Chemical Geology 310–311 (2012) 12–22

Contents lists available at SciVerse ScienceDirect

Chemical Geology

j ourna l homepage: www.e lsev ie r .com/ locate /chemgeo

Research paper

Bioindication of volcanic mercury (Hg) deposition around Mt. Etna (Sicily)

R.S. Martin a,⁎, M.L.I. Witt b, G.M. Sawyer a, H.E. Thomas c, S.F.L. Watt d, E. Bagnato e, S. Calabrese e,A. Aiuppa e, P. Delmelle f, D.M. Pyle b, T.A. Mather b

a Department of Geography, University of Cambridge, UKb Department of Earth Sciences, University of Oxford, UKc Department of Earth Sciences, University of Bristol, UKd National Oceanography Centre Southampton, University of Southampton Waterfront Campus, European Way, Southampton, UKe Dipartimento DiSTeM, Università di Palermo, Italyf Environmental Sciences, Earth & Life Institute, Université catholique de Louvain, Belgium

⁎ Corresponding author.E-mail address: rsm45@cam.ac.uk (R.S. Martin).

0009-2541/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.chemgeo.2012.03.022

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 December 2011Received in revised form 22 March 2012Accepted 23 March 2012Available online 4 April 2012

Editor: J.D. Blum

Keywords:VolcanoEmissionMercuryBioindicatorEtna

Mt. Etna is a major natural source of Hg to the Mediterranean region. Total mercury concentrations, [Hg]tot, inCastanea sativa (sweet chestnut) leaves sampled 7–13 km from Etna's vents (during six campaigns in2005–2011) were determined using atomic absorption spectroscopy. [Hg]tot in C. sativa was greatest onEtna's SE flank reflecting Hg deposition from the typically overhead volcanic plume. [Hg]tot also showed Hgaccumulation over the growing season, increasing with leaf age and recent eruptive activity. [Hg]tot in C.sativa was not controlled by [Hg]tot in soils, which instead was greatest on Etna's NW flank, and wascorrelated with the proportion of organic matter in the soil (% Org). An elevated [Hg]tot/% Org ratio in soils onEtna's SE flank is indicative of increased Hg deposition. This ratio was also found to decrease with local soilpH, suggesting that Hg deposited to the low pH and organic-poor soils on Etna's SE flank may not be retainedbut will instead be released to groundwater or re-emitted to the atmosphere. These results show that thedeposition of volcanic Hg has clear impacts and confirm that Etna is an important source of Hg to the localenvironment.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Mercury (Hg) is a metal of environmental concern; it is importantto characterise its emission sources and to understand its chemicaltransformations within the environment better. The major anthropo-genic sources of Hg to the atmosphere include coal combustion, metalproduction, waste incineration and artisanal gold mining, amountingto a total anthropogenic Hg flux of 2200–4000 Mg Hg yr−1 (Mason etal., 1994; Pirrone et al., 2009; Selin, 2009). The total natural Hg flux isof the same order (~4400–5200 Mg Hg yr−1; Schroeder and Munthe,1998; Mason, 2009) and the most significant contributors areemission from soils, vegetation, agriculture and the oceans. Many ofthese natural sources represent re-emission of previously depositednatural or anthropogenic Hg. In contrast, volcanic emissions are aprimary natural source of Hg to the atmosphere. While quiescentvolcanic degassing (~75–100 Mg Hg yr−1; Pyle and Mather, 2003;Bagnato et al., 2011) is a minor contributor to the global natural Hgflux, volcanic Hg emissions are limited to point sources such as

rights reserved.

Mt. Etna (Sicily; Bagnato et al., 2007) andMasaya volcano (Nicaragua;Witt et al., 2008a) so have greater potential for local environmentalimpact than Hg from more prodigious but diffuse natural sources.

Hg in the atmosphere exists predominantly as gaseous elementalmercury (GEM; Hg0). GEM has a long atmospheric lifetime (0.5–2 years, Lindqvist and Rodhe, 1985; Slemr et al., 1985) due to its lowsolubility and gaseous nature, which inhibits wet and dry depositionand promotes long-range global transport (Fitzgerald et al., 1998;Morel et al., 1998). A major loss process for GEM is oxidation toreactive gaseous Hg (RGM; HgII) and particulate Hg (Hgp; HgII). Thesecompounds are removed by wet and dry deposition and haveatmospheric lifetimes of days to weeks. RGM and Hgp generallymake up only a few percent of total atmospheric Hg, although theseproportions may be higher close to emission sources (Schroeder andMunthe, 1998). Once deposited into waters, soils and sediments,biological and chemical processes can convert Hg into toxicmethylated forms (Ullrich et al., 2001), which are lipid soluble andbioaccumulate and biomagnify in the food chain (Bloom, 1992;Jonnalagadda and Rao, 1993; Morel et al., 1998).

A major uncertainty regarding the impacts of volcanic Hg is theextent to which Hg emitted from a volcano is deposited locally ortransported globally. Recent measurement and modelling studies ofvolcanic Hg speciation indicate that near-source (i.e., where plumesages are on the order of a few minutes), RGM and Hgp typically

13R.S. Martin et al. / Chemical Geology 310–311 (2012) 12–22

amount to b5% of total Hg, with GEM the most abundant form of Hg(e.g., Bagnato et al., 2007; Witt et al., 2008a; Bagnato et al., 2009;Martin et al., 2011). This suggests that only a small fraction of volcanicHg (i.e., RGM and Hgp) is likely to be deposited close to the volcano.Volcanic plumes are, however, especially oxidising environments(e.g., Mather, 2008; von Glasow et al., 2009), as shown bymeasurements of oxidised species such as BrO (Bobrowski et al.,2003; Oppenheimer et al., 2006) and chemical modelling suggestsGEMmay be oxidised rapidly (~15 min) to RGM andHgp (von Glasow,2010). This chemistry, if it occurs, is analogous to the bromineexplosion and O3 depletion events observed in the Dead Sea and Polarenvironments (Steffen et al., 2008; Obrist et al., 2011). No fieldevidence is presently available to substantiate this result, althoughthis rapid oxidation would provide a mechanism for increased localdeposition of volcanic Hg.

Here, we use vegetation as a bioindicator of volcanic Hg depositionaround Mt. Etna (Sicily), which is a major natural source of Hg to theMediterranean region (Pirrone et al., 2001; Bagnato et al., 2007).Castanea sativa (sweet chestnut) leaves were sampled 7–13 km fromthe degassing vents of Etna volcano (Sicily) during a series of fieldcampaigns in 2005, 2006, 2007, 2008, 2010 and 2011. Approximately170 separate C. sativa samples were collected in total. Additionally,soils were collected at each of the 69 sampling sites in 2011. Allsamples were analysed for total Hg concentrations, [Hg]tot, usingatomic absorption spectroscopy. Soils were further analysed for bulkcomposition (% H2O, % Org, % Silicate, and pH). Our aims are todetermine the spatial and temporal variations of [Hg]tot in C. sativaand soils on Etna, and interpret these results in terms of theenvironmental transport and processing of volcanic Hg.

1.1. Uptake of Hg by vegetation

Vegetation may take up Hg in a range of chemical forms (Kabata-Pendias, 2001). Gases (GEM, RGM) are accumulated directly from theatmosphere by leaves through gas exchange across the stomata andby direct absorption onto the leaf surface (Suszcynsky and Shann,1995; Graydon et al., 2008; Bash and Miller, 2009; Stamenkovic andGustin, 2009; Converse et al., 2010). Particles (Hgp) are depositedonto the leaf surface, and dissolved, allowing for uptake of Hg. Undernormal conditions, the supply of GEM to vegetation greatly exceedsthat of RGM and Hgp so the most significant contributor to Hg uptakeis dry deposition of GEM (Graydon et al., 2008). These processes maybe important removal mechanisms of Hg from the atmosphere, as theHg is either taken up into the leaf or intercepted by the leaf beforesubsequently being washed to the ground by rainfall. Litterfall may bean important source of Hg to the soil since Hg is immobilised from theatmosphere within the leaves (Lindberg, 1996; Siwik et al., 2009).The soil is generally a minor source of Hg to the whole plant (Kabata-Pendias, 2001; Fay and Gustin, 2007) because Hg binds with organicspecies in the roots limiting translocation (Ericksen and Gustin,2004). However, in very contaminated soils (>10 μg g−1; e.g., minetailings, Moreno-Jimenez et al., 2006), soil uptake and translocationmay become a more important control on Hg concentrations inleaves. An additional route for soil Hg to affect leaf concentrations isabsorption of Hg by leaves following its emission from the soil. Thisprocess could be particularly important in volcanic areas where Hg issupplied to the soil by diffuse degassing (Varekamp and Buseck, 1983,1986; Gustin et al., 2000), in addition to the re-emission of previouslydeposited atmospheric Hg.

