IODP proposal VolTecArc aims at deep-sea drilling in and around the Christiana-Santorini-Kolumbo (CSK) marine volcanic field to investigate interactions and feedbacks between tectonics and volcanism and how volcanoes interact with their marine environments. The volcanic field lies in a rift system 100 km long and 45 km wide, oblique to the South Aegean volcanic arc, that is one of the most volcanically and seismically active regions of Europe. The volcanoes include three polygenetic and over 20 monogenetic centers that have jointly produced over a hundred explosive eruptions over the last few hundred thousand years.  The volcanoes pose important hazards to the Eastern Mediterranean region. Unrest at Santorini caldera in 2011-12 raised awareness of eruption threat at an island archipelago visited by 1.5 million tourists per year.

The results of onland volcanological research, eruption dating, multi-beam sea floor mapping, shallow sediment coring and dredge sampling, combined with a high-quality site-survey database of multichannel seismic profiles and a recent seismic tomography experiment, make deep drilling at the CSK volcanic field very timely. Deep drilling will enable characterization and interpretation of depositional packages visible on seismic images, chemical correlation of Santorini-derived volcanic layers in the rift fills with the dated onshore stratigraphy, and provide a tight chronostratigraphic framework for marine successions. Some objectives of drilling are to: (1) document the history of tectonics, subsidence, sedimentation and volcanism in an arc-rift environment, and how volcanism has evolved spatially and temporally since rift initiation; (2) determine how the genesis and compositions of magmas and their associated volatiles have evolved in time and space over the lifetime of the rift; (3) document the dynamics and environmental impacts of arc eruptions and calderas, including eruption frequencies, magnitudes and rates, the mechanisms of caldera collapse, and the origin of caldera unrest events.

How to cite: Druitt, T., Huebscher, C., Kutterolf, S., Nomikou, P., Papanikolaou, D., Karstens, J., and Preine, J. and the Participants of the 2017 Athens MagellanPlus workshop: Volcanism and tectonics in an island-arc rift environment: proposal to drill at Christiana-Santorini-Kolumbo marine volcanic field, Greece, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3132, https://doi.org/10.5194/egusphere-egu21-3132, 2021.

The Christiana-Santorini-Kolumbo (CSK) volcanic field in the South Aegean Sea is one of the most active volcanic-tectonic lineaments in Europe, having produced numerous explosive eruptions, catastrophic earthquakes, and disastrous tsunamis in the past 350,000 years. The present-day Santorini caldera is located in the centre of the NE-SW trending CSK field, which extends for more than 60 km from the extinct Christiana volcano in the southwest to the active submarine Kolumbo volcano northeast of Santorini. While the onshore architecture of Santorini has been well studied, little is known about its offshore architecture. Further, the past volcanism of Kolumbo is only known for its last eruption in 1650 AD and that of Christiana is completely unknown. Based on the available onshore datings of the volcanic formations, it has been proposed that volcanism in the CSK field initiated at Christiana, then migrated northeast towards Santorini and later to Kolumbo. This, however, has yet to be confirmed by offshore investigations. To fully constrain and understand the initiation and evolution of volcanism in the CSK field, we combine an extensive collection of high-resolution multichannel and vintage seismic data covering the entire zone.

With these seismic profiles, we are able to (1) correlate the seismo-stratigraphy of the Christiana basin west of Santorini with that of the Anhydros Basin east of Santorini, (2) identify major phases of extrusive and intrusive activity of individual volcanic vents, and (3) establish a regional chrono-stratigraphic framework in which we chronologically integrate these phases. We conclude that volcanism occurred repetitively during distinct phases of activity, which are separated from each other by periods with little or no volcanic activity. The onset of volcanism occurred without the generation of significant pyroclastic flows and was mainly characterized by shallow intrusions, clearly visible at Christiana, at its neighbouring volcanic cones, and at the southwestern flank of Akrotiri. The next phase saw the formation of the Kolumbo Volcanic Chain in the Anhydros Basin and the formation of sill intrusions in the Christiana Basin, which we find below a chaotic, presumably pyroclastic unit. This was followed by a major regional event on Santorini, during which a thick transparent subunit was deposited in all surrounding basins. The most recent phase was dominated by the volcanoclastic deposits from Santorini’s eruptive cycles and the recent eruption of Kolumbo. Using estimates of sedimentation rates, we convert this chrono-stratigraphic scheme into an approximate timeline, which implies that the initiation of volcanism occurred during Late Pliocene to Early Quaternary - much earlier than revealed by onshore dating.

How to cite: Preine, J., Hübscher, C., Karstens, J., Nomikou, P., Druitt, T., and Papanikolaou, D.: Chronostratigraphic Analysis of the Evolution of the Christiana-Santorini-Kolumbo Volcanic Field, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1702, https://doi.org/10.5194/egusphere-egu21-1702, 2021.

