ROV EXPLORATION OF SANTORINI CALDERA, GREECE

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1 ROV EXPLORATION OF SANTORINI CALDERA, GREECE Nomikou P. 1, Croff Bell K. 2, Bejelouk K. 1, Parks M. 3, Antoniou V. 1 1 University of Athens, Department of Geology and Geoenvironment, Panepistimioupoli Zografou, Athens, Greece, evi@ath.hcmr.gr, bejelouk@gmail.com, vantoniou@geol.uoa.gr 2 Graduate School of Oceanography, University of Rhode Island, Narragansett, kcroff@gso.uri.edu ³ University of Oxford, UK, michelle.parks@earth.ox.ac.uk Abstract The geology of the Santorini volcanic group has been described by a large number of researchers with petrological as well as geochronological data. The offshore area of the Santorini volcanic field has only recently been investigated with emphasis mainly inside the Santorini caldera and the submarine volcano of Kolumbo. In September 2011, new ROV exploration with the E/V Nautilus has been carried out inside the Santorini caldera. Submarine hydrothermal vents were found on the seafloor of the northern basin of the Santorini caldera with no evidence of high temperature fluid discharges or massive sulphide formations, but only low temperature seeps characterized by meter-high mounds of bacteria-rich sediment. ROV exploration at the northern slopes of Nea Kammeni revealed a fascinating underwater landscape of lava flows, lava spines and fractured lava blocks that have been formed as a result of the and AD eruptions. At the top of a volcanic dome, east of Nea Kammeni a crater with shimmery water was also found. The combination of multibeam data together with ROV exploration could provide new insights in the sea-bottom morphology of Santorini caldera. Keywords: ROV exploration, Santorini Caldera, multibeam mapping, hydrothermal vents. 1. Introduction The Santorini volcanic group form the central part of the modern Aegean volcanic arc, developed within the Hellenic arc and trench system, because of the ongoing subduction of the African plate beneath the European margin throughout Cenozoic (Mc Kenzie, 1972, LePichon & Angelier, 1979, Papanikolaou, 1993). It comprises three distinct volcanic structures occurring along a NE-SW direction: Christianna form the southwestern part of the group, Santorini occupies the middle part and Kolumbo volcanic rift zone extends towards the northeastern part (Nomikou et al., 2012). The current, exceptional landscape of Santorini caldera in the middle of the island, has been formed after the famous Minoan eruption in the 17th century BC (Friedrich, 2000) which has been considered as the second largest eruption in historical times, after the Tambora volcano eruption in Indonesia in 1815 (Sigurdsson et al., 1991). The caldera walls rise to over 300m above sea level while the maximum depth of the caldera seafloor is about 390m. The oldest volcanic products of Santorini are million years old (Druitt et al., 1989). Several eruptive cycles are distinguishable on the island with the Minoan eruption being the most recent. Minoan ash forms a white colored cap over the island up to 50m thick. Beneath that cap successive multicolored lava layers, pyroclastic flows, ash deposits and other volcanic formations are exposed on the upright caldera walls, providing evidence of the diverse volcanic history of the island. Since the devastating Minoan eruption, many, significantly weaker activity phases have occurred, and the landscape has continued to change even during very recent years. The Nea Kammeni island in the centre of the caldera forms the summit of the actively growing new volcanic center. The ancient geographer Strabo was the first to record volcanic eruptions inside the Santorini caldera. He described the birth of a newly formed small island in 197 BC in the middle of the caldera, which now forms Palea Kammeni. Several eruptive phases followed the birth of the Kammeni islands (Palea and Nea) in the years 46-47, 726, , , , , and 1950 AD as the most recent eruption. Technological advancements in seafloor exploration have now enabled researchers to observe products of submarine volcanism that were previously inaccessible (Schipper et al., 2010; Hekinian 2008). In particular, the use of Remotely Operated Vehicles (ROVs) have enabled the identification and characterization of submarine deposits that demonstrate the more common occurrence of explosive subaqueous volcanism than previously considered (Embley et al., 2007).

2 In September 2011, cruise NA-014 on the E/V Nautilus carried out new surveys on the submarine volcanism of the study area, investigating the seafloor morphology with high-definition video imaging. In this paper we report the results of ROV transects inside Santorini Caldera which have been overlaid on the detailed swath bathymetric map (Fig. 1) (Nomikou et al., 2012b). Fig. 1: Swath bathymetric map of Santorini caldera using 10m isobaths, where the three post-minoan caldera subbasins are indicated (the dotted borders show the basinal parts) (Nomikou et al., 2012b) The gray lines indicate the ROV transects of 2011 and VF: vent field. 1b: geographical index map. 2. Methods The multibeam bathymetric survey has been carried out on the research-vessel R/V AEGAEO of the Hellenic Centre for Marine Research (HCMR), during two successive cruises in 2001 and 2006, mapping the volcanic field of Santorini area (Nomikou et al., 2012). The SEABEAM 2120 (20 khz) multibeam system has been used during the two missions. It is a swath system that has been specifically designed to suit users with survey requirements exceeding 6000m water-depth, accomplishing a satisfactory resolution (up to 1 1 ) without mounting a very large array. The maximum swath coverage can reach 9 km at maximum depth and gives satisfactory data quality at speeds up to 11 knots. The multibeam data have been extensively processed by means of data editing, cleaning of erroneous beams, filtering of noise, processing of navigation data and interpolation of missing beams. The resulting bathymetric map of Santorini Caldera was originally compiled at 1:50.000, which was greatly reduced for publication with 10 different colors corresponding to 100 m depth intervals and with additional isobaths of 10 m (Fig. 1). This map permits the first detailed description of the overall topography of the sea floor as well as the mapping of the major morphotectonic structures within this area.

