This paper reviews the spatially and temporally varied air-sea CO₂ fluxes in the South China Sea (SCS), to show that its basin area is a weak source to the atmospheric CO₂, while its northern shelf is a CO₂ sink. On an annual average basis, the SCS emits carbon of (1.33±1.88)×10¹⁰ g. The northern shelf includes a River-dominated Ocean Margin (RiOMar) during the peak discharges, and an SCS basin as an Ocean-dominated Margin (OceMar). The OceMar is characterized by dynamic exchange with the open ocean via a two-dimensional or even three-dimensional process, i.e., the horizontal intrusion of the open ocean water and the subsequent vertical mixing and upwelling. Depending on the different ratios of the dissolved inorganic carbon (DIC) and nutrients from the source waters into the margins, the relative consumption or removal between the DIC and the nutrients, while being transported into the euphotic zones taken over by biogeochemical processes, determines the CO₂ fluxes. Thus, the excess DIC relative to the nutrients in the upper layer will lead to the CO₂ degassing. Similar diagnosis can also be made to the RiOMar systems with typical features of significant excess nutrients relative to the DIC. It is suggested that the framework of the carbon cycle revealed from the SCS has important implications in better understanding world's other coastal systems.

South China Sea  /  carbon fluxes  /  CO₂ source-sink nature  /  Ocean-dominated Margin  /  River-dominated Ocean Margin