RESEARCH ARTICLE

First 13C/12C isotopic characterisation of volcanic plume CO2

Giovanni Chiodini & Stefano Caliro & Alessandro Aiuppa & Rosario Avino &

Domenico Granieri & Roberto Moretti & Francesco Parello

Received: 31 March 2010 /Accepted: 28 October 2010 /Published online: 11 November 2010# Springer-Verlag 2010

Abstract We describe analytical details and uncertaintyevaluation of a simple technique for the measurement of thecarbon isotopic composition of CO2 in volcanic plumes.Data collected at Solfatara and Vulcano, where plumes arefed by fumaroles which are accessible for direct sampling,were first used to validate the technique. For bothvolcanoes, the plume-derived carbon isotopic compositionsare in good agreement with the fumarolic compositions,thus providing confidence on the method, and allowing itsapplication at volcanoes where the volcanic component isinaccessible to direct sampling. As a notable example, weapplied the same method to Mount Etna where we derived a

0.27‰ (Bocca Nuova and Voragine craters). The compar-ison of our measurements to data reported in previous work

values of Etna CO2 from~ −4‰, in the 1970’s and the1980’s, to~ −1‰ at the present time (2009). This shifttoward more positive δ13C values matches a concurrentchange in magma composition and an increase in theeruption frequency and energy. We discuss such variationsin terms of two possible processes: magma carbonateassimilation and carbon isotopic fractionation due tomagma degassing along the Etna plumbing system. Finally,our results highlight potential of systematic measurementsof the carbon isotopic composition of the CO2 emitted byvolcanic plumes for a better understanding of volcanicprocesses and for improved surveillance of volcanicactivity.

Keywords Volcanic plume . Carbon isotope . Etna .

Magmatic degassing

Introduction

Characterising the isotopic signature of carbon dioxide(CO2) emitted from active volcanoes has profound impli-cations on our ability to predict the origin of volcanic gases(Allard 1983), and as such contributes to shed light into thecycling of carbon among the atmosphere, hydrosphere,crust, and upper mantle (Berner and Lasaga 1989). Inaddition, the high abundance of CO2 in magmatic systems(Carroll and Halloway 1994; Wallace 2005), its lowsolubility in silicate melts favouring early exsolution atmantle to deep-crustal conditions (Holloway and Blank1994), and the relatively inert chemical behaviour (at leastcompared to more reactive species such as SO2, or HCl;Symonds et al. 2001) make CO2 an optimal chemical tracerof volcanic degassing processes. This fact, when combined

Editorial responsibility: J. Stix

G. Chiodini : S. Caliro (*) : R. Avino :R. MorettiIstituto Nazionale di Geofisica e Vulcanologia Sez. NapoliOsservatorio Vesuviano,Via Diocleziano 328,80124, Naples, Italye-mail: stefano.caliro@ov.ingv.it

A. Aiuppa : F. ParelloDipartimento CFTA, Università di Palermo,Via Archirafi 36,90123, Palermo, Italy

A. AiuppaIstituto Nazionale di Geofisica e Vulcanologia Sez. Palermo,Via Ugo La Malfa, 153,90146, Palermo, Italy

D. GranieriIstituto Nazionale di Geofisica e Vulcanologia, Sez. Pisa,Via della Faggiola, 32,56126, Pisa, Italy

Bull Volcanol (2011) 73:531–542DOI 10.1007/s00445-010-0423-2

δ13C of volcanic CO2 between −0.9±0.27‰ and −1.41±

highlights a temporal trend of systematic increase of δ13C

with the large mass-dependent fractionations among itscarbon stable isotopes (12C and 13C; Craig 1953; Chackoet al. 2001), has stimulated a large number of isotopicdeterminations in fumaroles and steaming grounds (Lyonand Hulston 1984; Allard et al. 1988; Giggenbach 1997),groundwaters (Allard et al. 1997; Caliro et al. 1999, 2005;Chiodini et al. 2000), and in soils on the surroundings ofactive volcanoes (Chiodini et al. 2008). These measure-ments have supported the conclusion of a large time-and-space variability of volcanic CO2 isotopic signature, whichin turn is generally ascribed to the significant chemicalheterogeneity of the upper mantle sourcing Earth’s magmas(Deines and Gold 1973; Kyser 1986), and/or to the additionof crustal, organic and atmospheric fluids to magma-derived volatiles (Sano and Marty 1995).

Notwithstanding this extensive research, a major draw-back of our current understanding of C isotopic systematicsis that none of the available 13C/12C measurements hasbeen undertaken in volcanic gas plumes (the atmosphericdispersion of volcanic gases). This is surprising, as quiescentplume degassing from open-conduit volcanoes is universallyaccepted as one of the major pathways of CO2 earth degassing(Gerlach 1991; Brantley and Koepenick 1995). This lack ofinformation is due to from the inherent analytical complexityof resolving the volcanic CO2 isotopic signature from theoverwhelming atmospheric CO2 signal (Gagliardi et al.2002). Thus, whilst CO2 concentrations in volcanic plumesare increasingly investigated thanks to the advent of modernIR (infrared) spectroscopic methods (Allard et al. 1991a,1994; Gerlach et al. 1997, 1998, 2002; Burton et al. 2000;Goff et al. 2001; Wardell et al. 2001, 2004; Aiuppa et al.2006, 2007, 2008; Oppenheimer et al. 2006; Hager et al.2008), there are no isotopic constraints on the origin of suchgas emissions, which prevents us from more in depthcharacterisation of CO2 exchanges among the differentgeochemical reservoirs.

