Diffuse emission of CO2 from Miyakejima volcano, Japan

Ž .Chemical Geology 177 2001 175–185www.elsevier.comrlocaterchemgeo

Diffuse emission of CO from Miyakejima volcano, Japan2

Pedro A. Hernandez a,), Jose M. Salazar b,1, Yoichi Shimoike a, Toshiya Mori a,´ ´Kenji Notsu a, Nemesio Perez b,1´

a Laboratory for Earthquake Chemistry, Graduate School of Science, The UniÕersity of Tokyo, Bunkyo 113-0033, Tokyo, Japanb EnÕironmental Research DiÕision, Instituto Tecnologico y de Energias RenoÕables, 38594 Granadilla, SrC de Tenerife, Spain

Received 1 September 1999; accepted 19 November 1999

Abstract

Two soil gas surveys were carried out in May and September 1998 at Miyakejima volcano, in the Izu Mariana arc, Japan.CO flux values for May and September surveys ranged from 0.1 to 18,150 g my2 dayy1 and from 0.1 to 9685 g my2

2

dayy1, respectively. Statistical graphical analysis showed three overlapping populations. The spatial distribution of theseemissions correlated quite closely with the geothermal and geological characteristics of the studied area. The structurereleasing higher CO is the summit cone Oyama and surrounding areas, where the most obvious geothermal features occur.2

A total output about 100–150 t dayy1 is estimated from this area. A good correlation was observed between soil CO flux2

and soil temperature at the summit caldera indicating extensive condensation of fumarolic steam within the upper part ofŽ 13 .Miyakejima. Carbon isotopic analysis of selected samples inside the summit caldera d C–CO sy0.90‰ to y5.70‰2

Ž 13suggests a mixing of carbon derived from marine limestone and magmatic CO while a clear biogenic origin d C–CO s2 2.y14.76‰ to y25.52‰ is observed for the diffuse degassing of CO outside summit caldera. q 2001 Elsevier Science2

B.V. All rights reserved.

Keywords: Miyakejima; Geochemistry; Soil gases; Carbon dioxide

1. Introduction

The measurement of CO from soils in diffuse2

form has been carried out extensively over volcanicand geothermal areas in the last 15 years, showingthat even during repose periods of volcanic activity,

Žvolcanoes release high amounts of CO Allard et2

al., 1987, 1991; Baubron et al., 1990; Farrar et al.,

) Corresponding author. Fax: q81-3-5841-4119.E-mail address: [email protected]

Ž .P.A. Hernandez .´1 Fax: q34-922-391001.

1995; Chiodini et al., 1996, 1998; Hernandez et al.,´.1998; Gerlach et al., 1998 . Carbon dioxide is usu-

ally the major constituent of the non-condensablefraction of the magmatic gas. Changes in the diffuseoutput of CO can be related to the level of volcanic2

Žactivity Baubron et al., 1991a; Giammanco et al.,.1995 . Measurements of CO flux can be carried out2

at safe distances from active craters. Mapping ofthese emanations may help to identify the active

Žstructural features of volcanic edifices faultrfrac-.tures and potential sites of future eruptions. The

measurement of soil gas emission have been usedalso to detect active structures such as fractures andfaults, showing that these structures act as preferen-

0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 00 00390-9

( )P.A. Hernandez et al.rChemical Geology 177 2001 175–185´176

tial pathways for the escape of gases towards theŽsurface Sugisaki et al., 1983; Klusman, 1993; Gi-

.ammanco et al., 1998 . These attributes and low costof soil gas surveys make this methodology an impor-tant tool for geochemical investigation in geothermaland volcanic areas.

In this paper, we present the first detailed study ofdiffuse degassing from a volcano in Japan based ontwo surveys carried out in May and September 1998.The goal of the present work was to delimit thoseareas with anomalous levels of CO diffuse de-2

gassing and estimate the total output of CO from2

Miyakejima volcano.

2. Geological setting

Miyakejima volcano is located in the Pacific, 200km south of Tokyo, on the of Izu–Mariana activevolcanic arc. This area is tectonically active becauseof the subduction of the Pacific plate beneath theEurasian plate. It is one of the most active basaltic

Ž .stratovolcanoes in Japan Fig. 1 . The main cone of

Fig. 1. Tectonic map around Miyakejima volcano, Japan.

