Export of particulate organic carbon estimated from234Th-238U disequilibria and its temporal variation in the South China Sea

Activities of234Th and nutrient concentrations in the upper 500 m water column were measured at a time-series station in the South China Sea over a time span of 12.3 d. Results showed a reduction of dissolved234Th and an overall increase of particulate234Th during the period. Meanwhile, activities of total234Th kept fairly constant, implying rapid transformation of234Th between the dissolved and particulate forms. Vertical profiles of total234Th showed evident deficit of234Th relative to238U in the upper 500 m water column. Using an irreversible steady-state model of thorium scavenging, export fluxes of particulate organic carbon (POC) corresponding to time pointsT 1 andT 2 were estimated to be 46.5 and 13.1 mmolC · m 2 · d 1. It was demonstrated that the estimation of POC export was greatly dependent on the POC/234ThP ratios and the bias caused by the different models of234Th scavenging, however, was considered to be of minor importance.