CO2 release variations during the last 2000 years at the Colli Albani volcano (Roma, Italy) from speleothems studies

Earth and Planetary Science Letters 243 (2006) 449 – 462 www.elsevier.com/locate/epsl

CO2 release variations during the last 2000 years at the Colli Albani volcano (Roma, Italy) from speleothems studies P. Tuccimei a,b,⁎, G. Giordano a , M. Tedeschi a Dipartimento di Scienze Geologiche, Università “Roma Tre”, L.go San Leonardo Murialdo 1, 00146 Roma, Italy Istituto di Geologia Ambientale e Geoingegneria, Sezione di Roma “La Sapienza”, P. A. Moro 5, 00195 Roma, Italy

a b

Received 4 July 2005; received in revised form 21 December 2005; accepted 4 January 2006 Available online 20 February 2006 Editor: G.D. Price

Abstract The Colli Albani is the quiescent volcano that dominates the southwestern skyline of Roma (Italy). The last eruption occurred during the Holocene, from the eccentric Albano maar, along its western slope. The volcano is presently affected by cyclic seismic swarms, ground uplift and diffuse CO2 degassing. The degassing has caused several deadly incidents during the last years and constitutes a major civil protection concern, as the volcano slopes are densely inhabited. Nevertheless, the volcano does not have a permanent monitoring network, and the background level and anomalous CO2 levels, the relationship between the gas release and the seismic and ground deformation activity at the Colli Albani are still to be defined. The aim of this work is to define the historical record of CO2 release. Evidences of deep CO2 periodic release during the last 2000 years in the area of Colli Albani volcano (Roma, Italy) are offered from speleothems studies. A Roman-age stone mine, now used for mushroom cultivation, is decorated with actively growing speleothems, characterised by depositional hiatuses. Different levels of four stalactites, separated by depositional unconformities, and several samples from a single depositional cycle belonging to a stalagmite have been dated by U/Th method and analysed for their O and C isotopic composition. Eight cycles of deposition have been identified from 90–110 A. D. to 1350–1370 A.D., some of which are recognised across different speleothems. The age gap dividing different growth layers is in the order of one to few hundred years giving a temporal span for periodic interruption of speleothems deposition. O and C isotopic analyses performed on the samples collected from a single cycle (the oldest) have shown that the composition of the mother solutions was initially mainly meteoric and that a progressive increase in the input of a deep component rich in CO2 (up to a proportion of 20–30%) occurred just before the interruption of the speleothem deposition. This could be due to a progressive increase of the acidity of the water solutions that caused the undersaturation of fluids. If we extrapolate this mechanism to the other cycles of deposition, being characterised by analogue isotopic compositions, we can hypothesise periods of deposition interrupted by episodes of CO2 release which in the Colli Albani volcano are often recorded in coincidence with earthquakes. Therefore we have correlated the hiatuses with some of the largest historical earthquakes interesting to the city of Rome. © 2006 Elsevier B.V. All rights reserved. Keywords: CO2; hazard; Colli Albani volcano; Roma; Italy; speleothems; U/Th dating; stable isotopes; water chemistry

1. Introduction ⁎ Corresponding author. Dipartimento di Scienze Geologiche, Università “Roma Tre”, L.go San Leonardo Murialdo 1, 00146 Roma, Italy. Tel.: +39 06 54888092; fax: +39 06 54888201. E-mail address: [email protected] (P. Tuccimei). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.01.009

Most of the active or dormant volcanoes around the world are affected by localised and diffuse degassing phenomena which can create health hazard and disaster

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potential (see for example Furnas volcano, São Miguel, Azores) [1]. Catastrophic gas exsolution may also occur if deep waters of crater lakes which have accumulated gases rich in CO2 and other carbon components are shifted towards the surface by an external cause (earthquakes, landslides, cold rains, strong winds) as occurred in the 1980s from the Cameroonian Lakes Nyos and Monoum [2 and references therein]. Moreover, magma degassing may be a trigger of bradyseismic events as in the case of Phlegrean Fields, as well as at other calderas in the world, such as Long Valley, Yellowstone and Rabaul [3]. These examples make clear that a detailed knowledge of degassing phenomena in active volcanic areas is very important to prevent different kinds of hazard. The Colli Albani is a quiescent volcano [4] located nearby the city of Roma. The presence at depth of recent magma chambers favours the occurrence of perivolcanic phenomena related to the available thermal

budget which triggers seismic swarms, ground deformation [5,6] and upwelling of deep seated magmatic fluids [7,8]. The most important areas with diffuse degassing and hydrothermal activity at the Colli Albani are (Fig. 1) [[9] and references therein, [10,11,7,8]]: 1) Acque Albule basin; 2) Cava dei Selci-Lago Albano; 3) Trigoria-Acqua Acetosa Laurentina; 4) La Zolforata; 5) Ardea; 6) Tor Caldara. On the origin of the CO2 there is still a lot of debate among authors, but there is a general agreement that most of the diffuse degassing is linked to the presence at shallow depth of thick successions of Mesozoic–Cenozoic carbonates that have hosted and still host most of the magma chambers of the Quaternary peri-Tyrrheninan volcanoes of Italy. The release of CO2 has caused several deadly incidents during the last few years and is therefore a major concern for civil protection. There are several cases recorded in the historical and modern literature that testify to a sudden rise of water temperatures up to 50–100 °C,

Fig. 1. Digital elevation model (DEM) of Colli Albani volcano, with gravimetric residual anomalies (contour interval 1 mGal) [55]. The map highlights the most important areas with diffuse degassing and hydrothermal activity (closed circles): 1) Acque Albule basin; 2) Cava dei Selci-Lago Albano; 3) Trigoria-Acqua Acetosa Laurentina; 4) La Zolforata; 5) Ardea. The location of Roma, Colli Albani caldera, Lago Albano, Ciampino positive gravimetric anomaly and “Grotta Centroni” cave (open circle) is also shown.

