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Bulletin of VolcanologyOfficial Journal of the InternationalAssociation of Volcanology andChemistry of the Earth`s Interior(IAVCEI) ISSN 0258-8900Volume 74Number 4 Bull Volcanol (2012) 74:861-871DOI 10.1007/s00445-011-0573-x

Time-dependent CO2 variations in LakeAlbano associated with seismic activity

G. Chiodini, F. Tassi, S. Caliro,C. Chiarabba, O. Vaselli & D. Rouwet

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RESEARCH ARTICLE

Time-dependent CO2 variations in Lake Albano associatedwith seismic activity

G. Chiodini & F. Tassi & S. Caliro & C. Chiarabba &

O. Vaselli & D. Rouwet

Received: 13 April 2011 /Accepted: 14 November 2011 /Published online: 1 December 2011# Springer-Verlag 2011

Abstract Lake Albano (Alban Hills volcanic complex,Central Italy) is located in a densely populated area nearRome. The deep lake waters have significant dissolved CO2

concentrations, probably related to sub-lacustrine fluid dis-charges fed by a pressurized CO2-rich reservoir. The ana-lytical results of geochemical surveys carried out in 1989–

2010 highlight the episodes of CO2 removal from the lake.The total mass of dissolved CO2 decreased from ∼5.8×107 kg in 1989 to ∼0.5×107 kg in 2010, following anexponential decreasing trend. Calculated values of both dis-solved inorganic carbon and CO2 concentrations along thevertical profile of the lake indicate that this decrease iscaused by CO2 release from the epilimnion, at depth <9 m,combined with (1) water circulation at depth <95 m and (2)CO2 diffusion from the deeper lake layers. According to thismodel, Lake Albano was affected by a large CO2 input thatcoincided with the last important seismic swarm at AlbanHills in 1989, suggesting an intimate relationship betweenthe addition of deep-originated CO2 to the lake and seismicactivity. In the case of a CO2 degassing event of an order ofmagnitude larger than the one that occurred in 1989, thedeepest part of Lake Albano would become CO2-saturated,resulting in conditions compatible with the occurrence of agas outburst. These results reinforce the idea that a suddenCO2 input into the lake may cause the release of a dense gascloud, presently representing the major volcanic threat forthis densely populated area.

Keywords Crater lakes . Limnic eruption . CO2 outburst .

Lake Albano

Introduction

Lake Albano, located 20 km SE of Rome, is a volcaniclake hosted within the youngest crater of the AlbanHills Quaternary volcanic complex (Fig. 1). The recentdiscovery of Holocene primary volcanic deposits demon-strates the persistence of eruptive activity up to recent times(Funiciello et al. 2002, 2003). This finding also indicates a

Editorial responsibility: M. Ripepe

Electronic supplementary material The online version of this article(doi:10.1007/s00445-011-0573-x) contains supplementary material,which is available to authorized users.

G. Chiodini : S. CaliroIstituto Nazionale di Geofisica e Vulcanologia,Osservatorio Vesuviano,Via Diocleziano 328,Naples, Italy

F. Tassi :O. VaselliDepartment of Earth Sciences, University of Florence,Via G. La Pira 4,50121 Florence, Italy

F. Tassi (*) :O. VaselliCNR-IGG Institute of Geosciences and Earth Resources,Via G. La Pira 4,50121 Florence, Italye-mail: franco.tassi@unifi.it

C. ChiarabbaIstituto Nazionale di Geofisica e Vulcanologia, CNT,Via di Vigna Murata 605,Rome, Italy

D. RouwetIstituto Nazionale di Geofisica e Vulcanologia,Sezione di Palermo,Via Ugo La Malfa 153,90146 Palermo, Italy

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potential volcanic hazard resulting from possible CO2 accu-mulation within the deep-water layers of Lake Albano(Funiciello et al. 2003). The lake has an ellipsoidal shape(3.5×2.3 km), a volume of 4.48×108 m3, a surface area ofabout 5.7 km2 and a maximum depth of 167 m. The deepestlayers of the lake can potentially host large amounts of CO2

and can trigger catastrophic events (limnic eruptions) similarto those that occurred atMonoun and Nyos lakes (Cameroon),in 1984 and 1986, respectively (Sigurdsson et al. 1987;Barberi et al. 1989; Giggenbach 1990; Evans et al. 1994;Rice 2000; Kusakabe 1996).

