Periodic Behavior of Deep Sea Current in the Hatoma Knoll Hydrothermal System

Abstract

Time series physical properties of water column were measured at the Hatoma Knoll hydrothermal field on the Okinawa Trough, using a multi-layered Acoustic Doppler Current Profiler (ADCP). We aimed (1) to provide a deep-sea current data within a small and complicated geographical feature of a caldera and (2) to show the possibility of hydrothermal plume observation by ADCP during the measurement in April 2004 and May 2005 cruises. The deep-sea current data were recovered at the depths between seafloor and 40-m or 60-m-layers above the seafloor. Moreover, acoustic reflection signals were sometimes observed up to 120-m layer above the seafloor due to the trail of hydrothermal plumes. A power spectrum density analysis by fast Fourier transform (FFT) showed the dominant time cycle of the horizontal current velocity, which corresponded to tidal periodicity over an approximate 12-hours period, at each layer. Residual currents, which play an important role in material transport, tended to northwest direction and suggested that most of material might keep retaining within the caldera. The causes of the measurement result of the flow which appeared into a pulse form may indicate fluctuation of the blowout of hydrothermal water. The verification of the fluid mechanism by the numerical model in the local field such as the caldera will be necessary in the future.

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

“…Because the gas plumes keep the same direction at the rising tide to the highest tide and the same direction is on the ebb and at lowest tide, there could be a time lag effect of the tidal rise and fall effect on the flow of the water layer. It agrees with a stable flow of ~3 h during the transition of the rising and ebb (Furushima and Yamamoto 2015).…”

“…Semi-diurnal tides also affect the deep ocean around the mid-Atlantic Ridge (Fujioka and Mitsuzawa 2001) and around the seabed near Oahu of Hawaii (Aucan et al 2006). In addition, Berdeal (2006) found significant inertial oscillations in the time series of horizontal flow in an 11-month near-bottom ADCP observation; similar result was obtained in the southern Okinawa Trough (Furushima and Yamamoto 2015). It is concluded that a semi-diurnal tide will affect the current flow direction and flow rate in the seawater.…”

“…The application of multi-beam echo sounder water-column imaging technology allows us to understand the relationship between the shape of the plume shape and the direction of current in the seawater (Urban et al 2017). Additionally, the mooring observation on the hydrothermal mounds of the mid-ocean ridge (Fujioka et al 1997) and the ADCP (Acoustic Doppler Current Profiler) observation in the Hatoma Knoll hydrothermal field of the southern Okinawa Trough (Furushima and Yamamoto 2015) have shown that the fluctuation cycle of temperature, pressure, and current is synchronized with semidiurnal tides.…”

Geochemical observations within the water column at the CO2‐rich hydrothermal systems Hatoma Knoll and Yonaguni Knoll IV, in the southern Okinawa Trough

“…Aside from temporal variation of the sources, the height and dispersal of the anomalies in the water column at YK and at HK is further controlled by the interplay between the flow field and the local bathymetry, which was found to differ significantly between the two research areas. At HK, the background flow within the caldera is restricted by the sidewalls and is on the order of 0.1 m/s [ Furushima and Yamamoto , ], with no preferred direction. Additionally, model predictions from TPXO7.2 indicate that tides are weak at this site, with amplitudes of less than 2 cm/s of tidal flow (see Figure S1 in the supporting information).…”

“…Up to 140 m high and 500 m wide caldera is almost completely closed except for a small breach at the southern rim, probably resulting from a former flank collapse and slumping [ Rehder and Schneider von Deimling , ; Shitashima et al ., ]. A total of at least 12 hydrothermal vents have been found in the HK area [ Furushima and Yamamoto , ; Watanabe , ]. They are located in the middle of the caldera close to a round‐shaped 30 m high mound and at the northwestern inner‐side rim of the caldera (Figure e).…”