Key lecture: Volcanological framework of Costa Rica and its volcanic lakes

G.E. Alvarado 1, 2, 3, 4, G.J. Soto 1, 2, 4, 5, R. Mora 3, 4, P. Ruiz 6, C. Ramírez 3, 4, A. Vargas 1, J.F. Fernández 7 1: Área de Amenazas y Auscultación Sísmica y Volcánica, Instituto Costarricense de Electricidad 2: Escuela Centroamericana de Geología, Universidad de Costa Rica 3: Centro de Investigaciones en Ciencias Geológicas, Universidad de Costa Rica 4: Red Sismológica Nacional (RSN: UCR-ICE), Costa Rica 5: Terra Cognita Consultores, Costa Rica 6: Rutgers University, New Jersey, USA 7: Laboratorio Químico, Instituto Costarricense de Electricidad

Costa Rica is located in a convergent margin, where three plates (Caribbean, Cocos and Nazca) and a microplate (Panama) interact. Hence, there is a complex young geology, since no rocks of pre-Mesozoic age are known. Magmatic provinces can be summarized in: a) ophiolitic complexes of different origins (200-40 Ma), b) volcano-sedimentary basins (100-0 Ma) including several primitive island arcs (125-100 Ma, 75-40 Ma), c) the first in situ arc (29-12 Ma), d) the second volcanic front (7-2 Ma), and the present volcanic front (2-0 Ma) with many eruptive foci, which have developed in three main stages. Orogenic plutonic rocks are Late Oligocene to Pliocene (29-2.1 Ma), mainly Miocene to Pliocene.

In the present volcanic front, at least 9 volcanoes are definitively known to have erupted during the last 10 ka, but additionally at least 4 more could have had Holocene activity. Monogenetic vents are both isolated or part of the huge present volcanic massifs, structurally aligned. Arcuate grabens on volcanic summits (often misinterpreted as calderas: e.g., Poás and Tenorio), and horseshoe-shaped sector collapse amphitheaters are present in several volcanoes (e.g., Cacao, Miravalles, Irazú and Turrialba). Volcanoes such as Poás and Rincón de la Vieja have shown periodical eruptions characterized by short-lived (few hours to several days), violent (vulcanian and phreatic) eruptions, or clusters in periods of 10-70 years (Rincón de la Vieja and Irazú), while others clearly erupted once, and after that, have had long periods of inactivity previous to next eruption, from more than one century (i.e., Turrialba, as today), to several centuries (as Barva) to several thousands years (i.e., Congo).

Hazard assessments have focused on the historically (after 1700 AD) active volcanoes (Rincón de la Vieja, Arenal, Poás, Irazú and Turrialba), and lesser on dormant volcanoes (Hule and Barva). Such hazard assessments, though, must be forwarded on other volcanoes without historical activity (like Orosí, Tenorio, Platanar and Porvenir), for a better volcanic risk management. At present, only six volcanoes (Rincón de la Vieja, Miravalles, Arenal, Poás, Irazú and Turrialba) have minimum monitoring systems in operation (seismic stations, geochemical and visual monitoring, geodetic control) for observing “normal” activity and unrest, though most monitoring is not in real time (only seismic records in some cases). Some volcanoes that could be classified as dangerous, and then represent an unpredictable potential hazard for future

1 eruptions, remain poorly understood and virtually unmonitored, situation that must be further tackled in a close future.

The necessity for a better volcano knowledge in Costa Rica is mainly because several volcanoes are near densely populated urban areas (>1.5 million people), and also because they have become major destinations for ecologically-curious tourists from throughout the world, who visit the volcanoes, their rain forests, volcanic lakes, hot springs and other related attractions. For instance, about 4.2 x 105 tourists (43% foreign) visit the Costa Rican volcanoes every year. Poás, Irazú and Arenal are the top three spots for tourism.

Other volcano-related products have also a tremendous positive impact in Costa Rican society and economy, as the exploitation of aggregates for construction, huge volcanic aquifers, and geothermal energy (up to 15% of the total electrical power produced in the country).

Costa Rica has at least 5% of the hot hyperacidic lakes in active volcanoes of the world: Poás (21- 84° C, pH 0-1.8), and Rincón de la Vieja (31-47° C, pH 0.2-1.2), with ever changing colors from milky white to mustard or aquamarine. There are also cold crater lakes, as Irazú presently (13-15° C, pH 3-5) which has been warmer previously (in 1991-92 was 25-29° C, pH 3.0-3.5) due to a mild fumarolic input. Irazú lake has shown as well, drastic changes of colors varying from blood-red, to green to mustard. Other cold crater lakes are Santa María, Tenorio, Chato, Botos, Barva and Danta, located at the summit of dormant volcanoes. There are also lakes residing into the Holocene maars of Hule and Río Cuarto, which have presented overturning events, with sudden changes in their color to reddish, cyclically repeated through the last decades, causing massive fish deaths. Many other lakes in volcanic environments though, have been formed by damming by lava flows, lahars, debris avalanches or other geological processes, as for instance Los Jilgueros, Peje, Cedeño, Copey and Bonilla, among several others.

2 Hydrogeochemical study of Azorean lakes: monitoring active volcanoes

P. Antunes , J. Cruz, R. Coutinho, F. Pedro, J. Fontiela Centro Vulcanologia e Avaliação de Riscos Geológicos (CVARG), University of the Azores [email protected]

The Azores archipelago is located in the North Atlantic Ocean, about 1600 km from Europe and 2200km from North America, between the latitudes 37º-40ºN and longitudes 25º-31º W. Made by nine islands and several islets of volcanic origin, the archipelago occupies a flat area of approximately 2332 km2 and presents, in general, an WNW-ESE orientation. In the majority of the islands numerous lakes can be observed, whose physical characteristics are conditioned by the specific geologic setting. However, these water bodies are located predominantly inside explosion craters. A total of 88 surface lakes are distributed throughout the islands of São Miguel, Terceira, Faial, Flores and Corvo as well an 2 small cave lakes (Graciosa and Terceira islands). The total water volume stored in the crater lakes is about 90x106 m3, 93% of which in São Miguel island. The lakes on Flores Island contribute with 5% of the total water and the remaining 2% correspond to the lakes located in Terceira, Faial and Corvo islands. Several depth profiles was made between 2002 and 2007 in 6 lakes in São Miguel island, 1 lake in Terceira and Graciosa Island (caves lakes), 6 lakes in Pico Island and 5 lakes in Flores island. In general, sampled water are cold (5.2 ºC – 23.5 ºC) and correspond mainly to sodium chloride and sodium bicarbonate types. The water from Furna do Enxofre cave lake (Graciosa) is mainly from the magnesian bicarbonate type. Some of these lakes are eutrophic and in summer, water density stratification of thermal origin appears, surface pH reaches 10 and the hipolimnion is slightly acid, with the minimum value of 5.93. During this period, the bottom of the lakes shows a maximum concentration of CO2 (80 mg/L in crater lakes and 456 mg/L in Furna do Enxofre lake), and in some cases is coincident with a slight increase of the electric conductivity. Results suggest, in general, three different mineralization processes that occur in these systems: (1) caves lakes of Furnas do Enxofre, Algar do Carvão (Terceira Island) and Furnas lake (São Miguel Island), characterize a group in which the dissolution of silicate minerals controls water chemistry. This process is related with the water contamination by volcanic fluids, situation that is obvious in Furna do Enxofre lake. (2) The multivariate analysis suggests a second group of lakes, without dominant mineralization process [Sete Cidades, Santiago Congro (São Miguel Island), Funda, and Negra lakes (Flores island)]. Nevertheless, water are influenced by sea salts of marine transport, as well as by water-rock interaction. (3) The third group consists of small lakes systems from Pico Island, Flores Island, and Corvo island, except the Fogo Lake (São Miguel Island). This group of lakes have the less mineralized water and the mechanism that control the chemical composition is related, mainly, with contamination of marine salts.

3 Origin of fish kill events at Lake Averno, Campi Flegrei, Italy

R. Avino, S. Caliro, G. Chiodini, C. Minopoli Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli, Osservatorio Vesuviano, Naples, Italy

Lake Averno is situated in the homonymous crater in the northwestern sector of the Campi Flegrei active volcanic system in Campania region, Italy. The lake was affected in the past by a series of a fish kill events, the last of which occurred in February 2005. The origin of the event was investigated by means of a geochemical survey performed few days after the occurrence of the phenomenon. The survey revealed that the lake water was unstratified chemically and isotopically as a result of lake overturn. In fact, in contrast to the February 2005 results, data collected in October 2005, shows the Lake Averno to be stratified, with an oxic epilimnion (surface to 6 m) and an anoxic hypolimnion (6 m to lake bottom at about 33 m). Chemical and isotopic compositions of Lake Averno waters suggest an origin by mixing of shallow waters with a Na– Cl hydrothermal component coupled with an active evaporation process. The isotopic composition of Dissolved Inorganic Carbon, as well as the composition of the non-reactive dissolved gas species again supports the occurrence of this mixing process. Decreasing levels of SO4 and increasing levels of H2S and

CH4 contents in lake water with depth, strongly suggests that anaerobic bacterial processes are occurring through decomposition of organic matter under anoxic conditions in the sediment and in the water column. Sulfate reduction and methanogenesis processes coexist and play a pivotal role in the anaerobic environment of the Lake Averno. Total gas pressure of dissolved gases ranges between 800 and 1400 mbar, well below the hydrostatic pressure throughout the water column, excluding the possibility, at least at the survey time, of a limnic eruption. Vertical changes in the density of lake waters indicate that overturn may be triggered by cooling of epilimnetic waters below 7 °C. This is a possible phenomenon in winter periods if atmospheric temperatures remain frosty for enough time, as occurred in February 2005. The bulk of these results strongly support the hypothesis that fish kill are caused by a series of events that began with the cooling of the epilimnetic waters with breaking of the thermal stratification, followed by lake overturn and the rise of toxic levels of H2S from the reduced waters near the lake bottom.

4 The monitoring of Volcanic lakes in Indonesia

A. Bernard1, B. Barbier1, C. Caudron1,2, D. Syahbana3, A. Solikhin3, S. Kunrat3, V. Hallet4 1: ULB, Université Libre de Bruxelles, Belgium 2: ORB, Observatoire Royal de Belgique, Belgium 3: CVGHM, Center of Volcanology and Geohazards Mitigation, Indonesia 4: FUNDP, Fondation Universitaire Notre Dame de la Paix, Belgium

More than 15 active indonesian volcanoes have their crater filled with an aqueous lake. These lakes present a hazardous situation because they contain large volume of waters like Dempo in Sumatra with 8.5 millions m3. These lakes are also ideal scientific platforms for the monitoring and prediction of volcanic eruptions. A large majority of these volcanic lakes have acid-sulfate-chloride (ASC) waters which reflect the direct absorption of magmatic gases into a sub-surface hydrothermal system. Two lakes have a contrasting composition where neutral chloride-sulfate waters dominate their chemistry: Kelud in Java and Segara Anak (Rinjani) in Lombok. Kelud volcano erupted on the 4th of November 2007 when a lava dome began to grow passively in the middle of its crater lake. The first precursor indicating a change in the activity of the volcano was observed in early July 2007 when an intense degassing of the lake floor was detected. Measurements of CO2 fluxes from the lake surface revealed a large increase in the total flux for the lake estimated at 330 Tons/day which corresponds to a tenfold increase compared to previous years (35 Tons/day). CO2 is notoriously relatively insoluble in hydrothermal waters and therefore can pass through the hydrothermal barrier. Because of that,

CO2 is probably one of the best magmatic tracer for forecasting an impending eruption. Rinjani volcano erupted on the 2th of May 2009. The massive Segara Anak lake covers an area of 11km2 with a maximum depth of 205m and a volume estimated before the eruption at 1.02km3.The first precursors were detected 3 weeks before the eruption when an acidification of the lake waters was recorded. pH profiles as a function of depth recorded with a SBE Seacat 19-Plus at several locations showed a clear acidification of Segara Anak lake especially at shallow depths (15-20 m). A chemical plume of low pH and dissolved oxygen was also observed at the lake surface extending up to several hundred meters away from hot springs discharging into the lake. The discharge of anoxic Fe-rich hot spring waters produced a spectacular yellowish-brown coloration of the lake waters due to the precipitation of ferric hydroxide

Fe(OH)3. The 2009 activity was characterized by mild eruptions that produced a lava flow covering an area of 0.65 km2. The lake surface area has been reduced by 0.46 km2.

5 Twelve years observation on geochemistry of the Albano crater lake

M.L. Carapezza1, , M. Lelli2, L. Tarchini3, M. Ranaldi3, T. Ricci1, F. Iacobelli3

1: Istituto Nazionale di Geofisica e Vulcanologia - Sezione Roma 1, Via Vigna Murata 605, 00143 Roma, Italy 2: Istituto di Geoscienze e Georisorse – Consiglio Nazionale delle Ricerche, Via G. Moruzzi 1, 56124 Pisa, Italy 3: Dipartimento di Scienze Geologiche - Università Roma Tre, L.go San L. Murialdo 1, 00146 Roma, Italy [email protected]

Albano lake is within the youngest polygenetic crater of Colli Albani quiescent volcano, from which several lahar-generating water overflows have occurred until early Roman times. The area has anomalous gas emissions and is affected by seismicity and uplift. The geochemistry of the lake have been systematically investigated by measuring physico-chemical parameters along vertical profiles with a multiparametric probe and by collecting water samples for chemical and isotopic analyses of both water and dissolved gases. The lake is thermally and chemically stratified, with an anoxic hypolimnion from -70 m to the bottom (-167 m). The isotopic composition of dissolved helium and total carbon is similar to that of the main gas emissions of Colli Albani and of the phenocryst inclusions of the Albani volcanics, suggesting that an endogenous gas of deep provenance is injected into the lake water. The dissolved CO2 content is however far from saturation and no Nyos-type hazardous gas cloud emission may presently occur in the lake. Temperature and chemical time variations indicate that water rollover episodes occur in harsh rainy winters when the surface lake temperature cools below 8.5 °C. Such rollovers tend to homogenize the physico-chemistry of the lake water and reduce the dissolved CO2 content. They may cause an environmental hazard because of related toxic algal blooms.

