INTRODUCTION
One of the best approaches for monitoring active volcanoes entails measuring the variations of volcanic gas emissions. Gas release is indeed a typical manifestation of the underground activity of volcanic systems and often the sole one during non-eruptive periods (Chiodini et al. Reference Chiodini, Cioni, Guidi, Raco and Marini1998). Degassing on active volcanoes occurs in the form of volcanic plumes, fumaroles, bubbles in thermal waters, and diffuse emanations from the soils (Allard et al. Reference Allard, Maiorani, Tedesco, Cortecci and Turi1991b; Baubron et al. Reference Baubron, Allard, Sabroux, Tedesco and Toutain1991; Farrar et al. Reference Farrar, Sorey, Evans, Howle, Kerr, Kennedy, King and Southon1995; Hernandez et al. Reference Hernandez, Perez, Salazar, Nakai, Notsu and Wakita1998; Delmelle and Stix Reference Delmelle and Stix1999; McGee et al. Reference McGee, Gerlach, Kessler and Doukas2000; Bergfeld et al. Reference Bergfeld, Goff and Janik2001; Werner and Cardellini Reference Werner and Cardellini2006). Diffuse soil degassing, in particular, occurs inside volcanic craters and on the external slopes of volcanic piles, during both quiescent periods and eruptions, and varies significantly in relation to the level of volcanic activity. Being also more accessible than hot crater emissions, it thus provides useful opportunities to survey the evolution of a volcanic system (Badalementi et al. Reference Badalamenti, Gurrieri, Hauser, Parello and Valenza1991; Baubron et al. Reference Baubron, Allard, Sabroux, Tedesco and Toutain1991; Toutain et al. Reference Toutain, Bachelery, Blum, Cheminee, Delorme, Fontaine, Kowalski and Taochy1992). Diffuse soil emanations on volcanoes are usually composed of almost pure carbon dioxide, plus trace amounts of H2, CH4, and rare gases (Allard et al. Reference Allard, Maiorani, Tedesco, Cortecci and Turi1991b; Baubron et al. Reference Baubron, Allard, Sabroux, Tedesco and Toutain1991; Chiodini et al. Reference Chiodini, Caliro, De Martino, Avino and Gherardi2012). Carbon dioxide (CO2) is the second-most abundant component of magmatic gases after water and also the first volatile species to form gas bubbles in magmas at depth owing to its low solubility. In low-temperature soil gas emanations, water vapor is previously depleted by condensation, leaving CO2 as the predominant compound. In such emissions CO2 concentrations approach 100%, in contrast to only 0.04% in standard atmosphere. Therefore, diffuse CO2 emanations are able to generate strong local CO2 anomalies in soils and in air, which can even become lethal when CO2 concentrations reach 12–15%.
Two different techniques, based on infrared gas sensing, are commonly used to measure CO2 emissions from volcanic soils: the accumulation chamber method (ACM) (Parkinson Reference Parkinson1981; Norman et al. Reference Norman, Garcia and Verma1992; Chiodini et al. Reference Chiodini, Cioni, Guidi, Raco and Marini1998; Allard et al. Reference Allard, Aiuppa, Beauducel, Gaudin, Di Napoli, Calabrese, Parello, Crispi, Hammouya and Tamburello2014) and the dynamic concentration method (DCM) (Gurrieri and Valenza Reference Gurrieri and Valenza1988; Camarda et al. Reference Camarda, Gurrieri and Valenza2006). These two techniques can be operated for either discrete measurements or permanent gas survey. However, both imply punctual measurement at a given site and require measuring, numerous sites in order to access the overall CO2 degassing pattern and flux. Alternative approaches to determine CO2 fluxes from soil degassing and fumaroles on volcanoes include the use of a CO2 Lidar (Pedone et al. Reference Pedone, Aiuppa, Giudice, Grassa, Cardellini, Chiodini and Valenza2015) or local eddy covariance, a key atmospheric measurement technique to calculate vertical turbulent fluxes within atmospheric boundary layers (Werner et al. Reference Werner, Chiodini, Granieri, Caliro, Avino and Russo2003; Lewicki et al. Reference Lewicki and Hilley2009, Reference Lewicki, Hilley, Dobeck and Marino2012).
