[HCG25-P06] Revisiting the meandering channel developed in the Okinawa Trough
Keywords:Okinawa Trough, Meandering Channel, Turbidity Current
In 1996, a French survey found that meandering channels existed on both north and south slopes of the trough north of Yonaguni Island, on the western edge of the Okinawa Trough, where active rifting in the north-south direction is taking place on the eastern margin of the Asian continent. Multibeam bathymetric survey during the ACT Cruise in the SPOT area east of Taiwan and the western end of the trough by the research vessel (R/V) 'l'Atalante' first revealed the channels (Sibuet et al., 1998). After that, LEQUIOS Cruise by R/V 'Yokosuka' and Submersible 'Shinkai6500' were conducted in 2000 (Matsumoto et al., 2001). The main purpose of the cruise was to study the formation and development of the central trough, hydrothermal activity in the trough, and wide-area tectonics and dynamics by use of the seafloor observation by the submersible. Marine geophysical observations including the detailed topographic survey in the eastern extension of the SPOT area was also carried out during the LEQUIOS Cruise. As a result, the entire meandering channel on both north and south slopes of the trough was revealed, and it was found that both channel complexes were almost the same size.
Kimura et al. (2001) pointed out that these channels were very similar to a fan formed on a flat land that transitions from a mountain to a plain, or a delta that can be formed at the mouth of a river, through an ROV (remotely operated vehicle) survey along these meandering channels. Then, the authors pointed out that it could have been formed in an environment such as the intertidal zone or the extremely shallow sea when part of the Okinawa Trough became land. However, considering that these meandering channels are developed on the elongated mounds of several kilometers wide in the east-west direction and are bordered by levees, these channels should be formed by turbidity currents from the continent of China. The channels should be in one stage of the process of forming a submarine fan. Larger ones can be found, for example, in the Amazon Fan drilled at ODP Leg155.
The University of the Ryukyus uses the Nagasaki Maru, a training ship of the Faculty of Fisheries of Nagasaki University, to offer undergraduate onboard training courses every year based on a credit exchange agreement with the Faculty of Fisheries. The ship is also used for research in the Ryukyu region. The old Nagasaki Maru was retired in 2017 and a new one entered service in 2018. The new ship is equipped with a multibeam echo sounder. Approximately 20 years have passed since the previous survey of the meandering channels on the Okinawa Trough, and during this time there might have been a supply of new turbidity currents from mainland China, and the shape of the existing meandering channel might have changed. Therefore, a re-survey of the precise bathymetry of these meandering channels was carried out during the one-week cruise of the training vessel 'Nagasakimaru' in May 2019. Due to rough sea conditions and time constraints, the authors focused on the channel on the northern slope of the trough during the cruise .
As a result, the position change of the flow path of the meandering channel developed on the mound for last 20 years were not detected. And also, no new mounds appeared. However, the water depth of the bottom of the meandering channel on the mound extending from 25° N, 122° 55’ E towards east-northeast is generally 5-15 m deeper than that of 20 years ago, while the levees on both sides of the channel is 15-20m shallower (higher). These suggest that the turbidity current supply from mainland China has continued for last 20 years, and that the erosion of the bottom of the channel and the growth of the levees has been taking place. On the other hand, meandering channels located in the area near 25° 08’N, 123° 25’E, which is not on a mound, had no change in water depth.
Future monitoring of the channel topography regularly and conducting a topographic survey again on the southern slope of the Okinawa Trough will help clarify the turbidity current supply mechanism at the western end of the Okinawa Trough.
Kimura et al. (2001) pointed out that these channels were very similar to a fan formed on a flat land that transitions from a mountain to a plain, or a delta that can be formed at the mouth of a river, through an ROV (remotely operated vehicle) survey along these meandering channels. Then, the authors pointed out that it could have been formed in an environment such as the intertidal zone or the extremely shallow sea when part of the Okinawa Trough became land. However, considering that these meandering channels are developed on the elongated mounds of several kilometers wide in the east-west direction and are bordered by levees, these channels should be formed by turbidity currents from the continent of China. The channels should be in one stage of the process of forming a submarine fan. Larger ones can be found, for example, in the Amazon Fan drilled at ODP Leg155.
The University of the Ryukyus uses the Nagasaki Maru, a training ship of the Faculty of Fisheries of Nagasaki University, to offer undergraduate onboard training courses every year based on a credit exchange agreement with the Faculty of Fisheries. The ship is also used for research in the Ryukyu region. The old Nagasaki Maru was retired in 2017 and a new one entered service in 2018. The new ship is equipped with a multibeam echo sounder. Approximately 20 years have passed since the previous survey of the meandering channels on the Okinawa Trough, and during this time there might have been a supply of new turbidity currents from mainland China, and the shape of the existing meandering channel might have changed. Therefore, a re-survey of the precise bathymetry of these meandering channels was carried out during the one-week cruise of the training vessel 'Nagasakimaru' in May 2019. Due to rough sea conditions and time constraints, the authors focused on the channel on the northern slope of the trough during the cruise .
As a result, the position change of the flow path of the meandering channel developed on the mound for last 20 years were not detected. And also, no new mounds appeared. However, the water depth of the bottom of the meandering channel on the mound extending from 25° N, 122° 55’ E towards east-northeast is generally 5-15 m deeper than that of 20 years ago, while the levees on both sides of the channel is 15-20m shallower (higher). These suggest that the turbidity current supply from mainland China has continued for last 20 years, and that the erosion of the bottom of the channel and the growth of the levees has been taking place. On the other hand, meandering channels located in the area near 25° 08’N, 123° 25’E, which is not on a mound, had no change in water depth.
Future monitoring of the channel topography regularly and conducting a topographic survey again on the southern slope of the Okinawa Trough will help clarify the turbidity current supply mechanism at the western end of the Okinawa Trough.