Hellenic Volcanic Arc Research Articles

Application of Time Series INSAR (SBAS) Method Using Sentinel-1 for Monitoring Ground Deformation of the Aegina Island (Western Edge of Hellenic Volcanic Arc)

This study employs advanced synthetic aperture radar (SAR) techniques, specifically the small baseline subset (SBAS) method, to analyze ground deformation dynamics on Aegina, a volcanic island within the Hellenic Volcanic Arc. Using Sentinel-1 satellite data spanning January 2016 to May 2023, this research reveals different deformation behaviors. The towns of Aegina and Saint Marina portray regions of stability, contrasting with central areas exhibiting subsidence rates of up to 1 cm/year. The absence of deformation consistent with volcanic activity on Aegina Island aligns with geological records and limited seismic activity, attributing the observed subsidence processes to settlement phenomena from past volcanic events and regional geothermal activity. These findings reinforce the need for continuous monitoring of the volcanic islands located in the Hellenic Volcanic Arc, providing important insights for local risk management, and contributing to our broader understanding of geodynamic and volcanic processes.

Magmatic evolution of the Kolumbo submarine volcano and its implication to seafloor massive sulfide formation

Seafloor massive sulfides form in various marine hydrothermal settings, particularly within volcanic arcs, where magmatic fluids may contribute to the metal budget of the hydrothermal system. In this study, we focus on the Kolumbo volcano, a submarine volcanic edifice in the central Hellenic Volcanic Arc hosting an active hydrothermal system. Diffuse sulfate-sulfide chimneys form a Zn-Pb massive sulfide mineralization with elevated As, Ag, Au, Hg, Sb, and Tl contents. These elements have similar behavior during magmatic degassing and are common in arc-related hydrothermal systems. Trace-element data of igneous magnetite, combined with whole rock geochemistry and numerical modelling, highlights the behavior of chalcophile and siderophile elements during magmatic differentiation. We report that, despite early magmatic sulfide saturation, chalcophile element contents in the magma do not decrease until water saturation and degassing has occurred. The conservation of chalcophile elements in the magma during magmatic differentiation suggests that most of the magmatic sulfides do not fractionate. By contrast, upon degassing, As, Ag, Au, Cu, Hg, Sb, Sn, Pb, and Zn become depleted in the magma, likely partitioning into the volatile phase, either from the melt or during sulfide oxidation by volatiles. After degassing, the residual chalcophile elements in the melt are incorporated into magnetite. Trace-element data of magnetite enables identifying sulfide saturation during magmatic differentiation and discrimination between pre- and post-degassing magnetite. Our study highlights how magmatic degassing contributes to the metal budget in magmatic-hydrothermal systems that form seafloor massive sulfides and shows that igneous magnetite geochemistry is a powerful tool for tracking metal-mobilizing processes during magmatic differentiation.

Boiling-induced extreme Cu isotope fractionation in sulfide minerals forming by active hydrothermal diffusers at the Aegean Kolumbo volcano: Evidence from in situ isotope analysis

Abstract We analyzed the first Cu isotopes in primary cupreous pyrite and orpiment, from modern CO2-degassing, seafloor massive sulfide diffuser vents (“KCO2Ds”), from the Kolumbo submarine volcano, Hellenic volcanic arc. Samples came from six KCO2Ds that are actively boiling. Pyrite comprises colloform pyrite-I and euhedral pyrite-II, which occur erratically distributed within the KCO2Ds and are contemporaneous with barite and spatially concurrent with the chalcopyrite that is lining narrow internal conduits, respectively. Orpiment occurs on the outer walls of the KCO2Ds with barite and stibnite. The δ65Cupyrite-I values show high variability, ranging from +2.93‰ to +6.38‰, whereas the δ65Cupyrite-II and δ65Cuchalcopyrite values vary from −0.94‰ to +0.25‰ and −0.45‰ to –0.09‰, respectively. The range of δ65Cuorpiment between +1.90‰ and +25.73‰ is the most extreme ever reported from any geological setting. Pyrite-I is concentrically layered, with a core comprising random crystallites, whereas the mantle crystallites have grain-size, shape, and orientation variability between layers. Pyrite-II forms aggregates of uniform euhedral pyrite crystals. Pyrite-I has higher concentrations of Cu (≤21,960 ppm) compared to pyrite-II (≤4963 ppm), and both have incompatible and volatile metal(loid)-rich composition and low Sb/Pb (<0.5) and Tl/Pb (<0.03) ratios. When combined with evidence for significant magmatic contributions at Kolumbo and geochemical and micro-textural evidence for recurrent intense boiling and/or flashing or gentle and/or non-boiling, the measured extreme δ65Cu values are consistent with transport of Cu by vapor that is preferentially enriched by heavy 65Cu and controlled by continuous Rayleigh distillation–type Cu fractionation. Boiling-induced Cu vapor transport can generate extreme Cu isotope fractionation.

