Abstract

We deployed a turbulent microstructure profiler, Vertical Microstructure Profiler-eXpendable (VMP-X) near a cluster of seafloor massive sulfide deposits to directly observe deep-sea turbulence. Level of near-bed turbulence (~10-9 to 10-7 W kg-1) roughly agreed with that indirectly estimated from Acoustic Doppler Current Profiler (ADCP) moored on the seabed (~1430 m). The result suggests the potential use of acoustic velocimeters for observing deep-sea turbulence.

Introduction

Turbulence is a chaotic flow resulted from advection due to tides and winds (Thorpe 2005). Sub-meter turbulent flows stretch and roll up portions of seawater, promoting diffusion of heat, salt, as well as any dissolved matter (Smyth and Moum 2019). Turbulence also homogenizes biotic and abiotic particles and colloids suspended in the seawater. Diffusion due to turbulent eddies largely modifies large-scale oceanic structures, forming the current condition of ocean (Ferrari and Wunsch 2009).

In-situ observation on marine turbulence have been started from the middle 20th century (Lueck et al. 2002). In those days, researchers used hot-film anemometers and cold-film thermometers installed on towed paravane to measure turbulent fluctuations of velocity and temperature*). Today those sensors are replaced with piezo-ceramic turbulent shear probes and FP07 fast-response thermistors (Lueck et al. 2002). The shear probe is composed of silicon rubber-covered piezo-ceramic beam, which generates an electrical charge in response to cross-axial forces caused by turbulent flows. Since micro-scale turbulent flow bends the beam, electric voltage digitally recorded reflects the fluctuation of turbulent velocity. The fast-response thermistor is composed of Fastip Probe (FP) 07 manufactured by General Electric Company (USA). The FP07 consists of small diameter (0.2 mm) glass-coated thermistor beads, by which micro-scale temperature fluctuation is measured with an ultra-fast response time of several milli-seconds. As turbulence modifies local temperature field in the ocean, microscale temperature fluctuation can be used as a proxy for turbulence. Shear probe and FP07 thermistor are typically installed on the head end of free-falling or free-rising microstructure profiler to detect turbulent fluctuations of velocity and temperature in the water column. Because of the measurement principle of the sensors, the microstructure profiler must move during measurements with travel speed of 0.2 to 0.8 m s-1 (depending on the type of sensors). Shear probe and FP07 thermistor, while there remain discussions on the uncertainties of those sensors (Gregg 1999), are defacto standard for measuring turbulence in the ocean (Lueck et al. 2002).

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