Mercury deposition is particularly suitable for bioindicator studybecause the dominant control on Hg concentrations in leaves isgenerally atmospheric Hg concentrations (Fay and Gustin, 2007). Alimitation of leaves as a bioindicator is that Hg may be accumulatedthroughout the growth season (Rasmussen, 1995; Ericksen et al., 2003;Dunagan et al., 2007; Fay and Gustin, 2007; Bushey et al., 2008; Bashand Miller, 2009), which complicates comparisons between samples

collected at different times (especially if leafingbegins at different timesacross the study area). Additionally, a significant fraction of leaf HgII

may be photo-reduced and re-emitted into the atmosphere as GEM(Graydon et al., 2006). Bioindicator studies of volcanic Hg depositionhave included study of lichens (Bargagli and Barghigiani, 1991; Daviesand Notcutt, 1996), mosses (Gramatica et al., 2006; Giudice andBonanno, 2009) and plants (Siegel and Siegel, 1982; Martin et al.,2010). The interpretation of results is often complicated by theintricacies of uptake from the atmosphere and soil, which may varyeven between individuals of the same species (e.g., Fay and Gustin,2007). Particularly relevant to studies in volcanic settings is that plantsexposed to SO2 may respond by a reduction in stomatal conductance(Tanner et al., 2007), increased exposure to SO2 would thereforedecrease uptake of GEM. Lichen [Hg]tot is regarded as a goodbioindicator for long-term Hg deposition, due to minimal uptake of Hgfrom the underlying rock. However lichens do not allow for investiga-tion of short-term variability (i.e., immediately after an eruption, orover a single year) and the sampling density is often limited by thespecies abundance. In contrast, it is generally possible to identify a plant(in this case, C. sativa) that grows abundantly throughout the studyarea, in principle, allowing for short-term variability in Hg deposition athigh spatial resolution to be investigated.

1.2. Study area

The activity of Etna volcano (elevation ~3300 m; Fig. 1A) ischaracterised by persistent degassing (providing a continuousemission source into the surrounding environment) punctuated byoccasional explosions and lava flows from one or more of the foursummit craters, and sporadic flank eruptions ranging from passivelava flows to violent fire-fountains that produce significant quantitiesof ash (Allard et al., 2006). Emissions from Etna (e.g., H2O, SO2, CO2,H2S, HF, HCl, HBr, HI; SO4

2−, Cl−, and metals; Toutain et al., 1995;Aiuppa et al., 2003, 2004a, 2005a, 2005b, 2006, 2008; Allen et al.,2006; Martin et al., 2008) are among the best characterisedworldwide, and measurements of near-source volcanic Hg indicatethat RGM+Hgp comprise ~1–2% of Hgtot (5.4 Mg yr−1) at Etna(Bagnato et al., 2007). Plumes from Etna are typically transported tothe SE sector of the volcano (Fig. 1B). Between 0.1 and 0.9% of theacidic gas emissions (i.e., SO2, HCl, and HF) from Etna are lost by drydeposition to the local environment (Aiuppa et al., 2006), withpotentially greater losses of particles (Allen et al., 2006; Calabrese etal., 2011). The activity at Etna over 2005–2011 began with a period ofquiescence (April 2005–June 2006), followed by a series of eruptions(starting in July 2006) ranging from short-lived (hours) andintermittent (periodicity of weeks to months) paroxysmal explosionsfrom the SE crater (being particularly intense in 2007–2008 and2011), to more long-lived fissure eruptions on the upper E flank (July2006, September–December 2006, May 2008–July 2009). A moredetailed record of eruptive activity of Etna can be found on thewebsite of the Istituto Nazionale di Geofisica e Vulcanologia (http://www.ct.ingv.it).

Recent bioindicator studies at Etna have focused on C. sativa(Fig. 1 C), which grows abundantly at altitudes of 1000–2000 m. Thisdeciduous tree species develops broad, green, oval leaves in March,which are lost in the following October and November. C. sativa hasbeen used as a bioindicator for the deposition of volcanic F (Bellomoet al., 2007), magnetic Fe-bearing minerals (Quayle et al., 2010) andvolatile trace metals (Watt et al., 2007; Martin et al., 2009; Quayle etal., 2010). While eruptions were found to result in increased metalconcentrations in C. sativa downwind of Etna's active vents (i.e.,typically the SE sector), no spatial differences were found duringquiescent periods, reflecting the reduced emission fluxes of metals(i.e., ~10× less than during eruptive periods; Gauthier and LeCloarec, 1998). Existing C. sativa samples from Sept. 2005, July 2006,May 2007 and Sept. 2008 (Martin et al., 2009; Quayle et al., 2010)

Fig. 1. (A) Sampling locations around Mt. Etna (Sicily) for C. sativa and soils. Samples from 2011 are shown separately from samples collected in other years. The sectors (NW, NE,SW, and SE) are defined here in terms of natural breaks in the abundance distribution of C. sativa. The major fault systems (PF = Pernicana, RN = Ripe della Naca, SV = SantaVenerina, T = Acireale, and R = Ragalna; adapted from Neri et al., 2004) and rift zones (i.e., NE, SW and W) are indicated. (B) Estimated plume transport direction frequenciesduring each year, based on atmospheric sounding data at 3000–3600 m from Trapani (Sicily). The results are shown overlaid on a simplified map of Mt. Etna. (C) Photograph ofC. sativa. Each leaf has typical dimensions of ~20 cm×~10 cm (which increase in size throughout the growing season).

14 R.S. Martin et al. / Chemical Geology 310–311 (2012) 12–22

were re-analysed for Hg, whereas samples from Sept. 2010 andSept. 2011 were collected specifically for the present study. Allsample locations are indicated in Fig. 1A, with GPS co-ordinatesgiven in Appendices A (2005–2008, 2010 campaigns) and B (2011campaign).

2. Methodology

For C. sativa sampling (2005–2008, 2010–2011), ~10 leaves (ofsimilar size, typically ~20 cm×~10 cm although increasing in sizethrough the growing season) were collected at each samplinglocation (from different plants, in so far as possible) and placedtogether in a clean polypropylene bag. In 2010, a further ~10 leaveswere collected from each of 3–6 trees at every site and these sampleswere placed in individual clean polypropylene bags. After transport tothe United Kingdom, the samples were cleaned with stiff plastic brushto remove dusts, dried, and crushed to a fine powder (i.e., b1 mm).The petiole was removed prior to drying. The powders were placed inopen glass vials and transferred to desiccators for several days toremove any residual water. To determine [Hg]tot, 50 mg of powderwas placed in a quartz sample boat and heated to 550 °C with aPyro915+ two chamber catalyst atomiser (Lumex, Russia). Hgreleased by the sample was measured with a Lumex915+ portable

mercury vapour analyser (e.g., Sholupov et al., 2004; Witt et al.,2008a,b) by Zeeman atomic absorption spectrometry using highfrequency modulation of light polarisation. We analysed 2–3replicates per sample and estimate an analytical uncertainty of ~8%(i.e., mean coefficient of variation between replicates). The within-site variability was b12% (i.e., maximum coefficient of variationbetween samples from each of 3–6 trees at every site in Sept. 2010,inclusive of the analytical uncertainty). The instrument was calibrat-ed using standard reference materials (NIST 2782, NIST 2587).

For soil sampling (2011), 5 samples of surface (0–10 cm) soil fromeach site were mixed in a plastic bucket and then sieved to retain theb2 mm fraction (any visible plant roots were discarded), which wasthen placed in a clean polypropylene bag. The mass of soil collectedfrom each site was ~10 g. The soil sampling method was adaptedfrom that described in Floor et al., (2011). [Hg]tot was determinedusing the same method as described above except at a highertemperature of 700 °C, with an analytical uncertainty of ~7% (i.e.,mean coefficient of variation between replicates). The bulk compo-sition of the soil was determined using loss-on-ignition analysis (e.g.,Heiri et al., 2001) with heating stages of 105 °C for 4 h (for % H2O bymass), 450 °C for 10 h (for % organic matter by mass, abbreviated %Org), and 950 °C for 10 h (for % CaCO3 by mass). We estimate ananalytical uncertainty of ~1% of total mass (i.e., a typical coefficient of

Fig. 2. Temporal variation of [Hg]tot in C. sativa, in terms of the minimum, mean,maximum and standard deviation for each sampling campaign. Also shown are theresults for [Hg]tot in soils in 2011. The sample size (n) and the activity of Etna areshown for each campaign.