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and gas bursts from the Colli Albani region [4]. The same authors have proposed that the more or less steady degassing may increase in correspondence with local and tele-seismic events which promote changes in the vertical hydraulic trasmittance, inducing large volumes of hot water or vapour and CO2 to rise into the shallow aquifers. Sudden hot fluid injection is described in the ancient literature also in Albano Lake; these phenomena may be responsible for possible roll over processes affecting the lacustrine water volume described in [4,8]. The studies conducted on the gas emission at the Colli Albani [[9] and references therein [10,11,7,8]] have illustrated the intense gas diffusion and the possible relationships with the main tectonic lineaments of the volcanic substrate but have not clarified nor quantified, due to the lack of a monitoring network, what at the Colli Albani has to be considered background, what anomaly and if there is any cyclicity in the gas emission rate or particular connection with geophysical phenomena that interest the region. This study is aimed at tracking the historical record of CO2 emission at the Colli Albani, by studying the age and the C and O isotopic composition of some carbonatic speleothems discovered in a Roman-age stone mine along the volcano slope. The speleothems precipitates from oversaturated groundwaters and show an internal stratigraphy made by intermittent depositional and nondepositional periods that we will demonstrate as due to periodic variations of deep CO2 release. 2. Activity of the Colli Albani volcano The Colli Albani (Fig. 1) is a quiescent volcano, which started its activity approximately at 600 ka [12]. The chemical composition of its products throughout its volcanic evolution is remarkably constant and mafic (≤50% SiO2; [13,14]). The volcano is made of the superposition of different edifices or lithosomes [15]. The older lithosome is a 1600 km2 plateau with a central caldera, named Vulcano Laziale, and is mainly constituted by large volume low aspect ratio ignimbrites (N350 ka; 10–100 km3 in volume), to which the caldera is related. After the eruption at approximately 350 ka [16,17] of the last large volume ignimbrite, the Villa Senni ignimbrite [12,18,19], two complex edifices were built within the caldera area: 1) the Tuscolano– Artemisio (TA) lithosome forms a horse-shoe shaped morphology (Fig. 1), made by coalescing fissurerelated lava flows, interbedded with scoria cones; the fissure system was fed by regionally controlled, pericalderic fractures and forms two distinct segments, one WNW-trending and one NE-trending, which form a

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sharp edge; 2) the Faete lithosome is a steep-sided stratovolcano which infilled the caldera (Fig. 1) and presently reaches 949 m a.s.l. The Tuscolano–Artemisio and the Faete edifices are partly interfingered and were emplaced between approximately 350 and 250 ka [16,20,21,17]. Their products indicate a remarkable reduction of erupted volumes with respect to the Vulcano Laziale lithosome and a change in eruption style to effusive and mildly explosive [15]. The most recent and still active, although quiescent, period of activity of the volcano has been characterised by eccentric phreatomagmatic activity, which has formed several overlapping monogenetic and polygenetic maars and tuff cones located along the western and northern slopes of the volcano. The products of this period of activity form the Via dei Laghi lithosome [15]. The older deposits have been dated at approximately 200 ka (age of the products from Ariccia maar; [17]. The most recent products, distributed around the Albano maar, have been only recently discovered, described and 14 C dated to the Holocene (Tavolato succession; [4]). The localisation of the maars, is related to the presence at relatively shallow depth (below 1–2 km) of the aquifer hosted in the Ciampino carbonatic horst [22], according to gravimetric data [23] and seismic data [24]. The intense diffuse CO2 degassing of the area is most likely related to the interaction of the magmatic bodies with the carbonates at depth [10,7,8]. The uprise of CO2 has been indicated as the main trigger for lake rollovers and overflows from the Albano maar crater, documented to have occurred during the Holocene and likely till ancient Roman times [4,25]. The discovery of previously unknown Holocene volcanic deposits has allowed the redefinition of the Colli Albani volcano to an active but quiescent volcano [4], making the study and the interpretation of the CO2 anomalies very important for civil protection purposes. 3. The Roman-age stone mine and its speleothems A Roman-age stone mine (Grotta Centroni), consisting of a network of galleries opened about 2000 years B. P. for the extraction of building materials for a residential house, is located to the SE of Roma, along the Anagnina statal road. The stone mine, now used for mushroom cultivation, is situated within the “tufo di Villa Senni” ignimbrite, the last caldera-forming ignimbrite erupted during the period of the Vulcano Laziale lithosome [15]. The roof of the stone mine is mostly made by the base of “Villa Senni” lava flow [13] belonging to the “Faete” lithosome. The “Villa Senni” lava flow, an augite-nepheline leucitite, is few metres

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thick above the mine and hosts a small aquifer containing supersaturated water with respect to calcium-carbonate. In their publication, Fornaseri et al. [13] reported the occurrence of an incrustating spring, locally called “Acqua Acetosa”, few metres downflow from the lava outcrop margin. Where the stone mine is covered by the lava flow, the waters circulating in the lava drip down from the roof, loosing CO2 and depositing abundant speleothems (Fig. 2). Most of them are stalactites because the stone mine floor has been cleared and is now covered by the cases where mushrooms grow. Stalactites mantle almost completely the galleries' roof and reach lengths of up to 10–15 cm and diameters of few centimetres. Four stalactites and a stalagmite have been collected in two different galleries and sectioned for studying their stratigraphy and selecting specimens for the analyses. 4. Materials and methods 4.1. Speleothems stratigraphy and deposition hiatuses The speleothems have been cut transversely relative to the growth axis close to the base (where the section was larger) using a diamond saw and the obtained sections have been accurately examined. The most relevant features were the unconformities separating the growth layer of all the speleothems, suggesting that there have been periodical interruptions of the deposition process during the last 2000 years, the age of the stone mine quoted from archeological evidences. The possible explanation of that, besides a strong periodic reduction of the rainfall amount, not recorded in any published pollen or isotopic record from Roma and

