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.

Unlike subaerial volcanic activity, deep submarine eruptions are difficult to detect, observe and monitor. The objective of this paper is to describe a large and complex volcanic region, named the Horseshoe area, recently discovered at ∼1500 m below sea level on the eastern upper submarine slope of Mayotte Island. The area is crucial because, since 2018, it has experienced an exceptionally deep seismic activity associated with the ongoing submarine eruption that formed a new volcanic edifice, Fani Maoré, about 40 km to the east. We present the results of a multiscale study, based on highresolution bathymetry and in-situ seafloor observations carried out with autonomous underwater vehicles (AUVs) and deep-towed camera systems. In-situ imagery provides ground-truth for the geological interpretation of seafloor textures mapped with the bathymetry. The combination of both datasets allows us to discuss the nature of the volcanic structures and to propose a relative chronology of previous eruptive events in the Horseshoe area. Based on our analyses, we propose the following chronology: (a) the emplacement of a large explosive volcanic cone, the Horseshoe edifice, (b) the later collapse of this edifice that resulted in the formation of an elongated, 2 km wide horseshoe-shaped depression, crosscutting older hummocky lava flows, (c) the development of an E-W eruptive fissure associated with numerous explosive craters, east of the Horseshoe edifice, and (d) late volcanism emanating from the rim of the horseshoe-shaped depression that fed elongated thin lava flows both towards and away from the depression. While all volcanic features mapped at the Horseshoe area were emplaced prior to the 2018 eruption, our study shows that this region has still been volcanically active in the recent past. Our results thus document a complex geological history at small spatial scales involved in the construction of major submarine edifices, and that are controlled by volcano-tectonic processes at larger scales.

IntroductionShallow hydrothermal systems share many characteristics with their deep-sea counterparts, but their accessibility facilitates their study. One of the most studied shallow hydrothermal vent fields lies at Paleochori Bay off the coast of Milos in the Aegean Sea (Greece). It has been studied through extensive mapping and its physical and chemical processes have been characterized over the past decades. However, a thorough description of the microbial communities inhabiting the bay is still missing.MethodsWe present the first in-depth characterization of the prokaryotic communities of Paleochori Bay by sampling eight different seafloor types that are distributed along the entire gradient of hydrothermal influence. We used deep sequencing of the 16S rRNA marker gene and complemented the analysis with qPCR quantification of the 16S rRNA gene and several functional genes to gain insights into the metabolic potential of the communities.ResultsWe found that the microbiome of the bay is strongly influenced by the hydrothermal venting, with a succession of various groups dominating the sediments from the coldest to the warmest zones. Prokaryotic diversity and abundance decrease with increasing temperature, and thermophilic archaea overtake the community.DiscussionRelevant geochemical cycles of the Bay are discussed. This study expands our limited understanding of subsurface microbial communities in acidic shallow-sea hydrothermal systems and the contribution of their microbial activity to biogeochemical cycling.

<p>The Milos hydrothermal field is one of the largest known shallow water hydrothermal systems, and shows both fluid and gas outflow through the seafloor. Recent studies based on imagery acquired by both aerial and submarine drones (Puzenat et al., submitted) reveal several types of fluid outflow associated with bacterial mats along the SE coast of the island (Paleochori, Spathi, and Agia Kyriaki bays). From these observations? include: a) zones of polygonal hydrothermal outflow and associated bacterial mats, b) extended white (bacterial) patches, and c) isolated ones. Subseafloor hydrothermal circulation is hosted in sediments with subseafloor temperatures >50°C, and there is a clear association between hydrothermal circulation and active degassing.</p><p>To understand the controls on and relationships between fluid and gas outflow in the area, we need to characterise: a) the nature of the subseafloor (sediment thickness, composition & permeability); b) the distribution of gas and subseafloor fluids, and c) the distribution of gas flares emanating from the seafloor. In November 2020, we conducted a short pilot geophysical study at Paleochori Bay, deploying a towed catamaran with a multibeam echo sounder (iXblue Seapix) to obtain seafloor bathymetry, acoustic backscatter and water column detection of gas flares. We also deployed a sub-bottom profiler (iXblue Echoes 3500 T1) to image sediment architecture and gas/fluid diffusion within the sediment. Our survey focused on Paleochori Bay, surveing areas from ~5 m (nearshore) to ~100 m waterdepth (offshore).</p><p>Preliminary results of this geophysical survey suggest that subseafloor gas accumulations play a major role on the nature and structure of hydrothermal activity at Milos. These gas accumulations within the sediments develop along an onshore/offshore fault system, and likely control the shallow subseafloor thermal structure, establishing a thin thermal conductive layer between the roof of gas pockets and the seafloor.[GJ1] [je2]   We will report on the link between the distribution and geometry (extent, depth, acoustic nature of the accumulations) of gas pockets, fluid outflows, and gas outflows, all of which will be characterised from both seafloor imagery and subsurface geophysical surveys. We will also discuss how gas pocket geometry may be linked to both fluid flow and subseafloor temperature structure. [HA3] </p><div> <div> <div> </div> </div> <div> <div> </div> </div> <div> <div> </div> </div> </div>