SANTORY: SANTORini’s Seafloor Volcanic ObservatorY

Submarine hydrothermal systems along active volcanic ridges and arcs are highly dynamic, responding to both oceanographic (e.g., currents, tides) and deep-seated geological forcing (e.g., magma eruption, seismicity, hydrothermalism, and crustal deformation, etc.). In particular, volcanic and hydrothermal activity may also pose profoundly negative societal impacts (tsunamis, the release of climate-relevant gases and toxic metal(loid)s). These risks are particularly significant in shallow (<1000m) coastal environments, as demonstrated by the January 2022 submarine paroxysmal eruption by the Hunga Tonga-Hunga Ha’apai Volcano that destroyed part of the island, and the October 2011 submarine eruption of El Hierro (Canary Islands) that caused vigorous upwelling, floating lava bombs, and natural seawater acidification. Volcanic hazards may be posed by the Kolumbo submarine volcano, which is part of the subduction-related Hellenic Volcanic Arc at the intersection between the Eurasian and African tectonic plates. There, the Kolumbo submarine volcano, 7 km NE of Santorini and part of Santorini’s volcanic complex, hosts an active hydrothermal vent field (HVF) on its crater floor (~500m b.s.l.), which degasses boiling CO2–dominated fluids at high temperatures (~265°C) with a clear mantle signature. Kolumbo’s HVF hosts actively forming seafloor massive sulfide deposits with high contents of potentially toxic, volatile metal(loid)s (As, Sb, Pb, Ag, Hg, and Tl). The proximity to highly populated/tourist areas at Santorini poses significant risks. However, we have limited knowledge of the potential impacts of this type of magmatic and hydrothermal activity, including those from magmatic gases and seismicity. To better evaluate such risks the activity of the submarine system must be continuously monitored with multidisciplinary and high resolution instrumentation as part of an in-situ observatory supported by discrete sampling and measurements. This paper is a design study that describes a new long-term seafloor observatory that will be installed within the Kolumbo volcano, including cutting-edge and innovative marine-technology that integrates hyperspectral imaging, temperature sensors, a radiation spectrometer, fluid/gas samplers, and pressure gauges. These instruments will be integrated into a hazard monitoring platform aimed at identifying the precursors of potentially disastrous explosive volcanic eruptions, earthquakes, landslides of the hydrothermally weakened volcanic edifice and the release of potentially toxic elements into the water column.

Moment tensor inversion of microearthquakes along the Santorini-Amorgos zone: Tensile faulting and emerging volcanism in an extensional setting

The Santorini-Amorgos zone is an area rich in microseismicity at the center of the Hellenic volcanic arc. The microseismicity of the zone is distributed along the Santorini-Amorgos ridge and Kolumbo submarine volcano. In this study, we utilized crustal events that were recorded by temporary networks during September 2002 to July 2004 and October 2005 to March 2007, and also by the permanent network from 2011 to 2020. These events were inverted for their moment tensors by using P-wave polarities as well as SV/P and SH/P amplitude ratios, yielding 74 well-constrained moment tensor solutions. Most of these moment tensors have significant CLVD and isotropic components that are positively correlated to each other (R2 = 0.68). Tensile faulting due to high pore pressure is considered as the most likely cause of the observed non-DC components. The positive and negative non-DC components observed in Kolumbo may be generated by the opening and closing of cracks beneath the shallow (6–7 km) magma chamber due to a steady migration of magmatic fluids from the deep reservoir into the chamber. In Anydros, most of the microearthquakes have positive non-DC components associated with the opening of cracks. It is possible that the extensional deformation and high pore fluid pressure in the area opens subvertical cracks that become pathways for upward migrating fluids. The upward migration of magmatic fluids in an extensional regime such as the Santorini-Amorgos zone can also be viewed as an indication of emerging volcanic activity in this area.