Central Italy [26–29], could be the periodic acidification, and in turn, the undersaturation of mother solutions by a dissolved CO2 increase, in a region of diffuse and localised degassing processes. Microscope studies have shown that the speleothems are entirely calcite and no aragonite is present. The hiatuses do not show evident signs of selective dissolution. 4.2. Selection of samples for MC-ICP-MS U/Th dating and O and C isotopic composition analyses In order to date the periods of speleothems growth and the phases of no deposition, seven samples from different growth layer of three stalactites (FS2, FS7, FS8) have been selected, taking care to choose growth layers limited by clear deposition hiatuses. In addition, with the purpose of calculating the duration of deposition processes, two couples of subsamples, each consisting of an inner and an outer subsample, have been extracted from two growth layers of two stalactites (the already mentioned FS7 and another one, FS9). Analogously, four subsamples from a single level of a stalagmite (FS6) have been selected (Fig. 3). The collection of samples has been carried out using a motor drill equipped with very subtle stainless steel points. The selection of four subsamples from the same layer has been possible only for stalagmite FS6 because growth levels of other speleothems were not large enough. The same samples and further fourteen from the same speleothems (Fig. 3) were selected for the analyses of their oxygen and carbon isotopic composition in order to document the origin and nature of mother solutions and any eventual isotopic trends within a single cycle (mainly from the stalagmite data). Samples have been labelled with the name of the speleothem, followed by a progressive number defining the growth layers, from the inner to the outer. In the case of layers FS6-1, FS7-1 and FS9-2, subsamples belonging to the same layer have been named, from the inner to the outer, with the letters of the Latin alphabet, added at the end of the label defining the relative growth layer. Finally, further seven subsamples have been selected from the inner growth layer of the stalagmite (FS6-1) using the mirror section of that previously consumed. The latter subsamples have been identified with Greek alphabet letters. 4.3. MC-ICP-MS U/Th dating

Fig. 2. The roof of “Grotta Centroni” decorated with abundant actively growing stalactites.

The 230Th/234U method is the most widely used dating technique applied to speleothems and is based on the extreme fractionation of the parent isotopes (238U and 234U) from their long-lived daughter 230 Th in

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Fig. 3. Transverse sections of stalagmite FS6 (two mirror slices) and stalactites FS2, FS7, FS8 and FS9 with location of studied samples. All of them have been analysed for the O and C isotopic composition and samples identified with a star have been also U-series dated. For sample labels see text (Section 4.2).

the hydrosphere. Uranium, markedly more soluble than thorium in the surface and near-surface environments, is readily mobilised as the highly soluble uranyl ion (UO22+) and its complexes, whereas thorium is easily hydrolised and precipitated or adsorbed on detrital particulates. Uranium and thorium concentrations in speleothems generally reflect relative abundances in the hydrosphere. Uranium is co-precipitated with CaCO3 on exsolution of CO2, while thorium is essentially absent and forms in situ by radioactive decay of co-precipitated U. In a closed system the extent to which the (230Th / 234 U) activity ratio has returned to unity is a function of time, taking into account also the state of disequilibrium between 234U and 238U [30]. If a detrital component is associated with the authigenic phase of calcite, a significant amount of thorium (232Th and 230 Th not produced by radioactive decay of uranium) is present. The extent of this Th contamination affects the (230Th / 234 U) activity ratio and thus the age of the speleothem and needs to be corrected for. Various methods are available to account for the presence of inherited thorium, all of which use the detrital 232Th as index of contamination. Total dissolution and isochron methodologies [31,32] are to be preferred when speleothems have observable differences in the amount of detrital material along episodes of continuous growth because, in contrast to leach–leach or leach–residue methodologies [33,34], total dissolution technique is not affected by isotope preferential leaching or readsorption of thorium onto detrital material. An alternative method assumes that the initial (230 Th / 232Th) activity ratio in

the sample is equal to that of the detrital fraction and on this basis corrects the (230 Th / 234U) activity ratio and thus the age [30]. Aliquots of 200 mg of powdered samples were spiked with about 0.1 g of a mixed 236U + 229Th spike solution and dissolved in 7.5 N HNO3. 236 U and 229 Th concentrations in the tracer solution were equal to 4.84 and 0.0457 pmol/g, respectively. The sample solutions were directly loaded onto anion exchange columns (Dowex 1 × 8) to separate U from Th. The following procedure of chemical purification has been designed by Jan Kramers (Institute of Geological Sciences, University of Bern) in order to improve the chemical yield of analyses. Uranium was eluted with 7.5 N HNO3 and thorium with 2 N HCl. The U fractions were dried, redissolved with 0.2 M H2SO4 and centrifuged in order to discard precipitated matrix elements (Ca, Mg etc.). Centrifuged solutions were then reloaded onto the anion exchange columns, washed with 0.2 M H2SO4, collected with 0.5 HNO3 and dried. The Th fractions were dried, redissolved in a mixture of 7.5 N HNO3 and H2O2 to fully oxidise the organic matter and evaporated. Finally, purified U and Th fractions were redissolved with about 0.5 mL of nitric acid and measured at the Laboratory for Isotope Geology (Institute of Geological Sciences), University of Bern (Switzerland) on “Nu Plasma” (Nu Instruments Ltd), an inductively coupled plasma double focusing magnetic sector multicollector mass spectrometer, equipped with three ion-counting electron multipliers, one of them placed behind a retardation (WARP) filter. Techniques for MC-ICP-MS

