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Vegetation history and the impact of tephra deposition during 7000 years based on pollen and tephra analysis of a Barasantou Bog sediment core, eastern Hokkaido, northern Japan
Quaternary International 503 (2019) 24–31
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Vegetation history and the impact of tephra deposition during 7000 years based on pollen and tephra analysis of a Barasantou Bog sediment core, eastern Hokkaido, northern Japan
T
Toshiyuki Fujikia,∗, Keiji Wadab, Eiichi Satoc, Mitsuru Okunod a
Department of Applied Science, Faculty of Science, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama, 700-0005, Japan Earth Science Laboratory, Asahikawa Campus, Hokkaido University of Education, 9 Hokumon-cho, Asahikawa, 070-8621, Japan c Institute for Promotion of Higher Education, Kobe University, Tsurukabuto 1-2-1, Nada-ku, Kobe, 657-8501, Japan d Department of Earth System Science, Faculty of Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka, 814-0180, Japan b
A B S T R A C T
To estimate the impact of a volcanic eruption on vegetation, we conducted a field survey and collected peaty sediments and made pollen and tephra analysis from Barasantou Bog, in eastern Hokkaido, northern Japan. Using an electron probe micro-analyzer, we identified five tephra layers in the core: Ma-g (7.6 cal ka BP), Ma-e (5.5 cal ka BP), Ma-d (4.0 cal ka BP), Ta-c (2.5 cal ka BP), and Ko-c2 (AD 1694). When the peat began to accumulate again after deposition of the Ma-g tephra, the groundwater level of this area was very low, and Alnus, Poaceae, Thalictrum, Artemisia, and ferns dominated. Gradually, the groundwater level increased, and the condition of this wetland approached the present conditions, about 5.0 to 4.5 cal ka BP. Pinaceae conifers increased 2.5 cal ka BP. It is presumed that Picea glehnii and Abiesspecies formed wetland forests. Myrica gale and Alnus increased again, and grew to the edge of the wetland. At the same time, Quercus subgen. Lepidobalanus decreased and Betula increased. This trend might be due to the influence of human activity. The wetland vegetation changed from Thalictrum and Artemisia to Sanguisorba, and then from Sanguisorba to Menyanthes after the deposition of Ma-g tephra or after 2.5 ka BP. It was thought that the wetland changed from low moor to middle or high moor. A tephra layer tens of cm thick is not expected to affect pollen composition, and may have no clear impact on the surrounding vegetation. Sediment with a thickness of a few cm requires a deposition time of approximately 10 years; pollen analysis results are on the scale of the average vegetation change during that period. We conclude that short-term vegetation changes were not detected.
1. Introduction Many volcanoes erupted in Hokkaido during the Holocene (Nakamura, 2016; Machida and Arai, 2003). Consequently, many tephra layers have been confirmed in sediments, and these are important for determining the sedimentation age. Most previous research on the influence of an eruption on vegetation has discussed how the climate change accompanying the eruption changed the vegetation (Tsuji, 1985; Ooi and Tsuji, 1989; Tsuji and Kosugi, 1991). Some studies have examined the direct influence of tephra fall on the surrounding vegetation (Tsuji and Kosugi, 1991; Hughes et al., 2013; Kito et al., 2017). Hughes et al. (2013) investigated wetland vegetation change after tephra fall using plant macrofossil analysis at Utasai Bog, Hokkaido, and showed that wetland plant communities shifted from Sphagnum to monocotyledons. They concluded that this vegetation change was affected by an increase in peat humification and a decrease in carbon accumulation. According to Kito et al. (2017), the amounts of Quercus subgen. Lepidobalanus and Poaceae pollen grains increased, and those of other tree pollen grains decreased, after tephra fall at Tashiro Bog,
∗
Aomori. These results are likely due to differences in resistance to tephra among tree species, and increases in the water level following tephra deposition. Igarashi (2016) and Endo et al. (1988) reported the pollen composition since Last Glacial Maximum (LGM) from peat layers on the coastal cliffs of Barasan, eastern Hokkaido. We think it possible that the peat might be mixed into the tephra layer on the coastal cliff. Thus, we attempted to sample peat sediments in Barasantou Bog, which is located upstream. We also conducted radiocarbon dating using accelerator mass spectrometry (AMS), and identified the tephra based on the main component chemical composition analysis of volcanic glass using an electron probe micro-analyzer (EPMA). We determined the age of the peat sediments and reconstructed the paleovegetation around Lake Barasantou using the changes in the fossil pollen grains contained in this sediment. We also considered the impact of tephra fall on the vegetation.
Corresponding author. E-mail address: [email protected] (T. Fujiki).
https://doi.org/10.1016/j.quaint.2018.10.013 Received 24 March 2018; Received in revised form 7 October 2018; Accepted 13 October 2018 Available online 15 October 2018 1040-6182/ © 2018 Published by Elsevier Ltd.
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Fig. 1. Maps showing the location of Lake Barasantou (black circle). (a) Map of Hokkaido Island. Black triangles indicate source volcanoes for the Holocene tephra layers. Ko: Mt. Komagatake (1131 m), Ta: Mt. Tarumae (1041 m), Ma: Mt. Mashu (857 m). Black squares indicate the quoted analysis point. 1: Utasai bog (Hughes et al., 2013), 2: Orochigahara bog (Morita, 1984), Kyogoku mire (Igarashi, 2000), Nakayama mire (Hoshino and Nakamura, 2003), 3: Uryu-Numa moor (Morita, 1985), 4: Toutsuru mire (Matsuda, 1983), 5: Ochiishi bog (Morita, 2001a), 6: Yururi Island (Morita, 2001b), 7: Habomai bog (Igarashi et al., 2001), 8: Barasan (Igarashi, 2016; Endo et al., 1988). (b) Map showing sampling site (black circle) around near Lake Barasantou (N 43° 25′ 23“, E 145° 15’ 13“, 4.4 m asl). Topographic map is part of 1/25,000 map sheet “Tokotan” issued from Geospatial Information Authority of Japan (GSI).
2. Site description Lake Barasantou is a dammed lake located in the middle reach of the Nishimarubetu River in Betsukai, eastern Hokkaido. It has an area of 0.3 km2 and the average water depth is 5 m. Barasantou Bog comprises the low to middle moors that formed around the lake (Fig. 1). The average temperature in Betsukai is 5–6 °C, and the mean annual rainfall is about 900 mm (Betsukai-cho, 2013). The coastal region receives little snowfall due to the marine climate (Betsukai-cho, 2013). Brasenia schreberi and Nuphar japonicum grow in the lake, and Phragmites australis, Carex sp., Alnus japonica, and Myrica gale grow in the low moors (Tsujii and Tachibana, 2003). Wetland grasslands, mainly comprising Carex spp. and secondary vegetation dominated by Rhododendron kaempferi and Quercus crispula, have established around the lake; however, a vast area has been converted to rangeland composed of Phleum pratense (Miyawaki, 1988). A 330-cm-long sediment core was obtained on September 3, 2014, at a location in Barasantou Bog (43°25′23″N, 145°15′13″E, 4.4 m a.s.l.) using a peat sampler. The sediment core was composed of undecomposed peat and five tephra layers were identified, at 38–41, 150–159, 207–216, 264–271, and 315–330 cm (Fig. 2).
3. Analytical methods 3.1. AMS radiocarbon dating Fig. 2. Columnar section showing the stratigraphy and age-depth profile of Barasantou Bog.
The leaf fragments were sub-sampled for age determination. Samples were cleaned by routine acid–alkali–acid (AAA) treatments to remove carbonates and secondary organic acids. After washing with distilled water and drying, each pretreated sample was converted to CO2 gas, and then reduced catalytically to graphite on Fe powder using H2 gas. The three carbon isotopes were measured in the samples and NIST oxalic acid (HOxII) standard using an NEC Peletron 9SDH-2 AMS system at the Institute of Accelerator Analysis Ltd (IAA). The 13C/12C ratios (δ13CPDB) obtained by AMS were used to correct carbon isotopic fractionation when calculating conventional 14C age. To estimate and remove the 14C background level, the 14C concentration of commercial
graphite powder was also measured in the same analytic sequence as the sample and standard measurements. We calibrated 14C dates measured by AMS to a calendar year timescale using the IntCal13 dataset (Reimer et al., 2013) and the program CALIB 7.1 (Stuiver and Reimer, 1993; Stuiver et al., 2015).
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than that inferred from identified tephra (Ma-e). Yamamoto et al. (2010) reported the eruption age of Ma-e tephra as 4720 ± 40 BP, but Miyata et al. (1988) described it as 4150 ± 40 BP. Their calendar ages were ca 5.5 cal ka BP and ca. 4.7 cal ka BP, respectively. The 14C age obtained in the current study was closer to that of Miyata et al. (1988). The eruption age of Ma-e tephra should be determined in a future study.
3.2. Tephra analysis Each tephra was identified from the chemical composition of the volcanic glass as follows. Tephra samples were embedded in resin and thin sections were obtained by double-sided polishing. Then, these were mirror-polished using diamond paste. The chemical composition of the volcanic glass was measured using wave dispersive WDS-EPMA (JEOLJXA 8600). The accelerating voltage was 15 kV and the current was 0.8 × 10−8 A. The scanning area of the electron beam was < 100 μm2. Ten oxide elements were measured: Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and Cl. The standard glass compositions of Ta-a, Ko-c2, Ta-c2, Ma-d, and Ma-g pumices were analyzed by WDS-EPMA at the Hokkaido University of Education at Asahikawa.
