New production based on Ra-228-derived nutrient budgets and thorium-estimated POC export at the intercalibration station in the South China Sea
- 海洋环境－已发表论文 
Ra-228, Th-228 and Th-234 in the upper 300 in 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 Ra-228 concentrations of about 210 dpm m(-3) in the surface mixed layer were observed. The concentration decreased considerably, from >200 dpm m(-3) at 25m to similar to 43 dpm m(-3) at 200m, then increased to 87 dpm m(-3) at 300m close to the bottom (460 m), coincident with the typical distribution pattern of Ra-228 in open oceans. Concentrations of dissolved and particulate Th-228 ranged from similar to 45 to similar to 27 dpm m(-3) and from similar to 40 to similar to 10 dpm m(-3), respectively, and showed a general decrease with increasing depth. A Th-234 deficit relative to U-238 was also observed in the upper 150 m, as quantified by beta -counting. The upward flux of nitrate into the euphotic zone was calculated by the coupled Ra-228-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 Th-234-U-238 disequilibrium and the measured ratio of POC to particulate Th-234 yielded a POC export flux of 5.7 minol C m(-2) d(-1), which is in general agreement with the new production derived from nutrient budgets. POC export based on the Th-228-Ra-228 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. (C) 2001 Elsevier Science Ltd. All rights reserved.