Previous studies have shown that the CO2 concentrationat Lake Albano increases with depth, although the maxi-mum measured CO2 concentrations are much lower than gassaturation (Martini et al. 1994; Cioni et al. 2003; Anzidei etal. 2008; Carapezza et al. 2008). More dangerous conditionsmay have occurred in the past. In the fourth century B.C.,Roman historians reported an episode of sudden lake over-flow that may have been caused by the injection of CO2-richfluids into the lake (Funiciello et al. 2002; Carapezza et al.2008). That event persuaded the Romans to keep the waterlevel 70 m below the lowest crater rim by means of adrainage tunnel. That event was probably last of a series ofoverflows, as suggested by deposits of Holocene lahars thatoverflowed the lake (Funiciello et al. 2003; Giordano et al.2005).

Intense seismic activity could theoretically represent crit-ical periods of CO2 release into the lake. Earthquakes cantrigger degassing from the buried structural high of thepermeable carbonate basement which is present beneaththe Albano area and which hosts a pressurized CO2-rich

reservoir (Chiodini and Frondini 2001). This scenario isnot unlikely, as this area is recurrently affected by seismicswarms that last from days to years (Amato et al. 1994;Amato and Chiarabba 1995; Chiarabba et al. 1997).

The last important seismic swarm occurred in 1989–1990in a NW–SE elongated area of 6×12 km2 with Lake Albanoin its center (Fig. 2). The aim of this paper is to provide anassessment of whether this seismic crisis caused a detectableand quantifiable variation of the CO2 content of the lake. Adecrease of the CO2 concentration was previously observedduring the period 1997–2006 and was attributed to lakewater overturn during the winter (Anzidei et al. 2008;Carapezza et al. 2008).

In this work, we computed the time-dependent CO2 var-iations for a longer period (1989–2010) in order to detail theeventual relationship of such a variation with the 1989–1990earthquakes. In particular, we discuss (1) the data fromliterature of nine geochemical surveys from 1989 to 2009,(2) the data of a new campaign performed at Lake Albano inFig. 1 Location of the Alban Hills Quaternary volcanic complex and

of Lake Albano

Fig. 2 Epicentral (a) and hypocentral (b) distribution of the best-located events recorded during the 1989–1990 seismic swarm (Amatoet al. 1994). The earthquake locations included in this figure have bothhorizontal and vertical computed errors less than 1 km. The red starsrefers to the five largest events of the swarm (MD≥3.6)

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May 2010, and (3) the first vertical profile of the carbonisotopic composition of the dissolved inorganic carbon(DIC).

The results show that since 1989 Lake Albano had a totalCO2 content of ∼5.8×107 kg, which has since decreased to0.5×107 kg (in 2010) following an exponential decay curve.This suggests that a large pulse of gas entered the lakeduring the 1989–1990 seismic swarm.

Geological and hydrogeological outlines

The Alban Hills are part of the high-K Roman Comag-matic Province (Gaeta et al. 2006; Boari et al. 2009).Volcanic activity started at 600 ka with the emplacementof an ignimbrite and subsequent caldera collapse. The lastactivity consisted in the formation of phreatomagmatic cra-ters. The polygenetic Albano maar, which hosts the homon-ymous lake, is the youngest of these craters (De Rita et al.1988; Giordano et al. 2006). Eruptions of the crater occurredfrom 69 to 36 ka (Freda et al. 2006) while the youngest lahardeposit originated at 5.8 ka (De Benedetti et al. 2008).Successively, Lake Albano was affected by dramatic levelchanges with occasional overflows similar to that describedby the Roman historian Titus Livius in 398 B.C. (e.g., DeBenedetti et al. 2008, and references therein). Funiciello etal. (2002) and Marra and Karner (2005) have describedsedimentary hiatuses and a variable CaCO3 isotopic com-position in the lake sediments to support Holocenic lakelevel fluctuations.

Since the 1960s, lake’s water level has progressivelylowered (average rate of the lake level drawdown 08.8 cm/year in the time span 1960–2005; Anzidei et al. 2008). Thelowering of lake level is attributed to excessive groundwaterwithdrawal (Capelli and Mazza 2005).

Three hydrogeological units were recognized in theAlban Hills (Boni et al. 1995). A multilayered aquifer ishosted within the volcanic units which overlie an aqui-clude related to Plio-Pleistocene sedimentary rocks (clayand marls). Below, a deep aquifer is hosted within theMesozoic–Cenozoic fractured limestone and dolomiteformations, which form the basement of the area.

Material and methods

This work is based on the results of a geochemicalsurvey carried out in May 2010 and on data fromliterature (nine campaigns performed from March 1989to September 2009). The list of available data is reported inTable 1 while the complete sets of the water chemistry areavailable in Table A of the Electronic supplementarymaterial.