Kolumbo represents one of the most hazardous, currently active, volcanoes in the eastern Mediterranean. Its last eruption in 1650 AD was associated with a vast explosion, causing a tsunami of regionally devastating impact. The eruption was also associated with the voluminous and rapid release of toxic gases asphyxiating humans and animals on the nearby Islands. Earthquake records from the recent decades document on-going unrest beneath the volcano. Remotely operated vehicle dives revealed several hydrothermal vent sites and bacterial mats at the crater floor, which are concentrated near the northern crater wall. The vents emit mainly CO₂, leading to the accumulation of acidic waters in the crater. Accordingly, one of the main volcanic hazards associated with Kolumbo is that rapid overturning of water in the crater may release harmful amounts of toxic gases. Monitoring the hydrothermal processes inside the Kolumbo crater will provide an important contribution to the understanding and evaluation of this and other volcanic hazards.

In October 2019, we deployed an ocean bottom seismometer and hydrophone (OBS/H) inside the Kolumbo crater. During the four days of passive recording we identified about 100 so-called short duration seismic events, which were only present on the seismometer channels, while generally being absent on the hydrophone channels. The events have durations of less than one second with dominant frequencies between 5 to 30 Hz. Most of the events represent well-polarized seismic phases, which enables us to determine their azimuth angle (with a 180-degree bias) and angle of incidence at the OBS/H. We cross-correlated all polarized seismic waveforms and subsequently used the cross-correlation coefficients for a hierarchical cluster analysis to elaborate whether the events have a random origin or originated from a common origin. Our analysis revealed that the majority of events is associated with two clusters. The azimuth angles of all events in the largest cluster coincide with the azimuth angle between station and the field of hydrothermal vents and bacterial mats inside the crater. This coincidence suggests that the origin of the short duration events is associated with the sub-seafloor migration of fluids or the fluid discharge process at the crater floor. In fact, short-duration events of similar characteristics, recorded by OBS/H, were previously attributed to sub-seafloor fluid migration and the discharge of fluids at the seafloor. Our analyses indicate that seismic monitoring of submarine volcanoes should include the detection and analysis of short duration events, which may act as a novel tool in the characterization of volcanic unrests and volcanogenic geohazard monitoring in general.

How to cite: Schmid, F., Karstens, J., and Nomikou, P.: Identification and interpretation of seismic short-duration events inside the Kolumbo submarine volcano in the Southern Aegean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-250, https://doi.org/10.5194/egusphere-egu21-250, 2020.

Santorini volcano in the central sector of the South Aegean volcanic arc is one of the most active and potentially dangerous magmatic systems in Europe having had twelve Plinian eruptions over the last 350 ka of which at least four eruptions were accompanied by caldera collapses. The well-known Late Bronze Age eruption (~3.6 ka^A) for example is considered to rank as one of the largest eruptions since the Late Miocene.

The main focus of research thus far has been on the comparatively young and subaerial deposits, whereas older stages of volcanism have been poorly studied. Our study comprises samples from the submarine caldera flanks and gives new insights into the early evolutionary stages of Santorini volcano, contributing to a better understanding of its eruptive history and potential risks. The submarine lava successions were sampled along the inner caldera wall by a remotely operated vehicle (ROV) during R/V POSEIDON cruise 511 in 2017.

The investigated lavas can be divided into two magmatic series: a low-K basaltic series overlain by medium- to high-K series, including basaltic andesites, andesites and occasional dacites. First results of ⁴⁰Ar/³⁹Ar dating reveal ages of ~250 ka for the andesites. For the presumably older basalts, no reliable age data could be obtained.

Major and trace element compositions and mineral zoning patterns suggest that fractional crystallization was the dominant process controlling magma evolution. In addition, repeated magma mixing played an important role as indicated by characteristic zonation patterns within plagioclase and clinopyroxene ante- and phenocrysts. Comparison of the major and trace element compositions with published data from subaerial deposits show a strong similarity between our lavas and the ~528-308 ka^A old deposits of Peristeria volcano, a composite stratocone in the north of the volcanic field and whose subaerial deposits are found on northern Thera only^B. This similarity is also supported by the Sr-Nd-Pb isotopic compositions of our lavas. Our results indicate both an extended age range of Peristeria activity and a much wider geographic distribution of its lava flows than previously recognized.

^A T. H. Druitt et al. (1999), Santorini Volcano, Geological Society of London Memoir

^B T. H. Druitt et al. (2015): Field guide to Santorini Volcano

How to cite: Pank, K., Hansteen, T. H., Geldmacher, J., Garbe-Schönberg, D., Jicha, B., Hauff, F., and Hoernle, K.: Origin and evolution of the lowermost lava successions at Santorini volcano (Greece): insights from major and trace element composition of rocks from the submarine caldera wall, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2096, https://doi.org/10.5194/egusphere-egu21-2096, 2021.

Volcanoes in island arcs can undergo edifice evolution that includes submarine and subaerial volcanism, providing a dynamic environment of magmatic heat and volatiles that drives hydrothermal fluid flow with potential inputs from sea and/or meteoric water. This, in turn, can generate significant hydrothermal alteration that can result in economic deposits of industrial minerals. One example includes bentonite, a smectitic rock composed dominantly of montmorillonite.

Economically viable bentonite deposits are typically only 0.5 – 5 meters thick and although Wyoming-type bentonites comprise 70% of the world’s known deposits, they are commonly no thicker than 8 m. The island of Milos is Europe’s largest and actively mined calcium bentonite resource from volcanic piles exceeding 80 m thickness. Here, we use the Milos island example to understand how magmatism, volcanic edifice evolution and hydrothermal activity interact. We integrate field relationships of volcanic stratigraphy and alteration zones, with clay mineralogy (XRD), stable (S, O and H) isotope analysis and high precision geochronology (CA-ID-TIMS zircon U-Pb, and alunite Ar-Ar) to elucidate the timescales, thermal drivers and fluid components that lead to the development of a globally important bentonite resource.

