234Th/238U disequilibrium and particulate organic carbon export in the northwestern South China Sea

²³⁴Th was utilized as a tracer of particulate organic carbon (POC) export in the northwestern South China Sea (SCS) on the basis of the data collected at four stations during a spring cruise. Depth profiles of dissolved and particulate ²³⁴Th activities were measured in the upper 60 m, showing a significant deficit relative to ²³⁸U over the investigated stations. A stratified structure of ²³⁴Th-²³⁸U disequilibrium was in general observed in the upper 60 m water column, indicating that the euphotic zone of the northwestern SCS in this reason can be divided into two layers, an upper and lower layer, with diffierent export rates. POC export fluxes were estimated from a one-dimensional steady state model of ²³⁴Th fluxes in combination with the measurement of POC/²³⁴Th ratios on suspended particles. The POC export in this region ranged from 8.2 to 20.0 mmol/(m²·d), with an average of 16.0 mmol/(m²·d), and was slightly higher than those previously reported in the southern SCS.