Apparent enhancement of 234Th-based particle export associated with anticyclonic eddies

Abstract

It is increasingly recognized that mesoscale eddies play an important role in modulating the variability of ocean biogeochemistry. It is commonly believed that contrary to cyclonic eddies, anticyclonic eddies are characterized by downwelling at their core regime, which may suppress particle export. Here, by considering submesoscale domains we demonstrate that particle export might be alternatively enhanced in anticyclonic eddies on the basis of a study carried out in the oligotrophic northern South China Sea basin. We examined particle fluxes associated with three coherent anticyclonic eddies using the naturally occurring radionuclide 234Th. When applying a 1D steady-state model, 234Th and its derived particulate organic carbon (POC) fluxes in all three eddy cores were 1.9- and 1.6-fold higher, respectively, relative to those in the non-eddy region. However, an eddy-resolving circulation numerical model showed complex submesoscale circulations associated with the anticyclonic eddy. Notably, dynamic interactions occurred at submesoscales that might induce advection into the eddy core from the edge, where the 234Th deficit was elevated owing to higher particle production and export, probably stimulated by upwelling at the edges. We suggest therefore that enhanced particle fluxes derived from the 1D model along the vertical horizon at eddy cores only appeared to be changes, and that horizontal advection between the eddy core and edge should be taken into consideration in the flux estimation. Indeed, by integrating the 234Th deficit among multiple profiles in the entire anticyclonic eddy system, we derived an average 234Th flux of 938 dpmm−2d−1 at the 100-m horizon, equivalent to a POC flux of 3.69 mmolCm−2d−1. This export level was 1.6-fold higher than that from the reference sites.