A vertical transect through bentonite-altered volcanic stratigraphy indicates multiple magmatic pulses ca. 2.8 Ma with a submarine andesitic cryptodome and accompanying pepperitic hyaloclastite. Cumulative volcanic and sub-volcanic processes occurred over ca. 170 kyrs, resulting in a vertically and laterally extensive volcanic pile overlain by an episode of magmatic quiescence and brackish-water diatomaceous sediments. It is overlain by a silicic pyroclastic flow host to pervasive silica-alunite-kaolinite alteration. Stable isotopic analyses of bentonite indicate a hydrothermal origin at around 70°C with the fluid being sourced from sea and meteoric waters. The timing of formation is defined by a maximum duration of ca. 170 kyrs, with clear geological evidence that a significant period of alteration occurred within < 20 kyrs at ~ 2.64 Ma. Alunite sulfur isotope compositions reflect steaming ground activity that could be interpreted as the oxidised, shallower level counterpart to a boiling geothermal system linked to development of extensive bentonite. However, the timing of alunite can be clearly resolved to > 1.5 myrs after bentonite formation to ~ 1.0 Ma, supporting a later overprint origin due to relatively recent steam heating of groundwater after emergence of the submarine system.

This study identifies key parameters that have resulted in the formation of an economic-scale bentonite resource on the emergent island of Milos. We conclude that the hydrology needed to form a bentonite deposit is not constrained to the marine environment and can be connected to emergent parts of the volcanic edifice. High precision geochronology indicates bentonite development happens on volcanic timescales (10 to 100 kyrs). A cumulative volcanic and sub-volcanic pile coeval with the formation of bentonite suggests multiple magmatic episodes over narrow timeframes provide and sustain the thermal driver for significant bentonite development. After emergence and development of a groundwater system, the subsequent steam heating is deleterious to grade and results in the development of alunite-kaolinite overburden.

How to cite: Miles, A. J., Tapster, S. R., Naden, J., Kemp, S. J., Barfod, D. N., and Boyce, A. J.: Forming an economic bentonite resource in a volcanic arc environment: Milos island, Greece, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-167, https://doi.org/10.5194/egusphere-egu21-167, 2020.

Over the last ~3 Ma, the volcanic complex of Milos Island has evolved from a shallow submarine into a subaerial edifice. It has erupted almost the entire range of calc-alkaline series compositions, but silicic units are volumetrically dominant (Fytikas et al., 1986; Stewart & McPhie, 2006). Although numerous studies have been published, data on the mineral record of the magmatic processes are absent. We examined amphiboles from 3 explosive and 4 effusive units, ranging from andesite to rhyolite, to gain insights into the structure and evolution of the plumbing system. Like many arc volcanoes worldwide, Milos products contain bimodal amphibole populations, often present within the same unit. Mg-hornblende (6.79-7.22 a.p.f.u. Si) forms macro-crysts (>600 μm; often partly decomposed) and crystal clots with plagioclase (An_47-51), orthopyroxene (Wo_1-2En_61-62Fs_37-38), and magnetite in the effusive units and phenocrysts (300-600 μm) in more evolved pumices. Mg-hastingsite occurs in effusive units as: (1) pristine micro-phenocrysts (<300 μm; 6.22-6.58 a.p.f.u. Si); (2) relics (6.22-6.46 a.p.f.u. Si) in the inner domains of pseudomorphs mostly replaced by coarse-grained orthopyroxene (Wo₂En₆₈Fs₃₀) rimmed by clinopyroxene (Wo₄₃En₄₇Fs₁₀), plagioclase (An₄₇), and magnetite; and (3) framework-forming crystals in quenched enclaves; and (4) the only amphibole (6.29-6.59 a.p.f.u. Si) phenocrysts in andesitic scoria.

Temperature (T) and pressure (P) conditions were calculated by applying hornblende-plagioclase (Holland and Blundy, 1994) and amphibole composition (Ridolfi and Renzulli, 2012) thermo-barometers. Amphibole compositions and calculated P-T conditions are in good agreement with experimentally grown amphiboles. Mg-hornblende compositions and their petrographic context are consistent with cold storage (780±24°C) in a near-solidus, upper crustal (1.7-2.8 kbar) silicic mush. This scenario is further supported by the rhyolitic (74±3.6 wt.% SiO₂) compositions of calculated melts in equilibrium with Mg-hornblende, in contrast with the less evolved bulk compositions of the host effusive units. Although the explosive eruptions likely originated from differentiated, crystal-poor melt pockets in the mush, the more common effusions of hybrid andesite-dacite magmas resulted from interaction between mafic recharge magma and the silicic mush. This interaction is preserved in the disequilibrium textures affecting both Mg-hornblendes and Mg-hastingsites, coupled with the growth of high-T (960-885°C) post-recharge Mg-hastingsite. Most of the recharge magmas in Milos are effectively dispersed, trapped, and hybridized in the upper crust, although in rare cases magmas from a deeper crustal storage region (T~960-885°C;P~3.8-5.1 kbar) erupted after limited interaction with the upper crustal storage system.