Here, we make use of recent technical advances in massspectrometry (Chiodini et al. 2008) to provide the first 13C/12C isotopic characterisation of CO2 in the atmosphericplumes of three Italian volcanoes: Solfatara (Campi Flegrei),Vulcano (Aeolian Islands) and Mount Etna (Fig. 1). Solfa-tara, a tuff cone of ~4 ka (Rosi and Sbrana 1987), is seat ofintense hydrothermal activity consisting on fumaroles, hotsoils and large areas of anomalous soil CO2 degassing(Chiodini et al. 2001). It is a part of Campi Flegrei, a nestedcaldera resulting from two large collapses related to theCampanian ignimbrite (37 ka) and the Neapolitan YellowTuff (12 ka) eruptions (Orsi et al. 1996). Vulcano is apolygenetic volcanic complex in the Aeolian Islands, whichmost recently active volcanic centre (the “La Fossa” cone) isdormant since its last explosive eruption in 1888–90. The“La Fossa” cone has been, since the late 1970 s, the site ofvigorous degassing activity from a fumarolic field located

inside the main crater, and along its northern rim (Chiodiniet al. 1993). This degassing unrest is still ongoing by thetime of writing. Etna is a 0.6 Ma old basaltic volcanocharacterized by persistent open-vent degassing activity fromits summit vents, and by recurrent effusive to mildlyexplosive eruptions from the summit and the volcano’sflanks (Branca and Del Carlo 2004). Etna is one of thelargest CO2 emitter volcanoes of the world (Allard et al.1991a; Aiuppa et al. 2006, 2008).

The Solfatara and Vulcano datasets are first used for insitu validation of the method, since gas plumes in bothareas are fed by fumaroles which are easily accessible fordirect sampling of the pure volcanic component. The samemethod is then applied at Etna, where the gas plume issustained by persistent degassing at the bottom of activevents, which are inaccessible for direct sampling. Hence,previous isotopic determinations have been limited tofumaroles (often of low-temperature and heavily contami-nated by air), which have provided controversial results(see Martelli et al. 2008 for a discussion). Starting from theacquired data, we reconstruct the air-volcanic gas mixingtrend, which we then use to derive the isotopic compositionof the pure volcanic component. These data, and theproposed technique, open the way for a better assessmentof the CO2 budget at active volcanoes.

Materials and methods

Dilute plume samples were taken at Solfatara (CampiFlegrei), Vulcano (Aeolian Islands), and Etna (Voragineand Bocca Nuova craters) in summer 2008 and spring2009. Pressurised samples (1.5–2 bar) were collected inglass vials of 25 cc volume equipped with two Teflonvalves. The plume was sampled with a plastic pipe fixed atthe top of a 5 m long fishing pole, passing the gas throughthe vials using a 50 cc syringe equipped with a tree wayvalve.

Gas plume samples were subsequently analyzed forchemical and isotopic composition at the Geochemistry

Fig. 1 Location of surveyedvolcanoes

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Laboratory of INGV-Osservatorio Vesuviano. CO2 concen-tration and carbon isotopic composition were determinedby coupling a gas-chromatograph (Agilent Technologies6890 N, GC) with a continuous flow mass spectrometer(Finnigan Delta plusXP, MS). The GC was equipped with acapillary column (HP-plotQ capillary, 30 m×0.53 mm×25 μm; oven at 40°C He as carrier gas flow 5 ml/min) thatpermits good separation between air components and CO2,a Thermal Conductivity Detector (TCD) and a post columnswitching device able to switch or split the column gas flowto the TCD and to the MS. During analysis, air componentswere completely switched to the GC detector (TCD),preventing the formation of undesirable species in the MSsource (NOx) that can lead to bias in the isotopicmeasurement. In addition, this switching device allowedinverting the column flow after the species of interest (airand CO2) had reached the detectors, keeping the systemclean. This analytical procedure allows simultaneous, singlegas injection determination of CO2 concentration and δ13Ccomposition. Precision of the system was determined byintroducing the reference standard (MAB δ13C=2.45‰ vs.V-PDB) using the same procedure as for the samples. Foreach batch of analyses, the reference standard was injectedin variable amounts (from 33% to 60%) with respect to thenumber of samples. The external reproducibility resultswas <0.1‰ for each set of analyses (Etna σ=0.099, n=15;Vulcano σ=0.095, n=15; Solfatara σ=0.096, n=7). Ana-lytical error on the CO2 concentration is±3%. Theanalytical results are reported in Table 1.