6 Hydroacoustic quantifications of CO2 bubbles in volcanic lakes

C. Caudron1, 2, A. Bernard1 1: Université Libre de Bruxelles 2: Observatoire Royal de Belgique

Echosounding surveys have been performed on Rinjani and Kelud volcanic lakes (Indonesia) with a SIMRAD ES60 single beam, dual frequency (50 and 200 kHz) echosounder during 2007 and 2008. CO2 gas bubbles are strong scatterers of acoustic waves. Echosounding methods might therefore be helpful for mapping gas emissions at the floor of volcanic lakes and to quantify volumetric density and fluxes of CO2 emitted through the lake. Analyses were carried out using 50 kHz data’s. Echointegration technique has been applied on the 40 log10 R(TVG) echograms using Sonar 4 post processing software (Balk and Lindem, 2007). A lower threshold of 60 dB has been used in order to avoid the backscattered noise. Each water column segment was analyzed ~5 m below the surface. The volumetric density of bubbles per cubic meter (N) in each water column is calculated by dividing the total backscattering volume (Sv) with the target strength (Ts = 10 log10 Bs, where Bs is the backscattering cross section). Standing profiles were also performed in order to calculate the speed and radii of ascending bubbles with different target strengths. This allowed us to derive an equation relating the average bubble volumes in water columns to the Ts values: V = 1964620.165 (exp 0.3308 Ts). The volume detection limit of bubbles is 0.01 ml. We can then easily calculate the volumetric concentration : VC = NV. In Rinjani, the volumetric concentrations of CO2 range between 0.001 and ~ 7 ml/m³. This exceeds degassing results of non volcanic lakes (Ostrovsky et al., 2008).

Taking into account the average rise of velocity of bubbles (0.22 m/s), the molar volume of CO2 at the bottom and VC, one can calculate the gaseous carbon dioxide fluxes. We found values ranging between 0 and ~ 900 g m-2d-1. These were compared to the fluxes measured with a floating accumulation chamber and give systematic lower values. This highlights an important CO2 diffuse flux which is not resolved by the echosounding technique. Further lab experiments are needed in order to refine and validate the empirical equations used.

7 Solute gases in Ruapehu Crater Lake waters

B.W. Christenson1, K. Britten2, A. Mazot2, J. Cole-Baker2 1: National Isotope Centre, GNS Science, Lower Hutt, New Zealand 2: Wairakei Research Centre, GNS Science, Taupo, New Zealand

Since the 2007 eruption of Ruapehu, we have been sampling the two major up-wellings (northern and central vents) in Ruapehu Crater Lake for analysis of their dissolved gases in addition to non-volatile solutes. Lake water is collected by helicopter into an under-slung, 25 litre bucket which is then delivered to persons on shore, who immediately take water into evacuated flasks fitted with teflon-stopcocks. The headspace gases are subsequently analysed by GC for CO2, H2, He, Ar, N2, O2, CH4, CO and C1-C6 hydrocarbons, with appropriate corrections made for distribution of gas between the vapour and water phases in the samples. The results show the expected influence of atmospheric and magmatic end member contributions to the dissolved gas species, and a systematic variation in composition with thermal cycling of the lake. CO2 is the predominant gas in solution, followed by N2, H2, Ar, O2, CO and trace amounts of hydrocarbons (both alkanes and alkenes). Waters range from close to saturation to mild super-saturation with respect to dissolved gases. Total dissolved gas contents typically show maximum values at the start of the heating cycles, at levels which exceed those expected from temperature-controlled solubility constraints alone. This pattern is in general agreement with airborne emissions data collected during the thermal cycles, typically pointing to maximum gas emissions at the onset of each heating cycle. Relative variations in solute concentrations provide interesting insights into equilibrium conditions and processes operating beneath the lake. Both H2/Ar and CO/CO2 ratios, for example, show coherence with measured lake temperatures, indicating changing equilibrium conditions in the hydrothermal environment during the heating cycles. H2/Ar ratios appear to adjust more rapidly to changing vent conditions than

CO/CO2 ratios. Although there is very little CH4 in the solute gases, CH4/CO2 ratios subtly increase during quiescent periods, pointing to the likely existence of a discreet hydrothermal component resident in the vent system. Evidence from the 2007 eruption suggests that degassing magma lies within a few hundred metres of the lake floor, which makes for a rather compact hydrothermal environment overlying the magma column.

Indicated CO2-CO-CH4 temperatures from this study range from ca. 370 °C to 650 °C during the cycles. Integrated finite difference (TOUGH2) modelling of the vent environment shows that such temperature and gas emission variations are likely brought about by intermittent release of heat (and gas) from the magma, followed convective circulation of lake water through the vent region.

8 Lake Monoun is degassed, Lake Nyos is still to be degassed

M. Kusakabe Department of Environmental Biology and Chemistry, University of Toyama, Gofuku 3190, Toyama-shi, 930-8555, Japan [email protected]

Lakes Nyos and Monoun in Cameroon experienced a “limnic eruption” in mid-1980s. The eruptions were caused by accumulation of excessive magmatic CO2 in the deep water of the lakes. Artificial degassing of the lakes which started in 2001 to remove dissolved CO2 is successfully going on. The latest CO2 profiles in both lakes are shown in Figs. 1 and 2 which illustrate that Lake Monoun has been degassed to safety, whereas Lake Nyos still needs to be degassed. Since the degassing system at Lake Monoun has already lost its gas self-lifting capability because pCO2 has been reduced to a very low level in the bottom water, it is strongly advisable to install a non gas self-lifting system to pump out the CO2-containing bottom water to the surface considering that natural CO2 recharge would continue. An idea of such a system will be presented. The recent results on noble gas systematics of the lakes will be also presented.

Fig. 1. Recent CO2 profiles at Lake Fig. 2. Recent CO2 profiles at Lake Nyos. Monoun.

9 Microbiological survey of El Chichón volcanic lake by cultivation –dependent and – independent techniques

F. Mapelli1, R. Marasco1, D. Daffonchio1, D. Rouwet2, G. Pecoraino2, S. Borin1 1: Università degli Studi di Milano, DiSTAM, Via Celoria 2, 20133 Milano, Italy 2: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, 90146 Palermo, Italy

Volcanic environments belong to the so-called “extreme environments” where the environmental stressors for the living biota are represented by acidic pH, moderate to high temperatures and high concentrations of chemicals like heavy metals. The investigation of microbial life in volcanic environments is of great interest. Thermoacidophilic microorganisms have potential biotechnological applications and, moreover, their ecophysiological study can give insights about the origin of life on our planet, since volcanic environments are considered analogous to the early Earth. In June 2009 five sites located into and in proximity of the volcanic lake El Chichón (Chiapas, Mexico) have been sampled from the acidic (pH 2.3) SO4-Cl type crater lake (LE), from the acidic (pH 4.1) SO4-type steam-heated pools (T 42°C) within the Soap Pool field (SP) and from acidic (pH 2.6-3.0) Cl-SO4 type saline thermal springs (T 53-59°C), located ~2 km outside the crater (AS1 and AS8). Molecular microbiology techniques are independent from cultivation and have the potential to overcome the limitations imposed by the low level of cultivability of environmental bacteria. Denaturing Gradient Gel Electrophoresis (DGGE) applied on the total DNA extracted from the samples demonstrated that i) all the sites were colonised by bacteria, ii) the different environments studied in El Chichón area contained a peculiar microbiota, probably adapted to the specific geochemical context. Basing on DGGE data, the microbiota inhabiting the crater lake resulted dominated by the species Acidithiobacillus thiooxidans, which has sulphur-oxidation energetic metabolism and plays a key role in the sulphur biogeochemical cycle. Sulphur-iron oxidizing bacteria were also cultivated from the SP and AS thermal springs. Bacteria of the order Aquificae were specifically retrieved in the thermal springs external to the crater, represented by Sulfurihydrogenibium azorense, a thermophilic, strictly chemolithoautotrophic species previously isolated from terrestrial hot springs in the Azores, Portugal. The molecular study was integrated by a cultivation-dependent approach in order to explore the physiological and metabolic properties of the organisms able to colonize such an extreme environment, with particular emphasis on bacteria having plant growth promoting (PGP) activity, since pioneer vascular plants were observed in SP site. Several bacterial species were associated to plant roots, some of them known for their capability to grow in extreme conditions. Tests about their survival at in situ conditions and potential PGP activity are at present undergoing. Several strains showed 1-aminocyclopropane-1-carboxylate (ACC)- deaminase activity, one of the best studied PGP activity, with a role in protecting the plant from environmental stress. The isolates showed, besides ACC-deamminase, also other PGP activities such as phosphate solubilization, dinitrogen fixation, and the production of siderophores.

10 Molecular and cultivation-based approaches demonstrated that El Chichón crater lake and the associated thermal springs are colonised by a rich microbiota mainly involved in the sulphur and iron biogeochemical cycles, which diversity is selected by the specific geochemical context. Plants adapted to live in this extreme environment are associated with several bacterial species having potential activity of plant growth promotion and stress protection.

11 Polythionates monitoring at the acid crater lake of Poás Volcano

M. Martínez 1,2, , M.J. van Bergen2, , E. Fernández 1, B. Takano3, 1: Observatorio Vulcanológico y Sismológico de Costa Rica (OVSICORI), Universidad Nacional Heredia, Costa Rica 2: Faculty of Geosciences, Utrecht University, Utrech,t the Netherlands 3: Department of Chemistry, Graduate School of Arts and Sciences, University of Tokyo, Japan, [email protected] [email protected] [email protected]

Polythionate (PT) concentrations are regularly monitored in the acid crater lake of Poás, which is usually hot (22-94°C in the last 30 years) and extremely acidic (pH≤0-1.8). Historically, its water level, temperature and chemistry have varied drastically in response to changes in energy release, the vigour of subaqueous fumarolic activity and rainfall. Together with published results, our data on tetrathionate, pentathionate and hexathionate abundances comprise a PT record for Poás over 30 years. During this period, the total concentrations of the measured PT ranged from undetectable up to 8000 mg/l. The levels dropped below detection limits shortly before phreatic eruptions resumed in 1987 after a period of relatively quiet activity. With few exceptions, the concentrations remained low until 1995, during a period that coincided with increased volcanic seismicity, repeated phreatic eruptions, increased subaqueous and subaerial fumarolic activity and occasional drying- out of the lake. Between 1996 and late 2003, the PT concentrations returned to relatively stable high levels that apparently mark more quiet conditions. The 1994-1996 transition was further accompanied by an increase in pH (from ~0 to 1-1.5), a decrease in lake-water temperature (from ~60 to 30-40oC) and a marked drop in sulphate and chloride contents. Afterwards, between late 2003 and early 2005, the PT were absent when the lake showed very weak subaqueous fumarolic activity (temperature fluctuated between 22-35°C and the pH between 0.6 and 1.3). After an absence for about a year, PT suddenly reappeared on the order of several thousands of ppm between April 2005 and April 2006, and showed a drop to a few hundreds of ppm in May-November 2006. This PT behaviour is consistent with the significant increased input of heat and volatiles into the lake that has been observed from early 2005 till present. Our findings suggest that polythionates are highly relevant for monitoring purposes at Poás, in particular because they may signal impending phreatic eruptions.

12 Fluxes of carbon dioxide from volcanic lakes: temporal evolution or highlighted structure

A. Mazot1, B. Christenson2, A. Bernard3, Y. Taran4 1: Wairakei Research Centre, GNS Science, Taupo, New Zealand 2: National Isotope Centre, GNS Science, Lower Hutt, New Zealand 3 : Université Libre de Bruxelles, Brussels, Belgium 4: Universidad Nacional Autónoma de Mexico, Mexico City, Mexico

Measurements of CO2 flux emitted by the surface of two volcanic lakes were performed using the so-called floated accumulation chamber method. After the last strong eruption of Kelud volcano (Indonesia) in 1990, a lake rapidly filled the crater. Prior to extrusion of a recent, mid-lake lava dome on the 4th November 2007, the lake contained near neutral waters with a pH of 6. The total CO2 emission rate estimated by stochastic simulation ranged from 105 t/day for 2001 to 35 t/day for 2006. In early July 2007, the total flux for the lake area was estimated at 330 t/day. The CO2 flux measured in early September reached 730 t/day. The 1982 eruption of El Chichón volcano, Mexico, ejected 1.1 km3 of anhydrite-bearing trachyandesite pyroclastic material to form a new 1-km-wide and 200-m-deep crater. Currently, intense hydrothermal activity, consisting of fumaroles (mainly at the boiling point), steaming grounds, a soap-pool and an acidic (pH≈2.6) and warm lake (30°C) occur in the summit crater. Carbon dioxide fluxes were measured in March,

December 2007 and April 2008 at the surface of the volcanic lake. Total CO2 emission rates from the lake were 164 ± 9.5 t/day (March 2007), 59 ± 2.5 t/day (December 2007) and 144 ± 5.9 t/day (April 2008).

Significant change in CO2 flux was not detected during the period of survey but the mapping of the CO2 flux on the lake area highlighted lineaments reflecting the main local and regional tectonic patterns. Frequent mild-to-moderate explosive eruptions have occurred in historical time from the crater lake of Ruapehu (New Zealand) with the last hydrothermal eruption occurred in September 2007. The pH of the o lake is around 1.1 with lake temperatures ranging from 10 to 60 C. CO2 emission measurements have been made since 2003 from an airborne platform at a constant distance from the summit and the data were processed using the plume contouring method. The total CO2 emission rate varies from not detectable to

2200 t/day. Future CO2 flux surveys at the surface of the crater lake using the floating accumulation chamber and an IR Laser Diode system will provide a better understanding of the Ruapehu magmatic- hydrothermal system.

13 Nakadake crater lake at Aso Volcano, SW Japan: its characteristics and subaqueous geothermal activity inferred from lacustrine sediments

Y. Miyabuchi1, , A. Terada2 1: Faculty of Education, Kumamoto University, Kumamoto 860-8555, Japan 2: Kusatsu-Shirane Volcano Observatory, Tokyo Institute of Technology, Gunma 377-1711, Japan [email protected]

The active crater of Nakadake at Aso Volcano, southwestern Japan, has been occupied by an acidic (pH=0.43) lake during its quiescent periods. To quantitatively estimate the geothermal activity of crater lake, we sampled lacustrine sediments from the nearly inaccessible active crater lake on 8 July 2008 and conducted textural and chemical analyses. The lacustrine sediments are characterized by an extremely high content (74 wt.%) of total sulfur, which exits as elemental sulfur, gypsum and anhydrite. The abundant elemental sulfur is believed to result from precipitation by the reaction of SO2 and H2S gasses and by the

SO2 disproportionation in the magmatic hydrothermal system beneath the crater lake. Based on the sulfur content of sediments and measured elevation change of the crater bottom, the sulfur accumulation rate at the

Nakadake crater lake was calculated as 250 tonne/day. This suggests that a considerable amount of SO2 is absorbed in lake water relative to total amount of SO2 emission (200-600 tonne/day) from the Nakadake crater. The sediments include a small amount (9 %) of apparently clear glass shards. Since the glasses are easily altered by hyperacid lake water, we suggest that the clear glass shards are fragments of recently emitted magmas from fumaroles on the bottom of crater lake even during quiescent periods.