May 2010 survey

The May 2010 geochemical survey included both (1) verti-cal profiles of temperature and pH (Fig. 3a), carried outusing a IDROMAR multi-parametric probe equipped witha Texas Instruments MSP430 processor, and (2) chemicaland isotopic (δ13CDIC) compositions of water and dissolvedgases of samples collected from the lake surface to thebottom every 10 m of depth (Figs. 4 and 5). The samplingequipment consisted of a plastic (Rilsan®) tube (6 mm indiameter) connected by steel connectors. Water was pumpedup through the sampling tube once it was lowered at thechosen depth by means of a 200-mL glass syringe anddirectly transferred into the storage containers after the dis-placement of a double water volume than the inner volume ofthe tube (about 0.03 dm3/m) (Tassi et al. 2009). The deter-mination of pH and the HCO3 concentrations (acidimetrictitration with 0.01 N HCl) were carried out in the field.Water samples were analyzed for major cations (Na, K,Ca, Mg) and anions (Cl, SO4, NO3) by atomic absorptionspectrophotometry (AAS; AAnalyst 100 PerkinElmer) andion chromatography (IC, Metrohm 761), respectively.

The dissolved gas samples were collected into one-way,pre-evacuated 250-mL glass flasks tapped with PTFE valves(Tassi et al. 2008, 2009). The analysis of CO2, N2, O2, andAr was carried out by gas chromatography (Vaselli et al.2006) using a Shimadzu 15A gas-chromatograph equippedwith a thermal conductivity detector. Methane was analyzedusing a Shimadzu 14A gas chromatograph equipped with aflame ionization detector. The complete composition of dis-solved gas compounds was calculated on the basis of theHenry’s law constants, regulating the liquid–gas equilibriumfor each volatile compound (Vaselli et al. 2006; Tassi et al.2008).

Furthermore, in the campaign of May 2010, separatesamples for the determination of δ13C of total DIC werecollected. All the dissolved carbon species were precipitatedas SrCO3 by adding to the water sample SrCl2 and NaOH insolid state. Carbonate precipitates were filtered and washedwith distilled water in a CO2-free atmosphere. Carbon iso-tope analyses were performed at the Geochemistry Labora-tory of INGV-Osservatorio Vesuviano using a FinniganDelta plusXP continuous flow mass spectrometer coupledwith a GasbenchII device (analytical errors: δ13C±0.06‰).Data of the water chemical and isotopic compositions arereported in Table 2.

Data from literature

The same water sampling technique of the May 2010 sam-ples was used for the April 1995, July 2000, March 2003,February 2007, and September 2009 surveys. In March1989 and August 1992, the sampling was carried out using

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a Niskin water sampler (Evans et al. 1993). Only pH andalkalinity determinations are available for the samples ofAugust 1992, April 1995, July 2000, March 2003, andFebruary 2007 (samples “pH-Alk” in Table 1), while thecomplete chemical composition of the waters is available forthe other campaigns (samples ‘C’ in Table 1).

CO2 and DIC computations

The DIC and the dissolved CO2 were computed for eachavailable sample based on pH, alkalinity, temperature, andconcentrations of major ions dissolved in the water (codePHREEQC, Parkhurst and Appelo 1999). When not avail-able, temperatures and major ion concentrations were esti-mated on the basis of the following assumptions: (1) watertemperature at depths >20 m was set at 9°C according to theaverage of the measured temperatures; (2) water strata shal-lower than 20 m were not considered for computation becauseat such depths the temperature is highly dependent on airtemperature; (3) Na, K, Mg, Cl, and SO4 concentrations,which do not show significant variations with time wereassumed equal to the average of the measured values (33.2,51.3, 17.0, 18.0, and 4.9 mg/L, respectively); (4) Ca con-centrations were computed as the difference between thesum of the main anions and of the main cations assumingelectrical neutrality for each analysis.

The uncertainties related to such assumptions were testedfor the complete datasets by comparing the measured DICvalues with those computed with the above assumptions.The results of this comparison show low uncertainties(<0.5% and <4% for DIC and CO2 values, respectively).

The subsequent step was the computation of the DIC andCO2 values every 10 m of depth for each survey by linearlyinterpolating the observed values (Tables B11 and B2 of theElectronic supplementary material).

Finally, the total amounts of dissolved CO2 and DIC forthe present and published profiles for the entire lake as wellas for different layers were computed by multiplying themean concentrations by the correspondent volume of waterderived from the depth–volume relation of Lake Albano (Fig. 6).