Machine Learning in Discriminating Active Volcanoes of the Hellenic Volcanic Arc

Identifying the provenance of volcanic rocks can be essential for improving geological maps in the field of geology and providing a tool for the geochemical fingerprinting of ancient artifacts like millstones and anchors in the field of geoarchaeology. This study examines a new approach to this problem by using machine learning algorithms (MLAs). In order to discriminate the four active volcanic regions of the Hellenic Volcanic Arc (HVA) in Southern Greece, MLAs were trained with geochemical data of major elements, acquired from the GEOROC database, of the volcanic rocks of the Hellenic Volcanic Arc (HVA). Ten MLAs were trained with six variations of the same dataset of volcanic rock samples originating from the HVA. The experiments revealed that the Extreme Gradient Boost model achieved the best performance, reaching 93.07% accuracy. The model developed in the framework of this research was used to implement a cloud-based application which is publicly accessible at This application can be used to predict the provenance of a volcanic rock sample, within the area of the HVA, based on its geochemical composition, easily obtained by using the X-ray fluorescence (XRF) technique.

Gravity observations on Santorini island (Greece): Historical and recent campaigns

Santorini is located in the central part of the Hellenic Volcanic Arc (South Aegean Sea) and is well known for the Late-Bronze-Age “Minoan” eruption that may have been responsible for the decline of the great Minoan civilization on the island of Crete. To use gravity to probe the internal structure of the volcano and to determine whether there are temporal variations in gravity due to near surface changes, we construct two gravity maps. Dionysos Satellite Observatory (DSO) of the National Technical University of Athens (NTUA) carried out terrestrial gravity measurements in December 2012 and in September 2014 at selected locations on Thera, Nea Kameni, Palea Kameni, Therasia, Aspronisi and Christiana islands. Absolute gravity values were calculated using raw gravity data at every station for all datasets. The results were compared with gravity measurements performed in July 1976 by DSO/NTUA and absolute gravity values derived from the Hellenic Military Geographical Service (HMGS) and other sources. Marine gravity data that were collected during the PROTEUS project in November and December 2015 fill between the land gravity datasets. An appropriate Digital Elevation Model (DEM) with topographic and bathymetric data was also produced. Finally, based on the two combined datasets (one for 2012–2014 and one for the 1970s), Free air and complete Bouguer gravity anomaly maps were produced following the appropriate data corrections and reductions. The pattern of complete Bouguer gravity anomaly maps was consistent with seismological results within the caldera. Finally from the comparison of the measurements made at the same place, we found that, within the caldera, the inner process of the volcano is ongoing both before, and after, the unrest period of 2011–2012.

Seismicity along the Santorini-Amorgos zone and its relationship with active tectonics and fluid distribution

The Santorini-Amorgos zone is located in the central part of the Hellenic volcanic arc and is hosting eight large faults as well as Kolumbo and Santorini volcanic centers. The largest earthquake (Mw ~ 7.1) in the southern Aegean during the 20th century also occurred in this area on 9 July 1956. A total of 1868 crustal events were recorded by temporary networks during September 2002 to July 2004 and October 2005 to March 2007, and also by the permanent network from 2011 to 2019. We relocated 1455 of these events by using HypoDD and revealed clusters of earthquakes beneath Kolumbo, Anydros graben, and Santorini-Amorgos ridge. Only the faults in the SW of Anydros, SE of Ios, and along the south coast of Amorgos were delineated by the relocated events. Nearly vertical clusters were observed beneath the island of Anydros, south of Amorgos, and in NE end of Amorgos fault, indicating possible pathways of upward migrating fluids. The seismogenic layer thickness calculated based on the depth distribution of the relocated events was 12.5 km. We combined this thickness with geometrical properties of the faults to calculate the expected moment magnitude of future earthquakes, resulting in a range of 6.3–7.2. In an effort to map the distribution of fluids, the Vp/Vs ratio distribution was estimated by utilizing the event-station travel time data along with crack density, fluid saturation, and Poisson's ratio. The petrophysical parameters observed in the northern part of the Santorini caldera suggest the existence of melt, while those observed in Anydros and in the NE of Amorgos fault support the suggestion of upward migrating fluids in these areas.

