Abstract
In this chapter, we document an extensive record of concentrations and speciation of polythionates (PTs: S4O62−, S5O62−, and S6O62−), which form in the warm (21–60 °C) and hyper-acidic (pH < 1.8) waters of the crater lake of Poás volcano (Costa Rica) through interaction with gaseous SO2 and H2S of magmatic origin. Our data set, together with earlier published results, covers the period 1980–2006 during which lake properties and behavior were marked by significant variations. Distinct stages of activity can be defined when combining PT distributions with geochemical, geophysical and field observations. Between 1985 and mid-1987, when fumarolic outgassing was centered on-shore, the total concentration of PTs in the lake was consistently high (up to 4,200 mg/kg). Mid-1987 was the start of a 7-year period of vigorous fumarolic activity with intermittent phreatic eruptions from the lake, which then dried out. Concentrations of PTs remained below or close to detection limits throughout this period. After mid-1994, when a new lake formed and fumarolic outgassing shifted to the dome, the total PT concentrations returned to relatively stable intermediate levels (up to 2,800 mg/kg) marking more quiescent conditions. Since early 1995, numerous weak fumarole vents started, opening up at several other locations in the crater area. During short intervals (November 2001–May 2002 and October 2003–March 2005), PTs virtually disappeared. After April 2005, PTs re-appeared in large amounts (up to more than 3,000 mg/kg) until February 2006, one month before the onset of the March 2006–2017 cycle of phreatic eruptions, when concentrations dropped and remained below 100 mg/kg. The observed behavior of PTs records changes in the input and SO2/H2S ratios of subaqueous fumaroles. The prevailing distribution of PTs is S4O62− > S5O62− > S6O62−, which is common for periods when total PT concentrations and SO2/H2S ratios of the gas influx into the lake are relatively high. PTs are virtually absent as a consequence of thermal or sulphitolytic breakdown during periods of strong fumarolic outgassing in response to shallow intrusion of fresh magma or fracturing of the solid envelope around a pre-existing body of cooling magma. They are also low in abundance or undetected during quiescent periods when subaqueous fumarolic output is weak and has low SO2/H2S ratios, resulting in a concentration sequence S5O62− > S4O62− > S6O62−. The onset of phreatic eruptions are preceded by an increase in PT concentrations, accompanied by a change in the dominance from penta- to tetrathionate, and followed by a sharp drop in total PT content, up to several months before. Periods of phreatic eruptive activity that started in 1987 and 2006 followed these PT signals of increased input of sulfur-rich gas, in both cases possibly in response to shallow emplacement of fresh magma or hydrofracturing.
Keywords
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Acknowledgements
We gratefully acknowledge the expert assistance of several colleagues with field and laboratory work, in particular Wendy Saénz Vargas (OVSICORI-UNA), Helen de Waard, Ronald Miltenburg, and Erick van Vilsteren (Utrecht University), as well as support from personnel of Poás Volcano National Park. MM thanks co-investigator Dr. Boku Takano for giving to a group of Costa Rica chemists and volcanologists the know-how on the chemistry of polythionates in acid lakes, and also for guiding us during field and laboratory work to learn on sampling and analysis of polythionate species using HPLC-UV techniques. Thanks also to Dr. Yasuyuki Miura (Tokai University) for advice on best practices to separate the thionates and other sulfur species by HPLC-UV. MM is very grateful with Dr. Takeshi Ohba (Tokai University) for training the chemists of OVSICORI-UNA on sampling and analyses of volcanic fluids from Poás and Turrialba volcano. Our sincere thanks also to the Japan International Cooperation Agency (JICA) for bringing to Costa Rica such a fine group of Japanese experts, Dr. Takano, Dr. Miura, and Dr. Ohba, to work with us at field and laboratory level. We are indebted to Dr. Juan Valdés González of the Laboratory of Atmospheric Chemistry of the School of Chemistry of Universidad Nacional (LAQAT-UNA) for providing access to its HPLC facility where part of the separation and analysis of polythionates were performed. We also thank the Instituto Costarricense de Electricidad (ICE) for providing rainfall data used in this chapter. MM acknowledges financial support from Utrecht University, Universidad Nacional, and the Ministry of Science, Technology, and Telecommunications of Costa Rica (MICIT). MM also thanks the financial contribution given by Dr. Karen McNally, Dr. Marino Protti, Dr. Ronnie Quintero, Lic. Jorge Brenes, Mr. Federico Chavarría Kopper, and the Roegiers Family (inSausalito California EEUU) for supporting this novel and exciting study.