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analysis are based on those of [35–37] and are fully detailed in [38]. In this study, such analyses were performed by aspirating U and Th through a Cetac Aridus desolvating nebulizer system into Nu MC-ICP-MS, which provides ion yields for both elements of ∼1‰. Full details for the MC-ICP-MS analysis are given below. The program measures 234 U, 235 U, 236 U and 238 U in static mode. 234U and 236 U are measured in ion counters. Measured 238U/235 U was compared to its true value of 137.88 to correct for mass discrimination. Mass discrimination was very stable during a single day, and from day to day, at 7–8‰ per amu. Ion counters gain calibration is done using the U050 standard (NIST), analysed either side of each sample at similar intensities. Ion counter gain was always better than 98% and remained stable during a day's analysis. In order to cancel out any memory effect, a 5–10 min wait between individual runs is required during which the system is flushed with clean 0.5 N nitric acid. For Th runs, the 235 U / 238 U ratio is used for fractionation correction and CRM-145 standard (formerly U-960) must be added to the sample. The program has two cycles in which 229Th and 230 Th are alternatively measured in the WARP-equipped multiplier which was calibrated with an in-house Th standard, Th from MOSS, best diluted 1 : 50 in 0.5 N HNO3. It has a 230Th / 232 Th ratio of 1.560 (± 0.003) × 10− 4 (1 σ, 24 runs). In order to prevent damage to the electron multipliers due to the high 232 Th beam, the 230Th / 232 Th ratio is not measured directly, but is calculated via 238U. If samples with little 232 Th are run after such a gain calibration run, it is important to allow time to let the 232 Th memory die

down. This mostly takes at least 10 min and can be helped by nebulizing some 2 N HCl for a while. Activity ratios for MC-ICP-MS data were calculated with the decay constants described in [39]. The age and the initial (234U / 238U) activity ratios of all samples were calculated by means of ISOPLOT, a plotting and regression program designed by [40] for radiogenicisotope data. U-series data are reported in Table 1. Errors are always quoted as 2σ. 4.4. Stable isotopes Carbon and oxygen isotope ratios were determined by mass spectrometry analysis (Finnigan MAT 252) of carbon dioxide obtained by reacting the calcite samples with 100% H3PO4 at 25 °C [41] and the results reported in the conventional δ ‰ notation; the reference standard is V-PDB for carbon and oxygen, but oxygen data have been also referred to V-SMOW, as required in the geothermometric calculations. They are based on the principle that, when studying the isotopic fractionation between two coexisting phases, the heavier isotope is preferentially concentrated or retained in the denser phase. The amount of such fractionation is determined by the ambient temperature. 4.5. Water chemistry Temperature, pH, electrical conductivity (EC) and alkalinity of present dripping waters were measured in the field using electric probes and, for the alkalinity, by titration with 0.02 N HCl. The measurements of pH were

Table 1 Results of U/Th analyses of speleothems Sample a

ppb U

FS2-1 FS2-3 FS2-4 FS6-1a FS6-1b

4508 ± 11 7844 ± 20 1647 ± 4 21,172 ± 58 22,599 ± 65

FS6-1c FS6-1d FS7-1a FS7-1d FS7-2 FS8-2 FS8-3 FS8-4 FS9-2b FS9-2c

21,734 ± 64 31,483 ± 88 2914 ± 8 3703 ± 10 10,011 ± 27 1624 ± 4 1814 ± 5 2460 ± 6 14,948 ± 40 5589 ± 15

(234U/238U) 1.043 ± 0.001 1.054 ± 0.001 1.071 ± 0.001

(230Th/232Th) 4.26 ± 0.04 6.11 ± 0.09 4.71 ± 0.10 8.40 ± 0.07 9.98 ± 0.09

1.0393 ± 0.0007 b

a b

1,044 ± 0,0006 1.0458 ± 0.0004 1.0458 ± 0.0007 1.037 ± 0.001 1.0385 ± 0.0007 1.0458 ± 0.0007 1.0348 ± 0.0009 1.0421 ± 0.0005

27.14 ± 0.27 69.36 ± 0.59 8.30 ± 0.09 1.68 ± 0.02 2.61 ± 0.03 1.49 ± 0.02 1.26 ± 0.01 1.34 ± 0.03 1.00 ± 0.02 8.73 ± 0.10

For details on samples label, see text ( Section 4.2). These data refer to the 232Th-free authigenic CaCO3 endmember of layer FS6-1. See Fig. 4.