4.2. Tephra identification Tephra identification was mainly based on the characteristics of the tephra particles, the chemical composition of tephra glass, and the sequence of the tephra layer. The first layer (38–41 cm) contained many well-foamed glass particles, and the glass composition was consistent with that of Komagatake-c2 tephra (Ko-c2; AD 1694; Katsui and Ishikawa, 1981b; Tokui, 1989) (Fig. 3and Table 2). However, the glass particles in 10% of samples were confirmed to be derived from Tarumai-a tephra (Ta-a; AD 1739; Katsui and Ishikawa, 1981a). The eruption ages of Ko-c2 and Ta-a were closer. Therefore, we concluded that some particles from Ta-a tephra from the upper layer were mixed into the first layer. The second layer (150–159 cm) also contained many well-formed glass particles. The glass composition was in the compositional field of Ta-a tephra and Tarumai-c2 tephra (Ta-c2; 2.5 cal ka BP; Furukawa et al., 2006); however, the composition of this layer was similar to that of Ta-c2 in the FeO-MgO diagram (Fig. 3). The second layer was at a lower stratigraphic position than Ko-c2 at AD 1694, and its age was consistent with that of Ta-c2 (Fig. 2). The third layer (207–216 cm) consisted of fallen pumice, indicating that this tephra was derived from a neighboring volcano. As the amount of K2O in the volcanic glass in the pumice was below 1.0 wt% (Fig. 3), this pumice was derived from the Mashu volcano (Okumura, 1991). Five plinian pumice-fall layers, Ma-b, Ma-d, Ma-g, Ma-h, and Ma-i, were confirmed to have come from the Mashu volcano in the past 10,000 years (Katsui et al., 1975). Ma-i, Ma-h, and Ma-g tephras were deposited continuously in that order during the caldera formation period of 7.5 cal ka BP (Kishimoto et al., 2009). Ma-b tephra is excluded as a candidate for the third layer due to its eruption age (ca. 1.0 cal ka BP; Katsui et al., 1975) and its northern distribution area. The third layer is likely either Ma-d or Ma-g tephra. The average standard chemical composition of volcanic glass in Ma-d pumice was higher in FeO, MgO, and CaO contents than that in Ma-g pumice (Table 2). As shown in the FeO-MgO diagram (Fig. 2), all glass compositions of the third layer belong to the compositional field of the Ma-d tephra. Thus, the third layer corresponds to Ma-d tephra of 4 cal ka BP (Kishimoto et al., 2009). The fifth layer (315–330 cm) consisted of fallen pumice; however, the bottom of this core did not reach the lower layer of this tephra. The amount of K2O in the volcanic glass was below 1.0 wt% (Table 2); therefore, this tephra was derived from the Mashu volcano. As the third layer was identified as Ma-d tephra, the fifth layer is likely Ma-g tephra,
3.3. Pollen analysis A few grams of sample were collected for pollen analysis at approximately 5-cm intervals of the core, and from just above and below the tephra. Fossil pollen and spores were extracted with 10% KOH treatment, ZnCl2 solution treatment, and Erdtman's acetolysis method (Erdtman, 1934). To determine pollen assemblages, the samples were dehydrated with an ethanol series (30%, 60%, and 99.5%) and then treated with xylene after acetolysis. The samples were mounted in Eukitt medium for observation under a light microscope (Fujiki et al., 2013). At least 500 pollen grains (excluding spores) were counted in each sample, including at least 300 arboreal pollen (AP) grains. The percentages for each taxon in the total AP were calculated. Alnus and Myrica gale pollen grains were excluded from the total AP because of their high frequencies and fluctuations that would hinder comparison among other taxa (Fujiki et al., 2013). 4. Results and discussion 4.1. AMS radiocarbon dating The AMS 14C dating of three samples yielded ages of 3945 ± 25 BP (IAAA-172589) for 310 cm, 4255 ± 25 BP (IAAA-172588) for 275 cm, 3765 ± 25 BP (IAAA-150504) for 230 cm, 935 ± 25 BP (IAAA150503) for 95 cm, and 370 ± 25 BP (IAAA-150502) for 65 cm (Table 1, Fig. 2). Their calendar ages were ca. 4.4 cal ka BP, ca. 4.8 cal ka BP, ca. 4.1 cal ka BP, 0.8 cal ka BP, and 0.4 cal ka BP, respectively (Table 1). The 14C data obtained from the lowest sample point yielded a younger age than that obtained by stratigraphy of the core. Therefore, we concluded that this leaf sample was included material mixed from above layers, and excluded this data to obtain the depositional ages (Fig. 2). The average sedimentation rate was estimated to be ca. 0.55 mm/y based on the modal points of the calibrated probability distributions. Although the upper three dates were consistent with the stratigraphy of this core, the 14C data for 275 cm yielded a younger age Table 1 Results of AMS14C dating using plant fragments (leaf). Depth (cm)
δ13CPDB (‰)
14
65
−25.2
370 ± 25
95 230
−27.6 −26.1
935 ± 25 3765 ± 25
275
−26.2
4255 ± 25
310
−27.9
3950 ± 25
C date (BP)
26
Age range (cal BP) (2σ probability %)
Laboratory code (IAAA-)
319 - 392 (39.6%) 426 - 500 (60.4%) 793 - 919 (100.0%) 4006 - 4033 (5.4%) 4080 - 4183 (78.9%) 4189 - 4191 (0.1%) 4196 - 4235 (15.5%) 4728 - 4735 (1.0%) 4740 - 4750 (1.4%) 4820 - 4863 (97.6%) 4295 - 4335 (17.9%) 4339 - 4444 (71.2%) 4482 - 4513 (10.9%)
150502 150503 150054
172588
172589
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Fig. 3. K2O-TiO2 and MgO-FeO variation diagrams for volcanic glasses from the tephra layers of Barasantou Bog. Table 2 Average chemical compositions of volcanic glasses from the tephra layers of Barasantou Bog. Analyzed sample
Standard composition (proximal pumice)
Tephra layer
1st layer
1st layer
2nd layer
3rd layer
5th layer
Ta-a
Ko-c2
Ta-c2
Ma-d
Ma-g
No. of analysis
1
9
9
12
13
21
34
9
36
24
Identified tephra
Ta-a
Ko-c2
Ta-c2
Ma-d
Ma-g
SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Cl
77.14 0.32 12.50 1.86 0.02 0.32 1.88 3.35 2.47 0.14
76.46 0.45 12.53 2.43 0.09 0.46 2.25 3.36 1.82 0.15
76.93 0.33 12.52 2.04 0.04 0.34 2.03 3.50 2.14 0.13
72.17 0.66 13.97 3.78 0.16 0.97 4.01 3.53 0.64 0.10
73.06 0.61 13.90 3.17 0.14 0.76 3.58 3.96 0.72 0.11
76.92 0.33 12.35 1.50 0.06 0.34 2.25 3.80 2.32 0.12
76.11 0.45 12.38 2.40 0.08 0.50 2.57 3.72 1.77 0.13
77.26 0.30 12.24 1.74 0.05 0.33 1.99 3.72 2.25 0.11
72.59 0.65 14.06 3.61 0.14 0.98 3.77 3.44 0.69 0.08
73.01 0.63 14.03 3.31 0.16 0.76 3.56 3.68 0.69 0.08
Value are in weight %.
whose stratigraphic position indicates the highest pumice-fall deposit in the Mashu caldera formation period. Since most of the glass chemical composition from the fifth layer belonged to the compositional field of Ma-g (Fig. 2), we conclude that the fifth layer is Ma-g tephra (7.6 cal ka BP). The fourth layer (264–271 cm) had few volcanic ash particles; however, we could not extract them. If the identifications of the third and fifth layers are correct, then the fourth layer must contain volcanic ash with eruption ages between 4.0 and 7.5 cal ka BP. If it was derived from the Mashu volcano, then the fourth layer is presumed to be Ma-e tephra from 5.5 cal ka BP (Kishimoto et al., 2009). Another possible candidate for the fourth layer, the widespread Komagatake-g tephra (Ko-g; 6.5–6.6 cal ka BP; Nakamura and Hirakawa, 2004) does not contradict the stratigraphy. If the fourth layer is Ko-g tephra from 6.5 to 6.6 cal ka BP, then the thickness of the peat layer between the fourth and fifth layers should be much thinner than that between the third and fourth layers. However, the material between the fourth and fifth layers is too thick; therefore, the fourth layer is likely a Ma-e tephra from an age of 5.5 cal ka BP.