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Appendices
Appendix: Sampling Strategy and Analytical Procedures
1.1 Sampling
Samples selected for PT analysis were routinely collected at a location on the NE side of the acidic crater lake (Fig. 1). Temperatures of the lake were measured at the same site using thermocouples. Most of the samples were stored in dark high-density polyethylene bottles at ambient temperature without filtration, dilution or addition of preservatives before the PT analyses were conducted. Between March 1990 and 1995 the sampling intervals were irregular in part because of the high activity of the volcano. From 1996 till November 2006, the data represent a monthly sampling frequency. A single available sample from 1980 was analyzed to record the earliest PT signal. Polythionate data documented by Rowe et al. (1992a) cover the period November 1984–March 1990 and represent an approximate sampling frequency of once every two months. Collectively, the currently available PT record thus covers the period 1980–2006. Some stored samples collected in 1988–1994 that contained grey muddy material were taken from mud pools that were formed during episodes when the lake volume was strongly reduced. These pools had a highly variable chemistry since they were largely mixtures of brine left during evaporation of the lake, fumarolic steam condensing at the bottom of the pools, and rainwater (Rowe 1994). Some pools contained unusual high concentrations of PTs (e.g. \( \Upsigma {\text{S}}_{\text{x}} {{\text{O}}_{6}}^{2 - } \) 6,740 mg/kg in a sample of 10 June 1994), but their results are not considered representative for the long-term evolution of the lake system and will therefore be ignored in this study.
1.2 Polythionate Analysis
About 190 lake water samples were analyzed for tetra-, penta-, and hexathionate ions at the Department of Chemistry of Tokyo University (Japan) and at the Laboratory of Atmospheric Chemistry (LAQAT) of the Universidad Nacional (Costa Rica) using similar ion-pair chromatographic techniques described below.
At Tokyo University tetra-, penta-, and hexathionate ions were determined following a high-performance microbore ion-pair chromatographic separation technique with ultraviolet absorption detection, and an ion-pair chromatographic separation technique with conductivity detection (Takano 1987; Takano and Watanuki 1988, 1990). The first technique allows the determination of tetra-, penta-, and hexathionate in excess of 0.2 ppm, and the second that of tetra-, and pentathionate in excess of 10 ppm. Standard solutions for penta- and hexathionate were prepared from synthesized salts (Takano and Watanuki 1988) following Goehring and Feldmann (1948). A commercially available potassium tetrathionate salt was re-crystallized for the preparation of tetrathionate standard solutions.
At LAQAT, separation and quantification of main PT species were performed using an ion-pair chromatographic technique with UV absorption detection following Miura and Kawaoi (2000) but using a silica ODS-2 chromatographic column as described below. A Shimadzu Model LC-10AS chromatographic system, equipped with a LC-10AS liquid delivery isocratic pump set at a flow rate of 0.6 mL/min, an automatic SIL-10A auto-injector with a 100 µl sampling loop, a SPD-10AV UV-VIS spectrophotometric detector set at 230 nm, and a SCL-10A system controller were employed. Chromatographic signals were obtained with the following settings: an Alltech/Allsphere ODS-2 analytical column 5 µm particle size (150 × 4.6 mm i.d.) coupled to a Brownlee guard column (50 × 3 mm i.d.); an acetonitrile-water (20:80 v/v) mobile phase (6 mM in tetra-propyl-ammonium hydroxide, TPA, and buffered at a pH = 5.0 by dropwise addition of glacial acetic acid 100%), filtered through a 0.45 µm membrane filter and degassed by vacuum. All of the reagents used for the mobile phase were analytical-grade; the acetonitrile was 99.93% HPLC grade, the TPA ion pair reagent used was an aqueous solution 20% (~1 M). Mobile phase, samples and standard solutions were diluted with distilled and deionized water. Samples were filtered and diluted immediately before injection into the chromatographic system. Chromatographic separations were carried out at ambient temperature (23 ± 2 °C).