Computation of the carbon isotopic compositionof the volcanic CO2 in diluted atmospheric plumes

Our data sets at Solfatara, Vulcano and Etna consist ofnumerous samples of dilute mixtures of Volcanic fluids andair. In terms of CO2 concentrations and δ13C(CO2) compo-sition (hereafter simply δ13C), the mixing between the twocomponents is described by the equations:

XCO2;p ¼ fXCO2;v þ 1� fð ÞXCO2;a ð1Þ

d13CpXCO2;p ¼ f d13CvXCO2;v þ 1� fð Þd13CaXCO2;a ð2Þ

where the terms XCO2 refer to CO2 molar fractions, f is therelative contribution (from 0 to 1) from the volcanic source,and the subscripts p, v and a stand for plume, volcanic, andair, respectively. Because of steam condensation, either inthe plume or in the vials, CO2 concentrations and thefraction f refer to gases on a water-free basis. For thisreason, XCO2,v can be generally assumed ~1.

Table 1 CO2 concentration and carbon isotopic composition of theplume samples. Concentration is expressed in ppmv, carbon isotopiccomposition in δ notation per mil relative to PDB

Locality CO2 δ13C‰

Solfatara 469 −7.43Solfatara 499 −7.22Solfatara 1171 −4.27Solfatara 2863 −2.78Solfatara 1761 −3.53Solfatara 1016 −4.35Solfatara 700 −5.90Solfatara 2914 −2.77Solfatara 828 −5.78Solfatara 1064 −4.52Solfatara 1214 −4.54Solfatara 1306 −3.84Solfatara 2755 −2.91Solfatara 1730 −3.65Solfatara 1944 −3.11Solfatara 819 −5.55Solfatara 2490 −3.26Solfatara 1035 −4.68Solfatara 1737 −3.06Solfatara 4597 −2.19Solfatara 1028 −4.55Solfatara 2283 −2.85Solfatara 1115 −4.58Solfatara 1417 −3.78Solfatara 1147 −4.09Solfatara 1180 −4.11Solfatara 2954 −2.59Solfatara 940 −5.72Air Solfatara 380 −9.30Vulcano 416 −7.82Vulcano 403 −7.96Vulcano 690 −5.51Vulcano 411 −8.03Vulcano 744 −5.24Vulcano 366 −8.10Vulcano 893 −4.47Vulcano 392 −8.32Vulcano 540 −6.02Vulcano 1099 −3.96Vulcano 2465 −2.61Vulcano 591 −6.34Vulcano 578 −5.98Vulcano 390 −7.75Vulcano 370 −8.44Vulcano 1024 −4.37Vulcano 420 −7.51Vulcano 1417 −3.77

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In order to derive the carbon isotopic composition of thevolcanic CO2, as well as to evaluate the uncertainty, wehave adopted the Monte Carlo method (Press et al. 2001).The computations have been done repeating 1000 times thefollowing procedure:

1) values of CO2 concentration and δ13C were randomlyextracted from the experimental measurements consid-ering a Gaussian distribution of the analytical errors (i.e.σ=3% and σ=0.1‰ for CO2 concentration and δ13C,respectively). In total we obtain 29 randomly extractedvalues for the air and the plume samples (see theexample in Fig. 2a referring to the Solfatara data set);

2) assuming the volcanic CO2 molar fraction XCO2,v~1,the randomly extracted values δ13Cp,δ

13Ca, and XCO2,a

were then used to solve the system of equations 1 and 2with respect to XCO2,p varying the carbon isotopiccomposition of the volcanic-CO2 (δ13Cv) with areiteration procedure. It is thus selected the δ13Cv valuewhich minimize the quadratic differences between thecomputed XCO2,p and the CO2 concentrations of thesynthetic data set. In the example illustrated in Fig. 2a,the resulting mixing curve between air and thecomputed volcanic CO2 is reported as the ‘best fit’curve of the randomly extracted values.

The isotopic composition of the volcanic end member ofthe plumes is given by the mean and the standard deviationof the one thousand δ13Cv values computed by applying theabove described procedure (Fig. 2b)

Results

At Solfatara, 28 samples were collected on the 7th ofJuly, 2008. Samples were taken from the plume formed by

Table 1 (continued)

Locality CO2 δ13C‰

Vulcano 369 −8.33Vulcano 1767 −3.20Vulcano 1806 −3.32Vulcano 36877 −1.76Vulcano 1904 −3.21Vulcano 524 −6.50Vulcano 1824 −3.31Vulcano 5321 −2.43Vulcano 529 −6.43Vulcano 1098 −4.29Vulcano 2187 −3.09Vulcano 3262 −2.73Vulcano 371 −8.10Vulcano 2450 −2.42Vulcano 4021 −2.36Vulcano 2388 −2.92Vulcano 798 −5.12Vulcano 443 −7.40Vulcano 1328 −3.89Vulcano 611 −6.05Air Vulcano 364 −8.60Etna BN 656 −4.93Etna BN 539 −5.81Etna BN 509 −6.01Etna BN 683 −5.08Etna BN 721 −4.64Etna BN 536 −5.92Etna BN 411 −7.28Etna BN 411 −7.12Etna BN 571 −5.84Etna BN 620 −5.14Etna BN 442 −7.03Etna BN 654 −4.96Etna BN 816 −4.04Etna BN 679 −4.61Etna BN 420 −6.92Etna BN 439 −6.79Etna BN 454 −7.24Etna BN 486 −6.45Etna BN 448 −6.64Etna BN 777 −4.39Etna BN 636 −5.21Etna BN 706 −4.48Etna BN 684 −4.94Etna BN 571 −5.50Etna BN 568 −5.62Etna BN 468 −6.53Etna BN 553 −5.71Etna BN 384 −7.64

