Report on field deployment of a permanent volcanic gas network at Nea Kameni Volcano, Santorini (Greece)

Transcription

1 Report on field deployment of a permanent volcanic gas network at Nea Kameni Volcano, Santorini (Greece) Prepared on September 2012 for the DCO, Reservoir and Fluxes Directorate Research carried out within the framework of the DECADE project A. Aiuppa 1,2 (PI), G. Tamburello 1 (Co-PI), G. Giudice 2, M. Liuzzo 2, E. Bagnato 1, R. Di Napoli 1, S. Gurrieri, (Co-investigators) 1 DiSTeM, Università di Palermo 2 INGV, Sezione di Palermo Work done in collaboration with the Institute for the Study and Monitoring of the Santorini Volcano (I.S.M.O.SA.V.); Co-investigators: Georges Vougioukalakis, IGME & Konstantinos Papazachos, Aristotle University of Thessaloniki NASA Image 1

2 PREMISE The Reservoir and Fluxes Directorate of the Deep Carbon Observatory has recently funded within the framework of the DECADE research network initiative (PI, Dr Tobias Fischer) - two field expeditions on Santorini Island, with the final goal to contribute establishing a network of fully automated instruments for volcanic gas observation on Nea Kameni. This initiative followed from the need of better improving our understanding on volcanic degassing processes at Santorini, which are yet poorly investigated; but was additionally motivated by starting of a volcanic unrest on 2011, which included inflation of the caldera floor and recurrent seismicity. This unrest stimulated an improvement of the monitoring network on the island. A first explorative field campaign on May 2012, conducted as a joint UniPa-ISMOSAV initiative, confirmed the potential for installation of permanent gas-sensing instruments on Nea Kamesi s summit. NEA KAMENI VOLCANO ON SANTORINI Santorini is situated in the central part of the Aegean arc on a km thick continental crust, at about 120 km north of Crete, and is the site two of the three active volcanic Aegean centers, Kameni and Kolumbo. Santorini is world-known for having produced one of the largest volcanic events in human history, the so-called Minoan eruption of the late Bronze Age (~3600 Figure 1 BP) (Bond and Sparks, 1976; Pyle, 1990; Cioni et al., 2000). This eruption discharged about 60 km 3 of rhyodacitic magma and ash which dispersed over a large area of eastern Mediterranean and Turkey (Sigurdsson et al. 2006). Post- Minoan activity mainly localized within the caldera area, with sub-aerial activity finally resuming on 197 BC (Fytikas et al., 1990). Since then, recurrent effusive to slightly explosive eruptions led to build up of Palea and Nea Kameni islets (between 197 BC and 1950 AD; Vougioukalakis and Fytikas, 2005; Zellmer et al., 2000) (Fig. 1), the sub-aerial expressions of a ~2 km 3 intra-caldera shield volcano which rises to ~500m above the caldera floor. The current volcanic activity of Santorini consists of weakly diffuse fumarolic degassing affecting the top of Nea Kameni volcanic field (covering an area of about 0.02 km 2 ). An anomalous sequence of earthquakes began inside the caldera early in 2011, accompanied by inflation (Newman et al., 2012); these represent the most significant events observed since the last eruptive sequence ended in

3 THE MAY 2012 EXPLORATIVE SURVEY According to previous studies, Nea Kameni fumaroles are made up to a large extent of heated atmospheric air and CO 2, with minor constituents (CH 4, H 2, CO) present in variable proportions (Vougioukalakis and Fytikas, 2005). A first geochemical survey was performed on the Nea Kameni volcanic/hydrothermal system on May 2012, leading to identification and chemical characterization of six main fumarolic areas (Fig. 1). During this field expedition, gas temperatures of C were measured. The concentrations of major volcanogenic constituents in the fumarolic plumes of Nea Kameni were monitored by using a portable version on the INGV-type Multi-component Gas Analyzer System (MultiGAS) (Aiuppa et al., 2007). This uses a CO 2 /H 2 O gas detector (Licor LI-840 NDIR closed-path spectrometer) and a series of electrochemical sensors for SO 2 (0 200 ppm; 3ST/F electrochemical sensor by City Technology Ltd.), H 2 S (0 50 ppm; EZ3H electrochemical sensor by City Technology Ltd.) and H 2 (0-200 ppm; EZT3HYT electrochemical sensor Easy Cal City Technology Ltd.) detection. The sensors are housed in a water-proof box mounted on a backpack frame, and were calibrated, before and after fieldwork, with standard calibration gases. The 0.5 Hz acquisition frequency of the MultiGAS allowed us to monitor short timescale changes in gas compositions, and to study the effects of mixing of fumaroles gases with background air. The simultaneous acquisition of CO 2, H 2 S, SO 2, H 2 O and H 2 by the portable MultiGAS allowed us to characterize the major species abundances in the fumarolic plumes. Our results confirm a distinct hydrothermal nature for Nea Kameni fumaroles, which are dominated by H 2 O, CO 2 (molar H 2 O/CO 2, 1-4) and H 2, with minor H 2 S (and SO 2 typically absent). All the fumaroles exhibit very low sulfur contents (mean atmospheric H 2 S concentration of ~102 ppb) with CO 2 /H 2 S molar ratios ranging from 3000 to 49000, suggesting sulfur scrubbing underneath the vents. Hydrogen is a significant component of the fumarolic gases (up to ~70 ppm in atmospheric plumes) and H 2 /H 2 O molar ratio range from Figure 2 The NEA2 fumarole, because of its easy access, high flux, and composition well averaging the field compositional variability, was selected as the best site for installation of a permanent fumarolic gas monitoring station (Plume station; Fig. 2). An explorative soil degassing survey was also conducted, allowing for identification of an appropriate site for continuous soil CO 2 measurements (soil gas station: Fig. 2). 3