A potassium tetrathionate calibration solution was injected several times during a 6 h sequence to monitor the response, the reproducibility of peak heights and retention times. Precision of retention times for standards and unknowns was ±0.9% within one day (n = 12) and ±1% within one week (n = 10) for the ion-pair UV chromatographic technique. This enabled unequivocal identification of the PTs by retention time only. Repeated analysis (n = 6) of the 21 June 2002 lake water sample to determine the reproducibility of the procedure yielded a relative standard deviation (RSD) of 5% at a mean concentration of 190 mg/kg for tetrathionate, 4% at 57 mg/kg for pentathionate, and 4% at 25 mg/kg for hexathionate. Reproducibility for the entire set of samples, analyzed in Costa Rica within a three-month period, was tested on randomly selected samples with a range of concentrations. Results showed average RSD values of 10, 7 and 8% for tetra-, penta-, and hexathionate, respectively. All samples were analyzed in duplicate and the results were averaged. Concentration differences between duplicates were usually <5%. Detection limits were 5 mg/kg for tetra-, 1.6 mg/kg for penta-, and 0.5 mg/kg for hexathionate.
One liter of lake water (original T = 32 °C, pH = 1.1, density = 1.02 g/mL), collected on 17 March 1999, was stored at 5 °C and used as a reference standard solution for PT calibration curves at LAQAT. This sample was periodically analyzed at Tokyo University to monitor its quality, in view of the chemical lability of PTs (Stamm et al. 1942). For instance, the analysis carried out at Tokyo University on 28 July 2002, 3 years and 4 months after collection, yielded 462 ± 8 mg/kg \( {\text{S}}_{4} {{\text{O}}_{6}}^{2 - } \), 182 ± 3 mg/kg \( {\text{S}}_{5} {{\text{O}}_{6}}^{2 - } \) and 120 ± 2 mg/kg \( {\text{S}}_{6} {{\text{O}}_{6}}^{2 - } \), concentrations that were in good agreement with those measured in April 1999 (455, 213 and 110 mg/kg, respectively). Polythionate concentrations in this reference sample remained at acceptable stable levels at least until August–October 2002 when analyses were run at LAQAT.
Some samples were analyzed within a few weeks after collection, but the vast majority was stored for several years at ambient temperature without preservation measures. Although it was demonstrated for natural and synthetic highly acid solutions (pH < 2), free from sulfur-oxidizing bacteria, that no significant decomposition of PTs occurs at concentrations of >100 ppm over at least eight years (Takano 1987; Takano and Watanuki 1988, 1990; Takano et al. 1994), the stability of PTs was verified in some lake water samples at LAQAT. As samples from the 1984–1990 period (Rowe et al. 1992a) were not available for re-analysis, a PT-rich mud pool sample and the reference sample (June 10, 1994 and March 17, 1999), were analyzed for tetrathionate on October 16, 2002 using a fresh synthetic aqueous solution of a 98% sodium tetrathionate dihydrate salt, Na2S4O6.2H2O, to prepare a calibration curve. Results were even somewhat higher than those obtained in Tokyo in 1999 and 2002 (RSD = 14 and 12%), indicating that there is no evidence for significant instability of this species over a period of at least three years. On the same date, all major PTs were also re-analyzed in the 10 June 1994 sample, using the 17 March 1999 reference solution. Original and newly measured values were in reasonable agreement with 2,825 and 3,185 mg/kg for tetra-, 2,067 and 2,600 mg/kg for penta-, and 725 and 880 mg/kg for hexathionate, respectively. Repeated analyses of aliquots of the reference solution in Tokyo in 1999 and 2002 confirmed that it had maintained its quality after three years.