(230Th/234U)

Age (years)

0.0135 ± 0.0002 0.0097 ± 0.0001 0.0059 ± 0.0001

1480 ± 12 1060 ± 15 641 ± 14

0.0173 ± 0.0001 b

1910 ± 11 b

0.0142 ± 0.0001 0.0141 ± 0.0001 0.0135 ± 0.0001 0.0116 ± 0.0002 0.0108 ± 0.0001 0.0067 ± 0.0001 0.0097 ± 0.0002 0.0098 ± 0.0001

1560 ± 15 1550 ± 16 1490 ± 12 1270 ± 19 1190 ± 12 740 ± 16 1070 ± 20 1070 ± 11

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Fig. 4. (230Th/232Th) versus (234U/232Th) (A) and (234U/232Th) versus (238U/232Th) (B) isochrone diagrams of subsamples FS6-1a, FS6-1b, FS6-1c and FS6-1d from the inner growth layer of stalagmite FS6. The gradients of the straight lines that best estimate the relationships in subsamples data of diagram A) and B) are respectively the (230Th / 234U) and (234U / 238U) activity ratios of 232Th-free authigenic CaCO3 endmember. The intercept on the Y axis of the straight line in diagram A) is the (230Th / 232Th) activity ratio in the detritus at time of speleothem formation. Error bars quoted as 2 σ are smaller than the sample label.

accurate and reproducible to 0.1 pH unit. The precision of the conductivity meter used for the analyses was between 0.1 an 1% and its reproducibility of about 1%. Ca2+ was measured using EDTA titration method. The saturation index with respect to calcite (SIc) of water samples has been determined as the difference from measured pH and the pH of waters if they were in equilibrium with CaCO3 at the existing calcium and bicarbonate ions concentrations. The adopted calculation estimates the ionic strength from the EC values of water samples with a precision within the range of ± 0.2. Water analyses were repeated for a period of 8 months, from June 2003 to February 2004 in order to evaluate the effect of climatic variations on SIc values. 5. Results 5.1. U/Th dating The results of MC-ICP-MS U/Th dating are reported in Table 1. Speleothems are characterised by high ura-

nium abundances (from 1.6 up to 31.5 ppm) and initial (234U / 238 U) activity ratios slightly higher than unity (1.03–1.07) in agreement with a short circuit within a volcanic aquifer, where the leaching of “Villa Senni” lava flow has weakly enriched groundwaters in 234 U with respect to 238 U for recoil phenomena. The elevated abundance of uranium resulted in low errors (2σ) associated to the age of speleothems, notwithstanding they are very young. The (230Th / 232Th) activity ratio measured in the four subsamples from the inner growth layer of the stalagmite (FS6) range from about 8 to 69 indicating, especially for samples with lower values of this ratio, the presence of a detrital component containing thorium not formed in situ by radioactive decay. Consequently a combined total dissolution-isochron methodology [31,32] has been applied to them in order to calculate the (230 Th / 234U) and (234U / 238U) activity ratios of the 232 Th-free authigenic CaCO3 endmember (used to calculate the 230Th/234U age). These ratios can be calculated from the gradients of the straight lines that best

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Table 2 O and C isotopic composition of speleothems, δ 18O of mother solutions, δ 13C of dissolved CO2, percentage of deep fluid in mother solutions from two-component mixing equation on the basis of O and C data Sample

FS2-1 FS2-2 FS2-3 FS2-4 FS6-1a FS6-1b FS6-1c FS6-1d FS6-2 FS6-1α FS6-1β FS6-1γ FS6-1δ FS6-1ε FS6-1ζ FS6-1η FS7-1a FS7-1b FS7-1c FS7-1d FS7-2 FS8-1 FS8-2 FS8-3 FS8-4 FS9-1 FS9-2a FS9-2b FS9-2c a b

δ

O‰

18

δ

O‰

18

δ

C‰

13

δ

O‰

δ

18

VPDB

VSMOW

VPDB

VSMOW, H2O

− 3.74 − 3.97 − 3.02 − 3.81 − 4.28 − 4.42 − 3.71 − 2.37 − 1.54 − 2.89 − 2.75 − 2.6 − 2.31 − 2.51 − 1.72 − 1.91 − 4.35 − 4.09 − 4.43 − 4.17 − 3.36 − 3.82 − 4.6 − 4.46 − 4.41 − 4.36 − 3.87 − 4.46 − 3.93

26.15 25.92 26.9 26.08 25.6 25.45 26.19 27.57 28.42 27.03 27.17 27.33 27.63 27.42 28.24 28.04 25.53 25.79 25.44 25.71 26.55 26.07 25.27 25.41 25.46 25.52 26.02 25.41 25.96

−6.06 −7.46 −4.39 −6.1 −10.15 −8.98 −8.89 −5.79 −4.3 −9.7 −8.21 −7.97 −7.56 −7.44 −5.48 −7.32 −9.8 −9.92 −8.94 −7.16 −5.78 −8.86 −8.63 −8.57 −8.03 −8.94 −9.26 −8.86 −8.28

− 4.19 − 4.42 − 3.47 − 4.26 − 4.73 − 4.87 − 4.16 − 2.82 − 1.99 − 3.34 − 3.2 − 3.05 − 2.76 − 2.96 − 2.17 − 2.36 − 4.8 − 4.54 − 4.88 − 4.62 − 3.81 −4.27 − 5.05 − 4.91 − 4.86 − 4.81 − 4.32 − 4.91 − 4.38

a

C‰

13

VPDB, CO2

b aq

−15.94 −17.34 −14.27 −15.98 −20.03 −18.86 −18.77 −15.67 −14.18 −19.58 −18.09 −17.85 −17.44 −17.32 −15.36 −17.2 −19.68 −19.8 −18.82 −17.04 −15.66 −18.74 −18.51 −18.45 −17.91 −18.82 −19.14 −18.74 −18.16

Percentage of deep fluid as from O data

Percentage of deep fluid as from C data

0.24 0.18 0.32 0.24 0.09 0.07 0.14 0.27 0.34 0.22 0.23 0.24 0.27 0.25 0.33 0.31 0.08 0.10 0.07 0.10 0.17 0.13 0.06 0.07 0.07 0.08 0.12 0.07 0.12

0.19 0.13 0.27 0.19 0 0.06 0.06 0.21 0.28 0 0 0.10 0.12 0.13 0.22 0.13 0.02 0.01 0.06 0.14 0.21 0.06 0.07 0.07 0.10 0.06 0.04 0.06 0.09

