New production based on 228Ra-derived nutrient budgets and thorium-estimated POC export at the intercalibration station in the South China Sea

228Ra, 228Th and 234Th in the upper 300 m of the water column were measured to quantify new production and export flux of POC at the intercalibration station in the South China Sea during November 1997. Surprisingly high 228Ra concentrations of about 210 dpm m−3 in the surface mixed layer were observed. The concentration decreased considerably, from >200 dpm m−3 at 25 m to ∼43 dpm m−3 at 200 m, then increased to 87 dpm m−3 at 300 m close to the bottom (460 m), coincident with the typical distribution pattern of 228Ra in open oceans. Concentrations of dissolved and particulate 228Th ranged from ∼45 to ∼27 dpm m−3 and from ∼40 to ∼10 dpm m−3, respectively, and showed a general decrease with increasing depth. A 234Th deficit relative to 238U was also observed in the upper 150 m, as quantified by β-counting. The upward flux of nitrate into the euphotic zone was calculated by the coupled 228Ra-nitrate approach and further converted into a new production of 4.4 mmol C m−2 d−1 based on a Redfield ratio of 6.6 for C : N. The 234Th–238U disequilibrium and the measured ratio of POC to particulate 234Th yielded a POC export flux of 5.7 mmol C m−2 d−1, which is in general agreement with the new production derived from nutrient budgets. POC export based on the 228Th–228Ra disequilibrium was estimated to be 1.7 mmol C m−2 d−1, significantly lower than the derived new production. DOC transport or accumulation, data uncertainties and the different time scales to which the various methods were applied could be reasons for the discrepancy. The vertical fluxes of nitrate and phosphate to the euphotic zone bear a N/P molar ratio of 35, which is significantly higher than the Redfield ratio and suggests P-limitation in the southern South China Sea.