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.