How to remove the seasonal atmospheric effect: An application on a surface volcanic temperature at Nisyros active volcano, Hellenic Volcanic Arc

Surface temperature at a volcanic system is the result of a mixture of two heat sources originating one from above and another from below the ground surface. The atmosphere significantly affects the surface's temperature of the volcano, and removal of the atmospheric effect is required for meaningful interpretations prior to any characterization of anomalous volcanic activity. Time series of meteorological data were acquired from the closest weather station to Nisyros volcano, and surface volcanic temperature from the southern part of Nisyros caldera were analyzed. Time series analysis took place at different time scales through a decomposition method. The decomposition of the time series into long-, seasonal-, and short-term components was performed by the application of the Kolmogorov-Zurbenko (KZ) filter to separate the noise that appears on the obtained data. The Time Series Regression (TSR) model was applied in all time scales. The long- and the short-term components of the surface volcanic temperature show no significant correlation with the atmospheric variables, while the seasonal term indicates a very strong association with an R2 equal with 99%. After the removal of the atmospheric effect, our model indicates that there is a normal activity of the seasonal heat emission in Nisyros with no anomalous signals in the temperature residuals during our survey. This methodology may facilitate the process of filtering a seasonal atmospheric effect and better comprehend the heat emission of a volcano-hydrothermal system from temperature surface readings.

Copernicus Sentinel-1 MT-InSAR, GNSS and Seismic Monitoring of Deformation Patterns and Trends at the Methana Volcano, Greece

The Methana volcano in Greece belongs to the western part of the Hellenic Volcanic Arc, where the African and Eurasian tectonic plates converge at a rate of approximately 3 cm/year. While volcanic hazard in Methana is considered low, the neotectonic basin constituting the Saronic Gulf area is seismically active and there is evidence of local geothermal activity. Monitoring is therefore crucial to characterize any activity at the volcano that could impact the local population. This study aims to detect surface deformation in the whole Methana peninsula based on a long stack of 99 Sentinel-1 C-band Synthetic Aperture Radar (SAR) images in interferometric wide swath mode acquired in March 2015–August 2019. A Multi-Temporal Interferometric SAR (MT-InSAR) processing approach is exploited using the Interferometric Point Target Analysis (IPTA) method, involving the extraction of a network of targets including both Persistent Scatterers (PS) and Distributed Scatterers (DS) to augment the monitoring capability across the varied land cover of the peninsula. Satellite geodetic data from 2006–2019 Global Positioning System (GPS) benchmark surveying are used to calibrate and validate the MT-InSAR results. Deformation monitoring records from permanent Global Navigation Satellite System (GNSS) stations, two of which were installed within the peninsula in 2004 (METH) and 2019 (MTNA), are also exploited for interpretation of the regional deformation scenario. Geological, topographic, and 2006–2019 seismological data enable better understanding of the ground deformation observed. Line-of-sight displacement velocities of the over 4700 PS and 6200 DS within the peninsula are from −18.1 to +7.5 mm/year. The MT-InSAR data suggest a complex displacement pattern across the volcano edifice, including local-scale land surface processes. In Methana town, ground stability is found on volcanoclasts and limestone for the majority of the urban area footprint while some deformation is observed in the suburban zones. At the Mavri Petra andesitic dome, time series of the exceptionally dense PS/DS network across blocks of agglomerate and cinder reveal seasonal fluctuation (5 mm amplitude) overlapping the long-term stable trend. Given the steepness of the slopes along the eastern flank of the volcano, displacement patterns may indicate mass movements. The GNSS, seismological and MT-InSAR analyses lead to a first account of deformation processes and their temporal evolution over the last years for Methana, thus providing initial information to feed into the volcano baseline hazard assessment and monitoring system.