14 Differential degassing magma beneath the crater lake Yugama on Kusatsu-Shirane volcano, Japan

T. Ohba Volcanic Fluid Research Center, Tokyo Institute of Technology, 641-36 Kusatsu, Agatsuma, Gunma 377- 1711 Japan [email protected]

The active crater lake, Yugama is located on the summit of Kusatsu-Shirane volano, Japan. The concentration of dissolved components in the lake water has varied responding the volcanic activity. The repeated sampling and analysis of the lake water was initiated by J. Ossaka in 1966. The duration since 1966 until 2005 was divided into three periods. In the first period, starting from 1966 ending 1981, the ─ 2─ temperature of lake water was relatively low. The concentration of Cl and SO4 was high in the early years then decreased gradually. In the period, the sealing zone surrounding a cooling magma would have been developed. The sealing zone is a region where secondary hydrothermal minerals such as pyrite, alunite and gypsum are deposited which prevents the dispersion of magmatic volatile emitted from the cooling magma. In the second period until 1989, several phreatic eruptions took place in Yugama crater in 1982 and 2─ 1983. The lake water temperature increased after the eruption. Synchronized with the eruption, SO4 concentration of lake water significantly increased but the Cl─ concentration did not changed. The above changes are interpreted to be the break of sealing zone and the subsequent dispersion of magmatic volatile stored within the region surrounded by sealing zone. Within the third period until 2005, a sharp increase in the Cl─ concentration of lake water accompanying the decrease in pH was observed around 1990. The changes would be produced by the interaction of a hot rock and ground water. The hot rock could be a solidified magma which had lost most of sulfur component until the second period. Due to the strong affinity of Cl with silicate, the large part of Cl was remained in the hot rock. By the interaction of the hot rock and ground water, the remained Cl was extracted to the vaporized ground water as HCl gas which was transported to Yugama lake causing the increase in Cl─ and decrease in pH. Summarizing the interpretation of lake water chemistry, the magma beneath Kusatsu-Shirane volcano has degassed differentially in terms of volatile component. In the early stage of degassing, sulfur component was emitted from magma which was followed by the release of Cl. Similar degassing trend was observed in the volcanic gas during the eruption of Unzen volacano in 1991 to 1995. The concept of differential degassing could be applied to the geochemical observation for the interpretation of magmatic activity.

15 Geochemical Surveys on Specchio di Venere Lake, Pantelleria Island, South Italy

G. Pecoraino, L. Brusca, W. D’Alessandro, M. Longo, P. Madonia Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy

Lake Specchio di Venere is an endorheic saline lake within a calderic depression on Pantelleria, a quiescent volcanic island in the Sicily Channel, between Tunisia and South Italy. This endorheic basin has been formed through upwelling of the water table, and that it is continuously fed by the thermal springs situated on its shores. In the shore of Lake Specchio di Venere, CO2 fluxes and concentrations were measured with the accumulation chamber method for a total of 136 measurements. Furthermore a vertical profile of main water chemical-physical characteristics (EC, pH, Eh and T) and a bathymetric survey have been made. Flux -2 -1 measurements for the whole surveys gave values ranging from 1 to 4700 g CO2 m day and a median of -2 -1 21.8 g CO2 m day . Organic activity and root respiration contributes can be distinguished on a probability plot evidencing statistically distinct populations. In this case a threshold of 30g m-2 day-1 to separate background (organic) from anomalous (magmatic/geothermal) values was chosen. The total CO2 output of the anomalous degassing areas was estimated through statistical method considering only values above the anomaly threshold. We have obtained a total output of about 0.349 kg s-1 over an area of about 0.103 km2.

CO2 concentrations in soils ranged from 0.035 (atmospheric value) up to 95% and two statistically distinct populations. The spatial distributions of CO2 concentrations closely resemble those of the CO2 fluxes. At sites where concentration and flux anomalies are both present, the C isotopic composition of CO2, shows the imprint of the magmatic/geothermal isotopic marker (5±1 ‰). The lake waters were inspected with one vertical profile. All analysed parameters (T, EC, pH, Eh) did not show any significant variation with depth. Such results exclude the presence at that time of any thermal or chemical stratification of the lake. Also the analysis of dissolved gases did not evidence anomalous gas accumulations in the lake waters. A bathymetric survey has been carried out using a fish-finder sonar coupled to a GPS. The obtained morphology of the lake bottom is quite irregular. The southern sector is characterized by very shallow waters (< 1 m), whereas the maximum depth of about 13 m is reached in the northern area. Tectonic seems to strongly influence the submerged morphology: a main lineation NW-SE oriented, coupled with a secondary conjugated direction SE-NW, is clearly revealed by the arrangement of the depth contour lines.

16 Total heat output estimation of El Chichón volcano-hydrothermal system, Mexico

L. Peiffer1, A. Mazot2, Y. Taran1 1: Instituto de Geofísica, Universidad Nacional Autónoma de México, México D.F., México 2: Institute of Geological and Nuclear Sciences, Taupo, 3352, New Zealand

Evidence of a vast hydrothermal system underneath El Chichón edifice are expressed by the presence of steam vents, boiling springs and steam-heated pools in the crater, as well as by numerous thermal springs located on the flanks of the edifice. This hydrothermal system is geochemically well characterized. However, there are still many questions left about how El Chichón works, including the structure of its hydrothermal system and its geothermal potential. In this paper we present the total heat discharge rate results from flank thermal springs as published by Taran and Peiffer (Geothermics 38 (2009) 370–378). Flank thermal springs mainly discharge near-neutral Na–Ca–Cl–SO4 waters (Cl~1500–2000mgkg−1). Low discharge of highly concentrated acidic waters is also observed on the NW flank of the volcano (10,000mgkg-1, >15 g/L). Flow rates of each thermal spring group were estimated by flow measurements at the junction points of thermal streams with the Rio Magdalena, the only drainage of El Chichón thermal water. Thermal streams discharge a total 468±47gs−1 of Cl into the Rio Magdalena, which represent a flux of 234 kgs−1 of hot water with Cl = 2000mgkg−1. This Cl flux is one of the largest measured for hydrothermal systems (Table 1). The convective heat output (W) from each hot spring group is calculated using the mass flow rate (Q) and the difference of enthalpy between hot spring water (Hs) and ambient temperature water (Ha), W = Q(Hs − Ha). For all thermal springs discharging at the slope of El Chichón volcano we obtain a total heat output of 45±5MWt. Adding the heat output estimation of the volcano crater (120±30MWt) we come to a total of 160MWt for the volcano-hydrothermal system of El Chichón. Such results attest to the high geothermal potential of El Chichón volcano. However, possible future volcanic activity may prevent the development of any geothermal plant.

17 Table 1. Chloride fluxes and natural heat flow through selected hydrothermal systems.

Cl Cl flux Source Natural heat System concentration (g/s) temperature flow in hot spring (Tg) based on (MWt) waters geothermometry (mg/L) (◦C) 1270– Yellowstone USA 400–450 340–360 1600–2100 1737 Cascades, USA 41–49◦ 721 Variable Variable 82 El Chichón, México 468 2000 200 45 Wairakei, New Zealand 300 1610 260 78 1951 Rotokawa, New Zealand 52 700 320 31 Lassen Peak, USA 42 2400 240 7.4 Mutnovsky, Russia 13 220 270 34 Table from Taran and Peiffer (Geothermics 38 (2009) 370–378).

18 Compact Instruments for Airborne In Situ Volcanic Gas Measurements

D. Pieri1, J.A. Diaz2, G. Bland3, M. Fladeland4, T. Griffin5 1: Jet Propulsion Laboratory, USA 2: University of Costa Rica, Costa Rica 3: NASA GSFC-Wallops Flight Facility, USA 4: NASA Ames Research Center, USA 5: NASA Kennedy Space Center, USA

The Hazardous Gas Detection Laboratory (NASA KSC) has developed an in situ mass spectrometer capable of autonomous in-flight operations from sea level to 44,000 feet (13.4 km) for monitoring ambient atmospheric gases as well as organics up to 200 amu, with self-calibration. In 2003, this system was deployed on the NASA WB-57 high-altitude (>20 km) research aircraft over volcanoes in Costa Rica (with the University of Costa Rica). To our knowledge, this was the first airborne mass spectrometer instrument capable of autonomous, quantitative gas detection. In 2005 this gas analyzer system was improved and provided the first in situ, NIST-traceable, 3-dimensional, quantitative chemical plot of emitted volcanic gases (e.g., CO2, SO2, H2S, H2O and others) from several volcanoes in Costa Rica, (Griffin, 2008). It is currently used at Kennedy Space Center for Shuttle launch ground support operations. We will deploy this instrument, now called ICAMS (In situ Compact Airborne Mass Spectrometer) onboard the SIERRA UAV, a 250kg robotic aircraft (~50kg payload—operated by NASA Ames Research Center) to rapidly and cost- effectively collect high caliber, low altitude, in situ, multi-gas characterization data near high-risk volcanoes, under hazardous flight conditions. A Costa Rica ICAMS-SIERRA deployment in Spring 2011 is being considered. In addition, we will deploy the JPL ultra-miniature SO2 radiosonde using tethered balloon and/or kite platforms in Costa Rica in late March 2010. Also, the University of Costa Rica is planning to deploy the JPL sensor and other micro-sensors on its own ultra-small UAV aircraft. We will discuss in situ airborne gas measurements over volcanic lakes and general volcanic gas sampling, including SO2 sub- orbital cal/val activities for ASTER. (This work was completed, in part, under contract to NASA at the Jet Propulsion Laboratory of the California Institute of Technology, Pasadena, California, USA).

Griffin T.P., Diaz J.A., Arkin C.R., Soto C., Curley C.H., and Gomez O., 2008, “Three-Dimensional Concentration Mapping of Gases using a Portable Mass Spectrometer System” J. Am. Soc. Mass Spectrom., 19, 1411-1418.

19 Physical changes before and during the new period of phreatic eruptions, Poás Volcano 2006-2010, Costa Rica

C. Ramírez-Umaña1,2,3, R.A. Mora-Amador1,2,3, G. González1,2,3 1: Escuela Centroamericana de Geología, Universidad de Costa Rica 2: Centro de Investigaciones en Ciencias Geológicas, Universidad de Costa Rica 3: Red Sismológica Nacional (RSN: UCR-ICE), Costa Rica

After 12 years of quiescence, Poás Volcano entered a new period of phreatic eruptions on 26 March 2006. Since months before, significant physical changes have been detected at Laguna Caliente and the intracrateric fumaroles. In January-February 2005, the Laguna Caliente crater lake reached its maximum level and corresponding depth of more than 60 metres. During this entire year, floating sulphur spherules were observed on the lake surface. In May 2005, the so far largest sulphur flow was reported at Poás. Its color suggested that it was emplaced in a low temperature-viscosity molten stage (estimated 113-160°C and 10-1 poise). The formed spatter cones, with a small molten sulphur pool inside, expelled pyroclasts, lapilli-like fragments, accretionary lapilli, Pelé’s hair and tears. This activity culminated in small phreatic eruptions in March 2006, after 12 years of eruptive dormancy. A yellow mass (slicks) of pure native sulphur formed spongy aggregates composed of hollow spherules, globules and fragments, and floated on the surface of the acidic lake. The lowest lake water temperature (22°C) corresponded with the highest lake level in January 2005. Since this moment the lake started to “lose” water and tended to increase its temperature. The highest lake water temperature was reported in September 2009 (57°C). During the same month, sulphur combustion at the dome south of Laguna Caliente occurred, forming a blue flame with a height up to 10 m. At this site temperatures higher than 500°C were measured by the end of 2009. During the study period, the meteoric precipitation remained within the annual average values of the pluviometer Vpsss1, registering since more than 25 years. Despite reaching record rainfall values of over 5000 mm/year, the general trend of the lake level in 2008 was a decline, with a small recovery in January 2009. The pH of the Laguna Caliente water during the study period varied between 0.5 and -0.2. In 2006, 17 phreatic eruptions were reported, while only 2 in 2007 and 2008 together. In 2009, 6 phreatic eruptions took place. During the first 10 weeks of 2010, 20 eruptions occurred. During the study period, a major tectonic earthquake (M 6.2 Richter, 8 January 2009, Cinchona, Red Sismológica Nacional 2009) was followed by 2 small phreatic eruptions at Poás Volcano (12 January and 21 March 2009).

20 1200

1000

800

600

400

Rainfall (mm) 200

0 20 18 may-05 oct-05 mar-06 ago-06 ene-07 jun-07 nov-07 abr-08 sep-08 feb-09 jul-09 dic-09 16 14 12 10 8 6 4 2 0 -2 -4

Lake level (cm) level Lake -6 -8 -10 -12 60 may-05 oct-05 mar-06 ago-06 ene-07 jun-07 nov-07 abr-08 sep-08 feb-09 jul-09 dic-09 55 50 45 40 35

Temperature 30 25 20 may-05 oct-05 mar-06 ago-06 ene-07 jun-07 nov-07 abr-08 sep-08 feb-09 jul-09 dic-09

21 Numerical modelling of the Boiling Lake in Dominica

D. Robertson1, N. Lewis1, E. Joseph1, N. Fournier2, F. Witham3 1: Seismic Research Centre, University of the West Indies, Trinidad 2: Institute for Geological and Nuclear Science (GNS), New Zealand 3: Department of Earth Sciences, University of Bristol, United Kingdom

The Eastern Caribbean remains one of the most volcanically active regions in the world with 21 live volcanoes spread over 11 volcanically active islands. The islands continually benefit from tourist driven revenue generated from associated volcanic features including crater lakes and active geothermal fields. Additionally, new interest has been shown in the use of regional geothermal systems as energy sources. Despite the obvious tourist and energy potential and hidden hazards, the dynamics controlling the regional geothermal systems are poorly understood. This presentation reviews the development of a multilayered numerical model of Dominica’s most renowned geothermal system, the Boiling Lake through the use of the finite element method. The numerical modelling is part of a postgraduate research project geared at fostering a better understanding of the dynamics of geothermal systems in the Eastern Caribbean. The model will be used to infer geothermal mass and energy fluxes into the Lake, and monitor variations in these parameters that may reflect changes in the underlying magmatic activity.

Keywords: Crater lakes, numerical modelling, finite element method,

22 A combined Cl and isotope balance approach for Laguna Caliente, Poás: precursory signals for single phreatic eruptions during the ongoing eruptive cycle?