The mineral chemistry reveals that a large, shallow, silicic reservoir has been the dominant component of the Pliocene plumbing system beneath Milos. Magma inputs from deeper crustal sources are preserved in enclaves and volumetrically minor explosive products. The plumbing system of Milos shares similarities with other Aegean arc volcanoes, where magmas experience storage, differentiation, and assimilation in different crustal levels, like Methana (Popa et al., 2020).

Acknowledgements

The research work was supported by the Hellenic Foundation for Research and Innovation (HFRI) under the HFRI PhD Fellowship grant (Fellowship Number: 364).

References

Fytikas, M. et al. (1986). JVGR,28(3-4),297-317.

Holland, T., & Blundy, J. (1994).CTMP,116(4),433-447.

Popa, R.G. et al. (2020).JVGR, 106884.

Ridolfi, F., &Renzulli, A. (2012).CTMP,163(5),877-895.

Stewart, A.L., & McPhie, J. (2006).BulV,68(7-8),703-726.

How to cite: Xydous, S., Baziotis, I., Bizimis, M., Klemme, S., Berndt, J., and Asimow, P. D.: Identifying the components of Milos island subvolcanic plumbing system (South Aegean Volcanic Arc, Greece): An amphibole perspective, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4922, https://doi.org/10.5194/egusphere-egu21-4922, 2021.

The Hellenic arc hosts several active volcanic centers, of which the Milos, Santorini-Kolumbo and Kos-Yali-Nisyros volcanic fields present particularly high threats due to recent unrest (2011-2012 and 1996-1997 at Santorini and Nisyros, respectively). These volcanic centers have repeatedly produced highly explosive eruptions (VEI 4 to 7) from ~360 ka into historic times. The marine tephra record provides information not only on the number of events, but also on their magnitudes and intensities inferred from tephra dispersal characteristics, and is thus essential to quantitatively assess future volcanic hazards and risks.

Here we complement earlier work on distal to ultra-distal east-Mediterranean sediment cores, which captured the largest eruptions. We present results from a grid of medial to distal sediment cores collected in 2017 during RV Poseidon cruise POS513 with core positions both comparatively close to and between the three volcanic fields, in order to record medium- to large-scale eruptions.

During this cruise, 47 gravity cores up to 7.4 m long, and 3 box cores of the uppermost 0.5 m sediment were recovered, which contain more than 220 primary ash layers. The compositions of glass shards from all layers were characterized by major (EMP) and trace-element (LA-ICPMS) analyses.

Geochemical fingerprinting supports correlations with 20 eruptions from all three volcanic fields as well as with the 39 ka Campanian ignimbrite eruption from the Campi Flegrei, Italy. Correlations with eleven eruptions from Santorini-Kolumbo (Kameni, Kolumbo 1650, Minoan, Cape Riva, Cape Tripiti, Upper Scoria 1 and 2, Middle Pumice, Cape Thera, Lower Pumice, Cape Therma 3) are established, and we newly identify two widespread tephras from eruptions on Milos (Lower and Upper Firiplaka). We have probably been able to solve some previous chronostratigraphic problems at Kos-Yali-Nisyros by correlating marine tephras with the Kos Plateau Tuff, and with the Yali 2 tephra, whereby we identify a second, less evolved facies produced by that eruption that has not yet been recognized on land. We also find tephras from four eruptions on Nisyros (Nisyros 1 to 4) including the previously established Lower (Nisyros 4) and Upper (Nisyros1) Nisyros Pumice eruptions.

These correlations also provide new age constraints for hitherto poorly or non-dated Aegean tephras based on sedimentation rates derived between multiple anchor points of dated terrestrial tephra ages. We deduce ages of ~22 ka and ~36 ka for Upper and Lower Firiplaka tephras from Milos (the latter overlying the Campanian ash) which are significantly younger than other eruption ages known from Milos, ~54 ka, ~62 ka, ~69 ka, and ~76 ka for the Nisyros 1 to 4 tephras, and ~52 ka for the Yali 1 tephra as well as a verified age of 33 ka for the Yali 2 tephra with its two contemporaneous facies.

These new tephrostratigraphic results help to improve quantifications of distribution and eruption characteristics for all these eruptions, and provide important pre-site survey data for the Santorini IODP proposal VolTecArc.

How to cite: Kutterolf, S., Freundt, A., Hansteen, T. H., Dettbarn, R., Hampel, F., Sievers, C., Wittig, C., Druitt, T., Nomikou, P., McPhie, J., Pank, K., Schindlbeck-Belo, J. C., Wang, K.-L., and Lee, H.-Y.: The medial offshore record of Plinian arc volcanism in the Eastern Aegean Sea: Implications for tephrostratigraphy, correlations, ages and volumes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3277, https://doi.org/10.5194/egusphere-egu21-3277, 2021.

The La Fossa volcano lies nearby the settled zone of the Island of Vulcano and its last eruption occurred in 1888-1890. Since then, the fumarolic-solfataric degassing accounted for both sulfur and carbon dioxide emissions at Vulcano Porto zone. Long exposure time to CO₂-polluted air causes severe health injuries, including suffocation. Since volcanic emissions expose people at risk, several international agencies fixed safety threshold figures based on both the gas concentration and the time of exposure.