Previous studies have demonstrated that volcanic CO2 emissions can also be indirectly assessed from their record in either living plants or the rings of trees growing on active volcanoes (Bruns et al. Reference Bruns, Levin, Münnich, Hubberten and Fillipakis1980; Allard et al. Reference Allard, Pasquier-Cardin, Fontugne, Hatté and Baubron1997; McGee et al. Reference McGee and Gerlach1998; Pasquier-Cardin et al. Reference Pasquier-Cardin, Allard, Ferreira, Hatté, Coutinho, Fontugne and Jaudon1999; Cook et al. Reference Cook, Hainsworth, Sorey, Evans and Southon2001; Mostacci et al. Reference Mostacci, Chiodini, Berti and Tinazzi2009; Evans et al. Reference Evans, Bergfeld, McGeehan, King and Heasler2010; Lewicki et al. Reference Lewicki, Hilley, Shelly, King, Mc Geehin, Mangan and Evans2014). Magma-derived CO2 emitted by volcanoes is devoid of 14C and thus dilutes the 14C fixed from normal atmosphere during photosynthesis.
We propose an alternative approach of the CO2 flux measurement based on the depletion of 14C concentration in the plants growing in active volcanic zones, due to their assimilation of a part of mineral CO2, 14C free, from the soil emanation (Allard et al. Reference Allard, Pasquier-Cardin, Fontugne, Hatté and Baubron1997; Pasquier-Cardin et al. Reference Pasquier-Cardin, Allard, Ferreira, Hatté, Coutinho, Fontugne and Jaudon1999). Our study will check the possible estimate of the intensity of local volcanic CO2 emission by measuring the degree of 14C depletion in living plants with respect to the contemporaneous standard atmospheric value: 14C analysis of selected plants in active volcanic areas could provide an indirect opportunity to quantify local CO2 fluxes averaged over periods of some weeks.
We present the results of a series of radiocarbon measurements on plants growing within and nearby the Solfatara crater (Figure 1), the most active vent of the Campi Flegrei caldera in southern Italy, which is the site of intense fumarolic and diffuse soil degassing.
Figure 1 Location map of Campi Flegrei in the Neapolitan area and map of Campi Flegrei showing the limit of the two calderas. The most recent volcanoes, post Neapolitan Yellow Tuff, are all situated along the rim of the youngest one, around the Gulf of Pozzuoli.
Volcanological Setting
Located north of the Gulf of Naples and extending partly into the town of Naples (Figure 1), Campi Flegrei is a large volcanic complex that developed over the last 50 ka (Cassignol and Gillot Reference Cassignol and Gillot1982; Scandone et al. Reference Scandone, Belluci, Lirer and Rolandi1991). Its evolution was marked by two huge ignimbritic eruptions, which generated two successive caldera collapses: the Campanian Ignimbrite and Neapolitan Yellow Tuff, dated to around 39 ka and 14 ka, respectively (Gillot and Cornette Reference Gillot and Cornette1986; De Vivo et al. Reference De Vivo, Rolandi, Gans, Calvert, Bohrson, Spera and Belkin2001; Blockley et al. Reference Blockley, Ramsey and Pyle2008).
Subsequent activity has been concentrated within and along the rim of the second caldera. The most important eruptive phase occured during the Bronze Age, between 4400 and 3600 BP (D’Antonio et al. Reference D’Antonio, Civetta, Orsi, Pappalardo, Piochi, Carandente, De Vita, Di Vito and Isaia1999). Most of the events were explosive (Agnano-Monte Spina, Solfatara, Astroni, Senga) and their pyroclastic products covered the Neapolitan area. The most recent eruption in Campi Flegrei happened, in 1538 AD and built Monte Nuovo, a volcanic cone located NW of the central town of Pozzuoli. The volcanic complex remains very active. Throughout historic times it has been affected by a “bradyseismic” activity including important ground deformations, up to several meters, and seismic crises. The phenomenon is well known since the Roman epoch. A strong bradyseismic crisis preceded the 1538 AD eruption of Monte Nuovo, followed by subsidence (Dvorak and Gasparini Reference Dvorak and Gasparini1991). Then, two new crises affected the central part of the caldera (Pozzuoli area), one between 1968 and 1972 (0.7 m of ground uplift) and a second one between 1982 and 1985 (1.8 m of uplift on average).