D. Rouwet1, , R.A. Mora-Amador2, C.J. Ramírez-Umaña2 1: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy, 2: Centro de Investigaciones en Ciencias Geológicas, Universidad de Costa Rica, San José, Costa Rica [email protected] or [email protected]

Poás Volcano (Costa Rica) entered a new cycle of phreatic activity on 24 March 2006, after more than a decade of quiescence. From March 2006 to December 2009, at least 25 phreatic eruptions were reported.

This study reports the temporal variations in the chemical (Cl- SO4 and major cations) and isotopic (δD and δ18O) compositions of Laguna Caliente crater lake, and of the hot spring waters and fumarole condensates in its active Main Crater. For the intensely evaporating, hot, and extremely acidic Laguna Caliente (T = 36.1- 56.0°C, pH = -0.28 to 0.30), the chemical and isotopic compositions of the lake water can be strongly modified by loss of Cl and water vapour from the lake surface. Therefore, a combined Cl and isotope balance approach has been developed, based on the δD, Cl content, temperature and lake level variations observed at Laguna Caliente. Temperature rises of the lake bottom water caused by the input of magmatic fluids into Laguna Caliente (>159°C), might be sufficient to increase the viscosity of the molten sulphur pool to seal the lake bottom. In turn, this process inhibits efficient mass and energy dissipation, which can result in phreatic eruptions. The combination of high input rates of a hot magmatic fluid and low evaporation rates at the surface of Laguna Caliente seem to be a precursory signal of phreatic eruptions, which can be useful in future monitoring programmes. The Cinchona tectonic earthquake near Poás (8 January 2009, M 6.2 Richter) might have triggered the phreatic eruptions of 12 January and 21 March 2009.

23 Geophysical and geochemical precursors to the current activity at Poás volcano, Costa Rica

H. Rymer1, C.A. Locke2, A. Borgia1,3, M. Martínez4, J. Brenes4, R. Van der Laat4, G. Williams-Jones5 1: Department of Earth and Environmental Sciences, The Open University, Milton Keynes, UK 2: School of Geography, Geology and Environmental Science, The University of Auckland, Auckland, New Zealand 3: SAS - Geological Sciences, Rutgers University, Piscataway, NJ 08854, USA 4: Volcanological and Seismological Observatory of Costa Rica, Universidad Nacional, Heredia, Costa Rica 5: Department of Geology, Simon Fraser University, Barnaby, Canada

Acidic crater lakes at persistently active volcanoes act as both a moderator and an index of volcanic processes. Phreatic eruptions through these lakes typically present a limited hazard, however activity associated with a catastrophic drop in lake level and consequent degassing directly to the atmosphere, poses a serious local environmental threat. Years before symptoms of enhanced activity manifest at the surface, mass changes at depth may occur, so detection of these could give vital warning of environmentally damaging future events. We integrate and interpret a unique time series, spanning more than two decades, of gravity, lake volume, temperature, sulfate concentration and power output data from Poás volcano, Costa Rica. The gravity data (corrected for tides, deformation and water table fluctuations), show significant variations that must be due to sub-surface mass changes. The observed data correlate closely, showing a remarkably coherent cyclicity with a ~20 year period. In the early 1990s, Poás erupted acid aerosols with devastating consequences for public health and the local environment and economy. Prior to this eruption, gravity and lake temperature increased and lake volume decreased; sulfate concentration also increased. The first warnings of this event can be retrospectively identified in the data from 1985, which indicate that there was a shallow magma intrusion. Between about 1995 and 2005, most measured parameters were stable and the lake re-established. Here we report recent data which show trends similar to those observed in the late 1980s and conclude that Poás has entered a similar pre-eruption period as a consequence of renewed magmatic intrusion. A repeat of the environmental damage that occurred in the early 1990s in this region might therefore be expected within the next 2 years if the current trends continue.

24 Acid floods and acidic aerosol release caused by non-eruptive, intermittent degassing at a glacier-covered volcano: Chiginagak volcano, Alaska, USA

J.R. Schaefer1, C.M. Kassel2, D.S. Kaufman2, W.C. Evans3, W.E. Scott4 1: Alaska Division of Geological & Geophysical Surveys, Alaska Volcano Observatory, USA 2: Northern Arizona University, USA 3: U.S. Geological Survey, Water Resources Discipline, USA 4: U.S. Geological Survey, Cascades Volcano Observatory, USA

Between November 2004 and May 2005, a mass of snow and ice 400-m wide and 105-m thick melted to form a lake in the summit crater of Mount Chiginagak volcano. In early May 2005, an estimated 3.8 x 106 m3 of sulfurous, clay-rich debris and acid water exited the crater and cascaded down the south flank and funneled into two valleys. Over 27 km downstream, the acidic flood waters inundated 0.5-km3 Mother Goose Lake and thoroughly acidified it (pH from 2.9 to 3.1); aquatic life, including all plankton, were killed. The rapid lowering of the crater lake reduced the hydrostatic load on the gas-charged deeper water, causing rapid gas exsolution and release of an ambioructic flow of acidic aerosols that followed the flood path, causing defoliation and necrotic leaf damage to vegetation in a ~30-km2 area along the flow path. The discharge represented only a partial draining of the crater-lake and currently over 1 x 106 m3 of water remains in the crater and continues to supply acidic metal-laden water to Mother Goose Lake. In 2005, over two months after the event, crater lake water sampled 8 km downstream of the outlet after considerable dilution from glacial melt-water was a weak sulfuric acid solution (pH = 3.2, [SO4] = 504 mg/L, [Cl] = 53.6 mg/L, and [F]= 7.92 mg/L, sp. cond. = 1280 µS/cm). In 2008, the water exiting the toe of the south flank glacier remained acidic with anomalously high concentrations of Fe and Al (pH = 3.3, [SO4] = 570 mg/L, [Cl] = 61.3 mg/L, and [F] = 6.8 mg/L, [Fe] = 39.4 mg/L, and [Al] = 38.2 mg/L, sp. cond. = 1475 µS/cm). In August of 2009, the pH of Mother Goose Lake had risen to 4.9. To assess the frequency of similar acidification events in the past, lake sediment cores were collected from Mother Goose Lake and analyzed for elemental composition, 239+240Pu and 14C dating, magnetic susceptibility, and slurry pH. Slurry pH proved most useful in distinguishing acidic depositional conditions from normal conditions. Volcanogenic acid floods cause the bicarbonate buffering capacity of the lake to be exceeded, thereby creating low pH conditions. Based on slurry pH, at least seven acidification events have impacted the lake during the past ~3800 years, including the event in 2005. Only one of these seven acidification events was associated with one of the 54 tephra layers located in the core, indicating that most events were likely initiated by non- explosive geothermal activity of Chiginagak volcano.

25 Lake Nyos and Lake Kivu - two contrasting examples of dangerous gas-rich lakes

M. Schmid1, N. Pasche1,2, M. Halbwachs3, A. Wüest1

1Surface Waters – Research and Management (Surf), Eawag: Swiss Federal Institute of Aquatic Science and Technology, Kastanienbaum, Switzerland 2current address: Lake Kivu Monitoring Program, Ministry of Infrastructure, Gisenyi, Rwanda 3Data Environnement, Z. I. de l'Erier, 73290 La Motte Servolex, France

Limnological gas eruptions are a rare natural hazard that was unknown before the catastrophic gas eruptions from the two Cameroonian crater lakes Monoun in 1984 and Nyos in 1986. They can only occur in permanently stratified deep lakes. The vertical exchange by turbulent diffusivity needs to be weak, and sufficient amounts of gases must be either produced within or injected into the deep water. Gases can then accumulate until the sum of the partial gas pressures approaches the hydrostatic pressure at some depth. Then, a relatively small disturbance of the stratification can cause local supersaturation, and the subsequent formation of bubbles can trigger the degassing of a large part of the gases stored in the lake. Lake Nyos has been artificially degassed since 2001 by draining deep water to the surface where the gases are emitted to the atmosphere. The system is self-sustaining, driven by the buoyancy of the gas bubbles within the pipe. It removes more gas than is supplied by the subaquatic springs, minimizing the probability of a gas eruption as long as the degassing system is maintained. The deep waters of Lake Kivu contain enormous amounts of dissolved carbon dioxide (250 km3 STP) and methane (60 km3 STP). The release of only a fraction of these gases could have catastrophic consequences for the densely populated region. Presently, the total gas pressure in the lake is below 60% saturation, and an extraordinary event would be needed to trigger a gas eruption. Contrary to Lake Nyos, an eruption from Lake Kivu would be triggered by methane rather than

CO2, but the erupted gas mixture would still mainly contain CO2. Recent measurements indicate increasing methane concentrations. Several potential causes for this have been discussed, including increased external nutrient inputs, the introduction of the sardine Limnothrissa miodon from Lake Tanganyika, and increased discharge of the subaquatic springs feeding the lake due to increased precipitation in the region. Since 2008, a first pilot plant is extracting methane from the lake, producing power and reducing the gas content of the lake at the same time. Large scale extraction is expected to build up in the next decades, but this needs to be carefully designed in order to avoid negative consequences for the safety and the ecology of the lake and its environment.

26 The Hule and Río Cuarto maars, Chapter One: The beginning

G.J. Soto1, 2, 3, G.E. Alvarado 1, 2, F.M. Salani 4, P. Ruiz 5, L. Hurtado de Mendoza1 1: Instituto Costarricense de Electricidad, Costa Rica 2: Universidad de Costa Rica, Costa Rica 3: Terra Cognita Consultores 4: Universidad de Buenos Aires, Argentina 5: Rutgers University, USA

The Hule and Río Cuarto maars are located 11 and 18 km northward of the active Poás volcano, on the Caribbean side of the Central volcanic range of Costa Rica. They lie along the northern part of a N-S trending volcanic fracture zone, ~20 km-long crossing Poás volcano. Along this “rift”, basaltic andesitic to andesitic aphyric lavas have been erupted through several vents along the last 200 ka. Both maars cut mid to distal volcanic facies. A prominent N70ºW fault scarp, 200 m high, is located between them, which is interpreted as a thrust propagation fault-fold. Hule is a sub-circular depression (2.3 km x 1.8 km, area ~3.5 km2). The volcanic products from the maar explosion are mainly pyroclastic surges (poorly vesiculated andesites with tiny plagioclases), acidic andesitic pumice flows, air fall deposits, ballistic blocks, and reworked deposits that overlie the regional basement and a first level of organic debris (BOD). Two intra- maar overlapping pyroclastic cones are into Hule maar, and at least 3 lava fields are related to them (high-Al basalt to basaltic andesite). Another explosion depression, Los Ángeles (400 m across), is located less than 1 km off the SE rim of Hule. Río Cuarto is a nearly circular crater (700-850 m across) and an area of 0.33 km2. Its inner lake has a maximum depth of 66 m (the deepest natural lake in Costa Rica). Río Cuarto products include surges and air-fall tephra. Its deposits show a narrow fan oriented westward, according to eastern-coming wind direction, indicating a directional surge (first 2 km), and latter to a relative low altitude explosive column. The air-fall deposits are not traceable farther than 5 km away. The stratigraphic sequence shows three main explosive phases. Radiocarbon dating and field work have confirmed that Hule was formed ~6.2 calendar ka ago and Pata de Gallo probably formed ~2.8 ka. Younger pyroclasts ~1.7 ka and ~0.7 ka would be related to eruptions from elsewhere in the neighborhood. There are no ages available yet for dating the formation of Río Cuarto maar, but archaeological data suggest that it erupted between 3-4 ka ago. The minimum volume of pyroclastic deposits associated with Hule maar can be estimated in 0.37 km3 and up to 0.51-0.53 km3 (0.36-0.40 km3 DRE), from which ~20% are juvenile material, therefore 0.07-0.08 of new DRE magma. The tephra from Río Cuarto is estimated in 4.4 x 107 m3, which makes 0.008 m3 of new DRE magma.

27 The security of the lake Nyos dam: Geological, geophysical, hydrological and geotechnical evaluation

G. Tanyileke Institute of Geological and Mining Research (IRGM), Yaounde, Cameroon [email protected]

In addition to the problem posed by the huge stock of CO2 gas in Lake Nyos, its security is further complicated by the specific geological and geotechnical characteristics of the dam at its outlet. The upper 40 m of this narrow natural dam is composed of poorly consolidated pyroclastic rocks. It’s estimated to be ≈ 6000 – 8000 years old and is apparently structurally weak, for subjected to mechanical and chemical erosion. Its collapse could result in the release of an estimated 55 million m³ of water resulting in a major flood with severe damage to communities downstream as far away as neighbouring Nigeria. Geological, geotechnical hydrological and geophysical studies indicate the need to reinforce it.

28

Rare Earth Elements and 87Sr/86Sr in Neutral and Acidic Waters of El Chichón Volcano, Chiapas-Mexico

Y. Taran1, D. Rouwet2, L. Peiffer1 1: Institute of Geophysics, Universidad Nacional Autónoma de México, México D.F., Mexico 2: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy

Natural manifestations of El Chichón volcano-hydrothermal system can be classified as belonging to four different groups: (1) near-neutral Na-Ca-Cl-SO4 hot waters discharging from numerous springs at the volcano slopes with moderate salinities (1500-2200 ppm of Cl); (2) acidic water from the crater lake with a variable salinity but a nearly constant pH (3 to 3000 ppm of Cl- and pH 2.3); (3) high salinity (10,000 ppm of Cl-) Na-Cl waters from the hottest springs at the NW flank of the volcano; (4) near-neutral water with the regularly decreasing salinity (SP spring, from 15,000 ppm in 1995 to 4,000 of Cl- in 2009) inside the volcano crater. The high-salinity springs of the NE flank (Agua Salada) have a lowest flow rate (~10 kg s-1) and variable pH from 2.5 to 7.8. It was found that the acidity of the Agua Salada springs is derived from the shallow oxidation of H2S – a thermal field with bubbling gas, steaming ground and steam-heated pools has been discovered in 2009 in la selva, some 100 m up from the main “acidic” group of Agua Salada. The most unusual characteristic of this gas is its low 3He/4He ratio (2.3 Ra, He/Ne=180), taking into account very high 3He/4He in steam of the crater fumaroles (up to 8 Ra). Here we report new data on geochemistry of thermal waters of El Chichón including the rare Earth elements and Sr-isotopes in all types of El Chichón thermal waters. The REE distributions for the lake water, SP spring and all neutral springs at the volcano flanks except Agua Salada are similar and almost coincide with the El Chichón volcanic rock REE pattern. In contrast, waters from Agua Salada neutral and acidic springs show a greater enrichment in LREE and a Eu-maximum , typical for some high-temperature and highly reduced thermal waters (Wood, 2003). Values of 87Sr/86Sr in all but Agua Salada waters are close to the El Chichón trachyandesite value of 0.7041. In contrast, Agua Salada waters, neutral and acidic, are characterized by 87Sr/86Sr ~ 0.7052, indicating a mixed, crustal-magmatic source of Sr. Along with the data on trace elements and mixing relationships between major ions (Cl, Na, Ca, Mg), these data evidence about the existence of at least two thermal aquifers feeding thermal springs of the volcano. A shallow aquifer beneath the crater is composed of volcanic rocks (pyroclastics and dome roots) and heated by shallow magma bodies associated with the plumbing system of the volcano dome complex. The aquifer feeding Agua Salada is probably much deeper and composed of sedimentary rocks exposed in the vicinity of the volcano. As rising to the surface, an additional contact with volcanic rocks adds some magmatic Sr to this deeper water; resulting 87Sr/86Sr reflects a mixing of magmatic and crustal Sr.

