soil gas co2, ch4, and h2 distribution in and around las cañadas caldera, tenerife, canary islands,...

Soil gas CO2, CH4, and H2 distribution in and around Las CanÄadascaldera, Tenerife, Canary Islands, Spain

P. HernaÂndeza,b, N. PeÂreza,b,*, J. Salazarb, M. Satoc, K. Notsua, H. Wakitaa,1

aLaboratory for Earthquake Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-Ku 113-0033, Tokyo, JapanbEnvironmental Research Division, Instituto TecnoloÂgico y de EnergõÂas Renovables (ITER), 38594 Granadilla, S/C de Tenerife, Spain

cMail Stop 956, National Center, USSG, Reston, VA 20192, USA

Received 14 April 1999; revised 27 January 2000; accepted 4 February 2000

Abstract

Diffuse degassing of CO2, CH4 and H2 was investigated at the surface environment of CanÄadas caldera, Canary Islands,

during the gas survey carried out in the summer of 1995. Soil CO2 concentration varied signi®cantly from atmospheric levels to

30%, while soil CH4 and H2 contents ranged from 5 to 851 ppm and from 0.5 to 620 ppm, respectively. Soil CO2, CH4 and H2

distribution suggests that high diffuse degassing at CanÄadas caldera is volcanic-structurally controlled. Anomalous soil H2

concentrations were identi®ed at the summit of Teide and outside caldera boundaries, where the most recent eruption of

Tenerife Island occurred. d 13C±CO2 data showed a magmatic, mixed magmatic±biogenic, and biogenic origin while a biogenic

origin is suggested for soil CH4 at CanÄadas caldera and its surroundings. By coupling the CO2/3He ratio with the 3He/4He ratio

of fumarolic gas samples from the summit of Teide, we propose three possible sources for carbon: MORB-type, organic carbon

and carbonate. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: soil gas; carbon dioxide; methane; hydrogen; isotopes; CanÄadas caldera; Teide volcano

1. Introduction

Soil gas prospecting has received much attention in

recent years as a useful tool for geothermal explora-

tion (Hinkle et al., 1978; Whitehead et al., 1983;

Bertrami et al., 1990; Finlayson, 1992) and for other

applications, such as volcanic surveillance (Barberi

and Carapezza, 1994; Toutain et al., 1992) and search-

ing for oil and gas reserves (Gregory and Durrance,

1985; Peachey et al., 1985). Studies carried out over

active faults/fractures have shown that these geo-

logical structures act as preferential pathways for the

discharge of gases from different origins toward the

surface, being proposed as useful geochemical indi-

cators for earthquake prediction (Irwin and Barnes,

1980; King, 1980; Sugisaki et al., 1980, 1983; Wakita

et al., 1980; Reimer, 1980; Satake et al., 1984).

CO2 is the most abundant gas, after water, in the

volatile phase exsolved from magma. The CO2

discharges occur as plumes and fumaroles from active

craters, as well as diffuse soil emanations from their

¯anks (Allard et al., 1991; Aubert and Baubron, 1988;

Badalamenti et al., 1988; Baubron et al., 1990; Chio-

dini et al., 1998; Farrar et al., 1995; Giammanco and

Gurrieri, 1997; HernaÂndez et al., 1998), being

released to the atmosphere even when they are in a

Journal of Volcanology and Geothermal Research 103 (2000) 425±438

0377-0273/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.

PII: S0377-0273(00)00235-3

www.elsevier.nl/locate/jvolgeores

* Corresponding author. Fax: 134-92291001.

E-mail address: [email protected] (N. PeÂrez).1 Present address: Gakushuin Women's College, Faculty of Inter-

cultural Studies, 3-20-1 Toyama, Shinjuku-Ku, Tokyo 162-8250,

Japan.

dormant stage. Carbon dioxide can be generated in

areas with anomalous geothermal gradients by ther-

mal metamorphism of carbonate rocks. The oxidation

of organic matter in rocks and soils can also produce

CO2 as well as the microbially aided oxidation of

sulphides (Lowell et al., 1980). Contents of CO2 in

hydrothermal systems can vary signi®cantly when the

gas is trapped by the groundwater and precipitates as

carbonate. The knowledge of the aquifer system is

very important to have a better understanding of the

gas distribution patterns.

Few studies about the origin, distribution and ¯ux

of CH4 from active volcanic and geothermal areas

have been reported. The main potential sources of

methane in terrestrial gases have been identi®ed as:

(1) reduction of CO2 in hydrothermal systems

(Hulston and McCabe, 1962); (2) decay of organic

matter due to thermogenic processes and bacterial

breakdown of organic materials in sediments

(Schoell, 1980); and (3) methane from magmatic

reservoirs. Methane is also a volatile component of

submarine and sub-aerial hydrothermal ¯uids

(Welhan and Craig, 1979: Welhan, 1988; Wakita

and Sano, 1983; Jean-Baptiste et al., 1990; Giggen-

bach et al., 1993; Ishibashi et al., 1994), where it is

associated with primordial helium enriched in 3He

that is still emanating from the Earth's interior, prin-

cipally through active volcano-tectonic regions

(Wakita et al., 1978; Sano et al., 1984; Poreda and

Craig, 1989). Methane is frequently present in soils

over a sedimentary cover where an anaerobic envir-

onment can develop, generating gas pockets due to the

accumulation of hydrocarbons originating from the

decomposition of organic matter and thermal altera-

tion of oil. Wakita et al. (1983, 1990) proposed an

abiogenic origin for CH4-rich commercial natural

gas in volcanoclastic reservoirs in Japan, based on

the observed high 3He/4He ratios.