Table 3 The list of fossil pollen grains and spores detected in the Barasantou Bog peat core. Plant type
Taxonomic group
Tree
Abies, Tsuga, Picea, Larix, Pinus, Cryptomeria, Juglans, Salix, Carpinus, Betula, Alnus, Fagus, Quercus subgen. Lepidobalanus, Ulmus, Celtis, Ligustrum, other Oleaceae, Tilia, Ilex, Myrica gale, Ericaceae Typha, Poaceae, Cyperaceae, Liliaceae, Eriocaulon, Chenopodiaceae, Caryophyllaceae, Thalictrum, other Ranunculaceae, Sanguisorba, Rubus, Apiaceae, Menyanthes, Lamiaceae, Artemisia, other Asteraceae, Drosera, Utricularia Lycopodiaceae, monolete type fern spore, trilete type fern spore
Herb
Fern
Among the tree pollen grains, Quercus subgenus Lepidobalanus pollen grains dominated in all layers. The appearance rates of Alnus pollen were high from the bottom to about 240 cm. The coniferous pollen grains, such as Abies, Picea, and Pinus, increased beginning at about 150 cm, while Quercus subgenus Lepidobalanus pollen decreased slightly. M. gale pollen increased from 100 cm, and Betula and Alnus pollen grains tended to increase and Quercus subgenus Lepidobalanus pollen tended to decrease from about 70 cm (Fig. 4). Among the herb pollen grains, those of Poaceae and Cyperaceae exhibited extremely severe fluctuations in all layers. These pollen grains tended to decrease in Ma-e tephra and to increase following deposition of Ta-c and Ko-c2 tephra. Only Poaceae pollen increased after
4.3. Fossil pollen composition The fossil pollen grains and spores detected from the Barasantou Bog peat core are shown in Table 3. The appearance tedencies of major fossil pollen grains and spores are shown in Figs. 4 and 5. The selected light microphotographs of fossil pollen grains are shown in Fig. 6. 27
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Fig. 4. Tree pollen diagram for cored sediments from Barasantou Bog.
deposition of Ma-d tephra. The appearance rates of Poaceae, Thalictrum, and Artemisia pollen grains were high from the bottom to about 300 cm. The appearance rates of Thalictrum and Artemisia pollen grains were high in the lowest layer. The appearance rate of Sanguisorba pollen was high from 230 to 100 cm, and those of Apiaceae and Menyanthes pollen grains were high from 100 to 80 cm. Drosera and Utricularia pollen grains were detected above 80 cm (Fig. 5). Among fern spores, the appearance rate of monolete-type spores was high under 230 cm, and became low above 230 cm. That of triletetype spores was high under 100 cm, and abruptly became low above 100 cm; these spores decreased after deposition of the Ma-e, Ma-d, and Ko-c2 tephra (Fig. 5). When peat began to accumulate again after deposition of the Ma-g tephra, the Barasantou Bog was a slightly dry environment with a low
Fig. 5. Herb pollen and fern spore diagram for cored sediments from Barasantou Bog.
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Fig. 6. Selected light microphotographs of the fossil pollen found in the Barasantou Bog peat core. 1: Abies, 2: Picea, 3: Pinus, 4: Cryptomeria, 5: Myrica gale, 6: Juglans, 7: Betula, 8: Alnus, 9: Fagus, 10: Quercus subgen. Lepidobalanus, 11: Ulmus, 12: Tilia, 13: Ericaceae, 14: Poaceae, 15: Cyperaceae, 16: Chenopodiaceae, 17: Thalictrum, 18: Sanguisorba, 19: Menyanthes, 20: Utricularia, 21: Drosera, 22: Artemisa, 23: other Apiaceae, scale bar is 10 μm.
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might be a resolution problem for the peat samples and pollen analysis. Peat samples are land sediments with a relatively high resolution. The commingling of Ta-a volcanic glass with Ko-c2 tephra in the first layer suggests the possibility of upward or downward movement of volcanic glass particles. During this time period, the sediments may have been disturbed by plant roots and animals. The eruption interval between the two volcanoes was 45 years. The subsampling interval of the pollen analysis was 5 cm, and 5 cm of sediments corresponds to 35–150 years, based on the average sedimentation rate in Fig. 2. Therefore, further high-density subsampling, such as the 2-mm intervals analyzed by Kito et al. (2017), is necessary to evaluate the impact of tephra fall on vegetation.
groundwater level, where Alnus, Poaceae, Thalictrum, Artemisia, and ferns grew. In Kyogoku Mire, Picea pollen sharply decreased and Quercus pollen increased; its age was 7650 ± 120 BP (8.5 cal ka BP) (Igarashi, 2000). The same phenomenon has been confirmed at Toutsuru Mire (Matsuda, 1983), Habomai Bog (Igarashi et al., 2001), Ochiishi Mire (Morita, 2001a), and Yururi Island (Morita, 2001b). Vegetation dominated by deciduous broadleaf forests, mainly composed of Q. crispula, continued after 7.6 cal ka BP around this bog also based on this study. Abies and Picea pollen grains have increased rapidly since 2.5 cal ka BP. The same phenomenon has been confirmed in Barasan (Igarashi, 2016) and Ochiishi Cap Bog (Igarashi et al., 2001), with ages of 2475 ± 235 BP (ca. 2.5 cal ka BP) in Barasan (Igarashi, 2016), and ca. 2.5 cal ka BP in Ochiishi Cap Bog (Igarashi et al., 2001). P. glehnii forest, established in lowland wetlands in the northern and eastern parts of Hokkaido (Tatewaki, 1943), and Abies species are mixed in the coastal areas of this forest (Tatewaki, 1945). P. glehnii wetland forest is thought to have established around Barasantou bog in a humid environment (Morita, 2001a). Sanguisorba pollen increased from ca 4.0 cal ka BP, and Thalictrum pollen decreased until ca 2.5 cal ka BP; then, Menyanthes pollen increased from ca. 1.0 cal ka BP. S. tenuifolia var. alba was present on the low moor (Yabe and Ito, 1982). Myrica gale, Menyanthes trifoliata, Utricularia intermedia, and Drosera rotundifolia were present in the middle to high moor, and M. gale established in flooded conditions or by watersides (Tachibana and Ito, 1980). It is thought that Barasantou Bog became a low moor environment from ca. 2.5 cal ka BP, and a middle to high moor environment from ca. 1.0 cal ka BP. As the groundwater level gradually became high, Quercus subgenus Lepidobalanus pollen decreased and Betula pollen increased after ca. 0.8 cal ka BP; this phenomenon has also been shown by data from Orochigahara moor (Morita, 1984), Uryu-Numa moor (Morita, 1985), Nakayama moor (Hoshino and Nakamura, 2003), and Yururi Island (Morita, 2001b). Hoshino (1998) reported that this phenomenon is an effect of climate warming. However, considering that Betula platyphylla is a pioneer plant (Watanabe, 2009), and Ainu people did not use the perishable B. platyphylla as housing material (Kodama et al., 1969), B. platyphylla likely increased following Q. crispula cutting. Further investigation of these events is required.
5. Conclusion This study investigated the vegetation changes on eastern Hokkaido, particularly those induced by tephra fall, using AMS 14C dates, tephra identification, and pollen analysis results for 330-cm core sediments from the Barasantou Bog. The peat of Barasantou Bog began to accumulate again after deposition of the Ma-g tephra in 7.6 cal ka BP, after which this bog became a slightly dry environment with low groundwater, and then a swamp environment at about 4.0 cal ka BP. Picea glehnii and Abies sachalinensis species formed wetland forests near this bog from 2.5 cal ka BP. Based on the topography and groundwater level, a low moor environment found from ca. 2.5 cal ka BP, and a middle to high moor environment formed after ca. 1.0 cal ka BP. Humans may have influenced the vegetation after ca. 0.8 cal ka BP. Tephra of 10–15 cm thickness may not have had a significant impact on the surrounding vegetation. However, a temporary increase in Poaceae and Cyperaceae pollen grains was confirmed after some tephra falls. Since 5 cm of sediment corresponds to 35–150 years resolutions, further high-density subsampling is necessary to evaluate the impact of tephra fall on vegetation. Acknowledgments This work was partly supported by a Grant-in-Aid (26350411) for Scientific Research from the Japan Society for the Promotion of Science (JSPS). Anonymous reviewers and editor helped to improve the manuscript.