This study accounts for the results of the survey performed in the summer of 2020 inside some buildings in the settled zone of Vulcano Porto. The survey aimed at identifying four suitable sites for deployment of continuous surveying stations of both soil CO₂ flux and air CO₂ concentration. This investigation targeted the anomalous soil CO₂ emissions at the Faraglione zone. A comparison between our results and previous studies shows the anomalous degassing zones at Vulcano have not changed their current position substantially. Several significant changes (i.e. independent from changes in atmospheric pressure and temperature) occurred instead in the emissions levels because of the volcanic gas addition. The indoor measurements aimed to verify the conditions where air CO₂ concentration achieves values higher than the safety thresholds, as the results of soil CO₂ flux.

The investigation targeted several types of environments including both outdoor and indoor sites, either accessed or not by people. The outdoor sites allowed the comparison with air CO₂ levels of the indoor environments. An infrared spectrophotometer enabled the air CO₂ measurements in the range of 0 - 10% vol. At least four measurements were performed at each site with 2 minutes sampling frequency. The results enabled evaluating the CO₂ concentration patterns in a time window consistent with sporadic exposure in the selected sites.

The results show indoor air CO₂ concentration > 1000 ppm vol in several selected sites. In a few specific sites, the air CO₂ concentration achieved 6% vol after a few minutes of measurement, which is higher than the Immediately Dangerous to Life and Health exposure limit (IDLH = 4% vol). Both the soil CO₂ emissions and air exchange, either normal or artificially induced, caused these air CO₂ values.

This study shows that gas hazard mitigation includes several actions in the settled zones of Vulcano Porto. The soil CO₂ flux and air CO₂ concentration surveying are both useful actions for risk decrease. However, it is unrealistic to design a network able to identify the risk level above a site-specific threshold and take timely mitigation actions. Comprehensive risk management includes the awareness of the gas hazard among people who live, work or arrive at the island of Vulcano. At the same time, people’s training aims to promote self-reliance in hazard identification and address taking suitable actions against risk in specific cases.

How to cite: Di Martino, R. M. R., Gurrieri, S., Diliberto, I. S., Vita, F., Camarda, M., Francofonte, V., Italiano, F., Longo, M., and Paonita, A.: Gas emissions in volcanic islands: establishing an early warning network for gas hazard management, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8422, https://doi.org/10.5194/egusphere-egu21-8422, 2021.

Colli Albani is an alkali-potassic quiescent volcano of Central Italy that last erupted 36 ka ago. Several lahar generating water overflows have occurred from Albano crater lake, the most recent in Roman times (IV Century B.P.) and the resulting deposits form a surficial impermeable cover on its north-western flank. An important NW-SE trending volcano-tectonic fracture extends from the volcano to the periphery of Rome city. This is a leaky fracture allowing deep magmatic gas to rise toward the surface. In zones where the impervious cover has been removed by excavations, as Cava dei Selci, the gas is freely discharged into the atmosphere creating local hazardous conditions. Elsewhere, the gas dissolves and pressurizes the shallow aquifer confined underneath the impervious cover. Any time this aquifer is reached by a drilling, a dangerous gas blowout may be generated, i.e. a sudden emission of a jet of gas, nebulized water and fine loose fragments of volcanic rocks. Since 2003 four gas blowouts, from ~ 45–50 m deep drillings, have occurred at the boundary between Rome and Ciampino municipalities, a site designed as the Rome gas blowout zone. Dangerous atmospheric CO₂ and H₂S concentrations killed some animals and several families had to be evacuated because of hazardous gas concentration inside their houses. The emitted gas consists mostly of CO₂ (>90 vol.%) and contains a low but significant quantity of H₂S (0.3–0.5 vol.%); it has the highest helium isotopic R/Ra value (up to 1.90) of all Colli Albani natural gas discharges. This He isotopic value is similar or even slightly higher than in the fluid inclusions of phenocrysts of the Colli Albani volcanic rocks, suggesting a likely magmatic origin of the gas. Colli Albani volcano is characterized by anomalous uplift, release of magmatic gas and episodic seismic crises. The Rome gas blowouts represent a geochemical window to investigate deep volcanic processes. Should a volcanic unrest occur, gas hazard would increase in this densely inhabited zone, as the input of magmatic gas into the confined aquifer might create overpressure conditions leading to a harmful phreatic explosion, or increase the emission of hazardous gas through newly created fractures.

How to cite: Carapezza, M. L., Tarchini, L., Ranaldi, M., and Barberi, F.: The Rome gas blowout zone (Central Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13359, https://doi.org/10.5194/egusphere-egu21-13359, 2021.

We used combined trace element and U-Pb isotopic data of zircon from dacitic to rhyolitic pyroclastic rocks and Si-rich ash-bearing deposits to assess their tephrostratigraphic potential. Data were collected using LA-ICP-MS analyses, a rapid and cost-effective method, to obtain simultaneously trace element contents and U-Pb ages of a large number of zircon grains. The rationale in using zircon crystals for characterizing tephra deposits is that zircon is a resistant mineral phase and is usually a late crystallizing mineral in highly evolved magmas. Therefore, they are assumed to be in equilibrium with the erupted melt phase represented by the volcanic glass. Knowing the zircon/melt partition coefficients, equilibrium melt composition can be calculated even in cases when the volcanic glass in the pyroclastic material has undergone severe post-depositional alteration.

