A high‐resolution study of particle export in the southern South China Sea based on 234Th:238U disequilibrium

Abstract

During a spring intermonsoon cruise in 2004, depth profiles of total and particulate 234Th in the upper 100 m were collected at 36 stations in the southern South China Sea (SCS), covering a surface area of ∼1.0 × 106 km2. Thorium‐234 was sampled by using a modified small‐volume MnO2 co‐precipitation technique, which allows mapping the 234Th distribution with a high spatial resolution. A stratified structure of 234Th/238U disequilibria was generally observed in the upper 100 m water column, suggesting that the euphotic zone of the southern SCS in this season can be separated into two layers: an upper layer with low export production rates and a lower layer with high export production rates. At the same time, we observed extensive zones of 234Th excess within the euphotic layer, which is possibly due to intense remineralization of particulate matter. Particulate organic carbon (POC) export was estimated from a three‐dimensional steady state model of 234Th fluxes combined with measurements of the POC/234Th ratio on suspended particles. The POC export for this region varied from a low of −10.7 ± 1.5 mmolC m−2 d−1 to a high of 12.6 ± 1.1 mmolC m−2 d−1, with an average of 3.8 ± 4.0 mmolC m−2 d−1. A negative flux of POC export is interpreted as the result of lateral input of particulate matter from nearby waters. Regional patterns in POC export show enhanced fluxes along the western and southern boundaries of the study region, and a “tongue” of low export extending northwestward from ∼7°N 116°E to ∼10°N 111°E. This geographic distribution is consistent with the overall surface circulation pattern of the southern SCS in this season.