Kohno et al. Technical Report of ISEI

Ser. A. No. 66 May 1996

VOLCANIC RECORDS AND DATING OF AN UPPER HALF OF THE
H15 ICE CORE FROM MIZUHO PLATEAU, EAST ANTARCTICA
Mika KOHNO1, Takaaki FUKUOKA2, Yoshiyuki FUJII3
and Minoru KUSAKABE1
1 Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori
682-01, Japan
2 Department of Chemistry, Gakushuin University, Toshima-ku, Tokyo 171, Japan
3 National Institute of Polar Research, Itabashi-ku, Tokyo 173, Japan
Submitted to Proceedings of the NIPR Symposium on Polar Meteorology
and Glaciology, 1996
Abstract: Two closely-spaced peaks of electrical conductivity were foundat depths between 45 and 50
m of the 120-m long ice core drilled at site H15 in Antarctica by the 32nd Japanese Antarctic Research
Expedition in 1991. Chemical analysis of the core containing these peaks revealed that the ice layers
were characterized by high acidity and high sulfate concentration suggesting volcanic signals. In order
to identify these characteristics, the core was dated using two methods: (1) counting the number ofhigh electrical conductivity peaks resulting from seasonal variations of SO42- and NO3-, and (2) anempirical model of firn densification. The dating results suggest that the high conductivity is related todeposition of acidic aerosols from the volcanic eruptions of Tambora in 1815 A. D. and of an
unknown volcano in 1809 A. D.
1. Introduction
Ice sheet of Arctic and Antactic regions contains atmospheric aerosols of volcanic, marine andartificial origins which deposit as dry fall-out and wash-out. They occur as aggregates in snow
particles. Ice cores drilled from ice sheet, therefore, include records of past volcanic activities, climaticand environmental changes.
A huge amount of volcanic gases and dust is emitted to the atmosphere at the time of explosive
volcanic eruption. These volcanic products, especially volcanic sulfur gases, are oxidized to form
aerosols of sulfuric acid droplets in the stratosphere. Such aerosols stay in the stratosphere for several
years after the eruption and reduce the insolation to the troposphere. Lowering of atmospherictemperature may follow the explosive eruption, lasting for a few years (LAMB, 1970; STOTHERS,
1984; RAMPINO and SELF, 1982; RAMPINO, 1988; SIGURDSSON, 1990). The amount of volcanic
sulfur added to the atmosphere by a volcanic eruption is important to assess its
effect on the earth's atmospheric environment.
The amounts of volatile emission by explosive volcanic activity in the past have been estimated by
two methods; petrologic method (DEVINE et al.,1984; PALAIS and SIGURDSSON, 1989; KOHNO,
1992) and chemical method based on hydrogen ion concentration of ice core under the assumption
which the global distribution pattern of the volcanic sulfuric acid aerosols and that of bomb-produced
total B activity are the same (HAMMER, 1977, HAMMER et al., 1980; CLAUSEN and HAMMER,
1988; LANGWAY et al., 1988). Global acid fallout by the past large volcanic eruptions in thenorthern hemisphere was estimated from the analysis of acidity of Greenlandic ice core samples from
Crete (71˚12'N, 37˚32'W) for the last 1500 years and Camp Century (71˚17'N, 61˚13'W) for the 10000
years (HAMMER et al., 1980). For example, the amounts of sulfuric acid emitted by volcanic
eruptions of Laki (1783 A. D., Iceland, 64˚42'N, 17˚33'W) and of Tambora (1815A. D., Indonesia,
8˚25'S, 118˚00'E) were both estimated to be ca. 300 Mt (CLAUSEN and HAMMER, 1988). The ice
cores from Dome C (74˚40'S, 125˚10'E) and Vostok (78˚28'S, 106˚48'E) in East Antarctica, Siple(75˚55'S, 85˚55'W) in West Antarctica and South Pole contain the records of several major volcanic
events that took place in the southern hemisphere in the last 220 years. They are Tambora 1815,
1