Hydrogen is an important component of volcanic

gases and its use as a tool for volcanic monitoring

started with Sato and McGee (1980). Because of its

chemical and physical characteristics, H2 moves to the

top of the atmosphere easily and is too light to be held

in the Earth's gravitational ®eld, so that it is

constantly lost to space from the atmosphere. The

atmospheric H2 concentration is very low (0.5 ppm)

and its small solubility in groundwater coupled with

its low atmospheric concentration, makes direct

contamination of this gas from meteorological sources

negligible. These characteristics make H2 an excellent

tracer for processes operating deep in the crust. H2 can

be generated abundantly by several chemical reac-

tions induced by a water±rock interaction. High

concentrations of H2 have been detected often in soil

gases and dissolved gases from fault zones (Sugisaki

et al., 1980; Wakita et al., 1980; Sato, 1988).

The interpretation of soil gas anomalies can be dif®-

cult because gas concentration depends on meteoro-

logical conditions as well as soil type at the sampling

site. The principal factors affecting soil gas concen-

trations include precipitation, soil and air tempera-

tures, relative humidity, barometric pressure, wind

speed and soil type (Hinkle and Ryder, 1987; Hinkle,

1990, 1994; Asher-Bolinder et al., 1990). The site

location and the soil permeability due to the sand,

clay and organic matter content can allow varying

amounts of atmospheric air to enter the soil and can

thereby also affect the concentrations of the monitored

gases. Moisture absorbed by organic matter or by clay

in soils can also modify the concentration of soil

gases.

The ®eldwork was carried out during dry periods to

eliminate the possible in¯uence of rainfall and soil

humidity. In this work, we attempt to identify those

areas of soil degassing of CO2, CH4 and H2 and to

evaluate the soil distribution of these gases at CanÄadas

caldera and its surroundings.

2. Geological setting of study area

Tenerife is the largest of the Canary Islands and,

together with Gran Canaria, is the only one that has

developed a central volcanic complex, characterized

by the eruption of differentiated magmas. The Tener-

ife central volcanic complex, the Las CanÄadas edi®ce,

started to grow about 3.5 Ma ago, immediately after

the construction of the basaltic shield, which forms

the basement of the island. The construction of the

Las CanÄadas edi®ce has involved several constructive

and destructive episodes, including caldera collapses

and large-scale landslides (Ancoechea et al., 1990;

MartõÂ et al., 1997).

One of the main morphological and structural

features of the Tenerife central volcanism is the Las

CanÄadas caldera, at the top of the Las CanÄadas

P. HernaÂndez et al. / Journal of Volcanology and Geothermal Research 103 (2000) 425±438426

edi®ce, in which interior stands the active complex

Teide±Pico Viejo. The Las CanÄadas edi®ce (Fig. 1)

is an elliptical depression measuring 16 £ 9 km2, open

to the sea by the north side. The depression has been

®lled with materials from intensive post-caldera activ-

ity that started about 0.17 Ma ago, coinciding with the

®rst episodes of the construction of the Teide±Pico

Viejo stratovolcanoes (MartõÂ et al., 1994). A pre-

caldera formation, the Roques de GarcõÂa, divides the

Las CanÄadas caldera into two morphological depres-

sions, the western depression being 150 m deeper than

the eastern one. Several families of phonolitic vertical

concentric dikes, radial dikes and cone-sheets,

together with concentric and radial fractures and

alignments of basaltic and phonolitic cones, constitute

the main structural features visible at he interior of the

caldera. The Las CanÄadas caldera connects at the east-

ern and western sides with the basaltic rift systems of

the Dorsal Ridge and the Santiago del Teide Ridge,

respectively (Ablay and Marti, 2000).

The stratigraphy of CanÄadas caldera shows two

main deposits: pre-CanÄadas and post-CanÄadas depos-

its. Pre-CanÄadas deposits, at the base of the caldera

edi®ce, are constituted by three series or units. One of

them, the series ªCanÄadas Inferiorº is characterized by

the existence of very compact and altered basalts with

null permeability. These basalts are accompanied by

large amounts of secondary materials such as carbo-

nates, zeolites and ¯uorites. This layer is several

hundreds of meters thick. Post-CanÄadas deposits are

constituted of four main units following the following

time sequence: (a) mortaloÂn (an impermeable and

thick deposit, originating from a debris avalanche

and located approximately 1 km underneath the CanÄa-

das caldera surface); (b) basaltic deposits; (c) trachy-

basaltic deposits; and (d) phonolytic and trachytic

deposits (Navarro, 1996). From a hydrogeological

point of view, the mortaloÂn is very important because

it ®ts the characteristics of the aquifer underneath

CanÄadas caldera.

The climate at CanÄadas caldera, type sub-alpine, is

characterized by strong climate contrasts during the

different seasons. Because of climatic conditions and

the range in age and type of eruptive materials, soil

conditions at CanÄadas caldera and its surroundings

vary markedly. The soils of the study area are typical

andosoils, strongly desaturated and developed over

pyroclastic and ash volcanic deposits, with an organic

matter content varying up to 25%.