Fig. 2. Structural map of Miyakejima volcano showing the erup-Ž .tive fissures and the radial dike pattern after Nakamura, 1977 .

the volcano has two nested calderas: the outerKuwanokitaira caldera which is 4 km in diameterand the inner Hatchodaira caldera, 1.8=1.6 km. Thecentral cone, Oyama, grew in the Hatchodairacaldera. The radial fissural system of the volcanoseems to be controlled by three stress orientations:s1 vertical, s2 the maximum horizontal principlestress and oriented NE, and s3 is the least horizontalprinciple stress and oriented NW. These stress orien-

Žtations are deduced from regional studies Nakamura,.1977 and the predominant NE-rend of fractures on

Ž .Miyakejima volcano Fig. 2 . During this century,Miyakejima erupted very regularly almost every 22

Žyears, and last erupted on October 1983 Tskui and.Suzuki, 1998 . The three most recent eruptions were

characterized by effusion of lavas and scoria fromŽ .fissures at the flank of the volcano Isshiki, 1964 ,

only the 1940 eruption was accompanied by a sum-mit eruption. At present, fumarolic activity is locatedat the central Oyama cone and surrounding areas,being mainly composed of water vapor and CO2

with a maximum temperature about 808CŽ .Hirabayashi et al., 1984 . Several geophysical andgeochemical studies have been carried out at this

( )P.A. Hernandez et al.rChemical Geology 177 2001 175–185´ 177

Žvolcano during the last years Utada et al., 1984;.Sasai et al., 1984 , showing significant changes in

the total force intensity as well as in the resistivity inassociation with the 1983 eruption. All these studieshave provided important information on wheremagma was stored within the volcano during theeruption.

3. Methodology

Two soil gas surveys were carried out during23–30 May and 18–20 September 1998, respectivelyat Miyakejima volcano. The sampling sites werechosen depending on the accessibility and geother-mal and structural features of the area. CO flux2

measurements in May and September 1998 werecarried out at 155 and 110 sampling sites, respec-tively, while CO concentration was measured only2

in September at 112 sites. Soil gas samples werecollected with a syringe from a probe inserted intothe soil at 40–50 cm depth and stored into pre-evacuated containers. The samples were analyzed inthe laboratory using a gas chromatograph with a

Ž .thermal conductivity detector Ohkura GC 103 anda Molecular 5A sieve column with oxygen as acarrier. The method used for CO flux measure-2

Fig. 3. Scheme of the accumulation chamber method used for CO2

flux measurements.

Žments, Aaccumulation chamber methodB Parkinson,.1981; Baubron et al., 1990, 1991b , was the same for

both surveys. The system consists of a cylindricalchamber opened at the bottom with a fan to improvegas mixing, a NDIR Riken Keiki spectrophotometerŽ . Ž .accuracyf5% and a data logger Fig. 3 . Eachmeasurement starts when the open side of the cham-ber is placed on the soil surface. A pump allows theair contained in the chamber to circulate through theIR spectrophotometer and then back into the cham-

Ž .ber. A trap filled with Mg ClO is placed between4 2

the chamber and the spectrophotometer to avoid anyanalytical interference by water vapor. Signals pro-portional to the CO concentrations in the chamber2

are recorded in the data logger. The accumulationchamber method was calibrated in the laboratory bymeasuring different CO flux rates several times.2

Based on these data, we assume a random error of"10% in emission rates.

4. Results and discussion

Analytical results are summarized in Table 1. SoilCO contents vary from atmospheric value to 1.4%2

and CO flux values for May and September surveys2

range from 0.1 to 18,150 g my2 dayy1 and from 0.5to 9685 g my2 dayy1, respectively. Statistical graph-

Ž .ical analysis Sinclair, 1974 was used to distinguishŽ .different geochemical populations Figs. 4 and 5 .

The log-normal distribution shows three overlappingpopulations. All the data showed a log-normallydistribution consisting of combinations of three pop-ulations. The geometric mean, x, was read at the50th percentile. The thresholds were chosen at the

Ž . Ž .2nd T and 98th T percentiles. Contour mapslow high

were made in order of multiples of backgroundvalue.