4.4. Impacts of vegetation by tephra fall The investigation of plant macrofossil records for Utasai Bog revealed that the peatland plant communities shifted from Sphagnum magellanicum to Cyperaceae species, including Moliniopsis japonica and Eriophorum vaginatum after deposition of the Ko-d tephra (Hughes et al., 2013). The investigation of fossil pollen records from Tashiro Bog revealed that Quercus subgen. Lepidobalanus and Poaceae pollen grains increased greatly, and Fagus and Ilex pollen grains decreased greatly, after deposition of To-Cu tephra (Kito et al., 2017). In this study, a temporary increase Poaceae and Cyperaceae pollen grains was confirmed after deposition of the Ta-c and Ko-c2 tephra, and a temporary increase in Poaceae pollen was confirmed after deposition of Ma-d tephra. However, some of our results were consistent with those of Hughes et al. (2013) and Kito et al. (2017), but the details are not clear. We were able to determine changes in the surrounding vegetation, but we could not confirm a clear effect of tephra fall on the vegetation in the current analysis. At thicknesses of 10–15 cm, tephra may have no clear impact on the surrounding vegetation, and it may only be possible to confirm a very temporary increase in pollen. However, it is thought that there was a great impact on wetland plants that were several centimeters high, such as Utricularia intermedia and Drosera rotundifolia. The pollen production of these plants is very low, and it is possible that changes in those wetland plants affected by tephra falls were not detected in our pollen analysis. We should further increase the number of measurement of fossil pollen grains in the samples just below and above the tephra layers to detect these wetland plant pollen properly. Furthermore, there
References Betsukai-cho, 2013. Biomass Industrial City Concept, Betsukai-cho. pp. 38 (in Japanese). Endo, K., Igarashi, Y., Sumita, M., Miyata, Y., 1988. Paleoenvironmens during the last 15,000 years from barasan peat layers, eastern Hokkaido. In: Izeki, K. (Ed.), Research-results Report of Grant-in-aid for Scientific Research (A) 61302084, pp. 45–52 (in Japanese). Erdtman, G., 1934. Über die Verwendund von Essigsäureanhydrid bei Pollenuntersuchungen. Svenska Botanica Tiddskrift 28, 354–361. Fujiki, T., Okuno, M., Nakamura, T., Nagaoka, S., Mori, Y., Ueda, K., Konomatsu, M., Aizawa, J., 2013. AMS radiocarbon dating and pollen analysis of core Ks0412-3 from Kashibaru Marsh in northern Kyushu, southwest Japan. Radiocarbon 55, 1693–1701. Furukawa, R., Nakagawa, M., Furukata, C., Yoshimoto, M., 2006. Pre-historic activities of Tarumai volcano, Hokkaido, north Japan. Chikyu Mon. 28, 302–307 (in Japanese). Hoshino, F., Nakamura, T., 2003. Vegetation history based on the pollen record and AMS-14C dating, at Nakayama mire of Loc.2 in south- western Hokkaido, Japan, since the late glacial substage. Sum. Res. AMS Nagoya Univ. 14, 91–97 (in Japanese with English abstract). Hoshino, F., 1998. Vegetation history of Hokkaido (II), southern Hokkaido. In: Yasuda, Y., Miyoshi, N. (Eds.), Illustration: Vegetation History of the Japanese Archipelago. 51–61. Asakura Publishing Co., Ltd., Tokyo (in Japanese). Hughes, P.D.M., Mallon, G., Browna, A., Essex, H.J., Stanford, J.D., Hotesc, S., 2013. The impact of high tephra loading on late-Holocene carbon accumulation and vegetation succession in peatland communities. Quat. Sci. Rev. 67, 160–175. Igarashi, Y., 2000. Vegetation history around the Kyogoku Mire, southwestern Hokkaido, during the last 13,000 ears. Jpn. J. Ecol. (Tokyo) 50, 99–110 (in Japanese with English abstract). Igarashi, Y., Igarashi, T., Endo, K., Yamada, O., Nakagawa, M., Sumita, M., 2001. Vegetation history since the late glacial of Habomai bog and Ochiishi cape bog, nemuro peninsula, eastern Hokkaido, north Japan. Jpn. J. Hist. Bot. 10, 67–79 (in Japanese with English abstract).
30
Quaternary International 503 (2019) 24–31
T. Fujiki et al.
g) in Hokkaido, northern Japan. Quat. Res. (Daiyonki-Kenkyu) 43, 189–200 (in Japanese with English abstract). Nakamura, Y., 2016. Stratigraphy, distribution, and petrographic properties of Holocene tephras in Hokkaido, northern Japan. Quat. Int. 397, 52–62. Okumura, K., 1991. Quaternary tephra studies in the Hokkaido district, northern Japan. Quat. Res. (Daiyonki-Kenkyu) 30, 379–390 (in Japanese with English abstract). Ooi, N., Tsuji, S., 1989. Palynological study of the peat sediments around the last glacial Maximum at hikone, the east shore of Lake biwa, Japan. J. Phytogeogr. Taxon. 37, 37–42. Reimer, R.W., Richards, D.A., Scott, E.M., Southon, J.R., Staff, R.A., Turney, C.S.M., van der Plicht, J., 2013. IntCal13 and Marine13 radiocarbon age calibration curves 0–50,000 years cal BP. Radiocarbon 55, 1869–1887. Stuiver, M., Reimer, P.J., 1993. Extended 14C data base and revised CALIB 3.0 14C age calibration program. Radiocarbon 35, 215–230. Stuiver, M., Reimer, P.J., Reimer, R.W., 2015. CALIB 7.1 [WWW program]. http://calib. org, Accessed date: 30 August 2015. Tachibana, H., Ito, K., 1980. Phytosocilogical studies of the sarobetsu mire in the northern part of Hokkaido, Japan. Environ. Sci. 3, 73–134 (in Japanese with English abstract). Tatewaki, M., 1943. Phytosociological study on the Picea glehnii forest. Res. Bull. Coll. Exp. For. Hokkaido Imper. Univ. 13, 1–181 (in Japanese). Tatewaki, M., 1945. Flora of Northern Great East Asia Saizenkan, Osaka (in Japanese). Tokui, Y., 1989. Volcanic eruptions and their effects on human activity, in Hokkaido, Japan. Annals of Ochanomizu Geographical Society 30, 27–33 (in Japanese). Tsuji, S., 1985. Volcanic activity and ancient environment. In: Kondo, Y., Amakasu, K., Sahara, M., Tozawa, M., Yokoyama, K., Kato, S., Tanaka, M. (Eds.), Iwanami Series: Japanese Archaeology. Vol. 2. Humans and Environment. Iwanami, Tokyo, pp. 290–317 (in Japanese). Tsuji, S., Kosugi, M., 1991. Influence of Aira-Tn ash (AT) eruption on ecosystem. Quat. Res. (Daiyonki-Kenkyu) 30, 419–426 (in Japanese with English abstract). Tsujii, T., Tachibana, H., 2003. Wetland Plants and Vegetation of Hokkaido. Hokkaido University Press, Sapporo (in Japanese). Watanabe, K., 2009. Why Does Acer pictum Blight the Shaft by Oneself? -Individuality of Tree and Survival Strategy. Tsukiji Shokan Publishing Co., Ltd., Tokyo (in Japanese). Yabe, K., Ito, K., 1982. Ecological studies of Utonaito Bog near Tomakomai, Hokkaido: Numerical analysis of plant communities. Environ. Sci. Hokkaido Univ. J. Grad. Sch. Environ. Sci. 5, 107–129 (in Japanese with English abstract). Yamamoto, T., Itoh, J., Nakagawa, M., Hasegawa, T., Kishimoto, H., 2010. 14C ages for the ejecta from Kutcharo and Mashu calderas, eastern Hokkaido, Japan. Bull. Geol. Surv. Jpn. 61, 161–170 (in Japanese with English abstract).
Igarashi, Y., 2016. Vegetation and climate during the LGM and the last deglaciation on Hokkaido and Sakhalin Islands in the northwest pacific. Quat. Int. 425, 28–37. Katsui, Y., Ando, S., Inaba, K., 1975. Formation and magmatic evolution of Mashu volcano, east Hokkaido, Japan. J. Facul. Sci. Hokkaido Univ. 16, 533–552. Katsui, Y., Ishikawa, T., 1981a. Study on eruptive history and products, and disaster map of Tarumai volcano for volcanic hazard assessment. In: Shimozuru, D. (Ed.), Report on the Characteristics Hazard Mapping, and Forecasting of Eruptive Hazards, Research Report of the Grant-in-aid for Scientific Research from Ministry Education. Science and Culture of Japan, pp. 9–13 (in Japanese). Katsui, Y., Ishikawa, T., 1981b. Komagatake, Hokkaido. In: Shimozuru, D. (Ed.), Report on the Characteristics Hazard Mapping, and Forecasting of Eruptive Hazards, Research Report of the Grant-in-aid for Scientific Research from Ministry Education. Science and Culture of Japan, pp. 23–29 (in Japanese). Kishimoto, H., Hasegawa, K., Nakagawa, M., Wada, K., 2009. Tephrostratigraphy and eruption style of Mashu volcano, during the last 14,000 years, eastern Hokkaido, Japan. Bull. Volcanol. Soc. Jpn. 54, 15–36 (in Japanese with English abstract). Kito, N., Ohtsuki, J., Tsuji, S., Tuji, K., 2017. The impact of tephra fall on vegetation and long-term recover processes: a case study of the mid-Holocene Towada-Chuseri Tephra fall. Jpn. J. Hist. Bot. 26, 15–26 (in Japanese with English abstract). Kodama, S., Inukai, T., Takakura, S., 1969. Ainu Ethnography. Daiichihouki Press (in Japanese). Machida, H., Arai, F., 2003. Atlas of Tephra in and Around Japan (Revised Edition). University of Tokyo Press, Tokyo (in Japanese). Matsuda, I., 1983. Pollen analytical study in Shari region I. Toutsuru Mire. Bull. Shiretoko Museum 5, 77–93 (in Japanese). Miyata, Y., Yamaguchi, S., Yazaki, K., 1988. Geology of the Kenebetsu District. Quadrangle Series, Scale 1:50000. Geological Survey of Japan, Tsukuba (in Japanese with English abstract). Miyawaki, A., 1988. The Vegetation of Japan, vol. 9 Hokkaido. Shibundo, Tokyo (in Japanese). Morita, Y., 1984. Preliminary palynological studies of the moors in the uplands in Hokkaido. Ecol. Rev. 20, 237–240. Morita, Y., 1985. Palynological study of the deposits from the uryu-numa moor in Mt. Shokanbetsu, Hokkaido. Annals of Tohoku Geographer Association 37, 166–172 (in Japanese with English abstract). Morita, Y., 2001a. Vegetation history in nemuro peninsula since the late pleistocene. Veg. Sci. 18, 39–44 (in Japanese with English abstract). Morita, Y., 2001b. Vegetation history of Yururi Island in easternmost Hokkaido since the late glacial. Jpn. J. Hist. Bot. 10, 81–89 (in Japanese with English abstract). Nakamura, Y., Hirakawa, K., 2004. Mid-holocene widespread tephra, Komagatake-g (Ko-
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Vegetation history and the impact of tephra deposition during 7000 years based on pollen and tephra analysis of a Barasantou Bog sediment core, eastern Hokkaido, northern Japan
T
Toshiyuki Fujikia,∗, Keiji Wadab, Eiichi Satoc, Mitsuru Okunod a
Department of Applied Science, Faculty of Science, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama, 700-0005, Japan Earth Science Laboratory, Asahikawa Campus, Hokkaido University of Education, 9 Hokumon-cho, Asahikawa, 070-8621, Japan c Institute for Promotion of Higher Education, Kobe University, Tsurukabuto 1-2-1, Nada-ku, Kobe, 657-8501, Japan d Department of Earth System Science, Faculty of Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka, 814-0180, Japan b
A B S T R A C T
To estimate the impact of a volcanic eruption on vegetation, we conducted a field survey and collected peaty sediments and made pollen and tephra analysis from Barasantou Bog, in eastern Hokkaido, northern Japan. Using an electron probe micro-analyzer, we identified five tephra layers in the core: Ma-g (7.6 cal ka BP), Ma-e (5.5 cal ka BP), Ma-d (4.0 cal ka BP), Ta-c (2.5 cal ka BP), and Ko-c2 (AD 1694). When the peat began to accumulate again after deposition of the Ma-g tephra, the groundwater level of this area was very low, and Alnus, Poaceae, Thalictrum, Artemisia, and ferns dominated. Gradually, the groundwater level increased, and the condition of this wetland approached the present conditions, about 5.0 to 4.5 cal ka BP. Pinaceae conifers increased 2.5 cal ka BP. It is presumed that Picea glehnii and Abiesspecies formed wetland forests. Myrica gale and Alnus increased again, and grew to the edge of the wetland. At the same time, Quercus subgen. Lepidobalanus decreased and Betula increased. This trend might be due to the influence of human activity. The wetland vegetation changed from Thalictrum and Artemisia to Sanguisorba, and then from Sanguisorba to Menyanthes after the deposition of Ma-g tephra or after 2.5 ka BP. It was thought that the wetland changed from low moor to middle or high moor. A tephra layer tens of cm thick is not expected to affect pollen composition, and may have no clear impact on the surrounding vegetation. Sediment with a thickness of a few cm requires a deposition time of approximately 10 years; pollen analysis results are on the scale of the average vegetation change during that period. We conclude that short-term vegetation changes were not detected.