15R.S. Martin et al. / Chemical Geology 310–311 (2012) 12–22

variation between replicates). Consistent with the volcanic origin ofthe soils, % CaCO3 was generally 1% or less (i.e., negligible). Soil pHwas determined using a pH meter for a 1:5 mixture of soil anddistilled deionised water (Delmelle et al., 2003).

3. Results and discussion

Results for [Hg]tot in C. sativa for each sampling campaign (Sept.2005, July 2006, May 2007, Sept. 2008, Sept. 2010, and Sept. 2011) aresummarised in Fig. 2, along with results for [Hg]tot in soils in 2011.Full results are given in Appendices A and B. The overall rangeof [Hg]tot in C. sativa was 19–190 ng g−1 and much of this rangewas found within a single sampling campaign (Sept. 2011,27–190 ng g−1). Concentrations are well below typical thresholdsfor mercury toxicity in vegetation (e.g., >1000 ng g−1; Kabata-Pendias, 2001). [Hg]tot in soils (4.6–73 ng g−1) was generally lowerthan in the corresponding C. sativa. The values of [Hg]tot shown inFig. 2 are comparable to those found in an earlier study of Hg in othervegetation species, e.g., Pinus sp.(mean 28 ng g−1) and Genistaaetnensis (mean 92 ng g−1), and soils (mean 64 ng g−1) from Etna(Barghigiani et al., 1988).

3.1. Correlation with seasonal growth and eruptive activity

The trends in mean and maximum [Hg]tot shown in Fig. 2 areconsistent with Hg accumulation over the growing season withsamples collected in Sept. (2005, 2008, 2010 and 2011) showinggreater [Hg]tot than samples collected in May (2007) or July (2006).This trend with leaf age is established in a wide range of vegetationspecies (Rasmussen, 1995; Ericksen et al., 2003; Fay and Gustin,2007; Bushey et al., 2008) and is also shown by comparison ofneedles of different ages from Abies alba on Etna (Barghigiani et al.,1988). The increase of the minimum [Hg]tot (~2 ng g−1 month−1)

between May (2007), July (2006) and Sept. (2005, 2008, 2010, and2011) is consistent with a typical rate of GEM uptake (i.e.,~1 ng g−1 increase in leaf [Hg]tot when exposed to 1 ng m−3 GEMfor 1 month, an estimate derived from measurements of Hg uptakein other species, Fay and Gustin, 2007) and the local atmosphericbackground of 3 ng m−3 (Barghigiani et al., 1988; Bagnato et al.,2007). The much larger differences in mean and maximum [Hg]totbetween Sept. 2005, 2008, 2010 and 2011 suggest that the time-integrated exposure of C. sativa to Hg was greater in Sept. 2008 and2011 than in either Sept. 2005 or 2010. Since minimum [Hg]tot doesnot vary significantly between Sept. 2005, 2008, 2010 and 2011, wemay also infer that at some locations there was little variationbetween the years.

These results mirror the changing volcanic activity of Etna. Theactivity over most of 2005, and in 2010, was characterised by aprolonged period of quiescence, and reduced magma feeding to theshallow plumbing system (as indicated by low CO2/SO2 ratios in thesummit emissions and a reduced SO2 flux; A. Aiuppa, unpublishedresults). In contrast, the activity in 2008 was characterised by a long-lived fissure eruption on the NE flank, with several summit eruptions,and in 2011 by a series of intense summit eruptions. While systematicSO2 flux time-series from Etna are presently unavailable for2005–2011, eruptive episodes generally correspond to enhancedSO2 emissions (e.g., Aiuppa et al., 2008). Furthermore, there is someevidence that the Hg/SO2 ratio increases during eruptive periods(Pyle and Mather, 2003), in line with the behaviour of other metals(Gauthier and Le Cloarec, 1998). Together, these factors suggest thatHg emissions increase during eruptive periods, which would result inincreased Hg deposition and accumulation into C. sativa.

The temporal trends for [Hg]tot in C. sativa contrast with those for[As]tot, [Cd]tot, and [Tl]tot in C. sativa (from the 2005–2008 campaigns;Martin et al., 2009; Quayle et al., 2010). [As]tot, [Cd]tot and [Tl]tot weregreater in May 2007 than in September 2005, reflecting recenteruptive activity rather than time-integrated exposure over the entiregrowing season (as for [Hg]tot). This difference could be explained ifmuch of the As, Cd and Tl is retained loosely on the leaf surfacewhereas Hg is immobilised within leaf tissues. Additionally, thesedifferences may also suggest that while As, Cd and Tl are highlyenriched in eruptive plumes relative to quiescent plumes (Gauthierand Le Cloarec, 1998), more modest increases in Hg/SO2 occur duringeruptive periods.

3.2. Spatial variation of Hg around Mt. Etna

The spatial variation of [Hg]tot in C. sativa is shown in Fig. 3,individually for Sept. 2005, July 2006, Sept. 2008, and Sept. 2011, andalso for all years combined (i.e., 2005, 2006, 2007, 2008, 2010 and2011). Sampling in May 2007 and Sept. 2010 was focused on specificsectors so the spatial variation of [Hg]tot for these campaigns is notshown individually. Results are normalised to maximum [Hg]tot fromeach campaign to remove the temporal influences on [Hg]tot. Forindividual campaigns, all show a distinct clustering of [Hg]tot maximain the SE sector, consistent with the most typical direction of plumetransport (Fig. 1). The spatial variation in [Hg]tot in Sept. 2005contrasts with most other metals (Martin et al., 2009), which showedno clear spatial variation during this period of quiescence. Whenresults from all years are combined, it also becomes clear that [Hg]totmaxima are generally found in more proximal locations, as might beexpected if [Hg]tot in C. sativa reflected exposure to volcanic Hgdeposited from the plume to the ground. The spatial distribution of[Hg]tot maxima (Fig. 3) is wider than expected simply based on thewind direction (Fig. 1), since [Hg]tot maxima are also found in the NEand SW sectors. This difference primarily reflects the influence ofproximity to Etna's vents, resulting in increased [Hg]tot over a widerangular range in more proximal locations.

16 R.S. Martin et al. / Chemical Geology 310–311 (2012) 12–22

These results contrast with those of Barghigiani et al. (1988) whofound no spatial variation in [Hg]tot in other vegetation species (i.e.,Pinus sp., G. aetnensis, and A. alba) from Etna, and led them toconclude that Etna does not influence the concentrations of Hg in thelocal environment. In the present study we analysed a large numberof coherent samples (i.e., 69 sites in 2011 and 138 sites in total for asingle plant species) while Barghigiani et al. (1988) analysed a widerrange of species but collected from only 7 sites. Local variability (i.e.,the differences in [Hg]tot between adjacent sites) often greatlyexceeds the within-site variability (b12%), indicating differencesdue to local topography (either increasing or decreasing exposure toHg) or other heterogeneities of similar scale (e.g., the time of leafing).This local variability may obscure wider spatial trends if only a smallnumber of sites are sampled. An additional possibility is that C. sativamay be inherently a more suitable bioindicator than any of thespecies analysed in the earlier study.

Between 0.1 and 0.9% of the acidic gas emissions from Etna are lostby dry deposition to the local environment (Aiuppa et al., 2006). Asimilar or greater proportion of particles may be deposited locally(Allen et al., 2006; Calabrese et al., 2011). Therefore, there existspotential for Hg deposition despite the ~2000 m vertical separationbetween the plume and ground. Diffusion tube measurements byAiuppa et al. (2004a) indicate SO2 concentrations at ground level inthe SE sector of ~1.8 d−2.3 (mg m−3), where d is the distance (in km)from the summit craters. The SO2/HCl and HCl/HF ratios from thesediffusion tube measurements were similar to those of the summitemissions, indicating comparable transport properties for SO2, HCland HF. Taking the mass ratio GEM/SO2=8.7×10−6 (Bagnato et al.,2007) and assuming comparable transport properties of GEM andSO2, we might predict additional (i.e., volcanic) GEM concentrationsof ~16 d−2.3 (ng m−3) due to the presence of plume overhead. Thelateral distance between Etna's summit and the most proximalsamples in the SE sector (~7 km) suggests additional GEM concen-trations of ~0.2 ng m−3 at ground level. As the typical localatmospheric background levels are 3 ng m−3 (Barghigiani et al.,1988; Bagnato et al., 2007) it is unlikely that transport of GEM fromthe plume to the ground could explain the spatial distribution andrange of [Hg]tot. Our estimate is consistent with earlier measurementsby Barghigiani et al. (1988) who found negligible differences betweenGEM concentrations in the SE sector compared to a site in the NWsector.

