Characteristics of deep-sea turbulent flow fluctuation near seafloor in Izena Cauldron, Okinawa Trough, Japan

Abstract

In this study, direct continuous measurements of deep-sea turbulent flows near the seafloor were made in the Izena Cauldron hydrothermal fields, Okinawa Trough, Japan, to understand the dynamics of the bottom mixed layer. The objectives of this study were to reveal time series fluctuations of deep-sea turbulence intensity, that is, the turbulence energy dissipation rate, using an expendable vertical microstructure profiler (VMP-X), and to correlate them with in situ environmental data (flow, turbidity, conductivity, temperature, and depth data) obtained from a monitoring station installed on the seafloor near the VMP-X observation point. The turbulence energy dissipation rate ($\boldsymbol{\varepsilon}$) value near the seafloor changed from 10−10 to 10−7 W kg−1, and $\boldsymbol{\varepsilon}$ maxima were observed from a flood tide between ebb tides (13:00–15:30 and 20:30–23:00) around a depth of 960 m, whereas the $\boldsymbol{\varepsilon}$ values decreased to 10−9 W kg−1 during ebb tide. In the former case, the water temperature increased only slightly, but in the latter case, water temperature increased by 0.1 °C. Furthermore, the turbulent flow was relatively strong from the seafloor to a height of around 50 m from the bottom. Turbulent flow observation points were located to the west of the hydrothermal vent area. The bottom topography of this area is closed. Deep-sea current data were successfully obtained for the layers between the seafloor and 60 m above the bottom, and periodic fluctuations with east–west and north–south components were synchronously observed with water-level variations. According to these environmental data and complex (closed) bottom topography data, the increasing water temperature fluctuation may occur as a result of hydrothermal venting (transport or intrusion of warm water). Therefore, $\boldsymbol{\varepsilon}$ may be affected by hydrothermal venting. However, to better understand the dynamics of vertical mixing processes, more observational data, such as continuous turbulent flow data, are necessary. In future studies, we will collect and analyze more spatiotemporal field data using VMP-X to understand the dynamics of the bottom mixed layer and improve the accuracy of numerical models.