The hydrogeological system of Tenerife Island is

strongly conditioned by the morpho-structural evolu-

tion of volcanism and speci®cally by that of the

P. HernaÂndez et al. / Journal of Volcanology and Geothermal Research 103 (2000) 425±438 427

5Km

N

Basaltic and trachy-basaltic emissioncenters

Phonolitic emissioncenters

Cinder cones(Valle de la Orotava)

Valle de Icod

Teide

RoquesdeGar cía

0

CAÑADASCALDE RA

Caldera wall

Atlantic Ocean

ValledelaOr otava

0 20km10500

1000

3000

2500

20001500

CañadasCaldera

13º13’

27º59’

TENERIF ISLAND

Fig. 1. Map of CanÄadas caldera and peripheral areas showing the main structural and geomorphological features (after Navarro, 1996).

central structure. Around CanÄadas caldera there are

numerous water galleries (sub-horizontal drillings)

with abnormal temperatures (up to 458C), gases

(CO2, H2, CH4, etc.) and water with considerable

concentrations of temperature indicators (SiO2, B,

NH41, etc.) and very high bicarbonate and sulfate

contents. These characteristics support the hypothesis

that the groundwater chemistry of the CanÄadas aquifer

is controlled by the input of deep-seated gases (espe-

cially CO2) from the Teide volcanic±hydrothermal

system (Valentin et al., 1990; Navarro, 1996).

3. Sampling and analytical methods

Soil gas samples were collected at CanÄadas caldera

and its surroundings, taking the local geology and

structure into careful consideration. PVC pipes were

inserted to a depth of 50 cm and left in the ground for

a 7-day period to allow soil atmosphere equilibration.

Soil gas samples were stored in evacuated containers

by using 10 cc hypodermic syringes (Hinkle and

Kilburn, 1979). CO2, CH4, Ar, N2, and O2

concentrations were measured using a gas chromato-

graph with a thermal conductivity detector (Ohkura

GC 103). CO2 was analyzed using a Porapaq column

with an operating temperature of 308C, with O2 as

carrier. CH4, N2 and Ar were analyzed with a 5 AÊ

molecular sieve column with oxygen as a carrier. O2

was analyzed with the same 5 AÊ molecular sieve

column but with argon as a carrier. Approximately

350 soil gas samples were analyzed for CO2 and 150

for CH4, N2, Ar, and O2.

Soil H2 measurements were carried out at the ®eld

by using a portable sensor instrument speci®cally for

H2 (Radd and Oertle, 1970). The measurement system

consists of a metallic membrane permeable only to H2

(palladium alloy) at the gas inlet placed inside a plas-

tic bottle with a septum. Samples were withdrawn

with 60 cc hypodermic syringes from the PVC pipes

by inserting a hypodermic needle through the septum,

and then injected into the plastic bottle. The instru-

ment was calibrated every day using air for zero

adjustment. Approximately 300 soil gas samples

were analyzed for soil H2.

Carbon isotopic ratios for CO2 and CH4 were

measured using a conventional mass spectrometer

(Finnigan Mat, Delta S) after separation of CO2

and CH4 from other chemical components using

traps held at liquid N2. Observed 13C/12C ratios

are expressed in the delta (d ) notation, as parts

per thousand deviation (per mil, ½) and were calibrated

against a running standard, whose isotopic composition


Table 1

Descriptive statistic of soil CO2, CH4 and H2 data from the surface

environment of CanÄadas caldera

Range (ppm) Mean s.d. No. of samples

CO2 430±296,423 2822 16.67 350

CH4 5±851.3 141.5 17.48 150

H2 0.5±619.7 22.98 40.32 300

Table 2

Observed d 13C values for soil CO2 and CH4 together with the

concentrations (ppm) from CanÄadas caldera, Tenerife

Sample CO2

(ppm)

CH4

(ppm)

d 13C±CO2

(½ vs. PDB)

d 13C±CH4

(½ vs. PDB)

3 7157.36 245.16 231.65 272.77

21 3164.03 298.56 233.63 269.92

28 1346.0 48.05 234.99 270.34

65 1524.39 20.25 225.47 263.25

67 1038.46 128.76 230.82 a

78 2363.93 a 233.96 269.76

88 2153.64 219.23 236.91 275.71

97 889.01 79.17 222.87 a

99 1430.99 163.81 225.34 a

130 5206.89 7.95 238.00 270.38

143 12152.43 292.8 218.77 a

147 2270.92 645.32 234.05 275.33

158 5564.87 407.49 230.71 269.55

164 4704.3 388.29 229.84 268.17

168 1402.89 29.6 230.23 a

179 952.7 5.00 227.84 268.53

201 6084 249.27 225.79 268.42

217 1535.15 217.05 233.85 267.7

225 2030.97 282.58 226.72 270.46

226 1699.2 377.56 217.98 266.42

256 2235.94 453.79 233.34 270.33

270 3236.29 60.32 230.26 270.08

276 5014 148.41 223.48 269.42

293 3887.61 24.33 230.14 268.85

300 3202.75 267.22 226.43 a

311 1990.06 5.00 220.89 265.14

354 3291 391.53 236.04 269.23

361 265.44 151.41 232.97 269.94

372 2166.12 427.62 234.33 267.79

T1 150638.3 104.23 212.00 270.35

T2 296423.3 381.05 28.54 270.25

a Not analyzed.

is calibrated against an international standard of PDB

carbonate. The 13C/12C ratios are accurate to ^0.1½

(1s). Soil gas samples for 3He/4He analysis were

collected by using lead-glass containers ®tted with

high-vacuum stopcocks at both ends. 4He/20Ne ratios

were measured using a quadrupole mass spectrometer

(QMG 112, Balzers) using air as a standard. Errors for

the 4He/20Ne ratio are estimated to be about 10%.

