Paraskevi Nomikou

Marine Investigations of Greece's Santorini Volcanic Field

The most recent major explosive eruption of the Santorini volcano in Greece—around 3600 years before present (B.P.), often referred to as the Minoan eruption—is one of the largest volcanic events known in historical time and has been the subject of intense volcanological and archeological studies [Druitt et al., 1999]. The submarine volcano Kolumbo, located seven kilometers northeast of Santorini and associated with Santorinis tectonic system, erupted explosively in 1650 A.D., resulting in fatalities on the island of Thera [Fouque, 1879]. A large fraction of the erupted products from the Minoan eruption has been deposited in the sea but, up to now, only has been studied in distal marine sediments. As part of a collaborative project between the University of Rhode Island (Narragansett), the Hellenic Centre of Marine Research (Athens, Greece), and the Institute of Geology and Mineral Exploration (Athens), a marine geological survey was conducted around Santorini from April to June 2006. he new work now shows that the volume of the Minoan eruption may be comparable to that of the largest known historical eruption, the 1815 eruption of Tambora in Indonesia [Sigurdsson and Carey, 1989]; provides insights into the depositional processes and size of the Minoan eruption; and led to the discovery of important submarine hydrothermal vents with active mineralization.

Morphotectonic structure of the western part of the North Aegean Basin based on swath bathymetry

Abstract The morphotectonic structure of the North Aegean Basin is studied on the basis of a new detailed swath bathymetric survey. The resulting bathymetric map is presented in reduction with 20-m isobaths. The slope analysis gives an accurate scheme of the geometry of the basin and distinction of several sub-basins (approximately 20). The overall basin geometry is a rectangular tetrahedron shaped by the major slope discontinuity separating the continental platform from the continental slope. The area distribution with depth shows a maximum at depths between 300 and 450 m along the sub-horizontal edge of the continental platform and at depths between 1000 and 1200 m at the basinal areas of the sub-basins. The separation of the western part of the North Aegean Basin from the eastern part (Saros Bay) is very clear in the area between Limnos and Thasos, with a maximum depth of 490 m. The 3.2% of the basin area is characterized by slope values >20%, which correspond to active fault zones. Their trend is NE–SW (N46°) and NW–SE (N136°). Some secondary E–W faults are also present within the basin with morphological expression only on the orientation of slopes

CO2 degassing from hydrothermal vents at Kolumbo submarine volcano, Greece, and the accumulation of acidic crater water

Discharge of volcanic gases in the marine environment can lead to local perturbations in ocean acidity, with consequences for biological communities and the potential for hazards related to depressurization and release of gases at the surface. Numerous hydrothermal vents in the crater of Kolumbo submarine volcano (Aegean Sea) are discharging virtually pure gaseous CO 2 together with clear fluids at temperatures up to 220 °C. Acoustic imaging of the ascending bubbles suggests that the gas is being dissolved into seawater within ∼10 m above the crater floor (500 m below sea level). Dissolution of the gas likely causes local increases in water density that result in sequestration of CO 2 within the enclosed crater, and the accumulation of acidic seawater. Lack of macrofauna at the Kolumbo hydrothermal vents, occurrence of carbonate-poor sediment in the crater, and pH values as low as 5.0 in recovered water samples point to acidic conditions within the crater. Buildup of CO 2 -rich water in the bowl-shaped crater of Kolumbo may be producing conditions analogous to some African volcanic lakes (Lake Monoun and Lake Nyos, Cameroon) where overturn of gas-rich bottom waters led to abrupt releases of CO 2 at the surface. A minimum estimate of 2.0 × 10 5 m 3 (STP) of excess CO 2 may currently exist in the bottom 10 m of the Kolumbo crater.

