K-Ak ash layer (7280 cal BP: Fukusawa, 1995). This was followed by a decreased rate of Poaceae and other herb pollen from 6500 to 5500 cal BP during the warming trend during of the Post Glacial Age, and then some scattered forests with pine and deciduous oak appeared. Vegetation present on the Aso Caldera rim plain from the late period of the Last Glacial Age suggests that some grassy species
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invaded from northern Japan in the Last Glacial Age. Some modern northern species such as Lily-of-the-valley (Convallaria majalis L.), Iris (Iris ensata var. spontanea (Makino) Nakai) and Lobelia (Lobelia sessilifolia Lamb.) exist in small numbers as relic species of the cool environment. After 5000 cal BP, the grassland was restored by human activity.
Acknowledgments We are very grateful to Dr. Takakazu Yumoto, a professor of the Research Institute for Humanity and Nature and chief organizer of this project, and to Dr. Kenji Iinuma, professor of Beppu University, and a leader of the Aso and Kuju Research Group. We are also thankful to Dr. Yashuo Miyabuchi, associate professor of Kumamoto University, who discussed and gave useful comments about vegetation of the grassland on the Aso Caldera rim plain. Dr. Ryoma Hayashi, who is a reviewer, had some discussion and gave us very useful advice for improvement of our manuscript.
References Fukusawa, H., 1995. Non-glacial varved lake sediment as a natural timekeeper and detector on environmental changes. The Quaternary Research 34, 135e149 (in Japanese with English Abstract). Hase, Y., Miyabuchi, I., Haruta, N., Sasaki, N., Yumoto, T., 2010. Change of sedimentary facies and topographic process after the late period of the Last Glacial Age of Asodani in central Kyushu, Japan. Bulletin of Goshoura Cretaceous Museum. No.11, pp. 1e10. (in Japanese, with English Abstract). Ito, S., 1981. In: Matsumoto, Y., Matsumoto, H. (Eds.), Plants in Aso. Aso Volcano. Tokaidaigaku-Syuppankai, pp. 28e33 (in Japanese). Iwauchi, A., Hase, Y., 1992. Vegetation changes after the Last Glacial Age in the Kumamoto plain and Aso caldera areas. Japanese Journal of Palynology 38, 116e133 (in Japanese, with English Abstract). Kira, T., 1949. Forest zone of Japan. Japan Forest Technology Association 41 (in Japanese). Machida, H., Arai, F., 1978. Akahoya ash e a Holocene widespread tephra erupted from the Kikai caldera, south Kyushu, Japan. The Quaternary Research 17, 143e163 (in Japanese with English Abstract). Matsumoto, A., Uto, K., Ono, K., Watanabe, K., 1991. K-Ar age determinations for Aso volcanic rocks: Concordance with volcanostratigraphy and application to pyroclastic flows, Programme and abstracts of the volcanological Society of Japan, No 2, p. 73. (in Japanese). Matumoto, T., 1933. The four gigantic caldera volcanoes in Kyushu. Japanese Journal of Geology and Geography 19, 1e57. Miyabuchi, Y., Sugiyama, S., 2006. A 30,000-year phytolith record of a tephra sequence, east of Aso caldera, southwestern Japan. The Quaternary Research 45, 15e28 (in Japanese, with English Abstract). Miyabuchi, Y., Sugiyama, S., 2008. A 30,000-year phytolith record of a Tephra sequence at the southwestern foot of Aso volcano, Japan. Journal of Geography 117, 704e717 (in Japanese, with English Abstract). Miyabuchi, Y., Sugiyama, S., Sasaki, N., 2010. Phytolith and macroscopic charcoal analyses of the Senchomuta drilling core in Asodani valley, northern part of Aso caldera, Japan. Journal of Geography 119 (1), 1e16 (in Japanese, with English Abstract). Nakamura, J., 1976. Pollen Analysis Kokinsyoin, Tokyo. (in Japanese).
