Using 228Th/228Ra Disequilibrium to Constrain Particulate Organic Carbon Export From the Upper Twilight Zone of the Northern South China Sea

We utilized the Th-228/Ra-228 and Th-234/U-238 disequilibrium methods to estimate the export of particulate organic carbon (POC) from the upper twilight zone in the northern South China Sea during four cruises from August 2009 to May 2011, covering an entire seasonal cycle of spring, summer, autumn, and winter. A significant finding of this study is that Th-228/Ra-228 disequilibrium method is highly sensitive for tracing POC export in the twilight zone, as demonstrated by the comparison of the "Minimum Detection Limit" (MDL) of various methods. We thus further conclude that Th-228/Ra-228 disequilibrium is suitable for the research of the twilight zone by comparing the POC export fluxes derived from Th-228/Ra-228 and Ra-228-NO3- disequilibrium. The mesopelagic transfer efficiency (T-eff), defined as the ratio of POC flux at 500-100 m, varied from 8% to 130%. The T-eff derived from >53 mu m Th-234 (Th-228) was higher than that based on 1-53 mu m. Large particles were observed to sink rapidly and exit the twilight zone. The T-eff was lower at the sampling station influenced by the Kuroshio waters in winter and spring, where the exported organic matter was relatively labile and prone to remineralization in the twilight zone. Conversely, the T-eff was relatively higher at other stations, indicating that the exported organic matter was refractory and underwent comparatively less degradation in the twilight zone.