Radiocarbon in Dissolved Organic and Inorganic Carbon of the South China Sea

We present the carbon isotope (14C and 13C), dissolved inorganic carbon (DIC), and dissolved organic carbon (DOC) concentration measurements in the South China Sea (SCS) to reveal the different sources and cycling time scales of the two major carbon pools in the SCS. The DIC concentrations ranged from 1,776 to 2,328 μmol kg−1, and they were lower at the surface and increased with depth. Conversely, the DOC concentrations ranged from 38 to 95 μM, and they were higher on the surface and decreased rapidly in the upper 500‐m water depth. The DIC Δ14C and DOC Δ14C values varied from −227‰ to 68‰ and −557‰ to −258‰, respectively, and both decreased with depth until 1,500 m and then remained relatively constant. DOC Δ14C values were −330‰ lower than DIC Δ14C, indicating that DOC has cycled for much longer than DIC in the SCS. The lower Δ14C‐DIC and Δ14C‐DOC values at depths shallower than 700 m were mainly influenced by intensified vertical mixing, which upwelled the deep water with low Δ14C‐DIC and Δ14C‐DOC values for thorough mixture with the upper layer water. Conversely, the small difference in the Δ14C signature in deep water (>1,500 m) between the SCS and the North Pacific confirmed the rapid water exchange through the Luzon Strait and rapid water mixing in the SCS basin, which plays an important role in controlling carbon cycling in the deep SCS. Plain Language Summary Dissolved inorganic carbon (DIC) is the largest carbon pool in the ocean and is closely linked to dissolved organic carbon (DOC), which is the largest exchangeable organic carbon pool in the ocean. Both DIC and DOC play important roles in the global carbon cycle, but their sources, distribution, and cycling time are different and controlled by different processes in the ocean. Here we report radiocarbon and stable carbon isotope measurements of DIC and DOC collected in the South China Sea (SCS) to reveal the sources and cycling time scales of the two major carbon pools in the SCS. The Δ14C values and 14C ages indicate that DOC has cycled for much longer than DIC in the SCS. The rapid water exchange and mixing between the SCS and the Kuroshio Current in the Northwestern Pacific play important roles in controlling the distributions and cycling of DIC and DOC in the SCS. Key Points The 14C levels in DOC and DIC of the South China Sea are equal to the levels in the western and central North Pacific The South China Sea is largely influenced by the Kuroshio Current extrusion The distribution and c