An alternative explanation for an increased supply of Hg to thegroundwithin the SE sector (as indicated by [Hg]tot in C. sativa) is thatGEM is oxidised to HgII (i.e., RGM) which then partitions into theparticle phase (Hgp) in Etna's plume, as suggested by the model ofvon Glasow (2010). Conversion of GEM to Hgp is predicted to occur

Fig. 3. Spatial variation of [Hg]tot in C. sativa normalised to maximum [Hg]tot in each samp2006, Sept. 2008, and Sept. 2011, and also for all years combined (i.e., 2005, 2006, 2007, 20

over a timescale of ~15 min, which corresponds to a lateral distanceof ~5 km (assuming a wind speed of ~5 m s−1). Hg deposition anduptake by C. sativa (sampled from lateral distances in >7 km) wouldthen be controlled by the relatively rapid wet and dry deposition ofHgp (e.g., Lindqvist and Rodhe, 1985) rather than the much slowerdeposition of GEM. While many uncertainties remain our results lendsupport to the model calculations of von Glasow (2010) and highlightthe need for further measurements of Hg speciation in volcanicplumes, both at the crater rim and downwind, and of Hg depositioninto the local environment.

In addition to Hg deposition from the plume, [Hg]tot in C. sativamay partly reflect Hg emission (in the form of GEM) throughfractures (e.g., Crenshaw et al., 1982), equivalent to diffuse degassingof CO2 (e.g., Giammanco and Bonfanti, 2009). Hg is expected toexsolve at high pressure (Varekamp and Buseck, 1981) so willpartition into the CO2-rich vapour phase as the magma becomes CO2-saturated (Giammanco et al., 1997, 1999; Azzaro et al., 1998; Neri etal., 2004). This CO2- and Hg-rich fluid could escape along fractures atthe surface. Hg concentrations have been shown to correlate with CO2

emissions at other volcanoes where low temperature geogenicemissions occur (Witt et al., 2008a, 2008b). The major fracturezones on Etna (e.g., Fig. 1A) are found on the Eastern flank, i.e., the NE,S and W rift zones (e.g., Andronico et al., 2005; Bonforte et al., 2011),the Pernicana Fault system in the NE sector (e.g., Giammanco et al.,1997; Giammanco et al., 1999), and the Santa Venerina Fault in the SEsector (b500 m; e.g., Aiuppa et al., 2004b; Burton et al., 2004).Although diffuse degassing of Hg makes a minor contribution to totalHg emissions from Etna (Bagnato et al., 2009), this process couldplausibly contribute to the spatial variation of [Hg]tot within the NE,SE and SW sectors (Fig. 3). It is unlikely that diffuse degassing of Hg isthe dominant source of Hg to C. sativa sampled in the SE sectorbecause [Hg]tot maxima tend to cluster at >1500 m within the SEsector (i.e., at much higher altitude than the Santa Venerina Faultsystem). However, the contribution of diffuse degassing of Hg may bemore significant around the Pernicana Fault system in the NE sector,where moderately high values of [Hg]tot in C. sativa are found alongthe entire length of the fault.

3.3. Correlation with soil properties

Sources of Hg to soils within the study area (Fig. 1) are expected toinclude litterfall, dry (GEM, RGM, and Hgp) and wet (Hgp) depositionfrom the volcanic plume, and diffuse degassing. The spatial variationof various soil characteristics is shown in Fig. 4. [Hg]tot in soils islowest within the SE sector (i.e., downwind) and highest within theNW sector (i.e., upwind). This result contrasts with those of

ling campaign (as shown in Fig. 2). Results are shown individually for Sept. 2005, July08, 2010 and 2011).

Fig. 4. Spatial variation of (A) [Hg]tot in C. sativa, (B) % Org in soils, (C) [Hg]org in soils and (D) [H+] in soils (defined here as 10−pH of a 1:5 mixture of soil and distilled deionisedwater).

17R.S. Martin et al. / Chemical Geology 310–311 (2012) 12–22

Barghigiani et al. (1988), who found no spatial variation in [Hg]tot insoils on Etna (again, this may reflect problems with local variabilityand the small number of sample sites). The percentage of soil organicmatter by mass (% Org) shows a similar spatial distribution to [Hg]totin soils (R2=0.73). This correlation potentially reflects the impor-tance of organic matter in Hg retention within the soil due tocomplexation (e.g., Kabata-Pendias, 2001; Tomiyasu et al., 2003).Assuming that soil Hg is contained within the organic component,then the concentration of Hg within the organic component is givenby [Hg]org=[Hg]tot /% Org (where % Org is expressed as a decimalfraction). [Hg]org provides a bioindicator of soil exposure to Hg andmaxima in [Hg]org (up to 700 ng g−1) were found primarily withinthe SE sector. [Hg]org in soils typically exceeds [Hg]tot in C. sativa (upto 200 ng g−1) due to mineralisation (causing Hg enrichment whenCO2 or CH4 is lost during leaf litter decomposition; Pokharel andObrist, 2011) and the supply of Hg to the soil from additional sources(e.g., dry and wet deposition, diffuse degassing). [H+] in soils(defined here as 10−pH of a 1:5 mixture of soil and distilled deionisedwater) was found to be highest within the SE sector. While this resultis consistent with the co-deposition of acidic species (e.g., HCl, SO2), awide range of heterogeneities (e.g., soil organic matter, amorphous Feand Al phases, and base cations; Kabata-Pendias, 2001) also influencesoil pH so a more detailed interpretation is not possible with thepresent data set. In any case, our results show clearly that [Hg]tot insoils does not control [Hg]tot in C. sativa, consistent with earlierstudies showing that the soil is generally a minor source of Hg tovegetation (e.g., Fay and Gustin, 2007).

Figs. 3 and 4 establish that the highest [Hg]tot in C. sativa, andhighest [Hg]org and [H+] in soils, are found within the SE sector ofEtna. Mean values for each metric in each sector (SE, SW, NE, and NW)are shown in Fig. 5A. The three metrics are positively correlated withthe SE sector having higher values than the NE and SW sectors, which

have higher values than the NW sector. This pattern is consistent withthe typical plume transport direction (Fig. 1) and supports ourargument that the main influence [Hg]tot in C. sativa is deposition ofHg from the volcanic plume (rather than the diffuse degassing of Hg).The relationship between [Hg]tot in C. sativa and [Hg]org in soils may becausative if [Hg]org primarily reflects Hg supplied by litterfall (e.g.,Lindberg, 1996; Siwik et al., 2009) rather than Hg supplied directly(i.e., from dry and wet deposition from the volcanic plume). Theinfluence of the plume in the NW sector (i.e., upwind) is minorsuggesting that absorption of background GEM may control [Hg]org insoils within the sector. A more complicated pattern emerges ifaveraging is not applied. Fig. 5B shows that although [Hg]tot in C.sativa and [Hg]org in soils generally increase together, severalsamples (primarily from the SE sector) have high [Hg]tot inC. sativa but low [Hg]org (~300–400 ng g−1) in soils. To understandthis result, consider Fig. 5C, which indicates that [Hg]org in soilsgenerally increases with soil pH (i.e., [Hg]org and [H+] in soils arenegatively correlated). The samples with high [Hg]tot in C. sativabut low [Hg]org in soils, also have low soil pH. Fig. 5D does notshow any clear relationships between [Hg]tot in C. sativa and soilpH.

These results suggest that while [Hg]tot in C. sativa around Etnaincreases with Hg deposition (which is greatest in the SE sector andduring eruptive periods), [Hg]tot in soils is primarily controlled by thesoil organic content (% Org). Thus, although [Hg]tot in soils is lowestwithin the SE sector (due to the higher ash input resulting in low% Org), [Hg]org is highest within the SE sector indicative of increasedHg deposition. The utility of [Hg]org in soils as bioindicator for Hgdeposition is, however, complicated by the decrease in [Hg]org at lowsoil pH (within each sector where soils are more homogenous).Assuming that this relationship is causative, it may reflect increasedHg mobility within low pH soils (e.g., Lumsdon et al., 1995). Hence,

Fig. 5. (A) Relationships between mean [Hg]tot in C. sativa, and mean [Hg]org (left-axis, closed symbols) and mean [H+] (right-axis, open symbols) in soils, for each of the foursectors (SE, SW, NE, and NW). (B) [Hg]tot in C. sativa versus [Hg]org in soils. (C) [H+] in soils versus [Hg]org in soils. (D) [Hg]tot in C. sativa versus [H+] in soils. [H+] is defined here as10−pH of a 1:5 mixture of soil and distilled deionised water.