Results

Geochemical profiles of Lake Albano in May 2010and origin of the CO2

Lake Albano waters have a HCO3–Ca, Na, Mg, K compo-sition and TDS values ranging from 409 to 507 mg/L.According to Cioni et al. (2003), the similarity of the rela-tive concentrations of Na, K, Mg, and Ca in lake waters,local groundwaters, and local volcanic rocks indicates that

Table 1 Time series (1989–2009) of physical–chemical measurements available at different depths for Lake Albano

Depth March1989

August1992

April1995

December1997

July2000

July2001

March2003

February2007

September2009

May2010

0 C pH-alk pH-alk pH-alk pH-alk pH-alk C C

−10 C pH-alk pH-alk C pH-alk C pH-alk pH-alk C C

−20 C pH-alk pH-alk pH-alk pH-alk pH-alk C C

−30 C C pH-alk C C

−40 C pH-alk pH-alk pH-alk pH-alk pH-alk C C

−50 C pH-alk C C

−60 C pH-alk pH-alk pH-alk pH-alk pH-alk C C

−70 C C pH-alk C C

−80 pH-alk C C

−90 C pH-alk pH-alk C pH-alk C pH-alk pH-alk C C

−100 pH-alk C C

−110 C pH-alk pH-alk C pH-alk C pH-alk pH-alk C C

−120 pH-alk C C

−130 C C pH-alk C C

−140 C pH-alk pH-alk pH-alk pH-alk pH-alk C C

−150 C pH-alk C C

−160 C pH-alk pH-alk pH-alk pH-alk pH-alk C C

Ref. 1 2 3 4 3 5 3 3 3 6

References are: (1) Tassi (1990), (2) Guiducci (1993), (3) unpublished data available at the University of Florence, (4) Cioni et al. (2003), (5)Carapezza et al. 2008, (6) this work

C complete chemical analysis, pH-alk availability only of pH and alkalinity data

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the concentrations of the main cations are mainly governedby incongruent dissolution of local volcanic rocks.

The water temperature decreases from ambient values at thesurface (17.2°C) to a minimum of 8.7°C from a depth of ∼95mand beyond (Fig. 3a). This depth almost corresponds to thelimit depth of oxic/anoxic water, below which O2 disappearsand CH4 concentration increases by ∼2 orders of magnitude,marking the separation of two main water layers (layers Iand II in Figs. 3, 4, and 5). In layer II, significant increases ofCa, HCO3, DIC, and CO2 concentrations are also observed,while the concentrations of Mg, Na, K, SO4, Cl, N2, and Arremain rather constant through the water column (Figs. 4and 5). Furthermore in the shallower layer (layer I), the pHstrongly decreases (from 8.5 to 7.2), whereas in the deepestpart of the lake (layer II) a slightly decrease is observed(from 7.2 to 7) (Fig. 3a).

On the whole, the geochemical data describe a situationalready known at Lake Albano (Martini et al. 1994; Cioni etal. 2003; Carapezza et al. 2008), i.e., the presence of a CO2-rich deep-water layer. The novelty in this study is theisotopic composition of the DIC along the vertical pro-file. The δ13CDIC value decreases from ∼+4.8‰ at the lakesurface to +3‰ at the bottom of layer I; the latter valueremains almost constant in the deepest layer II. This stronglypositive isotopic signature of δ13CDIC points to an inorganicsource of CO2. Previous interpretations in studies thatlack isotopic data (Cioni et al. 2003), stated that carbongases (CO2 and CH4) mainly originate from decomposi-tion processes of organic matter at anoxic conditions insediments, followed by upward diffusion in the overly-ing waters. However, CO2 from microbial activity is typ-ically characterized by strongly negative isotopic values(δ13 C<−20‰) (Cerling et al. 1991).

It is worth noting that the measured DIC isotopiccomposition of Lake Albano is even more positive thanthe one expected for a deep inorganic source. For example,Lake Nyos and Lake Monoun (Cameroon), and Lake Averno(Italy) are characterized by δ13CDIC values of −3.4‰to −6.7‰ and −5.3‰ to −6.3‰, respectively (Kusakabe etal. 1989; Caliro et al. 2008).