Oxygen composition of speleothem-forming solutions from geothermometric calculations [47]. Carbon composition of CO2 dissolved in speleothem-forming solutions from geothermometric calculations [48].

estimate the relationships in subsamples data respectively plotted on ( 230 Th / 232 Th)–( 234 U / 232 Th) and (234U / 232 Th)–(238 U / 232Th) space (Fig. 4A and B). The intercept on the Y axis of the straight line in (230Th / 232 Th)–(234U / 232Th) space (Fig. 4A) is the (230 Th / 232 Th) activity ratio in the detritus at time of speleothem formation (= 0.63). Data points in both diagrams (Fig. 4A and B) are well aligned showing they are really coeval and contain, to a different extent, the same homogeneous detrital phase. The resulting age, 1910 ± 11 years B.P., demonstrates that the stalagmite is the oldest among dated speleothems and closely approaches the age of the stone mine quoted from archeological evidences. The (230Th / 232Th) activity ratio measured for stalactites (FS2, FS7, FS8 and FS9), ranging from 1.0 to 8.7, indicates that also stalactites contain a detrital component with a significant amount of 232Th, accom-

panied by 230Th not formed by radioactive decay of coprecipitated U. In order to account for the presence of inherited 230Th, it is necessary to apply a correction method to all them. Since it was not possible to extract more than two coeval samples from episodes of continuous growth within the stalactites, the isochron technique using total sample dissolution [32] cannot be applied and consequently a possible correction approach is to consider the initial (230Th / 232Th) activity ratio in the sample equal to that of the detrital fraction and on this basis correct the (230Th / 234U) activity ratio of the sample [30]. The value chosen for this correction (0.63) is the intercept on the Y axis of the straight line that best estimates the relationship in the stalagmite data plotted on (230Th / 232Th)–(234 U / 232Th) space (Fig. 4A). This value falls within the range (0.5–0.7) reported in [42,43] for the (230 Th / 232Th) activity ratios of Colli Albani

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volcano products and finely approaches the values of this ratio (0.56–0.62) [44,45], measured in single mineral phases enriched in calcium (augite and leucite) and leached by groundwaters. These phases are abundant in the “Villa Senni” lava flow where the speleothems mother solution circulate. In order to demonstrate that this correction approach works, the method has been applied also to single subsamples of the stalagmite with the result that all of them fall within the range 1861 (±11)–1928 (±11) years B.P., in agreement with the age calculated from the isochron technique, 1910 ± 11 years B.P. Up to 8 cycles of speleothems deposition, interrupted by clear unconformities, have been identified (Table 1), including the data from the stalagmite. The ages calculated for different layers of the speleothems are always stratigraphically consistent, furtherly supporting the adopted correction method. They range from 1910 ± 11 years B.P. to 641 ± 14 years B.P., but it is worth noting that younger layers (always included between deposition hiatuses) occur in the studied speleothems, but have not been dated because they are too subtle to be isolated and sampled. Another evidence in favour of the applied correction is that the ages of two subsamples FS7-1a (1560 ± 15 years B.P.) and FS7-1d (1550 ± 16 years B.P.) from the same growth layer (FS7-1) correspond within the error limits. Statistically equivalent ages have been obtained also for subsample FS9-2b (1070 ± 20 years B.P.) and FS9-2c (1070 ± 11 years B.P.), both belonging to layer FS9-2. It is worth noting that some deposition cycles have been recognised in more than one stalactite: a first in layers FS2-1 (1480 ± 12 years B.P.) and FS7-2 (1490 ± 12 years B.P.) and a second in layers FS2-3 (1060 ± 15 years B.P.), FS9-2b (1070 ± 20 years B.P.) and FS9-2c (1070 ± 11 years B.P.). The age gap dividing different growth layer separated by clear unconformities is in the order of one to few hundreds of years giving a temporal scan for the frequency of periodic interruption of speleothems deposition. 5.2. Stable isotopes The values of the carbon and oxygen isotopic compositions of speleothems are reported in Table 2 and expressed in the conventional δ ‰ notation; the reference standard is V-PDB for carbon and oxygen, but oxygen data have been also referred to V-SMOW according to the equation of [46], as required in the geothermometric calculations. Geothermometric calculations calcite-H20 [47] and calcite-CO2 [48] have been respectively applied to the oxygen and carbon isotopic composition of speleothems,

457

to calculate the δ 18O of speleothem mother solutions and the δ 13C of dissolved CO2, giving the water temperature a value of 17 °C (the present value of dripping waters). The results of geothermometric calculations are reported in Table 2. Data relative to subsamples from an episode of continuous growth (stalagmite FS6-1) show an increasing proportion of a deep component starting from a mostly meteoric fluid (δ 18O ≅ − 4.80 per mil) with soilderived CO2 (δ 13C ≅ − 20.03 per mil) to a fluid with an heavier isotopic composition of both O and C values which can be due to the episodic input of deep fluids. In order to estimate the proportion of deep fluids to the speleothems mother solutions, a simple two-component mixing equation has been applied to the calculated oxygen isotopic composition of speleothem-forming drip waters, choosing the composition of present precipitation at Rome (δ 18O = − 5.65 per mil) [49] and that of the primary magmatic water (δ 18O = + 5.00 per mil) [50] as end-members. A similar approach has been applied to the calculated carbon isotopic composition of dissolved CO2 to estimate the proportion of deep CO2. The composition of soil CO2 (δ 13C = − 20.00 per mil) [51] and that of CO2 released in the nearby location of Cava dei Selci (δ 13C = + 1.20 per mil) [7] have been used as end-members. Given the natural variability of both δ 18 O and δ 13C in the local soils and water, the approach of choosing a two-component mixing model with rigidly fixed end member values is only a first approximation. Results of mixing calculations are reported in Table 2. As far as the episode of continuous growth FS6-1 is concerned, the input of deep fluids, nearly absent at the beginning of the deposition, reached values of about 20–30% approaching the depositional break, that is when that layer stopped forming. The likely interpretation for this evidence is that the increasing contribution of deep CO2 made the dripping waters more acid and therefore undersaturated with respect to CaCO3, causing the interruption of the deposition. The remarkable changes in isotopic composition occurred in a very short time period, as the isochrone dating of FS6-1 growth layer demonstrates (see perfect alignment of the four subsamples data points), and this makes difficult to invoke a climatic-induced isotopic changes of mother solutions. If this mechanism, well-constrained for the stalagmite growth layer FS6-1, is extended to the other growth layers of stalactites where a detailed sampling has not been possible, a model emerges of cyclic interruption of speleothems deposition due to episodic input of deep CO2, which makes drip waters undersaturated with respect to CaCO3 until the water chemistry changes again and a new layer starts growing.

