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226 Ra is commonly observed in the upper water column. Sinking fluxes of total mass, POC, PIC, PN, 234 Th, 210 Pb, and 210 Po measured at the euphotic depth were low in summer-fall and high in winter-spring, reflecting the seasonal variability of biological pumping. Excluding the suspiciously low primary productivity data point in July 2007, a relatively high e-ratio of 0.28-0.69 was estimated by the ratio of the POC flux at the euphotic depth and the integrated primary productivity. The ratios of 234 Th, 210 Pb, and 210 Po to organic carbon, inorganic carbon, and nitrogen in the sinking particles were combined with the disequilibria of 234 Th-238 U, 210 Pb-226 Ra, and 210 Pb to estimate export fluxes of POC, PIC, and PN from the euphotic layer. Compared with measured fluxes by the sediment trap and estimated fluxes by other approaches, it is concluded that the export production in the South China Sea, ranging from 1.8 to 21.3 mmol-C m −2 d −1 , can be reasonably estimated using 234 Th, 210 Pb, and 210 Po as carbon proxies.

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