Helium isotope measurements were carried out using

a high-precision gas mass spectrometer (VG5400, VG

Isotopes). Atmospheric helium was used as a running

standard and uncertainly for R/Ra ratios was about 1%.

All measurements were performed at the Laboratory for

Earthquake Chemistry, Tokyo University.

4. Results and discussion

Statistical descriptions of the soil gas chemical

compositions (CO2, CH4 and H2) are listed in Table

1. The soil CO2 concentrations showed a wide range

of values from atmospheric levels to 30%, while soil

CH4 and H2 contents ranged from 5 to 851 and from

0.5 to 620 ppm, respectively. d 13C±CO2 of soil gas

samples ranged from 28.54 to 236.92½ while

d 13C±CH4 values showed a smaller variation, from

263.25 to 275.71½ (Table 2).

Isotopic 3He/4He and 4He/20Ne ratios of soil gas

samples from the summit of Teide ranged from

0.98Ratm (where Ratm is atmospheric 3He/4He ratio of

about 1.4 £ 1026) to 3.71Ratm and from 0.39 to 0.69

(where atmospheric 4He/20Ne ratio is 0.314), respec-

tively. Both 3He/4He and 4He/20Ne ratios, which have

intermediate values between fumaroles (PeÂrez et al.,

1994) and air, suggest a deep contribution to the soil

gas samples.

4.1. Distribution of soil gas CO2, CH4 and H2 in and

around CanÄadas caldera

According to Sinclair (1974), a statistical±graphi-

cal analysis was carried out for both soil CO2 and CH4

data. This analysis allowed us to evaluate the statisti-

cal parameters of normal and log-normal unimodal

populations with adequate accuracy and distinguish

different geochemical populations.


Probability (% )

9850202 80

99.5%

III

II

96.8%

103

104

105

CO

(pp

m)

2

I

IV

4%

Fig. 2. Cumulative frequency plot of soil CO2 (ppm) at CanÄadas

caldera, Tenerife, Canary Islands.

Probability (% )

9850202 80

96.8%

20%

III

II

101

102

103

CH

(pp

m)

4

I

Fig. 3. Cumulative frequency plot of soil CH4 (ppm) at CanÄadas

caldera, Tenerife, Canary Islands.

The statistical±graphical analysis of total soil CO2

data showed four overlapping geochemical popula-

tions (Fig. 2). The background (population II) mean

was 2900 ppm and represents 92.8% of the total data.

The peak group (population IV) showed a mean of

110,000 ppm (11%) of CO2 and represents 0.5% of

the total data. An intermediate ªthresholdº population

(population III), which represents a mixing between

background and peak values, had a mean of 9000 ppm

of CO2 with 2.7% of the total data. An additional inter-

mediate population (population I) was observed and it is

mainly related to atmospheric disturbance of soil CO2

background levels (atmospheric CO2 is approximately

380 ppm). Population I showed a mean of 820 ppm CO2

and represents 4% of the total data.

Statistical±graphical analysis for the 150 measure-

ments of soil CH4 indicated three overlapping popula-

tions (Fig. 3): background (population II) and two

intermediate populations (populations I and III). The

background had a mean of 195 ppm CH4 and repre-

sents 63% of the total data. Population III showed a

mean of 410 ppm of CH4 and represents 16% of the

total data. Population I had a mean of 35 ppm of CH4

and might be related to atmospheric contribution to

the soil atmosphere.

The geometric mean, x, was read at the ®ftieth

percentile. The thresholds were chosen at the second

(Tlow) and ninety-eighth (Thigh) percentiles of the total

population. Figs. 4 and 5 show the locations of each

sampling site and the contour maps of soil CO2 and

CH4 concentrations for CanÄadas caldera, respectively.

The chemical characterization of the gas samples

showed higher N2/Ar molar ratios than atmospheric

value (85.21), suggesting a contribution of non-atmo-

spheric nitrogen to the soil gas samples.

Most of the caldera and its surroundings showed

background levels of diffuse degassing of CO2 and

CH4. The spatial distribution of the soil gases presents

the following features: (1) CO2, CH4 and H2 anoma-

lies are distributed around the caldera rim and outside

the caldera, with a few exceptions such as the summit

of Teide and Roques de GarcõÂa; and (2) relatively low

CO2 and CH4 and high H2 concentrations are located

inside CanÄadas caldera and in the western sector of the

caldera, where the last eruption in Tenerife Island

(Chinyero volcano, 1909) occurred. Outgassing is

greater from areas characterized by the existence of

fractures associated with the caldera structure and

surrounding areas. The highest soil CO2 values

(.100 £ background, 29%) were identi®ed with the

most obvious geothermal and volcanic feature at

CanÄadas caldera: the Teide summit crater. These

values are similar to the soil CO2 concentrations

measured in the summit crater of Etna volcano


16º40’W

28º15’N

CO (ppm;1992)2

0 1 2KmN

BackgroundStratovolcano

Caldera Wall

Road

Sampling Site

Background=2900ppm

Teide

Pico Viejo

RoquesdeGarcía

>1.5Background>4.5Background

>3Background>9Background

Fig. 4. Distribution of soil CO2 (ppm) anomalies at CanÄadas caldera, Tenerife, Canary Islands.

