2015 年 49 巻 6 号 p. 579-596
Polymetallic sulfides deposited in seafloor hydrothermal vents have recently attracted attention as potential deep-sea mineral resources for base, rare, and precious metals such as Cu, Zn, Pb, In, Ga, Ge, Au, and Ag. For future exploitation of this type of deep-sea mineral resources, development of effective methods for exploring seafloor hydrothermal activity is a key to provide the most promising list of fields. However, conventional exploration methods are likely laborious and time-consuming, and a more efficient methods for exploration of seafloor hydrothermal vents are to be further developed. In the last decade, water column observation using multibeam echo souder (MBES) systems have become successfully applied to exploration of seafloor hydrothermal vents. In 2013 and 2014, we conducted extensive water column surveys using MBES systems in the mid-Okinawa Trough. During the surveys, we detected 10 hydrothermal vent sites, including previously known sites, belonging to four relatively large hydrothermal vent fields, located at the Izena Hole, Iheya North Knoll, Iheya Small Ridge, and a seamount 15 km northwest of the Izena Hole. All of the hydrothermal vent sites are in groups of 2–3 vent sites belonging to a hydrothermal field. Morphological features of the acoustic water column anomalies (rising vertically up to ~1000 m from the seafloor without a significant change of width) implied that the acoustic water column anomalies were not caused directly by hydrothermal vent fluid flows. The depth of the top of the acoustic water column anomalies (~500 m) corresponded rather well to the depth of the CO2 phase transition from liquid/clathrate-hydrate to vapor. This suggests that the acoustic water column anomalies are attributed to water mass including dense liquid CO2 droplets with clathrate-hydrate crusts, which are originally derived from the seafloor hydrothermal fluid discharges.