When There Is No Offset: A Demonstration of Seismic Diffraction Imaging and Depth‐Velocity Model Building in the Southern Aegean Sea

AbstractA vast majority of marine geological research is based on academic seismic data collected with single‐channel systems or short‐offset multichannel seismic cables, which often lack reflection moveout for conventional velocity analysis. Consequently, our understanding of Earth processes often relies on seismic time sections, which hampers quantitative analysis in terms of depth, formation thicknesses, or dip angles of faults. In order to overcome these limitations, we present a robust diffraction extraction scheme that models and adaptively subtracts the reflected wavefield from the data. We use diffractions to estimate insightful wavefront attributes and perform wavefront tomography to obtain laterally resolved seismic velocity information in depth. Using diffraction focusing as a quality control tool, we perform an interpretation‐driven refinement to derive a geologically plausible depth‐velocity model. In a final step, we perform depth migration to arrive at a spatial reconstruction of the shallow crust. Further, we focus the diffracted wavefield to demonstrate how these diffraction images can be used as physics‐guided attribute maps to support the identification of faults and unconformities. We demonstrate the potential of this processing scheme by its application to a seismic line from the Santorini Amorgos Tectonic Zone, located on the Hellenic Volcanic Arc, which is notorious for its catastrophic volcanic eruptions, earthquakes, and tsunamis. The resulting depth image allows a refined fault pattern delineation and, for the first time, a quantitative analysis of the basin stratigraphy. We conclude that diffraction‐based data analysis has a high potential, especially when the acquisition geometry of seismic data does not allow conventional velocity analysis.

Compressional origin of the Aegean Orogeny, Greece

The Aegean Sea area is thought to be an actively extending back-arc region, north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean. The area shows extensive normal faulting, ductile ‘extensional’ shear zones and extensional S-C fabrics throughout the islands that have previously been related to regional Aegean extension associated with slab rollback on the Hellenic Subduction Zone. In this paper, we question this interpretation, and suggest the Cenozoic geodynamic evolution of the Aegean region is associated with a Late Cretaceous–Eocene NE-dipping subduction zone that was responsible for continent-continent collision between Eurasia and Adria-Apulia/Cyclades. Exhumation of eclogite and blueschist facies rocks in the Cyclades and kyanite-sillimanite grade gneisses in the Naxos core complex have pressures that are far greater than could be accounted for purely by lithospheric extension and isostatic uplift. We identify four stages of crustal shortening that affected the region prior to regional lithospheric extension, herein called the Aegean Orogeny. This orogeny followed a classic Wilson cycle from early ophiolite obduction (ca. 74 Ma) onto a previously passive continental margin, to attempted crustal subduction with HP eclogite and blueschist facies metamorphism (ca. 54–45 ​Ma), through crustal thickening and regional kyanite – sillimanite grade Barrovian-type metamorphism (ca. 22–14 ​Ma), to orogenic collapse (<14 ​Ma). At least three periods of ‘extensional’ fabrics relate to: (1) Exhumation of blueschists and eclogite facies rocks showing tight-isoclinal folds and top-NE, base-SW fabrics, recording return flow along a subduction channel in a compressional tectonic setting (ca. 50–35 ​Ma). (2) Extensional fabrics within the core complexes formed by exhumation of kyanite- and sillimanite gneisses showing thrust-related fabrics at the base and ‘extensional’ fabrics along the top (ca. 18.5–14 ​Ma). (3) Regional ductile-brittle ‘extensional’ fabrics and low-angle normal faulting related to the North Cycladic Detachment (NCD) and the South(West) Cycladic Detachment (WCD) during regional extension along the flanks of a major NW–SE anticlinal fold along the middle of the Cyclades. Major low-angle normal faults and ductile shear zones show symmetry about the area, with the NE chain of islands (Andros, Tinos, Mykonos, Ikaria) exposing the NE-dipping NCD with consistent top-NE ductile fabrics along 200 ​km of strike. In contrast, from the Greek mainland (Attica) along the SE chain of islands (Kea, Kythnos, Serifos) a SW-dipping low-angle normal fault and ductile shear zone, the WCD is inferred for at least 100 ​km along strike. Islands in the middle of the Cyclades show deeper structural levels including kyanite- and sillimanite-grade metamorphic core complexes (Naxos, Paros) as well as Variscan basement rocks (Naxos, Ios). The overall structure is an ~100 ​km wavelength NW–SE trending dome with low-angle extensional faults along each flank, dipping away from the anticline axis to the NE and SW. Many individual islands show post-extensional large-scale folding of the low-angle normal faults around the domes (Naxos, Paros, Ios, Sifnos) indicating a post-Miocene late phase of E–W shortening.

