地球環境研究，Vol.18（2016）

Eruption Age of Sakurajima-Satsuma Tephra
Using Thermoluminescence Dating
SHITAOKA Yorinao ＊MORIWAKI Hiroshi ＊＊AKAI Fumito ＊＊＊NAKAMURA Naoko ＊＊＊＊MIYOSHI Masaya ＊＊＊＊＊ and YAMAMOTO Junji ＊＊＊＊＊＊
キーワード：Sakurajima-Satsuma, Young Kitadake stage, thermoluminescence dating, 14C dating, eruption age
southern Kyushu and surrounding marine regions of

1. Introduction

Sakurajima volcano because this tephra, with the larg-

Sakurajima, located on the southern volcanic front inest volume（ca. 11 km3: Kobayashi and Tameike, 2002）

Kyushu, is a very active volcano in Japan. Tephra bedsof Sakurajima tephras, is widespread throughout south-from Sakurajima volcano are collectively designed as

ern Kyushu. Therefore, age determination of this key

the Sakurajima-tephra （Sz） group （Kobayashi, 1986）.

tephra layer（Sz-S）is expected to contribute greatly to

The eruption history of Sakurajima volcano is classifiedstudies in many fields.

tephrostratigraphically into the following three stages:

Regarding the ages of Sz-S, Machida and Arai（1992）

Old Kitadake, Young Kitadake, and Minamidake

reported a radiocarbon （14C） age of 10,500 BP as an（Kobayashi and Ezaki, 1997）. The first eruption of theaverage of several charcoal ages obtained using theYoung Kitadake stage is designated as Sakurajima-Sat-beta counting method. Okuno（1997）obtained accelera-suma （Sz-S or P14） tephra （Kobayashi and Ezaki,

tor mass spectrometry （AMS） 14C ages of charcoal

1997）, which is volcanic ash of the largest eruption ofsamples from tephra beds of Sz group, and soil materi-

the Sz group（e.g. Moriwaki, 1992）. Sz-S derived fromals related closely to the tephra. The reported 14C ages

Sakurajima volcano is a key marker for volcanic stratig-

are presented in Table 1. Okuno（2002）estimated it asraphy, palaeoenvironmental studies （e.g. Moriwaki,

12,800 cal yrBP based on the 14C dates shown in Table

1992）,and archaeological studies（e.g. Kodama, 2001）in

1.

14C
Reported
C agesages ofof samples
to Sz-S（Okuno,1997） 1997)
Table 1.Table
The1reported
samplesrelated
relating
to Sz-S (Okuno,
14

10,670±100

Calibrated age(cal yrBP)
12,700–12,400

NUTA-4634

Charcoal

11,050±120

13,100–12,700

NUTA-4642

Below Sz-S

Soil

10.910± 80

12,990–12,690

NUTA-3784

Below Sz-S

Soil

11,280± 80

13,300–13,000

NUTA-3878

Below Sz-S

Soil

11,330± 90

13,370–13,050

NUTA-4025

Below Sz-S

Soil

11,660±100

13,700–13,300

NUTA-3868

Below Sz-S

Soil

11,850± 90

13,940–13,460

NUTA-3561

Stratigraphic horizon

Material

In Sz-S

CharcoalIn Sz-S

14

C date (yrBP)

Lab no.

Calibrated ages were estimated using the IntCal13 data set (Reimer et al., 2013)				 Rissho University＊＊
			 Emeritus professor of Kagoshima University＊＊＊
			 Hokkaido Board of Education＊＊＊＊
		 Kagoshima University＊＊＊＊＊
		 University of Fukui＊＊＊＊＊＊The Hokkaido University Museum＊

29Eruption Age of Sakurajima-Satsuma Tephra Using Thermoluminescence Dating（Shitaoka・Moriwaki・Akai・Nakamura・
Miyoshi・Yamamoto）

Because of charcoal that is present in or below soil,
the possibility of contamination of the volcanic ash layer

ments of SAR take much more time than measurements of the MAAD protocol.

by old charcoal or a humic fraction from exotic materi-

The Sz-S tephra does not contain the quartz. There-

als cannot be eliminated completely. For such samples,fore, we addressed age determination of the sample

C dating sometimes misrepresents the true age.

using the TL method of PFG with the MAAD protocol.

14

Therefore, other dating methods are useful to confirm
the 14C age. Thermoluminescence（TL）dating is a candidate method for such an investigation because the TLage, obtained from the tephra itself, is the age of thelatest heating.