We studied Miocene silicic pyroclastic deposits in a broad area including the Pannonian Basin (eastern-central Europe) and its surroundings to characterize and correlate the explosive volcanic products. In regional scale, these deposits are usually assigned as important stratigraphic key horizons within sedimentary successions and thus, they help to understand better the chronostratigraphic framework and palaeoenvironmental changes having affected the highly-dynamic Mediterranean-Paratethys system.

The early to middle Miocene silicic pyroclastic deposits within the Pannonian basin are estimated to be more than 4000 km³ in volume within 4 Myr, suggesting an important ignimbrite flare-up event. At least 4 main eruption units were distinguished and characterized, each could have regional (>>100 km) effects. We demonstrate here the power of multivariate discriminant analyses as well as machine learning techniques in distinguishing the main eruptive units and their correlation with unclassified distal deposits based on zircon trace element data. The machine learning algorithms were trained using our zircon database with trace elements as input parameters. Both the discriminant analysis and the machine learning methods gave reliable results, i.e. distinguished the main 4 pyroclastic units and found the link of the distal deposits to them. As a result, we provide a robust zircon-based fingerprint that can be used as a proxy in tephrostratigraphy.

Zircon trace element compositions indicate distinct silicic magmas resided partly coeval in the upper crust. Using trace element content of zircon and glasses from the same samples of crystal-poor ignimbrites, we determined zircon/melt partition coefficients. The obtained values of the 4 main units are very similar and comparable with published data for silicic volcanic systems. This suggests that zircon/melt partition coefficients in calc-alkaline silicic systems are not significantly influenced by melt composition at >70 wt% SiO₂. These findings let us use these zircon/melt partition coefficients to calculate the equilibrium melt compositions for the pyroclastic occurrences even in case when no glass data were available. The zircon proxy approach can be limited by the non-existence of zircon in the rocks and also by the fact that no systematic compositional difference is found between eruption products, although the latter problem similarly stands for glass chemistry-based tephrostratigraphic studies.

This study was supported by the NKFIH FK-131869 project.

How to cite: Lukács, R., Petrelli, M., Guillong, M., Bachmann, O., Fodor, L., and Harangi, S.: The zircon proxy in tephrostratigraphy and magma evolution studies. Fingerprinting Miocene silicic pyroclastic rocks in the Pannonian Basin and its surroundins., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10353, https://doi.org/10.5194/egusphere-egu21-10353, 2021.

Large volumes of silicic tuffs deposited during highly explosive eruptions associated with caldera-collapses are widespread on the island of Sumatra (Indonesia) from north to south. Among them, only the Toba tuffs have been studied in great detail, while others have so far received less attention.

This study is aimed at determining the age and characteristics of a number of these tuffs from northern Sumatra (Sinkgut and Hopong-Sipirok), central Sumatra (Maninjau and Kerinci-Lempur) and southern Sumatra (Pasomah, Ranau and Lampong), in order to reconstruct the distribution and frequency of caldera-forming eruptions in the region. Furthermore, since most of the information about the volcanic activity in Sumatra comes from marine tephra layers with unknown sources, linking the temporal and compositional information to the terrestrial sources contributes a foundation for tephrostratigraphic correlations in south-east Asia.

We performed textural and geochemical analyses to characterize the crystallinity  and major and trace element compositions of bulk-rock, matrix glasses and mineral phases from the studied ignimbrites and derive information about the pre-eruptive conditions. We used a variety of geochronological methods (including U/Th, U/Pb and U/Th-He zircon dating together with ¹⁴C and ⁴⁰Ar/³⁹Ar) and statistical analyses to estimate the eruption ages and magma residence times. Multiple dating methods were often applied to the same deposits thus allowing comparison between independent age results.

Our research indicates that between ~1200 and 30 ka the region experienced at least 10 caldera-forming eruptions, in addition to 4 from Toba (between ~1.2 Ma and 74 ka) and 1 from Masurai (~33 ka): 3 from northern Sumatra at ~44 ka (Singkut), ~400 ka and ~580 ka (Hopong-Sipirok), 5 from central Sumatra at ~51 ka (Maninajau), ~150, 210, and 220 ka (Kerinci-Lempur) and 3 from southern Sumatra at ~35 ka (Ranau), ~480 ka (Pasomah), and ~1200 ka (Lampong). Each of these eruptions involved tens to hundreds of km³ of rhyolitic magmas (VEI>6) and produced calderas with diameters between ~5 and 30 km. Geothermobarometers and hygrometers indicate that prior to eruption, magmas were stored in the upper crust in similar conditions but the geochemical signatures (in particular the K₂O content), mineral assemblages and mineral chemistry define clear differences between the northern, central and southern sectors of the Sumatran volcanic arc, presumably linked to the regional geodynamics and structural setting.

This study allows to redefine the number of caldera-forming eruptions in Sumatra from 7 (previously dated) to 15 over the last 1.2 Ma. A similar frequency of VEI>6 eruptions during the Quaternary is reported for the Japan arc [1]. However, a significant number of eruptions, potentially better preserved in the marine record, might still be missing from our reconstruction.