Table 1 (continued)

Locality CO2 δ13C‰

Etna BN 609 −5.11Etna BN 589 −5.42Etna BN 440 −6.82Etna BN 468 −6.66Etna BN 406 −7.52Etna BN 410 −6.92Etna Voragine 613 −5.65Etna Voragine 648 −5.14Etna Voragine 790 −4.50Etna Voragine 796 −4.52Etna Voragine 524 −6.23Etna Voragine 434 −6.72Air Etna 357 −8.30

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the atmospheric dispersion of Bocca Nuova vent. Thisvent is characterised by a discharge temperature of 150°C,and a composition dominated by H2O (~ 80% by volume)and CO2 (~ 20%). The δ13C of the pure fumarolic CO2

was −1.7‰±0.1‰, a value consistent with those mea-sured during the last two years of systematic sampling ofthe vent (from −1.2‰ to −1.9‰) (Chiodini et al. 2010).The 28 plume samples showed CO2 concentrations from469 ppmv to 4600 ppmv, and δ13C values from −7.4‰to −2.2‰. The air sample, which was collected far fromthe degassing area, was characterised by a CO2 concen-tration of 380 ppmv and a δ13C of −9.3‰. The relativelynegative carbon isotopic composition of the analyzed airsample, may reflect the intense human activity in the area.In a δ13C vs CO2 diagram (Fig. 3a), the data plot along acurve representing the mixing between atmospheric

composition and a CO2-rich component characterised byheavier carbon isotopic composition, i.e. the volcanic CO2

discharged by BN vent. The isotopic composition ofvolcanic CO2 results of −1.68‰ with a standard deviationσ=0.07‰ (Fig. 2b) in good agreement with the CO2

isotopic composition of fumarolic fluids sampled in thesame period of the plume measurements and analyzedwith standard procedure (BN, δ13C= −1.7±0.1‰).

At Vulcano, the dataset consists of 38 samples collectedon the 13th of June, 2009. Samples were collected at the LaFossa crater rim from the plume formed by the F0 fumarole(Chiodini et al. 1993). The 38 plume samples display CO2

Fig. 3 Carbon isotopic composition plotted against CO2 concentra-tion of the volcanic plume samples from Solfatara (a), Vulcano (b),Etna Bocca Nuova (c) and Etna Voragine (d). The theoretical mixingcurves between air and the estimated composition of the volcanic CO2

are shown by the solid lines, while the dashed lines refer to±2σ of theestimated volcanic composition (see text)

Fig. 2 Computation of the carbon isotopic composition of thevolcanic CO2 (example from Solfatara data). a A set of CO2

concentrations and carbon isotopic values is randomly extracted(black dots, REV) starting from the measured values and their errors(grey points and errors bars). The REVs are used to derive the carbonisotopic composition of the volcanic CO2 (δ13C= −1.60‰) and thebest fit mixing curve. b Histogram of the one thousand δ13C valuescomputed by repeating 1000 times the procedure illustrated in Fig. 3a

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concentrations ranging from 366 to 36900 ppmv, and δ13Cvalues from −8.4‰ to −1.76‰. The air sample, which wecollected at La Fossa crater far from the degassing area, wascharacterised by a CO2 concentration of 364 ppmv, andδ13C of −8.6‰. Similarly to the Solfatara case, the plumesamples describe a mixing curve between air and volcanicCO2 (Fig. 3b). The isotopic composition of volcanic CO2,which was estimated applying the Monte Carlo proceduredescribed above, indicates an averaged δ13C of −1.70‰(σ=0.09‰), again in good agreement with the CO2

isotopic composition of fumarolic fluids. The CO2 of thefumarole, which was sampled during the survey andanalyzed with a standard procedure, was characterised byan isotopic signature of δ13C of −1.5±0.1‰.

At Etna, sampling was performed on the 11th ofOctober 2008 on two distinct plumes: (i) the plume ofVoragine crater where, because of a sudden weathervariation, it was possible to collect only 6 samples(Fig. 3d); (ii) the plume of Bocca Nuova crater, wherewe collected 34 samples (Fig. 3c). CO2 concentrations inthe two plumes ranged from 384 to 816 ppmv, while δ13Cvalues varied from −7.6‰ to −4.0‰. The air sample,which was collected far from the degassing areas, showeda CO2 concentration of 357 ppmv, and a δ13C of −8.3‰.Similar to the previous cases, the plume samples describea mixing curve between air and a component characterizedby heavier carbon isotopic composition and higher CO2