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Vegetation changes after the late period of the Last Glacial Age based on pollen analysis of the northern area of Aso Caldera in central Kyushu, Southwest Japan Yoshitaka Hase a, *, Akiko Iwauchi b, Utako Uchikoshiyama c, Eri Noguchi d, Naoko Sasaki e a
Goshoura Cretaceous Museum, Goshoura 4310-5, Goshoura Town, Amakusa City, Kumamoto Prefecture 866-0313, Japan Avance Co. Ltd., Ezu 1-3-48, Kumamoto City, Kumamoto Prefecture 862-0942, Japan c Kumamoto Prefectural Government, Culture Promotion Division, Suizenji 6-18-1, Kumamoto City, Kumamoto Prefecture 862-8570, Japan d Echo Electronic Industry Co. Ltd., Higashihie 3-1-2, Hakata-ku, Fukuoka City, Fukuoka 812-0007, Japan e Research Institute for Humanity and Nature, Kamogamo-motoyama 457-4, Kita-ku, Kyoto City, Kyoto-fu 603-8047, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 21 January 2011
Within the Aso Caldera in central Kyushu, Japan, the Asodani valley is located in the northern part of the inner area, encircled by a wide caldera rim plain consisting largely of grassland. Pollen analyses and radiocarbon dating of samples from the Senchomuta core of the upper part of the basal sediments in the inner area revealed a change in vegetation after around 14,000 cal BP. According to the analytic results from the Hosenbashi core, the forest was mainly formed under cool-temperate to sub-arctic conditions at about 24,000e17,000 cal BP, followed by a change to temperate conditions with predominance of deciduous trees, and subsequently followed by warm-temperate conditions to the present consisting largely of evergreen trees. Herb pollen abundance indicated that the grassland on the broad caldera rim plain 300 to 500 m above Asodani developed at the Last Glacial Age. During the climatic warming of the Post Glacial Age, the grassland basically continued with trees until just after the K-Ah ash layer (7280 cal BP) was deposited. At about 6500 to 5000 cal BP, the herb pollen rate decreased, and the grassland changed to some scattered low-density forest vegetation composed of pine, deciduous and/or evergreen Quercus, and willow. After that period, it is suspected that the grassland was restored by human activity. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Japan is a long, narrow archipelago lying in a northeast to southwest orientation, flanked by the Pacific Ocean to the east and the Japan Sea to the west (Fig. 1, A). The Aso area is located in the central part of Kyushu in Southwest Japan (Fig. 1, B). The Aso area has a caldera (Matumoto, 1933) formed by a depression after several large eruptions. The caldera rim has a diameter of 18 km from east to west and 25 km from north to south, and is mainly composed of three parts: the depression floor, the caldera rim plain and the central cones of Aso volcano. The study area is situated in the northern part of Aso Caldera (Fig. 1, C). The northern part of the caldera is divided into a wide valley named Asodani (Aso valley), which was depressed after the fourth huge eruption (Aso-4 pyroclastic flow) of the Aso Caldera activity about
* Corresponding author. Fax: þ81 96 383 5421. E-mail addresses: [email protected] (Y. Hase), [email protected] (A. Iwauchi), [email protected] (E. Noguchi), [email protected] (N. Sasaki). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.01.018
90,000 years ago (Matsumoto et al., 1991), and a broad caldera rim plain, separated by the steep cliffs of the caldera rim between 350 and 500 m above Asodani. The caldera rim plain radiates from the Aso Caldera rim, and consists of wide, flat grassland with many recently formed ravines. The study sites in Asodani are located at Senchomuta on the north side of the Kurokawa river in Mikubo, and at Ryozanbashi bridge on an alluvial fan in Ichinomiya town, Aso city, as well as three sites on the caldera rim plain: Sanno on the northeast, Zogahana on the north and Daikanbo at the entrance road to the Daikanbo viewpoint (Fig. 2). The strata in Asodani mainly consists of silt, sand and gravel deposited in lake, river, swamp and alluvial fan settings, and the intercalated vitric volcanic Kikai-Akahoya ash layer, commonly known as K-Ah (Machida and Arai, 1978). The stratified sediments on the caldera rim plain consist of yellowish to reddish brown ashy soil and black ashy Kuroboku humus soil in ascending order, with the K-Ah ash layer intercalated. There are four climatic and vegetation zones from north to south in Japan: sub-arctic, cool-temperate, warm-temperate and subtropic. Kira (1949) described the ‘Warmth-Index’ which correlates vegetation to temperature. The lowland area of Kyushu mainly
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too coarse for pollen analysis. This study describes pollen assemblages based on samples from the Senchomuta and Ryozanbashi drill cores in Asodani at an altitude of 450e500 m, as well as samples from the Sanno and Zogahana hand-drill cores, and an outcrop of the Daikanbo entrance road on the caldera rim plain at an altitude of 750e890 m (Fig. 2). The aim is to clarify and discuss the vegetation changes in the inner area of the caldera and on the caldera rim plain after the late period of the Last Glacial Age, combining data from this study with that of Iwauchi and Hase (1992). This study will verify the vegetation changes in the Aso Caldera area, and give evidence to suggest that human activity is implicated in the extended existence of grassland on the caldera rim plain. 2. Material and methods
Fig. 1. Map showing Japan (A), the Aso area in Central Kyushu (B) and the study area in the Aso Caldera (C).