[1] Schindlbeck, J. C. et al. One Million Years Tephra Record at IODP Sites U1436 and U1437: Insights into explosive volcanism from the Japan and Izu arcs. Isl. Arc. https://doi.org/10.1111/iar.12244 (2018).

How to cite: Forni, F., Oalmann, J. A., Fellin, G., Eisele, S., Phua, M., Guillong, M., Rifai, H., and Bouvet de Maisonneuve, C.: Quaternary caldera-forming eruptions from north to south Sumatra (Indonesia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7140, https://doi.org/10.5194/egusphere-egu21-7140, 2021.

Volcanic eruptions are considered as one of the primary natural drivers for changes in the global climate system and understanding the impact of past eruptions on the climate is integral to adopt appropriate responses towards future volcanic eruptions.

The Greenland ice core records are dominated by Icelandic eruptions, with several volcanic systems (Katla, Hekla, Bárðarbunga-Veiðivötn and Grimsvötn) being highly active throughout the Holocene. A notable period of increased Icelandic volcanic activity occurred between 500-1250 AD and coincided with climatic changes in the North Atlantic region which may have facilitated the Viking settlement of Greenland and Iceland. However, a number of these volcanic events are poorly constrained (duration and magnitude). Consequently, the Greenland ice cores offer the opportunity to reliably reconstruct past Icelandic volcanism (duration, magnitude and frequency) due to their high-resolution, the proximity of Iceland to Greenland and subsequent increased likelihood of volcanic fallout deposits (tephra particles and sulphur aerosols) being preserved. However, both the high frequency of eruptions between 500-1250 AD and the geochemical similarity of Iceland’s volcanic centres present challenges in making the required robust geochemical correlations between the source volcano and the ice core records and ultimately reliably assessing the climatic-societal impacts of these eruptions.

To address this, we use two Greenland ice core records (TUNU2013 and B19) and undertake geochemical analysis on tephra from the volcanic events in the selected time window which have been detected and sampled using novel techniques (insoluble particle peaks and sulphur acidity peaks). Further geochemical analysis of proximal material enables robust correlations to be made between the events in the ice core records and their volcanic centres. The high-resolution of these polar archives provides a precise age for the event and when utilised alongside other proxies (i.e. sulphur aerosols), both the duration and magnitude of these eruptions can be constrained, and the climatic-societal impacts of these eruptions reliably assessed.

How to cite: Gabriel, I., Plunkett, G., Abbott, P., Óladóttir, B., McConnell, J., Hörhold, M., and Sigl, M.: Tephra in the Greenland Ice cores: Insights into Icelandic volcano-climate impacts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9187, https://doi.org/10.5194/egusphere-egu21-9187, 2021.

The Main Ethiopian Rift (MER) is the northern portion of the East African Rift System and separates the Eastern and Western plateaus of Ethiopia. The recent volcanic and tectonic activity is largely focused within the rift basin along a 20 km wide zone on the rift floor. Large silicic volcanic complexes are aligned along this central rift axis but their eruptive histories are not well constrained.

The Bora-Baricha-Tullu Moye (BBTM) volcanic field is situated in the central Main Ethiopian Rift and has a different appearance than the other MER volcanic systems. The BBTM constitutes several late Quaternary edifices, the major ones are: Tullu Moye, Bora and Baricha. In addition, there are multiple smaller eruptive vents (e.g. Oda and Dima), cones, and domes across the ca. 20 X 20 km wide area. Currently, there is very little information on the frequency and magnitude of past volcanic eruptions. We present a new dataset of field observations, componentry, petrography, geochronology (⁴⁰Ar/³⁹Ar), and glass major and trace element chemistry. The data are assessed as potential fingerprints to assign diagnostic features and correlate units across the area, and establish a tephrostratigraphic framework for the BBTM volcanic field.

Two large-volume and presumably caldera-forming eruptions are identified, the younger of which took place at 100 ka. The volcanic products exposed in the BBTM area show that the volcanic field has undergone at least 20 explosive eruptions since then. The post-caldera eruptions have comenditic (Tullu Moye) and pantelleretic (Bora and Baricha) magma compositions. Other smaller edifices such as Oda and Dima also erupted pantelleritic magmas, and only differ slightly in composition than tephra of Bora and Baricha. Tullu Moye had two distinct explosive eruptions that dispersed tephra up to 14 km away and on to the eastern plateau. Bora and Baricha together had at least 8 explosive eruptions. Their deposits can be distinguished by their light grey color and unique lithic components. Oda had 7 eruptions, the most recent of which generated a pyroclastic density current that travelled up to 10 km away from the vent. Dima experienced at least 3 eruptions, generating tephra with a bluish-grey colour.

This mapping and compositional analysis of the deposits from the BBTM in the MER indicates that the region has been more active in the last 100 ka than previously thought, which has implications for hazards assessments for the region.

How to cite: Tadesse, A. Z., Fontijn, K., Melaku, A. A., Gebru, E. F., Smith, V., Tomlinson, E., Barfod, D., Gopon, P., Ayalew, D., Yirgu, G., and Gudbrandson, S.: Eruptive frequency of the Bora-Baricha-Tullu Moye (BBTM) volcanic system in the Central Main Ethiopian Rift, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13966, https://doi.org/10.5194/egusphere-egu21-13966, 2021.

