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

super(228)Ra, super(228)Th and super(234)Th in the upper 300m 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 super(228)Ra concentrations of about 210dpmm super(-3) in the surface mixed layer were observed. The concentration decreased considerably, from >200dpmm super(-3) at 25m to similar to 43dpmm super(-3) at 200m, then increased to 87dpmm super(-3) at 300m close to the bottom (460m), coincident with the typical distribution pattern of super(228)Ra in open oceans. Concentrations of dissolved and particulate super(228)Th ranged from similar to 45 to similar to 27dpmm super(-3) and from similar to 40 to similar to 10dpmm super(-3), respectively, and showed a general decrease with increasing depth. A super(234)Th deficit relative to super(238)U was also observed in the upper 150m, as quantified by beta -counting. The upward flux of nitrate into the euphotic zone was calculated by the coupled super(228)Ra-nitrate approach and further converted into a new production of 4.4mmol Cm super(-2)d super(-1) based on a Redfield ratio of 6.6 for C:N. The super(234)Th- super(238)U disequilibrium and the measured ratio of POC to particulate super(234)Th yielded a POC export flux of 5.7mmol Cm super(-2)d super(-1), which is in general agreement with the new production derived from nutrient budgets. POC export based on the super(228)Th- super(228)Ra disequilibrium was estimated to be 1.7mmol Cm super(-2)d super(-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.