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

We present the carbon isotope (C-14 and C-13), 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 mu mol kg(-1), and they were lower at the surface and increased with depth. Conversely, the DOC concentrations ranged from 38 to 95 mu M, and they were higher on the surface and decreased rapidly in the upper 500-m water depth. The DIC Delta C-14 and DOC Delta C-14 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 Delta C-14 values were -330 lower than DIC Delta C-14, indicating that DOC has cycled for much longer than DIC in the SCS. The lower Delta C-14-DIC and Delta C-14-DOC values at depths shallower than 700 m were mainly influenced by intensified vertical mixing, which upwelled the deep water with low Delta C-14-DIC and Delta C-14-DOC values for thorough mixture with the upper layer water. Conversely, the small difference in the Delta C-14 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 Delta C-14 values and C-14 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.