Volcanic ash (tephra) deposits are used to reconstruct past eruption parameters. The ways in which tephra deposits are modified between deposition and their long-term preservation in the stratigraphic archive are poorly understood. In particular, we don’t know if tephra layers preserved in lake sediments from small lakes accurately reflect the initial tephra fallout. We address this by re-surveying tephra deposits from the 1991 eruption of Volcán Hudson, Chile. We measured tephra thickness, mass-loading and grain-size distribution of tephra from multiple cores in six small (<0.2 km²) lakes at locations 76-110 km from the volcano and in areas of contrasting land cover and climate. We also measured tephra preservation in terrestrial sites within each lake catchment. These data were compared with measurements taken shortly (days to weeks) after the eruption to determine how the tephra deposits have changed in the 29 years since the eruption. Preservation is variable within and between lakes, and also varies with the vegetation cover at terrestrial sites adjacent to the lakes. Tephra thicknesses are broadly comparable to the original fallout, but the degree of similarity varied notably and is sensitive to preservation environment. These findings have implications for reconstructing eruption parameters from tephra deposits in small lakes, and where the fallout area crosses large environmental gradients and contrasting vegetation regimes.

How to cite: Streeter, R., Cutler, N., and Lawson, I.: Variable preservation of tephra from the 1991 eruption of Volcán Hudson in small lakes: implications for reconstructing past eruption parameters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-209, https://doi.org/10.5194/egusphere-egu21-209, 2020.

The climactic Los Chocoyos (LCY) rhyolitic eruption from Atitlán caldera (Guatemala) is a key chronostratigraphic marker for the Late Quaternary period that has been widely used for relative dating of paleoenvironmental, paleoclimate, and volcanic events throughout Central America and adjacent marine basins in the Pacific Ocean, the Caribbean Sea, and the Gulf of Mexico. Despite LCY tephra being an important marker horizon, a radioisotopic age for this eruption has remained elusive. LCY tephra has been dated at ca. 84 ka BP based on its occurrence in marine sediments with model δ¹⁸O ages, but this inferred age has not been independently confirmed through radioisotopic methods. This is due to the inherent limitations of radiocarbon dating (which is practically limited to ˂50 ka) and a lack of suitable materials for ⁴⁰Ar/³⁹Ar analysis in LCY tephra. To overcome this limitation, we applied ²³⁸U-²³⁰Th and (U-Th)/He zircon double-dating (ZDD). Due to zircon being alteration-resistant this method establishes absolute chronologies for and correlations between silicic tephra deposits, which are unaffected by glass alteration or complex compositional signatures within a single eruption. ²³⁸U-²³⁰Th zircon crystallization rim ages were obtained from LCY proximal tephras (~17 km from Atitlán caldera) including sub-units that may bear distinct glass compositions (e.g., fallout, ignimbrite, surge) as well as ultra-distal fallout tephra samples (~300 km from source) collected from drill cores at Petén Itzá Lake (ICDP) and the Pacific Ocean (IODP). All samples yielded zircon with statistically indistinguishable ²³⁸U-²³⁰Th zircon rim age spectra. These reveal continuous zircon crystallization from ca. 160 ka to ca. 74 ka, with peaks in zircon crystallization between 90-100 ka. ZDD eruption ages from two LCY fallout and one ignimbrite deposit are indistinguishable with error-weighted averages of 75.1 ± 3.2 ka (1σ; n = 16; MSWD = 4.1), 76.0 ± 2.5 ka (n = 16; MSWD = 2.5), and 72.8 ± 3.5 ka (n = 16; MSWD = 3.7). Considering all individual zircon results as a single population, a weighted average ZDD age of 74.8 ± 1.7 (1σ; n = 48; MSWD = 3.3) is obtained and considered as the best estimate for LCY eruption age. GIS-based reassessment of LCY eruptive volume uses thickness information from new 113 outcrops including 6–10 m thick pyroclastic density currents in Chiapas, Mexico (>130 km from the source) and suggests a minimum estimate volume of ~1200 km³, confirming the LCY eruption as the first‐ever recognized supereruption in Central America. The new ZDD age of 74.8 ± 1.7 ka for the LCY eruption is significantly younger than the commonly cited O-isotope stratigraphic age of 84 ± 5 ka. This age is close to the voluminous (2,800-5,600 km³) Young Toba Tuff (YTT) supereruption at ca. 73.8 ± 0.3 ka from Toba Caldera, Indonesia. Both YTT and LCY eruptions have been previously linked to prominent Quaternary climate excursions. Based on the new LCY eruption age, climate-forcing effects that are usually attributed to YTT may in fact be exacerbated by another supereruption occurring within a short time window of the YTT event.

How to cite: Cisneros de Leon, A., Schindlbeck-Belo, J. C., Kutterolf, S., Danišík, M., Schmitt, A. K., Freundt, A., Pérez, W., Harvey, J., Wang, K.-L., and Lee, H.-Y.: Timing and distribution of the Los Chocoyos supereruption from Atitlán caldera (Guatemala) by zircon 238U-230Th and (U-Th)/He double-dating, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16327, https://doi.org/10.5194/egusphere-egu21-16327, 2021.