4.1. May surÕey

The soil gas survey carried out in May covered anarea of 51.25 km2 with 155 sampling sites for CO2

flux measurements. Fig. 4 shows the cumulativeŽ y2 y1.frequency plot of CO flux g m day of the2

total data. Statistical graphical analysis of CO flux2

data showed three overlapping populations: back-


Table 1Statistics of soil gas data from the surveys carried out at Miyake-jima volcano, Japan

Range Average s.d. Number ofsamples

y2ŽCO g m 0.1–18,150 1011 2264 1552y1 .day ; May

y2ŽCO g m 0.5–9685 468 1205 1102y1 .day ; SeptŽ .CO ppm; Sept 340–14,306 2052 2620 1122

Ž . Ž .ground population II , peak population III andŽ .intermediate AthresholdB population I . The back-

ground mean is 94 g my2 dayy1 and represents 59%of the total data. The peak group showed a mean of9500 g my2 dayy1 and represents 2.7% of the totaldata. The mean of the intermediate threshold popula-tion is 1650 g my2 dayy1 with a 38.3% of the totaldata. The background value is relatively high com-pared with those reported from other volcanoesŽ .Chiodini et al., 1998 . Most of the data entered inthe graphical analysis are from the summit area.

Ž y2 y1Fig. 4. Cumulative frequency plot of CO flux g m day ;2.May survey at Miyakejima volcano, Japan.

Ž y2 y1Fig. 5. Cumulative frequency plot of CO flux g m day ;2.September survey at Miyakejima volcano, Japan.

Areas outside the summit caldera showed valuesunder the detection limit, and variations in these data

Žmay be related to meteorological conditions water.accumulation in soil . The main area releasing CO2

Žin diffuse form is central cone Oyama )5000 gy2 y1. Žm day . An isolated high soil CO flux 1915 g2y2 y1.m day is located outside the southwestern

caldera rim. This anomaly seems to be related withthe southwest flank fissure. No data are availablefrom the 1983 fault because the impossibility ofsampling along the recent lava flow. Other anoma-

Ž 2 y1.lous levels of diffuse CO )100 g m day were2

identified also outside the caldera rim and along theN–W and N–E flanks of Miyakejima.

4.2. September surÕey

The soil gas survey carried out in Septembercovered an area of 55 km2 with 112 and 110 sam-pling sites for both CO concentration and CO flux2 2


measurements, respectively. Statistical graphicalanalysis for total soil CO concentration data showed2

Žthree overlapping populations: background popula-. Ž .tion II , peak population III and intermediate

Ž .threshold population I . The background mean is470 ppm and represents 46.0% of the total data. Thepeak group showed a mean of 8100 ppm and repre-sents 5.8% of the total data. The mean of the inter-mediate threshold population is 1800 ppm with a48.2% of the total data. Statistical graphical analysisfor total CO flux data showed also three overlap-2

Ž .ping populations: background population II , peakŽ . Žpopulation III and intermediate threshold popula-

. Ž . y2tion I Fig. 5 . The background mean is 32 g mdayy1 and represents 62% of the total data. Themean background value is slower than that reported

Ž y2 y1.for the May survey 94 g m day . This changemay to be related to meteorological conditions be-cause the weather in September was dryer that inMay and more data were recorded outside the sum-mit caldera, decreasing the background value. Thepeak group showed a mean of 4100 g my2 dayy1

and represents 20% of the total data. The mean ofthe intermediate threshold population is 285 g my2

dayy1 with 18% of the total data. The central cone

of Oyama was also the area showing the highestŽ y2levels of CO diffuse degassing )5000 g m2

y1 .day .The variation of meteorological and environmen-

tal conditions, such as rainfall, soil humidity andbarometric pressure changes can also affect soil-gas

Ž .concentrations and fluxes Chiodini et al., 1996 .The survey of May was carried out after several daysof raining. The atmospheric pressure ranged duringMay and September surveys between 994.98 and1015.5 HPa and between 1008.0 and 1014.4 HPa,respectively. These variations on atmospheric pres-sure can influence individual measurements but areunlikely to explain the wide range of CO flux2

values measured at Miyakejima volcano.

