Vertical flux of particulate organic carbon in the central South China Sea estimated from 234Th-238U disequilibria

²³⁴Th-²³⁸U disequilibria were applied to examine the particle dynamics in the euphotic zone of the central South China Sea during the spring 2002 cruise. The particulate organic carbon (POC), ²³⁴Th (including both dissolved and particulate) and ²³⁸U in the water column at three stations were determined. The profiles of ²³⁴Th/²³⁸U activity ratio at the three stations all showed consistent ²³⁴Th deficit as compared to ²³⁸U in the upper 100 m water column. Based on the profiles of the dissolved and particulate ²³⁴Th and a steady state box model, the dissolved ²³⁴Th scavenging rates, the particulate ²³⁴Th removal rates and their resident times were quantified. It was found that the POC downward export fluxes out of the upper 100 m euphotic zone ranged from 9.40 to 14.78 mmol·m⁻²·d⁻¹. The results from this study provide new information for our understanding of carbon biogeochemical cycle in the South China Sea. Supported by the NSFC (key program No. 40531007) and general Programs (No. 40406014, 40276001 and 10705019), the State Social Commonweal Grant (No. 2004DIB5178), and the commonweal project sponsored by the Scientific Research Foundation of the Ministry of Science and Technology of China (No. 2007050228) and Chinese IPY Campaign found by MOF