belongs to the warm-temperate zone, having a Warmth-Index of 85e180. At the Aso-Otohime observation point (Automated Meteorological Data Acquisition System) at an altitude of 497 m in Asodani, the mean annual temperature observed is about 13 C, and the Warmth-Index is estimated at about 97 which suggests the warm-temperate zone. The caldera rim plain with its upper height at about 800 m in altitude has an estimated mean annual temperature of about 10 C with a Warmth-Index estimated at 80, placing the area in the cool-temperate zone. According to Ito (1981), the potential natural vegetation in the Aso Caldera area mainly belongs to the warm-temperate zone and is characterized by the forest on Kitamuki Mountain, located at the west part of the caldera rim at an altitude of 300e800 m, mainly covered by evergreen forest composed of oak and horse-chestnut with some deciduous species such as Japanese zelkova, Alangium and Helwingia. On the walls of the caldera around Asodani, artificial forests of Japanese cedar (Cryptomeria japonica) and cypress (Chamaecyparis obtuse) exist among natural evergreen forest, including pine. The broad grassland of the caldera rim plain mainly consists of eulalis (Miscanthus sinensis) and low bamboo (Pleioblastus chino). Iwauchi and Hase (1992) previously mentioned the vegetation change in Asodani based on pollen analysis of samples from the Hosenbashi core at Uchinomaki, but detailed change after ca. 9300 cal BP was not sufficiently explained, because sediments were
The samples were taken from machine drill and hand-drill cores at the sites shown in Fig. 2. The cores were mainly composed of silt, fine sand and tuffaceous fine material, including medium to coarse sand and gravel deposited after the late period of the Last Glacial Age (Hase et al., 2010) in Asodani, and ashy soil and Kuroboku on the caldera rim plain (Fig. 3). Columnar sections of the cores were correlated using the horizon of the K-Ah ash layer. Some samples were dated by radiocarbon age determination. In this study, pollen analyses were performed on 32 samples from the 25 m long Senchomuta core and 16 samples from the 34 m long Ryozanbashi core taken from Asodani, and 32 samples from the 7.3 m long Sanno core, and 24 samples from the 6.3 m long Zogahana core. Eight samples from a 1.1 m thick outcrop on the Daikanbo entrance road were also studied. Samples weighing 0.5e1.5 g in a dry condition were analyzed by the HFeKOHeAcetolysis method (Nakamura, 1976) with a microsphere count of 40,000 grains/cm3, and prepared on two slides per sample with glycerin jelly for observation under a microscope with a magnification of 600. Pollen grains and spores were observed and counted in all areas of each slide. If counted pollen on the first slide was less than 100 grains, the duplicate slide was used. The percentage of pollen grains of each taxon to the sum count of arboreal pollen grains was calculated. Samples with a count of less than 100 grains are shown by open lines in the diagrams (Figs. 5, 6 and 7). Table 1 shows the age determination of each core by the radiocarbon method and the horizon of the K-Ah ash layer. The age of a horizon was estimated by linear interpolation from dated positions in the core. 3. Results 3.1. Vegetation change in Asodani based on pollen analyses of samples from the Senchomuta and Ryozanbashi drill cores 3.1.1. Senchomuta core Miyabuchi et al. (2010) and Hase et al. (2010) observed the sedimentary facies of core samples at the Senchomuta drill site. Samples for pollen analysis were taken at intervals of 40 and/or 80 cm and consisted of silt and fine sand, often including organic matter. The resulting pollen analysis of the samples is shown in a pollen diagram with four local pollen assemblage zones in ascending order (Fig. 4). Pollen zone Se-A, LepidobalanuseUlmus/ZelkovaeCeltis zone, ca.14,000e8500 cal BP, was mainly composed of deciduous oak accompanied by Ulmus/Zelkova, Celtis/Aphanante and Castanea/ Castanopsis. Cyclobalanopsis gradually increased in the upper part of the zone. Poaceae and Artemisia pollen were evident. This type of forest is categorized as belonging to the lower part of the cooltemperate zone.
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Fig. 2. Locality map of the study area with the sites of drill cores. The Hosenbashi site indicates the Hosenbashi core in Iwauchi and Hase (1992).