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Krakatau 1883 (Indonesia) and Agung 1963 (Indonesia) eruptions with the relative proportion of acidemission by these eruptions being approximately 5 : 1 : 1 (LEGRAND and DELMAS, 1987).
Prominent bipolar excess H2SO4 signals found in ice cores from Byrd (80˚01'S, 119˚3'W) Station andSouth Pole in Antarctica and from Crete and Dye 3 (65˚18'N, 43˚49'W) in Greenland last 1000 years
were recorded in the layers whose ages were 1885, 1836, 1816, 1810, 1459, 1346, 1287, 1278, 1259,
1228 and 1168 A. D. The excess H2SO4 signals in the layers dated in 1885, 1836 and 1816 A. D. wereidentified to originate from volcanic eruptions of Krakatau 1883, Coseguina 1835 (Nicaragua) andTambora 1815. The other signals were regarded to be due to the aerosols released from volcanoes inlow latitudes (LANGWAY et al., 1995).
An ice core was drilled at site H15 (69˚04'46" S, 40˚46'54" E, 1050 m a.s.l.) to a depth of 120.2 m
using an electromechanical drill by the 32nd Japanese Antarctic Research Expedition in September to
October, 1991 (FUJII et al., 1992, Fig.1). The electrical conductivity measurement (ECM), a measureof acidity of ice core, was made at Syowa Station soon after the core recovery. It indicates that two
remarkably high peaks first appear at depths between 45 and 50 m. We have determined the chemicalcomposition and oxygen isotopic variations of the ice core samples between for 43 and 50 m to findthe causes for the high ECM peaks. We also attempted to date the ice core in order to obtainchronological information of the upper part of this core using two independent methods: (1) counting
the number of peaks resulting from seasonal variations of SO42- and NO3-, and (2) an empirical modelof firn densification of HERRON and LANGWAY (1980).
2. H15 ice core
The site H15 is located near the Soya coast, Dronning Maud Land, East Antarctica (Fig.1), where ahigh annual snow accumulation gives a high time resolution of the core. It is situated along the route
S-H-Z from Syowa Station to Mizuho Plateau.
Table 1 summarizes the surface mass balance of snow observed at site H15 (SHIMIZU, 1975;
FUJII, 1979; WADA et al., 1981; KOBAYASHI, 1982; SATOW et al., 1983; NAKAWO et al., 1984;NISHIO, 1984; FuJII et al., 1986; NISHIO et al. 1986; AGETA et al. 1987; NISHIO et al., 1988;
NISHIO and OHMAE, 1989; WATANABE et al., 1990; FUJII et al., 1992; MOTOYAMA et al.,1995). The observation has been continued since April 1971. The 1972-76, 1979 and 1992 data arelacking. The mean annual accumulation rate since 1971 is 32.0 ｱ 10.8 g/cm2/year. In this period, snow
accumulated every year without hiatus. The mean annual accumulation rate of snow there suggeststhat the whole core represents the records of the last 300 years.
Bulk density of the core was measured and ice structures such as the melt layers were describedsoon after recovery at the Syowa Station (FUJII, 1992). The site H15 is located above the dry snowline. The mean annual atmospheric temperature is -2l˚C. However, ten distinct melt layers, whichindicates presence of the hiatus of snow deposition, thicker than 3 mm were observed in the snow andfirn layers down to 60 m below the surface. Three thick melt layers out of the 10 melt layers wereobserved at depths of 15.3 m, 19.1 m and 26.0 m (water equivalent depth), their thickness being 8 mm,
7 mm and 25 mm (water equivalent), respectively. The melt layers were formed during warm summer
periods in the past. Existence of the melt layers may suggest that the atmospheric temperature was
warmer than that at present.
3. Sample preparation and analytical procedures
The core, 70 mm in diameter, was cut into half along the direction of depth after recovery, and has
been preserved in a low-temperature room (-20˚C) at National Institute of Polar Research. The core
was sliced into a wafer (7 ~ 10 cm thick) which covers a period of 4 ~ 6 months. Surface 5 ~10 mm ofthe ice core was removed using a grater in order to eliminate contaminants on the surface.
Water samples after melting the ice wafers were used for measurements of the pH, major chemicalcomposition and oxygen isotopic ratios (18O/16O ratio). The core samples were melted in a microwave
oven in a Teflon beaker that had been cleaned with ultra-pure water, and filtered with a 0.2 mm filter
paper (Nuclepore polycarbonate membrane, Corning Costar Corp.) in a clean room (class-1000) at the
National Institute of Polar Research. The residue on filter paper was preserved for observation of solid
residue.
Acidity, which is an index of acidic aerosols accumulated with snow, was measured using a pH
meter (HM-60S, TOA Electronics Ltd.). The pH meter consists of a glass electrode separated from a2

