Program     Carbon Cycle in the South China Sea: Budget, Controls and Global Implications       Poster Diagnosis of CO2 fluxes in global coastal oceans P-SPS3-02 Zhimian Cao* , State Key Laboratory of Marine Environmental Science, Xiamen University Wei Yang, State Key Laboratory of Marine Environmental Science, Xiamen University Yangyang Zhao, State Key Laboratory of Marine Environmental Science, Xiamen University Xianghui Guo, State Key Laboratory of Marine Environmental Science, Xiamen University Zhiqiang Yin, Chuanjun Du, State Key Laboratory of Marine Environmental Science, Xiamen University Huade Zhao, State Key Laboratory of Marine Environmental Science, Xiamen University Minhan Dai, Presenter Email: zmcao@xmu.edu.cn The bulk of global coastal oceans represents an important carbon sink, but due to high spatial-temporal variability within and between coastal systems, a mechanistic-based conceptualization of the coastal carbon cycle remains elusive, hindering its modelling and inclusion in earth system models. Here we show that the latitudinal distribution of global coastal surface pCO2 displays a strong mismatch to temperature, and its inter-seasonal changes are substantially regulated by water mass mixing and biological drawdown. These processes operate in both ocean-dominated and river-dominated margins, with carbon and nutrients sourced from ocean and land interfaces, respectively. These can be conceptualized by an analytical framework that assesses relative consumption between carbon and nutrients to determine how a coastal system is a carbon source or sink. The framework finds utility in identifying possible missing nutrients and testing hypotheses, and is therefore an essential first step toward comprehensive understanding and modelling of the global carbon cycle.