Particle export within cyclonic Hawaiian lee eddies derived from Pb–Po disequilibrium

Particle export from the upper waters of the oligotrophic ocean may play a crucial role in the global carbon cycle. Mesoscale eddies have been hypothesized to inject new nutrients into oligotrophic surface waters, thereby increasing new production and particle export in otherwise nutrient deficient regimes. The E-Flux Program was a large multidisciplinary project designed to investigate the physical, biological and biogeochemical characteristics of cold-core cyclonic eddies that form in the lee of the Hawaiian Islands. There, we investigated particle dynamics using ²¹⁰Pb–²¹⁰Po disequilibrium. Seawater samples for ²¹⁰Pb and ²¹⁰Po were collected both within (IN) and outside (OUT) of two cyclones, Noah and Opal, at different stages of their evolution as well as from the eddy generation region. Particulate carbon (PC), particulate nitrogen (PN) and biogenic silica (bSiO₂) export fluxes were determined using water-column PC, PN, and bSiO₂ inventories and the residence times of ²¹⁰Po. PC and PN fluxes at 150 m ranged from 1.58±0.10 to 1.71±0.16 mmol C m⁻² d⁻¹ and 0.22±0.02 to 0.30±0.02 mmol N m⁻² d⁻¹ within Cyclones Opal and Noah. PC and PN fluxes at OUT stations sampled during both cruises were of similar magnitudes, 1.69±0.16 to 1.67±0.16 mmol C m⁻² d⁻¹ and 0.30±0.03 to 0.26±0.03 mmol N m⁻² d⁻¹. The bSiO₂ fluxes within Cyclone Opal were 0.157±0.010 mmol Si m⁻² d⁻¹ versus 0.025±0.002 mmol Si m⁻² d⁻¹ at OUT stations. These results of minimal PC and PN export, but significant eddy-induced bSiO₂ fluxes, agree very well with other studies that used a variety of direct and indirect methods. Thus, our results suggest that using elemental inventories and residence times of ²¹⁰Po is another independent and robust method for determining particle export and should be investigated more fully.