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comparative reference electrode (FAR-101, TOA Electronics Ltd.), because melt water of ice cores
had a small buffer capacity due to very low ionic concentration (FUJII et al., 1989). Concentrations ofmajor anions (Cl-, NO3- and SO42-) and cations (Na+, K+, Mg2+ and Ca2+) were measured with ion
chromatography using a Dionex ion chromatograph (model 2000i) equipped with HIPC-AG4A and CG3 concentrators, AS4A and CS3 separator columns, and anion and cation micro membrane
suppressors to concentrate the ionic species that are dissolved in a ~100 ppb level. Replicate
measurements of standard solutions prepared from analytical grade reagents (NaCl, NaNO3, K2SO4
and KCl, Mg and Ca 1000ppm standard solutions) indicated the relative analytical error of 5% of ameasured value. The 18O/16O ratio (δ18O) of samples was measured with a stable isotope mass
spectrometer (MAT-262) using the H2O-CO2 exchange technique by EPSTEIN and MAYEDA(1957), and reported in a conventional δ18O expression with respect to SMOW.
4. Dating of the H15 core
Dating methods of ice cores proposed so far include (1) the stake method (annual accumulationrate), (2) the empirical model for firn densification (HERRON and LANGWAY, 1980), (3) thestratigraphical method based on seasonal variations of δ18O, particle concentration, chemicalcomposition, pH and ECM profiles (EPSTETN et al., 1965; HAMMER et al., 1978; FUJII, 1981;
THOMPSON et al., 1981; LEGRAND and DELMAS, 1984; WATANABE et al., 1988), (4) detectionof key volcanic signals such as tephra and high SO42- concentration from volcanic eruption of aknown date (HAMMER, 1977., HAMMER et al, 1980), and (5) the 210Pb method (CROZAZ and
LANGWAY, 1966; MASUDA and HARADA, 1986). Since the mean annual accumulation rate ofsnow at site H15 from 1971 to 1993 varies as much as ±30 % (Table.1), it is difficult to use the mean
accumulation rate for accurate dating of the H15 core. In this study, we thus adopted thestratigraphical method, i.e., counting the number of ECM peaks due to seasonal variations in snow
chemistry, and the snow densification model proposed by HERRON and LANGWAY (1980) for thecore dating.
Concetration of excess sulfate (or non sea-salt sulfate) of aerosols in the atmosphere at Mawson(67˚36'S, 62˚53'E) in East Antarctica is highest in January and relatively low from May to September(PROSPERO et al., 1991). The atmospheric excess SO42- concentration is governed by the oxidationof biogenic dimethylsulfide (DMS) to SO2 and methanestulfonic acid. The extent of oxidation varies
with atmospheric temperature (HYNES et al., 1986, KOGA et al., 1990). The sulfur dioxide of DMS
origin is oxidized to form sulfuric acid in the atmosphere (HATAKEYAMA et al., 1985) and thenaccumulates in snow by wash-out. Seasonal variations of SO42- and NO3- concentrations in driftingsnow which collected at Mizuho Station during February to December in 1986 increased significantly
in summer (OSADA et al., 1989; OSADA, 1994), and the pH values of drifting snow samples indicatethat snow in summer is more acidic than that in the other seasons (FUJII, 1989). Therefore, it isconceivable that an ECM peak reflects summer acidity of snow, and that ice core can be dated bycounting the ECM peaks.
When DC voltage is applied across a piece of ice through electrodes, a weak electric current is
induced. The current depends on the concentration of mobile ions, their mobility and efficiency of
charge exchange at the electrodes. Since the mobility of H3O+ ion is much greater than that of OHand other ions, the initial current is proportional to the H3O+ concentration or acidity. This is theprinciple of ECM (HAMMER, 1980; HAMMER, 1983). Figure.2 shows the ECM profile down to 50
m. The core depth has been converted to the water equivalent depth based on the density profile shownin Fig.3. The profile represents the ECM values after taking a running average of every 10 cm interval.
Above 50 m, we can find two high ECM peaks at depths of 46.7 m and 48.3 m (Fig.2).
Figure.4a compares the ECM profile of the top 4 m of the H15 core with the dates of firn given by
the stake observation (Table 1). We have assigned high ECM peaks to summer of an appropriate year
based on the stake observation. Summer peaks are indicated by a number by a high ECM peak,although there are a few exceptions. It should be noted that the highest ECM peak is found between
the two ECM peaks assigned to 1983 and 1984 summers. Excess SO42- concentrations shown in3