18 R.S. Martin et al. / Chemical Geology 310–311 (2012) 12–22

Hg deposited may not be retained by the low pH and low % Org soilswithin the SE sector but will instead be lost to groundwater or re-emitted to the atmosphere. While this process potentially makes animportant contribution to Hg in groundwater, in addition to sourcessuch as diffuse Hg degassing and water–rock interactions, ground-water Hg concentrations at Etna (e.g., typically b10 ng L−1; Bagnatoet al., 2009) are known not to exceed the WHO concentrationthreshold for human health (i.e., 1000 ng L−1).

4. Conclusions

Mt. Etna is a major natural source of Hg to the Mediterraneanregion (Pirrone et al., 2001; Bagnato et al., 2007). Total mercuryconcentrations, [Hg]tot, in C. sativa (sweet chestnut) leaves sampled7–13 km from Etna's vents (during six campaigns in 2005–2011)were determined using atomic absorption spectroscopy. [Hg]tot inC. sativa was greatest on Etna's SE flank reflecting Hg deposition fromthe typically overhead volcanic plume. [Hg]tot also showed Hgaccumulation over the growing season, increasing with leaf age andrecent eruptive activity (i.e., highest [Hg]tot in C. sativa was found inSept. 2008 and Sept. 2011, following periods of increased eruptiveactivity). These clear spatial and temporal trends support recentmodel calculations suggesting that gaseous elemental mercury

(GEM) may be oxidised rapidly (~15 min) in Etna's plume to RGMand Hgp (von Glasow, 2010), promoting local Hg deposition.

[Hg]tot in C. sativa was not controlled by [Hg]tot in soils, whichinstead was greatest on Etna's NW flank, and was correlated withthe proportion of organic matter in the soil (% Org). This trendreflects the importance of organic matter in Hg retention in soils.An elevated [Hg]tot/% Org ratio in soils on Etna's SE flank isindicative of increased Hg deposition. This ratio was also found todecrease with local soil pH, suggesting that Hg deposited to the lowpH and organic-poor soils on Etna's SE flank may not be retainedbut will instead be released to groundwater or re-emitted to theatmosphere. These results show that the deposition of volcanic Hghas clear impacts and confirm that Etna is an important source ofHg to the local environment.

Acknowledgements

The Hg analyses were funded under NERC grant NE/G01700X/1.RSM acknowledges Christ's College, University of Cambridge for aJunior Research Fellowship, and the University of London CentralResearch Fund for additional funding. GMS acknowledges supportfrom the European Commission funded projects MIAVITA andDEMONS. JE Martin, SJ Collins and RG Hilton are thanked forfieldwork assistance.

19R.S. Martin et al. / Chemical Geology 310–311 (2012) 12–22

Appendix A

Results for C. sativa samples from field campaigns in 2005, 2006, 2007, 2008 and 2010. GPS co-ordinates are given in the UTM system (Zone33 S). Samples are listed in order of decreasing leaf [Hg]tot in C. sativa.

Sample Campaign E N Group Leaf [Hg]tot (ng g−1)

S10 Sept. 2008 506076 4184584 NE 110S4 Sept. 2008 503671 4172613 SE 110S21 Sept. 2008 508141 4171168 SE 97S2 Sept. 2008 499116 4170872 SW 94S1 Sept. 2008 491547 4172176 SW 87S24 Sept. 2008 511054 4181273 SE 75C4 Sept. 2005 505059 4174100 SE 72S7 Sept. 2008 508729 4177467 SE 72C6 Sept. 2005 509245 4174690 SE 71C8 Sept. 2005 508303 4183672 NE 68S16 Sept. 2008 498427 4189277 NW 66S6 Sept. 2008 508805 4176791 SE 65S5 Sept. 2008 506651 4172016 SE 652010_2 Sept. 2010 498338 4171245 SW 65C7 Sept. 2005 505619 4184346 NE 64S13 Sept. 2008 509023 4185744 NE 64S22 Sept. 2008 507996 4173708 SE 62S18 Sept. 2008 490893 4186873 NW 622010_8 Sept. 2010 503425 4172196 SE 61C5 Sept. 2005 508301 4172849 SE 60S11 Sept. 2008 507214 4184674 NE 59C3 Sept. 2005 503432 4172144 SE 57C9 Sept. 2005 508765 4176740 SE 552010_3 Sept. 2010 498517 4170432 SW 55S25 Sept. 2008 504027 4191050 NW 54S12 Sept. 2008 508256 4185051 NE 53S19 Sept. 2008 492253 4175207 SW 53C13 Sept. 2005 515738 4178803 SE 52S9 Sept. 2008 506925 4178693 SE 522010_1 Sept. 2010 504700 4172504 SE 51S8 Sept. 2008 508312 4178472 SE 492010_9 Sept. 2010 505672 4173814 SE 49S15 Sept. 2008 505171 4189504 NW 48S23 Sept. 2008 510112 4174503 SE 48B13 July 2006 509336 4185574 NE 46B9 July 2006 504673 4172703 SE 46C1 Sept. 2005 496994 4171580 SW 462010_5 Sept. 2010 502051 4171066 SE 46C10 Sept. 2005 498884 4188054 NW 452010_4 Sept. 2010 497489 4168004 SW 45C11 Sept. 2005 491848 4185502 NW 442010_6 Sept. 2010 503361 4169526 SE 43B10 July 2006 505506 4173929 SE 42A15 May 2007 508891 4185750 NE 40B2 July 2006 492218 4186605 NW 40S3 Sept. 2008 496940 4171611 SW 40B11 July 2006 505663 4173811 SE 39S20 Sept. 2008 503312 4169427 SE 37A8 May 2007 503265 4169379 SE 36B7 July 2006 501614 4171083 SE 36B12 July 2006 508582 4177117 SE 35S17 Sept. 2008 496277 4189696 NW 35B14 July 2006 512789 4182916 NE 34B1 July 2006 492223 4175186 SW 342010_7 Sept. 2010 505673 4172199 SE 34A4 May 2007 511140 4180660 SE 32B8 July 2006 503667 4172607 SE 32C2 Sept. 2005 492979 4175373 SW 32A6 May 2007 508035 4171323 SE 30B5 July 2006 497984 4189584 NW 30A12 May 2007 507842 4177981 SE 28B3 July 2006 505800 4184258 NE 27B4 July 2006 507136 4184681 NE 25A16 May 2007 509419 4186493 NE 24A5 May 2007 509967 4176037 SE 23B6 July 2006 498255 4170445 SW 23A11 May 2007 508638 4178693 SE 22A3 May 2007 511491 4184500 NE 21A14 May 2007 508468 4185416 NE 19

20 R.S. Martin et al. / Chemical Geology 310–311 (2012) 12–22

Appendix B

Results for C. sativa and soil samples from the 2011 field campaign. GPS co-ordinates are given in the UTM system (Zone 33 S). Samples arelisted in order of decreasing leaf [Hg]tot in C. sativa. Soil [H+] (defined here as 10−pH of a 1:5 mixture of soil and distilled deionised water).