(27.9% CO2; Allard et al., 1991). High soil CO2

concentrations were also identi®ed along the NE

basaltic rift and at the NW side of CanÄadas caldera,

where there is intensive vegetation (pine grove) and

where soil samples also showed low pH values

(4 , pH , 5). Relatively high soil CO2 values might

be related to degradation of the organic matter, a

process enhanced by the acidity and permeability of

these soils (HernaÂndez et al., 1994).

Two factors may contribute to make the input of

deep gases to the CanÄadas caldera surface dif®cult:

®rst, the stratigraphy itself, and secondly, the hydro-

logic system. The CanÄadas caldera edi®ce is consti-

tuted of several deposits. One of them, the series

ªInferior CanÄadasº is characterized by the existence

of very compact and altered basalts with null perme-

ability, being several hundred of meters thick. The

other deposit, the ªmortaloÂnº, also presents a very

low permeability and is characterized by sealing

processes caused by hydrothermal alteration. Several

volcanic emission centers linked to the fracture

system seem to be the preferential pathways used by

deep gases to ascend to the surface. On the other hand,

the hydrological system of CanÄadas caldera, which is

characterized by the existence of high bicarbonate

groundwater, a product of the dissolved CO2, acts as

a barrier to those gases that ascend toward the surface.

Other factors such as barometric pressure, soil and air

temperature and wind speed could also contribute to

lowering the CO2 content in the soil atmosphere,

although their contribution has not been studied.

Soil CO2 levels at CanÄadas caldera are, in general,

relatively low with respect to other active calderas,

such as: (1) Long Valley (USA), where signi®cant

amounts of soil CO2 (.90%) of magmatic origin are

responsible for killing trees (Farrar et al., 1995); and

(2) Rabaul (Papua New Guinea) where soil CO2

measurements reach values up to 20% away from

active volcanic centers but just above the levels in

areas of high seismicity (PeÂrez and Wakita, 1995).

The different degree of volcanic activity might be a

plausible explanation for these observed differences

of soil CO2 levels in these macro-scenarios.

The spatial distribution of soil CH4 showed back-

ground levels with isolated positive anomalies. The

highest value of CH4 (851 ppm) was detected outside

CanÄadas caldera (southeast) where warm groundwater

(T . 308C) occurs. This anomalous high CH4 value is

geographically also well correlated with the existence

of horizontal drillings for groundwater exploitation

(galleries), which show high levels of CH4 in their

inner atmosphere. The origin of this CH4 could be


16º40’W

28º15’N

0 1 2Km

NCH (ppm;1992)4

Background

Caldera Wall

Road

Sampling Site

Background=195ppm

Teide

Pico Viejo

RoquesdeGarcía

Stratovolcano>1.5xBackground

>3xBackground>4xBackground

Fig. 5. Distribution of soil CH4 (ppm) anomalies at CanÄadas caldera, Tenerife, Canary Islands.

the degradation of organic matter from forest covered

by lava ¯ows. Groundwater from this area shows high

bicarbonate content; therefore, high soil CO2 levels

are not detected.

Soil H2 anomalies at CanÄadas caldera were distin-

guished in the base of different multiples (.15, .25,

.50, .100 ppm) of the atmospheric value (0.5 ppm).

75% of the samples had an H2 value higher than the

detection limit with a range from 0.5 to 620 ppm by

volume. Fig. 6 shows the location of each sampling

site and the contour map of H2 anomalies. Anomalous

soil H2 concentrations were identi®ed at the summit

area of Teide volcano where fumarolic degassing

occurs with 1.7 mmol H2/mole dry gas. Two addi-

tional areas outside the caldera boundaries also

showed high soil H2 concentrations, where the most

recent eruptions at Tenerife Island (Chinyero, 1909;

Siete Fuentes and Fasnia, 1704±1705) occurred. The

distribution pattern of high soil H2 (.50 ppm) in the

surface environment of CanÄadas caldera seems to be

controlled by volcano-tectonic lineaments of the study

area. Soil H2 anomalies at the studied area show, in

general, a good spatial correlation with those detected

for soil CO2, whereas for soil CH4 the correlation is

not clear with the exception of some speci®c areas.

Anomalous high soil H2 values are also spatially well

correlated with the existence of horizontal drillings

for groundwater exploitation (galleries), which show

relatively high levels of H2 in the inner atmosphere

(10±900 ppm) and temperatures up to 408C. Geother-

mometric techniques applied to the central system of

Teide (Albert-BeltraÂn et al., 1986) concluded that the

temperature of the residual magma body in the

Chinyero volcano and those associated with the erup-

tion of 1704±1706 are 350 and 2708C, respectively.

This observation suggests that H2 gas emanation is

produced by temperature-controlled redox reactions

between meteoric water and ma®c minerals at various

depths. In the other hand, northeast part of the caldera

showed lower soil gas H2 concentrations (,50 ppm),

relatively high soil gas CO2 concentrations

(.9000 ppm) and typical d 13C±CO2 and d 13C±CH4

values of biogenic gas. Koyama (1963) showed that

hydrogen can be produced by biological processes

associated with relatively high CO2 and CH4 concen-

trations. This suggests that biogenic H2 could contri-

bute to the total H2 output in these areas.