2. Stratigraphic positions of tephra samples
We collected samples of the Sz-S tephra bed fromtwo locations（Fig. 1）on the Osumi（Loc. 1）and Sat-The TL dating of quartz has clarified the history ofsuma Peninsulas（Loc. 2）. Loc. 1 is on the Aira caldera

young（quartz-bearing）silicic volcanisms（e.g. Gillot etwall at ca. 12 km east of Sakurajima volcano. Loc. 2 is atal., 1978; Guerin and Valladas, 1980; Ichikawa et al.,an archaeological site in the campus of the Faculty of1982）. Recently, Toyoda et al.（2006）clarified much of

Medicine, Kagoshima University, ca. 13 km southwest

Japanese Quaternary silicic volcanisms by dating ofof the volcano.

Isothermal-TL and ESR ages of quartz in several

At Loc. 1, a sequence of most Sakurajima marker

tephra layers. Ganzawa et al. （2007） estimated thetephras and two tephras from other volcanoes overlie

eruption age of Toya pyroclastic flow as 113–114 ka

the Ito pyroclastic flow deposit（Aramaki, 1984）. The

using the single aliquot regenerative-dose（SAR）proto-

five tephras were identified as Sz-Sy （P11）, Sz-Ub

col for quartz analysis（Murray and Wintle, 2000）. The（P12）, Sz-Tk3（P13）, Sz-S and Takano base surge（Tn-

TL dating of feldspar is a candidate for age determina-bs） from the upper to lower （Kobayashi and Fuku-

tion of young quartz-less volcanic rocks（Aitken, 1985）.

shima, 1984; Moriwaki, 1994）. Of these tephras in thisHowever, the applicability of TL dating of feldspar

section, Sz-S tephra is identified as a distinct tephra

remains controversial because the feldspar readily

occurring stratigraphically in the middle part of thecauses anomalous fading of luminescence signals（Win-sequence based on several distinct units including atle, 1973）.

fine-grained phreatomagmatic ash fall, in contrast to

Shitaoka et al.（2009a）reported that polymineral finegrains（PFG）have great potential for use as targets of
TL dating of quartz-less volcanic rocks. Shitaoka et al.
（2009a）reported that TL age obtained from PFG analysis, within error, to that obtained from quartz analysis.
Consequently, TL dating of PFG in quartz-less tephra isa powerful tool for establishment of details of the young
volcanic stratigraphy（e.g. Shitaoka et al., 2009b; Oishi
et al., 2011; Buvit et al., 2014）.
Palaeodoses estimations have used traditionally multiple-aliquot protocol（Aitken, 1985）or mainly singlealiquot protocol（Murray and Wintle, 2000）. The tephra
sample deposited near the source vent collected from athick and less-altered layer provides the most probableage of the tephra because it is a nearly closed system（Shitaoka and Nagatomo, 2011）. In this case, it is reasonable to use the multiple aliquot additive dose
（MAAD） protocol （Aitken, 1985） because measure30

Fig. 1 L ocality map of sampling points:
Loc. 1 is 31°35’ 18”N, 130°47’ 35”E:
Loc. 2 is 31°32’ 53”N, 131°31’ 45”E.

地球環境研究，Vol.18（2016）

for Sz-S（Moriwaki, 1994）, more than 6 units occur inthis section. A base surge deposit and two fine-grained
ash falls are intercalated with pumice falls. An analyzed
sample was collected from the lowermost coarse pumice fall unit （Fig. 2, Loc. 2）, because these fall units
were heaped up during a short period of eruption.

3. TL measurements
Sampling sites were covered with an opaque cloth.
All samples were collected after the top surface layerof ca. 5 cm thickness was removed.
After PFG（ca. 4–10 µm）were separated by suspension in acetone, they were treated with 10% hydrogen
peroxide for 16 hr and with 10% hydrochloric acid for
90 min. Then TL measurements were performed usinga Daybreak TL reader （model 1150; Shitaoka andNagatomo, 2011）.
A standard MAAD protocol was used to determine
palaeodoses. For each sample, three added doses（10,Fig. 2 eologic logs around Sz-S tephra and TL
Gsamples at Loc. 1 and Loc. 2. Sz-Sy（P11）,
Sz-Ub（ P12）
, Sz-Tk 6 （ P17 ）and Tn-bs（Kobayashi and Ezaki, 1997）are Sz group.
Arrows indicate sampling horizons.

