Influenced by land-ocean-atmosphere interactions, coastal ocean carbon cycling is an important component of the Earth's climate system. However, a mechanistic understanding of the coastal ocean carbon cycle remains limited, leading to an unanswered question of why some coastal systems are sources while others are sinks of atmospheric CO₂.

As the largest marginal sea of the North Pacific, the South China Sea (SCS) spans a wide range of latitudinal zones with distinct structures. The northern shelf, which receives land inputs from the Pearl River, can be characterized as a River-Dominated Margin (RiOMar) and a CO₂ sink to the atmosphere. The SCS basin, which exchanges with the Pacific, is identified as an Ocean-Dominated Margin (OceMar) and a CO₂ source.

Built upon the success of a five-year "973" project, CHOICE-C I on “Carbon cycling in the China Seas - budget, controls and ocean acidification”, CHOICE-C II is a five-year (January 2015 to August 2019) multi-PI interdisciplinary research project funded through National Basic Research Program of China (973 Program) by Ministry of Science and Technology. CHOICE-C II focuses on the northern SCS shelf (RiOMar) and the SCS basin (OceMar). Through an integrated study of the carbon cycling between field observations, remote sensing as well as numerical modeling in the SCS with a comparison strategy, CHOICE-C II aims to determine the source and sink terms of atmospheric CO₂ and their associated physical-biogeochemical controlling processes. What follows concentrates on the global implications and the future trends of carbon cycling in the SCS. Four subprojects are designed for CHOICE-C II: 1) Air-sea CO₂ flux and its biogeochemical controls in the SCS; 2) Primary productivity and carbon cycle in the SCS; 3) Recycling and export of organic carbon and its coupling with nitrogen and silicon in the SCS; 4) Carbon transport, simulations and future trends in the SCS.

Thus far, the project conducted three cruises in the SCS, one to the northern shelf in summer 2015 and two to the basin in summer and winter 2017. Three biogeochemical Argo floats were deployed to collect high resolution data to derive the distribution of Chla, particulate organic carbon and other related biogeochemical parameters.

Based on sea surface pCO₂ data collected from 36 cruises since 2000, the estimation of air-sea CO₂ fluxes in the SCS has been further improved, which is updated to be 13.1 ´ 10¹² g C yr^-1 as a CO₂ source to the atmosphere. A “mechanistic semi-analytic algorithm” (MeSAA) was applied to estimate sea surface pCO₂ in river-dominated coastal oceans using satellite data. Data synthesis further indicated that the input of exogenous dissolved inorganic carbon (DIC) and nutrients largely influences the temporal-spatial distribution pattern of CO₂ fluxes in the SCS.  The net DIC and dissolved inorganic nitrogen (DIN) flux transported through the Luzon strait was estimated to be 4295 Tg C yr^-1 and 13 Tg N yr^-1, respectively. Multi-scale carbonate system, nutrients and related biogeochemical cycles have been simulated through a high-resolution 3D circulation-biogeochemistry coupling model. The spatial variability of vertical DIC transport has been well examined showing the importance of mesoscale and submesoscale variations, which suggests that the horizontal divergence and/or convergence are the main controls. CHOICE-C II also examined the net diapycnal fluxes of DIC and nutrients to the euphotic zone. A higher DIC flux relative to phosphate than the Redfield stoichiometry infers DIC excess in the SCS. More results and progresses of CHOICE-C II will be presented at the 4th Xiamen Symposium on Marine Environmental Sciences (XMAS-IV).