29 Evidences of geochemical and isotopic vertical and horizontal heterogeneities in the Lake Kivu (Democratic Republic of the Congo)

F. Tassi1, , O. Vaselli1,2, D. Tedesco3,4, G. Montegrossi2, T. Darrah5, E. Cuoco3, M.Y. Mapendano6, A. Delgado Huertas7 1: Department of Earth Sciences, University of Florence, Via G. La Pira 4, 50121, Italy 2: CNR-IGG, Institute of Geosciences and Earth Resources, Via G. La Pira 4, 50121, Italy 3: Department of Environmental Sciences, 2nd University of Naples, Via Vivaldi 43, 81100 Caserta, Italy 4: United Nations, Office for the Coordination of Humanitarian Affairs, Switzerland 5: Environmental and Earth Sciences Department, University of Rochester, 227 Hutchison Hall, University of Rochester, NY 14627 Rochester, USA 6: Goma Volcano Observatory, Goma, Democratic Republic of the Congo [email protected]

Lake Kivu (1,460 m a.s.l.) is one of the Africans Great Lakes. It lies on the border between the Democratic Republic of Congo and Rwanda and is located in the tectonically and volcanically active Western Branch of the EARS (East African Rift System). The lake is characterized by the presence of a huge gas reservoir at 3 3 depth >200 m, mainly consisting of CO2 (256 km ) and CH4 (65 km ). The northern side of the lake is marked by the presence of two active volcanoes: Nyamulagira and Nyiragongo. During the two known historical eruptive episodes (1977 and 2002) Nyiragongo has produced lava flows that have occasionally reached and partly destroyed the city of Goma (2002) and the surrounding villages. In 2002 a lava flow entered the lake causing serious concerns about the possible occurrence of a (thermal driven) gas outburst from the lake, as those experienced at the lakes Monoun (1984) and Nyos (1986), both in (Cameroun). In order to provide insights into the associated risk of limnic gas eruptions and the likely impacts on the local population, the mechanisms regulating the dynamic equilibrium and stability of Lake Kivu have been studied over the last decade, mainly on the basis of water and dissolved gas chemical data provided by previous investigations ors. Despite all the efforts deployed over the years by the previous workers investigations, the main physical o-chemical processes controlling the peculiar geochemical features of this lake are not conclusively explained. In this work the distribution of chemical and isotopic compositions of the water and dissolved gas along vertical water columns within different areas of Lake Kivu was used to: i) compositionally characterize the lake into different morphological sectors into which Lake Kivu is divided; ii) assess the geochemical features of the hydrothermal fluid emissions discharging from the lake’s bottom; iii) provide a geochemical conceptual model of the lake by taking into account precipitation and dissolution processes, effects of biological activity and inputs of deep-seated fluids.

30 Chemical and isotopic features of gas reservoirs in Albano, Averno and Monticchio crater lakes (central-southern Italy)

F. Tassi1,2, , O. Vaselli1,2, J. Fiebig3, J. Cabassi1, M. Nocentini1, A. Delgado Huertas4 1: Deptartment of Earth Sciences, Via G. La Pira, 4, 50121 Florence, Italy 2: CNR – Institute Geosciences and Earth Resources, Via G. La Pira, 4, 50121 Florence, Italy 3: Institute for Geology and Paleontology, Bio-INCREMENTS Research Group, Goethe University, Senckenberganlage 32–34, 60325 Frankfurt/Main, Germany 4: CSIC – Estación Experimental de Zaidin, Prof. Albareda 1, 18008 Granada, Spain [email protected]

Deep waters of crater lakes hosted in non-active volcanoes are able to store huge amounts of gases, mainly

CO2 and CH4, that can be added from sub-lacustrine vents and/or produced by processes related to bacterial activity. Destabilization of deep lake strata may trigger massive release of dissolved gases producing “limnic eruption”, like those occurred at the Cameroonian Lakes Monoun and Nyos in 1984 and 1986, respectively. Volcanic systems of the central and southern Italian peninsula host several crater lakes. Among them, the Albano (Alban Hills volcanic complex; Central Italy), Averno (Phlegrean Fields; southern Italy), Monticchio Grande and Monticchio Piccolo (Vulture volcano; southern Italy) lakes are the only ones showing chemical and thermal stratification and presence of significant amounts of dissolved gases at depth. The distribution of the dissolved gas composition along the vertical profiles of these lakes is similar, being characterized by dominating N2 in the oxic epilimnion, while CO2 is the main gas species in the anoxic hypolimnion. The vertical patterns of CH4 concentrations resemble those of CO2, since both these 13 compounds show an increase from the surface to the bottom of 3-4 orders of magnitude. The δ C-CO2 values of Monticchio Grande, Monticchio Piccolo and Albano lakes (ranging between -0.4 and -5.8 ‰ V- 13 PDB) are consistent with those of mantle-derived CO2. Conversely, at the Averno lake the δ C-CO2 values 13 range between -8.2 and -13.4 ‰ V-PDB, supporting the occurrence of prevalent organic CO2. δ C-CH4 and

δD-CH4 values of all the investigated lakes (down to -67 ‰ V-PDB and -283 V-SMOW, respectively) suggest that bacterial activity is basically the main responsible of CH4 production. The carbon isotopic signature of the two main dissolved gas species along the vertical profiles seems to depend, besides of their origin, on 1) CO2-CH4 isotopic exchange, 2) CO2 reduction to CH4 at reducing conditions, 2) CH4 oxidation 13 to CO2 at oxidizing conditions. The δ C-CO2 values are indeed progressively more positive at increasing 13 depth, whereas an opposite trend is shown by the δ C-CH4 values. In conclusion, these results have shown that, although the morphometric features (water volumes of Monticchio Grande, Monticchio Piccolo, Averno and Albano lakes are 3.3 x106, 4 x106, 6 x106 and 450 x106 m3, respectively) and the relatively low gas concentrations (max 19.4 mmol/L at a depth of 39 m in the Monticchio Piccolo lake) suggest that the gas reservoirs of these lakes cannot presently represent a serious 31 13 13 hazard for limnic eruptions, the vertical patterns of the CO2/CH4 ratio and the δ C-CO2 and δ C-CH4 values may represent useful monitoring tools to control the rate of fluids discharged from the lake bottoms.

32 Triggers and precursors of the volcanic crisis at Turrialba Volcano (Costa Rica): Results from a geochemical and geophysical monitoring

F. Tassi1,2, , O. Vaselli1,2, E. Fernández3, E. Duarte3

1: Department of Earth Sciences, University of Florence, Via G. La Pira 4, 50121, Florence, Italy 2: CNR-IGG Institute of Geosciences and Earth Resources, Via G. La Pira 4, 50121, Florence, Italy 3: Volcanological and Seismological Observatory OVSICORI, Heredia, Costa Rica [email protected]

Turrialba (10º02N, 83º45W) is a 3,349 m high basaltic-andesitic stratovolcano that lies at the distance of 35 and 15 km from San José and Cartago, the two largest cities in Costa Rica. After the last eruption occurred in the West Crater in 1864-1866, volcanic manifestations were limited to weak fumarolic discharge (continuous since 1980) from the summit with outlet temperatures up to 93 °C. In 2001, seismic swarms, ground deformation and increasing fumarolic activity occurred. From 2005 to 2008, new fumarolic vents appeared in the Central and West summit craters, in the fracture system in between, and along the western and southwestern outer flanks of the volcanic edifice, showing sulphur deposits and progressively increasing degassing rate. In 2007, fumaroles and new fissures were also recognized at the base of the volcanic edifice along the WSW-ENE-trending Ariete fault. Seismic swarms recorded in the same period followed an increasing trend. The maximum seismic activity to date, up to thousands of events/day, was recorded in mid 2007. An inflationary trend was indicated in the crater area. These physical changes were accompanied by a drastic evolution in the fumarolic gas chemistry, which followed three distinct phases: 1) Hydrothermal (from 1998 to autumn 2001), characterized by the presence of H2O, CO2, H2S and to a very minor extent, HCl and HF; 2) hydrothermal/magmatic (autumn 2001-2007), 18 with the appearance of SO2, a significant increase of HCl and HF, and, since 2005, a change of the δ O and δD isotopic signature of steam, from meteoric to magmatic; 3) magmatic-dominated (2007 up to present), characterized by SO2/H2S>100 and fumarolic temperatures up to 282 °C (West crater, January 2009).

Accordingly, SO2 flux measured with mini-DOAS, has increased two orders of magnitude (1 t/day in 2002 to 740 t/day in January 2008). The volcanic plume produced by the increased fumarolic activity is presently affecting the W and SW flanks of Turrialba volcano causing heavy damages to vegetation and the local economy. Gas equilibrium in the CO2-CH4-H2 system suggests a progressive evolution of the deep fluid reservoir toward higher temperatures and more oxidizing conditions. The magmatic-dominated phase is still prevailing as evidenced by the fact that on the 4th of January 2010 at 16.57 (GMT) a loud explosion was heard from the West crater and followed by three other explosions spaced out every 10 minutes. These events were interpreted as associated to phreatic eruptions. The chemical-physical modifications of Turrialba in the last decade can be interpreted as part of a cyclic mechanism controlling the balance between the hydrothermal and the magmatic systems. In this context, the

33 risk of rejuvenation of the volcanic activity cannot be excluded, and an appropriate seismic, ground deformation and geochemical monitoring program is highly recommended.

34 Monitoring hydrothermal systems of Poás and Turrialba volcanoes, Costa Rica

R. Teasdale1, , J. Wenham1, S. Mendes1, R. Del Potro2, M. Martínez3, E. Fernández3 1: Geology, CSU Chico, Chico, CA 95929-0205 USA 2: Earth Sciences University of Bristol Bristol BS81RJ UK 3: Volcanology, OVSICORI-UNA, Heredia, 2346-3000, Costa Rica [email protected]

Recently installed temperature data loggers measure temperatures of the ultra acidic crater lake and fumaroles at Poás and Turrialba volcanoes of the Central Cordillera, Costa Rica. The active crater of Poás hosts a highly dynamic lake, Laguna Caliente, with low pH (-0.87 - 1.75) and temperatures that have ranged from 22 - 94°C in the last three decades. A recent compilation of over 20 years of gravity data indicates that Poás is currently in an episode of magma intrusion, which based on previous events, likely precedes temperature and chemical changes in the lake on order of years (Rymer et al., 2009). Poas had at least three phreatic events in 2009, including that of 18 September, which was preceded by a sharp decrease and then increase in lake water temperature. Degassing and seismic activity at Volcán Turrialba have increased since 2005, prompting increased monitoring efforts. Fumarole temperatures and gas compositions reflect further increased activity since 2007 which peaked in January 2010 with a phreatic eruption. Gas fumes dispersed by trade winds toward the NW, W, and SW flanks of Turrialba volcano have caused significant vegetation kill zones. Summit surface fractures are oriented parallel to the major fault system, so fumarole temperatures are monitored in fractures in an attempt to correlate fracture activity with magmatic pulses. Previously, water and fumarole temperatures and compositions at Poás and Turrialba volcanoes have been measured approximately once a month. This work records crater lake and fumarole temperature data every 15 minutes at each volcano. Using methods developed at Lassen Volcanic Center in the Cascades arc, temperature probes and data loggers were installed at Poás and Turrialba in June 2009. Initial data show that these methods are capable of providing high quality, meaningful data. Crater lake temperatures recorded at Poás volcano increased consistently from 55-57 °C prior to the September 2009 event. Once downloaded, we expect to correlate temperature data from Turrialba Crater Oeste with activity of January 2010. Temperatures continue to be recorded and are downloaded periodically at both volcanoes in an attempt to record small scale variations that can be correlated to other monitoring data sets (e.g. GPS, lake and fumarole geochemical compositional data, volcanic seismicity, etc.) for thorough monitoring of the hydrothermal and magmatic systems of Poás and Turrialba volcanoes.

35 Dynamic behaviour of the Poás crater-lake system from cation geochemistry

M.J. van Bergen1, M. Martínez1,2, E. Fernández2, J. Barquero2 1: Department of Earth Sciences, Utrecht University, Utrecht, the Netherlands 2: Volcanological and Seismological Observatory of Costa Rica, Universidad Nacional, Heredia, Costa Rica

Geochemical monitoring of Laguna Caliente during three decades demonstrates that magmatic- hydrothermal activity in the summit area of Poás volcano is highly dynamic and fluctuates on relatively short timescales. Alternating active and quiet intervals since the late 1970s suggests a periodicity in major activity cycles on the order of 6-10 years. Temporal variations in the concentrations of major and trace cations largely follow anion trends, except for those controlled by precipitation of saturated minerals in the lake. Variable proportions among cations reflect the degree of congruent rock dissolution, which reaches a maximum when the state of activity is highest. Systematic deviations are mostly due to formation or dissolution of secondary alteration minerals, notably alunite and other Al-bearing mineral phases. Their formation requires higher temperatures than observed in the lake, implying that cation budgets are largely controlled by influx of hot brine derived from a liquid-dominated part of the hydrothermal system. We infer that precipitation of alteration minerals in pore spaces, fractures and channels, reducing porosity and permeability in the subsurface, ultimately inhibits volatile and heat throughput, and is instrumental in regulating the properties of the lake as well as the vigour and location of subaerial fumarolic emissions elsewhere in the crater. Occasionally, sealing effects lead to almost complete shutdown of volatiles and heat input during brief intervals. Rare earth elements appear to be sensitive monitors of water-rock interaction and fluid cycling. A steady trend in Eu anomalies since the early 1990s points to extended interaction with progressively more altered rocks, reflecting maturation of the hydrothermal system, until the end of 2005 when a sudden change in Eu/Eu* reflects renewed exposure to fresh rock, presumably the result of magma intrusion below the crater. Other cation signatures are consistent with such an event. We conclude that successive stages of activity and quiescence in the lake area coincide with alternating vapour-phase and liquid-phase dominance in the hydrothermal domain between a cooling magma body and the crater area. Resurging input of heat and volatiles, eventually leading to sporadic phreatic activity, could be triggered not only by magma (fresh intrusion or fracturing of the brittle chilled margin around a cooling body) but also by rupture of an impermeable seal in the overlying sequence of altered lavas and pyroclastics.

