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

We utilized Th-234, a naturally occurring radionuclide, to quantify the particulate organic carbon (POC) export rates in the northern South China Sea (SCS) based on data collected in July 2000 (summer), May 2001 (spring) and November 2002 (autumn). Th-234 deficit was enhanced with depth in the euphotic zone, reaching a subsurface maximum at the Chl-a maximum in most cases, as commonly observed in many oceanic regimes. Th-234 was in general in equilibrium with 238 U at a depth of similar to 100 m, the bottom of the euphotic zone. In this study the Th-234 deficit appeared to be less significant in November than in July and May. A surface excess of Th-234 relative to U-238 was found in the summer over the shelf of the northern SCS, most likely due to the accumulation of suspended particles entrapped by a salinity front. Comparison of the Th-234 fluxes from the upper 10 m water column between 2-D and traditional 1-D models revealed agreement within the errors of estimation, suggesting the applicability of the 1-D model to this particular shelf region. 1-D model-based Th-234 fluxes were converted to POC export rates using the ratios of bottle POC to Th-234. The values ranged from 5.3 to 26.6 mmol C m(-2) d(-1) and were slightly higher than those in the southern SCS and other oligotrophic areas. POC export overall showed larger values in spring and summer than in autumn, the seasonality of which was, however, not significant. The highest POC export rate (26.6 mmol C m(-2) d(-1)) appeared at the shelf break in spring (May), when Chl-a increased and the community structure changed from pico-phytoplankton (<2 mu m) dominated to nano-phytoplankton (2-20 mu m) and micro-phytoplankton (20-200 mu m) dominated.