Comparison of concentration data obtained on aliquots of 8 samples (June 1994–September 2001) showed random differences between results from the two laboratories. Overall deviations from the mean values were better than 20% RSD (3–20% for tetra-; 2–15% for penta-, and 1–20% for hexathionate. As most analyses in Tokyo and Costa Rica were carried out with time differences of about 3 years, it is concluded that the PT time series presented here are largely unaffected by potential interlaboratory differences or chemical instability. Re-analysis of sulfate and chloride suggested that some of the oldest samples may have experienced a certain degree of evaporation or mineral precipitation during storage. According to enrichments of both anions found, this may have raised PT concentrations in these cases by 15–20% at most.
1.3 Determination of Major Anions, Dissolved Gases, pH and Other Data
Most of the samples collected between 1990 and June 2004 were analyzed for sulfate, chloride and fluoride at Utrecht University by suppressed ion chromatography, using a fully automated Dionex Model DX-120 system. Samples were filtered in the laboratory with 0.45 µm polycarbonate membrane filters prior to analysis. Repeated analysis (n = 14) of a lake water sample yielded relative standard deviations better than 4% for all of the anions. Precision was about 0.1, 0.3 and 4% for sulfate, chloride, fluoride, respectively, based on the analysis of a synthetic solution. Detection limits were 0.3, 0.1 and 0.05 mg/kg, respectively. Some samples collected in 2000 were filtered with 0.45 µm polycarbonate filters and diluted in the field to prevent precipitation of gypsum. From comparison with results of samples that were untreated in the field, it was inferred that precipitation of gypsum during storage might have lowered the sulfate concentrations by about 7%. Results of four untreated samples were on average 6% lower than those of filtered and diluted equivalents (Vaselli et al. 2003) that were collected on the same dates in 1998–2001. Similar effects from storage of untreated samples were also attributed to gypsum precipitation by Rowe et al. (1992b).
The samples collected between July 2004 and November 2006 were analyzed for sulfate, chloride and fluoride at OVSICORI-UNA in June–December 2006, using a fully automated microbore ion suppressed chromatographic system (Dionex ICS-3000) (Martínez 2008). The pH measurements were performed on untreated samples at ambient temperature (24 ± 2 °C) using a WTW Multi 340i potentiometer. Combination of the new results with previously available data (e.g. Casertano et al. 1985; Rowe et al. 1992a, b; Nicholson et al. 1992, 1993; Martínez et al. 2000) constitutes a record for major anion concentrations, pH and temperature that covers the period 1980–2006.
Dissolved unreacted SO2 and H2S gases in the lake water were measured in situ on an irregular basis during 1999–2006, using a gas detection tube method (Takano et al. 2008). Detection limits for dissolved SO2 and H2S were 1 and 0.2 ppm, respectively.
All field-related data (lake volume, temperature, color, seismic records, etc.) are from the database of OVSICORI-UNA (Venzke et al. 2002; Martínez et al. 2000; Martínez 2008). Most of the monthly rainfall data come from the Poás volcano summit rain gauge of the Centro de Servicios y Estudios Básicos de Ingeniería of ICE located at 2,564 m a.s.l.
Polythionates—Relationships with Lake Properties, Fumarolic Activity and Eruptive Phreatic Events
In the following sub-sections, variations in PT distributions and quantities are described in relation to the composition and behavior of the lake, on-shore fumarolic activity, supply of volatiles, eruptive phreatic (phreatomagmatic?) events, monitored seismicity, and microgravity survey for the lake activity stages distinguished by Martínez (2008).
