Particle-reactive radionuclides (234Th, 210Pb, 210Po) as tracers for the estimation of export production in the South China Sea

The time-series station, SEATS (18 degrees N, 116 degrees E) in the South China Sea was visited six times during October 2006-December 2008 to carry out seawater sampling and floating trap deployments for the determination of distributions and fluxes of POC, PIC, PN, Th-234, Pb-210, and Po-210 in the upper 200m of the water column. Radionuclide deficiencies resulted in removal fluxes from the euphotic layer of 1.1 x 10(3)-1.8 x 10(3) dpmm(-2) d(-1) and 7.1-40.2 dpmm(-2) d(-1) for Th-234 and Po-210, respectively. Due to atmospheric input, an excess of Pb-210 relative to Ra-226 is commonly observed in the upper water column. Sinking fluxes of total mass, POC, PIC, PN, Th-234, Pb-210, and Po-210 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 Th-234, Pb-210, and Po-210 to organic carbon, inorganic carbon, and nitrogen in the sinking particles were combined with the disequilibria of Th-234-U-238, Pb-210-Ra-226, and Po-210-Pb-210 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-Cm-2 d(-1), can be reasonably estimated using Th-234, Pb-210, and Po-210 as carbon proxies.