Sample E N Group Leaf [Hg]tot (ng g−1) Soil [Hg]tot (ng g−1) Soil % H2O Soil % Org Soil [H+] (μmol dm−3)

4.3 509244 4174729 SE 190 13 1.2 3.2 0.644.1 505690 4183880 NE 170 15 0.62 3.2 0.0435.13 511255 4185900 NE 160 31 2.1 9.2 0.0552.4 498342 4171222 SW 160 23 2.4 6.5 0.124.2 508742 4177181 SE 140 13 1.2 3 0.714.13 506652 4184585 NE 130 44 1.8 11 0.345.15 510991 4180076 SE 130 8.4 0.53 2.7 0.394.9 507584 4178969 SE 130 9.7 0.54 2.2 0.194.8 507733 4178013 SE 130 9.7 0.36 1.3 0.0954.4 509936 4176116 SE 130 22 1.8 7.7 0.73.7 505667 4173815 SE 130 21 1.6 4.8 0.393.5 505492 4173940 SE 130 14 2 3.5 0.143.2 506371 4172360 SE 130 12 0.64 1.8 0.132.1 503420 4172266 SE 130 17 1.1 4.1 0.0114.11 505839 4184331 NE 120 27 1.5 6.4 0.563.9 505574 4173061 SE 120 7.4 0.32 1.3 0.0323.4 505376 4174002 SE 110 23 1.6 3.9 0.0653.3 505143 4174072 SE 110 22 1.1 4.2 0.113.13 506716 4172775 SE 110 21 2.1 4.9 0.232.3 499098 4170846 SW 110 15 0.91 3.7 0.262.2 498503 4170435 SW 110 26 1.1 4.7 0.131.1 497509 4167947 SW 110 24 1.7 6.4 0.0234.14 507117 4184691 NE 100 11 0.36 1.7 0.125.14 511615 4184056 NE 100 24 1.9 6.6 0.323.8 505986 4173611 SE 100 19 2 4.1 0.0663.11 506442 4173035 SE 100 13 0.76 2.4 0.123.12 505643 4172207 SE 99 9.8 0.59 3 0.0312.5 496905 4171599 SW 95 67 2.1 12 0.0444.15 507896 4184758 NE 94 38 3.1 11 0.243.6 505576 4173867 SE 93 22 1.4 4.4 0.0721.5 492146 4174869 SW 93 21 2.1 6.1 0.175.1 501196 4189623 NW 92 42 4.6 15 0.0622.6 502057 4171073 SE 90 12 0.62 2 0.424.12 506076 4184587 NE 89 37 1.5 5.5 0.164.6 508630 4178230 SE 89 16 0.85 3.6 0.183.14 507058 4172198 SE 89 4.6 0.2 0.98 0.445.12 511815 4186876 NE 86 17 1.6 6.4 0.334.17 508476 4185415 NE 83 31 2.9 12 0.23.1 506027 4172984 SE 83 21 1.4 3.7 0.163.15 508061 4171230 SE 82 19 0.88 4.7 0.341.4 491917 4173208 SW 82 27 2.5 11 0.194.16 508198 4185059 NE 81 23 1.8 6.7 0.0744.21 509417 4186493 NE 78 35 2.6 9.5 0.21.2 495126 4170931 SW 75 25 2.7 7 0.0272.1 498551 4168662 SW 74 4.9 0.42 1.4 0.231.3 491497 4172148 SW 73 20 1.6 6.5 0.124.7 508553 4178550 SE 72 12 0.96 4.4 0.135.3 491770 4185507 NW 67 59 4.3 17 0.0182.8 502940 4168688 SE 66 21 1.3 5.3 0.245.11 508449 4187673 NE 65 45 3.3 14 0.222.11 501881 4166640 SE 65 13 0.71 2.3 0.0315.7 498445 4189308 NW 63 27 3.8 11 0.0912.9 503654 4172611 SE 62 11 2 3 0.753.1 504740 4172498 SE 58 5.1 0.33 0.88 0.161.6 498463 4167174 SW 58 16 1.7 5.6 0.341.7 492286 4172847 SW 58 53 3.2 12 0.184.1 508512 4172792 SE 56 26 1 4.4 0.525.5 494542 4188354 NW 55 31 3.1 12 0.0864.2 509311 4185671 NE 54 26 2.1 7 0.222.7 503352 4169484 SE 54 13 0.78 4.1 0.345.1 487320 4179084 SW 53 73 3.7 19 0.175.4 498756 4188685 NW 52 49 5 29 0.0595.8 499111 4188785 NW 50 40 4.3 16 0.0754.5 508837 4176759 SE 50 11 0.73 4.4 0.0994.18 508751 4185417 NE 49 34 2.1 10 0.0815.6 498014 4189545 NW 46 42 2.9 14 0.0315.9 499067 4188061 NW 42 21 2.9 9.1 0.174.19 508906 4185744 NE 31 23 1.4 4.6 0.0295.2 490194 4185459 NW 27 14 0.7 3.6 0.012

21R.S. Martin et al. / Chemical Geology 310–311 (2012) 12–22

References

Aiuppa, A., Dongarra, G., Valenza, M., Federico, C., Pecoraino, G., 2003. Degassing oftrace volatile metals during the 2001 eruption of Etna. In: Robock, A.,Oppenheimer, C. (Eds.), Volcanism and the Earth's Atmosphere. GeophysicalMonograph, 139. Am. Geophys. Union, Washington, D.C., pp. 41–54.

Aiuppa, A., Bellomo, S., D'Alessandro, W., Federico, C., Ferm, M., Valenza, M., 2004a.Volcanic plume monitoring at Mount Etna by diffusive (passive) sampling. Journalof Geophysical Research 109, D21308.

Aiuppa, A., Allard, P., D'Alessandro, W., Giammanco, S., Parello, P., Valenza, M., 2004b.Magmatic gas leakage at Mount Etna (Sicily, Italy): relationships with the volcano-tectonic structures, the hydrological pattern and the eruptive activity. Mt. Etna:Volcano Laboratory, Geophysical Monography Series, 143, pp. 129–145.

Aiuppa, A., Federico, C., Franco, A., Giudice, G., Gurrieri, S., Inguaggiato, S., Liuzzo, M.,McGonigle, A.J.S., Valenza, M., 2005a. Emission of bromine and iodine from MountEtna volcano. Geochemistry, Geophysics, Geosystems 6, Q08008.

Aiuppa, A., Inguaggiato, A., McGonigle, A.J.S., O'Dwyer, M., Oppenheimer, C., Padgett,M.J., Rouwet, D., Valenza, M., 2005b. H2S fluxes from Mt. Etna, Stromboli, andVulcano (Italy) and implications for the sulfur budget at volcanoes. Geochimica etCosmochimica Acta 69, 1861–1871.

Aiuppa, A., Bellomo, S., Brusca, L., D'Alessandro, W., Di Paola, R., Longo, M., 2006. Major-ion bulk deposition around an active volcano (Mt. Etna, Italy). Bulletin ofVolcanology 68, 255–265.

Aiuppa, A., Giudice, G., Gurrieri, S., Liuzzo, M., Burton, M., Caltabiano, T., McGonigle, A.J.S.,Salerno, G., Shinohara, H., Valenza, M., 2008. The total volatile flux from Mount Etna.Geophysical Research Letters 35, L24302.

Allard, P., Behncke, B., D'Amico, S., Neri, M., Gambino, S., 2006. Mt. Etna 1993–2005:anatomy of an evolving eruptive cycle. Earth-Science Reviews 78, 85–114.doi:10.1016/j.earscirev.2006.04.002.

Allen, A.G., Mather, T.A., McGonigle, A.J.S., Aiuppa, A., Delmelle, P., Davison, B.,Bobrowski, N., Oppenheimer, C., Pyle, D.M., Inguaggiato, S., 2006. Sources, sizedistribution, and downwind grounding of aerosols from Mount Etna. Journal ofGeophysical Research 111, D10302.

Andronico, D., Branca, S., Calvari, S., Burton, M., Caltabiano, T., Corsaro, R.A., Carlo, P.D.,Garfi, G., Lodato, L., Miraglia, L., Mure, F., Neri, M., Pecora, E., Pompilio, M., Salerno,G., Spampinato, L., 2005. A multi-disciplinary study of the 2002–03 Etna eruption:insights into a complex plumbing system. Bulletin of Volcanology 67, 314–330.

Azzaro, R., Branca, S., Giammanco, S., Gurrieri, S., Rosa, R., Valenza, M., 1998. Newevidence for the form and extent of the Pernicana Fault System (Mt. Etna) fromstructural and soil–gas surveying. Journal of Volcanology and Geothermal Research84, 143–152.

Bagnato, E., Aiuppa, A., Parello, F., Calabrese, S., D'Alessandro, W., Mather, T.A.,McGonigle, A.J.S., Pyle, D.M., Wangberg, I., 2007. Degassing of gaseous (elementaland reactive) and particulate mercury from Mount Etna volcano (Southern Italy).Atmospheric Environment 41, 7377–7388.

Bagnato, E., Aiuppa, A., Parello, F., D'Alessandro, W., Allard, P., Calabrese, S., 2009. Mercuryconcentration, speciation and budget in volcanic aquifers: Italy and Guadeloupe (LesserAntilles). Journal of Volcanology and Geothermal Research 179, 96–106.

Bagnato, E., Aiuppa, A., Parello, F., Allard, P., Shinohara, H., Liuzzo, M., Giudice, G., 2011.New clues on the contribution of Earth's volcanism to the global mercury cycle.Bulletin of Volcanology 73, 497–510.

Bargagli, R., Barghigiani, C., 1991. Lichen biomonitoring of mercury emission anddeposition in mining, geothermal and volcanic areas of Italy. EnvironmentalMonitoring and Assessment 16, 265–275.