36 Twelve years of volcanic fluid chemistry monitoring at Copahue Volcano, Argentina

J.C. Varekamp1, T. Kading1, A. Bermúdez2, D. Delpino3 1: Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459, USA 2: CONICET, National University of Comahue, Neuquen, Argentina 3: REPSOL-YPF, Dirección General de Exploración, Talero 360 – (8300) Neuquén, Argentina

Copahue Volcano (37.75oS, 71.17oW) had minor phreato-magmatic eruptions in 1992-1995 and a small magmatic eruption in 2000. An active volcano-hydrothermal system (pH~0, T~300 oC) at depth scrubs all volcanic volatiles, and SO2 disproportionates into bi-sulfate and liquid elemental Sulfur. The acid fluids are injected into a crater lake and acid hot springs, which feed a glacial meltwater river that drains into the large glacial Lake Caviahue. We have measured the water fluxes and river water and crater lake compositions for 12 years, including the 2000 eruptive period. This has provided a flux record of volcanic elements (VE: F, Cl, S, toxic trace elements like Pb, Cu, As) and major Rock Forming Elements (RFE: Al, Si, Fe, Mg, Ca, K, Na). The annual river flux measurements are complemented with analytical data from vertical water profiles through Lake Caviahue, which through its ~3.5 years water residence time has a ‘chemical memory’ of past element fluxes. A two-box non steady-state dynamic lake model was used to constrain the variations in element influxes into the lake for comparison with the annual influx measurements. Precipitation of jarosite / alunite since 2000 has strongly reduced the flux of K and Al into the lake. Recent increases in K, Al, and S fluxes indicate that these secondary precipitates have begun dissolving and are now being flushed out of the volcano. Furthermore, copper, lead, thallium, arsenic, and barium concentrations have increased considerably in the crater lake while the concentration of most other elements fell. The presence of high levels of lead, barium, and phosphorus in a K-Fe mineral found precipitating at the mouth of the hot spring suggest this mineral is forming within the volcano and its dissolution in the volcanic brine is the source of recent increases in trace metal concentration and fluxes. The S flux rate was translated into a volumetric magma degassing rate of ~ 3.5 108 m3/decade (using a high estimate of 200 ppm S released from the melt, based on glass inclusion and matrix glass analyses), whereas the mean RFE flux provides a rock dissolution rate of ~2 105 m3/decade. The degassing ultimately leads to magma crystallization because of water loss, suggesting that the magma solidification rate is at least 1000 times larger than the rock dissolution rate, leading to net-volcano growth. Additional growth stems from the small eruptions that emplace magma at the surface. The rock dissolution probably leads to periodic flank collapses, which has given Copahue its rounded shape and modest elevation of 3000 m.

37

POSTER SESSION

38

Geological and structural setting of the El Tatio geothermal field (Antofagasta region, Northern Chile) and environmental impact of thermal fluid discharges

B. Capaccioni1, F. Lucchi1, P.L. Rossi1, F. Tassi2, C.A. Tranne1, F. Aguilera3 1: Dept. of Earth Science and Environmental Geology, P.za di Porta S.Donato, 40126, Bologna, Italy 2: Dept. of Earth Sciences, University of Florence, Via La Pira 4, 50121, Firenze, Italy 3: Dept. of Geology, University of Atacama, Copayapu 485, Copiapò, Chile

The El Tatio active geothermal field, the largest and best known of northern Chile, is sited within the El Tatio volcanic area (central Andes, northern Chile). Its stratigraphic architecture is given by andesitic to dacitic stratovolcanoes and lava domes emplaced from Miocene to Holocene under control of NNE-SSW alignments, which are interbedded with regional ignimbrite sheets originated in the Altiplano Puna Volcanic Complex (APVC). Intense tectonic activity in the El Tatio area is shown by a series of roughly NNE-SSW oriented, east-dipping and west-verging thrust faults of middle-Late Pleistocene age, associated with a NW-SE oriented strike-slip fault system. This structural pattern likely indicates a recent (or present) development of NW-SE-directed contractional deformation processes at the western border of the APVC. These tectonic structures likely exert a control on the El Tatio geothermal field, the latter consisting of tens of thermal springs, fumaroles, geysers and boiling and mud pools subdivided in four main areas: a) “Central”, b) “West”, c) “Corfo”, and d) “Geyser Blanco”. Extended sinter deposits and polychrome algae mark the hydrothermalized terrains. Chemical and isotopic compositions of the discharged fluids appear to be mainly related to the boiling at depth of a meteoric-originated aquifer. Gas geoindicators suggest reservoir conditions of ~ 250 °C and fH2O ~ 40 bars corresponding to an hydrostatic depth of ~ 400 m. This appears in a good agreement with the thermal gradient measured during the drilling of geothermal wells performed between 1969 and 1971. Isotopic equilibrium between CH4 and CO2 appears to be established at higher temperatures, in the range of 300-380 °C, which implies a fH2O > 200 bars, i.e. at a depth of >2000 m. Hydrothermal fluids emerge at the surface as Na-Cl “mature” waters, showing very high concentrations of minor and trace components, such as B (up to 193280 ppb), Cs (up to 21310 ppb), Li (up to 37.70 ppm), total As (up to 31340 ppb), Rb (up to 4065 ppb), and Sr (up to 4037). Significant concentrations of these elements were also measured in superficial waters collected S (Purifica, Puritama, Machuca, S. Pedro, Tucle and Toconau rivers), SW (Corfo river) and E (Salado river) of the Central emission area. For instance, As and B concentrations in the Salado and Tucle rivers were one order of magnitude higher than the Maximum Acceptable Concentration (MAC) for drinking water, according to the EEC (European Economic Community) directive 98/83. Such strong anomalous concentrations of these contaminants were detected downstream along the Salado river for tens of km, and in general are a common feature for all the rivers and cold springs over a very large area surrounding the El Tatio geothermal field. On the whole, the main water sources for drinkable waters of the Calama and S. Pedro de Atacama villages,

39 i.e. Salado and S. Pedro rivers, have resulted significantly affected by the impact deriving from contribution from the endogenous vapours discharged at El Tatio, suggesting that the local authorities should adopt a severe monitoring of the water quality.

40 Geochemical characterization of the thermal and cold waters associated to Ubinas volcano, South Peru

V. Cruz , K. Gonzales Instituto Geológico Minero y Metalúrgico INGEMMET, Av. Canadá Nº 1470, San Borja Lima 41 - Perú - Apartado 889. Formerly: Instituto Geofísico del Perú, Lima. [email protected], [email protected] 1. Ubinas (16°22’S, 70°54’W; 5672 m. a. s. l.) is regarded as the most active volcano of the peruvian Central Volcanic Zone (CVZ) of the Andes. Since 1550, up to 23 degassing and ashfall events occurred. Thermal and cold water springs discharging in the area of the Ubinas volcano show NaCl and Ca(Mg)-

Cl(SO4) chemical compostion. These geochemical features are likely produced by mixing of 3 end-

members: 1) Cl-rich waters, 2) meteoric water, and 3) magmatic-related, SO4-rich fluids. Cl-rich waters, likely related to a deep geothermal fluid reservoir, seems to be fully equilibrated in the Na- K-Mg system. Conversely, waters seeping out at the edge of the Lagoon Salinas, where Boron salt is exploited, seem to attain a partial chemical equilibrium. Low salinity waters display a chemistry typical of immature waters showing relatively high Mg concentrations caused by isochemical rock dissolution. Therefore, a well-developed hydrothermal system, affected by conspicuous inputs from an active magmatic system, is likely the main source of fluids discharging at the Ubinas volcano.

41 Total CO2 budget discharged from Vulcano island (Aeolian Island, Italy) and continuous

monitoring of summit CO2 flux (2007-2009)

S. Inguaggiato1, , A. Mazot2, F. Vita1, D. Rouwet1, C. Inguaggiato3, S. Morici1 1: Istituto Nazionale di Geofisica e Vulcanologia - Sezione di Palermo, Italy 2: GNS Science Wairakei Research Centre 114 Karetoto Road, Wairakei, Private Bag 2000, Taupo,New Zealand 3: Università di Palermo – Facoltà di Scienze Geologiche - Palermo Italy [email protected]

Total CO2 output from fumaroles, soil gases, bubbling and water dissolved gases were estimated at Vulcano

Island, Italy. The CO2 output from fumaroles was estimated from SO2 plume flux, while CO2 discharged though diffuse soil emission was quantified on the basis of 730 measures of CO2 fluxes on the island surface, performed by means of the accumulation chamber method. Preliminary results indicate an overall output of 650 t/day of CO2 from the island. The main contribution to the total output is from the summit area of the active cone (609 t/day), being 408 t/day and 201 t/day from crater fumaroles and crater soil degassing, respectively. The release of CO2 from peripheral areas is 31 t/day by soil degassing (Palizzi and

Istmo areas mainly), and this measure is comparable to both the contribution of water dissolved CO2

(estimated as 6 t/day), and sea-bubbling CO2 (4 t/day measured in the Istmo area). The presented data (September 2007) refer to a period of moderate solphataric activity, when the highest temperature and gas/water ratio of fumaroles were 457 °C and 0.17, respectively. The calculated total CO2 emission allows the estimation of the background mass release and related thermal energy from the volcanic system. On the basis of the indications provided by this geochemical survey, an automated CO2 soil monitoring station was installed on the summit area of Vulcano island in September 2007. Here we report the first two years of

CO2 flux measured hourly, together with environmental parameters.

42 Watershed attenuation of volcanic Hg, As, and P at Copahue Volcano, Argentina

T. Kading1, J. Varekamp1, M. Andersson2, P. Balcom2, R. Mason2 1: Department of Earth and Environmental Sciences Wesleyan University, Middletown, CT 06459 USA 2: Department of Marine Sciences, University of Connecticut, Groton, CT 06340 USA

Copahue volcano (37.75oS, 71.17oW) is the primary source of the toxic elements mercury and arsenic and the nutrient phosphorus to the local watershed. These dissolved constituents occur in a hyper-acidic stream that is fed by spring fluids that scrub volcanic gaseous emissions at depth. Direct dissolved gaseous mercury measurements at the volcanic hot spring reveal that very little dissolved elemental mercury (~1%) is present in these fluids. Hg fluxes in the volcanic stream doubled from 2008 to 2009 from ~10 g/month to ~20 g/month, suggesting substantial temporal variability in Hg emissions. Hg/S mass ratios were 0.3 – 0.5 x 10-8, and these low values suggest that Copahue volcano is not a major global source of mercury. The dissolved Hg concentration decreases from 110 ng/l in the volcanic hot spring to less than 1 ng/l in Lake Caviahue and the Lower Rio Agrio. A statistically significant increase in dissolved mercury concentration is found below the thermocline in Lake Caviahue, which could be the result of Hg regeneration from the sediment bed. Two important sinks for mercury in the watershed may be the photoreduction of ionic mercury to DGM in the euphotic layer followed by evasion into the atmosphere, and adsorption to organic matter. The latter is then sequestered into the sediments of Lake Caviahue during the fluid’s long (3.5 year) residence time. Positive correlation between % carbon and total mercury in sediments suggest that even in low pH (2 - 3) conditions mercury sorbs strongly to organic matter. Arsenic emissions increased from 2008 to 2009 from 0.5 tonnes/month to 1.3 tonnes/month, while phosphate emissions increased slightly from 24.8 to 27.3 tonnes/month. Phosphate and arsenic concentrations decrease conservatively with meteoric dilution throughout the watershed until a red-orange ocher appears, which brings dissolved concentrations close to zero. This ocher has been identified as 3+ Schwertmannite (Fe 16O16(OH,SO4)12-13·10-12H2O) by NIR reflectance spectra, bulk chemical analysis, SEM-EDX, and TEM. The mineral is actively precipitating downstream from Lake Caviahue. MINEQL modeling of fluid chemistry suggests precipitation occurs when pH = 2.9, log aFe+++ = -5.7, and log aSO4-- = -3.0, which is similar to conditions described by Bigham et al. (1996). These samples have been found to be highly enriched in arsenic, phosphorus and vanadium (resp. 1000, 5000, and 1000 ppm ) suggesting removal occurs by adsorption and possibly by substitution of oxyanion species.

43 Behaviour of polythionates in the hyper-acidic crater lake of Rincón de la Vieja volcano (Costa Rica)

M. Martínez1,2, , M.J. van Bergen2, , B. Takano3, , W. Sáenz1, E. Fernández1 1: Observatorio Vulcanológico y Sismológico de Costa Rica (OVSICORI), Universidad Nacional Heredia, Costa Rica 2: Faculty of Geosciences, Utrecht University, Utrecht, the Netherlands 3: Department of Chemistry, Graduate School of Arts and Sciences, University of Tokyo Japan [email protected] , [email protected] [email protected]

Rincón de la Vieja, an active stratovolcano in NW Costa Rica (1805 m.a.s.l.) hosts a hot hyper-acidic crater lake, which has been a focal point of fumarolic degassing and various phreatic eruptions since Rincón´s latest phreatomagmatic activity in 1983. Phreatic eruptions were registered in 1983-87, 1991, 1995 and 1998, some of which triggered damaging lahars. Time-series trends for polythionates (PT) and other physico-chemical and geophysical data for the period February 1992-September 2006 point to large fluctuations in the influx of heat and sulphur-bearing magmatic volatiles into the lake. Total PT concentrations ranged between 3900 ppm and below detection limits within this monitoring period. Overall, tetrathionate was predominant, whereas hexathionate was always the least abundant of the species measured. From February 1992 till March 1998 and from March 2001 till August 2002 PT were absent or at very low concentrations, while the lake temperature was around 39°C and 43°C, and the pH about 0 and up to 1, respectively. Physico-chemical conditions were consistent with a significant input of heat and volatiles into the lake, which culminated in a series of strong phreatic eruptions in November 1995 and in February 1998. In each scenario, the virtual absence of PT likely resulted from sulphitolytic breakdown (i.e. fumarolic SO2/H2S molar ratios could have been much greater than unity). In contrast, PT concentrations were, in general, highest in August 1998-February 2001 and in December 2002-September 2006, ranging from several hundreds up to thousands of ppm. Maximum values were observed in August 1998 (a total of 3900 ppm) and between June 2005 and September 2006 (1500- 2000 ppm). The high concentrations in these two periods suggest a moderate fumarolic input into the lake, with SO2/H2S molar ratios that could have been around 0.13, i.e. with no excess of SO2. Our findings indicate that PT are sensitive to the state of activity of Rincón.