concentration (i.e. the volcanic CO2, Fig. 3c, d). In orderto estimate the carbon isotopic composition of volcanicCO2, the procedure previously described was applied,which is based on the generation of synthetic data and thereiterative use of equations 1 and 2. The term XCO2,v ofequations 1 and 2 (i.e. the CO2 concentration in theundiluted volcanic gas emanations, expressed on a waterfree basis) was assumed equal to 1. This assumption whichat Solfatara and Vulcano is supported by the chemicalanalysis of the fumaroles, is considered reasonable also atEtna where the gas forming the plumes is vented from thebottom of the open craters, which are not accessible fordirect sampling. Since CO2 is nearly always the mainincondensable gas species in volcanic fluids, this assump-tion, which is required for each volcano where the ventsare not accessible for direct sampling, does not affect theresults to any significant extent. For the Bocca Nuova andVoragine crater datasets, we then derive δ13C of thevolcanic CO2 of −0.9±0.27‰ and −1.41±0.27‰, respec-tively. No high-temperature gas vent was accessible todirect sampling during our 2008 survey on Etna, whichcould offer an independent validation to our results.However, a group of fumaroles on the outer Voraginecrater’s rim has recently been sampled and analysed byMartelli et al. (2008) (37 samples collected in the period2007–2008). These low temperature fumaroles (<100°C)

have CO2-rich (and air-free) compositions and are charac-terised by δ13C(CO2) ranging from −1.3‰ to −2.2‰.Notably they have an average δ13C value of −1.63±0.22‰, in good agreement with our Etna’s magmatic gasestimate based on Voragine crater plume samples (−1.41±0.27‰).

Uncertainty of the method

The statistical approach used to estimate the CO2 isotopiccomposition of the volcanic sources feeding the plumesindicate standard deviations (σ) of δ13C estimationranging from 0.07‰ at Solfatara to 0.27‰ at Etna. Weshow here that, at constant analytical errors, the mainfactor controlling the uncertainty of the estimation is thefraction of volcanic CO2 in the plumes, the errorincreasing in dilute plumes. Figure 4 contrasts the σ ofthe estimated δ13C values against the maximum value ofthe CO2, excess (CO2, excess=CO2, plume–CO2, air) in variousdatasets generated from the original data by step-to-stepexclusion of the highest CO2 concentrations (Table 2).Data reported in Figure 4 are well fitted by the followingsimple equation:

s ¼ 0:07þ 100 CO2;excess

� ð3Þ

Interestingly, equation 3, which has been derived fromthe experimental data, well describes the uncertaintydistribution also of a synthetic data set generated byequations 1 and 2, i.e., by considering a theoretical plumewhere air CO2 mixes with volcanic CO2. Practically, theempirical relation 3, which is dependent manly on theanalytical errors, and in minor extent on the differencebetween the isotopic composition of air and volcanic CO2,

Fig. 4 Dependence of the uncertainties of carbon isotopic estimation(σ13C estimation) on CO2 excess of the plume (see text)

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can be used to obtain a rough estimate of the uncertaintyexpected for plumes from other volcanoes of the worldwhere the CO2 concentrations are known.

Other possible causes of errors are the possibility ofmicrobial activity after sampling, which could modify theδ13C signal, and the presence near the plume of other CO2

sources with different isotopic compositions, as for examplefactories or cars, which can produce pulses of light CO2

(δ13C< −20‰). These processes can cause deviations fromthe mixing line producing unquantifiable errors in theestimations. In general the good agreement of measureddata with the theoretical mixing curves (Fig. 3) suggeststhat such problems, if present, were of limited extension.Only the dataset of Solfatara (see Fig. 3a), which is locatedwithin the densely populated area Campi Flegrei, ischaracterised by a few points which deviate from themixing line towards lighter CO2 isotopic compositionspossibly because either the presence near the plume of otherCO2 sources or the occurrence of microbial activity afterthe sampling. What ever is the cause, the effect is anunderestimation of the isotopic composition of the purevolcanic CO2 (see for example the systematic decrease inthe δ13C estimation in the data sets with low CO2, excess

values, Table 2).

Discussion

Scope and potential applicability of the proposed technique

The rates of CO2 release from subaerial volcanism haveremained until now poorly constrained, given that CO2

fluxes have only been estimated at a limited number ofvolcanoes (Brantley and Koepenick 1995). Indeed, therelatively weak CO2 emissions from quiescent volcanoes,where gas release occurs from easily accessible fumaroles,are relatively simple to derive, whilst the open-vent CO2

emissions from continuously degassing volcanoes, whichcontribute the most significant fraction of the globalvolcanic CO2 budget, are more difficult to characterize.However, the recent advent of more sophisticated (FTIR,MultiGAS) techniques is now enabling the measurementsof CO2 fluxes at an increasing number of open-ventvolcanoes (Table 3).