Pollen zone Se-B, PodocarpuseCyclobalanopsiseUlmus/ZelkovaeCeltis zone, 8500e5500 BP was composed of evergreen oak, increasing in the upper part. Lepidobalanus existed, while Ulmus/ Zelkova and Celtis/Aphanante were gradually reduced in number. This was categorized as a warm-temperate forest. Pollen zone Se-C, CyclobalanopsiseCastanea/Castanopsis zone, 5500e1800 cal BP, included abundant Cyclobalanposis and Castanea/Castanopsis. Therefore, this forest was also categorized as warm-temperate. Poaceae and Cyperaceae increased in this zone. In Pollen zone Se-D, PinuseCryptomeriaeCyclobalanopsiseCastanea/ Castanopsis zone, 1800 cal BP e present, coniferous Pinus and Cryptomeria characteristically increased in the upper part. It is assumed that human activity affected the forests at this time. Poaceae and Cyperaceae, along with Artemisia pollen grains markedly increased in this zone. 3.1.2. Ryozanbashi core The Ryozanbashi core is composed of coarse-grained materials (coarse sand to gravel) that were intercalated with fine-grained layers. Samples composed of fine sand, silt and clay with organic matter were taken from the core at irregular intervals.
The pollen diagram is displayed in Fig. 5. More than half of slides have counts of less than 100 grains of pollen, but it is assumed that the vegetation on the site was mainly composed of Pinus, Lepidobalanus, Cyclobalanopsis, Castanea/Castanopsis, Alnus of the arboreal variety, and Poaceae, Artemisia and other Compositae representing nonarboreal pollen with fern spores. The pollen composition shows vegetation of a warm-temperate forest. However, this represents a low density of trees, accompanied by some grass areas on the alluvial fan.
3.2. Vegetation change on the rim plain based on pollen analyses Pollen analyses of samples from two hand-drill sites at Sanno and Zogahana, and an outcrop of the entrance road to the Daikanbo viewpoint on the caldera rim plain, are described below. 3.2.1. Sanno core Based on pollen analysis of the samples from the Sanno handdrill core at intervals of 20 cm, four local pollen assemblage zones were recognized, in ascending order (Fig. 6).
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Fig. 3. Columnar sections of the drill cores correlated by the horizon of the K-Ah ash layer.
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In Pollen zone Sn-1, LipidobalanuseArtemisiaePoaceae zone, ca. 14,000e12,000 cal BP, Lipidobalanus represented 10e15% of the total arboreal pollen grains. Artemisia and Poaceae concentrations were more than 100% of the total arboreal pollen grains. In Pollen zone Sn-2, PinuseLepidobalanuseSalix zone, 12,000e 7500 cal BP, each prepared slide included a small amount of pollen less than 50 grains. It is assumed that the arboreal vegetation roughly consisted of pine, deciduous and/or evergreen oak and willow on the grassland. In Pollen zone Sn-3, LepidobalanuseCyclobalanopsiseCastanea/ Castanopsis-Salix zone, ca. 7500e5000 cal BP, Lipidobalanus was dominant and accompanied by Cyclobalanopsis. Castanea/Castanopsis and Salix were also dominant in the upper half of the zone. Pinus and Tsuga appeared in smaller amounts. Artemisia concentrations were more than 60% of the total arboreal pollen, with other Compositae and Poaceae in lesser amounts. In Pollen zone Sn-4, PinuseCryptomeriaeLepidobalanus zone, 5000 e present, Pinus mainly occurred as arboreal pollen with Lepidobalanus, Cyclobalanopsis, Castanea/Castanopsis and Salix. Cryptomeria slightly increased toward the top. In the lowermost part of the zone, arboreal pollen grains represented 90%, decreasing gradually to 10% at the uppermost horizon of the samples. Artemisia, Poaceae and other Compositae were dominant in occurrence. 3.2.2. Zogahana core The samples were irregularly taken at intervals of 10 cm from the top to 70 cm depth, and at intervals of 20 cm from 70 cm to 4 m depth. The K-Ah ash layer was present at 2.5 m in depth. The formation was divided into lower and upper parts. The lower part
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was tuff and tuffaceous material, and the upper half was Kuroboku soil above 3.3 m depth. The pollen diagram of the Zogahana hand-drilling core samples was characterized by an excellent count of herbaceous grains such as Poaceae, Artemisia and other Compositae with a high percent occurrence of Pinus (Fig. 7). Three local pollen assemblage zones are recognized. Pollen zone Zh-1, PinuseCryptomeriaeLepidobalanus-Cyclobalanopsis zone, ca. 10,000e7500 cal BP, is mainly composed of Conifer pollen such as Pinus, Cryptomeria and Abies, and deciduous and evergreen Quercus pollen. Herbaceous pollen such as Poaceae and Compositae including Artemisia are abundant. Pollen zone Zh-2, PinuseCyclobalanopsiseSalix zone, 7500e5000 cal BP, is mainly composed of Pinus, Cyclobalanopsis and Salix arboreal pollen. This zone is characterized by a decrease of herbaceous pollen, especially Poaceae and Artemisia. Pollen zone Zh-3, PinuseLepidobalanuseCyclobalanopsis zone, 5000 cal BP e present, shows an increase of coniferous pollen, mainly composed of Pinus, and Quercus. Lepidobalanus and Cyclobalanopsis existed with relatively equal counts. The Zh-3 zone is characterized by a large amount of herbaceous pollen, Poaceae and Compositae including Artemisia. 3.2.3. Daikanbo outcrop The Kuroboku soil crops out at the west side of the entrance road to the Daikanbo viewpoint. The K-Ah layer was included in the lower part at 80 cm depth in the outcrop. A pollen diagram was characterized by many counts of Lepidobalanus and Cyclobalanopsis with Castanea/Castanopsis. Importantly, herbaceous pollen of
Fig. 4. Pollen diagram of samples from the Senchomuta core.