2.1 Microgravity and PTs
Microgravity surveys detected an intriguing increase between 1996 and 2001 at stations near the N and NE lakeshores, possibly signaling a magmatic intrusion at depth, which must have been small and fairly localized since most of the other stations showed continuous decreases (Rymer et al. 2005, 2009). Gas release from this putative intrusion may first have induced the sudden re-appearance of large amounts of PTs in the lake in January 1996, followed by their sulfitolysis-induced disappearance due to substantial injection of fresh SO2 into the lake between early 1998 and late 2002. The occurrence of a new fresh intrusion is also supported by the high C/S ratios and high equilibrium temperatures determined in the volatile phase by Fischer et al. (2015), enhanced CO2 and H2 diffuse emission rates (Melián et al. 2010; Melián et al. Chapter “Diffuse CO2 Degassing and Thermal Energy Release from Poás Volcano, Costa Rica”.), the appearance of low temperature sub-aerial fumaroles onshore the lake, the dome and the eastern sector of the crater, and the sustained increased in microgravity since 2001 until at least 2009 (Fig. 5) (Rymer et al. 2009; Rymer H pers. comm. 2010). Contemporary with these observations is the occurrence of a hydrothermal explosion in April 1996, and a composition of the lake water which was unusually enriched in chloride respect to sulfate between late 1997 and early 2004 (Fig. 6). The enrichment of the lake water in chloride suggests the degassing of a “fresh body of magma” underneath the region that comprises the lake and the dome that started intruding at shallower levels sometime between the mid 1990s (1996–1999) and the early 2000s that might have triggered the 2006–2014 phreatic cycle (Stage V) (Rymer et al. Chapter “Geophysical and Geochemical Precursors to Changes in Activity at Poás Volcano”). A new magmatic intrusion might have started between 2015 and 2016, according to some seismic patterns (recording of discrete short duration harmonic tremor which is unusual at Poás) and chemical changes of the acid lake. We suggest that the phreatomagmatic eruptions of April 2017 is the surface manifestation of the 2015–2016 intrusion event.
2.2 Seismicity
Numerous swarms of A-type earthquakes and an intensification of AB-type seismicity between the end of 1996 and the first months of 1997 had preceded the initial increase in volatile concentrations, whereas remarkably sustained volcanic tremors between October 1997 and March 1998 preceded the disappearance of PTs in April 1998. In addition, a short-lived increase in A and B-type seismicity was observed (Fig. 5). Whereas at Poás tremors generally occur in short discontinuous events, a single episode on 21 February 1998 carried on for 2.5 h, and 55 h of tremor were recorded during the whole month. This incidence of sustained tremor possibly reflected a continuous rise of magmatic/hydrothermal fluids in conduits that on one hand reached the lake bottom, increasing the input of volatiles and heat, and on the other fuelled the new fumaroles onshore around the lake.
The enhancement of hydrothermal activity (sub-aerial fumaroles around the dome, at other locations around the lake and along the lower part of the eastern edge, appearance of thermal springs) within the crater area between 1999 and October 2001 coincided with increased levels of B- and AB-type seismicity after two and a half years of relative quiescence (Fig. 5), suggesting a stronger interaction between the subsurface heat source and the hydrous system beneath the crater. Tremor, which was rare in 1999, took place for less than 30 min per day in September–November 1999 when the plume from the dome reached maximum heights (Fig. 9). Several unusual low-frequency earthquakes with periods of 40–175 s were recorded in this period as well (V Barboza pers comm 2005). An exceptional increase in tremor (a total of 108 h were recorded in September 2001, of which 7% corresponded to monochromatic tremor) coincided with the peak in PT concentrations observed in September–October.
2.3 Upwelling
Although the lake had shown upwelling activity during most of sub-stages IVA and IVB, specifically in its central part and near the dome, with sulfur globules emerging from subaqueous vents, convection was more intense and sulfur globules appeared in larger abundances during September 1997–July 1998. In contrast, in September–November 1998 upwelling activity and evaporation were weak, although more vigorous sub-aerial fumaroles continued appearing on the eastern side of the dome. Throughout 2000 and 2001, the lake was entirely covered by an acidic whitish fog (HCl fumes?). In August–September 2001 large bubbles (ca. 3 m in diameter) were observed in the central part of the lake.
2.4 Fumarole Activity
From December 1997 and throughout 1998 when the PTs reached maximum concentrations, the gas plume was up to nearly 600 m above the dome. Between February 1999 and June 2000, an unusually strong plume rose from 0.7 to 2 km above the northern fractured side of the dome (the plume was ~2 km height in September 1999) (Fig. 9), and was seen from the capital city of San José at 35 km southeast of the crater.
