Geophysical site survey for drilling into a sedimented shallow-water hydrothermal system

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Marine shallow-water hydrothermal systems (MSWHS), as opposed to deep-sea hydrothermal systems, occur in shallow-water nearshore settings where they can have a considerable impact on the coastal ocean through discharge of reduced, hot hydrothermal fluids. One of the best-studied MSWHS is located in Paleochori Bay on the south side of Milos Island, which has been the focus of numerous scientific studies. However, despite more than 25 years of research, the subsurface processes and mechanisms that generate hydrothermal fluids with contrasting chemical compositions in Paleochori Bay are unclear. In contrast to most other locations, the MSWHS in Paleochori Bay is covered by sediments (of unknown thickness). Thus, the hydrothermal fluids percolate through the sediment cover before venting into Paleochori Bay seawater at low rates. This would allow for sediment-hydrothermal fluid interaction, which should cause a chemical overprint that obscures the true hydrothermal signal. Hence, it is a viable hypothesis that sediment thickness should be a controlling factor for the final chemical composition of hydrothermal fluids in Paleochori Bay. With this in mind, we propose to conduct a geophysical survey of the sediment cover and to generate a map of its thickness for Paleochori Bay. This map can be combined with existing maps of temperature and element distribution in Paleochori Bay pore fluids to evaluate the impact of sediment thickness on the physicochemical composition of the hydrothermal fluids. This new information will be the basis for collecting sediment cores, which are necessary for addressing the bigger questions and subsequent research for Paleochori Bay, such as, (1) Is the MSWHS a typical arc-hosted epigenetic hydrothermal system. These processes modulate the release fluxes of elements in significant ways and need to be understood to decipher the signatures of deeper-seated reactions. (2) Do sediment thickness and fluid flux rates determine the magnitude that sediment-hosted reactions in the shallow subseafloor play in modifying hydrothermal fluid composition? (3) Are fluid flux rates highest where fault planes come up through the basement?