At Alban Hills at least two deep inorganic sources cansupply gas to the lake: (1) a regional CO2 source, repre-sented by degassing processes of a mantle wedge enrichedin crustal fluids (Chiodini et al. 2004) and (2) at shallowerlevels, the interaction of magmas with limestone from thethick sedimentary carbonate basement that can produceadditional amount of CO2 (Iacono-Marziano et al. 2007) withan isotopic signature from −2‰ to +2‰ (e.g., Rollinson1993). The basement beneath Lake Albano is located over aNW-oriented carbonate basement structural high wheredeeply derived gases accumulate (Chiodini and Frondini2001). CO2-rich leakage from this buried structure suppliesgas to the lake and diffuse degassing areas at the AlbanHills, with an estimated CO2 flux of >500 t/day (Chiodiniand Frondini 2001).

The same NW trending structure also feeds the large gasemission at Cava dei Selci, a few kilometers NWof the lake,which is characterized by δ13CCO2 values +1.3‰. Takingthis latter composition as a good proxy for the original deepCO2 source, the more positive values measured at LakeAlbano suggest the occurrence of isotopic fractionationdue to the removal from the lake of isotopically lighterCO2 as expected during a degassing process.

Temporal variation of the CO2 content of the lakeand relation to earthquakes

Carbon dioxide removal from the lake in the last 20 years ishighlighted by the available field determination of pH and

Fig. 3 a Temperature (°C) and pH along the vertical profile of LakeAlbano measured in May 2010 using an Idromar multiprobe; b detailof the temperature profile of the first 20 m of the lake showing theposition of the thermocline

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alkalinity performed during the 10 surveys. All thedatasets (Table A of the Electronic supplementary material)are plotted against the depth in Fig. 7a (pH) and in Fig. 7b(alkalinity), where the different colors refer to the differentcampaigns. Each survey shows a general pH decrease andan alkalinity increase with depth in agreement with the CO2

vertical pattern, showing maximum concentrations in thedeepest layers. Time-related trends are also evident: thealkalinity has progressively decreased and the pH increasedat any depth (Fig. 7a, b). Considering that the alkalinity

(i.e., ∼ HCO3 concentrations) and pH are controlled by thedissolved CO2, these time-related variations clearly indicatethe occurrence of a process of CO2 removal from the lakefrom 1989 to 2010 in agreement with the indication from thecarbon isotopic composition.

The total mass of CO2 stored in the lake during eachsurvey was computed by (1) multiplying the CO2 concen-trations by the volume of the correspondent water level(Fig. 6, Table B of the Electronic supplementary material)and adding the different contributions.

The evolution in time of the total CO2 content of the lakeis compared with the seismic activity of the area. Seismicswarms at the volcano occur several times in a century.Periods of unrest, characterized by swarms and uplift, canlast for a few weeks up to years. The last unrest episodedates back to 1989–1990. Since then, there has been littleseismicity. The 1989–1990 seismic swarm consisted ofmore than 1,100 earthquakes with MD>1.5 and severalevents with MD of ∼4 (Amato et al. 1994). Data collectedby a temporary seismic network (Fig. 2) reveal that earth-quakes, confined in the uppermost 3–6 km of the crust,occurred within a Mesozoic sedimentary layer (Amato etal. 1994; Chiarabba et al. 1997, 2005) beneath the phreato-magmatic craters, i.e., the locus of the most recent volcanicepisodes (De Rita et al. 1988; Giordano et al. 2006). Thelargest earthquakes of this seismic crisis have normal fault-ing to strike-slip solutions coherent with a NE-trendingextension (Amato et al. 1994). Both the main events andthe rest of seismicity define two main clusters locatedaround the Lake Albano, surrounding a high Vp, high Vp/Vs body (Feuillet et al. 2004) interpreted as a fluid-filledover-pressured rock volume.

Fig. 4 Concentrations (in milligrams per kilogram) of the main ions (Ca, Na, K, Mg, SO4, Cl, HCO3), dissolved inorganic carbon (DIC hereexpressed as mg/kg of HCO3) and δ13CDIC values (in ‰ V-PDB) along the vertical profile of Lake Albano measured in May 2010

Fig. 5 CO2, CH4, N2, Ar, and O2 concentrations (in millimoles perkilogram) along the vertical profile of Lake Albano measured in May2010

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The maximum total CO2 content of the lake (∼5.8×107 kg) was registered in 1989, during this seismic activity(Fig. 8). In the succeeding years, characterized by lessseismic activity, the CO2 content decreased, reaching theminimum value of ∼0.5×107 kg in the 2010 survey (Fig. 8).We found that the CO2 removal is consistent with thefollowing exponential decay curve:

CO2 ¼ 6:017 � e�t=9:33 ð1Þ

Where CO2 is in kilograms, R200.98 and t in years from1989. Unfortunately, no CO2 data are available before 1989.