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Fig. 5. Pollen diagram of samples from the Ryozanbashi core.
Poaceae and Artemisia decreased in the middle part of the column, and the upper two horizons of the diagram included many counts of Pinus and Cryptomeria (Fig. 8). 4. Discussion 4.1. Stratigraphic correlation among the core samples 4.1.1. Hosenbashi, Senchomuta and Ryozanbashi cores in Asodani The Hosenbashi core was not dated in the work of Iwauchi and Hase (1992). In this study, the five samples from the core were dated by the radiocarbon dating method (Table 1). Samples at 69.1 m and 16.3 m in depth respectively indicated 23,770e22,930
and 9540e9450 cal BP. This indicates a deposition of the former sample during the Last Glacial Age, while the latter sample was deposited before the setting of the K-Ah ash layer. The K-Ah ash layer was not observed in the Hosenbashi core, but this horizon is radiocarbon dated in the correlative Senchomuta core (Fig. 3 in Hase et al., 2010). The Senchomuta core has more fine material than the Hosenbashi core after about 9000 cal BP. The Ryozanbashi core intercalates the K-Ah ash layer at 13.7 m in depth and also correlates to the Senchomuta core (Fig. 3). 4.1.2. Sanno and Zogahana cores and Daikanbo outcrop The base of the Sanno core originated from the Aso-4 pyroclastic flow deposit, about 90,000 a. The basal part of dark gray silt layer was
Fig. 6. Pollen diagram of samples from the Sanno core.
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Fig. 7. Pollen diagram of samples from the Zogahana core.
dated at 14,250e13,860 cal BP by the radiocarbon method (Table 1). The K-Ah ash layer was not recognized, but it is assumed that a horizon of the layer existed at nearly 2.7 m in depth in the lower part of the Kuroboku, because it was usually intercalated in the lower horizon of Kuroboku on the Aso Caldera rim plain. A sample of 3.5 m in depth of the Sanno core was dated at 2260 40 BP (Table 1), but it drastically differed from other data collected and is suspect and therefore omitted. The Zogahana core and Daikanbo outcrop can be correlated using the K-Ah ash layer (Fig. 3). 4.1.3. Asodani and the caldera rim plain There are different sedimentary environments between the inner floor area and the outer rim plain of the caldera. In the inner floor area, sedimentary environments included lake, river and swamp on the west side, and an alluvial fan with river and swamp conditions on the east side. On the outer rim plain, sedimentary conditions were usually aeolian, with occasional short rivers and small swamps. The
inner floor area and the outer rim plain were correlated using the KAh ash layer and some radiocarbon data (Fig. 3). 4.2. Vegetation changes in the Aso Caldera area after the late period of the Last Glacial Age 4.2.1. Asodani and surrounding area The upper part of the Hosenbashi drill core indicated river and/ or swamp conditions, due to its composition of coarse sand and granules, and sparse pollen grains. The samples from the Senchomuta core, correlated to the upper part of the Hosenbashi core (Hase et al., 2010), were mainly composed of fine materials deposited in a small lake existing on the outer side of a riverbank of the Kurokawa River, including pollen grains that were in abundance. The age of a sample of 69.1 m in depth of the Hosenbashi core was 23,770e22,930 cal BP (Table 1). Forest transformation in the
Table 1 14 C Age of samples from the Hosenbashi and Senchomuta cores in Asodani and the Sanno and Zogahana cores on the caldera rim plain. Core site Hosenbashi
Depth of sample (m) d13C (permil)
16.3 48.0 56.4 62.6 69.1 Senchomuta 1.45 2.26 4.69 6.6(K-Ah) 12.25 22.97 Sanno 0.96e1.0 2.35e2.39 6.62e6.66 Zogahana 0.28e0.29 2.36e2.70(K-Ah) 3.27e3.28
14
C Age (BP) (conventional R.A.)a,b Calibrated (calendar) agec (cal BP) Lab code no.