On the other hand, the measurement of the carbonisotope composition of CO2 in a volcanic gas plume hasnever been successfully attempted until now, so that carbonisotope systematics are unavailable for several open-ventvolcanoes, or are based on measurements in fumaroles ordegassing soils (Table 3). Even in these latter cases, issues

Table 2 Dependence of uncertainties on CO2 excess of the plume

Plume Maximum CO2 (ppmv) CO2 excessa (ppmv) Number of observations 13C estimationb δ ‰ Stand. Dev. δ ‰

Vulcano 36900 36536 39 −1.70 0.09

Vulcano 1904 1540 31 −1.94 0.09

Vulcano 1099 735 25 −1.89 0.17

Vulcano 893 529 22 −1.84 0.25

Vulcano 591 227 17 −1.68 0.52

Vulcano 540 176 15 −1.27 0.68

Vulcano 443 79 12 −1.18 1.16

Etna BN 817 460 35 −0.94 0.30

Etna BN 684 327 31 −1.03 0.35

Etna BN 589 232 23 −1.07 0.52

Etna BN 539 182 18 −1.01 0.69

Etna BN 486 129 15 −1.01 0.94

Etna BN 448 91 11 −0.33 1.24

Solfatara 4600 4220 29 −1.68 0.07

Solfatara 1944 1564 22 −1.72 0.10

Solfatara 1180 800 15 −1.87 0.15

Solfatara 828 448 6 −2.34 0.35

Etna V 796 439 7 −1.44 0.27

Etna V 648 291 5 −1.50 0.42

Etna V 524 167 3 −1.37 0.78

a CO2;excess ¼ CO2;plume � CO2;airb estimated δ13 C values on the maximum value of the CO2, excess with Monte Carlo procedure (see text)

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remain on whether or not these manifestations, whichintrinsically have lower fluxes than the main plume, havecarbon isotopic compositions which are representative ofthe magmatic isotopic signature, particularly in the case oflow-temperature (e.g. Erebus; Allard 1983) and/or air-contaminated gas sites (e.g. Masaya; Lewicki et al. 2003).In such circumstances, several processes may alter theoriginal magmatic gas phase composition, such as isotopicfractionations upon subsurface steam condensation, ormixing with atmospheric and/or organic CO2.

Here we have described analytical details, perfor-mance, and uncertainty evaluation of a simple techniquefor δ13C(CO2) measurement in a volcanic plume. Datacollected at Solfatara and Vulcano Island, where gasplumes are fed by fumaroles which are easily accessiblefor direct sampling of the pure volcanic component, werefirst used to validate the proposed technique. In bothvolcanic systems, the plume-derived carbon isotopiccompositions of volcanic CO2 (δ13C values of −1.68±0.07‰ and of −1.70±0.09‰ for Solfatara and Vulcano,respectively) are in good agreement with the directlysampled composition of fumarolic gases, thus providingconfidence for the developed method and allowing itsapplication in situations where the pure volcanic compo-nent is inaccessible to sampling. As a notable example, wehave presented the results of δ13C(CO2) measurements inMount Etna’s plume (Fig. 3c and d), which represent thefirst such determinations in an open-vent basaltic volcano.From our measurements, we derive δ13C compositions ofvolcanic CO2 at Etna between −0.9±0.27‰ (Bocca Nuovacrater) and −1.41±0.27‰ (Voragine crater). It is worth-while nothing that the δ13C value measured for the plume

of Voragine crater is in the range of the values measuredon a group of low temperature fumaroles (<100°C) locatedon the outer crater’s rim and which have been recentlysampled and analysed by Martelli et al. (2008). Also, thesmall but appreciable (~ 0.5 delta units) isotopic differencebetween Bocca Nuova and Voragine is reminiscent of thesystematic chemical diversity between the two vents(Aiuppa et al. 2006), which are remarkably only a fewhundred meters apart and separated by a relatively thinedge. During our observations, Voragine was emitting aCO2-depleted gas relative to Bocca Nuova (plume CO2/SO2 ratios averaged 6 and 10 at the two vents, respective-ly), likely suggesting that a more volatile-depleted magmawhich was likely isotopically modified by gas-meltfractionations was feeding gas emissions at the formervent.

We therefore propose that our newly developed methodoffers a potentially valuable tool for estimating the isotopiccompositions of volcanic CO2 at open vent volcanoes. Weemphasise that uncertainties in the estimated isotopiccomposition of the pure volcanic CO2 will mostly dependon plume CO2 “excess” (e.g. on the CO2 concentrationcontrast between the plume and the background atmospher-ic CO2; see equation 3), and thus ultimately on plumedensity, which in turn depends on source emissivity,distance of the sampling site from the source, andmeteorological conditions controlling the plume’s atmo-spheric transport. In particular, we calculate from equation 3that, for near-vent volcanic plumes where the CO2 excess ishigher than 100–150 ppmv, the expected uncertainties onδ13C are below 0.5‰. More specifically, we use the rangeof recently published in-plume CO2 concentrations in a

Table 3 CO2 fluxes, CO2 excess of the plume and expected uncertainty (1 ) on the estimation of the carbon isotopic composition of the plume ofvarious volcanoes

Volcano CO2 flux(kg/s)

Source CO2 excess(ppmv)