From quiescence to unrest: 20 years of satellite geodetic measurements at Santorini volcano, Greece

Periods of unrest at caldera-forming volcanic systems characterized by increased rates of seismicity and deformation are well documented. Some can be linked to eventual eruptive activity, while others are followed by a return to quiescence. Here we use a 20 year record of interferometric synthetic aperture radar (InSAR) and GPS measurements from Santorini volcano to further our understanding of geodetic signals at a caldera-forming volcano during the periods of both quiescence and unrest, with measurements spanning a phase of quiescence and slow subsidence (1993–2010), followed by a phase of unrest (January 2011 to April 2012) with caldera-wide inflation and seismicity. Mean InSAR velocity maps from 1993–2010 indicate an average subsidence rate of ~6 mm/yr over the southern half of the intracaldera island Nea Kameni. This subsidence can be accounted for by a combination of thermal contraction of the 1866–1870 lava flows and load-induced relaxation of the substrate. For the period of unrest, we use a joint inversion technique to convert InSAR measurements from three separate satellite tracks and GPS observations from 10 continuous sites into a time series of subsurface volume change. The optimal location of the inflating source is consistent with previous studies, situated north of Nea Kameni at a depth of ~4 km. However, the time series reveals two distinct pressure pulses. The first pulse corresponds to a volume change (ΔV) within the shallow magma chamber of (11.56 ± 0.14) × 106 m3, and the second pulse has a ΔV of (9.73 ± 0.10) × 106 m3. The relationship between the timing of these pulses and microseismicity observations suggests that these pulses may be driven by two separate batches of magma supplied to a shallow reservoir. We find no evidence suggesting a change in source location between the two pulses. The decline in the rates of volume change at the end of both pulses and the observed lag of the deformation signal behind cumulative seismicity, suggest a viscoelastic response. We use a simple model to show that two separate pulses of magma intruding into a shallow magma chamber surrounded by a viscoelastic shell can account for the observed temporal variation in cumulative volume change and seismicity throughout the period of unrest. Given the similarities between the geodetic signals observed here and at other systems, this viscoelastic model has potential use for understanding behavior at other caldera systems.

Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20'N and 13°30'N, Mid-Atlantic Ridge)

Microbathymetry data, in situ observations, and sampling along the 138200N and 138200N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high-angle fault, scarps show extensive mass wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the alongextension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension-parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 138200N OCC, and gabbro and peridotite at 138300N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 138300N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 138200N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution.

Tsunami hazard risk of a future volcanic eruption of Kolumbo submarine volcano, NE of Santorini Caldera, Greece

Kolumbo submarine volcano, located NE of Santorini caldera in the Aegean Sea, has only had one recorded eruption during historic times (1650 AD). Tsunamis from this event severely impacted the east coast of Santorini with extensive flooding and loss of buildings. Recent seismic studies in the area indicate a highly active region beneath Kolumbo suggesting the potential for future eruptive activity. Multibeam mapping and remotely operated vehicle explorations of Kolumbo have led to new insights into the eruptive processes of the 1650 AD eruption and improved assessments of the mechanisms by which tsunamis were generated and how they may be produced in future events. Principal mechanisms for tsunami generation at Kolumbo include shallow submarine explosions, entrance of pyroclastic flows into the sea, collapse of rapidly accumulated pyroclastic material, and intense eruption-related seismicity that may trigger submarine slope collapse. Compared with Santorini, the magnitude of explosive eruptions from Kolumbo is likely to be much smaller but the proximity of the volcano to the eastern coast of Santorini presents significant risks even for lower magnitude events.