5. Discussion

Fig. 6 shows the spatial distribution of soil CO2

concentration at Miyakejima Island in September1998. Background levels of diffuse CO degassing2

are found in most areas of Miyakejima volcano.Outgassing is greater from areas characterized by theexistence of faults andror fissures associated with

Ž .Fig. 6. Distribution of soil gas CO ppm anomalies at Miyakejima volcano, Japan.2


the volcanic edifice. The spatial distribution of thesoil CO concentration presents the following fea-2

Ž .tures: 1 the highest anomalies are located at thecentral cone Oyama were fumarolic degassing oc-

Ž .curs; 2 relatively high CO contents are identified2

along the S–W flank fissure system, where the lastŽ . Ž .eruption occurred 1983 , and 3 low CO concen-2

trations are identified at the north and south-eastflanks of the volcano. These relatively low CO2

contents can be attributed to subsurface sealing pro-cesses of fissures and the existence of numerous lavaflows covering the surface. Environmental parame-ters, such as wind velocity, atmospheric pressure andsoil moisture may also affect soil gas concentrationŽ .Reimer, 1980 . Wind could also force air into thesoil atmosphere, resulting in a lower CO fluxes.2

The soils of the study area are strongly desaturatedand developed over pyroclastic and ash volcanicdeposits, with variable organic matter content as highas 15%. Relatively high soil gas CO concentrations2

might be related to degradation of the organic matter.Degradation is enhanced by the acidity and perme-

Ž .ability of these soils Hinkle, 1994 .In order to quantify the total output of diffuse

CO from Miyakejima volcano, soil CO fluxes2 2

were calculated by multiplying the average level ofsoil CO flux and the area of each contour, which2

are established in order of multiples of the back-ground value. Based on the calculation of the CO2

output from the total studied area and consideringthe background values, we estimated a total output of

y1 Ž 2 . y1 Ž3200 t day 33.4 km and 4300 t day 41.442 .km for both May and September surveys, respec-

tively. These relatively large amounts of CO should2

not be considered as representative of CO released2

the magma chamber beneath Miyakejima volcanoŽbecause most of the CO is biogenic in origin see2

.Table 2 .In order to quantify the total output of diffuse

CO from Miyakejima volcano, the average level of2

soil CO flux was multiplied by the area of each2

contour, which is designed by multiples of the back-ground value. Therefore, we estimated a total output

y1 Ž 2 . y1 Žof 3200 t day 33.4 km and 4300 t day 41.442 .km for both May and September surveys, respec-

tively. These relatively large amounts of CO should2

not be considered as representative of CO released2

from Miyakejima volcanic–hydrothermal system be-

Table 2Observed d

13C–CO values together with CO concentration2 2Ž .ppm and soil CO flux from selected soil gas samples at2

Miyakejima volcano13Sample Temperature CO CO flux d C–CO2 2 2

y2 y1Ž . Ž . Ž .8C ppm g m day

1 23.3 1835 0.5 y18.9289 23.8 2971 29.9 y25.155

16 23.5 2839 27.8 y23.60919 25.3 1928 0.5 y22.52124 24.4 2646 0.5 y23.21631 23.1 2802 12.3 y24.87735 22.5 888 5.4 y14.76441 22.8 1850 19.8 y20.23049 22.8 7875 13.5 y22.85257 22.1 521 97.4 y20.89561 22.4 3134 48.8 y22.29567 23.3 14,306 n.a. y23.54584 49.5 8525 1687 y0.84585 65.5 5984 56.7 y0.15690 45.3 4443 103.3 y5.70393 90.9 10,574 9685 y0.95394 86.8 9266 8744 y0.896

100 46.5 6619 3377 y1.407101 71.7 5783 4551 y2.544103 67.6 3996 2186 y2.219118 55.3 5375 1249 y1.433124 21.8 5783 26.1 y22.347128 78.4 3836 1563 y1.484

n.a.: not analyzed.

Žcause most of the CO is biogenic in origin see2.Table 2 .

