Influence of a Decaying Cyclonic Eddy on Biogenic Silica and Particulate Organic Carbon in the Tropical South China Sea Based on .sup.234Th-.sup.238U Disequilibrium

Eddies play a critical role in regulating the biological pump by pumping new nutrients to the euphotic zone. However, the effects of cyclonic eddies on particle export are not well understood. Here, biogenic silica (BSi) and particulate organic carbon (POC) exports were examined inside and outside a decaying cyclonic eddy using .sup.234 Th-.sup.238 U disequilibria in the tropical South China Sea. For the eddy and outside stations, the average concentrations of BSi in the euphotic zone were 0.17±0.09 [mu]mol L.sup.-1 (mean±sd, n = 20) and 0.21±0.06 [mu]mol L.sup.-1 (n = 34). The POC concentrations were 1.42±0.56 [mu]mol L.sup.-1 (n = 34) and 1.30±0.46 [mu]mol L.sup.-1 (n = 51). Both BSi and POC abundances did not show change at the 95% confidence level. Based on the .sup.234 Th-.sup.238 U model, BSi export fluxes in the eddy averaged 0.18±0.15 mmol Si m.sup.-2 d.sup.-1, which was comparable with the 0.40±0.20 mmol Si m.sup.-2 d.sup.-1 outside the eddy. Similarly, the average POC export fluxes were 1.5±1.4 mmol C m.sup.-2 d.sup.-1 and 1.9±1.3 mmol C m.sup.-2 d.sup.-1 for the eddy and outside stations. From these results we concluded that cyclonic eddies in their decaying phase have little effect on the abundance and export of biogenic particles.