1. Introduction Many volcanoes erupted in Hokkaido during the Holocene (Nakamura, 2016; Machida and Arai, 2003). Consequently, many tephra layers have been confirmed in sediments, and these are important for determining the sedimentation age. Most previous research on the influence of an eruption on vegetation has discussed how the climate change accompanying the eruption changed the vegetation (Tsuji, 1985; Ooi and Tsuji, 1989; Tsuji and Kosugi, 1991). Some studies have examined the direct influence of tephra fall on the surrounding vegetation (Tsuji and Kosugi, 1991; Hughes et al., 2013; Kito et al., 2017). Hughes et al. (2013) investigated wetland vegetation change after tephra fall using plant macrofossil analysis at Utasai Bog, Hokkaido, and showed that wetland plant communities shifted from Sphagnum to monocotyledons. They concluded that this vegetation change was affected by an increase in peat humification and a decrease in carbon accumulation. According to Kito et al. (2017), the amounts of Quercus subgen. Lepidobalanus and Poaceae pollen grains increased, and those of other tree pollen grains decreased, after tephra fall at Tashiro Bog,
∗
Aomori. These results are likely due to differences in resistance to tephra among tree species, and increases in the water level following tephra deposition. Igarashi (2016) and Endo et al. (1988) reported the pollen composition since Last Glacial Maximum (LGM) from peat layers on the coastal cliffs of Barasan, eastern Hokkaido. We think it possible that the peat might be mixed into the tephra layer on the coastal cliff. Thus, we attempted to sample peat sediments in Barasantou Bog, which is located upstream. We also conducted radiocarbon dating using accelerator mass spectrometry (AMS), and identified the tephra based on the main component chemical composition analysis of volcanic glass using an electron probe micro-analyzer (EPMA). We determined the age of the peat sediments and reconstructed the paleovegetation around Lake Barasantou using the changes in the fossil pollen grains contained in this sediment. We also considered the impact of tephra fall on the vegetation.
Corresponding author. E-mail address: [email protected] (T. Fujiki).
https://doi.org/10.1016/j.quaint.2018.10.013 Received 24 March 2018; Received in revised form 7 October 2018; Accepted 13 October 2018 Available online 15 October 2018 1040-6182/ © 2018 Published by Elsevier Ltd.
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Fig. 1. Maps showing the location of Lake Barasantou (black circle). (a) Map of Hokkaido Island. Black triangles indicate source volcanoes for the Holocene tephra layers. Ko: Mt. Komagatake (1131 m), Ta: Mt. Tarumae (1041 m), Ma: Mt. Mashu (857 m). Black squares indicate the quoted analysis point. 1: Utasai bog (Hughes et al., 2013), 2: Orochigahara bog (Morita, 1984), Kyogoku mire (Igarashi, 2000), Nakayama mire (Hoshino and Nakamura, 2003), 3: Uryu-Numa moor (Morita, 1985), 4: Toutsuru mire (Matsuda, 1983), 5: Ochiishi bog (Morita, 2001a), 6: Yururi Island (Morita, 2001b), 7: Habomai bog (Igarashi et al., 2001), 8: Barasan (Igarashi, 2016; Endo et al., 1988). (b) Map showing sampling site (black circle) around near Lake Barasantou (N 43° 25′ 23“, E 145° 15’ 13“, 4.4 m asl). Topographic map is part of 1/25,000 map sheet “Tokotan” issued from Geospatial Information Authority of Japan (GSI).
2. Site description Lake Barasantou is a dammed lake located in the middle reach of the Nishimarubetu River in Betsukai, eastern Hokkaido. It has an area of 0.3 km2 and the average water depth is 5 m. Barasantou Bog comprises the low to middle moors that formed around the lake (Fig. 1). The average temperature in Betsukai is 5–6 °C, and the mean annual rainfall is about 900 mm (Betsukai-cho, 2013). The coastal region receives little snowfall due to the marine climate (Betsukai-cho, 2013). Brasenia schreberi and Nuphar japonicum grow in the lake, and Phragmites australis, Carex sp., Alnus japonica, and Myrica gale grow in the low moors (Tsujii and Tachibana, 2003). Wetland grasslands, mainly comprising Carex spp. and secondary vegetation dominated by Rhododendron kaempferi and Quercus crispula, have established around the lake; however, a vast area has been converted to rangeland composed of Phleum pratense (Miyawaki, 1988). A 330-cm-long sediment core was obtained on September 3, 2014, at a location in Barasantou Bog (43°25′23″N, 145°15′13″E, 4.4 m a.s.l.) using a peat sampler. The sediment core was composed of undecomposed peat and five tephra layers were identified, at 38–41, 150–159, 207–216, 264–271, and 315–330 cm (Fig. 2).
3. Analytical methods 3.1. AMS radiocarbon dating Fig. 2. Columnar section showing the stratigraphy and age-depth profile of Barasantou Bog.
The leaf fragments were sub-sampled for age determination. Samples were cleaned by routine acid–alkali–acid (AAA) treatments to remove carbonates and secondary organic acids. After washing with distilled water and drying, each pretreated sample was converted to CO2 gas, and then reduced catalytically to graphite on Fe powder using H2 gas. The three carbon isotopes were measured in the samples and NIST oxalic acid (HOxII) standard using an NEC Peletron 9SDH-2 AMS system at the Institute of Accelerator Analysis Ltd (IAA). The 13C/12C ratios (δ13CPDB) obtained by AMS were used to correct carbon isotopic fractionation when calculating conventional 14C age. To estimate and remove the 14C background level, the 14C concentration of commercial
graphite powder was also measured in the same analytic sequence as the sample and standard measurements. We calibrated 14C dates measured by AMS to a calendar year timescale using the IntCal13 dataset (Reimer et al., 2013) and the program CALIB 7.1 (Stuiver and Reimer, 1993; Stuiver et al., 2015).
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than that inferred from identified tephra (Ma-e). Yamamoto et al. (2010) reported the eruption age of Ma-e tephra as 4720 ± 40 BP, but Miyata et al. (1988) described it as 4150 ± 40 BP. Their calendar ages were ca 5.5 cal ka BP and ca. 4.7 cal ka BP, respectively. The 14C age obtained in the current study was closer to that of Miyata et al. (1988). The eruption age of Ma-e tephra should be determined in a future study.
3.2. Tephra analysis Each tephra was identified from the chemical composition of the volcanic glass as follows. Tephra samples were embedded in resin and thin sections were obtained by double-sided polishing. Then, these were mirror-polished using diamond paste. The chemical composition of the volcanic glass was measured using wave dispersive WDS-EPMA (JEOLJXA 8600). The accelerating voltage was 15 kV and the current was 0.8 × 10−8 A. The scanning area of the electron beam was < 100 μm2. Ten oxide elements were measured: Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and Cl. The standard glass compositions of Ta-a, Ko-c2, Ta-c2, Ma-d, and Ma-g pumices were analyzed by WDS-EPMA at the Hokkaido University of Education at Asahikawa.
