深海海底边界层原位观测系统研发与应用

为进一步研究南海北部陆坡海洋动力过程对深海海底边界层的影响,研发了“深海海底边界层原位观测系统(In-situ Observation System for Bottom Boundary Layer in Abyssal Sea)”,ABBLOS。观测系统主体为坐底式深海运载平台,最大工作水深可达6 700 m(实际工作水深取决于搭载设备的耐压水深),是研究深海海底边界层问题的重要技术创新。观测平台由上下两部分框架结构组成,上部框架用于搭载和回收观测设备,下部支撑架为配重,并且用于提供距离海底1 m的观测空间;同时创新性地设计了“卡槽定位-螺栓紧固”的连接方式连接上下两部分,连接方式简单可靠,保证了平台回收成功率。ABBLOS集成了75 k-ADCP、高频ADCP、ADV、高精度压力计、海底摄像机等设备,以及甲烷、温盐深、浊度、溶解氧、氧化还原电位等传感器,首次实现了内波、中尺度涡等海洋动力过程与海底边界层物理化学参数的动态变化同步观测,特别是可以观测距离海底1 m高度范围的水体流速剖面,并且达到7 mm一层的垂向空间分辨率。研制完成后,2020年在南海北部陆坡神狐海域655 m和1 405 m水深处分别成功布放并回收,观测时间共计34天,采集到观测站位上覆海水的流速剖面结构,捕捉到了平均周期为1天1次的内波作用过程,以及海底边界层的多种物理化学参数。初步分析655 m水深处的观测数据后,发现深海海底边界层的温度、压力、溶解氧、密度和盐度等参数受控于海洋潮汐过程,尤其是温度和压力的变化基本与潮汐周期同步。海底边界层氧化环境较为稳定,甲烷浓度由高变低,但是基本在海洋溶解甲烷平均浓度范围内。与潮汐相比,内孤立波对深海海底边界层水体的影响程度较小,但是明显可以引起沉积物的再悬浮,引起的海底边界层的海水浊度从背景值的0.01 NTU增大到48 NTU,海底摄像机也记录到了内孤立波期间深海底层海水突然变浑浊的过程,说明南海内孤立波可以影响海底沉积物的输运。

R&D and application of the Abyssal Bottom Boundary Layer Observation System (ABBLOS)

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.