Abstract:

In order to further study the influence of marine dynamics on the bottom boundary layer (BLL) of the northern slope, South China Sea, the Abyssal Bottom Boundary Layer Observation System (ABBLOS) was developed for in situ observations. The ABBLOS—a carrier platform with a maximum operating water depth of 6700 m (actual depth limits depend on the carried equipment)—is an important technological innovation for studying the deep-sea BBL. The platform is compose of upper and lower frames. The upper frame is used to carry and recover observation equipment, and the lower supporting frame is a counterweight and used to provide an observational space 1 meter from the seabed. A simple and effective “slot positioning with bolt fastening” connection was designed to connect the two frames to ensure a successful upper frame recovery. The ABBLOS integrates 75k-ADCP, high-frequency ADCP, ADV, high-precision pressure gauge, as well as sensors for methane, temperature, salinity, turbidity, dissolved oxygen, and ORP. For the first time, the ABBLOS achieves simultaneous observation of marine dynamic processes (such as internal waves and midscale vortices) and dynamic changes of physical and chemical parameters in the deep-sea BBL. In particular, the water flow velocity profile at 1 meter height above the seafloor can be observed with a 7 mm vertical layer resolution. The platform was successfully deployed and recovered in 2020 in the Shenhu sea area, northern slope of the South China Sea, at water depths of 655 and 1405 m. The total observation time was 34 days, and the seawater velocity profile structure was obtained. Also captured were a once-a-day (on average) internal wave action process and a set of physicochemical parameters for the BBL. After a preliminary analysis of the observation data at 655 m water depth, it was found that the temperature, pressure, dissolved oxygen, density and salinity in the BBL were controlled by the tidal process; especially, the change of temperature and pressure was in synch with tide. During a tidal cycle, the oxidative environment of the BBL was relatively stable, whilst the concentration of dissolved methane decreased with time but kept within the global background value range. Compared to the tidal process, internal waves had little impact on the abyssal BBL, but could obviously cause sediment resuspension. The turbidity of the BBL caused by internal waves increased from 0.01 to 48 NTU and meanwhile the submarine cameras also recorded turbid seawater in the BBL during the internal wave propagation, indicating the internal waves in the South China Sea can affect the transport of submarine sediment.

Key words: northern slope of South China Sea, bottom boundary layer, internal wave, in-situ observation