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Fig.4b were calculated using the following equation assuming that Na+ ion was entirely
derived from seaspray.
(SO42-)excess = (SO42-) - (SO42- / Na+)seawater.(Na+)(1)

where (SO42-) and (Na+) are the observed SO42- and Na+ concentrations of ice core samples (Table 2)and ((SO42-/Na+)seawater is the ratio of SO42- to Na+ concentrations of seawater that is 0.25 (weightratio). The excess SO42- concentration generally varies between 0 and 70 ppb (Fig.4b). High excess
SO42- peaks roughly agree stratigraphically with high ECM summer peaks. However, there are a few
sporadic negative peaks (not shown in Fig.4b), which suggests existence of excess Na+ of unknown
reasons. It should be noted that there is a layer that contains very high excess SO42- concentration at
2.3 m depth. Deposition of this excess SO42- occurred in the latter half of 1983, and is likely to reflect
deposition of sulfuric acid aerosols originated from an explosive volcanic eruption of El Chichon 1982
(Mexico, 17˚36'N, 93˚23'W) as will be discussed later. From the foregoing, counting the summer
peaks in the ECM profile is certainly usable to date the ice core.
The high ECM peaks shown in Fig. 2 were counted from the surface to 50 m depth. Above 50m,
there are two remarkably high peaks at 46.7 m and 48.3 m depths. They correspond to~1826 A. D.
(~165 B. P.) and -1821 A. D. (~170 B. P.), respectively, although this dating method may involve
errors due to over-counting of non-summer ECM peaks such as a volcanic signal at 2.3 m depth (EI
Chichon 1982) or under-counting of small summer peaks and undetected summer peaks caused bymelting of snow as observed at depths of 15.3 m, 19.1 m and 26.0 m (water equivalent depth).
An empirical model of firn densification was proposed by HERRON and LANGWAY (1980). Thismodel is based on the principle that air space of the firn is linearly related to stresses resulting fromweights of overlying snow. According to formulation by HERRON and LANGWAY (1980), there is alinear relationship between depth of firn and in (ρ/(ρi-ρ)), where ρi is a constant density of ice (0.917
g/cm3) and r is the density of the firn layer at a given depth. The plot of the ln(ρ/(ρi-ρ)) versus depth
relationship is divided into two segments based on the densification rate. The first segment is for ρ <0.55 g/cm3 and the second segment is for 0.55 < ρ < 0.82~0.84 g/cm3. These segments correspond to
the stages where settling and packing of snow grains and further compaction process take place. Since"pore close-off" will occur above the densities greater than 0.82~0.84 g/cm3, the above model cannot
be applied for ice whose density is greater than 0.84 g/cm3. The relationship between ln(ρ/(ρi-ρ)) and
raw depth that has been converted from the density profile (Fig. 3) is shown in Fig.5. HERRON and
LANGWAY (1980) related the annual accumulation rate A to the slope C of the ln(ρ/(ρi-ρ)) versus
depth relationship as,
A = (ρi.k1/C)2(2)

where k1 is a constant relating to the densification rate. The value of k1 is a function of the mean
atmospheric temperature and is given in HERRON and LANGWAY (1980). Taking the mean
atmospheric temperature of -21˚C at site H15 and the slope C of regression lines of the first andsecond stages in Fig. 5 to be 6.5 x 10-2 and 3.8 x 10-2, respectively, the accumulation rate A is
calculated to be 145 and 51 cm/year for the first and second stages, respectively. The time necessary
for snow to attain the maximum density of the first stage(ρ = 0.55 g/cm3; 6.2 m in raw depth or 2.9 m
in water equivalent depth) is calculated to be 2 years by dividing 2.9 m by the accumulation rate.
Indeed, the density of 0.55 g/cm3 is found at 7.8 m (Fig. 3). Similarly, the time required to attain the
maximum density of the second stage (ρ = 0.84 g/cm3; 58.8 m in raw depth or 42.9 m in waterequivalent depth) is calculated to be 148 ± 12 years. The bottom of firn in the second stage thus
deposited ~150 years ago, corresponding to 1841 ± 12 A. D. The error arises from the regression of4

