Shallow-water hydrothermalism at Milos (Greece): Nature, distribution, heat fluxes and impact on ecosystems

Abstract

Submarine hydrothermal activity is responsible for heat and chemical exchanges through the seafloor. Shallow-water hydrothermal systems (SWHS), while identified around the globe, are often studied in a way that is less comprehensive than their deep-ocean counterparts (e.g., along ridges), where systematic optical and acoustic mapping is more prevalent and coupled to in situ observations and sampling. Using aerial drones, an AUV, and temperature measurements at 10-40 cm subseafloor, we investigated in 2019 one of the most extensive SWHS known to date, in Paleochori (south of Milos, Greece). Hydrothermal venting, found from the shore to water depths of almost 500 m, shows emissions of gases and high-temperature fluids, often associated with bacterial mats and/or hydrothermal mineral precipitates. This study provides extensive drone mapping coupled with local AUV surveys for seafloor characterization and ground-truthing from the shore to ~20 m water depth. Seafloor photomosaics also provide a detailed context to samples, measurements and observations carried in situ. We interpret the photomosaics to define distinct seafloor types, linked to this hydrothermal activity. White hydrothermal patches (WHPs) often show a clear polygonal organization, together with outflow areas that are both more dispersed and distributed. Polygonal patterns likely result from fluid convection in a sandy porous medium heated from below. These WHPs display elevated subseafloor temperatures, typically >50 °C, with maximum values of ~75 °C. Photomosaics also display textures of biological origin, including seagrass and bioturbation patterns. Widespread bioturbation by burrowing shrimps is often associated with WHPs, bounding them, but also occurs on sandy seafloor away from hydrothermal patterns. Subseafloor temperatures at these bioturbated areas are of ~30-40 °C, and are thus transitional between hot WHPs and sedimented seafloor unaffected by hydrothermal activity (~24 °C). In addition to linking subseafloor temperature data and interpreted seafloor 4 photomosaics, our results provide a comprehensive general overview of this SWHS, of the organization of its hydrothermal outflow through the seafloor, and of the underlying subseafloor fluid circulation. This paper also gives the first perspectives on the heat fluxes of the system, and constitutes a background for other studies on the nature and distribution of microbial communities, controlled by this hydrothermal activity.

Metastable Iron (Mono)sulfides in the Shallow-Sea Hydrothermal Sediments of Milos, Greece

“…The volcanic island of Milos lies in the middle of the Hellenic Volcanic Arc (HVA), formed by the subduction of the African plate beneath the Aegean microplate (Figure a,b). Although the island of Milos has not exhibited explosive volcanic activity in the last 90 kya, it hosts a shallow-sea hydrothermal system (from 3 down to >500 mbsl) developed at the SE boundaries of the Fyriplaka volcano . This hydrothermal system is one of the largest in the Mediterranean Sea extending for more than 35 km 2 , mainly onshore and offshore Palaeochori, Spathi, and Agia Kyriaki bays − (Figure ).…”

“…Although the island of Milos has not exhibited explosive volcanic activity in the last 90 kya, it hosts a shallow-sea hydrothermal system (from 3 down to >500 mbsl) developed at the SE boundaries of the Fyriplaka volcano . This hydrothermal system is one of the largest in the Mediterranean Sea extending for more than 35 km 2 , mainly onshore and offshore Palaeochori, Spathi, and Agia Kyriaki bays − (Figure ). At Paleochori Bay, hydrothermal activity begins onshore and continues to the submarine environment, where it is manifested by diffuse gas venting and fluid flow through the marine sediments, associated with low outflow velocities (Figure b–d).…”

Integrating Multidisciplinary Observations in Vent Environments (IMOVE): Decadal Progress in Deep-Sea Observatories at Hydrothermal Vents

“…As part of repeated monitoring efforts, the use of non-invasive video sequence analyses could be key for volume, heat, and chemical flux estimations at smoker vents. Using pixel-based correlation methods (Particle Image Velocimetry or Diffuse Fluid Velocimetry; Crone et al, 2010;Mittelstaedt et al, 2010;Mittelstaedt et al, 2012;Escartı ́n et al, 2015;Puzenat et al, 2021), the flow of high-temperature and low-temperature fluids can be estimated with reasonable accuracy and in relatively short image capture sequences. To allow uniform, inter-comparable results using these techniques requires a common protocol with guidelines for image acquisition (camera distance; use of chessboard for image calibration; Escartin et al, 2013; minimum image resolution and frame rate guidelines), the implementation of a user-friendly computer program to process images as part of routine maintenance visits to vent sites, and sufficiently stable imaging platforms to avoid image motion related to currents and/or vehicle motions.…”

SANTORY: SANTORini’s Seafloor Volcanic ObservatorY

“…The least studied seafloor hydrothermal vent sites are associated with shallow submarine arc volcanoes and arcrelated rifts in subduction-related settings (Taran et al, 1992;Tsunogai et al, 1994;Lupton et al, 2006;Lupton et al, 2008;Lan et al, 2010), which typically occur at a much shallower water depth (<1000m). These vigorously degassing, submarine hydrothermal systems are associated with significant and dangerous volcanic and seismic activity and are more likely to impact the marine environment near-coastal populations (Dando et al, 1995;Zimanowski and Büttner, 2003;Puzenat et al, 2021;Mei et al, 2022). The cabled sea-floor observatory deployed off the coast of Panarea hydrothermal system (Aeolian Arc, South Tyrrhenian Sea, it was exploded in November 2002) at a depth of 24m, is at the moment the only monitoring system installed in the Mediterranean Sea which automatically transmits data of chemical and physical signals (T, EC, pH, dissolved CO 2 , acoustics) to shore (Caracausi et al, 2005a;Caracausi et al, 2005b).…”

“…Research on shallow submarine arc volcanoes is still in its infancy despite their potentially severe hazards. In the Mediterranean, the Aeolian Island Arc of the Tyrrhenian Sea (Caliro et al, 2004;Caracausi et al, 2005a;Chiodini et al, 2006;Capaccioni et al, 2007;Heinicke et al, 2009;Tassi et al, 2009;Monecke et al, 2014;Petersen et al, 2014;Tassi et al, 2014;Tassi et al, 2015;Esposito et al, 2018) and the Hellenic Volcanic Arc of the Aegean Sea (Dando et al, 2000;Nomikou et al, 2012;Nomikou et al, 2013;Carey et al, 2013;Kilias et al, 2013;Cantner et al, 2014;Christopoulou, et al, 2016;Ulvrova et al, 2016;Rizzo et al, 2016a;Rizzo et al, 2019;Puzenat et al, 2021;Daskalopoulou et al, 2022;Kilias et al, 2022) have attracted lots of attention, as very little is known about their volcanic and hydrothermal activity and impacts over intermediate and longtime scales. Nevertheless geological and historical records point to many catastrophic events that have had a broad impact throughout Southern Europe (Druitt et al, 1999).…”