4.2. Tephra identification Tephra identification was mainly based on the characteristics of the tephra particles, the chemical composition of tephra glass, and the sequence of the tephra layer. The first layer (38–41 cm) contained many well-foamed glass particles, and the glass composition was consistent with that of Komagatake-c2 tephra (Ko-c2; AD 1694; Katsui and Ishikawa, 1981b; Tokui, 1989) (Fig. 3and Table 2). However, the glass particles in 10% of samples were confirmed to be derived from Tarumai-a tephra (Ta-a; AD 1739; Katsui and Ishikawa, 1981a). The eruption ages of Ko-c2 and Ta-a were closer. Therefore, we concluded that some particles from Ta-a tephra from the upper layer were mixed into the first layer. The second layer (150–159 cm) also contained many well-formed glass particles. The glass composition was in the compositional field of Ta-a tephra and Tarumai-c2 tephra (Ta-c2; 2.5 cal ka BP; Furukawa et al., 2006); however, the composition of this layer was similar to that of Ta-c2 in the FeO-MgO diagram (Fig. 3). The second layer was at a lower stratigraphic position than Ko-c2 at AD 1694, and its age was consistent with that of Ta-c2 (Fig. 2). The third layer (207–216 cm) consisted of fallen pumice, indicating that this tephra was derived from a neighboring volcano. As the amount of K2O in the volcanic glass in the pumice was below 1.0 wt% (Fig. 3), this pumice was derived from the Mashu volcano (Okumura, 1991). Five plinian pumice-fall layers, Ma-b, Ma-d, Ma-g, Ma-h, and Ma-i, were confirmed to have come from the Mashu volcano in the past 10,000 years (Katsui et al., 1975). Ma-i, Ma-h, and Ma-g tephras were deposited continuously in that order during the caldera formation period of 7.5 cal ka BP (Kishimoto et al., 2009). Ma-b tephra is excluded as a candidate for the third layer due to its eruption age (ca. 1.0 cal ka BP; Katsui et al., 1975) and its northern distribution area. The third layer is likely either Ma-d or Ma-g tephra. The average standard chemical composition of volcanic glass in Ma-d pumice was higher in FeO, MgO, and CaO contents than that in Ma-g pumice (Table 2). As shown in the FeO-MgO diagram (Fig. 2), all glass compositions of the third layer belong to the compositional field of the Ma-d tephra. Thus, the third layer corresponds to Ma-d tephra of 4 cal ka BP (Kishimoto et al., 2009). The fifth layer (315–330 cm) consisted of fallen pumice; however, the bottom of this core did not reach the lower layer of this tephra. The amount of K2O in the volcanic glass was below 1.0 wt% (Table 2); therefore, this tephra was derived from the Mashu volcano. As the third layer was identified as Ma-d tephra, the fifth layer is likely Ma-g tephra,
3.3. Pollen analysis A few grams of sample were collected for pollen analysis at approximately 5-cm intervals of the core, and from just above and below the tephra. Fossil pollen and spores were extracted with 10% KOH treatment, ZnCl2 solution treatment, and Erdtman's acetolysis method (Erdtman, 1934). To determine pollen assemblages, the samples were dehydrated with an ethanol series (30%, 60%, and 99.5%) and then treated with xylene after acetolysis. The samples were mounted in Eukitt medium for observation under a light microscope (Fujiki et al., 2013). At least 500 pollen grains (excluding spores) were counted in each sample, including at least 300 arboreal pollen (AP) grains. The percentages for each taxon in the total AP were calculated. Alnus and Myrica gale pollen grains were excluded from the total AP because of their high frequencies and fluctuations that would hinder comparison among other taxa (Fujiki et al., 2013). 4. Results and discussion 4.1. AMS radiocarbon dating The AMS 14C dating of three samples yielded ages of 3945 ± 25 BP (IAAA-172589) for 310 cm, 4255 ± 25 BP (IAAA-172588) for 275 cm, 3765 ± 25 BP (IAAA-150504) for 230 cm, 935 ± 25 BP (IAAA150503) for 95 cm, and 370 ± 25 BP (IAAA-150502) for 65 cm (Table 1, Fig. 2). Their calendar ages were ca. 4.4 cal ka BP, ca. 4.8 cal ka BP, ca. 4.1 cal ka BP, 0.8 cal ka BP, and 0.4 cal ka BP, respectively (Table 1). The 14C data obtained from the lowest sample point yielded a younger age than that obtained by stratigraphy of the core. Therefore, we concluded that this leaf sample was included material mixed from above layers, and excluded this data to obtain the depositional ages (Fig. 2). The average sedimentation rate was estimated to be ca. 0.55 mm/y based on the modal points of the calibrated probability distributions. Although the upper three dates were consistent with the stratigraphy of this core, the 14C data for 275 cm yielded a younger age Table 1 Results of AMS14C dating using plant fragments (leaf). Depth (cm)
δ13CPDB (‰)
14
65
−25.2
370 ± 25
95 230
−27.6 −26.1
935 ± 25 3765 ± 25
275
−26.2
4255 ± 25
310
−27.9
3950 ± 25
C date (BP)
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Age range (cal BP) (2σ probability %)
Laboratory code (IAAA-)
319 - 392 (39.6%) 426 - 500 (60.4%) 793 - 919 (100.0%) 4006 - 4033 (5.4%) 4080 - 4183 (78.9%) 4189 - 4191 (0.1%) 4196 - 4235 (15.5%) 4728 - 4735 (1.0%) 4740 - 4750 (1.4%) 4820 - 4863 (97.6%) 4295 - 4335 (17.9%) 4339 - 4444 (71.2%) 4482 - 4513 (10.9%)
150502 150503 150054
172588
172589
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Fig. 3. K2O-TiO2 and MgO-FeO variation diagrams for volcanic glasses from the tephra layers of Barasantou Bog. Table 2 Average chemical compositions of volcanic glasses from the tephra layers of Barasantou Bog. Analyzed sample
Standard composition (proximal pumice)
Tephra layer
1st layer
1st layer
2nd layer
3rd layer
5th layer
Ta-a
Ko-c2
Ta-c2
Ma-d
Ma-g
No. of analysis
1
9
9
12
13
21
34
9
36
24
Identified tephra
Ta-a
Ko-c2
Ta-c2
Ma-d
Ma-g
SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Cl
77.14 0.32 12.50 1.86 0.02 0.32 1.88 3.35 2.47 0.14
76.46 0.45 12.53 2.43 0.09 0.46 2.25 3.36 1.82 0.15
76.93 0.33 12.52 2.04 0.04 0.34 2.03 3.50 2.14 0.13
72.17 0.66 13.97 3.78 0.16 0.97 4.01 3.53 0.64 0.10
73.06 0.61 13.90 3.17 0.14 0.76 3.58 3.96 0.72 0.11
76.92 0.33 12.35 1.50 0.06 0.34 2.25 3.80 2.32 0.12
76.11 0.45 12.38 2.40 0.08 0.50 2.57 3.72 1.77 0.13
77.26 0.30 12.24 1.74 0.05 0.33 1.99 3.72 2.25 0.11
72.59 0.65 14.06 3.61 0.14 0.98 3.77 3.44 0.69 0.08
73.01 0.63 14.03 3.31 0.16 0.76 3.56 3.68 0.69 0.08
Value are in weight %.
whose stratigraphic position indicates the highest pumice-fall deposit in the Mashu caldera formation period. Since most of the glass chemical composition from the fifth layer belonged to the compositional field of Ma-g (Fig. 2), we conclude that the fifth layer is Ma-g tephra (7.6 cal ka BP). The fourth layer (264–271 cm) had few volcanic ash particles; however, we could not extract them. If the identifications of the third and fifth layers are correct, then the fourth layer must contain volcanic ash with eruption ages between 4.0 and 7.5 cal ka BP. If it was derived from the Mashu volcano, then the fourth layer is presumed to be Ma-e tephra from 5.5 cal ka BP (Kishimoto et al., 2009). Another possible candidate for the fourth layer, the widespread Komagatake-g tephra (Ko-g; 6.5–6.6 cal ka BP; Nakamura and Hirakawa, 2004) does not contradict the stratigraphy. If the fourth layer is Ko-g tephra from 6.5 to 6.6 cal ka BP, then the thickness of the peat layer between the fourth and fifth layers should be much thinner than that between the third and fourth layers. However, the material between the fourth and fifth layers is too thick; therefore, the fourth layer is likely a Ma-e tephra from an age of 5.5 cal ka BP.