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Ser. A. No. 66 May 1996

the ln(ρ/(ρi-ρ)) versus depth relationship. As mentioned earlier, there are two high ECM peaks at 46.8
and 48.3 m (water equivalent depth, Fig.2). The dates of these peaks should, therefore, be older than1841 ± 12 A. D., and are roughly estimated to correspond to 1822~1828 (±12) A. D., assuming a
constant accumulation rate below the layer with ρ = 0.84 g/cm3.
5. Acidity, SO42- concentration and δ18O measurements
Acidity, chemical composition and δ18O of the H15 core samples were measured from surface to
50 m depth. As stated previously, the ECM and excess SO42- peaks at 2.3 m depth are present between
1983 and 1984 (Fig. 4) and are likely to be related to deposition of stratospheric aerosols released by
the volcanic eruption of El Chichon 1982. Other possible explosive volcanic activities prior to 1984
are the eruptions of Galunggung 1982 (Java, 7˚03'N, 108˚01') and Colo 1983 (Indonesia, 0˚02'N,
121˚06'E) whose Volcanic Explosivity Index (VEI) are 4(SIMKIN and SIEBERT, 1994). Of these
explosive eruptions, the El Chichon eruption of 1982 in April (VEI=5) is most likely to be responsible
for the high peaks because of its large magnitude. Its maximum column height is estimated to havereached 32 km (CAREY and SIGURDSSON, 1982), high enough to give a global impact. The
eruption injected a large amount of sulfur to the stratosphere, an estimate being 3.3 ~ 10 Mt by the
total ozone mapping spectrometer on the Nimbus 7 satellite (EVANCE and KERR, 1983; KRUEGER,
1983). The aerosols spread over the globe at the latitude of 20˚N for about 20 days and further tohigher latitude following the general atmospheric circulation. The solar radiation into the atmosphereobserved at the Syowa Station started to decrease from 1982 in November, and was obviously low inJanuary, 1983 (YAMAUCHI and SHIMURA, 1984). A time lag of over 1 year between the eruption(1982 in April) and the date (mid-1983) when the high excess SO42- peak was observed in Fig.4b is
explained by a long residence time of volcanic aerosols in the stratosphere. This is consistent with thefact that the stratospheric aerosols was present for about 2 years after the explosive volcanic eruptionof Mt. Pinatubo (Philippines, 15˚13'N, 120˚35' E) in June 1991 (McCORMIC et al., 1995). The PS1
and PS14 ice cores drilled at South Pole in 1983 ~ 1984 summers do not contain a prominent volcanic
signal of the 1982 El Chichon eruption (DELMAS et al., 1992). It may have taken longer for the
volcanic aerosols to reach South Pole compared to the site of H15 since the latitude of South Pole ishigher than that of H15, and the aerosols may not have deposited yet at South Pole where the ice cores
were drilled in 1983 ~ 1984 summers.
The H+, excess SO42- and δ18O variations were measured in detail for depths from 43 m to 50 m inorder to constrain the causes for the two high ECM peaks at 46.7 and 48.3 m depths (Fig. 2). The
results are shown in Table 3 and Fig. 6. It is obvious that the two high ECM peaks are accompanied by
H+ (Fig. 6b) and excess SO42- peaks (Fig. 6c). The excess SO42- concentrations of 350 ng/g at 46.7 m
and 200 ng/g at 48.3 m correspond to 7 and 4 µ equiv. H+/kg, respectively, assuming that the excess
SO42- deposited as sulfuric acid. These values are in reasonable agreement with the directly measured
H+ concentrations at corresponding depths (Fig. 6b). This is a good indication that excess SO42originates from acid fallout as sulfuric acid. From the discussion of dating, the layers containing thesehigh acidity peaks correspond to the beginning of 19th century. Two significant sulfate peaks havebeen found in the ice core drilled at Stations Dome C and Vostok (LEGRAND and DELMAS, 1987)
in East Antarctica, Siple (DAI et al., 1991) and Byrd (LANGWAY et al., 1994; LANGWAY et al.,1995) in West Antarctica and South Pole (DELMAS et al, 1992, LANGWAY et al., 1995). The twohigh SO42- peaks in the Dome C core are assigned to have originated from the unknown 1809 eruption
and Tambora 1815 eruption (MOORE et al., 1991; DAI et al., 1991; DELMAS et al., 1992;
LANGWAY et al., 1995), High SO42- layers are observed in ice cores of a similar age range at Siple
and Byrd in West Antarctica and South Pole and Site A (70˚38'N, 35˚49'W), Site T (72˚35'N,
38˚27'W), Dye 3 and Crete in Greenland. A comparison of SO42-concentrations of these coresindicates that an explosive volcanic eruption occurred in 1809 or earlier in the low latitude and theamount of stratospheric aerosol emission was about half that of Tambora 1815, although a volcanic
event in 1809 has not been described in the ancient literature (DAI et al., 1991; LANGWAY et al.,5