By late 1998, the first appearance of weak low-temperature fumaroles (around 83–95 °C) was observed in the eastern sector of the crater. In July 1999, long concentric cracks and some ambient and boiling point springs (15–95 °C) appeared here as well, coinciding with a magnitude 3.2 earthquake (Richter scale) that was felt at the summit of the volcano on 18 July. More fumaroles and springs continued appearing in the same area in 2000 and 2001 (Duarte et al. 2003), most of which remained active till 2006–2007. The widening of cracks produced instability and collapses at the eastern terrace. The fumaroles in the SW sector of the crater became more vigorous during the second half of 1999 when outgassing at the dome increased. In early 2000 these fumaroles weakened and eventually disappeared in June 2000 at the same time when degassing at the dome diminished and the gas-vapour plume returned to heights of 100–500 m (Venzke et al. 2002; Mora and Ramírez 2004). Gas condensates from the dome, collected in late 1999-early 2000, showed a significant increase in sulfate, chloride and fluoride concentrations, as the fumarole temperature did (188 °C in March 2000 on the north side of the dome) (Fig. 5) (Vaselli et al. 2003; Fernández et al. 2003).
2.5 Gas Fluxes
Infrequent gas remote-sensing measurements carried out during the IVB sub-stage suggest that the SO2 flux was of the same order of magnitude as the average of 90 tons per day recorded in February 1991 (Andres et al. 1992): In February 2001 the minimum average flux of SO2 was about 40 tons per day (Fournier et al. 2001, 2002), and in March 2002 the flux averaged 61 tons per day (Galle 2002). Between the second half of 1999 and the first part of 2000 an increase in Rn emission through the crater floor was observed (García et al. 2003). Significant increases in soil temperature and diffuse H2 and CO2 emissions in the eastern sector of the crater recorded in 2000–2002 were considered as possible precursor of pending volcanic unrest (Melián et al. 2001, 2003, 2004, Chap. 6). Alternatively, the invasion of deep hot permeable zones by meteoric water becoming enriched in volatiles and undergoing redox reactions may explain the strong fumarolic activity around the dome, the opening of the new field in the eastern sector and the H2 anomalies (Vaselli et al. 2003). Fracturing and subsequent increase in permeability of the volcanic edifice could have been triggered by ground deformation due to the presence of a dense, already crystallized magma body beneath the southern side of the lake (Fournier et al. 2001) or by local tectonic earthquakes (Mora and Ramírez 2004).
Relationship Between PTs and Seismic Activity
Seismic activity at Poás is complex, due to the dynamics of the magmatic and hydrothermal systems, their interactions and the possible combination with tectonic processes (Fernández 1990; Rowe et al. 1992b; Martínez et al. 2000; Rymer et al. 2000). Predominant seismic signals are the B-type or low-frequency earthquakes and the AB-type or intermediate-frequency quakes (Fig. 5). The B-type earthquakes are presumably generated within a few hundred meters from the surface, are concentrated below the crater floor, and may reflect interaction of liquid-gas phases within fracture conduits or bubble formation/collapse in the hydrothermal system. The AB-type quakes were attributed mostly to fumarole-opening events. The A-type or high-frequency earthquakes (volcano-tectonic quakes) and the T-type or volcanic tremor are not as frequently observed as the B- and AB-type quakes, confirming the shallow character of most of Poás’ seismic activity. The A-type quakes at Poás were related to tectonic readjustments within the local fault system that may or may not be triggered by regional subduction-related earthquakes (Fernández 1990). They were also associated with fracturing of the brittle carapace of a shallow magma body underlying the hydrothermal system (Casertano et al. 1987), or to mechanical fracturing of country rock, suggesting pressure build up in the system. The T-type seismicity was interpreted as a result of continuous movement of fluids or magma through a rigid medium (Aki et al. 1977; Fernández 1990).
Relationships between seismicity and PTs are difficult to evaluate for Stage II because of the scarcity of data. Despite high seismic activity and a high flux rate of SO2 from dome fumaroles in the early 1980s, the lake was probably characterized by low PT concentrations with a dominance of pentathionate (Fig. 5), indicative of H2S-enriched gaseous input and relatively modest activity. This illustrates a particular feature at Poás, namely that the location of major fumarolic outgassing may distort a straightforward relationship between the overall status and nature of degassing of the volcano and the behavior of the lake.