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Table 3 Temperature, pH, electrical conductivity, Ca 2+ and HCO−3 concentration and saturation index with respect to calcite (Sic) of speleothem-forming drip waters from June 2003 to February 2004 Date

Temperature pH Conductivity Ca2+ HCO−3 SIc (°C) μS/cm (mg/L) (mg/L)

June 30, 2003 October 17, 2003 November 5, 2003 November 21, 2003 December 19, 2003 January 8, 2004 January 23, 2003 February 6, 2004

17.5

7.1 1064

129

361

0.11

17.5

7.8

903

124

359

0.87

17.5

8.5

767

131

365

1.59

17.5

8.0

913

122

366

1.04

17.5

7.9

966

122

383

0.90

17.5

8.3

776

122

390

1.36

17.5

7.8

890

130

393

0.94

17.5

7.8

958

126

398

0.92

Sic values have been calculated using a computer program written by Paola Tuccimei on the basis of the equations reported in [56].

The values obtained for the growth layers of stalactites are coherent with data from the stalagmite, showing average deep fluids contribution to drip waters of about 10–14%, similar to average contribution calculated for the stalagmite (7–16%). Data from stalactites layers FS7-1 and FS9-1 (where two subsamples have been obtained from the respective layer) show increasing input of deep fluids, well constrained especially by C data. 5.3. Chemistry of speleothems mother solution The sampling and analyses of present drip waters have been repeated for a period of 8 months from June 2003 to February 2004 in order to evaluate the effect of seasonal climatic variations on Saturation Index values with respect to calcite (Sic) and detect any phase characterised by negative values of SIc and thus by no deposition. Data are reported in Table 3. The investigated period of time was characterised by a very low amount of precipitation (385 mm) in comparison with the average data for the same period (582 mm), with a deficit of about 34% [52]. Notwithstanding the exceptional dry period, with a maximum during the summer (only 6 mm of precipitation), SI values never fell down to 0. SIc range from a minimum

of 0.11 (June 30th 2003) to a maximum of 1.59 (November 5th 2003) with average values of about 0.9. The lowest value has occurred during the summer dry season when a reduction in the amount of precipitation and in turn in the aquifer volume caused an increase of ion concentration, H+ included, with a consequent reduction of SIc, not balanced by the contemporaneous increase of total dissolved salts and water temperature. This demonstrates that the system is naturally depositing, also in exceptional dry conditions, if no external phenomena of acidification occur. 6. Discussion The discovery of carbonatic speleothems at the Colli Albani volcano is very intriguing. Carbonatic speleothems are not, in fact, the most common occurrences in volcanic terrains. The circulation of CaCO3 rich fluid in the Colli Albani area is however well known; for example, the famous Tivoli travertine is a deposit related to the Colli Albani peripheral hydrothermal circuit [51]. Conditions for calcite precipitation in this volcanic area are mostly due to the mixing of uprising deep-seated fluids and the shallow hydrogeological system [10,51,8] changing the pH conditions of shallow waters. The data presented illustrate that the chemical conditions of the dripping waters at the Colli Albani have changed periodically causing the alternation of periods of oversaturation in CaCO3, i.e. deposition of calcite, and periods of undersaturation, i.e. non-depositional periods. This alternation is testified by the internal stratification of the speleothems. The study of the stable isotopic composition of C and O of the calcite indicates that Table 4 Correlation between the terminations of speleothem depositional episodes and earthquakes? Sample

Date

FS2-4 FS8-4 FS2-3 FS9-2b FS9-2c FS8-3 FS8-2 FS2-1 FS7-2 FS7-1a FS7-1d FS6-1 c

1350–1370 A.D. 1250–1280 A.D. 910–960 A.D. 910–960 A.D. 910–960 A.D. 800–820 A.D. 710–750 A.D. 500–550 A.D. 500–550 A.D. 430–460 A.D. 430–460 A.D. 90–110 A.D.

a

Seismic event 09/09/1349 a 04/30/1279 a − − − 04/29/801 b − 508 A.D. b 508 A.D. b 443 A.D. b 443 A.D. b 94 or 110 A.D. b

Data from [54]. Data from [53]. c Age of FS6-1 growth layer obtained by the analyses of 4 coeval subsamples: FS6-1a, FS6-1b, FS6-1c and FS6-1d. b

P. Tuccimei et al. / Earth and Planetary Science Letters 243 (2006) 449–462

459

Fig. 5. Schematic section of Ciampino (A) (see positive gravimetric anomaly in Fig. 1) and Centroni cave (B) areas illustrating a hydrothermal/ geothermal reservoir hosted in Mesozoic carbonate formations where the CO2 produced at depth accumulates, forming gas pockets at the top of the structure. From the gas pockets the gas escapes towards the surface generating CO2 manifestations, either localised or diffuse, and high PCO2's in shallow aquifers hosted in Quaternary volcanic products (V in (A)). STP, CQ, PC and K stands for Tufo pisolitico pyroclastic deposit, Quaternary cover (made up of marine and continental deposits), Plio-Pleistocene clays and Mesozoic carbonates, respectively. For (B) explanation, see text.

non-depositional periods are preceded by the uprise of CO2-rich deep seated fluids, that can be quantified as a 20–30% volume. This figure is a first indication of what can be expected at the Colli Albani as an anomaly, in terms of CO2 release.