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Ser. A. No. 66 May 1996

1995). Of these cores, hydrogen ion concentration of a layer containing aerosols from the Tambora
eruption is higher than that from the 1809 unknown eruption by a factor of 1.5 ~ 2 except at Dome C(Legrand and Delmas. 1987; Moore et al., 1991; Dai et al., 1991; Delmas et al., 1992; Langway et al.,1995). At H15, the total amounts of H2SO4 deposition were calculated by integrating the areas of theH+ peaks at 46.7 m from 46.46 to 46.90 m and at 48.3 m from 48.12 to 48.27 m, respectively (Fig. 6).
The amounts of H2SO4 were calculated to be 68 kg/km2 at 46.7 m and 44 kg/km2 at 48.3 m with theratio of 1.5. This fact, together with the observations at the other sites that the Tambora 1815 eruption
emitted more acid than the unknown 1809 cruption by a factor of 1.5 ~ 2, suggests strongly that the46.7 m peak corrsponds to the Tambora 1815 eruption and the 48.3 m peak corresponds to theunknown 1809 eruption.
The layers at 46.7 m and 48.3 m depths were initially dated to be 1828 and 1822 A. D.,
respectively, by the empirical densification model (HERRON and LANGWAY, 1980), and to be 1826
and 1821 A.D., respectively, by the counting the ECM highs. The difference of ~10 years may beascribed to the presence of unconformity as indicated by distinct three melt layers between 15 m and26 m (water equivalent depth). MACHIDA et al (1995) assumed that the ECM peaks at 46.7 m and48.3 m depths correspond to the Tambora 1815 and unknown 1809 eruptions in their estimation of theannual accumulation rate of 26.0 g/cm2/year. The present study supports their assumption.
Andesitic fine ash was found in layers dated to 1809-1810 and 1816 A. D. in South Pole and DomeC cores (DE ANGELIS et al., 1985; PALAIS et al., 1990). Fine ash-like materials (~10 mm) havebeen found in the corresponding layers of H15 core. If the materials are proven to be of volcanic origin
through chemical analysis, our interpretation will be further strengthened.
Except the high ECM peaks at 46.7 and 48.3 m, we can see ECM peaks that are significantly higher
than the background variations (Fig. 2). Sulfate concentrations were measured for samples of high
ECMs (> 2 µA above 15 m depth and >4 µA below 15 m depth). If the SO42- concentration is higher
than 100 ng/g (note that the background SO42- concentration is about 50 ng/g, see Fig. 4) and if thelayer is thicker than 0.1 m (water equivalent), we regard such peaks to represent volcanic signals.
There are altogether eight ECM highs satisfying the above conditions. Table 4 compiles these eight
ECM highs in the H15 ice core, together with the estimated dates, maximum SO42- concentration, andduration of ECM highs. Duration was calculated from the thickness of a SO42- concentration peak andthe annual accumulation rate of 26.0 g/cm2/year by MACHIDA et al. (1995). We attempted to relate
those ECM highs to possible volcanic eruptions. After 1815, two volcanic eruptions are known to have
the VELs greater than 5 (SIMKIN and SIEBERT, 1994), and therefore tp give large aerosol loading to
the atmosphere; Coseguina 1835 (Nicaragua, 12˚09'N, 87˚05'S) and Krakatau 1883 (Indonesia, 6˚01'S,
105˚04'E, RAMPINO and SELF, 1982; DEVINE et al., 1984; SELF et al., 1989). The volcanic signals
due to these eruptions have been found in the ice cores from Dome C, Vostok, Siple, Byrd and South
Pole (LEGRAND and DELMANS, 1987; DAI et al., 1991; DELMAS et al., 1992; LANGWAY et al.,
1994; LANGWAY et al., 1995). The ECM highs that are likely due to these eruptions occur at thewater equivalent depths of 39.57 and 26.29 m in the H15 core, and are dated to be 1839 and 1890,
respectively, based on the stake method using the annual accumulation rate of 26.0 g/cm2/year by
MACHIDA et al. (1995). These estimated dates are younger than the actual dates of respective
eruptions by 5-7 years. This probably arises from the formation of melt layers as will be discussed
later. Since there is no known big volcanic cruption around 1859 which corresponds to 34.4 m in the
H15 core, the causes of this ECM high is not clear. The ECM highs at 10.99 m (estimated to be 1949
A.D.) and at 9.09 m (estimated to be 1959 A.D.) may have resulted from the volcanic eruptions of
Nilahue 1955 (Chile, 40˚04'S, 72˚01'W) and Agung 1963 (Indonesia, 8˚03'S, 115˚05'E), respectively.
The difference of 6 years between the actual and estimated dates probably reflects the "unconformity"
effect as mentioned below.
In Table 4 were listed the estimated dates of the ECM (and SO42- ) highs and the dates of the
volcanic eruptions that might have caused the ECM highs. It is known that there are three thick melt
layers at 15.3, 19.1 and 26.0 m (water equivalent depth), which indicates presence of the hiatus ofsnow deposition. If the hiatus did occur, the accumulation rate of 26.0 g/cm2 by MACHIDA et al.
(1995) becomes an underestimate. With this value, the layers above the melt-containing layers would
be dated younger than the actual dates, whereas the layers below would be dated older, since theperiod for the hiatus would have been included in the calculation of dating. The difference between the6