Table 3 The list of fossil pollen grains and spores detected in the Barasantou Bog peat core. Plant type
Taxonomic group
Tree
Abies, Tsuga, Picea, Larix, Pinus, Cryptomeria, Juglans, Salix, Carpinus, Betula, Alnus, Fagus, Quercus subgen. Lepidobalanus, Ulmus, Celtis, Ligustrum, other Oleaceae, Tilia, Ilex, Myrica gale, Ericaceae Typha, Poaceae, Cyperaceae, Liliaceae, Eriocaulon, Chenopodiaceae, Caryophyllaceae, Thalictrum, other Ranunculaceae, Sanguisorba, Rubus, Apiaceae, Menyanthes, Lamiaceae, Artemisia, other Asteraceae, Drosera, Utricularia Lycopodiaceae, monolete type fern spore, trilete type fern spore
Herb
Fern
Among the tree pollen grains, Quercus subgenus Lepidobalanus pollen grains dominated in all layers. The appearance rates of Alnus pollen were high from the bottom to about 240 cm. The coniferous pollen grains, such as Abies, Picea, and Pinus, increased beginning at about 150 cm, while Quercus subgenus Lepidobalanus pollen decreased slightly. M. gale pollen increased from 100 cm, and Betula and Alnus pollen grains tended to increase and Quercus subgenus Lepidobalanus pollen tended to decrease from about 70 cm (Fig. 4). Among the herb pollen grains, those of Poaceae and Cyperaceae exhibited extremely severe fluctuations in all layers. These pollen grains tended to decrease in Ma-e tephra and to increase following deposition of Ta-c and Ko-c2 tephra. Only Poaceae pollen increased after
4.3. Fossil pollen composition The fossil pollen grains and spores detected from the Barasantou Bog peat core are shown in Table 3. The appearance tedencies of major fossil pollen grains and spores are shown in Figs. 4 and 5. The selected light microphotographs of fossil pollen grains are shown in Fig. 6. 27
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Fig. 4. Tree pollen diagram for cored sediments from Barasantou Bog.
deposition of Ma-d tephra. The appearance rates of Poaceae, Thalictrum, and Artemisia pollen grains were high from the bottom to about 300 cm. The appearance rates of Thalictrum and Artemisia pollen grains were high in the lowest layer. The appearance rate of Sanguisorba pollen was high from 230 to 100 cm, and those of Apiaceae and Menyanthes pollen grains were high from 100 to 80 cm. Drosera and Utricularia pollen grains were detected above 80 cm (Fig. 5). Among fern spores, the appearance rate of monolete-type spores was high under 230 cm, and became low above 230 cm. That of triletetype spores was high under 100 cm, and abruptly became low above 100 cm; these spores decreased after deposition of the Ma-e, Ma-d, and Ko-c2 tephra (Fig. 5). When peat began to accumulate again after deposition of the Ma-g tephra, the Barasantou Bog was a slightly dry environment with a low
Fig. 5. Herb pollen and fern spore diagram for cored sediments from Barasantou Bog.
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Fig. 6. Selected light microphotographs of the fossil pollen found in the Barasantou Bog peat core. 1: Abies, 2: Picea, 3: Pinus, 4: Cryptomeria, 5: Myrica gale, 6: Juglans, 7: Betula, 8: Alnus, 9: Fagus, 10: Quercus subgen. Lepidobalanus, 11: Ulmus, 12: Tilia, 13: Ericaceae, 14: Poaceae, 15: Cyperaceae, 16: Chenopodiaceae, 17: Thalictrum, 18: Sanguisorba, 19: Menyanthes, 20: Utricularia, 21: Drosera, 22: Artemisa, 23: other Apiaceae, scale bar is 10 μm.
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might be a resolution problem for the peat samples and pollen analysis. Peat samples are land sediments with a relatively high resolution. The commingling of Ta-a volcanic glass with Ko-c2 tephra in the first layer suggests the possibility of upward or downward movement of volcanic glass particles. During this time period, the sediments may have been disturbed by plant roots and animals. The eruption interval between the two volcanoes was 45 years. The subsampling interval of the pollen analysis was 5 cm, and 5 cm of sediments corresponds to 35–150 years, based on the average sedimentation rate in Fig. 2. Therefore, further high-density subsampling, such as the 2-mm intervals analyzed by Kito et al. (2017), is necessary to evaluate the impact of tephra fall on vegetation.
groundwater level, where Alnus, Poaceae, Thalictrum, Artemisia, and ferns grew. In Kyogoku Mire, Picea pollen sharply decreased and Quercus pollen increased; its age was 7650 ± 120 BP (8.5 cal ka BP) (Igarashi, 2000). The same phenomenon has been confirmed at Toutsuru Mire (Matsuda, 1983), Habomai Bog (Igarashi et al., 2001), Ochiishi Mire (Morita, 2001a), and Yururi Island (Morita, 2001b). Vegetation dominated by deciduous broadleaf forests, mainly composed of Q. crispula, continued after 7.6 cal ka BP around this bog also based on this study. Abies and Picea pollen grains have increased rapidly since 2.5 cal ka BP. The same phenomenon has been confirmed in Barasan (Igarashi, 2016) and Ochiishi Cap Bog (Igarashi et al., 2001), with ages of 2475 ± 235 BP (ca. 2.5 cal ka BP) in Barasan (Igarashi, 2016), and ca. 2.5 cal ka BP in Ochiishi Cap Bog (Igarashi et al., 2001). P. glehnii forest, established in lowland wetlands in the northern and eastern parts of Hokkaido (Tatewaki, 1943), and Abies species are mixed in the coastal areas of this forest (Tatewaki, 1945). P. glehnii wetland forest is thought to have established around Barasantou bog in a humid environment (Morita, 2001a). Sanguisorba pollen increased from ca 4.0 cal ka BP, and Thalictrum pollen decreased until ca 2.5 cal ka BP; then, Menyanthes pollen increased from ca. 1.0 cal ka BP. S. tenuifolia var. alba was present on the low moor (Yabe and Ito, 1982). Myrica gale, Menyanthes trifoliata, Utricularia intermedia, and Drosera rotundifolia were present in the middle to high moor, and M. gale established in flooded conditions or by watersides (Tachibana and Ito, 1980). It is thought that Barasantou Bog became a low moor environment from ca. 2.5 cal ka BP, and a middle to high moor environment from ca. 1.0 cal ka BP. As the groundwater level gradually became high, Quercus subgenus Lepidobalanus pollen decreased and Betula pollen increased after ca. 0.8 cal ka BP; this phenomenon has also been shown by data from Orochigahara moor (Morita, 1984), Uryu-Numa moor (Morita, 1985), Nakayama moor (Hoshino and Nakamura, 2003), and Yururi Island (Morita, 2001b). Hoshino (1998) reported that this phenomenon is an effect of climate warming. However, considering that Betula platyphylla is a pioneer plant (Watanabe, 2009), and Ainu people did not use the perishable B. platyphylla as housing material (Kodama et al., 1969), B. platyphylla likely increased following Q. crispula cutting. Further investigation of these events is required.
5. Conclusion This study investigated the vegetation changes on eastern Hokkaido, particularly those induced by tephra fall, using AMS 14C dates, tephra identification, and pollen analysis results for 330-cm core sediments from the Barasantou Bog. The peat of Barasantou Bog began to accumulate again after deposition of the Ma-g tephra in 7.6 cal ka BP, after which this bog became a slightly dry environment with low groundwater, and then a swamp environment at about 4.0 cal ka BP. Picea glehnii and Abies sachalinensis species formed wetland forests near this bog from 2.5 cal ka BP. Based on the topography and groundwater level, a low moor environment found from ca. 2.5 cal ka BP, and a middle to high moor environment formed after ca. 1.0 cal ka BP. Humans may have influenced the vegetation after ca. 0.8 cal ka BP. Tephra of 10–15 cm thickness may not have had a significant impact on the surrounding vegetation. However, a temporary increase in Poaceae and Cyperaceae pollen grains was confirmed after some tephra falls. Since 5 cm of sediment corresponds to 35–150 years resolutions, further high-density subsampling is necessary to evaluate the impact of tephra fall on vegetation. Acknowledgments This work was partly supported by a Grant-in-Aid (26350411) for Scientific Research from the Japan Society for the Promotion of Science (JSPS). Anonymous reviewers and editor helped to improve the manuscript.