20, and 30 Gy）were applied to sets of natural TL aliquots（generally five in each set）. The equivalent dose（De）of the sample can be estimated by fitting the dataassuming linear dose-dependence. Consequently, for correcting the nonlinear portion in the low-dose region

other tephras（Kobayashi, 1986; Moriwaki, 1992, 1994） （nonlinearity correction （Aitken, 1985; Shitaoka and（Fig. 2, Loc. 1）. This tephra overlies the Tn-bs deposit

Nagatomo, 2011）,∆ in Table 2）,we measured the TL of（Kobayashi and Ezaki, 1997）derived from the Aira cal-the samples annealed at 350℃ for 60 min and addeddera, of which the facies differ distinctly from those ofirradiation at 20, 30, and 40 Gy.

other fall tephras. The Sz-S tephra, which is 70 cm

The dose rate was estimated from uranium, thorium,

thick, consists mostly of pumice grains（Fig. 2, Loc. 1）.

and potassium concentrations measured using a high-The sample analyzed in this study was collected frompurity Ge detector（EGSP 8785; Eurisys Mesures）. The

the lowermost coarse pumice fall unit（Fig. 2, Loc. 1）.

dose rates（alpha, beta and gamma）were calculated

At Loc. 2, three marker tephras overlie the Ito pyro-using the dose-rate conversion factors （Adamiec andclastic flow deposit（Fig. 2, Loc. 2）. Each tephra bed

Aitken, 1998）.The dose rates were corrected using theshows distinct lithofacies: the upper consisting of a fine-present-day water contents（Zimmerman, 1971）. Con-

grained vitric ash bed, the middle one with many

tributions of the cosmic dose rate to the dose rate were

tephra units, and the lower one consisting of brown

assumed as 0.15 mGy/yr by following Prescott and Hut-pumice with bluish accidental lithics. Based on theseton（1994）and Shitaoka et al.（2009a）.

features, together with stratigraphic positions, the threetephras were identified as Kikai-Akahoya tephra（K-Ah; Machida and Arai, 1992）, Sz-S, and Sz-Tk6

4. Results and discussion（P17）from the upper to lower（Kobayashi and Fuku-

Fig. （3 a）presents TL measurement results. Minerals

shima, 1984; Moriwaki, 1994）. Of the more than 13 unitsof PFG samples were mainly plagioclase, as inferred
31Eruption Age of Sakurajima-Satsuma Tephra Using Thermoluminescence Dating（Shitaoka・Moriwaki・Akai・Nakamura・
Miyoshi・Yamamoto）

20

15

15

N+30 Gy

106

Relative TL Intensity

N+10 Gy

50

10

N+20 Gy

Natural (N)
0

100

200

300

400

Temperature (°C)

5b

ED: R=0.99

0-20

SPR=-0.9 ± 2.2 Gy

20

ED=17.8 ± 0.97 Gya

TL Intens ity (arb. units)

TL Intensity (arb. units)

25

-10

SPR: R=0.97

0

10

20

Dose (Gy)

30

40

c

5

plateau area (280-320°C)

43

N+30 Gy
N+20 Gy2
1

N+10 Gy
200

300

Temperature (°C)

400

Fig. 3 T L glow curves（a）and TL growth curve（b）（Loc. 2 ）. The equivalent dose（De）and the nonlinearity correction（SPR, ∆ in Table 2 ）were estimated using
the TL growth curve. The plateau area（Integrated area）is 280–320℃（c）.

from TL signals shape and intensity, and from results ofand Loc. 2 were 19.5 ± 1.8 Gy and 16.9 ± 2.4 Gy. The

XRD analysis. The De of each sample can be converted

respective dose rates of Sz-S samples of Loc. 1 and Loc.

to an apparent dose by extrapolation using a linear

2 were 1.41 ± 0.03 mGy/yr and 1.26 ± 0.07 mGy/yr.

model of TL growth（see Fig. 3（b））
. The TL growth

We obtained the TL ages, 13.8 ± 1.3 kyr in Loc. 1 andfor each sample was obtained from each plateau area

13.4 ± 2.0 kyr in Loc. 2. Uncertainties of each TL age（Loc. 1 and Loc.2 were 310–330℃ and 280–320℃（seewere ca. 9–15%; they were mostly attributable to scat-

Fig. 3（c））, respectively）, as determined using system-

tering of the glow curves of each aliquot and sensitivity

atically performed plateau tests（Aitken, 1985）. A lackchanges of annealing samples for nonlinearity correc-

of fading of TL signals within each plateau area wastion（∆）.