44 Chemical characteristics of phreatomagmatic explosion cones located at the eastern border of the Transmexican Volcanic Belt, Mexico

Y. Ocampo Uribe , A. Ramírez-Guzmán , L. Jerónimo Salgado, O. Talavera-Mendoza

Unidad Académica de Ciencias de la Tierra. Universidad Autónoma de Guerrero Ex- Hacienda de San Juan Bautista s/n. Taxco el Viejo, Guerrero CP 40223 [email protected]

[email protected]

The Serdan-Oriental drainage basin located some 23 km east of the city of Perote, Puebla State Mexico, is endoric, arid and the streams are ephemeral. The region is characterized by a volcanic complex with strato- volcanoes like Pico de Orizaba or Citlaltépetl, La Malinche and Cofre del Perote which have been active over the last 3 m.y (Pliocene-Quaternary). Nearby are rhyolitic domes like Cerro Pizarro, Cerro Pinto and Las Derrumbadas with ages varying from 0.51 to 0.24 Ma as well as small cinder, scoria, and six phretomagmatic (Alchichica, Quechulac, Atexcac, La Preciosa, Aljojuca y Tecuitlapa) cones known in the region as xalapascos (maars). This study describes the hydrochemical characteristics of two of these maars, Alchichica and Quechulac (located 6 km apart and some 32 km from the Los Humeros geothermal field). The results indicate a salinity (TDS) of 10,024-12,707 mg/L, Cl in the range of 4035-5067 mg/L, Na between 2530-4724 mg/L and high B values of 31.4-49.4 mg/L suggesting a Cl-HCO3-Na-type water for Lake Alchichica. Lake Quechulac on the other hand shows a TDS of 670-649 mg/L, Cl in the range of 89.6-97.5 mg/L; Na between 73.6-78.1 mg/L and low B contents in the order of 0.5 mg/L suggesting a HCO3-Cl-Mg-Na-type signature for Lake Quechulac. The springs around Los Humeros are quite variable with TDS ranging from 42-1169 mg/L, Cl concentrations between 2.1-75 mg/L; Na contents between 3.8-13.7 mg/L and very low B contents suggesting principally a HCO3-Na type. The geothermal wells though equally variable are slightly more enriched than the surrounding springs with TDS of 380- 5962 mg/L, Cl concentrations of 23-982 mg/L; Na contents of 30-1201 mg/L, B contents of 67-3168 mg/L and rather high As contents ranging from

0.5-73.6 mg/L suggesting a SO4-HCO3-Na-type water for the geothermal wells. The chemical characteristics of the four systems indicate that fluid circulation in the area is mainly regulated by meteoric waters that leach out boron from the volcanic rocks around Alchichica and Quechulac phretomagmatic cones.

45 Geochemical characterization of volcanic lakes in the Managua area (Nicaragua, Central America)

F. Parello1, A. Aiuppa1, E. Bagnato1, S. Calabrese1, D. Cellura1, H. Calderon2 1: Dipartimento CFTA, Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy 2: Centro para la Investigation en Recursor Acuaticos de Nicaragua, UNAN. Apartado postal 4598, Managua, Nicaragua

The Managua area, within which lies the Miocenic Nicaragua depression and the trans-extensive N-S trending Nejapa-Miraflores tectonic lineament running along its western edge, is the site of both volcanic and /geothermal activity. A hydro-geochemical study of some of its volcanic lakes (Jiloa, Apoyeque, Asososca de Leon and de Managua, Tiscapa, Apoyo, Nejapa) was undertaken, within the framework of scientific collaboration between the C.F.T.A. Department of the University of Palermo (Italy) and the CIRA of the UNAN University of Managua, Nicaragua, so as to determine their chemical composition as well as elucidate the potential interaction between surface and deep volcanic fluids. The main results indicate a wide compositional range in terms of their salinity (TDS < 1500 mg/l and > 1500 mg/l), mainly due to different genetic processes. Surface waters with TDS < 1500 mg/l show homogenous temperature values (~30°C) but a clear stratification of Eh which decrease with depth. Waters with TDS > 1500 mg/l are characterised by high chlorine contents, suggesting an intense recharge of deep Cl-rich fluids and expected decreasing pH values with depth, suggesting the input of magmatic fluid to the surface waters. Since most of the sampled lakes show evidence of intense geothermal activity, the geochemical monitoring of volcanic lakes could provide additional information on potential anomalies in depth magmatic/hydrothermal conditions. Chemical variations in water composition could consequently represent a serious threat for local people and their activities, because volcanic lakes might be a considerable water resource for nearby populated areas.

46 Global CO2 Emission from Volcanic Lakes

N. Pérez1, P. Hernández1, G. Padilla1, G. Melián1, E. Padrón1, J. Barrancos1, D. Calvo1, M. Kusakabe2, T. Mori3, K. Notsu3, D. Nolasco1 1: Environmental Research Division, Instituto Tecnológico y de Energías Renovables (ITER), Granadilla de Abona, Tenerife, Canary Islands, Spain 2: Department of Environmental Biology and Chemistry, The University of Toyama, Gofuku, Toyama-shi, Japan 3: Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo, Tokyo, Japan

During the last two decades, scientists have paid attention to CO2 volcanic emissions and its contribution to the global C budget. Excluding MORBs as a net source of CO2 to the atmosphere, the global CO2 discharge from subaerial volcanism has been estimated about ~ 300 Mt y-1 and this rate accounts for both visible (plume & fumaroles) and non-visible (diffuse) volcanic gas emanations (Mörner & Etíope, 2002). However,

CO2 emissions from volcanic lakes have not been considered to estimate the global CO2 discharge from subaerial volcanoes. In order to improve this global CO2 emission rate and estimate the global CO2 emission from volcanic lakes, an extensive research on CO2 emission of volcanic lakes from Phillipines, Nicaragua, Guatemala, Mexico, Indonesia, Germany, France, Cameroon, Costa Rica, El Salvador and

Ecuador had been recently carried out. In-situ measurements of CO2 efflux from the surface environment of volcanic lakes were performed by means of a modified floating device of the accumulation chamber method. To quantify the total CO2 emission from each volcanic lake, CO2 efflux maps were constructed 2 using sequential Gaussian simulations (sGs). CO2 emission rates were normalized by the lake area (km ), and volcanic lakes were grouped following classification in acid, alkaline and neutral lakes. The observed -2 -1 -2 - average normalized CO2 emission rate values increase from alkaline (5.5 t km d ), neutral (210.0 t km d 1 -2 -1 ), to acid (676.8 t km d ) volcanic lakes. Taking into account (i) these normalized CO2 emission rates from 31 volcanic lakes, (ii) the number of volcanic lakes in the world (~ 800), (iii) the fraction of the investigated alkaline (45%), neutral (39%), and acid (16%) volcanic lakes, and (iv) the average areas of the 2 2 2 investigated alkaline (36,8 km ), neutral (3,7 km ), and acid (0,5 km ) volcanic lakes; the global CO2 emission from volcanic lakes is about ~ 136 Mt year-1. This estimated value is about ~ 45% of the actual estimated global CO2 discharge from subaerial volcanism. This study highlights the importance of a deeper revision of the actual global CO2 discharge from subaerial volcanism. Mörner N.A. & Etíope, G. Global Plan. Chang, 33, 185–203, 2002.

47 “Suminagashi-like” sedimentation at El Chichón crater lake (Chiapas, Mexico)

D. Rouwet Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy [email protected] or [email protected]

“Suminagashi is the ancient Japanese technique of decorating paper with inks. It is believed to be the oldest form of marbling, originating in China over 2,000 years ago and practiced in Japan by Shinto priests as early as the 12th century. Suminagashi (sue-me-NAH-gah-she), which means literally "ink-floating" involves doing just that. Japanese Sumi-e inks were originally used, dropped carefully to float on a still water surface and then blown across to form delicate swirls, after which the ink was picked up by laying a sheet of white rice paper atop the ink covered water.” At El Chichón crater lake, the “ink” though rises from below. Since its formation in 1982, after the March-April Plinian eruptions, El Chichón crater lake (T 30°C, pH 2- 2.5) has become probably one of the most dynamic and largest steam-heated pools on Earth. Due its flat lake bottom, a minor decrease in the lake volume results in a major and often sudden decrease in the lake surface area. El Chichón’s lake bottom is sealed with clay minerals. In the centre of the lake, major upwelling occurs during fluid rise from the shallow hydrothermal system beneath the lake, while the northern sector of the crater lake manifests vigorous bubbling degassing. This turbulent convection is an ideal transport mechanism for the light phylosilicates, sedimented at the lake bottom. When reaching the lake surface, the clay minerals cohere to each other and adhere to the lake surface, changing the viscosity of the surface water layer. Due to the wind blowing over the lake, these floating clay films are transported towards the lake coast to be finally sedimented on the shore. Wind action can also differentiate the density of the surface clay film, resulting in swirling marble-like structures. Linear patterns are formed when the clay film is crossed by irregularities such as rocks. The high surface tension of the clay film is best demonstrated when a gas bubble rises through this “marbled” lake surface: the bubbles don’t break open but remain coated with moving marble-like clay clouds. An amalgamation of such unbreakable clay bubbles results in a foamy sedimentation when reaching the shore by wind transportation. Eight centuries ago, the Japanese Shinto priests were definitely inspired by similar natural phenomena when observing the lake surface of Noboribetsu or Yugama, dreaming to perfection their Suminagashi techniques.

48 Seepage of “aggressive” fluids reduce volcano flank stability: the Irazú and Turrialba case, Costa Rica

D. Rouwet1, , R.A. Mora-Amador2, C.J. Ramírez-Umaña2, G. González2 1: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy 2: Centro de Investigaciones en Ciencias Geológicas, Universidad de Costa Rica, Costa Rica [email protected] or [email protected]

The seepage of acidic fluids from summit hydrothermal systems towards the volcano flank can strongly affect the mechanical stability of the volcanic edifice. Rock is removed chemically as testified by high water discharges of thermal, often acidic and thus also metal-rich springs on volcano flanks. A decrease in the level of a crater lake seems to be the most obvious manifestation of such seepage of an “aggressive” fluid. A persistent fluid seepage can lead to volcano flank failure (collapse, lahars, debris avalanches) even during periods of quiescence of the volcano. The level of the Irazú crater lake has drastically decreased during the past years, suggesting an enhanced fluid loss out of the summit system. Although the cold, non-acidic and non-saline Irazú crater lake (3,080 m a.sl., pH 6.62, Cond 0.8 mS/cm, in December 2009) seems to be only the peripheral part of an active hydrothermal system, with the boiling temperature fumaroles as the centre of activity (< 1 km N-NW of the lake, 3,020 m a.s.l.), the lake is an important water reservoir for seeping fluids. Various groups of sulphate- rich, acidic, thermal springs discharge on the N-NE flank of Irazú: San Cayetano (36.8°C, pH 2.6, Cond 2.1 mS/cm, at 1,860 m a.s.l.), Santa Teresita (35.7°C, pH 4.5, Cond 2.2 mS/cm, at 2,240 m a.s.l.) and Ojo de Agua (28.8°C, pH 3.0, Cond 1.5 mS/cm, at 2,400 m a.s.l.). This type of waters originate from the absorption and near-surface oxidation of an H2S-rich vapour (steam-heated waters). The strong hydrothermal alteration on the N flank of Irazú probably reflects the dissolving capacities of the fluids circulating within the hydrothermal system. Nevertheless, the hottest, most acidic and most saline springs of Irazú discharge on the southern flank (Hervideros de Buenos Aires, T 56°C, pH 1.95, Cond 6 mS/cm, at 1,800 m a.s.l.). The neighbouring volcano Turrialba, 10.5 km NE-E of Irazú, increased fumarolic degassing since 2001 (Tassi et al. 2002), until culminating recently in phreatic activity (5 January 2010). Irazú’s “twin volcano” Turrialba shows a similar volcanic structure as its neighbour, with a horse-shoe shaped collapse structure towards the NE. Turrialba sometimes hosts a ephemeral crater lake in the central and less active crater (T

15.3°C, pH 2.7, Cond 1.7 mS/cm, in December 2009). The Bajo Las Peñas hot springs also discharge SO4- rich, slightly acidic steam-heated waters (T 68.5°C, pH 5.3, 0.7 mS/cm, at 2,420 m a.s.l.) on Turrialba’s NW flank, in a watershed very near Irazú’s Santa Teresita springs. The isotopic composition (δD and δ18O) of thermal spring and river waters permits to deduce the altitude of the meteoric recharge for both volcanoes. Applying the Cl inventory method (e.g. Ingebritsen et al. 2001;

49 Taran and Peiffer 2009) the discharge of the thermal springs can be estimated. When additionally knowing the major, minor and trace element content of the spring waters and of the volcanic rocks, the rock mass removal rate can be estimated for each spring. The red-ochre coloured and acidic Río Sucio (“Dirty River”, pH 3.1, Cond 0.9 mS/cm), the principal draining system of the northern springs of Irazú, proofs that rock removal is very effective at Irazú. The clear Río Toro Amarillo (pH 7.1, Cond 0.3 mS/cm) draining Turrialba is less “rock-rich”. With the ongoing Turrialba eruption it is extremely urgent to present a hydrogeochemical model for this volcano, in order to better estimate volcanic risk. Such a hydrogeochemical model is best presented together for both “twin volcanoes” Irazú and Turrialba, as hydrothermal aquifers could intersect in the main depression between both volcanoes. Despite the apparently lower volcanic activity of Irazú with respect to the present Turrialba, Irazú definitely is a more efficient rock-remover.