4.4. Impacts of vegetation by tephra fall The investigation of plant macrofossil records for Utasai Bog revealed that the peatland plant communities shifted from Sphagnum magellanicum to Cyperaceae species, including Moliniopsis japonica and Eriophorum vaginatum after deposition of the Ko-d tephra (Hughes et al., 2013). The investigation of fossil pollen records from Tashiro Bog revealed that Quercus subgen. Lepidobalanus and Poaceae pollen grains increased greatly, and Fagus and Ilex pollen grains decreased greatly, after deposition of To-Cu tephra (Kito et al., 2017). In this study, a temporary increase Poaceae and Cyperaceae pollen grains was confirmed after deposition of the Ta-c and Ko-c2 tephra, and a temporary increase in Poaceae pollen was confirmed after deposition of Ma-d tephra. However, some of our results were consistent with those of Hughes et al. (2013) and Kito et al. (2017), but the details are not clear. We were able to determine changes in the surrounding vegetation, but we could not confirm a clear effect of tephra fall on the vegetation in the current analysis. At thicknesses of 10–15 cm, tephra may have no clear impact on the surrounding vegetation, and it may only be possible to confirm a very temporary increase in pollen. However, it is thought that there was a great impact on wetland plants that were several centimeters high, such as Utricularia intermedia and Drosera rotundifolia. The pollen production of these plants is very low, and it is possible that changes in those wetland plants affected by tephra falls were not detected in our pollen analysis. We should further increase the number of measurement of fossil pollen grains in the samples just below and above the tephra layers to detect these wetland plant pollen properly. Furthermore, there
References Betsukai-cho, 2013. Biomass Industrial City Concept, Betsukai-cho. pp. 38 (in Japanese). Endo, K., Igarashi, Y., Sumita, M., Miyata, Y., 1988. Paleoenvironmens during the last 15,000 years from barasan peat layers, eastern Hokkaido. In: Izeki, K. (Ed.), Research-results Report of Grant-in-aid for Scientific Research (A) 61302084, pp. 45–52 (in Japanese). Erdtman, G., 1934. Über die Verwendund von Essigsäureanhydrid bei Pollenuntersuchungen. Svenska Botanica Tiddskrift 28, 354–361. Fujiki, T., Okuno, M., Nakamura, T., Nagaoka, S., Mori, Y., Ueda, K., Konomatsu, M., Aizawa, J., 2013. AMS radiocarbon dating and pollen analysis of core Ks0412-3 from Kashibaru Marsh in northern Kyushu, southwest Japan. Radiocarbon 55, 1693–1701. Furukawa, R., Nakagawa, M., Furukata, C., Yoshimoto, M., 2006. Pre-historic activities of Tarumai volcano, Hokkaido, north Japan. Chikyu Mon. 28, 302–307 (in Japanese). Hoshino, F., Nakamura, T., 2003. Vegetation history based on the pollen record and AMS-14C dating, at Nakayama mire of Loc.2 in south- western Hokkaido, Japan, since the late glacial substage. Sum. Res. AMS Nagoya Univ. 14, 91–97 (in Japanese with English abstract). Hoshino, F., 1998. Vegetation history of Hokkaido (II), southern Hokkaido. In: Yasuda, Y., Miyoshi, N. (Eds.), Illustration: Vegetation History of the Japanese Archipelago. 51–61. Asakura Publishing Co., Ltd., Tokyo (in Japanese). Hughes, P.D.M., Mallon, G., Browna, A., Essex, H.J., Stanford, J.D., Hotesc, S., 2013. The impact of high tephra loading on late-Holocene carbon accumulation and vegetation succession in peatland communities. Quat. Sci. Rev. 67, 160–175. Igarashi, Y., 2000. Vegetation history around the Kyogoku Mire, southwestern Hokkaido, during the last 13,000 ears. Jpn. J. Ecol. (Tokyo) 50, 99–110 (in Japanese with English abstract). Igarashi, Y., Igarashi, T., Endo, K., Yamada, O., Nakagawa, M., Sumita, M., 2001. Vegetation history since the late glacial of Habomai bog and Ochiishi cape bog, nemuro peninsula, eastern Hokkaido, north Japan. Jpn. J. Hist. Bot. 10, 67–79 (in Japanese with English abstract).
30
Quaternary International 503 (2019) 24–31
T. Fujiki et al.
g) in Hokkaido, northern Japan. Quat. Res. (Daiyonki-Kenkyu) 43, 189–200 (in Japanese with English abstract). Nakamura, Y., 2016. Stratigraphy, distribution, and petrographic properties of Holocene tephras in Hokkaido, northern Japan. Quat. Int. 397, 52–62. Okumura, K., 1991. Quaternary tephra studies in the Hokkaido district, northern Japan. Quat. Res. (Daiyonki-Kenkyu) 30, 379–390 (in Japanese with English abstract). Ooi, N., Tsuji, S., 1989. Palynological study of the peat sediments around the last glacial Maximum at hikone, the east shore of Lake biwa, Japan. J. Phytogeogr. Taxon. 37, 37–42. Reimer, R.W., Richards, D.A., Scott, E.M., Southon, J.R., Staff, R.A., Turney, C.S.M., van der Plicht, J., 2013. IntCal13 and Marine13 radiocarbon age calibration curves 0–50,000 years cal BP. Radiocarbon 55, 1869–1887. Stuiver, M., Reimer, P.J., 1993. Extended 14C data base and revised CALIB 3.0 14C age calibration program. Radiocarbon 35, 215–230. Stuiver, M., Reimer, P.J., Reimer, R.W., 2015. CALIB 7.1 [WWW program]. http://calib. org, Accessed date: 30 August 2015. Tachibana, H., Ito, K., 1980. Phytosocilogical studies of the sarobetsu mire in the northern part of Hokkaido, Japan. Environ. Sci. 3, 73–134 (in Japanese with English abstract). Tatewaki, M., 1943. Phytosociological study on the Picea glehnii forest. Res. Bull. Coll. Exp. For. Hokkaido Imper. Univ. 13, 1–181 (in Japanese). Tatewaki, M., 1945. Flora of Northern Great East Asia Saizenkan, Osaka (in Japanese). Tokui, Y., 1989. Volcanic eruptions and their effects on human activity, in Hokkaido, Japan. Annals of Ochanomizu Geographical Society 30, 27–33 (in Japanese). Tsuji, S., 1985. Volcanic activity and ancient environment. In: Kondo, Y., Amakasu, K., Sahara, M., Tozawa, M., Yokoyama, K., Kato, S., Tanaka, M. (Eds.), Iwanami Series: Japanese Archaeology. Vol. 2. Humans and Environment. Iwanami, Tokyo, pp. 290–317 (in Japanese). Tsuji, S., Kosugi, M., 1991. Influence of Aira-Tn ash (AT) eruption on ecosystem. Quat. Res. (Daiyonki-Kenkyu) 30, 419–426 (in Japanese with English abstract). Tsujii, T., Tachibana, H., 2003. Wetland Plants and Vegetation of Hokkaido. Hokkaido University Press, Sapporo (in Japanese). Watanabe, K., 2009. Why Does Acer pictum Blight the Shaft by Oneself? -Individuality of Tree and Survival Strategy. Tsukiji Shokan Publishing Co., Ltd., Tokyo (in Japanese). Yabe, K., Ito, K., 1982. Ecological studies of Utonaito Bog near Tomakomai, Hokkaido: Numerical analysis of plant communities. Environ. Sci. Hokkaido Univ. J. Grad. Sch. Environ. Sci. 5, 107–129 (in Japanese with English abstract). Yamamoto, T., Itoh, J., Nakagawa, M., Hasegawa, T., Kishimoto, H., 2010. 14C ages for the ejecta from Kutcharo and Mashu calderas, eastern Hokkaido, Japan. Bull. Geol. Surv. Jpn. 61, 161–170 (in Japanese with English abstract).
Igarashi, Y., 2016. Vegetation and climate during the LGM and the last deglaciation on Hokkaido and Sakhalin Islands in the northwest pacific. Quat. Int. 425, 28–37. Katsui, Y., Ando, S., Inaba, K., 1975. Formation and magmatic evolution of Mashu volcano, east Hokkaido, Japan. J. Facul. Sci. Hokkaido Univ. 16, 533–552. Katsui, Y., Ishikawa, T., 1981a. Study on eruptive history and products, and disaster map of Tarumai volcano for volcanic hazard assessment. In: Shimozuru, D. (Ed.), Report on the Characteristics Hazard Mapping, and Forecasting of Eruptive Hazards, Research Report of the Grant-in-aid for Scientific Research from Ministry Education. Science and Culture of Japan, pp. 9–13 (in Japanese). Katsui, Y., Ishikawa, T., 1981b. Komagatake, Hokkaido. In: Shimozuru, D. (Ed.), Report on the Characteristics Hazard Mapping, and Forecasting of Eruptive Hazards, Research Report of the Grant-in-aid for Scientific Research from Ministry Education. Science and Culture of Japan, pp. 23–29 (in Japanese). Kishimoto, H., Hasegawa, K., Nakagawa, M., Wada, K., 2009. Tephrostratigraphy and eruption style of Mashu volcano, during the last 14,000 years, eastern Hokkaido, Japan. Bull. Volcanol. Soc. Jpn. 54, 15–36 (in Japanese with English abstract). Kito, N., Ohtsuki, J., Tsuji, S., Tuji, K., 2017. The impact of tephra fall on vegetation and long-term recover processes: a case study of the mid-Holocene Towada-Chuseri Tephra fall. Jpn. J. Hist. Bot. 26, 15–26 (in Japanese with English abstract). Kodama, S., Inukai, T., Takakura, S., 1969. Ainu Ethnography. Daiichihouki Press (in Japanese). Machida, H., Arai, F., 2003. Atlas of Tephra in and Around Japan (Revised Edition). University of Tokyo Press, Tokyo (in Japanese). Matsuda, I., 1983. Pollen analytical study in Shari region I. Toutsuru Mire. Bull. Shiretoko Museum 5, 77–93 (in Japanese). Miyata, Y., Yamaguchi, S., Yazaki, K., 1988. Geology of the Kenebetsu District. Quadrangle Series, Scale 1:50000. Geological Survey of Japan, Tsukuba (in Japanese with English abstract). Miyawaki, A., 1988. The Vegetation of Japan, vol. 9 Hokkaido. Shibundo, Tokyo (in Japanese). Morita, Y., 1984. Preliminary palynological studies of the moors in the uplands in Hokkaido. Ecol. Rev. 20, 237–240. Morita, Y., 1985. Palynological study of the deposits from the uryu-numa moor in Mt. Shokanbetsu, Hokkaido. Annals of Tohoku Geographer Association 37, 166–172 (in Japanese with English abstract). Morita, Y., 2001a. Vegetation history in nemuro peninsula since the late pleistocene. Veg. Sci. 18, 39–44 (in Japanese with English abstract). Morita, Y., 2001b. Vegetation history of Yururi Island in easternmost Hokkaido since the late glacial. Jpn. J. Hist. Bot. 10, 81–89 (in Japanese with English abstract). Nakamura, Y., Hirakawa, K., 2004. Mid-holocene widespread tephra, Komagatake-g (Ko-
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