To explain the above processes, three different hypothe-ses are considered:

1. The CO2 removal from the lake started before 1989.Considering the exponential decay of the process, thishypothesis (line A in Fig. 8) would imply CO2 saturationconditions of Lake Albano at the end of 1960s;2. In the pre-1989 period, the lake was characterized by a highand constant CO2 content (line B in Fig. 8). This wouldimply a stationary CO2 flux from the lake bottom that wouldhave been stopped by the 1989–1990 seismic swarm;3. During the 1989–1990 seismic crisis, or shortly before, aCO2 pulse, of ∼5×107 kg, affected Lake Albano (line C inFig. 8).

This last case is considered to be the most probable, because(1) no evidences of lake gas saturation was reported during the1960s and (2) an increase in permeability, which could facilitatean increase in CO2 flux, is expected during seismic activityrather than a decrease of gas fluxes. Large pulses of CO2

associated with seismic activity have been observed in othervolcanoes, such as Mammoth Mt., California (Farrar et al.1995). Such a CO2 injection into Lake Albano during the1989–1990 seismic crisis was previously suggested byCarapezza et al. (2008). The temporal variation of the CO2

content of the lake reported in Fig. 8 provides direct evi-dence of such a seismicity-related degassing event.

Table 2 Water and dissolved gas compositions of Lake Albano in May 2010

Depth T (°C) pH Ca Mg Na K Alkalinity Cl SO4 δ13CDIC CO2 N2 Ar O2 CH4

0 17.2 8.32 24.8 16.4 34.9 48.9 269 17.5 6.3 4.77 0.02 0.57 0.014 0.229 nd

−10 15.7 8.40 24.7 16.3 34.7 55.3 275 16.7 6.8 4.72 0.03 0.66 0.017 0.150 nd

−20 10.0 8.20 25.0 16.5 35.4 51.7 276 17.2 6.1 4.27 0.05 0.70 0.017 0.114 nd

−30 9.4 7.84 25.3 16.4 35.3 51.1 279 16.7 6.1 3.77 0.15 0.75 0.018 0.102 nd

−40 9.2 7.73 24.8 16.1 34.7 52.0 280 16.5 5.4 4.39 0.17 0.78 0.019 0.077 nd

−50 9.1 7.71 24.7 16.2 35.4 50.8 281 16.9 5.2 3.83 0.19 0.69 0.016 0.066 0.0002

−60 9.1 7.53 25.3 16.4 35.2 51.2 282 16.9 5.5 3.69 0.21 0.73 0.017 0.011 0.0005

−70 9.0 7.50 26.3 17.0 34.6 50.5 281 16.5 5.0 3.75 0.27 0.75 0.018 0.012 0.0051

−80 9.0 7.35 25.6 16.2 36.6 49.8 282 17.1 5.3 3.05 0.42 0.82 0.020 0.007 0.0049

−90 8.8 7.20 26.5 16.8 35.7 50.6 294 16.7 5.3 3.02 0.66 0.84 0.020 0.003 0.0068

−100 8.7 7.22 29.3 16.7 35.7 49.1 303 16.8 5.4 2.83 0.79 0.75 0.018 0.002 0.082

−110 8.7 7.15 30.1 16.8 34.9 49.5 314 16.5 5.7 3.16 0.80 0.73 0.018 nd 0.142

−120 8.7 7.15 32.0 16.7 34.8 48.8 324 16.8 5.1 3.13 0.83 0.68 0.016 nd 0.223

−130 8.7 7.08 34.0 16.7 34.9 48.1 330 16.6 4.6 na 0.88 0.70 0.017 nd 0.312

−140 8.7 7.03 35.1 16.9 35.6 49.5 326 16.4 4.2 2.88 0.87 0.65 0.015 nd 0.408

−150 8.7 7.05 34.9 16.9 34.8 52.2 332 16.5 4.7 3.14 0.93 0.63 0.015 nd 0.449

−160 8.7 7.02 35.0 16.7 34.6 49.7 329 16.2 4.3 na 0.98 0.65 0.015 nd 0.484

Ion and dissolved gas concentrations are expressed in milligrams per kilogram and millimoles per kilogram, respectively. Alkalinity is expressed inmilligrams per kilogram of HCO3, δ

13 CDIC is in ‰ V-PDB

n.d. not detected, n.a. not analyzed

Fig. 6 Depth (meters) vs. water volume (cubic meter) of Lake Albano,computed from high-resolution bathymetry (Anzidei et al. 2008)

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A simple model for CO2 removal from 1989 to 2010

