Transient enhancement and decoupling of carbon vs opal export in cyclonic eddies
- 海洋环境－科研数据 
Given their ephemeral nature, eddies have proven difficult to study, with contrary results from field observations that typically sample at the center during a specific stage of an eddy lifespan. Using the natural occurring radionuclide 234Th, we examined particle fluxes within two cyclonic eddies (CEs) at different evolutionary stages (mature stage eddy, C2, and a decay stage eddy, C1) in the oligotrophic western South China Sea (SCS). Using a 1-D steady state model, 234Th derived particulate organic carbon (POC) and opal fluxes at 100 m were determined. Both POC and opal fluxes were significantly higher at the eddy edge relative to the eddy cores, with integrated eddy POC and opal fluxes of 6.2 (2.1) mmol C m-2 d-1 and 1.5 (0.28) mmol Si m-2 d-1 in C2 (C1). When compared to non-eddy regimes, both POC (by 2.6-fold) and opal fluxes (by 7.5-fold) in C2 were enhanced; they were reduced and decoupled (0.9-fold and 1.4-fold) in C1. The difference in enhancement of particle fluxes and the coupling/decoupling between POC and opal flux likely reflects changes in phytoplankton community structures resulting from eddy evolution. Scaling these results to the entire SCS basin, suggests that CEs contribute <4% of the net POC flux, but >15% of the opal flux. Therefore, CEs may regulate the biogeochemical cycling of silica to a much greater extent than carbon in the ocean.