Barghigiani, C., Bargagli, R., Gioffre, D., 1988. Mercury in the environment of theMt. Etna volcanic area. Environmental Technology Letters 9, 239–244.

Bash, J.O., Miller, D.R., 2009. Growing season total gaseous mercury (TGM) flux mea-surements over an Acer rubrum L. Stand. Atmospheric Environment 43, 5953–5961.

Bellomo, S., Aiuppa, A., D'Alessandro, W., Parello, F., 2007. Environmental impact ofmagmatic fluorine emission in the Mt. Etna area. Journal of Volcanology andGeothermal Research 165, 87–101.

Bloom, N.S., 1992. On the chemical form of mercury in edible fish and marine inver-tebrate tissue. Canadian Journal of Fisheries and Aquatic Sciences 49, 1010–1017.

Bobrowski, N., Honninger, G., Galle, B., Platt, U., 2003. Detection of bromine monoxidein a volcanic plume. Nature 42, 273–276.

Bonforte, A., Gugleilmino, F., Coltelli, M., Ferretti, A., Puglisi, G., 2011. Structuralassessment of Mount Etna volcano from Permanent Scatterers analysis. Geochem-istry, Geophysics, Geosystems 12, Q02002.

Burton, M., Neri, M., Condarelli, D., 2004. High spatial resolution radon measurementsreveal hidden active faults on Mt. Etna. Geophysical Research Letters 31, L07618.

Bushey, J.T., Nallana, A.G., Montesdeoca, M.R., Driscoll, C.T., 2008. Mercury dynamics ofa northern hardwood canopy. Atmospheric Environment 42, 6905–6914.

Calabrese, S., Aiuppa, A., Allard, P., Bagnato, E., Bellomo, S., Brusca, L., D'Alessandro, W.,Parello, F., 2011. Atmospheric sources and sinks of volcanogenic elements in abasaltic volcano (Etna, Italy). Geochimica et Cosmochimica Acta 75, 7401–7425.

Converse, A.D., Ricassi, A.L., Scanlon, T.M., 2010. Seasonal variability in gaseousmercury fluxes measured in a high-elevation meadow. Atmospheric Environment44, 2176–2185.

Crenshaw, W.B., Williams, S.N., Stoiber, R.E., 1982. Fault location by radon and mercurydetection at an active volcano. Nature 300, 345–346.

Davies, F., Notcutt, G., 1996. Biomonitoring of atmospheric mercury in the vicinity ofKilauea, Hawaii. Water, Air, and Soil Pollution 86, 275–281.

Delmelle, P., Delfosse, T., Delvaux, B., 2003. Sulfate, chloride and fluoride retention inAndosols exposed to volcanic acid emissions. Environmental Pollution 126,445–457.

Dunagan, S.C., Gilmore, M.S., Varekamp, J.C., 2007. Effects of mercury on visible/near-infrared reflectance spectra on mustard spinach plants (Brassica rapa P.).Environmental Pollution 148, 301–311.

Ericksen, J.A., Gustin, M.S., 2004. Foliar exchange of mercury as a function of soil and airmercury concentrations. Science of the Total Environment 324, 271–279.

Ericksen, J.A., Gustin, M.S., Schorran, D.E., Johnson, D.W., Lindberg, S.E., Coleman, J.S.,2003. Accumulation of atmospheric mercury in forest foliage. AtmosphericEnvironment 37, 1613–1622.

Fay, L., Gustin, M., 2007. Assessing the influence of different atmospheric and soilmercury concentrations on foliar mercury concentrations in a controlledenvironment. Water, Air, and Soil Pollution 181, 373–384.

Fitzgerald, W.F., Engstrom, D.R., Mason, R.P., Nater, E.A., 1998. The case for atmosphericmercury contamination in remote areas. Environmental Science and Technology32, 1–7.

Floor, G.H., Calabrese, S., Román-Ross, G., D'Alessandro, W., Aiuppa, A., 2011. Seleniummobilization in soils due to volcanic derived acid rain: An example from Mt. Etnavolcano, Sicily. Chemical Geology 289, 235–244.

Gauthier, P.J., Le Cloarec, M.F., 1998. Variability of alkali and heavy metal fluxesreleased by Mt. Etna volcano, Sicily, between 1991 and 1995. Journal ofVolcanology and Geothermal Research 81, 311–326.

Giammanco, S., Bonfanti, P., 2009. Cluster analysis of soil CO2 data fromMt. Etna (Italy)reveals volcanic influences on temporal and spatial patterns of degassing. Bulletinof Volcanology 71, 201–218.

Giammanco, S., Gurrieri, S., Valenza, M., 1997. Soil CO2 degassing along tectonicstructures of Mount Etna (Sicily): the Pernicana Fault. Applied Geochemistry 12,429–436.

Giammanco, S., Gurrieri, S., Valenza, M., 1999. Geochemical investigations applied toactive fault detection in a volcanic area: the North-East Rift on Mt. Etna (Sicily,Italy). Geophysical Research Letters 26, 2005–2008.

Giudice, R.L., Bonanno, G., 2009. Bioaccumulation of heavy metals in mosses from EtnaVolcano and Iblei Mountains (Eastern Sicily, Italy). Cryptogamie 30, 143–155.

Gramatica, P., Battaini, F., Giani, E., Papa, E., Jones, R.J.A., Preatoni, D., Cenci, R.M., 2006.Analysis of mosses and soils for quantifying heavy metal concentrations in Sicily: amultivariate and spatial analytical approach. Environmental Science and PollutionResearch 13, 28–36.

Graydon, J.A., St. Louis, V.L., Lindberg, S.E., Hintelmann, H., Krabbenhoft, D.P., 2006.Investigation of mercury exchange between forest canopy vegetation and theatmosphere using a new dynamic chamber. Environmental Science and Technol-ogy 40, 4680–4688.

Graydon, J.A., St. Louis, V.L., Hintelmann, H., Lindberg, S.E., Sandilands, K.A., Rudd, J.W.M.,Kelly, C.A., Hall, B.D., Mowat, L.D., 2008. Long-term wet and dry deposition of totaland methyl mercury in the remote boreal ecoregion of Canada. EnvironmentalScience and Technology 42, 8345–8351.

Gustin, M.S., Lindberg, S.E., Austin, K., Coolbaugh, M., Vette, A., Zhang, H., 2000.Assessing the contribution of natural sources to regional atmospheric mercurybudgets. Science of the Total Environment 259, 61–71.

Heiri, O., Lotter, A.F., Lemcke, G., 2001. Loss on ignition as a method for estimatingorganic and carbonate content in sediments: reproducibility and comparability ofresults. Journal of Paleolimnology 25, 101–110.

Jonnalagadda, S.B., Rao, P., 1993. Toxicity, bioavailability and metal speciation.Comparative Biochemistry and Physiology 106, 585–595.

Kabata-Pendias, A., 2001. Trace Elements in Soils and Plants. CRC Press, Boca Raton.Lindberg, S.E., 1996. Forests and the global biogeochemical cycle of mercury: the

importance of understanding air/vegetation exchange processes. In: Baeyens, W.(Ed.), Global and Regional Mercury Cycles: Sources, Fluxes and Mass Balances.Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 359–380.

Lindqvist, O., Rodhe, H., 1985. Atmospheric mercury — a review. Tellus 27B, 136–159.Lumsdon, D.G., Evans, L.J., Bolton, K.A., 1995. The influence of pH and chloride on the

retention of cadmium, lead, mercury and zinc by soils. Journal of Soil Contamination4, 137–150.

Martin, R.S., Mather, T.A., Pyle, D.M., Power, M., Allen, A.G., Aiuppa, A., Horwell, C.J.,Ward, E.P.W., 2008. Composition-resolved size distributions of volcanic aerosols inthe Mt. Etna plumes. Journal of Geophysical Research 113, D17211.

Martin, R.S., Mather, T.A., Pyle, D.M., Watt, S.F.L., Day, J., Collins, S.J., Wright, S.J., Aiuppa,A., Calabrese, A., 2009. Sweet chestnut (Castanea sativa) leaves as a bio-indicator ofvolcanic gas, aerosol and ash deposition onto the flanks of Mt. Etna in 2005–2007.Journal of Volcanology and Geothermal Research 179, 107–119.

Martin, R.S., Mather, T.A., Pyle, D.M., Day, J.A., Witt, M.L.I., Collins, S.J., Hilton, R.G., 2010.Major and trace element distributions around active volcanic vents determined byanalyses of grasses: implications for element cycling and biomonitoring. Bulletin ofVolcanology 72, 1009–1020.