In order to investigate process by which CO2 was removedfrom 1989 to 2010, we developed a simple model (Fig. 9)that can explain the observed variation in the total mass ofcarbon dissolved in the lake, in layers I and II (Mtot, ML1,and ML2 respectively, expressed as kilograms of CO2). Weconsider that annually, during the winter season, the laketurns over because the shallowest layer (epilimnion, volumeVep) becomes colder and denser than the deepest waters.This mixing process however should only affect layer I(volume VL1) but not layer II which is characterized by thelowest and constant temperatures (Fig. 3a), anoxic condi-tions (Fig. 5), and the highest densities. During the overturn,

the DIC value of the epilimnion increases by mixing withthe water richer in carbon of layer I. This excess of carbon isthen released during the subsequent year of lake stability assuggested by the constant DIC value of the epilimnion,which during winter is estimated to be ∼4.7 mmol/L, basedon water samples at <20 m depth for each dataset. The massof carbon of layer I (ML1) is related to this cyclical carbonloss and to the continuous diffusive addition of CO2 fromlayer II, which is characterized by much higher carbonconcentrations. Coherently with this diffusive CO2 loss,the deepest layer II is affected since 1989 by an exponentialdecrease of the mass of dissolved carbon ML2 that, accord-ing to the best fit equation (Fig. 10), is given by:

ML2;n ¼ 1:5937� 107 � 4:7182 � 107 � eðt=5:98Þ ð2Þ

where t is the time expressed in years since January 1, 1989and the mass of carbon M in kilograms.

For each t year, the total mass of dissolved carbon oflayer I (ML1,t) is given by:

ML1;t ¼ VL1 � DICL1;t�1 � VL1 þ DICep � Vep

� �= VL1 þ Vep

� �

þ ML2;t�1 �ML2;t

� �ð3Þ

Finally, the total mass of dissolved carbon of the lake in the tyear (Mtot, t), excluding the mass of carbon dissolved in theepilimnion which remained constant during time, is given by:

Mtot;t ¼ ML2;t þML1;t ð4Þ

The results of the model obtained by solving Eqs. 2, 3,and 4, well fit the observed data (Fig. 10), suggesting thatthe main processes which control the CO2 removal fromLake Albano are diffusion from layers II to I, mixing of layer Iwith the epilimnion, degassing of the epilimnion to the atmo-sphere. In detail, the computations started with the initial

Fig. 8 Computed CO2 total contents (in kilograms) of the AlbanoLake in 1989–2010 and number of earthquakes per years (MD>2)occurred in the Alban Hills area from 1983 to 2010 (available fromISIDE, Italian Seismological Instrumental and Parametric Data-Base,INGV; http://iside.rm.ingv.it). Lines A, B, and C refer to differenthypothesis on the origin of the CO2 anomaly (see the text forexplanation)

Fig. 7 Measured a pH and balkalinity (expressed as milli-grams per kilogram of CO2)along the vertical profile of LakeAlbano in 1989–2010. The time-related decrease of alkalinityand increase of pH are caused byCO2 removal

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condition of 1989 (ML1,1010.647×107 kg, ML2,106.076×

107 kg) and were repeated varying Vep, i.e., the thickness ofthe epilimnion h, until a good fit with measured data wasobtained for h09 m (Fig. 9). It is worth noting that this valueof h, for which the model well reproduces the observed data,

is actually a good estimation of the thickness of the LakeAlbano epilimnion as measured in May 2010 (see Fig. 3b).

This is a simplified model which does not consider thelake level drawdown of 8.8 cm/year which was observed inthe time span 1960–2005 (Anzidei et al. 2008). The effectsof this process are negligible if the water lost by the lake isthat of the layer I which is poor in CO2 (see dashed lines inFig. 10). On the contrary, if the water lost by the lake is thatof the CO2-rich deepest strata, i.e., at depths >95 m, theeffect would be significant. However, the very positiveδ13CDIC of the waters indicates that the CO2 removal occurswith isotopic fractionation, which is consistent with thedegassing process considered in our model. If CO2 wereremoved during a water loss through the lake bottom, noisotopic fractionation would be expected.