The existence of three overlapping geochemicalpopulations for both soil CO concentration and CO2 2

flux is clearly related to deep perturbations of thevolcanic–hydrothermal system on the surface envi-ronment. Log-probability plots for CO flux data2Ž .Figs. 4 and 5 shows that peak population dataŽ .population III represent a relative low percentage

Ž .of the total data 2.7% and 20% and has a strongmagmatic signature. Since fumarolic activity occurs

Ž .just at the summit of Miyakejima volcano Oyama ,we have taken into consideration only the diffuseemission of CO from this area. Figs. 7 and 8 show2

the spatial distribution of soil CO flux at Oyama for2

both surveys, and the estimated diffuse emission ofy1 Ž 2 .CO output is 146 t day 0.63 km and 93 t2

y1 Ž 2 .day 0.69 km for both May and Septembersurveys, respectively. Therefore, we consider a value


Ž y2 y1 .Fig. 7. Distribution map of soil CO flux anomalies g m day ; May survey at summit cone Oyama and surrounding areas,2

Miyakejima volcano.

between 90 and 150 t dayy1 of CO as representa-2

tive of the total CO discharge by diffuse degassing2

from the summit area of Miyakejima. This soil CO2Ž y1 .flux level between 90–150 t day is of the same

order of magnitude that those observed at otherŽ .active volcanoes Table 3 . Based on the chemical

and isotopic analysis of CO , we should consider as2

representative of CO released from deep origin just2

the soil CO flux values from the peak population,2

that is, those values over 30 times the backgroundvalue. This means that between 30 and 40 t dayy1 ofmagmatic CO are released from the summit area of2

Miyakejima volcano.No correlation was observed between CO fluxes2

and soil gas Rn and ThrRn ratios distribution at thesummit caldera. However, relatively high ThrRn

Žratios are observed inside this area Quintero et al.,.1998 , indicating high convective flow through this

permeable structures. Other areas with high ThrRnratios are located outside the summit caldera at theNE and SW flanks of the volcano, and seem to be

related to the existence of zones with better perme-Ž . Žability at depth fissures . Soil CO flux data May2

.and September showed a good correlation with thoseŽ .of soil temperature rs0.71 and 0.45 , soil gas CO2

Ž . Ž .rs0.40 and 0.1 and soil pH rs0.51 and 0.21 .The soil in the areas of high CO flux is character-2

Ž .ized generally by high temperatures T)608Cwhereas the areas with low CO flux are character-2

Ž .ized by relatively low temperatures Tf208C . Thispositive correlation between high CO flux and high2

temperatures in soils is supported by both spatialdistribution of CO flux and soil temperature maps2Ž .Fig. 9 , suggesting that part of the energy comingfrom the remnant heat source beneath Miyakejimavolcano is used to heat up the local meteoric waterpenetrated through fissures.

Ž .Sasai et al. 1997 studied the distribution ofelectrical potential at Miyakejima volcano reportingthat a high positive anomaly is located over the

Ž .summit area )700 mV whereas two negativeanomalies are found at the north and south flanks of


Ž y2 y1 .Fig. 8. Distribution map of soil CO flux anomalies g m day ; September survey at summit cone Oyama and surrounding areas,2

Miyakejima volcano.

the volcano, corresponding well with the distributionof centers formed in 1874 and 1763 eruptions, whichare connected by fissure zones. The positive CO2

Žflux anomalies found at the summit caldera sauna.area may be related to the upward movement of

heated water. This relation also suggests the occur-

rence of extensive soil degassing generated by con-densation processes of fluids originally similar to

Žthose discharged by the fumarolic system Chiodini.et al., 1996 . These observations indicate that there is

an appreciable release of CO from Miyakejima2

volcano.

Table 3Total CO fluxes observed at Miyakejima volcano and other volcanoes2

2 y1Ž . Ž .Date Area km CO flux t day Ref.2

Ž .Miyakejima summit caldera May 1998 0.62 146Ž .Miyakejima summit caldera Sept. 1998 1.10 98

Ž .Vulcano Italy April 1995 0.65 269 aŽ .Solfatara of pozzuoli Italy May 1994 0.09 133 b

Ž .Yanbajain China 1996 3.2 138 bŽ .Summit Teide Spain September 1996 0.53 380 c

Ž .Mammoth Mountain USA August 1995 0.145 350 dŽ .Mammoth Mountain USA September 1997 0.145 130 d

Etna October 1981 n.d. 55,000 e

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .a Chiodini et al. 1996 ; b Chiodini et al. 1998 ; c Hernandez et al. 1998 ; d Gerlach et al. 1998 ; e Allard et al. 1987, 1991 ;´ndsno data.