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Ser. A. No. 66 May 1996

estimated and actual dates should thus be negative below 26.0 m and positive above this depth. This isquite consistent with the above cpmsideration of the effect of the hiatus on the accumulation rates.
Different mean accumulation rates must be used below and above the melt layers.
The δ18O values of rain and snow vary with the average localatmospheric temperature(DANSGAARD, 1964). Consequently, seasonal changes of atmospheric temperature and long-termclimatic changes are reflected in the oxygen isotopic variations of ice cores (JOHNSEN et al., 1972;
HAMMER et al., 1980; DAI et al., 1991; DANSGAARD et al., 1993; GREENLAND ICE-CORE
PROJECT MEMBERS, 1993). Figure 6d shows the δ18O variations between 43 and 50 m depths ofthe core. Unfortunately, we cannot recognize the summer highs and winter-lows in the δ18O
variations. There is a tendency that the δ18O values decrcased after rhe eruption of the Tambora 1815
eruption (46.7 m depth), but it is not the case for the δ18O values after the 1809 unknown eruption(48.3 m depth). The δ18O profile of Siple core indicated that short-term temperature changes after
explosive volcanic eruptions such as Tambora 1815 were not recorded in the core (DAI et al., 1991).
Thus, interpretation of the δ18O variations as volcanic signals is not straightforward.
6. Conclusions
Two remarkably high peaks of ECM, acidity and excess SO42- concentrations were found at the waterequivalent depths of 46.7 m and 48.3 m of the 120-m ice core drilled at site H15 in East Antarctica.
These signals are considered to have originated from the volcanic eruptions of Tambora in 1815 A. D.
and unknown in 1809 A. D. The dates were estimated from two independent methods by counting thenumber of ECM peaks and an empirical model of firn densification. High SO42- peak detected at depthof 2.3 m most likely resulted from a volcanic eruption of El Chichon 1982. There is a possibility that
the ECM peaks associated with SO42- highs between Tambora 1815 and El Chichon 1982 eruptions
may represent the effect by the volcanic eruptions of Coseguina 1835, Krakatau 1883 and Agung
1963.

Acknowledgments
We wish to thank T. Kobayashi, T. Tominaga and Y. Wakaari of the National Institute of Polar
Research for their help in the chemical and oxygen isotopic analyses. We also thank H. Yamanouchi
of Nihon University for her help in sample preparation. We would also like to acknowledge the 32nd
Japanese Antarctic Research Expedition during which the H15 ice core was collected. This work waspartly supported by Grant-in Aid given to Y. F. No. 05302062 for Scientific Research from the
Ministry of Education, Science and Culture of Japan.
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