4 NETWORK INSTALLATION AND SETUP Instruments were permanently deployed in the field during the DCO-funded June campaign. The budget for instrumental costs ( 50 k ) was entirely covered by INGV. Fumarolic gas station This fully automated instrument was designed to measure the near-vent plume gases released by NEA2 fumarole (Fig. 3). The instrument inlet was positioned so that to collect a near-vent ( 30 cm) mixture of NEA2 gases and air (Fig. 4). The Figure 3 core of the fumarolic gas station is based on MultiGAS-type custom-made technology, and is driven by INGV-PA acquisition system based on STMicroelectronics microcontroller. It is equipped with a infrared spectrometer (Edimburgh sensors, Gascard, 0-1 %) for CO2 sensing, specific electrochemical sensors for H2S, SO2 and H2 sensing (0-50 ppm EZ3H, ppm 3ST/F and 0200 ppm EZT3HYT electrochemical sensor Easy Cal respectively; all electrochemical sensor by City Technology Ltd.), a Freescale Pressure sensor and Figure 4 Viasala Rh sensor (allowing for calculation of H2O concentration in the plume) (Fig. 5). The core station is located inside a rugged plastic box, which contains also a 12 V 60 Ah battery, a current regulator, and supports a 30 W solar panel on the top door, and the wlan antenna mast (Fig. 3). The probe, located near the NEA2 fumarole, is connected to the station via a 1m-long PTFE filter and a PTFE tube. Figure 5 The fumarolic gas station was configured to acquire in cycles of 200 samples, each being the median of 9 1 Hz (30 minutes per measurement cycle). A time interval of 12 hours among groups was set. The Santorini ISMOSAV Local wireless network was used for telemetry, (configured by K. Papazachos, University of Thessaloniki), with automatic download procedures running on INGVPA monitoring network system. 4

5 Soil gas station The CO2 soil gas station is also driven by an INGV-PA acquisition system, based on STMicroelectronics microcontroller, and is equipped with GasCard spectrometer (Edimburgh sensors). The core station is located inside a rugged plastic box, which contains also a 12 V 60 Ah battery, a current regulator, and supports a 30 W solar panel on the top door, and the GSM antenna mast (Fig. 6). The measurement principle is based on the Gurrieri and Valenza (1988) method. The probe, located close to the shelter box in a site selected after an accurate survey around the crater rim, is connected to the station via PTFE tube. Figure 6 The soil gas device acquires a group of 3 samples, each of which is the median of 9 1 Hz (3 minutes per group). A time interval of 6 hours among groups was set. A GSM cellular modem was used for telemetry, with automatic download procedures running on INGV-PA monitoring network system. As for the plume station, we are also able to download data manually, via GSM network, from the non volatile station backup memory. PRELIMINARY RESULTS Both permanent instruments are regularly collecting and transmitting data by the time of writing (September the 1st 2012). The good operational status of the instrument was also checked during a H2O/CO CO2/H2S 0 15/06/ /06/ /07/ /07/ /07/2012 Figure /08/ /08/ /08/ /09/2012 CO2/H2S H2O/CO2 7

6 second maintenance field expedition in late August 2012 (also funded by DCO). The acquired data, preliminarily shown in Figures 7 and 8, are currently under validation and elaboration. The permanent MultiGAS results (Fig. 7) reveal a relatively flat time evolution of both H 2 O/CO 2 and CO 2 /H 2 S ratios, indicating that no substantial modification in the source processes governing gas venting has occurred during the measurement interval. SO 2 has remained below detection during the observations, ruling out the eventuality of any pulse of more oxidized and hotter magmatic gas in the system. The MultiGAS data, combined with knowledge of individual fumarole plume s structure (obtained by CO 2 tomography) were also used to assess the order of magnitude of CO 2 flux emissions from Nea Kameni fumaroles (evaluated at t/d). The raw soil CO 2 fluxes, obtained by the permanent soil gas instruments, shown in Figure 8, range from 35.5 to 7719 gr m -2 day -1 with an average at about 827 gr m -2 day -1. These fluxes are typical of moderately degassing active hydrothermal system and are, for instance, one order of magnitude lower than those measured on the summit of Stromboli. The acquired data also reveal a high soil CO 2 flux variability, with degassing cycles of approximately one week period (red line on figure 8) CO 2 flux gr m -2 day giu giu giu lug lug lug-12 9-ago ago ago-12 6