Source Expected error

Ambrym 230 Allard et al. (2009) 75 Allard et al. (2009) 1.3

Erebus 15.4–22 Wardell et al. (2004); ~800a Oppenheimer and Kyle (2008) ~0.1Oppenheimer and Kyle (2008)

Erta-Ale 0.49–0.94 Sawyer et al. (2008a) ~120a Sawyer et al. (2008a) ~0.8

Etna 10–780 Allard et al. (1991a); 459 THIS WORK 0.3Aiuppa et al. (2006, 2008)

Kilauea 18.5–98.3 Gerlach et al. (2002) 250 Hager et al. (2008) 0.4

Masaya 32–36 Burton et al. (2000) 140 Witt et al. (2008) 0.7

Miyakejima 81–243 Shinohara et al. (2003) 40 INGV-Pa (2010) unpublished data 2.5

Nyiragongo 78–138 Sawyer et al. (2008b) 420 Aiuppa (2010) unpublished data 0.2

Stromboli 16.2 Aiuppa et al. (2009a) 680 Aiuppa et al. (2009b) 0.1

Villarica 2.4 Shinohara and Witter (2005) 100 Shinohara and Witter (2005) 1.0

Yasur 11.5 Métrich et al. (2010) unpublished data 30 Allard et al. (2008) 3.3

a background-corrected CO2 mixing ratios calculated from FTIR-sensed column amounts (in molecules cm-2 )

538 Bull Volcanol (2011) 73:531–542

(δ‰)

σ

significant subset of open-vent volcanoes (Table 3) tocalculate the range of expected errors on δ13C, and wedemonstrate that the proposed method would be suitable(errors <0.5‰) for application at divergent-plate (Erta Ale)or within plate (e.g. Erebus, Kilauea, Nyiragongo) volca-noes, which are typically associated with a CO2-richvolcanic gas plume. Application of our method to arcvolcanoes appears realistic at sites where CO2-rich gasesare erupted (e.g. Stromboli), while it could be more difficultfor the typically low CO2 contents in magmas (and thusultimately in plumes) of north (e.g. Miyakejima) and south(e.g. Ambrym, Yasur) Pacific arc volcanoes where theexpected errors exceed 1‰.

Contribution to C isotopic systematic in Italy, with a specialemphasis to Etna

Our observations here add new information on the isotopiccomposition of magmatic carbon in Italy, and thus offernew insights on its origin (a still controversial matter). Wenotice first that our derived magmatic CO2 compositions atthe three volcanoes fall in rather narrow range (−0.9to −1.7‰). This points to a rather homogeneous magmaticCO2 signature in Italy, suggesting that a regional-scalemechanism (e.g., a source process), rather than a localprocess, is most likely responsible for its departure from theaccepted MORB isotopic carbon range (−5‰ to −8‰;Deines and Gold 1973). In addition, while the isotopicsignatures of magmatic carbon at Campi Flegrei andVulcano have long been understood, and our determina-tions only offer further validation to earlier studies (Allardet al. 1991b; Capasso et al. 1997; Caliro et al. 2007), ourmeasurements open new views to the still-debated questionof the true magmatic carbon isotopic signature of CO2

released by Mount Etna. Past isotopic determinations onthis volcano have been limited to fumaroles (often of low-temperature and heavily air-polluted), providing somewhatcontroversial results, with the measured δ13C(CO2) valuesspanning more than 4 delta ‰ units (see Martelli et al.(2008) for a discussion). We observe, however, that whenour plume measurements are analysed in concert with datareported in previous work, an apparent temporal trendshowing a systematic increase of δ13C(CO2) values emerges(Fig. 5). This trend is particularly well determined by theunusually light carbon isotopic signature of gas samplescollected in the 1970’s and the 1980’s (δ13C(CO2)< −3.4‰),relative to post-1990 measurements (δ13C(CO2)> −2.8‰).We note that the 1975–76 samples reported in Allard(1983) refer to the typically CO2-poor (as shown by the lowCO2/SO2 ratio) gases released by lateral vents during flankeruptions. These “Type II” volcanic gases (sensu Gerlachand Graeber 1985) are derived from volatile-poor magmashaving previously experienced extensive CO2 loss through

the summit conduit systems and the central craters, aprocess which may have potentially altered the originalcarbon isotopic signature via gas-melt 13C/12C fraction-ation. This interpretation, however, does not hold for the1979–1986 samples of Allard et al. (1991a), which weretaken on the volcano’s summit from high-T (800–1090°C)gas vents on the North East crater. Hence the issue remainsopen on which process controls the temporal changes ofδ13C(CO2) Etna’s signature (Fig. 5).