These uplifts were attributed to either a pressure increase in the magma chamber (Bonafede et al. Reference Bonafede, Dragoni and Quareni1986), an increasing input of magmatic gas into the shallower hydrothermal system (De Natale et al. Reference De Natale, Pingue, Allard and Zollo1991; Chiodini et al. Reference Chiodini, Todesco, Caliro, Del Gaudio, Macedonio and Russo2003; Bonafede and Ferrari Reference Bonafede and Ferrari2009) or a magma intrusion (D’Auria et al.Reference D’Auria, Susi, Castaldo, Giudicepietro, Macedonio, Ricciolino, Tizzani, Casu, Lanardi, Manzo, Martini, Sansosti and Zinno2015). Campi Flegrei was affected by extensive CO2 degassing, thoughout its history, as revealed by a recent isotopic study of the calcite present in the volcanic products of different age (Chiodini et al. Reference Chiodini, Pappalardo, Aiuppa and Caliro2015a). The recent bradyseismic crises were actually accompanied or even preceded by an increased degassing activity and by a greater efflux of magma-derived CO2 in the fumarolic emissions (Chiodini et al. Reference Chiodini, Caliro, De Martino, Avino and Gherardi2012). CO2 degassing to the open air mainly occurs inside and around the central Solfatara volcano. After Mount Etna (Allard et al. Reference Allard, Maiorani, Tedesco, Cortecci and Turi1991a), Solfatara volcano supports the second largest emission of volcanic CO2 in Europe: 1500 t/day of CO2 (Chiodini et al. Reference Chiodini, Frondini, Cardellini, Granieri, Marini and Ventura2001; Granieri et al. Reference Granieri, Avino and Chiodini2010), together with 3300 t/day for H2O. The 13C content of the CO2 demonstrates its magmatic derivation (Caliro et al. Reference Caliro, Chiodini, Moretti, Avino, Granieri, Russo and Fiebig2007). The rising magmatic gas encounters hydrothermal meteoric aquifers at about 2 km depth, producing hydrothermal steam at 360ºC under 200–250 bar pressure (Gottsmann et al. Reference Gottsmann, Rymer and Berrino2006).
Geochemical studies indicate that the CO2 content and the temperature of Solfatara fumaroles have gradually increased since 2006, in coincidence with increasing ground deformation and seismicity, probably as a consequence of a greater influx magmatic gas at depth (Chiodini et al. Reference Chiodini, Avino, Caliro and Minopoli2011, Reference Chiodini, Caliro, De Martino, Avino and Gherardi2012).
The fumarolic activity at Solfatara volcano also extends outside the crater, especially in the eastern external zone of Pisciarelli (points 12 and 13 in Figure 2), where new and strongly degassing vents recently formed (December 2009). The increased fumarolic activity, together with important compositional variations of the discharged fluids, were recently interpreted as evidence of ongoing volcanic unrest (Chiodini et al. Reference Chiodini, Vandemeulebrouck, Caliro, D’Auria, De Martino, Mangiacapra and Petrillo2015b).
Figure 2 (a) The location of the sampling site; (b) the oblique view reveals the relief.
This intense activity raises an important challenge for civil defence because the Campi Flegrei area hosts close to a half million people in addition to the 2 million people living in the nearby Naples urban district up to Mount Vesuvius to the east. This makes the Naples agglomeration one of the most dangerous volcanic zones in the world (Barberi et al. Reference Barberi, Corrado, Inocenti and Luongo1984). Continuous volcano monitoring and assessing CO2 emissions, both in space and time, are therefore essential.