Soil H2 levels at CanÄadas caldera are relatively

higher than those reported for Sabatini volcanoes,

Italy (Bertrami et al., 1990). These authors carried

out a soil gas survey and found that the H2 distribution

showed isolated positive anomalies, and only 10% of


16º40’W

28º15’N

H (ppm;1995)2

0 1 2Km

N

>15ppmStratovolcano

Caldera Wall

Road

Sampling Site

Teide

Pico Viejo

Roques deGarcía

>25ppm >100ppm

>50ppm

Fig. 6. Distribution of soil H2 (ppm) anomalies at CanÄadas caldera, Tenerife, Canary Islands.

the samples had an H2 value higher than the detection

limit, ranging from 5 to 42 ppm by volume. The soil

H2 anomalies were in good agreement with those of

CO2, He and Rn, indicating zones with better perme-

ability at depth. High soil CO2, CH4 and H2 concen-

trations at CanÄadas caldera and its surroundings

present a very similar behavior, being well correlated

with those of Rn and Hg (HernaÂndez et al., 1993,

1994). This correlation rati®es the existence of high

heat ¯ow zones at CanÄadas caldera and surrounding

areas with a better permeability at depth. The presence

of several volcanic emission centers, linked to the

fracture system, which can act as preferential path-

ways to the surface for deep gases, support this

hypothesis.

Fig. 7 shows the distribution of CO2 and Rn along

the traverse established across the Roques de GarcõÂa

formation. The distribution pattern clearly re¯ects a

good correlation between both peaks of CO2 and Rn

(7100 ppm and 240 pCi/L, respectively) over the

fracture. These peaks coincide also in their position

with anomalous high He and Hg concentrations (200

and 13,320 ppb, respectively), which supports

the idea of high gas ¯ow through this permeable

structure.

The summit of Teide is the main structure releasing

CO2, where very high CO2 contents (.15 £Background) occur at the bottom of the crater as

well as in the SE and NE parts of the summit cone.

These high CO2 concentrations are well correlated

with high Rn activity (.700 pCi/L) and H2 content

(.100 ppm) at the summit crater, which supports the

existence of a high heat ¯ow in this area. Low diffuse

emission levels of CO2 are observed where lava ¯ows

exist, perhaps because they can act as impermeable

barriers for the gas leakage, or related to the intensive

air circulation which can mask CO2. Previous study on

diffuse CO2 emission from the summit of Teide

yielded soil CO2 and CH4 ¯ux levels about 380 and

1.61 t d21, respectively (HernaÂndez et al., 1998), by

using a variant of the accumulation chamber method.

This study revealed that the Teide volcano releases

abundant CO2 not only from its active crater, but

also from its ¯anks as diffuse emanations. Diffuse

emission of CO2 from the summit of Teide is of the

same order of magnitude as the ¯ux levels estimated

from Fossa, Vulcano, (Chiodini et al., 1996) and

lower than those observed at Kilauea and Etna volca-

noes (Gerlach and Graeber, 1985; Allard et al., 1991).

Fig. 8 shows the spatial distribution of soil CO2 and its

d 13C values along the summit of Teide, ranging from

1.11 to 42.65% and from 20.98 to 243.59½, respec-

tively. There is a clear trend of decrease in the CO2

concentration and d 13C isotopic values with increas-

ing distance from the summit crater. This pattern has

been observed at other volcanoes such as Fossa cone

at Vulcano Island (Baubron et al., 1991), and it is

associated with an elevated magmatic contribution

near the crater. The decrease of both parameters

with distance suggests a dilution with atmospheric

air, decrease in the diffuse degassing because of

minor surface activity and/or a mixing with an organic


250

0

100

150

200

50

0 200 400 600 1000800

Distance (m)

222 R

n(p

Ci/L

) CO

(pp

m)

2

8000

3000

5000

6000

7000

4000

1000

2000

Fracture

Fig. 7. Carbon dioxide and radon for the geochemical pro®le over

Roques de GarcõÂa formation, CanÄadas caldera, Tenerife, Canary

Islands.

Summitcrater of

Teide

CO

(%)

2

Distance (m)

δ13C

(CO

)2

200 4003001000 500

-50

-40

-30

-10

-20

048

40

24

32

16

0

8

Fig. 8. Relationship between soil CO2 (%) concentrations and d 13C

values with the distance of the sampling site from the summit crater

of Teide volcano, Tenerife, Canary Islands. Open circles and black

circles represent d 13C values and soil gas CO2 concentration,

respectively.

component as a result of the higher humidity and

biogenic activity.

In summary, high diffuse emission of CO2, CH4 and

H2 was detected at the summit of Teide and in those

areas where fracturing or faulting is known to exist.

The overall trend at CanÄadas caldera surface environ-

ment is of high positive anomalies associated with the

caldera structure: volcanic emission centers linked to

the fracture system and other well-developed perme-

able structures.