3 The ROV exploration of Santorini Caldera took place in September 2011 using the E/V Nautilus as a collaborative project (New Frontiers in Ocean Exploration 2011) between the Graduate School of Oceanography at the University of Rhode Island (URI-USA), the Dept. of Geology & Geoenvironment of University of Athens (NKUA-GREECE) and the Institute for Exploration (IFE- USA). The dives of ROVs Hercules and Argus were to observe the submarine volcanic structures detected north of Nea Kammeni Volcano. The Exploration Vessel (E/V) Nautilus of O.E.T. (Ocean Exploration Trust) is equipped with the ROVs Hercules and Argus which make up a dual-body deepwater system. Hercules and Argus are state-of-the-art deep-sea robotic vehicle systems capable of exploring depths up to 4000 m (Philips et al., 2011). Both vehicles have high-definition video cameras working with 1200-watt lamps, which provide a clear, high resolution imaging of the seafloor. Hercules is also equipped with two manipulator arms, one dexterous and the other a gripper, that work together to sample and move equipment around on the seafloor. Capable of working as a stand-alone system, Argus becomes a towed-body instrument for large-scale deepwater survey missions. E/V Nautilus has a high frequency satellite system which helps in distance learning and research during the journey, through Inner Space Center (ISC) in URI Graduate School of Oceanography and Exploration Command Consoles (ECCs) located around the world. 3. Results The present configuration of the caldera consists of three distinct basins that form separate depositional environments, divided by the Kammeni volcanic islands (Fig.1). The Northern Basin is the largest and deepest (389 m) developed between the Kammenes, Therassia and the northern part of the Santorini caldera. The Western Basin is the smaller and lies alongside the Aspronisi islet, Palaea Kammeni and Southern Therassia with a medium depth (325 m). The South Basin is developed between the Kammenes and the southern part of the Santorini caldera covering a medium area with the shallowest sea bottom (297 m). The continuation of lava flow can be observed only in the north-eastern part of Nea Kammeni in contrast to the southern part where abrupt volcanic cliffs up to 250m depth can be observed. The total volcanic relief of Kammenes islands reaches almost 470m. The floor of the Santorini caldera was explored with the ROV Hercules. A large number of hydrothermal vents was discovered (Fig. 2), but in contrast to the high-temperature venting found in the Kolumbo submarine volcano, only relatively low-temperature venting was observed within the Santorini caldera (Sigurdsson et al., 2006). They form a vent field in the NE part of the North Basin that is 200 to 300 m in extent at the depth around 340m. The vents form hundreds of 1 to 4 meter diameter mounds of yellowish bacterial mat that are up to 1 meter high. Temperatures in this area are approximately 15 to 17 C or about 5 C above ambient temperature. Two push cores were taken for further geochemical-biological analysis from the top of the venting mounts which were covered with yellowish bacterial mat (Fig. 3). In this area there is no evidence of high temperature fluid discharges or massive sulphide formations like those found in the Kolumbo crater floor (Carey et al., 2011), but only low temperature seeps characterized by meter-high mounds of bacteria-rich sediment. The North Basin hydrothermal vent field is located in line with the normal fault system of the Kolumbo rift, and also near the margin of a shallow intrusion that occurs within the sediments of the North Basin. Similar vent mounds occur in the South Basin, at shallow depths around the islets of Nea and Palaia Kammeni (Nomikou et al., 2012b). Hydrothermal activity within the caldera is known to temporally vary in composition and intensity, influenced by tidal and climatic variations (Varnavas and Cronan, 2005, Camilli et al., 2007). In the hydrothermal vent field has no significant changes were observed since the previous expedition in 2010 with the E/V Nautilus.

4 Fig. 2: Underwater photo taken by ROV Hercules showing the low temperature hydrothermal field at the northern part of the caldera. Fig. 3: Taking push core sample from the top of a vent mound in the NE part of Santorini Caldera. Submarine volcanic structures around the Kammenes islands comprise a number of lava flows towards the north reaching the sea bottom at 390 m depth, some dikes (Fig. 4) along the submarine part of the caldera in the northern walls of the North Basin belonging to the same system as the observed above sea level. Moving along a vertical wall in transect 2a from north to south, we observed lava rocks covered by pink sediments with red spots and some yellow coating at 172m depth (Fig. 5). The slopes in many places are steep, with fractured lava blocks along the submarine continuation of the Kammenes lava flows (Fig. 6) and lava spines covered by sediments (Fig 7). The exploration of the northern slopes of Nea Kammeni revealed no signs of hydrothermal venting.