METHODS
The approach developed here, based on the 14C deficit in grass, presents a double interest: first, it allows us to estimate the volcanic CO2 emission over a period of 2 to 6 months, depending on the growth rate of the plants; and secondly it makes it possible to determine the effect of very low volcanic CO2 concentrations added to the local atmosphere on apparent 14C plant age. Grass growing at low soil level is indeed more sensitive to CO2 soil degassing than higher trees and their leaves. Most of the grass sample selected for this study are poaceae, also commonly called graminae. It is a typical grass with leaves growing from the base, has a quite rapid growth, and does not exceed 50 cm in height. In order to get fresh samples, the grass was first mowed short (in autumn 2012). Two successive samplings were conducted, in March 2013 (plant growth between October and March), and in May 2013 (plant growth between March and May). We collected grass samples both inside and outside Solfatara crater, at 17 sites, distributed over a 1.5 km2 area (Figure 2). Sampling sites were chosen as a function of the variability of the soil CO2 flux already recognized by ACM measurements (Chiodini et al. Reference Chiodini, Frondini, Cardellini, Granieri, Marini and Ventura2001), taking account of their location with respect to the main fulmarolic vents and the topography of the area. Some sites were located close to the most active fumaroles inside the crater, where the vegetation is reduced due to the temperature and acidity of the soil (points 3 and 4), while others are near the very active new fumaroles of Pisciarelli (base of the eastern external slope), and on the rim of the Agnano-Monte Spina plain (12 and 13). Most of the other sites are remote from fumarolic zones and are located, either in suburban area (camping sites, gardens, or fields), or on the crater rim and out of the volcano.
Together with plant sampling, the soil CO2 flux was measured in situ with the ACM methodology at each site (Chiodini et al. Reference Chiodini, Cioni, Guidi, Raco and Marini1998). The ACM comprises an accumulation chamber covering 0.0314 m2 of the ground, and traps the soil gas emanation. It is connected to an infrared spectro-photometer which measures CO2 concentrations, in the range 0–20,000 ppm. The gas accumulating in the chamber is pumped through the spectrometer and is re-injected into the chamber to avoid chamber depressurisation. The slope of the temporal increase function provides the CO2 flux in g.m–2.day–1 (Chiodini et al. Reference Chiodini, Cioni, Guidi, Raco and Marini1998).
Our plant samples were analyzed in the Centre de Datation par le Radio-Carbone in Lyon (France). Before being processed for radiocarbon analysis, the samples were dried out for about one month. The 14C analysis was made on 30 g of dried plant material, using the classical chemical pretreatment (acid-base-acid); the CO2 was extracted from about 12 g of the sample, giving about 4 g of synthesized benzene. The radiocarbon activity is then measured by liquid scintillation counting (TRICARB – Packard, Perkin-Elmer), calibrated against international sample references (oxalic acid II and sucrose) (Mann Reference Mann1983; Stuiver and Polach Reference Stuiver and Polach1977; Mook and van der Plicht Reference Mook and van der Plicht1999). The 14C age is corrected by taking into account the 13C/12C ratio, referenced to PDB standard that was measured at ISA Villeurbanne (France).
RESULTS AND DISCUSSION
The soil CO2 flux, the δ13С of the plants and their 14C activity, are reported in Table 1. The standard 14C activity for plants grown in pure atmosphere was measured at 104.0 ± 0.3% or 14.1 dpm for 2013 samples and 103.0 ± 0.3% or 14.0 dpm for 2014 samples (internal laboratory reference). In 2014, we obtained an equivalent reference value of 102.5 ± 0.3% for a same plant growing on the top of Camaldoli Hills, on the external border of Campi Flegrei caldera. This value is slightly lower than the internal laboratory reference, but within the range of the analytical uncertainty.
Table 1 Flux measured, δ13С and 14C activities measured in the 17 selected sampling sites.
The locally measured soil CO2 fluxes range from 15 to 1412 g.m–2.day–1. The 14C activity in our plant samples varies between 97.4% and 52.4%, implying an assimilation of between 6.6% and 51.6% of volcanic carbon devoid of 14C. The highest proportion of volcanic CO2 results in an apparent age of 5194 BP for the corresponding plant samples.