4.2. Origin of carbon in the CO2 and CH4

In order to study the existence of different

geochemical reservoirs for the origin of carbon in

the soil and fumarolic gas CO2, ®rst the correlation

between soil CO2 and O2 was considered. Plotting soil

CO2 concentration vs. that of soil O2 (Fig. 9), we

observe a mixing between three geochemical reser-

voirs: (1) volcanic gas (CO2� 85% and O2� 0%);

(2) air (CO2� 0.03% and O2� 20.95%); and (3)

biogenic gas. The biogenic gas box boundaries are

selected, taking into consideration the range of CO2

and O2 concentrations corresponding to the background

population (1900 , CO2 , 4500 ppm). Although the

observed data suggest that most of the samples plot

along the mixing line between these three reservoirs,

isotopic analysis of carbon in CO2 is needed.

To constrain the origin of CO2 and CH4, their isoto-

pic ratios were measured. The d 13C value has often

been used to identify the origin of carbon in natural gas

samples (Schwarcz, 1969; Hoefs, 1980). Mid-ocean

ridge basalts (MORB) glasses have d 13C values

between 24 and 29½ with an average of 26.5½,

which is considered to represent the upper-mantle C

(Javoy et al., 1986; Marty and Jambon, 1987). In

contrast, d 13C values of crustal carbon can vary

signi®cantly, considering two main sources for

carbon: (1) marine limestone including slab carbo-

nate, which has an average d 13C value near 0½;

and (2) organic carbon from sedimentary rocks with

d 13C values lighter than 220½. Mixing of carbon

from marine limestone and organic sediment could

produce a d 13C value of 26.5½, since it is impossible

to identify the origin of carbon based on the d 13C

value only.

The carbon isotopic composition of CO2 and CH4 is

listed in Table 2. The following three groups of d 13C

values for soil gas CO2 were distinguished:

(1) Soil gas samples from the summit crater of

Teide with the heaviest signature of carbon isotopic

composition for soil CO2. The d 13C±CO2 in these

samples showed a range from 21.34 to 212.9½ rela-

tive to PDB, suggesting a magmatic origin for the soil

CO2. An atmospheric origin must be neglected

because of the high soil gas CO2 concentration of

these samples (4.1±29.6%). These isotopic values

are quite similar to the carbon isotopic composition

of the gases discharged by the fumarolic system from

Teide volcano with ranges of d 13C±CO2 from 23.7 to

28.1½ and high observed 3He/4He ratios, 7.5 Ra

(PeÂrez et al., 1994; 1996a). The wide range of

d 13C±CO2 observed for these emanations (between

21.34 and 212.9½) might suggest a potential mixing

between magmatic and biogenic soil CO2. Atmo-

spheric disturbance should also be neglected because

of the observed high soil gas concentrations.

(2) Soil gases with a d 13C±CO2 range from 212.9

to 218.0½ relative to PDB. This relatively heavier

isotopic signature for soil CO2 might be related also to

a mixing process between biogenic and magmatic

CO2 because of the relatively high CO2 concentrations

in these samples, which are not only located in areas

where there is vegetation.

(3) Soil gas samples with the lightest carbon isoto-

pic signature for the soil CO2 showing a range of

d 13C±CO2 from 218.8 to 238.0½, which suggests

a clear biogenic origin.

Fig. 10 shows the correlation diagram between the


VolcanicGas

BiogenicSoil Gas

Air

O (% )2

CO

(%)

2

102

101

100

10-1

10-2

0 2015105 25

Fig. 9. Correlation diagram between soil CO2 (%) and O2 (%)

concentrations. The line represents the mixing line between volca-

nic gas and atmospheric air reservoirs.

CO2 content of soil samples (ppm) and their d 13C±

CO2 values. This diagram indicates a con®rmation of

the existence of these three geochemical reservoirs (as

is suggested in Fig. 9): air (CO2� 380 ppm and d 13C±

CO2�28½), volcanic gas (85 , CO2 , 100% and

d 13C±CO2�22½) and biogenic gas (1900 , CO2

,4500 ppm and 220.89 , d 13C±CO2 , 236.92½).

It is clear that most of the samples are located between

the biogenic and volcanic gas reservoirs, suggesting a

mixing of both reservoirs. The soil gas samples with a

high CO2 content (.10%) and high d 13C isotopic ratios

(. 2 8½) are located at the summit of Teide, indicat-

ing an important magmatic contribution to their origin.

Sano and Marty (1995) used the combination of

d 13C values and CO2/3He ratios to identify the origin

of carbon in volcanic and geothermal discharges.

Marty and Jambon (1987) considered the CO2/3He

ratio of the relevant reservoirs in order to picture the

C geodynamics past and present, suggesting that this

ratio is unfractionated during magma outgassing. The

CO2/3He ratios are estimated by the observed 3He/4He

isotopic ratio, and helium and CO2 concentrations.

Atmospheric contamination in 3He is corrected

using the 3He/4He and 4He/20Ne ratios. This model

could allow us to estimate the fraction of carbon

from each geochemical reservoir, but only a qualita-

tive description of the different carbon sources is

possible because: (1) the isotope fractionation

processes affect the carbon in both soil and fumarolic

gases; and (2) content of CO2 in hydrothermal gases

may vary signi®cantly when the gas resolves in

groundwater or precipitates as a carbonate. Processes

such as evaporation±condensation and diffusion can

cause signi®cant isotope fractionations. The CO2/3He

ratio of soil gas samples from the summit of Teide and

the ªRoques de GarcõÂaº formation varies widely

(4.02 £ 109 to 1.39 £ 1010) as a result of strong frac-

tionation processes such as diffusion, contamination

and possibly diffusive loss of He, and has not been

considered. Hence, the most representative samples of

magmatic gas are likely to be the fumarolic emana-

tions although these should be considered with care.