Palaeoenvironmental evolution of a coastal lagoon as a response to climate oscillations during the Early Pleistocene: a case study from Sousaki Basin (Eastern Gulf of Corinth, Greece)

Climatic oscillations during the Pleistocene were controlled by glacial/interglacial cycles. Such oscillations are commonly imprinted into sediments and fossils from transitional marine environments, as is also the case in Sousaki Basin (Eastern Gulf of Corinth, Greece). Nevertheless, the records become scarcer as we go back in time. During the Lower Pleistocene, Sousaki Basin (Eastern Gulf of Corinth, Greece), was dominated by transitional marine environments. Micropalaeontological analysis has been carried out in sedimentary sequences to identify the evolution of the palaeoenvironments in an area where intense tectonic activity occurred, being at the western end of the Hellenic volcanic arc and at the eastern end of the Corinth rift. The recovered ostracod assemblage revealed a primarily brackish environment. Deposition took place in a coastal lagoon which was subjected to constant salinity changes. The palaeoenvironment in the basin evolved from a delta fan environment to an outer lagoon with possible connection to a sublittoral marine environment at the top. This evolution coincides and therefore is attributed to minor climate oscillations that occurred during the Early Pleistocene.

Hydrothermal Fluid Particle Geochemistry of Submarine Vents in Kos Island, Aegean Sea East Mediterranean

The study aims to fill the gap of knowledge in the literature concerning submarine hydrothermal activity on the eastern part of Hellenic Volcanic Arc. Hydrothermal fluids were collected by scuba diving in active shallow hydrothermal venting areas on Kos Island in the Aegean Sea in East Mediterranean. Analysis was carried out in suspended particles for Fe, Mn, Cu, Pb, Cd, Ca, Ba, Sr, Li, Al and Si. During sampling, gas and water flux was measured and gas/water ratios were estimated. The data presented significant time and spatial variability. The gas flux values in Kephalos Bay varied from 2 to 126 L/h and water flux varied from 60 to 122 L/h. In Bros Thermi the gas flux values varied from 4.5 to 60 L/h and the water flux was found to be from 16.5 to 96 L/h. The suspended particulate matter (SPM) flux varied from 0.93 to 8.64 mg/L and between 0.21 and 20.94 mg/L respectively at two sites studied. Metal/Al ratios also varied significantly within a short distance. The pH of hydrothermal waters was from 5.50 to 5.95 in Kephalos Bay and from 6.09 to 6.53 in Bros Thermi suggesting either different mixing of fluids with seawater deeper in the substrate or perhaps that gases CO2 and H2S control pH values. The main scope of this research is to evaluate the fluxes of gas and water and study the main particulate geochemistry of hydrothermal venting fluids. Hydrothermal particles from the submarine vents studied were dominated by Fe, Ca, Si and Al and strongly enriched in Mn, Cu, Pb and Ba. Based on Metal/Al ratios, it is suggested that there are two distinct hydrothermal reservoirs in Kos. The chemistry of the particles emitted from seeps strongly influence both the marine sediment geochemistry and element distribution locally on the sea bottom while subjected to further transportation, hydrothermal particulates could also affect a much greater region.