50 The volcanological evolution of Poás massif and its crater lakes Botos, Poás, Hule and Río Cuarto

P. Ruiz Cubillo1, , G.E. Alvarado2, 3, M.J. Carr1, G.J. Soto2,3,4, E. Gazel5 1: Department of Earth and Planetary Sciences, Rutgers University, 2: Instituto Costarricense de Electricidad (ICE) 3: Escuela Centroamericana de Geología, Universidad de Costa Rica. 4: Terra Cognita Consultores 5: Lamont-Doherty Earth Observatory, Columbia University [email protected]

The present study defines the stratigraphy of Poás volcano by using a geologic, petrographic, geochronological and geochemical analysis of the Poás units. The northern flank of the volcano is comprised of the following units: Río Sarapiquí, La Paz Andesites, Tiribí Formation (from Barva volcano, but intercalated with Poás’ stratigraphy), Río Cuarto Lavas, Von Frantzius, Cerro Congo, Bosque Alegre and Laguna Kopper (Río Cuarto maar). The units on the southern flank are the Colima Formation, La Paz Andesites, Tiribí, Achiote, Poasito, Sabana Redonda and Poás lapilli tuff. The central part of the volcano is the Poás Summit Unit, which includes the Main and Botos craters. The composition of the rocks spans the range from basalts to dacites. These units are geochemically correlated to magmatic components for Poás:

1. The Sabana Redonda component (TiO2 > 1%), enriched in HSFE and other trace elements, and present in Paleo Poás, Lavas Río Cuarto, Poasito, Sabana Redonda, Poás lapilli tuff and some of the Botos crater lavas; 2. The Von Frantzius component (TiO2 < 0.8 %), present in lavas of the main crater, Von Frantzius, Achiote, Bosque Alegre, Cerro Congo and some Botos crater lavas. During the last 600 ka the contents of

K2O and other oxides (TiO2 and P2O5) and traces (Zr, Ba) have varied significantly with time, suggesting the presence of these two geochemical end-members since the beginning of the magmatic activity of Poás. Within this time period, units with high and low values of these elements have coexisted; the latter is true for Botos lavas and the Main crater. For units that possibly shared a common vent, such as Paleo Poás,

Achiote and Main crater, the K2O and TiO2 contents decreased with time. More geochronological data are needed to improve the interpretations given above.

51 Fluids release at Taal Volcano

F. Sortino1, , J.P. Toutain2, J. Zlotnicki3 1: Istituto Nazionale di Geofisica e Vulcanologia,Sezione di Palermo, Italy 2 : CNRS, LMTG, F-31400 Toulouse, France 3 :CNRS, UMR6524, UMS 833-UBP Observatoire de Physique du Globe de Clermont-Ferrand [email protected]

Taal volcano is a complex stratovolcano of the Philippines archipelago 60 km south of Manila. It is located within a 16 x 27 km prehistoric caldera, partly filled by Lake Taal 3 meters above sea level. A 5 km diameter volcanic cone, 311 m a.s.l. occupies the center of the lake and concentrates historical activity. A 1 km2, 45x106 m3 lake (Main Crater Lake) fills the crater and shows seasonal water level fluctuations of about 1 m. Historical eruptions are phreatic to plinian with a 1 to 4-VEI index (Smithsonian, GVP). Chemical and isotopic evidences of sea-water incorporation within a developed hydrothermal system are supplied by Delmelle et al. (1998). Large interactions of surface waters with magmatic fluids are assessed through many 3 4 typical hydrothermal processes and characteristics: high He/ He ratios of 7.5 RA, extensive soil degassing

(CO2 up to 100%), shallow depth boiling temperatures, gas bubbling in the low pH (2-3)-high temperature (up to 37°C) MCL, hot-springs and fumaroles and episodic geyser activity in lake (Delmelle et al., 1998; Zlotnicki et al., 2007). Chemistry of fumarolic gases sampled in 1995 suggest a low equilibrium temperature of 200±60 °C, whereas thermal waters are not equilibrated with rocks (Delmelle et al., 1998). Opening of new fissures on the northern slope of the central crater followed by the migration of hydrothermal activity was triggered by a seismic crisis in 1992-1994. This new hydrothermal area is called Dang Kastila. Temperature gradients ([ΔT]) calculated by thermal profiling have been measured at 19 sites in soils. They indicate the existence of 4 main heat loss conditions according to their shape and ΔT values, matching well those described at other similar steaming sites. Very low ΔT-depth profiles are characterised by a decay in temperature in the first tens of cm due to solar diurnal radiations. Very high ΔT profiles show curves reaching asymptotically the 99°C isotherm. These typical shapes of ΔT-depth result from a basic model of heat transfer through steaming grounds involving both conductive and convective heat transports. Conductive heat transfer takes place in a first thin near-surface soil layer of tens of centimetres

Estimating reliable bulk CO2 fluxes which has not been performed at Taal by an extensive mapping of

(FCO2) and recognized the main area of degassing on soil and lake.

52 The Hule and Río Cuarto maars, Chapter Two: A degassing area and overturning lakes

G.J. Soto 1, 2, 3, G.E. Alvarado 1, 2, J.F. Fernández1 1: Instituto Costarricense de Electricidad, Costa Rica 2: Universidad de Costa Rica, Costa Rica 3: Terra Cognita Consultores

In the surroundings of these two maars, there are several identified spots of bubbling in cold and hot springs, and caverns with high CO2 concentrations (i.e., the called “Caverna de la muerte” or “Cavern of death”, because animals die in due to the accumulation of CO2), like Recreo Verde, on the left bank of Toro - = - river (35.0-40.7º C, pH 6.2-6.6; Cl 14.4 mmol/L; SO4 7.7 mmol/L; HCO3 42.2 4 mmol/L). This hot spring discharges CO2, SO2, N2, HCl, HF, CO, Ar and He. There are also several CO2-rich hot springs some 20 km NW of Río Cuarto lake (La Marina, La Palmera, San Rafael, 45.0-63.0ºC, pH 5.9-7.3), where travertine deposits are exploited. The gases of these two sites are typical of hydrothermal systems, with CO2/St ratios

>> 10. Sulfur is usually present as H2S in hydrothermal samples, but Recreo Verde has sulfur entirely present as SO2, which could be due to interaction with air-saturated groundwater (Zimmer et al., 2004).

Other sources of CO2-rich cold springs are in the Sardinal river, and hot springs in the Sarapiquí river (30.0° - = - C, pH 6.4-6.6; Cl 16.7 mmol/L; SO4 9.2 mmol/L; HCO3 30.8 mmol/L), about 5 km ESE from Río Cuarto and 5 km ENE from Hule. In Sardinal of Sarapiquí, ~20 km NNE from Río Cuarto, there are several boreholes where CO2 is extracted in commercial quantities from the CO2-saturated and confined aquifers that carry waters from unknown sources on their southern and southeastern edges. Additionally, under the construction of the tunnel of the Cariblanco hydroelectric project, 21 workers were affected (Sept. 19 – Oct.

8, 2004) mainly by CO and variable amounts of SO2, HCl, H2S and CH4. On the other hand, there is knowledge of fish death and change in the lake color at Río Cuarto at least since May 30th 1920, when a black and then white smoke plume rose up from its center. It seems that in Río Cuarto, at the beginning of some years, the green water switches to yellow-reddish coloration accompanied by massive fish mortality. The most likely explanation could be that during long periods of relatively cold weather between December and February, combined with strong winds, a physical and chemical mixture of hypolimnion and epilimnion happens. The lake may have turned over at least once between 1978 and 1991, and the owner of the land says that it occurs every 6-7 years. In Hule, at least 4 or 5 overturn events occurred during the last four decades: one prior to 1989, sometime between 1991 and 1996, in January 1996, and between December 2001 and January 2002, marked by sudden color changes (from dark blue to red), turbid and odorous water, and large-scale fish kills. These events are likely to occur when the air temperature is relatively cold and the weather is rainy and windy, so it is expected the lake to mix deeply between December to February. A deeper mixing of the water column was observed in Hule lake during February 1991, when a lowering of the layer of rapid oxygen decline was detected. The red coloration at that time was due to dense purple clumps floating of Merismopedia cf. chondroidea, which grow in the upper surface of the anoxic layer. At

53 Hule lake pH varies between 6.22 and 7.31 and in Congo lake (both into Hule maar) between 5.40 and 6.47, being slightly acid almost all the time, even at the surface. The question arises if the massive fish mortality is due to mixing of oxygenated and anoxic waters (the most accepted model) and/or accumulation and sudden emission of volcanic gases. Anyway, the dimensions of these reservoirs are limited by the continuous gas dispersion through lake surfaces, which is favored by the dominant convective regime resulting in frequent mixing of water strata. Therefore, in these systems the hazard related to the possible occurrence of Nyos-type gas eruptions can be considered negligible or very local, but important if tourists camp on or near the lake shorelines.

54 Degassing pathways through the shallow magmatic-hydrothermal system of Poás Volcano (Costa Rica)

L. Spampinato1,2, G.G. Salerno1,2, R.S. Martin3, G.M. Sawyer2, C. Ramírez4, E. Ilyinskaya2, C. Oppenheimer2 1: Istituto Nazionale di Geofisica e Vulcanologia, sezione di Catania, Piazza Roma, 2, 95123, Catania, Italy 2: Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, United Kingdom 3: Department of Earth Sciences, University of Cambridge, Downing Place, Cambridge, CB2 3EQ, United Kingdom 4: Centro de Investigaciones Geofisicas, University of Costa Rica (UCR), 1000 San José, Costa Rica

We report results from a multidisciplinary campaign carried out at Poás crater-lake (Costa Rica) on 17-18 March 2009. Thermal imagery of fumaroles on the north side of the pyroclastic cone and the lake surface revealed mean apparent temperatures of 25-40°C (maximum of 80°C), and 30-35°C (maximum of 48°C), respectively. Mean radiative heat output of the lake, uncorrected for downwelling flux, was estimated as -1 ~230 MW. The mean SO2 flux emitted by the crater measured by walking-traverses was 76 tonnes day , with approximately equal contributions from both the dome and the lake fumarole plumes. Gas measurements by active open-path FTIR spectroscopy indicated molar ratios of H2O/SO2 = 151 and

CO2/SO2 = 1.56. HCl and HF were not detected in measured spectra but based on the detection limits of these species, we calculated SO2/HCl > 40, and SO2/HF > 200. Particles were sampled from the plume by air filtration. The filters were analysed using ion chromatography, which revealed an abundance of K+ and 2- 2+ 2+ - SO4 , with smaller amounts of Ca , Mg and Cl . We discuss here the implications of the results for degassing pathways through the shallow magmatic-hydrothermal system.

55 Life returns to Lake Monoun

G. Tanyileke , N. Romaric, Issa, J. Hell Institute of Geological and Mining Research (IRGM), Yaounde, Cameroon [email protected]

Following the catastrophic explosions of Lakes Monoun and Nyos (Cameroon) in the 80s which killed close to 1800 people, controlled degassing was initiated at Nyos (2001) and Monoun (2003) to avert future gas disasters. Three years later (Feb., 2006), two additional pipes were installed in Lake Monoun at 93m and the 2003 pipe (73m) extended by 20m. Seven months degassing at Monoun resulted in a reduction in its clarity, dissolved oxygen content and aquatic life; signalling its death. But with all three pipes functional, the lake had lost most of its gas content by April 2008. The biphasic fountains initially at a height of 6m (April 2006) had either stopped functioning or been reduced to less than a meter (0.7m) by March 2008 coupled with a reduction of the total dissolved gas content to about 06%. In view of the success of the operation and the need to pre-empt future gas built up, one of the three pipes was lowered to lake bottom to purge it. Degassing resumed with a fountain of 5m which rapidly dropped and has over the past year reduced to a large bubble. Lake Monoun, which was virtually dead at the start of the degassing operation, is today safe, alive and the fishermen are back again. Here, we revisit Lake Monoun, highlight the results of this successful operation as well as present its post degassing structure.

56 A geochemical and isotopic overview of the crater lakes of Costa Rica

F. Tassi1,2, , O. Vaselli1,2, E. Fernández3, E. Duarte3, M. Martínez3, A. Delgado Huertas4, F. Bergamaschi1 1: Department of Earth Sciences, Via G. La Pira 4, 50121, Florence, Italy 2: CNR-IGG Institute of Geosciences and Earth Resources, Via G. La Pira 4, 50121, Florence, Italy 3: Volcanological and Seismological Observatory, Nacional University, Heredia, Costa Rica 4: Estación Experimental de Zaidin (CSIC), Prof. Albareda 1, 18008, Granada, Spain [email protected] The physical-chemical characteristics of crater lakes are often the source of useful information to forecast the volcanic hazard in active systems. Quiescent periods between eruptive phases allow these natural systems to store huge amounts of CO2(CH4)-rich gases that can be either added to the lakes from sub- lacustrine gas vents and/or produced by secondary processes (e.g. bacterial activity). External perturbations (i.e. earthquakes, landslides, heavy rains) may affect the lake stability. Consequently, an abrupt release of gases stored at depth may occur, producing to the so-called “limnic eruptions” like those recorded at Monoun and Nyos lakes (Cameroon) in 1984 and 1986, respectively. Geochemical investigations on crater lakes constitute an important tool not only for monitoring volcanic activity but also for the mitigation of the Nyos-type gas eruptions. Costa Rica is characterized by a relatively large variety of crater lakes whose dimensions are generally less than 1 km2. Some of them are associated with active volcanoes characterised by high fumarolic emissions and periodic phreatic eruptions (e.g. Rincón de la Vieja and Poas volcanoes), while others are hosted in volcanic systems showing moderate hydrothermal activity (e.g. Irazú and Tenorio volcanoes) or in a dormant state (e.g. Bosque Alegre caldera and Barva volcano). This situation offers a good opportunity to investigate and compare the geochemical features of crater lakes from volcanic systems pertaining to the same geodynamic environment but with different phases of volcanic activity. In this work the results of a geochemical study on water and dissolved gases and a detailed bathymetric survey of the Active Crater at Rincón de La Vieja volcano, Laguna Caliente and Botos at Poás volcano and Irazú, Hule, Congo and Tenorio are presented and discussed in order to investigate on the interactions with the hosting volcanic systems and evaluate the related potential hazards. The chemical-physical features of the Active Crater and Laguna Caliente mainly depend on the input rates of i) magmatic-related fluids, ii) meteoric water and iii) chemical compounds from water-rock interactions. Differently, Irazú lake seems to be mainly affected by the addition of hydrothermal CO2(H2S)-rich fluids, whereas the chemistry of Hule, Botos, Congo and Tenorio lakes is essentially regulated by meteoric precipitations and organic activity. At Botos lake, the pattern of chemical and isotopic composition along the vertical profile is practically constant, while relevant increases with depth of the dissolved-CO2 contents characterize Irazu and Hule lakes, likely related to gas inputs from lake bottom and degradation of organic material, respectively. - - 2- Laguna Caliente shows a peculiar F , Cl and SO4 stratification that may be produced by the combination of i) addition of high-temperature volatiles from sub-lacustrine fumaroles, ii) loss of HCl and H2S through

57 2- - evaporation and iii) SO4 and Cl consumption due to microbial activity at lake bottom. However, dissolved gases at Laguna Caliente have to be considered not able to produce Nyos-type gas eruptions, since their abundance is controlled by their dispersion through lake surface, being favoured by the convective regime controlling the heat transfer. At Irazú and Hule lakes, which are, at least partially, meromictic, the amounts of the dissolved CO2 is too low to represent a hazard in case of lake strata overturn.

58