Abstract
The Izena Cauldron in the Okinawa Trough has two sites of hydrothermal massive sulfide deposits named the Jade Site and the Hakurei Site. The present study conducted observation of ADCP/CTD towings and AUV dives to obtain the over-all distribution of bottom-currents in the caldera. In the ADCP observation, current velocity data and echo intensity data were obtained in multiple-layers between the seabed and the 100-150 m altitude. The analysis shows the characteristics of the bottom-currents; they have an organized structure with almost same directions in the whole region within the caldera, and averaged speed of horizontal velocity of approximately 10 cm s-1, and the directions change temporally with tidal frequency. There are descending currents with a 0-2 cm s-1 downward velocity component along the eastern slope of the caldera. The echo intensity data shows that high echo intensity anomalies distribute patchily at depths of approximately 1500 m. They are well-corresponded with turbidity maxima and upward velocity components, and two anomalies have an upward velocity component of more than 2 cm s-1. It is suggested that these high echo intensity anomalies show the distribution of particulate matter in hydrothermal plumes from the Hakurei Site. These results indicate a circulation pattern of hydrothermal plumes in the Izena Cauldron, as plumes rise to approximately 1500 m depth from the seabed in the Hakurei Site.