Ž .Fig. 9. Distribution map of soil temperature 8C; May survey at summit cone Oyama and surrounding areas, Miyakejima volcano.

Fig. 10 shows the correlation diagram betweenCO concentration and d

13C–CO data with the2 2

distance from the central cone Oyama. There is a

clear trend of decreasing on the CO concentration2

and d13C isotopic values with increasing distance

from the central cone Oyama. This pattern has been

Ž . 13Fig. 10. Relationship between soil–gas CO % concentrations and d C-values with the distance of the sampling site from the summit2

cone Oyama, Miyakejima volcano. Open circles and black circles represent soil gas CO concentration and d13C-values, respectively.2


observed at other volcanoes such as Fossa cone onŽ .Vulcano Island Baubron et al., 1990 , and it is

associated with an elevated magmatic contributionnear the crater. The decrease of both parameters withdistance suggests a dilution with atmospheric air anda decrease of diffuse degassing due to a minorsurface activity andror a mixing with an organicCO component associated with higher humidity and2

biogenic activity. The carbon isotopic composition ofsoil gases from Miyakejima volcano supports thishypothesis, showing a signature from y0.16‰ toy25.16‰ relative to PDB standard. Samples from

Ž . 13the summit cone Oyama showed a range of d C–CO from y0.16‰ to y5.70‰ and are similar to2

those found for the gases discharged by the fu-marolic system of Miyakejima volcano, which showsa d

13C–CO of y0.90‰ relative to PDB. These2

values suggest a mixing of two main sources for theŽ .CO emanating from Oyama area: 1 decarbonation2

reactions affecting the marine limestones of the vol-canic basement, assuming that elemental and isotopicfractionation is minimal in the hydrothermal system;

Ž .and 2 degassing of the magmatic chamber under-neath the summit caldera. The contribution of or-ganic carbon seems to be negligible for these emana-tions because the vegetation over Oyama cone isvery scarce and the soils are poorly developed. Out-side the caldera rim, d13C–CO values ranged from2

y20.23‰ to y5.70‰ indicating a mixing of bio-genic and volcanic CO . Deep CO is not released2 2

through the fissures and seems to be trapped by localmeteoric water. Fumarolic discharges inside the sum-mit caldera showed 3Her4 He isotopic ratios close to

Ž . 3 4air 0.877 Ra and similar to those Her He ratiosŽ .0.883 and 0.968 Ra observed in soil gases fromOyama cone, suggesting a strong air contaminationfor this diffuse degassing. Fumarolic and soil gas Hesamples from Oyama cone showed low concentra-

Žtion values 1.42–1.46 ppm and 1.29–1.48 ppm,respectively, being the nominal atmospheric concen-

.tration 5.248 ppm .

6. Conclusions

Soil gas CO flux results showed three different2

geochemical populations for both surveys carried outin May and September 1998. Most of the Miyake-

jima volcano showed background levels of diffuseemission of CO . The main structure releasing CO2 2

is the summit cone Oyama, where the most obviousgeothermal features are located. Based on multiplesof the background value, a total output between 100and 150 t dayy1 of CO is estimated for the summit2

caldera. Chemical and isotopic composition of dif-fuse soil gas CO from Miyakejima volcano reveal a2

clear organic origin for CO with the exception of2

that from Oyama area, where a mixing of carbonderived from marine limestone and magmatic CO is2

suggested by the observed d13C–CO . Continued2

monitoring of soil CO flux for volcanic surveillance2

seems desirable for Miyakejima volcano.

Acknowledgements

We thank Prof. Aotani of Miyakejima High Schooland P. Quintero for their assistance in the fieldwork.We also thank Dina Lopez who improved the earliest´version of this manuscript. Helpful reviews of themanuscript were provided by Fraser Goff, BrianTravis and Chuck Connor. We are grateful to M.Sato for helping in laboratory measurements. Thisresearch was carried out with grant from the EU-STF

Ž .Program in Japan P.H.P. .

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Diffuse emission of CO2 from Miyakejima volcano, Japan

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