5 Fig. 4: Underwater photos taken by ROV Hercules showing the lava in the form of dikes north of Nea Kammeni at 260m depth (transect 2a). Fig. 5: Moving upslope, we observed lava rocks covered in pink sediment at 172m depth (transect 2a).

6 Fig. 6: Underwater photo taken by the ROV Hercules showing the angular rocky lava blocks at the northern slopes of Nea Kammeni at 225m depth (transect 2b). Fig. 7: Lava spine with very steep slopes and lots of sedimentation at the northern slopes of Nea Kammeni Volcano (transect 2b). Following the transect No3 from south to north, a hummocky topography was observed in the area that lies between the town of Fira on the main island of Santorini and Nea Kammeni. The lower slopes were covered with landslide debris which consisted of lava blocks mostly mantled with soft sediment. At the upper slopes an abrupt cliff face was exposed that was highly covered with biological material. In general, the slopes were characterized by higher biodiversity than deeper areas (different types of sea stars, white sponges, anemones, urchins, fish, crabs, corals). After ascending the steep slope, we reached a rather flat area at the top of the bathymetric rise. It appeared that there was a crater at the top of this feature with its deepest part at 43m, its rim at about 34m with an approximately 8m diameter. Shimmery water with temperatures as much as 25ºC above ambient was observed there (Fig. 7), but the source of venting has not yet been found.

7 Fig. 8: Temperature measurement at the inner crater slopes, east of Nea Kammeni Volcano. 4. Discussion-Conclusions The combination of ROV video footage and multibeam data provide new information about the main morphological characteristics of Santorini Caldera. These results will be useful for the interpretation of understanding the offshore volcanic area and its linkage with onshore structures. Push cores have been collected from the vent mounds of a low temperature hydrothermal vent field in the northern part of the caldera which will provide insights for their geochemical characteristics and their relationship to the active vents of the Kolumbo underwater volcano. The flat top of the volcanic dome, east of Kammeni volcano needs further investigation for active craters and water sampling. In conclusion, this recent volcanic morphology of Santorini Caldera demonstrates the intense geodynamic processes occurring at the central part of the active Hellenic volcanic arc which can be understood only by studying the relief both in the offshore and onshore area. 5. Acknowledgements This work was supported by Institute for Exploration (IFE-USA) and the collaborative project New Frontiers in the Ocean Exploration 2011 between the Graduate School of Oceanography at the University of Rhode Island (URI-USA), the Dept. of Geology & Geoenvironment of University of Athens (NKUA-GREECE). The officers and the crew of the E/V NAUTILUS are gratefully acknowledged for their important and effective contribution to the field work and sampling. 6. References Camilli R., Sakellariou D., Foley B., Anagnostou C., Malios A., Bingham B., Eustice R., Goudreau J.& Katsaros K., Investigation of Hydrothermal vents in the Aegean Sea using an integrated mass spectrometer and acoustic navigation system onboard a human occupied submersible. Rapp. Comm. int. Mer Medit., 38, Carey S., Bell K. L. C., Nomikou, P., Vougioukalakis, G., Roman, C., Cantner, K., Bejelou, K., Bourbouli, M. & Martin Fero, J., Exploration of the Kolumbo Volcanic Rift Zone. Oceanography, Vol. 24, No.1, p , Supplement, March Druitt, T.H., Mellors, R.A, Pyle, D.M. & Sparks, R.S.J., Explosive volcanism on Santorini, Greece. Geological Magazine, 126, vol. 2: Embley, R., Baker, E., Butterfield, D., Chadwick, W., Lupton, J., Resing, J., DeRonde, C., Nakamura, K., Tunnicliffe, V., Dower, J.& Merle, S., Exploring the Submarine Ring of Fire Mariana Arc-Western Pacific. Oceanography: Special Issue on the NOAA Ocean Exploration Program 20(4), Friedrich, W., Fire in the Sea, The Santorini Volcano: Natural History and the Legend of Atlantis. Cambridge University Press, Cambridge. Hekinian, R., Mühe, R., Worthington, T.J.& Stoffers, P., Geology of a submarine volcanic caldera in the Tonga Arc: Dive results. Journal of Volcanology and Geothermal Research 176, Le Pichon, X. & Angelier, J., The Hellenic arc and trench system: a key to the neotectonic evolution of the eastern Mediterranean area. Tectonophysics 60, p McKenzie, D.P., Active tectonics of the Mediterranean region Geophys. J.R. Astron. Soc. 30, Nomikou P., Carey S., Papanikolaou D., Croff Bell K., Sakellariou D. & Alexandri M., Bejelou K.: Submarine Volcanoes of the Kolumbo volcanic zone NE of Santorini Caldera, Greece Glob. Planet. Change, doi: /j.gloplacha

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