Gas plumes and near-seafloor bottom current speeds of the southernmost Okinawa Trough determined from echo sounders

Using echo sounders to detect gas plumes in seawater is common, especially in the context of hydrothermal circulation areas or gas hydrate-bearing cold seeps. To understand the distribution of gas plumes in the southernmost Okinawa Trough, we have conducted 13 cruises with a 38 kHz single-beam echo sounder (EK60). A total of 266 gas plumes of acoustic image, associated with the hydrothermal circulation, are detected. To estimate the near-seafloor bottom current speeds, 201 gas plumes are further used. As a result, the gas plumes around the axial depression of the Okinawa Trough generally tilt to the northeast at rising tides and high tides, suggesting a northeastward flow of the bottom current. However, the gas plumes in the Keelung continental slope tilt to the southwest at ebb tides and low tides, suggesting a southwestward flow of the bottom current. Our results significantly show a good estimation of the near-seafloor bottom currents from EK plume images in the case of lacking real observations. The directions of the bottom currents depend on semidiurnal tides. Assuming a quasi-constant speed of upward gas bubbles out of seabed, we have estimated the bottom current speeds in 6 hydrothermal circulation zones near the rifting center of the southernmost Okinawa Trough. The estimated bottom current speeds in submarine volcanic areas vary largely from 2 to 160 cm s-1, but bottom current speeds in relatively flat region are between 20 and 50 cm s-1. The large variation of the bottom current speeds in the submarine volcanic areas could be due to the variable emissions of the gases out of the submarine volcanic areas.