Considering that CO2 is the main component (85±

99% V) in the fumarolic dry gas of Teide volcano,

no signi®cant dilution of magmatic gas by crustal

volatiles occurs (Urabe et al., 1985). However, CO2

is not as sensitive to the magmatic activity as the

helium isotopes. PeÂrez et al. (1996b) pointed out the

existence of a hot-spot deep-mantle source beneath

Tenerife Island in the base of the high 3He/4He ratios

observed at the Teide fumarolic discharges (.7.2 Ra).

This ®nding, together with the carbon isotopic data,

support the idea that part of the CO2 released from the

fumarolic system of Teide volcano is MORB-derived.

The CO2/3He ratio of fumarolic samples from the

summit of Teide ranges from 3.85 £ 109 to

8.11 £ 109 with an average of 7.0 £ 109. This value

suggests the existence of three possible sources for

carbon: MORB-type, organic carbon and carbonate.

If we assume that the steam is produced by a single-

step boiling process at a temperature of 1008C(Albert-BeltraÂn and co-workers, 1990) and that no

condensation occurs, elemental and isotopic fractio-

nation could be considered minimal (Sano et al.,

1995). Under this assumption, the estimated fraction

of each source of C can be estimated, considering it

always as an approximation. The results indicate that

a large portion of carbon from the fumarolic steam

samples is derived from carbonates (68%), while the

average MORB-type content for the carbon is 28%.

Chemical reactions between acidic magmatic-derived

volatiles (HCl, H2S, HF, etc.) and local carbonate

deposits located underneath CanÄadas caldera could

originate this large portion of carbon derived from

carbonates.

Valentin et al. (1990) reported isotopic values of

d 13C±CO2 from galleries associated to the Chinyero

and MontanÄa Negra volcanoes ranging from 25.0 to

26.7½, corresponding to a typical endogenous


-20

-40

-30

-10

0

105102103 104 106

CO (ppm)2

δ13C

(CO

) 2

Air

BiogenicSoilGas

VolcanicGas

Fig. 10. Correlation diagram between d 13C values and soil CO2 (%)

concentrations. The lines represent the mixing lines between atmo-

spheric air and soil gas reservoirs and between soil gas and volcanic

gas reservoirs.

source. These volcanic edi®ces are located several

kilometers away from the Teide volcano, and are

associated with the active fracture/fault system or

the ªrift zoneº of Tenerife Island. These observations

suggest that deep-mantle degassing also occurs far

from the summit of the Teide volcano, preferentially

along active structures such as fractures and/or faults.

PeÂrez et al. (1996b) studied the He-3 spatial distribu-

tion around Teide volcano, observing that 3He/4He

ratios are quite uniform and do not show any relation

to distance. These observations provide additional

geochemical evidence for a signi®cant advective

component due to the uprising of deep-seated gases

along this structure, which show high vertical perme-

ability levels.

Carbon isotope values for CH4 ranged from 263.25

to 275.33½. Schoell (1980) reported that biogenic

methane may be formed by the bacterial breakdown

of organic material in sediments, having a very

distinctive d 13C±CH4 composition of 255 to

285½. This suggests mainly a biogenic origin for

the methane at the CanÄadas surface environment.

5. Conclusions

The spatial distribution of soil CO2, CH4 and H2 at

the surface environment of CanÄadas caldera correlates

quite closely with that of thermal anomalies and mani-

festations of the study area. Convergence of soil gas

anomalies indicates areas where better permeability

can occur at depth, revealing zones of high heat

¯ow and also being in good agreement with those of

soil gas Rn and Hg. High soil CO2 and H2 levels

mainly reveal deep perturbations or magmatic origin

for both soil gases. Carbon isotopic ratios of CO2 vary

largely, suggesting different sources for the CO2. Soil

gas samples from the summit of Teide show the heavi-

est signatures of carbon isotopic composition (21.34

to 212.0½), suggesting a magmatic derivation of the

gas. Additional evidence is provided by elevated3He/4He ratios in both soil gas and fumarolic

discharges. Isotopic ratios of CH4 are comparatively

uniform and suggest mainly a biogenic origin at the

CanÄadas surface environment. Based on the d 13C

value and CO2/3He ratio, the origin of carbon was

assessed qualitatively in soil gases from the summit

of Teide fumarolic gases. Up to 28% of MORB-type

mantle is estimated for fumarolic discharges, whereas

the major part of the gas discharged from the summit

of Teide may be attributed to CO2 produced by a

decarbonation process.

Acknowledgements

We thank E. MeÂndez, E. PadroÂn, P. Quintero, J.A.

Cobas, G.V. MeliaÂn, L. Castro and R.N. Lima for their

assistance in the ®eldwork. We thank Y. Shimoike

and M. Sato for help in laboratory measurements.

We are also grateful to the Patronato del Parque

Nacional de Las CanÄadas del Teide, TelefeÂrico Pico

Teide S.A. and The Laboratory for Earthquake Chem-

istry at The University of Tokyo for their assistance

and logistical support. This research was supported by

the Cabildo Insular de Tenerife, NATO CRG #940882

(C.E.A. and N.C.S.) and with grants from the EU-STF

Program in Japan (P.H.P. and N.M.P.).

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