Evaluation of the Carbon Sink in the Coastal Sea

Principal Investigator: Tsung-Hung Peng, AOML
Collaborating scientist:
Richard Wanninkhof, AOML

Frank J. Millero, MAC/RSMAS, UM

Objective: To assess the magnitude of the potential carbon sink in the coastal zone. Based on the rich data bases collected since 1989 and the opportunity for taking samples for DIC and TA measurements, this project sets the first goal on the East China Sea.

Rationale: The continental shelf zone is the boundary area between land and ocean, and hence is affected heavily by the materials carried by the riverine run-off into the ocean. The nutrient load is one of the major anthropogenic materials that are transported into the coastal seas. The resulting enhanced biological activity tends to draw down the CO₂ dissolved in the coastal waters, and creates a potential sink for CO₂ in the atmosphere. The East China Sea (ECS) is located over a wide continental shelf, receiving tremendous amount of riverine run-off, sediments, and anthropogenic materials from a major river system in China, the Changjiang River. Investigating the carbon sink in ECS may shed some light on the role of coastal seas in global ocean carbon uptake.

Method: In addition to the discharge of nutrients to ECS from Changjiang, the other major nutrient source is the upwelling of nutrient rich subsurface water of Kuroshio warm current which flows northward along the continental slope to the east boundary of ECS. The Kuroshio Edge Exchange Processes (KEEP) program of Taiwan was launched in 1989 to study the physical, chemical, biological and geological processes taking place in the East China Sea as the Kuroshio warm current interacts with the ECS shelf water northeast of Taiwan. A recent KEEP geochemical survey cruise on board R/V Ocean Research I was carried out during the period May 2nd to May 15th, 1996. Water samples were taken at the selected stations in the survey network, and were shipped immediately after the cruise to Miami for determination of total dissolved inorganic carbon (DIC) at NOAA/AOML and for measurement of total alkalinity (TA) at MAC/RSMAS of the University of Miami. Additional measurements of TA for samples taken at stations mainly along the coast of mainland in this study area were carried out in Kaohsiung, Taiwan, at the Institute of Marine Geology and Chemistry of the National Sun Yat-Sen University.

Accomplishment: Results of DIC and TA measurements show that ECS is a CO₂ sink during the month of May. The mean difference of fCO₂ (fugacity of CO₂) between the atmosphere and surface water is 27 uatm , and the resulting net CO₂ invasion flux is 1.8 mol/m2/yr , which gives about 0.02 GtC/yr of CO₂ uptake in this continental shelf. If the global shelf area is taken to be 7.6% of the total ocean area, this observed fCO₂ deficiency would give an uptake of CO₂ on the order of 0.6 GtC/yr. This study supports the notion that the shelf regions could be a significant CO₂ sink. The riverine TA input to ECS is estimated to be about 1,769 umol/kg on the basis of a tight linear relationship between salinity-normalized TA and salinity. The observed TA of ECS is higher than that in the open sea, and this excess TA is estimated to be 42 umol/kg, which gives the total excess TA in the shelf of about 2x10¹² mol. With the Changjiang discharge of 28,900 m³ /sec (Perry et al., 1996), the mean residence time of the continental shelf water is thus estimated to be about 1.2 year. The DIC in the ECS is also found to be higher than that in the open sea. This excess DIC is estimated to be about 76 umol/kg, which is equal to a net carbon input to ECS of 3.3 mol/m2/yr. Based on the riverine TA input, the equivalent riverine carbon input from Changjiang discharge is estimated to be about 1.8 mol/m2/yr. This preliminary result implies that gas exchange and riverine carbon input are the main sources of excess carbon in the continental shelf.

Key reference:
Peng, T.-H., J.-J. Hung, R. Wanninkhof, and F. J. Millero, Evaluation of the carbon sink in the East China Sea, Tellus. (submitted).

Gong, G.-C., Y.-L. Lee Chen, and K.-K. Liu, 1996. Chemical hydrography and chlorophyll a distribution in the East China Sea in summer: implications in nutrient dynamics, Cont. Shelf Res. 16, 1561-1590.

Liu, K.-K., G.-C. Gong, S. Lin, C.-Z. Shyu, S.-C. Pai, C.-L. Wei and S.-Y. Chao, 1992. Response of Kuroshio upwelling to the onset of northeast monsoon in the sea north of Taiwan: observations and a numerical simulation, J. of Geophys. Res. 97, 12511-12526.

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