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Pacific Coastal and Marine Science Center

South China Sea Deep: In-situ observation of bottom flows and sediment dynamics in northeastern South China Sea

Background and Progress

Photo of the tripod at the Santa Cruz Wharf. Photo of the new FAT developed and designed in Santa Cruz, California by the Pacific Coastal and Marine Science Center.

Above: New USGS deepwater tripod, called the free-ascending tripod, or FAT. A, Sitting on the Santa Cruz Municipal Wharf in Santa Cruz, California, [larger version] and B, suspended from a crane before being lowered into water [larger version]. Some of the instruments mounted on the tripod have been labeled.

In 2010, the Natural Science Foundation of China (NSFC) funded the "South China Sea Deep" initiative, an interdisciplinary research project to study the tectonics, sedimentology and sediment transport, and biogeochemical cycling of the South China Sea. USGS Pacific Coastal and Marine Science Center (PCMSC) scientists were invited by colleagues in Tongji University (Shanghai, China) to collaborate on a proposal for studying the deep water sediment dynamics and sediment transport in the northeastern South China Sea. The proposal, "In-situ observation of bottom flows and sediment dynamics in northeastern South China Sea", has been funded for 4 years (2012-2015).

For the past 4 decades PCMSC has been at the forefront of sediment dynamics research and had the world-renowned expertise in designing and implementing field experiment for sediment dynamic studies from the continental shelves and slopes to submarine canyons. In this project, we apply this expertise to the northeastern South China Sea where bottom boundary layer studies in deep ocean have never been done before. A decade of fast economic growth has afforded China to substantially increase her investment in ocean science research. A successful collaboration in this pilot project can conceivably extend PCMSC's participation to international projects and strengthen its ocean research capability.

Through in-situ observation of sediment and oceanographic parameters by deep-water mooring and tripod deployments at several key locations, and water column profiling along the transects, as well as sediment and water sampling, the study tries to better understand the following processes that control the bottom circulations and sediment dynamics:

  1. the property and temporal and spatial distribution of bottom ocean currents in the study area;
  2. characterization and distribution of bottom nepheloid layer;
  3. sedimentation and resuspension in the bottom boundary layer;
  4. turbidity currents and their triggering mechanisms; and
  5. sources and transport pathways of bottom sediment that formed the subject high-sedimentation-rate deposit.

The results from this study, when combined with the analyses and interpretation of existing high-resolution sediment cores, are expected to help us understand the evolution of ocean bottom circulation since the last glacial maximum (LGM), and its control on the formation of the subject high-sedimentation-rate deposit.

Progress

The study area is located at the northeastern South China Sea, at 2000-3000 m water depth. In October 2013, the project deployed instruments attached to six deep-water moorings—lines with large flotation packages at the top and heavy anchors at the seabed—at multiple locations around the basin of the northeastern South China Sea. On April 19, 2014, FAT was deployed near site TJ-A-2 (see maps on main page). The moorings’ heavy anchors interfere with water and sediment movement near the seafloor, making FAT’s role particularly important: the tripod has a minimal effect on flow near the seabed and so can collect data about natural processes in the bottom boundary layer. The moorings and tripod were recovered in September 2014. Time-series data (measurements taken at regular intervals) from all the instruments will be analyzed by Tongji University scientists and USGS emeritus scientist Jingping Xu.

 

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Page Last Modified: 1 December 2014 (lzt)