We present in Figure 3 the overall variations of 14C activity in our samples against the in situ measured CO2 flux (ACM). The data points show some scattering, which can easily be explained by the differences in temporal recording of the two approaches and by the sensitivity of the both CO2 flux and the amount of air dilution to changing meteorological conditions. The gas flux has been shown to depend on the meteorological conditions (wind, rainfall and barometric pressure) (Carapezza et al. Reference Carrapezza, Ricci, Ranaldi and Tarquini2009, Reference Carrapezza, Barberi, Ranaldi, Ricci, Taquini, Barancos, Fisher, Perez, Weber, Di Piazza and Gattuso2011). However, nearly all the points plot in two envelopes near a regression line showing a relative correlation (r=0.88) between the 14C deficit in plants and the measured CO2 flux. We consider the lower envelope as best would representing the relationship between the CO2 emanation rate and the proportion of volcanic carbon integrated in the plants, which is inversely proportional to the measured 14C activity. The data points plotting well above that envelop in Figure 3 likely result from enhanced air dilution of the volcanic CO2 and its lower integration in the plants, due to a stronger site exposure to wind and meteorological dispersion. This defines an upper envelope in which, however, the proportionality between 14C activity and CO2 flux still persists.
Figure 3 Graph of the 14C activities measured in the plants against the Mean CO2 fluxes measured in each site. The central curve corresponds to the regression line (R = 0.88); the inferior line corresponds to the less contribution of the meteorological conditions; the upper one the larger contribution. Si: sampled in March 2013; Si*: sampled in May 2013.
We thus find that the 14C activity of plants growing at Solfatara volcano clearly records the volcanic emanations of CO2, with a large range of sensitivity. 14C depletion in each plant sample provides an average estimate of the local CO2 emission both in spaces (over a few m2) and, in time (between two successive sampling) and thus integrates the temporal variability of the gas flux. In order to check the sensitivity of our plant “sensor,” we collected and analyzed two plant samples distant by one meter at point S9 (in May 2014). The two results 90.03 ± 0.3% and 91.52 ± 0.32% (ACM: 29.6 g.m–2.day–1), rather comparable, appear nevertheless significantly different beyond the range of the analytical accuracy. The values are near to that measured (92.4 ± 0.2%) on the May 2013 sample and show that the signal can be consistently recognized even at the site with the lowest flux of any measured in our study (18–20 g.m–2.day–1). Most of the results, for a given site, stay within a ±5% range of variation, except for point 16 where an anomalous variation is observed for both the direct CO2 flux measurement and the radiocarbon activity in the plants. This point is located close to an active fracture system, linked to the Pisciarelli fumaroles. Such a particular location may be responsible important short-term and longer-term variations in the degassing rate, which could thus explain the variability in 14C depletion at that site.
Allard et al. (Reference Allard, Pasquier-Cardin, Fontugne, Hatté and Baubron1997) and Pasquier et al. (Reference Pasquier-Cardin, Allard, Ferreira, Hatté, Coutinho, Fontugne and Jaudon1999) demonstrated that 14C depletion in plants growing on volcanoes can also be associated with significant anomalies in δ13С. Here we observe the same effect at Solfatara crater: the bulk δ13С of the plants broadly increases with the local intensity of CO2 degassing, in agreement with the mixing between standard atmospheric CO2 (–8‰) and the volcanic CO2 feeding Campi Flegrei (–0.73 to 1.87‰) (Allard et al. Reference Allard, Maiorani, Tedesco, Cortecci and Turi1991b; Caliro et al. Reference Caliro, Chiodini, Moretti, Avino, Granieri, Russo and Fiebig2007). Figure 4 shows δ13С values against the 14C activity of our samples. We observe a difference between the two groups of C3 and C4 plants (as determined by δ13С measurements) except for three points corresponding likely to mixed sample. The mixing between atmospheric and volcanic CO2 is clear for the C4 group of plants but not apparent for the C3 group; indeed, as evidenced by Farquhar et al. (Reference Farquhar, Ehleringer and Hubic1989), the carbon isotope discrimination evolution versus the ratio of intercellular and ambient partial pressures of CO2 in the C3 and C4 group is different. Note that the δ13С was determined in order to correct the 14C activity measurements of the isotope fractionation not only due to plant photosynthesis but also to take account of chemical pretreatment and sample burning.