Geochemistry of Hydrothermal Particles in Shallow Submarine Hydrothermal Vents on Milos Island, Aegean Sea East Mediterranean

Abstract—The study aims to contribute to submarine hydrothermal particle geochemistry knowledge on the central part of the Hellenic Volcanic Arc. Hydrothermal fluids collected by scuba diving from active shallow hydrothermal venting areas on Milos Island in the Aegean Sea in East Mediterranean. Analysis carried out in suspended particles for Fe, Mn, Cu, Pb, Cd, Ca, Ba, Sr, Li, Al and Si. During sampling, gas and water flux was measured and gas/water ratios were estimated. The data presented significant time and spatial variability. The gas flux values in Palaeochori Bay varied from 4.5 to 63 L/h and water flux varied from 25 to 92 L/h. In Voudia Bay varied from 0.8 to 32 L/h and the water flux found from 7 to 118 L/h while in Adamas Bay the gas flux values were from 2 to 13 L/h and the water flux found from 24 to 72 L/h. The suspended particulate matter (SPM) flux varied from 0.07 to 47 mg/L in Palaeochori Bay, between 2 and 19 mg/L and from 0.8 to 4 mg/L respectively in the three sites studied. Metal/Al ratios also vary significantly within a short distance between seeps in the same area. The pH of hydrothermal waters was from 5.21 to 6.63 in Palaeochori Bay, from 5.45 to 6.39 in Voudia Bay and from 5.69 to 6.10 in Adamas Bay suggesting either different mixing of fluids with seawater deeper in the substrate or perhaps that gases CO2 and H2S control pH values. Based on the geochemical properties found a common geothermal reservoir is suggested located below the surface of the island and annual fluxes of elemental particulates have been estimated. The main scope of this research is to evaluate the fluxes of gas and water and the main particulate geochemistry of hydrothermal venting fluids. Hydrothermal particles from the submarine vents studied were dominated by Fe, Ca, Si and Al and strongly enriched in Mn, Cu, Pb, Ca and Ba. The chemistry of particles emitted from seeps can strongly influence the local sediment geochemistry and element distribution on sea bottom locally while subjected to further transportation can also affect a much greater region.

Tomographic imaging of the NW edge of the Hellenic volcanic arc

The broader South Aegean area is characterized by the convergence of the African and Eurasian lithospheric tectonic plates at a rate of approximately 30 mm/year, resulting in a complex and intense deformation of the Aegean (McClusky et al. Geophys. Res. 105: 5695–5719, 2000). The Saronikos Gulf, situated in the NW termination of the Hellenic volcanic arc (Central Aegean Sea), is an area of special geophysical interest due to the variety of the recorded seismicity, regarding both the origin (tectonic, volcanic) and the focal depths. The presence of Plio-Quaternary volcanic centers and the complex seismicity distribution urge the need of detailed research on the geophysical properties such as seismic velocities, revealing possible presence of magmatic bodies or geothermal fluids. In the present study, more than 3000 manually revised events were located using a local 1-D layered velocity model. The main spatiotemporal clusters are concentrated along the Leuces and Methana-Poros E-W striking neotectonic fault zones. A local earthquake tomography (LET) was performed, using body-wave (P and S) travel time data, to investigate small to medium scale (~ 10 km) anomalies that can be related to local neotectonic or volcanic patterns. Major shallow ~ E-W to NW-SE trending discontinuities between positive (to the north) and negative (to the south) VP perturbations, anticorrelated to the respective VS image, are identified south of Aigina Island. These results are mainly related to the principal local stress orientation and fault zones (~ E-W to NW-SE strike), such as the Moni, Anghistri, and Leuces, whereas the respective Vp/Vs ratio values imply the presence of possible magmatic features which can be linked to the Plio-Quaternary NW edge of the Hellenic volcanic arc and hydrothermal activity ENE of Poros Island.

Geochemistry of CO2-Rich Gases Venting From Submarine Volcanism: The Case of Kolumbo (Hellenic Volcanic Arc, Greece)