20.5 23.5 29.6 27.7 29.3 22.3 23.4 31.3
8480 12,500 13,140 14,190 19,560 1760 3520 5290
40 50 80 140 90 40 40 50
22.3 24.5 18.4 20.7 24.0 18.6
7970 10,010 2450 2260 12,220 910
60 40 30 40 50 40
19.7
7440 50
9540e9450 15,060e14,210 16,530e15,240 17,690e16,880 23,770e22,930 1810e1560 3900e3690 6200e5930 7280 9010e8610 11,710e11,300 2550e2360 2270e2160 14,250e13,860 920e730 7280 8370e8170
GX-29942-AMS GX-29943-AMS GX-29944-AMS GX-29945-AMS GX-29946-AMS Bata-274696 Bata-274697 Bata-248317
Material and analyzer Source
Silt # Silt # Silt # Silt # Silt # Pollen þ Pollen þ Charcoal þ Ash Bata-246468 Organic sediment þ Bata-247887 Organic sediment þ GX-28949-AMS Humus soil # GX-28950-AMS Humus soil # GX-28951-AMS Humus soil # Bata-269119 Organic sediment þ Ash Bata-269120 Organic sediment þ
Analyzer: þ BETA ANALYTIC INC., USA; # GEOCRON LABORATORIES, USA. Source: (1) Hase et al. (2010); (2) Miyabuchi et al. (2010); (3) This paper; (4) Fukusawa (1995). a 14 C ages were analyzed based on the Libby’s 14C half life of 5568 and 5570 years by the BETA and GEOCRON laboratories, respectively. b Conventional 14C ages were calculated using d13C values. c 2 Sigma calibrated result: 95% probability.
(1)
(3) (2) (4) (2) (3)
(3) (4) (3)
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Fig. 8. Pollen diagram of samples from an outcrop at the entrance road to the Daikanbo viewpoint.
Asodani area at the late period of the Last Glacial Age started with a cool-temperate coniferous forest that was accompanied by cooltemperate deciduous broad-leaved forest (Iwauchi and Hase, 1992). Cool-temperate deciduous forest including birch and beech was transitional into warm-temperate deciduous forest including hackberry, clearly followed by evergreen broad-leaved
forests such as evergreen oak in the Post Glacial Age in the Senchomuta core. After 1800 cal BP, an extreme increase of Poaceae pollen seems to suggest rice growing. In the youngest part of the zone, large contents of Pinus and Cryptomeria pollen show that vegetation change may have been artificial due to human involvement.
Fig. 9. Correlation of pollen counts in samples from the Senchomuta, Ryozanbashi in Asodani, and Sanno and Zogahana cores and the Daikanbo outcrop on the caldera rim plain.
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Fig. 10. Arboreal pollen/Non-arboreal pollen in the pollen samples of the Senchomuta, Ryozanbashi, Sanno and Zogahana cores and the Daikanbo outcrop.
The Ryozanbashi diagram shows that more than half of samples include less than 100 pollen grains. The numbers of grains per 1 g are less than 5000 (Fig. 9) where the percentage of arboreal pollen is highest (Fig. 10). This clarifies that the small amount of pollen grains in the Ryozanbashi site differ from the Senchomuta site which was mainly deposited under lake and swamp conditions, while its arboreal pollen rate was similar to that of the Senchomuta site (Fig. 10). The Ryozanbashi site lies on an alluvial fan plain composed of coarse material such as gravel and coarse sand with silt layers (Hase et al., 2010). The pollen diagram characteristically shows that the vegetation is composed of pine, shrubs and herbaceous plants such as Artemisia and other Compositae with fern spores, and existed on an alluvial fan plain deposited during the Post Glacial Age. 4.2.2. Formation process of the grasslands on the Aso Caldera rim plain The numbers of pollen grains per 1g of the Sanno samples are small compared with those of the Zogahana samples (Fig. 9), but their diagrams depict similar pollen compositions and rates (Fig. 10). Therefore, vegetation changes on the caldera rim plain can be represented by the Sanno pollen diagram. The result of pollen analysis of samples from the Sanno site showed that the grassland already existed at 14,000 cal BP. Vegetation of the late period of the Last Glacial Age suggests that some grassy species invaded the Aso Caldera rim plain from northern Japan in the Last Glacial Age. The vegetation, composing an open, low-density forest, was represented by deciduous oak with evergreen oak, pine, and hemlock ca. 7500 to 5000 cal BP. The forest
