Hydrothermal Vapor-Phase Fluids on the Seafloor: Evidence From In Situ Observations

Subseafloor phase separation is a common and significant process in hydrothermal systems and may result in a large of fluid composition differences. The temperatures of hydrothermal fluids are generally considered to be below the associated fluid boiling temperature due to mixing with ambient seawater and the phase separation process. However, we report here shimmering water with temperatures up to 383.3 degrees C observed in a hot overturned lake at the Yokosuka site, Okinawa Trough, East China Sea, where in situ Raman spectra suggest the presence of a superheated vapor phase. Hydrothermal vents similar to the low-density hydrothermal system found at the Yokosuka site have also been observed in many other regions. Therefore, much more attention should be given to the impacts of low-density hydrothermal fluid emanations on marine environments and resource distributions.