Particulate organic carbon export flux by 234Th/238U disequilibrium in the continental slope of the East China Sea

Abstract

234Th is widely used to quantify the magnitude of upper ocean particulate organic carbon (POC) export in oceans. In the present work, the rates of particulate organic carbon export were measured based on the distribution patterns of 234Th/238U disequilibrium in the water column within the continental slope of the East China Sea (ECS) during May 2011. The profiles of particulate and dissolved 234Th activities at all three stations showed a relative deficit with respect to 238U in the upper 100m of the water column. The dissolved 234Th scavenging rates and the particulate 234Th removal rates and their residence times were calculated by a one-dimensional steady state model. The results showed that the dissolved 234Th scavenging rates and the particulate 234Th removal rates ranged from 12.4–61.4 dpm/(m3·d) and from 3.8–21.8 dpm/(m3·d), respectively. The residence times of dissolved and particulate 234Th were in the range of 3.4–158 d and 63.7–96.5 d, respectively. Combined with the measurement of POC/234Th ratios of suspended particles, POC export flux (calculated by carbon) from the euphotic zone was estimated in the study region, which ranged from 4.14–14.7 mmol/(m2·d),with an average of 8.21 mmol/(m2·d), occupying 35% of the prime productivity in the study area. The results of this study can provide new information for better understanding the carbon biogeochemical cycle within the continental slope of the ECS.