The strong rise in PT concentrations and the shift from pentathionate to tetrathionate dominance in early 1986 were associated with an initial increase in A-, AB- and B-type earthquakes as well as short-duration harmonic tremors after a relatively quiet period. The sharp drop in PT concentrations to below detection limits in early 1987, approximately 3 months before renewal of phreatic activity (Rowe et al. 1992b), coincided with a more conspicuous increase in seismicity, in particular B-type earthquakes. High levels of B-type seismicity remained throughout Stage III when the locus of fumarolic activity was centered within the lake area, and PTs remained virtually below detection limits (Fig. 5). The enhanced heat and volatile output could have been the result of an ascending magma beneath the lake, some time between 1985 and 1986, and/or subsurface hydrofracturing (Fernández 1990; Rowe et al. 1992a, b; Rymer et al. 2005). Peaks in A-type seismicity and tremor in 1990 and 1991 were interpreted in terms of this hydrofracturing, which did promote degassing but without the intrusion of new magma at that time (Fernández 1990; Rymer et al. 2000).
Some of the swarms of A-type quakes in 1994 coincided with large peaks in tremor (Fig. 5), strong degassing and the occurrence of phreatic explosions that may have been associated with renewed magma ascent (Martínez et al. 2000) or fracturing of the brittle envelope of the cooling magma present. Subsequent large variations in PTs contents in the returning lake were accompanied by fluctuations in seismicity.
Changes in the contents and distribution of PTs during Stage IV, reflecting changes in the flow rate and/or in the SO2/H2S ratios of subaqueous fumaroles, coincided with periods of either high or low levels of seismicity. Most conspicuous is the fact that the transition from relatively low-to-much higher B- and AB-type activity around 1999 roughly coincided with the sulfitolytic decline of PTs. The enhanced input of SO2-rich volatiles may thus have been driven by stronger interaction between the magmatic and lake-hydrothermal systems. Following the brief quiet interval of Sub-stage IVC, the sudden rise of PTs, apparently prompted by an increase in volatile input, was accompanied by an increase in AB- and B-type seismicity and by uncommon tremors (Fig. 5). Some of these were also recorded in 1980 when magma was intruding, but in this case, given the lack of A-type earthquakes, the tremors was interpreted as a signal of fluid movement through pre-existing fractures (V Barboza pers. comm. 2005).
The reappearance of PTs in April 2005, their subsequent sulfitolytic or thermal breakdown and the renewal of phreatic explosions all point to the injection of hot SO2-rich gas promoted by the opening of conduits and the rise of fluids, as signaled by swarms of A-type events and by a dramatic increase in AB-type seismicity that preceded and accompanied the large numbers of volcanic tremors registered throughout Stage V (Fig. 5). The monochromatic character of some of these tremors is usually related to movement of magma or magmatic/hydrothermal fluids at depth.
In summary, changes in the concentrations and speciation of the PTs in response to fluctuations in the rate and composition of gas/volatile input are often accompanied by changes in the activity or type of seismic signals. Increases in seismicity often come together with enhanced input into the lake, leading either to the production of PTs or to their breakdown when the flow rate of volatile and heat release reaches maximum levels. In other cases, the disappearance of PTs can be caused by weakening or shutdown of subaqueous fumarolic gas release, which is then supported by a decrease of seismic activity and changes in the lake properties as clearly observed during sub-stage IVC.
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Martínez-Cruz, M., van Bergen, M.J., Takano, B., Fernández-Soto, E., Barquero-Hernández, J. (2019). Behaviour of Polythionates in the Acid Lake of Poás Volcano: Insights into Changes in the Magmatic-Hydrothermal Regime and Subaqueous Input of Volatiles. In: Tassi, F., Vaselli, O., Mora Amador, R. (eds) Poás Volcano. Active Volcanoes of the World. Springer, Cham. https://doi.org/10.1007/978-3-319-02156-0_7
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Publisher Name: Springer, Cham
Print ISBN: 978-3-319-02155-3
Online ISBN: 978-3-319-02156-0
eBook Packages: Earth and Environmental ScienceEarth and Environmental Science (R0)