Abstract
Abstract. The time-series station, SEATS (18° N, 116° 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, 234Th, 210Pb, and 210Po in the upper 200 m of the water column. Radionuclide deficiencies resulted in removal fluxes from the euphotic layer of 1.1×103–1.8×103 dpm m−2d−1 and 7.1–40.2 dpm m−2d−1 for 234Th and 210Po, respectively. Due to atmospheric input, an excess of 210Pb relative to 226Ra is commonly observed in the upper water column. Sinking fluxes of total mass, POC, PIC, PN, 234Th, 210Pb, and 210Po 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 234Th, 210Pb, and 210Po to organic carbon, inorganic carbon, and nitrogen in the sinking particles were combined with the disequilibria of 234Th–238U, 210Pb–226Ra, and 210Po–210Pb 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−2d−1, can be reasonably estimated using 234Th, 210Pb, and 210Po as carbon proxies.
Highlights
It is known that carbon dioxide sequestration from the atmosphere in the ocean depends, in part, on the magnitude of carbon removal via particle settling from the surface to the deep layer of the ocean
The SEATS is an ecosystem dominated by picoplankton with a relatively low primary productivity ranging between 300 mg-C m−2 d−1 in summer to 550 mg-C m−2 d−1 in winter (Chen et al, 2004; Chen and Chen, 2005)
The line showing vertical distribution of 226Ra was estimated from SiO2 concentration by 226Ra = 5.15 + 0.14 SiO2, which is based on the data determined at the site 300 km to the south of the SEATS by Nozaki and Yamamoto (2001)
Summary
It is known that carbon dioxide sequestration from the atmosphere in the ocean depends, in part, on the magnitude of carbon removal via particle settling from the surface to the deep layer of the ocean. The quantification of export flux from the euphotic zone is of great importance to the understanding of carbon cycling in the ocean. The SEATS site, a time-series station regularly visited by Taiwanese oceanographers since 1999, is located at 18◦ N 116◦ E in the central basin of the South China Sea (Wong et al, 2007). Being in a marginal sea, the site shows open-ocean characteristics with low productivity. The South China Sea has a thin surface mixed layer of 30–80 m and a constant euphotic depth of 100 m, which make the diffusive flux of nutrients an important mechanism of supporting the primary production in this oligotrophic system (Tseng et al, 2005)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