Figure 4 Graph of the 14C activities against δ13С measured in the plants for the C3 and C4 groups.
Finally, in 2014 we also sampled plants on the slope of Monte Nuovo, the most recent volcanic vent in Campi Flegrei, but far from any visible fumarolic emission. 14C activity in these plants is measured at 100.9 ± 0.3%, indicating a small 14C deficit of 1.5% compared to atmosphere, which is above our analytical uncertainty. Such a value, which corresponds to an apparent age of around 150 yr BP, indicates some contamination of the plants by a diffuse degassing of CO2 through the volcanic soil. We thus find that plants living within the central part of Campi Flegrei caldera display apparent radiocarbon ages ranging from 5194 yr BP to 156 yr BP. These new data raise again the possibility of biased 14C dating of volcanic deposits due to active 14C devoid emanations of magma-derived CO2 in volcanic areas (Saupé et al. Reference Saupé, Strappa, Coppens, Guillet and Jaegy1980; Rolandi et al. Reference Rolandi, Petrosino and Geehin1998; Passariello et al. Reference Passariello, Albore Livadie, Pierfrancesco, Lubritto, D’Onofrio and Terrasi2009).
To improve the detectability of 14C depletion in the plants as a sensor of the local CO2 soil degassing and to limit as far as possible the air dilution effect due to the wind, we positioned a greenhouse-like plastic box over an area about 1 m2 of grass within the point 2 zone. By doing this, we measured that the 14C activity in the plants decreased from 76.3% (sampled in October 2012) down to 38.0% while the δ13С was lowered from –12.62 to –10.79‰. Therefore, the use of such a plastic box can significantly increase the actual volcanic CO2 signal at a given site of emission. The sensitivity of such a “bio-sensor” can be extended to other applications (e.g. very low soil degassing or to detect possible leak in CO2 storage sites).
CONCLUSIONS
Our results are compared with the data obtained from contemporaneous ACM CO2 flux measurements at the same sites and thus allow investigating the impact of CO2 emissions in Solfatara crater. We have correlated the 14C deficit in plants with the mean CO2 flux measured. It demonstrates that grass and plants growing in degassing areas of the Campi Flegrei volcanic complex (Solfatara crater, Pisciarelli zone, Monte Nuovo) typically record the imprint of CO2 emanations in terms of both 14C depletion and δ13С mixing anomalies. Campi Flegrei is thus another volcanic area where such an effect of volcanic degassing is estimated using the 14C deficit in plants.
Grass sampling at the sites used for our study could be renewed with a few weeks’ periodicity between March and November; as such, they would provide the average degassing behavior making it possible to analyze more in detail the spatial and temporal variations of volcanic degassing. Moreover, the grass growing at low soil level is indeed more sensitive to CO2 soil degassing. Therefore, future studies on plants growing in active volcanic zones should be systematically focused on grass within a few centimeters above soil surface. Complementary to ACM repeated measurements, our approach gives a weighted mean integrating the daily variations; it allows the mapping of protracted degassing on active volcanoes and may thus usefully contribute to the surveillance of quiescent volcanoes during long and dangerous phases of unrest.
ACKNOWLEDGMENTS
Many thanks to Patrick Allard, William C Evans, and an anonymous reviewer whose corrections and suggestions are greatly appreciated and permit us to improve this work. The authors also wish to thank Viviane Belon for 14C sample treatment, and Stefano Caliro and Rosario Avino for their helpful presence in the field. Thank you to David Crawford-White for his English proofreading. The study has benefited from funding provided by the Italian Presidenza del Consiglio dei Ministri Dipartimento della Protezione Civile (DPC), projetto V2 precursori di eruzioni. This paper does not necessarily represent DPC official opinion and policies. This is LGMT contribution number 137.