Martin, R.S., Witt, M.L.I., Pyle, D.M., Mather, T.A., Watt, S.F.L., Bagnato, E., Calabrese, S.,2011. Rapid oxidation of mercury (Hg) at volcanic vents: insights from hightemperature thermodynamic models of Mt. Etna's emissions. Chemical Geology283, 279–286.

Mason, R., 2009. Mercury emissions from natural processes and their importance in theglobal mercury cycle. In: Pirrone, N., Mason, R. (Eds.), Mercury Fate and Transportin the Global Atmosphere. Springer, New York, pp. 173–191.

Mason, R.P., Fitzgerald, W.F., Morel, F.M.M., 1994. The biogeochemical cycling ofelemental mercury: anthropogenic influences. Geochimica et Cosmochimica Acta58, 3191–3198.

Mather, T.A., 2008. Volcanoes and the atmosphere: the potential role of the atmo-sphere in unlocking the reactivity of volcanic emissions. Philosophical TransactionsA 366, 4581–4595.

Morel, F.M.M., Kraepiel, A.M.L., Amyot, M., 1998. The chemical cycle and bioaccumula-tion of mercury. Annual Review of Ecology and Systematics 29, 543–566.

22 R.S. Martin et al. / Chemical Geology 310–311 (2012) 12–22

Moreno-Jimenez, E., Gamarra, R., Carpena-Ruiz, R.O., Millan, R., Penalcosa, J.M.,Esteban, E., 2006. Mercury bioaccumulation and phytotoxicity in two wild plantspecies of Almadén area. Chemosphere 63, 1969–1973.

Neri, M., Acocella, V., Behncke, B., 2004. The role of the Pernicana Fault System in thespreading of Mt. Etna (Italy) during the 2002–2003 eruption. Bulletin ofVolcanology 66, 417–430.

Obrist, D., Tas, E., Pelag, M., Matveev, V., Fain, X., Asaf, D., Luria, M., 2011. Bromine-induced oxidation of mercury in the mid-latitude atmosphere. Nature Geoscience4, 22–26.

Oppenheimer, C., Tsanev, V.I., Braban, C.F., Cox, R.A., Adams, J.W., Aiuppa, A.,Bobrowski, N., Delmelle, P., Barclay, J., McGonigle, A.J.S., 2006. BrO formation involcanic plumes. Geochimica et Cosmochimica Acta 70, 2935–2941.

Pirrone, N., Costa, P., Pacyna, J.M., Ferrara, R., 2001. Mercury emissions to theatmosphere from natural and anthropogenic sources in the Mediterranean region.Atmospheric Environment 35, 2997–3006.

Pirrone, N., Cinnirella, S., Feng, X., Finkelman, R.B., Friedli, H.R., Leaner, J., Mason, R.,Mukherjee, A.B., Stracher, G., Streets, D.G., Telmer, K., 2009. Global mercuryemissions to the atmosphere from natural and anthropogenic sources. In: Pirrone,N., Mason, R. (Eds.), Mercury Fate and Transport in the Global Atmosphere.Springer, New York, pp. 3–59.

Pokharel, A.K., Obrist, D., 2011. Fate of mercury in tree litter during decomposition.Biogeosciences 8, 2507–2521.

Pyle, D.M., Mather, T.A., 2003. The importance of volcanic emissions for the globalatmospheric mercury cycle. Atmospheric Environment 37, 5115–5121.

Quayle, B.M., Mather, T.A., Witt, T.A., Maher, B.A., Mitchell, R., Martin, R.S., Calabrese, S.,2010. Application and evaluation of biomagnetic and biochemical monitoring ofthe dispersion and deposition of volcanically-derived particles at Mt. Etna, Italy.Journal of Volcanology and Geothermal Research 191, 107–116.

Rasmussen, P., 1995. Temporal variation of mercury in vegetation. Water, Air, and SoilPollution 80, 1039–1042.

Schroeder, W.H., Munthe, J., 1998. Atmospheric mercury — an overview. AtmosphericEnvironment 29, 809–822.

Selin, N.E., 2009. Global biogeochemical cycling of mercury: a review. Annual Review ofEnvironment and Resources 34, 43–63.

Sholupov, S., Pogarev, S., Ryzhov, V., Mashyanov, N., Stroganov, A., 2004. Zeemanatomic absorption spectrometer RA-915+ for direct determination of mercury inair and complex matrix samples. Fuel Processing Technology 85, 473–485.

Siegel, B.Z., Siegel, S.M., 1982. Mercury content of Equisetum plants around MountSt. Helens one year after the major eruption. Science 216, 292–293.

Siwik, E.I.H., Campbell, L.M., Mierle, G., 2009. Fine-scale mercury trends in temperatedeciduous tree leaves from Ontario, Canada. Science of the Total Environment 407,6275–6279.

Slemr, F., Schuster, G., Seiler, W., 1985. Distribution, speciation and budget ofatmospheric mercury. Atmospheric Environment 3, 407–434.

Stamenkovic, J., Gustin, M.S., 2009. Nonstomatal versus stomatal uptake of atmospher-ic mercury. Environmental Science and Technology 43, 1367–1372.

Steffen, A., Douglas, T., Amyot, M., Ariya, P., Aspmo, K., Berg, T., Bottenheim, J., Brooks,S., Cobbett, F., Dastoor, A., Dommergue, A., Ebinghaus, R., Ferrari, C., Gardfeldt, K.,Goodsite, M.E., Lean, D., Poulain, A.J., Scherz, C., Skov, H., Sommar, J., Temme, C.,2008. A synthesis of atmospheric mercury depletion event chemistry in theatmosphere and snow. Atmospheric Chemistry and Physics 8, 1445–1482.

Suszcynsky, E.M., Shann, J.R., 1995. Phytotoxicity and accumulation of mercury intobacco subjected to different exposure routes. Environmental Toxicology andChemistry 14, 61–67.

Tanner, L.H., Smith, D.L., Allan, A., 2007. Stomatal response of swordfern to volcanogenicCO2 and SO2 from Kilauea Volcano. Geophysical Research Letters 34, L15807.

Tomiyasu, T., Okada, M., Imura, R., Sakamoto, H., 2003. Vertical variations in theconcentration of mercury in soils around Sakurajima Volcano, Southern Kyushu,Japan. Science of the Total Environment 304, 221–230.

Toutain, J.P., Quisefit, J.P., Briole, P., Aloupogiannis, P., Blanc, P., Robaye, G., 1995.Mineralogy and chemistry of solid aerosols emitted from Mt. Etna. GeochemicalJournal 29, 163–173.

Ullrich, S.M., Tanton, T.W., Abdrashitova, S.A., 2001. Mercury in the aquaticenvironment: a review of factors affecting methylation. Critical Reviews inEnvironmental Science and Technology 31, 241–293.

Varekamp, J.C., Buseck, P.R., 1981. Mercury emissions from Mount St. Helens duringSeptember 1980. Nature 293, 555–556.

Varekamp, J.C., Buseck, P.R., 1983. Hg anomalies in soils: a geochemical explorationmethod for geothermal areas. Geothermics 12, 29–47.

Varekamp, J.C., Buseck, P.R., 1986. Global mercury flux from volcanic and geothermalsources. Applied Geochemistry 1, 65–73.

von Glasow, R., 2010. Atmospheric chemistry in volcanic plumes. Proceedings of theNational Academy of Sciences of the United States of America 107, 6594–6599.

von Glasow, R., Bobrowski, N., Kern, C., 2009. The effects of volcanic eruptions onatmospheric chemistry. Chemical Geology 263, 131–142.

Watt, S.F.L., Pyle, D.M., Mather, T.A., Day, J.A., Aiuppa, A., 2007. The use of tree-rings andfoliage as an archive of volcanogenic cation deposition. Environmental Pollution148, 48–61.

Witt, M.L.I., Mather, T.A., Pyle, D.M., Aiuppa, A., Bagnato, E.M., Tsanev, V.I., 2008a.Mercury and halogen emissions from Masaya and Telica volcanoes, Nicaragua.Journal of Geophysical Research 113, B06203.

Witt, M.L.I., Fischer, T.P., Pyle, D.M., Yang, T.F., Zellmer, G.F., 2008b. Fumarolecompositions and mercury emissions from the Tatun Volcanic Field, Taiwan:results from multi-component gas analyser, portable mercury spectrometer anddirect sampling techniques. Journal of Volcanology and Geothermal Research 178,636–643.