A glimpse below the wine-dark sea: Multi-beam bathymetric survey around Despotiko and Strongylo islands (Cyclades, Greece)

Abstract

Our understanding and knowledge about Earth´s surface and processes benefitted greatly from the increasing availability of open accessible, large-scale remote sensing topographic data, including GTOPO30 (c. 1 km, 1996), ASTER (c. 30 m, 2009), SRTM 3.0 (c. 30 m, 2014) and TanDEM-X (c. 12 m, 2014). In comparison, the documentation of the sea floor at higher resolutions and/or its accessibility is scarce (e.g. https://emodnet.ec.europa.eu/en/bathymetry, https://www.ngdc.noaa.gov/mgg/bathymetry/lidar.html, Otero & Mytilineou 2022) and vast submarine areas are still a terra incognita. We have carried out a preliminary multi-beam bathymetric survey during 29-31 May 2023 with the 13.4 m research vessel “Alkyon” of the Hellenic Centre for Marine Research, funded by the Cheops Privatstiftung Wien and the Walter Munk Foundation for the Oceans (https://www.waltermunkfoundation.org). The Alkyon was equipped with a hull-mount Teledyne Reson T-50R multibeam echosounder. In total, an area of 26.8 km2 was surveyed west, south and east of Strongylo Island, south of Despotiko Island up to Cape Petalida at the southern tip of Antiparos and especially the Bay of Despotiko, between Despotiko and Antiparos islands with measured depth ranging between -6.7m to -105.7 m below sea-level. The survey aims include the (i) high-resolution documentation of this previously unknown sea-floor, (ii) information concerning local sea-level rise (see Lykousis 2009, Kolaiti & Mourtzas 2020, 2023), (iii) the possible continuation of tectonic features as well as coastal mass-movements investigated above sea-level. Processing and thorough geomorphological analysis of the high resolution bathymetric data provide valuable information on the extend of posidonia meadows on the seafloor  (e.g. Despotiko Bay), evidence for possible palaeo-sealevel indicators (palaeo-coastlines, wave-cut terraces) at various depths, palaeo-valleys and other geomorphological features belonging to the terrestrial landscape that was drowned during the post-glacial sea-level rise, as well as several deposits associated with the mass movements mapped on the adjacent rocky slopes of Strongylo, Despotiko and South Antiparos islands. Marine geological-geophysical research will be continued and complimented with high resolution sub-bottom profiling data and visual observation to unravel the recent geomorphological evolution of the survey area. Kolaiti, E. & Mourtzas, N. 2020. New insights on the relative sea level changes during the Late Holocene along the coast of Paros Island and the northern Cyclades (Greece). Annals of Geophysics, 63(6), https://doi.org/10.4401/ag-8504 Kolaiti, E. & Mourtzas, N. 2023. Late Holocene relative sea-level changes and coastal landscape readings in the island group of Mykonos, Delos, and Rheneia (Cyclades, Greece). Mediterranean Geoscience Reviews, 5, 99-128. https://doi.org/10.1007/s42990-023-00104-4 Lykousis, V. 2009. Sea-level changes and shelf break prograding sequences during the last 400 ka in the Aegean margins: Subsidence rates and palaeogeographic implications. Continental Shelf Research, 29(16), 2037-2044. Otero, M. & Mytilineou, C. (eds.) 2022. Deep-sea Atlas of the Eastern Mediterranean Sea: Current knowledge. IUCN-HCMR DeepEastMed Project. IUCN Gland, Málaga, 371 p. https://uicnmed.org/docs/deep-sea-eastern-med/DEEP-SEA-EASTERN-MEDITERRANEAN.pdf