Discussions and conclusions

Different geochemical surveys at Lake Albano havehighlighted a time-dependent loss of CO2 which decreasedfrom ∼5.8×107 kg in 1989 to ∼0.5×107 kg in 2010. TheCO2 removal follows an exponential decrease suggestingthat the lake was affected in 1989, or shortly before, by alarge pulse of CO2 which was likely coeval with the lastimportant seismic swarm in the area. The occurrence of sucha geothermal degassing event, injecting CO2 into the lake, isconsistent with the general volcano-tectonic setting of theregion. Powerful deep sources of gas are indeed present inthe subsoil of Alban Hills including both a regional deepCO2 source (Chiodini et al. 2004) and, at shallower levels,limestone assimilation by magmas (Iacono-Marziano et al.2007). This deep-originated CO2, during the migration to-ward the surface is trapped at the top of the carbonatebasement and then released into the atmosphere throughgas emissions and diffuse degassing processes (estimatedin at least ∼500 t/day) (Chiodini and Frondini 2001). LakeAlbano is located over a NW trending structural high of thecarbonate basement, which acts as a trap for the deep gasmigrating toward the surface (Chiodini and Frondini 2001).Most of the 1989–1990 earthquakes occurred within this gasreservoir. Although modeled in the past as the response of amagmatic supply intruded at a depth of 6 km (Chiarabba etal. 1997), more recent tomographic images (Feuillet et al.2004) and analogy with the Phlegrean fields Quaternaryvolcano (Avallone et al. 1999; Chiarabba and Moretti2006) indicate that the 1989–1990 unrest can be alsoexplained by the overpressure of a fluid reservoir hostedwithin the Mesozoic layer. Gas accumulation in the buriedstructure acting as a trigger of the seismic events seemsconsequently a process more realistic than an external trigger(tectonic or magmatic) which could have favored the suddenrelease of the gas and the CO2 anomaly in the lake. Whatever

Fig. 9 Conceptual model of CO2 removal from Lake Albano. TheCO2 (1) diffuse from layers II to I, (2) is transferred by mixing fromlayer I to the epilimnion, (3) is degassed from the epilimnion to theatmosphere

Fig. 10 Total mass of carbon (in kilograms of CO2) dissolved in thelake (a) and in layers I and II (b) compared with the results of a simplediffusion-mixing model. The model was repeated varying the thicknessof the epilimnion h (lines 5, 9, 15 m). A good fit with measured datawas obtained for h09 m. The dashed lines show the results of themodel considering a drawdown of the lake level of 8.8 cm/year(Anzidei et al. 2008) and that the water is lost by layer I (see the textfor explanation)

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is the cause–effect relationship between earthquakes anddegassing, the co-occurrence of seismic activity and degass-ing episodes is a well-known phenomenon at Alban Hills:according to Funiciello et al. (2003), up to 23 events ofanomalous hydrothermal activity (gas emission, temperaturesof waters, and so forth) seem to have occurred from 1754 to2000 in correspondence with the recurrent seismic crisis.

Currently, the input of deeply derived CO2 in Lake Albanois very low, or absent, preventing any dangerous gas accu-mulation in the deepest part of the lake. The latest datacollected during May 2010, being in agreement with previ-ous works, also show that CO2 concentrations at all depthsare much lower than gas saturation. However, the phenom-ena observed in 1989 highlights the possible occurrence ofmore dangerous scenarios during periods of intense seismicactivity. A CO2 degassing event of 1 order of magnitudelarger than that of 1989–1990 would saturate the deepestpart of Lake Albano causing a very dangerous condition.This condition was for example the main cause of the LakeNyos episode on August 21, 1986. At Lake Nyos, particularmeteorological conditions (heavy rain, low air temperature)caused convective overturn of the lake and both a waterwave as high as 75 m (Freeth and Kay 1987) and the releaseof a volume of CO2 vapor estimated from 0.15 to 1 km3

(Evans et al. 1994; Tuttle et al. 1987). The dense cloud of CO2

vapor suffocated more than 1,700 people and an uncountednumber of animals in just one night (Sigurdsson et al. 1987;Barberi et al. 1989; Giggenbach 1990; Evans et al. 1993;Rice 2000; Kusakabe 1996). At Albano, an area much moredensely populated than Nyos, a similar episode would prob-ably cause more catastrophic effects.

Such large degassing events in the past are inferred tohave triggered repeated Holocene overflows of the lake,including the event of the fourth century B.C. Finally, ourwork reinforces the idea of Funiciello et al. (2003) that thesudden input into the lake of CO2-rich fluids and the releaseof a dense gas cloud is presently the most important volca-nic hazard for this area.

Acknowledgments This work was partly supported by the Project ofCiudad de la Energia (Spain) and by MIUR/PRIN 2008,2008S889Y8R Project. C. Cigolini and S. Hurwitz are warmly thankedfor the comments and suggestions they provided in an early version ofthe manuscript.

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