confirmed using plateau tests. The method of estimat-

We calibrated the AMS-14C data（Okuno, 1997）tenta-

ing nonlinearity correction（∆ in Table 2）is analogous

tively in relation to the Sz-S bed using the IntCal13

to that of De （Aitken, 1985; Shitaoka and Nagatomo,

dataset（Reimer et al., 2013）（Table 1）. Okuno（2002）2011）. The TL growth for nonlinearity correction was

obtained a calibrated age of 12,800 cal yrBP for Sz-S.

obtained from each plateau area（e.g. 280–320℃）. The

The newly obtained TL ages in this study, 13.8 ± 1.3

palaeodose of the sample is the sum of the De and non-and 13.4 ± 2.0 kyr, are fairly consistent with the sevenlinearity correction（∆）.

calibrated-14C ages in this study（Table 1）within the

The dose rate was obtained as the sum of alpha,error limits（Fig. 4）.

beta, gamma, and the cosmic dose rates. The palaeodo-

Judging from the results and inferences presented

ses, dose rates, and TL ages are presented in Table 2.

above, age determination using TL method of PFG with

The respective palaeodoses of Sz-S samples of Loc. 1

the MAAD protocol is a powerful tool for quartz-less32

地球環境研究，Vol.18（2016）

Table 2.Table
Resultsof TL datingof Sz-S
2 Results
of TL datingof Sz-S
Locality

Equivalentdose (Gy)∆(Gy)

Paleodose(Gy)

Loc. 117.1±1.2 0

2.4±1.3

19.5±1.8

1.04±0.08 7.16±0.36 1.04±0.07

81±1Loc. 217.8±0.97-0.9±2.2

16.9±2.4

1.05±0.14 7.32±1.29 0.85±0.10

79±1Alpha ray(mGy/yr)U(ppm)

Beta ray Gamma+Cosmic ray(mGy/yr)(mGy/yr)

Th
(ppm)

K2 O
(wt%)

Dose rate(mGy/yr)

TL age(kyr)

0.37±0.01 0.53±0.03

0.51±0.01

1.41±0.03

13.8±1.3

0.32±0.04 0.45±0.04

0.48±0.04

1.26±0.07

13.4±2.0

Water
(wt%)

Acknowledgments:
We wish to thank Drs. Keiji Takemura, Tsutomu
Soda, Shinji Ohsawa, Masayuki Oishi, Tsuneto Nagatomo and Hiroaki Kamada, who provided us many useful comments and data. This study was financially
supported in part by research fellowships from theInstitute for Geothermal Sciences, Kyoto University,and a Grant-in-Aid for Scientific Research（A）and for
JSPS Fellows from the Ministry of Education, Culture,
Fig. 4

TL age of Sz-S compared to calibrated
C age of samples related closely to the
Sz-S layer.
14Sports, Science and Technology, Japan（No. 15200059
and No. 18-2078）.
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Miyoshi・Yamamoto）
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地球環境研究，Vol.18（2016）

熱ルミネッセンス年代測定による桜島薩摩テフラの噴火年代
中

下

岡

順

直＊

村

直

子＊＊＊＊

森
三

脇
好

広＊＊
雅

赤

也＊＊＊＊＊

山

井
本

文
順

人＊＊＊
司＊＊＊＊＊＊

立正大学
鹿児島大学名誉教授＊＊＊
北海道教育委員会
＊＊＊＊
鹿児島大学
＊＊＊＊＊
福井大学
＊＊＊＊＊＊
北海道大学総合博物館
＊

＊＊

要

旨：
桜島火山起源である桜島薩摩テフラ（Sz-S）の噴火年代を、熱ルミネッセンス（TL）年代測定法を用いて年代測定を行った。Sz-S は、２カ所の露頭で試料採取した。試料処理は多鉱物微粒子法（PFG）を用い、付加線量法で TL 測定を行った。得られた TL 年代は、13.8±1.3 kyr と13.4±2.0 kyr であった。Sz-S 中から採取された木炭や Sz-S 直下の土壌から得られた放射性炭素（14C）年代（奥野、1997）を、IntCal13データセットを用いて暦年較正した14C 年代と今回得られた TL 年代を比較したところ、誤差の範囲で整合性のある結果であった。よって、桜島火山の新期北岳段階の最初の噴火とされる Sz-S の噴火年代は、約13 kyr であり、既存の14C 年代データと矛盾がないことがわかった。

キーワード： 桜島薩摩テフラ、新期北岳段階、熱ルミネッセンス年代測定、放射性炭素年代測定、噴火年代

35