Studies of submarine hydrothermal systems in Mediterranean Sea are limited to the southern Italian volcanism, while are totally missing in the Aegean. Here we report on the geochemistry of high-temperature fluids (up to 220°C) venting at 500 m b.s.l. from the floor of Kolumbo submarine volcano (Hellenic Volcanic Arc, Greece), which is located 7 km northeast of Santorini Island. Despite the recent unrest at Santorini, Kolumbo submarine volcano is considered more active due to a higher seismicity. Rizzo et al. (2016) investigated the He-isotope composition of gases collected from seven chimneys and showed that are dominated by CO2 (>97%), with only a small air contamination. Here we provide more-complete chemical data and isotopic compositions of CO2 and CH4, and Hg(0) concentration. We show that the gases emitted from different vents are fractionated by the partial dissolution of CO2 in water. Fractionation is also evident in the C-isotope composition (13CCO2), which varies between –0.04‰ and 1.15‰. We modelled this process to reconstruct the chemistry and 13CCO2 of intact magmatic gases before fractionation. We argue that the CO2 prior to CO2 dissolution in water had 13C ~ –0.4‰ and CO2/3He ~ 1•1010. This model reveals that the gases emitted from Kolumbo originate from a homogeneous mantle contaminated with CO2, probably due to decarbonation of subducting limestone, which is similar to other Mediterranean arc volcanoes (e.g. Stromboli, Italy). The isotopic signature of CH4 (13C ~ –18‰ and D ~ –117‰) is within a range of values typically observed for hydrothermal gases (e.g. Panarea and Campi Flegrei, Italy), which is suggestive of mixing between thermogenic and abiotic CH4. We report that the concentrations of Hg(0) in Kolumbo fluids are particularly high (~61 to 1300 ng m–3) when compared to land-based fumaroles located on Santorini and worldwide aerial volcanic emissions. This finding may represent further evidence for the high level of magmatic activity at Kolumbo. Based on the geo-indicators of temperature and pressure, we calculate that the magmatic gases equilibrate within the Kolumbo hydrothermal system at about 270°C and at a depth of ~1 km b.s.l..

Gamma spectroscopy studies of the underwater hydrothermal vent field of the Methana Peninsula

A preliminary study of natural radioactivity was conducted on the thermal spas in Methana Peninsula. To carry out this research, a collection of 17 water samples were taken from thermal springs around and underwater of the volcanogenic Peninsula at depths ranging 0–5 m. The Methana peninsula belongs to the Hellenic Volcanic Arc and is characterized by hydrothermal vent activity. A NaI(Tl) scintillator (AMESOS) was used to carry out gamma–ray counting of the samples to deduce the activity concentrations of the natural radionuclides of the 238U and 232Th decay series, as well as 40K in spa waters. Results are expected to provide information on the geological setting of the Methana peninsula. The impact of naturally occurring radioisotopes to human health has also been assessed in terms of the radiation dose risk corresponding to the measured activities.

Arsenic and high affinity phosphate uptake gene distribution in shallow submarine hydrothermal sediments

The toxicity of arsenic (As) towards life on Earth is apparent in the dense distribution of genes associated with As detoxification across the tree of life. The ability to defend against As is particularly vital for survival in As-rich shallow submarine hydrothermal ecosystems along the Hellenic Volcanic Arc (HVA), where life is exposed to hydrothermal fluids containing up to 3000 times more As than present in seawater. We propose that the removal of dissolved As and phosphorus (P) by sulfide and Fe(III)(oxyhydr)oxide minerals during sediment–seawater interaction, produces nutrient-deficient porewaters containing < 2.0 ppb P. The porewater arsenite-As(III) to arsenate-As(V) ratios, combined with sulfide concentration in the sediment and/or porewater, suggest a hydrothermally-induced seafloor redox gradient. This gradient overlaps with changing high affinity phosphate uptake gene abundance. High affinity phosphate uptake and As cycling genes are depleted in the sulfide-rich settings, relative to the more oxidizing habitats where mainly Fe(III)(oxyhydr)oxides are precipitated. In addition, a habitat-wide low As-respiring and As-oxidizing gene content relative to As resistance gene richness, suggests that As detoxification is prioritized over metabolic As cycling in the sediments. Collectively, the data point to redox control on Fe and S mineralization as a decisive factor in the regulation of high affinity phosphate uptake and As cycling gene content in shallow submarine hydrothermal ecosystems along the HVA.

Correction to: Microbial community differentiation between active and inactive sulfide chimneys of the Kolumbo submarine volcano, Hellenic Volcanic Arc

In the original publication there is a mistake in the supplementary material. The correct supplementary material is provided in this correction article.