The recurrence times of such anomalies, according to the age determinations of individual growth layers reported in Table 1, are in the order of one to few hundreds of years, documenting that the Colli Albani deep-geothermal-hydrothermal system experiences

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periodic “crises” that allow significant volumes of deep fluids to escape and reach the surface. This indication can be considered the first order “geological record” of geochemical anomaly assessed at the Colli Albani, never attempted before. One possible agent of the recurrent release of deep CO2 in the last 2000 years at the Colli Albani volcano (Rome, Italy), as recorded by the speleothems from the Romanage stone mine, could be the periodic increase of dilatancy and crack development preceding the largest historical earthquakes affecting the area of Roma which have produced increase of springs and groundwater temperature, changes of groundwater level and occasionally also intense gas emissions ([4] and references therein). When examining a list of the earthquakes with effects in Rome [53,54] some chronological correspondences have emerged between seismic events and the speleothems depositional cycles dated by U/Th dating (Table 4). On such bases an interpretative model of all the collected data is presented in Fig. 5A,B. The area is affected by diffuse and localised degassing processes, generated by fluid leakage from buried reservoirs hosted in the structural highs of the Mesozoic carbonate basement which act as traps for the gases of deeper origin (probably produced by metamorphism of marine carbonate rocks) and become sources of high CO2 flux toward the surface [10]. Groundwaters circulating in the shallower volcanic aquifer show anomalous CO2 concentrations [10], but the small aquifer hosted in Villa Senni lava flow is affected by the flux of deep CO2 only when a strong earthquake is approaching. Changes in fluid flow and fluid–rock interaction due to stress-induced geometrical changes of pores and cracks are the source of the progressive increase of CO2 concentrations in speleothem-forming drip water, producing its progressive acidification, and in turn, the undersaturation with respect to calcium carbonate, followed by the interruption of speleothem deposition. With the restoration of normal permeability conditions, the shallow aquifer returns to its pristine chemical composition, typical of a fluid mostly meteoric with low soil-derived CO2 contents and capacity of depositing CaCO3. In other words the changes in the oxygen and carbon isotopic composition recorded by the speleothems and those inferred for their mother solutions could be considered as records of past degassing phenomena and also as precursors of past earthquakes. 7. Conclusions This paper provides data on the possibility of defining and quantifying the cyclicity of past deep CO2 gas release recorded by the interruption of speleothem

deposition probably in connection with geophysical phenomena, such as the development of cracks preceding the largest earthquakes. The study of the C and O isotopic composition of speleothems along with their U/Th dating, integrated with the definition of the present hydrogeological setting and chemical–physical characteristics of groundwater in sensitive sites, could be extended to other areas of Colli Albani volcano (natural and artificial cavities or galleries) where speleothems or other carbonatic deposits occur in order to collect additional information on recurrent phenomena of gas release and eventually document them all over the area. The bulk of these data represents a main contribution to model the hydrothermal circuit of Colli Albani area and to define short term hazards related to the upwelling of deep seated fluids (particularly CO2 and 222Rn) and the longer term hazards related to volcanic eruptions. This approach could be exported to other volcanoes in Italy or other countries, in association with the more conventional monitoring of present degassing phenomena. The knowledge of the timing of past gas releases could be extremely useful to assess the present hazard and predict future events. Acknowledgements Authors wish to thank Valerio Bettonti and his cooperators for the access to Centroni Cave and the help in drip water sampling. We are also grateful to Marco Mola for the O and C isotopic analyses and to Jan Kramers and Igor Villa for their help with U/Th MC-ICP-MS dating. R. Funiciello is particularly acknowledged for continuous discussion and encouragement and also for finding the speleothems. L. Porrit reviewed the English. We are grateful to Tim Atkinson for his review. References [1] P.J. Baxter, J.-C. Baubron, R. Coutinho, Health hazards and disaster potential of ground gas emissions at Furnas volcano, São Miguel, Azores, J. Volcanol. Geotherm. Res. 92 (1999) 95–106. [2] W.F. Giggenbach, Water and gas chemistry of Lake Nyos and its bearing on the eruptive process, in: F. Le Guern, G.E. Sigvaldson (Eds.), The Lake Nyos Event and Natural CO2 Degassing, J. Volcanol. Geotherm. Res., vol. 42, 1990, pp. 337–362. [3] G. Chiodini, M. Todesco, S. Caliro, C. Del Gaudio, G. Macedonio, M. Russo, Magma degassing as a trigger of bradyseismic events: the case of Phlegrean Fields (Italy), Geophys. Res. Lett. 30 (2003) 171–174. [4] R. Funiciello, G. Giordano, D. De Rita, The Albano maar lake (Colli Albani Volcano, Italy): recent volcanic activity and evidence of pre-Roman Age catastrophic lahar events, J. Volcanol. Geotherm. 123 (2003) 43–61.

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