doi: 10.3319/TAO.2019.07.07.01 * Corresponding author E-mail: hsu@ncu.edu.tw Gas plumes and near-seafloor bottom current speeds of the southernmost Okinawa Trough determined from echo sounders Ching-Hui Tsai 1 , Shu-Kun Hsu 1, 2, * , Yen-Fu Chen 2 , Hsiao-Shan Lin 1 , Shiou-Ya Wang 1 , Song-Chuen Chen 3 , Chin-Wei Liang 1 , and Yen-Yu Cho 1 1 Center for Environmental Studies, National Central University, Taoyuan City, Taiwan 2 Department of Earth Sciences, National Central University, Taoyuan City, Taiwan 3 Central Geological Survey, Ministry of Economic Affairs, Taipei City, Taiwan ABSTRACT 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 circu- lation, 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, sug- gesting 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 semi- diurnal 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 bot- tom 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. Article history: Received 26 January 2019 Revised 2 July 2019 Accepted 7 July 2019 Keywords: Gas plume, Bottom current, Okinawa Trough, Echo sounder Citation: Tsai, C.-H., S.-K. Hsu, Y.-F. Chen, H.-S. Lin, S.-Y. Wang, S.-C. Chen, C.-W. Liang, and Y.-Y. Cho, 2019: Gas plumes and near-seafloor bottom current speeds of the southernmost Okinawa Trough determined from echo sounders. Terr. Atmos. Ocean. Sci., 30, 649-674, doi: 10.3319/ TAO.2019.07.07.01 1. INTRODUCTION Relative to the Eurasian Plate, the Philippine Sea Plate moves northwestward at a speed of ~8 cm yr -1 around Taiwan (Seno et al. 1993; Yu et al. 1997). The Philippine Sea Plate is subducting beneath the Eurasian Plate and has formed the Ryukyu subduction zone (Letouzey and Kimu- ra 1985; Sibuet et al. 1987). Okinawa Trough is an active back-arc basin behind the Ryukyu Arc (Uyeda 1977; Lee et al. 1980; Kimura 1985; Sibuet et al. 1987) (Fig. 1). Our study area is located in the southernmost Okinawa Trough. Morphologically, Keelung Valley and Mienhua Canyon are loacated in the northern margin of the Okinawa Trough and transport sediments from the East China Sea to the Okinawa Trough (Hsu et al. 1996). The current rifting of the southern Okinawa Trough area could start at 0.1 million years ago (Sibuet et al. 1998). Kimura et al. (1986, 1988) and Shyu and Liu (2001) have pointed out unusually high geothermal flows in the Okinawa Trough, especially some hydrothermal hillocks and biota. It reveals an actively hydrothermal circula- tion system in the Okinawa Trough. Earthquakes and ac- tive submarine volcanoes are common in the southernmost Okinawa Trough (Lin et al. 2007, 2009). It suggests inten- sively magma activity associated with abundant submarine volcanoes. Base on the multi-beam bathymetric data from the ACT cruise of the Taiwan-French cooperation in 1996 (Lallemand et al. 1997), there are more than 70 submarine volcanoes in the southernmost area of the Okinawa Trough Terr. Atmos. Ocean. Sci., Vol. 30, No. 5, 649-674, Ocober 2019