Characteristics of the Environment Around a Massive Sea-Floor Sulfide Area in the Okinawa Trough

It is necessary to carry out a proper environmental impact assessment for not only the sea-floor massive sulfide area where a special hydrothermal-vent chemosynthetic ecosystem is observed, but also its surrounding area. This paper shows provisional results of the environmental baseline survey conducted at the Izena Cauldron in 2009. Environmental conditions differed clearly between inside and outside of the Cauldron. In the inside area, a high turbidity layer was recognized at ca. 200 m above the sea-floor. In the bottom layer close to the hydrothermal active region, water temperature was slightly higher than the surrounding area with the lower pH, higher concentrations of both carbon dioxide and the dissolved Mn. Inside the Cauldron, the sulfur isotope ratio of the sediment close to a sea-floor massive sulfide was similar to that of the hydrothermal sulfate. In addition, heavy metal concentrations of the sediment were higher inside than outside of the Cauldron. Especially, concentrations of Cd, As and Hg were as high as poisonous levels. These results show the possibility that most of the chemical materials derived from hydrothermal vent remain in the Cauldron. Pelagic shrimps of the family Oplophoridae were widely distributed in the bottom layer inside the Cauldron. The biomass of benthic shrimps which belong to infraorder Caridea was higher inside than outside of the Cauldron. These higher biomass was likely caused by the abundant organic matter. This hypothesis was reinforced by the result of the mooring sediment trap (placed at ca. 50 m above the seafloor) experiment, which demonstrated that the settling particle in the Cauldron was supplied from not only the euphotic zone but also the hydrothermal-vent ecosystem.