Transient Enhancement and Decoupling of Carbon and Opal Export in Cyclonic Eddies

Abstract

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 edges relative to the eddy cores, with integrated eddy POC and opal fluxes of 6.2 (2.1) mmol C m−2 day−1 and 1.5 (0.28) mmol Si m−2 day−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.