was formed in the upper part of a warm-temperate zone during the warming period in the Post Glacial Age. The grassland continued until deposition of the K-Ah ash layer, and afterwards Poaceae and other herb pollen rates decreased from 6500 to 5000 cal BP. From that point, some scattered forests with pine and deciduous and/or evergreen oak appeared. After 5000 cal BP, herbaceous pollen substantially increased, and therefore it is assumed that the grassland was restored. The research of Miyabuchi and Sugiyama (2006) which was based on phytolith analysis in the east of Aso Caldera mentioned that the intervals of ca. 32e30 cal ka of the east of Aso Caldera was dominated by a Sasa grassland (mainly Sasa sect. Crassinodi), suggesting a cool and dry climate. However during 30e13.5 cal ka, which corresponds to the Last Glacial Maximum, the Sasa grassland declined due to violent volcanic activity of Aso Volcano. During the Holocene, a Miscanthus grassland continued consistently for more than 10,000 years. The long continuation of the Miscanthus grassland might be related to artificial burning. Miyabuchi and Sugiyama (2008) noted vegetation at the southwest foot of Aso Caldera before 31 cal ka dominated by phytoliths of Sasa (mainly Sasa sect. Crassinodi), a cool-temperature dwarf bamboo. However, during 31e13.5 cal ka, which corresponds to the Last Glacial Maximum, vegetation composed mainly of Sasa dwarf bamboo had declined slightly. During the Holocene, the Sasa grassland dominated consistently, but phytoliths of Pleioblastus (a warm-temperature dwarf bamboo) and arboreal pollen such as Distylium increased from 7300 years ago. The reason for the restoration of the grassland is assumed to be the burning of forests mentioned by Miyabuchi and Sugiyama
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Table 2 Vegetation changes after the late period of the Last Glacial Age in the Aso Caldera area, central Kyushu, Japan.
Geologic Age (cal yBP) 1800
5,000
Asodani
Caldera rim plain
Iwauchi & Hase(1992) This study (Pollen)
Miyabuchi et al. (2010) (Phytolith)
Se-D: Pinus-Cryptomria -Cyclobalanopsis -Castanea/Castanopsis Z.
Miscanthus grassland
Sn-4: Pinus-Cryptomeria
existed continuously
-Lepidobalanus Z.
Se-C: Cyclobalanopsis -Castanea/Castanopsis Z. 7.3-6.6 cal ka:
5,500 8,500
10,000 12,300
Se-B: Podocarpus -Cyclobalanopsis -Ulmus/Zelkova-Celtis Z. U-V and Se-A: Lepidobalanus-Ulmus/Zeikova -Celtis Z.
U-IV: Lepidobalanus -Fagus-Carpinus Z. 15,000 16,500
Sasa increased 11-9 cal Ka: Sasa (cool-temperature dwarf bamboo) grassland and/or forests
This study (Pollen)
Sn-3: Lepidobalanus -Cyclobalanopsis -Castanea/Castanopsis -Salix Z.
Southwest foot of Caldera Miyabuchi and Sugiyama (2006) (Phytolith)
East of Caldera Miyabuchi and Sugiyama (2006) (Phytolith) Holocene: Sasa grassland dominated
Holocene:
consistently
a Miscanthus grassland
Pleioblastus and arboreal
continued
pollen (Distylium) increased
Sn-2: Pinus -Lepidobalanus-Salix Z.
from 7,300 years ago
Sn-1: Lepidobalanus -Artemisia-Poaceae Z.
U-III: Lepidobalanus -Fagus-Carpinus Z.
U-II: Pinus-Lepidobalanus -Carpinus-Betula Z. 20,000
31-13.5 cal ka: vegetation composed 30-13.5 cal ka:
mainly of Sasa dwarf
Sasa grassland declined
bamboo had declined
U-I: Pinus-Picea-Abies -Tsuga Z.
