What's at the bottom of the South China Sea?

December 3, 2014

A deepwater, free-ascending tripod (FAT) system designed and built at the U.S. Geological Survey (USGS) Pacific Coastal and Marine Science Center (PCMSC) in Santa Cruz, California, was used to acquire data from the northeastern South China Sea this year. It's the latest investigation in decades of research in the area, and one of the few non-proprietary studies of deep-sea sediment transport.

 
Red dots (TJ) mark sites where deepwater moorings and the FAT collected data. Green triangles designate ODP boreholes. Map from USGS.

Researchers are interested in deepwater circulation in the South China Sea, and how it controls sediment transport processes and deposition, but no one had ever directly measured deepwater circulation.

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 in the South China Sea (SCS).

Scientists at Tongji University in Shanghai invited colleagues at the PCMSC to collaborate, and the project, "In-situ observation of bottom flows and sediment dynamics in northeastern South China Sea," has been funded for 4 years (2012-2015). USGS participation is headed by Jingping Xu.

The team was looking at the layer of water just above the seafloor, called the bottom boundary layer, where seabed sediment and biological (pelagic) particles interact with the near-seabed currents.

 
The free-ascending tripod (FAT) being deployed in the South China Sea from the vessel Aquilla on 19 April 19 2014. Screenshot from video by Tongji University.

FAT tool

The free-ascending tripod was designed by George Tate, head of the Marine Facility (MarFac) at the PCMSC.

Dr. Jianru Li, assistant professor from the State Key Laboratory of Marine Geology, Tongji University, worked on  FAT development at PCMSC in 2012.

Tate and engineering technician Peter Harkins built the tripod at MarFac and tested the new system at the Santa Cruz Municipal Wharf in February 2013. Read more: Tripod test in Monterey Bay

The tripod is a stainless-steel frame about 2m (6.5ft) tall, with a triangular base about 4m on each side. It weighs about 900kg (~2000 pounds) in air and has a deployed buoyant weight of about 110kg. The tripod carries a suite of acoustic  and optical instruments to measure current velocity, temperature, and sediment concentration in the bottom boundary layer.

Tate and USGS marine geophysicist Joanne Ferreira traveled to China in April to familiarize collaborators with the equipment and taught them how to deploy the system.

USGS reported that the tripod was deployed in the South China Sea on 19 April 2014 and spent five months collecting data and images in remarkably deep water. The FAT was launched from the vessel Aquilla and descended to 1900m (6200ft) water depth.

Researchers focused on collecting data along the 2500m bathymetric contour in the South China Sea, at the foot of the slope. Three of the test sites were along the axis of a submarine canyon (Formosa Canyon), another was in the center of the basin. 

The tripod was recovered in late September 2014 and was able to freely rise to the surface after shedding its weighted footpads. Because of bouyant syntactic foam panels (orange on photo at right) it did not need to be pulled up with a line.

Instruments mounted on the tripod took bottom photographs and measured such variables as water temperature, current velocity, and suspended-sediment concentration. The cooperative project has provided scientists with a plethora of information to better understand how and where deep-seafloor sediment moves and accumulates.
Read more: Tripod brings data

HEBBLE program

South China Sea Deep researchers cited the High Energy Benthic Boundary Layer Experiment (HEBBLE) that took place in the mid-1970s, which also focused on sediment transport at the base of the continental rise.

The objective was to study the chemical, geological, physical, and biological processes occurring in the benthic boundary layer in the high-energy region of an undercurrent (although not in the South China Sea).

D.J. Collins of the Jet Propulsion Lab at the California Institute of Technology described the design of an autonomous Benthic Water Tunnel system for HEBBLE to examine in situ erosion and deposition.

 
Ocean Drilling Program (ODP) Leg 184, 1999.
BS = Bashi Strait, SCS = South China Sea

ODP Leg 184

In 1999, the JOIDES Resolution was used to explore hemipelagic (terrestrial + pelagic) sediments in the South China Sea (SCS) during Leg 184 of the Ocean Drilling Program. It began on 11 February and work ended on 10 April 1999, before the ship transit back to Hong Kong.

The shipboard scientific party investigated the paleooceanography, stratigraphy, and sedimentation through a series of core samples and downhole logs. Six sites were drilled in the basin and on the continental slope, and four were logged. They used a triple combo tool, a sonic tool (including Formation MicroScanner), and magnetic tool (with NMR).

Core sites in the northern SCS were close to the Pearl River Mouth basin

Researchers said that suspended matter from the East China Sea and the Pacific might be major sediment sources for the northern SCS. The highest particle fluxes occur during the winter monsoon, associated with high windspeed, rather than riverine transport, they said.

The continental slopes are blanketed with hemipelagic sediments and the deep-sea basin covered with abyssal clay.

"Modern sediments in the SCS consist mainly of terrigenous material, biogenic carbonate and opal, and a small portion of volcanic material. Clastic sediments are discharged mainly from the Mekong, Red, and Pearl Rivers."
Read more: Leg 184 summary

More than 150 wells had been drilled on the shelf before Leg 184 took place, and many more in the 15 years since then.

Related news:

UTEC Geomarine deploys soliton system in South China Sea, 14 October 2014

East China Sea studied, 16 September 2013



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