If real, such a shift toward more positive δ13C(CO2) gascompositions may be the result of deep processes takingplace through the plumbing system, down to the deepmagmatic source of Etna. It is worthwhile noting that theshift toward more positive δ13C(CO2) values (Fig. 5)matches a concurrent change in magma compositionstarting with the 1974 flank eruption (Corsaro et al.2009). As eruption frequency and energy have graduallyintensified at Etna over the past three decades (Allard et al.2006), a new volatile-rich basaltic magma component,which is more alkali-rich and with higher 87Sr/86Sr thanany other historical lava, has come to play an increasinglyimportant role in the plumbing system (see Métrich et al.(2004) for a thorough discussion and an updated list ofreferences). This new basaltic component is thought tohave progressively replaced the trachybasaltic magmaformerly residing in the volcano’s upper plumbing system(Métrich et al. 2004), and was first erupted “undiluted”during the 2001 eccentric eruption. This is the momentwhen Etna’s volcanic gas δ13C(CO2) values became

Fig. 5 Different measurements over time of the 13C isotopiccomposition of CO2 emitted by Etna. The numbers refer to the sourceof the data: 1—Allard (1983), 2—Allard et al. (1991a), 3—

1997), 4—Giammanco et al. (1998), 5—Aiuppaet al. (2004), 6—Aiuppa, unpublished data, 7—Pecoraino andGiammanco (2005), 8—average value of 37 data from Martelli et al.(2008), 9—this work

Bull Volcanol (2011) 73:531–542 539

D'Alessandro et al. (

systematically higher than −2‰ (Fig. 5). While theprocesses leading to these variations are still debated(Métrich et al. 2004), and alternative mechanisms such aseither an increasing crustal fluid contribution (Clocchiattiet al. 1988) or a change in magma source features(Schiano et al. 2001; Tonarini et al. 2001) have beenproposed, our carbon isotopic measurements offer anadditional line of evidence to solve this intricate puzzle.In this context, the heavier carbon isotopic signature ofpost-1990 samples relative to more MORB-like pre-1990samples may be ascribed to either (a) increasing carboncontributions from magma assimilation of carbonate rocksof the sedimentary basement, similar to what has beenrecently proposed for Vesuvius (Iacono-Marziano et al.2009). This process might indeed have been favoured bythe new magma component being more volatile-rich andhotter, and thus ultimately more prone to rock assimilationfrom the crust; or (b) to mantle metasomatism by slab-derived carbon-rich fluids (Frezzotti et al. 2009). It cannotbe excluded that processes (a) and (b) worked simulta-neously and in concert to alter the isotopic composition ofcarbon over the last few decades. As a final remark, weacknowledge that the interpretation of the time dependenttrend toward more positive δ13C(CO2) gas compositionsmay be complicated further by additional secondaryprocesses, such as variation in the conditions governingCO2 vapour-melt separation; these may ultimately lead tocarbon isotopic fractionations upon CO2 degassing frommelt. Mattey (1991) has shown that, in basalts at 1200°C,equilibrium gas-melt carbon isotopic exchange occurs,with a fractionation Δvapour-melt ~ +2‰. This is themaximum δ13C(CO2) shift expected for closed systemdegassing, although much higher differences can beproduced by open system degassing processes. Therefore,we cannot rule out the possibility that the δ13C magmaticsource has remained virtually unchanged since the 1970’sto the present. Hence the range of measured δ13C(CO2) gascompositions (Fig. 5) could be simply a reflection of thevariation in the mechanisms and depths of CO2 separationfrom melt, producing a variety of gas-melt carbon isotopicfractionations.

Whatever is occurring in terms of the two above end-member processes (carbonate assimilation/breakdown vs.carbon isotopic fractionation upon degassing), the evolvingfluid source of Etna’s magmatism, as suggested by Figure 5,may represent a key in understanding the changing activitytoward more CO2-rich eruptions (Spilliaert et al. 2006;Aiuppa et al. 2007, 2008). Finally, our observationshighlight the potential of systematic plume CO2 isotopicmeasurements for a better understanding of volcanicprocesses, and for implementing volcano monitoring pro-grams. A thorough appraisal of such a systematic record,accounting for fractionation processes between CO2, CO3

2--

bearing minerals and melts, could advance our knowledgeof how sedimentary carbon carried down to mantle depthsvia subduction (e.g. Kerrick and Connolly 1998, 2001) isrecycled upward through volcano plumbing systems.

Conclusions

Measuring the carbon isotopic composition of CO2 involcanic gas plumes has long been hampered by technicallimitations; yet, this type of measurement would providekey information on the origin of carbon, and potentiallycontribute to volcano monitoring. Here we have describeda simple low-cost technique for the collection of volcanicgas plume samples, and for their laboratory analysis to themeasurement of δ13C(CO2). We have demonstrated, using aMonte Carlo procedure, that the uncertainties associatedwith our novel technique are small enough to permit itsuse in a variety of volcanic systems in different tectonicand structural settings. We have also tested the perfor-mance of the technique at Solfatara and Vulcano Island,where our in-plume observations very closely reproducedthe directly sampled composition of fumarolic gases (δ13Cvalues of ~ −1.7‰ at both volcanoes). Finally, we havepresented the first ever-obtained δ13C(CO2) measurementsfor Mount Etna’s plume (δ13C between −0.9 and −1.4‰),and provided new evidence into the still-debated questionof the origin of carbon in this highly active volcano.

Acknowledgements We thank two anonymous reviewers for theirthoughtful reviews and valuable comments to the manuscript. G.Giudice, M. Liuzzo and N. Bobrowski (INGV-Pa) are acknowledgedfor support in the field.

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