slightly
24,000
25,000
30,000 32,000
(2006, 2008) and Miyabuchi et al. (2010). Some northern species of Honshu district, such as Lily-of-the-valley (Convallaria majalis L.), Iris (Iris ensata var. spontanea (Makino) Nakai), and Lobelia (Lobelia sessilifolia Lamb.) are reduced as relic species of the cool environment today (Ito, 1981). At this point, it is not clear whether the grassland existed in a natural state, however it is clear that the grassland is maintained at present by human burning. The pollen diagram of the Zogahana core is characterized by more than 10%Cryptomeria pollen rate in the lower to middle parts of the core. It is assumed that Japanese cedar grew on the caldera rim plain with pine and Quercus (deciduous and evergreen) forming some scattered open forests in the grassland before the K-Ah ash layer in the Holocene. The Kuroboku outcrop at the entrance road to the Daikanbo viewpoint is assumed to have been formed by aeolian activity. However, the Daikanbo pollen diagram indicates that the arboreal pollen has a higher percentage than that of both the Sanno and Zogahana sites (Fig. 10). It is assumed that arboreal pollen accumulation on the Daikanbo site, situated on the edge of the caldera rim, mainly originated by aeolian transport from the forest of the inner caldera. 5. Conclusion Vegetation change from cool-temperate forest to warmtemperate forest in the Asodani area during the late period of the
ca. 32-30 cal ka: dominated by a Sasa grassland (mainly Sasa sect. Crassinodi)
before 31 cal ka : dominated by of Sasa (mainly Sasa sect. Crassinodi)
Last Glacial Age and Post Glacial Age was recognized based on pollen analysis of samples from drill cores of the Hosenbashi, Senchomuta and Ryozanbashi sites. Vegetation change in the Uchinomaki Formation based on the Hosenbashi core (Iwauchi and Hase, 1992) and the Senchomuta core is summarized as follows in ascending order (Table 2). Pollen zone U-1: PinusePiceaeAbieseTsuga zone. ca. 24,000e 20,000 cal BP Sub-arctic coniferous forests, consisting of Picea, Abies, Tsuga and Pinus, widely covered the mountain slopes, accompanied by some deciduous broad-leaved trees, such as Betula, Lepidobalanus and Carpinus. Pollen zone U-II: PinuseLepidobalanuseCarpinus-Betula zone. 20,000e16,500 cal BP Cool-temperate deciduous forests developed; however, the coniferous forests were gradually reduced in the area. Pollen zone U-III: LepidobalanuseFaguseCarpinus-Betula zone. 16,500e15,000 cal BP Cool-temperate deciduous forests predominated in this period, accompanied by a few sub-arctic conifers and Betula trees.
Y. Hase et al. / Quaternary International 254 (2012) 107e117
Pollen zone U-IV: LepidobalanuseFaguseCarpinus zone. 15,000e 12,300 cal BP Cool-temperate forest continued to be widely distributed; however, sub-arctic elements did not appear any longer. In addition, in the later part of this period, the cool-temperate deciduous forests were slightly reduced in area. Pollen zone U-V and Se-A: LepidobalanuseUlmus/ZelkovaeCeltis zone. 12,300e8500 cal BP Warm-temperate deciduous broad-leaved forests, mainly composed of Lepidobalanus, Celtis and Ulmus/Zelkova, were widespread on the mountain slopes; in contrast, the cool-temperate deciduous forests were rapidly reduced in this period. Pollen zone Se-B: PodocarpuseCyclobalanopsiseUlmus/ZelkovaCeltis zone. 8500e5500 cal BP Warm-temperate deciduous forests, except for Ulmus/Zelkova and Celtis, were gradually reduced in this period; instead, the warm-temperate evergreen broad-leaved forests, mainly consisting of Podocarpus, Cyclobalanopsis and Castanopsis began to develop on the rim slopes. Pollen zone Se-C: Cyclobalanopsis-Castanea/Castanopsis zone. 5500e1800 cal BP Warm-temperate forest was composed of abundant Cyclobalanposis and Castanea/Castanopsis. Poaceae and Cyperaceae increased in this zone. Pollen zone Se-D: PinuseCryptomeriaeCyclobalanopsis-Castanea/Castanopsis zone. 1800 cal BP-present Coniferous trees of Pinus and Cryptomeria characteristically increased at the upper part of this zone. It is assumed that human activity affected the forests at this time. Poaceae and Cyperaceae, along with Artemisia pollen grains markedly increased in this zone, and it is suggested that this indicates rice growing through human activity. On the Aso Caldera rim plain, the grassland existed at least from 16,500 cal BP and continued until the K-Ak ash layer (7280 cal BP: Fukusawa, 1995). This was followed by a decreased rate of Poaceae and other herb pollen from 6500 to 5500 cal BP during the warming trend during of the Post Glacial Age, and then some scattered forests with pine and deciduous oak appeared. Vegetation present on the Aso Caldera rim plain from the late period of the Last Glacial Age suggests that some grassy species
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invaded from northern Japan in the Last Glacial Age. Some modern northern species such as Lily-of-the-valley (Convallaria majalis L.), Iris (Iris ensata var. spontanea (Makino) Nakai) and Lobelia (Lobelia sessilifolia Lamb.) exist in small numbers as relic species of the cool environment. After 5000 cal BP, the grassland was restored by human activity.
Acknowledgments We are very grateful to Dr. Takakazu Yumoto, a professor of the Research Institute for Humanity and Nature and chief organizer of this project, and to Dr. Kenji Iinuma, professor of Beppu University, and a leader of the Aso and Kuju Research Group. We are also thankful to Dr. Yashuo Miyabuchi, associate professor of Kumamoto University, who discussed and gave useful comments about vegetation of the grassland on the Aso Caldera rim plain. Dr. Ryoma Hayashi, who is a reviewer, had some discussion and gave us very useful advice for improvement of our manuscript.
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