Metagenomic investigation of the geologically unique Hellenic Volcanic Arc reveals a distinctive ecosystem with unexpected physiology

Hydrothermal vents represent a deep, hot, aphotic biosphere where chemosynthetic primary producers, fuelled by chemicals from Earths subsurface, form the basis of life. In this study, we examined microbial mats from two distinct volcanic sites within the Hellenic Volcanic Arc (HVA). The HVA is geologically and ecologically unique, with reported emissions of CO2 -saturated fluids at temperatures up to 220°C and a notable absence of macrofauna. Metagenomic data reveals highly complex prokaryotic communities composed of chemolithoautotrophs, some methanotrophs, and to our surprise, heterotrophs capable of anaerobic degradation of aromatic hydrocarbons. Our data suggest that aromatic hydrocarbons may indeed be a significant source of carbon in these sites, and instigate additional research into the nature and origin of these compounds in the HVA. Novel physiology was assigned to several uncultured prokaryotic lineages; most notably, a SAR406 representative is attributed with a role in anaerobic hydrocarbon degradation. This dataset, the largest to date from submarine volcanic ecosystems, constitutes a significant resource of novel genes and pathways with potential biotechnological applications.

Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system

Submarine volcanism represents ~80% of the volcanic activity on Earth and is an important source of mantle-derived gases. These gases are of basic importance for the comprehension of mantle characteristics in areas where subaerial volcanism is missing or strongly modified by the presence of crustal/atmospheric components. Though, the study of submarine volcanism remains a challenge due to their hazardousness and sea-depth. Here, we report 3He/4He measurements in CO2–dominated gases discharged at 500 m below sea level from the high-temperature (~220 °C) hydrothermal system of the Kolumbo submarine volcano (Greece), located 7 km northeast off Santorini Island in the central part of the Hellenic Volcanic Arc (HVA). We highlight that the mantle below Kolumbo and Santorini has a 3He/4He signature of at least 7.0 Ra (being Ra the 3He/4He ratio of atmospheric He equal to 1.39×10−6), 3 Ra units higher than actually known for gases-rocks from Santorini. This ratio is also the highest measured across the HVA and is indicative of the direct degassing of a Mid-Ocean-Ridge-Basalts (MORB)-like mantle through lithospheric faults. We finally highlight that the degassing of high-temperature fluids with a MORB-like 3He/4He ratio corroborates a vigorous outgassing of mantle-derived volatiles with potential hazard at the Kolumbo submarine volcano.

Post-eruptive flooding of Santorini caldera and implications for tsunami generation

Caldera-forming eruptions of island volcanoes generate tsunamis by the interaction of different eruptive phenomena with the sea. Such tsunamis are a major hazard, but forward models of their impacts are limited by poor understanding of source mechanisms. The caldera-forming eruption of Santorini in the Late Bronze Age is known to have been tsunamigenic, and caldera collapse has been proposed as a mechanism. Here, we present bathymetric and seismic evidence showing that the caldera was not open to the sea during the main phase of the eruption, but was flooded once the eruption had finished. Inflow of water and associated landsliding cut a deep, 2.0–2.5 km3, submarine channel, thus filling the caldera in less than a couple of days. If, as at most such volcanoes, caldera collapse occurred syn-eruptively, then it cannot have generated tsunamis. Entry of pyroclastic flows into the sea, combined with slumping of submarine pyroclastic accumulations, were the main mechanisms of tsunami production.

Evidence from cosmic ray exposure (CRE) dating for the existence of a pre-Minoan caldera on Santorini, Greece

Cosmic ray exposure (CRE) dating was performed on the caldera cliffs of Santorini with the aim of detecting cliff segments predating the Minoan eruption (17th century BCE). The methodology involved the determination of in situ-produced cosmogenic 36Cl concentration in basaltic-to-rhyodacitic whole rocks cropping out in the cliffs. After the samples were processed following the chemical protocol of 36Cl preparation for silicate rocks, 36Cl concentrations were measured by accelerator mass spectrometry (AMS). Important challenges during the implementation procedure were related to large amounts of radiogenic 36Cl, complex modeling of inherited 36Cl, and dominance of the thermal and epithermal (low-energy) neutron capture production pathway. Nevertheless, quantitative assessments on the basis of the contribution of the low-energy neutron capture pathway percent to the total production rate validated the calculated CRE dates. Current CRE ages demonstrate that an ancient caldera existed on pre-Minoan Santorini, occupying at least the northern half of the modern-day caldera.